GB2078251A

GB2078251A - System for Gasifying Coal and Reforming Gaseous Products Thereof

Info

Publication number: GB2078251A
Application number: GB8102751A
Authority: GB
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1980-06-19
Filing date: 1981-01-29
Publication date: 1982-01-06
Also published as: FR2485030A1; JPS5714691A; DE3103933A1; GB2078251B

Abstract

An improved, energy efficient process is disclosed for the catalytic hydrogasification of coal with steam and hydrogen at moderate temperature levels, and the steam reforming of gaseous products therefrom comprising methane with a non-coal heat source especially including high temperature gas cooled nuclear fission reactors to increase energy value and yield of gaseous products. The carbon monoxide produced throughout the process can be shift converted with steam to increase the amount of hydrogen gas product, and said hydrogen can be used to improve the efficiency of the coal gasification step. <IMAGE>

Description

SPECIFICATION System for Gasifying Coal and Reforming Gaseous Products Thereof This invention relates to coal gasification, and more specifically to an energy efficient, highly versatile composite system for catalytically converting coal into gaseous components and gas transformations that is readily adaptable to various heat sources, especially a high temperature gas cooled nuclear reactor, and is easily controllable or regulatable as to the qualitative and quantitative aspects of the gaseous products provided thereby.

The method or process system comprising the subject invention for the gasification or conversion of coal into several gaseous products, and the reforming or transforming of such gaseous products, or components thereof, into other more utilizable or valuable gaseous compositions or forms, as well as the cycling or returning of certain gases produced for use or reuse within the system, includes the following combination and sequence of operations or treatments.

A body of particulate coal is subjected to an atmosphere containing steam and hydrogen in the presence of a catalyst, whereby the coal is converted into several gases typically including methane, carbon monoxide, carbon dioxide and hydrogen. The several gases derived from the coal are thereupon processed such as by the separation or isolation of a single gas composition from the mixture as an end product or as a precursor ingredient for the synthesis of ancillary products; and/or for reforming or conversion within the system into another gas product or form such as a higher energy fuel or a material in greater demand or of more suitable attributes; and/or for cycling back into the system for use as a source of a needed ingredient or reagent for the activity of one or more phases of the system.

The said gasifier system is coupled to a heat source, preferably a high temperature gas cooled nuclear reactor, and a portion of the product gases comprised mainly of methane is reacted with steam thereby using the said source of heat to produce a modified product enriched in hydrogen and having a higher heat content.

Equations defining some of the more important fundamental reactions of the system and the energy entailed in the method of this invention include: Gasifier Steam gasification C+H2OeCO+H2 AH =31.4 Kcals/mole (1) Hydrogasification C+2H2oCH4 AH0=-1 7.9 Kcals/moie (2) Methane Reformer and Gasifier Water Gas Shift CO+H20oc02+H2 AH0=-9.9 Kcals/mole (3) Reforming CH4+H2Ooc0+3H2 AH =49.0 Kcals/mole (4) Shift Converter Water gas shift reaction (3).

Due to impurities or foreign material commonly contained in coal, the coal gasification operation will also result in the formation of incidental gaseous products comprising sulfurous gases such as hydrogen sulfide and carbonyl sulfide. Additionally, trace amounts of various higher hydrocarbons comprise an inherent product of the high temperature and pressure conditions imposed for coal gasification.

The system can, and preferably does, operate in a generally continuous manner with all components and conditions being provided or moving through the various operations or treatments, or stations for effecting the same, in a substantially uninterrupted progression of constant movement or of continual, frequently repeated incremental movements therethrough.

The present invention will be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 of the drawing is a simplified schematic flow diagram of the basic steps or operations for the process system of the present invention.

Figure 2 of the drawing is a schematic flow diagram comprising the interrelated steps or operations of one embodiment of the process system of the present invention.

Figure 3 of the drawing is a schematic flow diagram comprising the interrelated steps or operations of another embodiment of the process system of the present invention.

Referring to the drawings, and in particular to Figure 1 , there is provided a simplified schematic flow diagram illustrating the combined arrangement and sequence of the principal interrelated operations for the reactions and treatments constituting the basis for the system of this invention.

