Study On Coking And Regeneration Characteristics Of Catalysts In The Catalytic Pyrolysis Of Biomass

Biomass, as the only source of organic carbon in the post fossil fuels period, is a renewable resource which can be converted into liquid fuels and chemicals, realizing substitution of fossil fuel. Catalytic pyrolysis of biomass can convert biomass to hydrocarbons selectively with catalysts, among which olefins and aromatic hydrocarbons are all important chemical raw chemicals and platform compounds, and thus it has attracted lots of attention recently. However, severe catalyst coking happens during catalytic pyrolysis of biomass. Catalyst deactivates quickly and then needs regeneration, which determines its high operating cost. Thus catalytic pyrolysis of biomass has not been commercialized. Focusing on above problems, this paper aimed at catalysts coking and divided it into "active coke" which promoted the catalysis and "inert coke" which led to deactivation. First we identified in-situ chemical component of two kinds of coke, explored the formation and revolution mechanism of active coke and also its influence on products. Based on experimental and theoretical study, kinetic model of catalytic pyrolysis of biomass relating coke content with product distribution was built. Finally, catalyst regeneration was adjusted from two aspects of catalyst structure and process, realizing stable running of catalysis-regeneration cycles.The functional group of biomass pyrolysis vapor has a great effect on product and coking characteristics. In a gas flow reactor, pyrolysis temperature, weight hourly space velocity (WHSV) and partial pressure were considered to understand their influence on catalytic performance of biomass pyrolysis derivates with different functional groups (furans, saturated furans and straight chain). Furan was used as presentative species from pyrolysis of cellulose. Physicochemical property and its evolution of coke during catalytic conversion was explored, and the formation mechanism of coke in furan conversion was proposed. The results indicated that C-O breakage promoted aromatization and polymerization, and then coke deposition led to the decrease of conversion. Tetrahydrofuran (THF) has longer carbon chain compared with ethylene glycol (EG) and thus carbon yield of hydrocarbons and coke was relatively higher. In the initial stage, there was an induction period that carbon yield of hydrocarbons increased quickly in catalytic conversion of furan. In the viewpoint of the influence of coke on catalysis, carbon of small molecules (active coke) accumulated at the early stage that promoted catalysis, while carbon of large molecules (inert coke) blocked catalyst pores or covered active sites that led to the sharp decrease of hydrocarbon production at the late stage. Property of two kinds of coke were greatly related with pyrolysis temperature, especially coke at high temperature (600℃) showed obvious characteristics of polyaromatic hydrocarbons (PAH).Active coke provides good catalysis environment for the production of hydrocarbons in thae catalytic conversion of pyrolysis components. Combining off-line extraction with in-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), the effect of functional group and catalysts structure on coke formation mechanism and catalysis function were studied. The results showed that the length of induction period was greatly related with reactant, catalyst structure and contact time. Active coke were mainly polymethyl benzaldehyde and precursor of inert coke, namely alkyl naphthalene, was formed simultaneously. Finally, alkyl naphthalene polymerized to form coke of large molecules-PAH. The initial coke species were straight chain alkanes in the catalytic conversion of THF. It performed very differently in the catalytic conversion over kinds of catalysts. Polymethyl benaldehyde and polymethyl phenol were important composition of active coke over MCM-22 in the initial stage of catalysis. The above results provide good theoretical support for design of catalyst structure and process. On one hand, methyl benzaldehyde can be fed in in advance. On the other hand, pore structure with "large cages and small windows" like MCM-22 can be introduced into original ZSM-5 to improve probability of methyl benzaldehyde formation.Kinetic models of catalytic conversion of biomass considering coke deposition were built. One is a typical model relating coke formation and operation parameters on an improved thermogravimetric analyzer, and one is the quantitative description of the relationship between coke content and product distribution. The results indicated that coke content increased with the increase of pyrolysis temperature, WHSV and partial pressure. The above data helped building empirical model relating coke content with operating parameters. In the viewpoint of a catalyst particle, active coke accumulated inside pores and played a leading role in the early stage, and active coke was converted into inert coke partially. Large inert molecules deposited on the external surface layer by layer and inert coke accumulation led to catalyst deactivation gradually. Pyrolysis temperature played an important role on product distribution, secondly, it was change of pore shape and acid property by catalyst coking. Olefins, aromatic hydrocarbons and CO+CO2 can be considered as stable secondary products, coke was stable final product and benzofuran behaved as typical primary product. A kinetic model relating coke content and product distribution was built on the basis of product classification and pathway analysis, which provided a good theoretical basis for catalysis time.Catalyst regeneration was optimized from aspects of pore modification and reaction process. In terms of maximized and stable hydrocarbon yield, proper mesopores were postly introduced into the original ZSM-5 catalyst and several methods were then combined to optimize regeneration conditions. The results showed that appropriate alkali concentration (0.3 M) led to sheet-like mesopores on the external sphere and promoted mass transfer and anti-coking. Carbon yield of hydrocarbons was boosted by 21.6%. Homogeneous coke distribution caused fast diffusion of heat out of catalyst pores. After catalysis,15% oxygen/5% vapor was used to regenerate catalysts to lower catalyst temperature, and carbon yield of hydrocarbons was promoted by 28.6%. Controlled regeneration of deactivated catalysts can modify catalyst channel and pores, and cover some exterior acid sites. When coke content holded at 2.18%, carbon yield of hydrocarbon was improved by 27.4% compared with fresh catalyst, and olefin production was also promoted.