SOLAR THERMAL PRODUCTION OF ZINC AND SYNGAS VIA COMBINED ZNO-REDUCTION AND CH4-REFORMING PROCESSES

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 H-2 and CO. The overall reaction can be represented as: ZnO(s) + CH4 = Zn(g)+ 2H(2) + 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 (Delta H degrees(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 H-2 in a water-splitting scheme.