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Nuclear process heat applications for the modular HTR
Nuclear Engineering and Design 78 (1984) 137-145 North-Holland, Amsterdam
137
NUCLEAR PROCESS HEAT APPLICATIONS FOR THE MODULAR HTR W. J ~ . G E R , I. W E I S B R O D T a n d H. H I ~ R N I N G INTERA TOM, Internationale Atomreaktorbau GmbH, Bergisch Gladbach, Fed. Rep. Germany
Received September 1983
The advantages of the modular HTR as heat source for process heat are described below. Subsequently various possibilities for heat removal are presented, e.g. via a steam reformer, an He/He intermediate heat exchanger and a steam generator. The influence exerted by the process plant on pressures and temperatures in the reactor plant is explained. This is followed by a description of plants for the production of SNG (substitute natural gas) and methanol using both the hydro- and the steam gasification of coal, whereby synthesis gas is produced in the intermediate step. The production of synthesis gas from natural gas, the production of hydrogen and the application of steam generation for the production of electrical power and/or process steam are briefly considered. In conclusion, some remarks are made on the commercial efficiency and the commercial utilization of modular HTR plants.
1. Introduction
The HTR-module was designed as a universal high temperature heat source. Its performance and design are, for the most part, independent of its use for the generation of electrical power and process steam or for the production of process heat and coal gasification respectively. Of course, this does not exclude the fact that process-related requirements, which for example inevitably arise in the case of a plant for the production of process heat, must be taken into consideration when designing the module. It is therefore evident that, in the case of accidents, a reactor core, which is designed for a gas outlet temperature of 950°C, operates at higher fuel element temperatures than an identical reactor, which only requires a gas outlet temperature of 700°C. Nevertheless, when designing a modular HTR core, the fundamental rule is to limit the local, maximum fuel element accident temperature to 1600°C for all conceivable accidents. The core power must therefore be reduced to prevent this maximum permissible fuel element temperature from being exceeded in the event of an increase in the gas outlet temperature. The reactor power is therefore specified as being 200 MW for a gas outlet temperature of 700°C, while it is reduced to 170 MW for a gas outlet temperature of 950°C. The pressure in the primary loop is also dependent on the process-related application of the heat source.
Although high primary loop pressures have a favourable effect on the operating and accident behaviour of the reactor, it is nevertheless, desirable to have a relatively low pressure in the gasifier, for example in a process heat plant for steam gasification. As the primary, secondary and tertiary pressures should be more or less the same to meet the demand for low component loads, a compromise is made in the face of these opposing requirements, in that a pressure of 60 bar instead of 40 bar is selected for a plant for mere steam production. As the material exploitation in the steam reformer and H e / H e intermediate heat exchanger is already very high at reactor outlet temperatures of 950°C, additional loads resulting from hot or cold gas strands must be excluded wherever possible in the case of these components. Larger radial temperature differences are no longer acceptable here. For this reason, it is necessary to change over to two-zone refuelling of the core, as this permits a more even radial temperature distribution. Diverse plant behaviour, e.g. in the case of decay heat removal, is essentially compensated by designing the heat-removing components in a suitable manner. When designing the gas ducting and partitioning the reactor building, care is taken to find the optimum solution for the respective application purpose of the plant. In principle, the design and mode of operation of the HTR-module can be used for all fields of application without making any changes, but the safety characteris-
0 0 2 9 - 5 4 9 3 / 8 4 / $ 0 3 . 0 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g Division)
138
14/. Jgiger et al. / Nuclear process heat apphcations
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IV. Jiiger et aL / Nuclear process heat applications
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Fig. 2. H e / H e intermediate heat exchanger helix version of an HTR-module.
139
140
W. Jdger et aL / Nuclear process heat applications
tics of and the accident-related effects caused by the plant depend on its application. The accident potential of a coal gasification plant with a temperature of 950°C, which has combustible gases in the secondary resp. tertiary loop, must be classified as being higher than that of a power-generating plant with a gas temperature of only 700°C and water/steam in the secondary loop. The difference, however, are inherent in the processes and are not caused by the different designs of the HTR module. Precisely because the HTR-module has such a large safety potential, it seems possible to push forward to the technological limits in the case of process heat components, in order to obtain a more promising economic, nuclear-operated plant, without taking too great risks. Fig. 1 presents an HTR-module with a steam reformer. As shown in fig. 2, an H e / H e intermediate heat exchanger can be installed instead of the steam reformer.
