NO_x Formation in Counter-Flow Opposed-Jet Diffusion CH₄/Air Flames

Abstract

The present study focuses on the effect of pressure and strain rate on the formation of NO_x and NO to NO₂ conversion processes. The theoretical investigation was carried out using a stretched counter-flow opposed-jet diffusion flame code. The CHEMKIN thermodynamic and transport database were used in the model and a detailed reaction mechanism from GRI was employed. The fuel used was CH₄ with an initial inlet temperature of 298 K reacting with an opposed air flow with an initial temperature of 800 K. Axial profiles of temperature, velocity, species concentration and rate of formation of NO and NO₂ were predicted.

Theoretical results indicate that as the strain rate is increased and the pressure decreased, the maximum flame temperature decreased, leading to lower NO_x emission. The dominant pathway towards NO formation was found to be the prompt mechanism, especially at low pressure (1-5 atm) and high strain rate (400-1000 s⁻¹) where its contribution was over 90%.

NO₂ formed in very lean zones but also exhibited lower peaks in rich zones, indicating the importance of HO₂ radicals. The NO to NO₂ conversion process was found to be enhanced at high pressure and high strain rate with a maximum ratio of 11.5% at 30 atm and 1000 s^−l.

Keywords:

• NO_x emission
• CHEMKIN
• GRI
• air flames

Additional information

Notes on contributors

G. OSKAM

Now at Solar Gas Turbines Inc., San Diego, California, USA.