Effects of sequential extraction and thermal dissolution on the structure and composition of Buliangou subbituminous coal

Buliangou subbituminous coal (BSBC) was sequentially extracted under ultrasonication with petroleum ether, carbon disulfide, methanol, acetone, and isometric carbon disulfide/acetone mixed solvent to afford extracts and residue (RE) from the extraction. Then the RE was subjected to sequential thermal dissolution (TD) at 320 °C with cyclohexane, benzene, methanol, ethanol, and isopropanol to afford soluble portions and residue (RTD) from the TD. BSBC, RE, and RTD were characterized by Brunauer-Emmett-Teller method, X-ray photoelectron spectrometry (XPS), thermogravimetry, Fourier transform infrared (FTIR) spectrometry, and Curie-point pyrolysis (CPP)-gas chromatography/mass spectrometry to track the effects of the sequential extraction and TD on the coal structure. In total, 27.6 wt% of organic matter was released from BSBC by the ultrasonic extraction and subsequent TD. Both average pore diameter and total pore volume of RE and RTD increased, while specific surface area of RE and RTD decreased compared to the corresponding parameters of BSBC. No significant difference in the distribution of functional groups was observed in FTIR spectra between BSBC and RE, while the absorbances of aliphatic and COO moieties significantly decreased in FTIR spectrum of RTD compared to FTIR spectra of BSBC and RE. According to the CPP-gas chromatography/mass spectrometric analysis, significant differences in the distributions among the volatile species from the pyrolyses of BSBC, RE, and RTD at 500 °C were observed. Alkenes and oxygen-containing species were only identified in the volatile species from the pyrolyses of BSBC and RE rather than from the RTD pyrolysis. The cleavage of CαCβ bond in longer-chain alkylbenzenes and alkyltoluenes significantly proceeded during the pyrolysis.