U.S. patent application number 11/641088 was filed with the patent office on 2008-06-19 for gasification of sulfur-containing carbonaceous fuels.
This patent application is currently assigned to Simulent Inc.. Invention is credited to Arthur Lionel Kohl.
Application Number | 20080141591 11/641088 |
Document ID | / |
Family ID | 39204000 |
Filed Date | 2008-06-19 |
United States Patent
Application |
20080141591 |
Kind Code |
A1 |
Kohl; Arthur Lionel |
June 19, 2008 |
Gasification of sulfur-containing carbonaceous fuels
Abstract
A process for converting sulfur-containing carbonaceous fuel
into a combustible product gas is described. The fuel, in the form
of fine particles, is mixed with a first aqueous solution
containing dissolved alkali metal carbonate at approximately
atmospheric pressure to form concentrated slurry. The slurry is
pressurized and fed into a gasifier operating at an elevated
pressure. An oxygen-containing gas is also fed into the gasifier
where it reacts with the fuel to produce hydrogen and carbon
monoxide-containing gases and heat the reacting mixture to about
900-1400.degree. C. Oxygen combined with steam may also be used as
the oxidizer to produce hydrogen-rich product gas. Alkali metal
salts in the slurry are melted and absorb sulfur from the reacting
fuel to form a liquid smelt containing alkali metal sulfide. The
smelt is dissolved in water to form a second aqueous solution which
is regenerated by converting dissolved alkali metal sulfide into
dissolved alkali metal carbonate and hydrogen sulfide gas which is
released from the solution. The regenerated solution is recycled as
the principal ingredient of the first aqueous solution. The
hydrogen and carbon monoxide-containing gases are cooled, separated
and purified to produce combustible product gases.
Inventors: |
Kohl; Arthur Lionel;
(Woodland Hills, CA) |
Correspondence
Address: |
Nasser Ashgriz
Suite 302, 203 College Street
Toronto
ON
M5T 1P9
omitted
|
Assignee: |
Simulent Inc.
Toronto
CA
|
Family ID: |
39204000 |
Appl. No.: |
11/641088 |
Filed: |
December 19, 2006 |
Current U.S.
Class: |
48/197R |
Current CPC
Class: |
C10J 2300/0956 20130101;
C10J 2300/0973 20130101; C10L 5/366 20130101; C10K 1/004 20130101;
C10J 2300/0996 20130101; C10J 3/00 20130101; C10J 2300/0943
20130101; C10J 2300/0983 20130101; C10L 1/326 20130101; C10J
2300/1892 20130101; C10L 9/10 20130101; C10J 2300/0959 20130101;
C10J 2300/092 20130101; C10J 2300/093 20130101; C10J 2200/156
20130101; C10J 2300/1884 20130101; C10J 2300/0903 20130101; C10J
2300/0946 20130101 |
Class at
Publication: |
48/197.R |
International
Class: |
C10J 3/00 20060101
C10J003/00 |
Claims
1. A process for converting a sulfur-containing carbonaceous fuel
into a combustible product gas having a higher heating value of at
least 100 Btu/scf, dry basis, by reaction with oxygen in the
presence of a molten alkali metal salt comprising steps of: (a)
Mixing particles of said fuel with a first solution containing
about 5% to 35% by weight alkali metal carbonate, at approximately
atmospheric pressure to form slurry containing about 25% to 75% by
weight un-dissolved fuel particles; (b) Pressurizing and feeding
said slurry into a gasifier operating at an elevated pressure in
the range of 4 to 50 atmospheres, said gasifier containing a drying
zone and a gasification zone; (c) Feeding pressurized
oxygen-containing gas into said gasifier; (d) Drying said slurry in
said drying zone and gasifying the resulting dried mixture of fuel
and salts by reaction with said oxygen at a temperature in the
range of about 900 to 1400.degree. C. in said gasification zone to
produce a gasifier off-gas, containing carbon monoxide and water
vapor, and a hot liquid smelt containing molten alkali metal
sulfide; (e) Draining said smelt into a quench tank operating at
essentially the same pressure as the gasifier, and dissolving it in
an aqueous medium to form a second solution containing 5% to 35% by
weight, alkali metal salts including alkali metal sulfide; (f)
Recovering said gasifier off-gas as said combustible product gas;
(g) Reducing the pressure on said second solution and regenerating
it by converting said alkali metal sulfide to alkali metal
carbonate in said solution and hydrogen sulfide gas which is
released from solution; and (h) Utilizing the depressurized and
regenerated second solution as the principal ingredient of said
first solution.
2. The process of claim 1 wherein said alkali metal is sodium.
3. The process of claim 1 wherein said alkali metal is
potassium.
