Experimental Investigation of the Solar Carbothermic Reduction of ZnO Using a Two-cavity Solar Reactor

Abstract

Zinc production by solar carbothermic reduction of ZnO offers a CO2 emission reduction by a factor of 5 vis-a`-vis the conventional fossil-fuel-based electrolytic or Imperial Smelting processes. Zinc can serve as a fuel in Zn-air fuel cells or can be further reacted with H2O to form high-purity H2. In either case, the product ZnO is solar-recycled to Zn. We report on experimental results obtained with a 5 kW solar chemical reactor prototype that features two cavities in series, with the inner one functioning as the solar absorber and the outer one as the reaction chamber. The inner cavity is made of graphite and contains a windowed aperture to let in concentrated solar radiation. The outer cavity is well insulated and contains the ZnO-C mixture that is subjected to irradiation from the inner graphite cavity. With this arrangement, the inner cavity protects the window against particles and condensable gases and further serves as a thermal shock absorber. Tests were conducted at PSI’s Solar Furnace and ETH’s High-Flux Solar Simulator to investigate the effect of process temperature (range 1350-1600 K), reducing agent type (beech charcoal, activated charcoal, petcoke), and C:ZnO stoichiometric molar ratio (range 0.7–0.9) on the reactor’s performance and chemical conversion. In a typical 40-min solar experiment at 1500 K, 500 g of a ZnO-C mixture were processed into Zn(g), CO, and CO2. Thermal efficiencies of up to 20% were achieved.

Solar Aluminum Production by Vacuum Carbothermal Reduction of Alumina—Thermodynamic and Experimental Analyses

“…The use of concentrated solar energy as the source of high-temperature process heat significantly reduces the discharge of greenhouse gases and other pollutants derived from the combustion of fossil fuels. [12,13] Previous relevant metallurgical processes performed in solar furnaces include the carbothermal and methanothermal reductions of Fe 3 O 4 , MgO, ZnO, and SiO 2 to produce Fe, Mg, Zn, and Si, respectively [11,[14][15][16][17] ; the carbothermal reductions of Al 2 O 3 , CaO, SiO 2 , and TiO 2 in an Ar flow to produce Al 4 C 3 , CaC 2 , SiC, and TiC, respectively [17] ; and the carbothermal reductions of Al 2 O 3 , SiO 2 , TiO 2 , and ZrO 2 in a N 2 flow to produce AlN, Si 3 N 4 , TiN, and ZrN, respectively. [17][18][19] The present study thermodynamically examines the vacuum carbothermal reduction of Al 2 O 3 and demonstrates experimentally the production of Al using a biomass-based reducing agent and simulated concentrated solar energy.…”

Experimental Determination of the Extinction Coefficient for a Packed-Bed Particulate Medium

“…The reaction proceeds endothermically at above 1300 K and has been experimentally demonstrated using concentrated solar radiation as the energy source of process heat [2][3][4]. The solar chemical reactor design featured a packed bed of a ZnO-C mixture subjected to high-flux thermal irradiation and undergoing shrinking due to the carbothermic reduction.…”

Review of experimental investigation on directly irradiated particles solar reactors

“…The solar flux is distributed along the external walls of the tubes. Additional concepts of indirect reactors have been also developed for thermochemical applications, such as double-cavity reactors with a reaction chamber physically separated from the one that receives the radiation [8][9][10].…”