R&D Program on Thermochemical Water-Splitting Iodine-Sulfur Process at JAERI

Transcription

1 GENES4/ANP2003, Sep , 2003, Kyoto, JAPAN Paper 1072 R&D Program on Thermochemical Water-Splitting Iodine-Sulfur Process at JAERI Shinji KUBO, Hayato NAKAJIMA, Shunichi HIGASHI, Tomoo MASAKI, Seiji KASAHARA, Mikihiro NOMURA, Shintaro ISHIYAMA, Kaoru ONUKI*, Saburo SHIMIZU Department of Advanced Nuclear Heat Technology, Japan Atomic Energy Research Institute, 3607 Niihori, Narita-cho, Oarai-machi, Ibaraki-ken, JAPAN Studies on thermochemical water-splitting process of iodine-sulfur family (IS process) are being carried out at Japan Atomic Energy Research Institute (JAERI). IS process is composed of three chemical reactions and works like a chemical engine to produce hydrogen and oxygen from water. HTGR that can supply high temperature of ca o C will be the most suitable energy supplier to drive IS process. JAERI demonstrated the feasibility of continuous hydrogen production by combing the fundamental reactions and separations of IS process in laboratory. At present, hydrogen production test is continuing using a scaled-up glass apparatus. Corrosion-resistant materials for constructing a large-scale plant and process improvements by introducing advanced separation techniques such as membrane separation are under study, as well. Future R&D items are discussed based on these present activities. KEYWORDS: hydrogen production, water splitting, thermochemical cycles, Iodine-Sulfur process, high temperature gas-cooled reactors I. Introduction II. IS Process Hydrogen has ideal characteristics as an energy carrier. Hydrogen can be used as a clean fuel in a variety of energy end-use sectors including the conversion to electricity. It can be stored, and also can be transported long distance with lower loss compared to electricity. After combustion, it produces only water. Therefore, the concept of hydrogen energy system has attracted much interest worldwide, which recently has been strengthened by the growing concern on the environmental issues such as global warming due to the CO 2 emission and by the rapid progress of utilizing technologies, especially of fuel cells, as well. In order to develop the hydrogen energy system, in which huge hydrogen demand is expected, development of efficient hydrogen production system is an important and urgent problem. Nuclear energy that emits no carbon dioxide and can stably supply massive energy is one of the promising candidates of the primary energy for the hydrogen production. HTGR that features high level of inherent safety and has an ability to supply high temperature heat of ca o C is quite suitable among others for this purpose. Thermochemical water-splitting process is a method to make an effective use of the high temperature heat for hydrogen production. It works like a chemical engine to produce hydrogen and oxygen from water by combining high temperature endothermic chemical reactions and low temperature exothermic chemical reactions. 1) Based on these considerations, JAERI has been conducting R&D on HTGR and also on thermochemical hydrogen production. This paper describes the latter activity, where thermochemical water-splitting IS process is studied. IS process is composed of three chemical reactions shown in Fig.1. The raw material, water, reacts with sulfur dioxide and iodine to produce hydrogen iodide and sulfuric acid (the so-called Bunsen reaction), which are then thermally decomposed to produce hydrogen and oxygen, respectively. Thermal decomposition of sulfuric acid has favorable characteristics as high temperature endothermic reaction of thermochemical water-splitting process. It proceeds in two stages with no serious side reactions. First, H 2 SO 4 decomposes spontaneously into SO 3 and H 2 O in the temperature range of o C. By further heating up to over 800 o C, SO 3 decomposes into SO 2 and O 2 in the presence of solid catalyst. Both reactions are strongly endothermic and temperature range of the reactions is well Temperature ( o C) Sulfuric acid decomposition (endothermic) H2SO4 H 2 O SO 2 0.5O 2 H2O+SO2+0.5O2 H 2 SO 4 2HI H2+I2 SO2+I2+2H2O 2HI+H2SO4 0.5O2 H2O I 2 H2 2HI Hydrogen iodide decomposition Bunsen reaction (exothermic) * Corresponding author, Tel , Fax , onuki@popsvr.tokai.jaeri.go.jp Fig.1 IS process

