Formation mechanism of methane during coal evolution:A density functional theory study

The formation mechanism of methane(CH4) during coal evolution has been investigated by density functional theory(DFT) of quantum chemistry. Thermogenic gas, which is generated during the thermal evolution of medium rank coal, is the main source of coalbed methane(CBM). Ethylbenzene(A) and 6,7-dimethyl-5,6,7,8-tetrahydro-1-hydroxynaphthalene(B) have been used as model compounds to study the pyrolysis mechanism of highly volatile bituminous coal(R), according to the similarity of bond orders and bond lengths. All possible paths are designed for each model. It can be concluded that the activation energies for H-assisted paths are lower than others in the process of methane formation; an H radical attacking on β-C to yield CH4 is the dominant path for the formation of CH4 from highly volatile bituminous coal. In addition, the calculated results also reveal that the positions on which H radical attacks and to which intramolecular H migrates have effects on methyl cleavage.