Experimental study on the development and bidirectional propagation characteristics of spontaneous coal combustion in goaf

To effectively control the spontaneous combustion of coal in goaf, this study investigates the developmental mechanisms and bidirectional propagation characteristics of spontaneous coal fires under various influencing factors using oxidation kinetics tests and simulation experiments. The results show that the critical air leakage rate for self-sustained reverse propagation of coal fire is 16.4 m/h, significantly lower than that required for forward propagation. Consequently, under low air leakage conditions in the goaf, fire development exhibits distinct oxygen-seeking behavior. A distinct double-peak temperature phenomenon appears during reverse fire propagation for larger coal particle sizes. The high-temperature zone in the upstream fire area experiences two phases: reverse propagation and wind-following migration. The development of the upstream fire zone significantly affects forward fire propagation. Once the upstream fire reaches a certain extent, it suppresses or even halts further spread. As the particle size of residual coal decreases, the double-peak temperature phenomenon gradually disappears, while forward propagation is significantly inhibited. Upward air leakage direction with increased Inclination significantly accelerates forward propagation, with the fire front reaching a peak rate of 14.8 cm/h at 45°. Meanwhile, reverse fire propagation is suppressed and ceases when the Inclination exceeds 15°. Simultaneously, the fire zone expansion follows a non-linear trend, initially increasing, then decreasing, and rising again, with the largest fire zone and highest risk occurring at inclination angles of 10° and 45°. Downward air leakage direction with increased Inclination accelerates reverse propagation while inhibiting forward spread. The fire expansion rate is faster at steeper angles in the early fire stages(0-6 h). However, in the mid-to-late stages, the fire zone expands more extensively under lower inclination conditions.