Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS AND APPARATUS FOR GASIFICATION OF REFUSE
FIELD OF THE INVENTION
The present invention relates to synthetic fuel gas production from low cost
carbonaceous
materials and waste products such as biomass, municipal solid waste, plastic
and rubber
residues, wastewater treatment sludge, pulp and paper liquors, heavy petroleum
residues.
More specifically, the present invention relates to an apparatus and method
for production
a synthetic gas sufficiently clean to comply with current environmental
regulations.
BACKGROUND OF THE INVENTION
Synthetic gas has various uses such as:
~ fuel for gas burners/boilers for hot water or steam generation
~ fuel for internal combustion engines (Diesel, Otto, Stirling)
~ fuel for gas turbines to generate electricity
~ feedstock for chemical synthesis
~ feedstock for the production of hydrogen-rich gas by shifting the CO and
reforming any
hydrocarbons present in the synthetic gas.
Gasification is a known technique for converting carbonaceous materials to
valuable
combustible synthetic gas. In a gasification reactor, a fraction of the
feedstock is oxidized
thereby generating high temperatures inside the reactor (exothermic reaction).
The
remainder of the feedstock decomposes at the high temperatures generated by
the
oxidation reactions generating hot combustible synthetic gas and small amounts
of char
(endothermic reaction). The reactor is constantly fed with carbonaceous
material and
oxygen-containing gas to keep the exothermic reaction going so as to maintain
a high
temperature inside the reactor. Typically, the oxygen supply is about 25-30%
of
stochiometric values for total combustion. The partial oxidation regime
present in the
reactor renders the process self-sufficient in energy. Additional reaction
processes taking
place concurrently with thermal decomposition in the gasification reactor are
steam
reforming, the Boudouard reaction and the shift conversion reaction. The
extent of these
reactions determine, to a large degree, the composition of the synthetic gas
produced.
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At steady-state a typical gasification reactor operates at temperatures above
700°C and
pressures above 101 kPa.
Current gasification technology improvements focus on gasification reactor
design and gas
conditioning operations prior to its final use.
U.S. Patent 4,968,325 discloses a biomass gasification process and plant
design. This
process is commercially known as BIOSYN (trademark). The process uses a fluid
bed
gasification reactor containing fine sand fluidized by finely dispersed
bubbles, introduced
through a conveniently placed grid at the bottom of the reactor, of an oxygen-
containing
gas. Technical viability of this process has been demonstrated for wood
residues at
capacities as high as 10 tones/h. Moreover, the synthetic gas resulting from
wood residue
gasification is not sufficiently "clean" to be used as a fuel in modern energy
conversion
devices. What is meant by "clean" is a synthetic gas free of harmful
contaminants such as
tar and particulate material including chemically aggressive species as
defined by current
environmental regulations.
U.S. Patent 4,448,588 relates to a synthetic gas conditioning process. What is
meant by
"conditioning" is the treatment of the synthetic gas prior to its use as a
fuel or other
intended use. The process shows the use of the carbon-rich solids, i.e. char,
resulting from
gasification to remove organic vapors from the synthetic gas.
It is an object of the present invention to overcome the drawbacks of the
prior art by
providing an extremely versatile and commercially valuable process and
apparatus for
gasification of various sources of refuse and industrial by-products and
wastes.
A related object is to provide synthetic gas conditioning steps and related
apparatus for
purifying the synthetic gas and minimizing the final effluents to be disposed
of.
Other objects and further scope of applicability of the present invention will
become
apparent from the detailed description given hereinafter. It should be
understood,
however, that this detailed description, while indicating preferred
embodiments of the
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3
invention, is given by way of illustration only, since various changes and
modifications
within the spirit and scope of the invention will become apparent to those
skilled in the art.
SUMMARY OF THE INVENTION
In general terms, the present invention provides improvements relating to a
gasification
reaction vessel for converting carbonaceous feedstock to synthetic gas. The
reaction vessel
being of the type having a bottom portion with a grid for retaining a bed of
fluidizable and
heat-retaining particulate material. The latter is most preferably silica sand
of average
diameter of 200 to 700 Nm. The bed is fluidized by a stream of oxygen-
containing gas fed
through the grid tuyeres, the bottom portion also being adapted to receive,
laterally, a feed
of carbonaceous material to be gasified, the reaction vessel further having a
top portion for
recovering and evacuating the synthetic gas produced in the bottom portion.
