CA1171282A

CA1171282A - Coal conversion process

Info

Publication number: CA1171282A
Application number: CA000370278A
Authority: CA
Inventors: Charles A. Euker, Jr.
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1980-07-16
Filing date: 1981-02-06
Publication date: 1984-07-24
Also published as: AU6697881A; EP0044123A1; BR8100687A; JPS5734193A; AU538349B2; ZA81613B

Abstract

ABSTRACT OF THE DISCLOSURE
An improved gasification process for solid carbonaceous materials wherein reactor volume is reduced by elevating either the point or points at which the solid carbonaceous feed is introduced, the point or points wherein hydrogen and carbon monoxide are introduced or the point or points at which both the solid carbonaceous feed and the carbon monoxide and hydrogen are introduced. When the points of feed introduction and the points of carbon monoxide and hydrogen introduction are elevated, the reduction in volume is greater than would have been predicted from a relocation of either of these points separately.

Description

~ ~7128~

BAcKGRouND

2 This invention relates to an improved process

3 for converting solid carbonaceous materials. More par-

4 ticularly, this invention relates to a process ~or gasifying solid carbonaceous materials.
6 Before the turn of the century it was known 7 that hydrocarbon gases and liquids, tars and chemicals 8 could be obtained not only from petroleum, but from coal g and other carbonaceous liquids solids. Very early processes employed destructive distillation, coal bein~
11 transformed into gases and petroleum-like liquid products.
12 Primary emphasis in many of these processes is on gasifi-13 cation of the coal with the objective of improving 14 processes for the production of water gas, producer gas, lS or hydrogen, as opposed to the production of coal liquids.
16 For the past several decades, due to disallocations of 17 supplies, there have been reoccurring periods of interest 18 in the gasification of coal to produce fuel gases, first 19 primarily in Europe; and then, in thi~ country. The art reflects the various periods of interest in terms of peaks 21 defined by large numbers of patents, and literature.
22 Presently existing and projected shortages of natural gas 23 in this country have sparked a renewed and very keen 24 interest in the gasification of coal, and it appears that this will be a long-range trend. ~onsequently, intensive 26 research and development efforts are now underway to 27 produce synthetic high-BTU, intermediate-BTU and synthesis 28 fuel gases for commercial usages.
29 It was early recognized that some mineral and trace inorganic constituents naturally present in some 31 coal could exert favorable catalytic influences in gasi-32 fication reactions vis-a-vis thermal reactions, and a 33 variety of catalytic materials have been added to coal to 34 alter the natural chemistry inherent in various of the early coal gasification processes. The thrust of present 36 research is to develop processes for the production of 37 synthetic high-BTU gases with far higher efficiencies than 2~

1 was possible in the classical European, or early Euro-2 American processes. There are, however, inherent chemical 3 kinetic limitations in coal gasification processes which 4 have defied solution, and these problems are yet unsolved.
Yet, solution is essential, and ther~ remains a strong 6 interest in providing better coal gasification processes, 7 or catalysts for use in catalytic coal gasification 8 processes.
9 In a coal gasification process, i.e., one whose object is to produce a high BTU gas, an intermediate BTU
11 gas or a synthesis fuel gas; steam or a similar reagent 12 and particulate coal are fed to a gasifier at elevated 13 temperature and pressure and converted to a syn~hesis gas, 14 or gaseous mixture of high methane content, which contains signifisant amounts of carbon monoxide and hydrogen.
16 Recently, it has been proposed to separate the methane 17 from the carbon monoxide and hydrogen in a catalytic 18 process and to then recycle the carbon monoxide and 19 hydrogen to improve thermal efficiency where a high BTU
gas is desired or recycle a portion of the entire stream 21 when an intermediate BTU gas is desired. Generally, the 22 methane in the recycle stream may be reformed to carbon 23 monoxide and hydrogen prior to the recycling step. More-24 over, the entire hydrocarbon gas may be reformed when synthesis gas is the desired product. Processes of this 26 type are described in U. S. Patents Nos. 4,094,650 and 27 4,118,204. Practical objectives, however, require thermal 28 efficiencies coupled with a reduction of reactor size.
29 More recently, it has been discovered that the recycling of hydrogen to the reactor retards the steam 31 gasification of coal and similar liquid and solid carbona-32 ceous materials thereby increasing the size of the gasifi-33 cation reactor. The need for an improved gasification 34 process exhibiting the same thermal efficiently but re-quiring a smaller gasification reactor is, therefore, 36 believed to be readily apparent.