2 matched with that of HTGR. The process has been studied since 1970 s and General Atomics (GA) performed the most intensive work. 2-4) In Europe and in Japan, several variations of the process have also been studied which differed in the mode of reaction and methods to separate reaction products. 5-10) Initial efforts were concentrated on how to separate the HI and H 2 SO 4 produced in the Bunsen reaction. Researchers at GA proposed a method that utilized a liquid-liquid (LL) phase separation phenomena. 2) They found that, in the mixture of HI-H 2 SO 4 -H 2 O-I 2, the two acids separate into two aqueous solutions when excess amount of iodine is present. The main components of the phase-separated solutions are HI-I 2 -H 2 O (HIx solution) and H 2 SO 4 -H 2 O, respectively. Figure 2 shows the process scheme featuring the LL separation. Enthalpy changes shown in the figure indicate the potential of efficient hydrogen production, provided that effective scheme is adopted for the separation of SO 3 from sulfuric acid and that of HI from HIx solution. On these separation problems, several ideas have been proposed. As for the former problem in the sulfuric acid processing, utilization of conventional techniques such as steam recompression 4) or multi-effect flash evaporation 11) has been proposed. As for the HIx processing, extractive distillation using phosphoric acid 4) or reactive distillation 12) has been proposed. Flowsheeting studies based on these ideas indicated that the process thermal efficiency of hydrogen production in the range of 45-50% could be attained utilizing intensive and efficient heat recovery networks. 4,9,12) III. Present Activity at JAERI In JAERI, we have been undertaking studies on IS process in three research fields, i.e., experimental verification of continuous hydrogen production by IS process featuring the LL separation, investigation of process improvement, and that of corrosion-resistant materials of construction. 1. Closed-Loop Hydrogen Production Test Initial attempt was examined using a glass-made apparatus that was designed to include fundamental reactions and separations of IS process and to produce hydrogen of 1 NL/hr. Figure 3 shows the simplified flowsheet. At first, efforts were devoted to determine an appropriate composition of the phase-separated solutions, which enabled to minimize the operational instability due to the occurrence of sulfur forming side reactions etc. The LL separation behaviour and the Bunsen reaction were studied in detail. Purification method of the phase-separated acid solution was developed, which utilized the reverse reaction of Bunsen 10, 13-15) reaction. Then, continuous and stable production of hydrogen and oxygen was carried out for 48 hrs with stoichiometric ratio of water decomposition reaction. 16,17) In the experiments, every operation was carried out at ambient pressure and the iodine concentration in the product solution of Bunsen reaction was set to the saturating one at 0 o C. The 1NL/hr experiments clarified the importance of adequate design of reactors such as Bunsen Reactor and LL Separator, of the selection of adequate composition in the process solution, H 2 SO 4 Decomposition Step o C) H 2 (g,hhv=286kj/mol) Condenser O2 H 2 O H 2 +99kJ SO 3 (g) SO 2 (g) + 0.5O 2 (g) +231kJ H 2 SO 4 (aq;56%) H 2 O(g)+SO 3 (g) H 2 O(g) -44kJ H 2 O(l) 2HI(g) +148kJ 2HI(aq;10m) 2HI(g) HI Decomposition Step ( o C) +9kJ H 2 (g) + I 2 (g) I 2 (g) -47kJ I 2 (l) H2SO4 Decomposer H2SO4 Vaporizer Bunsen Reactor Acids Separator Condenser HI Decomposer HIx soln. -111kJ SO 2 (g) + 9I 2 (l) + 2H 2 O(l) {H 2 SO 4 (aq)+4h 2 O(l)} + HIx soln.{2hi(aq)+8i 2 (l)+10h 2 O(l)} Concentrator Purifier Purifier HIx Solution Distillation Column 0.5O 2 (g) H 2 O(l) Bunsen Reaction Step (ca.100 o C) Fig.2 Outline of IS process featuring a liquid-liquid phase separation proposed by General Atomics 4), which appears in the presence of excess iodine. Caloric figures denote enthalpy changes associated with the corresponding chemical changes at standard state. Here, no concrete unit operations are assumed for the separation of gaseous SO 3 and HI from the corresponding aqueous solutions. Fig.3 Schematic flowsheet of laboratory-scale test apparatus. Here, the separation of SO 3 from sulfuric acid is carried out by evaporation (concentrator and vaporizer), and that of HI from HIx solution (HI-I 2 -H 2 O mixture) is by distillation. Two types of apparatus have been set up in JAERI lab, both of which were made of glass. Hydrogen production capacity of the first one was 1 NL/hr and that of the second is 50 NL/hr.

