Unraveling the Nanopores Evolution: Chemical Structure Control in Coal during Coalification

Nanopores play a crucial role in the storage, desorption, and diffusion of coalbed methane within the coal. This study aims to clarify the evolution mechanism of nanopores controlled by the chemical structure of coal during coalification. By conducting low-pressure CO2 and N2 adsorption experiments, the nanopore structure of six distinct coal samples with varying ranks was identified. Additionally, the chemical structure of coal was characterized through 13C nuclear magnetic resonance and Raman spectroscopies. Through comprehensive analysis, the evolution mechanism of nanopores influenced by the chemical structure of coal was thoroughly investigated. The results show that during coalification (Ro range 0.6–3.0%), the aliphatic content in coal gradually decreases, while the aromatic content relatively increases. The aromatic structures become more ordered, and condensation leads to an increase in the size of aromatic rings. Due to the influence of the chemical structure, the evolution of nanopores in coal can be divided into three stages. In the first stage (0.6% < Ro < 1.5%), the cleavage of aliphatic side chains and the adjustment of chemical structure orientation lead to the transformation of micropores to mesopores in coal. In the second stage (1.5% < Ro < 2.0%), the strengthened condensation of aromatic rings leads to the transformation of mesopores to micropores in coal. In the third stage (2.0% < Ro < 3.0%), most oriented aromatic rings undergo significant condensation, leading to the formation of many micropores in coal. These findings can help understand the control mechanism of the chemical structure on the nanopores in coal and provide valuable theoretical insights for enhancing coalbed methane extraction efficiency.