Note: Descriptions are shown in the official language in which they were submitted.
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SMALL-SCALE CLEAN FUEL GAS PRODUCTION SYSTEM USING FLEXIBLE
FUEL GASIFICATION
REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to and the benefit of the Greek Patent
Application No.
20210100526, filed on August 02, 2021.
BACKGROUND
[0002] The process of gasification has been used for energy production for
more than 180
years. Initially, the supply consisted of coal and peat which were gasified in
gasification plants
to produce gas for lighting and cooking purposes. Subsequently, the method of
gasification was
used both to produce gas, which in turn was used in the production of
electricity, and in blast
furnaces. Currently, the most common use of gasification is for the production
of synthetic
chemicals. Through gasification and proper treatment of the process'
derivatives, high-quality
gases and/or liquid fuels are produced. This process was adopted on a large
scale during both
World Wars to counter-measure oil shortages during the Wars.
[0003] The gas produced by the gasification of biomass can be standardized in
terms of its
quality and used as a pure combustible gas for heating, power generation or as
feedstock for
chemical synthesis. The advantages of gasification over combustion are the
same as those
which characterize a gaseous fuel, compared to a solid fuel, i.e. higher heat
release rates, higher
combustion efficiency, reduced environmental impact, fewer ash-related
problems, direct
combustion of the gas in internal combustion engines and application in
combined cycles, as
well as easy distribution of the gas over short distances.
[0004] Due to the flexibility of the producer gas, for small-scale power
plants (solid fuel
consumption less than 150 kg/h on a dry basis) gas engine coupling is the
optimal solution in
terms of efficiency/cost ratio. Such small-scale plants are particularly
suitable for areas and
installations which produce solid residues (i.e., solid fuels of low energy
quality and high ash
content) from which quantities of combustible gas can potentially be produced
for energy
production. Such areas are agricultural areas where organic residues remain
after harvest
completion (i.e. harvesting of sugar cane, corn, cereals, etc.), areas with
developed logging,
plants where agricultural products are processed (i.e. olive mills, stone
fruit jam production
plants, etc.), livestock units (where feces of high straw content, such as
poultry, sheep, etc. are
produced), wood processing plants, organic waste management facilities (biogas
plants, waste-
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water treatment facilities) and more. Gasification of these solid residues
aims at converting a
difficult to manage and low-quality solid fuel into a fluid fuel that has a
higher energy density,
does not degrade over time and is easier to manage (transport, storage).
[0005] As the by-products of the agri-food sector are produced in local scales
and in a
distributed manner, it is economically disadvantageous to transfer them to
central exploitation
units (collection radius of more than 30 km). For this reason, it is
particularly useful to exploit
them on site. This is possible by installing easily transportable, small-scale
gasification units
directly at the locations where the solid residues are produced.
[0006] Basic gasification technologies and their disadvantages
[0007] Gasification technologies are distinguished according to the direction
of the supplied
air and fuel flow in the reactor (gasifier). The main gasifier concepts are:
the fixed bed of
updraft or downdraft current, the fluidized bed and the entrained flow
reactors. The fixed bed
reactor is the most common type in small-scale applications. Depending on the
flow direction
of the produced gas, reactors are classified as updraft, downdraft, or cross-
flow reactors.
[0008] In updraft reactors the fuel is supplied from the top part and the air
from the bottom
part of the reactor, through a support grate. The gasification solid residue
is concentrated on
the grate where it is burned at a temperature of 1000 C, the ash is
concentrated at the lowest
point while the hot gases move upwards, undergoing a reduction process. The
main advantages
of this type of reactor are its simplicity, the high conversion of carbon
residues and the internal
heat exchange rate, leading to low gas outlet temperature. Due to reactor
design, the incoming
fuel is dried at the top of the bed and it is therefore possible to use fuel
with high humidity (up
to 60%) without requiring pre-treatment. The main disadvantage is the
particularly increased
production of tars (50-100 g/Nm3).
[0009] In downdraft reactors the fuel and air flow in the same direction. The
gaseous products
exit the reactor after passing through the hot fuel particles zone, thus
facilitating the partial
decomposition of the tars generated during the pyrolysis step. Since gases
exit the reactor at a
high temperature (900-1000 C) the energy efficiency is low, as heat is
contained in the hot
gases. The gas tar content is low (1-2 g/Nm3). The design and operation of
this type of reactor
is relatively simple. The producer gas tar content is low, but it is
practically impossible to get
rid of it completely. The main disadvantage is considered to be the high ash
content in the
outgoing gas and the strict fuel particle size requirements, which must be
evenly shredded from
40-10 mm so as not to block the cross-section of the reactor and allow
pyrolysis gases to be
heated by the oxidation zone. The maximum humidity limit of the fuel is set at
25%.
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[0010] The fluidized bed reactor has been used extensively for the
gasification of fuels such as
lignite, coke, woody biomass and sludge. Its advantage over fixed bed reactors
is the uniform
temperature distribution in the gasification zone. This is achieved through
the use of a
pneumatically agitated, through the vertical upward flow of the process air,
fine-sand bed
which enhances mixing between the hot sand bed, the inlet fuel particles and
the produced
gases.
[0011] The bed temperature is set to 700-900 C and is maintained by
controlling the
air/fuel/sand mass ratios. In contrast to fixed bed reactors, in this reactor
type, there is no
separation into process zones due to the intense mixing of the fuel with the
oxidative fluid and
the fine-grained fluidization material. Drying, pyrolysis and gasification,
all take place
simultaneously throughout the volume of the reactor, where uniform mixing and
a constant
temperature prevail, achieving almost complete conversion of the fuel. For
these reasons
fluidized bed gasifiers can convert, without melting and agglomeration
problems, fuels with
high ash content and low ash melting points such as agricultural solid fuels.
The disadvantages
are summarized in the complex operation and control of the process as well as
the increased
pollutant load of the producer gas in suspended particulates and tar (10
g/Nm3). Due to their
complexity, fluidized bed reactors are usually applied on a large scale where
there is no space
limitation for the height requirement of the gasifier to avoid pneumatic
transport of the fuel
particles and for the inclusion of the bed removal and additional auxiliary
systems.
[0012] Main filtration technologies and their disadvantages
[0013] A filtration system for the purification of the producer gas is
necessarily placed
downstream the gasifier and upstream the producer gas utilization system such
as a power
production device. The main pollutants treated in the filtration system are
the particulate load
and the tar content as their presence in the gas stream is substantial and can
create critical
operational problems. Based on the operating temperature, cleaning
technologies are
distinguished in cold and hot methods. Cold methods, in turn, are divided into
'dry' and 'wet'
methods. Wet cleaning methods operate at temperatures around 150-250 C. They
have an
efficiency around 99%, in terms of particle separation, and around 20-80%
(depending on
temperature and active filter surface) in terms of tar retention. In wet
scrubber type methods,
the gas comes into contact with a jet of water or other liquid chemical (e.g.
diesel oil), and is
cooled at temperatures of 25-55 C. Thus, the scrubber cleanses the gas of
particles, tar, and
various nitrogen compounds (ammonia). Disadvantages of this technology include
the
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significant cooling of the gas as well as the need to install an additional
system for the recovery
of the washing liquid.
[0014] Regardless of gasification technology, usually the first stage of gas
cleaning is
categorized as hot and consists of a gravitational or centrifugal separator
(cyclone type). This
hot process removes much of the particulate load. Generally, the cyclone
filter can remove up
to 90% of particles with a diameter of more than 5 [tm, is partially efficient
for particles
between 1-5 [tm diameters, while such systems are generally unable to filter
particles less than
1 [tm in diameter due to their operating principle.