2. Main applications for HTR process heat The main field of application of the HTR-module is considered as being the direct utilization of the HTR heat for nuclear coal gasification. In addition, the mod-
ular HTR with steam generator is used for the production of electrical power and process steam in industrial plants or for the generation of electrical power and steam for district heating purposes in the municipal sector. At present, the steam reformer and the helium/helium intermediate heat exchanger with the corresponding gasification processes, namely hydro- and steam gasification of coal, are being developed within the scope of the PNP project. Using both processes it is possible to produce methane (SNG) for the gas grid and synthesis gas for industrial processing (see fig. 3). Today synthesis gas is the starting material for numerous chemical products, and it serves as reducing agent for the reduction of ore. As soon as a broader market opens for hydrogen, synthesis gas can be processed to hydrogen. Furthermore, synthesis gas is the intermediate product for methanol, which at present is almost exclusively used in the chemical sector for the production of plastics and as a solvent. In addition, the production of olefins from methanol is to be anticipated in the not too distant future. In the motor fuel sector, methanol could be added to gasoline instead of lead compounds to increase the knock rating in the short term, and, in the long term, it could be converted into gasoline with a higher octane
NaturalGas
/Distr~HeatG t
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ooess
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t Hydrogen,~ tReducinlll.S~
~ureMethano,~ t Petrol
~ LOlefins~
Fig. 3. Main fields of application for HTR-module.
IV. J?iger et aL / Nuclear process heat applications number using the Mobil Oil process. The important design features and the main data for plants for the production of methanol, SNG and hydrogen are presented as examples in the following.
2.1. Plants for the production of methanol The process for producing methanol from coal using HTR heat can be subdivided into 2 process steps: Production of the synthesis gas via coal gasification and reforming of the methane fraction in the steam reformer. Reaction of CO and CO2 with H E to form methanol in the methanol synthesis.
2.1.1. Hydrogasification of coal The entire process is presented in the flow sheet in fig. 4. The raw coal (hard coal or lignite) is dried, ground and fed into the gasifier via a system of locks. The gasifier, which operates in accordance with the fluidized bed process, is subjected to a flow of preheated pure hydrogen, obtained in the cryogenic gas separation facility. 55 to 65% of the carbon is converted at a pressure of 80 bar and a temperature of 920°C. The reaction C + 2H 2 = CH 4 taking place in the gasifier is exothermic, consequently it is not necessary to add any heat to the
gasification process. The char formed exists from the gasifier via a second system of locks; it is cooled and supplied to the consumer. The crude gas discharged from the gasifier is cooled, freed from dust and passed through a gas scrubber facility to remove sulfur compounds (H2S, COS) and CO 2. The purified gas is mixed with the gas discharged from the steam reformer and fed into the methanol synthesis. It is converted to methanol here in accordance with: CO + 2H 2 --~ CHaOH, CO 2 +
3H 2 --, CHaOH + H20.
The occurring crude methanol is condensed and the residual gas is split into a CH 4, an H 2 and a CO flow in the cryogenic gas separation facility. The H 2 gas flow is fed into the gasifier via the preheater, while the CO flow is added to the crude gas and the CH 4 flow is supplied to the steam reformer after the admixture of steam. The C H 4 / H 2 0 mixture is reformed in the tubes of the steam reformer, which are filled with catalysts, at approximately 800°C and 50 bar in accordance with: CH 4 + H20 "-~ CO + 3H 2, CO + H2 ° ~ CO 2 + H2. This endothermic reaction is maintained by the helium
Methanol
900 ° C. 50 U r
300* C
Sulphur,
CO2 OasiflerE ~ Data of a HTR-4-Module plant Reactor power
Hard coal input Residual coke Methanol production
Residual coke
680 MJ/$ 2690 t/d 1050 t/d 2530 t/d
141
Circuiting with further parallel lines
Condenser
Fig. 4. Block diagram of an HTR-module with steam reformer and hydrogasification of coal for methanol production.
