Thermochemical Water Splitting Cycle Research Articles

3D numerical investigation of ZnO/Zn hydrolysis for hydrogen production

The current research is focused on the hydrogen production through a two-step ZnO/Zn thermochemical water splitting cycle. In the present paper, numerical modeling of the second step is conducted using Computational Fluid Dynamics (CFD)2, where steam reacts with zinc to produce hydrogen. The parametric study shows that the hydrogen yield is relatively insensitive to the steam/zinc molar ratio and inversely proportional to the argon/steam molar ratio. For large argon to steam molar ratios, hydrogen yield is relatively insensitive to the inlet temperature of zinc and steam, and increases marginally with an increase in the argon inlet temperature. Five different reactor configurations were evaluated comprehensively. Among all configurations, a cylindrical reactor with a tangential inlet for argon and zinc, and a radial inlet for steam (both in the bottom plane of the reactor) and a tangential outlet in the top plane of the reactor produced the highest hydrogen yield of 88%. Copyright © 2013 John Wiley & Sons, Ltd.

Understanding Morphology-Controlled Synthesis of Zinc Nanoparticles and Their Characteristics of Hydrolysis Reaction

Two-step thermochemical water-splitting cycle based on a Zn/ZnO redox pair is considered as a potential route for carbon-free production of hydrogen because the first hydrolysis step of the cycle highly depends on the method of preparation and the resultant particle characteristics, such as size, morphology, surface state, and initial oxide content. Here, employing a conventional evaporation and condensation method, we successfully produce three types of Zn nanoparticles ranging from nanorods, mesoporous nanorods with nanospheres on their surfaces, and fully sintered nanocrystals. The achievement in morphology control is realized simply by changing the injection position of the quenching gas. We found that the resultant hydrolysis kinetics is highly dependent on the morphology and porosity of the Zn nanoparticles. Finally, a series of simple mathematical modeling is made in an effort to understand the formation mechanism of Zn nanoparticles.

Spinel Metal Oxide-Alkali Carbonate-Based, Low-Temperature Thermochemical Cycles for Water Splitting and CO2 Reduction

A manganese oxide-based, thermochemical cycle for water splitting below 1000 °C has recently been reported. The cycle involves the shuttling of Na+ into and out of manganese oxides via the consumption and formation of sodium carbonate, respectively. Here, we explore the combinations of three spinel metal oxides and three alkali carbonates in thermochemical cycles for water splitting and CO_2 reduction. Hydrogen evolution and CO_2 reduction reactions of metal oxides with a given alkali carbonate occur in the following order of decreasing activity: Fe_(3)O_4 > Mn_(3)O_4 > Co_(3)O_4, whereas the reactivity of a given metal oxide with alkali carbonates declines as Li_(2)CO_3 > Na_(2)CO_3 > K_(2)CO_3. While hydrogen evolution and CO_2 reduction reactions occur at a lower temperature on the combinations with the more reactive metal oxide and alkali carbonate, higher thermal reduction temperatures and more difficult alkali ion extractions are observed for the combinations of the more reactive metal oxides and alkali carbonates. Thus, for a thermochemical cycle to be closed at low temperatures, all three reactions of hydrogen evolution (CO_2 reduction), alkali ion extraction, and thermal reduction must proceed within the specified temperature range. Of the systems investigated here, only the Na_(2)CO_3/Mn_(3)O_4 combination satisfies these criteria with a maximum operating temperature (850 °C) below 1000 °C.

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.

HI decomposition over PtNi/C bimetallic catalysts prepared by electroless plating

The catalytic decomposition of hydrogen iodide (HI) has drawn increasing attention because it is the key reaction for hydrogen production in the Iodine–sulfur (IS) thermochemical water splitting cycle, which is considered one of the most promising alternative methods for massive hydrogen production with high efficiency and without CO2 emissions. Because it is very difficult for HI to decompose without the catalysts even at 500 °C, some catalysts have to be used to catalyze this reaction. In this study, four kinds of PtNi bimetallic catalysts supported on activated carbon (PtNi/C) were prepared by electroless plating and their catalytic activities were compared for HI decomposition in a fixed bed reactor at 400 and 500 °C under atmospheric pressure. Their differences in structures, surface areas, and morphology were characterized by XRD, BET and TEM, respectively. The used catalysts were also analyzed by TEM characterization in order to investigate the stability of catalysts. The results showed that the PtNi/C bimetallic catalysts are promising catalysts for HI decomposition because of their high activity and good stability, especially at high reaction temperature.