[0015] Additional components for the complete removal of particles and tar are
placed
downstream the cyclone filter. While in cold methods the tar is removed from
the gas through
condensation followed by separation or adsorption, in hot methods tar is
removed through
breaking it down into chemical compounds with lower molecular weight which do
not cause
clogging problems in producer gas utilization equipment. Tar breakdown is done
either
thermally (at temperatures above 1000 C i.e. by oxidation) or by means of a
catalyst (600 ¨
900 C). Due to the presence of carbon particles inhibiting the proper function
of the catalyst,
usually the particulate load of the gas is removed by a hot filtration
process, upstream of the
tar decomposition device. Due to the high temperatures, conventional filters
used in cold
methods (bag filters, sand/sawdust/straw bed filters) are not suitable. High
temperature filters
consist of ceramic or metal materials. They separate, through absorption,
sulfides and
chlorides, retaining even the smallest particles.
[0016] The retention of these particles gradually creates a solid filter cake
which partially clogs
the filter and causes an increase in pressure drop. For this reason, it is
necessary to periodically
clean the filters. Cleaning of the filters is usually done through channeling
a pulse of
compressed air or inert gas. This causes the detachment of the filter cake
which, due to gravity,
settles and is collected with appropriate mechanisms.
[0017] The most widely used high temperature filter technology concerns
ceramic and metal
"candle" type filters. According to publications, these types of filters
usually fail after 3,000
hours at temperatures above 400 C making their frequent change economically
unviable. The
main causes of failure are attributed to filter design, the manufacturing
material, temperature
variations and ash deposition. Notably, in the case of ash, since the
deposition of particles takes
place on the external side of the candle, the accumulation of material in
neighboring candles
leads to the linkage of these particle deposits, creating a "bridge" of
particles which due to
different thermal properties from the candle material quickly lead to candle
structural failures.
Various solutions have been proposed in the literature on the construction of
filters resistant to
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mechanical and thermal stress, with the adoption of ceramic filters seeming
more promising
but without providing a satisfactory solution to the related problems while
attention should also
be focused on filter support and fastening. More specifically, it is necessary
to ensure that their,
usually metallic, support base, does not create operational problems. These
problems, in turn,
arise from the mechanical stress that is applied on the ceramic filter when
the latter changes
dimensions during the contraction-expansion cycle attributed to the
temperature variations
during system operation.
[0018] In addition, the cleaning of filters, which is usually done through
channeling air under
pressure in the opposite direction to that of their operation, strains the
filters both mechanically
and thermally as a thermal shock is created due to the temperature difference
of the air that is
used for cleaning and the temperature of the filter itself.
[0019] Both these stresses are causes of filter failure, which in turn loses
part of its
properties, creating the need for its replacement with the respective cost for
the supply and
installation of the new filter but also for the interruption of the system's
operation. Due to
their geometry, candle-type filters have a restriction on the porosity of the
material, and
respectively on the filtering surface, in order to maintain structural
strength, deeming them
unsuitable for limited-space applications.
SUMMARY
[0020] The present disclosure provides methods and systems that address the
deficiencies and
technical problems associated with existing gasification systems and
processes.
[0021] The existing technologies which utilize solid ligno-cellulosic biomass
on a small scale
are limited to the utilization of fuel low in ash content (1-2% w/w) and
strict particle size and
moisture standards, which is practically applied only in the cases of
standardized chips and
pellets with the corresponding fuel cost. It is therefore necessary to design
an innovative small-
scale gas production system through gasification, utilizing solid fuels of
high heterogeneity and
ash content, offering the possibility of using low- or even negative-cost
biomass.
[0022] In certain implementations, an innovative small-scale system for the
production of
producer gas through gasification, utilizing solid fuels with high ash
content, is presented.
[0023] According to the present invention, the innovative system consists of a
fluidized bed
gasifier, a stage for removal of large suspended particles (> 5 lm), a
monolithic honeycomb
filters with plugged alternate channels unit suitable for cleaning of the
filter during the
operation of the system, a tar condensation and removal system, and a solid
fuel pre-drying
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system. The innovative system can be configured and dimensioned to be
contained in a
standard freight container properly configured for easy access to the
equipment, i.e. the sub-
units of the system. It has the capability to couple with a gas generator-set
utilizing the heat
produced either for the pre-drying needs of the solid fuel or to cover
external thermal needs.
[0024] The proposed solution introduces the following innovations: (a) the
operation of a
small-scale gasifier, designed for limited-space applications (for example a
standard freight
container), for the production of gas, from solid fuel high in ash content, at
operating
temperatures lower than the melting point of the solid fuel ash, (b) the
design of a filter unit
consisting of independent filters, connected in parallel, capable of
selectively isolating one or
more filters from the system, for the purpose of their cleaning through
providing a gaseous
pressurized medium at an appropriate temperature for filter or filters
cleaning (c) achieving a
desired filtration surface through the use of monolithic ceramic or metal
honeycomb filters, of
plugged alternate channels, at temperatures at which melting or softening of
the ash and its
consequent retention by the filter or filters is avoided, as well as the
condensation of the tar so
that the tar remains in gaseous form as it penetrates the filter, (d) tar
condensation, after removal
of the gas particulate load, and its easy extraction during operation, (e)
mechanical or
pneumatic ash removal units at the gasifier, at the first stage of particle
collection and at the
filter unit, and (f) a feeding and system operation control unit, for the
gasifier parameters
adjustment, the detection of the need to clean the filters and the
implementation of the cleaning
operation aiming at the uninterrupted clean gas production at the system
outlet, the protection
of the filters from failure during their cleaning and the effective tar and
other substances
removal from the producer gas. Various other sub-units may be added to or
removed from the
system. The indispensable sub-units for system operation include the gasifier,
the filters, and a
unit of controlled producer gas cooling which is located between the gasifier
and the filters.
DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 illustrates a diagram with an example of a clean gas production
system, by solid
fuel gasification, according to the state of the art.
[0026] FIG. 2 illustrates a simplified diagram of a clean gas production
system, by solid fuel
gasification, according to this invention.
[0027] FIG. 3 illustrates a simplified diagram of the system of this invention
and the operating
temperatures of its individual units.
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[0028] FIG. 4 illustrates examples of filters of the system of this invention
and their principle
of operation.
[0029] FIG. 5 presents simplified examples of filter mounting in FIG. 4 into
the system of the
present invention.
[0030] FIG. 6 presents a simplified system operation control flowchart.
DETAILED DESCRIPTION
[0031] System for the production of clean gas through solid fuel gasification
according to
the state of the art
[0032] FIG. 1 illustrates a diagram with an example of a small-scale clean gas
production
system (10-500 kWe) through solid fuel gasification according to the state of
the art. The
system (100) typically consists of a fixed-bed gasifier (110), a centrifugal
cyclone (120), a
candle filter system followed by a gas cooler, in case the cleaning is
performed at high
temperatures, or a heat exchanger system (130) for cooling the gas followed by
a fixed bed or
fabric filter (140) if the cleaning is performed at low temperatures.
[0033] The most common type of fixed bed gasifier at the above-mentioned
scales is that of
the downdraft due to the minimal tar amounts in the producer gas. In these
gasifiers the fuel
material (112) is typically imported from the upper part and fills the entire
volume of the
reactor. The oxidizing agent, typically atmospheric air (114) is also injected
at the upper part
at a slightly lower point than the supply height of the fuel material. At the
air supply point,
temperatures may rise to 1250 C as the oxidation stage of the contained
volatile matter,
including tar, is realized. The synthesized producer gas (111) exits from the
bottom after
passing through the reduction stage, at temperatures up to 1050 C. Due to the
consumption of
the solid fuel by the gas medium, the former shrinks and flows evenly to the
bottom of the
reactor. Consequently, the produced gas contains minimal tar amounts. However,
this type
shows increased sensitivity to changes in raw material humidity and particle
distribution. This
is due to the fact that the fuel does not rest on the grate, on account of the
high temperatures at
the bottom, but maintains the structure of the bed through the geometry of the
raw material.
For this reason, these gasifiers have a limitation to the minimum particle
size of the fuel
material. Also, due to the high temperatures, ash content melts and creates
agglomerates that
prevent the downward flow of the material. Therefore, there is a restriction
both in the amount
of ash in the incoming fuel and at the melting point of it, which should be
above 1100 C to
avoid problems.