3 Fig.4 Overview of the experimental apparatus (50 NL/hr). 18) It is equipped with an increased number of measuring devices, and internal recycling of unreacted acids in each process step is introduced. Handling of iodine-rich HIx solution is possible with the apparatus. and of precise control of the flow rate. Based on the experiences and chemical data acquired in the study mentioned above, a scaled-up glass apparatus was set up (Fig.4). 18) Its basic flowsheet is the same with the former one except that recycling of unreacted HI and H 2 SO 4 becomes possible within the HI decomposition step and the H 2 SO 4 decomposition step, respectively. It is equipped with newly devised pumps and sensors for monitoring process parameters such as flow rates and liquid levels. Also, process solution of high iodine concentration can be handled at elevated temperature, with which better separation is anticipated of hydrogen iodide and sulfuric acid. Operation tests in each reaction step have been carried out and the combined operation test will start in FY Studies on Process Improvement Process improvement has been pursued on the HI decomposition step, where HI is separated from the HIx solution supplied from the Bunsen reaction step and decomposed to produce hydrogen and iodine. Simple way to realize the chemical change is the distillation of HIx solution and the gas phase thermal decomposition of HI. However, because of the presence of azeotropic composition in HI-H 2 O system (the molar ratio of H 2 O/HI being ca 5), the distillation requires large thermal burden. Also, low equilibrium conversion of HI decomposition (ca 20% at 400 o C) induces excess circulation of HI within the process, which results in the increase of thermal burden. In order to overcome these difficulties, GA proposed an extractive distillation using phosphoric acid and a liquid phase HI decomposition using Pd catalyst. 4) Researchers at RWTH Aachen studied reactive distillation under high pressure. 12,19-21) However, there still remain spaces of improvement to reduce the complex process scheme or to remedy the very corrosive process environment. We are pursuing an application of membrane techniques. Volatility of HI over HIx solution drastically changes at the quasi-azeotropic composition. 19) So, by concentrating the HIx solution to over-quasi-azeotropic composition using membrane process that is not governed by phase equilibrium, pure HI could be separated by distillation. Also, by using membrane reactor equipped with hydrogen separation membrane, higher one-pass conversion than the equilibrium one could be attained. Concerning the former, application of electrodialysis has been examined. So far, the concentration of iodine-rich HIx solution, initial composition of which was HI/I 2 /H 2 O=1/4/5 (in mole) that was equivalent to the one anticipated in the efficient flowsheet proposed by GA 4), was demonstrated in beaker-scale experiments. 22,23) As for the latter, the hydrogen separation membrane should exhibit not only high separation performance but also high thermal and chemical stability in the environment of decomposition reaction. Therefore, ceramic membranes have been investigated. Silica membrane based on porous alumina substrate was prepared by chemical vapor deposition method on a trial basis. It showed good separation performance, the ratio of permeation rates of H 2 /HI being ca. 600/1 at 450 o C. Computer simulation of the membrane reactor was carried out using one-dimensional model that took the reaction rate, the permeation rate and mass balance into consideration. The results indicated that an ideal one-pass conversion of HI be over 90%. 24,25) Preliminary heat/mass balance analysis of the flowsheet featuring the membrane process indicated that a similar process thermal efficiency as that reported on extractive distillation process might be possible with simpler process scheme. 26) 3. Studies on Materials of Construction In order to select the candidate materials of construction for large-scale plant, corrosion tests of commercially available materials have been performed in representative process environments. Figure 5 summarizes the materials that showed corrosion resistance in the simulated process environments. Here, test results reported from several research groups are considered on the corrosion in the concerned environments ) As for the gas phase environment of H 2 SO 4 decomposition step, some refractory alloys that have been used in conventional chemical plants showed good corrosion resistance. Also, in the gas phase environment of HI decomposition step, a Ni-Cr-Mo-Ta alloy was found to show good corrosion resistance. As for the Bunsen reaction step, glass-lining materials, Zr, Ta showed corrosion resistance. In the environment of HIx distillation at 20bar, Ta showed excellent corrosion resistance. The severest environment is the boiling condition of concentrated sulfuric acid under high pressure (e.g. 20bar), where ceramic materials containing silicone such as SiSiC, SiC, and Si 3 N 4 were the only materials that showed excellent corrosion resistance.

