| Coal plays a dominant role in China's energy consumption structure for a long time.The efficient and clean utilization of coal are essential for guaranteeing our energy security and solving existing environmental problems.Coal structure determines its reactivity.The deep understanding into the structural characteristics of organic matters of coal is of great theoretical and practical significance for the development of clean and efficient coal conversion process.In recent years,researchers have accumulated a large number of methods to investigate coal structure.However,due to the restriction of complexity of coal structure,experimental conditions,analytical ability of instruments and basic theory,the existing methods have different limitations to some degree.The previous research has not yet reached the level of accurately understanding the structural characteristics of organic matters in coal.The research on coal structure still need further development.In this work,solvent extraction,in-situ pyrolysis,oxidative depolymerization and instrumental characterization were employed to investigate the structural feature of organic matters of low-rank Naomaohu(NMH)coal.The structural characteristics of NMH coal were obtained and physical as well as chemical structural model were constructed.This work provided certain guidance for the structure investigation and efficient utilization of low-rank coal.The main contents and results are as follows:(1)The solvent extraction of NMH coal was conducted to acquire macromolecular clusters which has representativeness for basic structural units of coal.The extraction behaviors of Microwave-assisted extraction with different solvents were studied.The structural characteristics of raw coal and extraction products were investigated by thermogravimetry analyzer,gel permeation chromatography,Fourier transform infrared spectroscopy,13C/1H nuclear magnetic resonance and vacuum ultraviolet single-photon ionization time of flight mass spectrometry(VUVPI-TOF-MS).The results showed that cyclohexanone(CYC)exhibits a higher total extract yield of 8.3 wt.%than THF,being 4.7 wt.%,and more efficiency in extracting condensed aromatics and macromolecular compounds in NMH coal.The molar fraction of aromatic bridgehead carbon(Xb)and the number-average molecular weight of CYC extract(Ecyc)are 0.232 and 786 amu,respectively.Alkenes,aromatics and phenols in the form of homologous series were detected during in-situ pyrolysis of NMH coal and ECYC on VUVPITOF-MS.The aromatic ring structures in ECYC mainly involve 1?4 rings compounds and 1-ring compounds are dominant.The various aromatic nuclei with different size in NMH parent coal are also exist in Ecyc.The chemical structure of ECYC could reflect the structural characteristics of NMH coal to some extent.(2)NMH coal was gradually degraded by a step-wise oxidation method.The irregular bridge bonds and side chains were first destroyed by hydrogen peroxide(H2O2)at mild conditions.Then,oxidation residue was further converted and ring-open reaction of condensed aromatic compounds occurred during sodium hydroxide(NaOH)alkali-oxygen(A-O)oxidation.As a result,the controllable oxidative depolymerization of orgnic matters in coal was achieved.The results showed that H2O2 is able to break bridge bonds and side chains connected to aromatic clusters at low temperature,while Car-Car bonds of arene keep unchanged at same conditions.Due to the lower bond energy of bridge bonds,aromatic structures,especially 1ring and 2-ring aromatic compounds are gradually enriched in water-soluble product(WSPH2O2),while aliphatic structures are retained in oxidation residues(RH2O2).After 10 h oxidation at 60?,the coal conversion is 83.4 wt.%and the aromaticity of RH2O2 decreases to 7.4%from 36.0%of raw coal.Meanwhile,Xb increases from 0.192 to 0.375.Apart from the basic reaction that reactive carbon atoms were oxidized to corresponding alcohols,ketones,aldehydes,carboxylic acids,H2O2 oxidation degradation of coal are also accompanied by generation of alkenes and decarboxylic reaction.The above degradation processes were verified by coal-related model compounds.NaOH A-O oxidation results of RH2O2 showed that almost all aromatic structures were degraded into liquid products(WSPalkali)without breakage of aromatic structures when oxidation temperature was at 180?.Then,the ring-open reaction of polycyclic aromatic structures occurred successively when oxidation temperature further increased.3?4 ring aromatic compounds were gradually degraded to 1?2 ring compounds and excessive oxidation of 1-ring compounds occurred subsequently.On the base of this phenomenon,the controllable production of high value-added benzene carboxylic acids(BCAs)from NMH coal was conducted.The results showed that the excessive or incomplete oxidation of benzene ring was avoided by controlling oxidation time.The maximum yield(17.3 wt.%)of BCAs was obtained at a condition of 240?,150 min,alkali/coal mass ratio of 3/1.Among which,the relative content of BCAs with three,four and five carboxyl groups are dominant,suggesting the higher substitution degree of the aromatic nucleus structure in NMH coal.(3)Based on the synchronous fluorescence spectroscopy analysis of liquid products of H2O2 oxidation and WSPalkali,it was found that 1-ring and 2-ring aromatic compounds are dominate in WSPH2O2,while WSPalkali contains more 3?5 ring compounds.The aromatic ring distribution of NMH coal was deduced from the aromatic structure characteristics of two kind of oxidastion products,where 1-ring compounds are dominant accounting for 54.4%,followed by 2-ring and 3-ring aromatic compounds accounting for 23.1%and 20.0%.The aromatic compounds with four and more rings are account for 2.4%.Moreover,it was inferred that 3ring and 4-ring compounds mainly existed in the form of anthracene and pyrene,respectively.Finally,the skeleton structure model of coal was proposed based on the complementary information from XPS,XRD and Raman.(4)Comprehensively analyzing the obtained structural information,we established the physical structural model for NMH coal,which reflect the aromatic ring distribution of mobile phase and clearly expound the solvent extraction and step-wise oxidation degradation process of coal.Moreover,the chemical structural models with molecular formulas of C526H535O117N5S and molecular weight of 8831 amu was conducted to reveal bonding structure characteristics of organic matters of NMH coal. |
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