CA1060652A - Process for production of low btu gas - Google Patents

Abstract

PROCESS FOR PRODUCTION OF LOW BTU GAS
ABSTRACT OF THE DISCLOSURE
A process for partial oxidation and complete gasification of a carbonaceous material to produce a combustible gas containing a substantial proportion of carbon monoxide, by introducing the carbonaceous material and air as a preferred source of oxygen into a molten salt containing an alkali metal carbonate and preferably also an alkali metal sulfide, the system being operated preferably above atmospheric pressure, up to about 20 atmospheres. The air is employed in a proportion to provide less than about 60% of the amount of oxygen stoichiometrically required for complete oxida-tion of the carbonaceous material. Sulfur and ash introduced with the fuel are retained in the molten salt. The gaseous effluent, containing a weight ratio of carbon monoxide to carbon dioxide substantially greater than 1, is a low BTU gas. This gas can be reacted or combusted outside the molten salt in a second reaction zone, such as a conventional boiler, to recover the heat value of such gaseous effluent.

Description

106~652 B~CKGROUND OF THE INVENTION
This invention relates to a process for the prsduction of low BTU gas by the combustion and gasification of carbonaceous materials, particularly solid sulfur-bearing carbonaceous fuel.
The invention particularly relates to a molten salt process for the ~ombustion and gasification of carbonaceous materials, par-ticularly coal, under conditions to obtain partial oxidation and production of a combustible gaseous effluent containing a high ratio of carbon monoxide to carbon dioxide, and, when employing o air as the source of oxygen, obtaining a low BTU gaseous effluen~ -of the above type which contains nitrogen, such gas being adapted for complete combustion in a secondary reaction zone.
The combustion of carbonaceous material such as solid carbonaceous fuel by reaction with a source of oxygen such as air, oxygen-enriched air, or pure oxygen is well known. In such a process, it is known to carry out the reaction employing an amount of air.or oxygen equal to or greater than that required for com-plete combus~ion, whereby the gaseous effluent contains carbon .: dioxide with little if any carbon monoxide. It is known also to carry out the gasification or partial oxidation of solid carbon-aceous materials or fuels employing a limited quantity of oxygen or air so as to produce some carbon monoxide together with car~on dioxlde .

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10~;0~;52 ~ low~ver, prior ~rt combu~tion re~ction~ generully h~ve been carr~e~ out ~n a single sta~e ~o obt~ln ~ubstanti~lly complete oxi~ntion of the carbon~ceou~ mater~al or fu~l, 80 that only a minor amount of carbon monoxide i~ present in the effluent gas.
In many ca~es, carbonaceous materials or fuels often contain impurities such as sulfur, and hence during gasification and com-bustion thereof undesirable acidic pollutants such as oxides of sulfur are formed.
The use of molten salts in the combustion and gasification 0 of carbonaceous materials is known. Thus, U.S. Patent No.

3,710,737 to Birk, directed to a method for producing heat, dis-closes carrying out the combustion of carbonaceous materials in a molten salt medium in the form of an alkali metal carbonate melt containing a minor amount of alkali metal sulfate or sulfide. In such combustion reaction, the combination of the oxygen and carbon occurs indirectly, as described in the above patent, and the alkali metal carbonate, such as sodium carbonate, provides a com-patible salt medium at practical operating temperatures, retains heat for conducting the combustion reaction, and also reacts with and neutralizes acidic or undesirable pollutants such as sulfur-containing gases which are formed during combustion of carbonaceous materials, e g. coal, containing impurities such as sulfur and sulfur-bearing compounds. A similar reaction in such molten salt medium is disclosed in U.S. Patent 3,708,270 to Birk et al, directed toa method of pyrolyzing carbonaceous material. A
carbonaceous feed is thermally decomposed in a pyrolysis zone by heating it in the absence of oxygen to fonm char and a gaseous effluen~. An optional steam input for gasification of the char material may also be utilized. In a heat generation zone, carbon and oxygen are reacted to form carbon dioxide to provide heat for the pyrolytic decomposition reaction.
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lO~V6~2 In both oE th~ nbove p~ltent~ th~ re~ct~on ~n the ~lkall melt i8 c~rried out to maximi~e heat gen~r~tlon 60 th~t the reac-tion product prlncipally contains C02, and al~o N2 where Air ls the source of oxyg~n. Thus, in these patents, particularly 3,710,737, it is noted that carbon monoxide formation iB und~-~l>le, and although provision is made for a separate furnace or burner to combust any carbon monoxide present, carbon monoxide is stated to be a minor product of the reaction.
The abo~e patents point out that an excess of carbon is lQ used, i.e., an amount of oxygen less than that stoichiometrically required for complete oxidation of the carbonaceous material is present in the melt, so that under steady-state operating condi-tions the suLfur present in the melt is maintained substantially all in the sulfide form. These conditions are employed in these patents not for purposes of obtaining incomplete combustion and formation of carbon monoxide, but in a manner so that subs~antial-ly complete combustion of the char or coal to C02 is achieved with as little production of C0 as possible. Thereby a maximum amount of heat is obtained from the char or coal, most of this heat being generated in the lten salt.
According to the present invention, C0 is obtained as the major product from the partial combustion and gasification process occurring in the molten salt. Subsequent heat generation is attained by combustion of the C0 to C02, carried out in a boiler separate from the molten salt.
U.S. Patent 3,567,412 to Lefrancois et al describes a process for the production of synthesis gas in which a two-zone furnace is utilized for the gasification of carbonaceous materials, in one zone of which steam and a carbonaceous material are added 3 to an alkali metal carbonate melt. The resulting char is trans-ferred to the second melt-conta~ning zone where it is catalytical-ly combusted to provide heat for the gasification reaction by 10ti~)6SZ
m~int~in~ng ~t le~st a critic~ll min~rDum concentration of ~.4 w~i~ht percent sodium sulfate.
U.S. Pat~nt 3,567,377 to Lefrnncois et al, i~ directed to absorption of sulfur from liquid and 801id carbon~ceous materials by contacting such material wlth a molten salt. rllel~ lc:~nvi~iol-led either a two-step process o~ gasification (steam pl~s carbonaceous material) plus combustion or a two step process involving sulfate reduction and pyrolysis, plus an oxidation step in which heat i8 produced by an exothermic oxidation of a carbonaceous material.
- lo U.S. Patent No. 3,252,773 to Solomon et al discloses a carbon-containing solid material and steam brought into contact wi~h a melt comprising an alkali metal compound under conditions such that a hydrogen-rich gas is formed along with a resultant char. As an adjunct, heat may be supplied for the gasification -reaction by combusting the resultant char with air, a requirement of the system being that any heat generation occur as the direct combustion of carbon by the reaction of carbon and oxygen to form carbon dioxide.
The Pelczarski et al Patents 3,533,739 and 3,526,478 dis-close the gasification of solid sulfur-bearing fuel wherein the fuel is injected into a molten iron bath maintained at a ~mpera~re .
above about 1400C, and a limited quantity of oxygen or air is also injected into the bath. Carbon contained in the fuel is absorbed by the iron and preferentially reacts with the air or oxygen to form carbon monoxide which then passes upwardly through the iron bath. A molten layer of lime-bearing slag is maintained on the surface of the molten iron bath to fu~ction as a fluxing agent for the ash and to cause the sulur absorbed by the molten iron to be desorbed and to react with the lime to form calcium sulfide. A portion of the slag is continuously withdrawn thereby continuously removing sulfur from the iron bath. The mixture of ga~es from the combustion reaction including carbon monoxide can ~, - 5 -1~360652 t~len be re~cte~ with ~xy~en to form (n~ )tl dioxide thereby g~n~rat~n~ a~ition~l heat.

