US2672410A - Gasification of carbonaceous solids - Google Patents

Description

March 16, 1954 w MATTOX 2,672,410

GASIFICATION OF CARBONACEOUS SOLIDS Filed D60. 1, 1949 EXCHANGER O2 INLT STEAM INLE7" Patented Mar. 16, 1954 2,672,410 .GASIFIGATION OF CARBONACEOUS SOLIDS William J. Mattox, Baton Rouge, La.,assignor to Standard Oil Development Company, a corporation of Delaware Application December 1, 1949, Serial No. 130,493

15 Claims.

The present invention relates to the production of gases from non-gaseous carbonaceous material, and specifically to the production of gas mixtures containing carbon monoxide and hydrogen, such as water gas, from such carbonaceous materials as coke and coals, wherein the formation of carbon dioxide is repressed by the addition of an alkali metal halide.

It has long been known that solid fuel materials, such as coke, coal, and the like, may be converted into more valuable gases which can more easily be handled and more efficiently utilized for a greater variety of purposes. One of the most widely practiced gas-generating conversions is the so-called water-gas process in which solid fuels, such as coal or coke of any origin, are reacted with steam at temperatures of about 1400" to 2000 F. to produc water gas mixtures of carbon monoxide and hydrogen in varying proportions, depending mainly on the time of contact, conversion temperatures, and steam feed rate. The overall water gas reaction being endothermic, heat must be supplied; this is usually accomplished by the combustion of a portion of the carbonaceous feed with an oxidizing gas, such as oxygen, at about 1600-2400 F. The combustion reaction may be carried out either simultaneously with the water gas reaction or alternately in a make-and-blow fashion.

The water gas process permits the production of gas mixtures of varying composition and B. t. u. content. The process as such, therefore, is suited not only for the production of fuel gases but also for the production of gases for hydrogenation processes and particularly for the catalytic synthesis of hydrocabons and oxygenated organic compounds from C and H2, which process requires H2:CO ratios, depending on the products desired and reaction conditions to be maintained, varying within limits of about 0.5 to 2.5 or more volumes of H2 per Volume of CO.

The technical utilization of the water gas process, particularly for hydrogenation and for production of synthesis gas, has been impeded by difficulties encountered particularly in heat supply, continuity of operation, and limitations in temperature imposed by low ash fusion or softening points. The problem of continuity of operation has been satisfactorily solved heretofore by the application of the fluid solids technique wherein the carbonaceous charge is reacted in the form of a dense turbulent mass of finely divided solids fluidized by the gaseous reactants and products. Heat is generated either by partial combustion of carbonaceous materials within the ga generator in a so-called single-vessel system or a continuous circulation of suspended solid carbonaceous material to a separate heater in which heat is generated by combustion of the carbonaceous constituents of the residue, and recirculation of the highly heated fiuidizable combustion residue to the gas generation zone to supply the heat required therein in a so-called twovessel system.

The single-vessel system would as such; be more desirable than the two-vessel system because the latter requires the circulation of tremendous quantities of solids between the two vessels, a factor which presents serious problems of design and equipment maintenance. However, single-vessel operation involves the disadvantage of product gas dilution with nitrogen and carbon dioxide when air is used as the combustion-supporting gas. Since technically pure oxygen has become available at relatively low cost, nitrogen dilution may be eliminated by the use of oxygen in a commercially feasible operation. However, carbon dioxide formation remains a problem seriously affecting the practicability of the otherwise preferable single-vessel system. Recent investigations have shown, for example, that in a gasification operation employing 2300 lbs. of powdered coal, 1700 cu. ft. of oxygen and 2000 lbs. of steam per hour to produce 70,000 cu. ft. of water gas, the product gas contains about 16% of CO2. This carbon dioxide represents a total loss as faras the output of hydrogen or synthesis gas is concerned. The significance of this loss will be appreciated when it is borne in mind that the present methods of synthesis gas production account for about of the cost of synthetic fuel production. Although CO2 formation may be reduced to a certain extent by adjusting operating conditions, this can be done only at the expense of generator capacity.

The present invention substantially alleviates these difficulties and afiords various additional advantages as will appear from the description given below wherein reference will be made to the accompanying drawing.

