Energy Evolution and Coal Crushing Mechanisms Involved in Coal and Gas Outburst

Coal and gas outburst is a dynamic disaster that poses a significant threat to coal mines. Energy analysis is an effective tool for understanding the mechanisms behind such outburst. The energy sources and dissipation during outbursts were explored using a multifunctional test system, and the energy transformation relationship was analyzed based on experimental results. Based on these, the crushing mechanism of outburst coal was further investigated. The results show that the adsorbed gas does not all participate in the work during an outburst. Thus, an effective desorption ratio was introduced to characterize quantitatively the percentage of volume of adsorbed gas involved in an outburst. The effective desorption ratios were calculated and found to be in the range of 5.07–13.36% under the test conditions and were primarily related to gas pressure and exhibited an exponential growth relationship. The energy of outbursts generally comes from gas expansion energy, which is 52.7 times that of the elastic strain energy. The gas expansion energy consists of adsorbed gas energy and free gas energy and accounts for 81.6–91.0% of the gas expansion energy, and it is the main energy source for continuous coal fragmentation and transport. The energy conversion of an outburst mainly occurs among gas expansion energy, coal crushing, and transport energy. The energy dissipation under high gas pressure conditions was found to be dominated by crushing energy, whereas under low gas pressure conditions, it was dominated by transport energy. The involvement of ground stress increased the breakage rate of the coal by 23%, whereas the breakage rate of the coal constantly increased with increase in gas pressure. The spatial distributions of the coal mass and crushing power in the roadway were consistent; however, the energy required in the initial stage of coal crushing was significantly larger than that in the middle and final stages. The release and consumption of energy during an outburst are processes of rapid decay from strong to weak. Finally, the processes of preparation, initiation, development, and termination of outbursts are described completely from the perspective of energy evolution, and the roles of ground stress and gas (free gas and adsorbed gas) during outbursts are clarified. The results provide reference ideas and theoretical basis for the accurate identification, effective prevention, and control of coal and gas outbursts.