U.S. patent application number 13/162241 was filed with the patent office on 2011-12-22 for producing low tar gases in a multi-stage gasifier.
Invention is credited to Paul Evans, Thomas J. Paskach, John P. Reardon, Jerod Smeenk.
Application Number | 20110308155 13/162241 |
Document ID | / |
Family ID | 45327419 |
Filed Date | 2011-12-22 |
United States Patent
Application |
20110308155 |
Kind Code |
A1 |
Paskach; Thomas J. ; et
al. |
December 22, 2011 |
Producing Low Tar Gases in a Multi-Stage Gasifier
Abstract
A system for gasifying solid matter uses multiple stages to
produce low-tar combustible gas includes a first reactor having a
fluidized bed to produce hydrogen containing gas, pyrolysis vapors,
tars, and char particles at temperature less than the exit of the
second reactor and a higher temperature partial oxidation combustor
zones. A second reactor includes a higher temperature partial
oxidation zone to activate hydrogen and cause cracking of aromatic
ring compounds, a co-flow moving granular bed with a char
gasification stage to catalyze tar reduction, and control char
residence time, and a media screen comprising a parallel wire
screen substantially vertically oriented supporting granular
media.
Inventors: |
Paskach; Thomas J.; ( Ames,
IA) ; Reardon; John P.; (St. Louis, MO) ;
Evans; Paul; (Miami, OK) ; Smeenk; Jerod;
(Ames, IA) |
Family ID: |
45327419 |
Appl. No.: |
13/162241 |
Filed: |
June 16, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61397765 |
Jun 16, 2010 |
|
|
|
Current U.S.
Class: |
48/77 ; 48/197R;
48/210; 48/62R |
Current CPC
Class: |
C10J 3/466 20130101;
Y02E 50/30 20130101; C10J 2300/0946 20130101; C10J 3/463 20130101;
Y02P 20/145 20151101; Y02E 50/32 20130101; C10J 2300/0993 20130101;
C10J 3/482 20130101; C10J 2300/0959 20130101; C10J 3/485 20130101;
C10J 2300/1807 20130101; C10J 3/84 20130101; C10J 3/721 20130101;
C10J 2300/0916 20130101; C10J 2300/0956 20130101 |
Class at
Publication: |
48/77 ; 48/62.R;
48/210; 48/197.R |
International
Class: |
C10J 3/48 20060101
C10J003/48; C10L 3/00 20060101 C10L003/00 |
Claims
1. A multi-stage reaction system for producing low-tar combustible
gas, the system comprising: a fluidized bed reactor that includes a
partial oxidation zone, in which a gas and a plurality of char
particles are created in said partial oxidation zone; and an
entrained flow partial oxidation reactor positioned downstream from
the fluidized bed reactor.
2. The system disclosed in claim 1 wherein said entrained flow
partial oxidation reactor includes a moving granular bed.
3. The system of claim 1, wherein the fluidized bed reactor further
comprises a freeboard, said freeboard operated at a velocity
controlled to create a generally consistent char particle size
feed.
4. The system of claim 2 wherein said plurality of char particles
is of generally consistent char particle size and at least a
portion of said plurality of char particles is provided to the
entrained flow partial oxidation reactor.
5. The system of claim 3 wherein provision of said generally
consistent char particle size comprises modulating a pressure of
the fluidized bed reactor.
6. The system of claim 1, further comprising a sand recovery
cyclone.
7. The system of claim 1, wherein the entrained-flow partial
oxidation reactor further includes a partial oxidation zone and
means for injecting a stream of combined gas and char into said
partial oxidation zone.
8. The system of claim 7, further comprising a cyclone for
concentrating the plurality of char particles out of a fed gas
stream prior to injection into said partial oxidation zone.
9. The system of claim 8, wherein said cyclone concentrates the
plurality of char particles out of the fed gas stream prior to
injection into said partial oxidation zone.
10. The system of claim 2, wherein the entrained-flow partial
oxidation reactor includes a plurality of blast inlet ports
configured around a periphery of a vessel enclosing the
reactor.
11. The system of claim 10, wherein said plurality of blast inlet
ports is arranged in an alternating pattern such that each of the
plurality of blast inlet ports targets a tangent curve of a tangent
circle.
12. The system of claim 10, wherein said plurality of blast inlet
ports is arranged in an alternating pattern such that a first one
of the plurality of blast inlet ports targets a first tangent curve
of a first tangent circle and a second one of the plurality of
blast inlet ports targets a second tangent curve of a second
tangent circle, wherein the first and second tangent circles are
located at the same elevation.
13. The system of claim 10, wherein said plurality of blast inlet
ports is arranged in an alternating pattern such that a first one
of the plurality of blast inlet ports targets a first tangent curve
of a first tangent circle and a second one of the plurality of
blast inlet ports targets a second tangent curve of a second
tangent circle, wherein the first and second tangent circles are
located at different elevations.
14. The system of claim 7, wherein the moving granular bed operates
in co-flow with respect to the stream of combined gas and char to
create a gas-char contacting zone.
15. The system of claim 14, wherein a gas-media disengagement
screen is oriented at an angle that is steeper than an angle of
repose of the combined media and char mixture.
16. The system of claim 15, wherein the gas-media disengagement
screen is oriented substantially vertically.
17. The system of claim 15, wherein the gas-media disengagement
screen includes a plurality of parallel wires extending between an
upper frame edge and a lower frame edge.
18. The system of claim 17, wherein each of the plurality of wires
includes a cross section partially defined by a first side that
converges with a second side at a vertex in the direction of the
disengaging gas flow.
19. The system of claim 18, wherein the cross section is wedge
shaped.
20. The system of claim 2 wherein the entrained-flow partial
oxidation reactor further includes a partial oxidation zone, means
for injecting a stream of combined gas and char into said partial
oxidation zone, and a plurality of blast inlet ports configured
around a periphery of a vessel enclosing the reactor.
21. The system of claim 20 wherein the moving granular bed operates
in co-flow with respect to the stream of combined gas and char to
create a gas-char contacting zone.
22. The system of claim 2 wherein the entrained-flow partial
oxidation reactor further includes a partial oxidation zone and
means for injecting a stream of combined gas and char into said
partial oxidation zone and said moving granular bed includes a
gas-media disengagement screen oriented at an angle steeper than
the angle of repose of the combined media and plurality of char
particles.
23. A method for controlling an operating pressure of a two-stage
gasification system, the method comprising: performing a partial
oxidation of a portion of biomass in a fluidized bed reactor,
wherein the partial oxidation creates a gas and a plurality of char
particles; elutriating at least a portion of said plurality of char
particles and gas from the fluidized bed reactor, wherein said
elutriating includes removing a mixture of gas and char particles
from the fluidized bed reactor; receiving the mixture into an
entrained flow reactor, wherein the entrained flow reactor includes
a moving granular bed of filtering media; allowing the mixture to
flow through the moving granular bed; and capturing a portion of
the plurality of char particles in the filtering media.
24. The method of claim 23 further comprising screening a portion
of the filtering media to remove captured char particles; and
returning the screened filtering media to the entrained flow
reactor.
