Thermochemical Cycle For Hydrogen Production Research Articles

Experimental investigation of heterogeneous hydrolysis with Zn vapor under a temperature gradient

The hydrolysis step of the Zn/ZnO thermochemical cycle for hydrogen production is experimentally investigated in a laboratory-scale tube-reactor. The current work uses a new approach in which the heterogeneous oxidation of gaseous Zn with steam is carried out under a negative axial temperature gradient in order to improve cycle efficiency by reducing the proportion of steam and inert carrier gas used. It is shown that complete conversion of Zn to ZnO is possible at steam-to-Zn stoichiometries greater than 5.0. As the steam-to-Zn stoichiometry approaches unity at reduced inert gas fractions, condensation of Zn on the reactor walls becomes more likely. In addition, the observed gas-phase equilibrium shift toward increased production of ZnO at temperatures under 800 K is consistent with earlier theoretical predictions. While complete conversion with low inert gas and steam usage was not achieved, our approach shows great improvement over previous aerosol-based approaches when considering the total amounts of steam and inert gas used per unit of hydrogen produced. Therefore, the current temperature gradient approach is promising for the design of an efficient reactor for water splitting via Zn vapor.

Conceptual design, process integration, and optimization of a solar Cu[sbnd]Cl thermochemical hydrogen production plant

A conceptual design of a solar CuCl thermochemical cycle with the capacity of 6000 kg day−1 was presented. To enhance thermal efficiency, the heat recovery within CuCl thermochemical hydrogen production cycle was proposed using the pinch analysis to design a heat exchanger network that recovers heat between hot and cold streams. This improves +10.2% in the thermal efficiency of the cycle compared to previous designs. The reformed cycle was assumed to be coupled to a solar installation that provides the required thermal energy for the cycle. For further improvement, the conceptual design was considered as the base case and four optimization scenarios were conducted on that. Three objective functions, including energy efficiency, exergy efficiency and the unit cost of hydrogen were optimized in three single-objective and one multi-objective scenario. Comprehensive thermodynamic, solar thermal, and exergoeconomic models were employed to obtain objective functions. Reaction temperatures, the number of the solar collectors, and volume of the solar storage tank were selected as design variables. The best alternative to five systems (one base case and four optimized systems) were selected using the TOPSIS method. It was found that thermal efficiency-optimized system has the preference over other four systems. It had 49.83% thermal efficiency, 58.23% exergetic efficiency and 6.33 $ kg−1 for produced hydrogen.

Thermodynamic analysis of solar driven SnO2/SnO based thermochemical water splitting cycle

There are many studies related on the SnO2/SnO based solar thermochemical water splitting cycle, however there are still no studies addressing on the detailed thermodynamic analysis of this process using HSC Chemistry software and its thermodynamic database. In this cycle, the first step belongs to the endothermic solar thermal reduction of SnO2 producing gaseous SnO and O2. The second step corresponds to the exothermic production of H2 via water splitting reaction using SnO produced in the first cycle, thereby regenerating SnO2 which can be recycled back to step 1. Thermodynamic equilibrium compositions associated with step 1 and 2 are identified as a function of reaction temperatures and partial pressures of O2 in the inert carrier gas. Furthermore, the thermodynamic efficiency analysis is performed by following the second law of thermodynamics to determine the cycle and solar-to-fuel energy conversion efficiencies associated with the SnO2/SnO based thermochemical water splitting cycle. Effects of thermal reduction and water splitting temperatures on various thermodynamic parameters are also investigated in detail. Obtained results indicate that the higher values of cycle efficiency (41.17%) and solar-to-fuel energy conversion efficiency (49.61%) are achievable by operating this cycle at a thermal reduction temperature of 1780K and water splitting temperature equal to 800K with 50% heat recuperation. This work gives a detailed thermodynamic and efficiency analysis of SnO2/SnO based two-step solar thermochemical water splitting cycle for hydrogen production.

