US2942959A

US2942959A - Process for the production of fuel gas

Info

Publication number: US2942959A
Application number: US701126A
Authority: US
Inventors: Harry V Rees; Clifford G Ludeman; Sellers Frederick Burton
Original assignee: Texaco Development Corp
Current assignee: Texaco Development Corp
Priority date: 1957-12-06
Filing date: 1957-12-06
Publication date: 1960-06-28
Anticipated expiration: 1977-06-28

Description

June 28, 1960 H. v. REEs ETAI- 2,942,959

RROcRss ROR THE PRODUCTION OR ROEL GAS Filed D60. 6, 3.957 ('aa/ [Z2/ nited States PROCESS FOR THE PRODUCTION F FUEL GAS Filed Dec. 6, 1957, Ser. No. 701,126

3 Claims. (Cl. 48'197) This invention relates to a method and apparatus for the production of fuel gas of high caloric value. More particularly, it is directed to a method which comprises contacting a gaseous mixture of carbon monoxide, hydrogen and water vapor with a carbonaceous solid at a pressure in excess of about 700 pounds per square inch gauge at a temperature to effect reaction of said carbon monoxide, hydrogen, water vapor and carbonaceous solid to form a product gas comprising methane. Advantageously a carbonaceous fuel may be converted to a fuel gas of high. caloric value by subjecting the carbonaceous vfuel to partiall combustion with an oxygen-containing gas at a pressure in excess of 700 pounds per square inch gauge to produce a synthesis gas' comprising carbon monoxide and hydrogen. Steam is adm'ixed with said synthesis gas and the mixture contacted with a carbonaceous solid at substantially the pressure of the syri-V thesis gas generation step forming a fuel gas of high caloric value.

Many processes have been employed -for the gasification of carbonaceous fuels. The simpler and cheaper methods have been characterized by the production of gas of low heating value. Methods for the production of higher heating value fuels have been complicated and expensive. It is known that carbon monoxide and hydrogen may be generated by the partial combustion of carbonaceous fuels. However, such mixtures 'are unsuited for general fuel uses because of the relatively low heating values of hydrogen and carbon monoxide which have gross heating values of 323 to 321 B.t.u.s per cubic foot respectively. It is also known thatvc'arbon monoxide and hydrogen may be reacted to form methane, which has a gross heating value of 1,009 B.t.u.s per cubic-foot, and heavier hydrocarbons, which have substantially higher heating values. The methanization of carbon monoxide and hydrogen mixtures is most effectively carried out at temperatures considerably below the temperatures etlec'- i tive for rapid and eicient partial oxidation. In the past, it has been observed that the cooling of synthesis gas mixtures from gas generation temperatures to lower temperatures at which methanization is substantial has resulted in the production of carbon deposits. We have found that contrary to the teachings of the prior art, the temperature of synthesis gas mixtures may be reduced without the production of carbon but with anet consumption of carbon by admixing steam with said synthesis gas and contacti-ng the aforesaid mixture with a carbonaceous solid at a pressure in excess of about 700 pounds per square inch gauge. Gaseous mixtures of carbon monoxide and hydrogen may be produced readily by the partial oxidation of gaseous, liquid, or solid carbonaceo'us fuels. Liquid or solid carbonaceous fuels for example, coal, lignite oil shale and the'like, residual oils, fuel oils, shale oils, tar oils, gas oils, kerosene and gasoline boiling range hydrocarbons and they like may be used depending upon-their relative availability and cost. The partial oxidation of: solid carbonaceouslfuels is described'in detail in the copending application of duBois-Eastman and Leon atei: i

Patented .inne 23, l

P. Gaucher, Serial No. 490,214, tiled February 24, 1955, now U.S. Patent 2,864,677. In the operation of a coal fired synthesis gas generator the reaction temperature is maintained vwithin the range of about 2000 to 3500 F. Partial oxidation of coal has heretofore usuallyrbeen conducted at pressures within the range of about to about 600 pounds per square inch gauge. However, the synthesis g'as generation process may be conducted advantageously at higher pressures and we prefer to effect synthesis' gasl generation at pressures Within the range of 4about 700 to 4500 pounds per square inch gauge.V Although pressure per se has only little eect on the synthesis gas generation process, the generation of synthesis gas at the pressure of utilization is a convenient method of providing synthesis gas at high temperature and high pressure without extensive compression and reheating facilities.