25. A multi-stage reaction system for producing low-tar combustible
gas, the system comprising: a fluidized bed reactor that includes a
partial oxidation zone in which a portion of the feedstock is
partially oxidized, wherein said partial oxidation creates a gas
and a plurality of char particles; an entrained flow partial
oxidation reactor situated downstream from the fluidized bed
reactor, the entrained flow partial oxidation reactor including a
moving granular bed; a media screening device that screens media
from the moving granular bed; and a media recycle system that
returns the screened media to the entrained flow partial oxidation
reactor.
Description
[0001] This application claims the priority of provisional
Application Ser. No. 61/397,765. The present invention relates, in
general, to gasifying materials such as biomass and waste to
produce high quality gas.
FIELD OF THE INVENTION
Background
[0002] "Tars are the Achilles heel of gasifiers, and many gasifier
projects have failed because of insufficient attention to low tar
production or efficient tar destruction"--Tom B. Reed (T. Milne
1998). The highly generic term "tar" was uniformly defined in 1998
(at the EU/IA/DOE conference in Brussels) as all organic
contaminants of gasification that have a molecular weight larger
than benzene. Several review articles have been published
discussing the nature, formation and destruction of tar from
biomass gasification. (Li 2009) (Han 2008) (T. Milne 1998).
[0003] A maturation process has been proposed for tar with
temperature, progressing from mixed oxygenates (400.degree. C.), to
phenolic ethers (500.degree. C.), then alkyl phenolics (600.degree.
C.), then heterocyclic ethers (700.degree. C.), then polycyclic
aromatics (800.degree. C.), and then larger Polynuclear aromatic
hydrocarbons (PAH), soot, and coke (900.degree. C.). Elliot, D. C.
"Relation of reaction time and temperature to chemical composition
of pyrolysis oils." Proceedings of the ACS Symposium Series 376,
Pyrolysis Oils from Biomass. American Chemical Society, 1988.
Polymerization and subsequent agglomeration of high molecular
weight PAH is described as a homogeneous pathway to "soot"
formation. Homann, K. H., Wagner, H. G., "Some new aspects of the
mechanisms of carbon formation in premixed flames." Eleventh
International Symposium on Combustion. Pittsburgh: The Combustion
Institute, 1967.
[0004] Several different classifications of tar have been
established. These classifications are related to temperatures.
Classification has been developed as follows: "primary tars" are
vapors produced at lower temperatures and are the first evolved in
thermal depolymerization of cellulose, hemicellulose, and
lignin--these are mainly oxygenated compounds. Next, the secondary
and tertiary reaction products of primary tars are termed
"secondary tar" and "tertiary tar". Tertiary tars were
sub-classified as tertiary-alkyl and tertiary-polynuclear aromatic
hydrocarbons (PAH). It is hypothesized that once tertiary tars are
formed these may require even higher temperatures and additional
residence time for thermal destruction.
[0005] There are several approaches to achieve adequate reduction
of tar after an initial stage of gasification including thermal
cracking, partial oxidation, and catalytic cracking using mineral
catalysts or reforming with metal catalysts. One such method is
indirect heat thermal cracking. This method has been discussed in
the open literature to reduce tars in raw product gas. In the
absence of char, a temperature of 900.degree. C. is insufficient to
achieve much tar destruction. Specifically, a slip stream was
filtered at 450.degree. C. to remove all char dust, but no
measurable difference was found (after a fluid bed gasifier (8000
mg/Nm.sup.3 in feed gas from CFB operating at 825.degree. C.) was
filtered at 450.degree. C. to remove all char dust). The
application of a homogeneous phase reactor demonstrated only
.about.25% reduction even with residence times as high as 12
seconds. Even at 1000.degree. C. with 12 second residence time,
only 75% reduction was achieved (.about.2000 mg/Nm.sup.3 in
product). (Houben, M. P. Analysis of tar removal in a partial
oxidation burner. PhD Dissertation, Eindhoven: Technical University
Eindhoven (Netherlands), 2004).
[0006] It is a common hypothesis that the minimal performance of
the char-free thermal treatment at 1000.degree. C. as compared to
the downdraft gasifier at the same temperature suggests a catalytic
role for char in tar reduction. It is possible that the nature of
the fed tars (refractory tertiary tars present in fluid bed gas
compared to primary or secondary tars in lower temperature
pyrolysis gases) may also play a role in determining the thermal
requirement for cracking. Even so, non-catalytic (homogenous phase)
tar conversion to below 200 mg tar per Nm.sup.3 of gas is possible,
starting with tar at 8000 mg/Nm.sup.3 by using .about.1150.degree.
C. for .about.4 seconds. (Houben 2004). It is also notable that
indirect heating only below 1100.degree. C. with short residence
times (say 1075.degree. for 2 seconds) initially increased the
amount of 2+ ring polycyclic aromatics--quantifiable tars with two
or more aromatic rings--but extended residence time mitigates this
effect.
[0007] Partial Oxidation has been explored as an alternate method
for achieving tar destruction. This method includes blast
containing oxygen subsequently added to raw generated gas. The
Energy Center of the Netherlands (ECN) performed experiments using
an atmospheric circulating fluidized bed gasifier (operated at
850.degree. C.) where air was added subsequently to increase the
product gas temperature to 1100.degree. or more. (Zwart, R. W. R.
Gas Cleaning, downstream of biomass gasification status report.
Public Report, Energy Center of the Netherlands (ECN), SenterNovem,
2009.) To achieve 100 mg tar/Nm.sup.3, a temperature of
1150.degree. C. was required, resulting in a cold gas efficiency
loss of 8%.
[0008] A custom low swirl number burner (swirl number less than
0.4) was employed by Houben to partially combust a relatively cool
(20.degree. and 200.degree. C.) synthetic gas feed, and so the peak
temperatures were also relatively low (less than 900.degree. C.).
This experiment isolated (somewhat) the partial oxidation effect
from thermal effect for tar destruction, and also included no
effect of char. The optimum amount of blast addition of
approximately 0.2 equivalence ratio, .lamda., relative to the fed
gas was reported to avoid growth in the PAH number (number of
aromatic rings). Further, adding no oxygen with indirect heat
promoted tertiary tar formation, but so did adding too much oxygen,
for example .lamda.>0.4, in Partial Oxidation.
[0009] The presence of hydrogen also seems to play a key role in
tar destruction. A PAH "cracking" scheme described in Jess, A.
"Mechanisms and kinetics of thermal reactions of aromatic
hydrocarbons from pyrolysis of solid fuels." Fuel 75, no. 12
(1996): 1441-1448 describes the alternate pathways of PAH growth or
PAH cracking (fewer aromatic rings and lower carbon numbers in tar
compounds) that may occur with varying hydrogen concentration.
Similarly, Houben (2004) found that if hydrogen concentration of
the inlet gas were more than about 20% vol., tar reduction was
optimized. Decreasing hydrogen at the inlet below this level
dramatically increased tar concentration in the products for the
same equivalence ratio, .lamda.. Naphthalene and tertiary PAH (3+
ring) were totally eliminated with an inlet hydrogen content
greater than 30% vol, but single ring aromatics, e.g. toluene and
benzene were retained. Therefore, gasifier operations that increase
the fed hydrogen concentration should result in beneficial tar
reduction for the same POX condition.
