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Nuclear energy as a primary energy source for hydrogen production
Int. J. Hydrooen Ener~yy Vol 5, pp. 281-292. Pergamon Press L t d 1980. Printed in Greal Britain
NUCLEAR ENERGY AS A PRIMARY ENERGY SOURCE FOR HYDROGEN PRODUCTION R. SCHULTEN Institute for Reactor Development, Nuclear Research Centre, Jiilich, West Germany
(Received for publication 16 October 1979) Abstract--New forms of energy are required to solve the future problems of the world energy market, especially as regards the substitution of mineral oil. High-temperature reactors can make an important contribution towards this goal. The prerequisite is a temperature availability of approx. 950°C, which has been demonstrated in the AVR reactor at Jiilich for 3 years. The 300-MW THTR is being constructed as a continuationof the German HTR programme. At present some processes for coal modification are being promoted by the Development Programme of the Federal Republic of Germany (FRG). The most ideal application of the high-temperature reactor could be the production of hydrogen from water with the aid of thermochemical methods and hybrid processes. INTRODUCTION ThE GROWTHof the world population and the desired rise in the standard of living of the developing countries obviously call for a doubling or trebling of the energy availability in the next few decades. Only a small portion of this increased demand can be supplied by an increase in the production of oil and gas. In the long run, even a decline in the percentage consumption of oil and gas is to be expected. The regenerative energy sources also can only cover a small part of the overall energy demand. Therefore, there seems no other choice to cover the energy deficit than the application of coal and in connection with it, nuclear process heat, for the wide range of heat energy and energy raw materials. The demand occurs mainly due to the production of basic materials in the steel industry, in the chemical industry and other industrial branches. Large efforts are required to realize the necessary energy amounts for industrial small-scale consumers, as well as for the production of household heating and fuel for traffic purposes. In the long run, the expected deficit between the production capacities existing today and the amounts of energy required in the future is so large, that this problem can only be solved with the greatest world-wide efforts. Every possible economic source of primary energy should, therefore, be followed up in order to achieve the goals aimed at. In the future energy-market, saturated presumably in the middle of the next century, liquid fuels and propellants will chiefly play a decisive role. The problems of storage and transport can be relatively simply solved only in this form. Theoretically there is the possibility of attaining this goal with the aid of conventional processes for coal gasification and liquefaction. Intensive investigations show, however, that a coal economy comes up against two limiting factors. On the one hand, coal reserves, which can be economically exploited, are concentrated in only a few countries in the world and therefore the availability is limited; on the other hand, a strongly increased coal consumption involves, in the long run, the problem of the production of carbon dioxide. The application of nuclear process heat for coal modification, for the liquefaction of coal and for the production of hydrogen, therefore, appear to be a necessity in the next century. It allows a more economic utilization of coal and reduces the problem of the production of carbon dioxide in the atmosphere. Nuclear process heat can be applied for the above-mentioned processes if it is produced within a temperature range 300-1000°C, in which a number of interesting chemical reactions can be carried out. According to today's status of knowledge, this temperature range is technically controllable and suffices for a broad range of application of nuclear process heat. THE AVR-REACTOR IN JI~ILICH Since 1975 the AVR reactor in Jiilich has been demonstrating that a maximum temperature of 950°C can be attained in a helium circuit of a high-temperature reactor. The operational experiences up to now show that such a temperature level can be maintained with a sufficiently low activation and contamination of the primary circuit by radioactive substances. The fundamental design of a reactor 281
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FIG. 1. Vertical cross section of the AVR. is represented in Fig. 1. The reactor core consists of 100,000 ball-shaped fuel elements, in which the fuel is in the form of coated particles. The whole circuit is enclosed in an inner pressure reactor vessel, which itself is surrounded by a second steel outer pressure reactor vessel. This additional safety measure was taken during the planning and design of the reactor as at that time there was not yet sufficient knowledge regarding leakage of fission products into the primary circuit. The reactor core, in the form of a pebble-bed, is enclosed within a graphite cylinder. The steam generator is situated above the reactor core. Helium, which flows upwards from the bottom to the top of this system, transfers the heat from the reactor core to the steam generator. The so-cooled gas is led downwards via a concentric gas duct and back to the reactor core by two circulators at the lower part of the steel outer pressure vessel. The diameter of the nuclear reactor is 3 m and that of the fuel elements is 6 cm. Extremely high hot-gas temperatures can be attained because in this system helium, which is inert and cannot be activated, is utilized as the heat-transfer medium and all the fuel elements and hot-gas containing components are made of ceramic materials. The structure of the fuel elements is shown in Fig. 2. About 6000-10,000 coated particles are pressed together with graphite powder to form a ballshaped fuel element; in the outer region of 5 mm there are no particles present. Each of these particles consists of a nucleus of uranium and thorium oxide, which itself is surrounded by many layers of pyrolytic graphite. This type of fuel coating has proved to be good for a high-temperature reactor even at extreme high temperatures of up to 1250'~C and a high burn-up of up to 160,000 MWd/t. The amount of radioactive fission products, which emerges, is extremely low. The residual radioactivity is mainly due to the fact that during the production of fuel elements, a small contamination of the inner graphite surface and the fuel elements occurs, caused by an evaporation of uranium, which cannot be completely prevented. The activation is, however, so low that a loss of coolant of the primary circuit, even for larger reactor systems, signifies no danger for the surroundings of a nuclear plant. Figure 3 represents the temperature profile of the helium flow dependent on the reactor height. From the figure it is seen that only the region between the reactor core and the steam generator is