Some carbonaceous materials require the `addition of steam for the production of hydrogen and carbon monoxide by reaction with oxygen at temperatures within the range of about 2000V to 3500 F. Other carbonaceous materials contain water in sufficient quantities or even in excess of that required. Anthracite coal is an example oi the former, requiring a considerable quantity of steam, for example, about 30 percent by weight. Lignite is an example of the latter, often containing more water than required for reaction. Water in excess of the normal requirements is not 4detrimental in the process of this invention. However, water in the gas generator feed effects theY generator temperature and oxygen requirements. While water may supply a part of the oxygen requirementsl of the system thereby increasing the hydrogen production, water is an endothermic reactant and lowers the reaction temperature. Advantage may be taken of this characteristic to effect reaction temperature control by regulation of the water content of the feed.

Advantageously particles of solid carbonaceous materials are admixed with aV liquid to form a suspension or slurry of particlesin a vaporizable liquid -for example, water. The aforesaid suspension is passed as a continuous stream through an externally heated tubular zone under turbulentiiow conditions. The slurry is heated in the heating zone to an elevated temperature suicient to vaporize the liquid thereby forming a suspension of solid particles in Vapor and preheating the solid particles. The dispersion of solid particles in vapor is then admixed with anl oxygen-containing gas in an unobstructed partial oxidation zone autogenously maintained at a temperature' within the range of about 2000 to about 3500 F. and at a pressure of above Iabout 700 pounds per square inch gauge.

A. preferred method of eecting the partial oxidation of heavy hydrocarbon oils is disclosed in the copending application of Dale M. Strasser, Serial No. 523,641, led iuly 21, 1955, and now abandoned. The partial oxidation of hydrocarbon oils is usually effected at a temperature within the range of about 1800 to about 3500 F. and at -a pressure within the range of about 20 to about 3000 pounds per square inch gauge. However, as in the case of carbonaceous solids, the partial oxidation orrr oils may be effected advantageously at pressures in excess of 700 pounds per square inch gauge and as high as 4500 pounds per square inch gauge if desired. Oil is readily introduced into a gas generator in the form of a dispersion in steam. In accordance with one method of introducingl oil feed, the oil is mixed with about l0 to about 200 weight percent of water. The mixture so formed is passed to a tubular heating zone under turbulent flow conditions to effect substantially complete vaporization of the-Water forming a dispersion of minute particles of oil in steam. The oil-steam dispersion in admixture with oxygen is introduced into a partial oxilower temperatures.

(lation zone maintained at an autogenous temperature within the range of about 1800 to 3500" F.

Product gases from the partial oxidation of solid carbonaceous fuels or of liquid hydrocarbons may have compositions falling Within the range of about 25 to about 70 percent of hydrogen, 25 to 70 percent carbon monoxide,

' 1 to 20 percent carbon dioxide,'3 to l0 percentrwater yapor, less than Vabout 0.5 percent methane, and from about to about 5 percent of other gases. Elluent gas is withdrawn from the gas generatorat substantially the temperature andrpressure of the gas generator and passed in admixture with Water or steam to a methanization zone wherein the aforesaid mixture is contacted with a carbonaceous solid at a reduced temperature. The carbonaceous solid presen-t may be that produced in situ upon cooling or may be fromvan external source.

In the methanization zone the hydrogen, carbon monoxide, water vapor, and carbonaceous solid react with one another to produce substantial quantities of methane. Methanized gas, after removal of carbon dioxide Vand Water vapor, is readily produced having heating values in excess of 430 B.t.u.s per s-tandard cubic foot and in excess of 5510 B.t.u.s per standard cubic foot under preferred operating conditions. vCarbon dioxide may be readily removed from gaseous mixtures by methods Well known inthe art, for example, by scrubbing with caustic or ethanolamine solutions. Most .ofthe water vapor is removed by condensation upon cooling to atmospheric temperatures and further drying is usually neither necessary nor desirable when the product gas is intended for fuel use. V

. We have found that important operating variables of the methanization process include pressure, terminal reaction temperature, and theV ratio of mols of water per atomof carbon contained in the synthesis gas-steam feed to the methanization zone. We have found that pressure exerts a marked elect upon the extent of methanization which may beobtained. At temperatures Within the range of about 1000 to 170091:., veryilittle methanization of synthesis gas mixtures may be obtained at pressures near atmospheric. However, as pressure is increased, the

' amount of methane which may be produced in synthesis gas rises rapidly, up to a pressure of about 700 pounds per square inch gauge. Further increase in pressure above 700 pounds per square inch gauge has relatively little effect upon the total amount of methane produced. It is therefore preferred lto effect the methanization processat a pressure in excess of about 700 pounds per square inch gauge and pressures within the range of about 700i to about 4500 pounds per square inch gauge or higher are suitable.