4. The process of claim 1 wherein said oxygen-containing gas is
air.
5. The process of claim 1 wherein said oxygen-containing gas is a
mixture of oxygen and steam.
6. The process of claim 1 wherein said sulfur-containing
carbonaceous fuel is petroleum coke.
7. The process of claim 1 wherein the amount of oxygen fed into the
gasification zone is less than about 60% of the amount needed for
complete combustion of the fuel.
8. The process of claim 1 wherein step (g) includes the removal of
ash components from said second solution.
9. The process of claim 1 wherein step (g) includes contacting said
second solution with a carbon dioxide-containing gas to form alkali
metal bi-sulfide and bicarbonate in solution, depressurizing the
solution, and stripping hydrogen sulfide gas from it to produce a
stream of hydrogen sulfide-rich gas and a stream of depressurized
and regenerated second solution.
10. The process of claim 1 wherein steps (f) and (g) include
treating said gasifier off-gas by contacting it with said second
solution at an elevated pressure, thereby purifying said
combustible product gas and absorbing carbon dioxide into said
second solution; depressurizing the carbonated second solution
and-stripping hydrogen sulfide gas from it at a reduced pressure in
the range of 0.1 to 2.0 atmospheres to produce a stream of hydrogen
sulfide-rich gas and a stream of depressurized and regenerated
second solution.
11. The process of claim 1 wherein step (g) includes contacting
said second solution with a carbon dioxide-containing gas and
absorbing sufficient carbon dioxide to convert alkali metal sulfide
and bi-sulfide into alkali metal bicarbonate thereby displacing
hydrogen sulfide gas from solution; and heating the resulting
bicarbonate solution to produce a stream of carbon dioxide gas and
a stream of depressurized and regenerated second solution.
12. The process of claim 1 wherein step (b) includes feeding said
slurry into the drying zone of said gasifier in the form of a
spray.
13. The process of claim 1 wherein step (c) includes feeding
pressurized oxygen-containing gas into the gasification zone of
said gasifier.
14. The process of claim 1 wherein step (d) includes discharging
said gasifier off-gas from the drying zone of said gasifier.
Description
BACKGROUND
[0001] 1. Field of Application
[0002] This application relates to the gasification of carbonaceous
fuels. More particularly, this technique of the application relates
to a process for converting a sulfur-containing carbonaceous fuel
into hydrogen and carbon monoxide-containing combustible gases by
reaction with an oxygen-containing gas, or by reaction with a
mixture of oxygen and steam, in the presence of a molten alkali
metal salt catalyst.
[0003] 2. Description of the Related Art
[0004] Numerous techniques are known for gasifying coal and other
carbonaceous fuels. In a review report published in 1983 by the
Electric Power Institute (EPRI AP-3109) 22 different coal
gasification processes are listed as being in commercial use, being
demonstrated, or under active development. In general, the known
coal gasification processes are categorized as moving bed,
fluidized bed, entrained flow, or molten bath.
[0005] The moving bed process employs a dense settled bed of large
fuel particles which move slowly downward in the bed while reacting
with gases that move upward. The Lurgi gasifier is an example of a
moving bed process. Difficulties with the moving bed gasifier, when
operated at elevated pressure, include feeding the large fuel
particles, handling fines, and handling hydrocarbon liquids which
are distilled out with the product gas.
[0006] The fluidized bed process employs a dilute bed of small
solid particles which are fluidized and continuously mixed by
up-flowing gases. The High Temperature Winkler (HTW) process is an
example of a fluidized bed system. Typical difficulties with
fluidized bed processes include entrainment of small particles in
the product gas and poor conversion efficiency with low reactivity
fuels.
[0007] The entrained flow process uses powdered fuel in a plug flow
reactor where the fine particles of coal react with the oxidant and
steam in high velocity co-current flow. Examples of entrained flow
systems are the Koppers-Totzek gasifier, which utilizes a solids
feed system, and the Texaco gasifier, which uses slurry of powdered
coal in water as feed. Difficulties with entrained flow gasifiers
include the need for an oxygen plant, materials corrosion due to
the high operating temperature and the presence of molten slag, and
handling the high temperature product gas.
[0008] The molten bath process uses a pool of molten metal or salt
in the reaction zone. U.S. Pat. No. 3,916,617 is an example of the
molten salt process. Common difficulties with molten bath processes
include excessive corrosion of materials used to retain the molten
bath and the need to feed solid materials into the gasifier as fuel
and makeup bath material.
[0009] U.S. Pat. No. 4,083,930 describes a process for removing and
recovering sulfur values from a molten mixture of alkali metal
sulfide and carbonate and producing a purified alkali metal
carbonate solid suitable for recycle to a gasifier of the type
described in U.S. Pat. No. 3,916,617. The process of U.S. Pat. No.