The
improvements generally consisting of: a top portion of the reaction vessel
having an
enlarged average internal cross-sectional area compared to the average cross-
sectional area
of the bottom portion so as to facilitate disengagement and removal of said
synthetic gas
from the bed of particulate material.
The present invention also provides an improved gasification process using a
reaction
vessel containing a fluidized bed for receiving a carbonaceous material
feedstock for
gasification and a fluidizing gas, the improvement in the gasification process
consisting of:
providing as the fluidization gas an oxygen-enriched air containing up to 50%,
preferably
- 40%, oxygen to said reaction vessel (percentages based on volume).
The present invention also provides an improved conditioning process for the
hot synthetic
25 gas exiting the reaction vessel. The conditioning process provides a clean,
cold synthetic
gas ready for use in various applications such as a fuel for boiler furnaces,
internal
combustion engines, gas turbines, etc. The process improvements consisting of
subjecting the synthetic gas to a sequential treatment in at least one cyclone
to remove
particulate material therefrom, followed by a treatment in at least one
counter-current wet-
30 scrubber tower to cool said gas and solubilize or entrain further
pollutants, followed by a
treatment in at least one venturi scrubber to remove further pollutants,
followed by
treatment in a at least one demister to remove residual liquid droplets.
Optional additional
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conditioning treatments are also provided. Recycling steps are also described
and are
aimed at revealing means to increase energy efficiency and minimize by-product
generation.
BRIEF DESCRIPTION OF THE FIGURES:
~ Figure 1 represents the gasifier, in accordance with the invention;
~ Figure 2 is a schematic representation of the process of the present
invention;
~ Figure 3 shows the cooling tower of the gas scrubbing system of this
invention;
~ Figures 4.1-4.5 present the composition of the producer gas as function of
the oxygen
content of the gasifying agent;
~ Figure 4.6 gives the producer gas flow rate as function of the oxygen
content of the
gasifying agent;
~ Figure 4.7 gives the gasification temperature as function of the oxygen
content of the
gasifying agent;
~ Figure 4.8 presents the HHV of the gas as function of the gasifying agent;
~ Figure 4.9 presents the cold gas efficiency as a function of the percentage
of oxygen
content of the gasifying agent;
~ Figures 5.1-5.5 present the composition of the producer gas as function of
the oxygen
content of the gasifying agent;
~ Figure 5.6 gives the producer gas flow rate as function of the oxygen
content of the
gasifying agent;
~ Figure 5.7 gives the gasification temperature as function of the oxygen
content of the
gasifying agent;
~ Figure 5.8 presents the HHV of the gas as function of the gasifying agent;
~ Figure 5.9 presents the cold gas efficiency as a function of the oxygen
content of the
gasifying agent;
~ Figure 5.10 presents the tar by-product generation as a function of the
oxygen content
of the gasifying agent.
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DETAILED DESCRIPTION OF THE INVENTION
This invention will be described herein below, by referring to specific
embodiments and
appended Figures, which purpose is to illustrate the invention rather than to
limit its scope.
5 The invention proposes a process to convert organic-rich wastes into a clean
synthetic gas
using an atmospheric or pressurized bubbling fluid-bed gasification reactor.
The
gasification is followed by gas conditioning which, will yield cold clean gas
for use as a
fuel in boilers, internal combustion engines, etc.
The present invention can advantageously be used to convert various forms of
refuse rich
in carbonaceous material. For example, wood residue, refuse derived fuel « RDF
» from
municipal solid waste, plastic and rubber residues, wastewater treatment
sludge as well as
residues from various industrial operations such as pulp and paper black
liquors, petroleum
heavy residues altogether with other non dangerous carbonaceous materials are
examples
of gasifiable residual streams.
These carbonaceous feedstocks may be predried or preheated in order to
increase the
efficiency of the overall process. Typically their moisture content does not
exceed 20 wt%
of the feedstock.
