Temperature-induced micropore structure alteration of raw coal and its implications for optimizing the degassing temperature in pore characterization

A reasonable degassing temperature setting is necessary for carrying out low-temperature nitrogen adsorption (LT-N2-A) experiments, and is also a powerful safeguard for the accurate determination of pore structure. This study investigates the dynamic variation characteristics of the nanopore structure of primary porous media (low-metamorphic coal) under different degassing temperature scenarios. These observations indicate that the degassing temperature alters the attributes of low-metamorphic coal determined using low-pressure gas adsorption. The increase in the degassing temperature leads to a decrease in the maximum adsorption capacity of the coal samples, and the adsorption “hysteresis loop” becomes smaller. Micropores are the most sensitive to degassing temperature in low-metamorphic coal pore systems. The pore volume contributed by the micropores in the normal test groups BD8 and JG82 decreased gradually with an increase in degassing temperature. When the degassing temperature reached the set interval of scenario 3 (413.15–433.15 K), the micropore volume of the JG82 coal sample decreased to 3.463 × 10−5 cm3/g. The increase in degassing temperature caused irreversible damage to the micropores, however, the effect of the change in degassing temperature did not have the same effect on the changing trend of mesopores of the two coal samples. The measurement results of the BD8 cycle testing group indicate that the ultra-low temperature environment created by the LT-N2-A experiment may play a vital role in protecting the microporous skeleton structure or micro-expanding the micropores. Therefore, the most suitable degassing temperature for low-metamorphic coal before the LT-N2-A pore structure determination experiment is the temperature range in scenario 1 (333.15–353.15 K). This provides the greatest protection to the original pore structure of the low-metamorphic coal, thus ensuring a reliable estimation of the pore structure of low-metamorphic coal. These findings have implications for setting the optimal degassing temperature for porous media.