- I }~ 2 2 It has now, surprisingly, been discovered that 3 the foregoing disadvantages of the prior art gasification 4 processes may be overcome with the method of the present invention and an improved gasification process provided 6 thereby. It is, therefore, an object of this invention 7 to provide an improved method for gasifying carbonaceous 8 materials. It is another object of this invention to 9 provide such an improved process wherein the thermal efficiency normally associated with the introduction of 11 carbon monoxide and hydrogen directly to the gasification 12 reactor is reali3ed. It is still another object of this 13 invention to provide such an improved process wherein a 14 smaller gasification reactor is required. These and other objects and advantages will become apparent from 16 the description set forth hereinafter.
17 In accordance with this invention, the foregoing 18 and other objects and advantages are accomplished by gasi-19 fying a carbonaceous material in a fluid bed at elevated temperatures and pressures such that either the carbona-21 ceous feed, added hydrogen and carbon monoxide or all 22 three are introduced into the gasification reactor at a 23 point generally above the bottom of the bed but suffi-24 ciently below the top thereof to permit substantial equilibration of the gas phase and to avoid tar break-26 through from the bed. Surprisingly, thermal efficiency 27 is maintained by introducing added carbon monoxide and 28 hydrogen into the gasification reactor and into the fluid 29 bed at an elevated point or points. As pointed out more fully hereinafter, raising the point or points of intro-31 duction of the carbonaceous feed and the carbon monoxide-32 hydrogen mixture results in a synergistic reduction in 33 gasification reactor size.

The Figure is a schematic flow diagram of a 36 gasification process within the scope of this invention 37 wherein a substitute natural gas is the principal product.

~ ~71282 1 DETAILED DESCRIPTION OF 'l'HE INVENTION
-2 As indicated, supra, the present invention 3 relates to an improved process for gasifying carbonaceous 4 materials. Thermal efficiency is enhanced by introducing a mixture of carbon monoxide and hydrogen into a fluid bed 6 comprising the carbonaceous material at varying degrees 7 of gasification~ The gasifier reactor size required is 8 reduced by elevating the point or points at which either 9 the carbon monoxide-hydrogen mixture or the carbonaceous feed is introduced to the fluid bed. Maximum reduction 11 in the gasiier reactor size required is realized when 12 both the carbon monoxide-hydrogen mixture and the carbon-13 aceous feed are introduced into the fluid bed at a point 14 or points above the bottom of the fluid bed and suffi-lS ciently below the top to permit substantial equilibration 16 of the gas phase and to avoid tar break through from the 17 bed.
18 In general, the process of this invention may be 19 used to gasify any carbonaceous material that will fluidize in a gas stream. The process is, therefore, particularly 21 suited to the gasification of solid carbonaceous materials 22 such as coal, coal char, metallurgical coke, petroleum 23 coke, charcoal, activated carbon and the like. In some 24 cases, inert carriers having carbon deposited on the surfaces thereof may also be gasified in the process of 26 this invention.
27 As indicated previously, the gasiication of coal 28 and similar carbonaceous materials normally produces a 29 syntllesis gas composed primarily of hydroyen and carbon monoxide. The principal reactions which take place in 31 such a system include the following:
32 C + H20 > C0 + ~2 (Endothermic) (1) 33 C + 2H2 ~ CH4 (Exothermic) (2) 34 C + C02 ---~ 2C0 (Endothermic) (3) 35 C0 + H20 ~_ ~ C02 + H2 (Exothermic) (4) 36 The reaction kinetics during conventional gasification 37 operations are such that the product gas normally contains ~ ~7.~2B~