NUCLEAR ENERGY AS A PRIMARY ENERGY SOURCE FOR HYDROGEN PRODUCTION 283
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subject to a high temperature load. As in the case of the AVR reactor, in all future plants also attention must be paid that this relatively small high-temperature region is laid out with ceramic materials as much as possible. All the other components of the reactor system, excepting the heat exchangers, can be held in a moderate temperature range, so that it is sufficient to utilize simple steels. Among the various reactor components, the charging and discharging system has to be especially mentioned. The fuel elements enter the reactor pneumatically through five ducts, so that it is possible for a differential charging of the inner and outer zone of the reactor to occur. Due to the continual charging and discharging during operation, a reactivity compensation for the burn-up is not necessary. The charging system also includes a removal of burnt-up fuel elements via a discharge channel in the lower part of the reactor core. During operation until now more than 1 million fuel elements have been transported in the reactor and have been removed again from the reactor core. Another important component of the reactor system is the gas purification system. Many years' operation of the AVR reactor has shown that it is necessary to remove non-radioactive gaseous impurities, e.g. hydrogen, carbon monoxide and methane. The original assumption, that it is necessary to continually remove the radioactive components of the gas circuit, has not been confirmed. The concentration of noble gases and iodine is relatively low. The solid radioactive substances, such as, e.g., cesium, silver and strontium, are deposited very soon in the circuit. On the whole, therefore there is no necessity of removing radioactive substances. The operation of the AVR reactor has furthermore shown that only
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FIG. 3. Temperatures in a vertical cross section of the AVR at 850 and 950°C average hot-gas temperature. very little graphite dust is produced in the primary circuit, so that only a very small amount has to be removed in a graphite filter provided for this. During this long-term operation of many years, the reactor has shown excellent operational behaviour. There were only a few interruptions in operation caused by small breakdowns. The most important components of the reactor, e.g. the charging and discharging machine or the circulators are accessible practically without any shielding. It could be proved, especially, that the question of emergency cooling for this reactor type can be easily solved. Due to the high heat capacity and the relatively good after-heat removal, an emergency after-heat removal system is not necessary for the AVR reactor. Even under extreme conditions, e.g. fall-out of all cooling and shut-down arrangements, there was no significant temperature increase in the course of 24 h. The negative temperature coefficient is sufficiently large, to regulate the power of the reactor through the amount of helium flow. Only the control rods of the reactor have to be utilized for adjusting the temperature level. At present, and in the next few years, the reactor will be employed to further investigate various types of fuel elements. Up to now five different types of fuel elements were successfully tested. In the next few years, fuel elements with 20 ~ enrichment (especially due to the possibly necessary proliferation safety) and elements with feed-and-breed particles are expected to be tested. The aim of this fuel element development is to raise the conversion factor by increasing the thorium content and ultimately the realization of a conversion factor of 1 for this reactor type. THE THTR (THORIUM HIGH-TEMPERATURE REACTOR) The good operational results of the AVR reactor led to the decision to construct a 300-MW reactor according to the same principle (see Fig. 4). In contrast to the AVR, a pre-stressed concrete vessel is utilized in the THTR as a pressure-carrying system. The overall primary system, including all components, are situated within this reactor pressure vessel. The pre-stressed vessel seems to be an im-
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provement on the steel vessel from the technical safety point of view, as a crack propagation during failure of the vessel can be excluded. The construction guarantees that the heat-transfer medium, helium, can only emanate slowly, in about 20 rain, through a crack in the primary system. The emission of this gas via a filter presents no danger for the surroundings. The pre-stressed concrete vessel, which has a height and diameter of 24 m, is gas-tight on the inside by means of a liner of steel. On the helium side, the liner has a thermal insulation and is so strongly cooled by water that the vessel is exposed to a maximum temperature of 50°C. In the middle of this vessel is the reactor core, which consists of a pebble-bed of 500,000 fuel elements. The helium gas ducting in the reactor core occurs from top to bottom. In the lower part of the reactor core, there is the ceramic hot-gas plenum, from where the hot gas is led to the steam generators arranged on the sides. After being cooled, the gas is led back to the
286
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reactor core by the circulator. The control and shut-down rods of the reactor are brought into the pebble-bed from the top. In normal operation, it is limited to the upper cold part of the reactor core. Solely for a total shut-down in a cooled-down condition, it is necessary for the rods to be brought right down to the bottom of the core. The fuel elements enter the reactor pneumatically through several ducts, similar to the case of the AVR; so it is possible for a charging of the reactor to occur with an inner and outer zone. By means of a differential charging of both of these zones, it is possible to considerably even up the exit temperature of helium. The deviation of the helium temperature over, the radius of the reactor can be held constant within a limit of + 50°C. The THTR was constructed in 1972. The critical experiments are expected to begin in 1980. This reactor represents a further important milestone in the realization of nuclear process heat.
