Note: Descriptions are shown in the official language in which they were submitted.
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A CONTROL SYSTEM FOR THE CONVERSION OF CARBONACEOUS
FEEDSTOCK INTO GAS
FIELD OF THE INVENTION
The present invention relates to control systems and, in particular, to a
control system for
the conversion of carbonaceous feedstock into gas.
BACKGROUND OF THE INVENTION
Gasification is a process that enables the conversion of carbonaceous
feedstock, such as
municipal solid waste (MSW) or coal, into a combustible gas. The gas can be
used to
generate electricity, steam or as a basic raw material to produce chemicals
and liquid
fuels.
Possible uses for the gas include: the combustion in a boiler for the
production of steam
for internal processing and/or other external purposes, or for the generation
of electricity
through a steam turbine; the combustion directly in a gas turbine or a gas
engine for the
production of electricity; fuel cells; the production of methanol and other
liquid fuels; as
a further feedstock for the production of chemicals such as plastics and
fertilizers; the
extraction of both hydrogen and carbon monoxide as discrete industrial fuel
gases; and
other industrial applications.
Generally, the gasification process consists of feeding carbonaceous feedstock
into a
heated chamber (the gasifier) along with a controlled and/or limited amount of
oxygen
and optionally steam. In contrast to incineration or combustion, which operate
with
excess oxygen to produce C02, H2O, SO,, and NOx, gasification processes
produce a raw
gas composition comprising CO, H2, H2S, and NH3. After clean-up, the primary
gasification products of interest are H2 and CO.
Useful feedstock can include any municipal waste, waste produced by industrial
activity
and biomedical waste, sewage, sludge, coal, heavy oils, petroleum coke, heavy
refinery
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residuals, refinery wastes, hydrocarbon contaminated soils, biomass, and
agricultural
wastes, tires, and other hazardous waste. Depending on the origin of the
feedstock, the
volatiles may include H2O, H2, N2, 02, C02, CO, CH4, H2S, NH3, C2H6,
unsaturated
hydrocarbons such as acetylenes, olefins, aromatics, tars, hydrocarbon liquids
(oils) and
char (carbon black and ash).
As the feedstock is heated, water is the first constituent to evolve. As the
temperature of
the dry feedstock increases, pyrolysis takes place. During pyrolysis the
feedstock is
thermally decomposed to release tars, phenols, and light volatile hydrocarbon
gases while
the feedstock is converted to char.
Char comprises the residual solids consisting of organic and inorganic
materials. After
pyrolysis, the char has a higher concentration of carbon than the dry
feedstock and may
serve as a source of activated carbon. In gasifiers operating at a high
temperature ( >
1,200 C) or in systems with a high temperature zone, inorganic mineral matter
is fused
or vitrified to form a molten glass-like substance called slag.
Since the slag is in a fused, vitrified state, it is usually found to be non-
hazardous and
may be disposed of in a landfill as a non-hazardous material, or sold as an
ore, road-bed,
or other construction material. It is becoming less desirable to dispose of
waste material
by incineration because of the extreme waste of fuel in the heating process
and the further
waste of disposing, as a residual waste, material that can be converted into a
useful
syngas and solid material.
The means of accomplishing a gasification process vary in many ways, but rely
on four
key engineering factors: the atmosphere (level of oxygen or air or steam
content) in the
gasifier; the design of the gasifier; the internal and external heating means;
and the
operating temperature for the process. Factors that affect the quality of the
product gas
include: feedstock composition, preparation and particle size; gasifier
heating rate;
residence time; the plant configuration including whether it employs a dry or
slurry feed
system, the feedstock-reactant flow geometry, the design of the dry ash or
slag mineral
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removal system; whether it uses a direct or indirect heat generation and
transfer method;
and the syngas cleanup system. Gasification is usually carried out at a
temperature in the
range of about 650 C to 1200 C, either under vacuum, at atmospheric pressure
or at
pressures up to about 100 atmospheres.
There are a number of systems that have been proposed for capturing heat
produced by
the gasification process and utilizing such heat to generate electricity,
generally known as
combined cycle systems.
The energy in the product gas coupled with substantial amounts of recoverable
sensible
heat produced by the process and throughout the gasification system can
generally
produce sufficient electricity to drive the process, thereby alleviating the
expense of local
electricity consumption. The amount of electrical power that is required to
gasify a ton of
a carbonaceous feedstock depends directly upon the chemical composition of the
feedstock.
If the gas generated in the gasification process comprises a wide variety of
volatiles, such
as the kind of gas that tends to be generated in a low temperature gasifier
with a "low
quality" carbonaceous feedstock, it is generally referred to as off-gas. If
the
characteristics of the feedstock and the conditions in the gasifier generate a
gas in which
CO and H2 are the predominant chemical species, the gas is referred to as
syngas. Some
gasification facilities employ technologies to convert the raw off-gas or the
raw syngas to
a more refined gas composition prior to cooling and cleaning through a gas
quality
conditioning system.
Utilizing plasma heating technology to gasify a material is a technology that
has been
used commercially for many years. Plasma is a high temperature luminous gas
that is at
least partially ionized, and is made up of gas atoms, gas ions, and electrons.
Plasma can
be produced with any gas in this manner. This gives excellent control over
chemical
reactions in the plasma as the gas might be neutral (for example, argon,
helium, neon),
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reductive (for example, hydrogen, methane, ammonia, carbon monoxide), or
oxidative
(for example, oxygen, carbon dioxide). In the bulk phase, a plasma is
electrically neutral.
Some gasification systems employ plasma heat to drive the gasification process
at a high
temperature and/or to refine the offgas/syngas by converting, reconstituting,
or reforming
longer chain volatiles and tars into smaller molecules with or without the
addition of
other inputs or reactants when gaseous molecules come into contact with the
plasma heat,
they will disassociate into their constituent atoms. Many of these atoms will
react with
other input molecules to form new molecules, while others may recombine with
themselves. As the temperature of the molecules in contact with the plasma
heat
decreases all atoms fully recombine. As input gases can be controlled
stoichiometrically,
output gases can be controlled to, for example, produce substantial levels of
carbon
monoxide and insubstantial levels of carbon dioxide.
The very high temperatures (3000 to 7000 C) achievable with plasma heating
enable a
high temperature gasification process where virtually any input feedstock
including waste
in as-received condition, including liquids, gases, and solids in any form or
combination
can be accommodated. The plasma technology can be positioned within a primary
gasification chamber to make all the reactions happen simultaneously (high
temperature
gasification), can be positioned within the system to make them happen
sequentially (low
temperature gasification with high temperature refinement), or some
combination thereof.
The gas produced during the gasification of carbonaceous feedstock is usually
very hot
but may contain small amounts of unwanted compounds and requires further
treatment to
convert it into a useable product. Once a carbonaceous material is converted
to a gaseous
state, undesirable substances such as metals, sulfur compounds and ash may be
removed
from the gas. For example, dry filtration systems and wet scrubbers are often
used to
remove particulate matter and acid gases from the gas produced during
gasification. A
number of gasification systems have been developed which include systems to
treat the
gas produced during the gasification process.
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These factors have been taken into account in the design of various different
systems
which are described, for example, in U.S. Patent Nos. 6,686,556, 6,630,113,
6,380,507;
6,215,678, 5,666,891, 5,798,497, 5,756,957, and U.S. Patent Application Nos.
2004/0251241, 2002/0144981. There are also a number of patents relating to
different
technologies for the gasification of coal for the production of synthesis
gases for use in
various applications, including U.S. patent Nos. 4,141,694; 4,181,504;
4,208,191;
4,410,336; 4,472,172; 4,606,799; 5,331,906; 5,486,269, and 6,200,430.
Prior systems and processes have not adequately addressed the problems that
must be
dealt with on a continuously changing basis. Some of these types of
gasification systems
describe means for adjusting the process of generating a useful gas from the
gasification
reaction. Accordingly, it would be a significant advancement in the art to
provide a
system that can efficiently gasify carbonaceous feedstock in a manner that
maximizes the
overall efficiency of the process, and/or the steps comprising the overall
process.
Therefore, there is a need for a control system for the conversion of
carbonaceous
feedstock into a gas that overcomes some of the drawbacks of known control
systems.
This background information is provided for the purpose of making known
information
believed by the applicant to be of possible relevance to the present
invention. No
admission is necessarily intended, nor should be construed, that any of the
preceding
information constitutes prior art against the present invention.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a control system for the
conversion of
carbonaceous feedstock into gas. In accordance with one aspect of the present
invention,
there is provided a control system for use in controlling a gasification
process for
converting a carbonaceous feedstock into a gas suitable for use in a selected
downstream
application, the system comprising: one or more sensing elements for sensing
one or
more characteristics of the gas; one or more computing platforms
communicatively
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linked to said one or more sensing elements for accessing a characteristic
value
representative of said sensed one or more characteristics; comparing said
characteristic
value with a predetermined range of such values defined to characterise the
gas as
suitable for the selected downstream application; and computing one or more
process
control parameters conducive to maintaining said characteristic value within
said
predetermined range; and a plurality of response elements operatively linked
to one or
more process devices operable to affect the process and thereby adjust said
one or more
characteristics of the gas, and communicatively linked to said one or more
computing
platforms for accessing said one or more computed process control parameters
and
operating said one or more process devices in accordance therewith.
In accordance with another aspect of the present invention, there is provided
a method for
controlling the conversion of carbonaceous feedstock into a gas suitable for
use in a
selected downstream application, the method comprising: providing a converter
for
converting the feedstock into a gas, said converter comprising a feedstock
input, one or
more additive inputs and one or more heat sources and an output; sensing one
or more
characteristics of the gas downstream from said output and comparing a value
representative thereof with a predetermined range of such values defined to
characterise
the gas as suitable for the selected downstream application; computing one or
more
process control parameters conducive to maintaining said characteristic value
within said
predetermined range; and operating one or more of said feedstock input, said
one or more
additive inputs and said one or more heat sources in accordance therewith.
In accordance with another aspect of the present invention, there is provided
a method for
controlling the conversion of carbonaceous feedstock into a gas, the method
comprising
providing a converter for converting the feedstock into a gas, said converter
comprising a
feedstock input, one or more additive inputs and one or more heat sources and
an output;
sensing one or more of a gas composition, a gas flow and a gas pressure and
comparing
values representative thereof with a respective predetermined range of such
values; and
when one or more of said representative values deviates from said respective
predetermined range, adjusting an additive input rate via said one or more
additive inputs
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to provide a fast response to the deviation; and adjusting a feedstock input
rate via said
feedstock input to provide a longer term response to the deviation.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features of the invention will become more apparent in the
following
detailed description in which reference is made to the appended drawings.
Figure 1 is a schematic diagram depicting a control system for controlling a
gasification
process implemented by a system for the conversion of carbonaceous feedstocks
into gas,
in accordance with one embodiment of the present invention.
Figure 2 is a schematic diagram depicting a system for the conversion of
carbonaceous
feedstocks into gas, in accordance with one embodiment of the present
invention.
Figure 3 is a schematic diagram depicting a system for the conversion of
carbonaceous
feedstocks into gas in accordance with one embodiment of the present
invention.
Figure 4 is a schematic diagram depicting a system for the conversion of
carbonaceous
feedstocks into gas in accordance with one embodiment of the present
invention.
Figure 5 is a schematic diagram depicting a system for the conversion of
carbonaceous
feedstocks into gas in accordance with one embodiment of the present
invention.
Figure 6 is a schematic diagram depicting a system for the conversion of
carbonaceous
feedstocks into gas in accordance with one embodiment of the present
invention.
Figure 7 is a schematic diagram depicting various downstream applications for
products
provided by a system for the conversion of carbonaceous feedstock into gas, in
accordance with one embodiment of the present invention.
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Figure 8 is a schematic diagram depicting various downstream applications for
products
provided by a system for the conversion of carbonaceous feedstock into gas, in
accordance with one embodiment of the present invention.
Figure 9 is a schematic diagram depicting various downstream applications for
products
provided by a system for the conversion of carbonaceous feedstock into gas, in
accordance with one embodiment of the present invention.
Figure 10 is a schematic diagram depicting various downstream applications for
products
provided by a system for the conversion of carbonaceous feedstock into gas, in
accordance with one embodiment of the present invention.
Figure 11 is a flow diagram depicting the use of a control system to control a
gasification
process for converting a carbonaceous feedstock into gas, in accordance with
one
embodiment of the present invention.
Figure 12 is a schematic diagram of a computing platform, and exemplary
components
thereof, of a control system to control a gasification process for converting
a
carbonaceous feedstock into a gas, in accordance with one embodiment of the
present
invention.
Figure 13 is a schematic diagram of a centralized control system, in
accordance with one
embodiment of the present invention.
Figure 14 is a schematic diagram of an at least partially distributed control
system, in
accordance with one embodiment of the present invention.
Figure 15 is a schematic diagram depicting exemplary sensing and response
signals
respectively received from and transmitted to a gasification system by a
control system to
control one or more processes implemented therein, in accordance with one
embodiment
of the present invention.
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Figure 16 is a schematic diagram depicting exemplary sensing and response
access points
of the integrated system control system to various devices, modules and
subsystems of a
system for the conversion of carbonaceous feedstocks to a gas of a specified
composition,
along with various possible downstream applications, in accordance with
various
exemplary embodiments of the present invention.
Figure 17 is a schematic diagram depicting a control system for controlling
inputs to a
converter of a system for the conversion of carbonaceous feedstock into a gas,
in
accordance with one embodiment of the present invention.
Figure 18 is a schematic diagram depicting an exemplary control sequence
implemented
by the control system of Figure 14.
Figure 19 is a schematic diagram of a converter for converting a carbonaceous
feedstock
into a gas, in accordance with one embodiment of the present invention.
Figure 20 is a schematic diagram of a converter for converting a carbonaceous
feedstock
into a gas, in accordance with one embodiment of the present invention.
Figure 21 is a schematic diagram of a converter for converting a carbonaceous
feedstock
into a gas, in accordance with one embodiment of the present invention.
Figure 22 is a schematic diagram of a converter for converting a carbonaceous
feedstock
into a gas, in accordance with one embodiment of the present invention.
Figure 23 is a schematic diagram of a converter for converting a carbonaceous
feedstock
into a gas, in accordance with one embodiment of the present invention.
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Figure 24 is a schematic diagram depicting a heat recovery subsystem of a
gasification
process for converting carbonaceous feedstock into a gas, in accordance with
one
embodiment of the present invention.
Figure 25 is a schematic diagram depicting a heat recovery subsystem of a
gasification
process for converting carbonaceous feedstock into a gas, in accordance with
one
embodiment of the present invention.
Figure 26 is a schematic diagram depicting a heat recovery subsystem of a
gasification
process for converting carbonaceous feedstock into a gas, in accordance with
one
embodiment of the present invention.
Figure 27 is a flow diagram showing the different regions of a gasifier of an
exemplary
gasification system controlled by a control system, in accordance with an
exemplary
embodiment of the present invention.
Figure 28 is a representation of gasification processes occurring in Regions
1, 2 and 3 of
the gasifier of Figure 27.
Figure 29 is an overview process flow diagram of a low-temperature
gasification facility
incorporating an exemplary gas conditioning system according to one embodiment
of
the invention, integrated with downstream gas engines.
Figure 30 is a site layout for an entire gasification system, in accordance
with an
exemplary embodiment of the present invention.
Figure 31 is a diagrammatic representation of a layout of a storage building
for municipal
solid waste.
Figure 32 is a perspective view of one embodiment of a gasifier, detailing a
feedstock
input, gas outlet, residue outlet, carrier-ram enclosure and access ports.
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Figure 33 is a side perspective view of the gasifier illustrated in Figure 32
detailing air
boxes, residue can and dust collector.
Figure 34 is a central longitudinal cross-sectional view through the gasifier
illustrated in
Figures 32 and 33, detailing the feedstock input, gas outlet, residue outlet,
lateral transfer
means, thermocouples and access ports.
Figure 35 illustrates a blown up cross sectional view detailing the air boxes,
carrier-ram
fingers, residue extractor screw and edge of step C.
Figure 36 is a sectional view of the gasifier of Figures 32 and 33 detailing
the refractory.
Figure 37 details the air box assembly of Step A and B of the gasifier
illustrated in
Figures 32 to 36.
Figure 38 illustrates a cross sectional view of the Step C air box of the
gasifier illustrated
in Figures 32 to 36.
Figure 39 illustrates a cross sectional view of the gasifier of Figures 32 to
36 detailing an
air box.
Figure 40 details the dust seal of the multi-finger carrier-ram of the
gasifier illustrated in
Figures 32 to 36.
Figure 41 showing the dust removal system of one embodiment of the gasifier
illustrated
in Figures 32 to 36 detailing the dust pusher, dust can attachment, shutter,
operator
handle and chain mechanism.
Figure 42 details the carrier-ram enclosure of the gasifier illustrated in
Figures 32 to 36
detailing the carrier-ram structure.
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Figure 43 is an illustration detailing the level switch locations in one
embodiment of the
invention.
Figure 44 shows the setup of the gasifier, gas reformulating chamber and the
residue
conditioning chamber.
Figure 45 is a cross-sectional view of the setup of the gasifier, gas
reformulating chamber
and the residue conditioning chamber.
Figure 46 is a schematic of the gas reformulating chamber.
Figure 47 is a view of the inner wall of the reformulating chamber.
Figure 48 is a top-down view of the reformulating chamber showing the position
of the
torches, and the air and steam nozzles.
Figure 49 shows the arrangement of the swirl inlets around the reformulating
chamber.
Figure 50 shows the attachment of the plasma torches on the reformulating
chamber.
Figure 51A is a cross-sectional view of the reformulating chamber of Figure
46.
Figure 51B is a diagram illustrating the air-flow within a gasifier comprising
the gas
reformulating system of the invention including the reformulating chamber of
Figure 46.
Figure 51C illustrates the injection of air from the air inputs into the
reformulating
chamber of Figure 46 and its effect on air-flow within.
Figure 52 is a functional block diagram of the residue conditioning system.
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Figure 53 shows a view of the actual implementation of the residue
conditioning system
and its connections to the gasifier and the baghouse filter.
Figure 54 shows a cross-sectional view of the residue conditioning chamber.
Figure 55 shows another view of the residue conditioning chamber.
Figure 56 shows a view of the residue conditioning chamber and the quench tank
with the
conveyor used for the transfer of vitrified slag to the slag stockpile.
Figure 57 shows the entire residue conditioning system from another angle and
also
shows the support structure used for the residue conditioning chamber.
Figure 58 shows the arrangement of the residue gas conditioning system with
the residue
conditioning chamber.
Figure 59 depicts a process flow diagram of the entire system, and in
particular the gas
conditioning system (GCS).
Figure 60 depicts the setup of the gas conditioning system integrated with a
syngas
regulation system according to one embodiment of the present invention.
Figure 61 is a more detailed drawing of the heat exchanger and shows the
process air
blower used for the control of the air input to the heat exchanger.
Figure 62 depicts a dry injection system whereby carbon is held in a storage
hopper and
is fed into the syngas stream by rotating screw; the syngas stream pipe is
angled so that
carbon not entrained in the gas stream rolls into the baghouse.
Figure 63 presents an exemplary schematic diagram of the dry injection system
in
combination with the baghouse.
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Figure 64 presents an exemplary schematic diagram of the HCl scrubber and
associated
components.
Figure 65 shows a system for collecting and storing waste water from the gas
conditioning system.
Figure 66 depicts a process flow diagram of an H2S removal process using a
Thiopaq-
based bioreactor, in accordance with one embodiment of the invention.
Figure 67 is an illustration of a gas homogenization system, in accordance
with one
embodiment of the invention, where gas is delivered from a single source to a
single
homogenization chamber and then delivered to multiple engines, each engine
having its
own gas/liquid separator and heater.
Figure 68 is an illustration of a fixed-volume homogenization chamber, in
accordance
with an embodiment of the invention.
Figure 69 is a high-level schematic diagram of a gasification system and
control system
therefore.
Figure 70 is an alternative diagrammatic representation of the gasification
and control
systems of Figure 69.
Figure 71 is a flow diagram of a control scheme for controlling the
gasification system of
Figures 69 and 70.
Figure 72 is a flow diagram of an alternative control scheme for controlling
the
gasification system of Figures 69 and 70, wherein this system is further
adapted for using
process additive steam in a gasification process thereof.
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Figure 73 is a flow diagram of an alternative control scheme for controlling a
gasification
process, in accordance with a further exemplary embodiment of the present
invention.
Figure 74 is a flow diagram of an alternative control scheme for controlling a
gasification
process, in accordance with a further exemplary embodiment of the present
invention.
Figure 75 is a flow diagram of an alternative control scheme for controlling a
gasification
process, in accordance with a further exemplary embodiment of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
Unless defined otherwise, all technical and scientific terms used herein have
the same
meaning as commonly understood by one of ordinary skill in the art to which
this
invention belongs.
As used herein, the term "about" refers to a +/-10% variation from the nominal
value. It
is to be understood that such a variation is always included in any given
value provided
herein, whether or not it is specifically referred to.
As used herein, the term "carbonaceous feedstock" and "feedstock" can be any
carbonaceous material appropriate for gasifying in the present gasification
process, and
can include, but are not limited to, any waste materials, coal (including low
grade, high
sulfur coal not suitable for use in coal-fired power generators), petroleum
coke, heavy
oils, biomass, sewage sludge, sludge from pulp and paper mills and
agricultural wastes.
Waste materials suitable for gasification include both hazardous and non-
hazardous
wastes, such as municipal waste, wastes produced by industrial activity (paint
sludges,
off-spec paint products, spent sorbents), automobile fluff, used tires and
biomedical
wastes, any carbonaceous material inappropriate for recycling, including non-
recyclable
plastics, sewage sludge, coal, heavy oils, petroleum coke, heavy refinery
residuals,
refinery wastes, hydrocarbon contaminated solid waste and biomass,
agricultural wastes,
tires, hazardous waste, industrial waste and biomass. Examples of biomass
useful for
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gasification include, but are not limited to, waste or fresh wood, remains
from fruit,
vegetable and grain processing, paper mill residues, straw, grass, and manure.
As used herein, the term "sensing element" is defined to describe any element
of the
system configured to sense a characteristic of a process, a process device, a
process input
or process output, wherein such characteristic may be represented by a
characteristic
value useable in monitoring, regulating and/or controlling one or more local,
regional
and/or global processes of the system. Sensing elements considered within the
context of
a gasification system may include, but are not limited to, sensors, detectors,
monitors,
analyzers or any combination thereof for the sensing of process, fluid and/or
material
temperature, pressure, flow, composition and/or other such characteristics, as
well as
material position and/or disposition at any given point within the system and
any
operating characteristic of any process device used within the system. It will
be
appreciated by the person of ordinary skill in the art that the above examples
of sensing
elements, though each relevant within the context of a gasification system,
may not be
specifically relevant within the context of the present disclosure, and as
such, elements
identified herein as sensing elements should not be limited and/or
inappropriately
construed in light of these examples.
As used herein, the term "response element" is defined to describe any element
of the
system configured to respond to a sensed characteristic in order to operate a
process
device operatively associated therewith in accordance with one or more pre-
determined,
computed, fixed and/or adjustable control parameters, wherein the one or more
control
parameters are defined to provide a desired process result. Response elements
considered
within the context of a gasification system may include, but are not limited
to static, pre-
set and/or dynamically variable drivers, power sources, and any other element
configurable to impart an action, which may be mechanical, electrical,
magnetic,
pneumatic, hydraulic or a combination thereof, to a device based on one or
more control
parameters. Process devices considered within the context of a gasification
system, and to
which one or more response elements may be operatively coupled, may include,
but are
not limited to, material and/or feedstock input means, heat sources such as
plasma heat
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sources, additive input means, various gas blowers and/or other such gas
circulation
devices, various gas flow and/or pressure regulators, and other process
devices operable
to affect any local, regional and/or global process within a gasification
system. It will be
appreciated by the person of ordinary skill in the art that the above examples
of response
elements, though each relevant within the context of a gasification system,
may not be
specifically relevant within the context of the present disclosure, and as
such, elements
identified herein as response elements should not be limited and/or
inappropriately
construed in light of these examples.
As used herein, the term "real-time" is used to define any action that is
substantially
reflective of the present or current status of the system or process, or a
characteristic
thereof, to which the action relates. A real-time action may include, but is
not limited to,
a process, an iteration, a measurement, a computation, a response, a reaction,
an
acquisition of data, an operation of a device in response to acquired data,
and other such
actions implemented within the system or a given process implemented therein.
It will be
appreciated that a real-time action related to a relatively slow varying
process or
characteristic may be implemented within a time frame or period (e.g. second,
minute,
hour, etc.) that is much longer than another equally real-time action related
to a relatively
fast varying process or characteristic (e.g. 1ms, 10ms, 100ms, 1s).
As used herein the term "continuous" is used to define any action implemented
on a
regular basis or at a given rate or frequency. A continuous action may
include, but is not
limited to, a process, an iteration, a measurement, a computation, a response,
a reaction,
an acquisition of data via a sensing element, an operation of a device in
response to
acquired data, and other such actions implemented within the system or in
conjunction
with a given process implemented therein. It will be appreciated that a
continuous action
related to a relatively slow varying process or characteristic may be
implemented at a rate
or frequency (e.g. once/second, once/minute, once/hour, etc.) that is much
slower than
another equally continuous action related to a relatively fast varying process
or
characteristic (e.g. 1KHz, 100Hz, 10Hz, 1Hz).
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As used herein, the term "reactant material" can mean feedstock or partially
or fully
processed feedstock.
As used herein, the term "composition of the product gas," refers to the
entire
composition of chemical species within a gas. In practice, however, this term
will
generally be used to express the species and concentrations of the chemical
constituents
that are most relevant to the downstream applications. For example, the gas
composition
desirable for a gas turbine will generally be described in terms of the amount
of nitrogen,
carbon monoxide, carbon dioxide, water and/or hydrogen in the synthesis gas.
The
chemical composition may also be identified as lacking specific chemical
species, i.e.
species that would be undesirable to transfer to the downstream application,
such as a gas
being, `free of H2S." The chemical composition of syngas can vary widely,
depending on
the composition of the feedstock used to generate the syngas and the manner in
which the
gasification process, the gas cleanup and conditioning were carried out.
Depending on
the context, which will be apparent to one skilled in the art, the composition
of the gas
will or will not contemplate trace elements.
As used herein, the term, "characteristics of the gas," refers to the chemical
and/or
physical qualities of the gas, which may include, but are not limited to its
chemical
composition, temperature, pressure, rate of flow, color, odor, etc.
As used herein, the term "off-gas" includes volatile molecules generated by
the
gasification of carbonaceous feedstock that can include carbon monoxide,
carbon
dioxide, hydrogen, light hydrocarbons and contaminating particulate matter
such as soot
and carbon black.
As used herein, the term "syngas" is a gaseous product of the gasification
process
comprising predominately carbon monoxide, carbon dioxide and hydrogen. Syngas
can
be derived from off-gas, or can be generated directly from the gasification
process if the
conditions in the converter enable the formation of this gas composition.
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For the purposes of the present invention, the term syngas (or synthesis gas)
refers to the
product of a gasification process, and may include carbon monoxide, hydrogen,
and
carbon dioxide, in addition to other gaseous components such as methane and
water.
The present invention provides a control system for the conversion of
carbonaceous
feedstock into a gas. In particular, the control system is designed to be
configurable for
use in controlling one or more processes implemented in, and/or by, a
gasification
system, or one or more components thereof, for the conversion of such
feedstock into a
gas, which may be used for one or more downstream applications. With reference
to the
exemplary embodiment of Figure 1, which is provided as an example only and not
meant
to limit the general scope and nature of the following disclosure, the
gasification
processes controllable by different embodiments of the disclosed control
system may
include in various combinations, a converter 110, a residue conditioner 410, a
recuperator
and/or heat exchanger system 510, one or more gas conditioners 610, a gas
homogenization system 710 and one or more downstream applications. Examples of
these components and subsystems will be described in greater detail below with
reference
to Figures 1 to 10, which depict exemplary embodiments of gasification systems
that may
be controlled by the present control system.
The control system operatively controls various local, regional and/or global
processes
related to the overall gasification process, and thereby adjusts various
control parameters
thereof adapted to affect these processes for a selected result. Various
sensing elements
and response elements are therefore distributed throughout the controlled
system, or in
relation to one or more components thereof, and used to acquire various
process, reactant
and/or product characteristics, compare these characteristics to suitable
ranges of such
characteristics conducive to achieving the desired result, and respond by
implementing
changes in one or more of the ongoing processes via one or more controllable
process
devices.
In one embodiment, the control system is used for controlling a gasification
process for
converting a carbonaceous feedstock into a gas suitable for use in a selected
downstream
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application. In one example, the gasification process is controlled such that
the product
gas thereof may be used in a continuous manner and/or in real-time for
immediate use.
Accordingly, the control system may comprise, for example, one or more sensors
for
sensing one or more characteristics of the gas to be used in the downstream
application.
One or more computing platforms are communicatively linked to these sensing
elements
for accessing a characteristic value representative of the sensed
characteristic(s), and
configured to compare the characteristic value(s) with a predetermined range
of such
values defined to characterise the gas as suitable for the selected downstream
application
and compute one or more process control parameters conducive to maintaining
the
characteristic value with this predetermined range. A plurality of response
elements may
thus be operatively linked to one or more process devices operable to affect
the process
and thereby adjust the sensed characteristic of the gas, and communicatively
linked to the
computing platform(s) for accessing the computed process control parameter(s)
and
operating the process device(s) in accordance therewith.
For example, the control system may be configured to control the conversion of
a
carbonaceous feedstock into a gas having one or more characteristics
appropriate for
downstream application(s), wherein the product gas is intended for use in the
generation
of electricity through combustion in a gas turbine or use in a fuel cell
application. In such
applications, it is desirable to obtain products which can be most effectively
used as fuel
in the respective energy generators. Alternatively, if the product gas is for
use as a
feedstock in further chemical processes, the composition will be that most
useful for a
particular synthetic application.
