Development of thermochemical and hybrid processes for hydrogen production

0360-319!$35 $3.00+ 0.w

Im. 1. Hydrogen Energy, Vol. 10, No. 7/S, pp. 431438, 1985.

Pnntcdm GreatBritain. @I 1985 International Assmation

DEVELOPMENT

Pcrgamon Press Ltd. for Hydrogen Energy

OF THERMOCHEMICAL AND HYBRID PROCESSES FOR HYDROGEN PRODUCTION G. E. BEGHI

Commission of the European Communities, Joint Research Centre-Ispra 21020 ISPRA (Varese), Italy

Establishment,

(Receiued for publicurion 15 October 1984) Abstract-Interest in water splitting using high-temperature heat as much as possible directly in the decomposition process started in a relatively recent period. An overview is given on these studies for a possible new method for hydrogen production following the evolution from the first calculations to the exploratory research and the more recent experimental realizations. Definition of thermochemical and hybrid cycles, and of the efficiencies of the processes, are given. The various phases of this research period are described. Major processes on which attention was concentrated in the more recent years are mentioned, i.e. the cycles of the so-called “sulfur family” based on the thermal decomposition of sulfuric acid. Experimental realizations at laboratory level have shown in 1978 the scientific feasibility of thermochemical water splitting, successive studies were concentrated on the more technological aspects. Major problems are pointed out: kinetics of reactions, separations and concentrations, materials handling, construction materials, chemical engineering solutions, availability of the heat sources, economics and safety aspects. Some recent results of main realization are mentioned. On the basis of the research performed up to now, trends for future research are given. together with main areas requiring development efforts in view of potential applications.

1. INTRODUCTION Twenty years after the first theoretical studies on decomposition of water by thermal energy, to produce hydrogen (J. Funk, 1964) [l, 21, and 10 y after the first International Conference on Hydrogen (THEME, the Hydrogen Economy Miami Energy Conference, 1974) [3] it is possible at this 5th World Hydrogen Energy Conference to survey the historical development of the research on thermochemical processes for hydrogen production: we can always learn some useful lessons from history, and develop guides for the future. The attention of researchers was concentrated on this subject, in the first period of the development, for a superposition of various reasons, which were sometimes interconnected. These included: (1) A scientific interest in the problem of demonstrating whether it is possible to dissociate water by operating a sequence of chemical steps with no network requirements at a temperature well below the temperature for single-step water splitting (2) An industrial interest in finding possible applications, directly in industrial processes, of the high-quality heat available in the High-Temperature Gas-Cooled Reactors (HTGRs) (3 ) An interest in the field of energy options, in synthetic fuels as alternatives to oil-derived fuels: this interest is oriented to all possible methods of producing hydrogen and hydrogen-rich compounds without using oil. Work in this research

area was, and still is, carried

out in several laboratories and organizations and is the subject of a collaborative programme, started in 1977, as part of the implementing agreement “Hydrogen” of the International Energy Agency [4]. Several papers are available in the literature, from the United States, Japan, Canada and European countries, particularly the F.R.G. and the Commission of the European Communities.

2. DEFINITIONS

AND CHARACTERISTICS

The theoretical energy requirements for water decomposition are known and it is possible to calculate by the thermodynamic laws the minimum theoretical voltage for water electrolysis (in this case primary heat must first be converted into electrical energy) or the minimum temperature of the heat energy input. For a single-step thermal conversion the working temperature is about 4000 K: if the available heat is at an insufficient high temperature, the same result can be obtained by absorbing heat in two or more steps (chemical reactions). The thermochemical process for water decomposition is a series of chemical reactions (or a “chemical cycle”) resulting in the production of hydrogen and oxygen from water with no net consumption of other chemical species. The energy input is in the form of “heat of reaction” for the endothermic reactions, and heat necessary for concentrations and separations. The primary heat which is needed is at a high temperature (normally it is considered to be in a range from 900 to 1250 K). When one of the chemical reactions is operated

