Nitrogen oxide formation in flames: The roles of NO 2 and fuel nitrogen

Abstract

Flat methane flames were probed in the presence and absence of nitrogen-containing compounds (referred to as fuel-N). Methylamine, pyridine and piperidine at about 120 ppm were added to the flames. The data, based on detailed NO and NO2 profiles, for flames with and without the fuel-N additives, indicate a sequence of reactions consistent with the folowing mechanism, NH and/or CH+O2=NO+OH and/or CO (1) NO+HO2=NO2+OH (2) NO2+O=NO+O2 (3) Spectroscopic data indicate that NH and CN are present in the visible flame. The NO produced from the N-containing radicals is rapidly consumed in the visible flame region by HO2 radicals, producing NO2 in accordance with step 2 of the mechanism. The NO−HO2 kinetics appear to be sufficiently rapid since NO was detected in the visible flame region only when fuel-N was added to the flames, i.e., only after saturation of Reaction 2. This is further supported by the fact that NO added to methane flames is also rapidly removed in the preflame region. The NO2 produced in the flame was subsequently converted to NO to varying degrees in a narrow reaction zone in the near postflame region where the O-atom concentration was rapidly increasing to its maximum level [Reaction (3)]. The extent to which NO2 was consumed depended on the oxygen content of the flame—complete consumption of NO2 occurring only in the fuel-rich flames. Profiles of the fuel-N compounds obtained from the probings indicate that methylamine produces more NO2 and NO in the combustion process than pyridine or piperidine. Piperidine however appeared least stable in terms of NO and NO2 produced via the preflame reactions. The relative stability of the three fuel-N compounds in the flames appeared to be pyridine, the most stable, followed by methylamine and piperidine. The fuel-N materials produce a thermally stable, as yet unidentified, intermediate during oxidation, which reacts readily with the O-atoms in the flame.