In one embodiment, the control system provides a feedback, feedforward and/or
predictive control of process energetics to substantially maintain a reaction
set point,
thereby allowing the gasification processes to be carried out under optimum
reaction
conditions to produce a gas having a specified composition. For instance, the
overall
energetics of the conversion of feedstock to gas can be determined and
achieved using an
appropriately configured gasification system, wherein various process
characteristics may
be evaluated and controllably adjusted to influence the determination of the
net overall
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energetics. Such characteristics may include, but are not limited to, the
heating value
and/or composition of the feedstock, the characteristics of the product gas
(e.g. heating
value, temperature, pressure, flow, composition, carbon content, etc.), the
degree of
variation allowed for such characteristics, and the cost of the inputs versus
the value of
the outputs. Continuous and/or real-time adjustments to various control
parameters,
which may include, but are not limited to, heat source power, additive feed
rate(s) (e.g.
oxygen, steam, etc.), feedstock feed rate(s) (e.g. one or more distinct and/or
mixed feeds),
gas and/or system pressure/flow regulators (e.g. blowers, relief and/or
control valves,
flares, etc.), and the like, can be executed in a manner whereby the net
overall energetics
are assessed and optimized according to design specifications.
Alternatively, or in addition thereto, the control system may be configured to
monitor
operation of the various components of a gasification system for assuring
proper
operation, and optionally, for ensuring that the process(es) implemented
thereby are
within regulatory standards, when such standards apply.
In accordance with one embodiment, the control system may further be used in
monitoring and controlling the total energetic impact of a gasification
system. For
instance, a gasification system for the conversion of a feedstock may be
operated such
that an energetic impact thereof is reduced, or again minimized, for example,
by
optimising one or more of the processes implemented thereby, or again by
increasing the
recuperation of waste heat generated by these processes. Alternatively, or in
addition
thereto, the control system may be configured to adjust a composition and/or
other
characteristics (e.g. temperature, pressure, flow, etc.) of a product gas
generated via the
controlled process(es) such that such characteristics are not only suitable
for downstream
use, but also substantially optimised for efficient and/or optimal use. For
example, in an
embodiment where the product gas is used for driving a gas engine of a given
type for the
production of electricity, the characteristics of the product gas may be
adjusted such that
these characteristics are best matched to optimal input characteristics for
such engines.
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In one embodiment, the control system may be configured to adjust a
gasification process
such that limitations or performance guidelines with regards to reactant
and/or product
residence times in various components, or with respect to various processes of
the overall
gasification process are met and/or optimised for. For instance, in an
embodiment where
municipal waste is used as a feedstock, it may be considered important to
adjust the
gasification process of such waste to account for a maximum residence time of
the waste
in a pre-processing and/or storage phase. For example, the waste and/or other
feedstock
may be transported to the controlled system facility periodically or on an on-
going basis,
wherein processing of such feedstock must be controlled so to avoid an
overstocking
thereof (e.g. increased pre-processing residence time) while allowing for
continuous
operation (e.g. reduced or avoided down-times). In such an example, a
processing rate of
a given feedstock may be controlled so to substantially match a delivery rate
of such
feedstock, thereby allowing for a substantially constant residence time of the
delivered
feedstock in a storage or pre-processing stage (e.g. a number of hours, days,
weeks, etc.).
Similarly, the residence time of the feedstock within the converter of a
gasification
system may be controlled to allow for sufficient processing, without depleting
resources
and thereby unduly reducing and/or limiting downstream processes and/or
applications.
For example, a given converter configuration may allow for a relatively stable
residence
time for which suitable processing of the feedstock is achieved (e.g. minutes,
hours, etc.).
Downstream components of the converter may equally be controlled such that a
residence
time appropriate therefor is also substantially respected. For example,
streaming gas
through a heat-exchange system, conditioning system and/or homogenisation
system may
be best processed by such components for a given gas flow and/or residence
time.
Similarly, variations in the gas flow and/or residence time may be addressed
and
compensated for by controlling various elements of such system components.
The control system of the present invention can be used to effectively convert
a feedstock
of substantially inhomogeneous characteristics and/or composition to produce a
gas
having substantially stable characteristics conducive for downstream
application.
Therefore, depending on a particular configuration of a gasification system
controlled by
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the present control system, fluctuations in feedstock characteristics may be
attenuated via
continuous and/or real-time control of this system, for example reducing long
term
process variability by at least 4 times. In an alternative embodiment,
fluctuations in
feedstock characteristics may be attenuated via continuous and/or real-time
control of this
system to reduce long term process variability by about 4 times. In an
alternative
embodiment, fluctuations in feedstock characteristics may be attenuated via
continuous
and/or real-time control of this system to reduce long term process
variability by about
3.5 times. In an alternative embodiment, fluctuations in feedstock
characteristics may be
attenuated via continuous and/or real-time control of this system to reduce
long term
process variability by about 3 times. In an alternative embodiment,
fluctuations in
feedstock characteristics may be attenuated via continuous and/or real-time
control of this
system to reduce long term process variability by about 2.5 times. In an
alternative
embodiment, fluctuations in feedstock characteristics may be attenuated via
continuous
and/or real-time control of this system to reduce long term process
variability by about 2
times. In an alternative embodiment, fluctuations in feedstock characteristics
may be
attenuated via continuous and/or real-time control of this system to reduce
long term
process variability by about 1.5 times.
Also, depending on a particular configuration of a gasification system
controlled by the
present control system, fluctuations in feedstock characteristics may be
attenuated via
continuous and/or real-time control of this system for example reducing short
term
process variability by at least 2.5 times. In an alternative embodiment,
fluctuations in
feedstock characteristics may be attenuated via continuous and/or real-time
control of this
system to reduce short term process variability by about 2.5 times. In an
alternative
embodiment, fluctuations in feedstock characteristics may be attenuated via
continuous
and/or real-time control of this system to reduce short term process
variability by about 2
times. In an alternative embodiment, fluctuations in feedstock characteristics
may be
attenuated via continuous and/or real-time control of this system to reduce
short term
process variability by about 1.5 times.
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The person of skill in the art will understand that the gasification system
and control
system, in their various embodiments, may be used in a number of processing
systems
having numerous independent and/or combined downstream applications. The
control
system is further capable, in various embodiments, of simultaneously
controlling various
aspects of a process in a continuous and/or real time manner.
Control system architecture
Referring to Figures 13 and 14, the control system may comprise any type of
control
system architecture suitable for the application at hand. For example, the
control system
may comprise a substantially centralized control system (e.g. see Figure 13),
a distributed
control system (e.g. see Figure 14), or a combination thereof. A centralized
control
system will generally comprise a central controller configured to communicate
with
various local and/or remote sensing devices and response elements configured
to
respectively sense various characteristics relevant to the controlled process,
and respond
thereto via one or more controllable process devices adapted to directly or
indirectly
affect the controlled process. Using a centralized architecture, most
computations are
implemented centrally via a centralized processor or processors, such that
most of the
necessary hardware and/or software for implementing control of the process is
located in
a same location.
A distributed control system will generally comprise two or more distributed
controllers
which may each communicate with respective sensing and response elements for
monitoring local and/or regional characteristics, and respond thereto via
local and/or
regional process devices configured to affect a local process or sub-process.
Communication may also take place between distributed controllers via various
network
configurations, wherein a characteristic sensed via a first controller may be
communicated to a second controller for response thereat, wherein such distal
response
may have an impact on the characteristic sensed at the first location. For
example, a
characteristic of a downstream product gas may be sensed by a downstream
monitoring
device, and adjusted by adjusting a control parameter associated with the
converter that is
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controlled by an upstream controller. In a distributed architecture, control
hardware
and/or software is also distributed between controllers, wherein a same but
modularly
configured control scheme may be implemented on each controller, or various
cooperative modular control schemes may be implemented on respective
controllers.
Alternatively, the control system may be subdivided into separate yet
communicatively
linked local, regional and/or global control subsystems. Such an architecture
could allow
a given process, or series of interrelated processes to take place and be
controlled locally
with minimal interaction with other local control subsystems. A global master
control
system could then communicate with each respective local control subsystems to
direct
necessary adjustments to local processes for a global result.
The control system of the present invention may use any of the above
architectures, or
any other architecture commonly known in the art, which are considered to be
within the
general scope and nature of the present disclosure.
The control system comprises response elements for controlling the reaction
conditions
and to manage the chemistry and/or energetics of the conversion of the
carbonaceous
feedstock to the output gas. In addition, the control system can determine and
maintain
operating conditions to maintain ideal, optimal or not, gasification reaction
conditions.
The determination of ideal operating conditions depends on the overall
energetics of the
process, including factors such as the composition of the carbonaceous
feedstock and the
specified characteristics of the product gases. The composition of the
feedstock may
range from substantially homogeneous to completely inhomogeneous. When the
composition of the feedstock varies, then certain control parameters may
require
continuous adjustment, via response elements, to maintain the ideal operating
conditions.
The control system can comprise a number of response elements, each of which
can be
designed to perform a dedicated task, for example, control of the flow rate of
one of the
additives, control of the position or power output of one of the one or more
heat sources
of the gasification system, or control of the withdrawal of by-product. The
control system
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can further comprise a processing system, (e.g. see Figure 12). In one
embodiment, the
processing system can comprise a number of sub-processing systems.
With reference to Figure 12, the control system generally comprises one or
more central,
networked and/or distributed processors 812, one or more inputs 814 for
receiving
current sensed characteristics from the various sensing elements, and one or
more outputs
816 for communicating new or updated control parameters to the various
response
elements. The one or more computing platforms of the control system may also
comprise
one or more local and/or remote computer readable media 818 (e.g. ROM, RAM,
removable media, local and/or network access media, etc.) for storing therein
various
predetermined and/or readjusted control parameters, set or preferred system
and process
characteristic operating ranges, system monitoring and control software,
operational data,
and the like. Optionally, the computing platforms may also have access, either
directly or
via various data storage devices, to plasma gasification process simulation
data 820
and/or system parameter optimization and modeling means, an exemplary
embodiment of
which is described in US patent 6,817,388, which a person of ordinary skill in
the art will
appreciate is readily applicable in the present context. Also, the computing
platforms may
be equipped with one or more optional graphical user interface and input
peripherals 822
for providing managerial access to the control system (system upgrades,
maintenance,
modification, adaptation to new system modules and/or equipment, etc.), as
well as
various optional output peripherals (e.g. 824) for communicating data and
information
with external sources (e.g. modem, network connection, printer, etc.).
As illustrated in this Figure, the control system may be further enhanced by
interactively
performing various system and/or process calculations defined to reflect a
current
implementation of a given gasification system. Such calculations may be
derived from
various system and/or process models, wherein simulation of process and/or
system
characteristics and control parameters may be used in a predictive and/or
corrective
manner to control the system or subsystem so modeled. US patent 6,817,388
provides an
example of such a system model, which may be used in conjunction with the
control
system to define various operational parameters, and predicted results based
thereon, for
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use as starting points in implementing the various processes of system. In one
embodiment, these and other such models are used occasionally or regularly to
reevaluate
and/or update various system operating ranges and/or parameters of the system
on an
ongoing basis. In one embodiment, the NRC, PLASCO and/or PLASCO/HYSYS
simulation platforms are used and can consider as inputs, waste type, any
combination of
input chemical composition, thermo-chemical characteristics, moisture content,
feed rate,
process additive(s), etc. The model may also provide various optional
interactive process
optimizations to consider, for example, site and feedstock type specifics,
maximization of
energy recovery, minimization of emissions, minimization of capital and costs,
etc.
Ultimately, based on the selected model options, the model may then provide,
for
example, various operational characteristics, achievable throughputs, system
design
characteristics, product gas characteristics, emission levels, recoverable
energy,
recoverable byproducts and optimum low cost designs. Various exemplary
representations are provided in US patent 6,817,388 which are readily
applicable in the
present context, as would be apparent to a person skilled in the art.
The processing system and any one of the sub-processing systems can comprise
exclusively hardware or any combination of hardware and software. Any of the
sub-
processing systems can comprise any combination of none or more proportional
(P),
integral (I) or differential (D) controllers, for example, a P-controller, an
I-controller, a
PI-controller, a PD controller, a PID controller etc. It will be apparent to a
person skilled
in the art that the ideal choice of combinations of P, I, and D controllers
depends on the
dynamics and delay time of the part of the reaction process of the
gasification system and
the range of operating conditions that the combination is intended to control,
and the
dynamics and delay time of the combination controller.
Important aspects in the design of the combination controller can be short
transient
periods and little oscillation during transient times when adjusting a
respective control
variable or control parameter from an initial to a specified value. It will be
apparent to a
person skilled in the art that these combinations can be implemented in an
analog
hardwired form which can continuously monitor, via sensing elements, the value
of a
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characteristic and compare it with a specified value to influence a respective
control
element to make an adequate adjustment, via response elements, to reduce the
difference
between the observed and the specified value.
It will further be apparent to a person skilled in the art that the
combinations can be
implemented in a mixed digital hardware software environment. Relevant effects
of the
additionally discretionary sampling, data acquisition, and digital processing
are well
known to a person skilled in the art. P, I, D combination control can be
implemented in
feed forward and feedback control schemes.
In corrective, or feedback, control the value of a control parameter or
control variable,
monitored via an appropriate sensing element(s), is compared to a specified
value or
range. A control signal is determined based on the deviation between the two
values and
provided to a control element in order to reduce the deviation. For example,
when the
output gas exceeds a predetermined H2:CO ratio, a feedback control means can
determine
an appropriate adjustment to one of the input variables, such as increasing
the amount of
additive oxygen to return the H2:CO ratio to the specified value. The delay
time to affect
a change to a control parameter or control variable via an appropriate
response elements
is sometime called loop time. The loop time, for example, to adjust the power
of the
plasma heat source(s), the pressure in the system, the carbon-rich additive
input rate, or
the oxygen or steam flow rate, can amount to about 30 to about 60 seconds, for
example.
In one embodiment, the product gas composition is the specified value used for
comparison in the feedback control scheme described above, whereby fixed
values (or
ranges of values) of the amount of CO and H2 in the product gas are specified.
In another
embodiment, the specified value is a fixed value (or range of values) for the
product gas
heating value (e.g. low heating value (LHV)).
Feedback control can be used for any number of control variables and control
parameters
which require direct monitoring or where a model prediction is satisfactory.
There are a
number of control variables and control parameters of the gasification system
that lend
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themselves towards use in a feedback control scheme. Feedback schemes can be
effectively implemented in aspects of the control system for system and/or
process
characteristics which can be directly or indirectly sensed, and/or derived
from sensed
values, and controlled via responsive action using adjusted control parameters
for
operating one or more process devices adapted to affect these characteristics.
It will be appreciated that a conventional feedback or responsive control
system may
further be adapted to comprise an adaptive and/or predictive component,
wherein
response to a given condition may be tailored in accordance with modeled
and/or
previously monitored reactions to provide a reactive response to a sensed
characteristic
while limiting potential overshoots in compensatory action. For instance,
acquired and/or
historical data provided for a given system configuration may be used
cooperatively to
adjust a response to a system and/or process characteristic being sensed to be
within a
given range from an optimal value for which previous responses have been
monitored
and adjusted to provide a desired result. Such adaptive and/or predictive
control schemes
are well known in the art, and as such, are not considered to depart from the
general
scope and nature of the present disclosure.
Feed forward control processes input parameters to influence, without
monitoring,
control variables and control parameters. The gasification system can use feed
forward
control for a number of control parameter such as the amount of power which is
supplied
to one of the one or more plasma heat sources, for example. The power output
of the arcs
of the plasma heat sources can be controlled in a variety of different ways,
for example,
by pulse modulating the electrical current which is supplied to the torch to
maintain the
arc, varying the distance between the electrodes, limiting the torch current,
or affecting
the composition, orientation or position of the plasma.
The rate of supply of additives that can be provided to the converter in a
gaseous or liquid
modification or in a pulverized form or which can be sprayed or otherwise
injected via
nozzles, for example can be controlled with certain control elements in a feed
forward
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way. Effective control of an additive's temperature or pressure, however, may
require
monitoring and closed loop feedback control.
Fuzzy logic control as well as other types of control can equally be used in
feed forward
and feedback control schemes. These types of control can substantially deviate
from
classical P, I, D combination control in the ways the plasma reformulating
reaction
dynamics are modeled and simulated to predict how to change input variables or
input
parameters to affect a specified outcome. Fuzzy logic control usually only
requires a
vague or empirical description of the reaction dynamics (in general the system
dynamics)
or the operating conditions of the system. Aspects and implementation
considerations of
fuzzy logic and other types of control are well known to a person skilled in
the art.
Process Control Overview
As presented above, the control system comprises sensing elements for sensing
one or
more process and/or system characteristics (e.g. gas composition (%CO, %C02,
%H2,
etc.), gas temperature, gas flow rate, etc.) and generating a characteristic
value from the
sensed characteristics, as well as one or more computing platforms, for
collecting and
analyzing the value(s) produced from the sensing elements and outputting
appropriate
control parameters to one or more response elements configured for controlling
one or
more process devices in accordance with the output control parameters.
In one embodiment, the control system ensures that the gas flow and gas
composition
from the converter, and optionally throughout the gasification system, remains
within
predefined tolerances to result in the optimum production of the product gas
and of
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system byproducts (commercial slag, gas recovery, steam generation, etc.),
irrespective
of the composition of different types of feedstock or any natural variability
in sources of
the same type of feedstock. The control system can thus recognize and make
necessary
adjustments to compensate for such variability. The parameters of the product
gas, such
as temperature, flow rate and composition, are monitored, and the relevant
process device
control parameters varied (e.g. via appropriate response elements) to maintain
the
characteristics of the product gas within predetermined tolerances as defined
by the end
use of the synthesis gas.
In one embodiment, the control system of the present invention provides
corrective
feedback by which one or more of the flow rate, temperature and composition of
the
product gas are monitored and corrections made to one or more of the
carbonaceous
feedstock input rate, the oxygen input rate, the steam input rate, the carbon-
rich additive
input rate and the amount of power supplied to the plasma heat sources. The
adjustments
are based on measured changes in the flow rate, temperature and/or composition
of the
product gas in order to ensure that these remain within acceptable ranges. In
general, the
ranges for the flow rate, temperature and/or composition of the product gas
are selected
to optimize the gas for a particular downstream application.
In one embodiment, the control system of the present invention simultaneously
uses the
controllability of plasma heat to drive the gasification process, and to
ensure that the gas
flow and composition from the process remains within an acceptable range even
if the
composition of the feedstock exhibits natural variability. In another
embodiment, the
control system allows for the total amount of carbon processed per unit time
to be held as
constant as possible, and utilizes the plasma heat to ensure that the total
heat that enters
and leaves the converter per unit time is kept within process limits. In other
embodiments the control system can make adjustments such as adjusting air and
or steam
input in order to respond to, for example, flow/pressure fluctuations and/or
fluctuations in
product gas heating value(s). The control system may also be configured to
monitor
and/or regulate processes occurring via any one of the solid residue
conditioner, the
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converter gas conditioner, the heat exchanger and/or the homogenization
system, as
schematically illustrated in Figure 1, for example.
With reference to the exemplary embodiment of Figure 1, which is provided as
an
example only and not meant to limit the general scope and nature of the
present
disclosure, in general, the gasification process controlled by the present
invention
generally takes place in a converter 110 comprising one or more processing
zones and
one or more heat sources, which may include in some embodiments one or more
plasma
heat sources (as in plasma heat sources 112 of Figure 1). The converter 110
also
generally comprises one or more feedstock feed mechanisms and/or devices for
inputting
the feedstock, which may include a single feedstock (e.g. municipal solid
waste (MSW)
as in MSW feed input 114, high carbon feedstock (HCF) as in HCF feed input
116, coal,
plastics, liquid wastes, hazardous wastes, etc.), distinct feedstocks, and/or
a mixed
feedstock into the converter 110, as well as means, for adding one or more
process
additives, such as steam, oxidant, and/or carbon-rich material additives (the
latter of
which is optionally provided as a secondary feedstock). The gaseous products
exit the
converter 110 via one or more output gas outlets. As will be described further
below, the
converter 110 may comprise a single zone and/or chamber converter (e.g. see
Figures 19
to 22), or a multi-zone and/or chamber converter, for instance comprising a
gasifier and
reformer wherein gasification and reformulation processes are implemented
respectively
(e.g. see Figure 23). These and other converter configurations will be
described in greater
detail below with reference to Figures 19 to 23, which provide various
exemplary
embodiments of such converters, and Figures 32 to 51 of Example 1.
In one embodiment, the composition and flow of product gas from the converter
110 is
controlled within predefined tolerances by controlling the reaction
environment. The
temperature is controlled at atmospheric pressure to ensure that the feedstock
that is
injected into the converter 110 encounters as stable an environment as
possible. The
control system can provide means to control the amounts of feedstock, steam,
oxygen
and/or carbon-rich material that are fed into the converter 110. Operating
parameters
which may be adjusted to maintain a selected reaction set point or range may
include, but
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are not limited to, feedstock feed rate, additive feed rate, power to
induction blowers to
maintain a specified pressure, and power to and position of the plasma heat
sources (e.g.
plasma heat sources 112). These control aspects will be discussed further
having regard
to each parameter.
In one embodiment, the application of plasma heat (e.g. via a plasma heat
source such as
a plasma torch or the like), in conjunction with the input of additives, such
as steam
and/or oxygen and/or carbon-rich material, helps in controlling the gas
characteristics,
such as flow, temperature, pressure and composition. The gasification system
may also
utilize plasma heat to provide the high temperature heat required to gasify
the feedstock,
reformulate the off-gas produced thereby, and/or to melt the by-product ash
and convert it
to a glass-like product with commercial value.
The gasification process controlled by the present invention may further
comprise means
for managing and controlling processing of the solid by-product of the
gasification
process. In particular, a gasification system may include a solid residue
conditioner 410
for the conversion of the solid by-products, or residue, resulting from
feedstock-to-energy
conversion processes, into a vitrified, homogenous substance having low
leachability.
The solid by-products of the gasification process may take the form of char,
ash, slag, or
some combination thereof.
Illustratively the solid residue conditioner 410 comprises a solid residue
conditioning
chamber or region, a plasma heating means (e.g. plasma heat source 118) or
other such
heating means adapted to provide sufficiently high temperatures, a slag output
means,
and a controlling means (which may be operatively linked to the overall
control system of
the gasification system), whereby plasma heat is used to cause solids to melt,
blend and
chemically react forming a dense silicometallic vitreous material that, when
poured out of
the chamber or region, cools to a dense, non-leachable, silicometallic solid
slag. In
particular, the control system disclosed herein may be adapted to optimize
processes
implemented in the SRC, namely by controlling the plasma heat rate and solid
residue
input rate to promote full melting and homogenization.
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The gasification process controlled by the present invention may also comprise
means for
the recovery of heat from the hot product gas. Such heat recuperation may be
implemented by various heat exchangers, such as gas-to-gas heat exchangers
(e.g.
recuperator 510), whereby the hot product gas is used to heat air or other
oxidant, such as
oxygen or oxygen enriched air, which may then optionally be used to provide
heat to the
gasification process. The recovered heat may also be used in industrial
heating
applications, for example. Optionally, one or more steam generator heat
exchangers may
be controlled as part of the gasification process to generate steam which can,
for example,
be used as an additive in the gasification and/or reformulation reaction(s),
or to drive a
steam turbine to generate electricity, for example.
Also, as seen in Figures 24 and 25, the heat exchanger may also include
additional heat
exchangers operatively extracting heat from various other system components
and
processes, such as via a plasma heat source cooling process, a slag cooling
and handling
process, converter gas conditioner cooling processes, and the like. The
control system of
the present invention may also comprise a control subsystem for controlling a
heat
recovery system, which may be operatively coupled to the system's overall
control
system, to optimize the energy transfer throughout the gasification system
(e.g. see
Figures 15 and 16).
The gasification process controlled by the present invention may further
include a
converter gas conditioner -GCS - (e.g. see Figures 1 to 10, and Figures 29, 59
to 66 of
Example 1), or other such gas conditioning means, to condition the product gas
produced
by the gasification process for downstream use. For instance, the product gas
may be
directed to a converter gas conditioner (e.g. converter gas conditioner 610 of
Figure 1), as
can gas generated from processing of the residue in the residue converter
discussed
above, where it is subjected to a particular sequence of processing steps to
produce an
output gas suitable for downstream use. In one embodiment, the converter gas
conditioner comprises components that carry out processing steps that may
include, for
example, removal of particulate matter (e.g., via a baghouse, cyclone or the
like), acid
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gases (HC1, H2S), and/or heavy metals from the synthesis gas, or adjusting the
humidity
and temperature of the gas as it passes through the gasification system. The
presence and
sequence of processing steps is determined by the composition of the synthesis
gas and
the specified composition of output gas for downstream applications. The gas
conditioning system may also comprise a control system, which may be
operatively
linked to the overall control system, to optimize the converter gas
conditioner process
(e.g. see Figures 15 and 16).
The gasification process controlled by the present invention may further
comprise a gas
homogenization system (e.g. homogenization system 710 of Figure 1) for
providing at
least a first level homogenization of the product gas. For instance, by
subjecting the
product gas to a given residence time within the homogenization system,
various
characteristics of the gas may be at least partially homogenized to reduce
fluctuations of
such characteristics. For example, the chemical composition of the product
gas, as well as
other characteristics such as flow, pressure, and/or temperature may be at
least partially
stabilized by the homogenization system to meet downstream requirements.
Accordingly,
a homogenization system may be used to promote increased stability in gas
characteristics for downstream application(s), such as a gas turbine or
engine, a fuel cell
application, and the like.
In one embodiment, the homogenization system of a gasification system provides
a gas
homogenization chamber or the like having dimensions that are designed to
accommodate a gas residence time sufficient to attain a gas of a sufficiently
consistent
output composition, pressure, temperature and/or flow. In general,
characteristics of the
homogenization system will be designed in accordance with requirements of the
downstream application(s), and, with respect to a capacity of the control
system to
attenuate fluctuations in product gas characteristics when the control system
is designed
with such intentions.
With reference now to Figures 5 to 10, the person of skill in the art will
understand that
the present control system may be used to control a number of gasification
processes,
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which may be used in a number of energy generation and conversion systems
having
numerous independent and/or combined downstream applications. For instance, in
the
exemplary embodiment of Figure 5, an Integrated Gasification Combined Cycle
(IGCC)
system can be controlled and used to produce output energy (e.g. electricity)
by providing
both a syngas for use in one or more gas turbines, and steam, generated by
cooling both
the syngas and exhaust gas associated with the gas turbine via one or more
steam
generator heat exchangers, for use in one or more steam turbines.
In the exemplary embodiment of Figure 6, the control system can be used to
control a
gasification system which combines an Integrated Gasification Combined Cycle
(IGCC)
system with a solid oxide fuel cell system, the latter of which using a
hydrogen-rich
byproduct of the syngas to produce energy (e.g. electricity).
In the exemplary embodiment of Figure 7, the control system can be used to
control a
gasification system which combines an Integrated Gasification Combined Cycle
(IGCC)
system with molten carbonate fuel cell system, the latter of which, as in
Figure 6, using a
hydrogen-rich byproduct of the syngas to produce energy (e.g. electricity).
In the exemplary embodiment of Figure 8, the control system can be used to
control a
gasification system which combines a solid oxide fuel cell system, as in
Figure 6, with
one or more steam turbines activated by steam generated by one or more steam
generator
heat exchangers recuperating heat from the syngas and the fuel cell output(s).
In the exemplary embodiment of Figure 9, a water-gas shift converter is added
to the
embodiment of Figure 8 to provide the hydrogen-rich syngas used in the solid
oxide fuel
cell system.
In the exemplary embodiment of Figure 10, the solid oxide fuel cell system of
Figure 9 is
replaced by a molten carbonate fuel cell system.
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As will be apparent to the person of skill in the art, the above exemplary
embodiments of
gasification systems controllable by various embodiments of the control system
of the
present invention are not meant to be limiting, as one of skill in the art
will understand
that other such system configurations and combinations can be provided for
which the
disclosed control system may be adapted, without departing from the general
scope and
spirit of the present disclosure.
With reference to Figures 15 and 16, and as discussed above, the control
system 800 may
be integrated throughout a given gasification system 10 to monitor, via
sensing elements
202, various system process or product characteristics, and implement, via
response
elements 206, various modifications to control parameters to manage the
energetics and
maintain each aspect of the process within certain tolerances. These
parameters, which
will be discussed in greater detail below, may be derived from processes
associated with
one or more of the plasma converter 100, the solid residue conditioner 400,
the plasma
heat source(s) 150 and slag processing heat source(s) 450, the heat exchanger
(e.g. gas-
to-air heat exchanger 500 and/or steam generator heat exchanger 599) and
additive inputs
associated therewith, the primary and/or secondary feedstock inputs (e.g.
carbon-rich
additives (HCF)), the converter gas conditioner 600, the homogenization system
700, and
any other processing element or module of the gasification system.
Furthermore, having access to these parameters and access, via the various
local and/or
remote storage devices, to the control system's one or more computing
platforms, to a
number of predetermined and/or readjusted system parameters, system operating
ranges,
system monitoring and control software, operational data, and optionally
plasma
gasification process simulation data and/or system parameter optimization and
modeling
means, the control system may further interact with the gasification system in
order to
optimize system outputs.
The exemplary embodiment of Figure 1 is provided as an example only and not
meant to
limit the general scope and nature of the present disclosure.
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Composition of product gas
The control system of the present invention can be used to sense, compare and
maintain
within predetermined range(s), one or more characteristics of the product gas.
As
discussed previously, if the product gas is intended for use in the generation
of electricity,
then it is desirable to obtain products which can be used as fuel to power
energy
generators.
The main components of the output gas as it leaves a converter amenable for
use in a
gasification process controlled by the control system of the present invention
are
generally carbon monoxide, carbon dioxide, hydrogen, steam, and nitrogen
(lesser
amounts of nitrogen are present when using oxygen-enriched air, oxygen, etc.).
Much
smaller amounts of methane, acetylene and hydrogen sulfide may also be
present. The
proportion of carbon monoxide or carbon dioxide in the output gas depends on
the
amount of oxygen which is fed into the converter. For example, carbon monoxide
is
produced when the flow of oxygen is controlled so as to preclude the
stoichiometric
conversion of carbon to carbon dioxide, and the process is so operated to
produce mainly
carbon monoxide.
The composition of the product synthesis gas may be optimized for a specific
application
(e.g., gas turbines and/or fuel cell application for electricity generation)
by adjusting the
balance between, for example, applied plasma heat, oxygen and/or steam and/or
carbon-
rich additives. Since addition of oxidant and/or steam additives during the
gasification
process affects the conversion chemistry, it is desirable for the control
system of the
present invention to provide sensing elements, for monitoring the syngas
composition.