Coupling the nickel-iodine-sulphur cycle with a nuclear reactor

Nuclear hydrogen production is a technically feasible and economically viable option for addressing future energy needs. Several projects have been started on the co-generation of hydrogen and electricity from nuclear energy. In this report, the nickel sulphur iodine (NIS) cycle, a thermochemical water splitting cycle originally developed in ENEA for solar hydrogen production was studied to be coupled with a new generation nuclear reactor for massive hydrogen production.

Phase separation characteristics of HI–I2–H2SO4–H2O mixture at elevated temperatures

The iodine–sulfur (IS) thermochemical water splitting cycle is one of the most promising massive hydrogen production approaches using nuclear or solar energy. Bunsen reaction is a crucial reaction of the process by which sulfuric and hydriodic acids form. The simultaneously phase separation of the Bunsen production products, composed of HI, I2, H2SO4, and H2O are essential for the smooth operation of the process. In this work, the phase separation characteristics of the quaternary mixtures were studied at elevated temperatures of 40–80°C to clarify the phase separation characteristics. The critical conditions of the phase separation and iodine solubility in the mixture, as well as the effects of the composition and temperature on the purity of the two phases, were investigated. Favorable concentration range of each component in the quaternary solutions at various temperatures was obtained and presented in a tetrahedron diagram. The solution separates into two liquid phases spontaneously without iodine precipitation within the range. The amount of I2 should be as high as possible, with no I2 precipitation, to keep impurities at minimum content in the two phases. The results of this study offer technical basis for the design and operation of the IS process.

Solar hydrogen production via pulse electrolysis of aqueous ammonium sulfite solution

A long-standing challenge for hydrogen production via solar water splitting is the efficiency of converting solar energy to hydrogen chemical energy. Thermolysis, photocatalysis and electrolysis are three basic solar water splitting processes that utilize solar thermal, photonic and electrical energies. A technology using a combination of these processes can utilize a wider spectrum of solar radiation, thereby enhancing the efficiency of solar energy conversion. Due to the simplicity and maturity of photovoltaic (PV) cells and electrolyzer cells, solar hydrogen production via PV cells plus water electrolysis has been implemented and widely used as a bench mark process. The present study focuses on solar hydrogen production via direct current pulse electrochemical oxidation of aqueous ammonium sulfite solutions, one important step in solar sulfur–ammonia (S-NH3) thermochemical water splitting cycles. The results show that pulsating electrolysis enhances the efficiency of hydrogen production. The effects of pulsating parameters (such as pulsating on time and off time, frequency and duty cycle) on hydrogen evolution rates are discussed in detail.

Exergy analysis of hydrogen production from different sulfur-containing compounds based on H2S splitting cycle

There is a tremendous demand for hydrogen production worldwide but the current H2 production routes from natural gas and other carbon fuels lead to large greenhouse gas emissions. Intentionally coupled with nuclear power, the sulfur–iodine (S–I) thermochemical water splitting cycle is one of the most widely studied cycles for the large-scale hydrogen production that has environmental benignity. Based on the inspiration of the S–I cycle, a novel chemical cycle called hydrogen sulfide splitting cycle has been proposed for hydrogen production. In addition to the SO2 production from the reaction of H2S and sulfuric acid, SO2 can be produced from the burning (direct oxidation) of hydrogen sulfide or elemental sulfur. And it can also be provided by SO2 capture from flue gas or other SO2-containing waste gases. This paper performs exergy analysis on the various SO2 provisions to the Bunsen reaction that make different routes for hydrogen production from waste sulfur-containing compounds as feedstock. It has been found that the route including SO2 from direct H2S oxidation potentially makes the best energy-efficient process of H2 production. The heat that is generated from H2S oxidation can be recovered and used to support the energy requirements for other steps of the cycle, making the entire hydrogen production cycle more energy-efficient.

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.

Low-temperature, manganese oxide-based, thermochemical water splitting cycle

Thermochemical cycles that split water into stoichiometric amounts of hydrogen and oxygen below 1,000 °C, and do not involve toxic or corrosive intermediates, are highly desirable because they can convert heat into chemical energy in the form of hydrogen. We report a manganese-based thermochemical cycle with a highest operating temperature of 850 °C that is completely recyclable and does not involve toxic or corrosive components. The thermochemical cycle utilizes redox reactions of Mn(II)/Mn(III) oxides. The shuttling of Na(+) into and out of the manganese oxides in the hydrogen and oxygen evolution steps, respectively, provides the key thermodynamic driving forces and allows for the cycle to be closed at temperatures below 1,000 °C. The production of hydrogen and oxygen is fully reproducible for at least five cycles.

