US3252773A

US3252773A - Gasification of carbonaceous fuels

Info

Publication number: US3252773A
Application number: US201534A
Authority: US
Inventors: Solomon Ernest; Joseph F Mcmahon
Original assignee: Pullman Inc
Current assignee: Pullman Inc
Priority date: 1962-06-11
Filing date: 1962-06-11
Publication date: 1966-05-24
Anticipated expiration: 1983-05-24
Also published as: GB994208A

Description

May 24, 1966 E. SOLOMON ETAL GASIFICATION OF CARBONACEOUS FUELS Filed June 11, 1962 N E T L O M N METAL SALT N m A c F S A G COK REAC TION RATE ALKALI imozouww JCE 22.202

N m m M m Tm R NOC O WSM W m R v A T w S EH H. NP Rfifi 0 J Y B 90 TEMPERATURE, I QyTK This invention relates to a novel and improved process for producing hydrogen-rich gas. In one aspect the invention relates to a process for gasification of solid carbon-containing material. In another aspect the invention relates to the production of synthesis gas from solid carbonaceous material at improved reaction rates.

The production of hydrogen-rich gas from solid carbonaceous material by various processes has been described. In accordance with modern methods, carbon is gasified under steam and oxygen under controlled conditions to produce primarily carbon monoxide and hydrogen. One drawback to such oxidative methods is that a large capital investment in an oxygen plant is necessitated. A method which does not require the use of oxygen is, therefore, highly desirable.

Older methods of gasifying carbon involve the reaction between steam and carbon with or without prior impregnation of the carbon with alkali metal carbonates. In such methods heat is first generated by partial combustion of the solid fuel, followed by passage of steam through the heated mixture. As the reaction proceeds, ash components build up in the gasifier, the removal of ash necessitating interruption of the process to discharge the high ash mixture and recharge the gasifier with fresh fuel or fuel impregnated with alkali carbonate. In addition to discontinuity of operation, another disadvantage is that satisfactory rates of reaction are realized primarily by high temperature operation.

It is an object of this invention, therefore, to provide an improved method for producing hydrogen-rich gas from solid carbonaceous materials.

Another object is to provide a method for gasification of solid carbonaceous fuels using steam, which method does not require the use of oxygen for the conversion of the fuel.

Another object is to provide a method for producing hydrogen-rich gas from coke or coal, whereby satisfactory reaction rates are obtained at relatively low temperatures.

A further object is to provide a process for gasification of solid fuels, which process is readily adapted to continuous operation.

A further object is to provide a coal gasification method from which ash residue can be removed as desired without interruption of the gasification reaction.

A still further object is to provide a medium for the gasification of solid carbonaceous fuels, which medium facilitates the removal of ash residue.

A still further object is to provide a method for the production of synthesis gas from a wide variety of solid carbonaceous materials including those coals which cannot be used in fixed bed gasification systems because of their tendency to become sticky during the coking stage.

Various other objects and advantages of this invention will become apparent to those skilled in the art from the accompanying description and disclosure.

The above objects are accomplished by the process of this invention which comprises contacting a carbon-containing solid material and steam with a melt comprising an alkali metal compound under conditions such that hydrogen-rich gas is produced. Although the process of this invention may be conducted in a discontinuous manner, it is readily adapted to continuous operation by Patented May 24, 1966 which the molten salt system is circulated between the one zone into which the carbonaceous material and steam are injected and from which hydrogen product is withdrawn, and a second zone wherein heat is generated to maintain the circulation salt system in the molten state.

In addition to being readily adapted to continuous operation, the present process possesses advantages over other prior art methods for effecting gasification of carbon in its various forms. For example, solid fuels are gasified by means other than combustion and thus the use of high purity oxygen is avoided. Another advantage is that the use of the particular molten salt system of this invention facilitates removal of ash residue and allows for such removal without interruption of the gasification reaction.

The gasification reaction of this invention is effected within the temperature range from about 800 F. to about 1800 F. From the standpoint of hydrogen content of the product gas, lower solubility of ash residue, and materials of construction, it is preferred that gasification be effected at a temperature from about 1000 to about 1700 F., a temperature below 165 0 F. being particularly preferred.

