Investigation on kinetic and thermodynamic characteristics of solar-radiation-driven copper oxychloride decomposition reaction for thermochemical cycle hydrogen generation

Abstract

In this work, a novel instrument for concentrating solar radiation to drive thermochemical reactions was designed. Based on it, the copper oxychloride (Cu₂OCl₂) samples were synthesized and controlled experiments were conducted on Cu₂OCl₂ thermolysis reaction in the copper-chlorine (Cu–Cl) cycle under direct radiation versus pure heat conditions. The isothermal decomposition mechanism function of Cu₂OCl₂ was obtained and validated through reaction kinetics analysis and thermogravimetric (TG) experiments, which conforms to the Avrami-Erofeev model (n = 2). The reaction activation energy is 61.34 kJ/mol under pure heat conditions and 34.88 kJ/mol under direct radiation. The high-energy photon promotes the thermolysis reaction, which is thermodynamically manifested by an increase in oxygen yield and a decrease in reaction initiation temperature, and kinetically by a significant reduction in the activation energy. A system model for one-dimensional heat transfer coupled with reaction kinetics was developed and the thermodynamic performances were investigated. The radiation uniformity and reaction tube dimensions directly affect the temperature distribution. Providing more uniform solar radiation and using a slender reaction tube are beneficial for enhancing heat transfer and improving Cu₂OCl₂ decomposition kinetics. When the length-to-diameter ratio is greater than 20 and diameter-to-thickness ratio is less than 85, the sample temperature at the center exceeds 500 °C. When the tube inner diameter is reduced from 0.2 to 0.06 $m$, the total reaction quantity increases from 8.52 to 19.10 mol. This study is of great significance for the kinetic analysis of thermochemical reactions driven by direct solar radiation and presents instructive insights for designing and optimizing the Cu–Cl cycle system.