Solar Thermal Reduction of ZnO Using $CH4:ZnO$ and C:ZnO Molar Ratios Less Than 1

The solar thermal reduction of ZnO, using solar process heat and $CH4$ or C as reducing agent, is investigated for $CH4:ZnO$ or C:ZnO molar ratios ranging from 0 (thermal decomposition at above about 2000°C) to 1 (stoichiometric reduction at above about 1000°C). At 1400°C, in thermodynamic equilibrium ZnO can be completely reduced using a $CH4:ZnO$ molar ratio of 0.3 and produces one fuel (Zn-metal) rather than two for the stoichiometric case (Zn and syngas). The maximal reactor thermal efficiency without heat recovery from the offgas, defined as the ratio of the heating-value of the zinc produced to the total thermal energy input, is 55%. $CO2$-emissions are reduced by a factor of 10–15 compared to fossil-fuel-based zinc-production technologies. For a closed materials cycle, in which power is extracted from the solar zinc using a fuel cell and the ZnO formed is recycled to the solar reactor, the total exergy efficiency, defined as the work output of the fuel cell to the thermal energy input, varies between 30 to 40% when based on the absorbed solar power in the reactor. These efficiency values are very encouraging, especially since the solar ZnO/Zn cycle allows—in contrast to other regenerative power plants—to store and transport solar energy.