Multiphase reactor scale-up for Cu–Cl thermochemical hydrogen production

This paper examines selected design issues associated with reactor scale-up in the thermochemical copper–chlorine (Cu–Cl) cycle of hydrogen production. The thermochemical cycle decomposes water into oxygen and hydrogen, through intermediate copper and chlorine compounds. In this paper, emphasis is focused on the hydrogen, oxygen and hydrolysis reactors. A sedimentation cell for copper separation and HCl gas absorption tower are discussed for the thermochemical hydrogen reactor. A molten salt reactor is investigated for decomposition of an intermediate compound, copper oxychloride (CuO·Cl₂), into oxygen gas and molten cuprous chloride. Scale-up design issues are examined for handling three phases within the molten salt reactor, i.e., solid copper oxychloride particles, liquid (melting salt) and exiting gas (oxygen). Also, different variations of hydrolysis reactions are compared, including 5, 3 and 2-step Cu–Cl cycles that utilize reactive spray drying, instead of separate drying and hydrolysis processes. The spray drying involves evaporation of aqueous feed by mixing the spray and drying streams. Results are presented for the required capacities of feed materials for the multiphase reactors, steam and heat requirements, and other key design parameters for reactor scale-up to a pilot-scale capacity.     

Recent Canadian advances in nuclear-based hydrogen production and the thermochemical Cu–Cl cycle

This paper presents recent Canadian advances in nuclear-based production of hydrogen by electrolysis and the thermochemical copper–chlorine (Cu–Cl) cycle. This includes individual process and reactor developments within the Cu–Cl cycle, thermochemical properties, advanced materials, controls, safety, reliability, economic analysis of electrolysis at off-peak hours, and integrating hydrogen plants with Canada's nuclear power plants. These enabling technologies are being developed by a Canadian consortium, as part of the Generation IV International Forum (GIF) for hydrogen production from the next generation of nuclear reactors.     

High performance ceramic carbon electrode-based anodes for use in the Cu–Cl thermochemical cycle for hydrogen production

A high performance ceramic carbon electrode (CCE) was fabricated by the sol–gel method to study the CuCl electrolysis in Cu–Cl thermochemical cycle. The electrochemical behavior and stability of the CCE was investigated by polarization experiments at different concentrations of CuCl/HCl system. The CCE displayed excellent anodic performance and vastly outperformed the bare carbon fiber paper (CFP) even at high concentrations of CuCl (0.5 M) and HCl (6 M), which is explained in terms of increased active area and enhanced anion transport properties. Further enhancement of activity was achieved by coating the CCE layer onto both sides of the CFP substrate.     

Progress of international program on hydrogen production with the copper–chlorine cycle

This paper highlights and discusses the recent advances in thermochemical hydrogen production with the copper–chlorine (Cu–Cl) cycle. Extended operation of HCl/CuCl electrolysis is achieved, and its performance assessment is conducted. Advances in the development of improved electrodes are presented for various electrode materials. Experimental studies for a 300 cm² electrolytic cell show a stable current density and production at 98% of the theoretical hydrogen production rate. Long term testing of the electrolyzer for over 1600 h also shows a stable cell voltage. Different systems to address integration challenges are also examined for the integration of electrolysis/hydrolysis and thermolysis/electrolysis processes. New results from experiments for CuCl–HCl–H₂O and CuCl₂–HCl–H₂O ternary systems are presented along with solubility data for CuCl in HCl–H₂O mixtures between 298 and 363 K. A parametric study of multi-generation energy systems incorporating the Cu–Cl cycle is presented with an overall energy efficiency as high as 57% and exergy efficiency of hydrogen production up to 90%.     

A study on the Fe–Cl thermochemical water splitting cycle for hydrogen production

Thermochemical water splitting cycles are recognized as one of the promising pathways for sustainable hydrogen production. In the present study, Iron-chlorine (Fe–Cl) cycle as one of the chlorine family thermochemical cycles where iron chloride is consumed for hydrogen production from water, is considered for a study. This four-step cycle is modelled by Aspen Plus software package and analyzed for performance investigation of each reaction step and system's components. The parametric studies are also performed to assess the effect of operation conditions such as temperature, pressure and steam to feed ratio on the reaction products and conversion rates. Results indicated that although the effect of pressure is not significant on reaction's production rates, an increase in temperature favors oxygen production in reverse deacon reaction and magnetite production in hydrolysis and lowers hydrogen production in the hydrolysis step. On the other hand, steam to chlorine (Cl₂) ratio is directly correlated with hydrochloric acid (HCl) and oxygen production in reverse deacon reaction and hydrogen production in hydrolysis.     

