Sulfur Iodine Research Articles

Stability of nickel catalyst supported by mesoporous alumina for hydrogen iodide decomposition and hybrid decomposer development in sulfur–iodine hydrogen production cycle

We propose a hybrid HI decomposer, which combines a high-temperature catalytic decomposition reactor with a dual bed temperature-swing decomposer. For the high temperature step, we screened and identified a catalyst that is stable above 700 °C by preparing nickel catalysts supported on mesoporous alumina and evaluating their activity toward the high-temperature catalytic decomposition reaction. The catalysts achieved HI decomposition yields up to 23% at 650 °C and maintained over 20% yield after 100 h of operation. For the temperature-swing process, we investigated the adsorption, desorption, and regeneration efficiency, and the optimal regeneration temperature of nickel catalysts supported on silica/alumina adsorbents. The optimal regeneration temperature was 400 °C. Based on these results, we propose hybrid HI decomposers in two configurations: 1) residual HI decomposer and 2) HI concentrator. The residual HI decomposer improves the overall conversion efficiency to levels above the thermodynamic limit, while the HI concentrator increases the concentration of HI by simple temperature control, even at below-azeotropic HI concentrations.

Affinity of ion-exchange membranes for HI–I2–H2O mixture

Electro-electrodialysis (EED) has been applied to thermochemical water-splitting hydrogen production iodine–sulfur process in order to enhance HI in a HI–I2–H2O mixture (HIx solution). In this paper, in order to understand the behavior of the components (I2, H+, I−, and H2O) of the HIx solution in the membranes on absorption equilibrium, each component absorbed in a styrene-grafted poly-(ethylene-co-tetrafluoroethylene) (ETFE-St) membrane and Nafion 212 were evaluated. The behavior of each component in the membrane was as follows. The I2 content in the ETFE-St membrane increased with an increase in the I2 concentration of the immersed HIx solution, whereas I2 absorption by Nafion 212 was minimal. The strong affinity of the ETFE-St membrane for I2 was attributed to the formation of a charge-transfer complex with electron-donating aromatic groups. The ETFE-St membrane absorbed I− (also H+ under the neutral condition) in a HI-molality-dependent manner, whereas I− (H+) absorption by Nafion 212 was negligible. This difference was attributed to the I3−-complex formation in the ETFE-St membrane, which was precluded in Nafion 212. The absorption of H2O was greater for the ETFE-St membrane than for Nafion 212. Incorporation into Nafion 212 would not greatly stabilize H2O while stabilizing H+ by a strong electrical attraction. These results accordingly clarified the affinity of the membranes for the HIx solution based on the excess Gibbs free energy calculated from the measured composition data and the estimated activity coefficients. The excess Gibbs free energy for I2 was larger in the membranes than in the immersed solution, indicating a destabilization of I2 in the membranes. Importantly, this difference agreed with the energy change estimated under the assumption that I2 could form I3− or the charge-transfer complex. The affinity for I2 as well as between the sulfonic acid groups and H+ or H2O is a factor determining the absorbability of the HIx solution.

Boiling heat transfer characteristics of a sulfuric-acid flow in thermochemical iodine–sulfur cycle

The Japan Atomic Energy Agency has been conducting research and development on the thermo-chemical iodine–sulfur (IS) process, which is one of the most attractive water-splitting hydrogen production methods that uses nuclear thermal energy. The sulfuric acid decomposer is one of the key components of the IS process. The boiling heat transfer coefficients of sulfuric acid solutions are required to design the sulfuric acid decomposer. These coefficients were measured in aqueous solutions where the mole fraction of H2O ranged from 0.17 to 0.37 (heat flux range from 16.9kW/m2 to 5.6kW/m2) and compared with the empirical correlations formulated for binary mixtures. A combination of the Stephan–Körner correlation, using the empirical constant A0=2.00, and the Nishikawa–Fujita correlation was used to predict the experimental results with an accuracy of ±10%.

