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

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 CuCl₂ 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 CuCl₂ to Cu₂OCl₂ 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 CuCl₂ 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.     

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

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 H₂/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.     

Greenhouse gas reduction in oil sands upgrading and extraction operations with thermochemical hydrogen production

This paper examines various methods of reducing CO₂ emissions by a thermochemical copper–chlorine (Cu–Cl) cycle of hydrogen production, for in-situ extraction and upgrading of bitumen to synthetic crude oil in Alberta’s oil sands. Particular focus is given to Canada’s SCWR (Supercritical Water-cooled Reactor) as a nuclear heat source for the Cu–Cl cycle, although other heat sources such as solar or industrial waste heat can be utilized. The feasibility of steam generation from supercritical water of a SCWR power plant is examined for bitumen extraction, as well as hydrogen production for bitumen upgrading via an integrated Cu–Cl cycle with SCWR. The heat requirements for bitumen extraction from the oil sands, and the hydrogen requirements for bitumen upgrading, are examined. A new layout of oil sands upgrading operations with integrated SCWR and a Cu–Cl cycle is presented. The reduction of CO₂ emissions due to the integrated SCWR and Cu–Cl cycle is quantitatively investigated based on the expected bitumen production capacity over the next two decades.     

Hydrogen production by the Cu–Cl thermochemical cycle: Investigation of the key step of hydrolysing CuCl2 to Cu2OCl2 and HCl using a spray reactor

One of the most challenging steps in the thermochemical Cu–Cl cycle for the production of hydrogen is the hydrolysis of CuCl₂ into Cu₂OCl₂ and HCl while avoiding the need for excess water and the undesired thermolysis reaction, which gives CuCl and Cl₂. Argonne National Laboratory has designed a spray reactor where an aqueous solution of CuCl₂ is atomized into a heated zone, into which steam/Ar are injected in co- or counter-current flow. The solid products of the reaction were analyzed by XRD and SEM. With a pneumatic nebulizer, the counter-current flow design gave high yields of Cu₂OCl₂ compared to the co-current flow design, but some CuCl₂ remained unreacted in both designs. With an ultrasonic nozzle, essentially 100% yields of Cu₂OCl₂ were obtained. Some CuCl was present in the products with both types of atomizers but this is believed to be due to decomposition of Cu₂OCl₂ rather than CuCl₂. Analyses of gaseous products from the hydrolysis reactions in a fixed bed were conducted at the Commissariat à L'Energie Atomique using ultraviolet-visible spectrometry and conductivity. At a reaction temperature of 390 °C, the desired HCl was formed while no Cl₂ was detected until the bed temperature was above 400 °C.     

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.     

太阳能制氢关键技术研究

围绕太阳能制氢技术展开论述,首先,介绍太阳能制氢技术的研究现状;其次,对于太阳能制氢技术尤其是光催化制氢技术及热化学循环分解水制氢技术,分别从技术原理、关键材料、技术难点等方面进行详细的论述;最后,对太阳能制氢技术研究给出结论及建议,旨在为未来太阳能制氢技术的研发布局和产业技术突破提供参考和思路。     

金属氧化物热化学循环分解水制氢热力学基础及研究进展

介绍了以金属氧化物为介质的热化学循环分解水制氢。与其它循环体系相比,金属氧化物循环仅由两步反应组成,过程简单、不向环境排放有害物质、避免了高温下分离气体的困难。研究发现,以铁酸盐为代表的复合体系有望在较温和的条件下进行反应,如果能与太阳能或者高温核反应堆耦合,则有望成为清洁的、具有经济性的制氢方法。     

Study of the miscibility gap in H2SO4/HI/I2/H2O mixtures produced by the Bunsen reaction – Part I: Preliminary results at 308 K

In the framework of the optimization of the sulfur–iodine thermochemical cycle for massive hydrogen production, investigations were performed in order to characterize the liquid phase (HIx and H₂SO_4(aq) phases) separation of solutions resulting from Bunsen reaction. Quaternary H₂SO₄/HI/I₂/H₂O mixtures were prepared at 308 K with different relative proportions of reactants and the chemical composition of each of the two phases formed was analyzed. An increase in iodine concentration and a decrease in water concentration appeared to improve the liquid–liquid equilibrium phase separation. However, a too low concentration of water also promoted the formation of byproducts. An increase in the [H₂SO₄]/[HI] ratio tended to favor the separation and seemed to lead to a dehydration of the HIx phase.     

Uranium thermochemical cycle: hydrogen production demonstration

In hydrogen production industry, thermochemical cycle technology for converting thermal energy into chemical storage energy of hydrogen owns absolute advantages. Compared with other thermochemical cycles, thermochemical cycle technology based on uranium (UTC) is safer and more efficient. This technology consists of three steps, where only the hydrogen production step is unique. In this paper, the verification has been done for this step. Solid products were characterized by XRD and Raman spectroscopy, which were confirmed to be α-Na₂U₂O₇. Gas chromatographic analyses were performed for gas samples, in which hydrogen output was obtained using an internal standard method.     

