Sulfuric acid decomposition in the iodine–Sulfur cycle using heat from a very high temperature gas-cooled reactor

Using the heat of high-temperature gas-cooled reactor to drive the thermochemical iodine-sulfur cycle is an important way to produce hydrogen on a large scale. The sulfuric acid decomposition is the key reaction affecting the hydrogen production efficiency in this method, so efficient sulfuric acid decomposition is needed. The present study focuses on the temperature rise, phase change and chemical reactions of sulfuric acid in a bayonet heat exchanger. The evaporation-condensation model in a steady temperature field was used to simulate the different stages of the phase change process with the species transport model used to calculate the product proportions. Finally, the effects of two catalyst arrangements on the decomposition fraction were analyzed. The results show that the temperature in the catalytic core area reaches 1100 K which provides the heat required for the decomposition reaction. When the sulfuric acid flow rate is 0.36 kg/h, the two-phase flow section is about 0.22 m long to promote better heat transfer. The simulations show that square and circular catalysts give sulfuric acid decomposition fractions of 65% and 57%. The pressure drop of the two catalyst arrangements is almost the same, while the square catalyst has a higher decomposition fraction, which can improve the economics. This study provides a reference for optimizing catalyst selection based on the flow and heat transfer rates.     

Ceria as a catalyst for hydrogen iodide decomposition in sulfur–iodine cycle for hydrogen production

In this work, ceria (CeO₂) prepared with different methods and at various calcination temperatures have been tested to evaluate their effect on hydrogen iodide (HI) decomposition in sulfur–iodine (SI) cycle at various temperatures. The CeO₂ catalysts' strongly enhance the HI decomposition by comparison with blank test, especially gel CeO₂ 300. TG–FTIR, BET, XRD, TEM and TPR were performed for catalysts' characterization. The results show that the CeO₂ catalyst synthesized by citric-aided sol–gel method and calcined at low temperature (<500 °C) shows more lattice defects, smaller crystallites, larger surface area and better reducibility. Oxygen can promote the significantly rapid surface reaction, but simultaneously consume hydrogen species (H) contained in HI. Lattice defects, especially the reduced surface sites, i.e., Ce³⁺ and oxygen vacancy, play the dominant role in surface reactions of HI decomposition. A new reaction mechanism for HI catalytic decomposition over CeO₂ catalyst is proposed.     

The application of membrane reactor technology in hydrogen production using S–I thermochemical process: A roadmap

Thermochemical cycle using water as raw material and nuclear/renewable energies as sources of energy is believed to be a safe, stable and sustainable route of hydrogen production. Amongst the well-studied thermochemical cycles, the sulfur–iodine (S–I) cycle is capable of achieving an energy efficiency of 50%, making it one of the most efficient cycles among all water-splitting processes.     

Thermochemical hydrogen production using Rh/CeO2/γ-Al2O3 catalyst by steam reforming of ethanol and water splitting in a packed bed reactor

This study is focused on investigating the dual performance of Rh/CeO₂/γ-Al₂O₃ catalyst for steam reforming of ethanol (SRE) and thermochemical water splitting (TCWS) using a packed bed reactor. The catalyst is designed to be thermally stable containing an active phase of Rh and the redox component of CeO₂ for oxygen exchange, supported on γ-Al₂O₃. The catalyst has been characterised by SEM, XRD, BET, TPR, TPD, XPS and TGA before testing in the reactor. The optimal temperature for SRE reaction over this catalyst is between 700 °C and 800 °C to produce high concentrations of hydrogen (~60%), and low CO and CH₄. The selectivity towards CO and CH₄ is higher at low temperatures and drops with rise in reaction temperature. Further, Rh/CeO₂/γ-Al₂O₃ is found to be active for TCWS at relatively low temperatures (≤1200 °C). At temperatures as low as 800 °C, this catalyst is especially found suitable for multiple redox cycles, producing a total of 48.9 mmol/g_cat in four redox cycles. The catalyst can be employed for large number of redox cycles when the reactor is operated at lower temperatures. Finally, the reaction pathways have been proposed for both SRE and TCWS on Rh/CeO₂/γ-Al₂O₃ catalyst.     

