[0001] This invention relates to the gasification of coal and similar carbonaceous materials
and is particularly concerned with a catalytic gasification process carried out in
the presence of a carbon-alkali metal catalyst to produce a chemical synthesis gas.
[0002] Existing and proposed processes for the manufacture of synthetic gaseous fuels from
coal or similar carbonaceous materials normally require the reaction of carbon with
steam, alone or in combination with oxygen, at temperatures between about 649°C (1200°F)
and about 1370°C (2500° F) to produce a gas which may contain some methane but consists
primarily of hydrogen and carbon monoxide. This gas can be used directly as a synthesis
gas or a fuel gas with little added processing or can be reacted with additional steam
to increase the hydrogen- to-carbon monoxide ratio and then fed to a catalytic methanation
unit for reaction with carbon monoxide and hydrogen to produce methane. It has been
shown that processes of this type can be improved by carrying out the initial gasification
step in the presence of a catalyst containing an alkali metal constituent. The alkali
metal constituent accelerates the steam-carbon gasification reaction and thus permits
the generation of synthesis gas at somewhat lower temperatures than would otherwise
be required. Processes of this type are costly because of the large quantities of
heat that must be supplied to sustain the highly endothermic steam-carbon reaction.
One method of supplying this heat is to inject oxygen directly into the gasifier and
bum a portion of the carbon in the feed material being gasified. This method is highly
expensive in that it requires the existence of a plant to manufacture the oxygen.
Other methods for supplying the heat have been suggested, but these, like that of
injecting oxygen, are expensive.
[0003] It has been recently found that difficulties associated with processes of the type
described above, can largely be avoided by carrying out the reaction of steam with
carbon in the presence of a carbon-alkali metal catalyst and substantially equilibrium
quantities of added hydrogen and carbon monoxide. Laboratory work and pilot plant
tests have shown that catalysts produced by the reaction of carbon and alkali metal
compounds such as potassium carbonate to form carbon-alkali metal compounds or complexes
will, under the proper reaction conditions, equilibrate the gas phase reactions occurring
during gasification to produce methane and at the same time supply substantial amounts
of exothermic heat within the gasifier. This additional exothermic heat of reaction
essentially balances the overall endothermicity of the reactions involving solid carbon
and thus results in a substantially thermoneutral process in which the injection of
large amounts of oxygen or the use of other expensive methods of supplying heat are
eliminated.
[0004] The catalytic effect of carbon-alkali metal catalysts on the gas phase reactions,
as distinguished from the solid-gas reactions or the reactions of carbon with steam,
hydrogen or carbon dioxide, allows the following exothermic reactions to contribute
substantially to the presence of methane in the effluent gas and drastically reduces
the endothermicity of the overall reaction:
(1) 2C0+2H2-->C02+CH4 (exothermic)
(2) CO+3H2→H2O+CH4 (exothermic)
(3) CO,+4H2--+2H20+CH4 (exothermic)
[0005] Under the proper operating conditions, these reactions can be made to take place
within the gasification zone and supply large amounts of methane and additional exothermic
heat which would otherwise have to be supplied by the injection of oxygen or other
means. Laboratory and pilot plant tests have shown that constituents of the raw product
gas thus produced are present in equilibrium concentrations at reaction conditions
and consist primarily of hydrogen, carbon monoxide, carbon dioxide, methane and steam.
It has been proposed (see US-A- 4,094,650) to utilize steam gasification in the presence
of a carbon-alkali metal catalyst to produce Btu product gas by treating the raw produce
gas for removal of steam and acid gases, principally carbon dioxide and hydrogen sulfide;
cryogenically separating carbon monoxide and hydrogen in amounts equivalent to their
equilibrium concentration in the raw product gas from the methane in the treated gas;
withdrawing methane as a high Btu product gas; and recycling the carbon monoxide and
hydrogen to the gasifier. The presence in the gasifier of the carbon-alkali metal
catalyst and equilibrium quantities of recycle carbon monoxide and hydrogen, which
tend to suppress reactions that would otherwise produce additional hydrogen and carbon
monoxide, results in a substantiall thermoneutral reaction to produce essentially
methane and carbon dioxide. Since the overall reaction is substantially thermoneutral,
only a small heat input is required to preheat the carbonceous feed material and to
maintain the reactants at reaction temperatures by compensating for heat losses from
the gasifier. This small amount of heat may be supplied by preheating the gaseous
reactants in a conventional preheat furnace.
[0006] It has also been proposed (see FR-A-238,820 or US-A-4.,118,204) to utilize steam
gasification of a carbonaceous feed material in the presence of a carbon-alkali metal
catalyst to produce an intermediate Btu product gas (i.e. a mixture of CH
4, H
2 and CO) by treating the raw product gas withdrawn from the gasifier for the removal
of steam and acid gases, principally carbon dioxide and hydrogen sulfide; recovering
a portion of the treated gas as the intermediate Btu product gas; contacting the remainder
of the treated gas with steam in a steam reformer under conditions such that the methane
in the treated gas reacts with the steam to produce additional hydrogen and carbon
monoxide; and passing the effluent from the reformer into the gasifier. The amounts
of hydrogen and carbon monoxide produced in the reformer compensate for the amounts
of those gases removed in the treated gas that is withdrawn as intermediate Btu product
gas. Thus the reformer effluent will normally contain carbon monoxide and hydrogen
in amounts equivalent to the equilibrium quantities of those gases present in the
raw product gas and will therefore supply the substantially equilibrium quantities
of hydrogen and carbon monoxide required in the gasifier along with the carbon-alkali
metal catalyst and steam to produce the thermoneutral reaction that results in the
formation of essentially methane and carbon dioxide.