Kinetics and modeling of hydrogen iodide decomposition for a bench-scale sulfur–iodine cycle

In this work, the decomposition of hydrogen iodide (HI) over platinum catalyst in a frame work of the development of a bench-scale Sulfur–Iodine (S–I) cycle is studied. The catalyst Pt/γ-alumina 1.0wt% is prepared by impregnation–calcination method. The experiments of HI decomposition over the as-prepared catalyst are conducted at the temperature range of 350–550°C and at the atmospheric pressure. The experimental data are then used to estimate new kinetic parameters for HI decomposition on the basis of Langmuir–Hinshelwood type where the surface reaction is considered as the rate-limiting step. The kinetics with the estimated parameters shows a reasonable agreement with the experimental data. It also reflects the fact that, HI conversion is significantly decreased with a small amount of iodine present in the feeding solution.Thereafter, the kinetic model is applied to the modeling of a HI decomposer for the hydrogen production rate of 1Nm3/h in which hot helium gas is used to provide heat for the decomposition. Effects of heat-exchanger reactor configuration and composition of the feeding solution on the reactor size and the heat consumed are examined using the proposed model. Calculation results show that heat consumed for the co-current configuration is less than that for the counter-current configuration of the reactor. I2 impurity and high water content in the feeding solution also result in an increase of reactor size and the heat required.

Process flow sheet evaluation of a nuclear hydrogen steelmaking plant applying very high temperature reactors for efficient steel production with less CO2 emissions

Recently, CO2 reduction is an important problem for steelmaking. Substitution of coal, presently used as a reducing agent of iron ore in blast furnaces, to hydrogen produced by non-fossil energy is a way to reduce CO2 emissions. In this study, the idea of nuclear hydrogen steelmaking (NHS) system was investigated using very high temperature reactor (VHTR) and thermochemical hydrogen production iodine–sulfur (IS) process. Heat input and CO2 emissions including material production, material transportation, and electricity generation were evaluation criteria. Results of the NHS system were compared with those of a conventional blast furnace steelmaking (BFS) system. Influence of heat input options to the steelmaking process and hydrogen production thermal efficiency of IS process were investigated for the NHS system. Though heat input to the NHS system was 130–142% of that to the BFS system, CO2 emissions of the system were 13–21%. Pre-heating of hydrogen by coal combustion before blowing to a shaft furnace was effective to decrease heat input, although CO2 emissions increased. Direct pre-heating by nuclear heat was also effective without increase of CO2 emissions if close location of the nuclear reactor to the steelmaking plant was publicly accepted. Hydrogen production thermal efficiency had a significant influence on the heat input. Conceptual design of a plant unit of the NHS system producing steel of 1.47×106t/y with 2 VHTRs of 600MW heat and a shaft furnace and an electric arc furnace was proposed. The steelmaking scale was a little smaller than those of the recent largest Midrex® plants.

Components development for sulfuric acid processing in the IS process

The Japan Atomic Energy Agency (JAEA) has been conducting research and development on a thermochemical iodine–sulfur (IS) process, one of most attractive water-splitting hydrogen production methods, that uses the nuclear heat of a high-temperature gas-cooled reactor (HTGR). An examination is planned to verify the integrity of the components in the sulfuric acid decomposition section. A bayonet-type sulfuric acid decomposer made of SiC ceramics, a key component in the section, was test-fabricated. In parallel, a direct-contact heat exchanger (DCHX) is contemplated for use in the sulfuric acid decomposition section to simplify the process. The application of the DCHX makes it possible to perform heat recovery and separate undecomposed sulfuric acid from the decomposed gaseous mixture in one reactor. Although the concept is very attractive, little is known about the heat and mass transfer behavior in the DCHX. Therefore, a test apparatus was constructed to measure the gas-phase mass transfer coefficients required for the optimal design of the DCHX. These coefficients of water were acquired and compared with an empirical correlation. The experimental data were in good agreement with those obtained from empirical correlation, and thus, the apparatus was confirmed to be reasonable.

