Study On Chemical Structure Change Of Lignite Char Druing Devolatilization Process Under Low Temperature Pyrolysis

Lignite possess the characters of low degree of coalification,low calorific value and highmoisture content.Direct combustion only utilize its properties as fuels,but does not make fully use of its essential value as chemical raw material.Therefore,Therefore,lignite must be treated with low temperature pyrolysis in order to enrich its utilization ways and improve its utilization efficiency.In this paper,Xilinguole lignite was taken as research object.The breakage of covalent bonds,the distribution characteristics and forming mechanism of pyrolysis volatile products and chemical structure were all studied by thermogravimetric analysis?TG?,fisher experiment,thermogravimetric coupled with Fourier transform infrared spectroscopy?TG-FTIR?,Fourier transform infrared spectroscopy?FTIR?and ¹³C nuclear magnetic resonance(¹³C NMR).By analyzing the correlation between the yield of tar,pyrolysis water and pyrolysis gaseous products and the chemical structure of char,the changes of chemical structure of char were investigated during devolatilization process under low temperature pyrolysis.The pyrolysis behavior of lignite and the breakage of covalent bonds in chars during lowtemperature pyrolysis were investigated by TG analyzer.The results shown that there were four kinds of covalent bonds which broke during low temperature pyrolysis.The release of bonded water and the decomposition of carboxylic acid occurred at around 180°C,the breakage of Cal-O/S/N bonds at about 376°C,the breakage of C_al-C_al/H/N bonds nearly at 432°C and the breakage of C_ar-C_al/O/S bonds approximately at 521°C.The yield of tar pyrolysis water and pyrolysis gaseous products at 376°C,432°C and 521°C were calculated by using fisher experiments.Based on the breaking information of covalent bonds during low temperature pyrolysis and TG-FTIR experiments,the distribution and formation mechanism of tar,pyrolysis water and pyrolysis gas were also discussed.The carbon structure of lignite and its chars were studied by ¹³C NMR and curve fittingmethods.The variation of carbon structure of char during devolatilination process were also investigated by combining the breakage of chemical bonds.The results shown that the breakage of C_al-O bond,C_al-C_al bond and C_ar-C_all bond during low temperature pyrolysis led to the loss of aliphatic side chains,aromatic structures of small molecules and derivatives of oxygen-containing groups from aromatic nuclei.Also the breakage of Cal-O,Cal-Cal and Car-Cal bonds resulted in the decrease of average length?C_n?of methylene side chains and degree of substitution???of aromatic rings.The aliphatic side chains are detached from char,leaving a large number of highly active aromatic substitution sites.The condensation of these substitution sites enhanced the degree of condensation?X_b?and aromatic carbon rate?f_a?of the char.Aliphatic side chains,small molecular aromatic structures and oxygen-containing group derivatives from char recombined with each other in the pyrolysis environment and evolved in in the form of tar,pyrolysis water and light gases during the pyrolysis process.The functional groups of lignite and its chars were studied by FTIR and curve fittingmethods.The variation of functional groups of lignite and its chars was analyzed by combining the breakage of covalent bonds and the precipitation of volatile products.The results shown that functional groups of lignite mainly include aromatic C-H structure,oxygen-containing functional group,aliphatic C-H structure and hydrogen bond of hydroxyl group.With the evolve of volatile products,the decrease of the degree of aromatic substitution were coupled with the increase of the relative content of 2H and 3H structures and the increase of the relative content of 4H structures in aromatic C-H structure.The oxygen-containing functional groups mainly include alkyl and aryl ether bonds?C-O?,alcoholic ethers and phenolic ethers?C-OH?,carboxyl?O=C-O?and conjugated C=O groups.The chemical reactivity of C-O and C-OH were lower than that of carboxyl?O=C-O?and conjugated C=O groups.