In the coal gasification phase for the system, particulate coal 10 is supplied to a catalyzed gasifier unit 1 2 provided with a source or means 14 for supplying steam and also with a source or means for supplying supplementary coal gasifying agents including hydrogen and carbon monoxide or optionally hydrogen. An appropriate catalyst, such as an alkali metal containing material, particulariy oxides, hydroxides, carbonates, or bicarbonates or mixtures thereof of potassium and/or sodium, is also supplied to the gasification unit 1 2. The said gasifying agents including steam are preheated to between 700 to 9500C to thereby decompose and gasify coal by reacting it with the hydrogen and steam.

Where the heat source is a high temperature gas cooled nuclear reactor, the chemical system pressure should balance the pressure within the reactor across the primary interface, the heat exchange tubes in the reformer. This avoids mechanically overstressing the said heat exchanger tubes.

Increasing pressure is detrimental to the conversion of methane in the reforming reaction, Equation (4).

Contrariwise, in the gasifier, increasing pressure is advantageous to the formation of methane via the reverse of the reforming reaction, Equation 4. Increase of pressure also is advantageous in increasing the rates of the gasification reactions, Equations (1) and (2). Consideration of the pressure requirements of the heat source and the chemicals system indicate acceptable pressure levels of between about 10 and about 60 bars. Current embodiment selects about 40 bars; however, this is an economic choice and is highly dependent on the temperature of the heat available from the source, 22, with increasing temperature allowing increasing pressure. Minor pressure variations in the chemical system naturally occur due to operating pressure drop.

The products of the catalyzed coal gasification derived from unit 12 are a gaseous mixture comprising methane, carbon monoxide, carbon dioxide and hydrogen along with some steam and typically incidental by products comprising hydrogen sulfide, carbonyl sulfide and some hydrocarbons.

Carbon dioxide, hydrogen sulfide and carbonyl sulfide are removed from the produced gaseous mixture in an acid gas absorber unit 16, and discharged from the system. Methane is then separated from the gas mixture in a cryogenic separating unit 1 8 for subsequent reforming. The cryogenic separation typically comprises the use of temperatures ranging down to about 2000 C. The remaining gases from the coal gasification operation, comprising hydrogen and optionally carbon monoxide, are cycled back into the coal gasification unit 12 to react together with steam upon coal under high temperatures and pressures in the presence of the catalyst, and thereby convert the coal into a mixtue of gaseous components comprising methane, carbon monoxide, carbon dioxide and hydrogen, along with some steam and incidental hydrogen sulfide, carbonyl sulfide and some hydrocarbons. These gases derived from the coal gasification can in turn be separated and employed in the same manner or used as described hereinbefore including a partial recycling, thereby repeating the cycle in a continuous operation.

In the reforming phase of the system of this invention, the methane gas removed from the gaseous mixture produced by coal gasification, is premixed with water and heated, using in part, steam generated by the heat content of the reformed gases, and transformed in the reformer unit 20 to a gaseous mixture comprising methane, carbon monoxide, carbon dioxide and hydrogen, along with the steam. The heat source 22 for the methane reformer unit 20 is of a capacity to provide adequate quantities of thermal energy at a maximum temperature of between about 700 to 9500C therein, and in the preferreed embodiment of this invention comprises a nuclear fission reactor, such as a high temperature gas cooled reactor. Alternative heat sources comprise solar means, such as devices or units that derive thermal energy by collecting and/or concentrating thermal radiation from the sun.

Optimum reforming reaction pressures comprise about 10 to about 60 bars.

The gaseous mixture produced by the methane reformer unit 20, is cooled and some steam condensed in condenser 24, and the condensation water is withdrawn whereby it can be mixed with incoming methane prior to vaporization in the reformer or is recycled to unit 14 to be fed to the coal gasifier. Carbon dioxide is then removed from the gaseous mixture by absorption as in the unit 1 6 and methane is subsequently separated from the mixture by cryogenic separation as in unit 1 8. The remaining gases of the mixture comprising hydrogen and carbon monoxide are then cycled into the coal gasification unit 12 to react with the coal and steam the coal gasification process.

