First-principles-based multiscale modelling of heterogeneous CoO oxidation kinetics in high-temperature thermochemical energy storage

Abstract

Metal oxides experiencing reversible oxygen release and uptake reactions involve the scale-span (surface→grain→particle→reactor) problem in a thermochemical energy storage reactor. This study developed a first-principles-based multiscale modelling to calculate the heterogeneous CoO oxidation kinetics. At the surface scale, density function theory (DFT) is used to compute the reaction mechanism, and the data from DFT calculation are integrated into the transition state theory (TST) to calculate the reaction rate constants. At the grain scale, a rate equation theory is proposed to couple the surface reaction with the lattice oxygen in bulk phase. At the particle scale, intraparticle diffusion and external diffusion of gas molecules are calculated by the effective factor and mass transfer coefficient. At the reactor scale, a two-phase fluidization model is adopted to describe the mass transfer of oxygen from bubble phase to emulsion phase. The model was used to simulate the oxidation kinetic characteristics of a newly prepared kaolin-supported Co-based oxide. The reaction mechanism and kinetic rates constants obtained from DFT and TST are directly used to predict the CoO oxidation kinetics instead of by fitting the experimental data. The oxidation kinetics were validated by comparing with the experimental data obtained using a micro-fluidized bed thermogravimetric analysis technology. Results show the oxidation will be fully finished within ~10 s in the temperature of 700–750 °C, and the reaction is first-order when the O2 partial pressure is within 21 vol%. The effect of particle size shows three-zone characteristics of chemical reaction and gas diffusion combined controlling mechanism. It is demonstrated the first-principle-based multiscale modelling can provide an accurate prediction of the CoO oxidation kinetics in thermochemical energy storage.