Investigation of coal char-slag transition during oxidation: Effect of temperature and residual carbon

The transition of coal char to molten slag at high conversion was studied for a bituminous coal using a laminar entrained-flow reactor under oxidizing conditions. Post-oxidized char particles were analyzed by various techniques including loss-on-ignition, gas adsorption analysis and scanning electron microscopy to determine carbon content, internal surface area and pore size distribution, and char morphology, respectively. These analyses provide information concerning the effect of temperature and residual carbon on the transition from porous char to molten slag. Results showed that at temperatures above ash flow temperature, the transition from porous char to molten slag occurred at about 90% conversion for the coal used in this study. No transition occurred at temperatures below ash flow temperature. This finding explains previous observations that there is a coal-dependent critical carbon conversion at which the ash stickiness increases dramatically. This result also indicates that surface area can be used as a criterion for determining the critical conversion of the transition. In addition, it was found that the randomly overlapping pore model can not be directly applied to predict the surface area evolution of char particles during the transition without considering the re-opening of closed micropores during initial reaction and the ash fusion effect.