An improved engineering design of a solar chemical reactor for the thermal dissociation of ZnO at above 2000K is presented. It features a rotating cavity receiver lined with ZnO particles that are held by centrifugal force. With this arrangement, ZnO is directly exposed to concentrated solar radiation and serves simultaneously the functions of radiant absorber, chemical reactant, and thermal insulator. The multilayer cylindrical cavity is made of sintered ZnO tiles placed on top of a porous 80%Al2O3–20%SiO2 insulation and reinforced by a 95%Al2O3–5%Y2O3 ceramic matrix composite, providing mechanical, chemical, and thermal stability and a diffusion barrier for product gases. 3D computational fluid dynamics was employed to determine the optimal flow configuration for an aerodynamic protection of the quartz window against condensable Zn(g). Experimentation was carried out at PSI’s high-flux solar simulator with a 10kW reactor prototype subjected to mean radiative heat fluxes over the aperture exceeding 3000suns (peak 5880suns). The reactor was operated in a transient ablation mode with semicontinuous feed cycles of ZnO particles, characterized by a rate of heat transfer—predominantly by radiation—to the layer of ZnO particles undergoing endothermic dissociation that proceeded faster than the rate of heat transfer—predominantly by conduction—through the cavity walls.
in Wiley InterScience (www.interscience.wiley.com).The two-step H 2 O-splitting thermochemical cycle based on the Zn/ZnO redox reactions is considered for solar H 2 production, comprising the endothermal dissociation of ZnO followed by the exothermal hydrolysis of Zn. A solar-driven thermogravimeter, in which a packed-bed of ZnO particles is directly exposed to concentrated solar radiation at a peak solar concentration ratio of 2400 suns while its weight loss is continuously monitored, was applied to measure the thermal dissociation rate in a set-up closely approximating the heat and mass transfer characteristics of solar reactors. Isothermal thermogravimetric runs were performed in the range 1834-2109 K and fitted to a zero-order Arrhenius rate law with apparent activation energy 361 AE 53 kJ mol À1 K À1 and frequency factor 14.03 Â 10 6 AE 2.73 Â 10 6 kg m À2 s
À1. Application of L'vov's kinetic expression for solid decomposition along with a convective mass transport correlation yielded kinetic parameters in close agreement with those derived from experimental data.
Rapid cooling for avoiding the recombination of Zn vapor and O 2 derived from the solar thermal dissociation of ZnO is investigated using a thermogravimeter coupled to a quenching apparatus. The ZnO sample, which is placed in a cavity receiver and directly exposed to concentrated solar irradiation, underwent dissociation in the temperature range 1,820-2,050 K at a rate monitored by on-line thermogravimetry. The product gases were quenched by water-cooled surfaces and by injection of cold Ar at cooling rates from 20,000 to 120,000 K/s, suppressing the formation of ZnO in the gas phase and at the walls. Zinc content of the collected particles downstream varied in the range 40-94% for Ar/Zn(g) dilutions of 170 to 1,500.
A transient heat transfer model is formulated for a shrinking packed-bed of reacting ZnO particles exposed to concentrated solar irradiation. The model combines conduction, convection, and radiation heat transfer with simultaneous sintering and reaction kinetics. Validation is accomplished in terms of temperatures and dissociation rates experimentally measured using a solar-driven thermogravimeter with ZnO packed-bed samples subjected to solar flux concentration ratios in the range 1225-2133 suns and surface temperatures in the range 1834-2109 K. Operating conditions are typical of an ablation regime controlled by the rate of radiative heat transfer to the first layers of ZnO undergoing endothermic dissociation.
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