Thermochemical Water-splitting Iodine Sulfur Process Research Articles

Present status of JAEA’s R&D toward HTGR deployment

Japan Atomic Energy Agency (JAEA) has been promoting the research and development (R&D) of High Temperature Gas-cooled Reactor (HTGR). HTGR having superior safety features can supply high temperature heat above 900 °C and it makes possible to use nuclear energy to heat source for various industries as well as to power generation. Promotion of R&D on HTGR and hydrogen production system is stated in Japanese policies, e,g, The 6th Strategic Energy Plan and The Green Growth Strategy Through Achieving Carbon Neutrality in 2050. R&D on reactor technologies is carried out by using High Temperature engineering Test Reactor (HTTR) which is the first HTGR in Japan. On July 30th 2021, the HTTR was resumed without significant reinforcements. On January 28th 2022, a safety demonstration test under the OECD/NEA LOFC project was carried out. JAEA is promoting R&D on a carbon-free hydrogen production by thermochemical water splitting Iodine-Sulfur process (IS process). Hydrogen production test for 150 h with 0.03 m3/h-H2 was successfully performed in January 2019. JAEA conducts design study for various HTGR systems toward commercialization. The major one is GTHTR300C for hydrogen and electric power cogeneration. JAEA has launched a new test program about demonstration of hydrogen production by the HTTR. Steam methane reforming hydrogen production system was selected for the first demonstration by 2030. The other carbon-free hydrogen production method will be demonstrated, later.

Introduction of loop operating system to improve the stability of continuous hydrogen production for the thermochemical water-splitting iodine–sulfur process

The thermochemical water-splitting iodine–sulfur process facilitates hydrogen production. This study proposes a new loop operation by subdividing the process configuration into four sections before transferring the continuous operation. The loop operation should define the section affecting the fluctuations to easily stabilize the system. The proposed loop operation was validated by analyzing the material and heat balances using a process simulator. The calculated results showed that the material balances of the respective loop sections were closed without component discharge to outside sections. The loop operation would transfer to the continuous operation by connecting all sections. Considering the switching of operation modes, the material and heat balance showed no or little difference, indicating that two operation modes can only be changed by switching the pipelines. Thus, the loop sections can be operated individually to stabilize the IS process system, and the loop operation can be transferred smoothly to the continuous operation.

Hydrogen production using thermochemical water-splitting Iodine–Sulfur process test facility made of industrial structural materials: Engineering solutions to prevent iodine precipitation

A thermochemical water-splitting iodine–sulfur process offers the potential for mass-producing hydrogen at high-efficiency levels, and it uses high-temperature heat sources, including high-temperature gas-cooled reactors, solar heat, and waste heat of industries. The raw material (H2O) is split into H2 and O2 by combining three chemical reactions using sulfur and iodine. Currently, R&D tasks are essential to confirm the integrity of the components that are made of practical structural materials and the stability of hydrogen production in harsh working conditions. A test facility for producing hydrogen was constructed from corrosion-resistant components that are developed using industrial materials. In addition, for stable hydrogen production, technical issues for instrumental improvements (i.e., stable pumping of the hydrogen iodide (HI)–I2–H2O solution without locking the shaft seal, prevention of leakage by improving the quality control of glass-lined steel, prevention of I2 precipitation using a water removal technique in a Bunsen reactor) were solved. The entire process was successfully operated for 150 h at the rate of ca. 30 L/h. The integrity of components made of practical structural materials and the operational stability of the hydrogen production facility in harsh working conditions were demonstrated.

Study of container using hybrid technique for sulfuric acid decomposition of thermochemical water-splitting iodine-sulfur process

Study of container using hybrid technique for sulfuric acid decomposition of thermochemical water-splitting iodine-sulfur process

Preparation and characterization of bimetallic Ni–Ir/C catalysts for HI decomposition in the thermochemical water-splitting iodine–sulfur process for hydrogen production

A series of bimetallic 10%Ni-xIr/C (x = 0.5, 1.0, 1.5, 2.0 wt%) and monometallic 10%Ni/C and 2%Ir/C catalysts were prepared through the impregnation–reduction method modified by adding the ionic surfactant hexadecyltrimethylammonium bromide (CTAB) as the stabilizing agent during the impregnation. Their catalytic performance was tested by HI decomposition under atmospheric pressure at 400 °C and 500 °C. X-ray diffraction, Brunauer–Emmett–Teller surface area, transmission electron microscopy, and X-ray photoelectron spectroscopy were adopted to characterize the structure, specific surface area, morphology, and surface chemical state, respectively. Results showed that the addition of Ir metal and the use of CTAB played important roles in enhancing the activity and stability of the Ni-based bimetallic catalysts. Among all the catalysts tested, the bimetallic 10%Ni-1.5%Ir/C catalyst presented excellent activity and stability for HI decomposition.