[0007] This invention provides a process for the generation of a high purity chemical synthesis
gas (i.e. a gas mixture comprising carbon monoxide and hydrogen) by the substantially
thermoneutral reaction of steam with coal, petroleum coke, heavy oil, residuum and
other carbonaceous feed materials in the presence of carbon-alkali metal catalyst
and added hydrogen and carbon monoxide. In accordance with the invention, it has now
been found that a chemical synthesis gas can be generated by reacting steam with a
carbonaceous feed material in a reaction zone at a temperature between 538° and 816°C
(1000°F and 1500°F), and a pressure in excess of 6,89 . 10
2 kPa (100 psia), preferably between 1,38. 10
3 (200) and 5,52.10
3 kPa (800 psia), in the presence of a carbon-alkali metal catalyst and sufficient
added hydrogen and carbon monoxide to provide substantially equilibrium quantities
of hydrogen and carbon monoxide in the reaction zone at reaction temperature and pressure
thereby producing an effluent gas consisting essentially of equilibrium quantities,
at reaction temperature and pressure, of methane, carbon monoxide, carbon dioxide,
steam and hydrogen; withdrawing the effluent gas from the reaction zone and treating
it for the removal of steam and acid gases to produce a treated gas containing primarily
carbon monoxide, hydrogen and methane; recovering carbon monoxide and hydrogen from
the treated gas as a chemical synthesis product gas; contacting at least a portion
of the remainder of the treated gas consisting primarily of methane with steam in
a steam reforming zone under conditions such that at least a portion of the methane
reacts with the steam to produce carbon monoxide and hydrogen; and passing the effluent
from the reforming zone into the reaction zone.
[0008] It is normally desirable that the reforming zone effluent contain carbon monoxide
and hydrogen in amounts equivalent to the equilibrium quantities of those gases present
in the effluent gas withdrawn from the reaction zone so that the effluent from the
steam reforming zone will supply the substantially equilibrium quantities of hydrogen
and carbon monoxide required in the reaction zone along with the carbon-alkali metal
catalyst and steam to produce the thermoneutral reaction that results in the formation
of essentially methane and carbon dioxide. If the reforming zone effluent contains
less than the desired amount of carbon monoxide and hydrogen, additional amounts of
these gases may be added to the gasifier. Preferably, a slip stream of the chemical
synthesis product gas is used for this purpose. If the reforming zone effluent contains
more than the desired amount of carbon monoxide and hydrogen, the excess can be mixed
with the reaction zone effluent, passed through the downstream processing scheme,
and withdrawn as a portion of the chemical synthesis gas product.
[0009] A sufficient amount of steam is normally fed to the reforming zone so that enough
unreacted steam is present in the steam reforming zone effluent to provide substantially
all the steam necessary to supply the reactions taking place in the reaction zone.
The reforming zone is normally operated at conditions such that its effluent may also
be used to supply the heat needed to preheat the carbonaceous feed material to reaction
temperature and compensate for heat losses from the reaction zone. This is normally
achieved if the temperature of the reforming zone effluent is between 38°C and 149°C
(100°F and 300°F) higher than the temperature in the reaction zone and the effluent
is passed without substantial cooling into the reaction zone.
[0010] The process of the invention, unlike similar processes proposed in the past, utilizes
the thermoneutral reaction of steam with a carbonaceous feed material to produce a
high purity chemical synthesis gas that has wide spread industrial applications.
[0011] The drawing is a schematic flow diagram of a process carried out in accordance with
the invention for the manufacture of a chemical synthesis gas by the gasification
of coal or similar carbonaceous solids with steam in the presence of a carbon-alkali
metal catalyst and added equilibrium quantities of hydrogen and carbon monoxide.
[0012] The process depicted in the drawing is one for the production of a chemical synthesis
gas by the gasification of bituminous coal, sub- bituminous coal, lignite, coal char,
coke or similar carbonaceous solids with steam at a high temperature in the presence
of a carbon-alkali metal catalyst prepared by impregnating the feed solids with a
solution of an alkali metal compound or mixture of such compounds and thereafter heating
the impregnated material to a temperature sufficient to produce an interaction between
the alkali metal and the carbon present. The solid feed material that has been crushed
to a particle size of about 8 mesh or smaller on the U.S. Sieve Series Scale is passed
into line 10 from a feed preparation plant or storage facility that is not shown in
the drawing. The solids intorduced into line 10 are fed into a hopper or similar vessel
11 from which they are passed through line 12 into feed preparation zone 14. This
zone contains a screw conveyor or similar device, not shown in the drawing, that is
powered by a motor 16, a series of spray nozzles or similar devices 17 for the spraying
of an alkali metal-containing solution supplied through line 18 onto the solids as
they are moved through the preparation zone by the conveyor, and a similar set of
nozzles or the like 19 for the introduction of a hot dry gas, such as flue gas, into
the preparation zone. The hot gas, supplied through line 20, serves to heat the impregnated
solids and drive off the moisture. A mixture of water vapor and gas is withdrawn from
zone 14 through line 21 and passed to a condenser, not shown, from which water may
be recovered for use as makeup or the like. The majority of the alkali metal-containing
solution is recycled through line 49 from the alkali metal recovery portion of the
process, which is described hereafter. Any makeup alkali metal solution required may
be introduced into line 18 via line 13.
[0013] It is preferred that sufficient alkali metal-containing solution be introduced into
preparation zone 14 to provide from about 1 to about 50 weight percent of an alkali
metal compound or mixture of such compounds on the coal or other carbonaceous solids.
