Experimental study and modelling of NOx formation in high pressure counter-flow premixed CH4/air flames

• NO mole fraction profiles measured by LIF in high pressure lean CH4/air flames.
• Three kinetic mechanisms: GDFkin®3.0_NCN, GRImech2.11 and GRImech3.0 are compared.
• Kinetic analysis to better understand the differences between the three mechanisms.
• Inclusion of the new prompt-NO formation pathway in the GRImech3.0 mechanism.

Nitric oxide (NO) is an atmospheric pollutant responsible for the destruction of the ozone layer and the creation of photochemical smog. As a result, NOx emissions from combustion sources are regulated in most industrialised countries. The need to control NOx emissions while also promoting more efficient use of fossil energy resources requires a better understanding of combustion processes, especially the chemical kinetics of NOx formation. NO formation in high-pressure flames is a research area of great practical interest as high pressure exists in practically all power-generation and propulsion engines and it is known that pressure influences the combustion chemistry. In the present work, NO mole fraction profiles were measured by Laser Induced Fluorescence in laminar high pressure (up to 0.7 MPa) counter-flow lean CH4/air (E.R. = 0.7) flames. Inherent problems linked to the application of the NO LIF technique in high pressure environment were addressed. The experimental NO profiles were then compared with modelling using the OPPDIF code and the three detailed kinetic mechanisms: the GDFkin®3.0_NCN mechanism developed by Lamoureux et al. and the two mechanisms from the Gas Research Institute: GRImech 2.11 and GRImech 3.0. A kinetic analysis based on rate of production/consumption analyses was performed to better understand the differences between the three mechanisms. Finally, the GRImech3.0 mechanism was modified with three updated prompt-NO submechanisms proposed in the literature and the consequences on the N-containing species mole fractions predictions are discussed.