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.     

Canada’s program on nuclear hydrogen production and the thermochemical Cu–Cl cycle

This paper presents an overview of the status of Canada’s program on nuclear hydrogen production and the thermochemical copper–chlorine (Cu–Cl) cycle. Enabling technologies for the Cu–Cl cycle are being developed by a Canadian consortium, as part of the Generation IV International Forum (GIF) for hydrogen production with the next generation of nuclear reactors. Particular emphasis in this paper is given to hydrogen production with Canada’s Super-Critical Water Reactor, SCWR. Recent advances towards an integrated lab-scale Cu–Cl cycle are discussed, including experimentation, modeling, simulation, advanced materials, thermochemistry, safety, reliability and economics. In addition, electrolysis during off-peak hours, and the processes of integrating hydrogen plants with Canada’s nuclear plants are presented.     

Exergoeconomic machine-learning method of integrating a thermochemical Cu–Cl cycle in a multigeneration combined cycle gas turbine for hydrogen production

Integrating new technologies into existing thermal energy systems enables multigenerational production of energy sources with high efficiency. The advantages of multigenerational energy production are reflected in the rapid responsiveness of the adaptation of energy source production to current market conditions. To further increase the useful efficiency of multigeneration energy sources production, we developed an exergoeconomic machine-learning model of the integration of the hydrogen thermochemical Cu–Cl cycle into an existing gas-steam power plant. The hydrogen produced will be stored in tanks and consumed when the market price is favourable. The results of the exergoeconomic machine-learning model show that the production and use of hydrogen, in combination with fuel cells, are expedient for the provision of tertiary services in the electricity system. In the event of a breakdown of the electricity system, hydrogen and fuel cells could be used to produce electricity for use by the thermal power plant. The advantages of own or independent production of electricity are primarily reflected in the start-up of a gas-steam power plant, as it is not possible to start a gas turbine without external electricity. The exergy analysis is also in favour of this, as the integration of the hydrogen thermochemical Cu–Cl cycle into the existing gas-steam power plant increases the exergy efficiency of the process.     

Performance investigation of magnesium–chloride hybrid thermochemical cycle for hydrogen production

In this paper, we study the yields of reactants in hydrolysis and chlorination chemical processes of the low temperature Mg–Cl hybrid thermochemical cycle to investigate the requirements of temperature, pressure and product ratios for individual reactors of the cycle. A simulation of both hydrolysis and chlorination processes is conducted using the Aspen Plus software. A Mg–Cl cycle is developed by considering the results obtained from the present simulations. Both energy and exergy efficiencies of Mg–Cl cycle are comparatively evaluated under varying system and environmental parameters, and an efficiency comparison of the cycle with other promising thermochemical water splitting cycles is conducted. The results show that, compared to other cycles, lower pressure, higher temperature and higher steam to magnesium–chloride ratio are required for full conversion of reactants in the hydrolysis step; and hence, lower temperature, higher pressure and higher chlorine to magnesium oxide ratio is required for full conversion in chlorination reactor. The efficiency results show that Mg–Cl cycle can compete with other low temperature thermochemical water splitting cycles and under influence of various internal and external parameters.     

Novel thermoelectric generator heat exchanger for indirect heat recovery from molten CuCl in the thermochemical Cu–Cl cycle of hydrogen production

The looming threat of global warming has elicited efforts to develop reliable sustainable energy resources. Hydrogen as a clean fuel is deemed a potential solution to the problem of storage of power from renewable energy technologies. Among current thermochemical hydrogen generation methods, the thermochemical copper-chlorine (Cu–Cl) cycle is of high interest owing to lower temperature requirements. Present study investigates a novel heat exchanger comprising a thermoelectric generator (TEG) to recover heat from high temperature molten CuCl exiting the thermolysis reactor. Employing casting/extrusion method, the performance of the proposed heat exchanger is numerically examined using COMSOL Multiphysics. Results indicate that maximum generated power could exceed 40 W at the matching current of 4.5 A. Maximum energy conversion efficiency yields to 7.1%. Results demonstrate that TEG performance boosts with increasing the inlet Re number, particularly at the hot end. For the molten CuCl chamber, findings denote that there is a 36% discrepancy between highest and lowest Re numbers. Similarly, the highest efficiency value pertains to the case with the highest inlet velocity. Moreover, the highest temperature difference between inlet and outlet of the cooling water is about 28 °C and 10 °C for the lowest and highest inlet Re numbers, respectively. Average deviation from anticipated friction factor and Nusselt number are 0.31% and 12.62%, respectively.     

