Combined shift and methanation reaction process for the gasification of carbonaceous materials

Abstract

A process for the gasification of coal and other carbonaceous materials to produce a methane rich fuel gas includes the combination of the shift and methanation reactions in a single reactor system. A hot raw synthesis gas comprising methane, hydrogen, hydrogen sulfide, and oxides of carbon passes from a coal gasification system into a combined shift and methanation reactor system where the shift reaction between steam and the product gas adjusts the hydrogen/carbon monoxide ratio. Simultaneously with the occurrence of the shift reaction in the combined reactor system, carbon monoxide and hydrogen are converted to methane and water. Steam formed by the methanation reaction promotes the shift reaction to, in turn, produce the hydrogen necessary to carry out the methanation reaction. After purification to remove the acid gases, the methane rich product gas is reacted in a cleanup methanator in the presence of a nickel catalyst to reduce the carbon monoxide content and increase the methane content to the pipeline standards required for synthetic natural gas.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the gasification of carbonaceous materials, and more particularly to a combined shift and methanation reaction process for providing a methane rich pipeline gas as the principal product.

2. Description of the Prior Art

The production of methane rich fuel gas by the gasification of coal or other carbonaceous materials is widely known in the art. Pyrolysis techniques are used to carbonize coal wherein coal is heated in the absence of air to obtain a solid char and gaseous products such as hydrogen, methane, and ammonia. The Lurgi process utilizes pressure and high temperature to recover synthetic natural gas from carbonaceous solids. All these processes yield product gas that contains carbon monoxide and hydrogen which can be methanated after the hydrogen to carbon monoxide ratio has been adjusted to about a 3 to 1 ratio to obtain high Btu. heating fuels having suitable pipeline quality. Generally the gasification processes use coal in fixed beds, fluidized beds or beds in suspension. Steam, hydrogen, and oxygen are used as the gasification media.

A two-stage gasification process, developed at Bituminous Coal Research, Inc., at Pittsburgh, Pa. combines the processes of coal gasification, shift conversion, acid gas removal and methanation to produce a methane rich fuel gas which meets the specification of a high Btu. pipeline gas. Particulate coal and steam are reacted in the second stage of the gasifier vessel with synthesis gas from the first stage of the gasifier vessel to produce char and a product gas containing hydrogen, hydrogen sulfide, methane and oxides of carbon. The char is recycled to the first gasification stage for reaction with steam and oxygen to produce a synthesis gas for reaction in the second gasification stage. The separated product gas is mixed with steam prior to entering a shift converter wherein the product gas passes over the shift catalyst. The shift converter adjusts the hydrogen to carbon monoxide ratio from about 1/1 to 3.1/1. The shift reaction raises the temperature of the product gas and the product gas flows to a waste heat broiler which supplies process steam and cools the product gas prior to removel of the acid gas in the purification unit.

In the purification unit the acid gas comprising principally hydrogen sulfide and carbon dioxide is removed from the product gas which is then reheated to, or above, 600.degree.F. and fed to the methanator. The catalytic methanation unit converts the hydrogen and carbon monoxide of the product gas to methane which is suitable for use as a high Btu. pipeline gas.

U.S. Pat. No. 3,600,145 describes a process for production of methane as a substitute natural gas by passing carbon monoxide and steam into contact with a metal catalyst supported on an alumina support and promoted with a barium salt. Prior to the conversion pf carbon monoxide and steam to form methane, substantially all the impurities contained in the feedstock are removed. Consequently, the shift reaction and the methanation reactions must be performed separately to permit removal of the carbon dioxide from the feedstock before the conversion process takes place.

There is need to provide a process for gasification of carbonaceous materials, including water gas shift reaction and a methanation reaction, to produce high methane content gas suitable for use as a pipeline gas in which the high costs of the process may be reduced and the process in general simplified. Specifically, by combining the shift and methanation reaction processes the volume of gas from which acid gas is removed may be reduced thereby reducing the size and cost of the acid gas removal unit associated therewith. Furthermore, by combining the water gas shift and methanation reaction processes, a methane rich fuel gas would be produced more efficiently at a lower unit cost.

