Mechanism of solid-partical combustion with simultaneous gas-phase volatiles combustion

Abstract

Data are presented on the effect of volatiles evolution on the combustion of solid-fuel particles. Analysis indicates that, if the surface flux of volatile pyrolysis products is small, gas-phase combustion occurs simultaneously with heterogeneous combustion on the surface of the particle; conversely, a large surface flux apparently forces the reaction zone away from the solid surface, thereby screening the solid material from oxygen attack. The experimental measurements were performed in a one-dimensional pulverized coal flame. Samples of solid and gaseous material collected along the axis of propagation of the flame, and measurements of flame temperature, provided information on the ignition, pyrolysis, and combustion of the coal particles. The size distribution of the experimental particles ranged from 0 to 200 microns; the rate of temperature rise in the preignition zone of the flame was about 104°C/sec; and the maximum temperature attained after ignition was about 1550°C. The surface flux of volatiles evidently increases with particle size: above a size of about 65 microns, the volatile flux is large enough to prevent oxygen from reaching the solid surface, below this size, heterogeneous and gas-phase combustion occur together on the solid surface, but the rate of the former process is small during rapid volatiles evolution unless the particle size is less than about 15 microns. The rate of heterogeneous combustion is apparently controlled by the rate of the chemical reaction, with mass transport being relatively fast. The heterogeneous reaction occurring simultaneously with rapid volatiles evolution exhibits an activation energy of 7 to 12 kcal/mole, which suggests an adsorption-controlled process. In the tail of the flame where solid residues burn with little volatiles evolution, the activation energy of the heterogeneous reaction appears to be 30 to 65 kcal/mole, which suggests rate control by desorption. This shift from adsorption to desorption control agrees with the expected influence of volatiles evolution on heterogeneous combustion.