142
14/. Jiiger et al. / Nuclear process heat applications
flow coming from the reactor, whereby this is cooled from approximately 950°C to approximately 680°C. After further cooling to 300°C in the steam generator, the helium is reheated to 950°C in the HTR-module. After cooling, the process gas discharged from the steam reformer is mixed with the purified crude gas as mentioned above. The steam present in the steam generator in the He-primary loop and the steam produced in the waste heat boilers is used to produce electrical power as well as process steam and steam for heating purposes. The plant is self-sufficient as regards electrical power and steam. An HTR 4-Module plant with a thermal power of 680 MW will produce 2530 t / d of methanol at a coal throughput of 2650 t/d. In addition, 1050 t / d of char will be produced. 2.1.2. S t e a m gasification o f hard coal
The reaction of steam with coal to form synthesis gas, which takes place during the steam gasification of coal, requires high temperature heat as it, in contrast to hydrogasification, is an endothermic reaction. This high temperature heat is fed into the gas generator via heating coils. The heating coils dip into the fluidized bed and the hot helium flows through them. As it is not possible to directly heat the steam gasification of coal using the helium heated up in the high temperature reactor for various reasons, an inter-
mediate loop is required when using the high temperature reactor as energy source. The secondary helium is heated to 900°C in the helium/helium intermediate heat exchanger and enters the gas generator at approximately this temperature. As a result of the carbon-steam reaction, the helium is cooled to approximately 810°C here. The gas generator itself is designed as a horizontal fluidized bed gasifier. Superheated steam flows against the fluidized bed from underneath, thus maintaining the fluidized bed. The dried and ground hard coal is introduced into the short side of the generator. The residue is removed at the other short side. A C-gasification degree of 95% is achieved. The crude gas discharged from the gasifier is cooled, freed from dust and passed through a gas scrubber facility to remove sulfur compounds. The gas, which now has synthesis gas quality, is compressed to approximately 75 bar and supplied to the methanol synthesis. After cooling, the methanol formed is condensed out of the discharged gas. The purge gas contains all the methane formed in the gasifier and can be fed into a gas grid as SNG. The process scheme corresponds to the process described in section 2.2.2 for the production of SNG, the only difference is that methanisation is replaced by methanol synthesis. A 4-module HTR plant with a thermal power of 680 MW will produce 2390 t / d of methanol at a coal
950° C, 50 bar HTR-Module ~ [
L iE
Methanol
~ f oesteem nln r
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a HTR-4-Module Plant Reactor power: 680 MJ/s Natural gas input: 1,15.105 Nrna/h Methano~ production: 132,6 t/h Hydrogen production: 9,55.104 Nm3/h Electr~,ty output: 5,9 MW
Data of
I
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Fig. 5. Block diagram of an HTR-module with steam reformer for methanol and hydrogen production.
W. Jigger et al. / Nuclear process heat applications
2.2.1. SNG production by means of the hydrogasification
throughput of 2680 t / d . Furthermore, the plant will produce 1 × 106 m 3 (i.N.)/d of SNG and 120 MW of electrical power.
of coal In contrast to the methanol plant, the gasifying agent is not pure hydrogen. On the contrary, it still contains some CH a from the treated process gas. The crude gas discharged from the gasifier is cooled and freed from dust and carbons with a higher boiling point in a water scrubber facility. It is then passed through a gas scrubber facility to remove the sulfur compounds HzS and COS and the carbon dioxide from the cooled gas. During the subsequent cryogenic gas separation, the gas is split into the fractions H 2, CO and CH 4. The carbon monoxide is subjected to conversion, in the course of which the carbon monoxide reacts with steam to form carbon dioxide and hydrogen. The resulting gas is fed into a molecular sieve in which pure hydrogen is recovered while N 2 and CO 2 are separated out. Together with the hydrogen discharged from cryogenic gas separation and the treated process gas, this hydrogen serves as gasifying agent. As much of the methane as is necessary to produce the hydrogen required in the gasifier is diverted from the methane produced by means of the cryogenic gas separation. After being mixed with steam in the ratio 1:4, it is supplied to the steam reformer heated with primary helium. The remaining methane is yielded as product.
2.1.3. Production of methanol from natural gas Instead of the methane produced by hydrogasification, natural gas can be directly reformed in the steam reformer to produce synthesis gas. The flow chart is presented in fig. 5. The primary loop of the reactor with steam reformer and steam generator corresponds to that described in section 2.1.1, Depending on the admixtures (CO2, sulfur compounds), the natural gas is purified and supplied to the steam reformer after being mixed with steam. The process gas discharged from the steam reformer is cooled and fed to the methanol synthesis as synthesis gas. This synthesis gas contains surplus hydrogen; consequently, after the methanol has been condensed out of the residual gas in a cryogenic gas separation facility, a hydrogen flow is produced as the byproduct. In addition, surplus electrical power is also supplied. 2.2. Plants for the production of SNG The following description only presents the differences between these plants and those for the production of methanol.
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4
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Fig. 6, Block diagram of an HTR-module with intermediate heat exchanger and steam gasification of coal for methane production.
144
W. ddger et aZ / Nuclearprocess heat applications 2.3. Hydrogen production
After leaving the steam reformer, the primary helium is further cooled in a steam generator before being refed into the reactor core via the primary loop blower. The process gas exiting from the steam reformer is passed to high temperature and low temperature conversion, where the carbon monoxide produced in the steam reformer reacts with steam to form carbon dioxide and hydrogen. The gas discharged from the conversion is freed from the CO 2 in a gas scrubber facility and from most of the steam in the subsequent cooler. The purified process gas is condensed and supplied to the gasifier.
At present increasing quantities of hydrogen are required for hydrogenation processes in the chemical industry and, in particular, for the processing of crude oil. In the long-term, hydrogen can be used as a highquality source of energy. All the plants described for the production of methanol can be used to produce hydrogen. The methanol synthesis is replaced by conversion, in which the CO reacts with H20 to form H 2 and CO~. A gas scrubber facility is also included to remove the CO 2.
