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

Abstract

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 dio.xide 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.

A Review of Solar Thermochemical CO2 Splitting Using Ceria-Based Ceramics With Designed Morphologies and Microstructures

“…The enhanced SSA of the 3-DOM ceria and its interconnected pore system, which facilitated the transport of reacting species to and from oxidation sites, was given as the cause of this increase in the kinetics of CO 2 splitting (Venstrom et al, 2012). This was the first work to clearly demonstrate the benefits of a regular 3-DOM network structure on CO production rates, albeit with quite small cells of only ~0.4 μm (Venstrom, 2012).…”

“…For an isothermal cycle at 800°C, CO production rates as a function of (B) time and (C) reaction extent for CO 2 splitting for commercial ceria, DS ceria and 3-DOM pure ceria. Adapted from Venstrom (2012), ©Luke J. Venstrom.…”

Mechanism of Zn Particle Oxidation by H2O and CO2 in the Presence of ZnO

“…Thermodynamic calculations have shown that it increases with an increase in the yield of fuel (H 2 and/or CO), thus the extent of both reactions 1 and 2 . 5 However, it has also been shown that achieving a high extent of reaction 1 requires ultrafast quenching of the product mixture to suppress the recombination of Zn-vapor and O 2 . 1 This task is generally performed by mixing substantial amounts of inert gas with the solar reactor product stream, 6 which adversely affects the cycle efficiency due to the additional energy required for purification and recycling of the inert gas quencher.…”

Splitting of CO2 via the Heterogeneous Oxidation of Zinc Powder in Thermochemical Cycles

“…Solar thermochemical cycles on the production of CO by splitting CO 2 with metal/metal-oxide redox pairs have been widely investigated, since the character of coping with the energy crisis and increasing carbon emissions. − Such redox pairs include volatile metal oxides (e.g., Zn/ZnO − and SnO/SnO 2 , ) and nonvolatile metal oxides (e.g., FeO/Fe 3 O 4 , ,, ferrite cycles, , CeO 2 /Ce 2 O 3 , , and doped ceria cycles ,, ). Venstrom et al stated that the maximum theoretical thermal efficiency for the dissociation of CO 2 with Zn vapor could reach 31% without heat recovery. Thus, the two-step thermochemical cycle for the splitting of CO 2 based on Zn/ZnO emerges as one of the promising cycles.…”