Gasification of coal with steam using heat from HTRs

NUCLEAR ENGINEERING AND DESIGN 34 (1975)59-63. © NORTH-HOLLAND PUBLISItlNG COMPANY

GASIFICATION

OF COAL WITH STEAM USING HEAT FROM HTRs

H. JUNTGEN and K.H. VAN HEEK Bergbau-Forschung GmbH, Essen, FederalRepublic of Germany Received 20 June 1975 In existing coal gasification processes a substantial part of the coal is used to provide the heat for the reaction, for the generation and superheating of steam and for the production of oxygen. By using heat from HTRs to substitute this part, the coal is then completely used as raw material for gas production. This offers the following advantages compared with the existing processes: a saving of coal, less CO2 emission and, in countries with high coal costs, lower gas production costs. A survey is given of the state of the project, discussing the first design of a commercial gasifier, the influence of the helium outlet temperature of the HTR, kinds of products, and the overall efficiency of the plant. The aim of. the development is to demonstrate the use of heat from an HTR for full scale coal gasification, starting in 1985.

1. Motives for the new gasification technique and problems for research and development Gasification of coal is a well-known technique which has been performed world-wide on an industrial scale and in many forms [1,2]. Steam gasification is highly endothermic. In all conventional plants coal serves as raw material for gas and as a source o f reaction heat, the heat for steam raising, and other energies needed at the plant. By using the process heat from gas-cooled high temperature nuclear reactors (HTRs) it is possible to avoid using coal as a supplier of heat and instead use it as a raw material for producing gas [ 3 - 5 ] . The advantages are lower costs of gas production compared with conventional processes in countries with high price rates for coal, the saving o f coal reserves and production of smaller amounts o f carbon dioxide in the gasification plant. In the special case of the production of synthetic natural gas (SNG) in Germany, it has been shown that the cost of gas falls by 25%, the coal input is reduced by 40%, and the CO 2 emission at the gasification plant is reduced by 33% [6]. Moreover, no oxygen is used in the new process. Figure 1 shows the manifold uses of an HTR and coal gasification combination [7]. Nuclear heat is fed into a steam gasifier, in which coal and steam are converted into water gas via a strong endothermic reaction. It consists mainly o f H 2, CO, CH 4 and CO 2 and can be

used directly as an energy carrier or as a reducing gas. By a well-known synthesis, methanol can be produced, which subsequently could be used as fuel for motor cars, and by a CO shift hydrogen is obtained which is regarded to be the energy carrier o f the future. Finally, partial methanation leads to town gas and complete methanation to CH 4 as synthetic natural gas. The transformation of water gas into other products involves only exothermic reactions which are performed at a temperature level which is far below that of gasification. Heat thus received must be regarded for electricity production or as waste heat and reduces the efficiency of the nuclear coal gasification as will be shown

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Fig. 1. Manifold uses of steam gasification of coal using nuclear heat.

60

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later. This is valid especially for the highly exothermic methanation reaction. But to produce SNG, which will continue to play an important role on the gas market, there is a more attractive possibility. Methane can be obtained by the hydrogasification of coal, in which case the necessary hydrogen comes froln steam gasification of the hydrogasification char. A heat balance shows that in this way 50% of the high temperature nuclear heat is saved, compared with the complete methanation of water gas for the production of equal amounts of methane. To summarize, the use of nuclear energy for the gasification of coal basically offers new opportunities for marked improvements of the transformation of coal into gas. Since 1971, therefore, research and development in these fields has been carried out in Germany by Kernforschungsanlage Jfllich GmbH, Julich, Rheinische Brankohlenwerke AG, Kt)ln and Bergbau-Forschung GmbH, Essen sponsored by Bundesministerium for Forschung und Technologie of the Federal Republic of Germany (FRG). The main problems which have to be solved before the new process can be operated on an industrial scale are: (a) the development of HTRs up to temperatures of at least 950°C: (b) the transfer of heat from the HTR, into the gas generator; and (c) the construction of an allothermal gas generator under the aspects of kinetics, heat transfer and materials for the heat exchanger. The work done by Bergbau-Forschung is devoted mainly to the solution of the last two points and this paper surveys the state of experimental and theoretical work. Concerning the HTR, there are two heliumcooled reactors under construction (or now in operation) in the FRG and the US, which have a He temperature of 750°C and an electricity production of 300 MW. It should be pointed out that the AVR has been operated at an average He outlet temperature of 950°C since February 1974 and at a thermal load of 45 MW [8]. It is the aim of a German cooperative industrial group, Arbeitsgemeinschaft Nukleare Prozessw~irme (ANP), to build a plant which demonstrates the use of process heat in connection with an HTR of 950°C He temperature and to operate it from 1985 [9]. Thus the process could be available on a commercial scale in the late 1980s.