From about 5 to about 30 percent is generally adequate. The dried impregnated solid
particles prepared in zone 14 are withdrawn through line 24 and passed to a closed
hopper or similar vessel 25 from which they are discharged through a star wheel feeder
or equivalent device 26 in line 27 at an elevated pressure sufficient to permit their
entrainment into a stream of high pressure steam, recycle product gas, inert gas or
other carrier gas introduced into line 29 via line 28. The carrier gas and entrained
solids are passed through line 29 into manifold 30 and fed from the manifold through
feed lines 31 and nozzles, not shown in the drawing, into gasifier 32. In lieu of
or in addition to hopper 25 and star wheel feeder 26, the feed system may employ parallel
lock hoppers, pressurized hoppers, aerated standpipes operated in series, or other
apparatus to raise the input feed solids stream to the required pressure level.
[0014] It is generally preferred to operate the gasifier 32 at a pressure between 6,89.
10
2 and 1,03 . 104 kPa (100 and 1500 psia), the most preferred range of operation being
between 1,38. 10-
3 and 5,52 . 103 kPa (200 and 800 psia). The carrier gas and entrained solids will
normally be introduced at a pressure somewhat in excess of the gasifier operating
pressure. The carrier gas may be preheated to a temperature in excess of about 149°C
(300°F), but below the initial softening point of the coal or other feed material
employed. Feed particles may be suspended in the carrier gas in a concentration between
about 0.2 and about 0.5 kg of solid feed material per kg of carrier gas. The optimum
ratio for a particular system will depend in part upon the particle size and density,
the molecular weight of the gas employed, the temperature of the solid feed material
and the input gas stream, the amount of alkali metal compound employed and other factors.
In general, ratios between about 0.5 and about 4.0 kg of solid feed material per kg
of carrier gas are preferred.
[0015] Gasifier 32 contains a fluidized bed of carbonaceous solids extending upward within
the vessel above an internal grid or similar distribution device not shown in the
drawing. The bed is maintained in the fluidized state by means of steam, hydrogen
and carbon monoxide introduced through line 33, manifold 34 and peripherally spaced
injection lines and nozzles 35 and through bottom inlet line 36. The particular injection
system shown in the drawing is not critical and hence other methods for injecting
the steam, hydrogen and carbon monoxide may be employed. In some instances, for example,
it may be preferred to introduce the gases through multiple nozzles to obtain more
uniform distribution of the injected fluid and reduce the possibility of channeling
and related problems. The space velocity of the rising gases within the fluidized
bed will normally be between about 2 and about 300 actual volumes of steam, hydrogen
and carbon monoxide per hour per volume of fluidized solids.
[0016] Within the fluidized bed in gasifier 32, the carbonaceous solids impregnated with
the alkali metal compound or mixture of such compounds are subjected to a temperature
within the range between 538°C and 816°C (1000°F and 1500°F), preferably between 649°C
and 760°C (1200°F and 1400°F). At such a temperature the alkali metal constituents
interact with the carbon in the carbonaceous solids to form a carbon-alkali metal
catalyst, which will under proper reaction conditions equilibrate the gas phase reactions
occurring during gasification to produce additional methane and at the same time supply
substantial amounts of additional exothermic heat in situ. Due to the gas phase equilibrium
conditions existing as a result of the carbon-alkali metal catalyst and due to the
presence of equilibrium quantities of hydrogen and carbon monoxide injected with the
steam near the lower end of the bed, the net reaction products will normally consist
essentially of methane and carbon dioxide. Competing reactions that in the absence
of the catalyst and the hydrogen and carbon monoxide would ordinarily tend to produce
additional hydrogen and carbon monoxide are suppressed. At the same time, substantial
quantities of exothermic heat are released as a result of the reaction of hydrogen
with carbon oxides and the reaction of carbon monoxide with stream. This exothermic
heat tends to balance the endothermic heat consumed by the reaction of the steam with
carbon, thereby producing an overall thermoneutral reaction. So far as the heat of
reaction is concerned, the gasifier is therefore largely in heat balance. The heat
employed to preheat the feed coal to the reaction temperature and compensate for heat
losses from the gasifier is supplied for the most part by excess heat in the gases
introduced into the gasifier through lines 35 and 36. In the absence of the exothermic
heat provided by the catalyzed gas phase reactions, these gases would have to be heated
to substantially higher temperatures than those employed here.
[0017] The carbon-alkali metal catalyst utilized in the process of the invention is prepared
by heating an intimate mixture of carbon and an alkali metal constituent to an elevated
temperature, preferable above 427°C (800°F). In the process shown in the drawing and
described above, the intimate mixture is prepared by impregnating the carbonaceous
feed material with an alkali metal-containing solution and then subjecting the impregnated
solids to a temperature above 427°C (800°F) in the gasifier itself. It will be understood
that the alkali metal catalyst utilized in the process of this invention can be prepared
without impregnation onto the carbonaceous solids to be gasified, and without heating
in the gasifier. The heating step, for example, may be carried out in a solid feed
preparation zone or in an external heater. The carbonaceous solids used will in most
instances be the ones which are to be gasified but in some variations of the process
carbonaceous materials other than the feed solids may be used. In some cases inert
carriers having carbon deposited on their outer surface may be used. Suitable inert
carriers include silica, alumina, silica-alumina, zeolites, and the like. The catalyst
particles, whether composed substantially of carbon and an alkali metal constituent
or made up of carbon and an alkali metal constituent deposited on an inert carrier,
may range from fine powders to coarse lumps, particles between about 4 and about 100
mesh on the U.S. Sieve Series Scale generally being preferred. The size selected for
use in a particular operation will normally depend in part on the gas velocities and
other conditions within the system in which the catalyst is to be used. In fluidized
bed systems, the particle size is in part dependent upon the conditions under which
the bed is to be operated. In fixed or moving bed systems, the catalyst particle size
is generally of less importance.
