Investigation of NO₂ Formation Kinetics in Dual-Fuel Engines With Lean Premixed Methane–Air Charge

A dual fuel engine concept with lean premixed methane–air charge ignited by a diesel pilot flame is highly promising for reducing NO_x and soot emissions. One drawback of this combustion method, however, is the high nitric dioxide (NO₂) emissions observed at certain operating points. The conditions leading to increased NO₂ formation have been investigated using a batch reactor model in cantera. It has been found that the high emission levels of NO₂ can be traced back to the mixing of small amounts of quenched CH₄ with NO from the hot combustion zones followed by postoxidation in the presence of O₂, requiring that the temperatures are within a certain range. NO₂ formation in the exhaust duct of a test engine has been modeled and compared to the experimental results. The well-stirred reactor model has been used that calculates the steady-state of a uniform composition for a certain residence time. An appropriate reaction mechanism that considers the effect of NO/NO₂ on methane oxidation at low temperature levels has been used. The numerical results of NO–NO₂ conversion in the duct at low temperature levels show good agreement with the experimental results. The partial oxidation of CH₄ can be predicted well. It can be shown that methane oxidation in the presence of NO/NO₂ at low temperature levels is enhanced via the reaction steps $CH3+NO2⇌CH3O+NO$ and $CH3O2+NO⇌CH3O+NO2$⁠. In addition, the elementary reaction $HO2+NO⇌NO2+OH$ is the important NO oxidizing step.