During gasification various chemical reactions take place. These chemical
reactions are
responsible for the conversion of the feedstock into 'synthetic gas'. The non-
condensable
portion of the synthetic gas is a mixture of CO, H2, CO2, NZ, H20 and light
hydrocarbons
of up to 7 carbon atoms. The relative concentration of each constituent
depends on the
feedstock but it is a strong function of the gasification agent (percentage of
oxygen in the
oxygen-containing gas fed to the gasification reactor), the ratio of
gasification agent to
feedstock, and the extent of reactions taking place in the freeboard of the
reactor. The
synthetic gas entrains small particles, known as particulate matter, and
contains low
amounts of higher molecular weight hydrocarbons in the gaseous phase, known in
the
literature as tar. The particles come from two sources: the inorganic matter
present in the
feedstock and hydrocarbons condensation reactions responsible also for the
formation of
tar present in the synthetic gas. The latter reactions lower the carbon
conversion of the
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6
process thus decreasing its total energetic efficiency. Appropriate conditions
are used as
function of the feedstock to minimize the condensation reactions.
In accordance with the present invention, the hot gas exiting the gasification
reactor, after
passing through a cyclone system (typically one or two cyclones) is sent to a
wet scrubbing
apparatus, generally multi-step, for gas cooling and conditioning. Generally
speaking, the
purpose of the wet scrubbing is to remove the particulate and the condensable
species
present in the synthetic gas.
Referring to FIG. 1, the gasification reactor of the present invention is
illustrated. In its
preferred embodiment, gasifier (10) is a cylindrical reaction vessel
internally lined with
appropriate insulation and refractory material layers, generally referred to
as insulation (12).
Gasifier (10) may operate either under atmospheric or above atmospheric
pressure. The
bottom section of gasifier (10) is equipped with an oxygen-rich gas
distribution grid (14).
Above the grid (14) there is the section known as the fluidization bed (16)
which is
advantageously filled with silica sand or alumina sand. However, other
granular material,
for example magnesia, chromia, etc., are also acceptable. Advantageously, the
mean size
of the granular material used in the bed will vary between 200 and 700,um. One
skilled in
the art will readily appreciate that the final choice will depend on
carbonaceous feedstock
reactivity, oxygen content fed to the gasification reactor, operation pressure
and reactor
geometry (defining the fluid-dynamics and the residence-time distributions).
Preferably, the height of bed at rest (16) is between 1,5 and 2 times the
internal cross-
sectional area of the gasifier (10). Gasifier (10) is preferably cylindrical.
However, one
skilled in the art will readily appreciate that gasifier (10) may be conical,
pyramidal, square
or rectangular or other suitable shapes. In all cases, the lower section (18)
is designed to
be filled almost entirely by the expanded fluid bed (16) during gasification.
The upper
section, i.e. the freeboard, (20) will advantageously have an enlarged
internal cross-
sectional area when compared to the lower section (18), in the case of a
cylindrical gasifier
(10), it will have up to about 1,5 times the cross-sectional area of the lower
section (18). In
the case of conical or pyramidal gasifier (10), it would be installed with the
wider portion
on top. The larger cross-sectional area at the top favors an appropriate
disengagement of
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7
the gas from the fluidized bed of granular solids and thus results in a
shorter vessel than a
reactor having an identical diameter in both the fluidization and the
freeboard sections for
the same levels of disengagement.
Once again, one skilled in the art will appreciate that the relative heights
of the two
sections (18) and (20) of the gasifier (10) will depend on feedstock physico-
chemical
properties and several other parameters among which gasification fluid-
dynamics and
kinetics as well as the desired synthetic gas composition.
Coarse solids, not entrained by the synthetic gas exiting the gasifier (10)
remain in the fluid
bed and can be removed continuously from the bottom of gasifier (10). The
separation of
the coarse solids from the bed material (i.e. silica sand or alumina) is
achieved naturally
during fluidization due to density differences. In general, particulate matter
of low
inorganic content material has lower density than sand and tends to stay at
the top of the
expanded bed. This leads to higher attrition rates which gradually decrease
the size of
these particles and facilitate the 'washing' of these solids off the fluid
bed. If high density
inorganic material accompanies the feedstock it may be removed from the bottom
of the
bed through appropriately designed exit ports.
Total residence times of the synthetic gas evolved in gasifier (10) is about
10 to 20 seconds,
usually half of it in the lower section (18) of the reactor. Advantageously,
the ratio of
lower section (18) height to top section (20) height is between 2 to 1 and 3
to 1.
The gasifier (10) advantageously operates at gas velocities ranging between 5
and 20 times
the minimum fluidization velocity (as defined in standard fluidization
engineering
I iteratu re).
