Study on energy instability mechanism of composite coal seam based on the coupling effect of damage and unsteady diffusion

•The gas migration control equations under the coupling effect were established.•The calculation models of energy considering the coupling effects were proposed.•Outburst mechanism of composite coal seam under the coupling effect was explained. As a fossil fuel, coal dominates global energy consumption. In the process of coal formation, due to the influence of geological structure, there are many phenomena of stratification of primary coal and tectonic coal. This also makes the occurrence and migration of gas in different coal seams different, which sharply increases the outburst risk of coal seams. In this paper, the control equation of gas migration was established according to the diffusion state of gas in damaged composite coal. The mechanical instability law of composite coal under the coupling damage and unsteady diffusion was simulated and analyzed. It is found that the existence of tectonic coal can effectively promote the gas desorption in the primary coal seam. Then, the calculation model of outburst energy under coupling effect was established, and the role of adsorption gas expansion energy in composite coal seam and single coal seam was analyzed from the perspective of energy. It is proposed that in the single coal seam of tectonic coal, the adsorption gas expansion energy is mainly used to transport coal and maintain the development of outburst. In the composite coal seam, it is mainly used to crush the coal, resulting in the occurrence of outburst. Finally, from the influence of tectonism, pore damage path and gas migration path, the outburst mechanism of composite coal seam under coupling effect was comprehensively explained. The research results provide a theoretical basis for the prevention and control of the outburst of composite coal seam.