The salt system employed as the gasification medium comprises an alkali metal compound which is molten within the aforesaid temperature range and which is of sufficiently low volatility such that loss of salt with the product gas is minimized. Suitable alkali metal compounds are, for example, the alkali metal carbonates, hydroxides, nitrates and oxides of which the carbonates and hydroxides are preferred. Such compounds may be used alone, but in order to operate within the aforesaid temperature range, appropriate mixtures of such salts are employed. Under the reaction conditions employed herein, and due to the presence of steam and carbon dioxide in the product gas, the charge is converted to an equilibrium mixture of alkali metal carbonates and hydroxides according to the following equation:

where M is an alkali metal. Thus, the reaction zone is charged either with a preformed carbonate-hydroxide mixture or with a mixture of any alkali metal compounds convertible to the desired carbonate-hydroxide mixture in situ. Typical examples of suitable combinations of such compounds which may be charged to the reaction zone are admixtures of: sodium carbonate and sodium hydroxide, potassium carbonate and sodium hydroxide, sodium carbonate and lithium carbonate, sodium carbonate and potassium hydroxide, sodium carbonate and sodium nitrate, as well as ternary systems such as an admixture of sodium carbonate, sodium hydroxide, and lithium carbonate. higher melting alkali metal compounds such as the carbonates of sodium and potassium are admixed with between about 10 and about weight percent of the lower melting compound, such as lithium hydroxide or carbonate. Also included within the scope of this invention is the use of a molten salt medium comprising the oxygen-containing alkali metal compound in admixture with other types of compounds such as the carbonates, oxides or hydroxides of the alkaline earth metals.

The feed which is contacted with the molten salt system comprises steam and a carbon-containing solid material, also referred to herein as solid carbonaceous material. Each of these terms as used herein, is intended to include within their scope, coal of various grades such as anthracite, sub-bituminous, bituminous and brown coal, cannel coal, lignite, lignitic coal; coke of various types such as coal coke and petroleum coke; peat; graphite; charcoal; wood and waste products comprising wood such as sawdust; non-woody plant materials such as In preparing the preferred salt systems, the 4 bagasse and cotton stalks, sugar and cellulose wastes. It is to be understood that the solid carbonaceous fuel employed as feed includes those that are naturally occurring as well as carbonaeous solids which are formed by coking and liquid hydrocarbons such as naphtha, whole and reduced crude oils, etc. The carbon-bearing material may be used in lump, granular or finely divided or pulverized form.

The carbonaceous material and steam are charged to the gasification reaction zone in admixture or individually at one or more points. Preferred operation is to use the carbon-containing solid material in finely divided form and introduce it in a stream of steam. This is conveniently achieved by charging the gasification reaction Zone containing the melt with a slurry of the carbon material in water.

The carbonaceous material is introduced into the gas generator at a rate within the range from about 0.01 to about 5 pounds of solid carbonaceous material per hour per pound of molten salt, more usually at a rate of from about 0.1 to about 1.0. Within these velocities for introducing the carbon-containing solid, the steam-carbon ratio is maintained between about 1 and about 5, although higher mol ratios may be employed without departing from the scope of this invention. Usually the steamcarbon ratio is between about 1.2 and about 2.0.

The gasification reaction is effected within the aforesaid temperature range at a pressure between about and about 1000 pounds per square inch absolute (p.s.i.a.) The particular pressure employed in any particular system depends largely upon economic considerations and the end use of the gaseous product. For commercial operation, therefore, the pressure is usually between about 200 and about 850 p.s.i.a.

The system employed by the process of this invention is readily distinguishable from systems in which the carbon-bearing material is impregnated with an alkali metal carbonate which is then gasified. The presently described process comprises a large excess of molten salt in relation to the carbon to be gasified, the weight ratio of salt to the solid carbonaceous material ranging between about 2 and about 25, although higher ratios may be employed without departing from the scope of this in vention. The use of such molten salt systems as the gasification medium is a highly efficient system inasmuch as the carbon-bearing solid material remains in contact with the molten medium until the carbon is completely consumed without the necessity of interrupting the process to impregnate carbon feed with additional alkali carbonate. In addition, the melt of the presently described system is such that the thermal efficiency of the process is also improved as compared with that of the prior art method. Thus, in accordance with one embodiment of the invention, a suflicient quantity of molten salt is circulated from the endothermic gasification zone at a temperature level therein to a heat generation Zone such that the temperature of the endothermic reaction zone is maintained at the desired level by liberation of the sensible heat of the circulating molten salt. Generally it is sulficient to maintain the exothermic zone at a temperature up to about 150 to 250 degrees Fahrenheit higher than the endothermic gasification reaction zone, and usually a 50 degree difference is sufficient. Heat may be generated in the exothermic zone by combustion of coal which may be accomplished by injecting coal and pre-heated air into the melt passing through the heat generator. Alternatively, the heat necessary for the endothermic gasification reaction may be supplied to the reactor by means of a gas furnace, electrical wiring, burning of fuel oil, and other suitable means.

