MODELING OF IGNITION AND CO OXIDATION IN THE BOUNDARY-LAYER OF A SINGLE CHAR PARTICLE

A model is developed for CO oxidation in the boundary layer of a singl e char particle. The model includes char oxidation and a 56 reaction g as phase kinetic scheme which is coupled with the diffusive properties of the 12 species involved in the CO oxidation. The model is compared with the experimental data of Tognotti and co-workers. The temperatur e reached on ignition, the resulting CO2/CO ratio, and the effect of c hanging water vapor concentrations are well. described. Studies on the effect of water concentration show that significant CO oxidation at l ow temperatures requires a high surrounding water concentration. The p resence of water vapor or hydrogen is found to be necessary to obtain a high degree of CO oxidation at low temperatures; however, there is a minimum temperature below which significant CO oxidation in the bound ary layer does not occur irrespective of how high the water concentrat ion is. In addition, CO oxidation is negligible even at a surface temp erature as high as 2500 K when water and hydrogen are absent. The mode l has also been applied to predict the effects of changing parameters. Catalytic acceleration of the rate of carbon oxidation, for example b y the addition of calcium, leads to both a high particle temperature o vershoot and a significant increase in CO oxidation over the particle surface.