Studying the micromechanism of water injection to suppress coal and gas protrusion

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.