Three-stage Gasification Of Coal

Abstract

A process for the production of methane-rich fuel gas by the gasification of coal in three stages. In the first stage, partially gasified char recycled from the process is reacted with oxygen and superheated steam at pressures greater than 50 atmospheres and temperatures greater than 2,500.degree.F. to yield a first stage synthesis gas containing hydrogen and carbon oxides. In the second stage, the first stage synthesis gas is reacted with superheated steam and coal at pressures in excess of 50 atmospheres and temperatures in excess of 1,600.degree.F. to yield a partially gasified char entrained in a second stage product gas containing methane, hydrogen and carbon oxides. In the third stage, char and gases from the second stage form a fluidized bed reacting at a pressure in excess of 50 atmospheres and at a temperature in excess of 1,500.degree.F. to form char entrained in a third stage product gas containing methane, hydrogen and oxides of carbon. The third stage char and third stage product gas are separated with the char recycled to the first stage and the product gas is purified by the removal of carbon oxides, hydrogen sulfide and other impurities and the purified product gas is methanated to produce the methane rich fuel gas. Slag formed in the first and second stages is collected in the third stage and is removed from the process after elutriation from the slag of char collected with the slag, which char is returned to the third stage.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the production of methane-rich fuel gas by the gasification of coal in three stages at high pressures and temperatures.

2. Description of the Prior Art

Methane-rich fuel gas is produced by a two-stage gasification process wherein particulate coal and steam are reacted in the second stage with synthesis gas from the first stage at a temperature in excess of 1,600.degree.F. and a pressure in excess of 50 atmospheres to produce char and a product gas containing hydrogen, methane and oxides of carbon. The char and product gas are withdrawn and separated and the product gas is thereafter treated to remove carbon oxides and other diluents and is ultimately methanated to produce a methane-rich fuel gas. The char is recycled to the first gasification stage for reaction with steam and oxygen at a temperature in excess of 2,500.degree.F. and a pressure in excess of 50 atmospheres to produce a first stage synthesis gas containing hydrogen and oxides of carbon for reaction in the second gasification stage. A portion of the char, which has low sulfur content, can be burned to produce process energy. Slag produced from the reactions in the first and second stages gravitates to the lower sections of the reactor in the first stage where the slag is cooled and removed from the process. Reaction times and residence times of the reactants in the first and second stages are limited and minimized to prevent softening and agglomeration of the coal in the second stage.

The two-stage gasification process described above was developed at Bituminous Coal Research, Inc. at Pittsburgh, Pennsylvania (BCR). The process is described in a publication of the Department of Interior, Office of Coal Research (OCR), dated 1965 and entitled, "Gas Generator-Research and Development Survey and Evaluation." That process was described also in a computer study entitled, "Computer Study of Stage-Two Reactions in the BCR Two-Stage Super Pressure Gasification Process," presented at the national meeting of the American Chemical Society, Division of Fuel Chemistry, Chicago, Ill. in September, 1967.

SUMMARY OF THE INVENTION

This invention resides in a process for three-stage gasification of coal to produce a methane-rich fuel gas. In the first stage, recycled char is reacted at high temperature with oxygen and steam at a pressure of at least 50 atmospheres to yield a first stage synthesis gas containing hydrogen and carbon oxides. In the second stage, synthesis gas from the first stage is reacted with superheated steam and coal at a high temperature and at a pressure of at least 50 atmospheres to produce a second stage product gas containing methane, hydrogen and oxides of carbon. In the third stage, partially gasified char is maintained as a fluidized bed by second stage product gas which reacts with the char to yield partially gasified char from the third stage entrained in a third stage product gas containing methane, hydrogen and oxides of carbon. The temperatures and residence times of reactants in the first and second stages are controlled to convert char ash to molten slag in the first stage and to convert coal partially into a gas containing methane, hydrogen and carbon oxides, at temperatures above the range of coal plasticity in the second stage. The temperature in the third stage is lower, and the residence time of the solid reactants in the third stage is longer, than those in the first and second stages to permit more complete utilization of the reactants in the third stage. The char and third stage product gas are separated and, while the char is recycled to the first stage for further reaction in the process, the third stage product gas is purified for conversion and removal of carbon oxides, hydrogen sulfide and other impurities and is thereafter methanated to produce a fuel gas containing at least 70 percent methane.

