The Study On The Evolution Mechanism Of Polycyclic Aromatic Hydrocarbons (PAHs) In Flames

Polycyclic aromatic hydrocarbons?PAHs?produced in incomplete combustion of hydrocarbon fuels are highly carcinogenic and mutagenic,and are considered as the precursor of soot.Understanding the mechanism of PAHs evolution in flame is beneficial to reduce the generation of soot particles from the combustion,and alleviate the severe fog and haze.Based on the quantum chemistry theory and reaction rate kinetics theory,the formation and oxidation process of PAHs in combustion process was studied from the molecular level.The effect of main combustion parameters on PAHs formation was investigated by laser induced fluorescence?LIF?technique.The main contents are as follows.?1?The addition reaction between CH₂-C₂H₂ and PAHs were studied based on density functional theory?DFT?.The results showed that CH₂ can be directly involved in PAHs growth by the following five reaction steps:1)the PAHs with vinyl group are formed by the reaction between PAH radical and C₂H₂,2)the internal H-transfer reaction,3)the adsorption of CH₂ to active C atom of PAH,4)the formation of C-C bond between CH₂ and C₂H₂ in PAHs,and 5)formation of stable local minimum structures by dehydrogenation.?2?The potential energy surface and kinetic results of the reaction of C₄H₄+C₄H₄?+H?showed that C₄H₄+C₄H₄?+H?reaction can give birth to styrene and phenylacetylene,which are important in the growth of PAHs.In the formation of styrene,C-C bond formation and fracture are the main themes.The H-transfer and the H-elimination reactions are the main features in the formation of phenylacetylene.?3?The chemisorption mechanism of CO₂ on pentacene and subsequent CO dissociation mechanism were studied.Carbonyl oxide,heterocyclic oxide and lactone oxide can be formed in the reaction of CO₂ and pentacene,their formation are all exothermic process.The order of released energy is carbonyl oxide<heterocyclic oxide<lactone oxide.Among CO dissociation elementary reactions,the rate coefficient of the rate-limiting reaction step is as high as 4.1×10⁶ s-1 at 1200 K,which means that the dissociation reaction rate of CO is fast at combustion temperature.?4?The thermal decomposition mechanism of 1,2-benzanthracene oxygen radical I-IV and 3H-cydopenta[a]anthracene oxygen radical were studied.The results showed that the decomposition rate coefficients are sensitive to temperature,pressure and surface type.A new PAHs evolution pathway is proposed:6-membered ring??? 5-membered ring??? 4-membered ring??? PAHs with vinyl ??? 6-membered ring or 5-membered ring.?5?The electron emission properties of 13 PAHs were investigated based on DFT,time-dependent density functional theory and PAHs chemical kinetics simulation.The results showed that the maximum emission wavelength of PAHs is sensitive to the presence of 5-membered ring structures.The maximum emission wavelength of PAHs with 5-membered ring locates in the visible region,and the number of C atoms in the structure has no effect on the emission wavelength.The fluorescence signal wavelength emitted by the PAHs without 5-membered ring is less than 450 nm,and sensitive to the structure.Further,its fluorescence wavelength increases with the number of C atoms.?6?The effect of temperature,equivalence ratio and CO₂ concentration on PAHs formation was studied based on LIF technique.The results showed that the PAHs are sensitive to the flame temperature.The concentrations of PAHs will reach the maximum at about 1730 K and decrease at the lower or higher temperature,resulting from the rate competition bewteen the PAHs oxidation reaction and the PAHs growth reaction.The flame with higher equivalence ratio contains more C₂H₂,promoting the formation of PAHs.While the chemical effect of CO₂ can inhibit the formation of C₂H₂ and C₃H₃,resulting in the reduction of PAHs production.?7?The LLM mechanism is developed on the basis of the ABF,toluene and methylnaphthalene reaction mechanism.The reactions of PAHs and CH_X,C₂H_X,C₃H₃,C₄H₄,C₅H₅,C₉H₈,c-C₆H₆O and A₁C₂HC₂H₂ were added into LLM mechanism based on the our previous study and literatures.The results showed that deviation between the predicted concentration of gas-phase small molecule species with LLM mechanism and corresponding experimental data is within 2 times,especially the concentration of CH₄ and C₂H₂,which plays a key role in the growth of PAHs.The deviation is within one magnitude when comes to PAHs concentration.It also showed excellent adaptability to different flame temperature and different CO₂ content.