SUMMARY OF THE INVENTION
It i8 an ob~ect of the present invention to provide an improved process for generating a low BTU gas from carbonaceous material. It is yet another ob~ec~ of the present invention to provide a process for heat generation from carbonaceous materials which obviates removal of pollutants formed in the combustion reaction. A particular object of the invention is the provision o of a process for partial oxidation of a carbonaceous material in a molten alkali metal salt medium for production of a gaseous effluent containing a high proportion of combustible gases, particularly carbon monoxide and hydrogen, such gaseous effluent then being adapted for further and complete combustion in a secondary reaction zone or combustor, to utilize the heat value of the gas.
In accordance with the broad aspects of the present in-vention, a carbonaceous material such as coal or a combustible waste material is introduced, together with a source of oxygen, suitably and preferably air, into a reaction zone preferably maintained at above atmospheric pressure. Pressures from 1 to about 20 atmospheres are preferred, pressures between 5 and lG
atmospheres being particularly preferred. This rPaction zone contains a molten salt mixture consisting essentially either only of an alkali metal carbonate or mixture of alkali metal carbonates, or preferably consisting essentially of a major portion of an alkali metal carbonate and a minor portion of an alkali metal sulfate or sulfide. The source of oxygen, preferably air, is employed in a proportion such as to proYide an amount of oxygen substantially below the amoun~ stoichiometrically required for complete combustion of the carbonaceous material. Generally the .

