Direct observation of coal–gas interactions under thermal and mechanical loadings

•A test system on coal-gas interaction has been developed.•Multi-physical process in intact coal under loadings was studied by the test system.•Increasing temperature can lead to coal deformation and desorption of adsorbed gas.•Thermal-induced rearrangement of porous structure degrades uniaxial strength.•Pre-existing adsorbed CO2 has a stronger affinity for the coal compared to CH4.•During the uniaxial compression, gas migration is controlled by the coal deformation.

In this study we directly observe the multi-physical process of coal–gas interaction induced by thermal impacts through a specially designed facility. This facility can be used to measure the evolution of coal deformation and migration of originally existing gas in coal. Tests were conducted through three stages, i.e., heating, uniaxial loading, and re-heating stage. In the first-stage, heating triggers two concurrent effects, coal expansion and gas migration. It was found that both the coal deformation and gas flow firstly increase with the rise of coal temperature. When the coal temperature is in isothermal equilibrium, the volume of coal is either recovered or maintained unchanged approximately. In the course of the process, the gas flow begins to peak off by a relatively low flow rate. The results at the uniaxial loading showed that the gas migration is significantly controlled by the progressive deformation of the stressed coal under isothermal conditions. From the relationship between the coal deformation and concurrent gas flow, the direction of gas migration depends upon the development of cracks. It consequently leads to either sucking of gas to the opening of cracks/fractures, or discharging of gas to the closure of cracks and pores. The re-heating at the last stage can accelerate gas discharge. The concentrations of released gas species increase proportionally with the coal temperature. In order to quantify the thermodynamic characteristics of gas migration, the concentration of desorbing gas is defined as a function of temperature by using the Arrhenius equation. The results statistically showed that CO2 has a stronger affinity for the coal than CH4. It was also revealed by the results that the deformation evolution of the heated coal is related to the competition between thermal swelling of solid coal and shrinking by gas desorption. Finally, it was indicated by the present tests that the complex deformation potentially makes it possible to deteriorate in the mechanical properties of coal. This work can help to understand the mechanism of coal–gas interaction induced by multi-physical fields including thermal-induced coal deformation, residual gas desorption and adsorption, and coal shrinkage.