A Theoretical Multiscale Approach to Study the Initial Steps Involved in the Chemical Reactivity of Soot Precursors

In the present study, bond formation reactions between soot precursors and their role in the soot inception process are investigated. The soot precursors were generated in macroscopic detailed gas-phase kinetic calculations and according to certain criteria introduced in simulation boxes to model bond formation between soot precursor molecules with reactive force field molecular dynamics modeling. The impacts of temperature, fuel mixture, and equivalence ratio have been investigated on the rate and structure of the newly formed molecules. The resulting structures compare well to previously reported experimental results. Furthermore, the bond formation rate between PAHs is found to be linearly correlated with the temperature at which the PAH precursors are generated, while fuel and equivalence ratio do not have a direct impact on the reaction rate. The generated growth structures are lumped in (1) directly linked, (2) aliphatically linked, and (3) pericondensed polycyclic hydrocarbons. It is found that the amount of aliphatically linked PAH increases with the amount of aliphatic content of the fuel mixture. Finally, a reaction scheme is presented displaying the most representative reaction pathways to form growth structures in each lumping class and their eventual interconversion. The present work-that applies a combined approach of macroscopic gas-phase kinetic calculations and atomistic reactive force field simulations-offers a good alternative to obtain structural differences of nascent soot for a broad range of thermodynamic conditions and detailed reaction mechanisms for the soot inception process.