SUMMARY OF THE INVENTION

The hereinafter described invention relates to a process for the gasification of carbonaceous materials that includes the combination of the shift reaction and methanation reaction in a single reactor system to ultimately produce methane rich fuel gas of pipeline quality. Hot synthesis gas comprising methane, hydrogen sulfide, hydrogen and oxides of carbon pass from a coal gasifier to a waste heat boiler for cooling. The cooled synthesis gas is introduced into a combined water gas shift and methanation system where the mixture comes on contact with a catalyst at a temperature between 500.degree.F. and 1050.degree.F. and at a pressure between 500 psig. and 2000 psig. to thereby increase the hydrogen/carbon monoxide ratio of the mixture and to accomplish methanation of carbon oxides, especially carbon monoxide and hydrogen. Thereafter, the methane rich product gas is recovered from the reactor.

The methane rich product gas then passes from the combined shift and methanation reactor to a purification unit having a selective solvent system to remove principally hydrogen sulfide and carbon dioxide. The purified product gas is treated in a final methanator containing a nickel based catalyst to reduct the carbon monoxide level in the product gas to less than 0.1% by volume and increase the methane content of the product gas to over 90% by volume and preferably over 95% by volume.

The combined shift and methanation reactor system can be characterized as a fixed bed catalytic reactor system or a reactor system which uses a catalyst suspended in a liquid. Preferably, a reactor system having a fluidized catalyst bed with internal cooling coils is used for the combined shift and methanation reactions.

In the catalytic system the shift and methanation reactions occur more or less simultaneously. The shift reaction increases the hydrogen/carbon monoxide ratio above that of the raw feed gas. The water produced as a result of the methanation reaction promotes the shift reaction by reacting with the carbon monoxide to increase hydrogen concentration required for the methanation reaction. The shift and methanation reactions which take place in the combined reactor system can yield a methane rich product gas comprising above 40% by volume methane. The catalytic material utilized may be selected from various metallic oxides or sulfides and is supported on an alumina base. Other catalyst supports such as sulica, magnesia, aluminum silicates, silica gel, magnesium silicate, or mixed silicates such as magnesium-aluminum silicate or molecular sieves can be used. It may be promoted with alkalai materials to retard carbon deposition.

Accordingly, the principal object of this invention is to combine the shift and methanation reaction processes in a single reactor system to produce methane plus higher hydrocarbons, principally ethane and propane.

Another object of this invention is to provide a combined shift and methanation reaction process in the gasification of carbonaceous materials which utilizes a fluidized catalyst bed comprising either a sulfur resistant catalyst or a sulfur sensitive catalyst.

Another object of this invention is to combine the shift and methanation reactions for the gasification of carbonaceous materials in a single reactor system for the purpose of reducing steam consumption and the operating costs associated therewith.

A further object of this invention is to provide a combined shift and methanation reaction process for the gasification of carbonaceous materials which permits sulfur recovery in a small unit with a high yield of sulfur and low emission of residual hydrogen sulfide.

These and other objects of this invention will be more completely disclosed and described in the following specification, accompanying drawing and appended claims.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a diagrammatic illustration of a combined shift and methanation reaction process used in the gasification of coal according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Throughout the specification, coal is utilized in the gasification process. It should be understood that the term "coal" is intended to designate carbonaceous material including all ranks of coal, lignite and the like, and further, that the gasification process is not limited to the gasification of coal and could also be used with oil shale, heavy oil residues, tars and the like.

The term "gasification" means the heating of coal in the presence of reacting agents, whereby all or part of the volatile portion of coal is liberated and the carbon in residual char is reacted with those agents or with other reactants present in the gasification process.

The term "synthesis gas" means a carbon monoxide, hydrogen and preferably methane containing gas such as the gas produced in the second stage of the two-stage gasification process described herein.