In one embodiment, the third stage and the fluidized char bed are in direct communication with a part of the second stage. In other embodiments, the third stage is separated from the first and second stages. Slag formed in the first and second stages is collected in, and removed from, a lower portion of the third stage.

Third stage gasification of char fluidized in product gas from a two-stage gasification process permits further reaction of the partially gasified char in the hydrogen-rich second stage product gas at lower temperatures and for longer residence times, thereby providing a higher methane yield with greater carbon utilization at lower oxygen requirements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of apparatus adapted for use in the process of this invention wherein the third stage portion of the gasification vessel communicates with the second stage portion.

FIG. 2 is a diagrammatic illustration of apparatus adapted for use in the process of this invention wherein the third stage is separate from the first and second stages.

FIG. 3 is a diagrammatic illustration of apparatus adapted for use with the process of this invention wherein the third stage is separate from the first and second stages and a portion of the char produced in the third stage is cooled and recycled to the third stage.

FIG. 4 is a diagrammatic illustration of apparatus adapted for use in the process of this invention wherein the third stage is separate from the first and second stages and wherein char separated from the second stage gas is moved into the third stage and is fluidized in the third stage by hydrogen-rich gas passed upwardly through the third stage to form a fluidized bed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In this specification, the term "coal" means any carbonaceous material including all ranks of coal, lignite, 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 the coal is liberated and at least part of the carbon in the residual char is reacted with those reacting agents or with other reactants present in the gasification process.

The term "product gas" means a methane containing gas such as the gas produced in the third stage of the three stage gasification process described herein.

THREE-STAGE GASIFICATION WITH PARTIALLY CONFINED SECOND STAGE REACTION

In the process illustrated in FIG. 1, a gasification vessel, indicated generally by reference numeral 10, includes a first stage portion 12, a second stage portion 14 and a third stage portion 16. Recycled partially gasified char is conducted through a conduit 18 to first stage portion 12 of gasification vessel 10. Oxygen and steam are conducted through a conduit 20 into the first stage portion 12 and are reacted therein with the partially gasified char while flowing downwardly through the first stage portion 12. That reaction yields a first stage synthesis gas containing hydrogen and oxides of carbon and other products including molten slag formed from char ash.

In first stage portion 12 of gasification vessel 10 the reaction is conducted at a pressure of at least 50 atmospheres and preferably at a pressure greater than 70 atmospheres. The temperature and residence time of reactants in first stage portion 12 are controlled to assure rapid gasification of the char at a temperature above the char ash melting point. A temperature of at least 2,500.degree.F. is preferred, and a residence time of less than 2 seconds is preferred in the first stage of the process.

The molten slag and first stage synthesis gas flow downwardly through a mixing portion, or throat, 22 in gasification vessel 10 wherein the velocity of the first stage synthesis gas and slag flowing into the upper portion of the second stage portion 14 is increased. Coal and steam are introduced through nozzles 24 into an upper area 26 of second stage portion 14 near the location at which mixing portion 22 opens into second stage portion 14. Alternatively, coal and steam nozzles 24 can open into mixing portion 22 above second stage portion 14. Feeding of coal and steam through nozzles 24 and feeding of first stage synthesis gas and molten slag through mixing portion 22 assure the rapid and intimate mixing of those reactants upon their entry into the upper area 26 of second stage portion 14, thereby assuring rapid reaction of the coal with the first stage synthesis gas and steam and rapid transfer of a substantial quantity of sensible heat from the molten slag to the reacting coal. As a consequence of the rapid transfer of heat from the molten slag to the coal, the molten slag cools and solidifies rapidly before any appreciable accumulation of molten slag on the walls of the gasification vessel 10 in second stage portion 14. The rapid transfer of heat from the slag to the coal also provides more complete and efficient utilization of the carbon.