In accordance with the present invention, carbon dioxide formation is considerably reduced and even substantially eliminated in water gas generators wherein carbonaceous solids are gasified with steam and oxygen, by the addition to the generator of small proportions of a halide which at the reaction conditions yields halogen and/or a volatile halogen compound.

, 'Alkali metal halides, such as the chlorides, bromides, and fluorides of sodium, potassium, lithium, rubidium, and cesium, or mixtures of such halides are particularly suitable for the purposes of the invention, but alkaline earth metal halides, such as those of barium, calcium, and magnesium, may also be used. Sodium chloride, because of its ready availability and low cost, is the preferred addition agent. Proportions of about 0.015%, preferably about 0.05-l%, by weight of alkali metal halide based on carbon supplied to the generator, are suitable to depress C02 formation from about 15-20% to less than about 5% of the product gas.

While it is not intended to limit the present invention by any theory of the reaction mechanism involved, it is believed that the effect of the addition of halides may be explained as follows. Alkali metal halides, such as NaCl, react with steam in the presence of oxygen at the gas generation temperatures of about 1800-2000 F., to yield free halogen and/or hydrogen halide and alkali metal oxide and carbonate. Small amounts of halogen or volatile halogen compounds are known to suppress the formation of CO2 in favor of an exclusive formation of CO in the combustion of carbonaceous materials. Since CO2 formation in the water gas generator is largely due to a complete combustion of carbon to CO2, the

halogen or halogen halide supplied by the alkali metal halide will act to suppress CO2 formation in the water gas reaction.

It will be appreciated from the foregoing that the addition of halides in accordance with the invention has nothing in common, regarding its purpose and effect, with the known addition of such agents catalyzing the water gas reaction as alkali metal carbonates, nickel, heavy metal oxides or sulfides, etc., or mixtures thereof which have no inhibiting effect on the formation of 00:. It is, however, a particular advantage of the present invention that such catalytic agents as alkali metal oxides or carbonates are formed as the solid residue of the liberation of halogen or hydrogen halide. As a result of this secondary catalytic eiiect, the gasification temperature may be substantially reduced and the 00 content of the product gas considerably increased. The latter effect is due mainly to an acceleration of the reaction which may be very appreciable at NazCOa concentrations as low as 0.01%, as has been shown by other researchers.

In accordance with the preferred embodiment of the invention, the alkali metal halide to be added is introduced into the gas generator by the injection of, or the impregnation of the carbonaceous charge with natural brines containin the desired halide, for example sea water. The alkali metal oxide or carbonate remaining on the gasification residue may be recovered as the corresponding hydroxide or carbonate by an aqueous extraction of the gasification residue. The aqueous extract may be used as a scrubbing solution to remove any excess halogen from the product gases, if desirable. The halide salt content of the used scrubbing solution may be reutilized in the process as a carrier of alkali metal halide to be added to the generator in accordance with the basic principle of the invention. In some cases, it may also be desirable to pass a certain quantity of halogen with the gas produced to a synthesis stage in order to promote a selectivity more favorable to certain desired products, in-

4 control the Hz/CO consumption ratio of the synthesis reaction.

The process of the invention may be carried out in any system wherein carbonaceous solids are reacted with steam and oxygen to produce gas mixtures of the type of water gas. Any conventional gas generator working on this principle may be adapted for the purposes of the invention, independent of the special technique involved, i. e. the essential advantages of the invention may be realized in fixed bed, moving bed, fluid bed or true suspension operation. However, fluid operation is preferred, because of its superior characteristics of gas-solids and solids-solids contact and its improved heat transfer characteristics, which greatly enhance the inhibiting and catalytic effects of the addition agents of the invention. A system of this preferred type is illustrated in the drawing, the single figure of which depicts schematically an expanded flow plan of the process.

Referring now to the drawing, the system shown consists essentially of a conventional fluid-type water gas generator [0 and metal halide recovering equipment 30 and 40, the functions and cooperation of which will be forthwith described using the gasification of a low temperature coke in the presence of NaCl as an example. It should be understood, however, that the system may be operated in a similar manner for the gasification of other carbonaceous solids, such as various coals, in the presence of other suitable halides.