The term "product gas" means a methane enriched gas produced in the combined shift and methanator.

Referring to the drawing, preheated coal is injected into the upper portion 10 of a two-stage gasification vessel generally designated by the numeral 12 as a reactant in the second stage of the gasification process. The practice of this invention is not limited to the use of a two-stage gasification process for the production of a synthesis gas containing hydrogen, hydrogen sulfide, methane and oxides of carbon, to be treated in a combined shift and methanator reactor system, ultimately yielding methane rich fuel gas of synthetic pipeline gas quality. any gasification process in which carbonaceous materials are converted to a synthesis gas containing hydrogen and oxides of carbon is acceptable for incorporation in the present invention. Therefore, reference in the present invention to a two-stage gasification process for the production of synthesis gas is made for the purposes of illustration and example only.

Steam and oxygen are introduced into the vessel lower portion 14 and are reacted with the preheated char in the first stage of the gasification process to produce a synthesis gas containing hydrogen and carbon oxides. The synthesis gas flows upwardly through the gasifier vessel 12 for reaction in the upper portion 10 as the second stage of the gasification process. The coal introduced in the vessel upper portion 10 is pulverized to sufficient particle size to permit entrainment of the pulverized coal with the synthesis gas flowing upwardly from the first stage to the second stage. The reaction in the second stage of the gasification is conducted at a temperature in excess of 1600.degree.F. and a pressure in excess of 50 atmospheres with residence time for the reactants in the second stage portion of the vessel 12 maintained to assure reaction of the coal.

The product of the reaction in the second stage between the preheated coal and synthesis gas comprises a low sulfur char entrained in a synthesis gas containing methane, hydrogen and carbon oxides. The sulfur content of the char is maintained at a minimum level by reacting the pulverized coal with the synthesis gas in the presence of hydrogen and steam at elevated temperatures and pressures.

The low sulfur char entrained in the synthesis gas is withdrawn from the upper portion of the vessel 12 and fed through conduit 16 into the cyclone separator 18. The partially gasified char separated in the cyclone separator 18 is withdrawn therefrom and fed through conduit 20 into the lower portion 14 of gasifier vessel 12 as a reactant in the first stage of the gasification process. Steam and oxygen are introduced into the vessel lower portion 14 and are reacted with char in the first stage of the gasification process procedure the to produce gas containing hydrogen and carbon oxides. The synthesis gas reacts in the upper portion 10 with the preheated coal as stage two of the gasification process. The reaction in the first stage is conducted at temperatures in excess of 2500.degree.F. and at a pressure in excess of 50 atmospheres. The molten slag formed in gasifier vessel 12 gravitates to the bottom of the vessel where the molten slag is cooled and withdrawn through conduit 22.

The hot synthesis gas exits from the top of the separator 18 through conduit 24 to a waste heat boiler 26 where the synthesis gas temperature is reduced from 1700.degree.F. to a temperature below 650.degree.F. During the cooling process in the boiler 26, feed water may be sprayed into the synthesis gas sufficiently to raise the moisture content within the product gas to a steam to dry gas ratio sufficient to provide hydrogen for the methanation synthesis. In cases when the hydrogen to carbon monoxide ratio of the cooled synthesis gas is less than 1.0, furhter provision further made for adding steam to the cooled synthesis gas in conduit 28 after is has left the boiler 26. The additional steam added to the cooled synthesis gas in conduit 28 produces a mixture of steam and cooled synthesis gas having a steam-gas ratio of about 0.5. Synthesis gases with hydrogen to carbon monoxide ratios of one or greater require no steam for combined shift and methanation.