Overall the gasification is "autothermal" meaning that once at steady state,
gasifier (10)
requires essentially no external heating or cooling. For the start-up,
gasifier (10) will require
preheating. This is preferably done with a gas burner to reach a temperature
of about
500°C. It is to be understood that steady state temperature inside
gasifier (10) is set by a
combination of the following parameters:
CA 02372111 2001-11-13
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8
~ reactor geometry;
~ operation pressure;
~ insulation and refractory lining thermal properties;
~ fluid-bed material and size;
~ feedstock nature, physico-chemical properties (composition, humidity, size)
and input
rate;
~ oxygen-containing gas composition, velocity and flow rate.
Due to the complexity of the numerous reactions taking place during
gasification it is
observed that for the same reactive mixture different operating parameters can
lead to
different synthetic gas compositions. This means that some steady states are
in fact
metastable and slight parameter fluctuations can lead to other steady states.
One key and surprising feature of the present invention is the discovery that
the use of an
oxygen-enriched fluidizing gas vastly improves the properties of the resulting
synthetic gas.
More specifically, it was discovered that using oxygen-enriched air, of about
30 to 50%vol
of oxygen, preferably 30 - 40 % vol. oxygen as a fluidizing agent greatly
improves the key
gasification parameters: cold gas efficiency, lower tar production and higher
calorific
values of the synthetic gas. Supporting data is found in Figures 4.1 to 5.10
and in Tables 1
to 7 herein below
CA 02372111 2001-11-13
WO 00/69994 PCT/CA00/00552
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16
The remaining apparatus and process steps relate to gas conditioning of the
synthetic gas
evolved from gasifier (10). These conditioning steps will now be described in
greater detail.
Referring now to FIG. 2, the synthetic gas exiting gasifier (10) passes
through cyclones (22)
and (24) connected in series. The number of cyclones requires will depend on
the
cleaning strategy. In this preferred embodiment, cyclones (22) and (24) will
generally
retain between 90 and 95% of the total amount of particles entrained by the
synthetic gas
as fly ash. Modern cyclones are indeed able to cut out of the synthetic gas
stream 99% of
particles with mean particle size higher than 10 ,um. This means that the
remaining
particles in the synthetic gas, estimated between 1000 and 2000ppm/v, have a
mean
particle size of less than 10 Nm.
The particles removed through cyclone (22) are split into two streams : C9 and
C10. C9 is
the solids purge of the gasification process while C10 is recycled back to the
gasifier to
increase the carbon conversion of the system. The ratio of these two streams
(or expressed
differently the recycle ratio) is a function of the carbon content of the
particles removed by
cyclone (22). These particles may eventually be mixed with any solids purged
directly from
the gasifier.
In an alternate embodiment, stream C9 may be partially directed to water
treatment
column (26) for use as an absorbent as described herein below. Indeed, the
carbon in the
particles has specific surface area and adsorption features resembling those
of activated
carbon. The proposed process utilizes this carbon-containing particle (also
known as ash)
together with activated carbon to treat, through adsorption, the wastewater
purge. Upon
saturation of the carbon by adsorption, part of the solids are recycled back
to the gasifier to
increase the carbon conversion while decreasing the amount of solids to be
disposed of.
Thus the synthetic gas exiting cyclones (22) and (24) is subject to further
conditioning prior
to its use as fuel. At this point there are two main conditioning options
~ Wet scrubbing as described in the present patent application if clean cold
gas is desired
(i.e. the case for use in burners/boilers or internal combustion engines);
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~ Hot gas conditioning if hot gas is required (i.e. the case for use in gas
turbines or in
integrated gasifier combined cycled: IGCC).
The hot gas conditioning module consists of a mobile granular filter and a
multitubular
fixed-bed tar catalytic reforming reactor. The mobile granular filter and the
catalysts are
proprietary inventions of the applicants and described in separate patents
and/or
applications.
Particles removed by cyclone (24) are fed to water treatment system (26) as
shown by
stream C36. These particles are characterized by sufficient surface area to be
used as
adsorbent. Nevertheless, additional amounts of activated carbon, stream C37,
are
necessary for compliance with environmental regulations for disposal of
treated water
effluent, stream C38. As will be apparent from the following description,
water treatment
system (26) is used to treat the water used to scrub remaining pollutants from
the synthesis
gas exiting cyclone (24), stream C7.
To remove the remaining particles from the synthetic gas, stream C7, and to
condense and
remove the tarry products, which are in the vapor state at temperatures higher
than 400°C,
the process includes the use of a three stage wet (water) scrubbing module
comprising
water spray column (28), venturi scrubber (30) and cyclonic separator (32).