Towards integration of hydrolysis, decomposition and electrolysis processes of the Cu–Cl thermochemical water splitting cycle

Abstract The thermochemical copper–chlorine (Cu–Cl) cycle is a promising technology that can utilize various energy sources such as nuclear and solar energy to produce hydrogen with minimal or no emissions of greenhouse gases. Past investigations have primarily focused on the design and testing of individual unit operations of the Cu–Cl cycle. This paper investigates the chemical streams flowing through each individual process from the aspect of system integration. The interactions between each of the two immediate upstream and downstream processes are examined. Considering the integration of electrolytic hydrogen production and cupric chloride hydrolysis steps, it is evident that an intermediate step to concentrate CuCl 2 and reduce HCl composition is required. Spray drying and crystallization, serving as the intermediate steps, are examined from the aspects of energy requirements and viability of engineering. Regarding the integration of the hydrolysis and oxygen production steps, thermodynamic and XRD analysis results are presented to study the mutual impacts of these two steps on each other. Within the hydrolysis reactor, high conversion of CuCl 2 to Cu 2 OCl 2 is preferable for the integration because it reduces the release of chlorine gas during the oxygen production. Considering the integration of the oxygen production step and electrolysis of CuCl, pulverization is needed for the solidified CuCl. The recovery of CuCl vapour entrained in oxygen gas requires further research. Residual CuCl 2 introduced from the hydrolysis step into the oxygen production step may be further entrained by CuCl into the electrolytic hydrogen production cell. Additionally, thermal energy integration patterns are briefly discussed while integrating the various chemical streams of the Cu–Cl cycle. Steam generated from the heat recovery of cuprous chloride can be introduced into the hydrolysis reactor to serve as a reactant.

Life cycle assessment of hydrogen production via thermochemical water splitting using multi-step Cu–Cl cycles

Abstract A comparative environmental impact study of the three-, four- and five-step copper–chlorine (Cu–Cl) thermochemical water splitting cycles is undertaken through life cycle assessment (LCA). This analytical tool is used to identify and quantify environmentally critical phases during the life cycle of a system or a product and/or to evaluate and decrease the overall environmental impact of the system or product. The LCA results for the hydrogen production processes indicate that the four-step Cu–Cl cycle has lower environmental impacts than the three- and five-step Cu–Cl cycles due to its lower thermal energy requirement. The global warming, acidification and eutrophication potentials of the system using the four-step Cu–Cl cycle are 0.56 kg CO 2 -eq, 0.00284 kg SO 2 -eq and 0.000232 kg Phosphate-eq, respectively.

Heat extraction from supercritical water-cooled nuclear reactors for hydrogen production plants

Abstract Many current and future hydrogen production methods, such as steam methane reforming and thermochemical water splitting cycles, require large amounts of heat as the major energy input. Using nuclear heat is a promising option for reducing emissions of greenhouse gases and other pollutants, thereby helping achieve clean and sustainable future energy systems. Various heat transfer fluids are compared and evaluation criteria are proposed for the selection of a heat transfer fluid. It is determined that helium is a promising option due to it being inert and chemically stable and having good heat transfer properties. The intermediate heat exchanger for the heat extraction is analyzed and designed using the log mean temperature difference (LMTD) method with helium serving as the heat transfer fluid to extract heat from the supercritical water. It is found that if the heat extraction load is in the range of 100–330 MW th , which approximately corresponds to a hydrogen production range of 40–125 tonnes per day, then a multi-tube and single-shell counter flow heat exchanger with a shell diameter of 0.7–1.3 m and length of 6.7 m encapsulating 420–1600 tubes of 0.025 m diameter would be appropriate according to the practical working conditions on the shell and tube sides. The analysis also shows that the diameter of the heat exchanger does not depend strongly on the heat transfer load if the load is smaller than 330 MW th (125 tonnes H 2 /day). This provides flexibility in case adjustments to the heat extraction load become necessary. However, if the heat load is larger than 330 MW th , for example, 500 MW th for 200 tonnes hydrogen per day, then a multi-tube and single-shell counter flow heat exchanger is not appropriate because the length-to-diameter ratio is outside of the recommended range.

Process Modeling and Thermochemical Experimental Analysis of a Solar Sulfur Ammonia Hydrogen Production Cycle

A solar thermochemical water-splitting cycle which utilizes the electrolytic oxidation of aqueous ammonium sulfite as the hydrogen production half cycle, and an all-liquid potassium sulfate/potassium pyrosulfate salt solution for the oxygen production half cycle was modeled using Aspen Plus. The thermochemistry of various molten salt mixtures for the oxygen production half cycle was studied. A robust Aspen Plus chemical plant model represents the thermodynamic characteristics of the cycle, as verified by experiment. The laboratory experiments showed that the melting point of the salt may be lowered by 100°C with the addition of sodium sulfate/pyrosulfate to the potassium sulfate/pyrosulfate and that the viscosities of the mixtures are low enough for pumping. There is a temperature separation of ∼25°C between evolution of ammonia and SO3 gases during molten salt decomposition.

