Thermal management of the copper-chlorine cycle for hydrogen production: analytical and experimental investigation of heat recovery from molten salt

Abstract

Hydrogen is known as a clean energy carrier which has the potential to play a major role in addressing the climate change and global warming, and thermochemical water splitting via the copper-chlorine cycle is a promising method of hydrogen production. In this research, thermal management of the copper-chlorine cycle for hydrogen production is investigated by performing analytical and experimental analyses of selected heat recovery options. First, the heat requirement of the copper-chlorine cycle is estimated. The pinch analysis is used to determine the maximum recoverable heat within the cycle, and where in the cycle the recovered heat can be used efficiently. It is shown that a major part of the potential heat recovery can be achieved by cooling and solidifying molten copper(I) chloride exiting one step in the cycle: the oxygen reactor. Heat transfer from molten CuCl can be carried out through direct contact or indirect contact methods. Predictive analytical models are developed to analyze a direct contact heat recovery process (i.e. a spray column) and an indirect contact heat recovery process (i.e. a double-pipe heat exchanger). Characteristics of a spray column, in which recovered heat from molten CuCl is used to produce superheated steam, are presented. Decreasing the droplet size may increase the heat transfer rate from the droplet, and hence decreases the required height of the heat exchanger. For a droplet of 1 mm, the height of the heat exchanger is predicted to be about 7 m. The effect of hydrogen production on the heat exchanger diameter was also shown. For a hydrogen production rate of 1000 kg/day, the diameter of the heat exchanger is about 3 m for a droplet size of 1 mm and 2.2 m for a droplet size of 2 mm. The results for axial growth of the solid layer and variations of the coolant temperature and wall temperature of a double-pipe heat exchanger are also presented. It is shown that reducing the inner tube diameter will increase the heat exchanger length and increase the outlet temperature of air significantly. It is shown that the air temperature increases to 190oC in a heat exchanger with a length of 15 cm and inner tube radius of 10 cm. The length of a heat exchanger with the inner tube radius of 12 cm is predicted to be about 53 cm. The outlet temperature of air is about 380oC in this case. The length of a heat exchanger with an inner tube diameter of 24 cm is predicted to be about 53 cm and 91 cm for coolant flow rates of 3 g/s and 4 g/s, respectively. Increasing the mass flow rate of air will increase the total heat flux from the molten salt by increasing the length of the heat exchanger. Experimental studies are performed to validate the proposed methods and to further investigate their feasibility. The hazards involving copper(I) chloride are also investigated, as well as corresponding hazard reduction options. Using the reactant Cu2OCl2 in the oxygen production step to absorb CuCl vapor is the most preferable option compared to the alternatives, which include absorbing CuCl vapor with water or CuCl2 and building additional structures inside the oxygen production reactor.