Feasibility study of hydrogen production for micro fuel cell from activated Al–In mixture in water

A safe and environmental-friendly method of hydrogen production from milled Al–In–Zn–salt mixture in water was proposed in this paper. The 10 h—milled Al–In–Zn–salt mixture had high reactivity and produced hydrogen in water at room temperature. Its improved reactivity came from that the additive Zn and salts facilitate to the negative shift of Al–In alloy and benefited the combination of Al, In and Zn in the milling process. Optimized the composition content, 1 g of 10 h—milled Al—5 wt%In—3 wt%Zn—2 wt%NaCl mixture had highest hydrogen yield of 1035 mL hydrogen/1 g Al in 4 min of hydrolysis reaction in water, corresponding to 9.21 wt% hydrogen (excluding water mass). Hydrogen supplying from milled Al–In–Zn–salt mixture was performed for micro fuel cell and 0.96 W was produced with the stable hydrogen supply rate. Therefore, the milled Al–In–Zn–salt mixture could be a feasible alternative for providing a source of CO₂ free hydrogen production to supply micro fuel cell.     

Enhancing the production of hydrogen via water–gas shift reaction using Pd-based membrane reactors

In this work, it is described an experimental study regarding the performance of a Pd–Ag membrane reactor recently proposed and suitable for the production of ultra-pure hydrogen. A dense metallic permeator tube was assembled by an innovative annealing and diffusion welding technique from a commercial flat sheet membrane of Pd–Ag. A “finger-like” configuration of the self-supported membrane has been designed and used as a packed-bed membrane reactor (MR) for producing ultra-pure hydrogen via water–gas shift reaction (WGS).     

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.     

A review of hydrogen production processes by photocatalytic water splitting – From atomistic catalysis design to optimal reactor engineering

‘Renewable energy is an essential part of our strategy of decarbonization, decentralization, as well as digitalization of energy.’ – Isabelle Kocher.Current climate, health and economic condition of our globe demands the use of renewable energy and the development of novel materials for the efficient generation, storage and transportation of renewable energy. Hydrogen has been recognised as one of the most prominent carriers and green energy source with challenging storage, enabling decarbonization. Photocatalytic H₂ (green hydrogen) production processes are targeting the intensification of separated solar energy harvesting, storage and electrolysis, conventionally yielding O₂/H₂. While catalysis is being investigated extensively, little is done on bridging the gap, related to reactor unit design, optimisation and scaling, be it that of material or of operation. Herein, metals, oxides, perovskites, nitrides, carbides, sulphides, phosphides, 2D structures and heterojunctions are compared in terms of parameters, allowing for efficiency, thermodynamics or kinetics structure–activity relationships, such as the solar-to-hydrogen (STH). Moreover, prominent pilot systems are presented summarily.     

Sustainable generation of hydrogen using chemicals with regional oversupply – Feasibility of the electrolysis in acido-alkaline reactor

An integral part of the concept of sustainable development and a serious challenge to be addressed is the demanding and costly disposal of the hazardous waste chemicals and/or overproduced chemicals remaining after various production cycles in the chemical industry. For example, the recently reported overproduction of sulfuric acid in China can affect base metal production rates. In this context the consideration of new technologies that can avoid environmental damage or even introduce an economically feasible framework for the utilization of hazardous waste chemicals becomes appealing.     

Continuous hydrogen production via the steam–iron reaction by chemical looping in a circulating fluidized-bed reactor

The steam–iron reaction was examined in a two-compartment fluidized-bed reactor at 800–900 °C and atmospheric pressure. In the fuel reactor compartment, freeze-granulated oxygen carrier particles consisting of Fe₃O₄ supported on inert MgAl₂O₄ were reduced to FeO with carbon monoxide or synthesis gas. The reduced particles were transferred to a steam reactor compartment, where they were oxidized back to Fe₃O₄ by steam, while at the same time producing H₂. The process was operated continuously and the particles were transferred between the reactor compartments in a cyclic manner. In total, 12 h of experiments were conducted of which 9 h involved H₂ generation. The reactivity of the oxygen carrier particles with carbon monoxide and synthesis gas was high, providing gas concentrations reasonably close to thermodynamic equilibrium, especially at lower fuel flows. The amount of H₂ produced in the steam reactor was found to correspond well with the amount of fuel oxidized in the fuel reactor, which suggests that all FeO that was formed were also re-oxidized. Despite reduction of the oxygen carrier to FeO, defluidization or stops in the solid circulation were not experienced. Used oxygen carrier particles exhibited decreased BET specific surface area, increased bulk density and decreased particle size compared to fresh. This indicates that the particles were subject to densification during operation, likely due to thermal sintering. However, stable operation, low attrition and absence of defluidization were still achieved, which suggest that the overall behaviour of the oxygen carrier particles were satisfactory.     

