Heterogeneous thermochemical decomposition of a semi-transparent particle under direct irradiation

Transient heat and mass transfer in a non-uniform emitting, absorbing and anisotropically scattering medium inside a semi-transparent, optically large and chemically reacting particle directly exposed to an external source of high-flux radiation is analyzed numerically. Thermal decomposition of calcium carbonate is selected as the model chemical reaction. The unsteady mass and energy equations are solved numerically using the finite volume technique and the explicit Euler time-integration scheme. Radiative transport is modeled using the Rosseland diffusion approximation and the Monte Carlo ray tracing method. Direct irradiation and internal radiative transfer in the particle are highly favorable for particle heating and the decomposition reaction, decreasing the total reaction time by a factor of 15, as compared to the case with external and internal radiation neglected in the analysis. In the latter case, the temperatures at the particle center and the particle surface increase monotonically to 1406 and 1417 K for the reacting particle, and 1428 and 1432 K for the non-reacting particle, respectively, after 179 s—the total reaction time of the reacting particle. With radiation included in the analysis, the surface temperatures of both reacting and non-reacting particle increase from the initial 300 to 1300 K in less than 2 s, and at the same rate until the onset of the endothermic chemical reaction at t=1.1 s. The surface temperature of the reacting particle increases further up to 2000 K after 12 s, when the whole particle is calcined. Weak dependence of the temperature, the overall reaction extent, and the total reaction time on the CaO grain size is observed in spite of strong dependence of the radiative properties of porous CaO on the CaO grain size.