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[0034] The gas exits at the bottom of the gasifier, dragging ash and fuel
particles that have not
reacted. It is then supplied either to a heat exchanger to cool the gas to a
temperature suitable
for conventional bag filters or fixed bed filters (e.g. W02018/037152 Al) or
cleaned at high
temperature in a candle type filter system (e.g. CA 2937445 Al).
[0035] Disadvantages of state-of-the-art systems
[0036] Cooling the gas before removal of the particles facilitates the
condensation of tar on the
flying particles that act as condensation nuclei. The produced waste has the
flow characteristics
of sludge and causes operational problems as it deposits on downstream
equipment while its
removal from the heat exchanger is a challenge and typically requires system
shut-down for
system cleaning and reassembly. After cooling, the gas typically passes
through a fixed bed
filter which is filled with low-cost filtration mediums, such as sawdust. To
achieve successful
retention of particles, low gas velocities are required within the filter to
prevent particle drag
by the gas leading to large filter cross-sections and space requirements.
Additionally, in order
to clean the filters it is necessary to interrupt the process of
cleaning/purifying the gas, and thus
utilization of the gas, with the corresponding economic effects. More
specifically, the filter
container is opened, and its contents (sawdust and particles) are removed and
replenished.
[0037] In the case which the gas is rid of the particulate load at high
temperatures using candle
type filters, cleaning is more direct and faster with the reverse flow method
mentioned.
However, apart from the structural failures of these filters as mentioned,
such systems are
practically only applied to large-scale installations (with capacities greater
than 1 MWe) in
which there are no special spatial constraints. Due to their geometry, candle-
type filters have a
limited filtration area thus a significant number of filters are required to
adequately refine the
gas.
[0038] Another reason why candle-type filters are not applied on small scale
systems concerns
the cleaning of the filter itself As mentioned, the cleaning is done by
reverse flow of
compressed gas at the filter outlet. Typically, a nozzle is placed at the edge
of each candle and
the cleaning is performed on each candle sequentially so as not to interrupt
the operation of the
rest of the filter array. In large-scale units where the candles which
constitute an array are many,
the addition of inert (e.g. of nitrogen) or not (e.g., air) gas due to the
pulse does not significantly
affect the quality of the produced gas. In the case of a small-scale system
however, where an
array of filters may contain from 4 to 8 candles, a pulse can significantly
affect the
instantaneous quality of the gas, obstructing the operation of the power
generation system. This
problem may be solved through isolating and operating the filters in parallel
as in the present
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invention but in the case of candle-type filters this would require an
extremely large space and
high cost of mechanical equipment such as isolation valves for each candle.
[0039] System for the production of clean producer gas through solid fuel with
high ash
content gasification according to the presented invention
[0040] Gasifier
[0041] FIG. 2 illustrates a simplified diagram of a clean gas production
system through solid
fuels gasification according to this invention. The system depicted in FIG. 2
may optionally
include additional elements which are not depicted and part of which is
described below. The
system (200) consists of three subsystems, the solid fuel supply subsystem
(dryer), the fuel
conversion to gas subsystem (fluidized bed gasifier) and the producer gas
treatment subsystem
(first combustible gas cooling device, filter unit, second combustible gas
cooling device)
[0042] The solid fuel supply subsystem consists of a storage tank for the
untreated solid fuel
at the base of which a screw conveyor or similar device which serves to
transport the solid
combustible material is located. The tank has a geometry suitable for
maximizing useful
volume and ensuring the vertical untreated fuel flow due to gravity without
forming
agglomerates and cavities. At the lower end of the tank in the flow direction
of the material
there is an opening through which the combustible material passes and is
supplied to an inclined
screw conveyor. The inclined screw conveyor rotates in a metal shell
compelling the fuel to
the highest point of the device where there is a discharge orifice at the
bottom of the shell. The
base of the middle part of the shell consists of a perforated metal sheet and
is encased in an
airtight, metallic duct. Drying air, heated by the heat from the gas cooling
system, passes
through a suitable orifice into the air duct and through the perforated plate
within the screw
conveyor which it scavenges heating the material and removing the moisture
content through
an orifice in the upper part of the shell through which it is dispersed to the
ambient. The two
screw conveyors have a fixed volumetric capacity ratio and rotate
simultaneously to avoid
supercharging or under-feeding of the inclined screw conveyor. The
simultaneous rotation is
ensured by a system of gears and chains. Alternatively, each conveyor is
coupled with its own
electric motor and rotates independently.
[0043] In alternative realization examples of the solid fuel supply system,
the storage tank is
replaced by a tank with a hydraulic floor which moves the material towards the
screw conveyor.
The hydraulic floor may be perforated allowing the drying air pass through. In
any case,
because the level of the material in the tank is not constant, part of the
drying air passes through
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the inclined screw conveyor in order to maximize the time of contact between
the bulk material
and the hot air stream.
[0044] The conversion subsystem consists of a gasifier device (210) in which
solid fuel is
inserted from solid fuel inlet (212) into the side of the gasifier (210) while
air (214) enters from
the bottom part of the gasifier (210) reacting with the fuel at high
temperature (650-950 C) and
converting the solid fuel into combustible gas (209) which exits the gasifier
at its upper part
(210). Fluidizing material (210) of specific particle size, is also injected
into the gasifier (216)
which is heated at the base of the gasifier (210) and the air (214) is
injected through the sand
bed (216) inside the gasifier (210). The high temperatures (about 650-950 C)
prevailing at the
base of the gasifier (210) and the air supply (214) make the sand (216) behave
like a fluid,
exchanging heat with solid combustible fuel material and air acting as a heat
storage medium.
The gasifier consists of a cylindrical metal tube internally lined with high
temperature
resistance refractory. The gasifier is divided into three inter-communicating
flow parts. The
upper part (201) is the main body or reactor, the middle part (202) is the air
distributor, and the
lower part (203) is the ash removal device. The ash removal device consists of
a cylindrical
cross-sectional duct with a blind metal plate at its base. A screw-conveyor,
or a similar bulk
material conveying system, is located at the center of the device, which
removes ash and other
inert and non-inert-material from the bottom of the gasifier by means of an
outflow duct and
transfers it out of the system into an airtight container. In order to
facilitate the flow of ash to
the screw conveyor, appropriate inclination is formed at the bottom of the
lower part (203).
The bottom (203) is connected to the air distributor (202) through a flange or
other appropriate
connection. The distributor (202) consists of two concentric cylindrical metal
ducts between of
which a heat-insulating material is placed or a cooling medium flows. The
outer cylinder is
welded to flanges or other equivalent connecting devices on each end. Before
it enters the
gasifier, the supply air is led into two identical collectors which are
oppositely positioned with
respect to the gasifier. Within each collector an electrical resistance is
coaxially mounted for
the rapid preheating of the air during the preheating phase of the gasifier.
The air enters the
collector where it is heated to temperatures higher than 200 C and then
distributed to three or
more air distribution pipes which are plugged at their end. Each pipe has
several orifices of a
maximum diameter of 3 mm through which air enters the gasifier in the form of
a jet. The
orientation of the orifices is preferably horizontal but may have any
inclination to the cross-
section of the gasifier. The orientation of the orifices of each distribution
pipe is preferably
vertical but may be in any other direction. The upper part (201) consists of
two cylindrical
ducts which are axially connected through an expansion. Considering an upward
flow, the
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diameter of the reactor upstream the expansion is always smaller than the
corresponding
diameter downstream the expansion. In the lower cross-section, a flange or any
other suitable
connection device is placed to connect to the air distributor. In the axial
distance between the
connection flange with the distributor and the expansion, there are two
openings which
communicate with the external environment through metal ducts. The first
opening is the solid
fuel inlet (212) from which the solid combustible material is inserted while
the second opening
(213) is used to regulate the active height of the fluidizing sand bed either
through mechanically
removing the content of the gasifier, or through mechanical addition of ash
that has already
been removed either from the bottom of the gasifier or from the collector of
the centrifugal
cyclone. Downstream the expansion there are also two or more openings while at
its upper
point the gasifier is connected to a blind flange or other isolation device.