Coupling of copper–chloride hybrid thermochemical water splitting cycle with a desalination plant for hydrogen production from nuclear energy

Energy and environmental concerns have motivated research on clean energy resources. Nuclear energy has the potential to provide a significant share of energy supply without contributing to environmental emissions and climate change. Nuclear energy has been used mainly for electric power generation, but hydrogen production via thermochemical water decomposition provides another pathway for the utilization of nuclear thermal energy. One option for nuclear-based hydrogen production via thermochemical water decomposition uses a copper–chloride (Cu–Cl) cycle. Another societal concern relates to supplies of fresh water. Thus, to avoid causing one problem while solving another, hydrogen could be produced from seawater rather than limited fresh water sources. In this study we analyze a coupling of the Cu–Cl cycle with a desalination plant for hydrogen production from nuclear energy and seawater. Desalination technologies are reviewed comprehensively to determine the most appropriate option for the Cu–Cl cycle and a thermodynamic analysis and several parametric studies of this coupled system are presented for various configurations.     

Life cycle greenhouse gases emission analysis of hydrogen production from S–I thermochemical process coupled to a high temperature nuclear reactor

A methodology for assessing the environmental impact of products and services is the life cycle analysis (LCA); which is a versatile tool to define the inclusion process and the scope of the production system, for different scenarios and selective comparison of environmental burdens. For the LCA developed in this work, the S–I thermochemical cycle coupled to a high temperature gas nuclear reactor was selected. The defined system function is the production of hydrogen using nuclear energy and the functional unit is 1 kg of hydrogen at the plant gate. The product system was defined by the following steps: (i) extraction and manufacturing of raw materials (upstream flows), (ii) external energy supplied to the system, (iii) nuclear power plant, and (iv) hydrogen production plant. Particular attention was placed to those processes where there was limited information from literature about inventory data, like the TRISO fuel manufacture, and the production of iodine from caliches, which is supplied to the thermochemical process for hydrogen generation. The environmental impact assessment focuses on the emissions of greenhouse gases as comparative parameter related to global warming. The results showed low emissions when electric power was supplied from nuclear energy. When the electric power supply was changed to a mix of fossil fuels, the emissions were significantly higher.     

Prediction of transient heat and mass transfer in a closed metal–hydrogen reactor

The metal–hydrogen reactor is usually composed of a porous medium (hydride bed) and an expansion volume (gaseous phase). During the sorption process, the hydrogen flow and the heat transfer in the expansion part are badly known and can have some effects on the sorption phenomena in the hydride medium. At our knowledge, the hypothesis that neglects those effects is typically used. In this paper, a 2D study of heat and mass transfer has been carried out to investigate the transient transport processes of hydrogen in the two domains of a closed cylindrical reactor. A theoretical model is conducted and solved numerically by the control-volume-based finite element method (CVFEM). The result on temperature and hydride density distribution are presented and discussed. Moreover, this paper discusses in detail the effects of some governing operating conditions, such as dimensions of the expansion volume, height to the radius reactor ratio, and the initial hydrogen to metal atomic ratio, on the evolution of the pressure, fluid flow, temperature and the hydrogen mass desorbed.     

Biogas reforming for hydrogen-rich syngas production over a Ni–K/Al2O3 catalyst using a temperature-controlled plasma reactor

Plasma-enhanced catalytic biogas reforming for hydrogen-rich syngas production over a Ni–K/Al₂O₃ catalyst was investigated using a tabular dielectric barrier discharge non-thermal plasma reactor. To better understand the plasma catalysis synergy at elevated temperatures, we compared different reaction modes: plasma catalysis, plasma alone, and catalysis alone in a reaction temperature range of 160–400 °C. The combination of Ni–K/Al₂O₃ and plasma produced synergistic effects. Notably, the plasma-catalytic synergy was temperature-dependent and varied at different reaction temperatures. Using plasma catalysis, the maximum conversion of CH₄ and CO₂ (31.6% and 22.8%, respectively) was attained over Ni–K/Al₂O₃ at 160 °C, while increasing the reaction temperature to 340 °C noticeably enhanced the H₂/CO ratio to 2.71. Moreover, compared to plasma-catalytic biogas reforming at 160 °C, increasing the reaction temperature to 400 °C suppressed biogas conversion with dramatically reduced coke formation on the Ni–K/Al₂O₃ surface from 6.81 wt% to 3.37 wt%.     

