Prediction and measurement of ignition temperatures of coal particles

Abstract

Ignition temperatures of one bituminous coal, one anthracite, and three chars have been measured at 5 particle sizes in the range of 60 to 230 microns by injecting small quantities into an electric tube furnace. The technique determined the lowest air temperature (Tg,i) required for ignition, and the results were used to verify a theoretical preiction for the variation of Tg,i with diameter based on a modification of the Semenov thermal explosion analysis. As predicted, Tg,i rose with decreasing diameter, and the quantitative verification permitted determination of the activation energies, frequency factors, and specific reaction rates for the different fuels. With these coefficients determined, the particle ignition temperatures, Ti, were calculated. The activation energies ranged from 19 to 29 kcal/mole; these are interpreted as implying heterogeneous ignition in carbon-reaction Zone II (partial penetration of the reaction zone). Heterogeneous ignition had to be true of the chars. It was notable that there was no evident differentiation of ignition temperature between the coals and chars, with more variation due to diameter than due to fuel type. There was a similar small variation in the specific reaction rates, with all being in the region of 10−4 g/cm2 sec, which is in agreement with a value extrapolated from data given by Smith (22). This supports the accuracy of the measured ignition temperatures which, being in the range of 700±50° K, are considerably lower than the few previously-published values of any validity; they are however, in line with commercial practice for safe operation of pulverized coal mills.