THE GASIFICATION OF WOOD-CHAR SPHERES IN CO2-N2 MIXTURES - ANALYSIS AND EXPERIMENTS

The gasification of charcoal spheres in an atmosphere of carbon-dioxid e-nitrogen mixture involving diffusion and reactions in the pores is m odelled 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) approximately d1.0 3 for large particles (> 5 mm), departing substantially from the t(c) approximately d2 law valid for diffusion limited conditions. The compu tational studies indicate that the kinetic limit for the particle is b elow 100 mum. The conversion time varies inversely as the initial char density as expected in the model. Predictions from the model show tha t there is no significant change in conversion time up to 60% N2 consi stent with the CO2-N2 experiments. The variation of diameter and densi ty with time are predicted. The peculiar dependence of conversion time on flow velocity in the experiments is sought to be explained by oppo sing free and forced convection heat transfer and the attempt is only partly successful. The studies also indicate that the dependence on th e CO concentration with low CO2 is significant, indicating the need fo r multistep reaction mechanism against the generally accepted single-s tep reaction.