Molecular Insight of Coal: A Spectroscopic Approach for Evaluating Maturity Parameters

Two Permian and one Tertiary Indian bituminous coal were characterized using conventional maturity parameters, viz. vitrinite reflectance (VRo) and Rock-Eval pyrolysis. Molecular characterization of the samples after solvent extraction was performed to study the presence of different structural moieties, using FTIR, UV–visible and fluorescence spectroscopic methods. For VRo, Rock-Eval pyrolysis and FTIR bulk coal samples were used whereas for UV–visible and fluorescence spectroscopic study, the coals were divided into solvent-extracted fraction and residual demineralized fraction. Both of the fractions were assessed in order to observe the molecular structures present within the coal samples to correlate thermal maturity of the coals with sequential rank-wise evolution. The UV–visible spectra of the solvent-extracted fractions depict the presence of alkenes, carbonyls, alkynes and polycyclic aromatic hydrocarbons (PAHs) and asphaltene porphyrins as the possible structural compounds, whereas the same spectra for the demineralized residual fractions show the presence of mostly carbonyls and aromatic traces. The fluorescence spectra for the solvent-extracted fractions of the coals indicate the presence of 4–5 PAH ring structures, viz. naphthacene and perylene, whereas the demineralized residue part is found to be enriched with smaller 3-ring PAHs, viz. anthracene. Observation of the different structural components in the coal samples of different ranks depicts their variation in terms of thermal evolution and can lead to produce a new maturity scale in future.