Kaoru Onuki

Thermochemical water-splitting cycle using iodine and sulfur

Research and development on the thermochemical water-splitting cycle using iodine and sulfur, a potential large-scale hydrogen production method, is reviewed. Feasibility of the closed-cycle continuous water splitting has been demonstrated by coupling the Bunsen reaction, thermal decomposition of hydrogen iodide and that of sulfuric acid. Studies are in progress to realize efficient hydrogen production. Also, development of chemical reactors made of industrial materials has been carried out, especially those used in the corrosive process environment of sulfuric acid vaporization and decomposition.

Simulation study on the catalytic decomposition of hydrogen iodide in a membrane reactor with a silica membrane for the thermochemical water-splitting IS process

Catalytic decomposition of hydrogen iodide in a membrane reactor was investigated theoretically for the application to the hydrogen production step in the thermochemical iodine–sulfur (IS) process. Characteristics of the membrane reactor were evaluated using observed permeances of H2 and HI in a homemade silica membrane that was prepared by chemical vapor deposition (CVD) method (selectivity of H2/HI: 650). The effect of the H2/I2 selectivity on the performance of the membrane reactor was evaluated by simulation since I2 permeance through the homemade silica membrane could not be determined so far because of the difficulty of the measurements. It was found from the simulation study that the conversion of over 0.9 would be attainable using the membrane reactor with the homemade silica membrane. Design criterion of the membrane reactor was discussed using the relationships between the ratio of reaction zone volume to the membrane surface area, the dimensionless reactor length and the conversion.

Thermochemical Water Splitting for Hydrogen Production Utilizing Nuclear Heat from an HTGR

Abstract A very promising technology to achieve a carbon free energy system is to produce hydrogen from water, rather than from fossil fuels. Iodine-sulfur (IS) thermochemical water decomposition is one promising process. The IS process can be used to efficiently produce hydrogen using the high temperature gas-cooled reactor (HTGR) as the energy source supplying gas at 1000°C. This paper describes that demonstration experiment for hydrogen production was carried out by an IS process at a laboratory scale. The results confirmed the feasibility of the closed-loop operation for recycling all the reactants besides the water, H2, and O2 . Then the membrane technology was developed to enhance the decomposition efficiency. The maximum attainable one-pass conversion rate of HI exceeds 90% by membrane technology, whereas the equilibrium rate is about 20%.

Preliminary process analysis for the closed cycle operation of the iodine-sulfur thermochemical hydrogen production process

Abstract In the iodine–sulfur thermochemical hydrogen production process, a separation characteristic of 2-liquid phase (H2SO4 phase and HIx phase) in the separator at 0°C was measured. Two-phase separation began to occur at about 0.32 of I2 molar fraction and over. The separation characteristic became better with the increase in iodine concentration in the solution. The effect of flow rate variations of HI solution and I2 solution from the HIx distillation column on the process was evaluated. The flow rate increase in HI solution from the distillation column did not have a large effect on the flow rate of HI solution fed to the distillation column from the separator. The decreasing flow rate of I2 solution from the distillation column decreased the flow rate of I2 solution fed to the distillation column from the separator. The variation of I2 molar fraction in the H2SO4 phase in the separator was sensitive to the variation in flow rate of both solutions from the distillation column. The tolerance level of the variation was investigated by considering I2 solubility, 2-liquid phase disappearance and SO2 reaction amount.

Development of Hydrogen Production Technology by Thermochemical Water Splitting IS Process Pilot Test Plan

Japan Atomic Energy Agency (JAEA) has been conducting a study on a thermochemical IS process for hydrogen production. A pilot test of IS process is under planning that covers four R&D subjects: (1) construction of a pilot test plant made of industrial materials and completion of a hydrogen production test using electrically-heated helium gas as the process heat supplier, (2) development of an analytical code system, (3) component tests to assist the hydrogen production test and also to improve the process performance for the commercial plant, (4) a design study of HTTR-IS system. Development of innovative chemical reactors is in progress, which are equipped with a ceramic heat exchanger. In the design of the IS plant, it is important to establish the system for “design by analysis”. Therefore, we have developed a multiphase flow analysis code that can analyze systems in which chemical reactions occur.

Studies on the nickel-iodine-sulfur process for hydrogen production—III

Abstract A thermochemical hydrogen production process utilizing nickel, iodine and sulfur (the NIS process) has been studied and suitable operation conditions examined. Nickel powder dissolution into acid mixtures, nickel iodide decomposition under iodine partial pressure, nickel sulfate decomposition and other steps were studied. Decomposition gas from the sulfate had near equilibrium composition as for the sulfur trioxide decomposition into sulfur dioxide and oxygen. A new process is also under study, utilizing methanol instead of nickel as the circulating reactant.

THERMODYNAMIC CONSIDERATIONS ON THE PURIFICATION OF H2SO4 AND HIX PHASES IN THE IODINE-SULFUR HYDROGEN PRODUCTION PROCESS

The reaction equilibrium and phase equilibrium in H2SO4 and HIx phases produced by the Bunsen reaction of the iodine-sulfur thermochemical hydrogen production process were examined using a chemical process simulator, ESP, with a thermodynamic database based on the mixed solvent electrolyte model. At temperatures lower than ca. 110°C, the reaction of HI and H2SO4 produced elemental sulfur in both phases. At higher temperatures, the reverse Bunsen reaction occurred, and SO2 was produced in the H2SO4 phase. In the HIx phase, conversely, SO2 formation predominated in a narrow temperature range and H2S was produced with the increase in temperature. The presence of N2 gas lowered the temperature of the predominant reaction change. A feed of O2 for purification was proposed to suppress the consumption of objective components in the H2SO4 phase purification, and an O2 feed to the HIx phase for the suppression of H2S and S impurities was proposed by the simulation.

Dehydration through pervaporation from HIx solution (HI-H2O-I2 mixture) using a cation exchange membrane for thermochemical water-splitting iodine-sulfur process

Pervaporation (PV) of water from HIx solution (HI-H2O-I2 mixture) using Nafion-117 was evaluated aiming at the application to dehydrate the azeotropic composition in HI decomposition reaction of thermochemical IS process. PV experiment was carried out by using HI solutions of 40–65 wt% and an I2/HI molar ratio of 0–3 in the feed at the room temperature. The permeation flux decreased with increasing HI weight fraction in the feed. The permeation flux is dependent on the I2 concentration in the feed having an I2/HI molar ratio. A long time PV experiment was carried out using I2/HI molar ratio of 1 (in HI solution of 55.9%) in the feed at room temperature. It is expected that the permeation component in the permeate zone using the PV process was mainly H2O, and H2O permeation was constant with increasing operation time.

Study of catalytic reduction of methanol for methane-methanol thermochemical hydrogen production cycles

Abstract Methanol was found to be reduced to methane by hydriodic acid solution containing platinum ions. The reaction was considered to proceed sequentially from methanol to methyl iodide and then to methane. Platinum ion acts as a catalyst in the methane formation. In the presence of iodine, the reaction was greatly suppressed and the reaction rate was almost independent of temperature. Methane-methanol thermochemical hydrogen production cycles utilizing the reaction are discussed.

Laboratory scale demonstration of CIS process

Abstract For the purpose of demonstrating the chemical practicability of CIS Process, a closed loop demonstration apparatus was constructed and operated. The loop consists of four key reactions of six reactions in CIS Process. Methanol was converted to methane and oxygen by the apparatus with conversions of 63–80%. 12 cyclic operations were successfully carried out with no serious troubles. Indications of side reactions, deposition of carbon-like compounds and formation of sulfur-like precipitates, were recognized.