Stage Characteristics and Grading Early Warning Method of Coal Spontaneous Combustion Process Based on Coal Structure and Gas Product Increment

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Coal spontaneous combustion (CSC) is one of the key problems in the process of coal mining. How to accurately predict the degree of CSC is still an important challenge in controlling the risk of CSC. To achieve detailed early warning and monitoring, the multi-element test system of CSC was conducted to study the evolution law of gas products in low-temperature oxidation of coal with different metamorphic grades. The Fourier transform infrared spectrometer was used to determine the content distribution of functional groups in raw coal. The variation characteristics of coal structure and gas product increment, gas product percentage increment ratio, chain alkane ratio, and Graham’s ratio were analyzed. The results show that the CSC process based on coal structure and gas product increment can be divided into seven temperature stages. T < 30°C is the latent stage, 30°C < T < 40 ~ 50°C is the chain initiation stage, 40 ~ 50°C < T < 80 ~ 90°C is the chemical oxygen absorption stage, 80 ~ 90°C < T < 100 ~ 110°C is the critical temperature rise stage, 100 ~ 110°C < T < 150 ~ 160°C is the olefin formation stage, 150 ~ 160°C < T < 210°C is the fission increase stage, T > 210°C is the combustion stage. On this basis, six warning levels were identified and applied in the field. This study provides a new method and scientific guidance for the field prediction and firefighting of CSC.