CO2 splitting via two-step solar thermochemical cycles with Zn/ZnO and FeO/Fe3O4 redox reactions:: Thermodynamic analysis

Two-step thermochemical cycles for CO2 reduction via Zn/ZnO and FeO/Fe3O4 redox reactions are considered. The first, endothermic step is the thermal dissociation of the metal oxide into the metal or a reduced valence metal oxide and O-2 using concentrated solar energy as the source of high-temperature process heat. The second, nonsolar, exothermic step is the reaction of the reduced metal/metal oxide with CO2, yielding CO and/or C, together with the initial form of the metal oxide that is recycled to the first step. Chemical equilibrium compositions of the pertinent reactions are computed as a function of temperature and pressure. A second-law thermodynamic analysis for the net reaction of CO2 = CO + 0.5O(2), indicates a maximal solar-chemical energy conversion efficiency of 39 and 29% for the Zn/ZnO and FeO/Fe3O4 cycle, respectively. Efficiencies are lower for both cycles yielding C. Major sources of irreversibility are associated with the re-radiation losses of the solar reactor operating at 2000 K and the quenching of its products to avoid recombination.