Mechanism of single coal particle ignition under microgravity condition

In order to simplify the complex coal ignition process, a microgravity condition (10^-3 g), in which natural convection is negligible, has been employed to observe ignition of single coal particles. The particle was heated and ignited by radiation from spot heaters. Three coals with different volatile matter contents were employed, having particle sizes in a range of 0.8 mm to 1.2 mm. As expected, under microgravity, a nearly spherical volatile cloud was found to form around the particle. Results clearly show that regardless of the particle size, the ignition is preceded by homogeneous gas-phase volatile ignition, followed by heterogeneous solid-phase char ignition after burn-out of the volatiles. The heterogeneous ignition temperature and time were higher and longer, respectively, under microgravity than under normal gravity condition. The homogeneous ignition also took place at a higher particle temperature under microgravity than under normal gravity. Further, the temperature for heterogeneous ignition increased with the absolute amount of volatiles contained in a coal particle, whereas the particle temperature for the homogeneous ignition decreased.