US3847567A

US3847567A - Catalytic coal hydrogasification process

Info

Publication number: US3847567A
Application number: US00391082A
Authority: US
Inventors: T Kalina; R Moore
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1973-08-27
Filing date: 1973-08-27
Publication date: 1974-11-12
Anticipated expiration: 1991-11-12

Abstract

Methane is produced by the thermoneutral reaction of steam with coal or other carbonaceous material in a hydrogasification zone containing an alkali metal catalyst and sufficient hydrogen to suppress competing endothermic reactions. The gas taken overhead from the gasifier is subjected to steam reforming and then processed for the removal of acid constituents, hydrogen which is recycled to the hydrogasification zone, and carbon monoxide which is used as fuel for the steam reformer.

Description

United States Patent Kalina et al.

[4 1 Nov. 12,1974

[5 l CATALYTIC COAL HYDROGASIFICATION 3,689,240 9/1972 Aldridge et al 43/202 PROCESS [75] Inventors: Theodore Kalina, Morris Plains, Primary 'l Bashore Ni; Roger E. Moore E1 P1150, Tex. ASSISHZIII ExammerPcter F. Kratz Attorney, Agent, or Firml. E. Reed [73] Ass1gnee: Exxon Research and Engineering Company, Linden, NJ. [57] ABSTRACT [22] Filed: Aug. 27, 1973 Methane is produced by the thcrmoneutral reaction of l l pp N03 391,082 steam with coal or other carbonaceous material in a hydrogasification zone containing an alkali metal cata- 15 us. Cl. 48/202 48/196 R and sumcicm hydrogen suPPfess Competing [51 Int. Cl. Cl0j 3/06 dothermic reactionsgas lake "Vcrheud {mm W L Field of Search 48/196 R 202 210 197 R gasificr is subjected to steam reforming and then prol cessed for the removal of acid constituents, hydrogen [56] References Cited which is recycled to the hydrogasification zone, and carbon monoxide which is used as fuel for the steam UNITED STATES PATENTS reformer. 3.004.839 10/1961 Tornquist 48/197 3,740,193 6/1973 Aldridge et al. 48/202 12 Claims, 1 Drawing Figure STEAM REFORMER COAL FURNACE CATALAYST '-lO sTEiAmzls H20 l2 28 7 3r 2s 7 l y \NNN crvn wo co m sm o 33 CARRIER 24 47'FQI/ f Y GAS 8 STEAM CH4+HZ+CO HYDROGASIFIERJ L2 45 8 SOLVENT H E ACID GAS REMOVAL STEAM I7 2 29 co m s SOLVENT 2| CRYOGENIC CATALYS; UNIT RECOVERY 23 H20 44 ASH CATALYTIC COAL HYDROGASIFICATION PROCESS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the gasification of coal and other carbonaceous materials and is particularly concerned with gasification processes carried out over alkali metal catalysts for the production of methane.

2. Description of the Prior Art Efforts to develop processes for the conversion of coal and similar carbonaceous materials into high Btu synthetic gases suitable for use as fuels have focused attention on the hydrogasification reaction: C 2H 2 CH Because this reaction is highly exothermic and requires the presence of hydrogen, it has been suggested that it be integrated with endothermic hydrogen-producing reactions such as the steam/carbon gasification reaction, C H 2 C0 H and themethane/steam reforming reaction, CH H 0 2 C0 3H in order to conserve heat and reduce the amount of hydrogen which must'be provided. It has been found that this can be done by reacting the coal or othercarbonaceous material with steam and hydrogen in a .hydrogasification zone to produce a methane-rich gas, passing at least a portion of this gas stream through a methane reforming zone where it is contacted with steam to reduce part of the methane and form hydrogen, and then recycling hydrogen and carbon monoxide recovered from the steam reformer overhead gas to the hydrogasification zone. Coal char or other carbonaceous solids are circulated between the hydrogasification and reforming zones to provide'heat integration. This system results in a substantially thermoneutral process which has numerous advantages over other processes suggested in the past.

SUMMARY OF THE INVENTION This invention provides an improvement over the process referred to above which permits significant savings in operating costs and has other advantages. In accordance with the invention, a high Btu gas suitable for pipeline purposes is produced by reacting finely divided coal or similar carbonaceous material with steam and hydrogen in the presence of an alkali metal catalyst, passing the resultant gas to a catalytic reforming unit where steam and methane react to form hydrogen and carbon monoxide, treating the reformer gas product for the removal of carbon dioxide and other acidic gases, and then separating the treated gas into a hydrogen stream which is recycled to the gasifier, a carbon monoxide stream which is used to fuel the reformer and provide heat for the process, and a product gas stream composed primarily of methane. The hydrogen and reactant steam concentrations in the gasifier are controlled so that the exothermic hydrogasification reactions provide sufficient heat for the endothermicsteam reactions, reactant preheat and reactor heat losses. This results in a substantially thermoneutral process which has economic advantages over processes employed in the past.

