Dissociation And Compositional Features Of Organic Matter In Hecaogou Low-rank Coal

The high oxygen content of low-rank coal severely limits its industrial application.However,low-rank coal has unique advantages as a raw material for obtaining high value-added oxygen-containing organic chemicals.Due to the complex molecular composition of coal,it is difficult to obtain complete composition information with a single method.Therefore,the combined method of ultrasonic-assisted fractional extraction,variable temperature thermal dissolution,and catalytic thermal dissolution is used to perform hierarchical dissociation and depolymerization of organic matter in low-rank coal to obtain information on the soluble components and carbon skeleton structure of the coal.The high value-added utilization of rank coal provides theoretical basis and technical support.This subject focuses on the gradual dissociation of organic matter in Hecaogou low-rank coal（HSC）.First,the HSC is subjected to ultrasonic-assisted fractionation and extract using CS₂,acetone and an equal volume mixed solution of acetone/CS₂ as the solvent.Secondly,the raffinate coal was sequential thermal dissolved in ethanol,isopropanol and n-hexane solvents,respectively,and the thermal dissolution temperature was 210℃,250℃,290℃and 330℃respectively.Finally,the residue of ethanol thermal dissolution at 300℃is catalytically thermal dissolved with Na OH to further dissociate and release the organic matter in the coal.GC/MS was used to analyze the composition and content of liquid phase products at all levels.Scanning electron microscope（SEM）,solid-state nuclear magnetic resonance(¹³C NMR),infrared spectroscopy（FTIR）,X-ray photoelectron spectroscopy（XPS）,element analyzer and thermogravimetric analyzer（TG）are used to analyze raw coal,extraction residues,thermal dissolution residue and catalytic thermal dissolution residue were directly characterized.The yields of the extracts obtained from HSC with CS₂,acetone and CS₂ mixed solvent of equal volume were 1.88%,0.94%,4.24%,respectively,and the total yield was7.06%.Through GC/MS analysis,extraction can dissociate the free small organic molecules in the macromolecular structure of the organic matter in the coal.E₁ mainly contains alkanes,oxygen-containing and aromatic substances,E₂ mainly contains oxygen-containing and alkanes.E₃ is mainly oxygen-containing,alkanes and nitrogen-containing substances.The dissociation ability of thermal dissolution of ER₃ in ethanol,isopropanol and n-hexane is in the order of ethanol>isopropanol>n-hexane.The total yields of each system are 32.72%,30.21%and 17.29%,respectively.GC/MS analysis shows that the main components of the soluble products are oxygen-containing compounds,indicating that the non covalent bond and weak covalent bond forces between organic matter in coal are destroyed by the thermal solution process.The organic matter is further dissociated during the process of TER₄ catalytic thermal dissolution,some of the weak covalent bonds such as Calk-O,Car-O,Car-Calk bonds were broken,and the yield of soluble matter up to 6.12%,GC/MS analysis,shows that the relative content of alkane substances in the catalytic thermal dissolution is the highest,followed by oxygen-containing substances.Solid ¹³C NMR analysis showed that the HSC carbon skeleton structure is mainly composed of aliphatic carbon（35.61%）and aromatic carbon（62.82%）,and the condensed aromatic ring structure is mainly composed of 2-3 aromatic rings.After fractional extraction,the proportion of aromatic carbon in the ER₃ carbon skeleton is63.12%,and the condensed aromatic ring structure is mainly composed of two aromatic rings.After being thermal in ethanol,isopropanol and n-hexane,the carbon skeleton structure of the residues TER₄,TIR₄ and THR₄ is still mainly composed of aliphatic carbon and aromatic carbon,and the aromatic carbon content increases to 73.05%,82.41%and 83.75%,respectively,the condensed aromatic ring structure is mainly composed of 2-3 aromatic rings.After TER₄ is catalyzed and thermally dissolved,the proportion of aromatic carbon in the carbon skeleton of CTER is 72.53%,and the average methylene chain length,Cn,is reduced from 1.96 of HSC to 0.52,indicating that weak covalent bonds such as Calk-O、Car-O and Car-Calk are broken,but the skeleton structure of the condensed aromatic ring is not destroyed.Elemental analysis shows that the carbon content and oxygen content of HSC are slightly reduced after the solvent fractionation extraction.After ER₃ is treated by thermal dissolution and catalytic thermal dissolution,the carbon content increases and the oxygen content decreases.XPS analysis shows that the surface oxygen of HSC,ER₃,TER₄,TIR₄,THR4 and CTER mainly exists in the form of C-O.Thermogravimetric analysis shows that the thermal weight loss rate of HSC,ER₃,TER₄,THR₄,TIR₄ and CTER decreases sequentially,and the temperature corresponding to the maximum thermal weight loss rate increases sequentially,indicating that solvent treatment can dissociate the organic matter of coal to a certain extent,dissociation improves the thermal stability of coal.