Insight on adsorption mechanism of coal molecules at different ranks

The adsorption mechanism of methane molecules and coal molecules at different ranks is of significant practical and theoretical importance in prevention gas disaster and methane storage. In this paper, four coal samples at different ranks were selected for experiments. The real coal molecules at different ranks were constructed. The relationship between the coal components and the gas adsorption capacity was analyzed. The adsorption mechanism was studied by performing combined Grand Canonical Monte Carlo(GCMC) and Density Functional Theory (DFT)simulation at temperatures of 298 K and at pressure range of 0-100 bar. The simulation results display a good agreement with the experimental results. In the process of methane adsorption simulation, methane molecules are preferentially adsorbed on the edge of the unit cell and then adsorbed inside the coal molecules. The methane molecules in the Up orientation have stronger adsorption capacity then Down orientation. The adsorption capacity order is as follows: aromatic structure > oxygen functional groups. In the aromatic structure, the adsorption capacity of adsorption sites is ranked as T > B > C. In the oxygen functional groups, the adsorption of methane by the coal molecule is caused by both the adsorption position and the adsorption direction. Under the influence of coal molecules, the structure and the Mulliken charge of methane molecules change significantly during the adsorption simulation. The adsorption forces between the coal molecules and methane molecules are mainly induced force, dispersion force, orientaion force and electrostatic force. This study enables us to better understand the adsorption mechanism of coal molecules at different ranks.