[0010] Catalytic tar reduction by contacting the gas with char in
temperatures in the range of 900 to 1000.degree. C.--notably lower
than necessary for thermal destruction in the absence of char, but
still elevated with respect to the typical biomass gasifier exit
temperature (750 to 850.degree. C.)--have also been disclosed.
(Chen 2009). The natural minerals in biomass ash (MgO, CaO,
K.sub.2O, etc.) are believed to contribute to the catalytic effect,
but the state of prior preparation (temperature history, surface
area or oxidative exposure) is also thought to impact performance.
Using commercial biochar (active carbon) and laboratory produced
biochars (using 500.degree. C. pyrolysis) blended with sand, it was
reported that naphthalene conversion was 99.6% and 94.4% at
900.degree. C. with 0.3 seconds residence time (25 cm.sup.3
catalyst bed, 2 cm bed height), starting with 90,000 mg
tars/Nm.sup.3 in fed gas, compared with the blank sand (2%),
natural olivine and sand (55%), and dolomite and sand (61%).
[0011] Fixed carbon gasifies much more slowly (orders of magnitude
more slowly) compared to the volatile matter fraction of a solid
fuel at the same temperature when under non-oxidizing conditions. A
portion of the initial fixed carbon feed is usually present as a
residue of gasification. If an appropriate technology were
available to capture this char and expose it to the flowing gas
stream it could be employed as a catalyst.
[0012] Fixed char beds with direct blast addition help to achieve
low tar by gaining elevated temperatures in the char bed and also
by presenting the char to the gas for possible catalytic benefit,
but this approach is prone to upsets such as high temperature
excursions. On the other hand, by separating the partial oxidation
zone to a location above the char bed, the hot gases can be exposed
to the active catalytic properties of char without blast input to
the delicate char bed. However, a mechanical grate supporting low
density char dust requires low superficial velocities and this is
also prone to its own possible solids flow upsets (bridging,
chanelling ("rat holing"), local hot spots, etc.) due to the
chaotic flow behavior of low density solids.
[0013] A partial oxidation zone that achieves higher temperatures
(1150.degree. C.) can be used to help effectively convert tars with
or without passing through a fixed bed of char. The presence of
hydrogen in the fed gas is an important feature to achieve maximum
POX performance where opening aromatic rings can be favored over
PAH growth. It is thought that the type of tars produced may also
impact their ability to be subsequently reformed on a bed of char,
and may impact the cracking performance in the POX stage; however,
this has remained an unproven possibility.
[0014] The combination of these theories and principles (hydrogen
rich gas production, followed by partial oxidation, exposure to the
catalytic properties of char, and supported char bed) in a robust
and scalable industrial design is not presently known to the state
of the art. The classic fixed bed downdraft gasifier (and other
techniques that employ a fixed bed of char alone) is not scalable
over about 10 MW.sub.th due to the anisotropic shape and chaotic
flow potentials in low density char beds. Blast addition directly
into the fixed bed of char would not be sufficiently robust for
commercial deployment and would not be scalable to industrial
capacities (>10 MW.sub.th). The fixed bed of char alone suffers
from solids flow irregularities ("bridging") and other process
upsets ("rat holes") that occur due its low bulk density and the
anisotropic nature and non-uniform particle size of the produced
char.
[0015] Generally known gasifiers are of several types. The
downdraft gasifier having an integrated fixed bed is a classic
technology that is well known to those skilled in the art for
low-tar gas production (<300 mg/Nm.sup.3). Increasing
superficial velocity through the downdraft gasifier, even when
there is no secondary air injection into the char bed, also results
in an increase of peak temperatures in the char bed. Lowest tar
yields are observed with high peak temperatures >1000.degree. C.
(Reed 1999) Referring now to FIG. 3, separate addition of blast
into the fixed bed of char is also known to improve
performance--this two stage downdraft is known to produce the
lowest tars (<100 mg/Nm.sup.3). The lowest tar performance
occurs when the secondary air (45) added to the char bed (44) is at
its maximum and thus primary air at its minimum. Setting the
secondary air flow such that it is slightly below the level where
"smoke" puffs out the open top describes the relativity desired of
primary and secondary air. Peak temperatures over 1000.degree. C.
were achieved in the char zone (46). The first stage of the
optimally operated two-stage downdraft gasifier functions as an
indirectly heated devolatilization or pyrolysis stage (43) and is
likely to produce simpler "primary" and "secondary" tar compounds.
These primary and secondary tar compounds occur at relatively low
temperatures (500 to 700.degree. C.) prior to encountering the hot
char bed (47). The hot char bed is believed to provide high
temperature thermal cracking opportunity as well as catalytic
benefit from the biomass ash minerals.
[0016] An alternative downdraft gasifier having a separated POX
zone is another possibility (See FIG. 4)._One of the challenges
with the classical two-stage, fixed bed, downdraft gasifier is that
injection of blast into a fixed bed of char can lead to difficult
operational problems--slagging (temperatures well over the ash
fusion point), clinkering (fused ash particles), chanelling ("rat
holing"), fuel bridging and material degradation in the blast input
tube.
[0017] A multi-stage gasifier system (Viking II) was developed by
the Danish Technical University (DTU) between 1980 and 1990 based
on the principles of the downdraft gasifier, but separated the
blast addition from the fixed bed to improve operability. The DTU
gasifier incorporates a separate low temperature pyrolysis stage
(52) (500 to 600.degree. C.) that is configured above a vortex flow
partial oxidation section (55) operated to achieve peak
temperatures .about.1150.degree. C.--this stage is the only zone of
direct blast addition. This partial oxidation zone (505) is
situated above a downdraft, dense "fixed" bed of char (57)
supported on a mechanical grate (58) comprised of pivoting angle
iron.
[0018] The DTU design suffers from limited scale-up potential (due
to the fixed bed of char (57) and indirectly heated feed auger
(51)). On the other hand, the DTU gasifier system proved to yield
very low tars (<25 mg/Nm.sup.3) and produced a rich gas with
.about.25% hydrogen without steam addition, and .about.35% hydrogen
with steam addition. The gas quality was greatly enhanced by the
recuperative indirect heat stage that can include indirect drying
(52). The main difficulty with the DTU system is that the gasifier
system still relied on a fixed bed of char, which consists of low
bulk density solids that are irregular in particle size and shape.
A relatively low superficial velocity is believed to be required
for achieving a char pile without disruption on the mechanical
grate (58) (previously described)--which indicates a costly
scale-up for this reaction stage. The low density bed of char
exhibits chaotic solids flow properties that would be unmanageable
in an industrial-scale system with commercial reliability
requirements.
[0019] Moving granular beds have also been used in prior art to
present char as a catalyst to produced gas but in a cross flow
moving granular bed filter, see FIG. 1 (Van der Drift 2005). The
Van der Drift article describes a laboratory experiment for
validating the theory that char can perform as a tar reduction
catalyst and only employed a slip stream from a larger gasifier.
The cross-flow design was reported to achieve high filtration
efficiency and about 75% reduction in tar at 900.degree. C., being
presented with gas from a fluid bed that was operated at
850.degree. C. The media retention screen (18) at the dust laden
gas inlet (16) of the moving granular bed of char was reported to
have dust fouling problems, and the system also suffered from media
agglomeration within the gas media retention screens (18) which
made media movement difficult.