4,083,930 includes quenching the molten salt, dissolving it in an
aqueous medium, reducing the pressure on the resulting solution to
evaporate water, carbonating the solution, vacuum stripping the
solution to release H.sub.2S, recarbonating the stripped solution
with pure CO.sub.2, to produce alkali metal bicarbonate crystals,
removing the crystals from the residual solution, and drying and
calcining the bicarbonate crystals to produce solid alkali metal
carbonate.
[0010] Any attempt to combine the processes shown in U.S. Pat. Nos.
3,916,617 and 4,083,930 results in an overall process which is much
more complicated than the subject technique of the application and
has the undesirable features of a solid feeding system and a molten
salt pool with air and solids injection beneath the surface.
[0011] U.S. Pat. Nos. 4,682,985, 4,773,918 and 5,984,987 describe
gasification processes designed to handle concentrated black
liquor, a liquid byproduct of the pulp and paper industry. Such
processes are not applicable to solid fuels and do not involve the
recycle of an alkali metal carbonate solution and the use of said
recycle solution to form a slurry with the carbonaceous fuel
feed.
[0012] The use of an alkali metal carbonate solution to remove and
recover H.sub.2S and. CO.sub.2 from gas streams is known. U.S. Pat.
Nos. 1,533,733; 2,094,070; and 2,243,323 describe typical
processes. However, the prior art does not suggest the integration
of gas purification steps with the gasification of
sulfur-containing carbonaceous fuel including the absorption of
sulfur into molten alkali metal salt within the gasifier and the
use of regenerated alkali metal carbonate solution to form a slurry
with the fuel feed for the purpose of conveying it into the
gasifier.
SUMMARY OF THE INVENTION
[0013] In its broadest aspects the process consists of mixing
sulfur-containing carbonaceous fuel, in particulate form, with an
aqueous solution of an alkali metal carbonate to produce a slurry;
pressurizing the slurry and feeding it into a gasifier operating at
superatmospheric pressure; feeding a pressurized oxygen-containing
gas into the gasifier and reacting the oxygen with the carbonaceous
fuel by exothermic partial combustion reactions in the presence of
molten alkali metal salts to form a carbon monoxide-containing gas
and an alkali metal sulfide-containing molten salt smelt;
recovering the carbon monoxide-containing gas as the combustible
product gas; dissolving the smelt in water and regenerating the
resulting solution by converting dissolved alkali metal sulfide
into dissolved alkali metal carbonate and gaseous hydrogen sulfide;
and recycling the aqueous solution of alkali metal carbonate to the
slurry preparation step.
[0014] In its more specific and preferred aspects the process
includes the following steps:
[0015] Sulfur-containing carbonaceous fuel, in the form of fine
particles, is mixed with a first aqueous solution of alkali metal
salts containing about 5% to 35% by weight, preferably 10% to 30%,
alkali metal carbonate and minor amounts of other salts such as
alkali metal bisulfide, thiosulfate, and sulfate to form a
concentrated slurry containing about 25% to 75% by weight,
preferably 35% to 65%, undissolved fuel particles. The slurry is
pressurized and fed into a gasifier operating at an elevated
pressure in the range of about 4 to 50 atmospheres, preferably 6 to
40 atmospheres, and containing a drying zone and gasification
zone.
[0016] The slurry contacts a hot gas in the drying zone producing
water vapor, which mixes with the gas, and a dried mixture of
carbonaceous fuel and salts. The dried mixture enters the
gasification zone where it is contacted with an oxygen-containing
gas (the oxidizer) which is fed into the gasification zone in an
amount less than about 60% of the amount required for complete
combustion of the carbonaceous fuel, and preferably in the range of
35% to 50%. The oxygen reacts with the carbonaceous fuel by
exothermic, partial combustion, reactions forming hydrogen and
carbon monoxide-containing gases and producing a temperature in the
gasification zone in the range of about 900-1400.degree. C.,
preferably in the range of about 950-1300.degree. C.
[0017] To prevent the presence of excessive nitrogen gas in the
product gas, pure oxygen can be used as the oxidizer. In this
process, the temperature of the gasification zone is moderated by
steam injection into the gasifier at above the molten pool. This
combined oxygen/steam oxidizer will produce hydrogen rich,
medium-Btu gas (higher heating value (HHV) of about .apprxeq.290
Btu/scf).