Membrane performance on electro-electrodialysis of HI–I2–H2O mixture for IS process

Research and development of a water-splitting hydrogen-production method, called the iodine–sulfur (IS) process, has been conducted as one of the heat applications of high temperature gas-cooled reactor. Among the unit operations in this IS process, we have investigated electro-electrodialysis (EED) using an ion-exchange membrane to increase a hydrogen iodide (HI) concentration in an HI–I2–H2O mixture. With the aim of maximizing EED performance, new membrane materials were prepared by the radiation-induced graft polymerization and were examined in terms of their proton permeability through the membrane, i.e., transport number, t+ and conductivity, σ at different I2 concentrations in the HI–I2–H2O mixture. t+ increased and σ decreased with an increase in the feed I2 molality. The EED model derived by the Nernst–Planck theory suggested that this trend could be explained exclusively by the variation of the diffusion coefficient of I−.

Development of strength evaluation method for high-pressure ceramic components

Japan Atomic Energy Agency is conducting R&D on nuclear hydrogen production by the Iodine-Sulfur (IS) process. Since highly corrosive materials such as sulfuric and hydriodic acids are used in the IS process, it is very important to develop components made of corrosion resistant materials. Therefore, we have been developing a sulfuric acid decomposer made of a ceramic material, that is, silicon carbide (SiC), which shows excellent corrosion resistance to sulfuric acid. One of the key technological challenges for the practical use of a ceramic sulfuric acid decomposer made of SiC is to be licensed in accordance with the High Pressure Gas Safety Act for high-pressure operations of the IS process. Since the strength of a ceramic material depends on its geometric form, etc., the strength evaluation method required for a pressure design is not established. Therefore, we propose a novel strength evaluation method for SiC structures based on the effective volume theory in order to extend the range of application of the effective volume. We also developed a design method for ceramic apparatus with the strength evaluation method in order to obtain a license in accordance with the High Pressure Gas Safety Act. In this paper, the minimum strength of SiC components was calculated by Monte Carlo simulation, and the minimum strength evaluation method of SiC components was developed by using the results of simulation. The method was confirmed by fracture test of tube model and reference data.

Concentration of HI in Iodine–Sulfur cycle using EED stack

Concentration and separation of HI is very important for the efficient hydrogen production of Iodine–Sulfur thermochemical cycle, because of the presence of HI–H2O azeotropy at HI:H2O=∼1:5 (molar ratio). Electro-electrodialysis (EED) has been proved to be very promising in concentrating HI, while the researches on stacked EED cells, which are essential for large scale application of EED process, has not been reported. In this study, EED stacks including 1, 2 and 4 cell units (50mm×50mm effective membrane area) were designed and applied for the concentration of HI in HIx solution (HI–H2O–I2 mixture), using Nafion membrane and graphite electrodes. At 60°C and 0.10A/cm2, the performance of the stacks was evaluated, and the effects of initial composition of HIx and EED unit quantity on the HI concentrating capacity and cell voltage were discussed.

Overview of the development of catalysts for HI decomposition in the iodine–sulfur thermochemical cycle at INET

The decomposition of HI which is the key reaction to produce hydrogen in the iodine–sulfur thermochemical cycle takes place hardly without the catalysts even at 500°C. Therefore, the catalysts play very important role during the HI decomposition. At INET, fundamental studies on the HI catalytic decomposition were initiated in 2006. This paper summarized our present research advances on the development of HI decomposition catalysts containing different supports (carbon nanotubes, active carbon, carbon molecular sieve, graphite and γ-Al2O3), supported monometallic (Pt, Ir and Ru) and bimetallic (Pt–Ir), and the preparation methods including impregnation-H2 reduction, impregnation–calcination, impregnation-hydrazine reduction, and electroless plating. In the course of the study, the physical properties, structure, and morphology of the catalysts were characterized by specific surface area (BET), X-ray diffractometer (XRD), and transmission electron microscopy (TEM), respectively. The catalytic activity for HI decomposition was investigated in a fixed bed reactor under atmospheric pressure. In addition, choice of catalysts for the future IS cycle was discussed.