Thus, the system provides for all phases of the process to be performed on an essentially continuous or uninterrupted basis, including the recycling of components, the introduction of feed material such as coal and thermal energy and steam, and the removal of end products such as hydrogen therefrom.

In the embodiment of this invention illustrated in detail in Figure 2 of the drawing, pulverized coal, for example Illinois &num;6 coal, is continuously fed into coal gasifier unit 1 2 which is provided with a catalyst, such as potassium carbonate, and therein maintained at a temperature of approximately 7000C. Hydrogen and carbon monoxide gas are preheated to about 7000C in unit 29 by helium as the heat transferring medium from heat exchanger 27 in an auxiliary heat exchange loop, then the preheated mixture passes into the gasifier unit 12 along with steam.

These conditions, as are further specified in the data given in Table I hereinafter, provide for the coal gasification such that reactions (3) and (4) are in equilibrium occurring substantially to an extent to cancel out the net thermal load of the reaction. Preferably the net reaction should be made slightly exothermic to avoid adding heat to the coal/catalyst (e.g. potassium carbonate) reaction at temperatures above 7000C that would encourage corrosion problems in any heat exchanger used to effect heat transfer to a net endothermic gasifier reaction. The gaseous product of this reaction is a mixture containing methane, carbon monoxide, carbon dioxide and hydrogen, sulfurous gases, unreacted steam and small quantities of higher hydrocarbons.

The coal gasification products are cooled in condenser 14 and 14' and the condensate therefrom is fed along with additional steam to provide steam for the gasifier unit 1 2, said steam is superheated to 7000C in unit 30 using helium from unit 27. The methane, carbon monoxide, carbon dioxide and hydrogen gaseous products are returned to the acid gas absorber unit 1 6 whereupon the carbon dioxide and sulfur containing gas are removed from the system, and then on to the cryogenic methane separator unit 1 8 whereupon the methane and higher hydrocarbons are separated for reforming, and the hydrogen and carbon monoxide are then recycled through the coal gasification unit 12, or taken out as product synthesis gas.

The methane gas and trace higher hydrocarbons separated from the gaseous mixture are mixed with water, evaporated and preheated using, in part, heat recouped from the heat content of the product gases from reformer 20 in unit 24 before introduction into the gas reformer unit 20. Additional heat for unit 24 is obtained from the power cycle unit 31 associated with the heat source, unit 22.

Heat exchangers 28 and 32 also provide heat to power cycle unit 31.

The power cycle unit 31 can be composed of standard power plant equipment, such as a heat exchanger as illustrated or a steam turbine, and in an embodiment of this invention includes the use of the bottom fraction of heat, for example from about 5750C to about 3000C, from unit 32 which produces steam at temperatures of up to about 5500C. The source of thermal energy for a power cycle unit can be the primary heat source 22, which in an embodiment of this invention comprises a high temperature gas cooled nuclear reactor. Power cycle units such as 31 can provide steam for a variety of useful chemical or processing services within the system including regeneration of an acid gas absorber, catalyst recovery, provide electrical power to drive compressors for plant recirculation flow or refrigeration.

Some additional heat generated from the heat content of the product gases from unit 20 is provided to the power cycle 31, being available at temperature levels incompatible with the thermal demands stream A. Water is recycled to the process from the condensate from unit 24 and make up water is also provided, for example, but not necessarily from power cycle 31. Reforming reaction temperatures of about 700 to 8500C can be provided with a high temperature nuclear reactor heat source 22, preferably a high temparature gas cooled nuclear reactor, a fossil fuel furnace or even solar heating means.

The reformed gas product is a mixture containing methane, carbon monoxide, carbon dioxide and hydrogen with steam. The product is cooled in unit 24 to condense excess steam, and the mixture is cycled through the system where the carbon dioxide is removed by adsorption in unit 16, the methane and trace hydrocarbons, separated in unit 1 8 and recycled back to the reformer, while the hydrogen and carbon monoxide are cycled into the coal gasification process.

The approximate material balances and temperatures for the reaction steps or treatments of the foregoing embodiment as illustrated in Figure 2 are all set forth in the following Table 1-1 and Table 1-2.