In the second stage portion 14, the coal reacts with the steam and first stage synthesis gas to yield a second stage product including partially gasified char entrained in a second stage product gas containing methane, hydrogen and oxides of carbon. The second stage reaction is conducted at a pressure of at least 50 atmospheres, and preferably greater than 70 atmospheres, and at a temperature and residence time controlled to assure rapid reaction of the coal at a temperature above the range of coal plasticity. It is preferred that the second stage temperature be at least 1,600.degree.F. It is preferred that the residence time for the coal in the second stage be less than 10 seconds, and more preferably, less than 4 seconds.

At the bottom of the second stage portion 14, the partially gasified char entrained in the second stage product gas, passes through a deflection portion 28 of gasification vessel 10, which changes the direction of flow of the second stage products and diverts them into the third stage portion 16 which extends upwardly around and partially encloses the second stage portion 14.

As a result of the rapid cooling of the molten slag in the upper area 26 of second stage portion 14 and the further loss of heat by the slag passing downwardly through second stage portion 14, the slag solidifies to form granules and larger particles which gravitate down through the entrained char and second stage product gas and collect, by reason of their greater density and larger size, in a slag collection portion 30 of gasification vessel 10 within third stage portion 16 below a deflection portion 28. The slag collected in portion 30 passes downwardly through a conduit 32 into an elutriation portion 34 of gasification vessel 10 where the slag is elutriated with steam, to separate the lighter particles of partially gasified char which are trapped in slag collection portion 30 from the slag. The elutriated char passes with a portion of the elutriation steam upwardly into third stage portion 16. Baffles or other deflecting means in the deflection portion 28 prevent the entry of the elutriated char into the second stage portion 14.

The heavier slag particles pass with a portion of the elutriation steam downwardly through a conduit 36 into a slag quenching portion 38 where the slag is quenched with water and ultimately removed from the process through conduit 40 as cool slag.

The dimensions of the third stage portion 16 are selected and maintained such that the char and second stage product gas form a fluidized bed in third stage portion 16 which partially encloses the second stage portion 14. The linear gas velocities in third stage portion 16 are controlled and maintained to assure the formation of the desired fluidized bed. Linear gas velocities in third stage portion 16 are suitable in the range from about 0.5 to about 20.0 feet per second and linear gas velocities in the range of from about 1.0 to about 10.0 feet per second are preferred.

The reaction in the third stage of the process is conducted near the pressure of the second stage. The temperature maintained in the third stage is lower than the temperature in the second stage and the residence time for char in the third stage is substantially longer than the residence time in the first and second stages of the process. A third stage temperature in the range of from about 1,500.degree.to about 1,650.degree.F. is suitable. In the third stage the residence time of the char should be at least five times, and preferably 20 or more times, the residence time of the second stage product gas and other gases therein. A char residence time in the third stage of 20 minutes or more is preferred, especially when high rank coals are used.

The elutriation of the reaction of carbon with hydrogen to form methane exhibits higher methane yields for lower temperatures. Therefore, the reaction in the third stage at a temperature lower than that in the second stage with longer residence and reaction times in the presence of the larger amount of char and larger volume of hydrogen rich product gas contained in the third stage, results in a higher gasification rate and a higher yield of methane with lower oxygen requirements than those for a two-stage gasification process.

Among the products from the third gasification stage, partially gasified char, entrained in a third stage product gas containing methane, hydrogen and oxides of carbon, is withdrawn through conduit 42 to a separator 44, such as a cyclone separator, where the partially gasified char is separated from the third stage product gas. The char is conducted through conduit 46 to a steam ejector 48 where the char is mixed with steam at a pressure at least slightly greater than the pressure in first gasification stage. The char and steam are conducted through conduit 18 for recycling to the first stage portion 12 for further reaction in the process.