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G.E. BEGHI

with an input of electricity, i.e. is an electrochemical reaction, we have a so-called "hybrid" thermochemical cycle; it is obvious that in this case the useful work required must be less than for the water electrolysis. At the beginning of the research on thermochemical cycles an attempt was made to "design" a chemical molecule suitable to build a two-step cycle [1], computing the ideal values of AH and AS of the chemical reactions of the theoretical cycle; however, no practical work was done in the study of such molecules. During the first period of the research, some effort was dedicated to the automatic, computer-guided search for chemical reactions suitable for thermochemical cycles: computer programs were used generating a very large number of cycles [5--7]. The assessment of thermochemical processes by the use of theoretical thermodynamic criteria is subject to severe limitations, due to uncertainty in the values of the thermodynamic properties of the chemical compounds. As a matter of fact the data are wanted for high temperatures, and often for elements not in current use. When comparisons have to be made between large numbers, and the differences are small, as usually happens, a relatively small uncertainty (a few kCal for instance) in the value of the free energy of formation of each compound can have a considerable influence on the conclusions. Uncertainty in the reaction model also leads to results which do not correspond to experimental findings. The activity of the condensed species, liquid and solid, is usually considered as unity, while the formation of solutions or eutectics is ignored; in some cases the effect can be very large. A comment can be made after this period of research: in spite of the very large amount of thermodynamic data existing in the literature a surprising amount of information on many of the chemical compounds used in the thermochemical cycles is missing for the calculations. Another comment, or conclusion from the studies carried out, is that the theoretical instruments provided by thermodynamics for the definition and the assessment of thermochemical cycles are necessary at the beginning of the research and are very useful tools, but that the results must be carefully considered and are not sufficient for a valid evaluation of a process for the thermochemical production of hydrogen. On the basis of theoretical analysis and computer programs hundreds of thermochemical cycles were published in the literature, particularly in the 1970s; some reviews are available, including those in the references [8--11]. Most of these cycles are only conceptual and have not been demonstrated in the laboratory; classifications are done using different criteria, usually considering families based on common elements and reactions. Many chemical elements are considered in these cycles and among the more usual elements, in addition naturally to hydrogen and oxygen, we can find in the published cycles Cu; Mg, Ca, Sr, Ba; Zn, Cd, Hg; V;

As, Bi; Cr; S, Se; Mn; C1, Br, I; Fe, Ni; C, N. Some less usual or exotic elements are also sometimes quoted such as Y, La, Ce, Eu, U. It can be remarked that nearly all of the cycles taken into consideration use inorganic compounds; very few processes are defined with only one organic compound, such as methane or methanol. Different reasons can be given for this; on the other hand, organic reactions very often have side-reactions and are not stoichiometric, which is a negative aspect when all products must be recycled. Moreover organic compounds are generally not stable at the high temperatures envisaged. Always referring to the cycles published in the literature, other considerations can be pointed out, such as, for instance, the number of chemical elements included in the process, apart from hydrogen and oxygen. According to an analysis of a large number of published thermochemical cycles, one third have two elements (in addition to H2 and Oz) and another third have three elements; the minimum is one element, the maximum is five, always in addition to H2 and O:. The distribution in percentage is given in Fig. 1, which also reports the number of elements for hybrid cycles (in this case on a much more limited statistical basis, about one order of magnitude): for the hybrid cycles we have a reduction in the number of elements used, and the cases with two elements are about the 60% of the total. The statistics of the number of chemical reactions give similar results: see Fig. 2. The number of reactions goes from 2 to 8 for the thermochemical cycles, is more limited, 2 to 4, for hybrid cycles: in this case energy is added as electrical energy in the electrochemical reaction. The selection of the most appropriate cycles for the thermochemical decomposition of water must be made according to various criteria: several criteria have been

NUMBER OF CHEMICAL ELEMENTS

% 3020-

thermochemieal

102

5

n

% 102030-

tc.hybrid

405060-

Fig. 1. Number of chemical elements (as percentages) in the processes for water decomposition as published in the literature, and subdivided in purely thermochermcal and hybrid cycles.