4 ( o C) Decomposition Vaporization SiSiC, SiC, Si 3 N 4, (SX) Bunsen Reaction Alloy 800, Hastelloy, etc. HI Decomposition Gas Phase MAT21, Hastelloy, etc. Distillation of HIx soln. Glass Lining, Zr, Ta, etc. Liquid Phase Fig.5 Candidates for materials of construction. This figure summarizes the reported results of corrosion tests carried out in the typical process environment ) Some new information obtained at JAERI is added on the environment of HI decomposition, as well. Here, SX and MAT21 are the brand names of a high-silicon stainless steel alloy (Edmeston/Sandvik) and a Ni-Cr-Mo-Ta alloy (Mitsubishi Materials), respectively. In summary, for gas phase service, there seem to exist little concern on the structural materials. As for the equipments used in the Bunsen reaction step, lining materials should be used. Special design consideration is required for the equipments to be used in the boiling and condensing conditions of the acids. IV. Future R&D Items The logical evolution of the above-mentioned studies is considered to be the pilot-scale hydrogen production test, the purpose of which is to demonstrate the continuous hydrogen production using an apparatus made of structural materials that can also be used in the commercial plant, and to acquire engineering data for scaling up the plant and for the detailed process evaluation. Considering the difficulties expected in fabricating the equipments for handling the corrosive process solutions, the next R&D stage may be divided into two phases as shown in Fig.6. Phase 1 prepares the techniques for constructing the test plant and the simulation codes of the plant, as well. In Phase 2, the plant is to be designed in detail, constructed, and operated to demonstrate the feasibility of continuous hydrogen production. The R&D items in the phase 1 are considered to be as follows. 1. Development of Equipments Purpose is to develop the methods and/or techniques for fabricating the equipments to be used in the pilot plant that is made of structural materials that can be used also in the commercial plants. Target figure of hydrogen production rate of the pilot plant may be in the range of 1-30m 3 /h. The concerned equipments include reactors, separators, pumps, valves, joints, piping etc. Selection and/or development of measuring instruments will also be considered such as pressure gauge, flow meter, and liquid level gauge. In this R&D, at first, design of the equipments is to be examined based on the knowledge obtained in the closed-loop test and in the studies on process improvement and on corrosion of materials of construction, as well. Then, fabrication of full sized equipments or small-scaled equipments or elements of the equipments is to be carried out on a trial basis in order to confirm the key techniques. Reactors such as SO 3 Decomposer that are supposed to be heated directly by secondary helium in the commercial system are designed as helium-heated reactors. Target specifications of each reaction section, equipments to be examined, and the candidate structural materials are as follows. As for the Bunsen reaction step, the flowsheet proposed by General Atomics 4) may be the basis of examination. The candidate materials of construction that show corrosion resistance in the concerned process environments such as glass, Ta is to be considered as the lining material. The equipments to be considered are Bunsen Reactor: Liquid-Liquid Phase Separator (LL Separator) HIx Purifier Purifier Pressure Gauge/ Flow Meter/ Level Gauge Pumps/Valves/Joints/Piping etc As for the HI decomposition step, the flowsheet including the membrane process may be the basis of examination. The equipments to be considered are HIx Concentrator (by electrodialysis.) HIx Distillation Column HI Decomposer Iodine Separator Hydrogen Separator Pressure Gauge/ Flow Meter/ Level Gauge Pumps/Valves/Joints/Piping etc As for the H 2 SO 4 decomposition step, the flowsheet proposed by RWTH Aachen 11) may be the basis of examination. The candidate materials of construction are silicon-containing ceramics such as SiSiC for liquid phase and refractory alloys such as alloy 800 for gas phase, respectively. The equipments to be considered are Concentrator Vaporizer SO 3 Decomposer Condenser Pressure Gauge/ Flow Meter Pumps/Valves/Joints/Piping etc