The inputs of the reactants are varied, e.g. via response elements, to
maintain the
characteristic values of the synthesis gas within predetermined ranges
suitable for the
selected downstream application(s).
With reference to the exemplary embodiment of Figure 1, which is provided as
an
example only and not meant to limit the general scope and nature of the
present
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disclosure, monitoring of the product gas can be achieved using various
sensing elements,
such as a gas composition sensing element (e.g. gas analyzer 801), a gas flow
sensing
element (e.g. flow sensing elements 802, 803 and 804), a gas pressure sensing
element
(e.g. pressure sensing elements 805, 806, 807 and 808), a gas temperature
sensing
element (e.g. temperature sensing elements 809, 810, and 811), and a gas
opacity sensing
element. The gas composition sensing element (e.g. gas analyzer 801) may be
used to
determine the hydrogen, carbon monoxide and/or carbon dioxide content of the
synthesis
gas, the value of which may be useable in various control steps, (e.g. see
exemplary
embodiments of Figures 18 and 71 to 75). Composition of the product gas is
generally
measured after the gas has been cooled and after it has undergone a
conditioning step to
remove particulate matter, although measurements may be taken at any point in
the
process.
The product gas can be sampled and analyzed using methods well known to the
skilled
technician. One method that can be used to determine the chemical composition
of the
product gas is through gas chromatography (GC) analysis. Sample points for
these
analyses can be located throughout the system. In one embodiment, the gas
composition
is measured using a Fourier Transform Infrared (FTIR) Analyser, which measures
the
infrared spectrum of the gas.
In one embodiment, the characteristics of the product gas, such as
temperature, flow rate
and composition, may be monitored via sensing elements located at the outlet
of the
converter. In another embodiment, sampling ports may also be installed at any
location in
the product gas handling system. As discussed previously, response elements
are
provided to vary the inputs of the reactants to maintain the characteristic
values of the
product gas within predetermined ranges suitable for the selected downstream
application(s).
An aspect of this invention may consist in determining whether too much or too
little
oxygen is being added during the gasification process by determining the
composition of
the outlet stream and adjusting the process accordingly. In one embodiment, an
analyzer,
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sensor or other such sensing elements in the carbon monoxide stream detects a
relevant
characteristic value such as the concentration of carbon dioxide or other
suitable
reference oxygen rich material.
It will be apparent that other techniques may be used to determine whether
mostly carbon
monoxide is being produced. In one alternative, the control system of the
present
invention may measure and analyze the ratio of carbon dioxide to carbon
monoxide. In
another alternative, the control system uses a sensor to determine the amount
of oxygen
and the amount of carbon downstream of the plasma generator, compares these
characteristic values to the predetermined ranges, computes one or more
process control
parameters conductive to maintaining the characteristic values within the
predetermined
range, and operates response elements in real-time to affect the process and
adjust the
characteristic values. In one embodiment, the values of CO and H2 are measured
and
compared to target values or ranges. In another embodiment, the product gas
heating
value (e.g. LHV) is measured and compared to target values or ranges, as
described
below.
The person of skill in the art will understand that these and other such
product gas
characteristic measurements, which may be carried throughout a given system
via the
above or other such sensing elements, may be used to monitor and adjust, via
response
elements, the ongoing process to maintain product gas characteristic values
within the
relevant suitable predetermined ranges, and should thus not be limited by the
examples
listed above and provided by the illustrative system and control system
configurations
depicted in the appended Figures.
Temperature at various locations in system
In one embodiment of the invention, there is provided means, as in sensing
elements, to
monitor the temperature (e.g. temperature sensing elements 809, 810, and 811)
at sites
located throughout the system, wherein such data are acquired on a continuous
or
intermittent basis. Sensing elements for monitoring the temperature in a
converter
amenable for use with the present control system, for example, may be located
on the
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outside wall of the converter, or inside the refractory at the top, middle and
bottom of the
converter.
Sensing elements for monitoring the temperature of the product gas may be
located at the
product gas exit, as well as at various locations throughout the product gas
conditioning
system (e.g. within a converter gas conditioner). A plurality of thermocouples
can be
used to monitor the temperature at critical points around the converter.
If a system for recovering the sensible heat produced by the gasification
process is
employed (such as a heat exchanger or similar technology), a sensing element
for
monitoring the temperature at points in the heat recovery system (for example,
at coolant
fluid inlets and outlets) may also be incorporated. In one embodiment, a gas-
to-air heat
exchanger, a steam generator heat exchanger or both are used to recover heat
from the hot
gases produced by the gasification process. In embodiments employing heat
exchangers,
the temperature transmitters are located to measure, for example, the
temperatures of the
product gas at the heat exchanger inlets and outlets. Temperature transmitters
may also
be provided to measure the temperature of the coolant after heating in the
heat exchanger.
These temperature measurements can be used by the control system to ensure
that the
temperature of the product gas as it enters a respective heat exchanger lies
within the
suitable operating temperatures or temperature ranges of that device. For
example, in one
embodiment, if the design temperature for a gas-to-air heat exchanger is 1050
C, a
temperature transmitter on the inlet gas stream to the heat exchanger can be
used to
control both coolant air flow rates through the system and plasma heat power
in order to
maintain the optimum product gas temperature. In addition, measurement of the
product
gas exit temperature may be useful to ensure that the optimum amount of
sensible heat
has been recovered from the product gas at all heat recovery stages.
A temperature transmitter installed on the air outlet stream to measure the
temperature of
the heated exchange-air ensures that the process is carried out under
conditions that
ensure the process air is heated to a temperature appropriate for use in the
gasification
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process. In one embodiment, the coolant air outlet temperature is, for
example, about
625 C, therefore a temperature transmitter installed on the air outlet stream
will provide
data that is used to determine whether adjustments to one or both of the air
flow rates
through the system and torch power in the plasma converter should be made in
order to
maintain the optimum product gas input temperature, which in turn can be used
to control
the temperature of the coolant air. It will be apparent to someone of skill in
the art that
temperature adjustments in any subsystem of the process will be determined not
only for
optimizing that particular subsystem, but also taking into account
requirements of the
downstream application(s). For instance, global requirements may be accounted
for when
controlling a particular local and/or regional process.
According to one embodiment wherein the gasification system used with the
control
system of the present invention comprises a steam generator heat exchanger,
the control
strategy sets a fixed set point for the optimum coolant air output
temperature, for
example, about 600 C, as well as a fixed value for the steam generator heat
exchanger gas
exit temperature, for example, about 235 C. Therefore, according to this
embodiment,
when the product gas flow is reduced, the product gas temperature at the exit
of the gas-
to-air heat exchanger gets cooler, resulting in decreased steam production
because the
steam generator heat exchanger gas exit temperature is also set to a fixed
value.
The same concept applies when the airflow through the system is reduced.
According to
one embodiment of the present invention, the exit coolant air temperature
remains fixed
therefore the exit product gas temperature for the gas-to-air heat exchanger
is hotter,
therefore producing more steam in the steam generator heat exchanger. However,
when
airflow through the system is reduced, product gas flow will consequently also
reduce, so
the increased inlet temperature to the steam generator heat exchanger will
only be
momentarily high. For example, if airflow is reduced to 50%, the maximum inlet
gas
temperature that the steam generator heat exchanger would momentarily see is
approximately 800 C, which is within the temperature limits of the heat
exchanger
design.
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In one embodiment of the invention, the sensing elements for monitoring the
temperature
is provided by thermocouples installed at locations in the system as required.
Such
temperature measurements can then be used, as described above, by the control
system.
The person skilled in the art will understand that other types of temperature
measurements carried throughout a given embodiment of the system, via the
above or
other such sensing elements, may be used to monitor and adjust, via response
elements,
the ongoing process to generate a product gas suitable for use in the selected
downstream
application(s), and optionally maximize process outputs and efficiencies, and
should thus
not be limited by the examples listed above and provided by the illustrative
system and
control means configurations depicted in the appended Figures.
Pressure of system
In one embodiment of the invention, there is provided sensing elements to
monitor the
pressure within the converter, as well as throughout the entire gasification
system
amenable for use with the present control system, wherein such data are
acquired on a
continuous or intermittent basis. In a further embodiment, these pressure
sensing
elements (e.g. pressure sensing elements 805, 806, 807 and 808 of Figure 1)
comprise
pressure sensors such as pressure transducers located, for example, on a
vertical converter
wall. Data relating to the pressure of the system is used by the control
system to
determine, on a real time basis, whether adjustments to parameters such as
plasma heat
source power or the rate of addition of feedstock or additives are required.
Variability in the amount of feedstock being gasified may lead to rapid
gasification,
resulting in significant changes in the pressure within the converter. For
example, if an
increased quantity of feedstock is introduced to the converter, it is likely
that the pressure
within the converter will increase sharply. It would be advantageous in such
an instance
to have sensing elements to monitor the pressure on a continuous basis,
thereby providing
the data required to make adjustments in real time, via response elements, to
process
control parameters (for example, the speed of the induction blower) to
decrease the
system pressure (as measured, for example, within the converter and at the
input to the
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recuperator). Another optional pressure sensing element (e.g. pressure sensing
element
807 of Figure 1) may be used with the solid residue conditioner and
operatively linked to
a control valve leading solid residue conditioner gas from the solid residue
conditioner to
the syngas conditioner. Another optional pressure sensing element (e.g.
pressure sensing
element 808 of Figure 1) may be provided with the homogenization system and
operatively linked to a control valve for release of syngas via the flare
stack, as well as
operatively linked to a control valve to increase an additive input flow to
the converter to
maintain continuous operation of the gas engine, for example. Also, flow
sensing
elements (e.g. flow sensing elements 802, 803 and 804 of Figure 1) may be used
throughout the system (for example to detect syngas flow to the homogenization
system),
for example, to regulate feedstock and additive input rates into the
converter.
In a further embodiment, a continuous readout of differential pressures
throughout the
complete system is provided, for example, via a number of pressure sensing
elements. In
this manner, the pressure drop across each individual component can be
monitored to
rapidly pinpoint developing problems during processing. The person of skill in
the art
will understand that the above and other such system pressure monitoring and
control
means can be used throughout the various embodiments of system via the above
or other
such sensing elements, to monitor and adjust, via response elements, the
ongoing process
to generate a product gas suitable for use in the selected downstream
application(s), and
optionally maximize process outputs and efficiencies, and should thus not be
limited by
the examples listed above and provided by the illustrative system and control
system
configurations depicted in the appended Figures.
Rate of gas flow
In one embodiment the control system comprises sensing elements (e.g. flow
sensing
elements 802, 803 and 804 of Figure 1) to monitor the rate of product gas flow
at sites
located throughout the system amenable for use with the present invention,
wherein such
data are acquired on a continuous or intermittent basis.
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The rate of gas flow through the different components of the system will
affect the
residence time of the gas in a particular component. If the flow rate of the
gas through
the reformulative region of the converter is too fast, there may not be enough
time for the
gaseous components to reach equilibrium, resulting in a non-optimum
gasification
process. Flow sensing elements may be used to detect syngas flow to the
homogenization
system/storage tank (e.g. flow sensing element 803 of Figure 1), for example,
to regulate
feedstock and additive input rates into the converter. The above and other
such gas flow
monitoring and control means located throughout the various embodiments of the
system
may be used, to monitor and adjust, via response elements, the ongoing process
to
generate a product gas suitable for use in the selected downstream
application(s), and
optionally maximize process outputs and efficiencies.
Process Converter
In general a converter amenable for use with the control system of the present
invention
may comprise one or more processing zones and/or chambers which form the
gasifier and
reformer. The gasifier and reformer may be within the same or distinct
chambers and/or
zones which may be of the same or different orientation. The converter may
also
comprise further process devices, such as feedstock input means for feedstock
comprising, for example, municipal solid waste (MSW), high carbon feedstock
HCF,
MSW and HCF together, or coal. Other further process devices may include means
for
adding one or more additives, including but not limited to steam, oxygen, air,
oxygen-
enriched air, oxidant, and carbon-rich additives (the latter of which may be
optionally
provided as a secondary feedstock), as required for maintaining one or more
characteristic values of the product gas within respective ranges suitable for
the selected
downstream application(s). The converter may also comprise one or more plasma
heat
sources and/or other heat sources coupled and optionally operatively
controlled by
response elements of the control system. The converter may also provide means
for gas
output and ash removal. The converter can be equipped with various sensing
means (i.e.
sensors) such as thermocouples, material height detectors, pressure sensors
and the like
for sensing various characteristics of the process. The control system of the
present
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invention can allow for control of various aspects of the converter processes,
including
but not limited to the input of feedstock, input of additives, plasma torch
power, waste
pile height and movement of waste through the converter.
The converter can have a wide range of length-to-diameter ratios and can be
oriented
either vertically or horizontally. The converter will have one or more gas
outlet means,
as well as means for removing solid residue (e.g., char, ash, slag or some
combination
thereof), which in some embodiments comprises an outlet disposed somewhere
along the
bottom of the chamber to enable the residue to be removed using gravity flow.
In one
embodiment, the converter will use physical transfer means to remove the solid
residue
from the bottom of the converter. For example, a hot screw may be used to
convey the
ash by-product into a solid residue conditioner. Means for processing and
handling slag
will be discussed in more detail below. Note that the slag may also be
processed in the
same chamber in which the gasification occurs (Figures 19 to 22), or in a
separate
chamber, as in solid residue conditioner of Figure 23.
In one embodiment of the present invention, the one or more sources of plasma
heat
assist in the feedstock-to-gas conversion process. In one embodiment the use
of one or
more plasma heat sources, in conjunction with the input of steam and/or oxygen
additives, helps in controlling the gas composition. Plasma heat may also be
used to
ensure the complete (or mostly complete) conversion of the offgas produced by
the
gasification process into the constituent elements, allowing reformulation of
these
constituent elements into the product gas having a specified composition (e.g.
in a
reformer distinct from or integrated within the gasifier). This reformulation
may take
place in the same zone or chamber as the gasification, or in a distinct zone
or chamber
within the converter, referred to herein as a reformer. The product gas may
then exit the
converter via one or more output gas outlets.
The gasification of carbonaceous feedstocks (i.e., the substantial conversion
of the
carbonaceous feedstocks to a syngas) takes place in the converter, and can
proceed at
high or low temperature, or at high or low pressure. A number of reactions
take place in
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the process of converting carbonaceous feedstocks to the syngas product. As
the
carbonaceous feedstock is gasified in the converter, the physical, chemical,
and thermal
processes required for the gasification may occur sequentially or
simultaneously,
depending on the converter design.
In the converter, the carbonaceous feedstock is subjected to heating, whereby
the
feedstock is dried to remove any residual moisture. As the temperature of the
dried
feedstock increases, pyrolysis takes place. During pyrolysis, volatile
components are
volatilized and the feedstock is thermally decomposed to release, for example,
tars,
phenols, and light volatile hydrocarbon gases while the feedstock is converted
to char.
Char comprises the residual solids consisting of organic and inorganic
materials.
The resulting char may be further heated to ensure complete conversion to its
gaseous
constituents, leaving an ash by-product that is later converted to slag. In
one
embodiment, the gasification of carbonaceous feedstocks takes place in the
presence of a
controlled amount of oxygen, optionally under the control of the control
system of the
present invention, to minimize the amount of combustion that can take place.
The combined products of the drying, volatilization and char-to-ash conversion
steps
provide an intermediate offgas product. This intermediate offgas may be
subjected to
further heating, typically by one or more plasma heat sources and in the
presence of a
controlled amount of additives such as air and steam, to further the
conversion of the
carbonaceous feedstocks to the syngas. This step is also referred to as a
reformulation
step and can take place within the same or a distinct chamber as the
gasification (e.g.
integrated or distinct gasifier/reformer).
The one or more plasma heat sources can be positioned to make all the
reactions happen
simultaneously, or can be positioned within the converter to make them happen
sequentially. In either configuration, the temperature of the pyrolysis
process is elevated
due to inclusion of plasma heat sources in the converter.
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The gasification reaction is driven by heat, which can be fueled by electrical
and/or fossil
fuel based (e.g. propane) heating means to heat the converter or by adding air
as a
reactant to drive the exothermic gasification reaction, which provides heat to
the reaction.
Some gasification processes also use indirect heating, avoiding combustion of
the feed
material in the converter and avoiding the dilution of the product gas with
nitrogen and
excess C02-
The design of the converter could allow for single-stage or multi-stage
conversion
processes. Various exemplary converter designs are provided in international
application
numbers WO/2006/128285 and WO/2006/128286 which are readily applicable in the
present context, as would be apparent to a person skilled in the art. In one
example, the
design of the converters is such that the process for converting the feedstock
to a syngas
may take place in a one-stage process, i.e., where the gasification (feedstock
to offgas)
and reformulation (offgas to syngas) steps both take place generally in a
single zone
within the system. In another example wherein the design of other converters
is such that
the feedstock to syngas conversion process takes place in more than one zone,
the process
occurs either in more than one zone within one chamber (e.g. the embodiments
of Figures
and 22 could be interpreted to represent multi-zone, single-chamber
converters), in
separate chambers (e.g. the embodiment of Figure 23) or some combination
thereof,
20 wherein the zones are in fluid communication with one another.
The converter optionally comprises one or more further process devices such as
additive
input means, which may be provided for the addition of gases such as oxygen,
air,
oxygen-enriched air, steam or other gas useful for the gasification process,
into the
converter. The additive input means may also provide means for the addition of
a carbon-
rich additive into the converter, which may also be provided via a secondary
feedstock
input means (e.g. Figures 19 to 23 define a process device comprising mixed
feedstock
input means which illustratively combines the primary feedstock input means
and
optional secondary feedstock input means). Thus, the additive input means can
include
air (or oxygen) input ports and/or steam input ports and/or carbon-rich
material input
ports, the latter of which is optionally provided via a secondary (or mixed)
feedstock
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option. These ports are positioned within the converter for the optimal
distribution of
additives throughout the converter. The addition of additives will be
discussed in greater
detail below.
The carbon-rich additive (or secondary feedstock) may be any material that is
a source of
carbon that can be added to the feedstock undergoing gasification to increase
the amount
of carbon available for the gasification process. Supplementing the feedstock
being
gasified with a carbon-rich material helps ensure the formation of a product
gas having a
specified composition. Examples of carbon-rich additives that can be used in
the
gasification process may include, but are not limited to, tires, plastics,
high-grade coal, or
a combination thereof.
With reference to the exemplary embodiment of Figure 23, the converter
depicted therein
comprises a horizontally oriented converter which is subdivided into three
gasification
zones which provide for the optimization of the extraction of gaseous
molecules from
carbonaceous feedstock by sequentially promoting, each in a respective zone,
drying,
volatilization and char-to-ash conversion (or carbon conversion). This is
accomplished by
allowing drying of the feedstock to occur at a certain temperature range (e.g.
300 to
900 C) in a first zone prior to moving the material to a second zone, where
volatilization
occurs at another temperature range (e.g. 400 to 950 C), prior to moving the
material to a
third zone where char-to-ash conversions (or carbon conversion) occurs at
another
temperature range (e.g. 600 to 1000 C). The main processes occurring at each
stage are
depicted generally in Figures 27 and 28 and described in greater detail in
Example 1
below.
The three zones are schematically represented in Figures 27 and 28, wherein
exemplary
reaction ratios are illustrated as progressing from a first zone where the
drying process is
most prominent over the volatilization and carbon conversion processes; a
second zone
wherein the volatilization process takes over; and a third zone where the
material is
practically completely dry, and the carbon conversion process takes over.
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The horizontal expansion of the gasification process allows for the
optimization of the
gasification process by regionally promoting one or more of the stages of the
gasification
process in response to the characteristics of the feedstock material at that
particular
location in the converter of Figure 23. It would be apparent to a worker
skilled in the art
that this converter could therefore be segregated into two, three, four or
more steps
depending on the characteristics of the feedstock used. The discussion below
describes
segregating the converter into three steps. The exemplary embodiment provided
by the
converter of Figures 23, however, is not technically restricted to three
steps.
In one embodiment, means are provided to move the material through the
converter in
order to facilitate specific stages of the gasification process (drying,
volatilization, char-
to-ash conversion). To enable control of the gasification process, means to
control the
material movement through the converter may also be provided. This movement of
material through the converter can be achieved via the use of one or more
material
transfer units. This is achieved with the material transfer means by varying
the
movement speed, the distance each material transfer means moves and the
sequence in
which the plurality of material transfer means are moved in relation to each
other. The
one or more material transfer means can act in a coordinated manner or
individual
material transfer means can act independently. In order to optimize control of
the material
flow rate, total residence time in the chamber and pile height, the individual
material
transfer means can be moved individually, at varying speeds, at varying
movement
distances, and at varying frequency of movement. The material transfer means
must be
able to effectively operate in the harsh conditions of the converter and in
particular must
be able to operate at high temperatures. The material transfer means can
include but are
not limited to augers, shelves, platforms, rams, and other such means readily
apparent to
the person of skill in the art.
Various exemplary material transfer means are provided in international
application
numbers WO/2006/128285 and WO/2006/128286 and readily applicable in the
present
context, as would be apparent to a person skilled in the art. For example, a
lateral transfer
means comprising lateral transfer units, motor means and actuators wherein the
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individual lateral transfer units comprise a moving element and a guiding
element. The
response element can operate transfer means via any means readily known in the
art, such
as motors, hydraulics and pneumatics.
In one embodiment, a material movement system is provided to move the MSW
along
the converter such that the appropriate processing occurs in the appropriate
stage of the
converter and spent residue is moved to the solid material outlet of the
converter. The
height of the pile at each stage may also be controlled, as is the total
residence time in the
converter. These functions are controlled by a system of carrier rams at the
floor of each
stage. Each carrier ram is capable of movement the full or partial length of
that stage, its
speed is also variable. This provides the capability of controlling material
pile height and
residence time; also the stage can be completely cleared if required. The
carrier rams
may be a single carrier ram or multiple fingers. Power for moving the carrier
rams can be
provided by electric motors which drive the carrier ram via a gearbox and
roller chain
system. The motors are controlled by the control system which can command
start and
stop position, speed of movement and frequency of movement. Each carrier ram
can be
controlled independently. In one embodiment, a roller chain is used. The
roller chain
provides high strength and tolerates a severe duty environment. In one
embodiment,
precision guides can be used to keep the carrier rams angularly aligned. In
another
embodiment, the use of two chains per carrier ram provides a means of keeping
the
carrier rams angularly aligned without the need for precision guides. To avoid
material on
top of the carrier ram being pulled back when the carrier ram is withdrawn,
the control
system can be programmed for a specific carrier ram movement sequence. For
example, a
sequence where the lowest carrier ram is extended first; the middle carrier
ram is then
extended which pushes material down onto the lowest carrier ram filling the
void created
by that carrier ram's movement; the lowest carrier ram is then retracted; the
upper carrier
ram is then extended filling the void at the back of the middle carrier ram;
the middle
carrier ram is then retracted; new material dropping from the feed port fills
any void on
the top carrier ram and the top carrier ram is retracted. All of these motions
can be
controlled automatically by the control system in response to system
instrumentation
data.
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In one embodiment temperature monitoring for the converter is achieved using
sensors
such as thermocouples. The temperature can be monitored at points along each
stage and
at various heights at each stage. Monitoring is achieved using thermocouples,
which tend
to need replacement during operation. In order to accomplish this without
shutting down
the process, each thermocouple can be inserted into the converter via a sealed
end tube
which is then sealed to the converter shell. This design allows the use of
flexible wire
thermocouples which are longer than the sealing tube so that the junction (the
temperature sensing point) of the thermocouple is pressed against the end of
the sealed
tube to assure accurate and quick response to temperature change. The sealed
tube can be
sealed to the converter and mechanically held in place by means of a
compression gland,
which can also accommodate protrusion adjustment into the converter. Where
temperature measurement is required into the material pile the sealed tube
could cause
holding back of the pile of MSW when movement is called for. To avoid this
problem
the end of the sealed tube can be fitted with a deflector which prevents MSW
from
getting blocked by the thermocouple tube.
The converter can be based on one of a number of standard converters known in
the art.
Examples of converters known in the art include, but are not limited to
entrained flow
converters, moving bed converters, fluidized bed converters, and rotary kiln
converters,
each of which is adapted to accept the feedstock(s) in the form of solids,
particulates,
slurry, liquids, gases or any combination thereof, through a feedstock input
means. The
feedstock(s) is introduced through one or more inlets, which are disposed to
provide
optimum exposure to heating for complete and efficient conversion of the
feedstock(s) to
the product gas.
In accordance with one embodiment of the present invention, the converter wall
is lined
with refractory material. The refractory material can be one, or a combination
of,
conventional refractory materials known in the art which are suitable for use
in a
converter for a high temperature (e.g., a temperature of about 1100 C to 1400
C) non-
pressurized reaction. Various exemplary converters, along with converter
compositions,
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configurations, etc., are described in detail in international application
numbers
WO/2006/128285 and WO/2006/128286 and readily applicable in the present
context, as
would be apparent to a worker skilled in the art.
The person of skill in the art will understand that by moving the one or more
plasma heat
sources, by adding other plasma heat sources, other sources of heat, and the
like, the
illustrated converters may be operated as single or multiple zone converters
without
departing from the general scope and nature of the present disclosure.
Furthermore, it will
be understood that the present control system may be implemented with any of
the above
or other such converter configurations. In fact, by monitoring one or more
direct or
indirect process characteristic relevant to the gasification and/or
reformulation processes
implemented within a given type of converter, whether these processes take
place in a
single zone or multiple zones within a single or multiple chambers, the
control system
may be used, via sensing elements, to monitor and adjust the ongoing processes
to
maximize, via response elements, process outputs and efficiencies. The control
system
may be implemented via direct control or via modular control wherein the
control system
comprises subsystems of control.
The person of skill in the art will further understand that, although the
above description
provides a number of exemplary converter types, configurations, and materials
to be used
therefor, other converter types, configurations and/or materials may be used
without
departing from the general scope and nature of the present disclosure.
Heating Means
The process for converting a carbonaceous feedstock into a product gas employs
one or
more plasma heating means which can be controlled by the control system of the
current
invention to ensure substantial conversion of the offgas to a product gas
suitable for use
in the selected downstream application(s). Plasma heating means may also be
optionally
provided to heat the carbonaceous feedstock to drive the initial gasification
process.
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In one embodiment, the one or more plasma heat sources will be positioned to
optimize
the offgas conversion to a suitable product gas. The position of the one or
more plasma
heat sources is selected according to the design of the gasification process,
for example,
according to whether the process employs a one stage or two stage gasification
process,
whether there are one or more converters, whether the reformer is integrated
or distinct,
or whether the converter is horizontally and/or vertically oriented.
A variety of commercially available plasma heat sources which can develop
suitably high
temperatures for sustained periods at the point of application can be utilized
in the
process. In general, such plasma heat sources are available in sizes from
about 100 kW to
over 6 MW in output power to produce temperatures, for example, in excess of
about 900
to about 1100 C as required for converting the offgas to the syngas product.
Examples may include inductively coupled plasma torches (ICP), and transferred
arc and
non-transferred arc torches (both AC and DC). Selection of an appropriate
plasma
heating means is within the ordinary skills of a worker in the art.
In one embodiment, the plasma heat sources are located adjacent to one or more
air/oxygen and/or steam input ports such that the air/oxygen and/or steam
additives are
injected into the path of the plasma discharge of the plasma heat source.
In a further embodiment, the plasma heat sources may be movable, fixed or any
combination thereof, and optionally, be operable by the control system of the
present
invention to adjust a position and/or orientation thereof.
In one embodiment, the gasification process uses the controllability of plasma
heat to
drive the conversion process and ensure that the gas flow and gas composition
from the
converter remain within predetermined ranges. Control of the plasma heat may
also assist
in the efficient production of the product gases, irrespective of the
composition of
different carbonaceous feedstock sources or any natural variability in sources
of the same
type of feedstock.
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In one embodiment, the control system of the present invention comprises
response
elements to adjust the power of the plasma heat sources to manage the net
overall
energetics of the reaction. In order to manage the energetics of the reaction,
the power to
the plasma heat source may be adjusted to maintain a constant gasification
system
temperature despite any fluctuations in the composition of the feedstock and
corresponding rates of feed of steam, air/oxidant and carbon-rich additives.
In one embodiment, the control system controls the power rating of the plasma
heat
source relative to parameters such as the rate at which the carbonaceous
feedstock and
additives are introduced into the converter, as well as the temperature of the
system as
determined by temperature sensing elements, and other such sensing elements,
located at
strategic locations throughout the system (e.g. temperature sensing elements
809, 810,
and 811 of Figure 1). The power rating of the plasma heat source must be
sufficient to
compensate, for example, for loss of heat in the converter and to process the
added
feedstock efficiently.
For example, when the temperature of the converter is too high, the control
system may
command a drop in the power rating of the plasma heat source (e.g. see Figures
18 and 71
to 75); conversely, when the temperature of the melt is too low, the control
system may
command an increase in the power rating of the plasma heat source.
In one embodiment of the invention, the control system comprises response
elements to
control the position of the torch to ensure the maintenance of the optimal
high
temperature processing zone as well as to induce advantageous gas flow
patterns around
the entire converter.
One or more plasma heat sources are also optionally provided to ensure
complete
processing of the solid residue of the gasification process, as will be
discussed later. In
some embodiments the converter comprises a distinct gasifier region where the
gasifying
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takes place in the absence of plasma heat, and a distinct reformer region
where plasma
heat is used for gas reformulation.
Feedstock input means
Referring to Figures 1 to 4, and 19 to 24, the gasification process includes
means, as in
input means, for introducing the carbonaceous feedstock (which may comprise,
for
example, coal, municipal waste and/or high carbon mixed feedstock) to the
converter,
optionally under the control of the control system of the present invention.
The high
carbon feedstock may optionally be input via secondary feedstock input means
or as an
additive via additive input means described below. The input means are located
to ensure
that the feedstock is deposited at an appropriate location in the converter
for optimum
exposure to the gasifying heat source.