The opening (215)
serves to supply fine-grained solids or liquids to the gasifier, such as bed
material (sand) or
liquid fuels respectively. Opening (211) is as close as possible to the upper
point of the gasifier
and through a metal duct the produced gas is discharged from the reactor.
[0045] The solid fuel material enters the gasifier from an opening (212)
through mechanical
bulk material transportation equipment such as screw feeder. The solid fuel
material, that may
optionally be dried, flows into a temporary storage tank which is connected to
the orifice at the
inlet of the solid fuel (212). The storage tank has an integrated moisture
meter suitable for use
in bulk raw materials which analyses in real time the moisture content of the
solid fuel after
the drying stage. The tank is isolated flow-wise from the screw conveyor
through a standard
hydraulic isolation device with or without pressure adjustment. The hydraulic
isolation device
consists of two or more valves of the knife gate type or other technology
(e.g. rotary valve)
suitable for use with powders and particles, between of each a vessel of given
volume is placed
equipped with solid fuel material detection sensors. This device enables a
pseudo-continuous
supply of fuel in small batches. The batch is transported to the screw
conveyor by the following
procedure. Initially, the first valve is connected to the screw conveyor at
the inlet of the
aforementioned vessel and remains open while a second valve at the outlet of
the vessel is
closed. The process starts with the closure of the first valve and the
subsequent opening of the
second valve, followed by the gravity flow of solid fuel accumulated in the
tank. When the
level sensor detects material, the first valve closes and then the second
valve opens releasing
the combustible material into the socket of the screw conveyor. In this way, a
controlled fuel
supply is achieved, preventing the return of the producer gas to the tank as
well as the adverse
effects that this backflow phenomenon entails (premature pyrolysis of fuel and
formation of
agglomerates). To help reduce the returning flow, it is possible to supply a
relatively small air
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supply or some inert gas downstream of the second valve to push the heated
gases, which have
the potential to return, back inside the gasifier. In the case of air use, the
flow rate shall be
known and subtracted from the total air supply to the process in order to keep
the air-fuel flow
ratio constant. The same principle is followed in case more than two isolating
valves are
included in the system as long as solid fuel material is present in each
independent volume.
[0046] The appropriate selection of the gasifier' s operating parameters
(210), i.e. pressure,
temperature, solid fuel supply (212), air (214) and additives (216) can be
used to produce gas
with specific characteristics (e.g. content of particulate matter, tar, etc.)
and at a desired
temperature (e.g. in the range of 650-950 C). The choice of the operating
parameters of the
gasifier (210) is not important in the case of the present invention, except
for the control of the
temperature of the producer gas (209), so that it does not approach or exceed
the threshold of
900 C, i.e. the fuel ash melting point.
[0047] Innovative Features and Operating Parameters of the proposed System
[0048] The operation of the gasifier in the selected temperature range is of
significance as, in
contrast to other known systems, the present invention achieves an easier,
more effective and
more efficient ash removal from the produced gas (209), by not melting the ash
contained in
the solid fuel. Thus, most of the ash settles due to gravity in the lower part
of the gasifier (210)
where it is mixed with the fluidized sand (216). Ash may be removed through a
special removal
unit, which has the form of a screw conveyor (217) and is mounted on the lower
part (218) of
the gasifier (210).
[0049] The layout of the gasifier allows for efficient processing in
applications of limited space
and especially limited height. It is an optimal compromise between the minimum
hydraulic
height to avoid pneumatic transport of fine particles and the maximum reactor
diameter to
ensure uniform distribution of solid fuel within the gasifier bed. The reactor
height
minimization is achieved through the combination of the horizontal mechanical
ash removal
and the specifically designed air distributor. In large-scale applications,
such as industrial
fluidized bed applications, removal of the bed material is achieved at the
bottom of the bed
with a vertical hydraulic isolation valve (lock hopper) system. The main
advantage of the ash
removal subsystem is the minimization of its height over other devices,
through its horizontal
orientation, ensuring reliable removal with mechanical means without requiring
vital space
under the bed. At the same time, by placing the air distributor collector
outside the reactor, the
amount of effective volume of gasification that is preserved, allows for the
total height of the
bed to be reduced. The basic principle of distributor design is the uniform
air supply throughout
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the reactor cross-section according to the criteria of V. E. Seneca! (V. E.
Seneca!, "Fluid
distribution in Process Equipment", Ind. Eng. Chem. 1957, 49, 6, 993-997).
[0050] In existing gasification systems (see CN 104093481 A) the gas plenum or
wind box is
located under the bed and is distinguished from it by perforated or porous
plates or rings which
constitute the air distributor. The placement of the plenum under the bed
balances the pressure
and allows the accumulation of air throughout the volume of the plenum and
therefore uniform
flow through the distributor orifices. In these cases, ash removal is a
challenge and is usually
achieved through a declined duct at a chosen height above the dispenser, so
that there is always
an inaccessible volume between the distributor and the entrance of the
declined duct, in which
ash accumulates.
[0051] In other applications (see CN 106336905 A, WO 2020071908 Al), for the
complete
removal of ash there is a vertical opening at the bottom of the gasifier. In
the opening perimeter,
and in order not to prevent the downward flow of the bed, the air distributor
consists of a
perforated conical sheet within the bed which internally separates the plenum
from the bed. In
these cases, ash is completely removed however achieving the uniform flow of
air from the
distributor is problematic.
[0052] The present invention places the collector out of the gasifier without
affecting the
internal diameter and the height of the bed. Through the symmetrical air
distribution, uniform
flow is achieved inside the gasifier while avoiding ash and sand deposition on
the heating
elements inside the plenum avoiding reduction in thermal efficiency and
service life reduction
of those heating elements.
[0053] By use of a moisture sensor in the temporary fuel supply tank, the
control system
calculates the actual energy content of the solid fuel material that enters
the gasifier and adjusts
the air supply accordingly in order to maintain the desired air equivalence
ratio (0.2 to 0.5)
enabling the utilization of non-standardized solid fuels with minimal effect
on the stability of
the gasifier's operation. The control of the air and solid fuel supply is
implemented through
appropriate variable speed drives on each feeder.
[0054] Alternative Realizations of the Gasifier
[0055] In alternative examples of gasifier realization (210), the cross-
section of the reactor may
be square, rectangular or of any other shape while piping for process flow air
preheating before
entering the distributor may run through the refractory lining. In an
alternative example the
gasifier is not internally lined and metal tube protection is achieved through
the preheating of
the process air. Additionally, internal baffles may be placed along the
gasifier height (i.e. means
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to increase the gas residence time in the gasifier) at an appropriate distance
and inclination to
create internal recirculation or to increase the actual distance traveled by
the producer gas at
high temperature which increasing residence time. In alternative examples the
opening (213)
may be inclined with an angle of < 90 to the ground and act as hydraulic
overflow protection
to control the bed level in the gasifier.
[0056] In alternative examples of gasifier realization, the air distributor
may consist of vertical
or inclined pipes. Instead of perforated pipes, perforated sheets or meshes
capable of holding
the weight of the bed may be used. In each realization example, there is a
sufficient gap
typically greater than 2 cm in the gasifier cross-section so as not to hinder
the free downward
flow of the material to the ash removal device located at the bottom of the
gasifier.
[0057] In alternative examples of the solid fuel supply subsystem, the screw
conveyor is
replaced by an inclined duct with an angle of > 90 to the ground through
which the fuel is
gravitationally transported into the gasifier bed. In other examples, the
screw conveyor may be
replaced by two or more screw conveyors with intermediate stages of isolation
which serve to
minimize returning hot gas flow from the inside of the gasifier to the supply
subsystem.
[0058] In alternative examples for the ash removal device, the ash extraction
unit may be
moved elsewhere in the lower part of the gasifier (210), while the screw
conveyor may be
replaced by another mechanism. An example of such a mechanism may be a gate or
other valve
mechanism which opens and closes accordingly, to allow ash and sand to be
extracted (216)
by gravity.
