Synergistic effects of moisture content and freezing temperature on mechanical-desorption responses of frozen gas-bearing coal and implications for outburst prevention

Cryogenic freezing technology can both reduce the energy storage of coal–rock mass and enhance its strength, showing broad application prospects for outburst prevention in deep outburst-prone coal seams. However, studies on the coupled effects of moisture content and freezing temperature on the mechanical strength and energy response characteristics of gas-bearing coal after freezing remain relatively scarce. In this study, briquette coal specimens and pulverized coal samples after adsorption equilibrium were subjected to quantitative water injection and cryogenic freezing. Uniaxial compression tests and gas desorption experiments were conducted, and mercury intrusion porosimetry (MIP) was used to characterize the pore structure changes before and after freezing. The results show that the uniaxial compressive strength and elastic modulus first increase and then decrease with increasing moisture content, peaking near 4.5% moisture content; both continuously increase with decreasing freezing temperature. The total gas desorption amount decreases with increasing moisture content and decreasing freezing temperature. MIP results indicate that at high moisture contents, frost heave increases the development of macropores and fractures, which may provide preferential seepage channels for free gas and lead to a rebound in the initial desorption rate. Compared with room-temperature conditions, both the increase in compressive strength and the decrease in gas expansion energy exhibit a unimodal trend with increasing moisture content, and the best response among the tested conditions occurs near 4.5% moisture content. At − 30 °C and 4.5% moisture content, the compressive strength increases by 292.23% and the gas expansion energy decreases by 62.78% compared with room temperature; at − 40 °C, these values reach 310.88% and 72.56%, respectively, although the growth rates of both improvements tend to slow down. Two-way ANOVA shows that moisture content, freezing temperature, and their interaction have significant effects on compressive strength and gas expansion energy, quantitatively verifying their synergistic effect. This study provides mechanistic insights and a theoretical reference for applying cryogenic freezing technology in deep outburst-prone coal seams.