Quantitative X-ray diffraction analysis and its application to various coals

A technique is presented to obtain the maximum structural information on ca rbonaceous materials from their X-ray scattering curves in the middle and h igh range of scattering angle. This technique involves a precise and system atic analysis of X-ray scattering curves. Four Australian black coals rangi ng in rank from semi-anthracite to HV bituminous are included in this study , and the results are compared with data in the literature. Based on qualit ative observations, a simplified coal structure, in which only two types of carbon structures including crystalline carbon and amorphous carbon are co nsidered, is suggested. The good agreement in intensity between experimenta l measurements and theoretical calculations demonstrates the validity of th e simplified model. The quantitative analysis yields three structural param eters. viz., fraction of amorphous carbon (x(A)), aromaticity (f(a)) as wel l as crystallite size and its distribution (L-a, L-c, d(002), p(n)). As exp ected, coal was found to contain a significant amount of highly disordered material, amorphous carbon, which gradually decreases during the coalificat ion process. In agreement with the TEM observations, coal crystallites are found to be around 6 Angstrom in diameter and piled up by 2-4 aromatic laye rs on average, with the high rank coal being more condensed in terms of the inter-layer spacing. However, our measurements suggest that the average cr ystallite height increases with coal rank from 7.5 Angstrom for Coal1-DD to 13.75 Angstrom for Coal5-YD. The measured aromaticity, which agrees with t he results of NMR spectroscopy, shows a good relationship with the hydrogen content in coal. (C) 2001 Elsevier Science Ltd. All rights reserved.