The cooled synthesis gas passing through conduit 28 is fed thereafter to a combined shift and methanation reactor vessel 30. The vessel 30 may be one of a plurality of vessels within the combined shift and methanation reactor system which utilizes fixed or fluidized catalyst beds or a catalyst suspended in a liquid. Preferably, the reactor vessel 30 includes a fluidized catalyst bed with internal cooling coils 31 for generation of high pressure steam as a by-product of the heat generated from the reaction. The combined shift and methanation process in the reactor vessel 30 is conducted at a temperature between the range of 550.degree.F. to 1050.degree.F. Preferably, the temperature is maintained in the range from 650.degree.F. to 850.degree.F. and a pressure from 500 to 2000 psig. Under these conditions the main methanation product is methane; however, significant amounts of ethane and higher hydrocarbons may be formed. Such formation improves the efficiency of the process. In the fluidized bed process, the synthesis gas fed into the reactor vessel 30, which is cooler than the catalyst bed, is heated to the desired reaction temperature, thus absorbing a portion of the heat liberated near the inlet distribution area affording protection from hot spots in this region of the vessel 30.

The catalyst employed in the reactor vessel 30 may be composed of various metals and their oxides or sulfides and supported in the reactor vessel 30 on an alumina or mixed alumina-silica base having a bulk density of preferably between 30 and 60 lb./cf. and a mean particle size of preferably 65 microns. A suitable catalyst employed in the combined shift and methanation reaction process may be selected from the group consisting of chromium oxide, molybdenum oxide or sulfide and iron oxide; mixtures of nickel oxide with oxides of chromium, molybdenum or tungsten; or mixtures of cobalt oxide with oxides of chromium, molybdenum or tungsten. In general, single metals, oxides, sulfides, or carbonates, or combinations of these selected from the group consisting of Groups I-B, VI-B or VIII, plus alkali-type promoters from Group I-A, II-A, or the Period 7 rare earths are suitable for use as catalysts in the present invention. Pursuant to the practice of this invention, carbon deposition in the reactor vessel 30 is preferably suppressed by providing catalysts containing oxides or sulfides or molybdenum with nickel or cobalt supported on alumina and activated with an alkali salt, such as potassium carbonate.

For successful conversion of the synthesis gas to methane having acceptable pipeline quality, the product gas entering the reactor vessel 30 must have a minimum hydrogen to carbon monoxide ratio of 1 to 1. In gas mixtures containing less than the requisite hydrogen to carbon monoxide ratio, steam is fed into the reactor vessel 30 along with the synthesis gas for the combined shift and methanation process. Thus, the optimum steam rate for a synthesis gas with a hydrogen to carbon monoxide ratio equal to or greater than one is zero. In the presence of the catalyst in the vessel 30 conversion of hydrogen and carbon monoxide takes place producing methane and water. At the same time the hydrogen to carbon monoxide ratio is altered by the shift reaction. The increased ratio of hydrogen to carbon monoxide provides for the hydrogenation of carbon monoxide to yield a methane rich product gas in the presence of the catalyst. The water present as a result of hydrogenation of carbon monoxide in the methanation reaction permits the shift reaction to occur. Accordingly, the shift reaction increases the concentration of free hydrogen for reaction with carbon monoxide in the methanation reaction.

The resultant product gas of the combined shift and methanation reaction process can contain more than 40% methane by volume and is withdrawn from the reactor 30 through conduit 32 to a waste heat boiler 34. The methanation reaction in the vessel 30 is highly exothermic, and a large amount of process heat is recovered as high pressure steam from the boiler 34 and the cooling coils 31 in the reactor vessel 30. To control the combined shift and methanation reactions and further to increase the methane content of the product gas, a portion of the dry product gas is recycled through conduit 35 to conduit 28 for mixture with the synthesis gas fed to the vessel 30. Preferably, not more than three volumes of dry product gas is added to one volume of synthesis gas fed to the reactor 30. The optimum recycle ratio of dry product gas to synthesis gas is determined on the basis of the highest methane yield for the lowest catalyst volume in the reactor vessel 30.

