Thermal Lag Characteristics During the Cooling Process of Coal Spontaneous Combustion Under Different Nitrogen Concentrations

Coal spontaneous combustion poses a major hidden danger to coal‑mine safety production, and the inhibition effect of nitrogen injection is closely related to the temperature response of coal during the cooling stage. In this study, Xiaolongtan (XLT) lignite was selected as the experimental coal sample. The evolution of coal surface physicochemical properties at different oxidation temperatures was characterized, and the dynamic variations in coal temperature and CO release were investigated after the atmosphere was abruptly changed from dry air to nitrogen-containing atmospheres with different nitrogen concentrations (83.5%, 89.0%, 94.5%, and 100%). The results show that, with increasing oxidation temperature, the coal surface became rougher, the pore structure gradually developed, the relative contents of C element and C–C/C–H groups decreased, while the contents of O element and oxygen-containing functional groups increased. These changes became more evident after 130°C. During the subsequent cooling process after atmosphere conversion, the XLT coal samples exhibited a temperature‑lag phenomenon, characterized by a short-term temperature increase followed by a decrease. The lag characteristics were influenced by both the oxidation temperature and nitrogen concentration, and clear differences among different nitrogen concentrations appeared from 130°C. The temperature‑change rate showed a “V-shaped” trend, with clear differences among nitrogen concentrations emerging from 180°C. Under the experimental conditions of this study, 94.5% and 100% nitrogen showed a more obvious cooling effect at the 230°C oxidation stage. At the end of cooling, the final coal temperature increased as the nitrogen concentration decreased, with a maximum temperature difference of 10.3°C among the tested atmospheres. CO release during cooling generally increased first and then decreased, except under pure nitrogen, and the differences in CO release among different nitrogen concentrations became more pronounced at higher oxidation temperatures. These results reveal the cooling‑stage response characteristics of XLT lignite after nitrogen‑atmosphere conversion and provide experimental reference for nitrogen inhibition of lignite spontaneous combustion under similar conditions.