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.     

热化学硫碘循环制氢系统研究进展

热化学硫碘循环制氢是目前最有前景的制氢方法之一.为了提高产氢效率,众多学者对硫碘循环制氢系统进行了优化.对热化学硫碘循环制氢系统及其各单元研究进展进行了综述,讨论了本生反应中水和碘的用量问题、相分离问题,以及硫酸分解与碘化氢分解反应的反应环境和反应催化等问题,并对目前研究该循环的部分团队的实验结果和流程计算进行了对比.     

Energy and economic assessment of an industrial plant for the hydrogen production by water-splitting through the sulfur-iodine thermochemical cycle powered by concentrated solar energy

The faster and faster global growth of energy consumption generates serious problems on its supply and about the pollution that may result. Through the use of thermochemical cycles it is possible to use renewable energy to produce hydrogen from water, with the dual purpose of having an unlimited source of energy without producing greenhouse gases.     

Chemical aspects of the water-splitting thermochemical cycle based on sodium manganese ferrite

Water thermolysis by means of the sodium manganese ferrite cycle for sustainable hydrogen production is reviewed, with particular focus on known elementary chemical processes taking place on solid substrates in the 600–800 °C temperature range. For the purpose, in-situ high temperature x-ray diffraction technique has been utilized to observe structural transformations produced by both temperature and reactive environment. The water-splitting reaction and the regeneration of initial reactants are described as multi-step reactions, in which the role of carbon dioxide, through carbonation and de-carbonation reactions is highlighted. A thermodynamic phase stability diagram is reported for the system MnFe₂O₄/Na₂CO₃/CO₂.     

Thermodynamic feasibility analysis of a water-splitting thermochemical cycle based on sodium carbonate decomposition

A novel water-splitting thermochemical cycle for the production of green hydrogen is introduced. This three-step cycle is based on the thermal decomposition of sodium carbonate. In this work, thermodynamic equilibrium analysis of the cycle's constituent reactions is performed, for a variety of temperatures, pressures, and dilution ratios. Feasible temperature/pressure/dilution operating windows are identified for the cycle's reactions such that the maximum temperature of the cycle can be generated using current concentrated solar power (CSP) technologies, thus enabling the replacement of steam methane reforming (SMR) as the major industrial production method of hydrogen.     

An entropy production and efficiency analysis of the Bunsen reaction in the General Atomic sulfur-iodine thermochemical hydrogen production cycle

An entropy production and efficiency analysis of the first reaction in the General Atomic sulfur-iodine thermochemical hydrogen production cycle has been carried out by simulating the reaction including the mixing of reactants and separation of the resulting phases. The reaction:

was simulated at 388 K, which is slightly above the melting point of I₂. Analysis of only this reaction shows that the reaction should be run at 15–25% I₂ reacted and the greatest excess of H₂O which will produce two product phases. Actual operating conditions are however dependent on the total processing scheme. An entropy production and efficiency analysis along with economic factors for the entire process is necessary to obtain these conditions.     

A critical review on integrated system design of solar thermochemical water-splitting cycle for hydrogen production

The development of clean hydrogen production methods is important for large-scale hydrogen production applications. The solar thermochemical water-splitting cycle is a promising method that uses the heat provided by solar collectors for clean, efficient, and large-scale hydrogen production. This review summarizes state-of-the-art concentrated solar thermal, thermal storage, and thermochemical water-splitting cycle technologies that can be used for system integration from the perspective of integrated design. Possible schemes for combining these three technologies are also presented. The key issues of the solar copper-chlorine (Cu–Cl) and sulfur-iodine (S–I) cycles, which are the most-studied cycles, have been summarized from system composition, operation strategy, thermal and economic performance, and multi-scenario applications. Moreover, existing design ideas, schemes, and performances of solar thermochemical water-splitting cycles are summarized. The energy efficiency of the solar thermochemical water-splitting cycle is 15–30%. The costs of the solar Cu–Cl and S–I hydrogen production systems are 1.63–9.47 $/kg H₂ and 5.41–10.40 $/kg H₂, respectively. This work also discusses the future challenges for system integration and offers an essential reference and guidance for building a clean, efficient, and large-scale hydrogen production system.     

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.     

