Filling characteristics of the micropores in coals with different metamorphic degrees

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To explore the occurrence state of coalbed methane (CBM) in coals of different metamorphic degrees and accurately predict CBM, we studied the micromechanism of coal and gas outbursts in a bid to avoid or reduce personnel injuries or property losses in such underground accidents. Low-temperature N2 adsorption experiments were conducted on three coal samples of different metamorphism degrees. We explored the critical filling characteristics of the packed N2 molecules in the coal. The Dubinin–Astakhov equation in the micropore filling theory and the density functional theory method were applied to obtain the critical filling pressure and critical filling aperture range for the N2 molecules to complete the micropore filling in the coal. The results showed that in the low-temperature nitrogen adsorption experiment, the pore structure of lignite was uniform, and the adsorption of N2 molecules was better. The ratio of nitrogen molecules in the adsorbed form was higher than those found in bituminous coal and anthracite. The initial filling of the micropores in coal started when the relative pressure was P/P0 ≈ 10−6–10−4 and completed when P/P0 ≈ 10−2. The order of completion was anthracite, bituminous coal, and lignite. The slit-type channel was more conducive to microhole filling than the ink bottle channel. The critical filling pore size of the coal samples with different levels of metamorphism was in the range of 1.7–2.2 nm. Finally, the critical filling pore size was divided into a primary filling stage with the range being 0.36–1.7 nm and a secondary filling stage with the range being 1.7–2.2 nm.