Another obstacle to the development of a commercially attractive and improved gasification process has been the problem of removal of ash, the principal constituents of which are silica, alumina, calcium oxide, magnesia, titania, iron oxide and other compounds which are not susceptible to gasification. Certain prior art methods, and especially those involving the use of oxygen, require temperatures as high as the fusion temperature of the ash, the fused ash, when cooled, forming a plastic or glass-like mass which causes operational difilculties, clogging of equipment, etc. Due to the nature of the gasification medium employed in the process of this invention, ash removal is facilitated. Certain constituents of the ash such as iron oxide are slightly soluble or insoluble in the molten gasification medium while other constituents such as silica are soluble. The ash content of the molten salt system is readily controlled by intermittent or continuous withdrawal of an ash-rich stream therefrom. If desired, the discharged ash-rich stream is subjected to further separation to recover the alkali metal compounds. This is readily accomplished by passing the discharged ash-rich stream to a recovery zone in which the melt is cooled and treated with water to dissolve the alkali metal compounds leaving ash residue. In this manner the ash content of the molten salt system is readily controlled and ash is discharged without interruption of the hydrogenproducing reaction.

The folowing examples are offered as a better understanding of this invention, and are not to be construed as unnecessarily limiting thereto.

In the following experiments, the solid carbonaceous fuel and the alkali metal salts were premixed in the solid phase and charged to a reactor (1 inch in diameter and 3 feet in length) positioned within a resistance furnace. The mixture so charged was then melted in the reactor. Steam was introduced by bubbling nitrogen saturated with water at 205-210 F. through the molten salt maintained at temperatures between 1300 F. and 1700 F. The quiescent depth of molten salt within the reactor was about 15 inches. Unreacted water was condensed and collected in a receiver and drying tube. .The dry gas product was collected and analyzed by mass spectrometer. Contact times reported in the following examples are nominal, having been calculated from gas flow rate and volume of quiescent melt. Water conversion was calcu lated from the amount of product water collected and the hydrogen content of the product gas.

Examples 1-4 In this series of experiments, coke (80 average mesh size) was used as the solid carbonaceous material and was charged to the above-described reactor in admixture with 350 grams of a mixture consisting of weight percent sodium carbonate and 25 weight percent sodium hydroxide. The salts were then melted. The specific operating conditions employed and results obtained are set forth in the following Table I. In Run Number 1, a 15 gram charge of coke was used, and .in each of Run Numbers 2-4, a 50 gram charge was used.

TABLE I Run Number l 2 3 4 Coke, grams 15 50 50 50 Operating Conditions:

Temperature, F 1, 300 1, 500 1, 650 1, 650 N2 Rate, ec./min 26. 3 76 57. 5 59. 3 1110 Rate, gIIL/ID 0.23 0.17 0.51 O. 28 Sat. Temperature, F 205 205. 5 215 215 Sat. Pressure, p.s.i.a."--. 15. 9 15. 9 1G. 7 17. 5 Contact Time, sec0nds 3. 5 3. 9 0. 9 1. 5 Run Time, minutes 20 19. 5 9 9 H1O Conversion, percent 78. 1 98. 5 93.5 98. 0 Product gas Composition, Mol percent, N z-free basis:

CO 16. 1 2. 1 39.0 42. 8 5. 6 3. 9 5. 4 3. 5 78. 3 03. 7 55. 5 53. (i 0.0 0. 3 0. 1 0. 1

Inspection of the above data shows that the coke feed was converted to hydrogen-rich gas at high conversions. The data of Table I were used as the basis of the graph of the accompanying figure which presents reaction rate data, based on pseudo-first order kinetics, of the gasification reaction. Inspection of the graph shows that the reaction rate was very rapid, the reaction rate constant at 1500 F. (lO /T K=9.2), for example, being 1.1 seconds Examples 5-8 In another series of experiments, 50 grams of graphite (average particle size of 0.1 mm.) was used as the solid carbonaceous feed and was charged to the above-described reactor admixed with 350 grams of a mixture consisting of 75 weight percent sodium carbonate and 25 percent sodium hydroxide which was then melted. The specific operating conditions employed and results obtained are set forth in the following Table II.

