Mass transfer coefficients considering boundary layer reaction in oxy-fuel combustion of coal char

摘要

In comparison with conventional air combustion, char particle reacts with higher concentrations of O_2 and CO_2 in the oxy-fuel combustion. At higher temperatures, strong Stefan flow is formed at the char surface and the CO gas-reaction occurs in the boundary layer, which have significant effects on the char reaction and the mass transfer. But Stefan flow and CO gas-phase reaction are often neglected in the study of conventional coal combustion. In this work, with the consideration of CO oxidation and Stefan flow in the boundary layer of a char particle, modifications to the mass transfer coefficients were presented for the coal oxy-fuel combustion. Better improvement was achieved in predicting the mass transfer coefficient by comparing with the experimental data. Further analyses indicated that the mass transfer coefficient of O_2 decreases and that of CO_2, increases in the presence of CO gas-phase reaction. Moreover, the closer the CO flame sheet is to the char surface, the smaller the correction factor of O_2 mass transfer coefficient and the larger the correction factor of CO_2. If only considering the oxidation reaction at the particle's surface, the minimum correction factor of O_2 is 0.5 which is 32.5% lower than that in the absence of CO gas-phase reaction. If only the surface gasification reaction is considered, the correction factor for CO_2 will increase rapidly at the flame location of about ten times the particle radius, and reach 1.0 at three times the particle radius which means that it is unnecessary to correct the diffusion coefficient. This is very different from the case neglecting the boundary layer reaction. If the surface oxidation and surface gasification are considered simultaneously, the increase of the number of surface reaction will decrease the mass transfer coefficients for both O_2 and CO_2, and will have greater influence on CO_2 transfer than on O_2. These changes of reactive gas mass transfer indicate the significant impacts of CO homogeneous reaction in the oxy-fuel combustion of coal char.