Table I, as shown below, indicates the percentage increase in the methane content of the product gas as a result of the recycle of the dry product gas to the synthesis gas for a 3 to 1 hydrogen/carbon monoxide ratio and a reactor vessel temperature of 800.degree.F.

TABLE I ______________________________________ Methane in Dry Carbon Monoxide Product Gas (% by volume) (% by volume) Recycle Ratio ______________________________________ 6.5 35.5 -- 3.4 55.0 1/1 2.3 67.0 2/1 ______________________________________

After recycling, the dry product gas containing a high concentration of acid gases, principally hydrogen sulfide and carbon dioxide, is conducted through conduit 36 to heat exchanger 38 for further cooling and passes thereafter through conduit 39 to cooler 40 for additional cooling to a temperature suitable for the selective removal of the acid gases. The product gas from the cooler 40 is conducted through conduit 42 to a hydrogen sulfide removal unit 44. The hydrogen sulfide mixed with the product gas contacts a selective solvent system for forming a hydrogen sulfide, rich stream. The solvent utilized in unit 44 for selectively removing hydrogen sulfide from the gaseous stream is preferably an organic compound containing basic groups, such as amino acids. The concentrated hydrogen sulfide stream is withdrawn from the bottom of the unit 44 through conduit 45 for routing to further recovery processes. The product gas, substantially free of hydrogen sulfide, passes from the unit 44 through conduit 46 for introduction into the carbon dioxide removal unit 48. In a similar manner, the product gas is contacted with a suitable solvent fed to the unit 48 for removing carbon dioxide from the product gas in the form of a carbon dioxide, rich solvent stream extracted from the bottom of the unit 48 through conduit 50 for routing to subsequent recovery processes. The purified product gas supplied from the removal unit 48 to conduit 52 realizes the essential complete hydrogen sulfide removal and up to 99% carbon dioxide removal as a result of the purification process and contains more than 90% methane by volume.

The washed product gas is passed to heat exchanger 38 through conduit 52 and passes thereafter through conduit 53 to heat exchanger 54 for additional heating before the methane rich product gas is fed through conduit 56 to a guard chamber (not shown) containing pelleted zinc oxide for the removal of traces of sulfur compounds that remained in the gas and then to a final conventional fixed bed methanator 58. The fixed bed methanator 58 uses a nickel catalyst for reacting the remaining carbon monoxide in the product gas with the available excess hydrogen. The methanator 58 converts approximately 95% of the remaining carbon monoxide and 50% of the remaining carbon dioxide to yield a pipeline gas containing over 90% methane and less than 0.1% carbon monoxide by volume. The fuel gas from the methanator 58 is passed to heat exchanger 54 through conduit 60 and after further cooling and drying is ready for delivery to the pipeline.

The composition of the synthesis gas leaving the gasification vessel 12 at the gas flow rate of 10,000 mols per hour having a steam to dry gas ratio of 0.5 was analyzed at various portions along the combined shift and methanation reaction process. The date compiled from the example shown in Table II indicates that a product gas rich in methane content is produced by the combined shift and methanation reaction process.

TABLE II ______________________________________ mol % mol/hr ______________________________________ Gas Composition before combined shift methanator reactor Carbon dioxide 16.7 1670 Carbon monoxide 40.1 4010 Methane 14.7 1470 Hydrogen 26.4 2640 Nitrogen 0.7 70 Hydrogen sulfide 1.4 140 Total 100.0 10,000 Water 5,000 Gas composition after combined shift and methanator reactor Carbon dioxide 53.8 4035 Carbon monoxide 0.2 15 Methane 41.2 3090 Hydrogen 2.0 150 Nitrogen 0.9 70 Hydrogen sulfide 1.9 140 Total 100.0 7500 Water 4255 Gas composition after acid gas removal Carbon dioxide 1.2 40 Carbon monoxide 0.4 15 Methane 91.8 3090 Hydrogen 4.5 150 Nitrogen 2.1 70 Hydrogen sulfide 0.0 0 Total 100.0 3365 Pipeline gas after final methanation Carbon dioxide 0.6 20 Carbon monoxide 0.0 1 Methane 96.3 3124 Hydrogen 0.9 28 Nitrogen 2.2 70 Hydrogen sulfide 0.0 0 Total 100.0 3243 ______________________________________

