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To explore the adsorption mechanism of CH4 and H2O molecules on the surface of low-rank coal (LRC) from the microscopic point of view, the electrostatic potential and frontier orbitals of each oxygen-containing functional group (OFGs) in LRC and adsorbent molecule, the adsorption energy, and Mulliken charge layout of CH4 molecules and H2O molecules with OFGs in LRC were investigated by density functional theory (DFT) simulation method. The results of DFT calculations showed that the order of adsorption strength of CH4 molecules on different OFGs was OCH3–LRC (-9.643 kJ/mol) > C=O–LRC (−8.625 kJ/mol) > OH–LRC (−7.241 kJ/mol) > COOH–LRC (−6.194 kJ/mol), which were all smaller than that of the C–LRC model without functionalization (−10.749 kJ/mol). The presence of OFGs reduces the adsorption strength of CH4 molecules on the surface of LRC. The order of strength of adsorption of H2O molecules on different OFGs was COOH–LRC (−69.836 kJ/mol) > OH−LRC (−46.442 kJ/mol) > C=O–LRC (−42.848 kJ/mol) > OCH3–LRC (−33.079 kJ/mol), and they were all greater than that of the C–LRC model without functionalization (−32.572 kJ/mol). The presence of OFGs improves the adsorption strength of H2O molecules on the surface of LRC. Both the LRC model modified with OFGs and the non-functionalized C–LRC model showed stronger adsorption of H2O molecules compared to that of CH4 molecules. Therefore, coal seam water injection can reduce the amount of gas gushing and mitigate coal seam gas protrusion.

This study aims at investigating the adsorption mechanism of CO2, CH4, CO, and N2 mine gases and oxygen-containing functional groups in lignite. Thus, density functional theory and grand canonical Monte Carlo simulation methods were used to determine the adsorption energy, configuration, isotherm, and isosteric heat as well as the diffusion coefficient of gas in lignite. The results showed that the adsorption capacity of CO2 molecules and oxygen-containing functional groups was greater than that of CH4, CO, and N2. The order of the absolute value of the adsorption energy of each oxygen-containing functional group of each gas molecule on the lignite surface model was as follows: carboxyl > hydroxyl > carbonyl > ether bond. The adsorption isotherms of the four gases (CO2, CH4, CO, and N2) in the lignite molecular structure model were consistent with the Langmuir adsorption isotherm model. The relationship between the adsorption amount of the four gases in the lignite structure model was CO2 > CH4 > CO > N2. In addition, the adsorption capacity of the four gases decreased as the temperature increased. The CO2 isosteric heat of adsorption was considerably greater than that of CH4, CO, and N2, indicating that the adsorption capacity of the lignite molecular structure model for CO2 was considerably stronger than that of CH4, CO, and N2. The CO2 diffusion coefficient showed the slowest change with the temperature increase, whereas the N2 self-diffusion coefficient showed the fastest change. The following diffusion activation energy sequence was obtained: CO2 < CH4 < CO < N2.

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.