Control of Heterogeneity in Nanostructured Ce1–xZrxO2 Binary Oxides for Enhanced Thermal Stability and Water Splitting Activity

To enhance the kinetics and overall production of renewable H2 fuel through a two-step thermochemical water splitting cycle, three-dimensionally ordered macroporous (3DOM) Ce1–xZrxO2 (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) materials were synthesized via colloidal crystal templating. The interconnected macropore system in these materials facilitates ready access to a relatively large active surface area (tens of m2/g), which benefits the heterogeneous reaction. Two different synthetic routes were employed, a methanolic solution of metal chloride salts, and a Pechini-type gel. These routes produced significant differences in the compositional homogeneity of the resulting mixed oxide. 3DOM Ce1–xZrxO2 synthesized with methanolic precursors had distinct CeO2- and ZrO2-rich domains, whereas the Pechini samples contained only a single phase. At higher Zr content, heterogeneities present in the samples from the methanolic synthesis increased both the productivity and peak production rates of H2 compared to the single-phase Pechini samples. Increasing the content of Zr in the mixed oxides also stabilized the 3DOM structure at 825 °C. All 3DOM Ce1–xZrxO2 materials exhibited significantly faster kinetics during water splitting compared to sintered, micrometer-sized CeO2 granules. Pechini-derived 3DOM Ce0.8Zr0.2O2 maximized both H2 production and peak production rates, offering better catalytic performance over 3DOM CeO2.

4-2-3 内循環流動層反応器による二段階水熱分解反応の一段階システム化に関する研究(4-2 太陽熱発電・水素製造2,Session4 新エネルギー,研究発表)

4-2-3 内循環流動層反応器による二段階水熱分解反応の一段階システム化に関する研究(4-2 太陽熱発電・水素製造2,Session4 新エネルギー,研究発表)

Prospects for solar only operation of the hybrid sulphur cycle for hydrogen production

The hybrid sulphur process is one of the most promising thermochemical water splitting cycles for large scale hydrogen production. While the process includes an electrolysis step, the use of sulphur dioxide in the electrolyser significantly reduces the electrical demand compared to conventional alkaline electrolysis. Solar operation of the cycle with zero emissions is possible if the electricity for the electrolyser and the high temperature thermal energy to complete the cycle are provided by solar technologies. This paper explores the possible use of photovoltaics (PV) to supply the electrical demand and examines a number of configurations. Production costs are determined for several scenarios and compared with base cases using conventional technologies. The hybrid sulphur cycle has promise in the medium term as a viable zero carbon production process if PV power is used to supply the electrolyser. However, the viability of this process is dependent on a market for hydrogen and a significant reduction in PV costs to around $1/W p.

Commercial activated carbon for the catalytic production of hydrogen via the sulfur–Iodine thermochemical water splitting cycle

Eight commercial activated carbon catalysts were examined for their catalytic activity to decompose hydroiodic acid (HI) to produce hydrogen; a key reaction in the sulfur–iodine (S–I) thermochemical water splitting cycle. Activity was examined under a temperature ramp from 473 to 773 K. No statistically significant correlation was found between the measured catalyst sample properties and catalytic activity. Four of the eight samples were examined for one week of continuous operation at 723 K. All samples appeared to be stable over the period of examination.

Development of direct resistive heating method for SO3 decomposition in the S–I cycle for hydrogen production

The Sulfur–Iodine (S–I) cycle has been considered as one of the efficient and promising thermochemical water-splitting cycles for hydrogen production using nuclear energy. However, the catalytic SO3 decomposition process in the S–I cycle demands high temperature heat (>800°C). Existing nuclear reactors cannot provide such heat for SO3 decomposition. AECL proposed a direct resistive heating concept to compensate for the requirement of high temperature heat. An experimental program was established at AECL to demonstrate the concept and to develop reliable catalyst structures for SO3 decomposition. Due to the high temperature and harsh chemical environment, Hastelloy C-276 was selected as the material for the heating element and reactor. The catalyst was directly applied on the surface of an electrical heating element. SO3 was produced online from H2SO4 in a pre-heated vessel. The SO3 decomposition percentage was determined using the measured O2 concentration in the exit gas stream. The results showed that SO3 decomposition can be successfully achieved with the direct resistive heating method. As much as 90% of the initial SO3 was decomposed under the experimental conditions explored. The Pt-based catalyst performed better than the Fe-based catalyst in the low temperature region (<700°C). The effect of carrier gas flow on SO3 decomposition was also considered.