NO_x-Hydrocarbon Kinetics Model Validation Using Measurements of H₂O in Shock-Heated CH₄/C₂H₆ Mixtures With NO₂ as Oxidant

One method frequently used to reduce NO_x emissions is exhaust gas recirculation, where a portion of the exhaust gases, including NO_x, is reintroduced into the combustion chamber. While a significant amount of research has been performed to understand the important fuel/NO_x chemistry, more work is still necessary to improve the current understanding on this chemistry and to refine detailed kinetics models. To validate models beyond global kinetics data, such as ignition delay time or flame speed, the formation of H₂O was recorded using a laser absorption diagnostic during the oxidation of a mixture representing a simplistic natural gas (90% CH₄/10% C₂H₆ (mol)). This mixture was studied at a fuel lean condition (equivalence ratio = 0.5) and at atmospheric pressure. Unlike in conventional fuel-air experiments, NO₂ was used as the oxidant to better elucidate the important, fundamental chemical kinetics by exaggerating the interaction between NO_x and hydrocarbon-based species. Results showed a peculiar water formation profile, compared to a former study performed in similar conditions with O₂ as oxidant. In the presence of NO₂, the formation of water occurs almost immediately before it reaches more or less rapidly (depending on the temperature) a plateau. Modern, detailed kinetics models predict the data with fair to good accuracy overall, while the GRI 3.0 mechanism is proven inadequate for reproducing CH₄/C₂H₆ and NO₂ interactions.