2. Basic concepts and feasibility of the process From the several possible ways of transferring the heat of the gas circuit of the nuclear reactor into the gasifier, the scheme shown in fig. 2 seems to be the best [7,81. The heat is discharged from the nuclear reactor first by a helium circuit, and is then transferred from it via an intermediate circuit of tte into the gas generator. This gas passes a heat exchanger immersed like an immersion heater in a fluidized bed of coal and steam, and provides the heat necessary for the gasification of the coal. It seems to be advantageous that the primary helium loop, the secondary gas circuit and the fluidized bed operate at the same pressnre of about 40 bar. The steam required is generated and superheated with helium at a lower temperatnre level. The gas produced in the gas generator can be processed to SNG or other products as shown in fig. l. The intermediate circuit separates the nuclear part of the plant from the gasification unit. Thus, it involves a higher degree Of security of the total plant, an inhibition of the permeation of hydrogen from the gasifier into the core of the reactor and of tritium from the nuclear part into the gas generator. Moreover, the possibility of an easier replacement of parts of the gasifier is given. But its disadvantages should not be overlooked, i.e. a loss in temperature of say 50°C, additional problems in building large H e - H e heat exchangers, and the enormous costs. On the whole, it is felt that an intermediate circuit is necessary to make the combination of an HTR and a gasification plant feasible. An industrial gasifier for 50 t/hr coal throughput needs a heat-exchanging area of say 4000 m 2. Our proposal is shown in fig. 3. The gasifier is a horizontal cylinder about 30 m long and 7 m in dia. The fluidized CO2,H2S

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single gas generator. bed will be operated in a tub in which the heat exchangr er that the hot helium flows through is located. Coal is fed in through the inlet on the right-hand side on the top of the reactor. Ash can be withdrawn through the opening on the left-hand side at the bottom. The bed is fluidized by high temperature steam, injected from below. The gas generator has an effective volume for the fluidized bed of 318 m 3. In co-operation with Mannesmannr6hren-Werke AG, Dtisseldorf, the feasibility of this proposal has been proved and confirmed. One of the priority tasks is the development of a suitable alloy for the heat exchanger, which withstands corrosion, shows sufficient creep rupture strength and allows tube forming. Under these aspects experiments underway have shown promising results [10].

The throughput of the gasifier is determined by the joint action of heat transfer and gasification kinetics. Considering the steady state of the gasifier the heat balance must hold, i.e. transferred heat = consumed heat. The transferred heat depends mainly on the overall heat transfer coefficient h whereas the heat consumed by gasification is mainly given by the gasification rate k [6]. The evaluation of the heat balance is described in detail in ref. [13]. The relevant data based on experimental results of Bergbau-Forschung at laboratory scale [11 ] and in a bench scale high pressure fluidized bed gasifier, are listed in fig. 4 for helium outlet temperatures in the range 900-1100°C. For the time being 1 l'00C'C seems out of reach, although it might be worthwhile to explain the advantages attainable by higher temperatures. Figure 4 shows in the first line the outlet temperatures of the HTR. The inlet and output temperatures T 1 and T 2 of the heat exchanger, immersed in the gasifier, have been determined on the assumption that a temperature difference of 50°C has to exist between T 1 and THe on the one hand and T2 and Tv on the other. The essential factors are the gasification temperature Tv and the corresponding throughput of coal. As to the temperature of the fluidized bed, it is most important that a certain rise of T 1 entails a much smaller rise of Tv. If, for instance, T 1 is rising by 100°C, say from 9 0 0 t o 1000°C, Tv will only rise by 24°C. This means that with rising inlet temperatures a bigger part

H. Ji~ntgen, K.II. van Heek, Gasification o f coal with steam

62

of the heat available at the inlet of the gasifier can be turned to account for the reaction, so as to improve the productivity of the generator. As can be seen from the listed throughput values, the amount of coal gasified, which is approx. 39 t/hr at 900°C, can be stepped up by 20, 40 or 80% with rising THe to 950, 1000 or 1100°C, respectively.

3. Overall efficiency of a combined plant Steam gasification of coal using nuclear energy requires heat for the performance of the steam-carbon reaction, the steam generation, and the electric power needed in the plant, as has been explained above. The reaction heat must be supplied at a temperature 50°C higher than that of the fluidized bed of coal and steam. Steam has to be superheated, as far as possible, to the level of the gasification temperature. On the other hand, the HTR, which is required supplies its output within a temperature interval from a maximum down to approx. 250°C, the inlet temperature of the core of the helium-cooled HTR with spherical fuel elements as developed in Germany. Thus it is to be expected that part of the output which exceeds the amount required for gasification has to be converted into electricity. For the calculation of overall efficiency it is most important that a rise of the helium temperature at the inlet of the heat exchanger enlarges the amount of coal gasified and causes a much smaller rise of the

