Splitting Water and Carbon Dioxide via the Heterogeneous Oxidation of Zinc Vapor: Thermodynamic Considerations

The heterogeneous oxidation of zinc vapor is proposed as a promising reaction path for the exothermic step in the two-step Zn/ZnO solar thermochemical water and carbon dioxide splitting cycles. This approach circumvents mass transfer limitations encountered in the oxidation of solid or liquid zinc, promising rapid hydrogen and carbon monoxide production rates concurrent with a complete conversion of zinc to zinc oxide. In this paper, a parametric thermodynamic analysis is presented to quantify the benefit of achieving a rapid and complete conversion of zinc via the heterogeneous oxidation of zinc vapor. The conversion of zinc in polydisperse aerosol reactors has been limited to 20% for reaction times on the order of a minute, resulting in a cycle efficiency of ∼6%. The benefit of completely converting zinc via the heterogeneous oxidation of zinc vapor is an increase in efficiency to 27% and 31% for water and carbon dioxide splitting, respectively. The cycle efficiency could be higher if heat recuperation is implemented.