A model for devolatilization and ignition of an axisymmetric coal particle

Abstract

A two-dimensional numerical model is developed to predict shape-dependent devolatilization and ignition of single coal particles exposed to radiant heat flux in an ambience at initially constant temperature. Intraparticle temperature gradients and radiation absorption are taken into account as well as the local release of volatiles and therefore the reduction of the raw coal. During pyrolysis, internal pressure gradients occur, which cause a viscous flow of gas through the pores described by Darcy's law. The model considers intrinsic char combustion depending on the local oxygen concentration and internal and external volatile oxidation. Ignition is identified either by an increase of gas-phase temperature or the inflection condition of the particle surface temperature for the homogeneous and the heterogeneous ignition mode, respectively. Comparison with a published one-dimensional model, which assumes an isothermal particle, provides reasonable agreement for the ignition delays of particles with diameters of 50–1000 μm inserted into hot air. In contrast to the published data, the calculated results show a different primary ignition phase for particles larger and equal to 300 μm due to intraparticle temperature gradients. Former experiments on coal particle ignition by radiation in cold air under microgravity conditions show different homogeneous ignition delays for spherical, cylindrical, and flat particles. These experimental results are confirmed by the present model: in addition, heterogeneous ignition points are detected in the calculations. Although the homogeneous ignition delays increase in the order of slabs, cylinders, and spheres, which is the order where specific surface decreases, ignition delays can not be sufficiently described in terms of particle-specific surface, especially not for the calculated heterogeneous ignition.