Studies of the sulfur-iodine thermochemical water-splitting cycle

Literature thermodynamic values were experimentally confirmed for the Bunsen reaction producing H₂SO₄- and HI-rich phases. The sulfur-iodine water-splitting cycle, which uses the Bunsen reaction, has been improved by enriching the H₂SO₄ solution to 57% in a system involving the H₂SO₄ product and counter-current liquid I₂ flow. The system was saturated with SO₂. The decomposition of H₂SO₄ was investigated. Pt/SiO₂, Pt/ZrO₂, Pt/TiO₂ and Pt/BaSO₄ were all good catalysts for H₂SO₄ vapor decomposition to SO₂ at high temperatures. Pt/Al₂O₃ was found to fail due to substrate sulfation. The importance of pressure to sulfation temperature is presented. A summary of catalyst studies for H₂SO₄ vapor decomposition compares catalyst effectiveness.     

A study on the dynamic behavior of a sulfur trioxide decomposer for a nuclear hydrogen production

The sulfur–iodine thermochemical water-splitting cycle (S–I cycle) is one of the most promising technologies for mass H₂ production. The S–I cycle is generally divided into three sections, one of which involves a H₂SO₄ concentration and decomposition. In the sulfuric acid processing section (Section 2), H₂SO₄ is decomposed into H₂O and SO₃, and then the produced SO₃ is further decomposed into SO₂ and O₂, which takes place in a H₂SO₄ decomposer and a SO₃ decomposer, respectively. The SO₃ decomposition requires heat of a high temperature and this suggests a heat-exchanger type reactor. To understand the temperature profiles and chemical reactions through a SO₃ decomposer, a dynamic model was developed by considering the heat and material balances in partial differential forms. A model was used to size the decomposer to a proposed design basis and it was also applied to simulate the responses corresponding to the changes of the operation conditions such as increased or decreased flow rates.     

Continuous Bunsen reaction and simultaneous separation using a counter-current flow reactor for the sulfur–iodine hydrogen production process

The sulfur–iodine (SI) process, which consists of three chemical reactions of the Bunsen reaction, a H₂SO₄ decomposition and a HI decomposition, is an important potential method for hydrogen production among thermochemical water splitting methods. For steady-state operation of the SI process, it is very important to provide information on the composition of each phase that passes from the Bunsen reaction section to the following H₂SO₄ and HI decomposition sections. In this study, the Bunsen reaction was carried out using a counter-current flow reactor, the Bunsen reaction and product separation steps were shown capable of being performed simultaneously, and the composition variation of each phase discharged at the top and bottom of reactor was investigated. The process variables were the SO₂ feed rate, temperature, I₂/H₂O molar ratio. As a result of constant reactant feed and continuous product discharge operation, it was found that the composition remained constant after 120 min of reaction time, indicating steady-state operation. The phase separation characteristics of the Bunsen reaction were minimally affected by the SO₂ feed rate. As the amount of I₂ introduced increased with increasing temperature, the volume of the H₂SO₄ phase discharged from the upper phase was unchanged, while that of the HI_x phase discharged from the lower phase increased proportionally. The average molar composition of the H₂SO₄ phase (H₂SO₄/H₂O/HI) obtained at a typical operation condition (353 K, I₂/H₂O molar ratio of 0.406) was 1/5.30–5.39/0.02–0.04, and the composition of the HI_x phase (HI/I₂/H₂O/H₂SO₄) was 1/2.81–3.09/5.67–6.40/0.04–0.06. These results could be used for the design and operation of H₂SO₄ and HI decomposition sections of the SI process.     

Pyrolysis of sugarcane bagasse pretreated with sulfuric acid

Pyrolysis is a promising technique for the recovery of useful gas, tar, and solid products from biomass waste. However, the low tar yields obtained from lignocellulosic biomass are a significant drawback. To enhance tar yields, sugarcane bagasse, which is the most abundant agricultural waste in Fiji, was pretreated at ambient temperature and atmospheric pressure using various sulfuric acid (H₂SO₄) concentrations. Here, the ether bonds of cellulose, hemicellulose, and lignin were partially hydrolyzed. The pretreated samples were then pyrolyzed at 500 °C, and it was confirmed that H₂SO₄-pretreatment disrupted the bagasse cell structure, with the thermogravimetry and differential thermogravimetry results confirming that decomposition occurred at lower temperatures after pretreatment. In addition, tar yields were significantly enhanced from 5.6 wt% to 13.4 wt% for the untreated and 3 M H₂SO₄-pretreated samples respectively. The main components detected in this tar product were levoglucosan, andcellulose-and hemicellulose-derived products, whose proportions were increased following pretreatment. Thus, our work demonstrates that dilute acid pretreatment enhances tar production from sugarcane bagasse due to the production of shorter chain components via the partial hydrolysis of ether bonds.     