Techno-economic analysis of thermochemical water-splitting system for Co-production of hydrogen and electricity

The two-step thermochemical metal oxide water-splitting cycle with the state-of-the-art material ceria inevitably produces unutilized high-quality heat, in addition to hydrogen (H₂). This study explores whether the ceria cycle can be of greater value by using the excess heat for co-production of electricity. Specially, this technoeconomic study estimates the H₂ production cost in a hybrid ceria cycle, in which excess heat produces electricity in an organic Rankine cycle, to increase revenue and decrease H₂ cost. The estimated H₂ cost from such a co-generation multi-tower plant is still relatively high at $4.55/kg, with an average H₂ production of 1431 kg/day per 27.74 MW_th tower. Sensitivity analyses show opportunities and challenges to achieving $2/kg H₂ through improvements such as increased solar field efficiency, increased revenue from electricity sales, and a decreased capital recovery factor from baseline assumptions. While co-production improves overall system efficiency and economics, achieving $2/kg H₂ remains challenging with ceria as the active material and likely will require a new material.     

Flowsheet study of the thermochemical water-splitting iodine–sulfur process for effective hydrogen production

The Japan Atomic Energy Agency (JAEA) is performing research and development on the thermochemical water-splitting iodine–sulfur (IS) process for hydrogen production with the use of heat (temperatures close to 1000 °C) from a nuclear reactor process plant. Such temperatures can be supplied by a High Temperature Gas-cooled Reactor (HTGR) process. JAEA's activity covers the control of the process for continuous hydrogen production, processing procedures for hydrogen iodide (HI) decomposition, and a preliminary screening of corrosion resistant process materials. The present status of the R&D program is reported herein, with particular attention to flowsheet studies of the process using membranes for the HI processing.     

Oxygen-releasing step of nickel ferrite based on Rietveld analysis for thermochemical two-step water-splitting

We investigated the thermal reduction (T-R) of NiFe₂O₄, either supported by m-ZrO₂ or unsupported, as the oxygen-releasing step of a solar thermochemical water splitting cycle based on a ferrite/wustite redox system, by performing the Rietveld analysis using powder X-ray diffraction. The solid materials obtained after the T-R step at 1300–1400 °C were subjected to Rietveld analysis. The amounts and chemical compositions of the wustite phase produced by the T-R step and the remaining ferrite phase were identified quantitatively. Chemical reaction formulas for the different T-R temperatures were determined from the results. Consistency for the chemical reactions of the thermal reduction was discussed and evaluated comparing the O₂ amounts predicted by the chemical reaction formulas and measured experimentally by mass spectrometry.     

Dynamic model of a solar thermochemical water-splitting reactor with integrated energy collection and storage

Water-splitting solar thermochemical cycles are important in meeting the challenges of global climate change and limited fossil fuels. However, solar radiation varies in availability, leading to unsteady state operation. We propose a solar receiver-reactor with integrated energy collection and storage. The reactor consists of a double-pipe heat exchanger placed at the focal line of a parabolic trough solar concentrator. Molten salt passes through the jacket, absorbing energy from the irradiated outer surface while driving the endothermic oxygen production step of the copper-chlorine water-splitting cycle in the reactor bore. Excess energy is stored in a thermal storage tank to buffer the reactor from changes in insolation. Dynamic simulation indicates that the reactor can sustain steady 100% conversion during 24/7 operation with a reasonable plant layout. The technology employed is extant and mature. This is important in view of the urgency to reduce dependency upon fossil fuels as primary energy sources.     

A spinel ferrite/hercynite water-splitting redox cycle

Cobalt ferrites are deposited on Al₂O₃ substrates via atomic layer deposition, and the efficacy of using these in a ferrite water splitting redox cycle to produce H₂ is studied. Experimental results are coupled with thermodynamic modeling, and results indicate that CoFe₂O₄ deposited on Al₂O₃ is capable of being reduced at lower temperatures than CoFe₂O₄ (200–300 °C) due to a reaction between the ferrite and substrate to form FeAl₂O₄. Although the reaction of FeAl₂O₄ and H₂O is not as thermodynamically favorable as that of FeO and H₂O, it is shown to be capable of splitting H₂O to produce H₂ if non-equilibrium conditions are maintained. Significant quantities of H₂ are produced at reduction temperatures of only 1200 °C, whereas, CoFe₂O₄ produced little or no H₂ until reduction temperatures of 1400 °C. CoFe₂O₄/Al₂O₃ was capable of being cycled at 1200 °C reduction/ 1000 °C oxidation with no obvious deactivation.     

Concentration of HIx solution by electro-electrodialysis using Nafion 117 for thermochemical water-splitting IS process

An experimental study of applying electro-electrodialysis (EED) for improved HI concentration in the HIx solution, a mixture of HI–I₂–H₂O of approximately quasi-azeotropic compositions has been carried out in the conditions of around 90 °C and using Nafion 117 and graphite electrodes. A range of 25–80% increase in initial current efficiency of HI molality in catholyte is measured with the use of EED. In general, the efficiency increases with increasing iodine molality and weight ratio of anolyte solution to catholyte solution. The EED performance degrades in time. In some cases, the HI concentration limits are observed. Electric conductivity of the HIx solution, overvoltage of electrode reaction, and the membrane voltage drop is measured in a temperature range of 20–120 °C. It is found that the EED cell voltage, which is an important cell performance parameter, is governed by the membrane voltage drop.     

