Abstract
A comparative study of coals of different ranks having different chemical structure and petrography would be desirable to gain an in-depth knowledge of Indian coals. Here, we report structural and petrographic analyses of three different coals: Coal A (coking coal), Coal B (semi-coking coal) and Coal C (non-coking coal) of Indian origin. The vitrinite reflectances of the three coals were measured to be 1.26, 1.38 and 0.46 for Coal A, Coal B and Coal C, respectively. Size fractionation (− 3.0 + 1.0 mm, − 1.0 + 0.5 mm and − 0.5 mm) followed by density gradient separation (1.2, 1.3, 1.4, 1.6, 1.8 g/cm3) were performed for all the coal samples to study mineral matter liberation and distribution of macerals in different density fractions. Petrographic analysis of different density fractions revealed a direct proportionality of vitrinite content to swelling properties. The highest free-swelling indices (7.5 and 6.5) were observed for 1.2 density fraction of Coal A and Coal B, respectively. Fourier transform infrared spectroscopy revealed crucial information relating the degree of aromaticity and aliphaticity with observed coking properties. Specifically, aromaticity (AR1) (CHar stretching/CHal stretching, (3000–3100 cm−1)/(2800–3000 cm−1) and mean reflectance (Ro in %) were found to have a strong positive linear correlation with each other, indicative of an increase in aromaticity with coal rank.
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