Numerical assessment of the energy instability of gas outburst of deformed and normal coal combinations during mining

Abstract The complexity of the coal and gas outburst process makes the gas outburst mechanism difficult to understand, leading to serious disasters of coal and gas outbursts in China. However, the presence of deformed coal may promote the occurrence of coal and gas outbursts, and several normal coal layers and deformed coal layers often occur at the same location. To better assess the energy instability of gas outbursts of coal combinations, the coal and rock were assumed to be homogeneous, isotropic mediums, and the deformation of coal and rock was assumed to be infinitesimal. Then, ignoring the influence of the change of gas pressure on the stress field during the process of gas emissions, the distribution of the horizontal stress, plastic failure and gas pressure in the coal in front of the roadway were analyzed in three different cases. The results show that there will be a large horizontal tensile stress between the normal coal and deformed coal, which may make plastic failure extend from the deformed coal to the normal coal through the interface. Due to plastic failure, the permeability of deformed coal in coal combinations will increase. Moreover, the deformed coal itself not only enables gas to migrate rapidly but also increases the gas emissions of the normal coal adjacent to the deformed coal. Then, a new model of gas outburst energy and an energy criterion of outburst were established. It was found that the total outburst energy of the normal coal in combinations is approximately 149 times that of a normal coal monomer, so the deformed coal increases the total outburst energy of the normal coal in coal combinations. In regard to the outburst energy of coal combinations, the desorbed gas expansion energy is the highest, which is 1–6 times the elastic energy and 5–17 times the gas expansion energy. In addition, measures should be taken to first reduce the expansion energy caused by desorbed gas, and second measures should be taken to reduce the elastic energy caused by ground stress. Finally, a series of control measures of gas outbursts for different coal seams were proposed. These research results, which were obtained under certain assumptions, can provide a theoretical basis for the selection of measures for the control of coal and gas outbursts of coal combinations.