Kinetic analysis of high-temperature solid-gas reactions by an inverse method applied to ZnO and SnO2 solar thermal dissociation

This study addresses the kinetic investigation of solid-gas reactions in a high-temperature solar chemical reactor. An inverse method was developed to identify the kinetics of metal oxide thermal dissociation as part of a two-step thermochemical redox cycle for solar splitting of H2O and CO2. This method was applied and further validated by studying ZnO and SnO2 solar thermal dissociation. A solar chemical reactor enabling continuous solid reactant processing was developed in which both the oxide reactant temperature at the front surface and the O-2 concentration in the off gas were measured dynamically. The aim of the inverse method was to identify the intrinsic kinetics of the dissociation reaction using only the available experimental data. Different approaches were proposed and compared to investigate the kinetics of solid-gas reactions. The activation energy of the reaction was first estimated roughly using an iso-conversional model-free approach, which can be used as an initialization value for further refinement with the inverse method. The inverse method consists in identifying the reaction kinetics from only the online diagnosis of outlet O-2 concentration and using a model enabling parameters fitting via an iterative process. Depending on the considered approach and assumptions for predicting the temperature profile within the reacting oxide rod (succession of stationary states assumption or unsteady state operation), the activation energy of the dissociation reaction was found to be 313 +/- 31 kJ/mol for ZnO and 353 +/- 18 kJ/mol for SnO2. Such a method may be implemented for the kinetic analysis of any kind of solid-gas reactions in high-temperature solar reactors. (c) 2012 Elsevier B.V. All rights reserved.