Flame-ion probe of intermediates leading to nitrogen oxide formation in methane combustio

A detailed investigation of the nitrogenous ions naturally occurring in both fuel-lean and fuel-rich CH$sb4$-O$sb2$-N$sb2$ flames was conducted by sampling the ions directly into a mass spectrometer. The flames were diluted interchangeably with ca. 10% N$sb2$ and Ar at constant temperature to determine the extent of nitrogen involvement. This diluent switching technique provided ion concentration profiles along the flame axis for CN$sp-$, NCO$sp-$, H$rmsb2CNsp+$, NH$sb2sp+$, NH$sb3sp+$, NH$sb4sp+$, NO$sp+$ and NO$sb2sp-$. The spatial resolution was sufficient to show distinct regions of the flames where a given ion is involved. The ions can be linked to their corresponding nitrogenous neutrals through chemical ionization (CI) processes, many of which are known from room temperature studies. A detailed discussion of the relevant CI processes is presented, providing strong evidence for the existence and sequential behaviour of the neutrals CN/HCN, NCO/HNCO, N, NH, NH$sb2$, NH$sb3$, NO and NO$sb2$. The fuel-lean results are consistent with the (extended) Zeldovich mechanism for NO$rmsb x$ formation. The fuel-rich data clearly show the involvement, both in the reaction zone and further downstream, of the variety of neutral intermediates invoked by Fenimore and others to explain 'prompt NO' formation. The interpretation of the fuel-rich data was supported by additive studies of fuel-rich CH$sb4$-O$sb2$-Ar flames doped separately with ca. 0.3% of HCN, NH$sb3$ and NO. Axial concentration profiles of the nitrogenous ions for these additive flames were found to maximize in the same sequence as those measured for he corresponding N$sb2$-diluted flame. These data suggest that the processes active in NO$rmsb x$ formation are the same, regardless of the form of the added nitrogen.