[0020] Another moving granular bed filter which is shown in FIG. 2
is disclosed in U.S. Pat. No. 7,309,384 to Brown et al., and does
not provide for disengagement screen scrubbing nor does it provide
the opportunity for extended gas residence time in the presence of
char. Instead, the gas residence time is shorter due to the rapid
char and media disengagement in the counter flow design, where gas
from the gas inlet (33) is admitted at the bottom of the media bed
(35) and then travels upward following path (34) mostly through a
bed of clean media which was admitted through the media inlet (31).
Extended gas-char contact that would otherwise benefit catalytic
tar reduction by char is therefore not provided. Further, although
not shown in the patent drawing, the claimed system requires a gas
barrier (downcomer) (39) for operation. The gas barrier is needed
to retain the media in a column and to direct the gas flow upward
through the downward moving media column.
[0021] The present invention differs from the above referenced
inventions and others similar in that these prior devices do not
provide features that can be readily scaled up to industrial
operational levels. What was needed was a gasifier system able to
meet the low tar requirements while producing high quality gases,
and which is feasible and operable in an industrial setting.
SUMMARY
[0022] This summary is provided to introduce a selection of
concepts in a simplified form that are further described below in
the Detailed Description. This summary is not intended to identify
key features or essential features of the claimed subject matter,
nor is it intended to be used in isolation as an aid in determining
the scope of the claimed subject matter. At a high level,
embodiments of the invention relate to a gasification system for
converting feedstocks such as biomass and waste to combustible
gases with low tar levels.
[0023] Embodiments of the invention include a gasifier wherein the
char bed can be scaled up while managing the low bulk density
solids and the irregularities in operation caused by variations in
superficial velocity. Some embodiments of the invention include a
gasifier that provides a partial oxidation zone(s) to allow maximum
advantage of the high temperatures required for lowest tar
production. Additionally, embodiments of the invention provide a
gasifier that fosters high quality gas production. Further
embodiments of the invention include a gasifier constructed to
provide disengagement screen scrubbing.
[0024] The utility of the present invention is to convert biomass
and waste feedstock (solids) into a combustible gas at elevated
temperature and pressure with substantially reduced tar
concentrations. There may be applicability of this invention to
gasification of other higher volatile matter solid fuels, including
for example, various low rank coals, brown coal, peat, and lignite.
Achieving low tar gas is the key to unlocking quantitative gas
conditioning needed for advanced high efficiency gas-to-power
systems (engines, combustion turbines, solid oxide fuel cells,
etc.) and advanced synthesis technology for biofuels (ethanol,
mixed alcohols, and Fischer-Tropsch liquids) and chemicals such as
hydrogen and ammonia.
[0025] Embodiments of the invention utilize an entrained flow
reactor coupled downstream of a fluidized bed reactor. An entrained
flow reactor is a reactor in which the reactant feedstock and
oxidant are fed into the top of the reactor so that the oxidant
stream surrounds (e.g., "entrains") the feedstock and carries the
feedstock through the reactor. A fluidized bed reactor is one in
which a fluid is forced upward through a granular bed at velocities
sufficient to cause the granular material to behave, in many
respects, as a fluid. In certain embodiments, the entrained flow
reactor incorporates a moving granular bed that captures and
supports a catalytic char bed.
[0026] A first illustrative embodiment of the present invention
relates to a multi-stage reaction system for producing low-tar
combustible gas. In certain embodiments, the system includes a
fluidized bed reactor that includes a partial oxidation zone, in
which a portion of the solid feedstock is partially oxidized,
thereby creating a gas and a plurality of char particles. The
illustrative embodiment further includes an entrained flow partial
oxidation reactor situated downstream from the fluidized bed
reactor, and where the entrained flow partial oxidation reactor
includes a moving granular bed.
[0027] A second illustrative embodiment of the present invention
relates to a multi-stage reaction system for producing low-tar
combustible gas. In certain embodiments, the system includes a
fluidized bed reactor that includes a partial oxidation zone, in
which a portion of the feedstock is partially oxidized thereby
creating a gas and a plurality of char particles. The illustrative
embodiment further includes an entrained flow partial oxidation
reactor situated downstream from the fluidized bed reactor. The
entrained flow partial oxidation reactor includes a moving granular
bed. In certain embodiments, a media screening device screens media
from the moving granular bed and a media recycle system returns the
screened media to the entrained flow partial oxidation reactor.
[0028] A third illustrative embodiment of the present invention
relates to a method for controlling an operating pressure of a
two-stage gasification system. In certain embodiments, the method
includes performing a partial oxidation of a portion of the
feedstock in a fluidized bed reactor; elutriating the resulting
plurality of char particles and the gas from the fluidized bed
reactor as a mixture of gas and char; receiving the mixture into an
entrained flow reactor that includes a moving granular bed of
filtering media; and allowing the mixture to flow through the
moving granular bed of char and media. As the mixture is pushed
through the granular bed, embodiments of the method further include
capturing a portion of the plurality of char particles in the
filtering media; screening a portion of the filtering media to
remove captured char particles; and returning the screened
filtering media to the entrained flow reactor.
[0029] These and other aspects of the invention will become
apparent to one of ordinary skill in the art upon a reading of the
following description, drawings, and the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] The present invention is described in detail below with
reference to the attached drawing figures, wherein:
[0031] FIG. 1 is a schematic drawing of tar reduction by tar
reduction equipment in accordance with the prior art;
[0032] FIG. 2 is a schematic drawing of a counter-flow moving
granular bed filter in accordance with the prior art;
[0033] FIG. 3 is a schematic drawing of a classical two-stage
downdraft gasifier in accordance with the prior art;
[0034] FIG. 4 is a schematic drawing of a downdraft gasifier
incorporating an indirect heat stage and separation of the partial
oxidation zone and char fixed bed in accordance with the prior
art;
[0035] FIG. 5 is a schematic drawing of a gasifier system in
accordance with a first embodiment of the invention described
herein;
[0036] FIG. 6 is a schematic drawing of a gasifier system in
accordance with a second embodiment of the invention described
herein;
[0037] FIG. 7 is a schematic drawing of a gasifier system in
accordance with a third embodiment of the invention described
herein;
[0038] FIG. 8 is a schematic drawing of a gasifier system in
accordance with a fourth embodiment of the invention described
herein;
[0039] FIGS. 9A and 9B are top-plan diagrammatic views of nozzle
placement and air flow;
[0040] FIG. 10A is a schematic drawing of a front view of a
screen;
[0041] FIG. 10B is a schematic drawing of an end view of a
screen
[0042] FIG. 11 is a flow diagram depicting an illustrative method
of the tar-reducing gasifier system in accordance with certain
embodiments of the invention described herein;
[0043] FIG. 12 is a chart depicting tar removal percentage vs.
equivalence ratios; and
[0044] FIG. 13 is a chart showing heating value vs. total
equivalence value.