[0059] Optionally the gasifier (210) is connected via the ash and valve
extraction mechanism
(219) to a sand recovery unit which separates the sand (216) from the ash and
feeds the
recovered sand (216) back to the gasifier (210). The sand cleaning unit (205)
may be in the
form of a sieve with an appropriate mesh size to retain the sand or
alternatively may be in the
form of a vortex or other known technology suitable for the gravitational
separation of sand
(216) from ash.
[0060] Innovative Features and Parameters of Operation of the proposed Cyclone
[0061] The gas treatment subsystem consists of three purification stages. In
the first stage (first
producer gas cooling device (220)) the combustible gas is cooled at a
temperature suitable for
supply to the next stage and large diameter particles are optionally removed
through a cyclone,
in the second stage (filter unit (240)) the entire particulate load of the gas
is removed using
barrier filters, while in the optional third stage (second fuel gas cooling
device (230))
condensate is removed by gas cooling and droplet separation.
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[0062] The produced combustible gas, which contains particles of different
diameters, is
supplied from the outlet (211) of the gasifier (210) through a pipe into a
centrifugal cyclone
unit (220) which retains at its lower part particles of larger selected
diameters (e.g. 5 [tm)
suspended in the combustible gas (209). The cyclone unit may be of wet or dry
type, while in
alternative realization examples it may be replaced by other types of large
particle retention
unit based on a different technology (e.g. electrostatic precipitator) or even
eliminate this
separation stage entirely.
[0063] The separated particles are extracted from the lower part of the
cyclone unit (220) using
a suitable removal device, which has the form of a screw conveyor (222) and is
based at the
lower part of the cyclone unit (220).
[0064] Alternative Realizations of the Cyclone (220)
[0065] In alternative realization examples, the cyclone may either be
completely eliminated or
operated in series with a second more efficient cyclone or with a filter of
other technology (e.g.
electrostatic precipitator). In alternative realization examples of the
cyclone unit (220), the
large particle extraction unit may be moved elsewhere in the lower part of the
cyclone unit
(220), while the screw conveyor may be replaced by another transport
mechanism. An example
of such a mechanism is a gate-type mechanism which allows the collection of
retained particles
through gravity flow. The valve (221) is connected to the output of the large
particle extraction
unit and specifically after the conveyor screw (222) when the latter is
connected to the cyclone
unit (220).
[0066] Innovative Characteristics and Operating Parameters of the proposed
Filter Unit
[0067] The combustible gas outlet, optionally free of large particles, is
performed through a
pipe (225) which feeds the gas into a filter unit (240) to remove the
remaining particulate load
consisting mainly of particles in the diameter range of 0.05-5 [tm. The filter
unit (240) consists
of at least two identical (or not) filter arrays in a parallel connection
layout. Through the use of
at least two filters, in parallel connected to each other, which may operate
simultaneously or
alternately, the creation of appropriate conditions for the uninterruptible
operation of the
system is achieved (200) and more specifically the possibility of controlled
cleaning of the
filters without affecting the unit productivity, that is, without interrupting
the operation of the
unit during the cleaning of one or more filters.
[0068] For the sake of a clearer illustration, the example in FIG. 2 presents
two filters, the first
filter (241) and the second filter (242) in parallel connection. Individuals
with technical domain
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knowledge of the invention may easily recognize that more than the two filters
depicted may
be connected in parallel without deviating from the intended purpose of the
invention. For this
purpose, i.e. to achieve the possibility of cleaning one (or more filters)
without interrupting the
operation of the system (200), each filter (241, 242) is placed in a container
independent of the
containers of the other filters. The container is connected at its entrance
with an independent
pipe (226, 227), respectively for the first (241) and second (242) filter, and
at its exit with an
independent pipe (236, 237), respectively for the first (241) and second (242)
filter. The pipes
(226, 227) comprise a branching of the pipe (225) connecting the outlet of the
cyclone unit
(220) with the entrance of the filter unit (240), and the tubes (236, 237)
converge in the pipe
(239) connecting the outlet of the filter unit (240) to the tar condenser
(230). The pipes (226,
227) include valves (246, 247) before filter inlets (241, 242), respectively,
and valves (234,
235) after filter outlets (241, 242), respectively.
[0069] By use of the filters of the filter unit (240), the filters (241, 242)
retain almost the entire
particulate load of the gas that is fed to the filter unit (240). Gradually,
the accumulation of
particles on the surface of the filters creates a "filter cake" mass, which
causes an increase in
the pressure drop at the filters and reduces the energy efficiency of the
system (200). For this
reason, filters (241, 242) must be cleaned periodically and preferably when
the pressure drop
exceeds a predetermined upper limit.
[0070] Innovative Features and Operating Parameters of the proposed Condenser
[0071] Particle-free gas exiting the filter unit (240) is channeled, through
pipe (239) in which
pipes (236, 237) converge, into the condenser (230) to remove tar and other
condensed
compounds. Tar, water vapors and other compounds condense as a consequence of
gas cooling
and deposit on the walls of the device and, due to gravity, flow towards the
lower part of the
condenser (230) where a collection tank is located. The condensate from the
collection tank is
evacuated using a simple liquids pump (232), which is located inside or
outside the collection
tank.
[0072] The condenser operating principle is the cooling of the gas from around
400 C to the
temperature of around 50 C in order to condense the moisture and tar content
and remove them
from the producer gas. Indicatively, a concentric tube-in-tube heat exchanger
is applied. The
cooled gas flows in the internal tube of the exchanger while in the external
tube, which encloses
the inner one, the coolant flows. In this application, gas cooling is achieved
in two stages. In
the first stage, the process air flows into the external tube and is preheated
before entering the
gasifier by recovering part of the heat. In the second stage, a coolant flows
in the external tube
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which delivers the recovered heat load to the heat exchanger responsible for
heating the drying
air. The double tube is preferably vertically orientated, and a certain number
of tube passes is
performed to achieve the required heat exchange surface. At the lower point of
each pass there
is a condensate collection duct connected to the condensate collection tank.
In the desired
implementation, the inner tube consists of straight tube sections without
internal configurations
to enable easy tube wall cleaning during maintenance. The external tube
contains fins which
amplify the heat transfer coefficient. In the desired realization, the fins
have the form of an
endless screw and force the coolant into a helical path around the internal
tube, increasing the
coolant speed and thus the convection coefficient. The fins may have any other
form that
amplifies the heat transfer coefficient between the fluid streams thin the two
tubes, for example
transverse rings or longitudinal bars.
[0073] By removing almost the entire particulate load of the produced gas, the
condensates
created in the condenser do not contain solid impurities and therefore have
more favorable flow
characteristics, compared to condensates that create muddy effluents as they
condense around
particles. The favorable characteristics allow easier flow into the condensate
collector tank by
minimizing agglomeration in the walls and valves of the condenser resulting in
lower need for
maintenance and thus, more economical operation. Also, due to the fluid form
of the
condensate, its removal is easily performed through a non-specialized pump
without need to
shut down the gas production plant.
[0074] Alternative Realizations of the Condenser
[0075] In alternative realization applications of the condenser, the heat
exchanger may be of
another type such as a tube-and-shell of one or more routes or a helical heat
exchanger. In
alternative condenser (230) realization examples, the condensate extraction
unit may be placed
elsewhere in the lower part of the condenser (230), while the pump may be
replaced by another
mechanism or device. The condenser (230) usually has the form of a tube-in-
tube heat
exchanger, but it may also take the form of an electrical cooler or gas
refrigerator or a venturi
type scrubber or a liquid precipitation device.
[0076] The gas is then discharged from the condenser exit (230) clean and is
channeled through
a pipe (260) to a utilization system such as an internal combustion engine for
electricity
generation or stored in a gas tank under pressure for future use. The engine
and the tank are not
shown in FIG. 2.