In operation, fresh coke ground to a particle size passing 4 mesh with a major proportion passing 60 mesh is fed from a feed hopper, preferably a lock hopper I, into a standpipe or other conveying means 3 into generator Ill. The coke may be preheated to temperatures of about 400- 800 F. in hopper I by means of product gases passed therethrough via lines 6 and 8, or in any other conventional means. Sea water is introduced through line 5 into standpipe 3 in amounts sufiicient to introduce about 0.054% by weight of NaCl, based on carbon, into the coke. About 25-100 gals. of sea water per ton of coke passing through standpipe 3 are normally adequate for this purpose. If the coal is preheated as described, the sea water evaporates quickly in the standpipe and the steam so generated acts as an aerating agent.

Simultaneously, a mixture of steam and Oz in a ratio of about 700-1800 lbs. of steam per 1000 cu. ft. of O2 is supplied from lines 12 and M, respectively, through a distributing device such as grid 16. About 1000-1500 cu. ft. of oxygen and about 700-2400 lbs. of steam per ton of coke are normally sufiicient to support the water gas reaction in the presence of NaCl at temperatures of about 1'7001800 F. In the absence of NaCl, the steam and oxygen consumption are somewhat higher for the production of a gas of comparable composition, and the process is much more sensitive to variations in the concentration of these reactants than one employing the alkali halide additive. Reactor [0 should be so designed that at the solids and gas feed rates involved a linear superficial gas velocity of about 0.5-1.5 ft. per second is maintained in reactor l0, at which its contents are converted into a highly turbulent fluidized mass M10 having an upper interface L10 and an apparent density of about 20-40lbs. per

cu. ft.

A dilute solids-in-gas suspension is withdrawn overhead from level L10 and passed through a gas-solids separator, such as cyclone it, from ,gas, at the conditions spec v d pcsitign about-as follows:

olsperce H2 A to 45 Total Cl (free and combined) 0.5

tb ir iulpur ti s T-Tg Any desired BQl'fiQQ-Qfi th ntellec gas may be hea exchanged. with case s ccer 1 via Y lines 6 and '8 as above described. The total gas efiluent in line 22 may then be further cooled in heat exchange with feed. gases and/or other media in a heat exchange and cooling system 24 to be passed through line 26 to a scrubber 30 at a temperature of about 60 to 200 F.

Returning now to generator l0, solid gasification residue is withdrawn through line 35 and passed to an extractor 40. Sea water may be supplied through line 42 to extractor 40 in amounts adequate to extract the soda content of the ash. All or part of the sea water to be supplied to line may be used for this purpose. Extracted ash may be discarded via line 44. Enriched sea water is passed through line 46 to scrubber 30 wherein it is used to remove chlorine and its compounds from the product gas in a manner obvious to those skilled in the art. Sea water which may now contain NaCl somewhat above its normal concentration may be returned via line 48 to feed line 5. The product gas recovered from scrubber 30 through line 50 is now substantially free of halogen and is ready as a feed gas to a conventional hydrocarbon synthesis or other hydrogenation unit, if desired after further desulfurization by conventional means. However, scrubber 38 may also be so operated that a substantial proportion of the sulfur content of the product gas is absorbed therein.

The above description and exemplary operations have served to illustrate specific embodiments of the invention but are not intended to be limiting in scope.

What is claimed is:

1. In the production of ga mixtures containmetal halide yielding a reaction product selected from the group consisting of elemental halogen and volatile halogen compounds at the reaction conditions said halide being present in amounts providing a halogen equivalent corresponding to that of about 0.01-1% by weight of alkali metal halide based on carbon present and said amounts being adequate to inhibit the formation of CO2 in favor of CO formation.

2. The process of claim 1 in which said halide is an alkali metal halide.

3. The process of claim 2 in which said halide is sodium chloride.

4. The process of claim 1 in which said halide is added to said materials prior to said reaction.

5. The process of claim a in which said halide is added in the form of a natural halide brine.

6. The process of claim 5 in which said'brine comprises sea water. 1

7. The process of claim 1 in which solid gasification residue withdrawn from said zone is ex- "6 treated with anaq eousm l m issolving compounds of the cation of said halide-to iorma solution enrichedin said compound.

.8. The method of. claim 7 which said gas mixture isscrubbedwith said solution to remove at lea-eta substantial portion of itscontent in halogen and volatile .halogen compounds from said gas mixture and to form .saidhalide in said. solution.