DETAILED DESCRIPTION
[0045] The subject matter of embodiments of the invention disclosed
herein is described with the specificity required to meet statutory
requirements. However, the description itself is not intended to
limit the scope of claims in this patent. Rather, the inventors
have contemplated that the claimed subject matter might also be
embodied in other ways, to include different features or steps, or
combinations of features or steps, similar to the ones described in
this document, in conjunction with other technologies. Moreover,
although the term "step" is used herein to connote different
elements of methods employed, the term should not be interpreted as
implying any particular order among or between various steps herein
disclosed unless and except when the order of individual steps is
explicitly described.
[0046] Referring to the drawings, and particularly to FIG. 5, there
is depicted an illustrative gasification system 100. The
gasification system 100 includes a first reactor 101 and, situated
downstream from the first reactor 101, a second reactor 102. As
shown in FIG. 5, the gasification system 100 also includes a media
screening device 103, a media recycle system 104, and a heat
recovery device 105. It should be understood that the illustrative
gasification system 100 is merely one example of a suitable
gasification system and is not intended to express or suggest any
particular limitations regarding implementations of aspects of
embodiments of the invention.
[0047] For example, in some embodiments, the gasification system
100 can include any number of additional components such as, for
example, those illustrated in FIGS. 6-8. In some embodiments, one
or more of the components described herein can be integrated with
one another and in other embodiments, one or more of the components
described herein can be separated into any number of desired
features, functions, and the like. According to various
embodiments, for example, the first reactor 101 is a fluidized-bed
reactor and the second reactor 102 is an entrained flow reactor. In
some embodiments, the second reactor 102 can also include
fluidized-bed technology, and in other embodiments, the first
reactor 101 can include entrained flow technology. All of these
various embodiments and implementation are considered to be within
the ambit of the invention.
[0048] With continued reference to FIG. 5, the first reactor 101
includes an upper portion 106 and a lower portion 108. The upper
portion 106 of the first reactor 101 includes a freeboard 110,
which provides a partial oxidation zone 112. As shown in FIG. 5,
the lower portion 108 of the first reactor 101 includes a fluidized
bed 114 and a port 116 used for adding heat. The first reactor 101
also includes, as illustrated in FIG. 5, a number of blast inlets
118, a solid fuel port 120, and a blast/steam inlet 122. A
fluidized-bed media discharge port 124 is situated at the bottom of
the lower portion 108 of the first reactor 101. The fluidized-bed
media discharge port 124 discharges media into a media discharge
system 125, which can carry the discharged media to any number of
various destinations such as, for example, a waste receptacle, a
storage tank, a recycling system, and the like.
[0049] In operation, the first reactor 101 creates a hydrogen-rich
partial oxidation zone 112 in its upper section 106 and preferably
includes direct blast addition and/or indirect heat addition
through the port 116. Embodiments of the invention allow for
influence and control of the hydrogen concentration in the raw gas,
thereby facilitating the subsequent cracking of tars, which occurs
in the partial oxidation zone 112 of the first reactor 101 and/or
in a partial oxidation zone 126 of the second reactor 102.
[0050] With continued reference to FIG. 5, the second reactor 102
is a two-stage entrained flow gasifier that is operated in a
non-slagging mode. The first stage is a partial oxidation stage and
is accomplished in the partial oxidation zone 126 of the second
reactor 102. As shown in FIG. 5, the partial oxidation zone 126 of
the second reactor 102 is situated within an upper portion 128 of
the second reactor 102. As illustrated, the partial oxidation zone
126 includes a low-swirl partial oxidation burner 136. A number of
blast inlets 138 and 140 are located in the upper portion 128 of
the second reactor 102 and will preferably include a multiple of
blast nozzles 142 at each level, as needed to achieve localized
"thermally intense zones," which will be described in more detail
below, with reference to FIGS. 9A and 9B. According to some
embodiments, the first reactor 101 can also include a partial
oxidation burner such as the burner 136. The second stage
associated with the second reactor 102 is a "dense-bed" stage and
is accomplished in a catalytic char-reduction zone 130 that is
situated within a lower portion 132 of the second reactor 102. As
shown in FIG. 5, the catalytic char-reduction zone 130 includes a
moving granular bed 134 that facilitates operation of the second
stage associated with the second reactor 102.
[0051] Embodiments of the invention can include a number of
different options for configuring blast nozzles 142 around the
periphery the first reactor 101 and/or the second reactor 102. The
configuration of the blast nozzles 142 facilitates forming
localized regions of oxidative thermal intensity (by virtue of the
mixing pattern), rather than achieving more uniform mixing patterns
achieved by typical approaches to designing gas burners for lean
fuel conditions. Turning briefly to FIGS. 9A and 9B, top-view
schematic drawings illustrate two different illustrative
configuration options for placement of the blast nozzles 142,
respectively. As illustrated in FIG. 9A, a vessel (e.g., reactor)
145 includes a number of inlet ports 147 and 149 having blast
nozzles 150 and 151, respectively. The blast nozzles 150 and 151
are configured in an alternating pattern such that the blast
nozzles 150 and 151 direct inputs toward tangent curves 152 and 154
associated with one or more target circles 153 and 156, the
diameter of which can be varied according to various embodiments of
the invention.
[0052] For example, as shown in FIG. 9A, the blast nozzle 150
targets the tangent 152 of a first target circle 153, which has a
first diameter 157a. Similarly, the blast nozzle 151 targets a
tangent curve 154 of a second target circle 156, which has a second
diameter 157b. As shown, the first diameter 157a can be smaller in
magnitude than the second diameter 157b. In other embodiments, the
first diameter 157a can be larger in magnitude that the second
diameter 157b. The targeting direction of each of any additional
blast nozzles is configured to alternate between tangent curves of
the first and second target circle 153 and 156. In the embodiment
depicted in FIG. 9A, the blast nozzles 150 and 151 are oriented at
the same elevation as one another, and therefore provide a coherent
flow direction. Other flow patterns can be achieved, in other
embodiments, by injection in a contrary flow direction at slightly
different elevations.
[0053] Turning to FIG. 9B, an alternative configuration option for
configuring blast nozzles to achieve desirable flow patterns in a
blast zone 162 situated within a reactor vessel 160 is depicted. As
shown, the vessel (e.g., reactor) 160 includes a number of inlet
ports 163 and 165 having blast nozzles 166 and 168, respectively.
The blast nozzles 166 and 168 are configured in an alternating
pattern such that the blast nozzles 166 and 168 direct inputs
toward tangent curves of target circles that are defined at
different elevations. For example, the blast nozzle 166 targets the
tangent curve 171 of a first target circle 169, which has a first
diameter 173a. Similarly, the blast nozzle 168 targets a tangent
curve 174 of a second target circle 170, which has a second
diameter 173b.
[0054] In the embodiment illustrated in FIG. 9B, the first target
circle 169 and the second target circle 170 are situated at
different elevations with respect to one another. That is, in
certain embodiments, the first target circle 169 can be situated at
a lower elevation than the second target circle 170, while in other
embodiments, the second target circle 170 can be situated at a
lower elevation than the first target circle 169. According to
various embodiments of the invention, the first diameter 173a can
be smaller in magnitude than the second diameter 173b. In other
embodiments, the first diameter 173a can be larger in magnitude
that the second diameter 173b. In further embodiments, the first
diameter 173a and the second diameter 173b can be substantially the
same. The targeting direction of each of any additional blast
nozzles is configured to alternate between tangent curves of the
first and second target circle 169 and 170.
