Kinetics of the thermal dissociation of ZnO exposed to concentrated solar irradiation using a solar‐driven thermogravimeter in the 1800–2100 K range

Abstract

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.

Thermal dissociation of compressed ZnO and SnO2 powders in a moving‐front solar thermochemical reactor

“…This kinetic analysis was restricted to the initial step of the dissociation reaction (roughly 2-3 min.) corresponding to the start of the O 2 concentration increase ( Figure 6) during the heating period from 1663 K. The estimated activation energy and pre-exponential factor for the ZnO dissociation were found to be 380 AE 16 kJ/mol and (20 AE 10) 27 This good agreement confirms that the surface reaction model (ablative mode) is suitable for the dissociation of ZnO shaped as a compressed rod.…”

Review of Heat Transfer Research for Solar Thermochemical Applications

“…For monolithic reactors with small reactive surface area or small samples used in thermogravitometers the species concentra- [65,[68][69][70]. However, for larger scale reactors, such as fluidized or stabilized beds with large reactive surface area, the species concentration gradient is significant in the flow direction, and a control volume approach combined with Eq.…”

“…The bulk kinetic reaction approach for modeling the reactor scale systems is described here. Experimental investigations of reaction kinetics are typically performed using thermogravimeters [64][65][66][67][68][69][70][71][72] or laboratory scale reactors, including fluidized beds [73][74][75], stabilized beds [76][77][78], and monolithic structured reactors [79][80][81][82][83]. Reaction kinetic modeling associated with each of these practices requires special attention to the corresponding experimental conditions.…”

“…They measured the kinetics of decomposition of zinc sulfate and pyrolysis of cellulose and discuss the necessity for accurate calibration and measurement of transient temperature and weight change. Schunk and Steinfeld [70] developed a solar TGA to study the kinetics of the solar thermal dissociation of zinc oxide. The device was used with a solar furnace capable of providing concentrated solar input up to 5000 suns.…”

“…13. The more accurate kinetic rate law derived from experimental measurements in a solar-driven thermogravimeter for directly irradiated ZnO samples [70] was incorporated into the model. The radiosity and Monte Carlo methods were applied to obtain the distribution of net radiative fluxes, and the finite volume method with the explicit Euler time integration scheme were applied to solve the energy equation over a shrinking domain.…”