An experimental and chemical kinetic modelling study of toluene oxidation with nitrous oxide

Abstract

The oxidation of toluene in the presence of nitrous oxide (N₂O) is investigated experimentally using shock tubes, and the results are simulated using an improved chemical kinetic model. The improved model is based on GalwayMech1.0 with updated rate constants for the reactions Ċ₆H₅ + $\dot{H}$ (+M) ↔ C₆H₆ (+M), N₂O (+M) ↔ N₂ + Ö (+M), N₂O + $\dot{H}$ ↔ N₂ + $\dot{O}$H, N₂O + Ö ↔ NO + NO, and N₂O + Ö ↔ N₂ + O₂. Additionally, the current model includes HNNO and NHNO intermediate chemistry. The proposed mechanism is validated over a wide range of temperatures and equivalence ratios, with validation targets including experimental data for toluene, H₂/N₂O, and newly generated toluene/N₂O blend data from shock tubes. High-pressure shock tube experiments reveal that the toluene/N₂O mixture is highly susceptible to pre-ignition at low temperatures. The chemical kinetic analysis indicates that the ignition of the toluene/N₂O mixtures is highly sensitive to the N₂O (+M) ↔ N₂ + Ö (+M) reaction. The heat released, along with the Ö atoms generated during the decomposition of N₂O, causes the rapid depletion of toluene at a substantially faster rate than N₂O. Similarly, $\dot{H}$ atoms, mostly produced from toluene chemistry, e.g., through benzyl radical breakup C₆H₅ĊH₂ ↔ Ċ₇H₆ + $\dot{H}$, help the decomposition of N₂O molecules via N₂O + $\dot{H}$ ↔ NO + $\ddot{N}$H. Moreover, other major nitric oxide (NO) producing reactions are identified, including N₂O + Ö ↔ NO + NO and $\ddot{N}$H + Ö ↔ NO + $\dot{H}$. Due to the rapid depletion of toluene, direct chemical interactions between N₂O and the aromatic ring have little influence on overall combustion chemistry. However, the enthalpy of formation of toluene and benzyl radical do influence N₂O decomposition significantly.