Metastability of CuCl2 in H2O-HCl for the Copper-Chlorine Thermochemical Cycle of Hydrogen Production

In this paper, metastability of CuCl2 in H2O-HCL for applications to the Cu-Cl thermochemical cycle is experimentally investigated to decrease crystallizing (precipitating) temperature and reduce thermal energy requirements of the cycle. Metastability delays the phase transition by altering the phase transition temperature, depending on direction (heating or cooling). The dissolving temperature is increased if the solution is heated and decreased if the solution is cooled, whereas a typical simple solubility model assumes dissolving and crystallizing occur at the same temperature. The solution temperature is controlled using an ethylene-glycol heater/chiller and measured with a thermocouple. The results of this paper show the metastable zone width increases with cooling rate and is unaffected by HCL concentration. The solubility product constant is calculated for CuCl2 dissolving in various concentrations of HCL and decreases with increasing temperature and HCL concentration.

System efficiency for two-step metal oxide solar thermochemical hydrogen production – Part 2: Impact of gas heat recuperation and separation temperatures

The solar-to-hydrogen (STH) efficiency is calculated for various operating conditions for a two-step metal oxide solar thermochemical hydrogen production cycle using cerium(IV) oxide. An inert sweep gas was considered as the O2 removal method. Gas and solid heat recuperation effectiveness values were varied between 0 and 100% in order to determine the limits of the effect of these parameters. The temperature at which the inert gas is separated from oxygen for an open-loop and recycled system is varied. The hydrogen and water separation temperature was also varied and the effect on STH efficiency quantified. This study shows that gas heat recuperation is critical for high efficiency cycles, especially at conditions that require high steam and inert gas flowrates. A key area for future study is identified to be the development of ceramic heat exchangers for high temperature gas–gas heat exchange. Solid heat recuperation is more important at lower oxidation temperatures that favor temperature-swing redox processing, and the relative impact of this heat recuperation is muted if the heat can be used elsewhere in the system. A high separation temperature for the recycled inert gas has been shown to be beneficial, especially for cases of lower gas heat recuperation and increased inert gas flowrates. A higher water/hydrogen separation temperature is beneficial for most gas heat recuperation effectiveness values, though the overall impact on optimal system efficiency is relatively small for the values considered.

SEM Analysis of Electrophoretically-Deposited Cobalt Ferrite Films

Cobalt ferrite nanoparticles were synthesized and electrophoretically deposited onto aluminum foil, graphite paper, and carbon felt. The resulting cobalt ferrite films were studied to determine their potential for use as an electrocatalyst for the oxidation of ammonium sulfite to ammonium sulfate for a proposed sulfur ammonia thermochemical cycle for hydrogen production. The effects of electrophoretic deposition conditions on deposit morphology and the effects of deposit morphology on electrochemical activity in 2M ammonium sulfite were investigated by SEM analysis.

Effects of the operation modes on the HIx phase purification in the iodine–sulfur thermochemical cycle for hydrogen production

In the iodine–sulfur thermochemical water-splitting process for hydrogen production, purification of hydriodic phase (HIx) had to be adopted to eliminate the minor components in the HIx phases (the mixture of mainly HI, H2O, I2 and small amounts of H2SO4), so as to avoid the undesirable side reactions between HI and H2SO4. In this work, experimental studies on HIx phase purification were carried out. Effects of the operation modes (such as batch operation, continuous operation and the mixed modes of the continuous and discontinuous operation), and process parameters (such as the reaction temperature, feed flowrate of the HIx solution, and the flowrate of the stripping gas N2) on the purification of HIx phase were investigated. The experimental results showed that increase of temperature and N2 flowrate, and decrease of feed flowrate of HIx solution favored the removal of H2SO4 from HIx phase. The purification efficiency would be improved by combining the continuous operation with the batch operation during the purification process. These results were favorable for achieving the closed operation of IS cycle.

Energy and exergy analyses of hydrogen production step in boron based thermochemical cycle for hydrogen production

This study deals with a thermodynamic assessment of hydrogen production step of the boron based thermochemical cycle. In addition, this step is assessed for its merits and demerits in terms of energetic and exergetic performances for various reference environment temperatures. In this regard, the energy and exergy efficiencies of this step are calculated as 11.00% and 20.34% and also the hydrogen production step of the cycle inlet, outlet exergy rates and exergy destruction are calculated as 1653.32 kJ/mol, 336.31 kJ/mol and 317.02 kJ/mol while the reference environment temperature is kept constant at 298 K, respectively. The technical and economic problems of the hydrogen storage and transportation find a possible solution provided that the hydrogen production step of this cycle is performed on-board of a vehicle.