1 varying amounts of methane. In steam gasification, the 2 methane which is present occurs primarily as a result of 3 devolatilization of the coal. The direct hydrogenation of 4 carbon in accordance with equation (2) above is known to be very slow as compared to the endothermic reactions of 6 steam and carbon dioxide with carbon as set forth in 7 equations (1) and`(3). The products of conventional steam 8 gaisification operations are thus primarily hydrogen and 9 carbon monoxide and such operations are highly endothermic.
As pointed out earlier, it has been proposed that this 11 endothermicity be reduced by carrying out the operation 12 in the presence of hydrogen to promote the exothermic 13 carbon-hydrogen reaction of equation (2) but this normally 14 requires a substantially higher reaction temperature than is needed for the steam-carbon reac~ion. Moreover, it is 16 now known that hydrogen inhibits the reaction of steam 17 with carbon to produce carbon monoxide and hydrogen. As 18 a result, gasifier reactors of a larger size than would 19 otherwise be required are required when hydrogen and/or carbon monoxide-hydrogen mixtures are introduced at or 21 near the bottom of the gasifier.
22 In general, the gasifier reactor size is reduced 23 when the feed point or points are raised above the bed 24 since the inhibiting effect of the devolatilization gases is limited to that portion of the bed at and above the 26 point of feed introduction. In this regard, it should be 27 noted that the devolatilization will occur rapidly after 28 feed introduction and the gases will flow upwardly. The 29 solid particles, on the other hand, will be distributed throughout the entire fluid bed since such a bed approaches 31 a perfectly mixed characterization. Ideally, the feed 32 point or points will be located at the highest elevation 33 possible without tar breakthrough from the bed.
34 Similarly, the gasifier reactor size is reduced when the point or points at which carbon monoxide and 36 hydrogen are introduced is raised in the reactor. In ~7 this regard, it should be noted that the introduction of ~ 1712~

carbon monoxide and hydrogen into the gasifier will improve thermal efficiency in gasification processes of the type described in U.S.
Patents No. 4,094,650 and No. 4,118,204. In addition to the gas phase reaction illustrated by equation (4) above, the following gas phase reactions occur:
2CO ~ 2H2 --~ C2 + CH4 (5) CO + 3H2 --~ H20 + CH4 (6) C2 ~ 4H2 ~ 2H2O + CH4 (7) All of these gas phase reactions are exothermic and, when an amount of carbon monoxide and hydrogen in excess of the equilibrium amount is introduced, heat is produced. Ideally, then, the point or points of introduction will be at the highest elevation which will permit the gas phase to substantially reach equilibrium at the top of ~he fluid bed. In this way, the heat generated will be distributed throughout the fluid bed by the well mixed solid particles compris-ing the bed.
In general, commercial scale gasification fluid beds will range in height from about 60 to about 125 feet and both the car-bonaceous feed and carbon monoxide and hydrogen feed to the bed will, independently and generally, be introduced at a point or points along the bed and within a range of distances ranging from about 10% of the ~otal bed height below the top of the bed and about 60% of the total bed height from the top. Such disposition will, of course, result in a maximum size reduction or a minimum bed height for any given gasification operation. It is, however, within the scope of this invention to position either the carbona-ceous feed or the carbon monoxide and hydrogen introduction within the specified range with the other being introduced at or near the bottom of the bed in a manner consistent with the prior art.
It is believed that the present invention will be better understood by reference to the appended drawing. Referring to the drawing, then, the process illustrated is 1 one for the production of a chemical synthesis gas by the 2 gasification of carbonaceous material such as coal, lig-3 nite, coal char, coke or similar carbonaceous material 4 with steam at an elevated temperature in the presence of a catalyst.
6 In the embodiment illustrated, a particularly 7 preferred catalyst is prepared by impregnating the feed 8 solids with a solution of an alkali metal compound or 9 mixture of such compounds and thereafter hea~ing the impregnated material to a temperature sufficient to pro-11 duce an interaction between the alkali metal and the 12 carbon present. Generally, the solid feed mate~ial will 13 be finely divided to a particle size suitable for fluidi-14 zation and a particle size of about 8 mesh or smaller on the U. S. Sieve Series Scale is particularly suitable. In 16 the embodiment illustrated, the feed is passed into line 17 10 from a feed preparation plant or storage facility that 18 is not shown in the drawing. The solids introduced into 19 line 10 are fed into a hopper or similar vessel 11 from which they are passed through line 12 into feed preparation 21 zone 14. This zone contains a screw conveyor or similar 22 device, not shown in the drawing, that is powered by a 23 motor 16, a series of spray nozzles or similar devices 17 24 for the spraying of an alkali metal-containing solution supplied through line 18 onto the solids as they are moved 26 through ~he preparation zone by the conveyor, and a similar 27 set of nozzles or the like 19 for the introduction of a hot 28 dry gas, supplied through line 20, which serves to heat 29 the impregnated solids and drive off the moisture~ A
mixture of watex vapor and gas is withdrawn from zone 14 31 through line 21 and passed to a condenser, not shown, from 32 which water may be recovered for use as makeup or the like.
33 The majority of the alkali metal-containing solution is 34 recycled through line 49 from the alkali metal recovery portion of the process, which is describsd hereafter. Any 36 makeup alkali metal solution required may be introduced 37 into line 18 via line 13.