Numerical Modeling of Bayonet-Type Heat Exchanger and Decomposer for the Decomposition of Sulfuric Acid to Sulfur Dioxide

The growth of global energy demand during the 21st century, combined with the necessity to master greenhouse gas emissions, has led to the introduction of a new and universal energy carrier: hydrogen. The Department of Energy (DOE) proposed using a bayonet-type heat exchanger as a silicon carbide integrated decomposer (SID) to produce the sulfuric acid decomposition product sulfur dioxide, which can be used for hydrogen production within a sulfur–iodine thermochemical cycle. A two-dimensional computational model of SID having a boiler, superheater and decomposer was developed using GAMBIT and fluid. The thermal and chemical reaction analyses were carried out in FLUENT. The main purpose of this study is to obtain the decomposition percentage of sulfur trioxide for the integrated unit. Sulfuric acid (H2SO4), sulfur trioxide (SO3), sulfur dioxide (SO2), oxygen (O2), and water vapor (H2O) are the working fluids used in the model. Concentrated sulfuric acid liquid of 40 mol% was pumped into the inlet of the boiler and the mass fraction of concentrated sulfuric acid vapor obtained was then fed into the superheater to obtain sulfur trioxide. The decomposer region, which houses the pellets, placed on the top of the bayonet heat exchanger acts as the porous medium. As the decomposition takes place, the mass fraction of SO3 is reduced and mass fractions of SO2 and O2 are increased. The percentage of SO3 obtained from the integrated decomposer was compared with the experimental results obtained from Sandia National Laboratories (SNL). Further, effects of various pressures, flow rates, and acid concentrations on the decomposition percentage of sulfur trioxide were studied.

Phase separation characteristics of the Bunsen reaction when using HIx solution (HI–I2–H2O) in the sulfur–iodine hydrogen production process

To continuously operate an integrated sulfur–iodine (SI) hydrogen production process, the HIx solution (HI–I2–H2O) could be recycled from the HI decomposition section as a reactant in the Bunsen reaction section. In this study, the temperature, iodine content and water content were varied to identify the phase separation characteristics of products from the Bunsen reaction using the HIx solution with SO2. Increasing the temperature increased the volume of the H2SO4 phase solution and decreased the impurity content in each phase. Increasing the iodine feed concentration somewhat decreased the volume of the H2SO4 phase solution, although the density difference between the phases increased. The amount of H2SO4 that separated into the H2SO4 phase was very small under most of these conditions, which significantly hindered the continuous operation of the integrated SI process. The feed of additional water in the separation step was suggested to improve the separation performance of the H2SO4 phase solution while minimizing side reactions.

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.

Exergy analysis of a simulation of the sulfuric acid decomposition process of the SI cycle for nuclear hydrogen production

This article details comprehensive energy and exergy analyses of the sulfuric acid decomposition process of the sulfur–iodine (SI) thermochemical cycle for hydrogen production. Energy and exergy efficiencies of the proposed process were evaluated over a variety of reaction temperatures and pressures. At an atmospheric temperature of 25 °C, the calculated values of exergy destruction of the H2SO4 decomposer ranged between 157 kJ/mol and 360 kJ/mol over reaction temperatures of 800–1000 °C and pressures between 1 and 50 atm. It was shown that the exergy efficiency of the H2SO4 decomposer improved with an increase in reaction temperature, while reaction pressure had a negative effect on exergy efficiency.

A simple thermo-physical model for vapor–liquid equilibrium of the reactive HIx (HI–H2O–I2–H2) system

Thermodynamic phase equilibrium of the reactive HIx system encountered in the sulfur-iodine (SI) cycle has been identified as a basic research need for the hydrogen economy. In this communication, a simple NRTL (nonrandom two-liquid)-based vapor–liquid equilibrium (VLE) model is devised for the HIx system considering the HI decomposition reaction in the gas phase only. We propose a new set of NRTL binary interaction parameters as well as a new expression for the HI decomposition chemical equilibrium constant as a function of temperature. This simple VLE model with the use of only one set of NRTL binary parameters is capable of reproducing the Larousse et al. (Int. J. Hydrogen Energy 34 (2009) 3258–3266) experimental data for a wide range of pressures (1–24bar) and HI concentrations beyond the azeotrope.