TABLE II Run Number 5 6 7 8 Graphite, grams 50 50 50 50 Operating Conditions:

Temperature, F 1, 300 1, 650 1, 700 1,700 N2 Rate, ec./min 43 47 50 100 H2O Rate, gm./min. 0.14 0.13 0.15 0.29 Sat. Temperature, F. c. 204. 5 205 206 204. 5 Sat. Pr ssur p- .i.a 15. 9 15. 9 15. 9 15. 9 Contact Time, seconds- 5. 8 3. 8 3. 1. 7 Run Time, Ininutes 20 20 20 20 E20 Conversion. percent 34.8 81.3 90.1 71.2 Product Gas Composition, Mol pcrcent. Nz-free basis:

CO 1. 29. 4 36. 4 29. 3 002-- 4.5 6.0 4.7 8.9 Hz 93. 8 64. 2 58. 5 G1. 8 CH4 0.2 0. 4 0.4 0. 0 Calculated Outlet Temp. F., Water as ft 1,300 1, 540 1, 680 1, 660

In each of the runs of Table II, water gas equilibrium was essentially maintained. Although the reaction rate was lower than that of the runs of Table I in which coke was employed, the reaction rate is nevertheless relatively rapid at temperatures of 1700 F. and less.

The hydrogen-rich gas obtained as product of the above examples, consists primarily of hydrogen and carbon oxides. When it is desired to produce hydrogen-rich gas containing methane as an additional component, such as when hydrogen gas of higher heating value is desired, coal having a substantial volatiles content is preferably employed as the solid carbonaceous reactant, the methane being derived from the volatiles content.

It is to be understood that although the above description of the invention is drawn primarily to the use of molten media comprising the alkali metal carbonates or hydroxides, any alkali metal compound convertible to an equilibrium mixture of the carbonate and hydroxide under the reaction conditions may be charged to the reaction zone initially. From the standpoint of operability, therefore, other salts such as the alkali metal sulfates, chlorides, etc., are usable. However, due to the fact that these latter compounds are highly corrosive they are usually not employed unless the equipment with which the molten medium is brought into contact is made of, or lined with, a high temperature material which is not corroded such as ceramics, brick, etc.

Various alterations and modifications of the process of this invention may become apparent to those skilled in the art without departing from the scope of this invention.

Having described our invention, we claim:

1. A process for the production of hydrogen-containing gas from a solid carbonaceous material and steam which comprises charging alkali metal carbonate to a reaction zone, maintaining said alkali metal carbonate in molten form in the reaction zone as the reaction medium, sai'd alkali metal carbonate constituting at least 75 weight percent of the reaction medium, maintaining said reaction medium at a temperature between about 800 F. and about 1800 F., introducing a solid carbonaceous ti material and steam to said molten reaction medium in which the solid carbonaceous material and steam react to produce gaseous product containing hydrogen and an oxide of carbon, and withdrawing gaseous efiluent containing said hydrogen and an oxide of carbon from the reaction zone.

2. The process of claim 1 in which said solid carbonaceous material is coal.

3. The process of claim 1 in which said solid carbonaceous material is coke.

4. The process of claim 1 in which said solid carbonaceous material is a non-woody plant material.

5. The process of claim 1 in which said solid carbonaceous material comprises wood.

6. The process of claim 1 in which said solid carbonaceous material is graphite.

7. The process of claim 1 in which said alkali metal carbonate is sodium carbonate.

8. The process of claim 1 in which said alkali metal carbonate is potassium carbonate.

9. The process of claim 1 in carbonate is lithium carbonate.

10. A process for the production of hydrogen-containing gas from a solid carbonaceous material and steam which comprises introducing a dispersion of a solid carbonaceous material in steam into a molten reaction medium comprising an alkali metal carbonate in an amount of at least weight percent and an alkali metal hydroxide maintained at a temperature between about 800 F. and about 1800 F. to produce hydrogen-containing gas, and recovering said hydrogen-containing gas as a product of the process.

11. A process for the production of hydrogen-containing gas from a solid carbonaceous material and steam which comprises reacting a solid carbonaceous material and steam in a molten reaction medium comprising between about and about 10 percent by weight of sodium carbonate and between about 10 and about 90 percent by weight of a lower melting alkali metal compound, at a temperature between about 800 F. and about 1800 F., the steam to carbon ratio being at least 1, to produce hydrogen-rich gas consisting essentially of hydrogen and carbon oxides, and recovering said hydrogen-rich gas as a product of the process.