Another important operating variable of the methanization process is the reaction temperature.v Temperatures in the methanization step of this process may vary from the high temperature at which the synthesis gas is produced to the final or terminal temperature of the reaction zone, thatis, the temperature of the reactants when they are separated from the carbonaceous solid as set forth hereinbefore. Synthesis gas may be produced at temperatures within the range of about 1800 to about 3500 F. Although reaotionyelocities are greatest at high temperatures, the production of substantial quantities of methane requires effecting methanization at substantially Since low temperatures favor compositions haying high caloriicvalues but reaction velocities are greatest at high temperatures, most efficient utilization of reactor volume is achieved by reducing the temperature gradually as the gases undergoing conversion pass through the reactor. In this manner, maximum rate is maintained as the gas composition changes until the final composition is reached. Reduction of the methanization temperature from the high temperature of the ffeed to the relatively lov'.7 terminal reaction temperature may be eEected Vby one or more of numerous means. lWater er stream introduced into the methanizer feed as a reactant may be employedras coolant as desired. Additionally, the Water vapor present in the methanizer is an endothermic reactant in that reaction of Water vapor with carbon monoxide and with carbon tends to absorb heat. The steam or water introduced into the'rmethanizer may readily be employed to control reaction temperatures by control of the rate of steam or Water added responsive to temperature measurements. A temperature gradient may be established in the bedv of carbonaceous solid by intro; ducing steam incrementally throughout the length of the reaction hed. The cooling capacity of the Water orrsteam employed maybe varied by control of the preheat'of Y the Water or steam employed. Obviously, heat exchange Vconsumption of carbonaceous solid.

of the synthesis gas feed with other processing streams, and the use of cooling coils disposedY Within the bed of carbonaceous solids may be used for temperature control and the recovery of useful heat. Some reduction in temperature is effected by radiation of heat from the Walls ofthe methanization zone. Terminal temperatures within the range of about 1000 to -l700 F. may Voe employed in the process of this invention to produce substantial quantif ties of methane. However, we prefer to employ terminal temperatures within the range of about 1300 to about 1500 F. At terminal reaction temperatures within the range of 1000 to 17010" F., We employ reaction times within the range of about Y5 to 1500 seconds, preferably within the range of about 25 to 500 seconds.

We have found that by maintaining a ratio of mols of Water to atoms of carbon in the methanizer feed above about 0.7 that the production of carbon deposits may be prevented in our process. We prefer to employ ratios of mols of water vto atoms of carbon in excess of about 0.7-

and preferably in excess of about `0.9 in order tofefect Ratios of mols of waterto atoms of carbon up to about 2.0! may be employed with good results to effect the consumption of substantial amounts of carbonaceous solid.

In the process of our invention the methanization reaction is effected in the presence of a bed' of carbonaceous solid. Variouscarbonaceous solids are suitable, for example petroleum coke, coal, lignite and coke oven coke. When fuels are employed containing volatilizable Vconstituents, for example, coal or lignite, the volatile constituents are distilledV from the methanization zone and appear in the product gas contributing to the heating value of the gas produced. Petroleum coke is a preferred carbonaceous solid because of the negligible ash content. Since Ithe carbonaceous solid is continuously consumed in our process, it is necessary to add carbonaceous solid to the methanization zone as make-up for that consumed. It is' preferred to maintain the carbonaceous solid in the form of a fixed bed so that a temperature gradient may be established to permit reduction in the temperature of the reactants to the desired terminal temperature.

An advantage of the process of this invention is that carbonaceous solids and heavy'fuel oils may be eiciently `converted to fuel gases of high caloric value.

Another advantage of this process is that gases comprising carbon monoxide and hydrogen produced by partialrcombustion may be methanized without the deposition offcarbon.

Another advantage of the process of this invention is that the high sensible heat resultant from the high .temperature partial oxidation of carbonaceous fuels may be efficiently utilized to convert steamand carbonaceous solids to gas of high caloriiicvalue.

