Catalytic Upgrading Of Lignite Pyrolysis Volatiles Over Modified HY Zeolites

Pyrolysis is an important technology for the thermochemical conversion of low-rank coal.However,tar usually contains a large number of heavy components,which could foul and block the subsequent equipment and limit the industrial application of pyrolysis technology.Moreover,high oxygen content and complex compositions of tar restrict the utilization of coal tar.To solve these problems,in this dissertation,modified HY zeolites were used for catalytic cracking of lignite pyrolysis volatiles.The heavy components were expected to be in-situ converted into light aromatics and then improve the quality of tar,and the upgrading mechanism of volatiles is obtained on pyrolysis electron impact/vacuum ultraviolet photoionization timeof-flight mass spectrometry(Py-EI/PI-TOFMS).To improve tar yield during the volatiles cracking,catalytic upgrading of lignite pyrolysis volatiles coupled with methane dry reforming was proposed.By this way,the cracking fragments of volatiles could be stabilized by small molecule free radicals from methane dry reforming,which can avoid the excessive cracking of volatiles,improve the yield and quality of tar and obtain more high value-added chemicals.The main work and results of dissertation are as follows:(1)The HY was modified by steam and acid solution treatments for the improvement of physicochemical properties.The effects of steam temperature,acid type and acid concentration on structure and property of HY zeolites were investigated,and the catalytic upgrading of pyrolysis volatiles from BYH lignite over modified HY zeolites was evaluated in a fixed-bed reactor.The results showed that the HY-650 modified by HNO3 and HCl acid solution had superior physicochemical properties and exhibited a good catalytic performance in the production of light aromatics.Over HY-650-HNO3 and HY-650-HCl,the average molecular weight of the upgraded tar decreased to 182 amu and 179 amu from 316 amu for non-upgrading tar,the light tar content increased to 81.5 wt.%and 82.0 wt.%from 55.5 wt.%,and the yields of naphthalenes were increased by 7.6 and 6.7 times.With increasing of HNO3 concentration,the non-framework A1 species were removed,then the SiO2/Al2O3 ratio,surface area and mesoporous volume were increased,and the total acid amounts were decreased.Modified HY by steam treatment at 650℃ and acid washing by 0.8 mol/L HNO3 showed the best upgrading performance.Compared with non-upgrading tar,the content and yield of light tar were increased by 46.8%and 11.0%over HY-650-0.8HNO3,and the total content of benzene,toluene,xylene,naphthalene and methylnaphthalene was increased from 5.1%to 45.6%,and the yields of naphthalene and methylnaphthalene were improved by 9.1 and 6.7 times,yet oxygenated polycyclic aromatic hydrocarbons yield was reduced by 81.0%via decarboxylation and decarbonylation reactions.(2)The novel Py-EI/PI-TOFMS was used to further investigate the influences of modified HY zeolites on the composition,distribution and temperature-evolved profiles of cracking products during catalytic upgrading.The results indicated that HY zeolites modified by sequential steam-HNO3 washing exhibited better catalytic cracking performances in lignite pyrolysis volatiles and were helpful for the formation of light aromatics and the conversion of organic oxygen species.Compared with non-catalytic pyrolysis,the addition of HY-650-HNO3 and HY-650’-HNO3 catalysts could obviously enhance the contents of benzenes and naphthalenes and selectivity of toluene,xylene,naphthalene and methylnaphthalene,reducing the contents of phenols and macromolecular compounds through condensation,dehydration,decarboxylation and decarbonylation.Especially,trimethylphenol,naphthalenol and naphthalenediol were totally converted into aromatics over HY-650’-HNO3 by dehydration and cracking.(3)To obtain high light tar yield during volatiles cracking,Ni supported over modified HY zeolites(Ni/MHY1,Ni/MHY2 and Ni/MHY3 catalysts)were prepared by precipitation method,and used as catalysts for the catalytic upgrading of BYH lignite pyrolysis volatiles under N2 or CH4/CO2 atmosphere.Ni/MHY3 exhibited better catalytic upgrading performance in terms of light tar yield and deoxygenation due to its high mesoporous volume,large surface area,small Ni nanoparticles and mild acidity.Under N2 atmosphere,the contents of pitch and organic oxygen species in tar over Ni/MHY3 decreased to 25.0 wt.%from 44.5 wt.%and 9.0%from 20.2%,respectively,and the light aromatics contents increased to 31.6%from 14.6%.The light tar yield under CH4/CO2 atmosphere over Ni/MHY3 was increased by 12.0%in comparison with that in N2 atmosphere.It is inferred that the catalytic cracking of pyrolysis volatiles and methane dry reforming could simultaneously occur over Ni/MHY3.Ni/MHY3 exhibited good activity and stability even after 4 cycles of reaction-regeneration.(4)To further improve the capability of coke resistance and acquire high light tar yield,Ni-xCeO2/MHY catalysts were prepared by the loading of Ni and CeO2 on modified HY zeolites and employed as catalysts for the upgrading of lignite pyrolysis volatiles under CH4/CO2 atmosphere.The addition of CeO2 promoter caused the decrease of surface area,volume and acid amounts,and enhanced the interaction of metal with support and the dispersion of metal particles.The results suggested that light tar yield over Ni-7CeO2/MHY increased by 15.6%in comparison with that of Ni/MHY.Compared with non-upgrading tar,the content and yield of light tar were improved by 30.6%and 21.9%respectively,and the content of naphthalenes were increased from 7.6%to 33.5%,yet the content of organic oxygen species decreased to 8.0%from 21.8%.When CD4 was used as the tracer,obvious deuterium peaks appeared in the 2H NMR spectrum of upgraded tar,indicating CD4 participated in the upgrading of lignite pyrolysis volatiles.Moreover,deuterium(D)was also detected in typical compounds by GC-MS,which further proved that tar upgrading was an integrated process of lignite pyrolysis volatiles cracking with methane dry reforming.