Structural Characterization of Indian Vitrinite-Rich Bituminous Karharbari Coal

The detailed investigation of the chemical structure of vitrinite-rich Karharbari coal was performed utilizing advanced analytical techniques. The salient objective of this work is the evaluation of various structural properties of coal, which is necessary for identifying the chemical and physical interactions between coal and various reactants during its utilization. Karharbari coal is a poorly organized coal with high aromatic content. The value of corrected aromaticity (f(a)') was found to be 0.82 by C-13 NMR spectroscopy and was also confirmed by XRD (aromaticity = 0.84) and FT-IR analysis (aromaticity = 0.82). The average molecular weight of the aromatic cluster was found to be 507 amu by the NMR result, which is closer to the result obtained by HRTEM (MW = 530 amu). The structural and lattice parameters of Karharbari coal were obtained by NMR spectroscopy and then compared with the similar rank Argonne Premium coal. The molecular weight distribution was obtained by LD-TOF-MS and compared with HRTEM fringe model analysis. The presence of different heteroatoms like carbon, oxygen, nitrogen, and sulfur with their functionalities was determined by using the XPS technique. Different carbon/oxygen functionalities present in the Karharbari coal were found to be (C-C) and (C-H) 68.5%, (C-O) 23.4%, (COO-) 1.9%, and (C=O) 6.0%. Nitrogen functionalities such as pyridine, pyrrolic, quaternary, and oxidized nitrogen and their compositions (mol %) were 19.3, 45.6, 31.2, and 3.7%, respectively. Different forms of sulfur were also found to be present, namely, thiophenes, sulfones, sulfuric acid, and sulfates with the molar contents of 16.4, 41.6, 21.3, and 20.5%, respectively. This information will be useful in improvising coal utilization techniques.