~ 0 60 6 S 2 Air enlplOyed i8 use~ ~n ~ pr~E~ortLon to provlde le~ th~n ~bout 60% of the ~mount of oxygen etoichiometric~lly requlred for ~ml~lr~
oxid~tlon or combustion. Other r~ction p~ram~ters are controlled so ~s to favor incomplete rombustion of th~ carbonaceous material, and maximize production of CO, con~istent with maintenance of the molten salt temperature at a desired value, as well as adequate throughput of coal or carbonaceous material ih the most economical manner.
It is particularly preferred that at least 1 wt.% of alkali metal sulfide, and up to 25 wt.%, be present in the molten salt under steady-state conditions, the sulfide serving to catalyze the rate of partial combustion of the carbonaceous material. Any sulfate initially present is converted to sulfide under steady-state conditions. In addition to the direct addition of sulfide to the melt, coal or other carbonaceous material containing sulfur can also serve as a source of the sulfide. The temperature of the molten salt is maintained between about 1400 and about 2000F
(about 760 to 1100C), particularly between about 1600 and about 1800F (about 870 - 980C) where coal is the carbonaceous material.
The result is a gaseous effluent from the gasification and com-bustion reactions which contains a substantially greater volume of CO than CO2, generally at least 5:1 and up to 20:1, and which also contains other combustible gases such as hydrogen and hydro-carbons .
The sulfur and sulfur-bearing contaminants and ash present in the carbonaceous materlal or fuel, e.g., coal, are retained in the molten salt The retention of the sulfur and ash from the fu~
in the melt eliminates the requirement for a stack gas sulfur oxide removal system and an electrostatic precipitator. These 3o materials can be removed from the reaction zone with a continuous stre~m of molten salt, the contaminants removed from such stream, and the regenerated stream of molten salt being returned to the eactor. Nitrogen oxide formation is negligible at the relatively low temperatures prevailing in the molten salt furnace.
The resulting combustible gaseous effluent from the salt furnace containing a substantial portion of combustible gases such as carbon monoxide and hydrogen can then be brought into a second combustion zone or unit, which may be in the form of a conventional utility boiler, and reacted therein with oxygen of the air to oxidize the combustible gases to CO2 and water with the release of heat.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be more clearly understood by reference to the detailed description below of certain preferred embodiments, taken in connection with the accompanying drawings wherein:
FIG. 1 is a flow chart illustrating generally the process of the present invention;
FIG. 2 is a schematic illustration of a preferred form of reactor containing the molten salt;
FIG. 3, which appears on the same sheet as FIG.l, is a flow chart of an alternative molten salt combustion process according to the invention, employing a pressurized gas feed; and FIG. 4 is a graph illustrating the effect of carbon content of the melt on CO/CO2 ratio obtained during partial combustion in the molten salt reactor.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
To illustrate the above noted mode of partial combustion of the carbonaceous material in the molten salt, in the case of carbon, for example, in the partial combustion reaction in the molten salt the carbon is partially oxidized to carbon monoxide with the release of its heat of combustion of about 26 kcal. The CO is then brought into a second reaction zone or combustion unit ,~, . . ' ': ' ' , ': -10~ 5'~
~uch A9 a ut~ y boller, whereln tlle C0 i~ oxidi~ed to C02 with the rele~se o~ 68 k~l of heat. Thus, of the overall he~t of combustion of carbun to C02 of 94 kcal, 26 Iccal or about 28% iB
rele~ed in the molten s~lt and 68 kcal or ~bout 72% i8 releaged in the secondary combus~ion ~one or unit. In the case of coal, hydrogen and hydrocarbons are also present in the gaseou~ effluent released from the salt, ~n addition to the C0 and C02 present.
An important advantage of the process of the present inven-t~on is that complex cooling equipment is not required for the 0 removal of heat from the molten salt, since the amount of heat generated by the oxidation reaction in the molten salt is essential-ly only that amount of heat which is required to maintain the salt in its molten condition. Most of the heat is generated by combus-tion of the gaseous effluent from the molten salt. This combustion takes place in a secondary combustion zone or boiler separate from the molten salt. Thus, a given size molten salt furnace will permit a much higher feed rate of fuel or carbonaceous material where only partial combustion is carried out therein for production of a combustible gas containing a substantial proportion of C0 and hydrogen in accordance with the present invention. Since the separate secondary combustor or boiler for large-scale combustion `is less expensi~e than a larger molten salt furnace which would be required for complete combustion, the overall economirs of the process are substantially improved. The application of the invention concept to the gasification particularly of coal is especially important since large throughputs of coal must be processed in the most economical m~nner.
In carrying out the invention process, in order to parti-cularly obtain a product gas having a volumetric ratio of carbon 30~ monoxide to carbon dioxide substantially greater than 1, the air is introduced into the molten salt in a proportlon to provide an amount of oxygen less than about 60% o~ that theoretically required , ~ _ g _ 10 ~0 6 5 2 for compl~te conl~ustion of the car~onAceous materlal to C02 ~nd ~2~ Gen~rally, the alr i9 ~mploy~d ~n an amount to provide from about 30% to about 60~/~, preferably from ~bout 35 to about 45%, o the amount of oxyg~n theoretically required for complete com-bu~tion of the carbonaceous material. If more than 60% of theoxygen needed for complete stoichiometric combustion is provided, then the resulting gas has a carbon monoxide.carbon dioxide ratio generally less than one, whlch is undesirably low. If less than about 30% of the oxygen stoichiometrically required for complete lo combustion is provided, then unburned coal or char begins to ac-cumulate in the molten salt until its viscosity becomes too high.
The invention process and the conditions of operation, particularly when using coal as ~he preferred carbonaceous material or fuel, and air as the preferred source of oxygen, are chosen so as to obtain from the partial combustion reaction in the molten salt, a combustible gas product containing as much C0 as possible and as high a BTU content as possible, with the minimNm amount of heat evolution in the molten salt as possible, sufficient to main--tain the salt in the molten state. Thereby the combustible product gas will provide a maximwm amount of heat in the secondary combus-tion zone. For ~his purpose, it is desîrable to employ a minimum amount of oxygen for carrying out the partial combustion reaction in the molten salt, consistent with efficient overall operation and heat requirements within the ranges noted above.
Air is the preferred source of reactant gaseous oxygen for use in the present process. While oxygen-enriched air or pure oxygen can be used~ thereby resulting in a combustible product gas of higher BTU content, the use of oxygen would ordinarily be economically undesirable for the production of such combustible product gas, since this would ordinarily require an oxygen plant.
Accordingly, the present invention will be particularly described and illustrated us~ng air as the source of oxygen.