712~

1 In general, sufficient alkali metal-containing 2 solution is introduced into preparation ~one 14 to provide 3 from about 1 to about 50 weight percent of an alkali metal 4 compound or mixture of such compounds on the coal or other carbonaceous solids. From about 5 to about 30 percent is 6 preferred. The dried impregnated solid particles prepared 7 in zone 14 are withdrawn through line 24 and passed to a 8 closed hopper or similar vessel 25 from which they are 9 discharged through a star wheel feeder or equivalent device 26 in line 27 at an elevated pressure sufficient to permit 11 their entrainment into a stream of high pressure steam, 12 inert gas or other carrier gas introduced into line 29 via 13 line 28. Moreover, it is within the scope of this inven-14 tion to use all or a portion of the carbon monoxide and hydrogen as a carrier gas. The carrier gas and entrained 16 soli`ds are passed through line 29 into manifold 30 and 17 introduced into the gasifier 32 through any one or more of 18 a plurality of feed points 31, 31' and 31''. As previously 19 indicated, the uppermost or highest feed point will be at least about 10% of the total fluid bed height below the 21 top of the fluid bed and the lowest feed point will be no 22 more than 60% of the total bed height below the top of the 23 fluid bed. In lieu of or in addition to hopper 25 and 24 star wheel feeder 26, the feed system may employ parallel lock hoppers, pressurized hoppers~ aerated standpipes 26 operated in series, or other apparatus to raise the input 27 feed solids stream to the required pressure level~
28 In general, the gasifier 32 will be operated at 29 a pressure between about 100 and 1500 psia, preferably at a pressure within the range of about 200 and 800 psia for 31 any desired product distribution. The carrier gas may be 32 preheated to a temperature in excess of about 300F, but 33 below the initial softening point of the coal or other 3~ feed material employed~ Feed particles~may be suspended in the carrie~ gas in a concentration between about 0.2 36 and about 5.0 pounds of solid feed material per pound of 37 carrier gas. The optimum ratio for a particular system 2~

g l will depend in part upon the particle size and density, 2 the molecular weight of the gas employed, the temperature 3 of the solid feed material and the input gas stream, the 4 amount of alkali metal compound employed and other factors.
In generalj ratios between about 0.5 and about 4.0 pounds 6 of solid feed material per pound of carrier gas are pre-7 ferred.
8 Gasifier 32 contains a fluidized bed of carbon-9 aceous solids extending upward within the vessel above an internal grid or similar distribution device not shown in ll the drawing. The bed is maintained in the fluidized state 12 by means of steam introduced through bottom inlet 36. The 13 bed may be partially maintained with carbon monoxide and 14 hydrogen introduced through line 33, manifold 34 and peri-pherally spaced injection lines and nozzles 35, when a part 16 or all of carbon monoxide and hydrogen are introduced at 17 the bottom o~ the bed. As previously indicated, however, 18 hydrogen retards the desirable gasification reactions and l9 is, therefore, preferably introduced into the fluid bed at a higher elevation. When this is done, the carbon monoxide 21 and hydrogen may be introduced through any one or more of 22 a plurality of injection points 35'-35'' which are supplied 23 by manifold 33' and which are independently positioned in 24 the same portion of the ~luid bed as the carbonaceous feed points 31, 31' and 31 ". Altexnatively, all or a part of 26 the carbon monoxide and hydrogen could be introduced into 27 the fluid bed with the carbonaceous feed, as previously 28 indicated, through line 28.
29 The particular injection system shown in the drawing is not critical and hence other methods for in-31 jecting the steam, hydrogen and carbon monoxide may be 32 employed. In some instances, for example, it may be pre-33 ferred to introduce the gases through multiple nozzles to 34 obtain more uniform distribution of the injected fluid and reduce the possibility of channeling and related 36 problems. The space velocity of the rising gases within 37 the fluidized bed will normally be between about 100 and 8~2 1 about 3000 actual volumes of steam, hydrogen and carbon 2 monoxide per hour per volume of fluidized solids.
3 With the fluidized bed in gasifier 32, the 4 carbonaceous solids are subjected to a temperature within the range between about 1000F and about 1500F, preferably 6 between about 1200F and 1400F. At such a temperature the 7 carbon-alkali metal catalyst will equilibrate the gas 8 phase reactions occurring during gasification to produce 9 additional methane and at the same time supply substantial amounts of additional exothermic heat in situ.
11 Due to the gas phase equilibrium conditions 12 existing as a result of the carbon-alkali metal catalyst 13 and due to the presence of added hydrogen and carbon mono-14 xide, there will be a net heat production. Moreover, competing reactions, that in the absence of the catalyst 16 and the added hydrogen and carbon monoxide would ordinarily 17 tend to produce additional hydrogen and carbon monoxide, 18 are suppressed. The heat produced tends to balance the 19 endothermic heat consumed by the reaction of the steam with carbon and as the amount of carbon monoxide and 21 hydrogen added increases, an overall thermoneutral reaction 22 can be approached. when the gasifier is basically in heat 23 balance, the heat required to preheat the feed to the 24 reaction temperature and compensate for heat losses from the gasifier is supplied for the most part by excess heat 26 in the gases introduced into the gasifier through line 36.
27 In the absence of the exothermic heat provided by the 28 catalyzed gas phase reactions, ~hese gases would have to 29 be heated to substantially higher temperatures than those normally employed here.
31 The gas leaving the fluidized bed in gasifier 32 32 passes through the upper section of the gasifier, which 33 serves as a disengagement zone where the particles too 34 heavy to be entrained by the gas leaving the vessel are returned to the bed. If desired, this disengagement zone 36 may include one or more cyclone separators or the like for 37 removing relatively large particles from the gas. The gas I 1731;~B~