2.2.2. The production of SNG by means of the steam gasification of coal The flow sheet is shown in fig. 6. The reactor plant, gasification plant and gas purification system correspond to those described in section 2.1.2 for the production of methanol. The pure gas, however, is subjected to methanisation after gas purification. As a result, the CO and H 2 discharged from the gasifier, and part of the CO 2 are converted into CH 4. The heat from this exothermic reaction serves to produce steam. The gas discharged from methanisation is dried and fed into the gas grid. The following data result for a 4-module plant with a thermal power of 680 MW: Coal throughput : 3470 t / d SNG production : 2690 × 103 m3 (i.N.)/d Electrical power yield : 84 MW
2.4. The production of electrical power and process steam Fig. 7 presents a modular HTR plant with steam generator as an example for the production of electrical power and process steam. The live steam discharged from the steam generator is supplied to a condensingextraction turbine. The process steam is produced in steam converters, which are heated by the steam extracted from the turbine. The remaining steam is expanded in the turbine until is reaches condenser pressure. The condensate from the turbine condenser and the steam converters is combined and, after preheating, it is returned to the steam generator. This circuit permits strict separation of the water-steam circuit of the turbine from that of the process steam, a demand which
>Processsteam(1) I
TO0° C, 60 l w
Water
:::
~.J
I
40 bar 360 ° C 200 t/h
~rocesssteam(2) Water
2,5 150 ° C 200 t/h
Data of a HTR-4-module plant t
l~m=IkN llno= ~
Tud ,I
Reactm" power I='ttx~P~ ~ o u t l e t (1) P R x ~ m l Iteam output (2) E ~ output of utlliz.mion
800 160 140 242 68
MJ/s MJ/s MJ/s MW %
t
I_
ltl
Fig. 7. Block diagram of an HTR-module with steam generator for the process steam and electricity production.
14I.. Jiiger et al. / Nuclear process heat applications
is often imposed by chemical firms. The main data of a 4-module HTR plant for two different process steam pressures may be taken from fig. 7. In addition, it is also possible to use circuits with back-pressure turbines to produce more process steam at a lower electrical output.
4. Economics and commercialization
With reference to the competitiveness of HTR-module plants when compared to conventional thermal power plants of identical size and with the same potential applications, the plant costs will be the decisive factor, since the fuel costs for the reactor are rather low. Competitive plant costs are achieved by: - simple technical solutions for the classical safety devices for shutdown and decay heat removal combined with the simple design of the confinement, - conventional layout of the whole secondary system, including the auxiliaries for the machinery and the electrical equipment, - use of steel pressure vessels for the reactor and the heat transferring component, - the possibility of series production due to standardization of all essential components. Thus, the economy of scale which is decisive for electricity-generating reactors will be replaced by an economy of series production for HTR-Module plants for the heat market. According to current estimates, the economic application of HTl~,module plants in conjunction with coal gasification plants can be expected within about 10 to 15 years. The time required to reach the economic
145
break-even-point will largely depend on the rise in future coal prices and on the prices to be achieved on the market for upgraded products such as methanol, hydrogen and methane. As regards HTR-module plants for process steam and electricity cogeneration, the economic calculations show that, even today, plants with 4 modules can compete with fossil-fired plants. In addition to the competitive plant costs, the short construction time of four years has a favourable effect on the investment costs. The fuel cycle costs for fuel elements and direct final storage are not higher than those of large nuclear plants. The HTR-Module plant with steam generator can be realized immediately without any R + D work. Planning and licensing will require about three to four years. An HTR-Module plant with steam reformer or H e / H e IHX can be started in about 3 years, as the development of the heat-exchanging components will be concluded by 1986. Coal gasification using the process heat generated in HTR Modules is interesting on a more longterm scale as it permits the substitution of oil and natural gas, thus stretching their availability. The growing environmental problems resulting from the emission of SO 2 and NO x and, increasingly, CO 2 with the stack gases from the combustion of fossil fuels point to the use of the modular HTR as a non-polluting heat source. Furthermore, the natural resources (coal, oil and natural gas) which are becoming increasingly scarce can be saved if they are exclusively used as raw materials in the process industry.