[0055] According to certain embodiments of the invention, one or
more auxiliary blast zones will include multiple nozzles configured
around the perimeter of the vessel so as to create at least two
tangent target circles. In some embodiments, as depicted in FIG.
9A, the nozzles can be configured such that the inputs are injected
coherent at the same elevation, while in other embodiments, the
nozzles can be configured such that the inputs are injected
convergent at slightly different elevations. These embodiments
provide differing thermal intensity patterns and it should be
understood that the configuration used in implementation can be
selected to achieve the desired thermal intensity patterns.
[0056] Moreover, according to embodiments of the invention, mixing
performance associated with the blast zones 146 and 162 can be
optimized through computational fluid dynamics (CFD) calculations.
For example, CFD software can be used to create 3-D patterns with
thermally intense zones having various peak temperatures. In
certain embodiments, mixing performance can be optimized by varying
relative diameters of the target circles, adjusting swirl number
(e.g., utilizing a swirl number less than 0.4), and by optimizing
the equivalence ratio of the total auxiliary blast addition. The
equivalence ratio, .lamda., is the blast to fuel ratio, relative to
the stoichiometric blast to fuel required to just burn the fed gas
and char. According to embodiments of the invention, the total
auxiliary blast input is less than about 25% of the stoichiometric
blast-to-fuel ratio calculated relative to the fed feedstock
analysis. Additionally, in some embodiments, the total auxiliary
blast can be about 50%, or more (and even up to 100%, particularly
when indirect heat is supplied during the first stage), of the
entire blast input to the reaction system.
[0057] For example, in one embodiment, the blast nozzle
configuration is developed using CFD software to model at least one
thermally intense zone having a peak temperature of
.about.1150.degree. C. The total auxiliary blast addition is
controlled such that it has an equivalence ratio, .lamda., of
approximately 0.2 (or less) in oxygen limited partial oxidation and
incorporates a majority (>50%) of the total blast addition
through auxiliary ports, configured to achieve localized zones of
peak temperature of approximately 1150.degree. C. Configuring the
blast nozzles accordingly can facilitate achieving desired
performance objectives during operation.
[0058] A peak temperature of between about 1000.degree. C. and
about 1200.degree. C. generally is sufficient for activating
hydrogen molecules in the manner necessary for cracking aromatic
ring compounds and providing the necessary termination to avoid
ring polymerization. Too high a temperature in the bulk gas may
cause melting of ash and slag formation that can interfere with
operation. Accordingly, the partial oxidation zones are configured
to include local thermally intense zones rather than high bulk gas
temperatures. These localized thermally intense zones facilitate
activation of hydrogen radicals that can subsequently initiate
chemical reactions in the adjacent bulk gas. For example, hydrogen
facilitates terminating the activated carbon atom in an aromatic
ring that has been thermally cracked open, thereby providing for
tar reduction rather than tar polymerization.
[0059] Returning to FIG. 5, a gas containing elutriated char 199,
formed in the first reactor 101, and containing various natural
catalytic minerals, escapes the first reactor 101 by an elutriation
mechanism 200 and travels from the elutriation mechanism 200
through a gas conduit 202. The elutriated char 199 is delivered,
via the gas conduit 202, to the second reactor 102 through a main
gas inlet 204. The maximum size and delivery rate of the elutriated
char 199 can be controlled, to a degree, by maintaining the
freeboard 110 superficial velocity through control of the operating
pressure of the gasification system 100. The elutriated char
particles 199 pass into the second reactor 102, either in a
dispersed manner, through the main gas inlet 204, as shown in FIG.
1, or in a separated and concentrated form, through an auxiliary
blast port 140 (e.g., see FIG. 7).
[0060] By controlling the operating pressure of the gasification
system 100 for a given gas production rate, it is possible to
control (to a degree) the maximum particle size and the rate of
release of char 199, and the maximum particle size of the char 199,
through the elutriation mechanism 200 associated with the first
reactor 101. The particle size of the char 199 affects the
catalytic performance of the char 199 for gas temperatures of less
than 1000.degree. C. In other words, a smaller particle size tends
to produce more tar reduction for the same temperature,
particularly if the temperature is less than 1000.degree. C. For
example, at 900.degree. tar reduction in one study was 88% for one
particle size range (1 to 2 mm) and 96% for another (0.1 to 0.15
mm). Accordingly, certain embodiments of the invention incorporate
a method of operating the gasification system to control the char
119 particle size and elutriation rate.
[0061] According to certain embodiments of the invention, the
method includes, at least in part, maintaining a target velocity in
the freeboard 110 of the first reactor 101 by modulating the
pressure set point. It will be appreciated by individuals having
skill in the relevant arts that pressure modulation can be
accomplished in a number of ways such as, for example, modulating
fuel and air inputs, modulating a downstream valve position (e.g.,
downstream from a particulate removal), and the like. For instance,
as illustrated in FIG. 5, the gasification system 100 can include
one or more modulating valves 210 that can be utilized to modulate
downstream particulate removals, thereby providing some level of
control over the operating pressure of the system 100.
[0062] Additionally, pressure control can be achieved by
controlling the flow of char 199 through the second reactor 102.
The gas 215 engaging the moving granular bed 134 in the second
reactor 102 moves in co-flow direction with granular material 135.
In certain embodiments, the granular material is input via the main
gas inlet 204 of the second reactor 102. In operation, the moving
granular bed 134 captures and dilutes char 199 in a matrix of
granular solids 135 that has a higher specific gravity, thereby
improving the solids' 135 flow properties. In this manner, a zone
of gas-char 215 is created such that the gas-char 215 contacts,
with sufficient residence time, the char 199 solids for catalytic
tar-reduction-by-char. The moving granular bed 134 captures and
mixes the low density char 199 (usually <190 kg/m.sup.3) with
other media (usually >1900 kg/m.sup.3), thereby improving the
char 199 flow properties. In this manner, the flow of char 199 can
be positively managed by its association with the co-flowing media
matrix 134.
[0063] With continued reference to FIG. 5, the concentration of
char 199 in the granular bed 134 can be managed through a screening
stage, accomplished by the media screening device 103. In one
particular embodiment, for example, a portion of char 199 and media
135 is removed, via a media discharge port 220 and provided to the
media screening device 103. As the char 199 and media 135 are
passed through the media screening device 103, the char 199 is
separated from the media 135. The media recycle system 104 is used
to return the screened media 135 to the second reactor 102. The
char 199 screened from the media 135 is discharged via a residue
discharge system 221. According to various embodiments, the
particle size of the moving granular bed 134 can be the same as the
particle size of the fluidized bed 114 in the first reactor 101
(e.g., see FIG. 8). In other embodiments, particle size of the
moving granular bed 134 can be much larger than the particle size
of the fluidized bed 114 in the first reactor 101 (e.g., 10 times
larger) to create a favorable pressure drop through the moving
granular bed 134 in the second reactor 102.
