Pore and fracture evolution characteristics and gas desorption law of coal-rock combination body under impact loading

The probability of occurrence of compound dynamic disaster triggered by impact ground pressure and gas protrusion in deep coal seams is greater. In order to investigate the desorption law of coal-rock assemblage under different impact loads, the pore and fracture evolution, cumulative desorption volume and desorption speed of coal-rock assemblage under different impact velocities were comparatively analyzed by using the impact damage-desorption test system of gas-containing coal rock, low-field nuclear magnetic resonance (NMR) instrument and industrial CT scanning system. The results show that with the increase of impact velocity, the pore size distribution of the coal-rock composite transitions from a “bimodal” curve to a “trimodal” curve, with the volume of fissures, surface area, and fractal dimension all exhibiting exponential growth. The impact load enhances the connectivity of pores, thereby improving the gas desorption capacity. When the impact velocity increased from 10.295 m/s to 16.077 m/s, the increment of post-impact gas desorption capacity (relative to pre-impact values) rose from 50% to 88%, and the increment of gas desorption rate increased from 83.04% to 95.5%. The amount and rate of gas desorption post-impact show a significant leap compared to pre-impact conditions. The cumulative gas desorption after impact exhibits a three-stage characteristic of “rapid rise−slow growth−stabilization”, while the desorption rate decreases rapidly at first, then more slowly, gradually approaching zero. Based on the Boltzmann equation, a gas desorption equation under impact load was established, accurately describing the gas desorption process in coal-rock composites under impact load. As the impact speed increases from 10.295 m/s to 16.077 m/s, the cumulative gas desorption expansion energy increased from 0.042 J/g to 0.635 J/g, representing a 14.1-fold increase, the cumulative gas release expansion energy increases, significantly raising the risk of coal and gas outburst. The characteristics of pore-fracture evolution and gas desorption leap in coal-rock composites under impact load are revealed, and a theoretical basis is provided for the prediction and prevention of compound dynamic disasters in deep coal seams.