The hydrogasification reaction is preferably carried out in a fluidized bed containing from about 1 to about 50 percent by weight, based on the carbonaceous feed material, of an alkali metal catalyst such as potassium carbonate, sodium carbonate, or the like. The use of such a catalyst permits operation of the gasifier at temperatures within the range of about l,200 to about l,500 R, which in turn favors the production of meth ane. The net hydrogen required for reaction with the carbonaceous material in the hydrogasification zone is generated by steam reforming the overhead gas, preferably in the presence of an alkali metal or similar methane reduction catalyst at a temperature in the range between about l,300 and about 1,700 F., in a steam reformer furnace. Here from about 15 to about 50 percent of the methane produced in the hydrogasification step is reduced to carbon monoxide and hydrogen. Some additional carbon dioxide is also formed. The reformer gases are treated with triethanolamine, hot potassium carbonate, or a similar solvent for the removal of carbon dioxide and hydrogen sulfide and then passed to a cryogenic separation unit where hydrogen, carbon monoxide, and methane are separately recovered. The hydrogen is recycled to the hydrogasification zone where it reacts with the carbonaceous material in the methane-forming reaction. The recovered carbon monoxide is not recycled as in earlier processes and instead is burned with ambient pressure air to fire the steam reformer furnace. This provides essentially all of the heat required in the process and eliminates the necessity for BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE in the drawing is a schematic flow sheet of a hydrogasification process carried out in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The process depicted in the drawing is one for the production of methane by the therrnoneutral reaction of coal or a similar carbonaceous solid with steam and hydrogen in a fluidized bed reactor. The solid feed material, which will normally be a bituminous or lower rank coal but may be oil shale, petroleum coke, char, charcoal or other carbonaceous material, is introduced into the system in finely divided form through line 10 from a' suitable storage facility or feed preparation plant which does not appear in the drawing. The use of a feed material which has been crushed and screened to a particle size less than about 8 mesh on the Tyler Screen Scale is normally preferred. The particulate feed material is passed from line 10 into hopper 11 where it is combined with a finely divided alkali metal catalyst introduced through line 12. Suitable alkali metal catalysts include the oxides and salts of cesium, potassium, sodium and lithium. The alkali metal carbonates are particularly effective. Cesium carbonate normally exhibits the greatest catalytic activity, potas sium carbonate and sodium carbonate being somewhat less active and lithium carbonate normally being the least active material. Because of its relative high activity, widespread availability and low cost, potassium carbonate is normally the preferred catalyst for use in the process. Other alkali metal compounds which can also be used but are less effective include the chlorides, hydroxides, sulfates, silicates, sulfides and the like. The catalyst employed may be introduced into the system as a finely divided powder or granular materialwhich is mixed with the feed material in hopper 11 to obtain relatively uniform distribution of the catalyst particles. In lieu of this, the carbonaceous solids employed as feed material can be impregnated with the catalyst during or following the feed preparation step or particles of coal char or a similar carbonaceous material having a high surface area can be treated with aqueous solutions of the catalyst and thereafter introduced with the feed material.

The amount of catalyst employed in the gasifler will normally range from about 1 to about 50 weight percent, preferably from about 5 to about 30 weight percent, of the combined total weight of the carbonaceous solids and catalyst on a moisture and ash-free basis. The use of from about to weight percent of potassium has been found to be particularly effective. The optimum amount of catalyst for a particular operation will depend upon the particular alkali metal compound selected and hence it may be preferable to employ somewhat greater amounts of sodium carbonate or lithium carbonate than would be used if cesium carbonate or potassium carbonate were employed.

The hydrogasifier employed in the process of the invention is operated at elevated pressure and hence the feed material and catalyst in hopper 11 must be pressurized prior to introduction into the hydrogasifier. The system depicted in the drawing involves the use of a star wheel feeder or similar device 13 through which the solid particles are discharged into line 14 containing steam, product methane, recycle hydrogen or a similar carrier gas at a pressure sufficiently high to permit entrainment and injection of the solids into the hydrogasifier 15. In lieu of or in addition to an arrangement of this type, parallel lock hoppers, pressurized hoppers, or aerated standpipes operating in series may be employed to raise the input coal stream to the system operating pressure. The use of such devices for handling coal and other finely divided solids at elevated pressures has been described in the patent and technical literature and will be familiar to those skilled in the art.