The synthetic gas stream C7 enters water spray column (28). Column (28) is
illustrated in
greater detail in Fig.3. Column (28) is a water-spray column preferably
counter-current,
cylindrical and with a conical bottom. Column (28) is used as a first stage
scrubber for (a)
cooling the synthetic gas down to about 90°C, b) removing about half of
all remaining
particles (targeting those comprised between 2 and 10 Nm) and c) condensing
all tarry
compounds having boiling points higher than 100°C (excluding some
volatile organic
compounds (VOCs)).
Referring still to FIG. 3, water stream W1 enters from the top of column (28),
under
pressure, and is dispersed using screw-shaped nozzles (34) producing wide
angle sprays
(36). Water stream W1 is the scrubbing/quenching solution.
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The pH of the scrubbing/quenching solution is adjusted in accordance with the
carbonaceous feedstock and the pollutants contained in the synthetic gas.
There may be
present: acid gases (NCI, HCN, SOX, NXOY), alkaline gases such as ammonia as
well as
volatile metal and salts. Acid gases are removed through alkaline scrubbing.
Ammonia is
removed through acidification of the scrubbing water. Volatile metals and
salts are
removed through quenching. Alkaline and acidic scrubbing produces
environmentally
neutral rejects. Condensed metals are subdivided into two categories : 1)
alkali and alkaline
earth metals giving non polluting salts and 2) heavy metals under their
elemental form or as
oxides if present in the gasified feedstock . The latter, being essentially
insoluble in water,
as shown in metal distribution studies, precipitate and can be recovered and
removed from
the process , streams C19 and C28.
Returning to FIG. 2, contaminated water stream C15 exists column (28) and is
routed to
heat exchanger (38) for further cooling to about 30°C. From there,
stream C17 enters
decanter/skimmer tank (40) to remove precipitated material. In tank (40), the
heavier
inorganics (and the organics sticking to the inorganics organics) decant and
can be
removed. Meanwhile the lighter organics float at the water surface and are
removed by
skimming.
The decanting and floating matter altogether with some entrained wastewater
are pumped
out , streams Cl8a, Cl8b, C19) and, after being mixed with other residual
streams (defined
later) or even separately, are sent back to gasifier (10). A purge, stream
C32, equal to about
10% of the recirculating matter, is removed for final disposal to insure the
stability of the
system.
The scrubbed synthetic gas, stream C16, still containing micronic particles,
water and tar
droplets, VOCs and some very volatile metals (if present in the initial solid
feedstock of the
gasifier) enters the second stage scrubbing process, namely venturi scrubber
(30).
Venturi (30) is preferably designed to operate at gas velocity range of 80-
100m/s and give a
total pressure drop between 0,07 and 0,15 atm. These conditions give a near
micronic
dispersion of the water stream entering the throat of the cyclone. This
provides an
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appropriate removal of the remaining particles with an efficiency of near 99%
for particles
of size as low as 0,10m. Ignoring the humidity of the synthetic gas, in this
venturi
scrubber, the ratio gas/water used is about 1 to 1 w/w. During the contact
time (usually of
less than 2s) the synthetic gas is cooled to about 35-40°C without
significant temperature
increase of the water stream.
From venturi (30), the synthetic gas enters the third and last stage of the
scrubbing module,
namely cyclonic separator (32). The water leaving the bottom of the gas-liquid
cyclonic
separator, stream C24 is fed into a second decanting/skimming tank (42) much
like tank
(40). Tank (42) is of significantly smaller size due to the lesser amounts of
water involved
in this recycle loop. Thus, streams C27 and C28 of Fig. 2 are pumped out to
join the
analogous streams C18 and C19 of tank (40). The cyclonic separator is a high
velocity
design unit able to remove droplets of mean size 5 ,um with an efficiency of
about 80%.
This means that the global separation efficiency is higher than 95%.
Nevertheless smaller
droplets are still present in the exiting gas stream (Fig. 2, stream C23) and
require further
removal.
Filter (44) is a high efficiency demister for completing droplet removal.
Commercially
available viscous filters as well as corrugated plate coalescers can be used
to accomplish
this task. Filter (44) can efficiently remove residual aromatics present in
minute quantities
(ppm or ppb). Removal is generally performed with a bed of activated carbon
which also
removes residual organic vapors. Once saturated the spent activated carbon may
be
recycled back to gasifier (10).