Figure l diagrammatically. illustrates au embodiment of the process of this invention.

'Figure 2 is a graphical representation of the effect of pressure upon the heating value of gases produced according to the process of this invention.

Although Figure l illustrates one embodiment of apparatus in which the process of this invention may be lpracticed it is not intended to limit the invention to the .particularapparatus or materials described.

Referring to Figure 1, coal from an external source, not shown, is introduced through line 2 into mixer 3. Water from an external source not shown, is admitted -to the mixer through line 4 in an amount sulicient to form a iluid slurry of coal particles in Water. Slurry is withdrawn from mixer 3 through line 5 and is forced under pressure by pump 7 to a heating coil 8 in heater 9. The slurry in heating coil 8 is heated to a temperature at least sufficient vto vaporize the water contained therein. The resultant dispersion of coal-in-steam is discharged through line 10 to gas generator 12. Synthesis gas generator 12 may comprise a pressure vessel provided with an internal refractory lining enclosing an essentially unobstructed reaction space. `A suitable gas generator for the partial combustion of ,solid fuels is described in the copending application of duBois Eastman, Serial No. 525,240, filed July 29, 1955, now U.S. Patent 2,871,114.

Oxygen for the reaction :is :supplied from an external source, not shown, through line 13 and is passed at a rate controlled by valve 14 to gas'generator 12. In gas generator 12 the coal, oxygen, and steam are reacted at a temperature within the range of about 2000 to 3500 F., Apreferably in the range of about 2200V to about 2800 E., to produce a gas comprising carbon monoxide and hydrogen. Euent synthesis gas from generator 12 is discharged through line 15 to methanizer 24. Water, either in the form of liquid water from line 16 at a rate 'controlled vby valve 17, or steam from line 19, heating coil 20 and lines 2.1 and 22 at a rate `controlled by valve 23, or both water and steam are passed through line 18 and ladmixed with the synthesis gas in line 15 to form the gaseous feed mixture passed to methanizer 24. The temperature of the gas mixture entering methanizer 24 may be readily controlled by varying the quantity of water, steam, or both introduced through line 18 as well as varying the temperature of the steam by control of heating coil 20. Y

Metallurgical coke is introduced through valve 28into lock hopper 27. Pressure in lock hopper 27 is then increased to the pressure of methanization zone 24, and rthe coke is introduced into the methanization zone 24 by opening valve 29. Coke, in the form of a dense bed, slowly ,gravitates from the top to the bottom of methanizer 24 as coke in the bottom of the methanizer is consumed by reaction with the carbon monoxide, hydrogen and water vapor present. Ash is withdrawn from the bottom of methanizer 24 by means of lock hopper 30 and

valves

32 and 33. Optionally the reaction tempera ture in methanizer '24 may be gradually reduced by introducing steam into the coke bed at various levels. Steam for this purpose is introduced through

lines

21, 34, and 34a, 34b, 34e and 34d at rates controlled by

valves

35a, 35b, 35e and 35d. Reactants reaching the top of methanizer 24 reach a terminal temperature within the range of about 1300 to 1500 F. and are withdrawn through line 37, cooled in heat exchanger 38, and are passed through line to water separator 45. Water is discharged through line 46. Separated gas is Withdrawn from separator through line 47 and passed to carbon dioxide and sulfur removal facility 48. Methods are well known which may be used for the removal of carbon dioxide and sulfur compounds from gaseous mixtures, for example, scrubbing with Water, caustic or ethanolamine solutions. r1`hese scrubbing methods also remove much of the sulfur present in the gas treated, however if substantially complete sulfur removal is desired, the scrubbed gases may be further treated, for example, by contact with iron or an iron oxide such as luxmasse. Carbon dioxide and sulfur compounds separated from the gas are discharged continuously or `intermittantly through line 49 either during processing or during regeneration. Product gas having a heating value in excess of 500 B.t.u. per thousand standard cubic feet is discharged through line 50.

The invention will be Ymore clearly understood with reference to the following example.