[0018] The salts, which enter the gasification zone in the dried
mixture, melt at the high temperature in the gasification zone,
forming a liquid smelt that catalyzes the gasification reaction and
absorbs sulfur from the reacting fuel resulting in the conversion
of alkali metal carbonate to the corresponding sulfide in the
smelt. The smelt drains out of the gasifier into a quench tank
operating at the same pressure as the gasifier where it is
dissolved in water to form a second aqueous solution containing
alkali metal sulfide, unconverted alkali metal carbonate, and minor
amount of other salts. The second aqueous solution is
depressurized; treated to convert dissolved alkali metal sulfide
into hydrogen sulfide gas, which is released from solution, and
alkali metal carbonate, which remains in the solution, and is then
recycled as regenerated solution for use as the principal
ingredient of the first aqueous solution.
[0019] The hydrogen and carbon monoxide-containing gases generated
in the high temperature gasification zone leaves the gasifier as
impure off-gas. Its carbon monoxide concentration may vary over a
wide range because of the effects of both the carbon content of the
fuel and the oxygen content of the oxygen-containing gas. It is
typically in the range of about 15 to 70% CO by volume, dry basis,
and is preferably in the range of 20 to 60% CO. The hydrogen
concentration, typically, ranges from 5% by volume--dry basis--when
oxidant agent is air and it is as high as 38% when gasification is
performed in combined oxygen and steam environment. The main
impurities in the off-gas are water vapor, hydrogen sulfide, and
salt fume particles. For some applications it may be used without
further treatment, but preferably it is cooled and purified to
produce the final product gas. In addition to hydrogen, carbon
monoxide and the above mentioned impurities, the product gas
typically contains nitrogen, hydrogen, carbon dioxide, and some
methane, and has a higher heating value (HHV) of at least 100
Btu/scf (3720 kJ/m.sup.3), preferably in the range of about 115 to
260 Btu/Std cu ft (4280 to 9670 kJ/m.sup.3).
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a block diagram of one embodiment of the
techniques of the present application.
[0021] FIG. 2 is a flow diagram of one embodiment of the techniques
of the present application.
[0022] The same reference numbers are used in the two figures to
denote similar elements.
DETAILED DESCRIPTION OF THE DRAWINGS
[0023] Referring to FIG. 1, particles of sulfur-containing
carbonaceous fuel, 1, are mixed with an aqueous solution containing
alkali metal carbonate, 5, in a slurry preparation step. The
resulting slurry, or suspension, 6, passes through a pressurization
step, which may be a simple liquid pump, and the pressurized
slurry, 8, is, fed into a pressurized gasification step.
Pressurized oxygen-containing gas is also fed into the gasification
step where it reacts with the slurry to produce a combustible
product gas and a molten smelt, 15. The smelt contains sulfur
absorbed from the fuel, in the form of alkali metal sulfide. The
smelt flows to a smelt dissolution step where it is dissolved in
water to form an aqueous solution at essentially the same pressure
as the gasification step. The aqueous solution, 20, flows through a
solution regeneration and depressurization step which uses the
reaction of CO.sub.2, and H.sub.2O with alkali metal sulfide in
solution to produce dissolved alkali metal carbonate and gaseous
H.sub.2S. The required CO.sub.2 may be absorbed from the product
gas stream or provided from an outside source. The H.sub.2S is
recovered as a byproduct acid gas stream which typically also
contains CO.sub.2 and water vapor. The regenerated and
depressurized aqueous solution containing alkali metal carbonate,
36, is recycled to the slurry preparation step.
[0024] Operation of the gasifier at elevated pressure is required
because it greatly reduces the volume of gas handled and,
therefore, the size of the gasifier vessel, gas lines, and
downstream gas handling equipment. Elevated pressure also increases
the efficiency of gas absorption operations and permits the product
gas to be used directly in gas turbines or other devices which
require a pressurized feed gas. The present technique of the
application avoids the problem of feeding solid particles into a
high pressure system by converting the fuel feed into a liquid
slurry form so that it can be pressurized and forced into the
elevated pressure gasifier using simple liquid pumps.
[0025] The sulfur-containing carbonaceous fuel may be any such
material that can be dispersed in a liquid aqueous medium to form a
concentrated pump-able suspension or slurry, including coal,
petroleum coke, and petroleum residuals. The process is most
advantageous when used with fuels that are solid at the temperature
of the feed slurry because liquid fuels do not need to be mixed
with an aqueous medium to become pump-able; however, a dispersion
of liquid or semisolid particles of sulfur-containing carbonaceous
fuel in the aqueous solution represents an acceptable feed slurry
for the practice of the present technique of the application.