Heat recovery from molten CuCl in the Cu–Cl cycle of hydrogen production

The copper–chlorine (Cu–Cl) cycle of thermochemical hydrogen production requires heat recovery from molten CuCl at various points within the cycle. This paper examines the convective heat transfer between molten CuCl droplets and air in a counter-current spray flow heat exchanger. This direct contact heat exchanger is analyzed as a proposed new method of recovering heat from the solidified molten CuCl. Effective thermal management within the Cu–Cl cycle is crucial for achieving high thermal efficiency. The cycle’s efficiency is improved drastically when all heat released by the products of reactions is recycled internally. Recovering heat from molten CuCl is very challenging due to the phase transformations of molten CuCl, as it cools from liquid to different solid states. In this paper, a spray column direct contact heat exchanger is analyzed for the heat recovery process. A predictive model of heat transfer and droplet flow is developed and then solved numerically. The results indicate that full heat recovery is achieved with a heat exchanger diameter of 0.13 m, and heights of 0.6 and 0.8 m, for a 1 and 0.5 mm droplet diameter, respectively. Additional results are presented and discussed for heat recovery from molten CuCl in the thermochemical Cu–Cl cycle.     

Solar thermochemical plant analysis for hydrogen production with the copper–chlorine cycle

In this article, a solar-based method of generating hydrogen from the copper–chlorine water-splitting cycle is developed and evaluated. An analysis is performed for solar plants with different hydrogen production capacities at three locations across Canada. Operating parameters of the solar field and the storage units are presented. The thermal efficiency and cost parameters of the hydrogen plant are also examined. A binary mixture of 60% NaNO₃ and 40% KNO₃ is used as the molten salt for solar energy storage. Different hydrogen production rates are analyzed. Since the solar irradiation in Calgary is much less than Toronto and Sarnia in the winter, it is found that a larger storage unit is required. The size of the storage unit increases for larger hydrogen production rates. The results support the feasibility of solar thermochemical Cu–Cl cycle as a promising and efficient pathway for large-scale production of hydrogen.     

Process integration of hydrolysis and electrolysis processes in the Cu–Cl cycle of hydrogen production

Process integration of the hydrolysis and electrolysis processes is one of the most important engineering challenges associated with the Cu–Cl cycle of hydrogen production. The kinetics of the hydrolysis reaction indicates the reversibility of this process which requires H₂O in excess of the stoichiometric quantity, which significantly decreases the overall thermal efficiency of the Cu–Cl cycle. Moreover, the HCl concentration in the produced gas mixture of H₂O and HCl in the hydrolysis reaction is in much lower concentration of the electrolysis reaction requirement for an effective electrolytic cell performance. This paper simulates an integrated process model of the hydrolysis and electrolysis processes by introducing intermediate heat recovery steam generator (HRSG) and HCl–H₂O separation process consisting of rectification and absorption columns. In the separation processes, the influence of operating parameters including reflux ratio, mole fraction of HCl in the feed stream, solvent flow rate and temperature, and column configuration variables such as the location of feed stage and number of stages on the heat duty requirements and the composition of products are investigated and analysed. It is shown that the amount of steam generated in the HRSG unit satisfies the extra steam requirement of the hydrolysis reaction up to 14 times more than its stoichiometric value and the separation process effectively provides HCl acid up to the concentration of 22 mol% for the electrolysis reaction.     

Activated carbon catalysts for the production of hydrogen via the sulfur–iodine thermochemical water splitting cycle

Seven activated carbon catalysts obtained from a variety of raw material sources and preparation methods were examined for their catalytic activity to decompose hydrogen iodide (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. Within the group of lignocellulosic steam-activated carbon catalysts, activity increased with surface area. However, both a mineral-based steam-activated carbon and a lignocellulosic chemically activated carbon displayed activities lower than expected based on their higher surface areas. In general, ash content was detrimental to catalytic activity while total acid sites, as determined by Boehm's titrations, seemed to favor higher catalytic activity within the group of steam-activated carbons. These results suggest that activated carbon raw materials and preparation methods may have played a significant role in the development of surface characteristics that eventually dictated catalyst activity and stability as well.     

