Structural Evolution of High-Rank Coals during Coalification and Graphitization: X-ray Diffraction, Raman Spectroscopy, High-Resolution Transmission Electron Microscopy, and Reactive Force Field Molecular Dynamics Simulation Study

The structure of organic matter in anthracite samples of different ranks and coal-derived natural graphites was investigated using XRD, micro-Raman spectroscopy, and HRTEM to determine XRD and Raman parameters that are suitable to characterize the structural evolution of anthracite throughout the coalification and natural graphitization processes. Additionally, the reactive force field molecular dynamics simulations were performed based on the molecular model of one anthracite analyzed in the present study to visualize the structural evolution of the carbon skeleton of anthracite at a molecular scale. The XRD parameters d(002)-spacing and full width at half maximum (FWHM) of (002) reflections of the short-range ordered carbon skeleton in anthracite do not show an obvious change trend with the coal rank increases before the transition stage between coalification and graphitization, which dispaly a decreasing change trend when entering the graphitization stage. This indicates the two XRD parameters may not be the reliable indicators to identify the structure of high-rank coals prior to meta-anthracite due to the fact that the microcrystalline structure of graphite has not been formed at the coalification stage, but are applicable to characterize the structural order of graphite with different graphitization degree. The Raman parameters D1-FWHM, G-FWHM, and their height and area ratios show a regular change trend for anthracite at the coalification and graphitization stages, which are effective to follow the structural evolution of high-rank coals during coalification and graphitization processes. The temperature plays an important role in facilitating the coalescence of carbon layers of anthracite and the ordered arrangement of carbon layers during coalification and graphitization processes by removing the heteroatomic cross-linkers and some hydrogen and reorienting the carbon layers. Besides the temperature, the pressure, shear stress, and time are also important factors promoting the graphitization of anthracite in nature.