Investigation on the structural features of Hecaogou subbituminous coal and its residues by multiple technical strategies

Abstract

The structural features (SFs) of Hecaogou subbituminous coal (HSBC) and its residues (Rs) were investigated with non-destructive tools, including scanning electron microscope (SEM), solid-state 13C nuclear magnetic resonance (SS 13C NMR), Fourier transform infrared spectrometer (FTIRS), and X-ray photoelectron spectrometer (XRPES), combined with thermogravimetric analyzer (TGA) and flash pyrolysis (FP). SS 13C NMR analysis shows that both HSBC and its Rs consist of aliphatic, aromatic, and carbonyl carbon species. The total content of aliphatic carbon species in the Rs is lower than that in HSBC, but the content of carbonyl carbon species is basically unchanged. In both HSBC and its Rs, aliphatic linkages connect 3–5 aromatic rings (ARs) and the number of substituents on each AR is up to 2 on average. Based on the semi-quantitative analysis with FTIRS and TGA, cleavaging weak- and medium-strength covalent bonds (>Cal-X-, X denotes H, N, S, or >Cal) should be the main reactions during the ethanolysis of HSBC and its inextractable portion. The analysis with XRPES indicates that the main oxygen-, nitrogen-, and sulfur-containing moieties are >C-O- groups, pyrrolic, and aromatic and sulfonic sulfurs, respectively, on the surface of HSBC and its Rs. Considerable amounts of alkanes and alkenes were released and non-substituted arenes are the main arenes released from the FP of HSBC and its Rs.