The gasification of wood-char spheres in CO₂-N₂ mixtures: analysis and experiments

Abstract

The gasification of charcoal spheres in an atmosphere of carbon-dioxide-nitrogen mixtures involving diffusion and reactions in the pores is modelled and the results are compared with experiments of Standish and Tanjung and those performed in the laboratory on wood-char spheres to determine the effects of diameter, density, gas composition and flow. The results indicate that the conversion time, t_c ~ d^1.03 for large particles (5 mm), departing substantially from the t_c ~ d² law valid for diffusion limited conditions. The computational studies indicate that the kinetic limit for the particle is below 100 μmm. The conversion time varies inversely as the initial char density as expected in the model. Predictions from the model show that there is no significant change in conversion time up to 60% N₂ consistent with the CO₂-N₂ experiments. The variation of diameter and density with time are predicted. The peculiar dependence of conversion time on flow velocity in the experiments is sought to be explained by opposing free and forced convection heat transfer and the attempt is only partly successful. The studies also indicate that the dependence on the CO concentration with low CO₂ is significant, indicating the need for multistep reaction mechanism against the generally accepted single-step reaction.