137
NUCLEAR PROCESS HEAT APPLICATIONS FOR THE MODULAR HTR W. J ~ . G E R , I. W E I S B R O D T a n d H. H I ~ R N I N G INTERA TOM, Internationale Atomreaktorbau GmbH, Bergisch Gladbach, Fed. Rep. Germany
Received September 1983
The advantages of the modular HTR as heat source for process heat are described below. Subsequently various possibilities for heat removal are presented, e.g. via a steam reformer, an He/He intermediate heat exchanger and a steam generator. The influence exerted by the process plant on pressures and temperatures in the reactor plant is explained. This is followed by a description of plants for the production of SNG (substitute natural gas) and methanol using both the hydro- and the steam gasification of coal, whereby synthesis gas is produced in the intermediate step. The production of synthesis gas from natural gas, the production of hydrogen and the application of steam generation for the production of electrical power and/or process steam are briefly considered. In conclusion, some remarks are made on the commercial efficiency and the commercial utilization of modular HTR plants.
1. Introduction
The HTR-module was designed as a universal high temperature heat source. Its performance and design are, for the most part, independent of its use for the generation of electrical power and process steam or for the production of process heat and coal gasification respectively. Of course, this does not exclude the fact that process-related requirements, which for example inevitably arise in the case of a plant for the production of process heat, must be taken into consideration when designing the module. It is therefore evident that, in the case of accidents, a reactor core, which is designed for a gas outlet temperature of 950°C, operates at higher fuel element temperatures than an identical reactor, which only requires a gas outlet temperature of 700°C. Nevertheless, when designing a modular HTR core, the fundamental rule is to limit the local, maximum fuel element accident temperature to 1600°C for all conceivable accidents. The core power must therefore be reduced to prevent this maximum permissible fuel element temperature from being exceeded in the event of an increase in the gas outlet temperature. The reactor power is therefore specified as being 200 MW for a gas outlet temperature of 700°C, while it is reduced to 170 MW for a gas outlet temperature of 950°C. The pressure in the primary loop is also dependent on the process-related application of the heat source.
Although high primary loop pressures have a favourable effect on the operating and accident behaviour of the reactor, it is nevertheless, desirable to have a relatively low pressure in the gasifier, for example in a process heat plant for steam gasification. As the primary, secondary and tertiary pressures should be more or less the same to meet the demand for low component loads, a compromise is made in the face of these opposing requirements, in that a pressure of 60 bar instead of 40 bar is selected for a plant for mere steam production. As the material exploitation in the steam reformer and H e / H e intermediate heat exchanger is already very high at reactor outlet temperatures of 950°C, additional loads resulting from hot or cold gas strands must be excluded wherever possible in the case of these components. Larger radial temperature differences are no longer acceptable here. For this reason, it is necessary to change over to two-zone refuelling of the core, as this permits a more even radial temperature distribution. Diverse plant behaviour, e.g. in the case of decay heat removal, is essentially compensated by designing the heat-removing components in a suitable manner. When designing the gas ducting and partitioning the reactor building, care is taken to find the optimum solution for the respective application purpose of the plant. In principle, the design and mode of operation of the HTR-module can be used for all fields of application without making any changes, but the safety characteris-
0 0 2 9 - 5 4 9 3 / 8 4 / $ 0 3 . 0 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g Division)
138
14/. Jgiger et al. / Nuclear process heat apphcations
-
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r-
~, 5 900
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1
o
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o
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~-
(~ (~)
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pebble bed pressure vessel fuel discharge boronafed spheres reflector rod
(~ ~ (~ (~ (~)
10 6 0 0
fuel loading steam generator (6 bundles) process gas inlet/outlet live steam line feed water line
Fig. 1. H T R - m o d u l e with steam reformer and steam generator.
(~ (~) ~) ~)
reformer tubes blower hot gas duct surface coolers insulation
IV. Jiiger et aL / Nuclear process heat applications
!
i i
i
(~ (~) (~ (~ (j~) (~
sec pipe connections support ptofe centro[ sec. hot gos return pipe tube bundle heat exchanger shell flow shroud
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pressure vessel prim. cold gas duct prim. hot gas duct sec. hot gas header blower
Fig. 2. H e / H e intermediate heat exchanger helix version of an HTR-module.
139
140
W. Jdger et aL / Nuclear process heat applications
tics of and the accident-related effects caused by the plant depend on its application. The accident potential of a coal gasification plant with a temperature of 950°C, which has combustible gases in the secondary resp. tertiary loop, must be classified as being higher than that of a power-generating plant with a gas temperature of only 700°C and water/steam in the secondary loop. The difference, however, are inherent in the processes and are not caused by the different designs of the HTR module. Precisely because the HTR-module has such a large safety potential, it seems possible to push forward to the technological limits in the case of process heat components, in order to obtain a more promising economic, nuclear-operated plant, without taking too great risks. Fig. 1 presents an HTR-module with a steam reformer. As shown in fig. 2, an H e / H e intermediate heat exchanger can be installed instead of the steam reformer.