[0064] Turning briefly to FIG. 11, a flow diagram depicts an
illustrative method 300 of controlling an operating pressure of a
two-stage gasification system. At a first illustrative step, step
310, the illustrative method includes performing a partial
oxidation of solid feedstock in a fluidized bed reactor. In certain
embodiments, the fluidized bed reactor can be similar to, for
example, the reactor 101 described above with reference to FIG. 5.
Performing the partial oxidation in the fluidized bed reactor
generates, among other things, gas and char. The char can be
elutriated from the fluidized bed reactor as a gas/char mixture, as
shown at step 312, using an elutriation device, or simply allowed
to elutriate naturally without any additional device. At step 314,
the mixture is received into an entrained flow reactor that has a
moving granular bed. In certain embodiments, for example, the
entrained flow reactor can be similar to the second reactor 102
described above with reference to FIG. 5.
[0065] As illustrated at step 316, the mixture is allowed to flow
through the moving granular bed and, as the mixture moves through
the moving granular bed, char particles are captured in the media
of the moving granular bed, as indicated at step 318. To control
the concentration of char particles in the moving granular bed (and
thereby, to facilitate control over the char flow rate and particle
size), a portion of the media of the moving granular bed is
screened to remove char particles, as shown at step 320. At a final
illustrative step, step 322, the screened media is returned to the
moving granular bed. According to certain embodiments, the
illustrative method 300 can be used alone, or in conjunction with
other methods, to affect control over the operating pressure of the
gasification system by controlling the char flow rate in the
entrained flow reactor.
[0066] Returning now to FIG. 5, it should be understood that
embodiments of the moving granular bed 134 do not require an inlet
screen for media retention at the gas engagement interface by
virtue of the geometric down-flow design. In contrast, the media
retention screen at the dust-laden gas inlet of the moving granular
bed of the prior art illustrated in FIG. 1, for example, was
reported to have dust-fouling problems in the gas engagement. To
the contrary, as a result of employing co-flow bed and gas and the
incorporation of a down-flow design along with various other
features of the moving granular bed 134, embodiments of the present
invention do not require any media retention screen at the gas
engagement thereby providing an improved method of employing char
as a catalyst. Additionally, whereas conventional moving granular
beds are tuned (e.g., adjusted and controlled) to provide optimum
filtration, the moving granular bed 134 of the present invention is
tuned to provide an optimal char contacting zone.
[0067] The moving granular bed 134 is operated to capture char 199
as a physical barrier. The media residence time is correlated with
the char residence time (the period of time that the average char
particle spends in the reactor), and this char residence time can
be modulated in a controlled manner with the media screening and
recycle subsystem (103/104). According to certain embodiments, the
moving granular bed 134 also can be configured to provide a zone of
narrow gas residence time distribution through the char bed 134 in
a conceptually plug flow reactor that allows for maximum tar
cracking. In certain embodiments, the gas residence time and char
residence time can differ by several orders of magnitude;
therefore, the differential velocity between the gas 215 and char
199 is very close to the local gas interstitial velocity through
the bed 134. In certain embodiments, the char in the char bed 134
is continuously refreshed by the char 199 supply from the first
reactor 101 thereby reducing or eliminating the need for high
performance filtration, even though some small particles of char
199 may slip through the bed with the gas 215.
[0068] With continued reference to FIG. 5, the moving granular bed
134 includes a substantially vertical gas disengagement screen 222.
Turning briefly to FIG. 10A, the disengagement screen 222 is
comprised of a plurality of wires 224, oriented in a substantially
parallel and vertical manner. The wires 224 are situated between an
upper frame edge 222a and a lower frame edge 222b. As illustrated,
the screen 222 can also include a pair of side frame edges 222c.
The disengagement screen 222 includes a number of gaps 225, each
gap 225 being defined between two adjacent wires 224. FIG. 10B
depicts a bottom, partial view of the screen 222. As shown in FIG.
10B, each wire 224 may be made of commercially available triangular
profile wire, or includes a wedge or V-cross section defined by a
first side 230 and a second side 232 that meet at a vertex 234. The
two sides 230 and 232 of the wire 224 converge (at the vertex 234)
in the direction of gas flow. Preferably, the gaps 225 between the
wires 224 are designed relative to the smaller cut size of the
granular media. The substantially vertical orientation (which, at a
minimum, is steeper than the angle of repose of the char-media
matrix 135 of FIG. 5) of the gas disengagement screen 222 provides
for scrubbing action with the moving bed 134 to maintain the gas
disengagement screen 222, without plugging.
[0069] Embodiments of the moving granular bed 134 of the present
invention include features that are not known to the art and that
have been described above. These features include, for example, a
co-flow design that is preferred for its gas-char contacting zone
for enhancing catalytic tar-reduction-by-char performance rather
than for its filter performance; the lack of a need for a media
retention screen for media retention at the gas engagement
interface; and a substantially vertically oriented gas
disengagement screen (e.g., which is steeper than the angle of
repose of the blended char and media matrix to scrub the
disengagement screen keeping it free of dust clogging).
[0070] In contrast, for example, the prior art moving granular bed
filter disclosed in U.S. Pat. No. 7,309,384 to Brown et al., and
illustrated herein in FIG. 2, does not provide for disengagement
screen scrubbing. The counter filter 304, 307 disclosed in the
Brown et al. patent also does not provide the opportunity for
extended gas residence time in the presence of char. Instead, the
gas residence time is shorter due to the rapid char and media
disengagement in the counter flow design which, in turn, shortens
the gas-char contact and reduces the catalytic reduction of tar by
char. To the contrary, embodiments of the present invention do not
include, or require, the presence of a gas barrier for media
retention whereas the Brown et al. invention will not work without
such a barrier.
[0071] According to various embodiments of the invention the
residence time (increased internal age distribution) of the trapped
char particles is controlled by modulating the media flow that
captures the char through an external recycle loop. With reference
to FIG. 5, a first embodiment employs a bubbling fluid bed reactor
101, the transport disengagement height of the freeboard 110
design, and the target operating velocity to achieve a mixture of
gas 215 and char 199 delivered to the second reactor 102. As
illustrated, the gasification system 100 of FIG. 5 includes an
"external" active media recycle system 104 (which can be, for
example, mechanical or pneumatic conveying). In certain
embodiments, the fluid bed reactor 101 can include processing of
the discharge stream 125 to remove foreign "tramp" materials
entering with the feedstock, and can optionally be recirculated and
reheated in a direct or indirect heating loop and returned to the
first reactor 101 via the port 116.
[0072] Turning to FIG. 6, another embodiment of a gasification
system 240 is illustrated. As shown, the gasification system 240
includes a first reactor 241, which is a fluidized bed reactor, a
second reactor 242 having a moving granular bed 243, a media
screening device 244, a media recycle system 246, and a heat
recovery device 248. This particular embodiment (illustrated in
FIG. 6) is designed to enable a higher, more turbulent, velocity in
the fluidized bed reactor 241 without excessive loss of fluid bed
sand. In this case, a sand recovery roughing cyclone 250 is an
elutriation device included between the first reactor 241 and the
second reactor 242.
