Thermodynamic analysis of the co-production of zinc and synthesis gas using solar process heat

We present a solar thermochemical process that combines the reduction of zinc oxide with the reforming of natural gas (NG) for the co-production of zinc and syngas. The overall reaction may be represented by ZnO+CH₄ = Zn+2H₂+CO. The maximum possible overall efficiency is assessed for an ideal, closed cyclic system that recycles all materials and also for a more technically-feasible open system that allows for material flow into and out of the system. Assuming that the equilibrium chemical composition is obtained in a blackbody solar reactor operated at 1250 K, 1 atm, and with a solar power-flux concentration of 2000, closed-cycle efficiencies vary between 40 and 65%, depending on recovery of the product sensible heat. Under the same baseline conditions, open-cycle efficiencies vary between 36 and 50%, depending on whether a Zn/O₂ or an H₂/O₂ fuel cell is employed. Compared to the HHV of methane for generating electricity, the proposed solar open-cycle process releases half as much CO₂ to the atmosphere. The process modelling described in this paper establishes a base for evaluating and comparing different solar thermochemical processes.