Mechanism of energy instability release during coal and gas outburst

As coal mining progresses deeper each year, the risks and hazards associated with coal and gas outbursts become increasingly severe. Therefore, this study employs a combination of physical experiments and numerical simulations to investigate the mechanisms of outbursts from the perspectives of coal failure and energy release. Based on physical experiments with four sets of permeability values (3.21, 7.62, 11.44, and 16.39 mD), numerical models for outburst-prone (3.21, 7.62, and 11.44 mD) and non-outburst-prone (16.39 mD) results were established. It is found that the gas pressure in both physical experiments and numerical simulations of outbursts shows high consistency, with gas pressure during outbursts exhibiting three distinct stages: delayed decline, rapid decline, and stabilization. The lower the coal permeability, the higher the peak seepage force and the longer its duration. Horizontal tensile stress decreases as permeability increases, with a shorter duration. Considering factors related to seepage force, such as gas pressure, horizontal stress, and friction between coal bodies, failure conditions of the coal were established, revealing that lower-permeability coal is more prone to failure. The coal with 16.39 mD never reached the failure condition, consistent with the absence of outburst in this case. During energy release in outbursts, the elastic energy and free gas expansion energy is of the same order of magnitude, while the desorbed gas expansion energy is approximately 1 ∼ 2 orders of magnitude higher than both. Gas expansion energy for pressure-dominated outburst is the primary energy source and decreases with the increase of permeability. The main energy during outbursts is used for coal fragmentation and transport, with the kinetic energy of coal exceeding the energy for fragmentation. The proportion of breakage work in energy dissipation increases with the permeability. The energy sources obtained from numerical simulations align closely with the energy dissipation calculated from physical experiments, validating the accuracy of the numerical model. Furthermore, the mechanisms of energy instability release during outbursts were explained from both microscopic, macroscopic and coal seam mechanics and energy evolution perspectives. The findings of this study provide valuable guidance for understanding outbursts.