CO2 splitting via two-step solar thermochemical cycles via metal oxide redox reactions:Thermodynamic and kinetic analyses


Peter G Loutzenhiser, peter.loutzenhiser@psi.ch1, Aron Graf2, Anastasia Stamatiou, anastasia.stamatiou@psi.ch1, and Aldo Steinfeld, aldo.steinfeld@eth.ch3. (1) Solar Technology Laboratory, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland, (2) Department of Mechanical and Process Engineering, ETH - Swiss Federal Institute of Technology, CH-8092 Zurich, Switzerland, (3) Department of Mechanical and Process Engineering, ETH Zurich, ETH Zentrum ML-J42.1, 8092 Zurich, Switzerland
Using concentrated solar energy as the source of high-temperature process heat, a two-step CO2 splitting cycle based on Zn/ZnO redox reactions is applied to produce renewable carbon-neutral fuels. The solar thermochemical cycle consists of: 1) the solar endothermic dissociation of ZnO to Zn and O2; 2) a non-solar exothermic reduction of CO2 with Zn to CO and ZnO; the latter is the recycled to the 1st solar step. The net reaction is CO2 = CO + O2, with products formed in different steps, thereby eliminating the need for their separation. A Second-Law thermodynamic analysis indicates a maximum solar-to-chemical energy conversion efficiency of 39% for a solar concentration ratio of 5000 suns. The technical feasibility of the first step of the cycle has been demonstrated in a solar furnace with a 10 kW solar reactor prototype. The second step of the cycle is experimentally investigated in a vertical quartz aerosol reactor, designed for in-situ quenching of Zn vapor, formation of Zn nanoparticles, and oxidation with CO2. Reaction extent and particle characterization are determined by gas chromatograph, X-ray diffraction, SEM, and BET. CO2 conversions of up to 45% are obtained for a residence time of ~ 1 s.