Differences in smoldering characteristics of coal piles with different smoldering propagation directions

Smoldering combustion frequently occurs in the mined-out gobs of coal mines. This can act as an ignition source, triggering gas explosions and resulting in serious accidents. In this study, a smoldering test device was constructed, in which horizontal, vertical, forward and reverse smoldering combustion experiments were performed to explore the smoldering characteristics (dynamic development process, smoldering rate and temperature, etc.) of a coal pile. It is found that owing to the evaporation moisture present in the coal and its re-condensation on the coal ahead of the smoldering front, a constant temperature of approximately 70-90 degrees C is maintained during the spread of forward smoldering. In the initial stage of reverse smoldering, both the pyrolysis and oxidation fronts exist and propagate backward; meanwhile, in the later stage, the intensity of the endothermic pyrolysis reaction is greatly reduced, resulting that the maximum smoldering temperature of reverse smoldering is slightly higher than that of the forward smoldering. The vertical smoldering temperature is 1.5-2 times that of horizontal smoldering, owing to the poorer oxygen supply in the horizontal spread. The propagation of the oxidation front is extremely slow, and is in the order of 0.1 mm/min; this makes it difficult to characterize and detect the smoldering fire, especially in inaccessible mined-out gobs with complex environments. This study provides useful insights into the identification of smoldering fire and assesses its development stage.