Understanding water-splitting thermochemical cycles based on nickel and cobalt ferrites for hydrogen production

Two step water-splitting cycles by using metal ferrites are considered as a clean and sustainable hydrogen production method, when concentrated solar energy is used to drive the thermochemical reactions. This process involves the reduction at very high temperature of the ferrite, followed by the water reoxidation to the original phase at moderate temperature, with the release of hydrogen. In order to decrease the temperature required to decompose the oxide, mixed ferrites of the type MFe₂O₄ with spinel crystal structure have been examined. In this sense, ferrites with the partial substitution of Co and Ni for Fe appear as successful materials in terms of hydrogen production and cyclability. In this work, commercial Ni and synthetic Co ferrites have been subjected to two water splitting cycles. The solid products obtained after thermal reduction and water decomposition reactions have been chemically and structurally characterized by WDXRF, XRD, XPS and SEM techniques, in order to get a deeper understanding of the mechanisms controlling the water splitting process. This knowledge contributes to improve the process involved in thermochemical cycles and to understand the lower efficiencies (H₂/O₂) for Co ferrite thermochemical cycles in comparison with those corresponding to Ni ferrite.     

Screening of water-splitting thermochemical cycles potentially attractive for hydrogen production by concentrated solar energy

Hydrogen, a promising and clean energy carrier, could potentially replace the use of fossil fuels in the transportation sector. Currently, no environmentally attractive, large-scale, low-cost and high-efficiency hydrogen production process is available for commercialization. Solar-driven water-splitting thermochemical cycles may constitute one of the ultimate options for CO₂-free production of hydrogen. The method is environmentally friendly since it uses only water and solar energy. First, the potentially attractive thermochemical cycles must be identified based on a set of criteria. To reach this goal, a database that contains 280 referenced cycles was established. Then, the selection and evaluation of the promising cycles was performed in the temperature range of 900–2000 °C, suitable to the use of concentrated solar energy. About 30 cycles selected for further investigations are presented in this paper. The principles and basis for a thermodynamic evaluation of the cycles are also given.     

硫碘热化学循环水分解制氢流程优化与模拟

选择大型化工流程模拟软件Aspen Plus对硫碘循环制氢系统进行流程优化设计与模拟,计算系统的质量平衡、能量平衡,并对系统热效率进行评估.碘化氢相中HI浓度采用本生反应实验中的过恒沸浓度,避免高能耗电渗析的使用,从而大大提高系统效率.在不考虑废热发电情况下,与文献值56.8％相比,系统产氢热效率高达68.46％.     

Chemical engineering assessment of the thermochemical cycle mark 9

The thermochemical cycles of the FeCl family are reviewed with special emphasis on the changes brought to the reaction sequence. The chemical equilibria involved in the hydrolysis, chlorination, decomposition and oxygen removal steps of the MARK 9, MARK 14 cycles are discussed in detail and resulting data are used in an engineering flow sheet of the MARK 9 cycle.The hydrolysis of FeCl₂ which is common to many FeCl cycles was examined from kinetics point of view and detailed mass and energy balances are given for different types of reaction conditions. The thermal efficiency is the limiting factor of this type of thermochemical cycle.     

Ferrite/zirconia-coated foam device prepared by spin coating for solar demonstration of thermochemical water-splitting

A Ferrite/zirconia foam device in which reticulated ceramic foam was coated with zirconia-supported Fe₃O₄ or NiFe₂O₄ as a reactive material was prepared by a spin-coating method. The spin-coating method can shorten the preparation period and reduce the coating process as compared to the previous wash-coating method. The foam devices were examined for hydrogen productivity and cyclic reactivity in thermochemical two-step water-splitting. The reactivity of these foam devices were studied for the thermal reduction of ferrite on a laboratory scale using a sun simulator to simulate concentrated solar radiation, while the thermally reduced foam devices were reacted with steam in another quartz reactor under homogeneous heating in an infrared furnace. The most reactive foam device, NiFe₂O₄/m-ZrO₂/MPSZ, was tested for successive two-step water-splitting in a windowed single reactor using solar-simulated Xe-beam irradiation with a power input of 0.4-0.7 kW_th. The production of hydrogen continued successfully in the 20 cycles that were demonstrated using the NiFe₂O₄/m-ZrO₂/MPSZ foam device. The NiFe₂O₄/m-ZrO₂/MPSZ foam device produced hydrogen at a rate of 1.1-4.6 cm³ per gram of device through 20 cycles and reached a maximum ferrite conversion of 60%.