[0077] The system also includes an operating control unit which is not shown
in FIG. 2.
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[0078] Cleaning of filters without interruption of the operation of the clean
gas
production system
[0079] In contrast to the state of the art, this invention allows filters
(241, 242) to be cleaned
by use of compressed air without interrupting the proper operation of the
system (200) while a
compressed air temperature control system ensures the minimization of thermal
stress on the
filtration elements during cleaning. The valves (246, 234) as well as the
valves (247, 235) are
open when the system operates so that the gas is led into the filters (241,
242).
[0080] Choosing as an example the cleaning of the first filter (241), while
the second filter
(242) is in operation, the control unit of the system (200) (not depicted in
FIG. 2) proceeds to
isolate the first filter (241) from the rest of the system (200) by closing
the valves (246) at the
entrance and (234) at the exit of the first filter (241) and its container.
After closing valves
(246, 234) the import and export of gas to the first filter (241) and its
container stops.
[0081] The filters (241, 242) are cleaned through compressed air injection
from the filter exit
towards the filter inlet, i.e. in the direction opposite to that of the gas-
flow during filter
operation. Compressed air is stored in a pressure vessel (251) with air
supplied by a compressor
or fan (250) through tubes (254, 255) which are connected to the filter
container outlets (241,
242), respectively. The pressure vessel is equipped with a stored gas
temperature control
system consisting of one or more electric heaters and thermostat. The tubes
(254, 255) are
connected to valves (252, 253) respectively, which valves (252, 253) are
closed during filter
operation (241, 242) so that the particle-free gas at the filter exit (241,
242) is not injected into
the compressor (250) but into the tubes (236, 237).
[0082] When cleaning the first filter (241), in addition to the closed valves
(246, 234), valve
(252) opens to supply compressed air to the filter outlet (241) in order to
clean it. By channeling
compressed air into the filter (241), the layer cake of material is detached
from the filter input
channels and hits the bottom surface of the filter container (241), while
closing the valve (252),
isolates the filter (241) and its container from the compressor (250). The
particulate mass
resulting from filter (241) cleaning is deposited in the lower part of the
filter container (241)
from where it is extracted using a suitable extraction unit, which is in the
form of a screw
conveyor (244). The air that has been released into the container then re-
penetrates the filter
and is extracted clean to the ambient through valves (272, 273) which open
while the exit valves
(234, 235) to the condenser remain closed. The valves (272, 273) in the
depicted realization
are connected to the extraction units (244, 245), while more commonly, in an
alternative
realization (which is not depicted), they are connected to branches of the
pipelines (236, 237),
which are located before the valves (234, 235), respectively.
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[0083] Alternative filter unit and filter cleaning process realizations
[0084] In alternative realization examples of the filter unit (240), the small
particles extraction
unit may be placed elsewhere in the lower part of the filter unit (240), while
the screw conveyor
may be replaced by another transport mechanism. An example of such a mechanism
may be a
gate-type mechanism which opens and closes to allow the removal of fine
particles by
pneumatic transfer or gravity.
[0085] The injection of compressed air for the back flushing of the first
filter (241) may be
repeated before the new charging cycle, if deemed necessary by the control
system and
specifically by the pressure drop's rate of increase in the filter during its
charging cycle.
[0086] Advantages of different realizations of the filter unit and their
cleaning process
[0087] The reason the filter (241) cleaning process with a single pressurized
air pulse is
effective is related to the way the system (200) and the filter (241) operate.
More specifically,
the system operates at temperatures lower than the ash melting or softening
point, while tars
and other volatile compounds are still in gaseous form as the temperature of
the gas while
entering the filter unit is between 350 and 550 C. The result of the operating
and design
conditions of the system (200) allow even a gas with a high ash content, such
as that produced
from solid agricultural etc. residues, to be filtered by the filter unit (240)
without tar
condensation and deposition on the pipes, the filter, and the particulate
layer on the filter
surface. Thus, the retained material does not contain viscous substances such
as tar or softened
ashes and may be easily detached by injecting compressed air (up to 6 bar) of
a single pulse.
More pulses may be used in cases where an anomaly is observed and more
specifically a sharp
increase in the pressure drop between two sequential charge cycles of the same
filter.
[0088] With the above-mentioned cleaning method, the pressurized air injected
into the filter
does not lead to mechanical stress on the filter elements capable of causing
damage either
during the cleaning of the filter or over time after a plethora of cleaning
cycles.
[0089] Moreover, the air injection for the filter (241) cleaning does not lead
to a thermal shock
capable of causing damage to the filter (241) during cleaning, nor over time
after a plethora of
cleaning cycles as the temperature of the compressed fluid entering the filter
(241) and its
container is controlled so that there is no critical temperature difference
between the filter to
be cleaned (241) and the air channeled through it. Depending on the material
and geometry of
the filtering element, the tolerable temperature difference between the filter
to be cleaned (241)
and the air injected into the filter (241) may be calculated experimentally so
as not to create a
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thermal shock that could cause a filter (241) material failure. Indicatively,
the temperature
difference between the filter to be cleaned (241) and the air injected into
the filter (241) is
50 C. Such a temperature difference is acceptable in order to avoid the
development of axial
stresses due to thermal strain greater than the strength limit of ceramic
filtering elements from
cordierite material, (Mg2A14Si5018) and wall thickness more than 1 mm.
[0090] In the present invention the problem of thermal shock is solved by
utilizing the above
experimental data for the selected filter type (241) and regulating the
injecting of compressed
air into the filter to be cleaned when said filter (241) is isolated from the
rest of the system
(200), when the filter (241) temperature is lower than a threshold which in
combination with
the temperature of compressed air do not make a temperature difference between
the filter
(241) and the compressed air capable of creating a thermal shock harmful to
the filter (241) as
this temperature difference has been experimentally calculated for the
selected filter (241).
[0091] The above procedure requires a specific amount of time for the filter
(241) to be cooled
to the desired temperature, the exact duration depends on the operating
temperature of the filter
(241), the temperature of the compressed air, the ambient temperature around
the filter unit
(240) and the rest of the filters and their containers contained in the filter
unit (240).
[0092] In an alternative system (200) realization, instead of compressed air,
the filter is purified
with the reverse flow expansion of inert gas such as nitrogen or argon at a
controlled
temperature. In another example, the filter may be cleaned with the use of
steam or purified
producer gas at the filter operating temperatures, during the cleaning cycle.
In another example,
an auxiliary supply of steam or air is injected into the filter during loading
in mixture with the
gas to be cleaned. Through steam or air supply, the oxygen necessary for the
oxidation of the
carbon particles is supplied to the filter, lengthening the loading cycle due
to the consumption
of part of the particulate load during filter loading. In an alternative
realization example,
cleaning is done in two stages. During the first stage, the cleaning described
in the desired
realization is carried out and during the second stage a high flow rate of air
at atmospheric
pressure is supplied to the filter for the oxidation of the particulate load
that has remained on
the filter surface.
[0093] In an alternative realization example of the system (200), filter
elements of the filter
system (240) are coated or impregnated with a catalytic material suitable for
breaking down tar
and/or other volatile compounds so that they, in the absence of condensation,
as the filter unit
is connected to the system (200) upstream the condenser (230), are broken down
into gaseous
components in order to reduce the amount of tar and the need to condense and
remove it in the
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liquid phase. In this realization example, the filter operating temperature
depends on the
catalytic material used and may be increased up to 800 C.
[0094] In a different realization example of the system (200), an optional
heating unit (251) is
placed per pipeline (254, 255) so that more than one filter may be cleaned
simultaneously, in
cases where the filter unit (240) contains more than two filters. This allows
the accurate control
of the compressed air desired temperature for the simultaneous cleaning of
each and every filter
within the filter unit (240).
