Experiments and computational modeling of pulverized-coal ignition. Semiannual report, Apr 1, 1998--Sep 30, 1998

Abstract

Under typical conditions of pulverized-coal combustion, which is characterized by fine particles heated at very high rates, there is currently a lack of certainty regarding the ignition mechanism of bituminous and lower rank coals. It is unclear whether ignition occurs first at the particle-oxygen interface (heterogeneous ignition) or if it occurs in the gas phase due to ignition of the devolatilization products (homogeneous ignition). Furthermore, there have been no previous studies aimed at determining the dependence of the ignition mechanism on variations in experimental conditions, such as particle size, oxygen concentration, and heating rate. Finally, there is a need to improve current mathematical models of ignition to realistically and accurately depict the particle-to-particle variations that exist within a coal sample. Such a model is needed to extract useful reaction parameters from ignition studies, and to interpret ignition data in a more meaningful way. The authors propose to examine fundamental aspects of coal ignition through (1) experiments to determine the ignition mechanism of various coals by direct observation, and (2) modeling of the ignition process to derive rate constants and to provide a more insightful interpretation of data from ignition experiments. They propose to use a novel laser-based ignition experiment to achieve their objectives. The heating source will be a pulsed, carbon dioxide laser in which both the pulse energy and pulse duration are independently variable, allowing for a wide range of heating rates and particle temperatures--both of which are decoupled from each other and from the particle size. This level of control over the experimental conditions is truly novel in ignition and combustion experiments. Laser-ignition experiments also offer the distinct advantage of easy optical access to the particles because of the absence of a furnace or radiating walls, and thus permit direct observation and particle temperature measurement. The ignition mechanism of different coals under various experimental conditions can therefore be easily determined by direct observation with high-speed photography. The ignition rate-constants, when the ignition occurs heterogeneously, and the particle heating rates will both be determined from analyses based on direct, particle-temperature measurements using two-color pyrometry.