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10f~65Z

The present process finds its principal and significant utility when integrated into a conventional coal-fired steam plant.
This involves the incorporation of the molten salt furnace and its associated auxiliary equipment into the coal feed system of the plant boiler. The molten salt furnace can thus be considered as an additional step in the treatment of the coal prior to its combustion in the boiler. This step takes the pulverized coal and converts it into a high temperature (about 980C.) low heat content (about 150 BTU/scf) fuel gas. This low BTU fuel gas is then burned in the boiler as a non-polluting fuel. The ash and sulfur are retained in the melt and removed in the auxiliary equipment associated with the molten salt furnace. Thus the low BTU gas which is generated is burned on site. However, various features of the present process may be utilized, using pure oxygen as the feed gas, together with certain significant modifications and additions to the process, to produce a high BTU pipeline gas, not intended for on-site heat generation. The process for producing such a synthetic pipeline gas does not form part of the present invention, but is the joint invention of K.M. Barkley, W.E. Parkins and J.R. Birk, disclosed in Canadian Patent Application No. 252,943, filed May 20, 1976.
The air and the carbonaceous material, preferably coal, are fed into the molten salt, which is maintained at a temperature --generally ranging from about 1400F to about 2000 F (about 760 C
to 1100C), and in preferred practice the temperature of the molten salt is maintained between about 1600 and about 1800F. (about 87QC to 980C). When the molten salt is principally sodium carbonate containing from about 1 to about 25 wt. %-sodium sulfide, a temperature preferably from about 1600 to about 2000F is utilized.
It is desirable to maintain the temperature low enough so that :, essentially no oxides of nitrogen are formed during the partial com-bustion reaction and so that particulate emission is minimized. ~ -106~6SZ
The initi~l molten ~alt mixture can contain either alknll met~l ~ul~Ate or sul~d~. It preferably con~lst~ es~entially of sodium carbonate contRining from ~bout 1 to 15 wt.% ~odium sulf~te, an amount between about 3 ~nd 10 wt.% sodlum eulfate being partl-cularly preferred. Alt~rnatively, a binary or ternary mixture ofthe carbonates of sodium, potas~ium and lithium can be employed, a preferred binary mixture being the Na2C03-K2C03 eutectic. The sul-fur compound may be added initially as sulfate, it being converted to sulfide under steady-state conditions. Any of the sulfates of 0 the foregoing alkali metals may be utilized. Sodium sulfate is generally preferred because of its ready a~ailability and low cost.
The sulfur (as sulfide~ content of the molten salt can also be furnished either wholly or partially from the sulfur content of the carbonaceous material, e.g., coal, employed, so that alkali metal sulfate or sulfide need not then be added initially to the alkali metal carbonate.
In the molten salt reaction medium, under the conditions of reaction in the present process, wherein a stoichiometric deficiency of oxygen is provided for initial incomplete combustion, the sodium sulfide is considered to catalyze the combustion reaction by a com~
plex reaction mechanism. While various exemplary intermediate reac-tions may be postulated, precise knowledge as to the details of the reaction mechanism is still lacking. Thus it is not intended that the present invention be considered limited by the following expla-nation.
The net overall reaction that occurs is the partial oxida-~ion of the carbonaceous material or coal. However, the combina-tion of the oxygen and carbon is believed to occur indirectily in that each of such components reacts separately with a component 3o present in the molten salt. Thus, the alkali metal sulfide, e.g., sodium sulfide, functions to increase the burning rate. The alkali metal carbonate provides a compatible salt medium at practical operating temperatures and acts as a dispersing medium for both lO~iV652the fuel bein~ combust~d nn~ the prlm~ry ~ir used for the combu~-tlon. In ~ltion, the c~rbon~te melt neutrali~es the acid~c pollut~nt~, ~uch n~ oxide~ of sulfur and chlorine-containin~ ga~e~, formcd in the p~rti~l oxid~tion reactlon, ~nd retnins the resulting products. The carbonate melt also acts as a heat sink, with high he~t transfer rates for absorbing and distributing the heat of combustion, as a heat source for the distillation of the volatile matter of the fuel, and as an absorbent for the ash from the fuel.
Many forms of carbonaceous materials, i.e., those providing lo an effective source of reactive carbon, can be used as the fuel or reductant in the invention process. Thus, all of the common forms of carbonaceous fuels can be used including coal, coke, fuel oil, petroleum residue, lignite, peat, wood9 photographic film, p~stics, pesticides and their containers, and municipal wastes such as household trash and garbage, and sewage sludge; industrial wastes such as polyvinyl chloride and scrap rubber, and agriculturalwas~s including plant and animal waste material. For purposes of the present invention, generally coal is the preferred carbonaceous material. The present process is further advantageous in its ability to handle a wide variety of coals, including lignite, sub-bituminous, bituminous, and an~hracite coals, wi~hout any need for pre-treatment of caking coals. Tar formation is also absent in the present process.
In addition to a feed of a carbonaceous material and a reactive source of oxygen, it is sometimes desirable to include an additional catalyst in the molten salt other than the alkali metal ~sulfide for the above reduction reaction. Iron compounds have been ~; found to be good catalysts for this reaction, employing an amount of iron rangingfrom about 0.5 to about 3 weight percent of the 30 melt. The iron can be added in the elemental form or preferably inthe form of compounds containing iron, such as iron sulfide or ^~ironsul~ate.
During operation o~ the partial combustion reaction in the r ~ ~ _ 1 3 ~
.~

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molt~n s~lt mlxture, impuritie~ pres~nt in the c~rb~naceou~
are retained in the melt. The ~mount ~nd type of impuritie~ pre-s~nt in the melt will vary depending upon the ~ource of cn~ nnwou~
material or feed. The most common impurities are a~h and ~ulfur, the sulfur generally being present as a sulfur compound such as sodium sulfide in the melt. To remove such impurities, a portion of the alkali carbonate melt is withdrawn continuously and pro-cessed in a regeneration system which removes the ash and sulfur compounds retained in the melt and returns the regenerated sodium carbonate back to the molten salt furnace. A typical impurity removal process for this purpose is described in above U.S.
Patents 3,710,737 and 3,708,270.
The effluent gas mixture from the partial combustion reac-tion in the molten salt contains carbon monoxide and carbon dioxide having a volumetric ratio of C0 to C02 substantially greater than 1, and generally ranging from about 5:1 to about 20:1. A combus~ble gaseous effluent according to the invention can contain from about 90 to about 95% C0 and about 5 to about 10% C02, by volume, based on these two components. Where coal is employed, the effluent gas will also contain hydrogen and hydrocarbons, together with nitrogen and water. It has been found that C0 concentrations in the gaseous effluent will increase with (1) reduction in the percentage of oxygen stoichiometrically required for complete combustion, (2) increasing carbon content in the melt, (3) higher temperatures of reaction and (4~ increasing sulfide content of the melt. The carbon content of the melt can range, for example, from about 1 to about 10%.
Where coal is employed as the carbonaceous ma~erial, and air is uset as a source of oxygen, the combustible effluent gas containing the above noted high ratios of carbon monoxide to carbon ;
dioxide, has a relatively low BTU heating value, which can range ~-- from about lO0 to about 200 BTU per cubic foot. When such a low _ 14 -106~652 ~TU ~a~ iG combu~ted in a secondary burner or c~mbusti~n zone, such ~s a boiler, th~ ma~or portion o the he~t of re~ction from the overall h~at of combuGtion by complete combustion of the carbonaceous material to C02, i5 r~le~sed in the secondary combus-tion zone~
The molten salt combustion system is operated at a pres-sure between 1 and ~o atmospheres, preferably between 5 and 10 atmospheres. By operating at pressures above atmospheric, a higher throughput of coal and air is obtained than at atmospheric pres-sure. Thereby the combustion reaction can be accomplished in asmaller vessel for a given rate of coal feed to the vessel.
Referring to FIG. 1 of the drawing, a carbonaceous feed material 10, such as coal or a waste material, and air 12, as a source of reactive oxygen, are supplied to the molten salt furnace or reactor 14 containing a Na2C03-Na2S melt. Furnace 14 is maintained at a pressure between 5 and 10 atmospheres. The air may be introduced in the bottom portion of the reactor zone so as to pass upwardiy through the melt and thereby provide for an intimate mixing of the air, coal, and molten salt. The heat generated by such oxidation reaction is sufficient to maintain the melt in the molten condition within the desired temperature ranges noted above for effective partial oxidation and substantially com~
plete gasification of the carbonaceous fuel according to the inven-tion.
A combustible gaseous effluent 16 from the molten salt furnace 14 contains C0 and C02, in the above noted volumetric ratio of C0 to C02 substantially greater than 1, and preferably at least 5 to 1, and also contains H2, H20, and N2 and small amounts of hydrocarbons. Such low BTU gaseous effluent, preferably having a heating value in excess of 100, and most desirably of the order of about 150 to about 200 BTU per standard cubic foot, is introduced-or injected into a secondary burner or boiler 18, ~,.. ,. .