X-ray diffraction of crystallization of copper (II) chloride for improved energy utilization in hydrogen production

Crystallization is an effective method to recover solids from solution, due to its relatively low energy utilization, low material requirements and lower cost compared to other alternatives. Hence, crystallization is of particular interest in the thermochemical copper-chlorine cycle for hydrogen production as an energy-saving means to extract solid CuCl2 from its aqueous solution. It has been determined from experiments that there is a range of concentrations that will demonstrate crystallization. If the initial concentration exceeds the upper bound of this range, the solution will be saturated and instantly become paste-like without forming crystals. Conversely, if the initial concentrations fall below the lower bound of a specified range, the solution will remain liquid upon cooling. As a result, it has been observed that crystallization does not occur for HCl concentrations below 3 M and above 9 M. Also, it has been found that anhydrous CuCl2 does not crystallize under any of the conditions tested. To analyze the composition of the recovered solids, X-ray diffraction (XRD) was employed. The samples were also analyzed using thermogravimetric analysis (TGA) in order to determine their thermochemical properties such as melting and decomposition temperatures. The stationary point on the TGA curve was found to be around 462 °C which is below the normal melting temperature of CuCl2. Also, the vaporization of the samples was found to be approximately 600 °C.

Catalytic performance and durability of Ni/AC for HI decomposition in sulfur–iodine thermochemical cycle for hydrogen production

This work reports the Ni content effect on the Ni/AC catalytic performance in the HI decomposition reaction of the sulfur–iodine (SI) thermochemical cycle for hydrogen production and the Ni/AC catalyst durability in a long-term test. Accordingly, five catalysts with the Ni content ranging from 5% to 15% were prepared by an incipient-wetness impregnation method. The activity of all catalysts was examined under the temperature range of 573–773K. The catalytic performance evaluation suggests that Ni content plays a significant role in the Ni dispersion, Ni particle size, and eventually the catalytic activity in HI decomposition. 12% is the optimal Ni content for Ni/AC catalysts in HI decomposition which is balanced between poor dispersion of Ni particles and increasing active center. The results of 24h durability test, which incorporated with BET and TEM investigations of the 12%Ni/AC catalyst before and after the reaction, indicate that establishing a better Ni particle dispersion pattern and improving the stability of Ni particles on the support should be considered in the future.

Effect of raw material sources on activated carbon catalytic activity for HI decomposition in the sulfur-iodine thermochemical cycle for hydrogen production

In order to study the effect of raw material sources on the catalytic performance of activated carbon, five activated carbon samples generated by steam activation of various raw materials (wood, coal, shell, bamboo and coconut shell) were investigated for their catalytic performance in HI decomposition for the sulfur–iodine (SI) thermochemical cycle. Their catalytic activities were evaluated from 573 K to 823 K. XRD, BET, SEM, proximate and ultimate analyses were used to characterize the catalysts. The results showed the raw materials of activated carbon played an important role in the catalytic activity of the activated carbon sample in HI decomposition. AC-CS and AC-SHELL exhibited the highest activity, followed by AC-BAMBOO, AC-COAL and AC-WOOD. AC-CS had the highest carbon content and the lowest ash content of the activated carbon samples. The catalytic activity of the activated carbon samples increased in coordination with increasing fixed carbon content and decreasing ash content. A long-term evaluation of catalytic performance was performed with AC-CS to assess its stability during extended use as an HI decomposition catalyst.

