Continuous Solar Carbothermal Reduction of Aerosolized ZnO Particles Under Vacuum in a Directly Irradiated Vertical-Tube Reactor

Abstract

A solar-driven aerosolized particle reactor under vacuum was tested for carbothermal reduction of zinc oxide using concentrated solar power. The reactor concept is based on the downward flow of zinc oxide and carbon particles, which are indirectly heated by an opaque intermediate solar absorption tube. The particles are rapidly heated to reaction temperature and reduced within residence times of less than 1 s. In the continuous feeding experiments, maximum sustained temperatures close to 2000 K and heating rates as fast as 1400 K min−1 could be achieved for pressures between 1 and 1000 mbar. Reactant conversions of up to 44% were obtained at 1000 mbar. It was found that a reduction in system pressure leads to a decreased particle residence time (as low as 0.09 s), and therefore low conversion (as low as 1%), thus partially diminishing the positive thermodynamic effects of vacuum operation. Experimental results validate the robust and versatile reactor concept, and simultaneously highlight the necessity of balancing the system design in order to optimize the conflicting influence of vacuum operation and reacting particle residence time.

Solar Metallurgy for Sustainable Zn and Mg Production in a Vacuum Reactor Using Concentrated Sunlight

“…Harnessing the plentiful, clean, and renewable solar energy resource via solar thermochemical conversion processes is a promising avenue to convert intermittent solar radiation into chemical fuels and commodities [1]. In particular, solar carbothermal reduction (CTR) routes for extracting oxygen from volatile metal oxides using solid carbon as a reducing agent in order to produce metals and CO have attracted attention [2]. These processes have the potential to both lower the thermodynamic barrier via utilization of solid carbon reductants and produce metals and CO in a single reaction while The possible solid-gas side reactions taking place during ZnO reduction with solid carbonaceous feedstock are related to Equations (3) and (4).…”

“…According to Equations (1) and (2), the enthalpy change of Zn formation is moderate, implying low operating temperature requirement, easy for scaled-up reactor operation, while that of Mg is quite high, indicating high potential for the storage of solar energy at the expense of higher operating temperature requirement. Therefore, different advantages arise between CTR of ZnO and MgO, which makes these systems attractive for further detailed investigation.…”

Heat Transfer Model of a 50 kW Solar Receiver–Reactor for Thermochemical Redox Cycling Using Cerium Dioxide

“…Ceria has emerged as an attractive redox material due to its fast reaction rates and crystallographic stability [7][8][9][10][11]. Various solar reactor concepts have been proposed to affect the ceria redox cycle and other two-step thermochemical cycles, including cavity receiver-reactors with rotating [12][13][14] or stationary redox materials [15], moving [16,17] and fluidized bed reactors [18], and aerosol flow reactors [19,20]. Previously, we developed a solar cavity-receiver at the 4 kW scale that featured a ceria reticulated porous ceramic (RPC) structure with dual-scale porosity: millimeter-scale pores with struts containing micrometer-scale pores [4].…”