A fully coupled hydro-thermo-mechanical model for the spontaneous combustion of underground coal seams

Abstract

The spontaneous combustion of underground coal seams involves complex interactions between geomechanical effects, oxygen transport and flow, and energy transport in the porous coal media. Prior studies normally ignore the thermo-mechanical effects such as gas and coal expansion due to the self-heating of coal, and have not implemented these complex interactions fully into their simulations. In this study, a fully coupled model of coal mechanical deformation, gas flow and transport, and heat transport is developed and their complex interactions are defined through a suite of coal property models and equation-of-states. These include (1) coal porosity model; (2) coal permeability model; (3) gas equation-of-state; and (4) self-heating model.Applying the model to quantitatively predict the time and locations of spontaneous combustion of underground gob-side entry in the Dongtan coal mine, the results are in good agreement with the in situ measurements. Besides, a significant self-accelerating-heating effect induced by the gas thermal expansion and subsequent gas pressure gradient increase is found in the self-heating process of coal through the comparison results from our model with other models. Furthermore, the self-heating susceptibilities of gob-side entry associated with extrinsic and intrinsic factors, incorporating coal permeability, pressure difference, oxygen-consumption rate, and reaction heat of coal oxidation, are gained insight using the verified model, which suggests the self-heating rate and gas velocity are positively correlated with above factors showing “S-type” upward trends, whereas the oxygen concentration has an “S-type” downward trend. The simulated results can provide some suggestions as to how to control the variables or parameters to retard or suppress the spontaneous combustion of porous coal media.