Desorption-oxidation characteristics of gas in coal and evolution of pore structureunder coupled temperature-pressure effects

Aiming at the coupled disasters of coal spontaneous combustion and gas in goafs under the “three highs and one disturbance” geological environment of deep mining, physical similarity simulation tests are performed to systematically investigate the synergistic effects of in-situ temperature and in-situ pressure on gas desorption performance, spontaneous combustion characteristics and pore evolution of residual coal in goafs. The results show that the rise of pre-adsorption temperature reduces the gas desorption capacity of coal samples, and the reduction becomes more obvious with increasing temperature. The increase of pre-adsorption pressure remarkably improves the desorption amount of gas, whereas the growth rate gradually slows down as pressure rises. CO is mainly produced from desorption of physically adsorbed gas below 110 °C, and its volume fraction increases exponentially above 110 °C, which can be adopted as an early warning indicator gas for coal spontaneous combustion. Due to multiple generation pathways, CO2 is not suitable for coal spontaneous combustion prediction. C2H6 originates from both desorption of inherent hydrocarbons in coal and coal–oxygen oxidation, and can serve as an auxiliary early-warning indicator gas for coal spontaneous combustion. C2H2 is first detected at 140 °C with a relatively low initial generation temperature. The isothermal adsorption curves of coal samples from working face 305 belong to type II, and the adsorption hysteresis loops are type H3. Increasing pressure leads to micropore closure in coal samples and the growth of macropore proportion, thereby reducing total pore volume and specific surface area of coal samples; rising temperature also decreases total pore volume and specific surface area. Pressure reduces surface fractal dimension but elevates pore fractal dimension, resulting in smoother coal surface and more complex pore structure. In contrast, temperature increases coal surface fractal dimension and decreases coal pore fractal dimension, contributing to rougher coal surface and more uniform pore distribution; temperature and pressure exert opposite influences on pore structure. Under coupled thermo-pressure effects, increased temperature facilitates gas desorption and aggravates coal spontaneous combustion propensity, while increased pressure strengthens gas adsorption and restrains coal spontaneous combustion development. Accordingly, the in-situ temperature and pressure of coal seams should be clarified during coal mining, and gas emission as well as spontaneous combustion hazards need continuous monitoring to guarantee safe production.