2. Main applications for HTR process heat The main field of application of the HTR-module is considered as being the direct utilization of the HTR heat for nuclear coal gasification. In addition, the mod-
ular HTR with steam generator is used for the production of electrical power and process steam in industrial plants or for the generation of electrical power and steam for district heating purposes in the municipal sector. At present, the steam reformer and the helium/helium intermediate heat exchanger with the corresponding gasification processes, namely hydro- and steam gasification of coal, are being developed within the scope of the PNP project. Using both processes it is possible to produce methane (SNG) for the gas grid and synthesis gas for industrial processing (see fig. 3). Today synthesis gas is the starting material for numerous chemical products, and it serves as reducing agent for the reduction of ore. As soon as a broader market opens for hydrogen, synthesis gas can be processed to hydrogen. Furthermore, synthesis gas is the intermediate product for methanol, which at present is almost exclusively used in the chemical sector for the production of plastics and as a solvent. In addition, the production of olefins from methanol is to be anticipated in the not too distant future. In the motor fuel sector, methanol could be added to gasoline instead of lead compounds to increase the knock rating in the short term, and, in the long term, it could be converted into gasoline with a higher octane
NaturalGas
/Distr~HeatG t
I
ooess
Ind. .l
t Hydrogen,~ tReducinlll.S~
~ureMethano,~ t Petrol
~ LOlefins~
Fig. 3. Main fields of application for HTR-module.
IV. J?iger et aL / Nuclear process heat applications number using the Mobil Oil process. The important design features and the main data for plants for the production of methanol, SNG and hydrogen are presented as examples in the following.
2.1. Plants for the production of methanol The process for producing methanol from coal using HTR heat can be subdivided into 2 process steps: Production of the synthesis gas via coal gasification and reforming of the methane fraction in the steam reformer. Reaction of CO and CO2 with H E to form methanol in the methanol synthesis.
2.1.1. Hydrogasification of coal The entire process is presented in the flow sheet in fig. 4. The raw coal (hard coal or lignite) is dried, ground and fed into the gasifier via a system of locks. The gasifier, which operates in accordance with the fluidized bed process, is subjected to a flow of preheated pure hydrogen, obtained in the cryogenic gas separation facility. 55 to 65% of the carbon is converted at a pressure of 80 bar and a temperature of 920°C. The reaction C + 2H 2 = CH 4 taking place in the gasifier is exothermic, consequently it is not necessary to add any heat to the
gasification process. The char formed exists from the gasifier via a second system of locks; it is cooled and supplied to the consumer. The crude gas discharged from the gasifier is cooled, freed from dust and passed through a gas scrubber facility to remove sulfur compounds (H2S, COS) and CO 2. The purified gas is mixed with the gas discharged from the steam reformer and fed into the methanol synthesis. It is converted to methanol here in accordance with: CO + 2H 2 --~ CHaOH, CO 2 +
3H 2 --, CHaOH + H20.
The occurring crude methanol is condensed and the residual gas is split into a CH 4, an H 2 and a CO flow in the cryogenic gas separation facility. The H 2 gas flow is fed into the gasifier via the preheater, while the CO flow is added to the crude gas and the CH 4 flow is supplied to the steam reformer after the admixture of steam. The C H 4 / H 2 0 mixture is reformed in the tubes of the steam reformer, which are filled with catalysts, at approximately 800°C and 50 bar in accordance with: CH 4 + H20 "-~ CO + 3H 2, CO + H2 ° ~ CO 2 + H2. This endothermic reaction is maintained by the helium
Methanol
900 ° C. 50 U r
300* C
Sulphur,
CO2 OasiflerE ~ Data of a HTR-4-Module plant Reactor power
Hard coal input Residual coke Methanol production
Residual coke
680 MJ/$ 2690 t/d 1050 t/d 2530 t/d
141
Circuiting with further parallel lines
Condenser
Fig. 4. Block diagram of an HTR-module with steam reformer and hydrogasification of coal for methanol production.