[0018] Any of a variety of alkali metal constituents can be used in preparing the carbon-alkali
metal catalyst. Suitable constituents include alkali metals themselves and alkali
metal compounds such as alkali metal carbonates, bicarbonates, formates, biphosphates,
oxalates, amides, hydroxides, acetates, sulfates, hydrosulfates, sulfides, and mixtures
of these and other similar compounds. All of these are not equally effective and hence
a catalyst prepared from certain alkali metal constituents can be expected to give
somewhat better results under certain conditions than do others. In general, cesium,
potassium, sodium and lithium salts derived from organic or inorganic acids having
ionization constants less than about 1 x 1 0-3 and alkali metal hydroxides are preferred.
The cesium compounds are the most effective, followed by the potassium, sodium and
lithium compounds in that order. Because of their high activity, relatively low cost
compared to cesium compounds, and ready availability, potassium compounds or sodium
compounds are generally employed. Potassium carbonate and potassium hydroxide are
especially effective.
[0019] In the embodiment of the invention shown in the drawing, the alkali metal constituent
and the carbonaceous solids are combined to form an intimate mixture by dissolving
a water soluble alkali metal compound in an aqueous carrier, impregnating the carbonaceous
solid with the resulting aqueous solution by soaking or spraying the solution onto
the particles, and thereafter drying the solids. It will be understood that other
methods of forming such an intimate mixture may be used. For example, in some cases
the carbonaceous material can be impregnated by suspending a finely divided alkali
metal or alkali metal compound in a hydrocarbon solvent or other inert liquid carrier
of suitably low viscosity and high volatility and thereafter treating the solids with
the liquid containing the alkali metal constituent. In other cases, it may be advantageous
to pelletize a very finely divided alkali metal or alkali metal compound with carbon
in an oil or similar binder and then heat the pellets to an elevated temperature.
Other catalyst preparation methods, including simply mixing finely divided carbonaceous
material with a powdered alkali metal salt and thereafter heating the mixture to the
desired temperature, can in some cases also be used.
[0020] The mechanisms which take place as the result of combining the carbonaceous solids
and alkali metal constituents and then heating them to elevated temperatures are not
fully understood. Apparently, the alkali metal reacts with the carbon to form carbon-alkali
metal compounds and complexes. Studies have shown that neither carbonaceous solids
nor the alkali metal constituents alone are fully effective for establishing equilibrium
conditions for gas phase reactions involving steam, hydrogen, carbon monoxide, carbon
dioxide and methane and that catalytic activity is obtained only when a compound or
complex of the carbon and alkali metal is present in the system. Both constituents
of the catalyst are therefore necessary. Experience has shown that these catalysts
are resistant to degradation in the presence of sulfur compounds, that they resist
sintering at high temperatures, and that they bring gas phase reactions involving
the gases normally produced during coal gasification into equilibrium. As a result
of these and other benefical properties, these catalysts have pronounced advantages
over other catalysts employed in the past.
[0021] Referring again to the drawing, the gas leaving the fluidized bed in gasifier 32
passes through upper section of the gasifier, which serves as a disengagement zone
where the particles too heavy to be entrained by the gas leaving the vessel are returned
to the bed. If desired, this disengagement zone may include one or more cyclone separators
or the like for removing relatively large particles from the gas. The gas withdrawn
from the upper part of the gasifier through line 37 will normally contain an equilibrium
mixture at reaction temperature and pressure of methane, carbon dioxide, hydrogen,
carbon monoxide, and unreacted steam. Also present in this gas are hydrogen sulfide,
ammonia and other contaminants formed from the sulfur and nitrogen contained in the
feed material, and entrained fines. This raw product gas is introduced into cyclone
separator or similar device 38 for removal of the larger fines. The overhead gas then
passes through line 39 into a second separator 41 where small particles are removed.
The gas from which the solids have been separated is taken overhead from separator
41 through line 42 and the fines are discharged downward through dip legs 40 and 43.
These fines may be returned to the gasifier or passed to the alkali metal recovery
portion of the process.
[0022] In the system shown in the drawing, a stream of high ash content char particles is
withdrawn through line 44 from gasifier 32 in order to control the ash content of
the system and permit the recovery and recycle of alkali metal constituents of the
catalyst. The solids in line 44, which may be combined with fines recovered from the
gasifier overhead gas through dip legs 40 and 43 and line 45, are passed to alkali
metal recovery unit 46. The recovery unit will normally comprise a multistage countercurrent
leaching system in which the high ash content particles are countercurrently contacted
with water introduced through line 47. An aqueous solution of alkali metal compounds
is withdrawn from the unit through line 48 and recycled through lines 49 and 18 to
feed preparation zone 14. Ash residues from which soluble alkali metal compounds have
been leached are withdrawn from the recovery unit through line 50 and may be disposed
of as land fill or further treated to recover added alkali metal constituents.