In one embodiment, the control system comprises response elements to adjust
the rate of
feedstock input via process devices for maintaining a product gas that is
suitable for use
in the selected downstream application(s). For example, the rate of feedstock
addition to
the converter can be adjusted to facilitate the efficient conversion of the
feedstock into a
suitable product gas. The rate of feedstock addition is selected according to
the design
specifications of the gasification process, in order to maintain a
characteristic value
representative of a sensed characteristic of the product gas within the
predetermined
range.
In one embodiment, the control system adjusts the feed rate via process
devices such as
input means to ensure that the feedstock is fed into the converter at an
optimum rate for
maintaining the gasification reaction as desired for the selected downstream
application(s).
The selection of the input means is made according to the requirements for
feed
dispersion, the operating pressure and the feedstock particle size. Input
means may
include, for example, a screw auger, a pneumatic transport system, a plunger
system, a
ram system, a rotary valve system, or a top gravity feed system.
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In one embodiment, municipal waste can be used as a feedstock for the
gasification
process. Municipal waste may be provided in solid or liquid form. For the
gasification
of solid wastes, the waste may be introduced to the converter through a solid
waste inlet
feed port. The converter may also be designed to optionally include liquid
waste feed
inlet ports for the processing of liquid waste.
A conditioning process for preparing the feedstock prior to introduction to
the converter
may also be utilized. In one embodiment, the feedstock, depending on its
nature and to
increase efficiencies and achieve a suitable product gas, can be pretreated,
for example, to
reduce its volume overall or increase its surface area to volume ratio by
shredding,
pulverizing, shearing, etc. In another embodiment, the feedstock may also
undergo a pre-
drying step to remove any residual moisture as required.
For example, in some embodiments, a gasification system amenable for use with
the
present control system additionally comprises a municipal solid waste (MSW)
shredding
system. The MSW shredding system may comprise an input conveyor, a shredder,
and a
pick conveyor. Stop, start and speed of the conveyor may be controlled
remotely by the
control system to match process demands. Sensors may be provided in the trough
to alert
the control system if material is not present. The shredder may be equipped to
automatically stop when a jam is sensed, automatically reverse to clear the
jam and then
restart. In one embodiment, if a jam is still detected the shredder will shut-
down and send
a warning signal to the control system. Shredded waste may optionally be
dropped from
the shredder system into a feed hopper, which provides a buffer of material
ready to feed
into the converter. The hopper may be equipped with sensors such as high and
low level
indicators which can be used to control flow from the shredding system into
the hopper.
The conveyor is optionally under the control of the control system to match
waste feed
rate to meet process demands. In some embodiments the MSW feed conveyor may
have
an additional entry to accept high carbon feedstock (for example, shredded
plastic) which
enables quick response to process demands for higher or lower carbon input to
meet the
required gas quality while avoiding the need for a second input point to the
converter.
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In some embodiments, the gasification system amenable for use with the present
control
system additionally comprises a plastics handling system to prepare plastic as
a high
carbon feedstock and/or additive. Municipal recycling programs currently
result in a
large quantity of non-recyclable plastic material which has had to be sent to
landfill; this
material, for example, can meet the high carbon material requirement of the
gasification
process. Plastic and the like may optionally be shredded before input into the
converter.
The plastics system can be designed to provide storage for the as-received
plastic, shred
it, place it into a stockpile and feed it, optionally under independent
control, into the
converter. The system may comprise a storage facility, a shredder with input
hopper, a
take-away conveyor and a stockpile. Also, a feed conveyor may be used to
introduce the
shredded plastic into the converter. Level detectors can be located in the
hopper to
indicate hi and lo condition. Motion of this conveyor may be under the control
of the
control system. Control of the plastics handling system may be implemented via
direct
control or via modular control wherein the control system of the current
invention
comprises subsystems of control.
Additive input means
Referring to Figures 1 to 4, and 19 to 24, additives may optionally be added
to the
converter (e.g. via additive ports) to facilitate efficient conversion of the
carbonaceous
feedstock into a suitable product gas. The type and quantity of the additives
may be
carefully selected to optimize the carbonaceous feedstock conversion while
maintaining
adherence to regulatory authority emission limits and minimizing operating
costs. Steam
input may also be used to help promote sufficient free oxygen and hydrogen to
maximize
the conversion of decomposed elements of the input waste into fuel gas and/or
non-
hazardous compounds. Air/oxidant input may be used to assist in processing
chemistry
balancing to maximize carbon conversion to a fuel gas (minimize free carbon)
and to
maintain the optimum processing temperatures while minimizing the relatively
high cost
of plasma heat input. Carbon-rich additives, which may also be provided as an
additional
and/or complimentary feedstock, may also be added to supplement the carbon
content of
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the feedstock undergoing gasification. The quantity of each additive is
established and
controlled for the selected downstream application(s). In some embodiments,
the amount
of oxidant injection may be carefully established to ensure a maximum trade-
off for
relatively high cost plasma arc input heat while ensuring the overall process
does not
approach any of the undesirable process characteristics associated with
combustion, and
while meeting and bettering the emission standards of the local area.
For those embodiments having the production of electrical energy as an
objective, it is
advantageous to produce gases having a high fuel value (e.g. measured by the
gas high
heating value (HHV) and/or low heating value (LHV)). The production of high
quality
fuel gases can be achieved by controlling reaction conditions, for example, by
controlling
the amount of additives that are added at various steps in the conversion
process.
The converter, therefore, can include a plurality of additive input ports,
which may be
provided for the addition of gases such as oxygen, air, oxygen-enriched air,
steam or
other gas useful for the gasification process. The additive input means can
include air
input ports and steam input ports. These ports may be positioned within the
converter for
the optimal distribution of additives through the converter. The steam input
ports can be
strategically located to direct steam into the high temperature processing
zone and into
the product gas mass prior to its exit from the converter. The air/oxidant
input ports can
be strategically located in and around the converter to enhance coverage of
additives into
the processing zone.
The additive input ports may also include input ports for the addition of
carbon-rich
materials, which may also be added via additional and/or complimentary
feedstock input
means. Feedstocks useful for the gasification process of the present invention
can
conceivably be any carbonaceous materials, and as such, may be inherently
highly
variable in their carbon content. In one embodiment of the invention, the
system
provides a means, as in a dedicated carbon-rich additive port, for the
addition of a
carbon-rich feedstock to supplement the carbon content of the feedstock
undergoing
gasification. The carbon-rich material may optionally be added by premixing
with the
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feedstock before addition to the converter (mixed feedstock input). The
provision of a
feedstock having a high carbon content increases the carbon balance in the
product gases.
In one embodiment, the control system comprises means to control the addition
of a
carbon-rich feedstock via process devices such as response elements to adjust
the
reactants to maintain one or more characteristic values of sensed
characteristics within
the respective predetermined ranges defined to characterize the product gas as
suitable for
the selected downstream application(s). For example, additives may be added to
the
converter to facilitate the efficient conversion of the feedstock into a
suitable product gas.
The type and quantity of the additives may be carefully selected for the above
mentioned
goal of suitable product gas. In another embodiment of the invention, the
control system
comprises response elements to control the addition of additives to maintain
production
of a suitable product gas. In another embodiment of the control system,
response
elements are provided to control the addition of two or more additives to
maintain
production of a suitable product gas. In yet another embodiment, response
elements are
provided to control the addition of three or more additives to maintain
production of a
suitable product gas.
In those embodiments comprising a one stage process, i.e., where the
gasification and
reformulation steps both take place in a single chamber converter, it may be
advantageous to strategically locate additive input ports in and around the
converter to
ensure adequate coverage of additives into the processing zone. In those
embodiments
wherein the process takes place in two stages, i.e., the gasification and
reformulation take
place in discrete regions within the system, it may be advantageous to locate
certain
additive ports (for example, steam inputs) proximal to the region where
reformulation by
the plasma heat source takes place.
In a further embodiment, the control system comprises response elements for
adjusting
the additive inputs based on data obtained from monitoring and analyzing the
characteristics of the product gas, via various sensing elements and computing
means
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whereby these data are used to estimate the composition of the feedstock. The
product
gas characteristics data may be obtained on a continuous basis, thereby
allowing the
adjustments to additive inputs such as air, steam and/or carbon-rich additives
to be made
on a real-time basis. The product gas characteristics data may also be
obtained and/or
analyzed on an intermittent basis.
The control system of the present invention, therefore, includes a means, as
in response
elements for introducing the additives into the system, based on
characteristic values as
monitored by various sensing elements, according to the predetermined range of
characteristic values defined to characterize the product gas as suitable for
the
downstream application(s). For example, in the event that a gas sensor detects
too much
carbon dioxide, the control system may reduce the delivery of oxidant into the
converter
to reduce the production of carbon dioxide.
In one embodiment of the invention, the process is adjusted to produce mostly
carbon
monoxide, rather than carbon dioxide. In order to expedite the production of
carbon
monoxide in such an embodiment, the system will include a sensor, analyzer or
other
such sensing elements for determining the amount of oxygen in the gaseous
output
stream. If a certain range (dependant upon the composition and rate of other
inputs such
as feedstock input) of oxygen input from steam or air/oxidant inputs is used
in the
gasification process, the product gas will be mainly carbon monoxide. If there
is too little
oxygen, a considerable amount of elemental carbon or carbon black may form
which may
ultimately plug up equipment downstream from the converter. If there is too
much
oxygen in the system, too much carbon dioxide will be produced, which is
undesirable if
the objective of the process is to produce a fuel gas. In response to too much
carbon
dioxide in the system, any steam or air/oxidant being injected may be reduced
or
eliminated by an appropriate signal from the control system (e.g. see Figures
1, 18 and 71
to 75).
In one embodiment, a syngas fuel value determination module can compute the
low
heating value LHV = c1*[H2] + c2*[CO], where c1 and c2 are constants and where
[H2]
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and [CO] are obtained from the syngas analyzer. The module may be operatively
coupled
to a Fuel:Air ratio controller, for example, for cooperatively controlling the
total
MSW+HCF feed rate and optionally the MSW/HCF ratio controller and carrier ram
sequence controller. In one embodiment, in order to determine the amount of
air additive
to input into the system to obtain a syngas composition within an appropriate
range for
the downstream application, or again within a range conducive to increasing
the energetic
efficiency and/or consumption of product gas, the control system may be
configured to
compute a control parameter based on an acquired characteristic value for the
LHV (e.g.
from analysis of [H2] and [CO] of syngas). For instance, by setting the
temperature and
pressure constant, or at a desired set point, a global system parameter may be
defined
empirically such that the air input parameter may be estimated with sufficient
accuracy
using a linear computation of the form [LHV ] = a[Air], wherein a is an
empirical
constant for a particular system design and desired output characteristics.
In another embodiment, in order to determine the amount of air additive and
steam
additive to input into the system to obtain a syngas composition within an
appropriate
range for the downstream application, or within a range conducive to
increasing the
energetic efficiency and/or consumption of product gas, the control system may
be
configured to compute control parameters based on acquired characteristic
values for
[H2] and [CO]. For instance, by setting the temperature and pressure constant,
or at a
desired set point, global system parameters may be defined empirically such
that the air
and steam input parameters may be estimated with sufficient accuracy using a
linear
computation of the form:
Hz = a b ][Air
CO c d Steam
wherein a, b, c and d are empirical constants for a particular system design
and desired
output characteristics. The person of skill in the art will appreciate that
although
simplified to a linear system, this embodiment may be extended to include
additional
characteristic values, and thereby provide for the linear computation of
additional control
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parameters. Higher order computations may also be considered to refine
computation of
control parameters as needed to further restrict process fluctuations for more
stringent
downstream applications.
The conversion of a carbonaceous feedstock into fuel gas within the converter
is an
endothermic reaction, i.e., energy needs to be provided to the reactants to
enable them to
reform into the suitable fuel gas product. In one embodiment of the invention,
a
proportion of the energy required for the gasification process is provided by
the oxidation
of a portion of the initial gaseous products or carbonaceous feedstock within
the
converter.
Introduction of an oxidant into the converter creates partial oxidation
conditions within
the converter. In partial oxidation, the carbon in the feedstock reacts with
less than the
stoichiometric amount of oxygen required to achieve complete oxidation. With
the
limited amount of oxygen available, solid carbon is therefore converted into
carbon
monoxide and small amounts of carbon dioxide, thereby providing carbon in a
gaseous
form.
Such oxidation also liberates thermal energy, thereby reducing the amount of
energy that
needs to be introduced into the converter by the plasma heat. In turn, this
increased
thermal energy reduces the amount of electrical power that is consumed by the
plasma
heat source to produce the specified reaction conditions within the converter.
Thus, a
greater proportion of the electricity produced by converting the fuel gas to
electrical
power in an electric power generating device (e.g. fuel cell application, gas
turbine, etc.)
can be provided to a user or exported as electrical power, because the plasma
heat source
requires less electricity from such an electric power generating device in a
system which
employs the addition of an oxidant.
The use of oxidant inputs as an additive therefore assists in maximizing the
conversion of
carbon to a fuel gas and helps to maintain the optimum processing temperatures
as
required while minimizing the relatively high cost plasma input heat. The
amount of
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oxidant injection may be carefully established to ensure maximum removal of
carbon in
gaseous form (CO and CO2). Simultaneously, because the gasification of carbon
reactions (combination with oxygen) are exothermic, substantial quantities of
heat are
produced. This minimizes the need for relatively high cost plasma input heat
while
helping to ensure the overall process does not approach any of the undesirable
process
characteristics associated with combustion
Although less fuel gas will be produced within the converter when partial
oxidizing
conditions exist (because some of the fuel gas or feedstock is oxidized to
liberate thermal
energy, and thus, less fuel gas is available to an electric power generating
device), the
reduction in electrical consumption by the plasma heat source(s) offsets a
possible loss in
electrical energy production. In one embodiment of the invention, the control
system
comprises means to adjust the addition of additives to maintain one or more
characteristic
values of sensed characteristics within predetermined ranges defined for a
product gas
suitable for the downstream application(s), while taking into account overall
energy
production from the process.
In one embodiment of the invention, the oxidant additive is selected from air,
oxygen,
oxygen-enriched air, steam or carbon dioxide. In those embodiments using
carbon
dioxide as an oxidizing additive, the carbon dioxide may be recovered from the
product
gases and recycled into the additive stream.
In some embodiments, an air feed system is provided for process air to be
distributed
fairly evenly over the area where the gasification process takes place. In one
embodiment, the heated air is introduced through a perforated floor. To avoid
blockage of
air holes during processing, the air hole size can be selected such that it
creates a
restriction and thus a pressure drop across each hole, sufficient to prevent
waste particles
from entering the holes. The holes may also be tapered outwards towards the
upper face
to preclude particles becoming stuck in a hole. While a multi-stage horizontal
design
example is presented here, any number of stages, as well as vertical
orientation, are also
included. In multi-stage gasification configurations, the flow at each stage
may be under
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independent control. In one embodiment of this example, there are three stages
of
processing with independently controllable air feed for each level.
Independent air feed
and distribution through a perforated floor may, in one embodiment, be
achieved by a
separate airbox which forms the floor at each stage.
The selection of appropriate oxidizing additive may be made according to the
economic
objectives of the conversion process. For example, if the economic objective
is the
generation of electricity, the oxidizing additive may be selected to provide
the optimal
output gas composition for a given energy generating technology. For those
systems
which employ a gas engine to generate energy from the product gases, a higher
proportion of nitrogen may be acceptable in the product gas composition. In
such
systems, air may be an acceptable oxidant additive. For those systems,
however, which
employ a gas turbine to generate energy, the product gases must undergo
compression
before use. In such embodiments, a higher proportion of nitrogen in the
product gases
will lead to an increased energetic cost associated with compressing the
product gas, a
proportion of which does not contribute to the production of energy.
Therefore, in certain
embodiments, it is advantageous to use an oxidizer that contains a lower
proportion of
nitrogen, such as oxygen or oxygen-enriched air.
In those embodiments of the present invention which seek to maximize the
production of
electrical energy using the fuel gases produced by the gasification process,
it may be
advantageous to minimize the oxidation of the fuel gas which takes place in
the
converter. In order to offset any decrease in the production of fuel gas due
to partial
oxidation conditions, steam may also be used as the oxidizing additive. The
use of steam
input as an additive may help promote sufficient free oxygen and hydrogen to
maximize
the conversion of decomposed elements of the input feedstock into fuel gas
and/or non-
hazardous compounds.
For those embodiments having the production of electrical energy as an
objective, it is
advantageous to produce gases having a high fuel value. The gasification of
carbonaceous feedstocks in the presence of steam produces a syngas composed
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predominantly of hydrogen and carbon monoxide. Those of ordinary skill in the
chemical arts will recognize that the relative proportions of hydrogen and
carbon
monoxide in the fuel gas product can be manipulated by introducing different
amounts of
additives into the converter, namely air, oxygen, oxygen-enriched air, other
oxidants,
steam, etc.
Steam input ports can be strategically located to direct steam into the high
temperature
processing zone and/or into the product gas mass prior to its exit from the
converter.
Solid Residue Conditioner
Referring to Figures 1 to 4, 19 to 23, and 52 to 58 of Example 1, the
carbonaceous
feedstock gasification system amenable for use with the present control system
may also
provide means for managing the solid by-product of the gasification process,
such as a
solid residue conditioner for the conversion of the solid by-products, or
other residues in
various phases, resulting from feedstock-to-energy conversion processes, into
a vitrified,
homogenous substance having low leachability.
The control system of the present invention may provide for the optimization
of the solid
residue-to-slag conversion by controlling the plasma heat rate and solid
residue input rate
to promote full melting and homogenization. In one embodiment, the solid
residue
conditioner comprises a solid residue conditioner having a solid residue
inlet, a plasma
heating means, a slag outlet, optionally one or more ports, and a downstream
cooling
means for cooling and solidifying the slag into its final form. The control
system of the
present invention may also provide sensing elements to monitor temperature and
pressure
throughout the solid residue conditioner, response elements to regulate the
efficient
conversion of the solid residue into slag, and via, for example, process
devices, means to
control such operational parameters as the power to the plasma heat source and
solid
residue input rate.
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The solid residue conditioner is adaptable to treat solid residue coming out
of any process
that converts the carbonaceous feedstock into different forms of energy. This
solid
residue is typically in a granular state and may come from one or more sources
such as
the converter and optionally the converter gas conditioner. The solid residue
from all
sources may be heated to a temperature required to convert the solids into a
vitrified,
homogeneous substance that exhibits extremely low leachability when allowed to
cool
and solidify. The solid residue conditioner therefore ensures that the solid
residue is
brought up to an adequate temperature to melt and homogenize the solid
residue. The
solid residue conditioner also promotes the capture of polluting solids (i.e.,
heavy metals)
in the slag, as well as the formation of a clean, homogeneous (and potentially
commercially valuable) slag product.
In order to ensure essentially complete processing of the solid residue, the
solid residue
conditioner may be designed to provide sufficient residence time in the solid
residue
conditioner. In one embodiment, the system provides a residence time of at
least 10
minutes. In another embodiment, the solid residue conditioner provides a
residence time
of up to 1 hour. In yet another embodiment, the solid residue conditioner
provides a
residence time of up to 2 hours.
The solid residue, which may take the form of char, ash, slag, or some
combination
thereof, will be removed, continuously or intermittently, from one or more
upstream
processes through appropriately adapted outlets and conveyance means as would
be
known to the skilled worker, according to the requirements of the system and
the type of
by-product being removed. In one embodiment, the solid residue is pushed into
the solid
residue conditioner through a system of hoppers and conveying screws.
The solid residue may be added in a continuous manner, for example, by using a
rotating
screw or auger mechanism. For example, in one embodiment, a screw conveyor is
employed to convey ash to a solid residue conditioner.
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Alternatively, the solid residue can be added in a discontinuous fashion. In
one
embodiment, the solid residue input means, attached to the solid residue
conditioner, may
consist of a system of conveying or carrier rams. In such an embodiment, limit
switches
may be employed by the control system to control the length of the carrier ram
stroke so
that the amount of material fed into the converter with each stroke can be
controlled.
The control system of the present invention may further include a control
means such that
the input rate of the solid residue can be controlled to ensure optimal
melting and
homogenization of the solid residue material.
In one embodiment, a plasma heat source, is employed to heat and melt the ash
into slag.
The molten slag, at a temperature of, for example, about 1300 C to about 1700
C, may be
periodically or continuously exhausted from the solid residue conditioner and
thereafter
cooled to form a solid slag material. Such slag material may be intended for
landfill
disposal. Alternatively, the molten slag can be poured into containers to form
ingots,
bricks tiles or similar construction material for use in, for example road
fill or concrete
manufacture. The solid product may further be broken into aggregates for
conventional
uses.
The solid residue conditioner, therefore, includes a slag output means,
optionally under
the control of the control system, through which in one embodiment molten slag
is
exhausted from the solid residue conditioner. The output means may comprise a
slag exit
port, which is typically located at or near the bottom of the converter to
facilitate the
natural flow of the molten slag pool out of the converter. The rate at which
the molten
slag flows out of the solid residue conditioner may be controlled in a variety
of ways.
For example, in one embodiment, the temperature differential between the point
closest
to the plasma heating means and the exit point may be adjusted to control the
re-
solidification time of the molten slag, e.g., through adjustments in the
volume of solid
residue material allowed to pool in the converter.
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The slag output means may further be adapted to minimize heating requirements
by
keeping the solid residue conditioner sealed. In one embodiment, the output
means
comprises a pour spout or S-trap.
As discussed previously, it may also be advantageous to aim the plume of one
or more of
the plasma heat sources towards the slag pool at, or around, the slag exit
port to maintain
the temperature of the molten slag and ensure that the slag exit port remains
open through
the complete slag extraction period. This practice will also aid in
maintaining the slag as
homogeneous as possible to guard against the possibility that some
incompletely-
processed material may inadvertently make its way out of the solid residue
conditioner
during slag extraction.
The molten slag can be extracted from the solid residue conditioner in a
number of
different ways. For example, the slag can be extracted by a batch pour at the
end of a
processing period, or a continuous pour throughout the full duration of
processing. The
slag from either pour method can be poured into a water bath, where the water
acts as a
seal between the external environment and the gasification system. The slag
can also be
dropped into carts for removal, into a bed of silica sand or into moulds.
The walls of the solid residue conditioner are lined with a refractory
material that can be
one, or a combination of, conventional refractory materials known in the art
which are
suitable for use in a converter for extremely high temperature (e.g., a
temperature of
about 1300 C to 1800 C) non-pressurized reactions. Examples of such refractory
materials include, but are not limited to, chromia refractories and high
alumina
refractories containing alumina, titania, and/or chromia.
The physical design characteristics of the solid residue conditioner can be
determined by
a number of factors. These factors may include, for example, the composition,
volume
and operational characteristics of the input of the solid residue to be
processed, efficient
heat transfer, adequate temperatures, molten slag flow, the residence time
required to
ensure that the solid residue is brought up to an adequate temperature to melt
and
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homogenize the solid residue, and the type of plasma heating means used, as
well as the
position and orientation, of the plasma heating means
The control system of the present invention may regulate the efficient
conversion of solid
residue into slag by providing sensing elements to monitor the temperature and
optionally
pressure at sites located throughout the solid residue conditioner, wherein
such data are
acquired on a continuous or intermittent basis. Sensing elements for
monitoring the
temperature in the conditioner, for example, may be located on the outside
wall of the
conditioner, or inside the refractory at the top, middle and bottom of the
conditioner. The
control system of the present invention also provides response elements
operatively
linked to process devices to control, for example, the power to the plasma
heat source
and solid residue input rate.
For example, when the temperature of the melt is too high, the control system
may
command a drop in the power rating of the plasma heat source; conversely, when
the
temperature of the melt is too low, the control system may command an increase
in the
power rating of the plasma heat source. Control of the solid residue
conditioner may be
implemented via direct control of the control system or via modular control
wherein the
control system comprises subsystems of control.
In one embodiment, the solid residue conditioner can also comprise a means for
recovering heat (e.g. plasma heat source cooling means and slag cooling means
of
Figures 24 and 25), which can reduce the amount of waste heat generated. Such
heat
recovery means can include, for example, heat exchangers. In such an
embodiment, the
control system can additionally control the operating conditions of the heat
exchanger.
The heat exchanger can have, for example, a number of temperature sensors,
flow control
elements, and other such monitoring and response elements.
In one embodiment, the solid residue is extracted from the primary converter,
fed into a
high temperature melting chamber, cooled and shattered into granules in a
quench tank
and transferred to a stockpile ready for removal from site. In another
embodiment, there
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is provided a solid residue feed system which extracts solid residue from the
converter by
means of a screw type conveyor. This can have serrated edges on the screw
flights to
break up any agglomerated material. The solid residue can then be taken to a
slag
melting chamber by means of a conveyor system. Additional sources of solid
residue can
also be catered for. In order for gasification to continue during solid
residue conditioner
downtime the solid residue may be diverted and later re-introduced into the
solid residue
conditioner feed system.
In one embodiment, the solid residue received from the feed system is
transferred into a
melting crucible and melted using a plasma torch. As molten slag rises within
the
crucible it reaches a weir and runs over the weir, dropping into a quench
tank.
The gases produced in the solid residue conditioner may be treated similarly
to the gases
produced in the converter (e.g. for downstream use in a same or alternate
downstream
application). Any metals which have not been removed during the MSW handling
system
stage may be transferred to the slag crucible and will not necessarily be
melted at the slag
normal vitrification temperature, thus the crucible could become clogged with
metal as it
is of higher density than the molten slag. To deal with this, the chamber
temperature may
in some embodiments be periodically raised to melt any metals and the molten
metals
may be tapped off from the bottom of the crucible to remove them. Due to the
very high
temperatures needed to melt the solid residue and particularly the metals in
the solid
residue, the refractory will be subjected to very severe operational demands.
These
include corrosion and erosion, particularly at the slag waterline in addition
to the high
temperature. The refractory may be selected to provide an inner lining of very
high
resistance to heat, corrosion and erosion. The layers of refractory outside
the lining may
then be selected to greater insulation.
In some embodiments, the solid residue may be provided to the solid residue
conditioner
from both the converter and the gas conditioner, the combination of which can
be
conditioned to yield a solid product (e.g. vitrified slag) and a syngas to be
conditioned
and combined with the converter syngas for further conditioning,
homogenisation and
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downstream use. In controlling the solid residue processing, the power of the
plasma
torch may be adjusted as needed to maintain temperatures adequate for the
melting
operation. The slag chamber may include various temperature sensing elements
and
pressure sensing elements. In some embodiments a control valve may be provided
in the
gas outlet line to restrict the flow of gas that is being removed by the
downstream
vacuum producer (syngas blower). The feed rate to the solid residue
conditioner may be
adjusted as required to ensure acceptable temperature control, within
capability of
melting rate of plasma torches, and to prevent high levels in the slag chamber
due to un-
melted material.
Heat Exchanger
Referring now to Figures 1 to 4 and 24 to 26, a carbonaceous feedstock
gasification
system amenable for use with the control system of the present invention may
also
provide means for the recovery of heat from the hot product gas via a heat
exchanger.
The heat exchanger may comprise one or more gas-to-air heat exchangers,
whereby the
hot product gas is used to provide heated exchange-air. The recovered heat (in
the form
of the heated exchange-air) may then optionally be used to provide heat to the
gasification process, as specifically illustrated in Figure 26, thereby
reducing the amount
of heat which must be provided by the one or more plasma heat sources required
to drive
the gasification process. The recovered heat may also be used in industrial or
residential
heating applications. In one example, the syngas temperature is reduced from
about
1000 C to about 740 C while increasing the air temperature from ambient to
about
600 C.
In another embodiment, the gas-to-air heat exchanger is employed to heat an
oxidant,
such as oxygen or oxygen-enriched air, which may then optionally be used to
provide
heat to the gasification process.
Different classes of gas-to-air heat exchangers may be used, including shell
and tube heat
exchangers, both of straight, single-pass design and of U-tube, multiple pass
design, as
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well as plate-type heat exchangers. In one embodiment, the product gas flows
inside the
tubes and the process air flows counter-currently on the shell side of the gas-
to-air heat
exchanger. The design of the heat exchanger may also take into account means,
such as
bellows, to avoid tube rupture. The selection of appropriate heat exchangers
is within the
knowledge of the skilled worker.
In order to minimize the hazard potential from a tube leak, the gasification
system may
further comprise one or more individual temperature sensing elements
associated with the
product gas outlet of the gas-to-air heat exchanger. These temperature sensing
elements
may be positioned to detect a temperature rise resulting from combustion in
the event of
having exchange-air leak into the syngas conduit. Detection of such a
temperature rise
can be used to effect an automatic shut down of the induction air blowers
which move the
coolant air through the heat recovery system. A lower predetermined limit may
also be
used as an indication that the tubes are starting to plug, which could in some
embodiments be used to indicate that the system should be shut down for
maintenance.
The heat exchanger may be under the direct control of the control system of
the present
invention and/or under control of a modular control subsystem.
Optionally, the heat exchanger additionally comprises one or more steam
generator heat
exchangers to generate steam, which can be used as an additive in the
gasification
reaction, as specifically illustrated in Figures 26 to drive a steam turbine,
or to drive
rotating process equipment, such as induction blowers. Heat from the product
gas is used
to heat water to generate steam using a heat exchanging means, such as a steam
generator
heat exchanger (e.g. see Figures 2, 3 and 25), a waste heat boiler (e.g. see
Figure 26), and
the like. In one embodiment, the steam produced using heat from the product
gas is
superheated steam.
With specific reference to Figure 26, the relationship between a gas-to-air
heat exchanger
and a steam generator heat exchanger is depicted in accordance with one
embodiment of
the invention. The exchange-steam can also be used as a steam additive during
the
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gasification process to help ensure sufficient free oxygen and hydrogen to
maximize the
conversion of the feedstock into the syngas product.
Steam that is not used within the conversion process or to drive rotating
process
equipment, may be used for other commercial purposes, such as the production
of
electricity through the use of steam turbines or in local heating applications
or it can be
supplied to local industrial clients for their purposes, or it can be used for
improving the
extraction of oil from the tar sands.
Optional Steam Generator Heat Exchanger
With reference to Figure 2, in one embodiment the steam recuperated from the
outputs of
the various steam turbines (e.g. a steam turbine operating from steam
generated by a
steam generator heat exchanger used to cool the syngas, a steam turbine
operating from
steam generated by a steam generator heat exchanger used to cool a gas
turbine/engine
and exhaust gas generated thereby, or any combination thereof), is cooled
through an
additional heat exchanger, which may also be controlled by the control system
of the
current invention, and is fed by a cooling tower pump or the like. In one
embodiment,
upon exit from the exchanger, the cooled steam/water is pumped through a
deaerator, fed
by a soft water source with appropriate chemicals, to remove air and excess
oxygen
therefrom, to then be processed back to the boiler feed water of the exhaust
gas steam
generator heat exchanger, the syngas steam generator heat exchanger, etc.
