Structure of the organic crystallite unit in coal as determined by X-ray diffraction

X-ray diffraction (XRD) was used to study the structure of the organic crystallite unit (L_a, L_c, d₀₀₂) in coals collected from Henan and Shanxi Provinces. XRD patterns of coal were collected in a step-scan mode (0.1°/step) over an angular range of 2–90° (2θ), allowing 8 s at each step. The structure of the crystallite unit was determined from the Scherrer equation and peak parameters deduced from whole pattern fitting. The results show that the structure of the crystallite unit in coal is mainly controlled by the coal rank. As the coal rank increases the average diameter of a coal crystallite unit (L_a) increases, the inter-layer spacing (d₀₀₂) decreases slightly, and the average height of a coal crystallite unit (L_c) increases at first but then decreases. A new diffraction peak from the crystallite unit in coal was found at a low scattering angle in the XRD pattern (2–10°). This suggests a structure with an inter-layer spacing from 1.9 to 2.8 nm exists in coal crystallites.