433

DEVELOPMENT OF THERMOCHEMICAL AND HYBRID PROCESSES NUMBER OF

REACTIONS

30 20 10 %10-

1213~41567 8 n

20' 30 405060-

Fig. 2. Number of chemical reactions (as percentages) in the processes for water decomposition as published in the literature. and subdivided in purely thermochemical and hybrid cycles.

defined and considered in preliminary analysis. The final decision must be taken on the basis of the most important "figure of merit" which is the production cost of hydrogen. But from the beginning of the evaluation of a cycle, and up to the final calculations of an industrial plant, a critical parameter is the overall efficiency of the process. The definition of the thermal efficiency of a thermochemical process (E) for hydrogen production as normally accepted, and also adopted by the International Energy Agency, is the following ratio: the Higher Heating Value of 1 m (H2) (285.9 kJ) to the total thermal energy input required for the decomposition process. Thus: E =

research it is possible to identify a few phases, which can be summarized as in Fig. 3. The first phase, as it has already been said, was mainly oriented towards searching and defining number of cycles; the bases were both thermodynamic calculations and/or preliminary experimental tests of chemical reactions. In some cases cycles were only speculative, in other cases well-verified chemical reactions were used. The period in which in practice almost all work was of this kind lasted for about 10 y, with the maximum effort in the first half of the '70s. The result was the definition of some typical "families" and the identification of several promising cycles. This period was characterized by many patents. A second phase can be identified: the demonstration on a laboratory scale. Attention was concentrated on the conversionlrates and kinetics of chemical reactions, the problems of separations and concentrations, i.e. the verification of the possibility of transforming speculative cycles into practical cycles. This period corresponds mainly to the second half of the '70s; main achievements, in 1978 and 1979, were the realization of some complete !cycles for water decomposition, on a laboratory scale [12-14]. Other bench-scale installations were built later on, such as [15] in 1983. A third phase is the period where the work is a progress towards prototypes, by technological experiments and chemical engineering studies. This corresponds to the orientation of the last years, at the beginning of the '80s. The work done in each of these phases (identified

285.9 ~

xQ

The term XQ includes the heat quantities required to produce all forms of work used in the decomposition; more particularly it can be written as:

~ Q = '~ qi + '~ Wi rli where qi are the direct heat inputs, Wi the direct useful work inputs and r/, the efficiencies of conversion of heat to useful work.

x\

"r \

3. D E V E L O P M E N T OF R E S E A R C H Activity on the thermochemical decomposition of water was in the beginning a theoretical study to show, and verify, the feasibility of a possible process; thermodynamic calculations were the basis of the studies. The transition to experimental verifications and engineering studies was rather rapid; in the evolution of the

ggmonst t||iot~ phlhl

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Fig. 3. Evolution and phases in the development of the studies on thermochemical processes for hydrogen production (see No. 3).

G. E. BEGHI

434

only to emphasize some characteristics) has some overlap. Technological experiments are carried out not because the process has been selected but mainly to show the practical feasibility of the method for watersplitting. The search for and study of new reactions and new cycles is always continuing, and there has even been an increase in recent years on the basis of the experience gained and the knowledge accumulated. 4. THERMOCHEMICAL AND HYBRID PROCESSES: EXAMPLES It is not the aim of the present report to make a classification and a list of the published thermochemical cycles: there are various reviews in the literature, and the World Hydrogen Energy Conferences Proceedings are comprehensive collections of the work done in this research area. We can look at some of the cycles which have been developed, as examples of processes and also to give an idea of the "state of the art" in the field. We refer here to the purely thermochemical cycles or to the "hybrid" cycles, i.e. those which include an electrochemical step but which also need high-temperatures for one of the chemical reactions. According to available information, one of the "families" of cycles on which a large amount of work has

Water

been carried out is the so-called sulfur family, based on the thermal decomposition of H2SO~ as common reaction. The first example of these cycles is the following "two-step", hybrid cycle: SO2 + 2H:O = H:SO~ + H2 (electrochem.) H2SO4 = H_,O + SO2 + ½0:. The cycle was first studied at the Los Alamos Scientific Laboratory (U.S.A.) and an important development has been made by the Westinghouse Electric Corporation (U.S.A.) [16]. Studies are also in progress at the JRC-Ispra (Commission of the European Communities), as the Mark 11 cycle, and at the KFA-J01ich (FRG), where the electrochemical reaction is receiving attention [17]. Another example of a "sulphur" cycle, in this case a "pure" thermochemical cycle, is the following: 2H20 + I2 + SO2 = H:SO4 + 2HI H2SO~ = H20 + SO2 + ½0., 2HI = H2 + I2. An important development of this cycle has been made by General Atomic (U.S.A.) [18] which found promising solutions to the critical problem of separating HI

