Solar thermal reduction of ZnO and SnO 2: Characterization of the recombination reaction with O 2

Abstract

This study addresses the high-temperature solar thermal reduction step of the ZnO/Zn and novel SnO 2/SnO thermochemical cycles for H 2 production. A new method was specifically developed to identify the kinetics of the reduced species recombination with O 2. 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 O 2 (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.