Mechanisms of methane generation from anthracite at low temperatures: Insights from quantum chemistry calculations

Abstract

For a long time, it has been debated if coals, especially the anthracite, could produce methane or other hydrocarbons under a temperature lower than pyrolysis temperature. In this study, to discuss the mechanism of methane production by anthracite under stress, we perform quantum chemistry calculations to investigate the dissociation mechanism of three different types of aliphatic structures. The following results are obtained: (1) The Gibbs free energy calculation shows it is the stress rather than the pyrolysis that produces methane in the deformation experiments at 200 °C; (2) For low-rank anthracite, the hydrogen atom contained in the coal can promote the dissociation of aliphatic carbon bonds and stress prefers to concentrate on aliphatic carbon bonds rather than other macromolecular structures of coal, which facilitate the gas generation; (3) For medium-rank anthracite, the hydrogen atom added to the aromatic ring can lead to more ordered and parallel orientation of the aromatic layers. Considering that the aromatic rings can rotate more easily than the bond stretching under stress, we think medium-rank anthracite may tend to enhance the degree of metamorphism, reducing the production of hydrocarbons. The gas and hydrocarbon generation under stress may be an important complement for the traditional hydrocarbon generation theory.