Chemical Structure Characteristics and Model Construction of Coal with Three Kinds of Coalification Degrees

The coal structure is extensively used for studying the properties of coal, and the construction of accurate and reliable coal structure models can promote these researches. In this study, Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) were used to analyze the changes in the coal structure as a function of the coalification degree, and a semiquantitative model construction method based on FTIR, XRD, and X-ray photoelectron spectroscopy (XPS) analyses was proposed. With an increase in the coalification degree, the size of the aromatic cores in the coal increased. During the conversion from lignite to bituminous coal, the content of aliphatic structures increased, while the content of oxygen-containing functional groups (OFGs) significantly decreased. Conversely, during the conversion from bituminous coal to anthracite, the content of aliphatic structures decreased while the content of OFGs slightly increased. The calculated FTIR spectra of the coal structure models were consistent with the experimental FTIR spectra, which confirmed the accuracy of the models. Furthermore, the models successfully explained the microscopic mechanism underlying the differences in the wettability of the coal samples with varying coalification degrees. The construction method and coal structure models in this study will be more widely applied in future research.