Research On The Mechanism Of CH₄、CO₂、H₂O And O₂ Adsorption On Coal Molecule

Under the background of engineering practice and application,like gas outburst in coal seam,CO₂ storage in abandoned coal seam,low-rank coal drying and spontaneous combustion,the adsorption mechanisms of gas molecule of CH₄,CO₂,H₂O and O₂ on coal molecular surface have been studied.Based on quantum chemistry methods,the optimized adsorption configurations have been carried out by energy calculation,electrostatic potential distribution analysis,reduced density gradient analysis,atoms in molecules analysis and interaction energy decomposition analysis,and adsorption sites of gas CH₄,CO₂,H₂O and O₂ on coal molecular surface have been defined,at the same time,the adsorption energy,type and strength of interaction between gas molecules and coal molecule have been evaluated.Building the basic coal molecular model,which the fundamental structure is organized by various contiguous and adjacent benzene rings.The adsorption configurations of CH₄,CO₂,H₂O and O₂ on coal molecular surface were optimized,the results indicated that all of CH₄,CO₂,H₂O and O₂ can stably adsorb on coal molecular surface,and corresponding adsorption stability satisfies the law of O₂>H₂O>CO₂>CH₄.Along with the increase of scale of coal molecular model,the adsorption stability of CH₄and other molecules becomes strong.The interaction between CH₄,CO₂,H₂O,O₂ and coal molecule belongs to van der Waals weak interaction.In detail,the interaction strength between O₂ and carbon atom which in coal molecular model is the strongest,the next is the interaction between H₂O and carbon atom,and the interaction strengths between CH₄,CO₂ and carbon atom are similar and the weakest.In the adsorption progress of CH₄ and other gas molecules on coal molecular model surface,the electrostatic and dispersion forces are attractive and make a great contribution to the attraction,conversely,the repulsion and exchange forces are repulsive,and repulsion plays a dominant role in the repulsive interaction.The adsorbed H₂O molecules and polymers which on the coal molecular model surface can increase the adsorption energy and stability of CO₂.Based on the basic coal molecular model,the surface characteristics of coal molecular model were created by adding the vacancy site and doping different atom groups.The adsorption configurations of CH₄ and other gas molecules on coal molecular model surface have been optimized,the results show that the vacancy site and doped groups decrease the adsorption capacity of CH₄.For CO₂ molecule,the vacancy site and doped groups can decrease and increase the adsorption ability,respectively.The vacancy site and doped groups in coal molecular model can increase the adsorption ability of H₂O molecule.O₂ molecule can form bonds with the vacancy site and doped carbonyl functional group in coal molecule surface,the doped hydrogen atom and hydroxyl and others oxygen containing functional groups can increase the adsorption stability of O₂ on coal molecule surface.In addition,the vacancy site and doped atom or group can increase the selective adsorption capacity of coal molecule to CO₂ molecule in mixed gas of CH₄ and CO₂.Along with the increase of interlayer spacing of double-layer coal molecular model,the adsorption stability of CH₄,CO₂,H₂O and O₂ decrease.Building tridimensional coal molecular model,the hydrocarbon chain,various carbon rings and functional groups were added in the coal molecular structure.The adsorption configurations of CH₄ and other gas molecules on adsorption sites in coal molecular model have been optimized,the results indicate that the CH₄ and other gas molecules adsorbed by the various groups or atoms at the same time on the coal molecular model surface,the structure characteristics of coal molecule have effect on the adsorption location and adsorption stability of CH₄ and other gas molecules.