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A Model of Lignite Macromolecular Structures and Its Effect on the Wettability of Coal: A Case Study

Cheng, Weimin, Xue, Jiao, Xie, Jun, Zhou, Gang, Nie, Wen
Energy & fuels 2017 v.31 no.12 pp. 13834-13841
Fourier transform infrared spectroscopy, X-ray diffraction, active ingredients, adsorption, aqueous solutions, aromatization, case studies, crystallites, hydrophilicity, ions, lignite, models, moieties, stereochemistry, wettability, China
Chemical and physical structure properties of coal macromolecules are the main microscopic factors affecting coal wettability. Numerous studies on coal structures have found that lignite, as a coal of low metamorphic degree, shows more-complicated macromolecular structures than other types of coal, and the macromolecular structure of lignite can better promote coal wettability than other low metamorphic coals with similar physical pore structures. To investigate the underlying reasons, lignite samples from the main coal producing areas of China were analyzed via X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy, and physical methods were used to construct two-dimensional and three-dimensional models of lignite macroscopic molecular structures to study its effect on coal wettability. The results indicated that aromatic layer spacing of lignite was considerably large and showed a low degree of aromatization with irregular arrangement and less directional degree. The macroscopic molecular structure of lignite was composed of 3–4 effectively stacked aromatic layers and alicyclic layers of hexagonal or pentagonal structures. The alicyclic ring structure was well-developed in the atom radial direction and had a high content of active ingredient, which was prone to attractive interaction with other molecules. The molecular formula of the lignite sample was determined to be C₁₈₀H₁₄₅O₃₁N₅S and the adsorption and spreading of wetting agents were found to be mainly dependent on the hydrophilic ability of coal surface. Furthermore, the microscopic pore stacking stereochemical structure of lignite indicated the existence of hydrophilic groups in surface functional groups around molecular nucleus, which resulted in the formation of infiltration points in the coal crystallite nucleus structure. Meanwhile, the absence of delocalized electron in the atom radial direction of amorphous lignite atoms could induce a relatively poor ability of attracting negative charge, and thus lessening the repulsive interaction to the electronegative ions in aqueous solutions, which promoted the wetting effect to some extent.