to~her w~th ~lr 20. ~leat ~rom the re~ction of c~rbon m~noxide in such combustibl~ ga~ with oxygen from the injected alr, fvrming carbon ~ioxlde, plu5 a ma~or portion of the s~nsibl~ heat content of th~ products o combustion passin~ throuh the boiler, are transferred to water within th~ boiler 18, converting it into steam, which can then be fed to a st~am turbine. The resulting gaseous combustion products pass out of the boiler 18 by way of a conduit 22. Such gaseous combustion products consist essentially of C02, H20, and N2. Such completely oxidized combustion products 0 can be vented or passed into a heat exchanger (not shown) for extraction of additional sensible heat, e.g., for prehèating boiler feed water. Operation of the molten salt furnace ~akes place preferably at a pressure between 5 and 10 atmospheres, although a pressure just high enough above ambien~ to allow the fuel gas generated to be injected into the fuel nozzles of the boiler is also suitable.
Referring now to FIG. 2 of the drawing, there is îllus-trated a type of molten salt gasification furnace which can be employed in the invention process. In FIG. 2, a reactor vessel 100 contains a body of molten salt 102, e.g. comprising sodium carbonate and 1 to 15 wt.% sodium sulfide. The reactor is pro-vided with an insulated air or water cooling jacket 104, and there is provided a primary air inlet 106 and an air manifold distributor system 108, and coal inlets 110, the air manifold and coal inlets being interconnected. The coal inlets can also serve for introduc-tion of alkali metal carbonate into the reactor. The reactor is also provided with a melt outlet 112 and a gaseous outlet 114. The outlet 114 is provided with a conventional demister 116 for re-moving liquid and solid particulates from the effluent gas. The reactor is also provided in the interior thereof with an overflow weir 118, to maintain a constant level of molten salt, and a drain 120. Air is supplied to the reaction or partial oxidation zone - 16 _ , ~ : ... .

10f~652 122 ~omprl~ of the 6alt melt 102 ~hroup,h the alr ~l~trlbutor ~ystem 108.
In th~ moltcn ~alt furn~ce, the c~rbon~c~ous material iB
partially combusted to CO, CO2 ~nd ~12O~ with rele~se of hydro~en and hydrocarbons into the resulting gas~. The partial combustion and the gasification take place rapidly at relatively low tem-peratures, e.g. of the order of 1700-1800F, because of the high contact areas and high heat transfer rateQ, and more importantly, because of the catalytic effect of the sodium sulide dissolved in the melt.
Under the conditions of reaction according to the present invention, employing a proportion of air to provide less than about 60% of the amount of oxygen stoichiometrically required for complete oxidation of the carbonaceous material, preferably about 35 to about 45% of such stoichiometric amount, partial oxidation of the coal occurs in the molten salt reaction zone 102. The gaseous effluent exiting the reactor at 114 contains at least 5 to 1 volumetric ratio of carbon monoxide to carbon dioxide, together with hydrogen, hydrocarbons and water, and also nitrogen from the air supply.
As the reaction proceeds in the molten salt body 102, acidic contaminants such as sulfur or sulfur-bearing materials in the carbonaceous material or coal pass into the molten salt, the sulfur-bearing materials forming alkali metal sulfides such as sodium sulfide. The capacity of the salt melt for retaining the sulfur and ash of the coal is limited by the maximum allowable concentration of these materials in the melt. When this concen-tration is reached, any undesirable buildup of sulfur and ash in the melt is prevented, and a steady-state condition is established by continuous withdrawal of the side stream 112 of sulfur-and ash-containing melt and addition of regenerated sodium carbonate and carbonate makeup back into the molten salt furnace.

