Polycyclic Aromatic Hydrocarbons Evolution and Interactions with Soot Particles during Fuel Surrogate Combustion: A Rate Rule-Based Kinetic Model

Modeling combustion of transportation fuels remains a difficult task due to the extremely large number of species constituting commercial gasoline and diesel. However, for this purpose, multi-component surrogate fuel models with a reduced number of key species and dedicated reaction subsets can be used to reproduce the physical and chemical traits of diesel and gasoline, also allowing to perform CFD calculations. Recently, a detailed surrogate fuel kinetic model, named C3 mechanism, was developed by merging high-fidelity sub-mechanisms from different research groups, i.e. C0-C4 chemistry (NUI Galway), linear C6-C7 and iso-octane chemistry (Lawrence Livermore National Laboratory), and monocyclic aromatic hydrocarbons (MAHs) and polycyclic aromatic hydrocarbons (PAHs) (ITV-RWTH Aachen and CRECK modelling Lab-Politecnico di Milano). In this work, the aromatic module of the combined model for PAHs chemistry is discussed, which was revised and updated to improve predictive capabilities in describing the combustion of gasoline TPRF surrogates in ideal reactors and laminar premixed flames, specifically focusing on PAHs soot precursors. Specifically, a rate rules- and reaction classes-based approach is used to tailor the kinetics of PAHs from that of MAHs, whose kinetics can be more easily obtained from ab-initio quantum chemistry methods and/or dedicated kinetic experiments and for which specific experimental data in controlled combustion environments (laminar flames, ideal reactors) are more generally available. The C3 mechanism satisfactorily predicts recently measured PAH formation from TPRF pyrolysis in jet-stirred and flow reactors. Also, it is shown that model performances can be strongly influenced by the inclusion of soot chemistry on top of the gas-phase mechanism, despite the lack of experimental data comprising both PAHs and soot yields limits an extensive validation of the surrogate fuel mechanism here proposed. For this purpose, the sectional soot model from CRECK was coupled to the C3 mechanism. Finally, a numerical analysis on the sooting tendency of gasoline surrogate combustion under high-temperature conditions of interest for engine operations is carried out. Specifically, laminar premixed TPRF flame simulations show that soot formation and particle dehydrogenation increase with toluene content in the fuel mixtures, independently of the related research octane number (RON).