The control system of the current invention can be used, in some embodiments
to
optimize the transfer of energy throughout the system, thereby managing the
energetics
of the feedstock-to-energy conversion. The energetics of the feedstock-to-
energy
conversion can be optimized using the heat exchanger, since the recycling of
the
recovered sensible heat back to the gasification process reduces the amount of
energy
inputs required from external sources for the steps of drying and volatilizing
the
feedstock. The recovered sensible heat may also serve to minimize the amount
of plasma
heat required to achieve a specified quality of syngas. Thus, the present
invention allows
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for the efficient gasification of a carbonaceous feedstock, wherein the
gasification heat
source is optionally supplemented by air heated using sensible heat recovered
from the
product gas.
In order to optimize the efficiency, the control system also optionally
provides a means
for controlling the conditions under which the heat exchanger process is
carried out.
These control means are provided to monitor one or more parameters, including,
but not
limited to, temperature and gas flow rates at specified locations throughout
the system,
and to adjust operating conditions accordingly, so as to maintain the system
within
defined parameters. Examples of operating conditions which may be adjusted by
the
control means, via response elements, include one or more of the exchange-air
flow rate,
the product gas flow rate, the rate of feedstock input, the rate of input of
additives such as
steam, and the power to the plasma heat sources.
For example, sensors such as temperature transmitters (and other such sensing
elements)
may be installed at specified locations throughout the gasification system
amenable for
use with the current invention. The temperature transmitters may be located to
measure,
for example, the temperatures of the product gas at the gas-to-air heat
exchanger inlet and
outlet, as well as the temperatures of the product gas at the steam generator
heat
exchanger inlet and outlet. Temperature transmitters may also be provided to
measure
the temperature of the process air after heating in the gas-to-air heat
exchanger, as well as
to measure the temperature of the steam as it exits the steam generator heat
exchanger.
These temperature measurements can be used to ensure that the temperature of
the syngas
as it enters a respective heat exchanger does not exceed the ideal operating
temperature of
that device. For example, if the design temperature for the gas-to-air heat
exchanger is
1050 C, a temperature transmitter on the inlet gas stream to the heat
exchanger can be
used to control both exchange-air flow rates through the system and plasma
heat power in
order to maintain the optimum syngas temperature. In addition, measurement of
the
product gas exit temperature may be useful to ensure that the optimum amount
of
sensible heat has been recovered from the product gas at both heat recovery
stages.
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In one embodiment a temperature sensing element such as a temperature
transmitter
installed on the air outlet stream to measure the temperature of the heated
exchange-air
helps ensure that the process is carried out under conditions that ensure the
process air is
heated to a temperature appropriate for use in the gasification process. In
one
embodiment, the exchange-air outlet temperature is, for example, about 600 C,
therefore
a temperature transmitter installed on the air outlet stream will be used to
control one or
both of air flow rates through the system and plasma heat source power in the
plasma
reformulating chamber in order to maintain the optimum syngas input
temperature, which
in turn can be used to control the temperature of the heated exchange-air.
According to one embodiment of the invention, the control strategy sets a
fixed set point
for the optimum heated exchange-air output temperature, for example, about 600
C, as
well as a fixed value for the steam generator heat exchanger gas exit
temperature, for
example, about 235 C. Therefore, according to this embodiment, when the syngas
flow is
reduced, the exit gas temperature of the gas-to-air heat exchanger gets
cooler, resulting in
decreased steam production because the steam generator heat exchanger gas exit
temperature is also set to a fixed value.
The same concept applies when the airflow through the system is reduced.
According to
one embodiment of the present invention, the exit exchange-air temperature
remains
fixed therefore the exit product gas temperature for the gas-to-air heat
exchanger is
hotter, therefore producing more steam in the steam generator heat exchanger.
However,
when airflow through the system is reduced, product gas flow will consequently
also
reduce, so the increased inlet temperature to the steam generator heat
exchanger will only
be momentarily high. For example, if airflow is reduced to 50%, the maximum
inlet gas
temperature that the steam generator heat exchanger 50 would momentarily see
is
approximately 800 C, which is within the temperature limits of the heat
exchanger
design.
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In addition, in some embodiments, the present control system optionally
provides
response elements for controlling an automatic valve for venting process air
to the
atmosphere if more air than required for the gasification process is
preheated. For
example, in some instances it is necessary to heat more gas than required for
the process
due to equipment considerations (e.g. when starting a shutdown procedure). In
such
instances, the excess exchange-air can be vented as required.
With reference to Figures 24 and 25, the heat exchanger described above may
also
provide for the cooling of the product gas as required for subsequent
particulate filtering
and gas conditioning steps, namely with regards to the converter gas
conditioner (e.g.
converter gas conditioner cooling means), as well as provide for the cooling
of the
plasma heat sources (e.g. source cooling means), slag handling and processing
means
(e.g. slag cooling means), etc.
Converter Gas Conditioner
With reference now to Figures 1 and 4, the control system of the present
invention is
amenable for use with a gasification system which optionally provides a
converter gas
conditioner, or other such gas conditioning means, to convert the product of
the
gasification process to an output gas of specified characteristics. Passage of
the product
gas through the converter gas conditioner helps ensure that the product gas is
close to free
of chemical and particulate contaminants, and therefore can be used in an
energy
generating system or in the manufacture of chemicals.
In one embodiment the product gas is directed to the converter gas
conditioner, where it
is subjected to a particular sequence of processing steps to produce the
output gas having
the characteristics required for downstream applications. The converter gas
conditioner
comprises components that carry out processing steps that may include, but are
not
limited to, removal of particulate matter, acid gases (HC1, H2S), and/or heavy
metals
from the synthesis gas, or adjusting the humidity and temperature of the gas
as it passes
through the system. The presence and sequence of processing steps required is
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determined by the composition of the synthesis gas and the specified
composition of
output gas for downstream applications. Optimization of the converter gas
conditioner
process can also be achieved via use the control system of the present
invention.
In one embodiment, under vacuum extraction conditions of the induction fan of
a
gasification system, the hot product gas is continuously withdrawn from the
gasification
system through an exit gas outlet(s) of the gasification system. A gas
transfer means,
such as a pipe or other conduit is used to transfer the gas from the converter
to the
converter gas conditioner.
It is also contemplated that one or more converter gas conditioners may be
used, such as
a primary converter gas conditioner and a secondary converter gas conditioner.
In this
case, the secondary converter gas conditioner may be used to process material
such as
particulate matter and heavy metals that are removed from the gas stream in
the primary
converter gas conditioner. The output gas from the converter gas conditioner
can be
stored in a gas storage tank (e.g. see Figure 3), fed through further
processing means such
as a homogenization system (e.g. see Figures 1 and 4) or alternatively, fed
directly to the
downstream application for which it was designed (e.g. see Figure 2).
As discussed above, it is advantageous to provide means for cooling the hot
product gas
prior to such a conditioning step. This cooling step may be required to
prevent damage to
heat-sensitive components in the system. In one embodiment, cooling step is
carried out
by the heat exchanger, whereby the heat recovered from the product gas may
also be
optionally recovered and recycled for use in the gasification process (e.g.
see Figures 1, 4
and 26).
In another embodiment, the gas from the gasification system is first cooled
down by
direct water evaporation in an evaporator such as a quencher (e.g. see Figures
1 and 4).
In yet another embodiment, evaporative cooling towers (e.g. see Figure 4) may
be used to
cool the syngas that enters the converter gas conditioner from the
gasification system.
The evaporative cooling tower is capable of cooling the temperature of the
syngas from
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about 740 C to about 150 to 200 C. This process may be achieved using
adiabatic
saturation, which involves direct injection of water into the gas stream in a
controlled
manner. The evaporating cooling process is a dry quench process, and can be
monitored
and controlled by the control system of the present invention to ensure that
the cooled gas
is not wet, i.e., that the relative humidity of the cooled gas is still below
100% at the
cooled temperature.
As mentioned above, the converter gas conditioner may comprise means for
removing
particulate matter from the optionally cooled gas, as well as gaseous
contaminants not
compatible with downstream uses of the product gas. A particulate removal
system may
be incorporated to remove particulates that may be entrained in the fuel gas
exiting the
converter. Particulate and dust removal systems 54 are widely available, and
may
include, for example, high-temperature (ceramic) filters, cyclone separators
(e.g. see
Figure 7), a venturi scrubber (e.g. see Figure 7), an electrofilter, a candle
filter, a
crossflow filter, a granular filter, a water scrubber, or a fabric baghouse
filter (e.g. see
Figure 4), and the like, which are well known to practitioners of gas
conditioning.
Alternative embodiments may make use of different orders of the various gas
clean-up
steps to use more efficiently the characteristics of alternative gas cleaning
devices.
Various exemplary embodiments are provided in international application
numbers
WO/2006/128285 and WO/2006/128286 which are readily applicable in the present
context, as would be apparent to a person skilled in the art.
There may also be provided means for removing mercury or other heavy metals
from the
product gas. For example, dry injection systems utilize a calculated amount of
activated
carbon which is injected in the gas stream with enough residence time so that
fine heavy
metal particles and fumes can adsorb in the activated carbon surface. Heavy
metals
adsorbed on activated carbon can be captured in, for example, a baghouse
filter or a wet
ESP system.
In one embodiment, the converter gas conditioner optionally comprises an acid
scrubbing
system to remove heavy metals. For example, this system may require the gas
containing
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heavy metals to be passed through a packed column with low pH (normally 1-2)
solution
circulation. Acid gas removal can be achieved by dry scrubbing or wet
scrubbing. The
main components of dry scrubbing may be, for example, a spray dry absorber and
soda
ash or lime powder injection before baghouse filtration.
In one embodiment, the mercury removal means are provided by an activated
carbon
mercury polisher (e.g. see Figure 4). An activated carbon filter bed can be
used as the
final polishing device for heavy metals.
An acid recovery subsystem may optionally be coupled to the converter gas
conditioner,
to recover sulfur or sulfuric acid and hydrochloric acid (from chlorinated
hydrocarbons),
which may have a marketable value. The acid removal system may include
scrubber
systems (e.g. HC1 scrubber), acid removal systems, and other conventional
equipment
related to sulfur and/or acid removal systems.
In yet another embodiment, a humidity control means can be provided. The
humidity
control means functions to ensure that the humidity of the output gas is
appropriate for
the downstream application required. For example, a humidity control means may
include a chiller to cool the gas stream and thus condense some water out of
the gas
stream. This water can be removed by a gas/liquid separator.
In another embodiment, the gas processing system can include means for the
recovery of
carbon dioxide and/or ammonia and/or chlorine and/or elemental sulfur.
Suitable means
are known in the art, and various exemplary embodiments are provided in
international
application numbers WO/2006/128285 and WO/2006/128286.
In one embodiment, the control system may sense decrease in efficiency or
alternate
functional deficiency in a process of the converter gas conditioner and divert
the gas
stream to a backup process or backup conditioning system. In another
embodiment, the
control system may provide a means for fine-tuning the steps of the converter
gas
conditioner and providing minimal drift from optimal conditions.
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The control system of this invention can include sensing elements for
analyzing the
chemical composition of the gas stream through the converter gas conditioner,
the gas
flow and thermal parameters of the process; and response elements to adjust
the
conditions within the converter gas conditioner to optimize the efficiency of
processing
and the composition of the output gas. Ongoing adjustments to the reactants
(for
example, activated carbon injection with sufficient residence time, pH control
for the HCl
scrubber) can be executed in a manner which enables this process to be
conducted
efficiently and optimized according to design specifications.
Homogenization System
The present gasification system also optionally provides means for regulating
the product
gas, for example, by at least partially homogenizing the chemical composition
of the
product gas and adjusting other characteristics such as flow, pressure, and
temperature of
the product gas to meet downstream requirements.
As is understood by those skilled in the art, the gasification process may
produce gases of
fluctuating composition, temperature or flow rates. In order to reduce the
fluctuations in
the characteristics of the product gas, there is optionally provided a
homogenization
system in the form of a capturing means useful for delivering a gas with
reduced
fluctuations to downstream equipment.
In one embodiment the present invention provides a homogenization system that
collects
the gaseous products of the gasification process and attenuates fluctuations
in the
chemistry of the gas composition in a homogenization system, or the like.
Other
elements of the system optionally may be used to help adjust characteristics
of the gas
such as flow, temperature and pressure
In particular, the homogenization system provides a gas homogenization system
(e.g. see
Figures 1 and 4, and Figures 60, 67 and 68 of Example 1) or the like having
dimensions
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that are designed to accommodate a residence time sufficient to assist in
attaining a
relatively homogeneous gas. Other elements of the homogenization system may be
designed to assist in meeting the gas performance requirements of the
downstream
application(s). As described above, the control system may be used to actively
control
various characteristics of the product gas before entering the homogenization
system such
that the gas, upon output therefrom, is of suitable characteristic(s) for the
downstream
application(s).
The composition of the product gas entering the homogenization system is
determined in
the gasification process. Adjustments made by the control system during the
gasification
process permit the product gas to be optimized for the specific downstream
application(s)
(e.g., gas turbines or fuel cell application for electricity generation).
Accordingly, the
composition of the product gas can be tailored for particular energy
generating
technologies (for example, for specific gas engines or gas turbines) and, for
best overall
conversion efficiency, according to the different types of feedstocks and
additives used,
by adjusting the operational parameters of the gasification process.
The product gas leaving the gasification system may be within a defined range
of a target
composition, however, over time the product gas may fluctuate in its
characteristics due
to variability in the gasification process such as feedstock composition and
feed rate, as
well as airflow and temperature fluctuations.
Similar to the control of the composition of the product gas, the flow rate
and temperature
of the product gas can be monitored, for example via sensing elements, and
controlled by
the control system, for example via response elements, in order to maintain
the
parameters of the gas within predetermined ranges suitable for the end use.
Adjustments
made by the control system can take into account the residency time of the
homogenization system to ensure that the product gas is suitable for the
downstream
application(s). The homogenization system helps attenuate remaining
fluctuations in flow
rate and temperature of the product gas. In the case of flow rate, these
fluctuations may
occur on a second to second basis; and with temperature on a per minute basis.
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The homogenization system comprises one or more gas homogenization chambers,
or the
like, having a product gas inlet means, a homogenized gas outlet means, and
optionally
an emergency exit port.
The homogenization system receives the product gas produced from a
gasification system
and encourages mixing of the product gas to attenuate any remaining
fluctuations in the
chemical composition of the product gas in the homogenization system.
Remaining
fluctuations in other gas characteristics, such as pressure, temperature and
flow rate, may
also be reduced during mixing of the product gas.
The dimensions of the homogenization system are designed according to the
performance
characteristics of the upstream gasification system and the requirements of
the
downstream machinery, with the objective of minimizing the size of the chamber
as
much as possible. The homogenization system is designed to receive product gas
from a
gasification process and retain the gas for a certain residence time to allow
for sufficient
mixing of the gas to attenuate remaining fluctuations.
The residence time is the amount of time that the product gas remains in the
homogenization system before being directed to the downstream equipment. The
residence time may be chosen to be proportional to the response time of the
related
gasification system under control of the present invention to correct for
remaining
variances in the fluctuations in the gasification reaction in order to achieve
a gas
composition that falls within predetermined range(s). For example, the gas is
retained in
the homogenization system long enough to determine whether its characteristics
fall
within the predetermined ranges allowed for the particular downstream
application(s) as
well as for the control system to make any adjustments to the gasification
process to
correct for the deviance.
Additionally, residence time of the product gas in the homogenization system
may be
determined by the amount of remaining variance in the product gas
characteristics. That
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is, the smaller the remaining variance in product gas characteristics, the
shorter the
residence time required in the homogenization system to correct for the
remaining
variance.
The control system of the present invention can be used to control the
gasification
process so that when using a homogenization system of a given residency time,
the
product gas will have stabilized characteristics that meet the specifications
of the
downstream application(s). Typically, machine manufacturers will provide the
requirements and tolerances allowed by the specific machinery and would be
known to
the person skilled in the art.
The invention will now be described with reference to specific examples. It
will be
understood that the following example is intended to describe an embodiment of
the
invention and is not intended to limit the invention in any way.
EXAMPLE 1
In this example, with reference to Figures 27 to 72, details of one exemplary
embodiment of the invention, including various options, are provided. This
example
presents details for each subsystem of a gasification system amenable for use
with the
control system of the present invention and demonstrates how they work
together to
function as an integrated system for the conversion of municipal solid waste
(MSW) into
electricity. The subsystems discussed in this example are: a Municipal Solid
Waste
Handling System; a Plastics Handling System; a Horizontally Oriented Gasifier
with
Lateral Transfer Units System; a Gas Reformulating System; a Heat Recycling
System;
a Gas Conditioning System; a Residue Conditioning System and a Gas
Homogenization
System.
Figure 1 shows a functional block diagram overview of the entire gasification
system
120, amenable for use with the control system of the present invention,
designed
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primarily for the conversion of MSW to syngas, with the associated use of
reformulated,
conditioned, and homogenized syngas in gas engines for the generation of
electricity.
Municipal Solid Waste (MSW) Handling System
The initial MSW handling system 9200 is designed to take into account: (a)
storage
capability for supply of four days; (b) avoidance of long holding periods and
excess
decomposition of MSW; (c) prevention of debris being blown around; (d) control
of
odour; (e) access and turning space for garbage trucks to unload; (f)
minimization of
driving distance and amount of turning required by the loader 9218
transporting MSW
from the MSW stockpile 9202 to the MSW shredding system 9220; (g) avoidance of
operational interference between loader 9218 and garbage trucks; (h)
possibility of
additional gasification streams to allow for plant expansion; (i) minimum
intrusion by
trucks into the facility, especially into hazardous areas; (j) safe operation
with minimum
personnel; (k) indication for the loader operator of the fill levels in the
conveyor input
hoppers 9242; (1) shredding the as-received waste to a particle size suitable
for
processing; and (m) remote controllability of MSW flow rate into the processor
and
independent control of the plastics feed rate (described below).
The MSW handling system 9200 comprises a MSW storage building 9210, a loader
9218, a MSW shredding system 9220, a magnetic separator 9230 and a feed
conveyor
9240. A separate system 9250 is also designed for storing, shredding,
stockpiling and
feeding a high carbon material (non-recyclable plastics in this example), the
feed-rate of
which is used as an additive in the gasification process. Figure 30 shows an
overall
layout of the entire system site. All storage and handling of MSW until it is
fed into the
gasification system 120 is confined in MSW storage building 9210 to contain
debris and
odour.
A first-in-first-out (FIFO) scheduling approach is used to minimize excessive
decomposition of the MSW. FIFO is enabled by having access for trucks and
loaders
9218 at both ends of the MSW storage building 9210. MSW is unloaded from the
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at one end of the building while the material is being transferred by the
loader 9218 at
the other end of the MSW storage building 9210, thus also allowing the loader
9218 to
operate safely and without interference by the trucks. When the loader 9218
has
removed the material back to the approximate mid point of the MSW stockpile
9202 i.e.
the `old' material has all been used, the operations are then changed to the
opposite ends
of the MSW storage building 9210.
To minimize the size of MSW storage building 9210, space for manoeuvring the
garbage trucks is outside the MSW storage building 9210. This also minimizes
the size
of door 9212 required as it needs only to allow a truck to reverse straight
in, thus
providing the best control of the escape of debris and odour. Only one door
9212 needs
to be open at any time and then only when trucks are actually unloading.
Receipt of
MSW will normally take place during one period per day so that a door 9212
will only
be open for about one hour per day.
Figure 31 shows a layout of the MSW storage building 9210. The MSW storage
building
9210 has a bunker wall 9214 to separate the MSW stockpile 9202 from the aisle
9216
where the loader 9218 must drive to access the input conveyor 9222 of the MSW
shredding system 9220. The bunker wall 9214 stops short of the ends of the MSW
storage building 9210 to allow the loader 9218 to travel from the MSW
stockpile 9202
to the input conveyor 9222 without leaving the MSW storage building 9210.
Thus, the
doors 9212 at one end of the MSW storage building 9210 can be kept closed at
all times
while the other end is open only when trucks are unloading or when a loader
(described
below) for transferring material from the stockpile to the shredding system
needs to exit
to move plastic.
By having the MSW storage building 9210 located adjacent and parallel to the
road
9204 and allowing for truck manoeuvring at both ends of the MSW storage
building
9210, as shown in Figure 28, both space requirements and truck movements
within the
facility is reduced. The space layout design allows a truck to drive into the
facility,
reverse into the MSW storage building 9210, dump its load and drive directly
back onto
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the road 9204. At no times do they get near any of the process equipment or
personnel.
The two road entrance concept also avoids the need for an additional roadway
within the
facility to enable the trucks to access both ends of the MSW storage building
9210.
A mechanized, bucket-based loader 9218 is used to transfer material from the
stockpile
to the shredding system. A skid steer loader design is used due to its compact
size,
manoeuvrability, ease of operation etc. A standard commercially available skid
steer has
adequate capacity to feed the MSW, clean up the stockpile floor after the
trucks have
unloaded and also handle the waste plastics system shredder and process feed.
The MSW shredding system consists of an input conveyor 9222, a shredder, a
pick
conveyor and a magnetic pick-up conveyor. The input conveyor 9222 transports
the
MSW from inside the MSW storage building 9210 upwards and drops it into a
shredder.
The feed hopper for this conveyor is located entirely inside the MSW storage
building
9210 to prevent debris being blown around outdoors. The conveyor has a deep
trough
which, combined with the capacity of the feed hopper holds sufficient material
for one
hour of operation. The portion of the trough outside the MSW storage building
9210 is
covered to control escape of debris and odour. The conveyor is controlled
remotely by
the process controller to match process demands. Mirrors are provided to allow
the
loader operator to see the level of MSW in the hopper from either side.
Detectors
provided in the trough alert the process controller that material is absent.
The shredder ensures that the as-received MSW is suitable for processing, by
breaking
any bags and cutting the larger pieces of waste into a size able to be
processed. As the
received MSW may include materials too large and hard for the shredder to
handle, thus
causing the shredder to jam, the shredder is equipped to automatically stop
when a jam
is sensed, automatically reverse to clear the jam and then restart. If a jam
is still detected
the shredder will shut-down and send a warning signal to the controller.
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The shredded waste is dropped onto a belt conveyor to be transported under a
magnetic
pick-up system and then to be dropped into the feed hopper of a screw conveyor
which
will feed the waste into the gasifier 2200.
To avoid inadvertent feeding of excessive amounts of ferrous metals through
the gasifier
2200, a magnetic pick-up is located above the pick conveyor, which attracts
ferrous
metals that may be present in the shredded waste. A non-magnetic belt runs
across the
direction of the pick conveyor, between the magnet and the waste so that
ferrous metals
attracted to the magnet get moved laterally away from the waste stream. The
ferrous
metal is later removed from the magnet and dropped onto a pile for disposal.
The MSW feed system consists of a hopper and screw conveyor to transport
shredded
waste from the shredder system to the gasification chamber 2202. Shredded
waste is
dropped from the shredder system into the feed hopper, which provides a buffer
of
material ready to feed into the processor. The hopper has high and low level
indicators
which are used to control flow from the shredding system into the hopper. The
conveyor
is under the control of the process controller to match waste feed rate to
meet process
demands. The use of a screw conveyor with integral feed hopper also provides
gas
sealing for the processor. The input hopper is connected to the shredder
system with
covers to control debris and odour. The MSW feed conveyor has an additional
entry to
accept shredded plastic.
Plastics Handling System
The gasification system 120 provides for the addition of plastics as a process
additive.
The plastics are handled separately from the MSW, before being fed to the
gasifier 2200.
The system for handling plastics is designed to provide storage for as-
received bales of
plastic, shred it, place it into a stockpile and feed it under independent
control into the
processor. The system comprises a storage facility, a shredder with input
hopper, a take-
away conveyor and a stockpile, all located in a common building to control
debris. A
feed conveyor moves the shredded plastic into the processor.
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The plastics storage building has the capacity to store two truck loads of
plastic bales. It
is closed on three sides and opens on one side, thus providing containment of
the
material with access for stacking and removing bales. The building also
provides
protection for the shredding system and debris control and protection for the
shredded
material.
The shredder facilitates the plastic material meeting the process
requirements. As-
received plastic is loaded into the feed hopper of the shredder with a loader.
The
shredded material drops onto a belt conveyor that transports it up and drops
it into a
stockpile.
The shredded plastic is picked up by a loader and dropped into the input
hopper of the
feed conveyor. As the conveyor is outdoors, the hopper incorporates an
integral roof and
upwardly extended walls to minimize escape of plastic during filling of the
hopper. The
conveyor trough is sealed to the trough of the MSW conveyor such that the
plastic is
introduced into the gasifier 2200 via the MSW conveyor to reduce openings into
the
gasifier 2200. The conveyor is a screw conveyor with the hopper sealed to it
to provide
gas sealing when it contains material. Detectors are located in the hopper to
indicate
high and low levels and a mirror is provided for the skid steer operator to
monitor fill
level. Motion of this conveyor is under the control of the process controller.
Converter
The converter 1200 comprises a gasifier 2200 and a Gas Reformulating System
(GRS)
3200. The MSW and plastics are fed into the gasifier 2200 and the resulting
gas is sent
to the GRS 3200 where it is reformulated. Any resulting residue from the
gasifier 2200
is sent to the residue conditioning system 4200.
The gasifier 2200 is designed to take into account the requirements to: (a)
provide a
sealed, insulated space for primary processing of the waste; (b) introduce hot
air and
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steam in a controlled and distributed manner throughout the gasifier 2200; (c)
enable
control of the height and movement of the waste pile through the gasifier
2200; (d)
provide instrumentation for controlling the gasification process; (e) transfer
the gas to
the GRS 3200; (f) remove residue for further processing; and (g) provide
access to the
interior for inspection and maintenance.
Referring to Figures 32 to 35, the gasifier 2200 comprises a horizontally
oriented
refractory-lined gasification chamber 2202 having a feedstock input 2204,
inputs for hot
air used for heating the gasification chamber, input for steam which serves as
a process
additive, a centrally-located gas outlet 2206 to which the GRS is directly
coupled, a
residue outlet 2208 and various service 2220 and access 2222 ports. The
gasification
chamber 2202 is built as a steel weldment having a stepped floor with a
plurality of floor
steps 2212, 2214, 2216. A system comprising carrier rams 2228, 2230, 2232 is
used to
facilitate the lateral movement of the material through the gasifier 2200.
Provision is
also made for installation of instrumentation, such as thermocouples, material
height
detectors, pressure sensors and viewports.
The refractory lining of the gasification chamber 2202 protects it from high
temperatures, corrosive gases and also minimizes the unnecessary loss of heat
from the
process. Referring to Figure 36, the refractory is a multilayer design with a
high density
chromia layer 2402 on the inside, a middle high density alumina layer 2404 and
an outer
very low density insulboard material 2406. The refractory lines the metal
shell 2408 of
the gasification chamber. The gasification chamber 2402 is further lined with
a
membrane to further protect it from the corrosive gases.
Each step 2212, 2214, and 2216 of the stepped floor of gasification chamber
2402 has a
perforated floor 2270 through which heated air is introduced. The air hole
size is
selected such that it creates a restriction and thus a pressure drop across
each hole
sufficient to prevent waste materials from entering the holes. The holes are
tapered
outwards towards the upper face to preclude particles becoming stuck in a
hole.
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Referring to Figures 27 & 28, the conditions at the three individual steps are
designed
for different degrees of drying, volatilization and carbon conversion. The
feedstock is
introduced into the gasification chamber 2202, onto the first stage via the
feedstock
input 2204. The targeted temperature range for this stage (as measured at the
bottom of
the material pile) lies between 300 and 900 C. Stage II is designed to have a
bottom
temperature range between 400 and 950 C. Stage III is designed to have a
temperature
range between 600 and 1000 C.
The three steps 2212, 2214 & 2216 of the stepped-floor, that separate the
gasification
chamber 2202 into three stages of processing have their own independently
controllable
air feed mechanism. The independence is achieved by using separate airboxes
2272,
2274, and 2276 which form the perforated floor 2270 at each stage. The system
of
carrier rams 2228, 2230 & 2232 used for movement of material in the
gasification
chamber 2202 prevents access from below steps 1 & 2, 2212 & 2214. Thus for
these
stages, the airboxes 2272 & 2274 are inserted from the side. The third stage
airbox 2276
is however inserted from below, as shown in Figures 33 & 34.
The perforated top plate 2302 of the airboxes 2272, 2274, 2276, in this design
and
referring to Figures 37 & 38, is a relatively thin sheet, with stiffening ribs
or structural
support members 2304 to prevent bending or buckling. To minimize stress on the
flat
front and bottom sheets of the boxes, perforated webs are attached between
both sheets.
To allow for thermal expansion in the boxes they are attached only at one edge
and are
free to expand at the other three edges.
As shown in Figure 37, the fixed edge of the Step 1 & 2 airboxes 2272 and 2274
is also
the connection point of the input air piping 2278. Thus, the connection flange
2280 will
be at high temperature and must be sealed to the cool wall of the gasifier
2200. A shroud
is used, as shown in Figure 37, to achieve this without creating stress and
without using
a complex expansion joint. The hot air box 2272 and pipe 2278 are attached to
one end
of the shroud 2282 and the other end of the shroud 2282 is connected to the
cool gasifier
2200. As a temperature gradient will occur across the length of the shroud
2282, there is
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little or no stress at either connection. The other advantage of this
arrangement is that it
positions the airbox rigidly in the required position without causing stress.
The space
between the shroud 2282 and the internal duct of the air box 2272 is filled
with
insulation to retain heat and to ensure the temperature gradient occurs across
the shroud.
When the airbox is in its operating location in the gasification chamber 2202,
the top
plate opposite to the air connection is extended beyond the airbox to rest on
a shelf of
refractory. This provides support to the airbox during operation and also acts
as a seal to
prevent material from falling below the airbox. It also allows free movement
to allow for
expansion of the airbox, as shown in Figure 39.
