Study on Catalytic Upgrading Process of Pyrolysis Volatiles from Low Rank Bituminous Coal

Author:Zhang Zhuang Zhuang

Supervisor:ma xiao xun


Degree Year:2019





China’s primary energy production capacity structure and consumption structure are dominated by coal The energy supply faces problems such as huge oil consumption gap and insufficient conventional natural gas reserves.The coal pyrolysis is an effective way to make full use of its structural potential rich in aromatics to achieve partial oil and gas substitution and carry out grading and qualitative utilization of coal.Coal catalytic pyrolysis could regulate the generation of free radicals and the secondary reaction behavior of volatiles in the pyrolysis process,which can effectively improve the content of light aromatics in tar and improve the quality of tar.In this paper,catalytic upgrading process of Shendong coal(a typical low rank bituminous coal)pyrolysis volatiles is taken as the research object,and the light aromatic hydrocarbons in low temperature pyrolysis products of Shendong coal are target products.By adjusting the dosage,acid properties,grain size and pore structure distribution of ZSM-5 zeolite,and coupling metal active center,the structure-property relationship between the structural properties of catalyst and the distribution and properties of gas,liquid and solid three-phase products obtained by pyrolysis was investigated in a fluidized bed(catalytic)pyrolysis reactor.In order to provide a theoretical basis for the design and preparation of coal pyrolysis catalysts,the mechanism of catalytic upgraiding of coal pyrolysis volatiles was discussed.First of all,the catalyst dosage was optimized at different bed heights and different fluidization medium in this study.The results show that the primary pyrolysis conversion rate of coal is independent of the fluidized bed height and the type of fluidized medium.The zeolite mainly acts on the catalytic upgrading process of coal pyrolysis volatiles.The more the amount of catalyst is used,pyrolysis volatiles are easily adsorbed and catalyzed on the surface of catalyst,and the catalytic reaction is deepened.Hence,it is essential for improving the yield of light aromatics to choose the appropriate catalyst dosage,which allowing the product to escape in time to become the final pyrolysis product.Secondly,a series of ZSM-5 zeolites with different acidity and the same crystal size were prepared by seed-induced method.The ZSM-5 zeolite with a Si/Al of 50 has a large amount of acid,a large acid density,a short distance between active site,resulting in consecutive catalytic reactions of pyrolysis volatiles.The aromatization of aliphatic hydrocarbons and the deoxygenation reaction of phenolic compounds are promoted.The relative contents of total aromatics and the yields of light aromatics in tar are the highest,and more carbon deposition is formed.On this basis,micron-and nano-ZSM-5 zeolites with highly dispersed crystals and similar acid properties were prepared by seed-induced method.The nano-zeolite has a short diffusion path,exposes more outside surface active sites.More intergranular mesopores and available pores are formed between the small crystal grains.Catalytic conversion of tar volatiles was promoted.The benzene series compounds can escape the pores in time,which improving the selectivity of monocyclic aromatic hydrocarbons by avoiding excessive residence in the pores.In addition,a series of hierarchical ZSM-5 zeolites composited by micro-mesoporous structure were prepared by desiliconization with alkali treatment.Hierarchical ZSM-5zeolites expose more outer surface and increase the relative acid amount of catalyst.Catalytic conversion of tar volatiles was promoted.It is beneficial for the diffusion of carbon deposition precursors during the reaction process,and alleviating the formation of carbon deposits.It is important to maintain a suitable micro/mesoporous ratio.Finally,a metal/acid double active center catalytic system was established in this paper.Dehydrogenation of aliphatic hydrocarbons together with hydrodeoxygenation of phenolic compounds and stabilization of macromolecular radicals was coupled by metal/acid synergistic catalysis.This hydrogen cycle process effectively promoted the formation of light aromatics by using the hydrogen existed in original system in the absence of external H2.