Comparative assessment of various chlorine family thermochemical cycles for hydrogen production

This study deals with a comparative assessment of various chlorine family cycles, namely copper–chlorine (CuCl), magnesium–chlorine (MgCl), iron–chlorine (Fe–Cl) and vanadium–chlorine (V–Cl) cycles, which are driven by heat and/or electricity. Hydrogen production through thermochemical and/or hybrid cycles can play a significant role in reducing greenhouse gas emissions and hence offering opportunities for better environment and sustainability. In this paper, we conduct energy and exergy analyses of the VCl cycle and examine both energy and exergy efficiencies of the cycle. We also undertake a parametric study to investigate how the overall cycle performance is affected by changing the reference environment temperature and cycle operating conditions. The performance of VCl cycle are evaluated and compared with CuCl, MgCl and FeCl cycles. Furthermore, these cycles are discussed and compared with each other through their advantages and challenges. As a result, VCl cycle offers a good potential due to its high efficiency over 40% based on a complete reaction. In this regard, VCl cycle appears to be one of the most promising low temperature cycles. It may, therefore, compete with other low temperature cycles such as copper–chlorine.

Process integration of material flows of copper chlorides in the thermochemical Cu–Cl cycle

The copper–chlorine (Cu–Cl) thermochemical hydrogen production cycle consists of three chemical reactions, i.e., electrolyisis of copper(I) chloride (CuCl) and hydrogen chloride (HCl), hydrolysis of copper(II) chloride (CuCl2), and thermolysis of copper oxychloride (Cu2OCl2). The outlet stream of the electrolysis includes aqueous CuCl2, CuCl, and HCl. The CuCl2 product of the electrolysis is the reactant of downstream hydrolysis. In this paper, three integration pathways for the copper chloride flows between electrolysis and hydrolysis reactors are investigated in terms of energy saving and reduction of auxiliary operations for the processing of the flows. The integration pathways include solid precipitation of CuCl2 using a crystallization process, water vaporization in the hydrolysis reactor by introducing the electrolyzer outlet stream directly to the reactor, and vaporization in an intermediate spray dryer.

Role of the physicochemical properties of hausmannite on the hydrogen production via the Mn3O4–NaOH thermochemical cycle

In this work, hausmannite (Mn3O4) is used as the key component of a modified Na–Mn thermochemical cycle for hydrogen production. Although this cycle has a lower theoretical thermodynamic efficiency compared with that using MnO (54% instead of 76%), the reduction of Mn2O3 to Mn3O4 occurs at temperatures of around 850 °C (about 550 °C lower than for MnO), and these less stringent operation conditions can be advantageous for increasing the durability of materials and reactors. The present study is mainly focused on determining how some relevant physicochemical properties affect the solid reactivity and, consequently, to the efficiency of hydrogen production. For this purpose, Mn3O4 samples with different morphological and textural characteristics were prepared by changing the conditions of the thermal treatment. These materials were characterized by XRD, SEM and temperature programmed reduction (TPR). The obtained results indicate that the surface area of Mn3O4 is the main factor affecting the hydrogen production yield, while the crystalline domain size has minor influence on the efficiency of the process. In order to explore the viability of the thermochemical cycle, the Mn3O4 sample that exhibited the highest hydrogen production was submitted firstly to hydrolysis and subsequently to a thermal reduction to recover the initial oxide. This study reveals that, although the removal of Na is not complete in the low temperature stage, reducibility of the solid does not appear to be significantly modified by the presence of these impurities. Overall, these results suggest that the Mn3O4–NaOH can be a viable alternative for the production of solar hydrogen.

Experimental investigation of particle dissolution rates in aqueous solutions for hydrogen production

Results of reaction kinetics studies of chemical processes related to materials integration of the thermochemical copper-chlorine cycle for hydrogen production are reported. The reaction rate of solid cuprous chloride (CuCl) in liquid hydrochloric acid is investigated experimentally for various acid concentrations. A rate constant—a function of constituent concentrations—describes how quickly the reactants are converted into products in satisfying the activation energy to enable the reaction to move forward. In this paper, the change in area of a solid CuCl particle is examined, rather than concentration in previous studies. New predictive models are developed to describe the characteristics of the chemical reaction in terms of its transition states and reaction mechanisms.

