Solar thermal reduction of ZnO and SnO2: Characterization of the recombination reaction with O2

This study addresses the high-temperature solar thermal reduction step of the ZnO/Zn and novel SnO2/SnO thermochemical cycles for H2 production. A new method was specifically developed to identify the kinetics of the reduced species recombination with O2. Experimental tests were performed in a solar vessel subjected to high-flux solar irradiation to investigate the influence of pressure, neutral gas flow-rate, and reduction rate on the solar step yield. As a result, the benefic effect of high dilution ratio or reduced pressure operation was pointed out, which thereby eliminates the need for a complex quenching device that may induce significant material losses by condensation of the vapors released by the reaction. In addition, the first detailed kinetic analysis of the recombination reaction with O2 (reverse oxidation of SnO and Zn) was proposed. The kinetic parameters (global reaction order and activation energy) were determined by using an original inverse method combining a non-isothermal plug-flow reactor model and ex-situ powders characterization. A global reaction order of about 1.5 corresponding to the reaction stoichiometry was numerically identified for both SnO and Zn recombination, and activation energies were estimated to be 42±4 kJ/mol for SnO and 35±3 kJ/mol for Zn.