5 2. Performance Tests of Equipments Purpose of the tests is to confirm the design specification of the equipments or the elements of equipments, which was designed and fabricated on a trial basis in the work of Development of Equipments, in the concerned process conditions in terms of reaction and separation characteristics, corrosion, etc. Three test apparatus will be needed to carry out the tests in the three reaction sections, i.e. the Bunsen reaction step, the HI decomposition step, and the H 2 SO 4 decomposition step. The tests are to be carried out of the equipments or the elements of equipments one by one and/or in the mode of combined operation. 3. Development of Simulation Codes Purpose is to develop static and dynamic simulation codes that are to be used in the heat/mass balance analysis and the simulation of operation procedure of the pilot plant. The codes will be developed using a commercial chemical process simulator. Improvement of chemical database is also to be carried out by extensive literature survey and/or experimental study on selected data such as vapor-liquid equilibrium. 4. Basic Design of Pilot Plant The work is to make and improve the plant flowsheet and PID that covers heat/mass balance, sizing of equipments etc. Considerations of practical measures on safety, maintenance, legal formalities etc. shall be carried out, as well. The design is to be dynamically modified year by year by incorporating the results of the works in Development of Equipments and Performance Tests of Equipments. Also, conceptual consideration of commercial HTGR-IHX-IS system is to be carried out and the results shall be reflected in the design work. Pilot Test: 1-30Nm 3 -H 2 /h Phase 2 - Detailed design, construction, and operation of pilot plant Phase 1 - Development of equipments for pilot plant - Performance tests of equipments - Development of simulation codes (incl. acquisition of supplmental physco-chemical data) - Basic design of pilot plant, Scheduling of Phase2 Study on Advanced HI Processing - Separation of HI from HIx solution - Enhancement of HI conversion Lab-Scale Demo: 50NL-H 2 /h - Atmospheric pressure operation - Bunsen Step: elevated temperature - Method of meas&control - Process simulator (preliminary) - System concept (preliminary) Screening of the Corrosion Resistant Materials of Construction - Corrosion tests of commercially available materials Lab-Scale Demo: 1NL-H 2 /h (1) Bunsen Step: LL separation at 10 o C (2) H 2 SO 4 Step: decomp at 850 o C (3) HI Step: conventional distill & gas phase decomp Fig.6 R&D plan on IS process considered based on the present activity at JAERI

6 V. Conclusion Present status of studies on IS process at JAERI was described, which is planned to complete in FY2004. R&D items in future pilot test that is to be followed by the present studies was discussed briefly. By accomplishing the pilot test, the thermochemical hydrogen production will acquire engineering reality and practical evaluation of the process will become possible. 11) K.F. Knoche, H. Schepers, K. Hesselmann, Second law and cost analysis of the oxygen generation step of the General Atomic Sulfur-Iodine cycle, Proc. 5th World hydrogen Energy Conf., Toronto, Canada, July 1984, vol.2, pp (1984). 12) H. Engels, K.F. Knoche, M. Roth, Direct dissociation of hydrogen iodide - an alternative to the General Atomic proposal, Proc. 6th World hydrogen Energy Conf., Vienna, Austria, July 1986, vol.2, pp (1986). Acknowledgement 13) M. Sakurai, H. Nakajima, K. Onuki, K. Ikenoya, S. The closed-loop hydrogen production test (50NL/hr) is Shimizu, Preliminary process analysis for the closed being carried out under the contract of research between cycle operation of the iodine-sulfur thermochemical JAERI and the Ministry of Education, Culture, Sports, hydrogen production process, Int. J. Hydrogen Energy, Science and Technology of Japan. 