Solar Thermochemical Process Research Articles

The Solar-Driven Coal/Fe3O4 Redox System

The solar-driven endothermic reaction of coal and magnetite was studied for mixing solar and fossil energies. The overall reaction can be represented by CH x + Fe 3 O 4 = CO + 3FeO+1/2xH 2 where x depends on the coal(x= 0.2 in our study). Laboratory experimental studies with an equimolar mixture of anthracite coal and Fe 3 O 4 powder using an infrared furnace showed rapid gas evolution above about 1200°C and 1bar, producing FeO(s) and a gas mixture containing a CO/CO 2 molar ratio of 4.5. Solar-driven experiments were conducted using a high-flux solar furnace. Samples were directly exposed for short time intervals to a solar flux irradiation of 300 W/cm 2 . The carbon content decreased rapidly after only 1 second exposure, suggesting efficient heat transfer and chemical conversion by direct absorption of concentrated solar energy at the reaction site. The proposed solar thermochemical process offers the possibility of performing simultaneously the gasification of coal and reduction of iron oxide, and for producing a fuel with an upgraded calorific value.

Decomposition of limestone in a solar reactor

Abstract The thermal decomposition of limestone has been selected as a model reaction for developing and testing an atmospheric open solar reactor. The reactor consists of a cyclone gas/particle separator which has been modified to let the concentrated solar energy enter through a windowless aperture. The reacting particles are directly exposed to the solar irradiation. Experimentation with a 60 kW reactor prototype was conducted at PSI's 90m 2 parabolic solar concentrator, in a continuous mode of operation. A counter-current flow heat exchanger was employed to preheat the reactants. Eighty five percent degree of calcination was obtained for cement raw material and 15% of the solar input was converted into chemical energy (enthalpy). The technical feasibility of the solar thermal decomposition of limestone was experimentally demonstrated. The use of solar energy as a source for high-temperature process heat offers the potential of reducing significantly the CO 2 emissions from lime producing plants. Such a solar thermochemical process can find application in sunny rural areas for avoiding deforestation.

Thermodynamic analysis of the co-production of zinc and synthesis gas using solar process heat

We present a solar thermochemical process that combines the reduction of zinc oxide with the reforming of natural gas (NG) for the co-production of zinc and syngas. The overall reaction may be represented by ZnO+CH 4 = Zn+2H 2+CO. The maximum possible overall efficiency is assessed for an ideal, closed cyclic system that recycles all materials and also for a more technically-feasible open system that allows for material flow into and out of the system. Assuming that the equilibrium chemical composition is obtained in a blackbody solar reactor operated at 1250 K, 1 atm, and with a solar power-flux concentration of 2000, closed-cycle efficiencies vary between 40 and 65%, depending on recovery of the product sensible heat. Under the same baseline conditions, open-cycle efficiencies vary between 36 and 50%, depending on whether a Zn/O 2 or an H 2/O 2 fuel cell is employed. Compared to the HHV of methane for generating electricity, the proposed solar open-cycle process releases half as much CO 2 to the atmosphere. The process modelling described in this paper establishes a base for evaluating and comparing different solar thermochemical processes.

Solar thermal production of zinc and syngas via combined ZnO-reduction and CH 4-reforming processes

The combined thermal reduction of ZnO and reforming of CH 4 has been thermodynamically and kinetically examined. The chemical equilibrium composition of the system ZnO + CH 4 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) + CH 4 = Zn(g) + 2H 2 + CO. Thermogravimetric measurements on ZnO powder were conducted at various temperatures and CH 4 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 (Δ H∘ 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 CH 4, 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.