Ultrasonic Radiation Extraction And Fractionated Thermal Dissolution Of Zhaotong Lignite

As low rank coals, lignites retain the complex macromolecular structures of coal-forming plants to a great extent. Directionally converting organic matters in lignites into value-added chemicals is not only an ideal way but also a pressing issue for efficient utilization of lignite resources. Therefore, it is of great significance for efficient conversion and utilization of lignites to reveal the compositional and structural features of soluble organic species in lignites at the molecular level.In this paper, Zhaotong lignite(ZL) was used as the research sample and sequentially extracted with cyclohexane, carbon disulfide, methanol, acetone, and isometric carbon disulfide/acetone mixed solvent under ultrasonic radiation at room temperature. The extraction residue(ER) was subjected to fractionated thermal dissolution in cyclohexane and methanol at the temperature of 100-300 oC, and the total yield of extracts and thermally soluble portions(SPs) is 3.00% and 47.40%, respectively. The extracts and SPs were characterized with a Fourier transform infrared(FTIR) spectrometer, gas chromatograph/mass spectrometer(GC/MS), and Fourier transform ion cyclotron resonance mass spectrometer(FTICR MS). The results show that extraction process can isolate the unbonded small molecular compounds, mainly including alkanes, arenes, and heteroatom-containing species, the molecular mass and double bond equivalent distributions of which center between 300 and 600 u and within 2-6, respectively. Arenes with 1-2 rings are dominant, such as benzenes and naphthalenes, while heteroatom-containing species primarily exist in the forms of pyridines, imidazoles, piperazines, thiophenes, and thioethers. Inextractable small molecules, mainly alkanes, were separated from the ER through thermal dissolution at low temperatures. However, more small molecules were obtained at high temperatures owing to the cleavage of bridged bonds. Phenols are the main components and predominantly concentrated in SPM(> 200 oC). According to FTICR MS analysis, the basic N1 and N1Oy(y = 1-3) class species were mainly detected in SPM-250, which could be attributed to the reason that pyridines are difficult to be dissolved at temperatures below 250 oC.Three solid samples, ZL, ER, and thermal dissolution residue(TDR), were directly characterized with FTIR spectrometer, solid-state 13 C nuclear magnetic resonance(NMR), X-ray photoelectron spectrometer, thermogravimetry analyzer, and Curie-point pyrolyzer-GC/MS(CPP-GC/MS). According to the characterization by 13 C NMR, aliphatic carbons dominate ZL and ER. The average length of methylene chain in ZL and ER is relatively long and each aromatic cluster contains 2-3 aromatic rings on average. Compared with ZL and ER, the TDR is dominated by aromatic carbons and the average number of aromatic rings per cluster in the TDR is 4-5. Oxygen in ZL, ER, and TDR mainly exist as C-O moieties, carbonyls, and furan rings. The nitrogen-containing species primarily consist of pyrrolic, pyridinic, amino, and quaternary nitrogen species, and sulfur forms include mercaptan, sulfone, sulfoxide, thiophene, and inorganic sulfur. The analysis by CPP-GC/MS suggests that the pyrolysis products of ZL and ER are extremely similar and differ largely from those of TDR. On the basis of the above results, only a part of unbonded small molecules in ZL were extracted through ultrasonic radiation extraction, which cannot destroy the macromolecular skeleton structure of ZL. However, the breakage of covalent bonds, such as Cal-Cal, Cal-O, Cal-N, and Cal-S, in the ER occurs during thermal dissolution, resulting in significantly increased yields of SPs.