[0023] The gas leaving separator 41 is passed through line 42 to gas-gas heat exchanger
51 where it is cooled by indirect heat exchange with a gaseous mixture of methane
and steam introduced through line 77. The cooled gas is then passed through line 53
into waste heat boiler 54 where it is further cooled by indirect heat exchange with
water introduced through line 55. Sufficient heat is transferred from the gas to the
water to convert it into steam, which is withdrawn through line 56. During this cooling
step, unreacted steam in the gas from exchanger 51 is condensed out and withdrawn
as condensate through line 57. The cool gas leaving waste heat boiler 54 through line
58 is passed to water scrubber 59. Here the gas stream passes upward through the scrubber
where it comes in contact with water injected into the top of the scrubber through
line 60. The water absorbs ammonia and a portion of the hydrogen sulfide in the gas
stream and is withdrawn from the bottom of the scrubber through line 61 and passed
to downstream units for further processing. The water scrubbed gas stream is withdrawn
from the scrubber through line 62 and is now ready for treatment to remove bulk amounts
of hydrogen sulfide and other acid gases.
[0024] The gas stream is passed from water scrubber 59 through line 62 into the bottom of
solvent scrubber 63. Here the gas passes upward through the contacting zone in the
scrubber where it comes in contact with a down-flowing stream of solvent such as monoethanolamine,
diethanolamine, a solution of sodium salts of amino acids, methanol, hot potassium
carbonate or the like introduced into the upper part of the solvent scrubber through
line 64. If desired, the solvent scrubber may be provided with spray nozzles, perforated
plates, bubble cap plates, packing or other means for promoting intimate contact between
the gas and the solvent. As the gas rises through the contacting zone, hydrogen sulfide,
carbon dioxide and other acid gases are absorbed by the solvent, which leaves the
scrubber through line 65. The spent solvent containing carbon dioxide, hydrogen sulfide
and other contaminants is passed through line 65 to a stripper, not shown in the drawing,
where it is contacted with steam or other stripping gas to remove the absorbed contaminants
and thereby regenerate the solvent. The regenerated solvent may then be reused by
injecting it back into the top of the scrubber via line 64.
[0025] A clean gas containing essentially methane, hydrogen, and carbon monoxide in amounts
substantially equivalent to the equilibrium quantities of those gases in the raw product
gas withdrawn from gasifier 32 through line 37 is withdrawn overhead from the solvent
scrubber via line 66. The methane content of the gas will normally range between about
20 and about 60 mole percent and the gas will be of an intermediate Btu heating value,
normally containing between 14800 and 27750 kJ/m
3 (400 and 750 Btu's per standard cubic foot).
[0026] The intermediate Btu gas withdrawn overhead from solvent scrubber 63 through line
66 is introduced into heat transfer unit 67 where it passes in indirect heat exchange
with liquid methane introduced through line 68. The methane vaporizes within the heat
transfer unit and is discharged as methane gas through line 69. The vaporizing methane
chills the intermediate Btu gas, which is primarily composed of methane, hydrogen
and carbon monoxide, to a low temperature approaching that required for liquefaction
of the methane contained in the gas, after which the chilled gas is passed through
line 70 into cryogenic unit 71. Here the gas is further cooled by conventional means
until the temperature reaches a value sufficiently low to liquefy the methane under
the pressure conditions existing in the unit. Compressors and other auxiliaries associated
with the cryogenic unit are now shown. The amount of pressure required for the liquifaction
step will depend in part upon the pressure at which the gasifier is operated and the
pressure losses which are incurred in the various portions of the system. A substantially
pure stream of liquefied methane is taken off through line 72 and passed through line
68 into heat transfer unit 67 as described earlier. Hydrogen and carbon monoxide are
withdrawn overhead from cryogenic unit 71 through line 80 and recovered as a chemical
synthesis product gas. Normally, the cryogenic unit is operated and designed in such
a manner that less than about 10 mole percent of methane, preferably less than about
5 mole percent, remains in the product gas removed through line 80. Thus, the chemical
synthesis gas produced in the process is one of extremely high purity and therefore
has many industrial applications.
[0027] The recycle methane gas removed from heat transfer unit 67 through line 69 is passed
to compressor 73 where its pressure is increased to a value from 1,72 . 10
2 to 1,03. 10
3 kPa (25 psi to 150 psi) above the operating pressure in gasifier 32. The pressurized
gas is withdrawn from compressor 73 through line 74 and passed through tubes 75 located
in the convection section of steam reforming furnace 76. Here, the high pressure gas
picks up heat via indirect heat exchange with the hot flue gases generated in the
furnace. The methane gas is removed from the tubes 75 through line 77 and mixed with
steam, which is generated in waste heat boiler 54 and injected into line 77 via line
56. The mixture of methane gas and steam is then passed through line 77 into gas-gas
heat exchanger 51 where it is heated by indirect heat exchange with the raw product
gas removed from separator 41. The heated mixture is removed from exchanger 51 and
passed through line 78 to steam reforming furnace 76.
[0028] The preheated mixture of steam and methane gas in line 78 is introduced into the
internal tubes 79 of the steam reforming furnace where the methane and steam react
with one another in the presence of a conventional steam reforming catalyst. The catalyst
will normally consist of metallic constituents supported on an inert carrier. The
metallic constituent will normally be selected from Group VI-A and the iron group
of the Periodic Table and may be chromium, molybdenum, tungsten, nickel, iron, and
cobalt, and may include small amounts of potassium carbonate or a similar compound
as a promoter. Suitable inert carriers include silica, alumina, silica-alumina, zeolites,
and the like.