The downstream edge of the airbox is also dealt with in the same way. The
upstream
edge of the airbox is sealed with a resilient sheet sealing 2306 between the
carrier ram
and the top plate of the airbox 2302.
The airbox is connected to the hot air supply piping using a horizontal
flange. Therefore,
only the flange has to be disconnected to remove an airbox.
The third stage airbox 2276 is inserted from below and also uses the shroud
concept for
sealing and locating the box to the gasifier 2200.
Sealing against dust falling around the edges of the third stage airbox 2276
is achieved
by having it set underneath a refractory ledge at the edge of the second stage
2214. The
sides can be sealed by flexible seals protruding from below recesses in the
sides of the
refractory. These seals sit on the top face of the box, sealing between the
walls and the
box. The downstream edge of the air box is dust sealed to the side of an
extractor trough
using a flexible seal. The box is reinforced with stiffeners and perforated
webs between
the flat faces of the air boxes to permit the use of thin sheet metal for the
boxes.
The hot air pipe connection is vertical to permit removal of the third stage
airbox 2276
after disconnecting the pipe connection.
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Referring to Figure 42, a series of a system of carrier rams 2228, 2230, 2232
is used to
ensure that the MSW is moved laterally along the gasifier 2200 for appropriate
processing in each of the three steps 2212, 2214 & 2216, and that the spent
residue is
moved to the residue outlet 2208. Each of the three stage floors is serviced
by its own
carrier ram. The carrier rams control both the height of the pile at each
stage as well as
the total residence time in the gasification chamber. Each carrier ram is
capable of
movement over the full or partial length of that step, at variable speeds.
Thus, the stage
can also be completely cleared if required.
Each carrier ram comprises an externally mounted guide portion, a carrier ram
having
optional guide portion engagement members, externally mounted drive system and
an
externally mounted control system. The carrier ram design comprises multiple
fingers
that allow the air-box air-hole pattern to be arranged such that operation of
the carrier
rams does not interfere with the air passing through the air-holes.
In the multiple finger carrier ram design, the carrier ram is a structure in
which fingers
are attached to the body of the carrier ram, with individual fingers being of
different
widths depending on location. The gap between the fingers in the multiple
finger carrier
ram design is selected to avoid particles of reactant material from bridging.
The
individual fingers are about 2 to about 3 inches wide, about 0.5 to about 1
inch thick
with a gap between about 0.5 to about 2 inches wide.
The air box air hole pattern is arranged such that operation of the carrier
rams do not
interfere with the air passing through the air holes. For example, the pattern
of the air
holes can be such that when heated they are between the fingers (in the gaps)
and are in
arrow pattern with an offset to each other. Alternatively, the air hole
pattern can also be
hybrid where some holes are not covered and others are covered, such that even
distribution of air is maximized (i.e. areas of floor with no air input at all
are
minimized). In choosing the pattern of the air holes, factors to consider
include avoiding
high velocity which would fluidize the bed, avoiding holes too close to
gasifier walls
and ends so that channeling of air along refractory wall is avoided, and
ensuring spacing
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between holes was no more than approximately the nominal feed particle size
(2") to
ensure acceptable kinetics.
A multi-finger carrier ram can have independent flexibility built-in so that
the tip of each
finger can more closely comply with any undulations in the air-box top face.
This
compliance has been provided by attaching the fingers to the carrier ram main
carriage
using shoulder bolts, which do not tighten on the finger. This concept also
permits easy
replacement of a finger.
The end of the carrier ram finger is bent down to ensure that the tip contacts
the top of
the air in the event that the relative locations of the carrier ram and airbox
changes (for
example, due to expansions). This features also lessens any detrimental effect
on the
process due to air holes being covered by the carrier ram, the air will
continue to flow
through the gap between the carrier ram and the airbox.
Referring to Figure 39, the guide portion comprises a pair of generally
horizontal,
generally parallel elongated tracks 2240 (a), 2240 (b) mounted on a frame.
Each of the
tracks has a substantially L-shaped cross-section. The moving element
comprises a
carrier ram body 2326 and one or more elongated, substantially rectangular
carrier ram
fingers 2328 sized to slide through corresponding sealable opening in the
gasification
chamber wall.
The carrier ram fingers are constructed of material suitable for use at high
temperature.
Such materials are well-known to those skilled in the art and can include
stainless steel,
mild steel, or mild steel partially protected or fully protected with
refractory. Optionally,
specific individual carrier ram fingers or all carrier ram fingers can be
partially or fully
covered with refractory. Optionally, cooling can be provided within the
carrier ram
fingers by fluid (air or water) circulated inside the carrier ram fingers from
outside the
gasification chamber 2202.
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The carrier ram fingers are adapted to sealingly engage the gasification
chamber wall to
avoid uncontrolled air from entering the gasifier 2200, which would interfere
with the
process or could create an explosive atmosphere. It is also necessary to avoid
escape of
hazardous toxic and flammable gas from the gasification chamber 2202, and
excessive
escape of debris. Gas escape to atmosphere is prevented by containing the
carrier ram
mechanisms in a sealed box. This box has a nitrogen purge facility to prevent
formation
of an explosive gas mixture within the box. Debris sealing and limited gas
sealing is
provided for each finger of the carrier ram, using a flexible strip 2308
pressing against
each surface of each finger of the carrier rams, as shown in Figure 40.
Alternatively, the
seal can be a packing gland seal providing gas and debris sealing for each
finger.
The design of this sealing provides a good gas and debris seal for each
carrier ram finger
while tolerating vertical and lateral movements of the carrier ram. The seals
at the sides
of the fingers were the greatest challenge as they must be compliant to the
vertical and
lateral motions of the carrier ram while remaining in close contact with the
carrier ram
and the seals of the upper and lower surfaces of the carrier ram. Leakage of
debris can
be monitored by means of windows in the sealed box and a dust removal facility
is
provided if the debris build-up becomes excessive. This removal can be
accomplished
without breaking the seal integrity of the carrier ram box, as shown in Figure
41.
The dust removal facility 2310 comprises a metal tray 2312 having a dust
outlet 2314
equipped with a shutter 2316 and attachment site 2318 for a dust can 2332, and
a
manual-operated, chain 2320 driven dust pusher 2322. Dust is pushed to the
dust outlet
2314 by the pusher 2322 when the operator handle 2324 is used.
Power for moving the carrier rams 2228, 2230 & 2232 is provided by electric
motors
which drive the carrier ram via a gearbox and roller chain system. Briefly,
the power to
propel the carrier rams along the tracks is supplied by an externally mounted
electric
variable speed motor 2256 which drives a motor output shaft 2258 selectably in
the
forward or reverse direction allowing for extension and retraction of the
carrier ram at a
controlled rate. Position sensor (sensors) 2269 transmit the carrier ram
position
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information to the control system. Optionally, the motor may further comprise
a gear
box. Two driver sprocket gears 2260 are mounted on the motor output shaft. The
driver
sprockets 2260 and corresponding driven sprockets 2262 mounted on an axle 2264
operatively mesh with chain members 2266 which are secured by brackets 2268 to
the
elongated rectangular block 2244.
The motors are controlled by the overall system control means which can
command start
and stop position, speed of movement and frequency of movement. Each carrier
ram
can be controlled independently. Roller chain is used for this implementation
as it
provides high strength and tolerates a severe duty environment. The use of two
chains
per carrier ram provides a means of keeping the carrier rams angularly aligned
without
the need for precision guides. There is a tendency for the material on top of
the carrier
ram to be pulled back when the carrier ram is withdrawn. This can be dealt
with by
sequencing the carrier rams where the lowest carrier ram 2232 is extended
first; the
middle carrier ram 2230 is then extended which pushes material down onto the
lowest
carrier ram 2232 filling the void created by that carrier rams movement; the
lowest
carrier ram 2232 is then retracted; the upper carrier ram 2228 is then
extended filling the
void at the back of the middle carrier ram 2230; the middle carrier ram 2230
is then
retracted; new material dropping from the feed port fills any void on the top
carrier ram
2228 and the top carrier ram 2228 is retracted. All these motions are
controlled
automatically and independently by the system control means in response to
system
instrumentation data.
Referring to Figure 43, a staggered carrier ram sequence control strategy was
implemented to facilitate movement of the carrier rams, as summarized below:
carrier ram C 2232 move fixed distance (with adjustable setpoint), creating a
pocket at the start of step C 2216;
carrier ram B 2230 follows as soon as carrier ram C 2232 passes a trigger
distance (trigger distance has adjustable setpoint) carrier ram B
pushes/carries
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material to immediately fill the pocket at the start of step C 2216. Feedback
control is to stroke as far as necessary to block level switch C 2217, or
minimum setpoint distance if already blocked, or maximum setpoint distance
if blocking does not occur. At the same time as carrier ram B 2230 is filling
the pocket at the start of Step C 2216, it is creating a pocket at the start
of Step
B 2230;
carrier ram A 2228 follows as soon as carrier ram B 2230 passes a trigger
distance. carrier ram A 2228 pushes/carries material to immediately fill the
pocket at the start of Step B 2214. Feedback control is to stroke as far
necessary to block level switch B 2215, or minimum setpoint distance if
already blocked, or maximum setpoint distance if blocking does not occur. At
the same time as carrier ram A 2228 is filling the pocket at the start of Step
B
2214, it is also creating a pocket at the start of Step A 2212. This typically
triggers the feeder to run and fill the gasifier 2200 until level switch A
2213 is
blocked again;
all carrier rams reverse to home position simultaneously.
Access is provided to the gasifier 2200 using a manhole at one end. During
operation,
this is closed using a sealable refractory lined cover. Further access is also
possible by
removing the third stage air-box 2276.
The residue (e.g. char or ash) remaining after gasification must be removed
from the
gasifier 2200 and passed to the residue conditioning system (RCS) 4220. As the
material
is processed and moved in the gasifier 2200, the heat generated within the
pile can cause
melting, which will result in agglomeration of the residue. Agglomerated
residue has
been shown to cause jamming in drop port type exits. In order to ensure that
any
agglomerations do not create jamming at the exit from the gasification chamber
2202, a
screw conveyor 2209 is used to extract the residue from the gasification
chamber 2202.
The carrier ram motion pushes the residue into the extractor screw 2209 which
pushes
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the residue out of the gasification chamber 2202 and feed it into a residue
conveyor
system. Rotation of the extractor screw 2209 breaks up agglomerations before
the
residue is fed into the conveyor system. This breaking up action is enhanced
by having
serrations on the edge of the extractor screw flights.
For implementing process control, various parameters have to be monitored
within the
gasification chamber 2202. For example, the temperature needs to be monitored
at
different points along each stage and at various heights at each stage. This
is achieved
using thermocouples, which tend to need replacement during operation. In order
to
accomplish this without shutting down the process, each thermocouple is
inserted into
the gasification chamber 2202 via a sealed end tube which is then sealed to
the vessel
shell. This design allows the use of flexible wire thermocouples which are
procured to
be longer than the sealing tube so that the junction (the temperature sensing
point) of the
thermocouple is pressed against the end of the sealed tube to assure accurate
and quick
response to temperature change. The sealed tube is sealed to the gasification
chamber
2202 and mechanically held in place by means of a compression gland, which can
also
accommodate protrusion adjustment into the gasification chamber 2202. For
temperature
measurements within the MSW pile, the sealed tube can result in the pile being
held
back when its movement is needed. To avoid this problem the end of the sealed
tube is
fitted with a deflector which prevents the MSW pile from getting blocked by
the
thermocouple tube.
The off-gas produced in the gasifier 2200 then moves into the Gas
Reconstituting
System (GRS) 3200. The GRS 3200 is designed to satisfy a wide range of
requirements:
(a) provide necessary volume for the required gas refining residence time; (b)
provide
insulation for heat conservation and protection of the outer steel vessel; (c)
provide inlets
for addition of air and steam; (d) enable mixing of the gases; (e) process the
gases at
high temperature using plasma torches 3208; (f) provide instrumentation for
monitoring
the gas composition for process control and for enhanced performance of the
plasma
torch 3208; and (g) output the processed gas to a downstream heat exchanger
5200.
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The gas reformulating system (GRS) 3200 provides a sealed environment with
mounting
and connection features for process air, steam, plasma torches 3208 and torch
handling
mechanisms, instrumentation and exhaust of the output syngas. As shown in
Figure 46,
the GRS 3200 comprises a substantially vertically mounted refractory-lined
cylindrical
or pipe-like reformulating chamber 3202 having a single conically shaped off-
gas inlet
3204 to which the gasifier 2200 is connected to via a mounting flange 3214.
The GRS
3200 has a length-to-diameter ratio of about 3:1. The residence time within
the GRS
3200 is 1.2 seconds. The GRS 3200 further comprises three levels of
tangentially
positioned air nozzles, two tangentially located plasma torches 3208, six
thermocouple
ports, two burner ports, two pressure transmitter ports and several spare
ports. The high
temperatures created in the GRS 3200 by the plasma torches 3208 ensure that
the
molecules within the off-gas disassociate into their constituent elements, and
then
combines together to form syngas. The hot crude syngas exits the GRS 3200 via
the
syngas outlet 3206.
As mentioned earlier, the GRS 3200 incorporates supports for refractory
lining. The
major support feature for the refractory is a series of shelves 3222 around
the interior of
the GRS 3200. During operation, these shelves 3222 will be at considerably
higher
temperature than the shell of the reformulating chamber 3202. Therefore, it is
necessary
to avoid any waste of heat by conduction to the GRS 3200, while providing
allowance
for differential expansion. Also, the shelves 3222 must be capable of
supporting the
considerable weight of the refractory. These requirements were met by making
the
shelves 3222 segmented with expansion gaps between segments to allow for the
expansion. Also, there is a gap between the shelf 3222 and the wall to avoid
heat
transfer. To take the weight of the refractory, each shelf segment is
supported by a
number of gussets welded to the wall, as shown in Figure 47. Expansion along
the shelf
3222 along its length would create stress and possibly failure in the gussets
if they were
welded to the gussets. However, by resting the shelf 3222 on the gussets
without
welding, the shelf 3222 is allowed to expand freely. To hold the segment into
its correct
location, it is welded to the center gussets only where the expansion is small
and even
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then only the outer portion is welded. This minimizes any stress on the
gussets and
potential buckling of the shelf 3222.
The top of the reformulating chamber 3202 is capped with a refractory-lined
lid 3203
thereby creating a sealed enclosure. The whole GRS 3200 is coated with a high
temperature resistant membrane internally to prevent corrosion by the
unrefined off-gas.
It is painted on the exterior surfaces with a thermo-chromic paint to reveal
hot spots due
to refractory failure or other causes.
The refractory used is a multilayer design with a high density layer on the
inside to resist
the high temperature, erosion and corrosion that is present in the GRS 3200.
Outside the
high density material is a lower density material with lower resistance
properties but
higher insulation factor. Outside this layer, a very low density foam board
material with
very high insulation factor is used because it will not be exposed to abrasion
of erosion.
The outside layer, between the foam board and the vessel steel shell is a
ceramic blanket
material to provide a compliant layer to allow for differential expansion
between the
solid refractory and the vessel shell. Vertical expansion of the refractory is
provided for
by means of a compressible refractory layer separating sections of the non-
compressible
refractory. The compressible layer is protected from erosion by overlapping
but
extendible high density refractory.
As shown in Figures 48 & 49, air is injected into the off-gas stream by three
levels of air
nozzles that include four jets at the lower level, and another six jets at
upper level, in
which three jets are slightly higher than other three to create cross-jet
mixing effects to
achieve better mixing. Angular blowing of the air into the GRS 3200, achieved
using
deflector at the tip of the input nozzle, also results in better mixing while
allowing the
inlet pipes and flanges to be square with the reformulating chamber 3202. The
improved
mixing of the gases in the GRS 3200 allows for optimal refining of the syngas.
This is
achieved by inducing a swirling action at the base of the reformulating
chamber 3202 by
making use of the process air velocity. Air is injected into the off-gas
stream through
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swirl ports 3212 to create a swirling motion or turbulence in the off-gas
stream thereby
mixing the off-gas and creating a re-circulating vortex pattern within the GRS
3200.
As mentioned earlier, the GRS 3200 also includes two tangentially mounted
300kW,
water cooled, copper electrode, NTAT, DC plasma torches 3208 mounted on a
sliding
mechanism. The DC plasma torches 3208 are powered from a DC power supply.
Thermocouples are positioned at various locations within the GRS 3200 to
ensure that
the temperature of the syngas is maintained at about 1000 C.
The plasma torches 3208 require periodic maintenance and it is most desirable
that they
are replaceable with the process still running. As mentioned earlier, this
implementation
uses two torches 3208 in the GRS 3200 when strictly only one is needed for
operation.
Removal and replacement of the plasma torches 3208 have to be done in the
presence of
high temperature toxic and flammable gas in the GRS 3200. In addition, the
torch 3208
will also need to be removed in the event of failure of the torch cooling
system to protect
it from the heat in the GRS 3200.
These challenges are met by mounting the torch 3208 on a sliding mechanism
that can
move the torch 3208 into and out of the reformulating chamber. The torch 3208
is
sealed to the reformulating chamber 3202 by means of a sealing gland. This
gland is
sealed against a gate valve, which is, in turn, mounted on and sealed to the
vessel. To
remove a torch 3208, it is pulled out of the reformulating chamber 3202 by the
slide
mechanism. Initial movement of the slide disables the high voltage torch power
supply
for safety purposes. The gate valve shuts automatically when the torch 3208
has
retracted past the valve and the coolant circulation is stopped. The hoses and
cable are
disconnected from the torch 3208, the gland is released from the gate valve
and the torch
3208 is lifted away by a hoist.
Replacement of a torch 3208 is done using the reverse of the above procedure;
the slide
mechanism can be adjusted to permit variation of the insertion depth of the
torch 3208.
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For the sake of simplicity and safety, all the above operations except for the
closing of
the gate valve are carried out manually. The gate valve is operated
mechanically so that
operation is automatic. A pneumatic actuator is used to automatically withdraw
the torch
in the event of cooling system failure. Compressed air for operating the
actuator is
supplied from a dedicated air reservoir so that power is always available even
in the
event of electrical power failure. The same air reservoir provides the air for
the gate
valve. An electrically interlocked cover is used a further safety feature by
preventing
access to the high voltage torch connections.
Residue Conditioning System
The residue remaining after the gasification must be rendered inert and usable
before
disposal. This is done by extracting it from the gasifier 2200 into a plasma-
based residue
conditioning chamber (RCC) 4220, melting it and rendering it into an inert
vitreous slag
4203, cooling and shattering the slag 4203 into granules using a quench tank
4240 before
transfer to a slag stockpile 4204 ready for removal from the site. The final
by-product is
suitable for use as road fill or concrete manufacture.
As mentioned earlier, the movement of residue from the gasifier 2200 is
complicated by
the potential for agglomeration caused due to the heat generated within the
pile. This
problem is solved by using a screw type conveyor 2209 at the outlet end of the
gasifier
2200. The conveyor has serrated edges on the screw flights to break up any
agglomerated
material.
The residue is then taken to the RCC 4220 by means of a main conveyor 4210
system
comprising a series of screw conveyors. This conveyor system 4210 also takes
the
residue from the GCS baghouse filter 6230 downstream and passes it onto the
RCC 4220.
To minimize the number of entry ports to the RCC 4220, the residue from all
sources is
combined before introduction to the RCC 4220. This avoids enlarging the RCC
4220 to
cater to multiple feed sources. An additional source of residue may also have
to be
catered for. In order for gasification to continue during RCC 4220 downtime
the residue
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may be diverted. In which case it must be re-introduced into the RCC feed
system. The
overall schematic of the residue conditioning system is shown in Figure 52.
As shown in Figure 54, the residue is dropped into the RCC 4220, where it
accumulates
in a reservoir 4222 whose depth is determined by the height of a weir 4224,
and
undergoes heating by a plasma torch 4230. As the level of the molten slag
rises within the
reservoir 4222 it runs over the weir 4224, dropping into a quench tank 4240.
The water
tank 4240 ensures that the RCC 4220 is sealed to the atmosphere. Any metals
which have
not been removed during the MSW handling system stage is transferred to the
RCC 4220
and will not necessarily be melt at the slag's normal vitrification
temperature. Thus, the
crucible could become clogged with metal as it is of higher density than the
molten slag.
To avoid this, the RCC temperature is periodically raised to melt any metals
and the
molten metals are tapped off from the bottom of the crucible.
Due to the very high temperatures needed to melt the residue and particularly
the
constituent metals in it, the refractory is subjected to very severe
operational demands.
These include corrosion and erosion, particularly at the slag waterline, in
addition to the
high temperature. Also the refractory must provide good insulation to conserve
heat and
the RCC 4220 must be as small as possible. The refractory is selected to
provide an inner
lining of very high resistance to heat, corrosion and erosion. The layers of
refractory
outside the lining are then selected to greater insulation.
It is anticipated that the crucible refractory in particular will require
periodic
maintenance. To allow for this, the bottom of the RCC with the crucible can be
removed
without disturbing any connections to the RCC. This is accomplished by
suspending the
RCC from its support structure 4270 rather than setting it onto a structure,
as shown in
Figure 57. Thus the lower portion of the RCC with the crucible can be dropped
away
from the top without having to disconnect any connections. Also the entire RCC
can be
removed by disconnecting the connections and lowering it. This avoids the need
to lift
the conveyor 4260 and piping out of the way.
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When the molten slag drops into the quench tank 4240 it is cooled and
shattered into
granular form. A slag conveyor 4260 then removes the granular slag 4203 from
the
quench 4240 and places it into a stockpile 4204 for disposal or further use,
as shown in
Figure 56. The slag drop port is sealed to the environment by means of a water
trap
consisting of a shroud sealed to the RCC 4220 at the top and with its lower
edge
submerged in the quench medium. The same quench medium seals the slag conveyor
4260 from the RCC 4220.
The gases produced in the RCC 4220 are treated similarly to the gases produced
in the
converter 1200. The residue gas exits the RCC 4220 via the gas outlet 4228 and
is
directed to a residue gas conditioner (RGCS) 4250. It undergoes a pre-cooling
step in an
indirect air-to-gas heat exchanger 4252 prior to being passed through a
baghouse filter
4254 that removes particulates and heavy metal contaminants. The residue gas
is then
cooled using a second heat exchanger 4256 before it is passed through an
activated
carbon bed 4258 for the further removal of heavy metals and particulate
matter. The
cleaned and conditioned residue gas is diverted back to the downstream GCS
6200 to
feed back with the syngas stream from the converter 1200.
The raw syngas exits the converter 1200 and passes through a Syngas-to-air
Heat
Exchanger (HX) 5200 where the heat is transferred from the syngas stream to a
stream of
air. Thus, the syngas is cooled while the resulting hot stream of air is fed
back to the
converter 1200 as process air. The cooled syngas then flows into a Gas
Conditioning
System (GCS) 6200, where the syngas is further cooled and cleaned of
particulates,
metals and acid gases sequentially. The cleaned and conditioned syngas (with
desired
humidity) is stored in the syngas HC 7230 before being fed to gas engines 9260
where
electricity is generated. The functions of the major components (equipment) in
the system
after the converter 1200 and RCS 4200 are outlined in Table I, in the sequence
in which
the syngas is processed.
Table 1 Steps after Converter 1200 and RCS 4200
Subsystem or equipment Main Function
Heat Exchanger 5200 Cool down syngas and recover sensible heat
Evaporative Cooler 6210 Further cooling down of syngas prior to baghouse
Dry Injection System 6220 Heavy metal adsorption
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Baghouse 6230 Particle or dust collection
HCL Scrubber 6240 HCl removal and syngas cooling/conditioning
Carbon Filter Bed 6260 Further mercury removal
H2S Removal System 6270 H2S removal and elemental sulfur recovery
RGCS 4250 RCC off-gas cleaning and cooling
Syngas Storage 7230 Syngas storage and homogenization
Chiller 7210; Gas/Liquid Separator 7220 Humidity control
Gas Engines 9260 Primary driver for electricity generation
Flare Stack 9299 Burning syngas during start-up
Syngas-to-air Heat Exchanger (Recuperator)
The output syngas leaving the GRS 3200 is at a temperature of about 900 C to
1100 C.
In order to recover the heat energy in the syngas, the raw syngas exiting from
GRS 3200
is sent to a shell-tube type syngas-to-air heat exchanger (HX) 5200. Air
enters the HX
5200 at ambient temperature, i.e., from about -30 to about 40 C. The air is
circulated
using air blowers 5210, and enters the HX 5200 at a rate between 1000 Nm3/hr
to 5150
Nm3/hr, typically at a rate of about 4300 Nm3/hr.
The syngas flows vertically through the tube side 5202 and the air flows in a
counter-
clockwise fashion through the shell side 5206. The syngas temperature is
reduced from
1000 C to between 500 C and 800 C, (preferably about 740 C) while the air
temperature
is increased from ambient temperature to between 500 C and 625 C (preferably
about
600 C). The heated exchange-air is recirculated back into the converter 1200
for
gasification.
The HX 5200 is designed specifically for high level of particulates in the
syngas. The
flow directions of the syngas and the air are designed to minimize the areas
where build
up or erosion from particulate matter could occur. Also, the gas velocities
are designed to
be high enough for self cleaning while still minimizing erosion.
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Due to the significant temperature difference between the air and syngas, each
tube 5220
in the HX 5200 has its individual expansion bellows 5222. This is essential to
avoid tube
rupture, which can be extremely hazardous since the air will enter the syngas
mixture.
Possibility for tube rupture is high when a single tube becomes plugged and
therefore no
longer expands/contracts with the rest of the tube bundle.
Multiple temperature transmitters are placed on the gas outlet box of the gas-
to-air heat-
exchanger 5200. These are used to detect any possible temperature raise that
occurs due
to combustion in the event of an air leak into the syngas. The air blower 5210
is
automatically shut down in such a case.
The material for the gas tubes in the HX 5210 has to be carefully selected to
ensure that
corrosion is not an issue, due to concerns about sulphur content in the syngas
and its
reaction at high temperatures. In our implementation, Alloy 625 was selected.
Gas Conditioning System (GCS)
In general, a gas conditioning system (GCS) 6200 refers to a series of steps
which
converts the crude syngas obtained after the heat exchanger 5200 into a form
suitable for
downstream end applications. In our implementation, the GCS 6200 can be broken
down
into two main stages. Stage 1 comprises of: (a) an evaporative cooler (dry
quench) 6210;
(b) a dry injection system 6220; and (c) a baghouse filter (used for
particular
matter/heavy metal removal) 6230. Stage 2 comprises of (d) a HCl scrubber
6240; (e) a
syngas (process gas) blower 6250; (f) a carbon filter bed (mercury polisher)
6260; (g) a
H2S (sulphur) removal system 6270; and (h) humidity control using a chiller
7210 and
gas/liquid separator 7220.
The heat exchanger 5200 before the GCS 6200 is sometimes considered as part of
Stage
1 of the GCS 6200. The syngas (process gas) blower 6250 typically includes a
gas cooler
6252 which is sometimes mentioned separately in Stage 2 of the GCS 6200. Also,
humidity control mentioned here as part of Stage 2 of the GCS 6200 is often
considered
part of the syngas regulation system 7200 further downstream to the GCS 6200.
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Figure 59 shows a block diagram of the GCS 6200 implemented in our system.
This is
also an example of a converging process in which the GCS 6200 is integrated
with the
RGCS 4250. Figure 60 shows a view of the layout of the GCS.
After initial cooling in the heat exchanger 5200, the input syngas is further
cooled by dry
quenching, which lowers the syngas temperature and also prevents condensation.
This is
achieved using an evaporative cooling tower (a.k.a `dry quench') 6210 by
direct injection
of water into the gas stream in a controlled manner (adiabatic saturation).
The water is
atomized before it is sprayed co-currently into the syngas stream. As no
liquid is present
in the cooling, the process is also called dry quench. When the water is
evaporated, it
absorbs the sensible heat from syngas thus reducing its temperature from 740 C
to
between 150 C and 300 C (typically about 250 C). Controls are added to ensure
that
water is not present in the exiting gas. The relative humidity at the exiting
gas
temperature is therefore still below 100%.
Once the gas stream exits the evaporative cooling tower 6210, activated
carbon, stored in
a hopper, is pneumatically injected into the gas stream. Activated carbon has
a very high
porosity, a characteristic that is conducive to the surface adsorption of
large molecular
species such as mercury and dioxin. Therefore, most of the heavy metals
(cadmium, lead,
mercury etc.) and other contaminants in the gas stream are adsorbed on the
activated
carbon surface. The spent carbon granules are collected by the baghouse 6230
and
recycled back to the RCS 4200 for further energy recovery as described in the
next step.
For obtaining efficient adsorption, it is necessary to ensure that the syngas
has sufficient
residence time in this stage. Other materials such as feldspar, lime, and
other absorbents
can also be used instead of, or in addition to, activated carbon in this dry
injection stage
6220 to capture heavy metals and tars in the syngas stream without blocking
it.
Particulate matter and activated carbon with heavy metal on its surface is
then removed
from the syngas stream in the baghouse 6230, with extremely high efficiency.
The
operating parameters are adjusted to avoid any water vapour condensation. All
particulate
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matter removed from the syngas stream forms a filter cake which further
enhances the
efficiency of the baghouse 6230. So while new non-coated bags have a removal
efficiency of 99.5%, the baghouse 6230 is typically designed for 99.9 %
particulate
matter removal efficiency. The baghouse 6230 employs lined fiber glass bags,
unlined
fibre glass bags or P84 basalt bags and is operated at a temperature between
200 C and
260 C.
When the pressure drop across the baghouse 6230 increases to a certain set
limit, nitrogen
pulse-jets are used to clean the bags. Nitrogen is preferred to air for safety
reasons. The
residue falling from the outside surface of the bags are collected in the
bottom hopper and
are sent to the residue conditioner 4200 for further conversion or disposal.
Special
reagents can be used to absorb the high molecular weight hydrocarbon compounds
(tars)
in order to protect the baghouse 6230. Figure 63 & 62 shows the schematic and
design of
the baghouse respectively.
The baghouse uses cylindrical filters which do not require support.