Preliminary results from bench-scale testing of a sulfur-iodine thermochemical water-splitting cycle

Portions of a bench-scale model of a sulfur-iodine thermochemical water-splitting cycle have been operated at General Atomic Company as part of a comprehensive program to demonstrate the technology for hydrogen production from non-fossil sources. The bench-scale model consists of three subunits which can be operated separately or together and is capable of producing as much as 4 l/min^?1 (6.7 × 10^?5m³s^?1) at standard conditions of gaseous hydrogen. One subunit (main solution reaction) reacts liquid water, liquid iodine (I₂) and gaseous sulfur dioxide (SO₂) to form two separable liquid phases: 50 wt % sulfuric acid (H₂SO₄) and a solution of iodine in hydroiodic acid (HI_x). Another subunit (H₂SO₄ concentration and decomposition) concentrates the H₂SO₄ phase to the azeotropic composition, then decomposes it at high temperature over a catalyst to form gaseous SO₂ and oxygen. The third subunit (HI separation and decomposition) separates the HI from water and I₂ by extractive distillation with phosphoric acid (H₃PO₄) and decomposes the HI in the vapor phase over a catalyst to form I₂ and product hydrogen. This paper presents the results of ongoing parametric studies to determine the operating characteristics, performance, and capacity limitations of major components.     

The utilization of ZnSO4 decomposition in thermochemical hydrogen cycles

We find that the ZnSO₄ decomposition reaction can be used as the high temperature step in a number of thermochemical cycles. It is especially applicable as a substitute for the H₂SO₄ decomposition step in H₂SO₄ based cycles, and significantly improves the efficiency of such cycles. In this study, we have taken an initial look at the effects of heat-up rate on the decomposition of ZnSO₄ as it is heated through an α-β transition at 1015 K. We find that a rapid heat-up of fine ZnSO₄ particulates through this transition leads to fracturing of the ZnSO₄ crystallites and significantly enhances its subsequent decomposition rate at ～1043 K. We also find evidence for an autocatalytic decomposition process in fine ZnSO₄ particulates with either rapid or slow heat-up rates. We believe that a combination of (1) fracturing of the crystallites and (2) surface catalysis by ZnO to equilibrate the SO₃/SO₂/O₂ gaseous products contributes to the observed autocatalytic behaviour.     

Energy and exergy analyses of a novel sulfur–iodine cycle assembled with HI–I2–H2O electrolysis for hydrogen production

A novel sulfur–iodine (SI or IS) cycle integrated with HI–I₂–H₂O electrolysis for hydrogen production was developed and thermodynamically analyzed in this work. HI–I₂–H₂O electrolysis was used to replace the conventional concentration, distillation, and decomposition processes of HI, so as to simplify the flowsheet of SI cycle. And then the new cycle was divided into Bunsen reaction, H₂SO₄ decomposition and HI–I₂–H₂O electrolysis sections. Through incorporating the user-defined module of HI–I₂–H₂O electrolysis with Aspen Plus, the cycle was simulated and 0.448 mol/h (10 L/h) of H₂ was produced. The overall energy and exergy efficiencies of the novel SI system were estimated to be 15.3–31.0% and 32.8%, respectively. Most exergy destruction occurred in the H₂SO₄ decomposer and condenser for H₂SO₄ decomposition and Bunsen reaction sections, which accounted for 93.0% and 63.4%, respectively. A high exergy efficiency of 92.4% for HI–I₂–H₂O electrolysis section with less exergy destruction was determined, mostly due to the transformation of the overall electricity in electrolytic cell to exergy. Appropriate internal heat exchange and waste heat recovery will favor improving the energy and exergy efficiencies.     

