Evolution of Coal Spontaneous Combustion Characteristics in Goaf During Working Face Retreat and Optimization of Nitrogen Injection Conditions

During the working face retreat, ventilation structure changes induce significant goaf airflow fluctuations, reshaping the coupling of oxygen supply, heat accumulation, and pore structure, thereby greatly increasing residual coal spontaneous combustion risk. This study symmetrically investigated the effects of ventilation flow variations and different nitrogen injection conditions on coal oxidation and goaf spontaneous combustion three zones via TG-DSC thermal analysis, PFC3D overlying strata collapse simulation, and Fluent flow-temperature field coupling calculation. Results show that weakened ventilation raises coal’s characteristic temperature, enhances high-temperature heat release, and intensifies low-wind-speed self-heating. Nitrogen injection mainly affects high-temperature oxidation, with 70 C pre-oxidized coal showing the highest combustion index. Under normal caving, the goaf oxidation zone distributes asymmetrically, expanding to 50–72 m (intake side) and 30–45 m (return side) during retreat. Seven-day continuous nitrogen injection reduces the oxidation zone width by over 50%, with optimal inerting at 40–45 m/s injection rate and 50–60 m intake side position. This study reveals the "airflow disturbance-pore evolution-oxygen supply-heat accumulation" coupling mechanism and proposes an optimal nitrogen injection strategy, providing theoretical and technical support for goaf coal spontaneous combustion control during retreat.