Performance assessment of solar-driven integrated Mg–Cl cycle for hydrogen production

The present study develops a new solar energy system integrated with a Mg–Cl thermochemical cycle for hydrogen production and analyzes it both energetically and exergetically for efficiency assessment. The solar based integrated Mg–Cl cycle system considered here consists of five subsystems, such as: (i) heliostat field subsystem, (ii) central receiver subsystem, (iii) steam generation subsystem, (iv) conventional power cycle subsystem and (v) Mg–Cl subsystem. Also, the inlet and outlet energy and exergy rates of all of subsystems are calculated and illustrated accordingly. We also undertake a parametric study to investigate how the overall system performance is affected by the reference environment temperature and operating conditions. As a result, the overall energy and exergy efficiencies of the considered system are found to be 18.18% and 19.15%, respectively. The results show that the Mg–Cl cycle has good potential and attractive overall cycle efficiencies over 50%.     

Preliminary process analysis for the closed cycle operation of the iodine–sulfur thermochemical hydrogen production process

In the iodine–sulfur thermochemical hydrogen production process, a separation characteristic of 2-liquid phase (H₂SO₄ phase and HI_x phase) in the separator at 0°C was measured. Two-phase separation began to occur at about 0.32 of I₂ molar fraction and over. The separation characteristic became better with the increase in iodine concentration in the solution. The effect of flow rate variations of HI solution and I₂ solution from the HI_x distillation column on the process was evaluated. The flow rate increase in HI solution from the distillation column did not have a large effect on the flow rate of HI solution fed to the distillation column from the separator. The decreasing flow rate of I₂ solution from the distillation column decreased the flow rate of I₂ solution fed to the distillation column from the separator. The variation of I₂ molar fraction in the H₂SO₄ phase in the separator was sensitive to the variation in flow rate of both solutions from the distillation column. The tolerance level of the variation was investigated by considering I₂ solubility, 2-liquid phase disappearance and SO₂ reaction amount.     

Sulfur trioxide electrolysis studies: Implications for the sulfur–iodine thermochemical cycle for hydrogen production

In this paper we describe our efforts to develop a sulfur trioxide (SO₃) electrolyzer that could lower the temperature of the SO₃ decomposition step in the sulfur–iodine and hybrid sulfur thermochemical cycles. The objective is to develop an alternative to the standard process of converting SO₃ to SO₂, which is thermal decomposition at 830 °C and above. Thermodynamic calculations show that high SO₃ conversions can be obtained at 590 °C if oxygen is removed during the SO₃ decomposition stage. One way of achieving oxygen removal during SO₃ decomposition is electrolysis, if suitable electrode and electrolyte materials can be found. Active oxygen electrode materials are already developed and we have demonstrated suitability of a thin doped-zirconia electrolyte in this study. The main difficulty came in the development of an active and stable SO₃ electrode. Using Ga–V–O/NbB₂/Au electrodes we demonstrated high catalytic activity, but could not achieve acceptable electrochemical performance.     

Solid particle decomposition and hydrolysis reaction kinetics in Cu–Cl thermochemical hydrogen production

This paper examines cupric chloride solid conversion during hydrolysis in the thermochemical copper-chlorine (Cu–Cl) cycle of hydrogen production. The hydrolysis reaction is a challenging step, due to the excess steam requirement and decomposition of cupric chloride (CuCl₂) into cuprous chloride (CuCl) and chlorine (Cl₂). In this paper, the hydrolysis and decomposition reactions are analyzed with respect to chemical equilibrium conversion and the reaction kinetics. The effects of operating parameters are examined, including the temperature, pressure and excess steam, on equilibrium conversion. It is shown that the reaction kinetics expression that represents a reversible reaction reflects the equilibrium limitation on the solid conversion, rather than first-order kinetics.     

Comparison of different copper–chlorine thermochemical cycles for hydrogen production

Copper–chlorine thermochemical cycles for hydrogen production are very promising water splitting cycles. In this paper, different types of copper–chlorine cycles with various numbers of steps are compared. The factors that determine the number and effective grouping of steps are analyzed. It is found that the water requirement in the hydrolysis step is affected by a combination of drying and hydrolysis steps. It is also found that hydrogen can be produced either from electrolysis of cuprous chloride, or from chlorination of copper by hydrogen chloride, which indicates a potential combination of disproportionation and chlorination steps. The major engineering advantages and disadvantages of these cycle variations with different amounts of steps will be analyzed and discussed.     

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.