The use of a dehumidification unit (46) depends on specifications imposed by
the synthetic
gas end-use devices. The synthetic gas leaving the filter/demister will have
an average
temperature of about 25°C and will be saturated with water. This means
that it will contain
between 2 and 3%w/w of water. Usually this level of humidity is not
prohibitive for final
synthetic gas use in commercial burners/boilers, internal combustion engines
and gas
turbines. Dehumidification units (46) work by adsorption, for example by
passing over a
bed of alumina, and can reduce the moisture content of the gas to the desired
levels.
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EXAMPLES OF GASIFICATION PERFORMANCE
Description of the unit
The runs have taken place in a Process Development Unit (or pilot plant). The
gasification
reactor is a 4 m high refractory-lined cylinder having 60 cm as outside
diameter in the
5 lower section, expanded to 74 cm in the upper section. The inner diameters
are 30 cm
and 45 cm, respectively.
Fluidizing gas (air or oxygen-enriched air) enters the reactor through a
distribution grid
formed by an assembly of 9 tuyeres. The grid, of proprietary design, also
serves as support
10 for the sand bed.
The gasifier is fed from a 1.3 m3 live-bottom hopper by a system consisting of
a transfer
screw, a chute and an injection screw which can be located in either one of
two ports
situated at 48 cm or 29 cm above the grid.
Air (or oxygen-enriched air) is supplied by a compressor. The main flow (about
80%)
enters the reactor through the grid. The rest (20%) is directed to the feeding
system to
prevent back flow of gases from the reactor to the hopper. The sand (Si02 )
bed height (at
rest) has been varied between 60 and 45 cm. The average diameter of the sand
particles is
0.5 mm (2.6 kg/I as density). Fluidizing velocities are comprised between 0.3
and 0.75
m/s. The biomass solids in the bed are estimated at less than 10% of the total
bed solids.
The volumetric bed capacity is 1.12 tones of biomass (mbed3 ~ h).
The gas produced exits the gasifier and passes through two cyclones in series
and the wet
scrubbing module described in the present patent application. A bypass system
permits to
either send the entire gas produced to a granular filter and then to the
flare, or go directly
to the flare. Char and ash are collected in two reservoirs connected to the
cyclones as well
as in the filter media used.
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The installation is equipped with more than 40 thermocouples and 20 pressure
transducers.
Air and producer gas flows are measured by thermal dispersion and orifice flow
meters,
respectively.
The waste feed rate is monitored by a system of 3 load cells. The feed rate is
adjusted by
the rotation of the screws at the bottom of the hopper.
All measured variables are recorded by a Hewlett Packard data acquisition
system taking
samples at 10s intervals.
Feedstocks that have been successfully gasified
(1 ) Woody biomass
(2) Forest and agricultural residues
(3) Biological treatment sludge
(4) Refuse Derived Fuel (RDF) from urban wastes (MSW);
(5) Rubber residues containing 5-15% KEVLAR residues;
(6) Granular polyethylene and polypropylene residues;
Analytical strategies
Sampling trains, designed following EPA's and Canadian standard methods, each
consisting
of a particulate material filter, a tar-water condenser, a demister and a
drying/absorber
allow collection and analysis of the gases.
When hot gas conditioning is applied three isokinetic samplers are used to
determine the
particulate, tar, VOC and water contents in the off gases before and after gas
conditioning.
Each isokinetic gas sampling probe has a heated filter, a water/tarNOCs
condensing heat
exchanger/collector, a gas impact demister, a drierite gas dryer, a regulated
pump, a dry
gas flow meter and an appropriate orifice/manometer arrangement insuring
isokinetic
conditions.
When wet scrubbing is applied two isokinetic sampling trains are sufficient to
measure the
efficiency of the gas conditioning. An HP 5890 gas chromatograph analyses dry,
tar free
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gases by separation of components on Porapak Q and Molecular Sieve 13X columns
in
series followed by TC detection. The sampling combined with the appropriate
analysis
methods allows to:
~ Calculate the gross composition of the stack gas (the H20, H2~ CO, C02, N2,
and 02
content) using Gas Chromatography (combination of Porapak Q & Molecular sieve
column packings) as well as the particulate matter load of the stack gas.