Evaluation of an iron-chlorine thermochemical cycle for hydrogen production

In this paper, a theoretical and experimental study on an iron-chlorine thermochemical cycle for hydrogen production is presented. The study was addressed to confirm the occurrence of the thermochemical reactions originally proposed and to investigate the influence of kinetic parameters for improving the overall performance of this cycle. Firstly, a thermodynamic analysis was done for determining whether this cycle is attractive for hydrogen production at reaction temperatures below 1223 K, in terms of both energy efficiency and yield of hydrogen. Following, proof-of-concept experiments using a batch reactor were performed at different reaction temperatures, pressures and holding times. Experimental results showed that the reaction temperature is expected to have a small effect for increasing the hydrogen production, while an increase of the system pressure was observed to raise markedly the conversion degree achieved. Based on experimental results, it was possible to confirm the reaction pathway of thermochemical reactions originally proposed, to identify the rate determining step of the overall process, and to explain the beneficial effect of increasing the system pressure on the hydrogen yield. Finally, a modified cycle is proposed for increasing its overall energy efficiency, by lowering the reaction temperature of two thermochemical reactions from 1198 K to 923 K in order to avoid the phase change of FeCl2 that melts at 950 K. Comparative calculations of enthalpy balance and external heat and work requirements for the original and modified cycles showed that the limiting energy efficiency could be theoretically increased from the range 24–28% to 32–37% and this significant increment appears to be promising for further investigations.

Hazard analysis and safety assurance for the integration of nuclear reactors and thermochemical hydrogen plants

The challenges and approaches of the safety and risk management for the hydrogen production with nuclear-based thermochemical water splitting have been far from sufficiently examined, as the thermochemical technology is still at a fledgling stage and the linkage of a nuclear reactor with a hydrogen production plant is unprecedented. This paper focuses on the safety issues arising from the interactions between the nuclear heat source and thermochemical hydrogen production cycle, and also between the proximate individual processes in the cycle. As steam is utilized in many thermochemical cycles for the water splitting reaction, and heat must be transferred from the nuclear source to hydrogen production plant, this paper particularly analyzes and quantifies the heat hazard for the dynamic scenarios of start-up and shutdown of the hydrogen production plant. Potential safety impacts on the nuclear reactor are discussed. It is concluded that one of the main challenges of safety and risk management is efficient rejection of heat in a shutdown accident. Several options for the measures to be taken are suggested. Chemical hazards that may propagate to the nuclear plant zone due to the integration to thermochemical hydrogen production cycles are also examined, and hazard prevention approaches are proposed from the aspects of control at the source, control along the path, and control at the nuclear workplace. It is concluded that linking to high temperature electrolysis (HTE) is safer than to other cycles for the minimization of chemical hazards, and most non-redox thermochemical cycles involve corrosive and very toxic gases that may enter the nuclear zone. It is preferable that the thermochemical processes are confined to a closed building and the vent for the building serves as a hazard absorber. Another option is to protect the infrastructure of the nuclear plant from ingress of hazardous gases. It is expected that the newly reported results of the heat and chemical hazard analysis in this paper could help predict potential hazards and take appropriate measures to prevent risks arising from the linkage of different nuclear reactors and thermochemical cycles.

(Invited) Benchmarking a Metal Oxide-Based Thermochemical Cycle for Solar Hydrogen Production

