NOX MECHANISMS IN RICH METHANE-AIR FLAMES

Local production rates of NO have been derived from experiments in ver tical names on a burner commonly used in central heating equipment. Th e fuel-air equivalence ratio of around 1.9 caused a double-flame struc ture to develop above the burner, a phenomenon reflected by the temper ature and OH concentration fields. The production rare of NO was a max imum in regions of low temperature and low OH concentration, located i n the inner premixed name and prior to the higher temperatures and OH concentrations of the outer diffusion name. In addition, the peak rate of disappearance of NO was at the core of the premixed flame. These e xperimental results supported the assumption that the main mechanism o f NO formation was the Fenimore Prompt route; also, an important inter nal ''reburn'' mechanism was operating. The opposed now flame code was used here in the largest part of the simulations with the GRI 2.11 me chanism and with two additional mechanisms for a particular condition of flow and fuel-air equivalence ratio. The code was applied for fuel streams containing methane in air and oxidizer streams containing air only; the aim was to draw analogies with the experimental two-dimensio nal flame. Results from local rates of production and sensitivity anal yses and quantitative reaction path diagrams (QRPD) for the nitrogenou s species are presented for conditions representative of domestic appl iances (cookers, space and water heaters, and central heating units) f ired with natural gas. The co-flow experimental flame and the counter- flow theoretical flame at similar strain rates and fuel richness showe d significant similarities. (C) 1998 by The Combustion Institute.