Transient heating of coal particles undergoing pyrolysis

Abstract

Most of the studies reported to date have viewed the coal devolatilization process as occurring isothermally throughout the coal particle, and transient processes occurring in the interior of coal particle have been largely ignored. However, in pulverized coal combustion processes involving several heating rates and several particle sizes, large temperature gradients are often produced within the coal particles that are undergoing pyrolysis. Large temperature gradients inside the coal particle result in pyrolysis of the coal particle in the following manner. The pyrolysis front first initiated at the particle surface propagates inward into the particle, leaving behind a char layer that gradually thickens as the front moves inward. The present work uses the method of line to theoretically investigate the competition between the motion of the pyrolysis front and the intraparticle heating for coal particles as it undergoes heating and pyrolysis under the effects of heating rate, particle size, particle thermal conductivity, and heat of devolatilization. Two intraparticle heating mechanisms have been identified in this study. The first is the intraparticle diffusion heating where the motion of the pyrolysis front is faster than the rise in the surface temperature but is slower than the rise in the temperature of the unreacted core. As a result, the coal particle is totally pyrolized before the surface temperature reaches its oxidation value. The second mechanism is due to the thermal wave moving with the pyrolysis front where the motion of the pyrolysis front is faster than the rise in the temperature of the unreacted core but is slower than the rise in the coal surface temperature. This may lead to a situation where the surface temperature reaches its oxidation value and only a narrow outer layer close to the surface is pyrolized.