[0029] The reforming furnace is operated under conditions such that the methane in the feed
gas will react with steam in the tubes 79 to produce hydrogen and carbon monoxide
according to the following equation:
[0030] The temperature in the reforming furnace will normally be maintained between 649°C
(1200°F) and 983°C (1800°F), preferably between 38°C (100°F) and 149°C (300°F) above
the temperature in gasifier 32. The pressure will range between 69 and 207 kPa (10
and 30 psi) above the pressure in the gasifier. The mole ratio of steam to methane
introduced into the reactor will range between about 2:1 and about 15:1, preferably
between about 3:1 and about 7:1. The reforming furnace may be fired by a portion of
the methane gas removed from heat transfer unit 67 via line 69, a portion of the intermediate
Btu gas removed from solvent scrubber 63 through line 66, or a similar fuel gas.
[0031] The gaseous effluent stream from the steam reforming furnace, which will normally
be a mixture consisting primarily of hydrogen, carbon monoxide, and unreacted steam,
is passed, preferably without substantial cooling, through lines 81, 36, and 33 into
gasifier 32. This stream is the primary source of the hydrogen, carbon monoxide, and
steam required in the gasifier in addition to the carbon-alkali metal catalyst to
produce the thermoneutral reaction that results in the formation of essentially carbon
dioxide and methane. It is therefore desirable that the reforming furnace effluent
contain sufficient carbon monoxide and hydrogen to supply the substantially equilibrium
quantities of those gases required in the gasifier and sufficient unreacted steam
to provide substantially all of the steam required by the reactions taking place in
the gasifier.
[0032] As pointed out previously, substantial quantities of exothermic heat are released
in the gasifier as a result of the reaction of hydrogen with carbon oxides and the
reaction of carbon monoxide with steam. Thus, the carbon monoxide and hydrogen in
the reformer effluent stream comprises a substantial portion of the heat input into
the gasifier. To supply the desired amounts of hydrogen and carbon monoxide in the
effluent, sufficient methane should normally be present in the feed to the reforming
furnace so that enough carbon monoxide and hydrogen is produced by steam reforming
the methane to compensate for the amount of hydrogen and carbon monoxide removed in
the chemical synthesis product gas withdrawn from the process overhead of cryogenic
unit 71 through line 80. If there is insufficient methane in the feed to the reforming
furnace, the conditions in the gasifier may be altered so that additional methane
is produced in the raw product gas. Alternatively, a slip stream of the chemical synthesis
product gas may be used to make up any deficiency in the amounts of carbon monoxide
and hydrogen required. If, on the other hand, there is more than the desired amount
of methane in the feed to the reforming furnace, the conditions in the gasifier may
be altered to decrease the amount of methane produced in the raw product gas, the
excess methane may be withdrawn as a byproduct stream from line 74 prior to subjecting
it to steam reforming, or the excess methane may be reformed to produce additional
carbon monoxide and hydrogen that can be passed from line 81 into line 42 and recycled
through the downstream portion of the process. If the amount of steam added via line
56 to the reforming furnace feed stream in line 78 is not sufficiently in excess of
the amount consumed in the furnace so as to provide the desired quantity of unreacted
steam in the reformer effluent, additional steam may be injected into line 78 through
line 82.
[0033] For the purposes of thermal efficiency, it is preferable that the steam reforming
step of the process be utilized in such a manner as to obviate the need for a separate
preheat step. This may be achieved by operating the reforming furnace so that the
heat content of the effluent is sufficient to preheat the carbonaceous feed material
to reaction temperature and maintain all of the reactants at such temperature by compensating
for heat losses during gasification. Normally, this may be accomplished if the temperature
of the effluent is between 38°C (100°F) and 149°C (300°F) higher than the operating
temperature in the gasifier. For optimum thermal efficiency it is important that the
effluent from the steam reforming furnace be passed to the gasifier in such a manner
as to avoid substantial cooling. As used herein "heat content" refers to the sum of
the heats of formation plus the sum of the sensible heats for each component in the
reforming furnace effluent.
[0034] It will be apparent from the above discussion that the effluent from the reforming
furnace 76 will supply substantially all of the heat required in gasifier 32. The
effluent will not only contain sufficient sensible heat to preheat the carbonaceous
feed material to reaction temperature and maintain all the reactants at such temperature
by compensating for heat losses during gasification, but it will also contain sufficient
amounts of carbon monoxide and hydrogen which react in the gasifier to produce enough
exothermic heat to substantially balance the endothermic heat consumed by the reaction
of the steam with carbon.
[0035] It will be apparent from the foregoing that the invention provides a process for
producing a high purity chemical synthesis gas from the steam gasification of a carbonaceous
material such as coal in the presence of a carbon-alkali metal catalyst and substantially
equilibrium quantities of added hydrogen and carbon monoxide. The process of the invention
has advantages over existing coal gasification processes that may be used to generate
a chemical synthesis gas in that its gasifier operates at lower temperature, it is
more energy efficienct, and it does not require the injection of oxygen to supply
heat, thereby obviating the need for an expensive oxygen plant.