Separated third stage product gas is withdrawn through conduit 50 and conveyed to purification apparatus 52 wherein third stage product gas is purified by conversion and the removal therefrom of carbon oxides, hydrogen sulfide and other impurities. The purified third stage product gas passes through conduit 54 to methanation apparatus 56 wherein the purified third stage product gas is catalytically methanated and removed through conduit 58 to yield a fuel gas containing at least approximately 70 percent methane by volume, and preferably at least 90 percent methane by volume.

THREE-STAGE GASIFICATION WITH SEPARATE THIRD STAGE

In the process illustrated in FIG. 2, certain process modifications within the concept of this invention are illustrated. Partially gasified recycle char mixed with steam is introduced through conduit 60 into a first gasification stage portion 62 of a two-stage gasification vessel indicated generally by reference numeral 64. Oxygen and steam are introduced through conduit 66 into the first stage portion 62 and reacted with the char therein to produce a first stage synthesis gas.

The process illustrated in FIG. 2 does not include passage of the first stage synthesis gas and other first stage reaction products through a mixing portion or throat as was employed in the process illustrated in FIG. 1. In the instant process, the synthesis gas and other reaction products from the first stage pass directly into the upper portion 68 of a second gasification stage portion 70, where rapid and substantial mixing between and among the synthesis gas and other reaction products from first stage are effected by the introduction into the upper portion 68 of coal and superheated steam through nozzles 72 controlled to provide the necessary reactant velocities and direction to achieve the desired mixing and rapid heat transfer reaction. The first stage and second stage reaction temperatures, pressures and residence times are substantially the same as for those described in the process illustrated in FIG. 1.

In the instant process, a third gasification stage portion 74 is provided and maintained separate from the first stage portion 62 and second stage portion 70. Partially gasified char from the second stage, entrained in second stage product gas containing methane, hydrogen and oxides of carbon, is conducted through a conduit 76 to a lower portion 78 of third stage portion 74, where the second stage char is maintained in a fluidized bed by the hydrogen-rich second stage product gas. Third stage temperature, pressure and residence times are substantially the same as those described for the third stage of the process illustrated in FIG. 1. It should be noted, however, that in the instant process, residence times can be longer and methane yield and carbon utilization can be higher while oxygen consumption is lower because separation of the third stage portion 74 from contact with the second stage portion 70 avoids the transfer of heat from second stage portion 70 to the fluidized char bed in the third stage portion 74, which heat transfer might raise the temperature, especially in the upper portion of the fluidized char bed in the third stage portion 74, thereby tending to reduce the yield of methane from the reaction of carbon with hydrogen.

In the instant process, slag is transported by the second stage product gas and entrained char through conduit 76 for collection in the lower portion 78 of third stage portion 74. The elutriation, quenching and removal of slag from third stage portion 74 is conducted as previously described for the process illustrated in FIG. 1. Hereinafter, those process elements which are identical with process elements described in previous embodiments and drawings will be identified by identical reference numerals as used in said previous embodiments and drawings.

The removal of third stage char and third stage product gas, including their subsequent recycling and purification and methanation, respectively, is the same as described for the process illustrated in FIG. 1, with the exception that, in the instant process, hydrogen sulfide gas, removed from the third stage product gas in the purification process, is withdrawn through conduit 80 and reacted in a sulfur reclamation reactor, such as a Claus reactor 82, to produce elemental sulfur.

SEPARATE THIRD STAGE GASIFICATION OF RECYCLED THIRD STAGE CHAR

In the process illustrated in FIG. 3, the process steps through the first gasification stage and the second gasification stage are identical with the process steps for the process illustrated in FIG. 2 with the exception that the char and steam recycled to the first stage has a slightly different composition, which will be explained more fully below.

Second stage product gas and char, introduced into the third stage portion 74 through conduit 76, are reacted in the third stage to produce partially gasified char entrained in a third stage product gas containing methane, hydrogen and oxides of carbon, while slag collection, elutriation and removal from the third stage portion 74 are conducted as previously described.

