Construction Of In-situ Mass Spectrometer And Its Application In The Study Of Pyrolysis Behavior And Reaction Mechanism Of Coal And Model Compounds

Pyrolysis is an important mean for the efficient clean utilization of coal.The complexity,variability and heterogeneity of coal structure and the lack of accurate description of coal pyrolysis at the molecular level lead to the lack of relevant theoretical knowledge about the dissociation law of chemical bonds and free radical reaction mechanism in coal pyrolysis process.The reasonably developed pyrolysis time-of-flight mass spectrometer can realize the in-situ detection of primary pyrolysis products,intermediate and radicals.In this thesis,the pyrolysis EI/VUVPI time-of-flight mass spectrometer system（Py-EI/VUVPI-TOFMS）and pyrolysis EI/VUVPI molecular-beam-sampling time-of-flight mass spectrometer system（Py-EI/VUVPI-MBMS）were constructed to study the pyrolysis characteristics of coal and model compounds（aromatic ether and long-chain n-alkane）,and the reaction mechanism of specific structure in coal was speculated by DFT theoretical calculation,which provided experimental and theoretical support for further comprehension on coal pyrolysis behavior and mechanism.A laser indued acoustic desortion EI/VUVPI time-of-flight mass spectrometer system（LIAD-EI/VUVPI-TOFMS）was established for qualitative analysis and molecular weight distribution research of NMH extract,which provides a new method for understanding the structure of macromolecular clusters in coal.The major contents and results in this work are summarized as follows:The basic principles and development experience of the instrument were obtained by assembling and adjusting a set of Py-EI/VUVPI-TOFMS system that meets the experimental requirements.On this basis,a set of Py-EI/VUVPI-MBMS system with a mass range less than1200 amu.,an EI sensitivity 3.8 ppb,a PI sensitivity 51.6 ppb and a resolution greater than 1330was constructed.The test results showed that the design of the sampling system,pyrolysis furnace and molecular beam sampling device is reasonable and can meet the experimental requirements.A set of LIAD-EI/VUVPI-TOFMS system was established and can be used for component identification and qualitative analysis of complex systems.The composition and evolution behavior of primary pyrolysis products of five coal with different ranks were investigated by the Py-EI/VUVPI-TOFMS system.Small molecule products（H₂,CH₄,H₂O,CO and CO₂）and primary pyrolysis organic volatiles were analyzed by EI-MS and VUVPI-MS,respectively.Results indicated that the main evolution temperature range of volatile products from coal pyrolysis is 300-600℃.The hydrocarbon products consist of mainly arenes of 1-3 rings,the oxygen-containing compounds are dominated by phenols with content of less than 2%,and nitrogen/sulfur-containing compounds are mainly heterocyclics with content of less than 1%.The peek temperature of pyrolysis products is found to be positively correlated with the volatile content and negatinely correlated with the carbon content of coal.The the pyrolysis behavior and mechanism of three aromatic ether model compounds（diphenyl ether,phenethoxybenzene,benzyl ether）were investigated by the Py-EI/VUVPI-MBMS system.Results indicated that the photodissociation reaction occurs during the pyrolysis of phenethoxybenzene and benzyl ether at 200℃.The pyrolysis of aromatic ether compound starts from the cleavage of C-O bond.The cracking of benzyl and phenoxyl radicals generate cyclopentadienyl radical around 800℃,and the reaction energy profiles of benzyl cracking to cyclopentadienyl is analyzed by DFT theory calculation.Bibenzyl,diphenylmethane,dibenzofuran,phthalan,fluorene,9,10-dihydrophenanthrene,naphthalene,phenanthrene and biphenylene,etc.are generated from dehydrocyclization and rearrangement of some intermediate products and radicals.In addition,the mass peak of m/z=79,an intermediate product of methylcyclopentadiene to benzene,confirms the reaction path of methyl cyclopentadiene（m/z=80）to benzene during the pyrolysis of diphenyl ether and benzyl ether.The pyrolysis behavior and mechanism of six long-chain n-alkanes(C_nH_2n+2,n=10,12-16)were investigated by the Py-EI/VUVPI-MBMS system.Results indicated that the pyrolysis of n-alkanes is dominated by a chain reaction mechanism,beginning with the C-C bond homolysis of n-C_nH_2n+2 to produce primary alkyl radicals and termination byβ_C-H homolysis and mutual combination of radicals.The chain is propagated by theβ_C-C homolysis and H-abstraction of primary alkyl radicals to produce secondary alkyl radicals,theβ_C-C homolysis of the secondary alkyl radicals to form alkyl/alkenyl radicals and alkenes(C_kH_2k/C_kH_2k-2)and the hydrogen-shift to form free-radical isomers.It was also found that the position of the produced alkyl radical is closer to the central carbon at low temperature.With temperature rises,alkyl radical with free radical on the second carbon atom,marked as 2-C_mH_2m+1,will gradually become dominant.The qualitative analysis experiment of different model compounds proved that the constructed LIAD-EI/VUVPI-TOFMS system is feasible for qualitative analysis of multiple components.On this basis,the molecular weight distribution of NMH extract was obtained by LIAD-EI/VUVPI-TOFMS and the synchronous fluorescence spectroscopy analysis.The results indicated that the LIAD-EI/VUVPI-TOFMS system combined with other characterization and sample pretreatment methods can provide a new idea and method for the study of coal chemical structure.