A. Frei

Solar thermal production of zinc and syngas via combined ZnO-reduction and CH4-reforming processes

Abstract The combined thermal reduction of ZnO and reforming of CH4 has been thermodynamically and kinetically examined. The chemical equilibrium composition of the system ZnO + CH4 at 1200 K and 1 atm consists of a single gas phase containing Zn (vapor) and a 2:1 mixture of H2 and CO. The overall reaction can be represented as: ZnO(s) + CH4 = Zn(g) + 2H2 + CO. Thermogravimetric measurements on ZnO powder were conducted at various temperatures and CH4 concentrations of the reducing gas. The apparent activation energy obtained was 146 kJ mol−1. By aplying a shrinking-particle model, the reaction mechanism was found to be controlled by gas film diffusion in the Stokes regime. The reaction was also studied in a solar furnace using concentrated radiation as the energy source of high-temperature process heat (ΔH∘1200K = 440 kJ mol−1). Its technical feasibility was demonstrated. The solar receiver consisted of a fluidized-bed tubular quartz reactor coupled to a compound parabolic concentrator. Directly irradiated ZnO particles, fluidized in CH4, acted as heat absorbers and chemical reactants, while the Zn vapor produced was trapped in a cold-finger condenser. The proposed solar combined thermochemical process offers the possibility of simultaneous production of zinc and synthesis gas from zinc oxide and natural gas, without discharging greenhouse gases and other pollutants to the atmosphere. Furthermore, it provides an environmentally clean path for either recycling Zn-air batteries or producing H2 in a water-splitting scheme.

Vacuum Carbothermic Reduction of Al2O3, BeO, MgO-CaO, TiO2, ZrO2, HfO2 + ZrO2, SiO2, SiO2 + Fe2O3, and GeO2 to the Metals. A Thermodynamic Study

Thermochemical equilibrium calculations are carried out to elucidate improved conditions for the production of Al, Si, FeSi, Ti, Mg, Hf, Zr, Be, and Ge by the high-temperature carbothermic reduction of their oxides, and for the production of Mg by the silicothermic reduction of MgO–CaO. The onset temperature for the formation of free Al, Be, Si, Ti, Mg, Hf, and Zr in the gas phase is considerably lowered by decreasing the total pressure, enabling their vacuum distillation. An important prediction of vacuum operation is the suppression of undesired by-products, such as Al-carbide, Al4C3, and the Al-oxycarbides Al2OC and Al4O4C. These species considerably interfere in the carbothermic Al production at an ambient pressure, as shown in preliminary experiments using induction furnace irradiation. CO coproduced in these reactions may be water-gas shifted to syngas and further processed to hydrogen and liquid fuels.

Thermoanalysis of the combined Fe3O4-reduction and CH4-reforming processes

The chemical equilibrium composition of the system Fe3O4 + 4CH4, at 1300 K and 1 atm consists of solid Fe and a 2:1 gas mixture of H2 and CO. Thermogravimetric (TG) analysis combined with gas Chromatographic measurements was conducted on the reduction of Fe3O4 (powder, 2-µm mean particle size) with 2.3, 5, 10, and 20 pct CH4 in Ar, at 1273, 1373, 1473, and 1573 K. The reduction proceeded in two stages, from Fe3O4 to FeO, and finally to Fe. CR, conversion and H2 yield increased with temperature, while the overall reaction rate increased with temperature and CH4 concentration. C (gr) deposition, due to the cracking of CR,, was observed. By applying a topochemical model for spherical particles of unchanging size, the reaction mechanism was found to be mostly controlled by gas boundary layer diffusion. The apparent activation energy reached a maximum at 30 pct reduction extent and decreased monotonically until completion. When compared with the results using instead either H2 or CO as reducing gas, the reduction achieved completion faster using CH4, at temperatures above 1373 K.

Thermo-neutral production of metals and hydrogen or methanol by the combined reduction of the oxides of zinc or iron with partial oxidation of hydrocarbons

Stoichiometry and temperature requirements are determined for combining the endothermic reduction of metal oxides (ZnO, Fe2O3, and MgO) with the exothermic partial oxidation of hydrocarbons (CH4, n-butane, n-octane, and n-dodecane) in order to co-produce simultaneously metals and syngas in thermo-neutral reactions. Thermogravimetric and GC measurements on the combined reduction of ZnO and Fe2O3 with the partial oxidation of CH4 were conducted at 1400 K to experimentally verify the products predicted by equilibrium computations, and resulted in the complete reduction to Zn and Fe, respectively, while producing high quality syngas. A preliminary economic assessment that assumes a natural gas price of 11.9 US

CO2 Splitting via Two-Step Solar Thermochemical Cycles with Zn/ZnO and FeO/Fe3O4 Redox Reactions II: Kinetic Analysis

/MWh and credit for zinc sale at 750 US

Carbothermal reduction of alumina : Thermochemical equilibrium calculations and experimental investigation

/metric ton, indicates a competitive cost of hydrogen production at 6.0 US

Solar hydrogen production via a two-step thermochemical process based on MgO/Mg redox reactions : Thermodynamic and kinetic analyses

/MWh, based on its high heating value. The proposed combined process offers the possibility of co-producing metals and syngas in autothermal non-catalytic reactors, with significant avoidance of CO2 emission.