Spontaneous Combustion Hazard Zoning and Ventilation Optimization in an Adjacent Goaf Considering Zonal Porosity of Narrow Coal Pillars

The degree of damage to a narrow coal pillar during mining directly affects its permeability, thereby influencing air leakage patterns and spontaneous combustion hazard areas in the adjacent goaf. However, existing studies commonly overlook zonal porosity variations caused by mining disturbances. This study focuses on the intrinsic relationship between coal pillar damage and zonal porosity. FLAC3D numerical simulation was employed to analyze the stress-displacement evolution of narrow coal pillars during mining. Based on fracture development characteristics and displacement magnitude, the pillar was divided into crushed, fracture development, and intact zones, with porosity ranges of 0.19–0.23, 0.06–0.18, and 0.01–0.05, respectively. A numerical model incorporating the adjacent goaf, narrow coal pillar, and working face was established. Using the obtained porosity parameters, air leakage pathways and spontaneous combustion hazard areas under different wind speeds were analyzed. The results indicate that crushing of the narrow coal pillar creates three distinct flow zones in the adjacent goaf: a high-ventilation velocity mainstream zone, a transitional diversion zone, and a large-scale vortex zone. As wind speed increases, the oxygen diffusion distance expands, and the spontaneous combustion hazard areas gradually enlarge. At a wind speed of 1.6 m/s, the spontaneous combustion hazard areas reach a minimum of 7,619.0 m2, accounting for approximately 6.6% of the total goaf area. The distribution of spontaneous combustion hazard areas was validated through field measurements. The proposed FLAC3D-Fluent coupled method, which incorporates zonal porosity, can accurately predict fire risks in the adjacent goaf, providing a theoretical and technical reference for fire prevention in coal mines.