CHRIS

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T32

Tll

H D

T31 ' T32 S1G C15 Tll EI1 E12/13 T15 E15

Air ve~wl Washingtower Feed system Comprwsor S03 decomposition tower Air suplr hmtw Heat rlcuplrator SO3 $ynthosit reactor Salt heat exchanger

Fig. 4. Schematic view of the experimental circuit for thermal decomposition of HzSO4 built at the JRC--Ispra (maximum temperature 900°C, pressure 25 bars).

DEVELOPMENT

from the sulfuric acid solution. of iodine to the use of bromine, hybrid cycle:

OF THERMOCHEMICAL

Shifting from the use we have the following

2HrO + Br? i SO2 = H~SOI + 2HBr H2SOJ = Hz0 + SO2 + 40, 2HBr = Hz + Brz. This cycle is being developed at the JRC-Ispra (Commission of the European Communities) as Mark 13 [19] and is the most advanced from the point of view of continuous bench-scale realizations. Several other cycles use sulfur compounds, with different schemes and different chemical elements; as an example one can mention the so-called NIS process, developed in Japan [20]:

AND HYBRID PROCESSES

43s

concentration and decomposition of HrS04 by using advanced materials for the high-temperature heatexchangers, one based on a flowsheet with direct heating of H2S04 in a flow of hot air, decreasing the severity of working conditions for heat exchangers. The first solution was studied by Westinghouse [21,22]; experimental tests are also being performed by General Atomic Technologies (231; the second solution is being developed by the JRC-Ispra [24,25], where a technological plant was designed and built: see Fig. 4. As a result of all the studies performed in various laboratories it can be said that the thermal decomposition of HzS04 is feasible, and proven with commercially available materials; the information available on the basis of experimental results is sufficient to design an industrial pilot-plant.

2HzO + I: + SO2 + 2Ni = NiI2 + NiS04 + 2Hz 5. PROBLEMS NiIz = Ni + 11 NiSOJ = NiO + SO? + +02 NiO + Hz = Ni + HzO. Also in Japan there are studies on other cycles, such as the magnesium-sulfur-iodine cycles, or, with other elements, the UT3 cycle (15): CaBr2 + Hz0 = CaO + 2HBr CaO + Brr = CaBr2 + $0, Fe30j + 8HBr = 3FeBr2 + 4HzO + Brz 3FeBrz t 4HzO = Fe304 + 6HBr + HZ. To give a better idea of the large diversification of research orientations, it can be mentioned that some interesting, original thermochemical cycles are being studied at the University of Aachen (F.R.G.), in the Institutes of Professors Schulten and Knoche. Promising chemical reactions which are useful for thermochemical processes, are being studied theoretically and experimentally at the Los Alamos National Laboratory (U.S.A.); among others the reactions include sulfites, sulfates, and oxides. As it has already been said, these examples are not at all sufficient to give a complete picture of all the possibilities considered in this research area. Several thermochemical cycles have been tested on a laboratory scale. with a demonstration of their technical feasibility. Experimental studies on the chemical reactions and measurements of physico-chemical properties, were sufficient for some processes to know the more important parameters for a design of technological plants, a flowsheeting of industrial processes, and economic evaluations. As an example of the stage of the development one can mention the step for the thermal decomposition of HzSO~. the common reaction of the sulfur family and the more critical step, due to high temperature and corrosive conditions. For the practical realization, two solutions were studied and tested: one based on the

