Numerical and experimental Investigation of coal pillar crushing and spontaneous combustion in shallow and closely spaced coal seams

Coal spontaneous combustion is one of the major hazards in coal mines. During shallow and closely spaced coal seam mining, the coal pillar crushing and oxidation inside the goaf are difficult to determine, making it impossible to accurately predict coal pillar spontaneous combustion and determine goaf fire prevention and extinguishing indicators. Therefore, this paper analyzes the influence of lower working face mining on the plastic zone distribution and oxidation characteristics of the coal pillar in the upper coal seam based on numerical simulation, experimental research, and other means. The study found that as the closely spaced coal seam advances, the deformation characteristics of the upper coal seam coal pillar can be roughly divided into three stages: intact and stable, deformed and damaged, and crushed and unstable. The damaged and oxidized coal pillars have an earlier critical temperature point for CO and C₂H₄ by 10–20 °C during secondary oxidation, and the lower oxygen concentration limit of the coal decreases. After the coal pillar is crushed and oxidized, the surface structure becomes rougher, increasing the contact area with oxygen. The proportion of oxygen-containing functional groups inside the coal increases, and the oxidation activity of the coal increases. This shows that the coal pillar crushing process has a certain promoting effect on coal oxidation. The research results reveal the influence of shallow and closely spaced coal seam mining on the crushing and oxidation spontaneous combustion of the upper coal seam, which has certain guiding significance for fire prevention and extinguishing work in shallow and closely spaced coal seams.