Influence of the initial HI on the multiphase Bunsen reaction in the sulfur–iodine thermochemical cycle

In the sulfur–iodine cycle flowsheet, HI may exist in the feeds of Bunsen reaction. The effects of the initial HI and the operating temperature on the kinetic process and thermodynamic equilibrium of the multiphase Bunsen reaction were investigated. Increasing initial HI concentration (HI/H2O = 0–1/18) or temperature (303 K–358 K) amplified the reaction kinetic rate, and led to the earlier appearance of liquid–liquid separation and less time to reach the thermodynamic equilibrium. But the separation became difficult for further increase of the initial HI content. The liquid–liquid equilibrium (LLE) phase separation was enhanced with rising temperature. An increase in the initial HI content slightly weakened the LLE phase separation at a lower temperature, while at 345 K and 358 K, the LLE phase separation characteristics showed little variation in the HI/H2O molar ratio range of 0–1/18. A hyper-azeotropic HI concentration in the HIx phase was obtained with feeding HI. The conversion of SO2 lowered as the initial HI content and the temperature increased.

An Investigation of the Performance of the HI Concentration By Elctrodialysis Cell for Hydrogen Production in Sulfur Iodine Process

not Available.

Influence of the structural and surface characteristics of activated carbon on the catalytic decomposition of hydrogen iodide in the sulfur–iodine cycle for hydrogen production

Coal-based activated carbon (AC-COAL) catalysts subjected to acid treatment were tested to evaluate their performance on hydrogen-iodide (HI) decomposition for hydrogen production in sulfur-iodine (SI or IS) cycle. The effects of acid treatment on catalysts and the relations between sample properties and catalytic activities were discussed. The AC-COAL obtained by non-oxidative acid treatments had the best catalytic activity. However, the catalytic activity of AC-COAL decreased after the treatment of nitric acid. Higher surface area, higher carbon contents, lower ash contents and fewer surface oxidation groups contributed to the catalytic activity of ACs. HI decomposition on the AC surface itself may be due to high densities of unpaired electrons associated with structural defects and edge plane sites with similar structural ordering. Moreover, the oxygen-containing groups reduced the electron transfer capability associated with the basal plane sites.

Energy consumption in iodine section of I–S: Effect of operating parameters of electro-electrodialysis on overall efficiency of cycle

Reducing heat demand for increasing concentration of HI in the HIx solution of the iodine circuit of the Iodine–Sulphur cycle is considered the most effective way of increasing efficiency of the cycle. Electro-electrodialysis has emerged as an energy efficient way of increasing the HIx concentration above azeotropic value. Simulation of the iodine circuit consisting of an EED, a flash and a decomposer was carried out in Aspen Plus™ simulation platform to study the effect of EED current density and outlet HI concentration on the efficiency of the cycle. Efficiency reduced strongly with increase in current density. For EED current density of 5 A/dm2, maximum efficiency was ∼35.9% and the optimal range of EED catholyte's exit HI concentration, iodine-free, mole fraction was 0.19–0.21. Simulation results showed that reducing EED resistance was most effective, among all EED parameters, in increasing the cycle's thermal efficiency and if the EED resistance is completely eliminated the thermal efficiency value would increase to 39.4%.

Study on Apparent Kinetics of the Reaction between Sulfuric and Hydriodic Acids in the Iodine-Sulfur Process

The iodine–sulfur (IS) thermochemical process for hydrogen production is one of the most promising approaches in using high-temperature process heat supplied by a nuclear reactor. This process includes three reactions that form a closed cycle: the Bunsen reaction, in which iodine, water, and sulfur dioxide react to form sulfuric acid and hydriodic acid (HI); HI decomposition; and sulfuric acid decomposition. However, the side reactions between H2SO4 and HI may disturb the operation of the IS closed cycle. For optimal process conditions, the reaction kinetics between H2SO4 and HI should be examined. In this work, a preliminary kinetic study was conducted. Using the initial reaction rate method, the kinetic parameters of the reaction between sulfuric acid and HI, such as the apparent reaction orders and rate constant were determined. For I−, the apparent reaction order was approximately 1.77, whereas the orders for H+ and SO42− were 7.78 and 1.29, respectively. The apparent rate constant at 85 ± 1°C was approximately 2.949 × 10−11 min−1 (mol/L)−9.84. The H+ concentration had more significant influence on the reaction rate than those of SO42− and I−. Such basic data provide useful information for related process design and further kinetics study.