1 withdrawn ~rom the upper part o~ the gasifier th~ough line 2 37 will normally contain an equilibrium mixture at reaction 3 temperature and pressure of methane, carbon dioxide, 4 hydrogen, carbon monoxide and unreacted steam. Hydrogen sulfide, ammonia and other contaminants formed from sulfur 6 and nitrogen contained in the feed material may also be 7 present in this gas and entrained fines may also be 8 present.
9 As is well known, basically the same gaseous effluent will be produced in the gasifier when steam is 11 used to effect the gasification. As is also well known, 12 the ultimate gaseous product depends upon the further 13 processing to which this effluent is subjected. As a 14 result, the improvement of this invention is equally applicable to any catalytic process wherein a carbonaceous 16 material is gasified in the presence of steam.
17 In the embodiment illustrated, the effluent gas 18 is introduced into cyclone separator or similar device 38 19 for removal of the larger fines. The overhead gas then passes through line 39 into a second separator 41 where 21 smaller particles are removed. The gas from which the 22 solids have been separated is taken overhead from separator 23 41 through line 42 and the fines are discharged downward 24 through dip legs 40 and 43. These fines may be returned to the gasifier or passed to the alkali metal recovery 26 portion of the process.
27 In the system shown in the drawing, a stream of 28 high ash content char particles is withdrawn through line 29 44 from gasifier 32 in order to control the ash content of the system and permit the recovery and recycle of alkali 31 metal constituents of the catalyst. The solids in line 44, 32 which may be combined with fines recovered from the gasi-33 fier overhead gas through dip legs 40 and 43 and line 45, 34 are passed to alkali metal recovery unit 46. The recovery unit will normally comprise a multistage countercurrent 36 leaching system in which the high ash content particles 37 are countercurrently contacted with water introduced ~ ~7~282 1 through line 47. An aqueous solution of alkali metal 2 compounds is withdrawn from.the unit through line 48 and 3 recycled through lines 49 and 18 to feed preparation zone 4 14. Ash residues from which soluble alkali metal compounds have been leached are withdrawn from the recovery.unit 6 through line 50 and may be disposed of as land ~ill or 7 further treated to recover added alkali metal constituents.
8 The gas leaving separator 41 is passed ~hrough 9 line 42 to gas-gas heat exchanger 51 where it is cooled by indirect heat exchange with a gaseous mixture of methane 11 and steam introduced through line 77. The cooled gas is 12 then passed through line 53 into waste heat hoiler 54 13 where it is further cooled by indirect heat exchange with 14 water introduced throu~h line 55. Sufficient heat is transferred from the gas to the water to convert it into 16 steam, which is withdrawn through line 56. During this 17 cooling step, unreacted steam in the gas from exchanger 18 51.is condensed out and withdrawn as condensate through 19 line 57. The cool gas exiting waste heat boiler 54 through line 58 is passed to water scrubber 59. Here the 21 gas stream passes upward through the scrubber where it 22 comes in contact with water injected into the top of the 23 scrubber through line 60. The water absorbs ammonia and 24 a portion of the hydrogen sulfide in the gas stream and is withdrawn from the bottom of the scrubber through line 61 26 and passed to downstream units for further processing.
27 The water scrubbed gas stream is withdrawn from the 28 scrubber through line 62 and is now ready for treatment 29 to remove bulk amounts of hydrogen sulfide and other acid gases.
31 The gas stream is passed from water scrubber 59 32 through line 62 into the bottom of solvent scrubber 63.
33 Here the gas passes upward through the contacting zone in 34 the scrubber where it comes in contact with a downflowing 3S stream of solvent such as monoethanolamine, diethanolamine, 36 a solution of sodium salts of amino acids, methanol, hot 37 potassium carbonate or the like introduced into the upper t ~7~82 1 paxt of the solvent scrubber through line 64. If desired, 2 the solvent scrubber may be provided with spray nozzles, 3 perforated plates, bubble cap plates, packing or other 4 means for promoting intimate contact between the gas and the solvent. As the gas rises through the contacting zone, 6 hydrogen sulfide, carbon dioxide and other acid gases are 7 absorbed by the solvent, which exits the scrubber through 8 line 65. The spent solvent containing carbon dioxide, 9 hydrogen sulfide and other contaminants is passed through line 65 to a stripper, not shown in the drawing, where it 11 is contacted with steam or other stripping gas to remove 12 the absorbed contaminants and thereby regenerate the 13 solvent. The regenerated solvent may then be reused by 14 injecting it back into the top of the scrubber via line 64.
A clean gas containing essentially methane, 16 hydrogen, and carbon monoxide in amounts substantially 17 equivalent to the equilibrium quantities of those gases 18 in the raw product gas withdrawn from gasifier 32 through 19 line 37 is withdrawn overhead from the solvent scrubber via line 66~ The methane content of the gas will normally 21 range between about 20 and about 60 mole percent and the 22 gas will be of an intermediate BTU heating value, normally 23 containing between about 400 and about 750 BTUs per 24 standard cubic foot.
As will be readily apparent, this intermediate 26 BTU gas could be withdrawn as a product. When this is 27 done a portion of the product could be separated and then 28 subjected to steam reforming to produce the carbon monoxide 29 and hydrogen required for improved thermal efficiency.
Alternatively, the carbon monoxide and hydrogen could be 31 provided from any of the sources therefor known in the 32 prior art.
33 The intermediate BTU gas withdrawn overhead from 34 soLvent scrubber 63 through line 66 is introduced into heat transfer unit 67 where it passes in indirect heat 36 exchange with liquid methane introduced through line 68.
37 The methane vaporizes within the heat transfer unit and is 2~;~

