Effects of Seam Inclination and Ventilation on the Three-Dimensional Redistribution of Spontaneous-Combustion Hazard Zones in a Composite Goaf

Layered extraction of extra-thick, steeply inclined coal seams can connect active and abandoned goafs across large elevation differences, creating leakage pathways that planar oxygen criteria and homogeneous seepage models cannot resolve. A three-dimensional heterogeneous porous-medium model was therefore developed for the 1502–2 panel of Chicheng Coal Mine. The model explicitly represented the active goaf, a 5-m interlayer protective pillar, and the overlying abandoned goaf, and coupled airflow, oxygen transport, low-temperature coal oxidation, and energy conservation. Sulfur hexafluoride tracer measurements confirmed the principal cross-layer pathway, with a 6.67% error in mean velocity. Three cases separated the effects of inclination and ventilation: 36° at 1.5 m.s–1, 36° at 2.5 m.s–1, and 0° at 1.5 m.s–1. A joint oxygen and low-velocity criterion was used to reconstruct the three-dimensional hazard zone. Relative to the horizontal case, inclination displaced the hazard zone from a near-bottom, approximately symmetric distribution toward the high return side and the interlayer pathway, increasing its volume from 3.50 × 104 to 4.42 × 104 m3. Increasing the inlet velocity reduced the maximum normal height from 12.0 to 6.2 m and the hazard volume to 2.65 × 104 m3, but shifted the deep oxidation-zone boundary from 85 to 115 m. Thus, stronger ventilation limited buoyancy-driven high-level deflection and enhanced convective cooling while sustaining oxygen transport farther into compacted material. Fire-prevention measures should account for both the high-level cross-layer pathway and the downstream oxidation front.