[0026] Other processes for gasifying sulfur-containing carbonaceous
fuels in the presence of molten salts have the requirement to
provide solid alkali metal carbonate feed as make-up for a molten
salt pool. The present technique of the application avoids this
requirement by recycling alkali metal carbonate as an aqueous
solution. The recycled solution need not be highly purified because
the presence of unconverted sulfide or bi-sulfide in the feed will
not adversely affect gasifier operation. With solid salt recycle
alkali metal sulfide and bi-sulfide cannot be included in the
recycle stream in significant quantities because these salts are
very difficult to crystallize out or solution and the solid forms
tend to absorb water from the air causing sticking and plugging
problems in the solids feed system. In one mode of operation of the
present technique of the application, the carbonaceous fuel is a
low ash material such as petroleum coke, the oxidant is air, and
the alkali metal is sodium or potassium. The fuel, in the form of
small particles, is mixed with a first aqueous solution to form
concentrated slurry which is pumped into the gasifier. The key
reaction in the gasification of carbonaceous fuel with oxygen
is:
C+1/2O.sub.2.dbd.CO
[0027] Other known reactions that result in the formation of
CO.sub.2, H.sub.2, H.sub.2O, H.sub.2S, CH.sub.4, and other products
may also occur.
[0028] The overall gasification reactions are exothermal,
generating a temperature in the gasification zone in the range of
900-1400.degree. C. which is sufficiently high to melt the alkali
metal salts. The molten salt acts as a catalyst for the
gasification reactions and also serves to absorb sulfur from the
reacting fuel, by forming alkali metal sulfide. The molten salt, or
smelt, is drained from the gasifier and dissolved in water at the
gasifier operating pressure to form a second aqueous solution. The
second aqueous solution is depressurized and regenerated for use as
the principal ingredient of the first solution by converting
dissolved alkali metal sulfide into hydrogen sulfide gas which is
released from solution and alkali metal carbonate which remains in
the recycled solution.
[0029] Solution regeneration may be accomplished, by various known
techniques; however, the overall chemical reaction is generally
represented by the equation:
M.sub.2S+CO.sub.2+H.sub.2O=M.sub.2CO.sub.3+H.sub.2S (where M is Na
or K)
[0030] The required CO.sub.2 may be absorbed from a dilute gas,
injected as pure CO.sub.2, or generated in situ by decomposing
MHCO.sub.3.
[0031] The regeneration system typically includes using the second
solution to absorb carbon dioxide from the gasifier off-gas at
approximately the gasifier pressure; depressurizing and stripping
hydrogen sulfide gas from the solution; separating at least a
portion of the ash components such as silicon, aluminum, and
vanadium compounds, if present, from the solution; and recycling
the resulting solution as the principle ingredient of the first
aqueous solution. Valuable ash components, such as vanadium, may be
recovered as a byproduct as part of the ash separation step.
[0032] The solution leaving the quench tank is very alkaline with a
pH typically in the 12-14 range. Its principal ingredients are
alkali metal sulfide and carbonate; however it typically contains
small amounts of other compounds such as alkali metal hydroxide,
sulfate, and thiosulfate. When it is used to absorb acid gases from
the gasifier off-gas or other source of CO.sub.2, the alkalinity is
neutralized to a considerable extent by reactions such as:
M.sub.2CO.sub.3+CO.sub.2+H.sub.2O=2MHCO.sub.3, (where M is Na or
K)
M.sub.2S+H.sub.2O+CO.sub.2=MHS+MHCO.sub.3
[0033] As a result of such reactions, the carbonated solution from
the absorber contains alkali metal bicarbonate and bi-sulfide and
is only mildly alkaline, with a pH in the range of about 7.5 to
10.5. The absorption reactions are carried to the point where the
solution contains essentially no hydroxide or sulfide, but it may
contain a significant fraction of un-reacted carbonate.
[0034] Typically, the carbonated solution is fed into a stripper
where the combined effects of heat, reduced pressure, and stripping
vapor cause the following reactions to occur:
MHS+MHCO.sub.3=M.sub.2CO.sub.3+H.sub.2S
2MHCO.sub.3=M.sub.2CO.sub.3+CO.sub.2+H.sub.2O
[0035] The regenerated solution from the stripper has an alkalinity
intermediate between the quench solution and the carbonated
solution with a pH typically in the range of about 9 to 13. It
contains primarily alkali metal carbonate with small amounts of
other salts such as alkali metal bi-sulfide, sulfate, and
thiosulfate. The regenerated solution is recycled to the feed
preparation step as the principal ingredient of the first aqueous
solution. Additional alkali metal carbonate or water may be added
prior to recycle to adjust the concentration and make up for salt
losses that may occur in the system.
[0036] The separation of ash components from the solution may be
carried out at any point in the solution flow circuit and may be
accomplished by settling, filtration, centrifugation, or other
means. When essentially ash free fuel is used, no separate ash
removal step is required.
