Role of coal deformation energy in coal and gas outburst: A review

Coal deformation energy and gas expansion energy are the main power sources for coal and gas outbursts (called outbursts for short). Although the combined effect hypothesis of the outbursts has been widely recognized, the contribution of the coal deformation energy to the outbursts is always despised compared with the gas expansion energy. In this paper, the famous energy criterion for outburst trigger proposed by Hodot was reviewed systematically. It was found that Hodot's energy criterion pays more attention to coal deformation energy and almost ignores gas expansion energy, different from the views in most literatures related to outburst energy. The characterization methods of coal deformation energy and gas expansion energy were summarized. Experimental results show that the deformation energy of tectonic coal with high outburst risk has a power function relationship with stress and the power exponent is between 1 and 2, indicating that the deformation properties of tectonic coal is between soil and elastomer. The releasing mode of the deformation energy of tectonic coal is different from that of intact coal, that is, the deformation energy of intact coal and tectonic coal mainly releases outward and dissipates inward respectively after they are damaged. The coal deformation energy was proved to have the same order of magnitude with the gas expansion energy, about 102 to 103 kJ/t. A new energy release model for whole process of outbursts was established to discuss the role of coal deformation energy in actual outbursts. It was finally determined that the pre-release of deformation energy of tectonic coal is the necessary condition for the accumulation of the free gas expansion energy and outburst trigger.