Unraveling coalification dynamics: a comprehensive spectroscopic study on the chemical and microstructural evolution from lignite to semi-anthracite.

Raman spectroscopy can play a crucial role in coal rank identification by providing direct insights into the structural evolution of carbon during coalification. Unlike any traditional method such as vitrinite reflectance, which rely on specific macerals and often face limitations in low-vitrinite or compositionally altered samples, Raman offers a non-destructive and comprehensive assessment of all organic matter types. It detects changes in chemical bonding, aromaticity, and the degree of structural order-key indicators of coal maturity-by analyzing vibrational energy levels. Its ability to differentiate between sp- and sp-hybridized carbon and monitor the transition from amorphous to graphitic structures makes it especially valuable for evaluating thermal evolution. This research investigates the evolution of coal's chemical composition and microstructure during maturation using Raman spectroscopy, supported by Fourier Transform Infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS). The transformation of peat into coal, driven by microbial activity, thermochemical degradation and burial, increases carbon concentration and releases volatiles. Traditional methods such as vitrinite reflectance suffer from limitations, particularly in samples with low or absent vitrinite content, or when vitrinite is suppressed by bitumen or enhanced by recycled particles. Measurement errors may arise from polishing issues, anisotropy or instrument calibration, but Raman spectroscopy offers more profound insights into carbon structure changes during coalification and graphitization. Initially, coal and lignite samples of varying ranks (from lignite to semi-anthracite) were analyzed using traditional methods such as proximate analysis and vitrinite reflectance to determine coal rank. Subsequently, spectroscopic techniques were employed to evaluate carbon structure, functional groups, and sp/sp carbon ratios. FT-IR identified functional groups, while XPS examined surface elements. Raman spectra revealed a clear relationship between coal rank and carbon structure, showing higher-rank coals with greater sp carbon ordering, resembling graphite. Gaussian fitting confirmed that sp content increases while sp content decreases with rank, consistent with XPS and IR findings. The G-band shift and broadening indicated increased nanocrystalline graphite from sub-bituminous to anthracite, while lignite displayed more sp content and amorphous carbon. XPS confirmed that higher-rank coals have more CC bonds, while lignite contains CO and COOH groups. The study provides a novel framework for assessing coal rank and maturation, enhancing understanding of coal's metamorphic history, and offering insights for optimizing coal utilization and expanding geological knowledge of organic sediment metamorphism.