Research and development on the thermochemical decomposition of water has produced significant results, such as the demonstration of the feasibility of this new method for hydrogen production, and the selection of few promising cycles. One of the results is also the identification of the more critical problems, and of some guidelines for the development of competitive processes. Eficiency is one of the critical parameters, also taking into account the fact that the optimum utilization of heat is essential in a process where thermal energy is the raw material. All aspects affecting efficiency are critical and here we must consider some of the more important problems. Efficiency must first at all be carefully calculated taking into account not only the reversible energy used but accounting also for all irreversibilities (entropy production) which are present in a practical process and a technological plant: all irreversibilities associated with chemical reactions, mixing, heat exchanges, distillations, etc. Separation of chemical products is an important aspect influencing the efficiency of a process. The first and often the most critical product is water which is present in a cycle not only as raw material to be decomposed but usually also as a solvent. The presence of excess water is useful and sometimes necessary in hydrolysis steps, usually run with steam at high temperature; and as a solvent in some reactions in liquid phase. As an example we recall the formation of H+SOs in the SO2 electrochemical reaction with H20; the potential of this reaction is strongly dependent upon the HzSOd concentration, Water removal often absorbs a large amount of primary energy, because of the large amount of heat necessary for the evaporation of H20. Separations are also necessary for reaction products; if they are already in different phases, the separation does not require additional energy. If they are in the same phase (frequently the gaseous one) one of them must be separated by a phase change, or must be absorbed or dissolved into another product.

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It can be mentioned that, because of the internal recirculation of the products in the cycle, it is not necessary to go to a very high level in the separation of products. This has the advantage of avoiding the last steps in the separation processes, which are the most difficult; there is a disadvantage in a loss of the overall efficiency, but in a limited quantity. Oxygen and hydrogen, however, as products leaving the process, must be thoroughly purified. Together with these problems which arise from the use of solutions of aqueous media, another aspect influencing efficiency is the low-temperature side of the cycle. As usual the theoretical efficiency increases as the temperature of the heat-sink temperature decreases; but this requirement is in contradiction with the advantage of suitable kinetics; fast reaction rates decrease the plant investments, but reactions are usually slow at low temperature. Another critical aspect, not linked to the theoretical basis of the thermochemical cycles but very important for the realization of industrial plants, is the problem of construction materials. This problem of materials is common to all processes working with fluids at high temperature: the chemical-mechanical resistance of the materials is often the limiting parameter for the temperature in these processes. The thermochemical processes, and particularly the high-temperature step, are in the same conditions; only corrosion tests can give information on suitable materials, and rather long times are necessary for reliable data, or for a development of improved, well-adapted alloys. Attention should be paid to the matching heat source/chemical reactions; heat sources can be isothermal or non-isothermal. Isothermal heat is more suitable for heterogeneous reactions, run at constant temperature. When the available heat is not isothermal, homogeneous reactions are more suitable. A critical problem is the heat source availability: the most suitable source is the High-Temperature Gas Reactor (HTGR), and most of the thermochemical processes were selected using criteria adapted to this source. Unfortunately the development of these reactors was not as expected and it is now impossible to foresee when commercial HTRs will be available, with the maximum temperature corresponding to the needs for thermochemical processes. Another possible heat source is the solar tower, but the economics in this case are much more critical. Moreover, due to the characteristics of this source (discontinuity, smaller size, etc.) other criteria must be taken into account for the selection of the most convenient processes. It should be mentioned that thermonuclear fusion reactors could also be used, but this solution can only be considered for the long-term future. As a consequence, as far as the heat source is concerned, no practical, economic technology is available today. Last in this list, but not least, is the problem of safety, particularly when the nuclear heat source is considered. Some chemicals used in the process may require special measures including an intermediate heat exchanger,