Table 1-1 Product Streams in Short Ton moles/hr for 2000 STD Illinois &num;6 Coal Feed to Gasifie Operation &commat; 700 C Stream A B C C' D D' E E' F G H CH4 5.472 1.936 1.926 1.926 3.546 3.546 - - - 3.546 5.472 H2O 16.415 11.239 - - - - - - 0.564 trace CO - 1.910 1.910 1.910 1.784 1.784 2.971 0.722 - 1.784 CO2 - 1.631 1.631 - trace - - - - trace H2 - 12.267 12.267 12.267 0.004 0.004 7.654 4.618 - 0.004 Nuclear Heat &commat; 950 C 215.6 MW (Reformer Heat &commat; 825 C) Btu/day (HHV) in syn gas 6.254x1010 Work in Cryogenic Separation 32 MWe Table 1-2 Temperature Levels Corresponding to Figure 2 Nuclear Heat &commat; 9250C Gasifier &commat; ? 700 7000C Temperature Stream C A 450 B 600 C 40-70 C' 40-70 D 40-70 D' 40--70 E 25 E' 25 F 700 G 700 H 40 Note: 1) 21 5.6 MWth of nuclear heat provided in reformer per 2000 short tons per day of coal feed to gasifier.

2) Temperature of stream B is that out of internal heat exchanger in the reformer. The peak process temperature is 8250C.

3) Cryogenic separation operates down to approximately 200C C.

The embodiment of this invention shown in Figure 3 includes a modification for an improved gasifier system and a typical application for hydrogen production comprising a cryogenic separation of carbon monoxide and shift reactor incorporated within the system. This variant in the invention diverts carbon monoxide away from the coal gasification operation and unit and directs it through a water gas shift (see equation 3 above) conversion, thereby producing more hydrogen. Advantages or attributes of this embodiment with respect to the former include the following 1) The reduction of carbon monoxide which is undesirable in the gasification reaction within the gasifier unit 12, enables the use of a smaller gasification unit.

2) Reaction rates in the gasifier will be higher as there is an increase in the partial pressure of hydrogen with respect to the former embodiment.

As is apparent from the drawings, the embodiment of Figure 3 is substantially the same as that of Figure 2 with respect to the coal gasification phase and the methane reforming phase, and further detailed description thereof is not needed.

However, in this embodiment the carbon monoxide and hydrogen passing through the cryogenic separator 18, or from other appropriate locations within the system, is diverted through a shift reactor 26 for conversion of the carbon monoxide with steam to hydrogen. Because the reaction is incomplete at temperatures at which the shift reaction can be catalyzed, a large amount of steam is included in excess of that required for stoichiometry according to CO+H20=CO2+H2.

An optimum temperature range for the shift conversion reaction is about 250 to about 4500C.

Steam for this purpose, as well as for other phases of the system of the invention can be, generated in a power plant, unit 31, heated by means such as a high temperature gas cooled nuclear reactor, a light water nuclear reactor, a fossil powered or a solar powered plant. A portion of the heat required to generate said steam may be obtained from the heat content of the product stream from the shift reactor.

The shift reaction is slightly exothermic (-9.9 Kcal/gmole C) so that conversion is best effected at a relatively low temperature over a suitable iron oxide or zinc/copper oxide based catalyst which operates down to 2500C.

As shown in the drawing, the carbon monoxide and hydrogen mixture is cycled through shift reactor unit 26 to convert the carbon monoxide with steam to carbon dioxide and hydrogen according to the above equation.

The approximate material balances and temperatures for the reaction steps or treatments of this latter embodiment including the shift reaction as illustrated in Figure 3 are all set forth in the following Table 2-1 and Table 2-2.