Effect of HIx solution concentration on ion-exchange membrane performance in electro-electrodialysis

Electro-electrodialysis (EED) has been applied to the thermochemical water-splitting iodine-sulfur process for hydrogen production to enhance the hydrogen iodide (HI) component in the HI–I2–H2O mixture (HIx solution). In this work, the HIx solution concentration dependence of conductivity and transport number as the membrane-performance indexes of Nafion 212 and a styrene-grafted poly(ethylene-co-tetrafluoroethylene) (ETFE) membrane was investigated experimentally. From a theoretical analysis of measurements based on the new EED model, the effects of the H+ and I− contents and the diffusion coefficients of H+ and I− in the evaluated membranes on conductivity and transport number were as follows: HIx solution concentration dependence of the H+ and I− contents exhibited a significantly small or different variation in comparison with that of conductivity and transport number, indicating that the dominant factors for conductivity and transport number would not be the H+ and I− contents; thereby, the effect of the diffusion coefficients of H+ and I− should not be ruled out. In terms of the conductivity of Nafion 212, the diffusion coefficient of H+ is the dominant factor because the existence of I− was negligible, whereas in the case of the ETFE-St membrane, the effect of the diffusion coefficient of I− should be considered. The diffusion coefficient of I− could more strongly affect transport number for the evaluated membranes rather than the diffusion coefficient of H+, indicating that the H+ selectivity of the membranes could be determined relatively based on variations of I− behavior in the membranes.

Module design of silica membrane reactor for hydrogen production via thermochemical IS process

The potential of the silica membrane reactors for use in the decomposition of hydrogen iodide (HI) was investigated by simulation with the aim of producing CO2-free hydrogen via the thermochemical water-splitting iodine-sulfur process. Simulation model validation was done using the data derived from an experimental membrane reactor. The simulated results showed good agreement with the experimental findings. The important process parameters determining the performance of the membrane reactor used for HI decomposition, namely, reaction temperature, total pressures on both the feed side and the permeate side, and HI feed flow rate were investigated theoritically by means of a simulation. It was found that the conversion of HI decomposition can be improved by up to four times (80%) or greater than the equilibrium conversion (20%) at 400 °C by employing a membrane reactor equipped with a tubular silica membrane. The features to design the membrane reactor module for HI decomposition of the thermochemical iodine-sulfur process were discussed under a wide range of operation conditions by evaluating the relationship between HI conversion and number of membrane tubes.

Corrosion Resistance of Nickel-Based Alloy to Gaseous Hydrogen Iodide Decomposition Environment in Thermochemical Water-Splitting Iodine-Sulfur Process

Corrosion Resistance of Nickel-Based Alloy to Gaseous Hydrogen Iodide Decomposition Environment in Thermochemical Water-Splitting Iodine-Sulfur Process

Hydrogen production tests by hydrogen iodide decomposition membrane reactor equipped with silica-based ceramics membrane

The decomposition of hydrogen iodide in the thermochemical water splitting iodine–sulfur process at an intermediate temperature (400 °C) using a catalytic membrane reactor was reported here, for the first time. The performance of a catalytic membrane reactor based on a hexyltrimethoxysilane-derived silica membranes (H2 permeance of 9.4 × 10−7 mol Pa−1 m−2 s−1 and H2/N2 selectivity of over 80.0.) was evaluated at 400 °C by varying the HI flow rates of 2.6, 4.7, 6.9, 8.4, and 9.7 mL min−1. The silica membranes were prepared by counter-diffusion chemical vapor deposition method on γ-alumina-coated α-alumina tubes. Hydrogen was successfully extracted from the membrane reactor using the silica membrane at 400 °C. A significant increase in HI conversion was achieved. The conversion achieved at an HI flow rate of 2.6 mL min−1 was approximately 0.60, which was greater than the equilibrium conversion in HI decomposition (0.22).

Preparation of an H2-permselective silica membrane for the separation of H2 from the hydrogen iodide decomposition reaction in the iodine–sulfur process

A high-performance, H2-permselective silica membrane derived from hexyltrimethoxysilane (HTMOS) was developed for application in the thermochemical water-splitting iodine–sulfur process. Silica membranes, referred to here as HTMOS membranes, were prepared via counter-diffusion chemical vapor deposition on γ-alumina-coated α-alumina support tubes with outer diameters of 10 mm. Special attention was devoted to obtain high H2/HI selectivity, high H2 permeance, and good stability in the presence of corrosive HI gas. The effects of the deposition conditions, temperature, and period were investigated. The HTMOS membrane prepared at 450 °C for 5 min exhibited high H2/HI selectivity (>175) with H2 permeance on the order of 10−7 mol Pa−1 m−2 s−1. On the basis of stability experiments, it was found that the HTMOS membrane was stable upon HI exposure at a temperature of 400 °C for 11 h.

