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 ¹³C 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.

In the recent past, researchers have paid much attention to HF, HNO₃, and HCl acids for demineralization of various types of coals with the assumption that they bring negligible amount of change in the coal structure. Here we report the changes occurring in the structural parameters and the nature of organic functional groups present in an inertinite-rich lignite coal during acid demineralization processes. The investigation has been carried out by analyzing the coal using multiple analytical tools, namely, Fourier transform infrared spectroscopy (FTIR), solid-state ¹³C nuclear magnetic resonance (¹³C-NMR), and X-ray diffraction techniques (XRD). The maximum demineralization (90.82%) and surface area (252m²/g) of coal was achieved using HF/HCl/HF process. A broad peak of =N–OH functional group was observed in FTIR spectrum of HF/HNO₃ process which can be associated with the interaction of coal and nitric acid. This result is also consistent with an increase in nitrogen (from 2.0% to 5.6%) and oxygen (from 12.6% to 19.3%) content of HF/HNO₃-treated coal. The NMR results indicate that the coal structure contains two aromatic rings per cluster, which remains intact even after demineralization. Some increase in the amount of phenolics or phenolic ester (from 1.4% to 3.6%), oxy-methylene (from 1.96% to 2.4%), and oxymethine (1.0% to 1.5%) in the coal structure was observed by demineralization of coal using HF/HNO₃ process whereas it remains constant when HF/HCl/HF process is used, indicating that the presence of HNO₃ can remove organic sulfur (up to 53.96%) from the coal sample during acid treatment processes. The aromaticity (f_a) of raw coal was found to be in the range of 0.71–0.75, which increases with HF/HNO₃ treatment but remains constant with HF/HCl/HF treatment. The other crystallite structural parameters obtained from FTIR, ¹³C-NMR, and XRD were influenced to a relatively greater extent by HF/HNO₃ treatment when compared with HF/HCl/HF treatment of coal. This is the first detailed analysis of the structural change in the macromolecular arrangement of coal due to demineralization, which will aid toward new techniques for utilization of Indian coal in clean coal technology.