[0095] At the end of the filter cleaning process (241) through compressed air
injection,
collection of the detached particles at the lower part of the filter container
(241), and their
extraction from the lower part of the filter container (241) using the
extraction unit (e.g. the
screw conveyor (244)) valve (272) closes, while valve (252) and valves (234,
246) are already
closed, so that the filter (241) and its container are isolated from any other
unit of the system
(200), from external systems and the external environment, while the filter
(242) operates
normally, having the valves (247, 235) open and the valves (273, 253) closed
and remain in
this state until the new loading cycle begins.
[0096] Process of restoring a cleaned filter to the operation of the system
[0097] To return the filter (241) to working condition after cleaning it,
valves (246, 234) open
and gas is channeled into the filter (241), in the same way already being
channeled into the
filter (242), through pipe (225) connecting the cyclone unit exit (220) to the
filter unit (240)
entrance.
[0098] The same cleaning procedure described for the first filter (241) is
followed,
respectively, for the second filter (242), and for each additional filter in
case several filters are
included in the filter unit (240). In the case of the second filter (242) the
valves (247, 235, 253,
273), the pipes (227, 237, 255) and the screw conveyor (245) are used.
[0099] Parameters of operation of a clean gas production system
[00100] FIG.3 illustrates a simplified diagram of the system in this invention
and the operating
temperatures of its individual units. The clean gas production system (300)
consists of, inter
alia, a gasifier (310), a first stage fuel gas cooler (320), a filter unit
(340), a condenser (330),
and an air compressor (350).
[00101] The gasifier (310) is of the fluidized bed type and may be adjusted so
that in its lower
part it operates at a temperature range around 650-950 C. This temperature
decreases axially
towards the upper part of the gasifier (310) from which it exits in the
temperature range of 500-
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800 C. The temperature drop is due to the endothermic chemical reactions that
take place inside
the gasifier (310) between the solid fuel and the air introduced into the
gasifier (310). The
temperatures within the gasifier (310) may be adjusted by regulating the flow
rate, the flow
rate ratios of solid fuel, air and sand supply and the ash outlet supply, so
that the producer gas
temperature in the upper part of the gasifier is approximately 500-800 C.
These adjustments
allow (with an accuracy of some C, e.g. 10-50 C) to regulate the desired gas
temperature so
that, on the one hand, no ash melting occurs in the bed and on the other hand
the bed
temperature is maintained at the desired levels to maximize the rate of solid
fuel conversion
into gaseous fuel. To maximize the conversion rate, the gasifier (210) is
adjusted to deliver fuel
gas to the lower part (203) at a superficial speed less than 3m/sec.
[00102] With the flow of the gas from the gasifier (310) to the first gas
cooling device (320)
large flying particles (> 51.tm) are removed from the gas and at the same time
the gas
temperature is reduced so that the gas is channeled to the filter unit at
temperatures around
400 C. In alternative realization examples of the system (200, 300) the
temperature of the gas
channeled to the filter unit may be adjusted using an optional heat exchanger
(not depicted) to
cool the gas to the desired temperature.
[00103] The gas introduced into the filter unit is at temperatures much lower
(about 350-550 C
or even lower in alternative realization examples) than the melting or
softening point of the ash
and at the same time higher than the condensation temperature range of the tar
and other
gasified compounds so that they do not deposit on the filters, causing
clogging and making the
cleaning operation of the filters difficult, costly and damaging to the
filters.
[00104] After passing through the filter unit and subsequently cleansed from
suspended
particles, the gas passes through the condenser (330), which cools it down to
a temperature of
around 50 C in order for the tars to be condensed also dissolving other
gasified compounds
contained in the gas such as hydrogen sulfide. At the condenser exit (330),
the gas is now clean
and suitable for use in an engine for electricity generation, or for storage
and future use. In
different realization examples, the gas is not cooled but is utilized at high
temperature with tars
remaining in gaseous form (e.g. gas turbine or gas burner).
[00105] To clean the filters, the compressor (350) channels compressed air
into one or more
filters of the filter unit (340) and approximately at the temperature of the
filter to be cleaned.
To avoid thermal shock on the filters during their cleaning, the system (200,
300) allows
compressed air supply to the filter (or filters) for cleaning, after isolating
a priori the specific
filter (or filters) from the gas flow, at a controlled temperature.
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[00106] Alternative examples of system operating conditions
[00107] In alternative examples of system operating conditions, the air
supplied to the process
is replaced by steam or oxygen or carbon dioxide or a mixture of air with
steam and/or
oxygen/carbon dioxide in order to produce a gas with a lower nitrogen content
and therefore a
higher calorific value.
[00108] In an alternative example of operating conditions, the oxidizing agent
is replaced by an
inert gas such as nitrogen or argon. Under these conditions there is no
oxidation, and the gasifier
operates under pyrolysis conditions to produce bio-oil and char. The bio-oil
is condensed and
collected in the condenser while the char is collected from the ash removal
screw conveyor at
the bottom of the gasifier. During pyrolysis, a small amount of gas is still
produced which
follows the cleaning stages of the invention and is used for energy
production. The
corresponding operating temperatures of the gasifier under pyrolysis
conditions are in the range
of 500 ¨ 700 C.
[00109] Selection and use of filters in the clean gas production system
[00110] FIG. 4 illustrates examples of filters of the system in the present
invention and their
principle of operation. Filters (400) are examples of filters configurations.
Different
configurations are possible without deviating from the scope and intended
protection of the
present invention. The different configurations are obvious to persons of
ordinary skill in the
art related to the present invention and are therefore not specifically
mentioned.
[00111] The preferred filter option for the system (200, 300) is monolithic
ceramic honeycomb
filters with plugged alternate channels. Alternatively, other filter types can
be used depending
on the temperature of the gas channeled to the filter inlet. The following
paragraphs give
examples of the preferred filters, but similar conditions apply to the
alternative types of filters
that could be used in alternative realization examples of the system (200,
300).
[00112] The filter (410) is illustrated in a side-frontal view having a square
cross-section and,
in this example, consisting of longitudinal channels, in an AxB matrix
arrangement, where A>2
and B>2, and where one channel in relation to the other channels is positioned
in parallel and
with the input and output of each channel aligned with the filter input and
output or vice versa.
The channel cross-section is depicted square, but it may also be circular,
rectangular or any
other shape or size and is formed into monolithic ceramic material. At the
input (401) of the
filter (410) the channels (442) are sealed, while the channels (443) are open,
and at the filter
exit the channels (442) are open and the channels (443) are sealed. The
sealing or plugging of
the channels (442, 443) may be done with any suitable material, resistant to
high temperature
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and with a low expansion coefficient, such as inert cement, etc. Each channel
(442, 443) may
be open only at one end and is alternately placed inside the filter (410) so
that when the gas
enters the open channels (443) it penetrates the length of the channels (443)
up to their closed
end at the exit (402) of the filter (410). The gas, unable to exit the closed
end of the channels
(443) (nor to enter through the clogged entrance of the channels (442)) and
due to the gas
pressure, is forced through the porous walls of the channels (443) and enters
the neighboring
channels (442) from which it cannot exit through their plugged end at the
entrance (401) of the
filter and thus finds an outlet from the open end of the channels (442) at the
exit (402) of the
filter (410). The arrows (445) indicate the gas flow path when filtered by the
filter walls (410),
where the particles contained in the gas are deposited since the ceramic
material wall pores of
the filter (410) have a smaller cross-section than the cross-section of the
smallest particle they
may retain.
[00113] The outer surface of the filter (410) has no pores and may comprise of
the same or
different material from the material of the channels (442, 443). The choice of
material of the
outer filter surface (441) can be made to offer protection to the filter (410)
during use, its
connection to the filter unit, etc.
[00114] The filter (420) is depicted in cross-section view. It is the same as
the filter (410) both
in terms of structural elements and function. The only difference lies in the
fact that the
channels (448, 449) are in a circular matrix layout, as opposed to the filter
(410). The cross
section of the channels is depicted square but may be of any shape and size,
as analyzed for the
filter (410) and its method of manufacturing is the same as that of the filter
(410).
