A transient three-dimensional heat and mass transfer model is developed for a semi-transparent optically large, non-uniform and non-isothermal particle exposed to an external source of high-flux irradiation and undergoing chemical reaction. The model couples radiation, conduction and convection heat transfer modes to mass transfer inside the porous particle. The transient mass and energy equations for the solid and gas phases are solved by employing the finite volume method and the explicit Euler time integration scheme. The selected model chemical reaction is the thermal decomposition of CaCO3, which is assumed to proceed at a well-defined CaO-CaCO3 interface between an unreacted CaCO3 spherical core and a porous CaO spherical shell. Radiative heat transfer analysis accounts for emission, absorption and anisotropic scattering inside the particle, with radiative properties obtained from the electromagnetic theory and the Mie scattering theory. The radiative transport inside the particle is modeled using the Monte Carlo ray tracing and the Rosseland diffusion approximation. The results show the effect of non-uniform particle irradiation on the transient temperature and composition of the particle, the total reaction time, and on the relative contributions of the three heat transfer modes to the overall energy balance. The direct irradiation and internal radiative transfer in the particle are highly favorable for the particle heating and the decomposition reaction.
The research presented has significance to the development of industrially relevant high-temperature thermochemical processes involving directly-irradiated semi-transparent particles. In particular, the numerical analyses presented in this study find application to the development of solar thermochemical CO2 capture technologies and solar thermochemical production of lime and cement with reduced CO2 emissions.
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