~ Detect and quantify toxic and acid components via colorimetric methods
(Matheson-
Kitagawa's Kits) in the stack gas (i.e. SOx, NOx, HCI) .
~ Identify and quantify the main organic components dissolved in the scrubbing
water by
means of GC/MS analysis.
~ Measure the inorganic anions retained by scrubbing water using ion
chromatography.
~ Identify and quantify the distribution of metals in the output streams of
the gasifier and
its modules using X-rays fluorescence, AES/ICP, SEM and AA analyses.
Other analyses have been used in order to gather additional information as
follows:
~ Calorimetry for the different feedstocks tested.
~ TGA for the study of the thermal stability/instability of the different
feedstocks tested.
~ TOC, COD and BODS analyses for the scrubbing water before and after
treatment
~ BET analysis for the porosity of the carbon-rich gasification ashes used for
the adsorption
tests.
~ GC/MS analysis for tar and halogenated hydrocarbons.
EXAMPLE 1
Results with residual wood as gasification feedstock
The four (4) runs reported herein below have the following common
parameters/variables:
~ Stoichiometric (or equivalence) ratio: ~,= 30% (refers to the stoichiometric
amount of
oxygen needed for complete combustion of the carbon and hydrogen fed to the
gasifier);
~ Mean Solids feed rate: 35 kg/h;
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~ Mean Solids humidity: 12%w/w;
~ Mean Solids size: 1cm;
~ Oxygen content of the fluidizing air was 20,9; 30; 40; and 50%vol. for the
four runs
respectively.
The results obtained are presented as follows:
~ Figures 4.1-4.5 present the composition of the producer gas as function of
the oxygen
content of the gasifying agent. We can conclude that as oxygen content. In the
fluidizing gas increases the inert gases NZ and Ar are decreasing while all
other gases
coming from organic matter gasification increase.
~ Figure 4.6 presents the synthetic gas flow rate as function of the oxygen
content of the
gasifying agent. As expected the gas flow rate is decreasing. It is preferably
for the
commercialization of the technology to have flow rates as low as possible in
order to
decrease the cost associated with gas conditioning, piping and handling in
general.
Moreover, for the same gasification reactor, lower gasifying agent and
producer gas
flow rates lead to lower linear velocity profiles and higher residence times
in the fluid-
bed gasification vessel. This in turn leads to lower particle entrainment
(carry-over) and
allows heavy tar gasification and CO shift reactions to proceed at higher
conversion
rates.
~ Figure 4.7 presents the gasification temperature as function of the oxygen
content of the
gasifying agent. As the available heat is transferred to a smaller gas flow
rate the
temperature increases nearly linearly with the oxygen content.
~ Figure 4.8 presents the HHV of the synthetic gas as function of the
gasifying agent. It is
also an increasing function. Values between 9 and 12 MJ/Nm3 are obtained. Such
values classify the synthetic gas as a medium calorific value gas and render
its use in
end-use devices easier and more efficient than low calorific value producer
gas.
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EXAMPLE 2
Results with RDF as gasification feedstock
The runs reported herein below have the following common parameters/variables:
~ Stoichiometric (or equivalence) ratio: ~,= 30%;
~ Mean Solids feed rate: 35 kg/h;
~ Mean Solids humidity: 12%w/w;
~ Mean Solids size: 1cm;
~ Oxygen content of the fluidizing air was 20,9; 30; and 40%v/v for the three
runs
respectively.
The following observations can be made from the results:
~ Figures 5.1-5.5 present the composition of the producer gas as a function of
the oxygen
content of the gasifying agent. We can also conclude, as in the wood case,
that as
oxygen content increases in the fluidizing gas the inert gases NZ and Ar are
decreasing
while all other gases coming from organic matter gasification increase.
~ Figure 5.6 gives the producer gas flow rate as a function of the oxygen
content of the
gasifying agent. The trend and the comments are the same as in the wood case.
~ Figure 5.7 gives the gasification temperature as function of the oxygen
content of the
gasifying agent. The trend is the same as in the wood case. Here we observed
however
a levelling-off behavior around 35% of oxygen content. As the RDF calorific
value is
lower than that of wood it is probable that the balance between heat
production over
heat transfer and losses has attained its equilibrium faster than in the case
of wood
gasification.
~ Figure 5.8 presents the HHV of the gas as function of the gasifying agent.
It is also an
increasing function. As in the case of wood gasification, values between 9 and
12
M)/Nm3 are measured.