[00115] How to secure filters in the filter unit + advantages
[00116] FIG. 5 presents simplified examples of securing the filters in FIG. 4
into the system in
the present invention. The filter (510) depicted in a side-frontal view, is
the same as the filter
(410) and contains plugged alternate channels (542, 544) in a square matrix
layout. When
placed in the filter unit (240, 340), the filter (510) is placed in a (usually
metallic) quadrilateral
frame (545) of dimensions larger than the filter (510), and between the filter
(510) and the
frame (545) a sealing material is inserted (543) suitable for securing the
filter (510) in the frame
(545), where the material (543) is wrapped around the periphery of the filter
(510) and is made
of a material resistant to the filter operation (510) high temperatures,
capable of absorbing
vibrations and suitable for expanding to allow the space between the filter
(510) and the frame
to be sealed (545). In this way the material (543) absorbs any forces that
could be applied on
the filter (510) when moving the system (200, 300), securing and releasing the
filter (510) in
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the frame (545), during filter operation (510), and during contraction and
expansion cycles of
the frame (545). An example material for the construction of the sealant (543)
is a layer of
needle-shaped polycrystalline fiber.
1001171 Alternative filter realizations
[00118] The filter (520) depicted in a cross-section, is the same as the
filter (510) and contains
plugged alternate channels (547, 548) in a circular matrix arrangement. When
placed in the
filter unit (240, 340), the filter (520) is placed in a (usually metallic)
cylindrical frame (550) of
dimensions larger than the filter (520), and between the filter (520) and the
frame (550) a
material (549) suitable for securing the filter is inserted (520) in the frame
(550), said material
(549) is wrapped around the outer surface of the filter (520) and is made of a
material resistant
to high temperatures and capable of absorbing vibrations and deformations,
thus allowing the
filter to be secured (520) in the frame (550). In this way the material (549)
absorbs any forces
that could be applied on the filter (520) when moving the system (200, 300),
securing and
releasing the filter (520) in the frame (550), during filter operation (520),
and during the
contraction and expansion cycles of the frame (550). An example of a material
for the
construction of the sealant (549) is a layer of needle-shaped polycrystalline
fiber.
[00119] Filters (410, 420) may consist of arrays of identical filters grouped
into square, circular
or other shapes and size matrices, and contain a different number of channels
than those shown
in FIG. 5. It is also possible that the matrices and/or arrays of filters are
encased within frames
of a similar shape for ease of transport, securing and protecting from
external forces. The
monolith or array of monoliths that make up the filter (510, 520) is placed
vertically within the
container in which it is anchored at one end, while the other end is free to
move to prevent the
formation of compressive or tensile stresses on the ceramic material due to
contraction and
expansion of the metallic housing.
[00120] Due to its compact form, the filter (410, 420) possesses high
mechanical strength in
combination with a high filtration area thanks to the dense channel structure.
Such ceramic
filters are widely used in the automotive industry for the filtration of soot
particles from diesel
engine exhaust gases called Diesel Particulate Filters or DPFs. Space savings
in the automotive
industry are of critical importance, so DPF ceramic filters, through their
high filtration surface
and small volume, are ideal for small spaces and therefore small-scale
gasification units in
limited space applications. More specifically, they occupy at least two times
less space than
ceramic and metallic candle type filters used in similar larger-scale
applications. In DPF filters
the lack of empty space between adjacent channels increases the static
strength of the ceramic
CA 03228189 2024-02-02
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filter while the empty space between adjacent candles in other applications is
a source of
problems regarding the structural strength of the filter and the lack of
protection against the
accumulation of ash on the outer surfaces of candles.
[00121] Control of operation of a clean gas production system
[00122] FIG. 6 shows a simplified system operation control flowchart. The
operation control
(600) of the system (200, 300) begins with the reception of data (610) from
temperature and
pressure sensors at various points of the system (200, 300), position sensors
on the valves, and
motion sensors in other moving parts of the system (200, 300) as well as
level, weight and
humidity sensors mounted on the solid raw material feed system or at the
dryer's outlet. The
sensors may be selected among all known temperature, pressure, position and
motion sensor
technologies as well as other sensor technologies which may be used in ways
that provide data
equivalent to temperature, pressure, distance and humidity sensors.
[00123] Step measurements (610) are received from a control unit (e.g.
microprocessor,
computer, programmable logical controller (PLC), etc.) which executes software
stored in a
memory drive or hard disk or other physical storage medium (e.g. magnetic or
optical medium)
or in a the cloud or on another computer to which it is connected.
Alternatively, the
computational unit is of the type of an Application-Specific Integrated
Circuit (ASIC) or
equivalent and/or executes firmware.
[00124] The control unit, hereinafter the computer, proceeds to adjust and
control the operation
of one or all of the following: the gasifier (620), the cyclone unit (630),
the filter unit (640) and
the condenser (650). Steps (610)-(650) are performed until the system is shut
down (660) (200,
300) and undertake to implement all the necessary functions for the
gasification of the solid
fuel, the cleaning of the combustible gas and the cleaning of the filters.
More specifically, the
control unit may be designed or programmed to implement the adjustment of at
least one of the
following: the fluidized bed gasifier (210) supply with solid fuel and air,
the operation of the
first combustible gas cooling device (220), the operation of the filter unit
(240), and the
operation of the second combustible gas cooling device (230). The computer may
also record
the data of the sensors and the actions performed in the steps (610)-(650) in
order to process
them (or assign the data processing to an external computer) for the purpose
of their statistical,
or other, analysis and the use of the results of the analysis to optimize the
actions of the steps
(610)-(650). The data analysis and the optimization of the steps (610)-(650)
may be done using
appropriate software, firmware, hardware, or a combination of all and may
include amongst
others, artificial intelligence techniques and machine learning.
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[00125] The examples used above to describe the present innovative solution
should not be
considered as restrictive to the scope of this innovative solution. This
innovative solution may
be applied to scenarios and arrangements other than those described in the
examples presented
above. This innovative solution should be considered as applicable to any
system of
gasification of solid fuels of all types and cleaning of the gas produced.
[00126] The average person with relative knowledge of the state of the art
understands that the
shape and dimensions of the parts of the present invention, as presented in
the exemplary
embodiments, may be modified without deviating from the scope and intended
protection of
the present invention.
[00127] The above exemplary embodiment descriptions are simplified and do not
include parts
that are used in the embodiments but are not part of the current invention,
are not needed for
the understanding of the embodiments, and are obvious to any user of ordinary
skill in related
art. Furthermore, variations of the described exemplary embodiments are
possible, where, for
instance, some parts of the exemplary embodiments may be rearranged, omitted
and replaced
with equivalent, or new parts may be added, as well as, the existing parts may
be interconnected
differently than the described manner, under the provision that the different
interconnection is
compatible with the technical effect of the parts of the invention, being
characteristics of the
invention. Similarly, the modification of the shape and dimensions of the
presented parts is
assumed to fall within the scope of protection of the present innovative
solution to the degree
that these modifications are equivalent to the described exemplary
embodiments, or do not add
tangible and unanticipated or non-obvious improvements to the technical effect
they offer.
Thus, the present text is not intended to be limited only to the presented
exemplary
embodiments but it should be given the broadest possible scope according to
the principles and
the new characteristics it discloses.
[00128] Unless specifically otherwise noted, it is the intention of the
inventor that the words and
phrases in the specification and claims be given the ordinary and accustomed
meanings to those
of ordinary skill in the applicable art(s).
[00129] The foregoing description of a preferred embodiment and best mode of
the invention
known to the applicant at this time of filing the application has been
presented and is intended
for the purposes of illustration and description. It is not intended to be
exhaustive or limit the
invention to the precise form disclosed and many modifications and variations
are possible in
the light of the above teachings. The embodiment was chosen and described in
order to best
explain the principles of the invention and its practical application and to
enable others skilled
in the art to best utilize the invention in various embodiments and with
various modifications
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as are suited to the particular use contemplated. Therefore, it is intended
that the invention not
be limited to the particular embodiments disclosed for carrying out this
invention, but that the
invention will include all embodiments falling within the scope of the
appended claims.
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