PNP (PROJECT N U CLEA R PROCESS HEAT) Since about 5 years ago, a group of firms [Bergbauforschung (BF), Gesellschaft fiir Hochtemperaturreaktor-Technik (GHT), Hochtemperatur-Reaktorbau GmbH (HRB), Rheinische Braunkohlenwerke (RBW) together with the Kernforschungsanlage Jiilich (KFA)] have been working on the realization of a process heat reactor of large power. The first step was the investigation of the feasibility of such a reactor having 3000 MW(th) and an outlet temperature of 950°C. The concept worked out is shown in Fig. 5. An integrated system was chosen for the design, which is so characterized that all the components of the primary circuit are situated within a pre-stressed concrete vessel. For the reactor and the most important components, the same system was utilized as in the THTR. To attain the abovementioned power, the reactor core contains about 3 million fuel elements, which are utilized for about 3 years in the reactor core. The helium gas ducting was also chosen to be the same as that in the THTR, i.e. from top to bottom in the region of the reactor core. From the hot-gas plenum situated at the bottom, helium passes through the hot-gas ducting to the steam reformer and here it is cooled from 950 to 750°C. This helium then enters a steam generator, where it is further cooled to about 300°C. Then follows a helium circulator which pumps the cooled helium concentrically back through the reactor, so that all the metallic parts of the components excepting the heat exchanger ducts of the steam reformer and the steam generator, are exposed to a temperature of less than 300°C. The shut-down system, the circulator and the charging and discharging system can be taken over from the THTR technology. The most important new component is the steam reformer, which is utilized for the process of hydrogenating coal gasification. The design and construction of this novel component closely follows that of corresponding conventional plants, which are utilized in the chemical industry. A first choice of material was the centrifugally cast ducts usually used for steam reformers. The main difference compared with conventional systems is that helium is utilized as the heat-transfer medium at a pressure of 40 atm whereas conventional plants are operated by flue gases at 1 atm pressure. This means that the material problems are facilitated, as no temperatures above 950°C occur. In contrast to the conventional design, there is a counter-current heat exchanger in the nuclear steam reformer, so that only relatively small temperature differences are required between helium and the gases utilized in the steam reformer. The individual ducts of the steam reformer can be tested. In case of failure of a duct, it can be disconnected at the upper end which is easily accessible. The pressure difference between the helium and the gas circuit on the secondary side is practically zero. There is only a small excess pressure on the secondary side, so that a leakage of helium to the outside surroundings is definitely prevented. Parts of this steam reformer have already been tested in technical experiments in the last few years. For example, a number of ducts have been tested in the EVA plant (see Fig. 6) at a temperature of 1000°C and a pressure of 40 bar. The experiments carried out show that it is possible to have an optimum design of the heat transfer conditions and the kinetics of the desired reaction. In extensive accompanying material research programmes, a large number of material samples for the steam reformer are being tested in long-term experiments. The biggest problem at present is the permeation of hydrogen from the secondary side into the helium circuit as well as vice versa, the permeation of tritium from helium into the secondary circuit. However, from an appropriate number of test set-ups, it has been possible to successfully clarify the behaviour of hydrogen, so that the feasibility of this concept can be predicted. The construction of the SU PER-EVA, which has been started during the course of the last yeai'.(see Fig. 7), should be able to accomplish the demonstration of a characteristic bundle of the steam reformer system in long-term operation.
NUCLEAR ENERGY AS A PRIMARY ENERGY SOURCE FOR HYDROGEN PRODUCTION 287
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FIG. 6. Simplifiedflow sheet of the pilot plant EVA (Einzelrohr-Versuchs-Anlage)singletube experimental plant. PHYSICAL DESIGN OF THE REACTOR CORE The high outlet temperature in the reactor core of a high-temperature reactor should be achieved with the lowest possible temperature ate the coated particles. To fulfill this condition, the so-called OTTO(Once-Through-Then-Out) charing of the pebble-bed reactor was developed. The principle is as follows: Fresh fuel elements are loaded into the upper cold part of the reactor core and after slowly flowing through the reactor core in about 3 years' time they are removed at the bottom as burnt-up fuel elements. Through the consumption of fissionable fuel and the accumulation of fission products, an unsymmetrical neutron flux distribution is obtained. As a result of this type of neutron flux, the power density of the reactor in the axial direction is also strongly unsymmetrical as is shown in Fig. 8. At the top, cold part of the reactor, the power density can be made to be five times higher than that at the reactor bottom. This type of reactor charging has big advantages especially for the temperature distribution. It is thus achieved, as shown in Fig. 8, that there is only a small deviation between the helium outlet temperature and the maximum temperature within the fuel elements. Thus for a maximum coated particle temperature of 1050°C, a mean helium outlet temperature of 950°C can be attained. From today's status of knowledge, it can be predicted that, for this maximum temperature of 1050°C, the release of radioactive fission products is sufficiently low. This is of importance with respect to the technical safety problems as well as for maintenance and repairs. A further advantage of this type of charging is that the burnt-up elements which are removed have considerably lost their residual heat, so that it is not necessary to have water cooling, but air cooling suffices for their storage. This charging scheme has been investigated for a number of different fuel cycles. It is possible to
