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Thermochemical equilibrium calculations indicate the possibility of significantly lowering the onset temperature of aluminum vapor formation via carbothermal reduction of Al 2 O 3 by decreasing the total pressure, enabling its vacuum distillation while bypassing the formation of undesired by-products Al 2 O, Al 4 C 3 , and Al-oxycarbides. Furthermore, the use of concentrated solar energy as the source of high-temperature process heat offers considerable energy savings and reduced concomitant CO 2 emissions. When the reducing agent is derived from a biomass source, the solar-driven carbothermal reduction is CO 2 neutral. Exploratory experimental runs using a solar reactor were carried out at temperatures in the range 1300 K to 2000 K (1027°C to 1727°C) and with total pressures in the range 3.5 to 12 millibar, with reactants Al 2 O 3 and biocharcoal directly exposed to simulated high-flux solar irradiation, yielding up to 19 pct Al by the condensation of product gases, accompanied by the formation of Al 4 C 3 and Al 4 O 4 C within the crucible. Based on the measured CO generation, integrated over the duration of the experimental run, the reaction extent reached 55 pct at 2000 K (1727°C).

The efficient transfer of high-temperature solar heat to the reaction site is crucial for the yield and selectivity of the solar-driven gasification of biomass. The performance of a gas-solid trickle-bed reactor constructed from a high thermal conductivity porous ceramic packing has been investigated. Beech char particles were used as the model feedstock. A two-dimensional finite-volume model coupling chemical reaction with conduction, convection, and radiation of heat within the packing was developed and tested against measured temperatures and gasification rates. The sensitivity of the gasification rate and reactor temperatures to variations of the packing's pore diameter, porosity, thermal conductivity, and particle loading was numerically studied. A numerical comparison with a moving bed projected a more uniform temperature distribution and higher gasification rates due to the increased heat transfer via combined radiation and conduction through the trickle bed.