Composition and structure characteristics of soluble organic matter from Naomaohu lignite by sequential extraction and thermal conversion performance of the corresponding residue

Carbon disulfide (CDS), methanol, acetone and isometric carbon disulfide/acetone mixture (IMCDSAM) were used as solvents to sequentially extract Naomaohu lignite (NL) via ultrasonic-assisted extraction to obtain extracts (E1–E4) and final extraction residue (ER). Composition and structure of E1–E4 were analyzed by GC-MS. It is found that the main compounds in E1 are alkanes, aromatics, alcohols and esters. Alkanes, alcohols and esters are the main compounds in E2. Alcohols, phenolics and esters are the main components in E3, and esters are mainly phthalic diester compounds. Affected by synergistic effect of the two solvents CDS and acetone, the relative content of alkenes in E4 is relatively high. FT-IR was used to characterize functional groups in NL, E1–E4 and ER. The results show that the ultrasonic extraction process only extracts free small compounds from macromolecular skeleton of the NL and some other molecules, which connect the macromolecular skeleton by weak covalent bonds, and the process does not destroy the macromolecular skeleton structure. In addition, peak fitting results from FT-IR show that types of infrared absorption peaks in ER do not change after ultrasonic extraction, while intensity of the peaks varies. TG-DTG profiles of NL and ER indicate that after ultrasonic extraction weight loss of NL increases from 47.09% to 51.04%, and peak of the maximum weight loss rate is advanced from 450 to 430°C. Pyrolysis kinetic analysis of NL and ER based on Coats-Redfern model show that after ultrasonic extraction activation energy of ER in rapid pyrolysis stage is lower than that of NL, and the pyrolysis process is easier to proceed.