1 discharged as the intermediate BTU gas, which is primarily 2 composed of methane, hydrogen and carbon monoxi~e, is 3 cooled to a low temperature approaching that required for 4 liquefaction of the methane contained in the gas, after which ~he chilled gas is passed ~hrough line 70 into 6 cryogenic unit 71. Here the gas is further cooled by 7 conventional means until the temperature reaches a value 8 sufficiently low to liquefy the methane under the pressure 9 conditions existing in the unit~ Compressors and other auxiliaries associated with the cryogenic unit are not 11 shown. The amount of pressure required lor the liquefac-12 tion step will depend in part upon the pressure at which 13 the gasifier is operated and the pressure losses which are 14 incurred in the various portions of the system. A sub-stantially pure stream of liquefied methane is taken off 16 through line 72 and may be withdrawn as product. In the 17 In the embodiment illustrated, however, the methane is 18 passed through line 68 into heat transfer unit 67 as 19 described earlier. Hydrogen and carbon monoxide are withdrawn overhead from cryogenic unit 71 through line 80 21 and recovered as a chemical synthesis product gas.
22 Normally, the cryogenic unit is operated and designed in 23 such a manner that less than about 10 mole percent of 24 methane, preferably less than about 5 mole percent, remains in the product gas removed through line 80. Thus, 26 the chemical synthesis gas produced in the process is one 27 of extremely high purity and therefore has many industrial 28 applications.
29 As previously indicated, the methane could be withdrawn as product and the carbon monoxide and hydrogen 31 separated in the cryogenic separator returned to the 32 gasifier to facilitate thermal efficiency. In the embodi-33 ment illustrated, however, the recycle methane gas removed 34 from heat transfer unit 67 and through line 69 is passed to compressor 73 where its pressuxe is increased to a value 36 from about 25 psi to about 150 psi above the operating 37 pressure in gasifier 32. The pressurized gas is withdrawn ~ 1 7~282 l from compressor 73 through line 74 and passed through 2 tubes 75 located in the convection section of steam 3 reforming furnace 76. Here, the high pressure gas picks 4 up heat via indirect heat exchange with the hot flue gases generated in the furnace. The methane gas is removed from 6 the tubes 75 through line 77 and mixed wlth steam, which 7 is generated in waste heat boiler 54 and injected into 8 line 77 via line 56. The mixture of methane gas and 9 steam is then passed through line 77 into gas-gas heat exchanger 51 where it is heated by indirect heat exchange ll with the raw product gas removed from separator 41. The 12 heated mixture is removed from exchanger 51 ana passed 13 through line 78 to steam reforming furnace 76.
14 The preheated mixture of steam and methane gas in-line 78 is introduced into the internal tubes 79 of 16 the steam reforming furnace where the methane and steam 17 react with one another in the presence of a conventional 18 steam reforming catalyst. The catalyst will normally l9 consist of metallic constituents supported on an inert carrier. The metallic constituent will normally be 21 selected from Group VI-B and the iron group of the 22 Periodic Table and may be chromium, molybdenum, tungsten, 23 nickel, iron, and cobalt, and may include small amounts af 24 potassium carbonate or a similar compound as a promoter.
Suitable inert carriers include silica, alumina, silica-26 alumina, zeolites, and the like.
27 The reforming furnace is operated under condi-28 tions such that the methane in the feed gas will react 29 with steam in the tubes 79 to produce hydrogen and carbon monoxide according to the following equation:
31 H 0 + CH ~ 3H + C0 32 The temperature in the reforming furnace will normally be 33 maintained between about 1200F and about 1800F, prefer-34 ably between about 100F and about 30QF above the temperature in gasifier 32. The pressure will range 36 between about 10 and about 30 psi above the pressure in 37 the gasifier. The mole ratio of steam to methane 1~7~1282 1 introduced into the reactor will range between about 2:1 2 and about 15:1, preferably between about 3:1 and about 3 7:1. The reforming furnace may be fired ~y a portion of 4 the methane gas removed frvm heat transfer unit 67 via line 69, a portion of the intermediate BTU gas removed 6 from solvent scrubber 63 through line 66, or a similar 7 fuel gas.
8 The gaseous effluent stream from the steam 9 reforming furnace, which will normally be a mixture con-sisting primarily of hydrogen, carbon monoxide, and un-11 reacted steam, is passed, preferably without substantial 12 cooling, through lines 81 to manifolds 33 and/or 34' and 13 ultimately into gasifier 32. This stream will be the 14 primary source of the hydrogen, carbon monoxide and steam required in the gasifier. In a preferred embodiment, 16 therefore, it is desirable that the reforming furnace 17 effluent contain sufficient carbon monoxide and hydrogen 18 to provide the desired thermal balance.
19 As pointed out previously, substantial quantities of exothermic heat are released in the gasi~ier às a 21 result of the reaction cf hydrogen with carbon oxides and 22 the reaction of carbon monoxide with steam. Thus, the 23 carbon monoxide and-hydrogen in the reformer effluent 2~ stream comprises a substantial portion of the heat input into the gasifler. To supply the desired amounts of 26 hydrogen and carbon monoxide in the effluent, sufficient 27 ` methane should normally be present in the feed to the 28 reforming furnace so that enough carbon monoxide and 29 hydrogen is produced by steam reforming the methane to compensate for the amount of hydrogen and carbon monoxide 31 removed in the chemical synthesis product gas withdrawn 32 from the process overhead of cryogenic unit 71 through 33 line 80.
3 d, PREFERRED EMBOD IMENT
In a preferred embodiment of the present inven-36 tion, coal will be gasified with steam in the presence of 37 an alkali metal catalyst and at a temperature withln the - ~ 17~2~