142
14/. Jiiger et al. / Nuclear process heat applications
flow coming from the reactor, whereby this is cooled from approximately 950°C to approximately 680°C. After further cooling to 300°C in the steam generator, the helium is reheated to 950°C in the HTR-module. After cooling, the process gas discharged from the steam reformer is mixed with the purified crude gas as mentioned above. The steam present in the steam generator in the He-primary loop and the steam produced in the waste heat boilers is used to produce electrical power as well as process steam and steam for heating purposes. The plant is self-sufficient as regards electrical power and steam. An HTR 4-Module plant with a thermal power of 680 MW will produce 2530 t / d of methanol at a coal throughput of 2650 t/d. In addition, 1050 t / d of char will be produced. 2.1.2. S t e a m gasification o f hard coal
The reaction of steam with coal to form synthesis gas, which takes place during the steam gasification of coal, requires high temperature heat as it, in contrast to hydrogasification, is an endothermic reaction. This high temperature heat is fed into the gas generator via heating coils. The heating coils dip into the fluidized bed and the hot helium flows through them. As it is not possible to directly heat the steam gasification of coal using the helium heated up in the high temperature reactor for various reasons, an inter-
mediate loop is required when using the high temperature reactor as energy source. The secondary helium is heated to 900°C in the helium/helium intermediate heat exchanger and enters the gas generator at approximately this temperature. As a result of the carbon-steam reaction, the helium is cooled to approximately 810°C here. The gas generator itself is designed as a horizontal fluidized bed gasifier. Superheated steam flows against the fluidized bed from underneath, thus maintaining the fluidized bed. The dried and ground hard coal is introduced into the short side of the generator. The residue is removed at the other short side. A C-gasification degree of 95% is achieved. The crude gas discharged from the gasifier is cooled, freed from dust and passed through a gas scrubber facility to remove sulfur compounds. The gas, which now has synthesis gas quality, is compressed to approximately 75 bar and supplied to the methanol synthesis. After cooling, the methanol formed is condensed out of the discharged gas. The purge gas contains all the methane formed in the gasifier and can be fed into a gas grid as SNG. The process scheme corresponds to the process described in section 2.2.2 for the production of SNG, the only difference is that methanisation is replaced by methanol synthesis. A 4-module HTR plant with a thermal power of 680 MW will produce 2390 t / d of methanol at a coal
950° C, 50 bar HTR-Module ~ [
L iE
Methanol
~ f oesteem nln r
300 ~C ~
a HTR-4-Module Plant Reactor power: 680 MJ/s Natural gas input: 1,15.105 Nrna/h Methano~ production: 132,6 t/h Hydrogen production: 9,55.104 Nm3/h Electr~,ty output: 5,9 MW
Data of
I
I~rO0UG-'I[IOtnIII~S
Fig. 5. Block diagram of an HTR-module with steam reformer for methanol and hydrogen production.
W. Jigger et al. / Nuclear process heat applications
2.2.1. SNG production by means of the hydrogasification
throughput of 2680 t / d . Furthermore, the plant will produce 1 × 106 m 3 (i.N.)/d of SNG and 120 MW of electrical power.
of coal In contrast to the methanol plant, the gasifying agent is not pure hydrogen. On the contrary, it still contains some CH a from the treated process gas. The crude gas discharged from the gasifier is cooled and freed from dust and carbons with a higher boiling point in a water scrubber facility. It is then passed through a gas scrubber facility to remove the sulfur compounds HzS and COS and the carbon dioxide from the cooled gas. During the subsequent cryogenic gas separation, the gas is split into the fractions H 2, CO and CH 4. The carbon monoxide is subjected to conversion, in the course of which the carbon monoxide reacts with steam to form carbon dioxide and hydrogen. The resulting gas is fed into a molecular sieve in which pure hydrogen is recovered while N 2 and CO 2 are separated out. Together with the hydrogen discharged from cryogenic gas separation and the treated process gas, this hydrogen serves as gasifying agent. As much of the methane as is necessary to produce the hydrogen required in the gasifier is diverted from the methane produced by means of the cryogenic gas separation. After being mixed with steam in the ratio 1:4, it is supplied to the steam reformer heated with primary helium. The remaining methane is yielded as product.
2.1.3. Production of methanol from natural gas Instead of the methane produced by hydrogasification, natural gas can be directly reformed in the steam reformer to produce synthesis gas. The flow chart is presented in fig. 5. The primary loop of the reactor with steam reformer and steam generator corresponds to that described in section 2.1.1, Depending on the admixtures (CO2, sulfur compounds), the natural gas is purified and supplied to the steam reformer after being mixed with steam. The process gas discharged from the steam reformer is cooled and fed to the methanol synthesis as synthesis gas. This synthesis gas contains surplus hydrogen; consequently, after the methanol has been condensed out of the residual gas in a cryogenic gas separation facility, a hydrogen flow is produced as the byproduct. In addition, surplus electrical power is also supplied. 2.2. Plants for the production of SNG The following description only presents the differences between these plants and those for the production of methanol.
950 ° ~ 40 bar
Cool
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I / / I ~ ~r
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II II
2690x10 s Nm3/d 84 IdW
4
__| (~rcutting with further raw.Not lines
Fig. 6, Block diagram of an HTR-module with intermediate heat exchanger and steam gasification of coal for methane production.
144
W. ddger et aZ / Nuclearprocess heat applications 2.3. Hydrogen production
After leaving the steam reformer, the primary helium is further cooled in a steam generator before being refed into the reactor core via the primary loop blower. The process gas exiting from the steam reformer is passed to high temperature and low temperature conversion, where the carbon monoxide produced in the steam reformer reacts with steam to form carbon dioxide and hydrogen. The gas discharged from the conversion is freed from the CO 2 in a gas scrubber facility and from most of the steam in the subsequent cooler. The purified process gas is condensed and supplied to the gasifier.