INET's catalysts study for HI decomposition in the thermochemical water-splitting iodine–sulfur process for hydrogen production

The iodine–sulfur thermochemical cycle (IS cycle) can be applied to realize water decomposition and hydrogen production by using the solar energy or the nuclear heat from High Temperature Gas-cooled Reactor as heat source. Among the three reactions of the IS process (Bunsen reaction, H2SO4 decomposition and HI decomposition), the HI decomposition has the slowest reaction rate and the lowest thermodynamic equilibrium conversion (about 23% at 500 °C). In order to make HI decompose at the workable reaction rate, catalysts had to be applied in the reaction system. This paper outlines our study on the monometallic and bimetallic catalysts for HI decomposition at INET (Institute of Nuclear and New Energy Technology, Tsinghua University, China). The influences of different supports, transition metals, bimetallic compositions, reaction time on the catalytic performance for HI decomposition were investigated. The optimized catalyst of 2.5 wt.%Pt–2.5 wt.%Ir/C was proposed and adopted to catalyze the HI decomposition in the close-cycle operation of IS-100 facility with the H2 production rate of 100 NL/h. Both the conversion of HI (21%) and the H2 production rate (60 NL/h) reached the technical targets of the nuclear hydrogen project (National S&T Major Projects 2010zx06901: Research on key technologies of hydrogen production from high temperature gas cooled reactor).

Characteristics of hybrid tube with Fe-high Si alloy lining by centrifugal casting for thermochemical water-splitting iodine-sulfur process

Characteristics of hybrid tube with Fe-high Si alloy lining by centrifugal casting for thermochemical water-splitting iodine-sulfur process

Safety design consideration for HTGR coupling with hydrogen production plant

Safety requirements and design considerations are examined for a nuclear hydrogen production system that consists of High Temperature Gas-cooled Reactor (HTGR) and a hydrogen production plant by thermochemical water splitting iodine–sulfur process (IS process). Requirements in order to construct hydrogen production plants under conventional chemical plant regulation are identified in order to take into account a fundamental difference in safety philosophy between the nuclear facility and chemical plant and meet requests from the potential users of nuclear heat. In addition, safety requirements for the collocation of the nuclear facility and hydrogen production plant utilizing IS process (IS plant) are investigated. Furthermore, design considerations to comply with the requirements are suggested and the technical feasibility of the design considerations is evaluated. The evaluation results for a reference plant showed that safe distance determined by the chemical plant regulation against combustible gas and hazardous chemical leakages comply with the plant layout design. Furthermore, the results demonstrated the feasibility of IS plant construction under non-nuclear regulation by showing that the tritium concentration in IS plant can be maintained below the regulation limit and reactor normal operation can be achieved during abnormal conditions in the IS plant. These results clarified that design considerations suggested for coupling the IS plant to HTGR are reasonably practicable. The proposed criteria can be used not only for coupling hydrogen production plants but also for other chemical plants such as steam reforming plants, etc.

Effect of sulfuric acid on electro-electrodialysis of HIx solution

The effect of sulfuric acid on the concentration of HIx solution by electro-electrodialysis (EED) was examined for the thermochemical water-splitting iodine–sulfur process. Presence of sulfuric acid in the anolyte HIx solution did not affect the concentration behavior. However, sulfuric acid in the catholyte solution caused side reaction(s) producing whitish precipitates, which indicates that the sulfur compound should be removed prior to the EED operation.

Corrosion resistance of structural materials in high-temperature aqueous sulfuric acids in thermochemical water-splitting iodine–sulfur process

Very harsh environments exist in the iodine–sulfur process for hydrogen production. Structural materials for sulfuric acid vaporizers and concentrators are exposed to high-temperature corrosive environments. Immersion tests were carried out to evaluate the corrosion resistance of ceramics and to evaluate corrosion-resistant metals exposed to environments of aqueous sulfuric acids at temperatures of 320, 380, and 460 °C, and pressure of 2 MPa. The aqueous sulfuric acid concentrations for the temperatures were 75, 85, and 95 wt%, respectively. Ceramic specimens of silicon carbides (SiC), silicon-impregnated silicon carbides (Si–SiC), and silicon nitrides (Si3N4) showed excellent corrosion resistance from weight loss measurements after exposure to 75, 85, and 95 wt% sulfuric acid. High-silicon irons with silicon content of 20 wt% showed a fair measure of corrosion resistance. However, evidence of crack formation was detected via microscopy. Silicon enriched steels severely suffered from uniform corrosion with a corrosion rate in 95 wt% sulfuric acid of approximately 1 g m−2 h−1. Among the tested materials, the ceramics SiC, Si–SiC, and Si3N4 were found to be suitable candidates for structural materials in direct contact with the considered environments.

