Study On Catalytic Upgrading Of Shengli Lignite Pyrolysis Volatiles Over Metal-modified HZSM-5

The upgrading of lignite through pyrolysis is one of the important ways for the processing and utilization of low rank coal.But the high portion of heavy materials and different classes of organic oxygen species(OOSs)in the pyrolysis tar seriously restrict its commercial storage and utilization.Upgrading of coal-tar to obtain light products such as BTEXN is one of the key techniques urgently to solve during coal pyrolysis.In the dissertation,catalytic upgrading of pyrolysis volatiles from Shengli lignite(SL)were conducted in a drop tube reactor in order to obtain high-quality light-tat rich in BTEXN.This study provides a theoretical basis for developing new technology for directional conversion of lignite to light aromatics.The pyrolysis behaviors of SL was firstly investigated by using thermogravimetric analysis and the kinetic data were evaluated with Kissinger(KAS)and Ozawa-Flynn-Wall(OFW)models.The activation energy values were 292.7 and 277.6 kJ/mol for thermal decomposition of SL based on KAS and OFW models,respectively.Moreover,the effects of temperature and space velocity on the yields of pyrolysis products were studied.In particular,the chemical compositions of pyrolysis oil were analyzed.The temperature was found to be the main parameter that affect thermal decomposition of SL.The tar achieved a maximum yield of 21.4%(daf)at a temperature of 600 oC and a gas residence time of 1.0 s.The chemical compounds detected in the tar by CG/MS include organic oxygen,nitrogen and sulfur species(OOSs,ONSs and OSSs),aliphatics and arenes,in which the relative content of aliphatics and arenes are 50.0% and 28.9%,respectively.In situ catalytic upgrading of SL pyrolysis volatiles was conducted over HZSM-5,Co-,Moand Ni-modified HZSM-5 prepared via wet impregnation.The catalysts were characterized by X-ray diffraction,N2 adsorption-desorption analyzer,temperature programmed desorption of ammonia,Fourier transform infrared spectrometer and transmission electron microscope.When the pyrolysis volatiles passed through HZSM-5 catalyst bed,the resulted gas yield increased and the tar yield decreased.And the upgrading tar achieved a maximum yield of 11.9% at 600 oC.The relative content of arenes is 87.3% and the OOSs content is only 8.1%.Besides,the BTEXN amount in the upgrading tar by HZSM-5 at 600 oC reached to 16.1 mmol/g,which increased 3.0 times compared with non-catalytic test.This should be attributed to the special pore structure and acidic sites of HZSM-5,which could promote the catalytic cracking,aromatization of hydrocarbons as well as dehydroxylation of phenols.Due to the participation of metal and acid sites,the bifunctional metal-loaded HZSM-5 showed comparable catalytic activity for aromatic production from SL.The relative content of arenes in the upgrading tar by Co/HZSM-5,Mo/HZSM-5 and Ni/HZSM-5 is 90.5%,88.8% and 94.2%,respectively.And the amount of main light aromatics is 33.0,26.6 and 39.7 wt.%,respectively,which is 2.1,1.7 and 2.6 times than that with HZSM-5.Among the catalysts,Ni/HZSM-5 exhibited the best performance for production of high quality tars with highest aromatic yield and H2-rich gas,which can be used as a potential candidate for catalytic fast pyrolysis of lignite.