The overall reaction which takes place in the process of the invention can be represented by the equation: 2H O 2C CH CO This overall reaction is carried out in two separate steps, an initial step for the formation of methane in the hydrogasifier in accordance with the equation: 2C 4H 2CH and a second step for the formation of hydrogen in the steam reformer in accordance with the equation: 2H O CH, 4H CO Concurrently, in the first reactor the reaction C H O C0 H is carried out to balance the exothermic hydrogasification and produce carbon monoxide for use as fuel. The hydrogasifier is operated under conditions which are thermodynamically favorable to the production of methane in high concentrations, normally at relatively high pressure and a temperature as low as possible without sacrificing high reaction rates. The temperature in hydrogasifier 15 will therefore generally be maintained in the range between about 1,200 F. and about l,500 F. and the pressure will generally range from about 100 psig to about 5,000

psig, preferably from about 500 psig to about 1,000 psig.

The coal or other carbonaceous solids and catalyst injected through line 14 into hydrogasifier 15 form a fluidized bed in which the solid particles are supported by up-flowing steam introduced intothe lower end of the reaction vessel through line 16 and by recycle hydrogen admitted by way of line 17. The steam and hydrogen react with the carbon in the fluidized bed to produce a methane-rich synthesis gas containing from about 30 to about 60 mole percent methane. This gas is taken overhead through one or more cyclone separators or similar devices 18 by means of which entrained solids are removed from the gas stream and returned to the fluidized bed. A solids stream composed primarily of ash and catalyst, but also containing some char, is withdrawn from the fluidized bed through line l9 near the lower end of the hydrogasifier and passed to catalyst recovery zone 20. Here the hot solids are quenched with water introduced through line 21 in order to permit heat recovery and leaching out of the catalyst. The resulting aqueous catalyst solution can then be concentrated and recycled through line 22 for use with the fresh make-up catalyst introduced into the system through line 12. Alternatively, the catalyst solution can be sprayed on the incoming coal introduced into the system through line 10 or used to pretreat coal or other carbonaceous solids in the feed preparation plant. The ash containing a small amount of char is withdrawn from the catalyst recovery unit as indicated by line 23 and may be employed for the manufacture of building materials or used for other purposes;

The gas stream taken overhead from hydrogasifier 15 is passed through line 24 into the tubes 25 of steam reformer furnace 26. This furnace also contains steam coils 27 from which water introduced through line 28 is withdrawn as steam by means of line 29. A portion of the steam thus generated is injected through line 31 into the gas stream from the hydrogasifier for reaction with methane in the reformer furnace tubes. The remaining steam is passed through

lines

32 and 16 to the lower end of the hydrogasifier for reaction with the coal feed.

The steam-methane reaction carried out in reformer furnace 26 is conducted in the presence of a methane reduction catalyst at a temperature in the range between about l,300 F. and about l,700 F. and at a pressure between about and about 5,000 psig, preferably between about 500 and about 1,000 psig. The catalyst may be a transition metal of Group VIII of the Periodic Table such as iron, nickel or cobalt or an oxide, carbonate, nitrate, carbide, chloride, sulfate or other compound of such a metal. Preferably, however, an alkali or alkaline earth metal catalyst similar to that employed in the hydrogasifier will be used. The catalyst selected, in the form of metal shot, metallic turnings, ore particles, nitrited steel wool, or similar particles, is disposed within the tubes of the reformer furnace. The use of potassium carbonate particles or particles of alumina, kieselguhr or a similar inert high surface material on which potassium carbonate has been deposited is generally preferred. In the presence of such a catalyst, a portion of the methane in the gas stream from the hydrogasifier reacts with the steam in the furnace tube to produce hydrogen, carbon monoxide and carbon dioxide. The gas discharged from the reformer furnace will thus contain methane, hydrogen, carbon monoxide,

carbon dioxide, unreacted steam, and small amounts of nitrogen and hydrogen sulfide derived from impurities in the coal.