HIx concentration by electro-electrodialysis using stacked cells for thermochemical water-splitting IS process

In the thermochemical water-splitting iodine–sulfur process for hydrogen production, efficient concentration/separation of HI from HIx solution, a mixture of HI–H2O–I2, is very important. In this paper, an experimental study on concentrating HI in HIx using stacked electro-electrodialysis (EED) cells was carried out under the conditions of 1atm, 80 °C and the current density of 0.10 A/cm2. The performance of EED stacks including 1, 2 and 4 EED units was evaluated. The results showed that multi-unit EED cells could concentrate HI in catholyte much faster than single-unit cells. The apparent transport number (t+) of all the experiments were very close to 1, while the ratio of permeated quantities of water to H+ (β) changed in a relatively larger range of 1.98–2.89. Although the current efficiency will degrade faster when using a multi-unit stack than a single-unit cell at the late stage of EED process, at high iodine content multi-unit stack could maintain quite high current efficiency.

熱化学水素製造法ISプロセスに用いる装置用金属材料の気相HI分解環境に対する適用性

熱化学水素製造法ISプロセスに用いる装置用金属材料の気相HI分解環境に対する適用性

Effect of temperature on electro-electrodialysis of HI–I2–H2O mixture using ion exchange membranes

Abstract In the thermochemical water-splitting iodine–sulfur process for hydrogen production, an electromembrane process (electro-electrodialysis) was applied to increase the HI molality of a HI–I 2 –H 2 O mixture. In order to investigate the temperature dependence of the membrane performance for increasing the HI concentration, a theoretical formula for the performance indexes (proton transport number, initial cell voltage, and ratio of water permeation to proton permeation in the membrane) was derived on the basis of the Nernst–Planck equation and electrophoresis theory. The obtained formula could reproduce the experimental values of the grafted membranes and Nafion 212 in the temperature range 313–373 K and was validated. The temperature dependence of the performance indexes was then clarified as follows: (1) The initial cell voltage decreased with temperature upon increasing the diffusion coefficient of H + and (2) the proton transport number decreased with temperature because the diffusion coefficient of I − increased more markedly than that of H + . For the ratio of water permeation to proton permeation in the membrane, we find the optimum regression parameters for achieving low water permeation, and controlling the ion exchange capacity might help in optimizing the membrane structure so that water permeation can be minimized.

Equilibrium potential across cation exchange membrane in HI–I 2–H 2O solution

The equilibrium potential across Nafion 117 in a HIx solution (HI–I 2–H 2O mixture) was measured for an electro-membrane process (electro-electrodialysis, EED) in the thermochemical water-splitting iodine–sulfur process. The experimental results were correlated with a theoretical formula derived on the basis of the electrochemical reaction, the Donnan equilibrium and the Nernst–Planck equation. The obtained empirical formula could reproduce the experimental equilibrium potential within ±0.03 V. Also, the obtained regression parameter agreed well with that calculated from the vapor liquid equilibrium data of a HIx solution reported by Hodotsuka et al. (2008) [3].

Electro-electrodialysis of HI–I 2–H 2O mixture using radiation-grafted polymer electrolyte membranes

In the thermochemical water-splitting iodine–sulfur process for hydrogen production, new polymer electrolyte membranes were applied in an electro-membrane process (electro-electrodialysis, EED) to increase the HI molality of HIx solution (HI + I 2 + H 2O mixture) to be over quasi-azeotropic. Radiation grafting of a styrene monomer into a poly(ethylene- co-tetrafluoroethylene) base film and subsequent sulfonation provided electrolyte membranes that had ion exchange capacities (IECs) of 1.1–1.6 mmol/g. With the EED of the HIx solutions using [HI] = [I 2] = 10 mol/kg at 40 °C the transport number of protons, ratio of permeated quantities of water to the protons, and current efficiency all appeared to depend on the IEC of the resulting membranes. When compared to Nafion, the self-made membranes exhibited lower electric cell resistance, and thereby decreasing up to 32% of the overall energy required in the concentration operation.