Effects of particle size on the self-ignition behaviour of a coal dust layer on a hot plate

Knowing the sizes of coal particles is vital for understanding the real ignition behaviour of coal dust deposits on a hot plate. Furthermore, such knowledge facilitates model construction for assessing industrial coal dust explosion risk. Three particle sizes (0.10-0.15, 0.15-0.20, and 0.20-0.30 mm) of coal samples were selected to analyse the influence of particle size on ignition and smouldering behaviour. Characteristic parameters, such as the minimum ignition temperature of a dust layer (MITL), highest temperature in a layer (HTL), ignition delay time, and heating rates were tested and analysed using a dust layer ignition apparatus. The results revealed that the MITL was 200-210 °C and that the MITL tended to decrease with particle size. The aggregation effect improved the MITL containing fine particles. The optimal ignition position was 7.2 mm from the hot plate. As the particle size increased, the ignition delay time of the sample became longer. However, the HTL and the time to reach the maximum temperature reduced accordingly. For corresponding geometrical shapes of a coal dust layer, the results demonstrated that particle size had a limited effect on the void fraction but had a considerable effect on the number of particles. The finer coal dust had a substantial number of particles, thus producing a larger overall specific surface area in which to oxidize and devolatilize. Small coal dust particles pose a severe hazard in industrial sites, and thus, sites should be cleaned regularly to avoid the possibility of unwanted fires or explosions.