Simulation of coal self-heating processes in underground methane-rich coal seams

Abstract

In this work, we revised our previous fully coupled model of coal deformation, methane diffusion from matrix and compositional gas flow in fractures, and thermal transport to investigate coal self-heating processes in underground methane-rich coal seams. The coal self-heating in underground coal seams involves a chain of physico-chemical reactions, which are linked together through compositional gas flow and diffusion, reaction kinetics, energy transport and coal deformation mechanisms. Although coal–oxygen reactions have been comprehensively investigated, fewer studies consider the collective impacts of methane-desorption induced coal deformation and compositional gas concentration changes on the methane-rich coal self-heating. We included these self-heating mechanisms of underground methane-rich coal seams in the revised model.The validation and superiority of the new model were demonstrated against previous results. The model was applied to quantify the self-heating susceptibilities associated with methane desorption diffusion time, coal permeability, differential pressure of leakage, coal-oxidation reaction heat and coal-oxidation rate. The simulation results show that (1) The compositional gas flow in fractures is related not only with coal permeability and methane desorption rate, but also with gas thermal expansion induced self-acceleration effect. (2) The desorption methane in coal matrix infiltration into the fractures can not only dilute oxygen concentration and inert combustible coal, but also hinder outside air leakage into porous coal media, even promote the coal self-heating at the later stage due to gas self-accelerating effect and matrix shrinkage. (3) The larger differential pressure of leakage, coal permeability, coal-oxidation reaction heat and pre-exponential factor can enhance coal self-heating, whereas the larger coal-oxidation activation energy may retard coal self-heating. The simulation results can be used to better understand the coupling mechanism of coal–gas-heating interactions associated with methane and coal spontaneous combustion, and provide some suggestions as to how to control the variables or parameters to retard or suppress the symbiosis disasters of gas and coal spontaneous combustion in underground porous coal media.