Study on the Effect of Surface Air Leakage on Coal Spontaneous Combustion in Shallow-Buried Composite Goafs: A Case Study of Huojitu Coal Mine

Coal spontaneous combustion is a severe hazard in the goafs of shallow-buried coal seams, particularly under the condition of continuous surface air leakage. This study conducted an integrated experimental and 3D multi-field coupled numerical investigation based on the Huojitu Coal Mine. Experimental kinetic analyses revealed that the upper seam coal exhibits a significantly higher oxygen consumption rate and CO generation capacity than the lower seam coal, characterized by an earlier initial CO generation temperature of 40 °C compared to 60 °C. Subsequent simulations indicated that the flow field and oxygen distribution within the overlying goaf exhibit a distinct “U-shaped” profile governed by surface air leakage. The sequential extraction of the lower coal seam significantly expands the oxidation zone on the return side of the overlying goaf, leading to the formation of a critical high-temperature zone exceeding 100 °C near the return side of the setup entry. Guided by these findings, a three-phase foam technology was implemented in the field, effectively encapsulating the residual coal and drastically reducing the CO concentration at the upper corner from a peak of 221 ppm to a stable 5 ppm. The findings highlight the role of surface air leakage in coal mining and provide corresponding strategies to mitigate spontaneous combustion risks in shallow-buried coal seams.