Characterization of Chemical Structure with Relative Density of Three Different Ranks of Coal from India
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Original Paper
Characterization of Chemical Structure with Relative Density of Three Different Ranks of Coal from India Sanchita Chakravarty,1,4 K. Chakravarty,2 Vivek Mishra,1,3 Saswati Chakladar,1 Ashok Mohanty,1 and Mamta Sharma1 Received 12 August 2019; accepted 31 January 2020
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 freeswelling 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 cm1)/(2800–3000 cm1) 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. KEY WORDS: Indian coal, Petrography, FT-IR spectroscopy, Swelling index, Coal rank.
INTRODUCTION The chemical composition and macro-molecular structure of coal have raised substantial interest due to their importance in the development of novel and improved coal conversion practices (Larsen et al. 1986; Marzec 2002). Coal structure is inher1
CSIR-National Metallurgical Laboratory, Jamshedpur, Jharkhand 831007, India. 2 R&D Division, Tata Steel Ltd, Jamshedpur, Jharkhand 831007, India. 3 Hebei Collaborative Innovation Center of Coal Exploitation, Hebei University of Engineering, Handan 056038, Hebei, China. 4 To whom correspondence should be addressed; e-mail: [email protected]
ently complex; it varies widely in their chemical (molecular) and physical (conformational) properties, making detailed structural characterization of coal extremely difficult. Therefore, research on coal structure is still a stimulating task and continues to be pursued (Mathews and Chaffee 2012). It is well known that during coalification, the organic structure of coal consisting of heterogeneous aliphatic and aromatic compounds keep changing with aromaticity increasing from low rank to high rank coals (Oikonomopoulos et al. 2013). In our present study, three coals of Indian origin with different ranks, namely Coal A (ranked 1.26%), Coal B (ranked 1.38%) and Coal C (rank
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