Solar Syngas Production from H2O and CO2 via Two-Step Thermochemical Cycles Based on Zn/ZnO and FeO/Fe3O4 Redox Reactions: Kinetic Analysis

Abstract

Syngas production via a two-step H2O/CO2-splitting thermochemical cycle based on Zn/ZnO and FeO/Fe3O4 redox reactions is considered using highly concentrated solar process heat. The closed cycle consists of: (1) the solar-driven endothermic dissociation of ZnO to Zn or Fe3O4 to FeO; (2) the nonsolar exothermic simultaneous reduction of CO2 and H2O with Zn or FeO to CO and H2 and the initial metal oxide; the latter is recycled to the first step. The second step was experimentally investigated by thermogravimetry for reactions with Zn in the range 673−748 K and CO2/H2O concentrations of 2.5−15% in Ar, and for reactions with FeO in the range 973−1273 K and CO2/H2O concentrations of 15−75% in Ar. The reaction mechanism was characterized by an initial fast interface-controlled regime followed by a slower diffusion-controlled regime. A rate law of Langmuir−Hinshelwood type was formulated to describe the competitiveness of the reactions based on atomic oxygen exchange on active sites, and the corresponding Arrhenius kinetic parameters were determined by applying a shrinking core model.