Current R&D status of thermochemical water splitting iodine-sulfur process in Japan Atomic Energy Agency

Current R&D on the thermochemical water splitting iodine-sulfur (IS) process in Japan Atomic Energy Agency (JAEA) is summarized. Reactors were fabricated with industrial materials and verified by test operations: a Bunsen reactor, a H₂SO₄ decomposer, and a HI decomposer. Component materials of the reactors were stable in the operation environment. Small amount of H₂SO₄ in the anolyte solution in an electro-electrodialysis (EED) cell had no negative impact on cell performance parameters. Relationship between cell solution composition and temperature and cell parameters was formulated by experimental data. Demonstration of the test facility with process design of 100 L/h hydrogen production is performed to verify integrity of process components and stability of hydrogen production. Tests of sections were first conducted individually to show material processing rates were controllable. Based on the result, an 8-h continuous operation of the total IS process was performed in February 2016 with H₂ production rate of 10 L/h. Demonstrations are planned for longer operation period and higher H₂ production rate after improvement of components to prevent troubles.     

H2SO4 poisoning of Ru-based and Ni-based catalysts for HI decomposition in SulfurIodine cycle for hydrogen production

The sulfur–iodine (SI) cycle is deemed to be one of the most promising alternative methods for large-scale hydrogen production by water splitting, free of CO₂ emissions. Decomposition of hydrogen iodide is a pivotal reaction that produces hydrogen. The homogeneous conversion of hydrogen iodide is only 2.2% even at 773 K [1]. A suitable catalyst should be selected to reduce the decomposition temperature of HI and attain reaction yields approaching to the thermodynamic equilibrium conversion. However, residual H₂SO₄ could not be avoided in the SI cycle because of incomplete purification. The H₂SO₄ present in the HI feeding stream may lead to the poisoning of HI decomposition catalysts. In this study, the activity and sulfur poisoning of Ru and Ni catalysts loaded on carbon and alumina, respectively, were investigated at 773 K. HI conversion efficiency markedly decreased from 21% to 10% with H₂SO₄ (3000 ppm) present, which was reversible when H₂SO₄ was withdrawn in the case of Ru/C. In the case of Ru/C and Ni/Al₂O₃, catalyst deactivation depends on the concentration of H₂SO₄; the higher the concentration of H₂SO_4, the greater the severity of deactivation. Catalysts before and after sulfur poisoning were characterized by transmission electron microscopy (TEM), energy-dispersive X-Ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Experimental results and characterization of poisoned and fresh catalysts indicate that the catalyst deactivation could be ascribed to the competitive adsorption of sulfur species and change in its surface properties.     

The evolution characteristic of sulfur-containing gases during coal thermal conversion

In this study, the evolution characteristics of sulfur-containing gases during thermal conversion of two coals under different atmospheres were studied through temperature-program decomposition (TPD) and rapid-heating decomposition (RHD) coupled with online mass spectrum (MS). The releasing profiles of H₂, CH₄ and CO were also measured. Results showed that the effect of atmosphere and heating rate on evolution of sulfur-containing gases was very significant. It was found that Ar atmosphere was more favorable to the formation of sulfur-containing gases than CO₂ atmosphere by using TPD-MS. In CO₂, the formation of H₂S and SO₂ was restrained in 260–650 °C, but was promoted in 880–980 °C; the formation of COS was promoted during the whole process. In Ar, high releasing intensity of H₂ and CH₄ could stabilize sulfur-containing radicals which led to high amount of H₂S and SO₂; while high releasing intensity of CO in CO₂ resulted in high amount of COS. By using RHD-MS, it was found that the steam atmosphere was highly favorable for the transformation of H₂S, SO₂ and COS during the entire reaction period. However, the CO₂ atmosphere was disadvantageous to the transformation of H₂S, SO₂ and COS at the initial stage, but slight favorable for the transformation of H₂S, SO₂ and COS during the later stage. These was resulted from the gasification reaction of steam/CO₂ with coal. The key factor was the releasing amount of H₂ and CO, which promoted the formation and transformation of H₂S, SO₂ and COS.     

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 HI_x 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 HI_x 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.     

Preparation and surface modification of anhydrous calcium sulfate whiskers from FGD gypsum in autoclave-free hydrothermal system

The anhydrous calcium sulfate (AH CaSo₄) whiskers were prepared in H₂O–H₂SO₄ autoclave-free hydrothermal system by using flue gas desulfurization gypsum as raw materials. The influences of solid/solution ratio, H₂SO₄ concentration, and surface modification agent (stearic acid) on AH whiskers growth were detected. The whiskers, with a smooth surface and high aspect ratio, were obtained in a 20 mass% H₂SO₄ solution for hydrothermal 1 h with 1:60 ratio of solid/solution. The whiskers diameter hardly changed, but the amount increased in the sample by adding stearic acid. The whiskers were not obtained in 10 mass% H₂SO₄ solution for hydrothermal 1 h with 1:60 ratio of solid/solution. Only a little of whiskers were obtained in 30mass% H₂SO₄ solution at the same hydrothermal condition.     

