Carbon Structure Evolution of Middle- and High-Rank Coal during Coalification: Implication for Methane Production and Adsorption

Most coal-accumulating basins in the world experience regional thermal metamorphism to varying degrees, which results in significant changes in the carbon structure and gas content of coal. In this study, a comprehensive investigation employing Fourier transform infrared (FTIR) spectroscopy and high-resolution transmission electron microscopy (HRTEM) was conducted to characterize the evolution of the carbon structure during coalification. The FTIR results show that in the bituminous stage, the evolution of chemical structure is mainly reflected in the increase of aromatization degree. However, in the anthracite stage, the increase of aromatic rings in the coal plays a dominant role, resulting in the reduction of the CH bond content. The HRTEM results suggest that the lateral length, the number of stacking layers, and the height of the basic structural unit in anthracite increased significantly, and the aromatic fringe was straight and highly arranged. The spacing between adjacent aromatic fringes is close to 0.335 nm, indicating the transition of carbon structure from coal toward graphite. The relationships between Langmuir volume and functional groups indicate that when the contents of aromatic hydrocarbons and aliphatic groups are roughly equal, the coal matrix shows a strong adsorption capacity. However, the high content of aromatic hydrocarbons or aliphatic groups is not conducive to methane adsorption. The change in methane adsorption in coal is due to the evolution of molecular structure during organic matter pyrolysis. Pyrolysis causes the alkane side chains and oxygen-containing functional groups to break off, leading to the orientation of the aromatic nucleus, which improves the polarity of organic molecules and enhances the adsorption capacity.