Study on the characteristics and conversion mechanism of gas generation stage in closed fire areas coal spontaneous combustion process

Many mining areas are facing problems such as reignition during the process of unsealing fire areas. The generation laws of gases and the transformation process of coal surface functional groups during the coal spontaneous combustion-extinction-reignition (CER) were studied by using programmed temperature rise for coal spontaneous combustion(CSC) and in-situ infrared spectroscopy experiments. Results show that: during the first heating, the concentrations of various gases and oxygen consumption increased slowly first and then exponentially with rising temperature. When cooled with pure N₂, they dropped rapidly; however, in atmospheres of 3% O₂, 5% O₂, 10% O₂, and 21% O₂, there was a lag effect in the early stage of extinguishment, and the gas concentrations decreased as the temperature continued to drop. Compared with the first heating stage, oxygen consumption was higher when cooling; additionally, a lower oxygen concentration corresponded to a higher level of oxygen consumption. For coal samples cooled in various atmospheres, the gas concentrations during the second heating were all higher than those in the first heating. When cooled in 3% and 5% O₂, the consumption of active functional groups exceeded their generation; in 10% and 21% O₂, the generation was slightly stronger than consumption. After cooling in various atmospheres, the storage amounts of functional groups were all higher than those in raw coal, in the order: pure N₂>10% O₂, 21% O₂>3% O₂, 5%O₂>Raw coal. The findings of this research offer a theoretical foundation for curbing the rapid spread and preventing the reignition of fires, and provide technical support for environmental protection in fire-affected areas. © 2026 Elsevier Ltd.