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utilize both low-enriched and highly-enriched uranium. In the latter case, the cycle can be optimized either to a high burn-up with fuel elements which can be discarded or to a high conversion factor with reprocessing. Furthermore, it was found that cycles with 20 ~ enriched uranium produced only extremely small amounts of plutonium and that this plutonium is unsuitable for proliferation purposes due to its composition. Such elements have the advantage that they can be stored without reprocessing as middle-active waste. The introductory strategy of the reprocessing technology for high-temperature reactors will probably be relatively simple. In the next 20 years, in which there will be few hightemperature reactors, cycles should be utilized in which the fuel elements can directly be brought to a final storage. In this case, the uranium consumption, even without reprocessing, would be able to compete with that of light-water reactors. At a later date, i.e. when there are a large number of hightemperature reactors, a reprocessing plant of medium size could be utilized, which would make it possible to raise the conversion factor from 0-6 to 0.95. St~ch a stepwise approach to a near-breeder does not require any constructive alteration in the reactor, but is solely a specification of the reprocessing technology. In this way the high-temperature reactor can adapt itself in an ideal manner to a possible future uranium shortage. Further investigations have ensured that in the distant future the pebble-bed reactor can be operated with a breeding factor of 1 if a breeder blanket in the form of a pebble haze is applied. This would, however, mean a constructive alteration of the charging system. Instead of the usual two zones up to now of the reactor core in the axial direction, three zones would be necessary. It cannot be predicted today what the uranium situation will be like in 50 years' time. But it is certain today that nuclear process heat utilizing the thorium cycle can be applied long-term and that it is possible to sufficiently raise the conversion factor of this reactor system. PROCESS APPLICATIONS In the project "'Prototyp Nukleare ProzeSw~irme" (PNP) of the Federal Republic of Germany two coupled processes of nuclear plants with subsequent processes have been developed up to now. One process is the reforming (or splitting) of methane with steam and nuclear heat, the other is the gasification of coal with the aid of steam and nuclear heat. Methane reforming can be utilized for a number of
NUCLEAR ENERGY AS A PRIMARY ENERGY SOURCE FOR HYDROGEN PRODUCTION 291 processes. The simplest process is the hydrogenating coal gasification, for the demonstration of which a semi-technical plant has been in operation for over 3 years. In this process, hydrogen and coal are converted to methane in a known process in an exothermic reaction. Half of the methane produced is led to a steam reformer of the nuclear system where it is converted to carbon monoxide and hydrogen. By a further conversion process it is completely converted to hydrogen. The hydrogen produced can be led back to the hydrogenating coal gasification. The reaction scheme is shown in Fig. 9. The same L H 2 + 2C - - - - " CHz, + H 2 0 CO
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coupled processes can be used for the liquefaction of coal. In the Bergius-Pier method, developed 40 years ago, hydrogen and coal are made to react under high pressure in the oil sump. Besides the desired hydrocarbons, a number of light gaseous hydrocarbons are produced. These are led to the steam reformer, where they are reformed by nuclear heat and steam. By a conversion process, the gases produced in the reforming process are converted to hydrogen. The reaction scheme of this coupled process is shown in Fig. 10. methanol can also be obtained from coat by a similar process. Here half of the gas
(C>+(H2> ~ CnH2n÷2(Benzin,01} +C,H2,.2{Kohlenwosserstoff, C, H2,.2+ ill20 ~ ,CO+ (2,+1)H2 Gas} iCO+iH20 ~ ,C02+ iH2 Summe: (C>+
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produced in the steam reformer is utilized for hydrogenating coal gasification, and the other half is utilized for producing methanol. The methane produced in the first part as a product of the hydrogenating coal gasification is led back to the steam reformer to produce reformer gases, so that new reformer gas is available for the production of methanol or for the hydrogenating coal gasification. The reaction scheme is shown in Fig. 11. The steam reformer can also be utilized to transfer large amounts of energy into gas ducts. The chemical equilibrium of methane reforming is utilized for this. Through the reforming of methane about 50 kcal/mole nuclear heat is brought into the gas circuit. These reformer gases can be led by a pipeline to the consumer where they are reconverted at a somewhat lower temperature. The energy input is thus freed again. In this way, it is possible from a large plant to supply a number of consumers with heat energy in a temperature range of up to 500°C. This system is especially appropriate for the production of process heat and household heating.
292
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FIG. 11. The production of methanol using nuclear heat.
The direct gasification of coal with the aid of steam is shown in the following reaction scheme {see Fig. 12). The required reaction temperature depends on the type of coal and varies from 750 to 900°C. For this reaction about 30 kcal/mole are required on the whole. The gas mixture of carbon monoxide and hydrogen, which practically always contains methane, has to be purified first. The purified gases
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FIG. 12. Steam gasification using nuclear heat. can be utilized to produce natural gas and liquid hydrocarbons. Compared with conventional gasification processes, the above-mentioned processes have the advantage that the coal input per unit of heat obtained is lower by a factor of 1.6. The balance sheet of the carbon dioxide produced also is lower by this factor. Extensive economy studies have shown that, on the basis of coal prices in the FRG, it is to be expected that this process is able to economically compete with mineral oil and natural gas in the long run, providing that realistic conditions are taken for the prices of mineral oil, natural gas and coal. An important result is that large plants with a capacity of approx. 3 to 4 million t SKE/yr do not require much higher investment costs than conventional autothermal gasification plants, if their capacity is considered relative to the amount of natural gas or liquid hydrocarbons produced. An especially important field of application of nuclear process heat in future will certainly be the splitting of water with the aid of thermochemical methods or hybrid processes. At present, however, economically applicable solutions have not been found yet. It is to be hoped that new successes in this area will be pointed out at this conference. The He/He-heat exchange required for these processes is being developed within the scope of our project.
