Mass Transfer Coefficients for the Combustion of a Char Particle in O2/CO2

Abstract

As contrasted with the conventional coal combustion, there are higher O2 and CO2 contents in the oxy-fuel combustion. At higher temperature, Stefan flow, due to strong nonequimolar counter-diffusion of the gas component, occurs at the surface of char and has significant effects on the char combustion. But Stefan flow is often neglected in the study of conventional coal combustion. In this work, two correction factors to the case of neglecting the Stefan flow are presented for the mass transfer coefficients related to the oxidation reaction, 2C + O2 → 2CO, and the reduction reaction, C + CO2 → 2CO, respectively. According to the discussions for various limiting combustion conditions, it can be seen that the correction to the mass transfer of the reduction reaction is notable, comparing to that of oxidation reaction. The correction to reduction reaction can be up to 74%, depending on the combustion rate ratio of two reactions and the ambient CO2 content. The correction to oxidation reaction also depends on the content of the product CO except for the combustion rate ratio and ambient O2 content. From the comparisons with the rigid continuous-film model considering Stefan flow and the single-film model neglecting Stefan flow, it is concluded that the corrections greatly improve the predictions for the particle temperature, combustion rates, and burnout time. Comparisons to the experimental results also exhibit better improvement in predicting the burnout of the char particle. The correction factors have simple forms and are fairly easy to incorporate into the char combustion model that neglects the Stefan flow.