STRUCTURAL AND CHEMICAL UPSHOTS WITH PROGRADING COAL RANK: A PERSPICACITY FROM SPECTRAL PROBES

The low-rank coals from northeastern India are explored for the first time for microstructural and chemical characterization using Raman spectroscopy, FTIR, XPS, and ¹³C NMR. The samples are grouped based on the R̄_r values into lignite (R̄_r: 0.26%), sub-bituminous (R̄_r: 0.47 & 0.48%), and high volatile bituminous (R̄_r: 0.53-0.75%). Further, these low-rank coals are compared with anthracites: (R̄_r: 4.15-5.36%) to understand the effects of coalification on microstructural and chemical parameters. The low-rank coal samples exhibit strong infrared spectra for hydroxyls, aliphatic -CH₂ and –CH₃, aromatic C=C, C-H stretching, and C=O stretching vibrations. The oxygen is present mainly in aromatic ester, carboxyl/carbonyl, and hydroxyls, whereas nitrogen is observed in –NH hydroxyl and pyridines. The organic sulfur is noticed in thiols and thioether units, whereas inorganic sulfur is observed as disulfides and polysulfides. Conversely, the anthracites are characterized by inner surface hydroxyl, absence of aliphatic C-H bonds, weak absorbance intensities of oxygenated groups, and aromatic C=C vibrations. XPS parameters further compliment the FTIR data for low-rank coals that show the inorganic forms of sulfur are present in fair quantity, followed by considerable aliphatic, aromatic, and oxy-sulfur compounds. Further, the O1s spectra of lignite suggest that about 80% of the oxygen is attached to carbon in the form of carbonyl and carboxyl groups. Additionally, the C1s spectra of anthracite show a sharp rise in the area of C-O, C=O, and –COOH groups from the lower rank coals. Besides, the FTIR data exhibit that the lower iso-rank samples are enriched in aliphatic structures, while relatively higher iso-rank samples contain condensed aromatic structures with improved microstructural ordering. The FTIR data implies that the low-rank coal samples possess significant hydrocarbon potential. Most of the samples comprise type II-III admixed kerogen with abundant sulfur content suggesting admixed autochthonous marine organic matter with the allochthonous terrestrial organic matter within a topogenous mire, which would have experienced the burial heating at a range from ~74 °C to 113 °C. Conversely, for anthracite samples, the calculated peak temperature for metamorphism fluctuates from ~334 °C to 367 °C.

These findings may bring a comprehensive understanding of coal's chemical reactivity during various coal conversion processes as well as its molecular model for better coal utilization.