The gas discharged from the reformer furnace 26 is passed through line 33 to heat exchanger 34 where it is cooled and the steam initially present is recovered as condensate by means of line 35. Cooling water supplied to vessel 34 through line 36 is withdrawn as steam through line 37. The overhead gas stream from which steam has been removed is passed through line 38 to vessel 39 for the removal of acid gases, principally carbon dioxide and small amounts of hydrogen sulfide. Here the gas stream is scrubbed with a conventional carbon dioxide-absorbing solvent introduced through line 40. The solvents which may be employed for this purpose include n-methyl pyrrolidone, triethanolamine, propylene carbonate, and hot potassium carbonate. Spent solvent containing absorbed carbon dioxide and hydrogen sulfide is withdrawn from the lower end of the scrubber by means of line 41 and may be treated before removal of the acidic constituents and then recycled. Most of the carbon dioxide and hydrogen sulfide can generally be removed from the solvent by flashing and the rest can usually be taken out by stripping with an inert gas. Cryogenic separation or other conventional methods for removal of the acidic constituents from the gas stream can also be employed. The overhead gas from vessel 39 is passed through line 42 to cryogenic separation unit 43 where it is fractionated to produce a product methane stream that is withdrawn through line 44, a hydrogen stream which is taken off overhead and recycled through lines 45 and 17 to the hydrogasifier, and a carbon monoxide stream which is passed through line 46 to reformer furnace 26. Here the carbon monoxide is injected into the furnace burners 47 with low pressure air and employed as fuel for the furnace.

The process of the invention can be further illustrated by considering a plant for the production of 250 million standard cubic feet of methane per stream day. The feed to this plant will consist of about 500 tons of coal per hour and about 59 tons potassium carbonate as a catalyst. The solids feed stream to the hydrogasifier will thus comprise about 89 weight percent of finely divided coal screened to less than about 8 mesh on the Tyler Screen Scale and about 1 1 weight percent potassium carbonate. Following the initial startup, substantially all of the catalyst withdrawn from the system will be separated from the ash in the catalyst recovery unit and recycled so that only make-up quantities of potassium carbonate need be added. About 77 tons of ash per hour containing about 14.6 percent char is withdrawn from the catalyst recovery unit for use in the manufacture of building materials or for other purposes.

The coal fed to the hydrogasifier is supported in a fluidized bed by steam introduced at the rate of about 468,300 pounds per hour and recycle hydrogen injected into the lower end of the gasifier at a rate of about 31.5 million standard cubic feet per hour. At the gasifier temperature of about 1,200 F. and pressure of about 100 psig, these materials react to form about 41.2 million standard cubic feet per hour of a methanerich synthesis gas having the following composition:

The methane-rich gas taken overhead from the hydrogasifier at a temperature of about l,200 F. is combined with 990,810 pounds of steam per hour from the steam reformer furnace. The resulting mixture of steam and methane-rich synthesis gas is passed through the reformer furnace tubes where it is contacted with a potassium carbonate on alumina catalyst. At the furnace temperature of about l,300 F., the steam and synthesis gas react to reduce a portion of the methane and produce additional carbon monoxide, carbon dioxide and hydrogen. This results in about 74 million standard cubic feet per hour of gas having the following composition:

TABLE ll Reformed Gas Composition Constituent Mole 7:

CO; 5.6 H: 43.6 H O 24.1 CH4 15.5 N, 0.2 H 5 0.6

The reformed gas is cooled to permit the removal of 844,182 pounds of water of condensate per hour and then scrubbed with triethanolamine for the removal of acid gases. This results in the separation from the gas of about 4. 1 million standard cubic feet per hour of carbon dioxide and about 0.4 million standard cubic feet per hour of hydrogen sulfide. The scrubbed gas, about 51.6 million standard cubic feet per hour, contains about 62.6 percent hydrogen, 22.3 percent methane, 14.9 percent carbon monoxide and about 0.2 percent nitrogen. This gas stream is passed to the cryogenic unit where about 250 million standard cubic feet of methane per stream is recovered as product gas. About 31.5 million standard cubic feet of hydrogen per hour is recycled to the hydrogasifier. The remaining gas stream, about 9.7 million standard cubic feet per hour of a mixture of 79.1 percent carbon monoxide, 8.3 percent hydrogen, 11.1 percent methane and 1.5 percent nitrogen, is transmitted to the steam reformer furnace for use a fuel. This burning of carbon monoxide, together with lesser quantities of hydrogen and methane not recovered in the cryogenic separation unit, provides all of the heat which is required for the process served to generate the by-product stream required for the hydrogasification and reforming steps.

it will be apparent from the foregoing that the process of the invention has numerous advantages over processes for the production of methane which have been employed in the past. The use of a steam reformer furnace to simultaneously carry out the steam refonning reaction and generate process. steam eliminates the need for supplying heat to the process from an external source and does away with the need for the air compressors or high pressure oxygen facilities which are normally needed for carrying out endothermic gasification reactions requiring high heat input levels. Because the process is substantially thermoneutral, it does not require removal of the large quantity of heat that would otherwise have to be recovered. Moreover, the process maximizes the quantity of carbon dioxide which is rejected from the system with the flue gases and minimizes the quantity which must be taken out by scrubbing of the gas. The use of the carbon monoxide as fuel, rather than recycling it to the hydrogasifier to suppress carbon monoxide production as in earlier processes, eliminates the need for burning solid fuel or product methane to generate process steam and satisfy the other heat requirements of earlier processes. This in turn simplifies pollution control problems and avoids the removal of large quantities of hydrogen'as water vaor in the flue gas. These and other advantages make the process of the invention attractive for a variety of applications.

