Investigation Of The Coke Formation Characteristics Of Bio-oil During Pyrolysis Treatment Based On The Radical Reaction Behavior

Bio-oil belongs to a kind of liquid fuel with great potential to replace fossil fuels such as petroleum.Heat treatment process is the most commonly used in the conversion and utilization of bio-oil.However,bio-oil is easy to form coke during heat treatment process,which is the main obstacle to its large-scale utilization.Therefore,elucidating the coking reaction mechanism of bio-oil was a prerequisite for formulating coking control strategies and suppressing the negative effects caused by coking.This thesis paper aims to analyze the bio-oil coking formation process under different temperatures(200-500℃)and reaction times(0-60 min).The evolution characteristics of the main components and the reaction characteristics of radicals in this coking formation process of bio-oil was studied.Meanwhile,the kinetics model of the coking formation process of bio-oil was established in this work.It could be clarified to understand the coking mechanism of bio-oil based on the analysis of radical reaction behavior.The results indicate that the decomposition of oil and the formation of coke occur simultaneously in the pyrolysis process of bio-oil,both of them were affected by radical reaction.The pyrolysis reaction of bio-oil was dominated by the decomposition of oil and the generation of coke when it was in the initial stage.This process mainly occurs the polymerization reaction of single aromatic ring substances to generate polycyclic aromatic substances,and produces a small number of stable radicals.And then the polycyclic substances further condensed to produce larger ring substances with the increase of temperature,which increased the concentration of large aromatic ring structure in the generated coke.On the other hand,the decomposition of polycyclic substances was happened when the temperature went up,which caused the formation of small aromatic rings substances and lighter components.And the temperature increase will also promote the ring condensation reaction of non-aromatic components to produce aromatic ring substances.The radicals always exist in the process of bio-oil coking formation.And stable radicals mainly exist in solid products and its concentration was correlated with temperature and reaction time.There was a kind of strong connection between the radicals and the components of pyrolysis products,but there was no linear relationship between the concentration of stable radicals and the coke yield.Hydrogen donor DHP was used to capture radicals in the pyrolysis process of biooil,and the active radicals in the reaction process were quantified by the consumption amount of DHP.It was found that the content of stable radicals accounted for 0.25%-1.5% of the total radicals during this pyrolysis process of bio-oil.There was no coking phenomenon occurred in the pyrolysis process of bio-oil when the content of DHP hydrogen donor in bio-oil was sufficient(>30 wt.%),which effectively inhibited the coking process.Additionally,through analyzing EPR spectrum,it was found that the increase of temperature would reduce the content of some oxygen-containing radicals and heteroatoms.However,it was more conducive to the formation of oxygencontaining radicals and aromatics radicals with the increase of temperature and the extension of reaction time.It was found that reaction kinetics combined with autocatalytic kinetics could describe the coking process of bio-oil when we consider using kinetics model to analyze this process.The activation energy of bio-oil decomposition and recondensation was329kJ/mol,and for coke autocatalytic coking was-159kJ/mol.It could be indicated that the coking process was dominated by the decomposition and recondensation reactions of bio-oil.The generation of radicals during pyrolysis could be described by two first order kinetics: oil cracking reaction and coke cracking reaction.Their activation energies were 481kJ/mol and 57kJ/mol,respectively.It could be seen from the above that the categories of coking kinetics and radical generation kinetics in the pyrolysis process of bio-oil were significant difference.Therefore,it would be not able to quantitatively analyze the coke yield through the change of radical concentration.