Table 2-1 Approximate Product Streams in Short Ton moles/hr for 2000 ST/D Illinois &num;6 Coal Feed to Gasifier Operating &commat; 700 C Stream A B C C' D D' E F G H CH4 4.963 1.747 1.747 1.747 3.216 3.216 - - 3.216 4.963 H2O 14.889 10.194 - - - - - 1.191 trace CO - 1.732 1.732 1.732 0.968 0.968 - - 0.968 CO2 - 1.479 1.479 - 0.001 - - - 0.001 H2 - 11.127 11.127 11.127 0.020 0.020 4.199 - 0.020 Stream I I' J J' K K' L M N CH4 - - - - - - - - H2O - - 15.345 4.968 13.330 4.313 17.643 0.301 0.236 CO 2.045 0.655 2.045 0.655 0.025 0.002 0.027 0.027 0.027 CO2 - - - - 2.020 0.653 2.673 2.673 H2 6.948 - 6.948 - 8.698 0.653 9.611 9.611 9.611 Nuclear Heat &commat; 925 C 215.6 MW Gasifier &commat; 700 C 5.673x1010 Btu/Day (HHV) in Syn Gas Work in cryogenic separation 25 MWe Table 2-2 Temperature Levels Corresponding to Figure 3 Nuclear Heat &commat; 9250C Gasifier &commat; ? 700 7000C Temperature Stream C A 450 B 600 C 40-70 C' 40-70 D 40---70 D' 40--70 E 40-70 F 700 G 700 H 40-70 40-70 1' 40-70 J 250 J' 250 K 220 K' 220 L 220 M 40-70 N 40-70 Several modifications of the above process are possible involving substitution of some functions of units 1 8, 26 and a second shift reactor 26'. The hydrogen for unit 12 may be produced by shifting synthesis gas to hydrogen without the cryogenic separation of carbon monoxide in unit 18; contrariwise, as illustrated in Figure 3, all of the carbon monoxide present in streams C' and D' may be cryogenically separated in unit 1 8 and fed to a carbon monoxide shift reactor and a portion of the hydrogen product may be fed as stream E to unit 29. Combinations of these strategies will now be obvious to those skilled in the art. For example, product hydrogen may be removed directly from the cryogenic separator.

Claims

1. A method of catalytically converting coal into gaseous components and reforming the gaseous products thereof using a non-coal external heat source consisting of a system comprising the combined steps of: a) subjecting a mass of particulate coal to steam, hydrogen, and carbon monoxide under high temperatures and pressures to thereby decompose and gasify the coal, producing a gaseous mixture comprising methane, carbon dioxide, carbon monoxide, and hydrogen:: b) removing carbon dioxide and any sulfurous gases from the gaseous mixture; c) drawing off a portion of the hydrogen and carbon monoxide of the gaseous mixture as a product; d) separating the methane from the gaseous mixture for reforming; e) applying another portion of the remaining hydrogen and carbon monoxide to act upon particulate coal in the presence of a catalyst and steam for continuing decomposition and gasification of the coal; f) combining the separated methane with water under high temperatures and moderate pressures to reform the methane using a non-coal external heat source into a gaseous mixture comprising methane, carbon dioxide, carbon monoxide and hydrogen; and g) employing gases of the mixture from the reformed methane within the system, including removing a portion of the carbon dioxide from the said gaseous mixture, drawing off a portion of the hydrogen and carbon monoxide from said gaseous mixture as a product, cycling a portion of the methane from said gaseous mixture with water for reforming under high temperatures and moderate pressures, and applying another portion of the hydrogen and carbon monoxide from said gaseous mixture to act upon particulate coal in the presence of a catalyst and steam for continuing decomposition and gasification of the coal.

2. A method of catalytically converting coal into gaseous components and reforming the gaseous products thereof using a non-coal external heat source consisting of a system comprising the combined steps of: a) subjecting a mass of particulate coal to steam and hydrogen under high temperatures and pressures to thereby decompose and gasify the coal, producing a gaseous mixture comprising methane, carbon dioxide, carbon monoxide, and hydrogen; b) removing the carbon dioxide and any sulfurous gases from the gaseous mixture; c) drawing off a portion of the hydrogen of the gaseous mixture as a product; d) separating the methane from the gaseous mixture for reforming; e) Applying another portion of the remaining hydrogen to act upon particulate coal in the presence of a catalyst and steam for continuing decomposition and gasification of the coal;; f) combining the separated methane with water under high temperatures and moderate pressures to reform the methane using a non-coal external heat source into a gaseous mixture comprising methane, carbon dioxide, carbon monoxide and hydrogen; and g) employing gases of the mixture from the reformed methane within the system, including removing a portion of the carbon dioxide from said gaseous mixture, drawing off a portion of the hydrogen from said gaseous mixture as a product, cycling a portion of the methane from said gaseous mixture with water for reforming under high temperatures and moderate pressures, and applying another portion of the hydrogen from said gaseous mixture to act upon particulate coal in the presence of a catalyst and steam for continuing decomposition and gasification of the coal.