We claim:

1. A process for the manufacture of methane which comprises:

a. reacting finely divided carbonaceous solids wit steam and hydrogen in the presence of an alkali metal catalyst in a hydrogasifier and withdrawing a methane-rich gas overhead from said hydrogasifier; 7 H V b. reacting said methane-rich gas with steam in a catalytic steam reforming furnace and withdrawing a methane-rich gas of increased hydrogen content from said reforming furnace;

c. treating said gas of increased hydrogen content for the removal of acidic constituents;

d. separating the treated gas into a product methane stream, a hydrogen stream, and a carbon monoxide stream;

e. recycling at least a portion of said hydrogen stream to said hydrogasifier; and

f. burning at least a portion of said carbon monoxide stream as fuel in said steam reforming furnace.

2. A process as defined by claim 1 wherein carbonaceous solids are reacted with said steam and hydrogen in said hydrogasifier at a temperature in the range between about l,200 F. and about 1,500 F. and at a pressure in the range between about and about 5,000 psig.

3. A process as defined by claim 1 wherein said alkali metal catalyst is an alkali metal carbonate.

4. A process as defined in claim 1 wherein said methane-rich gas is contacted with said steam in said catalytic steam reforming furnace at a temperature in the range between about 1,300 F. and about l,700 F. and at a pressure in the range between about 100 and about 5,000 psig.

5. A process as defined by claim 1 wherein said steam reforming catalyst is an alkali metal.

6. A process as defined by claim 1 wherein said separation is performed cryogenically.

7. A process as defined by claim 1 wherein said catalyst in said hydrogasifier comprises from about 5 to about 30 weight percent, on a moisture and ash-free basis, of the total solids in the hydrogasifier.

8. A process as defined by claim 1 wherein said carbonaceous solids comprise a bituminous or lower rank coal.

9. A-process as defined by claim 2 wherein said solids are reacted with said steam and hydrogen in said hydrogasifier at a pressure between about 500 and about 1,000 psig.

10. A process as defined by claim 3 wherein said catalyst comprises potassium carbonate.

11. A process as defined by claim 4 wherein said gas is reacted with said steam in said reforming furnace at a pressure between about 500 and about 1,000 psig.

12. A process as defined by claim 5 wherein said catalyst comprises potassium carbonate.

Claims

1. A PROCESS FOR THE MANUFACTURE OF METHANE WHICH COMPRISES: A. REACTING FINELY DIVIDED CARBONACEOUS SOLIDS WITH STEAM AND HYDROGEN IN THE PRESENCE OF AN ALKALI METAL CATALYST IN A HYDROGASIFIER AND WITHDRAWING A METHANERICH GAS OVERHEAD FROM AID HYDROGASIFIER; B. REACTING SAID METHANE-RICH GAS WITH STEAM IN A CATALYTIC STEAM REFORMING FURNACE AND WITHDRAWING A METHANE RICH GAS OF INCREASED HYDROGEN CONTENT FROM SAID REFORMING FURNACE; C. TREATING SAID GAS OF INCREASED HYDROGEN CONTENT FOR THE REMOVAL OF ACIDIC CONSTITUENTS; D. SEPARATING THE TREATED GAS INTO A PRODUCT METHANE STREAM, A HYDROGEN STREAM, AND CARBON MONOXIDE STREAM;

2. A process as defined by claim 1 wherein carbonaceous solids are reacted with said steam and hydrogen in said hydrogasifier at a temperature in the range between about 1,200* F. and about 1, 500* F. and at a pressure in the range between about 100 and about 5,000 psig.

4. A process as defined in claim 1 wherein said methane-rich gas is contacted with said steam in said catalytic steam reforming furnace at a temperature in the range between about 1,300* F. and about 1,700* F. and at a pressuRe in the range between about 100 and about 5,000 psig.

9. A process as defined by claim 2 wherein said solids are reacted with said steam and hydrogen in said hydrogasifier at a pressure between about 500 and about 1,000 psig.