12. A process for the production of hydrogen-containing gas from a solid carbonaceous material and steam which comprises charging a solid carbonaceous material and steam to a reaction zone maintained at a temperature between about 800 F. and about 1800 F. and containing a molten reaction medium comprising at least 10 Weight percent of an alkali metal carbonate, in said reaction zone contacting the molten reaction medium with said solid carbonaceous material and steam such that a hydrogen-containing gas is produced, Withdrawing an ash-rich stream from said reaction zone, and recovering said hydrogen-containing gas as a product of the process.

13. A process for the production of hydrogen-containing gas from a solid carbonaceous material and steam which comprises introducing a solid carbonaceous material and steam to a reaction zone maintained at a temperature between about 800 F. and about 1800 F. and containing a molten reaction medium comprising at least 10 weight percent of an [alkali metal carbonate such that said solid carbonaceous material and steam are contacted with said molten medium to produce hydrogen-rich gas, supplying the endothermic heat of reaction by circulation of said molten medium between said reaction zone and I3. heating Zone, and recovering said hydrogen-rich gas as a product of the process.

14. A process for the production of hydrogen-containing gas from a solid carbonaceous material and steam which comprises charging a reaction zone With (1) an alkali metal carbonate of the group consisting of sodium carbonate and potassium carbonate, and (2) another alkawhich said alkali metal li metal compound having a melting point below that of said alkali metal carbonate, heating said alkali metal carbonate and lower melting compound to obtain a molten admixture thereof, said alkali metal carbonate (1) being present in an amount of at least 10 weight percent of the molten admixture, introducing a solid carbonaceous material and steam at a steam to carbon ratio of at least one to said molten reaction medium, the ratio of the total weight of said alkali metal compounds to the solid carbonaceous material in the reaction zone being at least 2, supplying heat to said reaction zone to maintain the reaction medium in the molten state and at a temperature between about 800 F. and about 1800 F. and withdrawing normally gaseous eflluent from said reaction zone containing hydrogen gas and an oxide of carbon.

15. The process of claim 14 in which said lower melting alkali metal compound is an alkali metal hydroxide.

16. The process of claim 15 in which said alkali metal compound is sodium hydroxide.

17. Te process of claim 14 in which said lower melting alkali metal compound is lithium carbonate.

18. A process for the production of hydrogen-containing gas from a solid carbonaceous material and steam which comprises contacting a solid carbonaceous material and steam in a molten reaction medium maintained at a temperature between about 800 F. and about 1800 F. and comprising at least 10 weight percent of an alkali metal carbonate and at least one other alkali metal compound of the group consisting of an alkali metal hydroxide, an alkali metal nitrate and an akali metal oxide, said alkali metal carbonate and other alkali metal compound being in molten form, said solid carbonaceous material and steam reacting in the molten reaction medium to produce hydrogen-containing gas as a product of the process.

References Cited by the Examiner UNITED STATES PATENTS 701,186 5/1902 Faulkner 4892 1,921,711 8/1933 Wangemann 48206 2,031,987 2/1936 Sullivan. 2,647,045 7/ 1953 Rummel 48206 2,794,709 6/1957 Kirkbride 23212 X FOREIGN PATENTS 7,718 1910 Great Britain. 8,426 6/1892 Great Britain. 322,959 12/ 1929 Great Britain. 465,548 5/ 1937 Great Britain.

MORRIS O. WOLK, Primary Examiner.

MAURICE A. BRINDISI, Examiner.

A. J. STEWART, D. E. GANTZ, I. SCOVRONEK,

Assistant Examiners.

Claims

12. A PROCESS FOR THE PRODUCTION OF HYDROGEN-CONTAINING GAS FROM A SOLID CARBONACEOUS MATERIAL AND STEAM WHICH COMPRISES CHARGING A SOLID CARBONACEOUS MATERIAL AND STEAM TO A REACTION ZONE MAINTAINED AT A TEMPERATURE BETWEEN ABOUT 800*F. AND ABOUT 1800*F. AND CONTAINING A MOLTEN REACTION MEDIUM COMPRISNG AT LEAST 10 WEIGHT PERCENT OF AN ALKALI METAL CARBONATE, IN SAID REACTION ZONE CONTACTING THE MOLTEN REACTION MEDIUM WITH SAID SOLID CARBONACEOUS MATERIAL AND STEAM SUCH THAT A HYDROGEN-CONTAINING GAS IS PRODUCED, WITHDRAWING AN ASH-RICH STEAM FROM SAID REACTION ZONE, AND RECOVERING SAID HYDROGEN-CONTAINING GAS AS A PRODUCT OF THE PROCESS.