Mechanisms of pore structure evolution during coal heating: Insights from the size and direction of aromatic rings

Coalbed methane (CBM) is stored and transported in coal pores, and the size, shape, and connectivity of coal pores directly affect the CBM endowment state and transport process, which have direct implications for gas disaster prevention and CBM mining. However, previous studies on the characterization and genesis of coal nanopores have mainly focused on mineral composition and molecular structure, paying relatively little attention to the effect of the size and directionality of aromatic structures on pore formation. This study determined the nanopore characteristics and the relationship of coal nanopores with aromatic ring size and ordering. To this end, coal samples of different maturity levels, which were obtained through heating under an open-exchange system, were analyzed through Scanning Electron Microscopy, Mercury Intrusion Porosimetry, Low Temperature Nitrogen Adsorption, and High-Resolution Transmission Electron Microscope. The results showed that the pores transitioned from organic matter pores to microfractures with the increase of coal maturity. Moreover, the size of aromatic rings gradually increases and the directionality is also gradually enhanced. The diameter of pores with the smallest throat gradually decreases with the increase of the coal rank, and the volume of mesopores exhibits a trend of initial increase followed by a decrease. The volume of macropores exhibits a trend of initial slow increase followed by a rapid increase with the rise of coal rank. The average fractal dimension of macropores decreases with increasing coal maturity, indicating that the non-homogeneity of pore structure gradually decreases and the pore-fracture system tends to homogenize. The average fractal dimension of mesopores shows a fluctuating change trend of low–high-low–high with the increase of coal rank. The relationship between aromatic ring sizes and nanopores shows that pores of 2–9 nm may be controlled by aromatic rings of 5.5–14.4 Å, and 9–10 nm pores may be controlled by 5.5–7.4 Å and 7.5–11.4 Å aromatic rings. The control of 10–15 nm pores is unclear, and 15–50 nm pores may be controlled by 3.0–5.4 Å aromatic rings.