NUCLEAR ENERGY AS A PRIMARY ENERGY SOURCE FOR HYDROGEN PRODUCTION R. SCHULTEN Institute for Reactor Development, Nuclear Research Centre, Jiilich, West Germany
(Received for publication 16 October 1979) Abstract--New forms of energy are required to solve the future problems of the world energy market, especially as regards the substitution of mineral oil. High-temperature reactors can make an important contribution towards this goal. The prerequisite is a temperature availability of approx. 950°C, which has been demonstrated in the AVR reactor at Jiilich for 3 years. The 300-MW THTR is being constructed as a continuationof the German HTR programme. At present some processes for coal modification are being promoted by the Development Programme of the Federal Republic of Germany (FRG). The most ideal application of the high-temperature reactor could be the production of hydrogen from water with the aid of thermochemical methods and hybrid processes. INTRODUCTION ThE GROWTHof the world population and the desired rise in the standard of living of the developing countries obviously call for a doubling or trebling of the energy availability in the next few decades. Only a small portion of this increased demand can be supplied by an increase in the production of oil and gas. In the long run, even a decline in the percentage consumption of oil and gas is to be expected. The regenerative energy sources also can only cover a small part of the overall energy demand. Therefore, there seems no other choice to cover the energy deficit than the application of coal and in connection with it, nuclear process heat, for the wide range of heat energy and energy raw materials. The demand occurs mainly due to the production of basic materials in the steel industry, in the chemical industry and other industrial branches. Large efforts are required to realize the necessary energy amounts for industrial small-scale consumers, as well as for the production of household heating and fuel for traffic purposes. In the long run, the expected deficit between the production capacities existing today and the amounts of energy required in the future is so large, that this problem can only be solved with the greatest world-wide efforts. Every possible economic source of primary energy should, therefore, be followed up in order to achieve the goals aimed at. In the future energy-market, saturated presumably in the middle of the next century, liquid fuels and propellants will chiefly play a decisive role. The problems of storage and transport can be relatively simply solved only in this form. Theoretically there is the possibility of attaining this goal with the aid of conventional processes for coal gasification and liquefaction. Intensive investigations show, however, that a coal economy comes up against two limiting factors. On the one hand, coal reserves, which can be economically exploited, are concentrated in only a few countries in the world and therefore the availability is limited; on the other hand, a strongly increased coal consumption involves, in the long run, the problem of the production of carbon dioxide. The application of nuclear process heat for coal modification, for the liquefaction of coal and for the production of hydrogen, therefore, appear to be a necessity in the next century. It allows a more economic utilization of coal and reduces the problem of the production of carbon dioxide in the atmosphere. Nuclear process heat can be applied for the above-mentioned processes if it is produced within a temperature range 300-1000°C, in which a number of interesting chemical reactions can be carried out. According to today's status of knowledge, this temperature range is technically controllable and suffices for a broad range of application of nuclear process heat. THE AVR-REACTOR IN JI~ILICH Since 1975 the AVR reactor in Jiilich has been demonstrating that a maximum temperature of 950°C can be attained in a helium circuit of a high-temperature reactor. The operational experiences up to now show that such a temperature level can be maintained with a sufficiently low activation and contamination of the primary circuit by radioactive substances. The fundamental design of a reactor 281
282
R. SCHULTEN
inner
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outer reactor vessel
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FIG. 1. Vertical cross section of the AVR. is represented in Fig. 1. The reactor core consists of 100,000 ball-shaped fuel elements, in which the fuel is in the form of coated particles. The whole circuit is enclosed in an inner pressure reactor vessel, which itself is surrounded by a second steel outer pressure reactor vessel. This additional safety measure was taken during the planning and design of the reactor as at that time there was not yet sufficient knowledge regarding leakage of fission products into the primary circuit. The reactor core, in the form of a pebble-bed, is enclosed within a graphite cylinder. The steam generator is situated above the reactor core. Helium, which flows upwards from the bottom to the top of this system, transfers the heat from the reactor core to the steam generator. The so-cooled gas is led downwards via a concentric gas duct and back to the reactor core by two circulators at the lower part of the steel outer pressure vessel. The diameter of the nuclear reactor is 3 m and that of the fuel elements is 6 cm. Extremely high hot-gas temperatures can be attained because in this system helium, which is inert and cannot be activated, is utilized as the heat-transfer medium and all the fuel elements and hot-gas containing components are made of ceramic materials. The structure of the fuel elements is shown in Fig. 2. About 6000-10,000 coated particles are pressed together with graphite powder to form a ballshaped fuel element; in the outer region of 5 mm there are no particles present. Each of these particles consists of a nucleus of uranium and thorium oxide, which itself is surrounded by many layers of pyrolytic graphite. This type of fuel coating has proved to be good for a high-temperature reactor even at extreme high temperatures of up to 1250'~C and a high burn-up of up to 160,000 MWd/t. The amount of radioactive fission products, which emerges, is extremely low. The residual radioactivity is mainly due to the fact that during the production of fuel elements, a small contamination of the inner graphite surface and the fuel elements occurs, caused by an evaporation of uranium, which cannot be completely prevented. The activation is, however, so low that a loss of coolant of the primary circuit, even for larger reactor systems, signifies no danger for the surroundings of a nuclear plant. Figure 3 represents the temperature profile of the helium flow dependent on the reactor height. From the figure it is seen that only the region between the reactor core and the steam generator is