24, (1999). References 1) S. Sato, Thermochemical Hydrogen Production, in Solar-hydrogen energy systems, (ed.) T. Ohta, Oxford, Pergamon Press, (1979). 2) J.H. Norman, G.E. Besenbruch, D.R. O Keefe, Thermochemical water-splitting for hydrogen production, GRI-80/0105, Gas Research Institute (GRI), (1981). 3) P.W. Trester and H.G. Staley, Assessment and Investigation of Containment Materials for the Sulfur-Iodine Thermochemical Water-Splitting Process for Hydrogen Production, GRI-80/0081, Gas Research Institute (GRI), (1981). 4) J.H. Norman, G.E. Besenbruch, L.C. Brown, D.R. O Keefe, C.L. Allen, Thermochemical Water-Splitting Cycle, Bench-Scale Investigations and Process Engineering, GA-A 16713, General Atomics (GA), (1982). 5) G. De Beni, G. Pierini, B. Spelta, The reaction of sulfur dioxide with water and a halogen. The case of iodine: Reaction in presence of organic solvents. Int. J. Hydrogen Energy, 5, (1980). 6) S. Sato, S. Shimizu, H. Nakajima, Y. Ikezoe, Nickel-Iodine-Sulfur process for hydrogen production, Proc. 3rd World Hydrogen Energy Conf., Tokyo, Japan, June 1980, vol.1, pp (1980). 7) T. Kumagai and S. Mizuta, Laboratory scale demonstration of the Mg-S-I cycle for thermochemical hydrogen production. Ind. Eng. Process Des. Dev., 24, (1985). 8) S. Shimizu, K. Onuki, H. Nakajima, Y. Ikezoe, S. Sato, Laboratory scale demonstration of CIS process. Int. J. hydrogen Energy, 12, (1987). 9) I.T. Oeztuerk, A. Hammache, E. Bilgen, A new process for oxygen generation step for the hydrogen producing sulfur-iodine thermochemical cycle, Trans. IChemE., 72 Part A (1989) ) K. Onuki, H. Nakajima, I. Ioka, M. Futakawa, S. Shimizu, IS process for thermochemical hydrogen production, JAERI-Review , Japan Atomic Energy Research Institute (JAERI), (1994). 14) M. Sakurai, H. Nakajima, K. Onuki, S. Shimizu, Investigation of 2 liquid phase separation characteristics on the iodine-sulfur thermochemical hydrogen production process, Int. J. Hydrogen Energy, 25, (2000). 15) M. Sakurai, H. Nakajima, R. Amir, K. Onuki, S. Shimizu, Experimental study on side-reaction occurrence condition in the iodine-sulfur thermochemical hydrogen production process, Int. J. Hydrogen Energy, 25, (2000). 16) H. Nakajima, K. Ikenoya, K. Onuki, S. Shimizu, Closed-cycle continuous hydrogen production test by thermochemical IS process, Kagaku Kogaku Ronbunshu, 24, (1998), [in Japanese]. 17) H. Nakajima, M. Sakurai, K. Ikenoya, G.-J. Hwang, K. Onuki, S. Shimizu A study on a closed-cycle hydrogen production by thermochemical water-splitting IS process, Proc. 7th Int. Conf. Nuclear Engineering (ICONE-7), Tokyo, Japan, April 1999, ICONE-7104 (1999). 18) S. Kubo, S. Shimizu, H. Nakajima, K. Onuki, S. Higashi, N. Akino, Construction of apparatus with thermochemical hydrogen production process, presented at the 11th Canadian Hydrogen Conf., Victoria, Canada, June, (2001). 19) D. Neumann, Phasengleichgewichte von HJ/H 2 O/J 2 - Loesungen, Diplomarbeit, RWTH Aachen, (1987). 20) M. Roth, K.F. Knoche, Thermochemical water splitting through direct HI-decomposition from H 2 O/HI/I 2 solutions, Int. J. Hydrogen Energy, 14, (1989). 21) C. Berndhaeuser and K.F. Knoche, Experimental investigations of thermal HI decomposition from H 2 O-HI-I 2 solutions, Int. J. Hydrogen Energy, 19, (1994). 22) K. Onuki, G.-J. Hwang, Arifal, S. Shimizu, Electro-electrodialysis of hydriodic acid in the presence of iodine at elevated temperature, J. Membr. Sci., 192, (2001). 23) G.-J. Hwang, K. Onuki, M. Nomura, S. Kasahara, H.-S. Choi, J.-W. Kim, Improvement of the thermochemical water-splitting IS process by electro-electrodialysis, submitted to J. Membr. Sci.

