Study on Dynamic Evolution of the Influence of Nitrogen Injection on Spontaneous Combustion Process of Residual Coal in Goaf

In order to investigate the dynamic evolution of spontaneous combustion of residual coal in goaf during the entire nitrogen injection process, a self-built similarity simulation platform was utilized to simulate the spontaneous combustion conditions in an actual goaf. The study examined the dynamic evolution of temperature, indicator gas concentration and three zones of spontaneous combustion in the goaf before, during, and after nitrogen injection. The experimental results demonstrate that a spatially coupled “gas-heat-zone” model forms during the spontaneous combustion of residual coal. Within this model, a high-temperature zone develops on the intake side of the oxidation zone, driven by suitable oxygen concentration and heat accumulation. Meanwhile, CO generated from rapid oxidation migrates with the airflow, ultimately forming a high‑concentration zone on the return side. Nitrogen injection effectively suppressed combustion development through dual chemical‑physical mechanisms. After 30 min of injection, the peak temperature dropped from 79°C to 36°C, and the medium‑temperature zone decreased by 82.7%. Concurrently, the maximum CO concentration declined from 39.3ppm to 10.7ppm, with the medium‑CO-concentration zone shrinking by 97.2%. However, this indicates that spontaneous combustion risk persists once inerting is interrupted: within 10 min after injection ceased, the temperature rebounded to 41°C and the medium-temperature zone expanded by 51.5%, while CO concentration rose to 13.8ppm and the medium-CO-concentration zone increased by 376.1%. The study reveals asynchronous changes between the temperature and gas concentration fields during nitrogen injection. The rapid decline of high-temperature zones directly results from reaction suppression. In contrast, the delayed reduction of high-CO-concentration zones is controlled by nitrogen dilution and CO transport processes.