3. A method of catalytically converting coal into gaseous components and reforming the gaseous products thereof using a non-coal external heat source consisting of a system comprising the combined steps of: a) subjecting a mass of particulate coal to steam, hydrogen, under high temperatures and pressures to thereby decompose and gasify the coal, producing a gaseous mixture comprising methane, carbon dioxide, carbon monoxide, and hydrogen; b) removing the carbon dioxide and any sulfurous gases from the gaseous mixture; c) drawing off a portion of the hydrogen of the gaseous mixture as a product; d) separating the methane from the gaseous mixture for reforming; e) drawing off a portion of carbon monoxide and hydrogen and combining said gases with steam to shift convert the carbon monoxide with steam to carbon dioxide and hydrogen to thereby increase the hydrogen ratio;; f) applying another portion of the remaining hydrogen to act upon particulate coal in the presence of a catalyst and steam for continuing decomposition and gasification of the coal; g) combining the separated methane with water under high temperatures and moderate pressures to reform the methane using a non-coal external heat source into a gaseous mixture comprising methane, carbon dioxide, carbon monoxide and hydrogen; and h) employing gases of the mixture from the reformed methane within the system, including removing a portion of the carbon dioxide from said gaseous mixture, drawing off a portion of the hydrogen from said gaseous mixture as a product, cycling a portion of the methane containing trace hydrocarbons from said gaseous mixture with water for reforming under high temperatures and moderate pressures, and applying another portion of the hydrogen from said gaseous mixture to act upon particulate coal in the presence of a catalyst and steam for continuing decomposition and gasification of the coal.

4. A method as claimed in any one of claims 1 to 3 wherein the particulate coal is decomposed and gasified in the presence of an alkali metal containing catalyst.

5. A method as claimed in any one of claims 1 to 4 wherein the particulate coal is decomposed and gasified in the presence of a potassium-containing catalyst, a sodium-containing catalyst or a catalyst containing mixtures in varying proportions of sodium and potassium-containing materials.

6. A method as claimed in any one of claims 1 to 5 wherein the high temperatures for the reforming of the methane are provided by heat derived from a nuclear reactor or by heat derived from a solar source.

7. A method as claimed in any one of claims 1 to 6 wherein the reforming of the methane is effected at temperatures of about 700 to about 9250C with heat derived from a nuclear reactor.

8. A method as claimed in any one of claims 1 to 7 wherein the reforming of the methane is effected at a temperature of approximately 8250C.

9. A method as claimed in any one of claims 1 to 8 wherein the coal is gasified at temperatures of about 650 to about 8000C.

10. A method as claimed in any one of claims 1 to 9 wherein the coal is gasified at a temperature of about 7000C.

11. A method as claimed in any one of claims 1 to 10 wherein the coal is gasified at a pressure of between about 10 and about 60 bars.

12. A method as claimed in any one of claims 1 to 11 wherein the coal is gasified at a pressure of about 40 bars.

13. A method as claimed in any one of claims 1 to 12 wherein the methane is separated from the gaseous mixture with cryogenic temperature conditions.

14. A method as claimed in any one of claims 1 to 1 3 wherein the reforming of the methane is effected at a pressure of about 10 to about 60 bars.

1 5. A method as claimed in any one of claims 1 to 1 4 wherein the reforming of the methane is effected at a pressure of about 40 bars.

16. A method as claimed in any one of claims 1 to 1 5 wherein the carbon monoxide and steam are shift converted at temperatures of about 250 to about 4500C.

1 7. A method as claimed in any one of claims 1 to 3, substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.