The following Table III is a compilation of experimental data illustrating various product conversions as the result of the combined shift and methanation of the product feed gas, having various hydrogen/carbon monoxide ratios, at a preselected space velocity into the reactor vessel operating at preselected temperatures and a pressure of 1000 psig. with an 11% molybdenum trioxide (MoO.sub.3) and an 89% alumina (Al.sub.2 O.sub.3) catalyst.

TABLE III __________________________________________________________________________ RUN A B C D __________________________________________________________________________ Temperature (.degree.F.) 850 820 850 980 Pressure (psig) 1000 1000 1000 1000 Space velocity 1500 1250 1350 1250 (standard volumes feed volume catalyst/hr.) Synthesis gas composition (% vol.) Carbon monoxide 25.69 14.35 39.62 14.38 Carbon dioxide 0.07 30.12 16.41 29.94 Hydrogen 73.85 42.87 43.23 42.38 Nitrogen 0.22 1.35 0.36 1.58 Methane 0.17 11.31 0.38 10.95 Hydrogen sulfide 0.00 0.00 0.00 0.77 Useful conversion of carbon monoxide and hydrogen (% (CO+H.sub.2 .fwdarw.hydrocarbons) ) (CO+H.sub.2 FED) 61.0 38.4 73.0 16.5 Selective conversions of carbon monoxide to higher hydrocarbons (% (CO.fwdarw.C.sub.1 .sup.+)/(CO.fwdarw.hydrocarbons) ) 24.0 23.8 41.7 8.8 __________________________________________________________________________

Reference in the above Table III to useful conversion of carbon monoxide and hydrogen to hydrocarbons, expressed by percentage, indicates the percentage of carbon monoxide and hydrogen which was converted to methane, ethane and traces of other hydrocarbons. The selective conversion of carbon monoxide to higher hydrocarbons, expressed by percentage, represents the amount of carbon monoxide in the synthesis gas converted to hydrocarbons more complex than methane compared to the total carbon in the synthesis gas converted to hydrocarbons. As illustrated in Runs B and D, the increase in the percentage of carbon dioxide added to the synthesis gas reduced the conversion of carbon monoxide and hydrogen to hydrocarbons to 38.4% and 16.5% respectively. Even with a content of 0.77% of hydrogen sulfide in the synthesis gas for Run D, a conversion of 16.5% of carbon monoxide and hydrogen to hydrocarbons was achieved. Thus, it is possible to provide a catalyst which will function in a sulfided environment and combine the shift and methanation reactions. Furthermore, the result was achieved without the addition of an alkali promotor, such as potassium salt, to the catalyst.

An economic survey was made comparing the operating costs of the combined shift and methanation reaction process with the conventional separate shift and methanation process in a plant having a capacity of 250 million s.c.f.d. of pipeline gas. The operating costs of the overall gasification process is substantially reduced by replacing separate shift and methanator reactors by a single shift and methanation reactor system. Furthermore, the size of the acid gas removal units are substantially reduced when the product gas is purified after completion of the combined shift and methanation process as compared to purifying the product gas after the shift conversion and prior to entry in the methanator. The capacity requirements of the acid removal units decrease as a result of the volumetric shrinkage from the methanation reaction to produce a smaller quantity of the gas stream for acid gas purification treatment. Also, the steam requirements of the combined process are substantially reduced in comparison with those of the separate process thereby providing additional savings in operating costs. A net investment savings of 17 million dollars is realized for the combination process over the separate process resulting in a reduction of 6.0 cents/MM Btu. in pipeline gas cost, based upon economic study prepared by Air Products and Chemicals, Inc., as reported in "Engineering Study and Technical Evaluation of the Bituminous Coal Research, Inc. Two-stage Super Pressure Gasification Process," (R&D Report No. 60) prepared for the Office of Coal Research, Department of the Interior, 1971.