In the instant process, third stage char and third stage product gas are removed through a conduit 82 from third stage portion 74 and quenched with water or other suitable fluid, and passed to a primary separator 84. Separator 84 separates the third stage product gas, steam and fine char entrained therein from the coarse heavier char particles from the third stage. The cooled coarse char is conducted through a conduit 86 to a steam ejector 88. The steam and coarse char are introduced through a conduit 90 into the lower portion 78 of third stage portion 74 to enable further temperature control of the reactants in that stage and to provide additional carbon and other reactants in the char for the third stage reaction. Recycling of additional quantities of char, at low temperatures, to third stage portion 74 provides further increase in methane yield and carbon consumption at low oxygen requirements.

The third stage product gas and entrained fine char separated in primary separator 84 is withdrawn therefrom through a conduit 92 and passed to a secondary separator 94, wherein the fine char is separated from the third stage product gas which is conducted through a conduit 76 to purification and methanation with production of elemental sulfur as previously described in FIG. 2. The separated fine char withdrawn from secondary separator 94 is conducted through a conduit 98 to a steam ejector 100 for recycling through conduit 102 as a mixture of steam and char at a pressure greater than the pressure existing in the first stage.

SEPARATE THIRD STAGE GASIFICATION WITH A COUNTER FLOW FLUIDIZED BED

In the process illustrated in FIG. 4, recycle char and steam are introduced through a conduit 104 into a first gasification stage portion 106 of a gasification vessel 108. Steam and oxygen are introduced through a conduit 110 into first stage portion 106 and flow upwardly therethrough with the recycled char into a second gasification stage portion 112. The reaction conditions in the first and second stages are similar to those described for the other processes described herein except that flow through the first and second gasification stages is in the upward direction and a mixture of coal and steam is introduced through a conduit 114 into second stage portion 112 above the first stage portion 106 for entrainment in the first stage synthesis gas and other first stage products as the synthesis gas and other products travel upwardly into second stage portion 112.

Molten slag formed from char ash produced in the first and second stages accumulates on the walls of the first and second stage portions 106 and 112 of gasification vessel 108. The accumulated molten slag gravitates to the lower portion 116 of the first stage portion 106 where the molten slag collects and is removed through a conduit 118 for quenching and disposal from the system.

Second stage product gas with entrained char is removed through a conduit 120 and passed to a primary char separator 122. In separator 122 the fine second stage char mixed with the second stage product gas is separated and removed from coarse char which is conducted through a conduit 124 to an upper portion 126 of third gasification stage portion 128. As coarse char moves downwardly through third stage portion 128, a hydrogen-enriched gas is conducted through a conduit 130 to a lower portion 132 of third stage portion 128 and travels upwardly through the third stage portion 128 to form a fluidized bed with the downwardly moving coarse char. The instant process enables the use of a smaller size third stage portion 128 of gasification vessel 108 because the use of hydrogen or a hydrogen-rich fluidizing gas permits higher methane yields with smaller volumes of fluidizing gas.

The hydrogen-rich gas is compressed to a pressure of at least 1,100 psig. before introduction into the third stage and maintains the gravitating coarse char in a fluidized bed. As the gravitating coarse char passes the location of the point of introduction of the hydrogen-rich gas at lower portion 146, the gravitating char enters an elutriation portion 134 wherein the hydrogen-rich gas is replaced by an elutriating fluid such as steam. Some char particles are carried upwardly by the elutriating steam from elutriation portion 134 and returned to the reaction in third stage.

The remaining char particles elutriated in elutriation portion 134 pass downwardly therefrom through a conduit 136 to a steam ejector 138. A mixture of steam and char is then recycled through conduit 104 to the first stage portion 106 as described above.

The second stage product gas with entrained fine char removed from primary separator 122 is passed through a conduit 138 and mixed with quenching water for introduction into secondary separator 140. The third stage product gas with entrained char is conducted through a conduit 142 from the upper portion 126 of third stage portion 128 and mixes in conduit 138 with the quenched char and second stage product gas from primary separator 122 for introduction into secondary separator 140.

