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

A technique is presented to obtain the maximum structural information on carbonaceous materials from their X-ray scattering curves in the middle and high range of scattering angle. This technique involves a precise and systematic analysis of X-ray scattering curves. Four Australian black coals ranging 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 qualitative observations, a simplified coal structure, in which only two types of carbon structures including crystalline carbon and amorphous carbon are considered, is suggested. The good agreement in intensity between experimental measurements and theoretical calculations demonstrates the validity of the simplified model. The quantitative analysis yields three structural parameters, viz., fraction of amorphous carbon (xA), aromaticity (fa) as well as crystallite size and its distribution (La, Lc, d002, pn). As expected, coal was found to contain a significant amount of highly disordered material, amorphous carbon, which gradually decreases during the coalification process. In agreement with the TEM observations, coal crystallites are found to be around 6 Å in diameter and piled up by 2–4 aromatic layers on average, with the high rank coal being more condensed in terms of the inter-layer spacing. However, our measurements suggest that the average crystallite height increases with coal rank from 7.5 Å for Coal1-DD to 13.75 Å for Coal5-YD. The measured aromaticity, which agrees with the results of NMR spectroscopy, shows a good relationship with the hydrogen content in coal.