Laboratorial Investigation of Coal Fire Extinguishing and Re-burning Risk in Underground Coal Mines

The extinguishing and re-burning of the closed fire area in an underground coal mine were investigated by laboratory-scale physical simulation. Temperatures in the center of the fire source were recorded, and the typical cooling process was observed to include the rapid cooling stage (900-400 °C) and dilatory cooling stage (400-100 °C). With the increase of coal mass from 20 to 80 kg, the rate of cooling decreases and the time required for fire extinguishing increases by 69.5%-193.2%. At temperatures ranging between 500 and 100 °C, yields of CO and H₂ show strong correlation with the attenuation of the coal fire, and the trend in the yield of H₂ might be used as the optimal indicator considering the different amounts of coal. A significant difference appears in the concentration of H₂ released by samples of different dosages of coal in the early stage of cooling, especially when the temperature exceeds 200 °C. During the extinguishing process, micropores in coal fused into mesopores and macropores, while the content of O-containing groups fluctuated significantly. Variations of elemental C and O also indirectly reflect the combustion state in the fire cooling. Taking the experimental reactor as a physical model, the time required for the fire area from closure to safe re-opening is deduced, that is, t = Cm ln (T ₁ - T _∞)/(T ₂ - T _∞ ). The calculated results were compared with the changes in measured temperatures, providing a theoretical foundation for the re-opening prediction of mine fire areas.