A typical operational specification of the baghouse 6230 (assuming the input
is fly-ash
with heavy metals) is as follows:
Design Gas flow rate - 9500 Nm3/hr
Dust loading - 7.4 g/Nm3
Cadmium - 2.9 mg/ Nm3
Lead - 106.0 mg/Nm3
Mercury - 1.3mg/ Nm3
Guaranteed filtration system outlet:
Particulate matter - 1lmg/Nm3 (about 99.9% removal)
Cadmium - 15 g/Nm3 (about 99.65% removal)
Lead - 159 g/Nm3 (about 99.9% removal)
Mercury - 190 g/Nm3 (about 90% removal)
The quantity of residue contaminated with heavy metals exiting the baghouse
6230 is
large. Therefore, as shown in Figure 59, this residue is sent to the plasma-
based RCC
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4220 for conversion into vitreous slag 4203. The secondary gas stream created
in the
RCC 4220 is then treated in a separate residue gas conditioner (RGCS) 4250
with the
following Stage 1 processes: cooling in an indirect air-to-gas heat exchanger
4252 and
removal of particulate matter and heavy metals in a smaller baghouse 4254. The
smaller
baghouse 4254 is dedicated to treating the secondary gas stream generated in
the RCC
4220. As shown in Figure 59, additional steps carried out by the RGCS 4250
include
cooling the gas further using a gas cooler 4256, and removing heavy metals and
particulate matter in a carbon bed 4258. The processed secondary syngas stream
is then
diverted back to the GCS 6200 to feed back into the primary input syngas
stream prior to
the baghouse filter 6230.
The quantity of residue removed from the bag-house 4254 of the RGCS 4250 is
significantly less compared to the baghouse 6230 in the GCS 6200. The small
baghouse
4254 acts as a purge for the heavy metals. The amount of heavy metals purged
out of the
RGCS 4250 will vary depending on MSW feed composition. A periodic purge is
required
to move this material to hazardous waste disposal, when the heavy metals build-
up to a
specified limit.
Below is a typical design specification for the smaller RGCS baghouse 4254,
once again
assuming that the input is fly-ash with heavy metals:
Design Gas flow rate - 150 Nm3/hr
Dust loading - 50g/Nm3
Cadmium - 440mg/ Nm3
Lead - 16.6 mg/Nm3
Mercury - 175mg/ Nm3
Guaranteed filtration system outlet:
Particulate matter - 10mg/Nm3 (about 99.99% removal)
Cadmium - 13 g/Nm3 (about 99.997% removal)
Lead - 166 g/Nm3 (about 99.999% removal)
Mercury - 175 g/Nm3 (about 99.9% removal)
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The GCS 6200 may comprise direct and indirect feedback or monitoring systems.
In our
implementation, both the GCS and RGCS baghouse filters have a dust sensor on
the exit
(direct monitoring) to notify of a bag rupture. If a bag rupture occurs, the
system is
shutdown for maintenance. Optionally, the water stream in the HC1 scrubber
6240 can be
analyzed at start-up to confirm particulate matter removal efficiency.
The particulate-free syngas stream exiting from the baghouse 6230 is scrubbed
in a
packed tower using a re-circulating alkaline solution to remove any HC1
present. This
HC1 scrubber 6240 also provides enough contact area to cool down the gas to
about
35 C. A carbon bed filter 6260 is used to separate the liquid solution from
potential
soluble water contaminants, such as metals, HCN, ammonia etc. The HCI scrubber
6240
is designed to keep the output HC1 concentration at about 5ppm. A waste water
bleed
stream is sent to a waste water storage tank 6244 for disposal.
For metallurgical considerations, the HC1 scrubber 6240 is located upstream of
the gas
blower 6250. An exemplary schematic diagram of an HCI scrubber 6240 including
associated components such as heat exchangers 6242 is shown in Figure 64.
Figure 65
shows an exemplary system for collecting and storing waste water from the GCS
6200. A
carbon bed 6245 is added to the water blowdown to remove tars and heavy metals
from
the wastewater. Typical specification for the HC1 scrubber 6240 is as follows:
Design Gas flow rate - 9500 Nm3/hr
Normal Inlet / Max HC1 loading to scrubber - 0.16 % / 0.29 %
HC1 outlet concentration - 5 ppm
After HCI removal, a gas blower 6250 is employed which provides the driving
force for
the gas through the entire system 120 from the converter 1200 to the gas
engines 9260
downstream. The blower 6250 is located upstream of the mercury polisher 6260
as the
latter has a better mercury removal efficiency under pressure. This also
reduces the size
of the mercury polisher 6260. Figure 29 shows a schematic of the entire system
120 and
the position of the process gas blower 6250.
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The blower 6250 is designed using all upstream vessel design pressure drops.
It is also
designed to provide the required pressure for downstream equipment pressure
losses to
have a final pressure of -2.1 to 3.0 psig (typically 2.5psig) in the HC 7230.
As the gas is
pressurized when passing through the blower 6250, its temperature rises to
about 77 C. A
built-in gas cooler 6252 is used to reduce the temperature back to 35 C, as
maximum
operating temperature of the H2S removal system 6270 is about 40 C.
A carbon bed filter 6260 is used as a final polishing device for any heavy
metal
remaining in the syngas stream. Its efficiency is improved when the system is
under
pressure instead of vacuum, is at lower temperature, gas is saturated, and
when the HC1 is
removed so that is does not deteriorate the carbon. This process is also
capable of
absorbing other organic contaminants, such as dioxins from the syngas stream
if present.
The carbon bed filter 6260 is designed for over 99% mercury removal
efficiency.
The performance of this system is measured by periodically analyzing the gas
for
mercury. Corrections are made by modifying the carbon feed rate and monitoring
the
pressure drop across the polisher 6260, and by analyzing the carbon bed
efficiency via
sampling.
Typical specification for the carbon bed filter 6260 is as follows:
Design Gas flow rate - 9500 Nm3/hr
Normal/Max Mercury loading - 190 g/Nm3 / 1.3mg/Nm3
Carbon bed life - 3-5 years
Guaranteed mercury carbon bed outlet - 19 g/Nm3 (99%)
The H2S removal system 6270 was based on SO2 emission limitation outlined in
A7
guide lines of the Ministry of Environment, Ontario, Canada, which states that
syngas
being combusted in the gas engines will produce SO2 emission below 15ppm. The
H2S
removal system 6270 was designed for an output H2S concentration of about
20ppm.
Figure 66 shows the details of the H2S removal system 6270.
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The Shell Paques Biological technology was selected for H2S removal 6270. This
technique consists of two steps: First, syngas from the carbon bed filter 6260
passes
through a scrubber 6272 where H2S is removed from syngas by re-circulating an
alkaline
solution. Next, the sulphur containing solution is sent to a bioreactor 6274
for
regeneration of alkalinity, oxidation of sulfide into elemental sulphur,
filtration of
sulphur, sterilization of sulphur and bleed stream to meet regulatory
requirements. The
H2S removal system 6270 is designed for 20 ppm H2S outlet concentration.
Thiobacillus bacteria are used in the bioreactor 6274 to converts sulfides
into elemental
sulphur by oxidation with air. A control system 8200 controls the air flow
rate into the
bio-reactor to maintain sulphur inventory in the system. A slip stream of the
bio reactor
6274 is filtered using a filter press 6276. Filtrate from filter-press 6276 is
sent back to the
process, a small stream from this filtrate is sent as a liquid bleed stream.
There are two
sources of discharge; one solid discharge - sulphur with some biomass and one
liquid
discharge - water with sulphate, carbonate and some biomass. Both streams are
sterilized
before final disposal.
Typical specification for the H2S removal system 6270 is as follows:
Design Gas flow rate - 8500 Nm3/hr
Normal / Max H2S loading - 353 ppm/666 ppm
Guaranteed H2S outlet for system - 20ppm
After the H2S removal, a chiller 7210 is used to condense the water out of the
syngas and
reheat it to a temperature suitable for use in the gas engines 9260. The
chiller 7210 sub-
cools the gas from 35 C to 26 C. The water condensed out from the input gas
stream is
removed by a gas/liquid separator 7220. This ensures that the gas has a
relative humidity
of 80% once reheated to 40 C (engine requirement) after the gas storage prior
to being
sent to the gas engines 9260.
The following table gives the major specifications of the entire GCS 6200:
Quench Tower 6210 quench gas from 740 C to 200 C in 2 sec residence time
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Dry Injection 6220 90 % mercury removal efficiency
Baghouse Filter 6230 99.9 % Particulate removal efficiency
99.65% Cadmium removal efficiency
99.9% Lead removal efficiency
HCI Scrubber 6240 99.8 % HCI removal efficiency
Gas Blower 6250 Zero leak seal rotary blower
Gas Cooler 6252 0.5 MBtu/hr cooling load
Carbon Bed Filter 6260 99 % mercury removal efficiency
H2S Scrubber 6270 H2S at scrubber outlet - 20 ppm
Bioreactor 6274 Maximum regeneration efficiency with minimum blow-
down
Filter Press 6276 2 days sulphur removal capacity
Homogenization Chamber 2 min gas storage capacity
7230
As noted above, the GCS 6200 converts an input gas to an output gas of desired
characteristics. Figure 59 depicts an overview process flow diagram of this
GCS system
6200 which is integrated with a plasma gasification system and downstream
application.
Here, the secondary gas stream generated in the RCS 4200 is fed into the GCS
6200.
The Residue Gas Conditioner (RGCS)
As mentioned earlier, the residue from the GCS baghouse 6230 which may contain
activated carbon and metals is purged periodically by nitrogen and conveyed to
the RCC
4220, where it is vitrified. The gas coming out of the RCC 4220 is directed
through a
residue gas conditioner (RGCS) 4250 baghouse 4254 to remove particulates and
is cooled
by a heat exchanger 4256 before entering an activated carbon bed 4258. The
baghouse
4254 is also periodically purged based on pressure drop across the system. The
residue
collected in the RGCS baghouse 4254 is disposed by appropriate means. The
combustible
gas exiting from the RGCS 4250 as a secondary gas stream is sent back to the
main GCS
system 6200 to fully utilize the recovered energy.
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Syn ag s Regulation System
The cleaned and cooled syngas from the GCS 6200 enters a syngas regulation
system
7200 designed (SRS) to ensure that the syngas flowing to the downstream gas
engines
9260 is of consistent gas quality. The SRS 7200 serves to smooth out short-
term
variations in gas composition (primarily its low heating value -LHV) and its
pressure.
While the downstream gas engines 9260 will continue to run and produce
electricity
even with short-term variations in the LHV or pressure of the syngas, it may
deviate
from its threshold emission limits due to poor combustion or poor fuel to air
ratio.
The SRS 7200 comprises a chiller 7210, a gas/liquid separator 7220 and a
homogenization chamber (HC) 7230. The gas is heated on the exit of the gas
storage
prior to the gas engines 9260 to meet engine temperature requirements.
Two types of homogenization chambers (HC) are available: a fixed volume HC and
a
variable volume HC. The latter is typically more useful to reduce flow and
pressure
fluctuation while the former is more useful to reduce LHV fluctuations. LHV
fluctuations are more prominent in our application due to the nature of the
MSW
feedstock. A fixed volume HC is also typically more reliable than variable
volume in
terms of its construction and maintenance.
Figures 68 show the schematic of the homogenization chamber (HC) 7230 used in
this
implementation. It is designed to hold about 2 minutes of syngas flow. This
hold up time
meets the gas engine guaranteed norms on LHV fluctuation specifications of
about 1%
LHV fluctuation/30 sec. The residence time up to the gas analyzer 8130 is
typically
about 30 sec (including analysis and feedback). The maximum LHV fluctuation is
typically about 10%. Thus, to average this out and get 3% LHV fluctuation,
>1.5 min
storage is needed. The 2 min storage allows for some margin.
The HC 7230 is operated at a range of 2.2 to 3.0 psig to meet the fuel
specifications of
the downstream gas engines 9260. The exiting gas pressure is kept constant
using a
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pressure control valve. The HC 7230 is designed for a maximum pressure of
5psig and a
relief valve is installed to handle unusual overpressure scenarios.
The 2 min hold up time of the HC 7230 also provides enough storage to reduce
pressure
fluctuations. For our design, the allowable pressure fluctuation for the gas
engine 9260 is
0.145 PSI/sec. In the case of a downstream failure of the gas engine 9260, a
buffer may
be required (depending on control system response time and 30-35 sec gas
resident
times) to provide time to slow down the process or to flare the excess gas.
Typical syngas flow rate into the HC 7230 is at - 8400 Nm3/hr. Therefore, for
a hold up
time of 2 min, the HC's volume has to be about 280 m3.
The HC 7230 is free-standing and is located outside where it will be exposed
to snow,
rain and wind. Therefore, the dimensions of the HC 7230 are designed to meet
mechanical engineering requirements. Its support structure 7232 interfaces
with a
concrete foundation.
As some water will condense out of the syngas, a bottom drain nozzle is
included in the
design of the HC 7230. To assist in the drainage of the HC 7230, its bottom is
intentionally designed to not be flat, but as a conical bottom with a skirt.
Traced/insulated drain piping is used to form the drain flange. As the water
within the
HC 7230 has to gravity drain to the floor drain, the HC 7230 is kept slightly
elevated.
The HC 7230 is designed to meet the following design requirements.
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Normal / Maximum Inlet Temperature 35 C / 40 C
Normal / Maximum Operating Pressure 1.2 psig / 3.0 psig
3 3
Normal / Maximum Gas Inlet Flow Rate 7000 Nm
/hr / 8400 Nm /hr
3 3
Normal / Maximum Gas Outlet Flow Rate 7000 Nm
/hr / 8400 Nm /hr
Relative Humidity 60%-100%
Storage Volume 290 m
Mechanical Design Temperature -40 C to 50 C
Mechanical Design Pressure 5.0 psig
The material used for the HC 7230 has to take into account both the mechanical
design
requirements above and the typical gas composition given below. Corrosion is
particularly a concern due to the presence of water, HC1, and H2S.
N2 47.09%
CO2 7.44%
H2S 20 ppm
H2O 3.43%
CO 18.88%
H2 21.13%
CH4 0.03%
HC1 5 ppm
The following openings are provided in the HC 7230:
One 36" manhole near the bottom for accessibility;
One 6" flange at the top for relief;
One 16" flange on the shell for gas inlet;
One 16" flange on the shell for gas outlet;
Six 1" flanges on the shell (2 for pressure, 1 for temperature and 3 as
spares);
One 2" flange at the bottom of HC (drain); and
One 1" flange on the bottom cone for level switches.
In addition to satisfying the design requirements, the HC 7230 also provides:
Openings, manhole covers, and blind flanges for all spare nozzles.
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A ladder allowing safe access, (e.g. with railing) to the roof and relief
valve.
Required lifting hooks and anchor bolts.
A concrete ring wall.
Interior and exterior coatings of the HC 7230, if required.
Insulation and heat tracing of the bottom of the HC 7230.
A concrete slab for support.
The gas engine 9260 design requires that the inlet gas be of a specific
composition range
at a specified relative humidity. Therefore, the cleaned gas that exits the
H2S scrubber
6270 is sub-cooled from 35 C to 26 C using a chiller 7210. Any water that is
formed
due to the condensation of the gas stream is removed by the gas/liquid
separator 7220.
This ensures that the syngas has a relative humidity of 80% once reheated to
40 C, a
typical requirement for gas engines 9260.
A gas blower 6250 is used to withdraw syngas from the system by providing
adequate
suction through all the equipment and piping as per specifications below. The
blower
design took heed to good engineering practice and all applicable provincial
and national
codes, standards and OSHA guidelines. Operation of the blower 6250 was at
about 600
Volts, 3 phase, and 60 Hz.
The gas blower 6250 was designed to meet following functional requirements.
Normal gas inlet temperature 35 C
Normal gas suction pressure -1.0 psig
Normal gas flow rate 7200 Nm3/hr
Maximum gas flow rate 9300 Nm3/hr
Maximum gas suction temperature 40C
Normal discharge pressure 3.0 psig
Normal discharge temperature (after gas cooler) <35 C
Mechanical design pressure 5.0 psig
Relative Humidity of gas at blower inlet 100 %
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Gas Molecular Weight 23.3
Cooling water supply temperature (product gas cooler) 29.5 C
Maximum acceptable gas discharge temperature (after product gas cooler) 40 C
Turn down ratio 10%
The typical gas composition (wet basis) drawn is as follows:
CH4 0.03%
CO 18.4%
CO2 7.38%
H2 20.59%
Normal / Max H2S 354 / 666 ppm
H2O 5.74%
Normal / Max HCl 5 ppm / 100 ppm
N2 47.85%
As the syngas is flammable and creates an explosive mixture with air, the
blower 6250 is
configured such that there is minimal to no air intake from the atmosphere,
and minimal
to no gas leak to the atmosphere. All service fluids, i.e., seal purges are
done with
nitrogen and a leak-free shaft seal is used. Advanced leak detection systems
are
employed to monitor leaks in either direction.
In addition to the design criteria above, the blower 6250 also provides:
An explosion proof motor with leak-free blower shaft seal.
A gas cooler 6252.
A silencer with acoustic box to meet noise regulation of 80 dBA at 1m.
A common base plate for the blower and motor.
An auxiliary oil pump with motor, and all required instrumentations for
blower auxiliary system.
All instruments and controls (i.e. low and high oil pressure switch, high
discharge pressure and temperature switch, differential temperature and
pressure switch). All switches are CSA approved discharge pressure gauge,
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discharge temperature gauge, oil pressure and temperature gauge. All
instruments are wired at a common explosion proof junction box and the VFD
is controlled by a pressure transmitter installed upstream of the blower.
A zero leaks discharge check valve.
Equipment safety system to prevent blower from excessive pressure /vacuum/
shut off discharge (e.g. systems like PRV and recycle line).
As the gas blower 6250 is located outside the building, exposed to rain, snow
and wind.
The gas blower 6250 is configured to withstand the following environmental
conditions.
Elevation above mean sea level - 80 m
Latitude - 450 24' N
Longitude - 750 40' W
Average atmospheric pressure - 14.5 psia
Maximum summer dry bulb temperature - 38 C
Design summer dry bulb temperature - 35 C
Design summer wet bulb temperature - 29.4 C
Minimum winter dry bulb temperature - 36.11 C
Mean wind velocity - 12.8 ft/sec
Maximum wind velocity - 123 ft/sec
Design wind velocity - 100 mph/ 160 kph
Prevailing wind direction - Mainly from south and west
Seismic Information - Zone 3
Since the blower 6250 works in an environment where explosive gases may be
present,
all instruments and electrical devices installed on syngas pipes or within
about 2 meter
distance are designed for the classification of Class 1, zone 2.
For ensuring reliability, proper access for inspection and maintenance is
provided, as is
access to isolate and correct faults quickly. While the blower 6250 can be
operated
continuously (24/7), frequent start/stop operation is more common during
process
stabilization are contemplated.
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The material of construction was chosen based on design conditions and gas
composition. For example, electrical circuit boards, connectors and external
components
were coated or otherwise protected to minimize potential problems from dirt,
moisture
and chemicals. Control panels and switches are of robust construction,
designed to be
operated by personnel with work gloves.
Generally, variable speed drive (VSD) with a flow range of 10% to 100% is
employed
for motor control. Over-voltage and overload protection are included. The
motor status,
on/off operation and change of speed are monitored and controlled remotely
through the
distributed control system (DCS).
Once the regulated gas exits the HC 7230, it is heated to the engine
requirement and
directed to the gas engines 9260.
Gas Engines
Five reciprocating GE Jenbacher gas engines 9260 with 1MW capacity each are
used to
produce electricity. So, the full capacity of electricity generation is 5 MW.
Optionally,
any of the gas engines 9260 can be turned off depending on the overall
requirements.
The gas engine 9260 is capable of combusting low or medium heating value
syngas with
high efficiency and low emissions. However, due to the relatively low gas
heating value
(as compared to fuels such as natural gas) the gas engines 9260 have been de-
rated to
operate around 700kW at their most efficient operating point.
Flare Stack
An enclosed flare stack 9299 will be used to burn syngas during start-up, shut-
down and
process stabilization phases. Once the process has been stabilized the flare
stack 9299
will be used for emergency purposes only. The flare stack 9299 is designed to
achieve a
destruction efficiency of about 99.99%.
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Control System
In this implementation, the gasification system 120 of the present example
comprises an
integrated control system for controlling the gasification process implemented
therein,
which may include various independent and interactive local, regional and
global
processes. The control system may be configured to enhance, and possibly
optimise the
various processes for a desired front end and/or back end result.
A front-to-back control scheme could include facilitating the constant
throughput of
feedstock, for example in a system configured for the gasification of MSW,
while
meeting regulatory standards for this type of system. Such front-to-back
control scheme
could be optimised to achieve a given result for which the system is
specifically
designed and/or implemented, or designed as part of a subset or simplified
version of a
greater control system, for instance upon start-up or shut-down of the process
or to
mitigate various unusual or emergency situations.
A back-to-front control scheme could include the optimisation of a product gas
quality
or characteristic for a selected downstream application, namely the generation
of
electricity via downstream gas engines 9260. While the control system could be
configured to optimise such back-end result, monitoring and regulation of
front-end
characteristics could be provided in order to ensure proper and continuous
function of
the system in accordance with regulatory standards, when such standards apply.
The control system may also be configured to provide complimentary results
which may
be best defined as a combination of front-end and back-end results, or again
as a result
flowing from any point within the system 120.
In this implementation, the control system is designed to operate as a front-
to-back
control system upon start-up of the gasification process, and then progress to
a back-to-
front control system when initial start-up perturbations have been
sufficiently attenuated.
In this particular example, the control system is used to control the
gasification system
120 in order to convert feedstock into a gas suitable for a selected
downstream
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application, namely as a gas suitable for consumption by a gas engine 9260 in
order to
generate electricity. In general, the control system generally comprises one
or more
sensing elements for sensing various characteristics of the system 120, one or
more
computing platforms for computing one or more process control parameters
conducive
to maintaining a characteristic value representative of the sensed
characteristic within a
predetermined range of such values suitable for the downstream application,
and one or
more response elements for operating process devices of the gasification
system 120 in
accordance with these parameters.
For example, one or more sensing elements could be distributed throughout the
gasification system 120 for sensing characteristics of the syngas at various
points in the
process. One or more computing platforms communicatively linked to these
sensing
elements could be configured to access characteristic values representative of
the sensed
characteristics, compare the characteristic values with predetermined ranges
of such
values defined to characterise the product gas as suitable for the selected
downstream
application, and compute the one or more process control parameters conducive
to
maintaining these characteristic values within these predetermined ranges. The
plurality
of response elements, operatively linked to one or more process devices and/or
modules
of the gasification system operable to affect the process and thereby adjust
the one or
more characteristics of the product gas, can be communicatively linked to the
one or
more computing platforms for accessing the one or more computed process
control
parameters, and configured to operate the one or more processing devices in
accordance
therewith.
The control system may also be configured to provide for an enhanced front-end
result,
for example, for an enhanced or constant consumption and conversion rate of
the input
feedstock, or again as part of start-up, shut-down and/or emergency procedure,
or again,
configured to implement the process of the gasification system 120 so to
achieve a
predetermined balance between front-end benefits and back-end benefits, for
instance
enabling the conversion of the feedstock to produce a product gas suitable for
a selected
downstream application, while maximising throughput of feedstock through the
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converter. Alternative or further system enhancements could include, but are
not limited
to, optimising the system energy consumption, for instance to minimise an
energetic
impact of the system and thereby maximise energy production via the selected
downstream application, or for favouring the production of additional or
alternative
downstream products such as consumable product gas(es), chemical compounds,
residues and the like.
A high-level process control schematic is provided for this example in Figure
69,
wherein the process to be controlled is provided by the gasification system
120
described above. Figure 70 provides an alternative depiction of the system 120
and
control system of Figure 69 to identify exemplary characteristics and sensing
elements
associated therewith. As described above, the system 120 comprises a converter
1200,
comprising a gasifier 2200 and GRS 3200 in accordance with the present
example, for
converting the one or more feedstocks (e.g. MSW and plastics) into a syngas
and a
residue product. The system 120 further comprises a residue conditioning
system (RCS)
4200 and a heat exchanger 5200 conducive to recuperating heat form the syngas
and, in
this example, using this recuperated heat for heating the air input additive
used in the
converter 1200. A gas conditioning system (GCS) 6200 for conditioning (e.g.
cooling,
purifying and/or cleaning) the syngas is also provided, and a regulation
system 7200
used for at least partially homogenising the syngas for downstream use. As
depicted
herein, residue may be provided to the RCS 4200 from both the converter 1200
and the
GCS 6200, the combination of which being conditioned to yield a solid product
(e.g.
vitrified slag 4203) and a syngas to be conditioned and combined with the
converter
syngas for further conditioning, homogenisation and downstream use.
In Figures 69 and 70, various sensing and response elements are depicted and
configured
to provide various levels of control for the system 120. As discussed
hereinabove,
certain control elements may be used for local and/or regional system
controls, for
example in order to affect a portion of the process and/or subsystem thereof,
and
therefore, may have little or no effect on the overall performance of the
system. For
example, while the GCS 6200 may provide for the conditioning and preparation
of the
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syngas for downstream use, its implementation, and variations absorbed
thereby, may
have little effect on the general performance and output productivity of the
system 120.
On the other hand, certain control elements may be used for regional and/or
global
system controls, for example in order to substantially affect the process
and/or system
120 as a whole. For example, variation of the feedstock input via the MSW
handling
system 9200 and/or plastics handling means 9250 may have a significant
downstream
effect on the product gas, namely affecting a change in composition and/or
flow, as well
as affect local processes within the converter 1200. Similarly, variation of
the additive
input rate, whether overall or discretely for different sections of the
converter 1200, may
also have a significant downstream effect on the product gas, namely to the
gas
composition and flow. Other controlled operations, such as reactant transfer
sequences
within the converter 1200, airflow distribution adjustments, plasma heat
source power
variations and other such elements may also effect characteristics of the
product gas and
may thus be used as a control to such characteristics, or again be accounted
for by other
means to reduce their impact on downstream application.
In Figures 69 and 70, various sensing elements are depicted and used in the
present
example to control various local, regional and global characteristics of the
gasification
process. For instance, the system 120 comprises various temperature sensing
elements
for sensing a process temperature at various locations throughout the process.
In Figure
69, one or more temperature sensing elements are provided for respectively
detecting
temperature variations within the converter 1200, in relation to the plasma
heat source
3208, and in relation to the residue conditioning process in RCS 4200. For
example,
independent sensing elements (commonly identified by temperature transmitter
and
indicator control 8102 of Figure 69) may be provided for sensing temperatures
Ti, T2
and T3 associated with the processes taking place within Stages 1, 2 and 3 of
the gasifier
2200 (e.g. see Figure 70). An additional temperature sensing element 8104 may
be used
to sense temperature T4 (e.g. see Figure 70) associated with the reformulating
process of
the GRS 3200 and particularly associated with the output power of the plasma
heat
source 3208. In this example, a temperature sensing element 8106 is also
provided for
sensing a temperature within the RCC 4220 (e.g. temperature T5 of Figure 70),
wherein
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this temperature is at least partially associated with the output power of the
residue
conditioner plasma heat source 4230. It will be appreciated that other
temperature
sensing elements may also be used at various points downstream of the
converter 1200
for participating in different local, regional and/or global processes. For
example,
temperature sensing elements can be used in conjunction with the heat
exchanger 5200
to ensure adequate heat transfer and provide a sufficiently heated air
additive input to the
converter 1200. Temperature monitors may also be associated with the GCS 6200
to
ensure gases conditioned thereby are not too hot for a given sub-process, for
example.
Other such examples should be apparent to the person skilled in the art.
The system 120 further comprises various pressure sensing elements operatively
disposed throughout the system 120. For instance, a pressure sensing element
(depicted
as pressure transmitter and indicator control 8110 in Figure 69) is provided
for sensing a
pressure within the converter 1200 (depicted in the example of Figure 70 as
particularly
associated with GRS 3200), and operatively associated with blower 6500 via
speed
indicator control, variable frequency drive and motor assembly 8113 for
maintaining an
overall pressure within the converter 1200 below atmospheric pressure; in this
particular
example, the pressure within the converter 1200, in one embodiment, is
continuously
monitored at a frequency of about 20Hz and regulated accordingly. In another
embodiment, the blower is maintained at a frequency of about 20Hz or above in
accordance with operational requirements; when blower rates are required below
20Hz
an override valve may be used temporarily. A pressure sensing element 8112 is
also
provided in operative association with the RCC 4220 and operatively linked to
a control
valve leading residue conditioner gas from the RCC 4220 to the GCS 6200.
Pressure
sensing element 8116, is also provided for monitoring input air pressure to
the heat
exchanger 5200 and is operatively linked to blower 5210 for regulating same
via speed
indicator control, variable frequency drive and motor assembly 8120. A
pressure control
valve 8115 is optionally provided as a secondary control to override and
adjust pressure
within the system when the syngas blower speed 6250 falls below the blower's
minimum operating frequency
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Another pressure sensing element 8114 is further provided with the syngas
regulation
system (SRS) 7200 and operatively linked to control valve 7500 for controlled
and/or
emergency release of syngas via flare stack 9299 due to excess pressure, for
example
during start-up and/or emergency operations. This pressure sensing element
8114 is
further operatively linked to control valve 8122 via flow transmitter and
control
indicator 8124 to increase a process additive input flow to the converter 1200
in the
event that insufficient syngas is being provided to the SRS 7200 to maintain
continuous
operation of the gas engines 9260, for example. This is particularly relevant
when the
control system is operated in accordance with a back-to-front control scheme,
as will be
described in greater detail below. Note that in Figure 70, the air flow
sensing element
8124 and control valve 8122 are used to regulate the additive air flows to
Stages 1, 2 and
3 of the gasifier 2200, as depicted by respective flows F1, F2 and F3, and
additive air
flow to the GRS 3200, as depicted by flow F4, wherein relative flows are set
in
accordance with a pre-set ratio defined to substantially maintain pre-set
temperature
ranges at each of the process stages. For example, a ratio F1:F2:F3:F4 of
about
36:18:6:40 can be used to maintain relative temperatures Ti, T2 and T3 within
ranges of
about 300-600 C, 500-900 C and 600-1000 C respectively, or optionally
within ranges
of about 500-600 C, 700-800 C and 800-900 C, respectively, particularly
upon input
of additional feedstock to compensate for increased combustion due to
increased
volume, as described below.