gasification temperature. Due to these facts, a bigger part of the total thermal output of the HTR can be used for coal gasification with higher outlet temperatures. In line with the findings about the temperature drop it can be stated that the part of nuclear heat consumed for gasification, including steam raising, will increase from about 20% at 900°C to 50% at 1000°C and to approx. 80% at 1100°C. Whereas the role of power generation is dominating at lower helium temperatures, the share of power generation is of no consequence at temperatures in the range of 1100°C [12]. Consequently the efficiency of the total plant as given in fig. 5 for different products is increased with temperature. As is shown, the efficiency for the production of water gas which can be used without further exothermic processing, improves from 54% at 900°C to some 75% at 1100°C. This is to be expected as electricity is produced with an efficiency up to some 40% while by steam gasification up to 100% of the process heat is used and bound in the water gas. This heat is lost less or more completely, by subsequent exothermic reactions, which have to be performed for the production of town gas, methanol or SNG. The loss is highest in the latter case. Thus, the corresponding overall efficiencies range between 47 and 60%. In the case of H 2 production, endothermal CH4 reforming and exothermal CO shift require or produce nearly equal amounts of heat.

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The overall efficiency of a plant which combines an HTR with the steam gasification of coal is higher than the value for the production of electricity only. Efforts should be made to reach helium temperatures beyond 1000°C, as far as this can be achieved at reasonable costs. The real way to profit from the possibilities offered by nuclear heat is to produce water gas or hydrogen.

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Fig. 5. Overall efficiency of a plant combining an H T R and steam gasification for various products.

The work described in this paper is performed within the framework of co-operation between Bergbau-Forschung GmbH, Essen; Rheinische Braunkohlenwerke AG, KOln and Kernforschungsanlage Jfdich GmbH,

H. Jftntgen, K.H. van Heek, Gasification o f coal with steam

Ji~lich concerning the project'Entwicklung yon Verfahren zur Umwandlung fester fossiler Rohstoffe mit W~irme aus Hochtemperatur-Kernreaktoren', sponsored by the FRG, Bundesministerium ffir Forschung und Technologie.

References [1] H.W. yon Gratkowski, Kohlvergasung-Verfahren, Ullmanns Encycl. Techn. Chem. 10 (1958) 376-458. [2] G.G. yon Fredersdorff and M.A. Elliot, In: H.H. Lowry, Chemistry of Coal Utilization, Wiley, London (1963) 892. [3] R. Schulten, Die Anwendung von nuklearer Energie fiir die Energie erzeugung der Zukunft, Erd61 Kohle 24 (1971) 334-337. [4 ] W. Peters, Neue Technologien, Gliickauf 105 (1969) 1 2 8 3 1286. [5] K.ti. van Heek, H. JiJntgen and W. Peters, Stand der Gaserzeugung aus Kohle durch Wasserdampfvergasung unter Nutzung von Hochtemperaturreaktorw~irme, ErdSl Kohle 26 (1973) 701-703. [6] H. J~intgen, K.H. van Heek and J. Klein, Vergasung yon Kohle mit Kernreaktorw~irme, Chem. lng. Techn. 46 (1974) 937-943.

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[7] H. Kriager, K.H. van Heek and H. Ji~ntgen, Wasserdampfvergasung von Kohle mit Kernreaktorw~irme, gwf/Gas/ Erdgas 115 (1974) 538-542. [8] W. Cautius, J. Engelhard and G. Ivens, Der Betrieb des Versuchskraftwerks AVR und die HTR-Entwicklung. Atomwirt. Atomtech. 19 (1974) 4 4 4 - 4 5 0 . [9] Nukleare Prozessw~irme - MOglichkeiten und Programmvorschl~ige, Memorandum der Arbeitsgemeinschaft Nukleare Prozessw~irme (ANP), Jan. (1974) Bensberg. [10] G. Kalwa and K.H. van Heek, Development of alloys for the transfer of heat into fluidized coal beds with regard to steam gasification, BNES Int. Conf., London, 2 6 - 2 8 Nov. 1974, Session VII, No. 42. [11] K.H. van Heek, H. Jiintgen and W. Peters, Fundamental studies on coal gasification in the utilization of thermal energy from nuclear high-temperature reactors, J. Int. Fuel 46 (387) (1973) 240-258. [12] D. Wiegand, K.H. van Heek and H. Jiintgen, The significance of the HTR temperature for the economic use of nuclear heat for coal gasification, BNES Int. Conf. London, Nov. 1974, Session III, No. 13. [13] P.P. Feistel, R. DiJrrfeld, K.H. van Heek and H. Jtintgen, Layout of an internally heated gas generator for the steam gasification of coal, Nucl. Eng. Des. 34 (this issue) 147-155.