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

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 N-2 from 600 degrees C to 1100 degrees 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 A(D)/A(G) (A(D1)/A(G), A(D2)/A(G), and A(D3)/A(G)) increased from 600 degrees C to 900 degrees C and then decreased after 900 degrees C, indicating that the degree of order of coal char first decreased and then increased with increasing pyrolysis temperatures (600-1100 degrees 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 (L-a) generally increased between 600 degrees C and 1100 degrees 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.