Estimation of mass transfer rate of oxidant to coal char particle surface with partial oxidation reaction in O₂/CO₂ system

In this study, the analytical solution of the mass transfer rate of oxidant to coal char particle surface with partial oxidation in an O₂/N₂ system was extended to that in an O₂/CO₂ system. The difference in the mass transfer mechanism of oxidant in the two systems was investigated by comparing the mass transfer rate of oxidant to char particle surface in these systems. As a result, the mass transfer rate of oxidant to char particle surface was found to be smaller in the O₂/CO₂ system than the O₂/N₂ system, corresponding to the diffusion coefficients when mass transfer is considered to occur by diffusion alone. The effect of convection due to the product of partial oxidation in the O₂/CO₂ system was found to be smaller in O₂/N₂ with an increase in volume fraction of oxidant. Therefore, the mass transfer rate of oxidant in O₂/CO₂ is greater than the difference in diffusion coefficient when both diffusion and convection due to the product of partial oxidation are taken into account. The model parameter in the analytical solution for mass transfer rate of oxidant with diffusion and convection due to the product of oxidation cannot be treated as constant in the O₂/CO₂ system and needs to be calculated according to volume fraction of oxidant, whereas it can be treated as constant in the O₂/N₂ system. The transition temperature from chemical reaction rate-controlling to mass transfer rate-controlling of oxidant is lower in the O₂/CO₂ system than the O₂/N₂ system.