A reduced toluene reference fuel chemical kinetic mechanism for combustion and polycyclic-aromatic hydrocarbon predictions

A reduced toluene reference fuel (TRF, n-heptane, iso-octane and toluene)-polycyclic-aromatic hydrocarbon (PAH) chemistry mechanism with 109 species and 543 reactions is proposed for combustion, PAH and soot formation predictions. The mechanism has been validated with shock tube ignition delays, laminar flame speeds, species profiles in premixed flames, and with homogeneous charge compression ignition (HCCI) and direct injection compression ignition (DICI) engine combustion data. Sensitivity and reaction pathway analyses were used to identify the important reactions and reaction pathways for PAH formation for both non-aromatic and aromatic fuels. The results show that the mechanism yields good agreement under all validation conditions. The PAH species concentrations are well predicted for eight flame configurations (C2H4, n-heptane, iso-octane, benzene (C6H6-A1) and toluene). For the non-aromatic fuels, reactions related to C2H4, C2H3 and C2H2 show the highest sensitivity for PAH formation, while with the aromatic fuel, PAH formation depends highly on reactions related to the ring structure. Benzene (A1) is mainly produced through the combination of C4 + C2 and two C3 radicals for non-aromatic fuels, and for aromatic fuels A1 mostly comes from the fuel molecule (toluene). The formation pathways of higher PAH species are quite similar for both non-aromatic and aromatic fuels, including C5H5 + C5H5 = A2 + 2H, the hydrogen abstraction acetylene addition (HACA) scheme, addition reactions between radicals and molecules and reactions between aromatic radicals and molecules. The mechanism is also capable of capturing combustion and emissions in HCCI and DICI engine simulations. The toluene content greatly enhances the PAH and soot formation with TRF fuels. However, improving mixing with TRF fuels due to their longer ignition delay also favors soot oxidation, and the final soot emissions of TRF fuels can be lower than diesel. The good agreement in the predictions of PAH species and soot emissions with the various fuels suggests that the current mechanism can be used reliably for combustion and PAH predictions for surrogate fuels of practical interest.