XRD characterization of structural evolution in low-middle rank coals

The structural parameters including L_a, L_c and d₀₀₂ of 28 coal samples with the maximum vitrinite reflectance R_{o, max} varying from 0.30% to 2.05% were analyzed using X-ray diffraction (XRD). The change in the XRD parameters associated with increasing vitrinite reflectance was obtained. The results indicate that, for the test coals with R_{o, max} less than 1.0%, the values of L_a and L_c sharply increase while d₀₀₂ shows a reverse trend, and the decrease of oxygen-containing functional group and the length of branching aliphatic side chains are dominated. For those with R_{o, max} from 1.0% to 1.6%, the value of L_a continuously increases, d₀₀₂ increases first and then decreases, and L_c decreases first and then remains steady. Moreover, dehydrogenation and adjusting the space steric hindrance are simultaneous. The value of d₀₀₂ continues to decrease while L_a and L_c increases, and the variation of coal structure is dominated by aromatization process during the R_{o, max} range of 1.6% to 2.0%. The variation of XRD parameters presents close relation with the first and the second coalification jump phase.