The Combustion Rates of Coal Chars: a Review

A brief description is given of devolatilization of raw coal and combustion of the residual char. Major achievements in the study of devolatilization and important areas for further experimentation are noted; however, attention is focussed on the rate processes involved in char combustion. Means of calculating reaction rates are shown which account for temperatures of particles, mass transfer and diffusion of oxygen into the particle's pore structure, and reaction on the pore walls. Theoretical consideration is then given to the changes in the size, density, and pore structure of chars as they burn. Experimental data are shown for observed reactivities of coal chars corrected for external mass transfer effects, and intrinsic reactivities are reported for various coal chars and other carbons corrected for pore diffusion effects. Measured data on mass-transfer rates and changes in particle structure during combustion are given. Recommendations are made for further research on char reactivity and the manner in which pore structure develops during combustion. The need for combustion-oriented kinetics studies of coal devolatilization is outlined.