Mechanisms behind thermal evolution of the structural alignment of polycyclic aromatic hydrocarbons in coals: Implications for structural transition of coal macromolecules

ObjectiveCoals are highly sensitive to variations in temperature and pressure conditions. The structural evolution of coal macromolecules, driven by tectono-thermal processes, is the key to understanding coal rank transition and geological evolution. The structural alignment of polycyclic aromatic hydrocarbons (PAHs) in coals represents a critical indicator for characterizing coal rank transition and geological evolution. However, due to limited studies, there is a lack of systematic experimental demonstration and theoretical support for the structural alignment and microstructural characteristics of PAHs in coals, as well as their intrinsic relationships with the structural transition of coal macromolecules, during pyrolysis. MethodsThis study investigated bituminous coal samples from the Wuguantun Coal Mine in the Datong Coalfield, Shanxi Province. Using pyrolysis experiments at 400–1000 ℃, high-resolution transmission electron microscopy (HRTEM), laser Raman spectroscopy (LRS), and Fourier-transform infrared spectroscopy (FTIR), as well as comparison with actual geological evolution processes, this study revealed the mechanisms behind the thermal evolution of the structural alignment of PAHs, aiming to provide certain implications for the structural transition of coal macromolecules. ResultsThe results indicated that the raw coals exhibited an increasing proportion of short PAHs and a decreasing proportion of long PAHs at approximately 400 ℃. In contrast, the raw coals showed a significantly elevated proportion of long PAHs at 400–1000 ℃. The PAH orientation decreased under low-temperature pyrolysis (≤ 400 ℃), increased slightly at 600–800 ℃, and increased markedly at 800–1000 ℃. These variations demonstrate pronounced stepwise characteristics. Curvature analysis reveals that with increasing pyrolysis temperature, coal samples generally showed a declining proportion of high-curvature aromatic fringes. This finding implies that thermal evolution would ultimately render the fringes progressively straighter. The PAH stacking ratio of raw coals changed slightly at approximately 400℃ and gradually increased at 400–1000 ℃. In contrast, under high-temperature pyrolysis at 800–1000 ℃, the PAH stacking tended to show a concentric layered distribution of graphene- and carbon onion-like structures.ConclusionThe results of this study pose challenges to the conventional view that coal graphitization merely corresponds to the repair of structural defects. Instead, this study proposes that during the critical transition window from coalification to graphitization, the intense reorganization of the internal structures of PAHs induces significant secondary structural defects. These defects essentially represent the lattice distortions occurring as aromatic laminae evolve toward graphene- or carbon onion-like structures driven by thermal-kinetic processes. This insight also accounts for the microscopic strain heterogeneity ubiquitously observed in tectonically deformed coals. The presence of overall PAH orientation in pyrolyzed coals at 800–1000 ℃ marks the end of coalification and the onset of the graphitization stage (Rmax>6.22%).