NUCLEAR ENERGY AS A PRIMARY ENERGY SOURCE FOR HYDROGEN PRODUCTION 283
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subject to a high temperature load. As in the case of the AVR reactor, in all future plants also attention must be paid that this relatively small high-temperature region is laid out with ceramic materials as much as possible. All the other components of the reactor system, excepting the heat exchangers, can be held in a moderate temperature range, so that it is sufficient to utilize simple steels. Among the various reactor components, the charging and discharging system has to be especially mentioned. The fuel elements enter the reactor pneumatically through five ducts, so that it is possible for a differential charging of the inner and outer zone of the reactor to occur. Due to the continual charging and discharging during operation, a reactivity compensation for the burn-up is not necessary. The charging system also includes a removal of burnt-up fuel elements via a discharge channel in the lower part of the reactor core. During operation until now more than 1 million fuel elements have been transported in the reactor and have been removed again from the reactor core. Another important component of the reactor system is the gas purification system. Many years' operation of the AVR reactor has shown that it is necessary to remove non-radioactive gaseous impurities, e.g. hydrogen, carbon monoxide and methane. The original assumption, that it is necessary to continually remove the radioactive components of the gas circuit, has not been confirmed. The concentration of noble gases and iodine is relatively low. The solid radioactive substances, such as, e.g., cesium, silver and strontium, are deposited very soon in the circuit. On the whole, therefore there is no necessity of removing radioactive substances. The operation of the AVR reactor has furthermore shown that only
284
R. SCHULTEN f l a n g e of out,
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FIG. 3. Temperatures in a vertical cross section of the AVR at 850 and 950°C average hot-gas temperature. very little graphite dust is produced in the primary circuit, so that only a very small amount has to be removed in a graphite filter provided for this. During this long-term operation of many years, the reactor has shown excellent operational behaviour. There were only a few interruptions in operation caused by small breakdowns. The most important components of the reactor, e.g. the charging and discharging machine or the circulators are accessible practically without any shielding. It could be proved, especially, that the question of emergency cooling for this reactor type can be easily solved. Due to the high heat capacity and the relatively good after-heat removal, an emergency after-heat removal system is not necessary for the AVR reactor. Even under extreme conditions, e.g. fall-out of all cooling and shut-down arrangements, there was no significant temperature increase in the course of 24 h. The negative temperature coefficient is sufficiently large, to regulate the power of the reactor through the amount of helium flow. Only the control rods of the reactor have to be utilized for adjusting the temperature level. At present, and in the next few years, the reactor will be employed to further investigate various types of fuel elements. Up to now five different types of fuel elements were successfully tested. In the next few years, fuel elements with 20 ~ enrichment (especially due to the possibly necessary proliferation safety) and elements with feed-and-breed particles are expected to be tested. The aim of this fuel element development is to raise the conversion factor by increasing the thorium content and ultimately the realization of a conversion factor of 1 for this reactor type. THE THTR (THORIUM HIGH-TEMPERATURE REACTOR) The good operational results of the AVR reactor led to the decision to construct a 300-MW reactor according to the same principle (see Fig. 4). In contrast to the AVR, a pre-stressed concrete vessel is utilized in the THTR as a pressure-carrying system. The overall primary system, including all components, are situated within this reactor pressure vessel. The pre-stressed vessel seems to be an im-
NUCLEAR ENERGY AS A PRIMARY ENERGY SOURCE FOR HYDROGEN PRODUCTION 285 10
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FIG. 4. THTR-300 metal reactor internals.
provement on the steel vessel from the technical safety point of view, as a crack propagation during failure of the vessel can be excluded. The construction guarantees that the heat-transfer medium, helium, can only emanate slowly, in about 20 rain, through a crack in the primary system. The emission of this gas via a filter presents no danger for the surroundings. The pre-stressed concrete vessel, which has a height and diameter of 24 m, is gas-tight on the inside by means of a liner of steel. On the helium side, the liner has a thermal insulation and is so strongly cooled by water that the vessel is exposed to a maximum temperature of 50°C. In the middle of this vessel is the reactor core, which consists of a pebble-bed of 500,000 fuel elements. The helium gas ducting in the reactor core occurs from top to bottom. In the lower part of the reactor core, there is the ceramic hot-gas plenum, from where the hot gas is led to the steam generators arranged on the sides. After being cooled, the gas is led back to the
286
R. SCHULTEN
reactor core by the circulator. The control and shut-down rods of the reactor are brought into the pebble-bed from the top. In normal operation, it is limited to the upper cold part of the reactor core. Solely for a total shut-down in a cooled-down condition, it is necessary for the rods to be brought right down to the bottom of the core. The fuel elements enter the reactor pneumatically through several ducts, similar to the case of the AVR; so it is possible for a charging of the reactor to occur with an inner and outer zone. By means of a differential charging of both of these zones, it is possible to considerably even up the exit temperature of helium. The deviation of the helium temperature over, the radius of the reactor can be held constant within a limit of + 50°C. The THTR was constructed in 1972. The critical experiments are expected to begin in 1980. This reactor represents a further important milestone in the realization of nuclear process heat.