[0037] In one alternative regeneration step, the solution from the
quench tank is carbonated by the absorption of carbon dioxide from
a source other than the gasifier off-gas. The resulting carbonated
solution is stripped to remove H.sub.2S then recycled to the feed
preparation step. In another alternative regeneration system the
quench solution, which may first be pre-carbonated by contact with
dilute CO.sub.2 gas, is regenerated by contact with pure CO.sub.2
gas. In this technique, H.sub.2S is displaced from the dissolved
salts by CO.sub.2 because of the greater effective acidity of
CO.sub.2 relative to H.sub.2S. The overall chemical reaction
is:
MHS+CO.sub.2+H.sub.2O=MHCO.sub.3+H.sub.2S
[0038] The product acid gas is a mixture of released H.sub.2S and
un-reacted CO.sub.2. This technique may result in the precipitation
of alkali metal bicarbonate crystals, particularly when sodium
salts are used, but these can be re-dissolved in a separate step
where the mixture is heated to decompose the bicarbonate and
produce the pure CO.sub.2 required by the previous step while
forming the more soluble carbonate salt for recycle to the feed
preparation step.
[0039] The molten salt catalyzes gasification reactions in the
gasifier and absorbs sulfur from the reacting fuel; however a
portion of the sulfur normally leaves the gasifier as H.sub.2S in
the off-gas due to reactions such as:
M.sub.2S+H.sub.2O=M.sub.2O+H.sub.2S
[0040] The H.sub.2S may be removed, from the off-gas by absorption
in the quench solution, absorption in a recycle stream of
regenerated solution, or use of an auxiliary gas purification
system.
[0041] Referring to FIG. 2, a sulfur-containing carbonaceous fuel,
1, such as petroleum coke, in the form of a fine powder is fed into
a hopper, 2, from which it is fed into slurry tank, 3, which is
equipped with a mixing device, 4. A first aqueous solution of
alkali metal salts, 5, is also fed into the slurry tank to form a
suspension, or slurry, of fuel particles in alkali metal salt
solution. The slurry flows out of the slurry tank through line 6 to
pressurizing pump, 7, which, forces it through line 8 to gasifier
9. The gasifier shown is of a type that has a drying zone, 10, and
a gasification zone, 11. Inside the gasifier the slurry is dried in
the drying zone by contact with hot gas originating in the
gasification zone. The dried slurry, a mixture of fuel and alkali
metal salt solids, enters the gasification zone where it is
gasified by reaction with oxygen. Alternative gasifier designs that
bring about the reaction of the feed slurry with an oxygen
containing gas to produce a molten salt smelt and a CO-containing
product gas may be employed.
[0042] Air, 12, is compressed by compressor, 13, to a pressure
slightly higher than the gasifier pressure, and flows into the
gasifier through high pressure air line, 14. The air enters the
gasification zone of the gasifier where the oxygen reacts, with the
carbonaceous fuel, raising the temperature in the gasification zone
to about 900 to 1400.degree. C. The amount of air added is less
than about 60% of the amount required for complete combustion of
the carbonaceous fuel, and preferably in the range of 35 to 50%. An
excessive amount of air will raise the gasification zone
temperature above the desired range and also reduce the heating
value of the product gas. Too little air will cause excessive
build-up of un-reacted fuel in the gasifier.
[0043] At the high temperature in the gasification zone, the alkali
metal salt solids that enter with the dried slurry are melted and
the resulting molten salt, or smelt, wets the reacting fuel
particles, catalyzing the gasification reactions. Sulfur in the
fuel reacts with the molten salt to form alkali metal sulfide. A
portion of the fuel sulfur may also form alkali metal sulfate and
thiosulfate in the smelt and hydrogen sulfide in the gas phase.
[0044] The smelt, consisting primarily of alkali metal sulfide and
un-reacted alkali metal carbonate flows out of the gasifier through
line 15 into quench tank, 16, which operates at the same elevated
pressure as the gasifier. Water is added to the quench tank through
line 17. All or a portion of the water may be produced by
condensing water vapor out of the gasifier and stripper off-gas
streams as shown in the figure. The amount of water added to the
quench tank is controlled to dissolve the smelt and produce a
quench solution, or second aqueous solution, containing about 5% to
40% total dissolved solids.
[0045] The quench solution is withdrawn from the quench tank
through line 18 to pump 19 which forces it to flow through line 20
to the gas purification system, 21. For simplicity, the gas
purification system is shown as a single vessel with solution feed
near the top and gas entry near the bottom. In practice the system
may involve multiple contactors or sections, split stream liquid or
gas feed, solution recycle loops, and other features known to
improve gas purification and absorption operations. The purpose of
this system is to carbonate the solution by the absorption of
carbon dioxide and also to reduce the hydrogen sulfide content of
the gas to a level that is acceptable in the final product gas.