1. A process for the production of a chemical synthesis product gas from a carbonaceous
feed material and steam comprising:
(a) reacting said steam with said carbonaceous feed material in a reaction zone at
a reaction temperature between 538°C and 810°C (1000°F and 1500°F) and at a reaction
pressure in excess of 6,89.101 kPa (100 psia), in the presence of a carbon-alkali metal catalyst and sufficient
added hydrogen and carbon monoxide to provide substantially equilibrium quantities
of hydrogen and carbon monoxide in said reaction zone at said reaction temperature
and pressure;
(b) withdrawing from said reaction zone an effluent gas containing substantially equilibrium
quantities, at said reaction temperature and pressure, of methane, carbon dioxide,
steam, hydrogen and carbon monoxide;
(c) treating said effluent gas for the removal of steam and acid gases to produce
a treated gas containing primarily carbon monoxide, hydrogen and methane;
(d) recovering substantially all of the carbon monoxide and hydrogen from said treated
gas as a chemical synthesis product gas;
(e) contacting at least a portion of the remainder of said treated gas comprising
substantially methane with steam in a steam reforming zone under conditions such that
at least a portion of the methane present reacts with said steam to produce hydrogen
and carbon monoxide; and
(f) passing the effluent from said steam reforming zone into said reaction zone without
substantial cooling, thereby supplying said added hydrogen and carbon monoxide required
in said reaction zone, and wherein said reforming zone is operated at conditions such
that the heat content of said effluent from said steam reforming zone is sufficient
to supply substantially all of the heat needed to preheat said carbonaceous feed material
to said reaction temperature.
2. A process according to claim 1 in which the carbon-alkali metal catalyst is prepared
by treating said carbonaceous feed material with an alkali metal compound and thereafter
heating the treated coal to said reaction temperature in said reaction zone.
3. A process according to claim 1 or claim 2 in which the carbonaceous feed material
comprises coal.
4. A process according to any one of claims 1-3 in which the reaction pressure is
between 1,38 . 103 and 5,52. 103 kPa (200 psia and 800 psia).
5. A process according to any of claims 1-4 in which the reaction temperature is between
649°C and 760°C (1200°F and 1400°F).
6. A process according to any one of claims 1-5 in which the chemical synthesis product
gas contains less than 10 mole percent methane.
7. A process according to any one of claims 1-6 in which sufficient steam is contacted
with said methane in said steam reforming zone so that the effluent from said zone
will contain enough unreacted steam to supply substantially all the steam required
in said reaction zone.
8. A process according to any one of ctaims 1-7 in which the temperature of said effluent
from said steam reforming zone is between 38°C and 149°C (100°F and 300°F) higher
than said reaction temperature in said catalytic gasification zone.
9. A process according to any one of claims 3-8 in which the coal is impregnated with
an aqueous solution of a potassium compound and dried prior to the introduction of
said coal into said catalytic gasification zone.
10. A process according to claim 9 in which the aqueous solution comprises alkali
metal compounds recovered from char withdrawn from said catalytic gasification zone.
1. Procédé pour la production d'un gaz de synthèse chimique à partir d'une matière
première carbonée et de vapeur d'eau, caractérisé en ce qu'il comprend:
(a) la réaction de la vapeur avec la matière première carbonée dans une zone de réaction
à une température de réaction comprise entre 538 et 816°C et à une pression de réaction
supérieure à 686x 1 03 Pa, en présence d'un catalyseur de carbone et de métal alcalin
et d'une quantité suffisante d'hydrogène et d'oxyde de carbone ajoutés, pour fournir
des quantités pratiquement en équilibre d'hydrogène et d'oxyde de carbone dans la
zone de réaction aux température et pression de réaction indiquées;
(b) l'extraction de la zone d'un gaz effluent contenant des quantités pratiquement
en équilibre aux température et pression de réaction indiquées, de méthane, dioxyde
de carbone, vapeur d'eau, hydrogène et oxyde de carbone;
(c) le traitement de ce gaz effluent pour éliminer la vapeur d'eau et les gaz acides
de manière à produire un gaz traité contenant surtout de l'oxyde de carbone, de l'hydrogène
et du méthane;
(d) la récupération de pratiquement la totalité de l'oxyde de carbone et de l'hydrogène
à partir du gaz traité sous forme d'un gaz de synthèse chimique;
(e) la mise en contact d'au moins une partie du gaz traité restant comprenant en substance
du méthane avec de la vapeur d'eau dans une zone de reformage, dans des conditions
telles qu'au moins une partie du méthane présent réagisse avec la vapeur d'eau pour
produire de l'hydrogène et de l'oxyde de carbone; et
(f) le passage de l'effluent de la zone de reformage à la vapeur d'eau dans la zone
de réaction sans refroidissement substantiel, apportant ainsi l'hydrogène et l'oxyde
de carbone ajoutés, requis dans la zone de réaction, et la zone de reformage fonctionnant
dans des conditions telles que la teneur en chaleur de l'effluent provenant de la
zone de reformage à la vapeur soit suffisante pour fournir pratiquement toute la chaleur
nécessaire pour le préchauffage de la matière première carbonée à la température de
réaction.
2. Procédé selon la revendication 1, caractérisé en ce qu'on prépare le catalyseur
de carbone-métal alcalin en traitant la matière première carbonée avec un composé
de métal alcalin, et en chauffant ensuite le charbon traité à la température de réaction
dans la zone de réaction.
3. Procédé selon l'une des revendications 1 ou 2, caractérisé en ce que la matière
première carbonée comprend du charbon.
4. Procédé selon l'une quelconque des revendications 1 à 3, caractérisé en ce que
la pression de réaction est comprise entre 1 372x 103 et 5 488x 103 Pa.
5. Procédé selon l'une quelconque des revendications 1 à 4, caractérise en ce que
la température de réaction est comprise entre 649 et 760°C.
6. Procédé selon l'une quelconque des revendications 1 à 5, caractérisé en ce que
le gaz de synthèse chimique contient moins de 10% en moles de méthane.
7. Procédé selon l'une quelconque des revendications 1 à 6, caractérisé en ce qu'on
met en contact avec le méthane dans la zone de reformage à la vapeur d'eau une quantité
de vapeur d'eau suffisante pour que l'effluent provenant de ladite zone contienne
assez de vapeur d'eau intacte pour fournir pratiquement la totalité de la vapeur d'eau
nécessaire dans la zone de réaction.