PNP (PROJECT N U CLEA R PROCESS HEAT) Since about 5 years ago, a group of firms [Bergbauforschung (BF), Gesellschaft fiir Hochtemperaturreaktor-Technik (GHT), Hochtemperatur-Reaktorbau GmbH (HRB), Rheinische Braunkohlenwerke (RBW) together with the Kernforschungsanlage Jiilich (KFA)] have been working on the realization of a process heat reactor of large power. The first step was the investigation of the feasibility of such a reactor having 3000 MW(th) and an outlet temperature of 950°C. The concept worked out is shown in Fig. 5. An integrated system was chosen for the design, which is so characterized that all the components of the primary circuit are situated within a pre-stressed concrete vessel. For the reactor and the most important components, the same system was utilized as in the THTR. To attain the abovementioned power, the reactor core contains about 3 million fuel elements, which are utilized for about 3 years in the reactor core. The helium gas ducting was also chosen to be the same as that in the THTR, i.e. from top to bottom in the region of the reactor core. From the hot-gas plenum situated at the bottom, helium passes through the hot-gas ducting to the steam reformer and here it is cooled from 950 to 750°C. This helium then enters a steam generator, where it is further cooled to about 300°C. Then follows a helium circulator which pumps the cooled helium concentrically back through the reactor, so that all the metallic parts of the components excepting the heat exchanger ducts of the steam reformer and the steam generator, are exposed to a temperature of less than 300°C. The shut-down system, the circulator and the charging and discharging system can be taken over from the THTR technology. The most important new component is the steam reformer, which is utilized for the process of hydrogenating coal gasification. The design and construction of this novel component closely follows that of corresponding conventional plants, which are utilized in the chemical industry. A first choice of material was the centrifugally cast ducts usually used for steam reformers. The main difference compared with conventional systems is that helium is utilized as the heat-transfer medium at a pressure of 40 atm whereas conventional plants are operated by flue gases at 1 atm pressure. This means that the material problems are facilitated, as no temperatures above 950°C occur. In contrast to the conventional design, there is a counter-current heat exchanger in the nuclear steam reformer, so that only relatively small temperature differences are required between helium and the gases utilized in the steam reformer. The individual ducts of the steam reformer can be tested. In case of failure of a duct, it can be disconnected at the upper end which is easily accessible. The pressure difference between the helium and the gas circuit on the secondary side is practically zero. There is only a small excess pressure on the secondary side, so that a leakage of helium to the outside surroundings is definitely prevented. Parts of this steam reformer have already been tested in technical experiments in the last few years. For example, a number of ducts have been tested in the EVA plant (see Fig. 6) at a temperature of 1000°C and a pressure of 40 bar. The experiments carried out show that it is possible to have an optimum design of the heat transfer conditions and the kinetics of the desired reaction. In extensive accompanying material research programmes, a large number of material samples for the steam reformer are being tested in long-term experiments. The biggest problem at present is the permeation of hydrogen from the secondary side into the helium circuit as well as vice versa, the permeation of tritium from helium into the secondary circuit. However, from an appropriate number of test set-ups, it has been possible to successfully clarify the behaviour of hydrogen, so that the feasibility of this concept can be predicted. The construction of the SU PER-EVA, which has been started during the course of the last yeai'.(see Fig. 7), should be able to accomplish the demonstration of a characteristic bundle of the steam reformer system in long-term operation.
NUCLEAR ENERGY AS A PRIMARY ENERGY SOURCE FOR HYDROGEN PRODUCTION 287
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288
R. SCHULTEN
EVA D- 1 steam -reformer E - 1 evaporator, superheater E-2 CH~-heater E-3 H20/CH 4 -superheater E -4 helium heat exchanger E -5 helium heater Product gas
E - 6 cooler E-8 heat exchanger
G - I feed wafer pump G-2 CHx -compressor G- He helium-circulatx)r G-He
FIG. 6. Simplifiedflow sheet of the pilot plant EVA (Einzelrohr-Versuchs-Anlage)singletube experimental plant. PHYSICAL DESIGN OF THE REACTOR CORE The high outlet temperature in the reactor core of a high-temperature reactor should be achieved with the lowest possible temperature ate the coated particles. To fulfill this condition, the so-called OTTO(Once-Through-Then-Out) charing of the pebble-bed reactor was developed. The principle is as follows: Fresh fuel elements are loaded into the upper cold part of the reactor core and after slowly flowing through the reactor core in about 3 years' time they are removed at the bottom as burnt-up fuel elements. Through the consumption of fissionable fuel and the accumulation of fission products, an unsymmetrical neutron flux distribution is obtained. As a result of this type of neutron flux, the power density of the reactor in the axial direction is also strongly unsymmetrical as is shown in Fig. 8. At the top, cold part of the reactor, the power density can be made to be five times higher than that at the reactor bottom. This type of reactor charging has big advantages especially for the temperature distribution. It is thus achieved, as shown in Fig. 8, that there is only a small deviation between the helium outlet temperature and the maximum temperature within the fuel elements. Thus for a maximum coated particle temperature of 1050°C, a mean helium outlet temperature of 950°C can be attained. From today's status of knowledge, it can be predicted that, for this maximum temperature of 1050°C, the release of radioactive fission products is sufficiently low. This is of importance with respect to the technical safety problems as well as for maintenance and repairs. A further advantage of this type of charging is that the burnt-up elements which are removed have considerably lost their residual heat, so that it is not necessary to have water cooling, but air cooling suffices for their storage. This charging scheme has been investigated for a number of different fuel cycles. It is possible to