1 range from about 1200 to about 1400F and at a pressure 2 within the range from about 200 to about 600 psia. The 3 gasification will be accomplished in a fluid bed having a 4 bed height within the range from about 60 to about 130 feet.
The fluid bed will be maintained with steam introduced at 6 the bottom of the gasification vessel and distributed 7 through a suitable grid. The coal feed will be introduced 8 into the fluid bed at one or more points located within the g range from about 20 percent o the total height below the top of the bed to about 50 percent of the total height 11 below the bed. The catalytic process will be operated 50 12 as to produce a substitute natural gas and substantially 13 all of the carbon monoxide and hydrogen contained in the 14 gaseous effluent from the gasifier will be recovered and recycled to the gasification vessel, The recycled carbon 16 monoxide and hydrogen will be introduced into the fluid 17 bed at one or more points positioned along the fluid bed 18 within from about 20 percent of the total height from the 19 top of the bed to about 50 percent of the total height from the top of the bed.
21 In the preferred embodiment, the amount of 22 carbon monoxide and hydrogen recycled will be equal to the 23 amount of carbon monoxide and hydrogen which would be 24 produced as a result of the steam gasification of the coal if no carbon monoxide and hydrogen were introduced and when 26 sufficient nominal holding time is provided to permit 27 equilibration of the gaseous effluent from the gasifier.
28 Also in the preferred embodiment, the exact point or points 29 of the coal feed introduction will be optimized as a function of the activity of the coal to steam gasification.
31 Having thus broadly described the invention and 32 set forth a preferred embodiment thereof, it is believed 33 that the invention will be even better understood by 34 reference to the following example. The example is presented, howevèr, solely for the purpose of illustration 36 and should not be construed as limiting the invention 37 in any way.