The effect of acid treatment on pyrolysis of Longkou oil shale

The effect of acid treatment on mineral removal and pyrolysis of Longkou oil shale were investigated. X-ray diffraction (XRD) and X-ray fluorescence (XRF) indicated that the HCl treatment can remove the calcite, the H₂SO₄ treatment can convert the calcite to CaSO₄, and the HF treatment can remove the quartz and convert the calcite to CaF₂; moreover, all three treatments cannot remove the pyrite in the oil shale. Oil shale was individually treated with HCl, H₂SO₄, and HF before conducted the pyrolysis experiment. The pyrolysis results showed that oil shale treated with H₂SO₄ or HF almost equally enhanced the oil yield, while HCl treatment had a negative effect on the oil yield. Thermogravimetry (TG) analysis indicated that the carbonates had a catalytic effect, sulfates may also had a catalytic effect and the silicates had an inhibitive effect on the decomposition of kerogen. Combining the TG analysis, oil yield and the price of every acid, the H₂SO₄ treatment was considered to be the best method to treat oil shale.Moreover, the carbonate minerals can be removed after H₂SO₄ treatment, so it would reduce the amount of pyrolysis feed to increase production efficiency.     

Catalyzed thermal decompositon of H2SO4 and production of HBr by the reaction of SO2 with Br2 and H2O

Decomposition of H₂SO₄ and production of HBr have been studied as a part of the research and development of the thermochemical hydrogen production from water. The catalytic activities of various metals and metal oxides on a porous alumina support were studied for the thermal decomposition of sulfuric acid in a fixed bed reactor. A Pt-Al₂O₃ catalyst gave a conversion of SO₃ close to the equilibrium at temperatures from 1073 to 1173 K and at a space velocity below 10 000 h^?1. It was also found that metal oxide catalysts such as CuO and Fe₂O₃ were as active as the Pt catalyst. To prepare SO₂-free HBr gas by the reaction between SO₂, Br₂ and H₂O, vapor-liquid equilibrium determinations for SO₂/Br₂/HBr/H₂SO₄/H₂O system were carried out at 298 K under atmospheric pressure. The unconverted SO₂ can be effectively removed by contacting the effluent gases with a HBr saturated aqueous solution containing an excess of Br₂.     

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.     

Simulation and experimental study on the sulfuric acid decomposition process of SI cycle for hydrogen production

This work is concerned with customization of the existing electrolyte thermodynamic model for process simulation and experimental studies on the sulfuric acid decomposition process of the SI cycle, using a commercial steady state simulator. An electrolyte thermodynamic model for the sulfuric acid–water system was tailored with four candidates available in commercial software, utilizing data from Perry's Handbook. Simulation of the sulfuric acid decomposition process comprising a flash separator, distillation column and decomposer was validated with the experimental results. To facilitate the lumped-parameter steady-state model-based simulation of sulfuric acid decomposition, the decomposer was conceptually decoupled into three sections: evaporation, H₂SO₄ dissociation, and SO₃ catalytic reduction to SO₂. The process simulation results exhibited good agreement with experimental data. This work contributes to future work on simulation and experimental study of a scaled-up process system and exergy analysis for an optimal energy-efficient sulfuric acid decomposition process in the SI cycle.     

Design of a Compact Ceramic High-Temperature Heat Exchanger and Chemical Decomposer for Hydrogen Production

This article describes a compact silicon carbide ceramic, high-temperature heat exchanger for hydrogen production in the sulfur iodine thermochemical cycle, and in particular, to be used as the sulfuric acid decomposer. In this cycle, hot helium from a nuclear reactor is used to heat the SI (sulfuric acid) feed components (H₂O, H₂SO₄, SO₃) to obtain appropriate conditions for the SI decomposition reaction. The inner walls of the SI decomposer channels are coated with platinum to catalytically decompose sulfur trioxide into sulfur dioxide and oxygen. Hydrodynamic, thermal, and the sulfur trioxide decomposition reaction were coupled for numerical modeling. Thermal results of this analysis are exported to perform a probabilistic mechanical failure analysis. This article presents the approach used in modeling the chemical decomposition of sulfur trioxide. Stress analysis of the design is also presented. The second part of the article shows the results of parametric study of the baseline design (linear channels). Several alternate designs of the chemical decomposer channels are also explored. The current study summarizes the results of the parametric calculations whose objective is to maximize the sulfur trioxide decomposition by using various channel geometries within the decomposer. Based on these results, a discussion of the possibilities for improving the productivity of the design is also given.     