;9. The process of claim 8, in which the halide termed in said solution isv introduced into said zone.

10. The process of claim lin which said halide is an alkaline earth metal halide.

11. The process of claim 10- in which said halide is selected from the group consisting of the halidesof barium, calcium and magnesium.

12. In the production of gas mixture containing Hz and CO by reacting solid carbonaceous materials with steam and oxygen in a gasification zone at gasification conditions, the improvement which comprises contacting said steam and oxygen simultaneously at said conditions in said zone adding to said zone an extraneous metal halide forming at said conditions a. reaction product selected from the group consisting of elemental halogen and volatile halogen compounds, with a dense turbulent mass of subdivided carbonaceous solids fluidized by upwardly flowing gases to resemble a boiling liquid, said halide being present in amounts providing a halogen equivalent corresponding to that of about 0.0l1% by weight of alkali metal halide based on carbon present and said amounts being adequate to inhibit the formation of CO2 in favor of CO formation.

13. In the production 01 gas mixtures containing H2 and CO by reacting solid carbonaceous materials simultaneously with steam and oxygen in a gasification zone at gasiflcation conditions, the improvement which comprises contacting said steam and oxygen at said conditions in said zone with a dense turbulent mass of subdivided carbonaceous solids fluidized by upflowing gases to resemble a boiling liquid and adding to said mass a natural metal halide brine containing a halide forming at said conditions a reaction product selected from the group consisting of elemental halogen and volatile halogen compounds, said metal halide being present in amounts providing a halogen equivalent corresponding to that of about 0.01-1% by weight of alkali metal halide based on carbon present and said amounts being adequate to inhibit the formation of CO2 in favor of CO formation.

14. The process of claim 13 in which said brine comprises sea water.

15. In the production of gas mixtures containing H2 and CO by reacting solid carbonaceous materials simultaneously with steam and oxygen in a gasification zone at gasification conditions, the improvement which comprises impregnating subdivided carbonaceous solids of fluidizable particle size with sea water of enriched metal halide content, said metal halide forming at said conditions a reaction product selected from the group consisting of elemental halogen and volatile halogen compounds, said metal halide being present in amounts providing a halogen equivalent corresponding to that of about 0.01-l% by weight of alkali metal halide based on carbon present, said amounts being adequate to inhibit the formation of CO2 in favor of CO formation contacting said impregnated solids with steam and oxygen at said con- 7 ditions in said zone in the form of a dense turbulent mass of solids fluidized by upwardly flowing gases, withdrawing a product gas containing volatile halogen compound and free halogen from said zone, separately withdrawing solid gasification residue from said zone, leaching said residue with sea water to extract soluble alkali and alkaline earth metal compounds and to produce sea water enriched in said last-named compounds, scrubbing said product gas with said lastnamed enriched sea water to extract volatile halogen compounds and free halogen from said product gas and to produce said sea water of enriched metal halide content, and using said last- References Cited in' the file of this patent UNITED STATES PATENTS OTHER REFERENCES named sea water for said impregnating treat- 15 Grant: Chemical Dictionary. 3rd

ment.

WILLIAM J. MATTOX.

edition, page 685.

Claims (1)

1. IN THE PRODUCTION OF GAS MIXTURES CONTAINING H2 AND CO BY REACTING SOLID CARBONACEOUS MATERIALS SIMULTANEOUSLY WITH STEAM AND OXYGEN IN A GASIFICATION ZONE, THE IMPROVEMENT WHICH COMPRISES ADDING TO SAID REACTION AN EXTRANEOUS METAL HALIDE YIELDING A REACTION PRODUCT SELECTED FROM THE GROUP CONSISTING OF ELEMENTARY HALOGEN AND VOLATILE HALOGEN COMPOUNDS AT THE REACTION CONDITIONS SAID HALIDE BEING PRESENT IN AMOUNTS PROVIDING A HALOGEN EQUIVALENT CORRESPONDING TO THAT OF ABOUT 0.01-1% BY WEIGHT OF ALKALI METAL HALIDE BASED ON CARBON PRESENT AND SAID AMOUNTS BEING ADEQUATE TO INHIBIT THE FORMATION OF CO2 IN FAVOR OF CO FORMATION.

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