which increases the investment costs considerably and leads to a reduction in the maximum useful temperature. 6. T H E S I T U A T I O N T O D A Y Research on the thermochemical production of hydrogen is in progress in several laboratories in the world, and most of the activities are performed as part of the collaborative work organized in the frame of the International Energy Agency. Various organizations and universities are active in Japan, working on different processes; in the United States various laboratories are studying different aspects of the thermochemical processes, from specific reactions to technological solutions for the interface with the heat source. In Canada evaluations are being carried out in academic Institutes. In the F.R.G. some Institutes and laboratories are active, in different directions and with experimental programmes; the activities of the European Community Commission, particularly at the JRC--Ispra, have reached a conclusion and have been discontinued. With emphases which are changing from one country to another, activities which are in progress today, or are planned, fall within two main research areas: (1) Studies on chemical reactions, including the search for new cycles, with updated criteria for selection, and experimental verifications of chemical reactions suitable for thermochemical processes. (2) Technological work, with experimental demonstration of process steps, design and construction of technological units, corrosion tests with construction materials, flowsheet calculations and improvements. Economic evaluations have been made on the basis of available data, using preliminary flowsheets for industrial plants; usually a plant capacity of 100000 Nm3(H2) h -t is considered. The hydrogen production cost has been calculated for thermochemical processes, and, to evaluate the competitivity, for advanced electrolysis and for high-temperature electrolysis: with the assumptions which must be made for these new technologies which are not yet industrially developed, the costs are rather similar, in most of the available calculations, and are within the limits of error and uncertainty. To be able to identify the most competitive technology the investment costs must be evaluated more carefully. To do this, one must wait for the results of the first technological experiments in order to have more reliable and detailed, engineering data. The objective of all the studies is to determine an economic, competitive hydrogen production cost, optimizing investment and efficiency. The target for the thermochemical processes is to have an overall plant efficiency of about 40%, and an investment cost lower than 1500 $ Nm-3(H2) h -~. With these values thermochemical hydrogen could

DEVELOPMENT OF THERMOCHEMICAL AND HYBRID PROCESSES be competitive with the other, advanced production methods; but it cannot at present be competitive with hydrogen from natural gas, although the difference between the costs is not too large. An increase of the price of natural gas by about 50% could change the situation. 7. AREAS

FOR FUTURE

(9) Research area is not yet fully explored: improvements of processes and new cycles are definitely possible. Further studies could confirm the potential of thermochemical hydrogen in the future energy system. REFERENCES

INTEREST

To progress in the development of thermochemical processes, on the basis of present knowledge, some main subjects for further attention and work can be pointed out, as follows: (1) Improvements in technological selected processes:

solutions

437

for the

(a) Chemical engineering design of process units. (b) Study of construction materials. (c) Problems of separation and concentration. (2) Design, construction and operation of demonstration units, suitable for the evaluation of chemical engineering and thermal parameters (3) Measurements of thermodynamic data for the chemicals used, in the conditions defined in the processes (4) Detailed cost analysis for plant investments (5) High-temperature heat exchange and coupling with HT’GR, including safety aspects (6) Research for new chemical reactions and new cycles (7) Definition of criteria, and selection of chemical reactions suitable for solar heat sources 8. CONCLUSIO& As a result of the work done so far on thermochemical processes some conclusions can be drawn, which are summarized in the following points. (1) The chemical feasibility of the thermochemical production of hydrogen is demonstrated (laboratory circuits for complete cycles). (2) A few candidate processes are selected (mainly in the “sulfur family”) with general consensus. Promising processes are “hybrid”, i.e. including an electrochemical reaction. (3) Construction materials for selected processes have been identified, both commercially available and advanced. (4) Thermochemical processes are technologically and industrially feasible. (5) The overall thermal efficiency of industrial processes can be in the range 36-40%. (6) Detailed cost analysis is not yet possible, but the hydrogen production cost, with rough estimates, is nearly the same as that of advanced water electrolysis. (7) Economic competitivity is likely, in the medium term, using nuclear heat sources (HTGR) and dedicated plants of very large size. (8) Small size plants (-100 MW) are not competitive.

Funk and R. N . Reinstrom, System study of hydrogen generation by thermal energy. Allison Division of General Motors, Report EDR 3714, Vol. 11, Supplement A (1964). 2. J. E. Funk and R. N. Reinstrom, Energy requirements in the production of hydrogen from water. ZEC Proc. Des. and Deu. 5, 336 (1966). 3. The Hydrogen Economy Miami Energy (THEME) Conf. Proc. (T. N. Veziroglu. ed.) Miami Beach, Florida (18-20 March 1974). 4. International Energy Agency, Annual Renort on Enerev Research, Development and Demonstration Activities xf the IEA 1978-1979. OECD-Paris. 5. K. F. Knoche and J.‘Schubert, Theoretische Untersuchung zur Erzeugung von Wasserstoff durch thermische Zersetzung von Wasser in Mehrstufenprozessen. Research contract of the European Communities. Nr. 045/72/7/ECID (August 1973). 6. J. L. Russel and J. T. Porter, A search for thermochemical watersplitting cycles. THEME Conf. Miami Beach, Florida (1?3-20March 1974). 7. K. Yoshida, H. Kameyama and K. Toguchi, A computer-aided search procedure for thermochemical water decomposition processes. Int. J. Hydrogen Energy 1. J . E.