[0046] Gasifier off-gas leaves the drying zone of the gasifier via
line 22 and passes through gas cooler-condenser system 23, which
serves to reduce the temperature and water vapor content of the
gasifier off-gas. The gas then flows through line 24 to the gas
purification system, 21 where carbon dioxide and hydrogen sulfide
are absorbed. It leaves this system as the final product gas via
line 25.
[0047] The carbonated solution flows out of the gas purification
system through line 26 and pressure reduction valve, 27, into the
acid gas stripper 28 which may include a heating coil as shown in
the figure. The stripper operates at a significantly lower pressure
than the gas purification system and gasifier. The stripper
operates at a pressure between about 0.1 and 2.0 atmospheres, and
preferably in the range of 0.2 to 1.0 atmospheres. The acid gases,
hydrogen sulfide and carbon dioxide, and water vapor produced in
the stripper flow through line 29 into condenser 30 where the gas
stream is cooled, to condense water. The resulting liquid water may
be returned to the stripper or used in the quench tank as shown in
the figure. The acid gas stream leaves the condenser via line 31
and is typically sent to a sulfur recovery plant (not shown on the
drawing) for recovery of sulfur values. In this embodiment of the
technique of the application the gas purification system and acid
gas stripper combined represent the solution regeneration and
depressurization step of FIG. 1.
[0048] Regenerated solution flows out of the acid gas stripper
through line 32 to pump 33 which forces it through separator 34 and
line 36 back to the slurry tank where it serves as the aqueous
solution of alkali metal salts, 5, referred to previously. Ash
components, removed from the solution by separator 34, are
discharged from the system via line 35 for disposal or recovery of
byproducts. A small amount of un-gasified carbonaceous material may
be suspended in the quench solution. This material may be removed
with the ash components but preferably is recycled to the slurry
tank and ultimately to the gasifier.
[0049] All or a portion of the product gas may flow out of the
gasification zone with smelt. This gas is separated from the molten
smelt and flows through vent line 37 to the product gas line.
EXAMPLE
[0050] Sound prediction of the techniques of the present
application is based, on small scale test data for individual
operations and computer model calculations. The feed material is
petroleum coke of the composition shown in Table 1. Performance
data are based on 100 grams of dry coke feed.
[0051] The coke is mixed with an equal weight of aqueous solution
containing approximately 18% sodium carbonate and 2% sodium
bi-sulfide. The resulting slurry is fed into a gasifier operating
at a pressure of 20 atmospheres (294 psia) and a temperature of
about 1000.degree. C. in the gasification zone. An approximate
material balance around the gasifier is given in Table 2.
[0052] Molten smelt flows from the gasifier into a quench tank
operating at the same pressure as the gasifier and is dissolved in
approximately 80 grams of water to yield about 98.7 grams of quench
solution. The quench solution is regenerated by carbonating it in
an absorber used to scrub the gasifier off-gas then stripping
H.sub.2S and CO.sub.2 from it in a sub-atmospheric pressure
stripper. The stripper produces an acid gas stream containing
approximately 50% H.sub.2S and 50% CO.sub.2 by volume, dry basis.
Ash is removed from the regenerated solution by filtration and the
filter cake is washed with water to remove soluble salts. The
filtered regenerated solution is recycled, to the slurry
preparation step.
[0053] A material balance around the quench tank and solution
regeneration system is given in Table 3. The material balance
envelope for this table includes all steps relating to solution
processing including smelt dissolution; product gas cooling, water
condensation, and scrubbing; solution stripping; acid gas cooling
and water condensation, and ash separation.