-- t J 7~82 ExAMpLE
2 In this example, a series of steam gasifications 3 were completed over a range of gasification temperatures 4 of 1275F and pressures of 5000 psia and at a steam to coal ratio of 1.58. From these tests, a kinetic model 6 -~as developed and from this model, it has been predicted 7 that the optimum fluid bed volume can be reduced by 11 8 percent by raising the coal feed point to a height within g the range from about 10% below the top of the bed to about 60 percent o the total height below the top of the bed.
11 It has also been predicted from this model that the reactor 12 volume can be reduced by 27 percent if an equilibrium 13 mixture of carbon monoxide and hydrogen is introduced at a 14 point or points located at a point below the top of the bed by an amount equal to about 10 percent of the total 16 height. It has further been predicted that the total 17 volume can be reduced by 42 percent if both the feed 18 point and the carbon monoxide and hydrogen point or 19 points of introduction are both relocated. Based on predictions from the results obtained by relocating 21 each feed point separately, it was anticipated that only 22 a 35 percent reduction would have been realized by re-23 locating both feed points simultaneously.
24 From the foregoing, it is believed readily apparent that elevation o either the feed point or the 26 carbon monoxide and hydrogen introduction poin`t will 27 result in a significant reduction in reactor size or 28 required bed height. It is also believed readily apparent 29 that if both of these feed points are elevated a signifi-cant and synergistic reduction in total bed height is 31 realized.

32 While the present invention has been described 33 and illustrated by reference to a particular embodiment 34 thereof, it will be appreciated by those of ordinary skill in the art that the same lends it$elf to variations not 36 necessarily illustrated herein.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A gasification process wherein a solid carbonaceous material is gasified in a fluidized bed reactor in the presence of steam, added carbon monoxide and hydrogen, and a catalyst characterized in that said carbonaceous material and said hydrogen and carbon monoxide are separately and independently introduced into said fluidized bed reactor, and wherein said hydrogen and carbon monoxide are introduced into the fluidized bed in said reactor at a point or points along the bed between about 10 percent and about 60 percent of the total bed height below the top of said fluidized bed.

2. A process according to claim 1 further characterized in that an alkali metal catalyst is present during gasification.

3. A process according to claim 1 further characterized in that said added hydrogen and carbon monoxide are recovered from the gaseous effluent from said fluidized bed reactor.

4. A process according to claim 1 further characterized in that substantially all of the hydrogen and carbon monoxide contained in the effluent from said fiuidized bed reactor is separated and recycled to the fluidized bed.

5. A process according to claim 1 further characterized in that said added hydrogen and carbon monoxide are obtained by reforming at least a portion of the methane contained in the effluent from said fluidized bed reactor.

6. A process according to claim 1 further characterized in that the effluent from said fluidized bed reactor is treated for the removal of steam and acid gases to produce a treated gas containing primarily carbon monoxide, hydrogen and methane, the carbon monoxide and hydrogen are recovered from the treated gas as a chemical synthesis product gas thereby producing a gas comprised substantially of methane, the methane is contacted with steam in a steam reforming zone under conditions such that at least a portion of the methane present reacts with the steam to produce hydrogen and carbon monoxide, and the effluent from the steam reforming zone is used to supply the carbon monoxide and hydrogen which is introduced into said fluidized bed.

7. A process according to claim 1 further characterized in that the solid carbonaceous material is introduced into said fluidized bed reactor at a point or points along the fluidized bed between about 20 percent and about 50 percent of the total bed height below the top of said fluidized bed.

8. A process according to claim 1 further characterized in that said solid carbonaceous material is introduced into said fluidized bed reactor near the bottom of the fluidized bed.