1~6065Z
ide At~e~m 1~ gu~nche~l in w~lter, which dl~ulves the ~odium carbonat~ and sul~ur compoun~. The in~olubl~ ash ~n~ ~ny un-combusted cnrbon ~re removed from the solution by clurlfic~tion ~nd/or filtration, preferably in ~he presence o~ C02 to decre~se S silicate formation. Carbonation of the filtrate with flue gas and steam stripping are employed to regenerate the sodium carbonate and release hydrogen ~ul~ . The hydrogen sulfide is processed in a conventional manner for recovery of elemental sulfur or sulfuric acid. The sodium carbonate is crystalliæed out of its 0 water solution, and after addition of makeup, is returned to the molten salt furnace.
Although the combustion of the combustible gaseous product from reactor 102 has been described above and illustrated in FIG. 1 of the drawing as being further combusted in a separate combustor or burner 18, it will be understood that such combustible gaseous product may be combusted, e.g., in the reactor vessel, in a zone above the body of the melt 102.
As a further feature> integration of the molten salt com-bustion and gasification process of the present invention into a conventional coal-fired steam plant can be achieved by inc~ra~ng the molten salt furnace and its associated auxiliary equipment into the coal feed system of the boiler. The molten salt furnace can thus be considered as an additional initial step in the treat-ment of the coal prior to combustion of the product gas in the b~ler rhe integration of the molten salt furnace system into convention~ .
power plant can be done in various ways, the simplest involving the installation of the molten salt furnace as a supplementary unit upstream of the boiler.
Operation of the molten salt furnace at a pressure just lligh enough above ambient to allow injection of the gas generated into the boiler, as in conventional operation, has the disadvantage that it requires a large cross section molten salt furnace, since 0 ~0 6 5 2 the controlline paranleter involve~ ~A the superflcia] velocity of th~ fuel ~ns ~encrate~. To d~crea~e th~ cro6s s~ction of the molten salt furnace, op~rntion of this furnace can be carried out und~r pressure. Typic~lly ~ pressure oE 5 ~tmosphere~ will de-crea~e the diameter of the furnace by a factor of 2.2. The amountof energy required to compress the primary air feed is however appreciable and, for economic reasons, it is important that thiR
energy be recovered by expanding either the ~uel gas produced or the off-gas from the system, through a~gas turbine. A process and system employing such concept is illustrated in FIG. 3 of the drawing.
In FIG. 3, air is compressed in a compressor 200, and fed together with coal at 202 into a molten salt furnace 204. The combustible flue gas from the molten salt furnace and containing a volumetric ratio of CO to CO2 substantially greater than 1 according to the invention, is fed to a secondary combustion cham-ber 206 where the gas is completely combusted. The combustion gases at 208 are introduced into a gas turbine 210, generating power for operation of co~pressor 200, and the expanded gases from the gas turbine are introduced at 212 into a waste heat boiler ~ 214 which generates the steam for a steam turbine 216 for the : steam cycle portion of the plant. The process also includes a melt regeneration system 218, described above. Thus, in this process a combination molten salt furnace and secondary combustion ~hamber can be used as a substitute for the combustion chamber of a conventional gas turbine.
In an alternative method to the method illustrated in FIG. 3, the fuel gas from the molten salt furnace can be fed ~ direc~lyto a gas turbine to generate power, and the turbine dis- -:~charge gas which is still uncombusted and containing a major por--tionof CO with respect to C02 according to the invention, is fed to a power plant boiler functioning a the secondary combustion -- 19 _ ~.0 ~0 6 S Z

æon~ to ~ffec~ compl~t~ combustion oE tlle fu~ 9 from the molten salt furn~ce. A m~or advanta~ of ~his ~mbodim~nt iG th~t the ~as turbine expansion lowers th~ t~mpcrature of th~ fuel ga~ by several hundred degrees, with a consequent decrease in the combus-tion temperatur~ in the secondary combustor and therefore a reduc-tion in the oxides of nitrogen present in the stack gases.
Thus, the advantages of the above-noted alternative embo-diments employing a compressed air feed and a gas turbine include a significant reduction in molten salt furnace cross-section as lo compared with the unpressurized operation, and generation of electric power in excess of that required for compression of the primary air feed, thereby permitting a significant improvement in the overall net heat rate of the plant.
The following examples illustrate the practice of the inven-tion, it being understood that such examples are not intended as limitations of the invention.

.

Gasification of Kentucky No. 9 Seam Coal at 1800F.
A series of tests were run employing Kentucky No. 9 Seam Coal having the following analysis:
Moisture 6.18%
Car~on 62.19 Hydrogen 4.29 Nitrogen 1.31 Chlorine 0.04 : :~
. Sulfur 4.22 Ash 15.34 . Oxygen (by difference~ 6.43 Heating Value: BTU/lb 11,266 '1`11~ snlt 1)~ tl.l..l;~ ` t~ .q ~In~l n ~oml)o.qitlotl nt the start o~ the~e tests of ~0.3% ~odium c~rbonate, 13.4% sodium sulfate and 6.3% ash.
The coal was ground and dried before being fed by means of a screw feeder to a bench-scale reactor containing the molten salt mixture. Air was fed at a rate of about 1.6 to about 2.1 scfm tft.3/min. at standard conditions of 70F and 14.7 psia) to the reactor, and the coal feed at a rate of about 11 to about 19 glmin. Air rates and coal rates were chosen to give 1 ft/sec lO superficial velocity for the product gas exiting the salt bed.
Each run was made employing the same time pattern, the runs being one hour in length, with one-half hour being allowed to reach steady state. This series of tests was carried out at a melt temperature of approximately 1800F (about 980C) using four dif-ferent air stoichiometries, as noted in Table 1 below.
The moisture content of the gas exiting the reaction zone was calculated assuming saturation at the tem~erature measured.
Furnace heat was used to maintain the desired temperature of the melt. `
All gas analyses except those for hydrogen were made using a Beckman GC-2A gas chromatograph and helium as carrier gas. The carbon dioxide and hydrocarbons were determined using a Poropak Q
column at 130C. The oxygen, nitrogen and carbon monoxide were determined using a Molecular Sieve 13X column at room temperature.
25 Xydrogen was determined using a Perkin Elmer 820 gas chromatograph, a Molecular sievP 5A column at 190C and argon as a carrier gas.
The results and data of these tests are set forth in Table 1 below:

, ~ , __ .