EXAMPLE Table yI Synthesis Gas Generation Test `1 Test 2 Test 3 Steam and oil temperature, F-. 710 710 710 Oxygen temperature, F- 80 80 80 Reaction temperature, F 2, 600 ,2, 690 2, 780 Pressure, p.s.i.g p .27,9 720 2, 925 Clflarge and product rates, per pound of ,oil

Steam, pounds 382 .332 382 Oxygen, SCF 11.61 11.61 11.61 Synthesis gas, as produced, SCF 53.1 53. 0 52. 6

Analysis, volume percent:

Hz 42.1 41g 4. 8 4. 0 V5.3 2 2 2 2 2 .2 51. 0 51. 1 51-0 CO2 1.6 1.6` ,1.6 Heating value, higher, .after treatment for sulfur, OO2 and Water ret maval, BTU/SCF 321 '321 '325 It will be vnoted that Tests l, 2 andV 3 vare conducted at essentially the same conditions with the exceptionof the synthesisgas generationy pressure whichis respectively 279, 720 and 2,925 pounds per squareinch gauge. ;It Will be noted that the synthesis gas composition is substantially thesame `for all three tests. The heating values shown in Table I are obtained for comparison by treatment of the synthesis gas with an ethanolarnine solution and iron-iron oxide for removal of sulfur and water is -separated by condensation.

The synthesis gas of Tests l, 2 and v3 is passed directly to a-methanization reactor in admixture with steam and a carbonaceous solid for a contact time of about sec'- onds in each case With the results and at 'the conditions shown in Table II, following:

Table II Methauizaton Test Test Test Test Test 1A 1B 2A 2B 3 Water/carbon ratio 0. 50 1. 00 0. 91 1. 82 1. 00 Terminal temperature, F l, 430 1,430 l, 340 1, 340 1, 520 Pressure, psig 279 279 720 720 2, 925 Charge and product rates, per

pound of synthesis gas generator oil feed:

Water or steam added,

pounds 1. 09 1. 086 2. 294 l. 196 Coke consumed, pounds- 0.235 0.046 0.499 0.153 Coke produced, pounds. .046 Raw product gas, SCF 53.1 71. 9 59. 3 88. 9 62.0

Analysis:

H 28. 3 28.9 17. 6 18. 0 l5. 2 CH4. 9. 4 9. 8 15. 2 15. 9 17.0 H10 21.3 21. 5 32.6 32. 5 31. 5 H2S-l-COS 0.2 0.2 0.2 0.1 0.2 0. 2 0.2 0.2 0.1 0.2 CO 21.8 21. 4 9. 7 9.5 12.1 CO2.- 18.8 18.1 24.5 23.8 23.8 Methanized fuel gas, treated for removal of C02, water and sulllII SCF/pound of synthesis gas generator oil feed 31. 9 43. 6 25. 7 39. 1 27. 9 Heating value, higher, BTU/ SCF 426 430 559 566 577 Effluent product from Test 1 in Table I is employed at Water to -carbon'ratios of'0.50 and 1.00 identified as Test 1A and 1B respectively in Table II. Similarly the synthesis gas from Test 2Table I, is employed at water to carbonY ratios at 0.91 and 1.82 identied as Test 2A and'ZBrespectively in Table II. The efuent gas from Test 3, Table I, is employed in Test 3, 'Table II. It will be noted yfrom the result in Test'lA, wherein the water to carbon ratio was below the critical level of 0.7, that carbon wasnotconsumed buton the contrary there was a depositionl of about 0.046 pound of carbon per pound of synthesis gas generator oil feed. Test l-B shows that when the water to carbon ratio'is increased to a point above the ''riticyaluf"0.7,"Cfbn 'isvv nt produced but Vis C011'- sumed at 'the rate of 0.235 pound' per pound of synthesis l gas Vgenerator oil feed; "The effect of pressure'upon the methanization reaction andhence upon the heating value of the fuel gas producedfisV evident by comparison of the heating values of the methanizredlfuel `gas from Tests 2A, 2B and 3 with Tests 1A and 1B. In the former the pressures are in excess of 720 pounds per square inch gauge and heating values `in exe'ss'of 559 B.t.u.s per standard cubic foot are obtained. Y In Tests =1A and 1BV pressures less than flflflpndfslierffsqlate inch gauge are employed and heating -values ofV 430 B.t.u. per standard cubic feet or loW'er vareV obtained. kThe increase in heating value of the fuels produced by methanization is evidentV by comparison of the heating values of the methanized fuel gas in Table II with the corresponding heating values of thesynthesis g'las Ain TableAI.