In secondary separator 140 the product gases from the second and third stages are separated from the char from the second and third stages. The separated char is conducted through a conduit 144 to an intermediate portion 146 of the elutriation portion 134 to mix with the coarse elutriated char in elutriation portion 134. The mixture in elutriation portion 134 of coarse elutriated char and fine char from separator 140 is introduced through the conduit 136 into steam ejector 138 and pressurized to a pressure in excess of the pressure existing in the first gasification stage. A resulting mixture of char and ejector steam is recycled through the conduit 104 to the first stage portion 106 for further reaction in the process.

The mixture of second and third stage product gases separated in secondary separator 140 is conducted through conduit 148 to purification apparatus 150 wherein the gases are passed through a water-gas shift reactor and the hydrogen sulfide and carbon dioxide are removed as separate streams. The hydrogen sulfide gas is conducted through conduit 152 to a sulfur conversion reactor 154, such as a Claus reactor, for conversion to elemental sulfur.

Part of the purified product gases may be conducted through conduits 156 and 158 to a compressor 160 and then conducted through a conduit 130 to the third stage portion 128 while the remaining purified product gases are conducted through a conduit 162 to methanation apparatus 164. Alternatively, a portion of the purified product gases may be conducted through a conduit 156 to a hydrogen separator 168, which may be a cryogenic separator, wherein hydrogen or a hydrogen-rich gas is separated and withdrawn through a conduit 170 and passed to compressor 160 for delivery and recycling through conduit 130 of the hydrogen-rich gas to the third gasification stage portion 128 at a pressure greater than the pressure of the third stage, and preferably approximately 1,100 psig.

As shown in FIG. 4, there are many possible modifications in the ultimate use of the purified product gas transported in conduit 156. The purified product gas leaving the hydrogen separator 168 is conducted through a conduit 172 to the methanation apparatus 164 where the purified product gas is catalytically methanated to yield fuel gas containing at least 70 percent methane by volume and preferably about 90 percent methane by volume.

Inordinate or undesirable temperature increase in the third stage is prevented by steam entering the third stage portion 128 from the elutriation portion 134 because the vaporized water in the steam entering the third stage reacts endothermically with carbon to produce carbon monoxide plus hydrogen gas.

The process of this invention, with certain modifications, can be employed for the production of producer gas from the three stage gasification of coal. In the production of producer gas, the combination of oxygen and steam fed into the first gasification stage is replaced by air as the gasification medium. The air should be preheated to within a range of about 900.degree.F. to about 1,300.degree.F. or more. The air is introduced into the first gasification stage for reaction with the recycle char therein. Reaction pressures in the three separate stages are maintained between approximately 5 and approximately 60 atmospheres. The rest of the reaction conditions and process steps are as described hereinabove. The producer gas obtained by this process using preheated air as the first stage gasification medium is particularly suitable as an industrial fuel or as a fuel for power generation wherein emission of sulfur dioxide is not desirable.

The advantages to be gained with the use of the three stage gasification process of this invention as compared to the use of two-stage gasification process, can be illustrated by a comparison of the following tabulated calculations of the composition of fuel gases which may be obtained by each process and by calculation of the possible coal and oxygen requirements for each process.

TABLE I ______________________________________ Present Process Two-Stage 3 Stages Process ______________________________________ Analysis of gas from Gasifier, dry basis: % by volume CH.sub.4 28.2 20.5 H.sub.2 17.9 24.1 CO 27.5 34.4 CO.sub.2, H.sub.2 S 24.8 19.6 N.sub.2 1.6 1.4 For 1000 scf pipeline gas required: Coal, lb. 88.5 92.5 Oxygen, lb. 28.7 37.2 ______________________________________

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

Claims

I claim:

