Experimental and numerical study of the role of NCN in prompt-NO formation in low-pressure CH4–O2–N2 and C2H2–O2–N2 flames

We report an experimental and modeling study on prompt-NO formation in low-pressure (5.3 kPa) premixed flames. Special emphasis is given to the quantitative detection (and prediction) of NCN, whose role in prompt-NO formation has recently been confirmed in alkane flames. Here a rich (Φ = 1.25) CH4–O2–N2 flame and rich (Φ = 1.25) and stoichiometric C2H2–O2–N2 flames have been investigated. Absolute concentration profiles of CH and NCN radicals and NO species are obtained by combining laser-induced fluorescence (LIF) and cavity ring-down spectroscopy (CRDS). Temperature profile is determined in each flame using OH and NO–LIF thermometry. Flame modeling is performed to determine the role of NCN in prompt-NO formation and to test the capacity of the present chemical mechanisms to predict some intermediate species involved in prompt-NO formation. The methane flame is modeled using GDFkin®3.0_NCN mechanism [El Bakali et al., Fuel 85 (2006), 896–909]. The acetylene flames are modeled using the Lindstedt and Skevis C/H/O mechanism [Lindstedt and Skevis, Proc. Combust. Inst. 28 (2000), 1801–1807], completed by the submechanism issued from GDFkin®3.0_NCN for nitrogen chemistry. This submechanism includes the initiation reaction CH + N2 = NCN + H. Rate constants of NO-sensitive reactions of the submechanism are modified by taking into account the recent literature. In particular, the C2O route could be explored thanks to a significant presence of C2O in acetylene flames. Globally, the modified submechanism of nitrogen chemistry coupled with the two hydrocarbon mechanisms leads to a satisfying prediction of NCN and NO mole fraction profiles, even though refinements of rate constant determination is still required. The role of NCN in prompt-NO formation in acetylene flames is demonstrated.