Dissociation And Composition Characterization Of Organic Matter In Two Low-rank Coals

Understanding the composition and structure of organic matter in low-rank coals at the molecular level is a main research topic of molecular coal chemistry. It is not only a key problem needed to be solved urgently for coal chemistry, but also the scientific foundation for efficient, clean and value-add utilization of low-rank coals.Buliangou subbituminous coal(BSBC) and Huozhou lignite(HL) were sequentially extracted under ultrasonication to afford extracts(E1–E5) and extraction residue(ER). Then the ER were subjected to sequential thermal dissolution at 320 °C for 2 h to afford soluble portions(SP1–SP5) and thermal dissolution residue(TDR), meanwhile, extracts and soluble portions were characterized with kinds of analytical tools. Finally, TDR was pyrolyzed by using Curie-point pyrolyzer(CPP) to release more organic matters which were detected by GC/MS on-line.The yields of extracts from BSBC and HL are about 1% and 18%, respectively. The synergistic effects of carbon disulfide and acetone during mixed solvent extraction are greatly influenced by the coal quality, which is more effective during HL extraction. The compounds detected by GC/MS in coal extracts mainly are alkanes and arenes, some heteroatomcontaining compounds were also detected. These compounds are mainly in the extracts of petroleum ether and carbon disulfide; although the yields of methanol, acetone, and mixed solvent extracts are higher, there is few GC/MS-detectable compound in these extracts. Electrospray ionization(ESI) Fourier transform ion cyclotron resonance mass spectrometer(FTICRMS) analysis shows that the m/z of detected compounds in EBSBC-3–EBSBC-5 mainly distributes from 200 to 600. Main basic organonitrogen compounds(ONCs) detected are N1Ox(x = 0–1) class species in EBSBC-3–EBSBC-5. The core structures of most basic N1Ox(x = 0–1) class species should be aromatic rings, including benzene, pyridine, quinoline, and acridine rings.Arenes and phenolic compounds are dominant in the soluble portions from ERHL and ERBSBC, and some O-containing compounds were also detected. Phenolic compounds are from the release of those exist in coals and the crack reaction of aryl ether structures. Moreover, there are molecular interactions, such as hydrogen bond and π–π interaction, between those existence phenols and other structures in coals. Esters mainly are from the transesterification during alcoholysis with ethanol. GC/MS and ESI FTICRMS analyses imply that SPs from ERHL are of higher molecular weight and aromaticity than those from ERBSBC. Main basic ONCs detected are also N1Ox(x = 0–1) class species in SPs. These N1 Ox class species are concentrated in DBE values of 4–20 and CNs of 12–32 for BSBC but 4–26 and 11–39 for HL.Many GC/MS-detectable compounds were released during the pyrolyses of E3–E5 from BSBC and HL at 445 oC, including alkanes, arenes, kinds of O-containing compounds, and a few N/S-containing compounds. Compared with SPBSBC-1–SPBSBC-5, there are more GC/MS-detectable compounds in the products from SPs pyrolyses at 445 oC. These added compounds mainly are from the pyrolysis of aryl ethers and longer-chain alkylarenes. Aryl ether structures in ERBSBC have a wider distribution than ERHL. CPP-GC/MS analysis of SPs from ERHL shows that the introduction of N atom to the aromatic ring of N-containing heterocycles, phenols, and ketones will facilitate the pyrolysis of these compounds.Both average pore diameter and total pore volume of ERBSBC and TDRBSBC increased, while specific surface area of ERBSBC and TDRBSBC decreased compared to the corresponding parameters of BSBC. No significant difference in the distribution of functional groups was observed in FTIR spectra between raw coal and ER, the absorbances of aliphatic and COO moieties significantly decreased in FTIR spectrum of TDR compared to FTIR spectra of raw coals and ER. According to the CPP-gas chromatography/mass spectrometric analysis, significant differences were observed during BSBC and HL pyrolyses. Alkanes, alkenes, arenes, and phenols were detected during BSBC pyrolysis at 500 °C. Otherwise, alkenes and oxygen-containing species were only identified in the volatile species from the pyrolyses of BSBC and ERBSBC rather than from the TDRBSBC pyrolysis. The cleavage of C?–Cβ bond in longer-chain alkylbenzenes and alkyltoluenes significantly proceeded during the pyrolysis.