.,.~ ~ h u~ .
~ X
t-l ~_~
~rl .,~ ~ ~ . I~
~l ~ O~ oo h 0 td o .
= ,~ O .
~D ~ ~ ~ ~
Q~ ~ 00 00 ~ 0 ~
~D 0 -I ~ ~
t-.~ ' _ . ~
~1 N ~ ~ ~
O 3~ ~ ~ C\i ~
_~ V ~ 0 E ~D ~1oo u~ u~ .
0 U~ ~ -I O o O 1 . . . . o ~3 u~ ~ b~ O O O ~ t-. ~ ~ C`~ O ~0 :.
z; ~ ~R ~ o o al :
X ~ 0~ U~ ~ :' ~ ~ ~
~ O ~0 0 O ' ~- ~ -: ' "
~Q V . O
~ O t~ D
O-rl~ ~C`l ~ h ~ ~ ~. _ ~ ::
~~ ~. C'~ ~ ~ '.''-~
O~ V x01~O~) 00 VG~ G~ O O~
O ~~ ~
O ~0 ~ ~ .
0 U~ ~ ~ ~ .~
bO-"a td 13 hN ~ :
~ ~ h ~ O ~ .
:~ ~ ~) ~ ~ ~ h ~ O ~O:i~ ~
c~ u~ ~ ~ t) '~

- - .:
For each of the four stoichiometrie~ ~et forth in Table 1 above, data were obtained at certain time intervals ~:: during the one-hour period of the te~t, the data for the stoichiometry of 33.2% average actual theoretical air being :-gi~en in Table 2 below as illu~trati~e.
' ' .

~ 2 2 ~.

. : .

b~ . .
~

h 0 O ~0 ct~ ) ~ ;1-. I ~ ~ U~ ~ ~ ~
~1 ~ ~ .
L~ .
,~
m . . . _ o ~ o ~ oo ~ I ct~ ~
;~ o . -~D , O ~ ~ -I 00 co ~
.. . .~, .
,~~ ~ C'~ ~ ~ ~
I O
~ . ~
~ ~D ~ l ~
C~ . .
, o o o o o o ~, ~
o . . ` ,~., C) ~ ,, ~ o ,o ~ o D ~ F ~ J ~ i ~D
~n v ~ . ~D E
o~ ~ ~ ~ ~ , . ~o ~ ~ I ~ I o Z . _ . ~ I ~
~1 ~ O ~ C~ O ~ ~ ~
. O . . . . . . ~ ~ V
~1 1 1 ~ I ~o ~ tO
___. . , , bD
O ~ C~t O O ~0'N C~l t-- O
~ ~ ~ O . O~ C~l X tl5 ~ C2 I ~ O O O_~ -i . a.~ o ~: ~ ~ _l ~ ~ ~
o ~ al . . 0 o ..
O ~ p, . C~l ~ ON ~ 0 rl ..
,:: -1 ~ X~ 00 X 000000 ~ E~ O
0 ~ ~ o ~ 3 0 c~
,:: ~-1 . g C~ tn .: C~ ~ .,~ ~I
':', ~ ~D ~ ~ ~ ., ~ - ~ ~q E ~:: oU~ ~o o o o h ~
~ ~ ~ o 0 ~ Z

-'` ' - ~;

~ ~ - 23 -, . .

It is seen from Table 1 above that when employing from about 33 to about 47~ average theoretical air, the ratio of CO/C02 is substantially greater than 1 and ranges from about 3 to 1 for 46~ theoretical air to about 9 to 1 for 33%
theoretical air. On the other hand, when employing about 75%
average theoretical air the ratio of CO/C02 is substantially less than 1.
It is also noted that the exit gas heating value varied from 36 BTU/scf. for 75% average theoretical air, to 118 BTU/scf. for about 47~ theoretical air, and for the runs employing 33% theoretical air according to the invention, it is noted that the heating value of the effluent gas was 151 BTU/scf. -It is particularly noted that a 150 BTU gas is produced when the amount of air employed is about 33% of theoretical. i-When producing such a 150 BTU gas~, the particulate loading is about 2 grams/scf~ with close to 50% of the particulates being carbon. The bench scale equipment employed in these tests was ~
not designed for demisting of the melt nor the de-e~trainment --of air-borne solid~ 90 that the particulate emission is not an absolute value to be expected in actual plant operations.
It is also noted that no H2S, COS or S02 was detected in the exit gas, the limits of detection in these tests being about 80 ppm.

GasifiGation of Kentuck~ No. 9 Seam Coal at 1700F. (927C.) The procedure of Example 1 was substantially followed ~ `
in running another series of tests at four different air ~` stoichiometries, similar to Example 1 in which steady-state 3 conditions were attained, but in this case the molten salt bed temperature was maintained at about 1700F. The data and results of these tests are set forth in Table 3 below:

_24-_ , ~^ U~
~ ~ ~ . ~ oo ~o oo ~ ~ ~ ~ ,, ~ ~ U~
,~ ~ a~ ~ ~I
O ~ ~ 1 h :~ ^~
~3 r/ Q) ~ . r~ ~ ~ N
~ ~ rl ~ O ~ 00 0 h td t~S ;) . . . .
O ~ ~q N ~1 O --I

h :' h ~ o oo ~ It~ a~
~ ~ 0 1~ ~
.~ ~ m ~o~ _ t~
c~ ~n ~ ~ ~ o o ~ a~ u~ ~ O o'o ~/
~ z ~ _~ I-'ooo X ~. I
~ ~ o~ 0~ . .
~ 0 ~ L~
~4 C~ o I ~ . ~
O C) u~ ~ N
O ~_ N N,~
~n ~ __ .~ CO l~\ ;t u~
~ o ~ E ~ o ~ ~ ~
~ ~ a~
0u~ 0 rl ~ bD_I
h o .. .
a a ~ ~ ~ ~ ~ o~
c~u~ ~ ~

From Table 3 above, it iil noted that for about 80~
theoretical air the CO/C02 content of the effluent gas was substantially less than 1 to 1. On the other hand, when employing air stoichiometries ranging from about 33 to 50%
: according to the invention, the ratio of CO/C02 in the ~- ~ effluent gRs ranged from about 2 for 50~ theoretical air to almost 5 for about 34% theoretic~l air.

j~,; .

