Investigation On The Formation Mechanism Of Polycyclic Aromatic Hydrocarbons(PAHs)in The Process Of Soot Nucleation

Fossil fuels will still play important roles in the energy demand of China in future decades.The emission of soot and PAHs from the incomplete combustion of fuel is extremely harmful to the environment and human.Due to PAHs are precursors of soot formation,the accurate prediction of PAHs is better for the simulation of soot formation in flame.At present,the H-abstraction-C₂H₂-addition?HACA?mechanism is the most popular PAH formation mechanism in combustion simulation models,and other PAHs formation pathways initiated by the addition of C₁,C₃ or C₄ were added into the chemical reaction model for specific demand from modeling.However,these chemical reaction models could not always predicting the formation of PAHs completely.Besides,most of the PAHs produced from the present mechanism are A₂,A₃,A₄ or other Pericondensed-PAHs?PC-PAHs?.Few mechanisms lead to PAHs within 4-membered,5-membered,7-membered or aliphatic chain,and these PAHs are also harmful to human.Hence,with G3?MP2,CC?method,the main objective of this work is to explore the formation pathways of PC-PAHs?A₂,A₃ and A₄?and CP-PAHs?PAHs within 4-membered,5-membered,7-membered or aliphatic chain?from the addition of non-acetylene type organic compounds,such as C₂H₃,C₄H₄,C₅H₄ and C₆H₅.Among these,multiplicity from electron configuration,ground states and excited states in flame,and the influence of tunnel effect were considered.And the rate constants of reantions in above pathways were calculated with transition state theory and Rice-Ramsperger-Kassel-Marcus?RRKM?theory from the results of quantum chemistry calculation.The influences of temperature,pressure,reaction types,reactive sites and H atoms from flame to rate constants were explored.And then,these PC-PAHs formation pathways were added to improve a C₄ chemical reaction kinetics model with the modeling of butane or butadiene laminar premixed flame.The studies gave the following main results:?1?Some non-acetylene addition pathways were proposed in this study,which could lead to PC-PAHs,such as A₂,A₃ and A₄,and non-PC-PAHs within 4-membered,5-membered,7-membered or aliphatic chain involving 124 elementary reactions and 104 structures,which reriched our understanding of PAHs formation pathways from non-acetylene additions in flame.Compared with HACA pathways,the energies required to the formation of PC-PAHs were a little higher,but much more irreversible.The energies of pathways leading to non-PC-PAHs in this study were with little difference from those of HACA pathways,and non-PC-PAHs pathways were more irreversible.?2?Excited states,CH₃ and H atoms in flame were important for accelerating the formation of PAHs,and these mainly shown in: Excited states were easier to form PAHs than ground states;CH₃ could replace one H from C₄H₄,which could then decrease the energies required by ring enlargement,H-migration,and accelerate the formation of PAHs;H atoms reduced the energy requirements for the formation of radicals and ring enlargement reactions,which facilitates the formation of PAHs greatly.?3?Reaction types and reactive sites affect rate constants.C-addition,and ring formation reactions were the fastest,H-abstractions with H atoms the second,andcis-transisomerizations the slowest,andcis-transisomerizations are always the rate-limiting step of its pathway;reactive sites have little influence on rate constants of H-abstraction,ring formation,H-migration andcis-transisomerization reactions.However,in C-addition reactions,only those reactions with same additions and same reactive sites own close rate constants.?4?Tunnel effect,temperature and pressure effect rate constants.Tunnel effect has great impacts on rate constants of reactions with the participating or producing of H atoms.Rate constants of elementary reactions climbed with the increase of temperature and pressure,and were close to a value at certain temperature or pressure.?5?The addition of our calculated PAHs formation pathways improved the the formation of PAHs in butane or butadiene laminar premixed flame,such as A₂,A₃ and A₄,without influencing concentrations of reactants and products.From the modeling results,PAHs formation pathways initiated by C₂H₃ and C₄H₄ additions were good for PAHs formation in butadiene flame,and in butane flame,pathways initiated by C₄H₄ additions were better for PAHs formation than those initiated by C₂H₃ additions.