Thermal Kinetics of Coal Spontaneous Combustion Based on Multiphase Fully Coupled Fluid–Mechanical Porous Media Model

Coal spontaneous combustion (CSC) is a complex physical and chemical process. Numerical simulation is an efficient method for visualizing and quantifying CSC. However, in previous models, the property parameters of coal are frequently assumed to be constant and hence cannot be used to predict accurately the thermodynamic behaviors during CSC. In this study, a fully coupled porous media model considering water migration, heat transfer, and thermal deformation was developed to investigate the thermal kinetics of CSC. The porosity and thermal strain during CSC in the model were consistent with the experimental results. Moreover, a method based on nuclear magnetic resonance (NMR) was used to calculate the residual saturation of fluids in porous media during heat treatment. The results showed that porosity increased with temperature during CSC, and the NMR transverse relaxation cutoff values of coal depended nonlinearly on temperature. In the initial stage of CSC, moisture was the dominant factor affecting CSC. As time progresses, moisture and temperature exhibited a synergistic effect. Temperature was the leading factor contributing to CSC in the final stage. The temperature increase during CSC exhibited a ‘slow and then fast’ trend. In addition, the deformation of coal during CSC caused thermal shrinkage, and its magnitude depended on the temperature increase rate. The results can be used to identify the thermodynamic behaviors during CSC and contribute to a better understanding of the mechanism of CSC.