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This paper aims to investigate the evolution of the chemical structure of coal char during pyrolysis. Two bituminous coals (coals A and B) and one lignite (coal C) were pyrolyzed in a fixed bed in N2 from 600 °C to 1100 °C. The chemical structure of coal char was characterized by Raman spectroscopy and X-ray diffraction (XRD). The carbon and oxygen functionalities of coal char were analyzed by X-ray photoelectron spectroscopy (XPS). The Raman spectroscopic parameters AD/AG (AD1/AG, AD2/AG, and AD3/AG) increased from 600 °C to 900 °C and then decreased after 900 °C, indicating that the degree of order of coal char first decreased and then increased with increasing pyrolysis temperatures (600–1100 °C). The content of graphite-like microcrystalline carbon decreased and then increased with an increase in temperature. Prominent diffraction peaks of microcrystalline carbon for coal chars A and B were observed, but only minerals were shown in diffraction patterns of coal char C since the ash content of coal chars A and B is much lower that that of coal char C. The lateral size of the crystallite plane (La) generally increased between 600 °C and 1100 °C. The relative content of C=O and COOH in coal chars A and B generally decreased as the temperature increased, suggesting an increase in the degree of order at higher temperatures. The oxygen functionalities of coal char were composed of organic oxygen and oxygen-containing bonds of minerals.

Investigation of the Evolution of the Chemical Structure of Bituminous Coals and Lignite during Pyrolysis