Experimental investigation of hydrogen production by CH4–CO2 reforming using rotating gliding arc discharge plasma

Hydrogen produced from CH₄–CO₂ reforming by an optimized rotating gliding arc discharge plasma reactor is investigated in this study. The effect of CH₄/CO₂ ratio (mole ratio), total input flow rate, discharge gap, voltage, and discharge frequency are analyzed. The results show that H₂ yield increases with the increase of CH₄/CO₂ ratio. Arc can be stretched effectively by increasing total input flow rate, then the discharge region is enlarged. Increasing discharge gap can enlarge the discharge region, but the reaction of the gas mixture would be suppressed if the discharge region was excessively large. The discharge region decreases with the increased discharge frequency to a certain degree. Based on the experimental results, the optimal experimental condition is concluded as applied voltage 60 V, discharge frequency 20 kHz, and minimum discharge gap 3 mm. It is anticipated that the results would serve as a good guideline to the application of hydrogen production from hydrocarbon fuels by plasma reforming onboard.     

Modeling of a catalytic membrane reactor for CO removal from hydrogen streams – A theoretical study

Typical industrial hydrogen streams arising from reforming processes contain about 1% of carbon monoxide (CO). For fuel cell applications hydrogen should contain less than 10 ppm of CO, since it poisons the platinum catalysts in the electrodes. Traditionally, this is carried out through a selective oxidation reactor – PROX reactor. However, the parallel oxidation of hydrogen to water should be avoided. This work proposes the use of a catalytic membrane reactor (MR) whose design is based on a CO permselective membrane containing the selective catalyst loaded in the permeate side. It is considered plug-flow pattern and segregated feed of CO and oxygen. This strategy should improve the selective oxidation, as the permselective membrane enhances the CO/H₂ ratio at the catalyst surface.     

Temperature-induced selectivity enhancement in hydrogen generation from Rh–Ni nanoparticle-catalyzed decomposition of hydrous hydrazine

In this work, we found that Rh–Ni bimetallic nanocatalysts, with a rhodium content as low as 10 mol %, prepared by the coreduction of the corresponding metal chlorides, exhibit excellent catalytic activity to the decomposition of hydrous hydrazine, producing hydrogen with 100% selectivity at 323 K. The present catalyst with low noble-metal content promotes the practical use of the hydrogen-storage system based on the catalytic complete decomposition of hydrazine in aqueous solution at ambient conditions.     

Hydrolysis of CuCl2 in the Cu–Cl thermochemical cycle for hydrogen production: Experimental studies using a spray reactor with an ultrasonic atomizer

The Cu–Cl thermochemical cycle is being developed as a hydrogen production method. Prior proof-of-concept experimental work has shown that the chemistry is viable while preliminary modeling has shown that the efficiency and cost of hydrogen production have the potential to meet DOE's targets. However, the mechanisms of CuCl₂ hydrolysis, an important step in the Cu–Cl cycle, are not fully understood. Although the stoichiometry of the hydrolysis reaction, 2CuCl₂ + H₂O ↔ Cu₂OCl₂ + 2HCl, indicates a necessary steam-to-CuCl₂ molar ratio of 0.5, a ratio as high as 23 has been typically required to obtain near 100% conversion of the CuCl₂ to the desired products at atmospheric pressure. It is highly desirable to conduct this reaction with less excess steam to improve the process efficiency. Per Le Chatelier's Principle and according to the available equilibrium-based model, the needed amount of steam can be decreased by conducting the hydrolysis reaction at a reduced pressure. In the present work, the experimental setup was modified to allow CuCl₂ hydrolysis in the pressure range of 0.4–1 atm. Chemical and XRD analyses of the product compositions revealed the optimal steam-to-CuCl₂ molar ratio to be 20–23 at 1 atm pressure. The experiments at 0.4 atm and 0.7 atm showed that it is possible to lower the steam-to-CuCl₂ molar ratio to 15, while still obtaining good yields of the desired products. An important effect of running the reaction at reduced pressure is the significant decrease of CuCl concentration in the solid products, which was not predicted by prior modeling. Possible explanations based on kinetics and residence times are suggested.