It is estimated that growth in global population and continued industrialization of developing countries will double world energy consumption by 20351. This rising demand for energy will be largely met by burning fossil fuels, thereby increasing anthropogenic carbon in the atmosphere and fuelling the socioeconomic strife associated with climate change. Developing transformational energy technologies that efficiently and cost-effectively convert solar energy into simple chemical fuels is therefore necessary in order to realize a carbon-neutral, sustainable energy future. In this talk we will critically assess a two-step, high temperature thermochemical water splitting (WS) cycle powered by concentrated solar energy. This conceptually simple technology, outlined in Figure 1, can theoretically achieve greater solar-to-hydrogen (STH) conversion efficiency than alternative WS routes such as photosynthesis (natural or artificial) or photovoltaic-driven electrolysis2. However, many engineering and material science challenges must be overcome to demonstrate economic viability at large scales. Cost and scalability issues for solar thermal hydrogen production will be addressed within the context of a particle-based solar receiver reactor concept3–5developed at Sandia through a multi-year research initiative sponsored by the US Department of Energy. Key design principles will be reviewed and create a framework for discussing the synergistic interplay of materials chemistry, reactor design, and system operation necessary for this technology to achieve optimal STH conversion efficiency. The successful implementation of this solar energy conversion technology is predicated on meeting stringent requirements for cost and scalability that are deeply rooted in STH efficiency. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000. (1) International Energy Outlook 2011; DOE/EIA-0484(2011); U.S. Energy Information Administration, DOE/EIA-0484(2011) | September, 2011. (2) Siegel, N. P.; Miller, J. E.; Ermanoski, I.; Diver, R. B.; Stechel, E. B. Ind. Eng. Chem. Res. 2013, 52, 3276–3286. (3) Ermanoski, I.; Siegel, N. P.; Stechel, E. B. J. Sol. Energy Eng. 2013, 135, 031002. (4) Ermanoski, I.; Miller, J. E.; Allendorf, M. D. Phys. Chem. Chem. Phys. 2014, 16, 8418. (5) Ermanoski, I. Int. J. Hydrog. Energy 2014, 39, 13114–13117. Figure 1

Thermodynamic and environmental impact assessment of steam methane reforming and magnesium-chlorine cycle-based multigeneration systems

In this study, thermodynamic analyses and environmental impact assessments of two integrated systems are comparatively performed for multigenerational applications. The first system consists of a heliostat solar heat and power generation field as energy source, a magnesium–chlorine (Mg–Cl) hybrid thermochemical cycle for hydrogen production, a steam Rankine cycle for power production, and a LiBr–water absorption chiller cycle for space cooling application. The second system is same as the first system except that the steam methane reforming (SMR) which considered for hydrogen production instead of Mg–Cl cycle. The SMR method is commonly used to produce hydrogen; however, it has disadvantages such as releasing CO2 emissions and utilizing fossil fuels for reforming. Mg–Cl cycle can be considered as a promising thermoelectrochemical cycle for water splitting because of its simplicity and applicability as well as possessing higher energy and exergy efficiencies and stability of chemical reactions throughout the cycle. Comparative energy and exergy performance assessments of both integrated systems are performed, and their assessment results are presented and discussed. In addition to the performance evaluation, environmental impact and sustainability assessments of both integrated systems are performed. Furthermore, the study compares two hydrogen production systems in terms of their performances and environmental impacts at varying conditions. Energy and exergy efficiencies of the present systems are found to be 28.4–18.4% for the system I and 43.2–27.4% for the system II, respectively. Exergetic sustainability of SMR-based system is higher than that of Mg–Cl-based system. However, Mg–Cl system does not emit greenhouse gases during production and is more environmentally benign in terms of greenhouse gas emissions. Copyright © 2015 John Wiley & Sons, Ltd.

Equilibrium conversion and reaction analysis in sulfur‐iodine thermochemical hydrogen production cycle

Equilibrium conditions of the main reactions of the SI cycle were studied in this paper. The extent of reaction for two main reactions of this cycle including sulfuric acid decomposition and hydrogen iodide decomposition was evaluated as a function of temperature, pressure, and initial mole number of reactants. In the reaction modelling of sulfuric acid and sulfur trioxide decompositions, two models including simple and multiple reaction models were considered. In the simple model, it was assumed that two decomposition reactions proceeded independently. However, in the multiple reaction model, both reactions were considered simultamneously. It was shown that the Bunsen reaction completely proceeded in the range of operating conditions of the Bunsen reactor. Also, sulfuric acid was demonstrated to be decomposed at a higher rate when the reaction temperature and initial concentration of the sulfuric acid and sulfur trioxide increased. Furthermore, the operating pressure had to be maintained at lower values and the generated sulfur dioxide had to be withdrawn from the reactor in order to have a higher yield of product in this reactor. Finally, in the hydrogen iodide decomposition, the operating pressure, temperature, and concentration of hydrogen iodide had to be maintained at higher values to have a complete reaction. Moreover, the generated hydrogen was shown to be withdrawn from the hydrogen iodide reactor to prevent from backward reaction.