Solar metallurgical process for high-purity Zn and syngas production using carbon or biomass feedstock in a flexible thermochemical reactor

Abstract

Co-production of high-purity Zn and syngas via carbothermal reduction of ZnO was performed in a directly-irradiated concentrated solar reactor, thereby converting and storing intermittent sunlight into high-value chemical fuels and commodities. On-sun experiments were carried out by varying operating parameters including solid carbonaceous feedstocks (either solid carbon or beech wood biomass) in batch and continuous modes at 950–1350 °C, demonstrating solar reactor flexibility and robustness. Decreasing pressure (150–400 hPa) promoted both ZnO reduction rate and net ZnO conversion above 78%, thus enhancing Zn production yield. Nevertheless, CO selectivity decreased because of rising CO2 due to the residence time decrease. A remarkable increase in gas production rates/yields, CO selectivity, and reaction extent was highlighted when increasing temperature during continuous pellets reactant injection. Furthermore, utilizing wood biomass as a sustainable green reducer was proved to be an attractive choice to produce both metallic Zn and high-quality syngas in a single process consisting of biomass gasification with solid ZnO. Zn content exceeding 90 wt% was demonstrated for both batch and continuous tests, unveiling high reactor performance for the metallurgical process. The energy content of the feedstock was upgraded by the solar power input (maximum energy upgrade factor up to 1.2), and the maximum solar-to-fuel energy conversion efficiency up to ∼ 6% was achieved with continuous reactant injection. The high-purity Zn can be further used to produce fuel via CO2-splitting in a complete and fast reaction.