Effects of Stefan Flow on Ignition and Heat Transfer of a Char Particle in O2/CO2 Atmospheres

Abstract

Stefan flow, occurring at the surface of a carbon particle, is often neglected or simply regarded as a mass transfer phenomenon in pulverized-coal combustion. In this work, considering the Stefan flow and the CO oxidation in the boundary layer, a heat balance equation of the char particle is developed to analyze the effects of Stefan flow on the ignition and heat transfer in oxy-fuel combustion. Better agreement is achieved between the experimental data and the predictions with the novel equation. When the Stefan flow is considered, the ignition delay is observed, and a higher gas temperature and a larger oxygen mole fraction are required for the ignition of the char particle. The neglect of Stefan flow would lead to larger combustion rate, higher surface temperature, and smaller burnout diameter under the same conditions, implying the overestimation of char burnout rate. For the situation of considering Stefan flow, the contribution of CO2 reduction reaction to the total reaction heat flux is smaller since the Stefan flow blocks the diffusion of CO2 toward the particle surface. It results in the increase of total reaction heat flux in the boundary layer with the gas temperature increasing. In addition, the presence of Stefan flow greatly weakens the heat transfer by conduction and accelerates the heat loss from the particle, which makes the particle temperature lower. It can be concluded that the effects of Stefan flow on the heat balance of a char particle cannot be neglected in oxy-fuel combustion.