The system 120 also comprises various flow sensing elements operatively
disposed
throughout the system 120. For instance, as introduced above, a flow sensing
element
8124 is associated with the air additive input to the converter 1200 and
operatively
linked to the control valve 8122 for adjusting this flow, for example in
response to a
detected pressure drop within the SRS 7200 via sensing element 8114. A flow
sensing
element 8126 is also provided to detect a syngas flow to the SRS 7200, values
derived
from which being used to regulate both an air additive input rate as a fast
response to a
decrease in flow, and adjust a feedstock input rate, for example in accordance
with the
currently defined fuel to air ratio (e.g. the (MSW+plastics):(Total additive
air input)
ratio currently in use), via MSW and/or plastics feeding mechanisms 9200 and
9250
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respectively, for longer term stabilisation; this again is particularly useful
when the
system is operated in accordance with a back-to-front control scheme, as
described
below. In this example the air to fuel ratio is generally maintained between
about 0 to 4
kg/kg, and during normal operation is generally at about 1.5 kg/kg. A flow
sensing
element 8128 may also be provided to monitor flow of excess gas to the flare
stack
9299, for example during start-up, emergency and/or front-to-back control
operation, as
described below.
Figures 69 and 70 also depict a gas analyser 8130 for analysing a composition
of the
syngas as it reaches the SRS 7200, the control system being configured to use
this gas
composition analysis to determine a syngas fuel value and carbon content and
adjust the
fuel to air ratio and MSW to plastics ratio respectively and thereby
contribute to regulate
respective input rates of MSW and plastics. Once again, this feature is
particularly useful
in the back-to-front control scheme implementation of the control system,
described in
greater detail below.
Not depicted in Figures 69 and 70, but described above with reference to an
exemplary
embodiment of the gasifier 2200, is the inclusion of various sensing elements
configured
for detecting a height of reactant within the gasifier 2200 at various
locations, namely at
steps 1, 2 and 3 2212, 2214 & 2216. These sensing elements may be used to
control the
motion of the lateral transfer means, such as carrier rams 2228, 2230 & 2232
to enhance
effective processing within the gasifier 2200. In such an example, a carrier
ram sequence
controller would both affect computation of an actual feedstock input rate, as
would
variation in the desired feedstock input rate need to be communicated to the
carrier ram
sequence controller. Namely, the carrier ram sequence controller can be used
to adjust a
feedstock input rate, and the control system, in communication with the
carrier ram
sequence controller, may be used to compensate for variations induced by
changes in the
carrier ram sequence (e.g. to address issues raised due to various detected
reactant
distributions) in downstream processes.
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Figure 71 provides a control flow diagram depicting the various sensed
characteristic
values, controllers (e.g. response elements) and operating parameters used by
the control
system of the present example, and interactions there between conducive to
promoting
proper and efficient processing of the feedstock. In this figure:
a converter solids levels detection module 8250 is configured to cooperatively
control a transfer unit controller 8252 configured to control motion of the
transfer
unit(s) 8254 and cooperatively control a total MSW+HCF feed rate 8256;
a syngas (product gas) carbon content detection module 8258 (e.g. derived from
gas analyser 8130) is operatively coupled to a MSW:HCF ratio controller 8260
configured to cooperatively control an MSWIHCF splitter 8262 for controlling
respective MSW and HCF feed rates 8264 and 8266 respectively;
a syngas (product gas) fuel value determination module 8268 (e.g. LHV =
cl*[H2] + c2*[CO], where cl and c2 are constants and where [H2] and [CO] are
obtained from the syngas analyser 8130) is operatively coupled to a Fuel:Air
ratio
controller 8270 for cooperatively controlling the total MSW+HCF feed rate 8256
directed to the MSW:HCF splitter 8262 and the transfer unit controller 8252;
a syngas flow detection module 8272 is operatively coupled to a total airflow
controller 8274 for controlling a total airflow 8276 and cooperatively control
the
total MSW+HCF feed rate 8256; and
a process temperature detection module 8278 is operatively coupled to a
temperature controller(s) 8280 for controlling an airflow distribution 8282
and plasma heat 8284 (e.g. via PHS 1002).
In this configuration, in order to determine the amount of air additive to
input into the
system 120 to obtain a syngas composition within an appropriate range for the
downstream application, or again within a range conducive to increasing the
energetic
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efficiency and/or consumption of product gas, the control system may be
configured to
compute a control parameter based on an acquired characteristic value for the
LHV (e.g.
from analysis of [H2] and [CO] of syngas). For instance, by setting the
temperature and
pressure constant, or at a desired set point, a global system parameter may be
defined
empirically such that the air input parameter may be estimated with sufficient
accuracy
using a linear computation of the following format:
[LHV] = a[Air]
wherein a is an empirical constant for a particular system design and desired
output
characteristics. Using this method, it has been demonstrated that the system
120 of the
present example may be operated efficiently and continuously to meet
regulatory
standards while optimising for process efficiency and consistency.
Figure 72 provides an alternative control flow diagram depicting the various
sensed
characteristic values, controllers (e.g. response elements) and operating
parameters that
can be used by a slightly modified configuration of the control system 8000,
and
interactions therebetween conducive to promoting proper and efficient
processing of the
feedstock. In this figure:
a converter solids levels detection module 8350 is configured to cooperatively
control a transfer unit controller 8352 configured to control motion of the
transfer
unit(s) 8354 and cooperatively control a total MSW+HCF feed rate 8356;
a syngas (product gas) carbon content detection module 8358 (e.g. derived from
gas analyser 8130) is operatively coupled to a MSW:HCF ratio controller 8360
configured to cooperatively control an MSW/HCF splitter 8362 for controlling
respective MSW and HCF feed rates 8364 and 8366 respectively;
a syngas (product gas) [H2] content detection module 8367 (e.g. obtained from
the
syngas analyser 8130) is operatively coupled to a Fuel:Air ratio controller
8370
for cooperatively controlling the total MSW+HCF feed rate 8356 for
cooperatively controlling the transfer unit controller 8352, the MSW/HCF
splitter
8362, the steam flow calculation 8390 and the total airflow 8376;
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a syngas (product gas) [CO] content detection module 8369 (e.g. obtained from
the syngas analyser 8130) is operatively coupled to a Fuel:Steam ratio
controller
8371 for cooperatively controlling the steam flow calculation 8390 for
controlling
the steam addition rate 8392 (note: steam additive input mechanism may be
operatively coupled to the converter 1000 (not shown in Figures 69 and 70) and
provided to compliment air additive and participate in refining the chemical
composition of the syngas);
a syngas flow detection module 8372 is operatively coupled to a total airflow
controller 8374 for cooperatively controlling a total airflow 8376 and
cooperatively controlling the total MSW+HCF feed rate 8356; and
a process temperature detection module 8378 is operatively coupled to a
temperature controller 8380 for controlling an airflow distribution 8382 and
plasma heat 8384.
In this configuration, in order to determine the amount of air additive and
steam additive
to input into the system 120 to obtain a syngas composition within an
appropriate range
for the downstream application, or again within a range conducive to
increasing the
energetic efficiency and/or consumption of product gas, the control system may
be
configured to compute control parameters based on acquired characteristic
values for
[H2] and [CO]. For instance, by setting the temperature and pressure constant,
or at a
desired set point, global system parameters may be defined empirically such
that the air
and steam input parameters may be estimated with sufficient accuracy using a
linear
computation of the following format:
r Hz a b ][Air
CO c d Steam
wherein a, b, c and d are empirical constants for a particular system design
and desired
output characteristics. The person of skill in the art will appreciate that
although
simplified to a linear system, the above example may be extended to include
additional
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characteristic values, and thereby provide for the linear computation of
additional control
parameters. Higher order computations may also be considered to refine
computation of
control parameters as needed to further restrict process fluctuations for more
stringent
downstream applications. Using the above, however, it has been demonstrated
that the
system 120 of the present example may be operated efficiently and continuously
to meet
regulatory standards while optimising for process efficiency and consistency.
It will be appreciated that the various controllers of the control system
generally operate
in parallel to adjust their respective values, which can include both absolute
(e.g. total air
flow) and relative values (e.g. feed to air ratio), although it is also
possible for some or
all of the controllers to operate sequentially.
As discussed above, a front-to-back (or supply-driven) control strategy is
used in the
present example during start-up operation of the system 120 where the
converter 1200 is
run at a fixed feed rate of MSW. Using this control scheme, the gasification
system 120
allows for process variations to be absorbed by the downstream equipment such
as gas
engines 9260 and flare stack 9299. A small buffer of excess syngas is
produced, and a
small continuous flare is hence used. Any extra syngas production beyond this
normal
amount can be sent to the flare, increasing the amount flared. Any deficiency
in syngas
production first eats into the buffer, and may eventually require generator
power output
to be reduced (generators can be operated from 50 - 100% power output via an
adjustable power set point) or further system adjustments to be implemented by
the
control system, as described below. This control scheme is particularly
amenable to
start-up and commissioning phases.
The main process control goals of this front-to-back control scheme comprise
stabilizing
the pressure in the HC 7230, stabilizing the composition of the syngas being
generated,
controlling pile height of material in the gasification chamber 2202,
stabilizing
temperatures in the gasification chamber 2202, controlling temperatures in the
reformulating chamber 3202, and controlling converter process pressure.
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When using GE/Jenbacher gas engines 9260, the minimum pressure of product gas
is
about 150 mbar (2.18 psig), the maximum pressure is about 200 mbar (2.90
psig), the
allowed fluctuation of fuel gas pressure is about +/- 10% (+/- 17.5 mbar, +/-
0.25 psi)
while the maximum rate of product gas pressure fluctuation is about 10 mbar/s
(0.145
psi/s). The gas engines 9260 have an inlet regulator that can handle small
disturbances in
supply pressure, and the holdup in the piping and HC act somewhat to deaden
these
changes. The control system however still uses a fast acting control loop to
act to
maintain suitable pressure levels. As mentioned above, the converter 1200 in
this
control scheme is run at sufficient MSW feed rate to generate a small buffer
of excess
syngas production, which is flared continuously. Therefore the HC 7230
pressure control
becomes a simple pressure control loop where the pressure control valves in
the line
from HC 7230 to the flare stack 9299 are modulated as required to keep the HC
pressure
within a suitable range.
The control system generally acts to stabilize the composition of the syngas
being
generated. The gas engines 9260 can operate over a wide range of fuel values,
provided
that the rate of change is not excessive. The allowable rate of change for
Lower Heating
Value (LHV) relevant in this example is less than 1% fluctuation in syngas LHV
per 30
second. For hydrogen based fuels, the fuel gas is adequate with as little as
15%
hydrogen by itself, and the LHV can be as low as 50 btu/scf (1.86 MJ/Nm3). The
system volume and HC 7230 aid in stabilizing the rate of change of LHV by
providing
about 2 minutes of syngas production.
In this control scheme, the product gas composition can be measured by the gas
analyzer
8130 installed at the inlet of the HC 7230, or proximal thereto. Based on this
measurement, the control system can adjust the fuel-to-air ratio (i.e.
slightly
increase/decrease MSW feed rate relative to air additive input air) in order
to stabilize
the gas fuel value. Increasing either the MSW or plastics feed relative to the
air addition
increases the fuel value of the gas. It will be appreciated, however, that
this control
action may have a relatively long response time depending on the overall
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implementation of the system 120, and as such, may be tuned to prevent long-
term drift
rather than respond to short-term variation.
While the plastics feed is by itself a much richer fuel source (e.g. LHV of
about twice
that of MSW), it is typically added in a ratio of about 1:20 (0 to 14%) with
the MSW,
and therefore, in accordance with this example, it is not the dominant player
in terms of
fuel being added to the system. Since it can be uneconomical to add too much
plastics
to the system 120, the plastics feed may be used as a trim rather than as a
primary
control. In general, the PLASTICS FEED is ratioed to the total feed with the
ratio
optionally adjusted to stabilise the total carbon exiting the system 120 in
the syngas, as
measured by the gas analyzer 8130. This may thus have for affect to dampen
fluctuations in MSW fuel value.
In addition, a reactant pile level control system may be used to aid in
maintaining a
stable pile height inside the converter 1200. Stable level control may prevent
fluidisation
of the material from process air injection which could occur at low level and
to prevent
poor temperature distribution through the pile owing to restricted airflow
that would
occur at high level. Maintaining a stable level may also help maintain
consistent
converter residence time. A series of level switches in the gasification
chamber 2202
may be used, for example, to measure pile depth. The level switches in this
example
could include, but are not limited to, microwave devices with an emitter on
one side of
the converter and a receiver on the other side, which detects either presence
or absence
of material at that point inside the converter 1200. The inventory in the
gasifier 2200 is
generally a function of feed rate and carrier ram motion (e.g. carrier ram
motion), and to
a lesser degree, the conversion efficiency.
In this example, the Stage 3 carrier ram(s) sets the converter throughput by
moving at a
fixed stroke length and frequency to discharge residue from the gasifier 2200.
The Stage
2 carrier ram(s) follows and moves as far as necessary to push material onto
Stage 3 and
change the Stage 3 start-of-stage level switch state to "full". The Stage 1
carrier ram(s)
follows and moves as far as necessary to push material onto Stage 2 and change
the
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Stage 2 start-of-stage level switch state to "full". All carrier rams are then
withdrawn
simultaneously, and a scheduled delay is executed before the entire sequence
is repeated.
Additional configuration may be used to limit the change in consecutive stroke
lengths
to less than that called for by the level switches to avoid excess carrier ram-
induced
disturbances. The carrier rams may be moved fairly frequently in order to
prevent over-
temperature conditions at the bottom of the converter. In addition, full
extension carrier
ram strokes to the end of each stage may be programmed to occur occasionally
to
prevent stagnant material from building up and agglomerating near the end of
the stage.
It will be apparent to the person skilled in the art that other carrier ram
sequences may be
considered herein without departing from the general scope and nature of the
present
disclosure.
In order to optimize conversion efficiency, in accordance with one embodiment
of the
present invention, the material is maintained at as high a temperature as
possible, for as
long as possible. Upper temperature limits are set to avoid the material
beginning to melt
and agglomerate (e.g. form clinkers), which reduces the available surface area
and hence
the conversion efficiency, causes the airflow in the pile to divert around the
chunks of
agglomeration, aggravating the temperature issues and accelerating the
formation of
agglomeration, interferes with the normal operation of the carrier rams, and
potentially
causes a system shut down due to jamming of the residue removal screw 2209.
The
temperature distribution through the pile may also be controlled to prevent a
second kind
of agglomeration from forming; in this case, plastic melts and acts as a
binder for the
rest of the material.
In one embodiment, temperature control within the pile is achieved by changing
the flow
of process air into a given stage (i.e. more or less combustion). For
instance, the process
air flow provided to each stage in the bottom chamber may be adjusted by the
control
system to stabilize temperatures in each stage. Temperature control utilizing
extra carrier
ram strokes may also be used to break up hot spots. In one embodiment, the air
flow at
each stage is pre-set to maintain substantially constant temperatures and
temperature
ratios between stages. For example, about 36% of the total air flow may be
directed to
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stage 1, about 18 % to Stage 2, and about 6% to Stage 3, the remainder being
directed to
the GRS (e.g. 40% of total air flow). Alternatively, air input ratios may be
varied
dynamically to adjust temperatures and processes occurring within each stage
of the
gasifier 2200 and/or GRS 3200.
Plasma heat source power (e.g. plasma torch power) may also be adjusted to
stabilize
exit temperatures of the GRS 3200 (e.g. reformulating chamber output) at the
design set
point of about 1000 degrees C. This may be used to ensure that the tars and
soot formed
in the gasification chamber 2202 are fully decomposed. Addition of process air
into the
reformulating chamber 3202 may also bear a part of the heat load by releasing
heat
energy with combustion of the syngas. Accordingly, the control system may be
configured to adjust the flow rate of process air to keep torch power in a
good operating
range.
Furthermore, converter pressure may be stabilized by adjusting the syngas
blower's
6250 speed, in the embodiment of Figure 69, depicted proximal to the
homogenisation
subsystem input. At speeds below the blower's minimum operating frequency, a
secondary control may override and adjust a recirculation valve instead. Once
the
recirculation valve returns to fully closed, the primary control re-engages.
In general, a
pressure sensor 8110 is operatively coupled to the blower 6250 via the control
system,
which is configured to monitor pressure within the system, for example at a
frequency of
about 20Hz, and adjust the blower speed via an appropriate response element
8113
operatively coupled thereto to maintain the system pressure within a desired
range of
values.
A residue melting operation is also performed in a continuous operation in a
separate
vessel (e.g. RCC 4220) which is directly connected to the outlet of the
converter 1200.
The residue is removed from the gasification chamber 2202 by a toothed screw
conveyor
(residue extraction screw) or the like mounted at the end of the gasifier 2200
and fed
into the top of the RCS 4200 via a series of screw conveyors, for example. A
small
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stream of particulate from the bag house filters 6230 may also join the main
stream of
residue via screw conveyors, for example, for further processing.
The RCS 4200 is a small, refractory-lined residue conditioning chamber (RCC)
4220
with a 300 kW plasma torch 4230 mounted into the top, a process gas outlet
4228
connecting a gas treatment skid, and a molten slag outlet 4226. The gas
exiting the gas
treatment skid may be directed to join the main stream of syngas from the
converter
1200 at the inlet to the main baghouse 6230, or directed alternatively for
further
processing. In this example, the residue drops directly into the top of the
RCC 4220
where it is melted by close contact with the plasma torch plume 4230. The
molten slag
is held-up, for example, by a vee-notch weir 4224 inside the RCC 4220. As
additional
residue particles flow into the RCC 4220 and are melted, a corresponding
amount of
molten material overflows the weir 4224 and drops into a water-filled quench
tank 4240
integral with a screw conveyor where it solidifies, shatters into small pieces
of glass-like
slag, and is conveyed to a storage container.
In controlling the residue processing, the power of the plasma torch 4230 may
be
adjusted as needed to maintain temperatures adequate for the melting
operation. The
RCC 4220 temperature instrumentation (e.g. temperature sensing element 8106)
may
include, for example, two optical thermometers (OT's) which measure the
surface
temperature of the surface upon which they are aimed, 3 vapour space
thermocouples
mounted in ceramic thermo wells above the melt pool, and 5 external skin
mounted
thermocouples mounted on the outer metal shell. The RCC 4220 may also include
a
pressure transmitter for measuring process pressure (e.g. pressure sensing
element 8112)
inside the RCC 4220.
One melt temperature control strategy contemplated herein is to measure the
delta
temperature being observed by the two optical thermometers. One OT is aimed at
the
melt pool below the torch 4230, the other at the melt pool near the weir 4224.
If the
temperature near the weir 4224 is cooling off compared to the temperature
below the
torch 4230, then more torch power is applied. An alternative is to use the OT
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temperatures directly. A set point in the range of 1400 - 1800 C, known to be
above the
melting temperature of most MSW components is entered into the controller.
Torch
power is then adjusted as required to meet this set point.
In general, the level is not measured directly, but is inferred by both OT
temperature and
vapour space thermocouples. If the temperature falls below the temperature set
point,
this is an indication of un-melted material and interlocks will be used to
momentarily
slow the feed rate of residue, or to shut down the RCS 4200 as a last resort.
The rate of
material flow may be controlled by adjusting the RCC feed screw conveyor speed
via
drive motor variable frequency drives (VFD's), for example. The feed rate may
be
adjusted as required to ensure acceptable temperature control, within
capability of
melting rate of plasma torches 4230, and to prevent high levels in the RCC
4220 due to
un-melted material. In general, there may be some hold-up capacity for residue
beyond
Stage 3 in the gasification chamber 2202, but sustained operation will depend
on the
RCC 4220 having adequate melting capacity matching the steady-state production
of
residue.
The pressure in the RCC 4220 may be monitored by a pressure transmitter tapped
into
the vapour space of the vessel (e.g. element 8112). In general, the operating
pressure of
the RCC 4220 is somewhat matched to that of the converter gasification chamber
2202
such that there is minimal driving force for flow of gas through the screw
conveyors in
either direction (flow of solid residue particles only). A control valve 8134
is provided
in the gas outlet line which can restrict the flow of gas that is being
removed by the
downstream vacuum producer (syngas blower). A DCS PID controller calculates
the
valve position needed to achieve the desired operating pressure.
Beyond the start-up phase, a back-to-front control, or demand-driven control
can be used
where the gas engines 9260 at the back-end of the system 120 drive the
process. The gas
engines 9260 consume a certain volume/hr of fuel depending on the energy
content of
the fuel gas (i.e. product gas) and the electrical power being generated.
Therefore the
high level goal of this control system is to ensure that adequate MSW/plastics
feed
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enters the system 120 and is converted to syngas of adequate energy content to
run the
generators at full power at all times, while adequately matching syngas
production to
syngas consumption such that flaring of syngas is reduced, or even eliminated,
and the
electrical power produced per ton of MSW consumed is enhanced, and preferably
optimized.
In general, the front-to-back control scheme described above comprises a sub-
set of the
back-to-front control scheme. For instance, most if not all process control
goals listed in
the above scheme are substantially maintained, however the control system is
further
refined to reduce flaring of syngas while increasing the amount of electrical
power
produced per ton of MSW, or other such feedstock, consumed. In order to
provide
enhanced control of the process and achieve increased process efficiency and
utility for a
downstream application, the flow of syngas being produced is substantially
matched to
the fuel being consumed by the gas engines 9260; this thus reduces reduce
flaring or
otherwise disposition of excess product gas from the system 120, and reduces
the
likelihood of insufficient gas production to maintain operation of the
downstream
application. Conceptually, the control system therefore becomes a back-to-
front control
(or demand-driven control) implemented such that the downstream application
(e.g. gas
engines/generators) drive the process.
In general, in order to stabilize syngas flow out of the converter 1200 in the
short term,
the air additive input flow into the converter 1200 may be adjusted, providing
a rapid
response to fluctuations in gas flow, which are generally attributed to
variations in
feedstock quality variations (e.g. variation in feedstock humidity and/or
heating value).
In general, effects induced by an adjustment of airflow will generally
propagate within
the system at the speed of sound. Contrarily, though adjustment of the MSW
and/or
plastics feed rate may also significantly affect system output (e.g. syngas
flow), the
feedstock having a relatively long residence time within the converter 1200
(e.g. up to
45 minutes or more for this particular example), system response times
associated with
such adjustment will generally range at about 10 to 15 minutes, which on the
short term,
may not be sufficient to effect the product gas in a timely manner to avoid
unwanted
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operating conditions (e.g. flared excess gas, insufficient gas supply for
optimal
operation, insufficient gas supply for continuous operation, etc.). While
still having a
slower response than an increase in airflow, an increase in MSW feed rate may
result in
a faster response than an increase in PLASTICS FEED because the moisture
content of
MSW may produce steam in about 2 to 3 minutes.
Accordingly, adjusting total airflow generally provides the fastest possible
acting loop to
control pressure and thereby satisfy input flow requirements for the
downstream
application. In addition, due to the large inventory of material in the
converter 1200,
adding more air, or other such additive, to the bottom chamber does not
necessarily
dilute the gas proportionately. The additional air penetrates further into the
pile, and
reacts with material higher up. Conversely, adding less air will immediately
enrich the
gas, but eventually causes temperatures to drop and reaction rates/syngas flow
to
decrease.
Therefore, total airflow is generally ratioed to material feed rate
(MSW+plastics) as
presented in Figure 71, whereby an increase in additive input will engender an
increase
in feedstock input rate. Accordingly, the control system is tuned such that
the effect of
increased air is seen immediately, whereas the effect of the additional feed
is eventually
observed to provide a longer term solution to stabilizing syngas flow.
Temporarily
reducing generator power output may also be considered depending on system
dynamics
to bridge the dead time between increasing the MSW/plastics feed rate and
seeing
increased syngas flow, however, this may not be necessary or expected unless
faced with
unusual feedstock conditions. While adjustments to airflow (the fastest acting
control
loop) and adjustments to the fuel to air ratio and the total fuel rate (both
longer term
responses) are preferred in this example to maintain suitable gas
characteristics for the
downstream application, the MSW to plastics feed ratio control is not
necessary, but
may act as an additional control used to help smooth out long term
variability.
In this example, MSW moisture content generally varies between 0 and 80%, and
heating
values vary between about 3000 and 33000kJ/kg, and the HC has a 2 minute
residency
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time and generally a pressure of about 210 mbar. A variation of about +/- 60
mbar is
possible without exceeding the minimum supply pressure for the engine of about
150
mbar. Without the control system, the pressure can vary by up to about 1000
mbar, hence
the long term flow fluctuations are actively reduced by the control system by
up to 4
times (or 75%) in order to run the gas engine 9260 at constant load as
desired.
Furthermore, pressure fluctuations of the converter gas can reach about 25
mbar/s
without the control system, which is about 2.5 times the maximum of about 10
mbar/s for
the engine of this example (or about 60%). Hence, the control system of the
present
invention may reduce short time process variability by at least 2.5 times
(60%) and long
term process variability by about 4 times (75%). Use of the HC 7230 in this
example can
help reduce the short term variations.
Accordingly, in view of the foregoing results, it will be appreciated that the
control
system of the present invention can be used to effectively convert a feedstock
of
substantially inhomogeneous characteristics and/or composition to produce a
gas having
substantially stable characteristics conducive for downstream application.
Therefore,
depending on a particular configuration of a gasification system controlled by
the present
control system, fluctuations in feedstock characteristics may be attenuated
via continuous
and/or real-time control of this system for example reducing long term process
variability
by at least 4 times. In an alternative embodiment, fluctuations in feedstock
characteristics
may be attenuated via continuous and/or real-time control of this system to
reduce long
term process variability by about 3 times. In an alternative embodiment,
fluctuations in
feedstock characteristics may be attenuated via continuous and/or real-time
control of this
system to reduce long term process variability by about 2 times.
EXAMPLE 2
In this example an alternative control scheme is presented for a gasification
system such
as the one presented in Example 1. This alternative control scheme is
presented in Figure
73 and is a variant of the control scheme presented in Figure 71, wherein a
syngas
pressure detection module is used instead of a syngas flow detection module.
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EXAMPLE 3
In this example an alternative control scheme is presented for a gasification
system such
as the one presented in Example 1. Figure 74 provides an alternative control
flow
diagram depicting the various sensed characteristic values, controllers (e.g.
response
elements) and operating parameters used by the control system 8000 of the
present
example, and interactions therebetween conducive to promoting proper and
efficient
processing of the feedstock. In this example:
a converter solids levels detection module 8550 is configured to cooperatively
control a transfer unit controller 8552 configured to control motion of the
transfer
unit(s) 8554 and cooperatively control a total MSW+HCF feed rate 8556;
a syngas (product gas) carbon content detection module 8558 (e.g. derived from
a
gas analyser) is operatively coupled to a MSW:HCF ratio controller 8560
configured to cooperatively control the total MSW+HCF feed rate 8556 for
controlling an MSW/HCF splitter 8562 (for controlling respective MSW and HCF
feed rates 8564 and 8566 respectively) and cooperatively controlling both a
transfer unit controller 8552 and a total airflow controller 8574;
a syngas (product gas) flow value detection module 8572 is operatively coupled
to
the total feed controller 8596 for cooperatively controlling both the total
airflow
controller 8574 and the total MSW+HCF feed rate 8556;
a syngas (product gas) fuel value determination module 8568 (e.g. LHV =
cl*[H2] + c2*[CO], where c1 and c2 are constants and where [H2] and [CO] are
obtained from a syngas analyser) is operatively coupled to a Fuel:Air ratio
controller 8570 for cooperatively controlling the total airflow controller
8574 for
controlling a total airflow 8566 and a total MSW+HCF feed rate 8556; and
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a process temperature detection module 8578 is operatively coupled to
temperature controllers 8580 for controlling an airflow distribution 8582 and
plasma heat 8584.
EXAMPLE 4
In this example an alternative control scheme is presented for a gasification
system such
as the one presented in Example 1. Figure 75 provides another alternative
control flow
diagram depicting the various sensed characteristic values, controllers (e.g.
response
elements) and operating parameters that can be used by a slightly modified
configuration
of the control system 8000, and interactions therebetween conducive to
promoting proper
and efficient processing of the feedstock. In this figure:
a converter solids levels detection module 8650 is configured to cooperatively
control a transfer unit controller 8652 configured to control motion of the
transfer
unit(s) 8654 and cooperatively control a total MSW+HCF feed rate 8656;
a syngas (product gas) carbon content detection module 8658 (e.g. derived from
a
gas analyser) is operatively coupled to a MSW:HCF ratio controller 8660
configured to cooperatively control an MSW/HCF splitter 8662 for controlling
respective MSW and HCF feed rates 8664 and 8666 respectively;
a syngas (product gas) [H2] content detection module 8667 (e.g. obtained from
a
syngas analyser) is configured to cooperatively control a Fuel:Air ratio
controller
8670 for cooperatively controlling the total MSW+HCF feed rate 8656;
a syngas (product gas) opacity detection module 8698 for cooperatively
controlling both the Fuel:Air ratio controller 8670 and the Fuel:Steam ratio
controller 8671;
a syngas (product gas) [CO] content detection module 8669 (e.g. obtained from
a
syngas analyser) is configured to cooperatively control a Fuel:Steam ratio
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controller 8671 for cooperatively controlling a steam flow calculation 8690
for
controlling a steam addition rate 8692;
a syngas flow detection module 8672 is operatively coupled to a total airflow
controller 8674 for cooperatively controlling a total airflow 8676 and
cooperatively controlling the total MSW+HCF feed rate 8656; and
a process temperature detection module 8678 is operatively coupled to a
temperature controller(s) 8680 for controlling an airflow distribution 8682
and
plasma heat 8684.
EXAMPLE 5:
Figures 17 and 18 provide a further example of how the control system can be
used to
control conversion of a carbonaceous feedstock into a gas. In this example,
water is
preheated into steam as an additive input, oxygen preheated as an air additive
input, and
carbonaceous feedstock preheated for feeding into a converter for conversion.
On the
output of the converter, the flow rate, temperature and composition of the
product gas is
monitored using one or more sensing elements.
As shown in Figure 18, the sensed gas flow rate, %CO and %C02 is used to
estimate a
carbon content of the product gas and thereby adjust a feedstock feed rate.
The sensed
%CO and %C02 are further used, as well as the sensed %H2, to estimate a new 02
and
steam input rate to achieve a desired gas composition. Finally, the sensed gas
temperature
is used to adjust a plasma heat source power, if needed.
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