1, 123 (1976).

8. A hydrogen energy carrier. (R. L. Savage, L. Blank, T. Cady, K. Cox, R. Murray and R. D. Williams, eds) NASA Grant NGT 44-005-114 (September 1973). 9. R. E. Chao, Thermochemical water decomposition processes. IEC Proc. Res. Deo. 13, 94 (1974). 10. C. E. Bamberger and D. M. Richardson, Hydrogen production from water by thermochemical cycle;. Cr$og&ics 197, 197 (April 1976). 11. G. E. Beghi, Review of thermochemical hydrogen production. Int. J. Hydrogen Energy 6, 555 (1981). 12. D. Van Velzen, H. Langenkamp, G. Schuetz, D. Lalonde, J. Flamm and P. Fiebelmann, Development, design and operation of a continuous laboratory-scale plant for hydrogen production by the Mark 13 process. 2nd World Hydrogen Energy Conf., Zurich (1978). 13. G. Besenbruch et al., Development of a sulphur-iodine thermochemical water-splitting cycle for hydrogen production. 14th IECEC. Boston (1979). 14. G. H. Parker and P.‘W. T. Lu, Laboratory model and electrolyzer development for the sulphur cycle hydrogen production process. 14th IECEC, Boston (1979). 15. T. Nakayama, H. Yoshioka, H. Furutani, H. Kameyama and K. Yoshida, Mascot-a bench-scale plant for producing hydrogen by the UT-3 thermochemical decomposition cvcle. Int. J. Hvdroaen Encrnv 9. 187 (19841. 16. 6. H. Parker,* G. ‘ii. FarbGan’ and ‘W. A. Summers, Near-term applications and critical components design for the sulphur cycle hydrogen production process. 3rd World Hydrogen Energy Conference, Tokyo (1980). 17. B. D. Struck, A three compartment electrolytic cell for anodic oxidation of sulphur dioxide and cathodic production of -. hydrogen. ..___. 3rd World Hydrogen Energy Conterence, Tokyo (1980).

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18. J. H. Norman, D. R. O'Keefe, G. E. Besenbruch, L. C. Brown and J. B. Gibson, Improvements in the GA sulphur-iodine water-splitting cycle: alternative methods of processing HI-H20--I2 solution. 4th World Hydrogen Energy Conference, Pasadena, California, U.S.A. (1982). 19. D. Van Velzen and H. Langenkamp, Status Report on the operation of the bench-scale plant for hydrogen production by the Mark 13 process. 3rd World Hydrogen Energy Conf., Tokyo (1980). 20. S. Sato, S. Shimiau, H. Nakajima, K. Onuki and Y. Ikezoe, Studies on the Nickel--Iodine~Sulfurprocess for hydrogen production. 4th World Hydrogen Energy Conf., Pasadena California, U.S.A. (1982). 21. S. S. Lin and R. Flaherty, Design studies of the sulphur trioxide decomposition reactor for the sulphur cycle hydrogen production process. 4th World Hydrogen Energy Conf., Pasadena California, U.S.A. (1982).

22. R. L. Ammon, Status of materials evaluation for sulfuric acid vaporization and decomposition applications. 4th World Hydrogen Energy Conf., Pasadena California, U.S.A. (1982). 23. G. E. Besenbrnch, K. H. McCorkle, J. H. Norman, D. R. O'Keefe, J. R. Schuster and M. Yoshimoto, Hydrogen production by the G.A. sulfur-iodine process. A progress report. 3rd World Hydrogen Energy Conf., Tokyo (1980). 24. A. B r o w , H. Langenkamp, G. Mertel and D. Van Vetzen, Decomposition of sulfuric acid by the Cristina process A status report. 4th World Hydrogen Energy Conf., Pasadena California, U.S.A. (1982). 25. F. Coen-Porisini, Long-term corrosion tests of materials for thermal decomposition of sulphuric acid. 4th World Hydrogen Energy Conf., Pasadena California, U.S.A. Pergamon Press, New York (1982).