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utilize both low-enriched and highly-enriched uranium. In the latter case, the cycle can be optimized either to a high burn-up with fuel elements which can be discarded or to a high conversion factor with reprocessing. Furthermore, it was found that cycles with 20 ~ enriched uranium produced only extremely small amounts of plutonium and that this plutonium is unsuitable for proliferation purposes due to its composition. Such elements have the advantage that they can be stored without reprocessing as middle-active waste. The introductory strategy of the reprocessing technology for high-temperature reactors will probably be relatively simple. In the next 20 years, in which there will be few hightemperature reactors, cycles should be utilized in which the fuel elements can directly be brought to a final storage. In this case, the uranium consumption, even without reprocessing, would be able to compete with that of light-water reactors. At a later date, i.e. when there are a large number of hightemperature reactors, a reprocessing plant of medium size could be utilized, which would make it possible to raise the conversion factor from 0-6 to 0.95. St~ch a stepwise approach to a near-breeder does not require any constructive alteration in the reactor, but is solely a specification of the reprocessing technology. In this way the high-temperature reactor can adapt itself in an ideal manner to a possible future uranium shortage. Further investigations have ensured that in the distant future the pebble-bed reactor can be operated with a breeding factor of 1 if a breeder blanket in the form of a pebble haze is applied. This would, however, mean a constructive alteration of the charging system. Instead of the usual two zones up to now of the reactor core in the axial direction, three zones would be necessary. It cannot be predicted today what the uranium situation will be like in 50 years' time. But it is certain today that nuclear process heat utilizing the thorium cycle can be applied long-term and that it is possible to sufficiently raise the conversion factor of this reactor system. PROCESS APPLICATIONS In the project "'Prototyp Nukleare ProzeSw~irme" (PNP) of the Federal Republic of Germany two coupled processes of nuclear plants with subsequent processes have been developed up to now. One process is the reforming (or splitting) of methane with steam and nuclear heat, the other is the gasification of coal with the aid of steam and nuclear heat. Methane reforming can be utilized for a number of
NUCLEAR ENERGY AS A PRIMARY ENERGY SOURCE FOR HYDROGEN PRODUCTION 291 processes. The simplest process is the hydrogenating coal gasification, for the demonstration of which a semi-technical plant has been in operation for over 3 years. In this process, hydrogen and coal are converted to methane in a known process in an exothermic reaction. Half of the methane produced is led to a steam reformer of the nuclear system where it is converted to carbon monoxide and hydrogen. By a further conversion process it is completely converted to hydrogen. The hydrogen produced can be led back to the hydrogenating coal gasification. The reaction scheme is shown in Fig. 9. The same L H 2 + 2C - - - - " CHz, + H 2 0 CO
+ H20---"
2 CH4 " C0+3H2 CO2+ H2
I Hydrierende-Vergasung I FIG. 9. Hydrogenating gasification.
coupled processes can be used for the liquefaction of coal. In the Bergius-Pier method, developed 40 years ago, hydrogen and coal are made to react under high pressure in the oil sump. Besides the desired hydrocarbons, a number of light gaseous hydrocarbons are produced. These are led to the steam reformer, where they are reformed by nuclear heat and steam. By a conversion process, the gases produced in the reforming process are converted to hydrogen. The reaction scheme of this coupled process is shown in Fig. 10. methanol can also be obtained from coat by a similar process. Here half of the gas
(C>+(H2> ~ CnH2n÷2(Benzin,01} +C,H2,.2{Kohlenwosserstoff, C, H2,.2+ ill20 ~ ,CO+ (2,+1)H2 Gas} iCO+iH20 ~ ,C02+ iH2 Summe: (C>+
(]l/Benzin-Produkt ion mtt nuklearer Wdrme FIG. 10. Production of gasoline and synthetic oil using nuclear heat.
produced in the steam reformer is utilized for hydrogenating coal gasification, and the other half is utilized for producing methanol. The methane produced in the first part as a product of the hydrogenating coal gasification is led back to the steam reformer to produce reformer gases, so that new reformer gas is available for the production of methanol or for the hydrogenating coal gasification. The reaction scheme is shown in Fig. 11. The steam reformer can also be utilized to transfer large amounts of energy into gas ducts. The chemical equilibrium of methane reforming is utilized for this. Through the reforming of methane about 50 kcal/mole nuclear heat is brought into the gas circuit. These reformer gases can be led by a pipeline to the consumer where they are reconverted at a somewhat lower temperature. The energy input is thus freed again. In this way, it is possible from a large plant to supply a number of consumers with heat energy in a temperature range of up to 500°C. This system is especially appropriate for the production of process heat and household heating.
292
R. SCHULTEN 6C+ 12H2 ~ 6CH4
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FIG. 11. The production of methanol using nuclear heat.
The direct gasification of coal with the aid of steam is shown in the following reaction scheme {see Fig. 12). The required reaction temperature depends on the type of coal and varies from 750 to 900°C. For this reaction about 30 kcal/mole are required on the whole. The gas mixture of carbon monoxide and hydrogen, which practically always contains methane, has to be purified first. The purified gases
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FIG. 12. Steam gasification using nuclear heat. can be utilized to produce natural gas and liquid hydrocarbons. Compared with conventional gasification processes, the above-mentioned processes have the advantage that the coal input per unit of heat obtained is lower by a factor of 1.6. The balance sheet of the carbon dioxide produced also is lower by this factor. Extensive economy studies have shown that, on the basis of coal prices in the FRG, it is to be expected that this process is able to economically compete with mineral oil and natural gas in the long run, providing that realistic conditions are taken for the prices of mineral oil, natural gas and coal. An important result is that large plants with a capacity of approx. 3 to 4 million t SKE/yr do not require much higher investment costs than conventional autothermal gasification plants, if their capacity is considered relative to the amount of natural gas or liquid hydrocarbons produced. An especially important field of application of nuclear process heat in future will certainly be the splitting of water with the aid of thermochemical methods or hybrid processes. At present, however, economically applicable solutions have not been found yet. It is to be hoped that new successes in this area will be pointed out at this conference. The He/He-heat exchange required for these processes is being developed within the scope of our project.