Impact of particle size on autoignition and smoldering of bituminous coal dust layer driven by thermal radiation

Autoignition and smoldering of coal dust have gained great safety concern in both industry and mining due to their high susceptibility to heating. To better understand their controlling mechanism, autoignition and smoldering of bituminous coal particles (1-mm diameter) and coal powders (0.1-mm diameter) in the form of bulk layer heated by 4–45 kW m⁻² thermal radiation levels were systematically studied. The results showed that the critical heat flux was between 4 and 5 kW m⁻² for both samples. Under 5 kW m⁻², no morphology change was detected on surface, but internal ignition and smoldering occurred. Partial oxidation and intense glowing ignition were observed under heat fluxes of 10 and 15–45 kW m⁻², respectively. The average glowing temperature was 782.4 ± 19.6 K, while the maximum temperature during smoldering changed in 976–1206 K. Smoldering spread rate of powder sample was larger than that of particle sample due to emerged cracks during heating and larger specific surface area, and both spread rates linearly depended on heat flux. The linear dependency of ignition time to the power of − 0.5 on heat flux in classical ignition theory was found still valid for glowing ignition of both samples, and apparent thermal inertia and critical heat fluxes were quantitatively estimated. In all smoldering tests, mass loss rates exhibited a bimodal feature, with first peak charactering moisture evaporation, while the second peak indicating coal-oxygen reaction. Under 35 and 45 kW m⁻² heat fluxes, a unique faint blue flame appeared which depended more on surface temperature than on mass flux of pyrolyzates. This contribution may deliver useful reference for evaluation of coal fire risk and help mitigate hazard of smoldering in practical applications.