[0073] Turning now to FIG. 7, another embodiment of a gasification
system 260 of the present invention is illustrated. As shown, the
illustrative gasification system, according to this particular
embodiment, includes a first (fluidized bed) reactor 261, a second
(entrained flow) reactor 262 having a moving granular bed 263, a
media screening device 264, a media recycle system 265, and a heat
recovery device 266. In this embodiment (illustrated in FIG. 7),
the sand recovery cyclone 268 and turbulent fluid bed 263 are both
included, but a char-concentrating cyclone 270 is also included to
give an opportunity for separate, controlled char injection into
the partial oxidation zone 276 of the second reactor 262. The
embodiment, illustrated in FIG. 7, includes the char fines cyclone
270 to create an opportunity for a majority of char to bypass an
upper partial oxidation zone 277 in the second reactor 262. This
configuration affords separate control of char injection using a
steam motivated eductor 272 for delivery into the partial oxidation
zone 277 through an auxiliary partial oxidation blast port 274.
According to certain embodiments, the gasification system 260 can
be optimized for char oxidation heat release by adding blast along
with the char feed, therefore creating a hot zone through which the
fed gas passes, in which case a lesser amount (or, in some
implementations no amount) of blast gas is fed through an upper
blast port 279.
[0074] Turning to FIG. 8, another embodiment of a gasification
system 280 is illustrated. The illustrated embodiment of FIG. 8 is
a new form of a circulating fluidized bed reactor system 280. The
illustrative gasification system 280 illustrated in FIG. 8 is quite
different from the previously described embodiments herein in
several respects. For example, this particular embodiment (shown in
FIG. 8) requires no external media recycle system. Instead, the
same media 283 is cycled between the first reactor 281 and the
second reactor 282, and char concentration is modulated by
adjusting the portion of discharged sand 290 that either returns to
the first reactor 281 or that passes through the media screen 284
before recycling. There is no separate media recycle loop for the
second reactor 282 in the embodiment depicted in FIG. 8. Circulated
media 292 is the same particle size and same material used in the
fluidized bed reactor 281. Bed material is circulated in a closed
loop between the fluid bed reactor 281 and the second reactor 282.
According to embodiments, the bed can have a ratio of superficial
velocity relative to the minimum velocity required to fluidize
(U/Umf) of between 6 and 12 to cause increased sand elutriation.
Dust-laden media discharging flow (indicated as 290) is split by
gravity assist and/or pneumatic push to return a portion into the
fluid bed 281a and the balance is processed to remove fines and
dust by the media screening device 284 as needed, to produce a
cleaned media stream 296. The media stream from the fluid bed 281a
and the media screening device 284 combined 298 can optionally be
used in a direct or indirect heating loop or cleaned of tramp
materials and returned via the port 281b.
[0075] To recapitulate, embodiments of the invention include a
gasification system having a fluidized bed reactor situated
upstream from an entrained flow reactor. The entrained flow reactor
includes a moving granular bed that holds up char and so presents
catalytic properties of char to the gas for the purpose of tar
cracking. In some embodiments, char concentration in the granular
solids matrix is controlled with the media screening and recycle
system (such as the media screening device 103 and the recycle
system 104 illustrated in FIG. 5). Tests have shown that a
gasification system having at least some of the features described
herein performs as desired.
[0076] For example, tests were performed under various operating
conditions in a laboratory-scale entrained flow reactor, configured
according to embodiments of the invention, using yellow seed cord
as a model feedstock. Air or oxygen was used as blast as indicated
in FIG. 12. The stoichiometric air/fuel requirement is 5.45 kg
air/kg biomass (dry), or 1.26 kg oxygen/kg biomass (dry). Air blown
gasification tests used 3.7 kg/hr biomass, and oxygen blown tests
used 6 kg/hr biomass with .about.0.45 kg steam/kg biomass. Inert
rock material and low grade iron ore (taconite) were used as media
in the entrained flow reactor, sized to approximately
1/8''.times.1/4'' granules. The first stage (fluidized bed)
gasifier was operated at various equivalence ratios, .lamda.,
(Air/Fuel relative to the stoichiometric Air/Fuel requirement):
0.11, 0.14, and 0.18, as indicated in FIG. 12 and achieved fluid
bed temperatures from 600 to 700.degree. C.
[0077] The entrained flow reactor consisted of a small partial
oxidation burner that injected blast laterally through 6 small
holes (having an internal diameter of 1.4 mm) with slight swirling
action, in the configuration illustrated in FIG. 9A, with a tangent
circle that is 13 mm diameter, inside of a 40 mm inside diameter
pipe, that subsequently expanded into an 100 mm (inside diameter)
pipe. Test results indicate that tars were converted with
increasing blast (and increasing temperatures) up to a point, which
correlated with delivering .about.50% of the total blast into the
secondary reactor. The lowest tar condition was .about.560
mg/Nm.sup.3 dry gas. The temperature in the partial oxidation zone
was not measured, but the maximum measured temperature in the zone
below was 980.degree. C.
[0078] As another example, further tests were performed in which
the same test conditions discussed above were analyzed for lower
heating value--indicate that the heating value stabilized at
.about.5 MJ/Nm.sup.3 (dry) for air blown tests, and 10 MJ/Nm.sup.3
(dry) for oxygen blown tests, so long as the total equivalence
ratio was less than 0.35, as reflected in FIG. 13.
[0079] A reactor, configured in accordance with embodiments of the
invention, that combines a partial oxidation burner for tar
reduction above a "pebble grate" (which is previously described as
a bed of granular media supported by a vertical wire screen
supporting the media at the gas disengagement) to support a bed of
char has not heretofore been known to the art. The integration of a
first reactor that operates at lower temperatures in sequence with
a higher temperature partial oxidation stage (sub-stoichiometric
combustion) with a subsequent heat recuperative device that
transports thermal energy back to the first reactor to drive
pyrolysis reactions is also not previously known. Because most
moving granular beds are designed for filtration, the co-flow
design of the present invention is also not known because it does
not impart ideal filtration conditions but rather it imparts ideal
gas-char contact conditions for tar cracking. Low density
char--that otherwise has chaotic solids flow properties--is
dispersed into a granular bed material that has approximately ten
(10) times the bulk density, which imparts improved solids flow
properties. This is an improvement over prior art and contrasts
with any reactor that includes a fixed bed of char in downdraft
type gasifiers (integrated or separated from partial oxidation
zones) that have been known to experience upsets associated with
poor solids flow, "solids bridging", and "rat hole" formations due
to the chaotic movement and anisotropic nature of low density char
beds.
[0080] The present invention has been described in relation to
particular embodiments, which are intended in all respects to be
illustrative rather than restrictive. Alternative embodiments will
become apparent to those of ordinary skill in the art to which the
present invention pertains without departing from its scope. For
example, different contact times and variations in temperatures for
certain zones may be employed. Connections between reactor vessels
and return lines can vary in general position. It will be
appreciated by individuals having skill in the relevant arts that
certain optimizations will be necessary depending on the source of
feedstock.
[0081] From the foregoing, it will be seen that this invention is
one well adapted to attain all the ends and objects set forth
above, together with other advantages, which are obvious and
inherent to the system and method. It will be understood that
certain features and subcombinations are of utility and may be
employed without reference to other features and subcombinations.
This is contemplated by and is within the scope of the claims.
* * * * *