~ls~ WllOll ellll)lOy~ 80% t}looro~lc~ r, tho ~oating valuo Or tl1o o~l`luont ~r~s iS onLy 3~ ~l~/sc~. On thc otllor hnnd, WhOII omploying from a~out 34 to 50% thoorotical air according to the invontion, tho hoating value of the effluent gas ran~ed from 75 ~TU/scf. for 50% theorctical air to l38 BTU/scf. for about j4% theoretical air.

Conditions for Obtainin~ High_CO/C02 Ratios - Tests were carried out employing coal char (coal from which the volatiles have been removed) on a laboratory batch scale for producing a combustible gas containing carbon monoxide according to the in~ention, in a bed of molten sodium carbonate containing 12% sodium sulfide. Air waa introduced into the molten salt maintained at a temperature f about 1700F. One set of tests was carried out at a superficial air velocity of l ft. per second, and another set of tests was carried out at a superficial air velocity of 3 ft. per second.
In each of these two sets of tests the CO/C02 volu-metric ratio in the effluent gas was measured based on ~ varying percentage~ of carbon in the melt. --~ FIG. 4 shows the plot of such CO/C02 ratio against - ~ ~ percent carbon in the melt~ for superficial air velocities of l and 3 ft. per second, as represented by curves A and B, ~25 respectively. It was noted that during these runs there was a gradual decrease in the CO concentration and in the CO/C02 ratio with time apparently a9 a result of decrease in the ~ ~ .
carbon content of the melt with time.
~' The plot ~hows that the CO/C02 ratio in the effluent gas resulting from partial combustion o~ the char increase~
,'~ .
-26_ ~ . ~ . ...
, :.

n~ tllo nlllo~ult ol corb~n i~ tho mclt in~ro~l~e~ us~ I'O-ferrlng to curvo A, wllen oml~loyllltr ln uir voloclty O.r 1 rt.
por second, wlth pcrcent cnr~on ln tllo molt increasing irom about 3 to 7.5%, tho C0/C02 volullletrlc ratio in tho ~ffluent gas increasod from 5 to 25. As ~con from curves A and B~ the C0/C02 ~olumo ratios at superficial air vclocity of 1 ft/sec were substantially higher than at 3 ft/sec, for corresponding percentages of carbon in the melt.
From the foregoing, it is seen that the invention provides an efficient improved procedure for the production of low BTU gas by a molten salt combustion process for carbonaceous materials, particularly coal. This is accom-plished by employment of an amount of air providing less than about 6 ~ of the oxygen that is stoichiometrically required for complete oxidation of the carbonaceous material. There is provided partial combustion and complete gasification of the carbonaceous material or coal in the salt melt, thus per-mitting recovery of a combustible gaseous effluent free of acidic pollutants~ and containing a high ratio of carbon monoxide to carbon dioxide, substantially greater than 1.
The resulting low BTU gas can be readily combusted to complete oxidation for extraction of the heating value of the gas.
The process permits economical processing of large coal throughputs while providing a pollution-free exhaust gas.
It will be of course realized that various modifica-tions can be made in the design and operation of the present invention without departing from the spirit thereof. Thus, - while the principle~ preferred construction, and mode of operation of the invention have been explained and what is now COn61(IOrOd to ropro.sollt lt~ ~o.~t o~ odllllotlt 11LI~g ~oon lllu~trnto~ n(l do.scri~o~ it ~sllould ~o undor6-tood tllat wlthin t]lo ~copo of tho npporldod clnlm~ tho invontlon may bo practlcod o-thorwi~o than a9 ~pocifically illu~tratod and doscribod, ~ ' ' - .-~ 28_ ::

Claims (4)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A continuous process for gasifying coal which comprises:
a. providing in a reaction zone a molten salt consisting essentially of a major portion of sodium carbonate and a minor portion of from about 1 to about 25 wt. percent sodium sulfide;
b. maintaining said molten salt at a temperature of from about 1600°F to about 2000°F.
c. maintaining from about 1 percent to 10 percent carbon in said molten salt;
d. maintaining said reaction zone under a pressure of from about 1 to 20 atmospheres;
e. concurrently introducing into said carbon-containing molten salt a sulfur and ash-containing coal and air, the air being introduced into the lower portion of the reaction zone at a super-ficial velocity of less than about 3 ft/sec and in an amount to provide from about 35 percent to about 45 percent of the amount of oxygen stoichiometrically required for complete oxidation of the introduced coal; and f. controlling the conditions set forth in paragraphs (a)-(e) to promote carbon monoxide production and thermally decompose the coal to produce a substantially sulfur and ash-free combustible, low BTU gaseous effluent containing a volume ratio of carbon monoxide to carbon dioxide of at least 5:1 and having a heating value within a range of from about 100-200 BTU/scf, the sulfur and ash content of the coal being retained in the molten salt.

2. The process as defined in claim 1, which includes passing said combustible gaseous effluent into a secondary combustor in the presence of air and completing combustion of said combustible gaseous effluent to convert carbon monoxide and hydrogen present substantially to carbon dioxide and water vapor, and recovering the heat of combustion in said secondary combustor.

3. The process as defined in claim 1, which includes initially compressing said air prior to introduction thereof into said molten salt, passing said combustible gaseous effluent from said molten salt into a secondary combustor in the presence of air and completing combustion of said combustible gaseous effluent, and employing said gaseous effluent from said molten salt or the combustion gases from said secondary combustor in a gas turbine to generate power.

4. The process as defined in claim 1 wherein in paragraph (c) there is maintained from about 3 percent to 7.5 percent carbon in said molten salt.