TABLE-US-00001 TABLE 1 Typical Petroleum Coke Composition
Components Concentration (wt. %, dry basis) Carbon 88.9 Hydrogen
3.9 Nitrogen 2.2 Sulfur 2.1 Oxygen 1.3 Ash 1.6 Total 100
TABLE-US-00002 TABLE 2 Gasifier Material Balance Input Output Feed
Slurry Off-Gas Solid Phase CO 185.1 grams Carbon 88.9 grams
CO.sub.2 35.8 Hydrogen 3.9 H.sub.2 4.8 Nitrogen 2.2 N.sub.2 403.4
Sulfur 2.1 CH.sub.4 1.2 Oxygen 1.3 H.sub.2O 71.9 Ash 1.6 H.sub.2S
0.1 Aqueous Phase Smelt Na.sub.2CO.sub.3 18.0 Na.sub.2S 7.7 NaHS
2.0 Na.sub.2CO.sub.3 9.4 H.sub.2O 80.0 Ash 1.6 Compressed Air
Oxygen 119.8 Nitrogen 401.8 Total Input 721.0 grams Total Output
721.0 grams
TABLE-US-00003 TABLE 3 Quench Tank and Solution Regeneration System
Material Balance Input Output grams grams Vol % Gasifier Purified
Gas Off-Gas CO 185.1 CO 185.1 27.1 CO.sub.2 35.8 CO.sub.2 29.3 2.7
H.sub.2 4.8 H.sub.2 4.8 9.8 N.sub.2 403.4 N.sub.2 403.4 59.1
CH.sub.4 1.2 CH.sub.4 1.2 0.3 H.sub.2O 71.9 H.sub.2O 4.3 1.0
H.sub.2S 0.1 H.sub.2S negligible 0.0 Smelt Acid Gas Na.sub.2S 7.7
H.sub.2S 2.2 50 Na.sub.2CO.sub.3 9.4 CO.sub.2 2.9 50 Ash 1.6
Regenerated Solution Make-up Water Na.sub.2CO.sub.3 18.0 H.sub.2O
15.4 NaHS 2.0 H.sub.2O 80.0 Ash Cake Ash 1.6 H.sub.2O 1.6 Total
Input 736.4 grams Total Output 736.4 grams
[0054] Table 3 also presents the approximate composition of the
product and byproduct gas streams. The product gas is calculated to
have a higher heating value (HHV) of about 124 Btu/scf (4600
kJ/m.sup.3), dry basis, which is suitable for fuel to a gas
turbine. The material balances are approximate and include only the
principal components in each stream. For example, a small amount of
alkali metal salt may be present in the ash filter cake, and minor
amounts of oxidized sulfur compounds such as sulfate and
thiosulfate may be present in the smelt and aqueous solutions.
Also, traces of higher hydrocarbons and other sulfur compounds may
be present in the product gas stream.
[0055] Although certain embodiments of the techniques of the
present application have been described, the spirit and scope of
the application is by no means restricted to what is described
above. Persons having ordinary skill in the art will be able to
make variations, permutations, and combinations, in view of the
above description, all of which are within the scope of the present
application.
Process Advantages:
[0056] Significant technical advantages are offered by this
gasification process in both production cases of either low- or
medium-Btu gas: [0057] 1. A wide variety of carbonaceous materials
can be and indeed have been handled, including anthracite,
bituminous and lignite coals; several types of petroleum coke;
organic waste; photographic film; wood chips, and heavy residual
hydrocarbons. Fuel particles are pneumatically injected into the
melt and are immediately wetted and dispersed so there is no
opportunity for caking or swelling to affect operation. [0058] 2.
Feed need not be closely sized. Pulverization and fines removal are
unnecessary. The only requirement for solid feed is that it be
pneumatically conveyable, which generally requires particles of
less than 4 mesh. [0059] 3. The product gas is essentially free of
sulfur. Approximately 90 to 99 percent of feed sulfur is recovered
in the regeneration system. This level of removal is generally
adequate to meet environmental requirements for combustion use of
product gas. However, if product gas is to be used in synthesis of
chemicals, further sulfur removal may be required to prevent
poisoning catalyst. [0060] 4. Negligible amounts of tar, heavy
hydrocarbons and NO.sub.x are produced. The catalytic effect of the
melt apparently causes almost complete destruction of heavy
hydrocarbons and organic nitrogen compounds at the gasifier
operating temperature. [0061] 5. Oxygen requirement is relatively
low. For a typical petroleum coke, about 0.8 pounds of oxygen per
pound of coke is required. [0062] 6. The gasifier turndown
capability is excellent because a gas-sparged molten pool is
relatively insensitive to gas velocity. [0063] 7. There is no
explosion hazard when the fuel feed stops. Any oxygen that flows
into the molten salt pool is absorbed by an inventory of reduced
compounds (sodium sulfide, residual carbon). [0064] 8. Thermal
efficiency is high. Although actual efficiency is a function of
fuel composition and other design parameters, a relatively high
efficiency is possible because no char is produced and carbon
utilization is generally over 98%; heat losses are low because the
reaction zone is not actively cooled, and product gas is amenable
to efficient heat recovery because of its moderate temperature and
relative freedom from tar, ash and other objectionable impurities.
[0065] 9. Valuable byproducts such as sulfur and vanadium can be
recovered relatively easily. All ash constituents are retained in
the melt which is processed to return sodium carbonate to the
gasifier. A vanadium-recovery step may be added to the
melt-regeneration system.
* * * * *