Membrane distillation of and mixtures for the sulfur–iodine thermochemical process

The concentration of hydriodic and sulfuric acid aqueous solutions by membrane distillation (MD) was experimentally investigated. Two commercial hydrophobic membranes, with two different recirculation batch configurations, were tested: direct contact membrane distillation (DCMD) with a polypropylene (PP) capillary membrane, and air-gap membrane distillation (AGMD) with polytetrafluoroethylene (PTFE) flat-sheet membrane. Feed temperatures were 58 °C for DCMD and 80 °C for AGMD, with cooling water at 15 °C and 1 atm operating pressure in both cases. H₂SO₄ concentration in the feed solution increased from 1.1 up to 7.0 mol/L with the DCMD and from 0.9 up to 10.1 mol/L with the AGMD. HI concentration increased from 0.3 up to 7.0 mol/L with the DCMD and from 0.3 up to 8.0 mol/L with the AGMD. The latter value is higher than the azeotropic concentration of the HI/H₂O (7.57 mol/L) mixture and, hence, HI further separation from water can be easily achieved with conventional distillation units. Durability of PTFE membrane in acid solutions was assessed too.     

Conceptual design of a two-step solar hydrogen thermochemical cycle with thermal storage in a reaction intermediate

This paper presents the conceptual design for a two-step thermochemical cycle producing hydrogen continuously, even off-sun, with the concentrated solar energy as the heat source. For a case study, the two-step iron oxide cycle (Fe₃O₄/FeO) is selected to illustrate the design concept. Two reactors, one storage tank and the solar collector comprise the system. Molten wustite (FeO) is accumulated in the storage tank on-sun. The FeO is not only involved in the reactions but also acts as the heat transfer medium, obtaining the energy from the solar insolation and delivering energy to support the thermal decomposition of magnetite (Fe₃O₄). In this way, the temperature limitation (<800 K) of molten salt is solved, and the intermittency problem of variable insolation is circumvented. A simple feedback scheme is used to control the flow rate between the storage tank and the reactors in order to minimize the temperature fluctuations. For the wustite hydrolysis reaction, the volumetric flow rate of water is regulated to control the temperature in the reactor. We derived the kinetics of the two-step iron oxide cycle from previous experimental reports. We simulated the dynamics of the system over 50 days with mass and energy balances. The simulation results show that the storage tank temperature will be stationary at 2250 K. After five days, the decomposition temperature at 2100 K, and the hydrogen production stabilized at 7 kg/min. Admitting the difficulty of high temperature operation, this design is still promising due to the high efficiency of two-step cycle itself, the process intensification of the FeO acting as the reactant/product/heat transfer medium (no need of heat exchangers), and the continuous operation/production of hydrogen.     

太阳能制氢技术及其进展

阐述了光解水制氢的原理,介绍了光解水制氢技术的现状,分析了目前光解水制氢技术存在的问题以及提高光解水效率的有效途径,指出了利用光热化学循环进行光解水制氢的新途径.     

Determining parameters of heat exchangers for heat recovery in a Cu–Cl thermochemical hydrogen production cycle

A heat exchanger is a device built for efficient heat transfer from one medium to another. Shell and tube heat exchangers are separated wall heat exchangers and are commonly used in the nuclear and process industry. The CuCl cycle is used to thermally crack water in to H₂ and O₂. The present study presents the heat exchanger thermal design using analysis of variance for heat recovery from oxygen at 500 °C, coming from the molten salt reactor. Polynomial regressions in terms of the amount of chlorine in the oxygen, the mass flow rate on the tube side, and the shell's outlet temperature are estimated for various exchanger parameters and the results are compared with the bell Delaware method. Based on energy and exergy analysis, this study also discusses the best possible path for the recovered heat from oxygen. Optimal heat exchanger parameters are estimated by Design-Expert^® Stat-Ease for most effective heat recovery.     

Steam flow effects on hydrolysis reaction kinetics in the Cu–Cl cycle

In this paper, the effects of an inert carrier gas and steam flow on the reaction kinetics of a CuCl₂ hydrolysis reactor are examined for the thermochemical copper-chlorine (Cu–Cl) cycle of hydrogen production. Experimental data from two packed bed reactors, at three separate vapour pressures of H₂O in the gaseous input stream, are investigated in terms of the transient conversion efficiencies and reaction kinetics. The results show that the transient reaction rate reduces by over 75% as the reaction progresses and physical resistances develop in the reactor. The effects of system temperature and reactant flowrate on the reaction rate are also investigated with experimental data. The results of this paper show that by reducing the steam density, the variability in reaction rate can be decreased. These results can be used to predict the reaction kinetics, allowing residence time and transport properties to be more effectively considered.     

Natural gas usage as a heat source for integrated SMR and thermochemical hydrogen production technologies

This paper investigates various usages of natural gas (NG) as an energy source for different hydrogen production technologies. A comparison is made between the different methods of hydrogen production, based on the total amount of natural gas needed to produce a specific quantity of hydrogen, carbon dioxide emissions per mole of hydrogen produced, water requirements per mole of hydrogen produced, and a cost sensitivity analysis that takes into account the fuel cost, carbon dioxide capture cost and a carbon tax. The methods examined are the copper–chlorine (Cu–Cl) thermochemical cycle, steam methane reforming (SMR) and a modified sulfur–iodine (S–I) thermochemical cycle. Also, an integrated Cu–Cl/SMR plant is examined to show the unique advantages of modifying existing SMR plants with new hydrogen production technology. The analysis shows that the thermochemical Cu–Cl cycle out-performs the other conventional methods with respect to fuel requirements, carbon dioxide emissions and total cost of production.