According to the provisions of the patent statutes, we have explained the principle, preferred construction and mode of operation of our invention and have illustrated and described what we now consider to represent its best embodiments. However, we desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

Claims

We claim:

1. In a process for gasification of carbonaceous materials to produce high methane content gas, including a water gas shift reaction and a methanation reaction comprising,

gasifying carbonaceous materials and generating a hot synthesis gas comprising a mixture of methane, hydrogen sulfide, hydrogen and oxides of carbon,

cooling said hot synthesis gas,

adding steam to said cooled synthesis gas when the hydrogen to carbon monoxide ratio of said cooled synthesis gas is less than 1.0 to produce a mixture of steam and cooled synthesis gas having a steam/gas ratio of about 0.5,

introducing said cooled synthesis gas into a combined water gas shift and methanation reactor into contact with a catalyst at a temperature between 550.degree.F. and 1050.degree.F. and at a pressure between 500 psig. and 2000 psig. whereby simultaneously the hydrogen/carbon monoxide ratio is increased by carbon monoxide reacting with water produced from the methanation reaction and methanation of carbon monoxide and hydrogen is accomplished,

recovering from said reactor a methane rich product gas which includes above 40% by volume methane, said methane rich product gas also including hydrogen sulfide, carbon oxides and other higher hydrocarbons such as ethane and propane,

passing said methane rich product gas through a waste heat boiler to thereby cool and dry said methane rich product gas and thereafter,

introducing said methane rich gas into a purification unit for removal of acid gas such as hydrogen sulfide and carbon dioxide to produce a purified methane rich product gas, and

recycling a portion of said cooled and dried methane rich product gas for mixture with said mixture of steam and cooled synthesis gas introduced into said combined water gas shift and methanation reactor in an amount which is not more than three volumes for each one volume of said mixture of steam and cooled synthesis gas.

2. The process as set forth in claim 1 in which said catalyst is selected from the group consisting of chromium oxide, molybdenum oxide, molybdenum sulfide, iron oxide; mixtures of nickel oxide with oxides of chromium, molybdenum or tungsten; and mixtures of cobalt oxide with oxides of chromium, molybdenum or tungsten.

3. The process as set forth in claim 1 comprising, passing said mixture of cooled synthesis gas in said combined water gas shift and methanation reactor over a fluidized catalyst bed having internal cooling coils arranged therewith to generate high pressure steam from the heat of the combined water gas shift and methanation reaction.

4. The process as set forth in claim 3 wherein said fluidized catalyst bed includes a catalytic material selected from the group consisting of Groups I-B, VI-B and VIII plus alkali-type promoters selected from the group consisting of Groups I-A, II-A and the period seven rare earths.

5. The process as set forth in claim 1 wherein said catalyst is supported on an alumina base having a density of 30 to 60 lb. per cubic foot and a mean particle size of about 65 microns.

6. The process as set forth in claim 1 wherein the said catalyst includes an alkali salt selected from the group consisting of potassium or rubidium salts.

7. The process as set forth in claim 1 comprising,

introducing said purified methane rich product gas into a fixed bed methanator having a nickel catalyst therein to reduce the carbon monoxide level of said purified methane rich product gas to yield a final product gas comprising methane and carbon monoxide in a concentration acceptable for synthetic pipeline gas quality.

8. The process as set forth in claim 1 wherein said purification unit for removal of acid gas from said methane rich product gas includes,

contacting said methane rich product gas with a selective solvent system to form a hydrogen sulfide rich solvent stream,

withdrawing said hydrogen sulfide solvent stream from said methane rich product gas,

contacting said methane rich product gas with a selective solvent system to form a carbon dioxide rich solvent stream, and

withdrawing said carbon dioxide rich solvent stream from said methane rich product gas.