7 24) G.-J. Hwang, K. Onuki, S. Shimizu, Separation of hydrogen from a H 2 -H 2 O-HI gaseous mixture using a silica membrane, AIChE J., 46, (2000). 25) G.-J. Hwang and K. Onuki, Simulation study on the catalytic decomposition of hydrogen iodide in a membrane reactor with a silica membrane for the thermochemical water-splitting IS process, J. Membr. Sci., 194, (2001). 26) S. Kasahara, G.-J. Hwang, H. Nakajima, H.-S. Choi, K. Onuki, M. Nomura, Effects of process parameters of the IS process on total thermal efficiency to produce hydrogen from water, J. Chem. Eng. Jpn., in press 27) K. Onuki, H. Nakajima, S. Shimizu, S. Sato, I. Tayama, Materials of construction for the thermochemical IS process, (I), Journal of the Hydrogen Energy Systems Society of Japan, 18, (1993), [in Japanese]. 28) K. Onuki, I. Ioka, M. Futakawa, H. Nakajima, S. Shimizu, S. Sato, I. Tayama, Materials of construction for the thermochemical IS process, (II), Journal of the Hydrogen Energy Systems Society of Japan, 19, (1994), [in Japanese]. 29) K. Onuki, I. Ioka, M. Futakawa, H. Nakajima, S. Shimizu, I. Tayama, Screening tests on materials of construction for the thermochemical IS process, Corrosion Engineering, 46, (1997). 30) R.L. Ammon, Status of materials evaluation for sulfuric acid vaporization and decomposition application, Proc. 4th World Hydrogen Energy Conf., California, USA, June 1982, vol.2, pp , (1982). 31) F. Coen Porisini, Selection and evaluation of materials for the construction of a pre-pilot plant for thermal decomposition of sulfuric acid, Int. J. Hydrogen Energy, 14, (1989). 32) Y. Imai, Y. Kanda, H. Sasaki, H. Togano, Corrosion resistance of materials in high temperature gases composed of iodine, hydrogen iodide and water, Boshoku Gizyutsu, 31, (1982), [in Japanese]. 33) I. Ioka, K. Onuki, M. Futakawa, Y. Kuriki, M. Nagoshi, H. Nakajima, S. Shimizu, Corrosion resistance of Fe-Si alloys in boiling sulfuric acid, Zairyo (J. Soc. Mat. Sci., Japan), 46, (1997), [in Japanese]. 34) M. Futakawa, K. Onuki, I. Ioka, H. Nakajima, S. Shimizu, Y. Kuriki, M. Nagoshi, Corrosion test of compositionally graded Fe-Si alloy in boiling sulfuric acid, Corrosion Engineering, 46, (1997). 35) N. Nishiyama, M. Futakawa, I. Ioka, K. Onuki, S. Shimizu, M. Eto, T. Oku, M. Kurabe, Corrosion resistance evaluation of brittle materials in boiling sulfuric acid, Zairyo (J. Soc. Mat. Sci., Japan), 48, (1999), [in Japanese]. 36) I. Ioka, J. Mori, C. Kato, M. Futakawa, K. Onuki, The characterization of passive films on Fe-Si alloy in boiling sulfuric acid, J. Materials Sci. Lett., 18, (1999). 37) Y. Kurata, K. Tachibana, T. Suzuki, High temperature tensile properties of metallic materials exposed to a sulfuric acid decomposition gas environment, J. Japan Inst. Metals, 65, (2001), [in Japanese]. 38) S. Kubo, M. Futakawa, I. Ioka, K. Onuki, S. Shimizu, K. Ohsaka, A. Yamaguchi, R. Tsukada, T. Goto, Corrosion test on structural materials for iodine-sulfur thermochemical water splitting cycle, Proc. 2nd Topical Conf. on Fuel Cell Technology, AIChE 2003 Spring National Meeting held at New Orleans, AIChE, N.Y., (2003).