Transport limitations in pyrolysing coal particles

Pyrolysis of coal is controlled by chemical kinetics and several transport processes to and inside a coal particle. In nearly all calculations on combustion of solid fuels, the particles are assumed to be so small as to have no internal temperature or concentration gradients. The boundary conditions for which this assumption is permissible are compiled in this paper on the basis of accurate simulation. A one-dimensional mathematical model was used to determine the influence of mass and heat transport processes on the overall devolatilization rate of a coal particle. The delay of volatiles evolution due to transport effects increases with increasing particle diameter. Up to a 'limit diameter', transfer processes have no considerable influence on the course of devolatilization. Depending on temperature and relative gas velocity, limit diameters were calculated for radiative and convective heat transfer to the particle surface and for heat conduction inside the particle. Thus operating conditions (bulk gas temperature, gas velocity, particle diameter) are revealed under which the individual heat transfer processes must not be neglected in simulation. Even for particles <100 mu m in diameter, pyrolysis can be limited by radiative heat transfer under certain boundary conditions. Copyright (C) 1996 Elsevier Science Ltd.