At present increasing quantities of hydrogen are required for hydrogenation processes in the chemical industry and, in particular, for the processing of crude oil. In the long-term, hydrogen can be used as a highquality source of energy. All the plants described for the production of methanol can be used to produce hydrogen. The methanol synthesis is replaced by conversion, in which the CO reacts with H20 to form H 2 and CO~. A gas scrubber facility is also included to remove the CO 2.
2.2.2. The production of SNG by means of the steam gasification of coal The flow sheet is shown in fig. 6. The reactor plant, gasification plant and gas purification system correspond to those described in section 2.1.2 for the production of methanol. The pure gas, however, is subjected to methanisation after gas purification. As a result, the CO and H 2 discharged from the gasifier, and part of the CO 2 are converted into CH 4. The heat from this exothermic reaction serves to produce steam. The gas discharged from methanisation is dried and fed into the gas grid. The following data result for a 4-module plant with a thermal power of 680 MW: Coal throughput : 3470 t / d SNG production : 2690 × 103 m3 (i.N.)/d Electrical power yield : 84 MW
2.4. The production of electrical power and process steam Fig. 7 presents a modular HTR plant with steam generator as an example for the production of electrical power and process steam. The live steam discharged from the steam generator is supplied to a condensingextraction turbine. The process steam is produced in steam converters, which are heated by the steam extracted from the turbine. The remaining steam is expanded in the turbine until is reaches condenser pressure. The condensate from the turbine condenser and the steam converters is combined and, after preheating, it is returned to the steam generator. This circuit permits strict separation of the water-steam circuit of the turbine from that of the process steam, a demand which
>Processsteam(1) I
TO0° C, 60 l w
Water
:::
~.J
I
40 bar 360 ° C 200 t/h
~rocesssteam(2) Water
2,5 150 ° C 200 t/h
Data of a HTR-4-module plant t
l~m=IkN llno= ~
Tud ,I
Reactm" power I='ttx~P~ ~ o u t l e t (1) P R x ~ m l Iteam output (2) E ~ output of utlliz.mion
800 160 140 242 68
MJ/s MJ/s MJ/s MW %
t
I_
ltl
Fig. 7. Block diagram of an HTR-module with steam generator for the process steam and electricity production.
14I.. Jiiger et al. / Nuclear process heat applications
is often imposed by chemical firms. The main data of a 4-module HTR plant for two different process steam pressures may be taken from fig. 7. In addition, it is also possible to use circuits with back-pressure turbines to produce more process steam at a lower electrical output.
4. Economics and commercialization
With reference to the competitiveness of HTR-module plants when compared to conventional thermal power plants of identical size and with the same potential applications, the plant costs will be the decisive factor, since the fuel costs for the reactor are rather low. Competitive plant costs are achieved by: - simple technical solutions for the classical safety devices for shutdown and decay heat removal combined with the simple design of the confinement, - conventional layout of the whole secondary system, including the auxiliaries for the machinery and the electrical equipment, - use of steel pressure vessels for the reactor and the heat transferring component, - the possibility of series production due to standardization of all essential components. Thus, the economy of scale which is decisive for electricity-generating reactors will be replaced by an economy of series production for HTR-Module plants for the heat market. According to current estimates, the economic application of HTl~,module plants in conjunction with coal gasification plants can be expected within about 10 to 15 years. The time required to reach the economic
145
break-even-point will largely depend on the rise in future coal prices and on the prices to be achieved on the market for upgraded products such as methanol, hydrogen and methane. As regards HTR-module plants for process steam and electricity cogeneration, the economic calculations show that, even today, plants with 4 modules can compete with fossil-fired plants. In addition to the competitive plant costs, the short construction time of four years has a favourable effect on the investment costs. The fuel cycle costs for fuel elements and direct final storage are not higher than those of large nuclear plants. The HTR-Module plant with steam generator can be realized immediately without any R + D work. Planning and licensing will require about three to four years. An HTR-Module plant with steam reformer or H e / H e IHX can be started in about 3 years, as the development of the heat-exchanging components will be concluded by 1986. Coal gasification using the process heat generated in HTR Modules is interesting on a more longterm scale as it permits the substitution of oil and natural gas, thus stretching their availability. The growing environmental problems resulting from the emission of SO 2 and NO x and, increasingly, CO 2 with the stack gases from the combustion of fossil fuels point to the use of the modular HTR as a non-polluting heat source. Furthermore, the natural resources (coal, oil and natural gas) which are becoming increasingly scarce can be saved if they are exclusively used as raw materials in the process industry.