The oxidation of sulphur dioxide by bromine and water

The reaction of SO₂, Br₂ and H₂O, forming HBr and H₂SO₄, is one of the reactions of the hybrid cycle for thermochemical decomposition of water, Mark-13.Experimental work on this reaction is described in this paper.Equilibrium measurements show that high sulfuric acid concentrations (higher than 80 wt%) are attainable. At low sulfuric acid concentrations only traces of Br₂ and/or SO₂ remain in the gaseous phase.Besides the equilibrium determinations, development work by dynamic experiments has also been carried out.A simple mathematical model for the reaction rate in packed columns has been developed which satisfactorily fits the available experimental data.     

Process optimization and safety assessment on a pilot-scale Bunsen process in sulfur–iodine cycle

This study investigates Bunsen reaction in the sulfur-iodine (SI) cycle for optimal conditions and specification of equipment in terms of the maximum HI yield and the least impurities in HIx (mixture of HI, I₂ and H₂O), the reaction safety, and dispersion of SO₂ gas and HI_X solution for leakage accident. The pilot-scale Bunsen process was simulated and validated. The optimization of the Bunsen reactor, 3-phase separator, and HI_X purifier have been investigated in order to parameterize the operating conditions and equipment specification for three cases: (1) Maximize the HI yield for the final product (2) Minimize the H₂SO₄ impurities (3) Multi-objective case of both maximum HI production and minimum impurities. The gas reactivity safety was investigated on HI, H₂SO₄, I₂, SO₂, H₂O, and O₂. Also, the SO₂ gas dispersion distance for 30 ppm, 0.75 ppm, and 0.2 ppm and HI dispersion distance for 120 ppm, 25 ppm, and 1 ppm was investigated for targeted unit operators at each optimization scenario. The deviation between pilot-scale experiment and simulation case falls within 1–3% for Bunsen reactor, 6~8% for 3-phase separator, and 2~4% for HI_X purifier. The maximized HI production was increased by 17% for the maximum HI yield case from the designed case. The size and temperature of the Bunsen reactor was increased to enhance the reaction. However, the HI_X purifier size was reduced since reverse Bunsen reaction causes loss in HI product. The H₂SO₄ impurities in the minimize H₂SO₄ impurities case were reduced by 71% from the designed case. The size of the Bunsen reactor remained the same as design case, but the HI_X purifier size was increased to enhance the reverse Bunsen reaction. For multi-objective case, the HI productivity was increased by 16% and the H₂SO₄ impurities were reduced by 67% simultaneously. According Chemical Reactivity Worksheet (CRW) result, O₂ should therefore not be stored with any components except iodine. For SO₂ and HI_X dispersion assessment, the maximum HI yield case reveals the maximum dispersion of SO₂ gas and HI_X solution from the Bunsen reactor. The dispersion from 3-phase separator was almost the same for all the cases. For HI_X purifier, the minimum H₂SO₄ case exhibited the longest distance of SO₂ gas and HI solution dispersion. At 3 bar and 140 °C, the maximum SO₂ and HI_X dispersion distance were occurred.     

Overview of nuclear hydrogen production research through iodine sulfur process at INET

Thermochemical water-splitting cycle is a promising process to produce hydrogen using solar or nuclear energy. R&D on hydrogen production through iodine sulfur (IS) thermochemical cycle was initiated in 2005 at INET. Fundamental studies on the three reactions of IS cycle, i.e., Bunsen reaction, HI decomposition reaction, sulfuric acid decomposition reaction, and related techniques, such as separation, concentration and purification, were carried through. In Bunsen section, the reaction kinetics and separation characteristics of H₂SO₄ and HIx phases were studied. In HI section, Pt catalysts were loaded on different supporters by various methods and used for HI decomposition; and electro-electrodialysis(EED) was developed for concentration of HI acid. In sulfuric acid section, non-Pt catalysts were developed for SO₃ decomposition. Based on fundamental researches, a closed-loop test apparatus of 10 NL/h H₂ was designed and established. The current status of IS process research is summarized in this paper. In addition, R&D plan of IS process at INET is presented.