1. A process for gasifying coal comprising,

introducing a partially gasified recycle char into a first gasification zone,

introducing oxygen and steam into said first gasification zone,

reacting said oxygen and steam with said partially gasified recycle char at a pressure of at least 50 atmospheres to produce a first zone synthesis gas comprising hydrogen and oxides of carbon and a molten slag formed from a molten ash of said partially gasified char,

introducing said first zone synthesis gas and said molten slag into a second gasification zone below said first zone,

introducing coal and steam into said second gasification zone,

reacting said coal, steam, first zone synthesis gas and molten slag in said second gasification zone at a pressure of at least 50 atmospheres to produce a second zone product comprising solidified slag and a second zone partially gasified char entrained in a second zone product gas comprising methane, hydrogen and oxides of carbon,

introducing said second zone product gas and said second zone partially gasified char into a third gasification zone,

forming in said third gasification zone a fluidized bed of said second zone partially gasified char in a fluidizing stream of said second zone product gas,

reacting said second zone partially gasified char in said third gasification zone with said second zone product gas in said third gasification zone to produce a third zone partially gasified char entrained in a third zone product gas comprising methane, hydrogen and oxides of carbon,

removing from said third gasification zone said third zone partially gasified char entrained in said third zone product gas,

separating said third zone partially gasified char from said third zone product gas,

mixing said third zone partially gasified char with steam to produce said partially gasified recycle char,

recycling said partially gasified recycle char to said first gasification zone,

purifying said third zone product gas separated from said third zone partially gasified char to remove carbon dioxide, hydrogen sulfide and other impurities to produce a purified third zone product gas, and

methanating the purified third zone product gas to produce a fuel gas containing at least 70 percent methane by volume.

2. A process for gasifying coal as set forth in claim 1 which includes,

conducting the reaction in said third gasification zone at a temperature within the range of from about 1,500.degree.F. to about 1,600.degree.F.

3. A process for gasifying coal as set forth in claim 1 which includes,

maintaining a residence for said second zone partially gasified char in said third gasification zone of at least five times the residence time of said second zone product gas in said third gasification zone.

4. A process for gasifying coal as set forth in claim 1 which includes,

passing said first zone synthesis gas and said molten slag from said first gasification zone through a mixing zone prior to introducing said first zone synthesis gas and said molten slag from said first gasification zone into said second gasification zone to assure rapid mixing and rapid reaction in said second gasification zone between said coal, said first zone synthesis gas and said steam and to assure the rapid transfer of heat from said molten slag from said first gasification zone to said coal.

5. A process for gasifying coal as set forth in claim 1 which includes,

moving downwardly through said first gasification zone said oxygen, said steam and said partially gasified recycle char,

moving downwardly through said second gasification zone said first zone synthesis gas, said solidified slag, said coal and said steam, and

moving upwardly through said third gasification zone said second zone partially gasified char and said second zone product gas.

6. A process for gasifying coal as set forth in claim 5 which includes,

mixing said molten slag with said coal in said second gasification zone to transfer heat from said molten slag to said coal and thereby form a solidified particulate slag in said second gasification zone,

collecting said solidified particulate slag and a portion of said second zone partially gasified char in a lower portion of said third gasification stage,

removing said solidified particulate slag and said second zone partially gasified char from said third gasification zone,

contacting said solidified particulate slag and said second zone partially gasified char with elutriating steam,

returning a portion of said elutriating steam with said second zone partially gasified char entrained therein to said third gasification stage for reaction therein, and

quenching said solidified particulate slag after elutriation.

7. A process for gasifying coal as set forth in claim 1 wherein,

at least part of said second gasification zone is enclosed by said third gasification zone, and

introducing said second zone partially gasified char, said second zone product gas and said slag directly into a portion of said third gasification zone from said second gasification zone.

8. A process for gasifying coal as set forth in claim 1 which includes,

mixing said third zone product gas and said third zone partially gasified char with water to cool said third zone partially gasified char,

separating said third zone partially gasified char into a heavier coarse portion of said third zone partially gasified char and a lighter fine portion of said third zone partially gasified char entrained in said third zone product gas,

mixing said heavier coarse portion of said third zone partially gasified char with steam,

recycling said heavier coarse portion of said third zone partially gasified char with steam to said third gasification zone,

separating said lighter finer portion of said third zone partially gasified char from said third zone product gas,

mixing said lighter fine portion of said third zone partially gasified char with steam, and

recycling said lighter fine portion of said third zone partially gasified char with steam to said first gasification zone.