8. Procédé selon l'une quelconque des revendications 1 à 7, caractérisé en ce que
la température de l'effluent provenant de la zone de reformage à la vapeur d'eau est
de 38 à 149°C au-dessus de la température de réaction dans la zone de gazéification
catalytique.
9. Procédé selon l'une quelconque des revendications 3 à 8, caractérisé en ce que
le charbon est imprégné avec une solution aqueuse d'un composé de potassium et séché
avant introduction dudit charbon dans la zone de gazéification catalytique.
10. Procédé selon la revendication 9, caractérisé en ce que la solution aqueuse comprend
des composés de métaux alcalins récupérés à partir de l'extraction de charbon dans
la zone de gazéification catalytique.
1. Verfahren zur Herstellung von Synthesegas aus einem kohlenstoffhaltigen Einsatzmaterial
und Dampf, dadurch gekennzeichnet, daß man
a) den Damp mit dem kohlenstoffhaltigen Einsatzmaterial in einer Reaktionszone bei
einer Reaktionstemperatur zwischen 538 und 816°C und einem Reaktionsdruck über 7 kg/cm2 in Gegenwart eines Kohlenstoff-Alkalimetall-Katalysators und ausreichend zugesetztem
Wasserstoff und Kohlenmonoxid, um im wesentlichen Gleichgewichtsmengen an Wasserstoff
und Kohlenmonoxid in der Reaktionszone bei der Reaktionstemperatur und dem Reaktionsdruck
aufrecht zu erhalten, umsetzt;
b) aus der Reaktionszone ein Gas abzieht, das im wesentlichen bezogen auf die Reaktionstemperatur
und den Reaktionsdruck Gleichgewichtsmengen an Methan, Kohlendioxid, Dampf, Wasserstoff
und Kohlenmonoxid enthält;
c) das Gas zur Entfernung von Dampf und sauren Gasen behandelt, um ein behandeltes
Gas zu erzeugen, das hauptsächlich Kohlenmonoxide, Wasserstoff und Methan enthält;
d) im wesentlichen alles Kohlenmonoxid und allen Wasserstoff aus dem behandelten Gas
als Synthesegas gewinnt;
e) mindestens einen Teil des Rückstandes des behandelten Gases, das im wesentlichen
aus Methan besteht, mit Dampf in einer Dampfreformierzone unter solchen Bedingungen
kontaktiert, daß mindestens ein Teil des vorhandenen Methans mit dem Dampf unter Erzeugung
von Wasserstoff und Kohlenmonoxid reagiert; und
f) das aus der Dampfreformierzone austretende Gas ohne wesentliche Kühlung in die
Reaktionszone einleitet und dadurch den zugesetzten Wasserstoff und zugesetztes Kohlenmonoxid,
die in der Reaktionszone erforderlich sind, liefert, wobei die Reformierzone unter
solchen Bedingungen betrieben wird, daß der Wärmegehalt des aus der Dampfreformierzone
austretenden Gases ausreicht, um im wesentlichen die gesamte Wärme zu liefern, die
erforderlich ist, um das kohlenstoffhaltige Einsatzmaterial auf die Reaktionstemperatur
vorzuwärmen.
2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß man den Kohlenstoff-Alkalimetall-Katalysator
herstellt, indem man das kohlenstoffhaltige Einsatzmaterial mit einer Alkalimetallverbindung
behandelt und anschließend die behandelte Kohle auf die Reaktionstemperatur in der
Reaktionszone erhitzt.
3. Verfahren nach Anspruch 1 oder 2, dadurch gekennzeichnet, daß das kohlenstoffhaltige
Einsatzmaterial Kohle enthält.
4. Verfahren nach jedem der Ansprüche 1 bis 3, dadurch gekennzeichnet, daß der Reaktionsdruck
zwischen 14 kg/cm2 und 56 kg/cm2 liegt.
5. Verfahren nach jedem der Ansprüche 1 bis 4, dadurch gekennzeichnet, daß die Reaktionstemperatur
zwischen 649 und 760°C beträgt.
6. Verfahren nach jedem der Ansprüche 1 bis 5, dadurch gekennzeichnet, daß das gewonnene
Synthesegas weniger als 10 Mol % Methan enthält.
7. Verfahren nach jedem der Ansprüche 1 bis 6, dadurch gekennzeichnet, daß man ausreichend
Dampf mit dem Methan in der Dampfreformierzone kontaktiert, so daß das aus der Zone
austretende Gas ausreichend nicht umgesetzten Dampf enthält, um im wesentlichen den
gesamten in der Reaktionszone erforderlichen Dampf zu liefern.
8. Verfahren nach jedem der Ansprüche 1 bis 7, dadurch gekennzeichnet, daß die Temperatur
des aus der Dampfreformierzone austretenden Gases zwischen 38 und 149°C höher als
die Reaktionstemperatur in der katalytischen Vergasungszone liegt.
9. Verfahren nach jedem der Ansprüche 3 bis 8, dadurch gekennzeichnet, daß die Kohle
mit einer wässrigen Lösung einer Kaliumverbindung imprägniert und vor der Einbringung
in die katalytische Vergasungszone getrocknet wird.
10. Verfahren nach Anspruch 9, dadurch gekennzeichnet, daß die wässrige Lösung Alkalimetallverbindungen
enthält, die aus Verkohlungsprodukten gewonnen worden sind, die aus der katalytischen
Vergasungszone abgezogen worden sind.