The heatingvalues of the methanized -fuel gas'f VTable II and the synthesis gas of Table I are plotted in Figure 2. Figure2 clearly shows the eiect of methanization and the relative effect of Vpressure upon the heating value of the methanized fuel gas. YIt is evident that heating value increases rapidly as pressure is increased up to a critical pressure yof about 700 pounds per square inch and Vincrease of pressure above this level has little eifect upon the heating value of the fuel gas produced.

' Obviously, many modifications and variations of the invention as hereinbefore set forth may lbe made without departing from ythe spirit and scope thereof and only suchlimitations should be imposed as are indicated in the appended claims. Y

lus

Y We claim: Y 1 Ar process for the manufacture of fuelgas which comprises subjecting a carbonaceous fuel Yto partial combustion with an oxygen-containing gas ata temperature 'Within vtherange of about 1800 to 3500 F.Y andl at fa pressure in excess of about 700 pounds per square inch gauge forming a raw synthesis gas comprising mainly carbon monoxide and hydrogen, passing said raw synthesis gas together with suicient steam to provide a gas mixture comprising in excess of 0.7 mol of Water vapor per atomof carbon at an elevated pressure above 700 pounds per square inch gauge and a temperature above 1800 F. into' contact with a fixed Ibed of solid carbon, passing said 'gas mixture through said Ibed and introducing steam into Y comprising methane and having a ,higher heating value than said raw synthesis gas.

2. A process according'to claim l-wherein said bed of carbon is petroleum coke. r 3. A process according to claim l wherein said terminal reaction temperature is within the range of labout 1300 to 1500 F.

References Cited in the file of this patent UNITED STATES PATENTS Mm., ww,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,942,959 June 28 1960 Harry V. Rees et a1.

yIt is hereby certified that error appears in the above numbered pateent requiring correction and that the said Letters Patent should read as corrected below..

Column 2, line 53, for "523,641" read 523Y6O1 e; line 54, strike out and now abandoned".

Signed and sealed this 14th day of November 1961.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Commissioner of Patents Attesting Officer USCOM M-DC

Claims

1. A PROCESS FOR THE MANUFACTURE OF FUEL GAS WHICH COMPRISES SUBJECTING A CARBONACEOUS FUEL TO PARTIAL COMBUSTION WILTH AN OXYGEN-CONTAINING GAS AT A TEMPERATURE WITHIN THE RANGE OF ABOUT 1800 TO 3500*F. AND AT A PRESSURE IN EXCESS OF ABOUT 700 POUNDS PER SQUARE INCH GAUGE FORMING A RAW SYNTHESIS GAS COMPRISING MAINLY CARBON MONOXIDE AND HYDROGEN, PASSING SAID RAW SYNTHESIS GAS TOGETHER WITH SUFFICIENT STEAM TO PROVIDE A GAS MIXTURE COMPRISING IN EXCESS OF 0.7 MOL OF WATER VAPOR PER ATOM OF CARBON AT AN ELEVATED PRESSURE ABOVE 700 POUNDS PER SQUARE INCH GAUGE AND A TEMPERATURE ABOVE 1800*F. INTO CONTACT WITH A FIXED BED OF SOLID CARBON, PASSING SAID GAS MIXTURE THROUGH SAID BED AND INTRODUCING STEAM INTO SAID BED ALONG THE PATH OF FLOW OF SAID GAS AND RESULTING REACTION PRODUCTS, SAID ADDED STEAM BEING SUPPLIED IN RELATIVE PROPORTIONS SUFFICIENT TO REMOVE HEAT OF REACTION AND REDUCE THE TEMPERATURE OF THE GAS STREAM TO A TERMINAL REACTION TEMPERATURE WITHIN THE RANGE OF ABOUT 1,000 TO 1,700*F. WHEREBY A TEMPERATURE GRADIENT IS MAINTAINED THROUGH SAID BED OF CARBONACEOUS SOLIDS EFFECTING GRADUAL COOLING OF SAID GASEOUS MIXTURE AND GASEOUS REACTION PRODUCTS PASSING THERETHROUGH TO SAID TERMINAL REACTION TEMPERATURE EFFECTING REACTION OF SAID CARBON MONOXIDE, HYDROGEN, WATER VAPOR AND CARBON TO FORM A PRODUCT GAS COMPRISING METHANE AND HAVING A HIGHER HEATING VALUE THAN SAID RAW SYNTHESIS GAS.