Heterogeneous Thermochemical Decomposition of a Semi-Transparent Particle Under High-Flux Irradiation—Changing Grain Size Versus Shrinking Core Models

Abstract

An unsteady numerical model coupling radiative-conductive-convective heat and mass transfer to chemical kinetics of heterogeneous decomposition is developed for a semi-transparent and optically large CaCO3 reacting particle exposed to direct high-flux irradiation. Two decomposition models are studied: a shrinking core model, with a well-defined CaO–CaCO3 interface between the spherical unreacted CaCO3 core and the porous CaO spherical shell around the core, and a volumetric model, with changing grain size throughout the particle. The Rosseland diffusion approximation is employed to solve for internal radiative transfer in the particle. The mass and energy equations are solved numerically by employing the finite volume method and the explicit Euler time integration scheme. For fixed CO2 partial pressure and total gas phase pressure, the computed temperature profiles show the features typical for a heat transfer limited reaction. The volumetric reaction model leads to a faster chemical conversion as compared to that for the shrinking core model, by 41.3%. For CO2 pressure increasing due to the chemical reaction and diffusion of CO2 being the only mass transfer mode inside the porous particle, the reaction becomes mass transfer limited. The increasing partial pressure of CO2 inhibits the chemical reaction, leading to an increase of the total reaction time by a factor of 433 and 734 for SCM and VM, respectively, compared to the case with fixed partial pressure of CO2.