Influence of C 2 and C 3 compounds of natural gas on NO formation: an experimental study based on LIF/CRDS coupling

Abstract

This study reports CH and NO mole fraction profiles measured in 12 premixed flames representative of natural gas combustion. Four mixtures CH 4/O 2/N 2, CH 4/C 2H 6/O 2/N 2, CH 4/C 3H 8/O 2/N 2, and CH 4/C 2H 6/C 3H 8/O 2/N 2 have been investigated at three equivalence ratios (0.7, 1, and 1.25). The aim of this work was to study the influence of C 2 and C 3 compounds of natural gas on NO formation in flames. Flames were stabilised on a flat flame burner at low pressure (33 torr). The experimental approach was exclusively based on non-intrusive techniques: laser induced fluorescence (LIF) for OH-LIF thermometry and species concentration profiles determination, and cavity ring-down spectroscopy (CRDS) for absolute calibration. Thus, an extended database including a large dynamic of combined CH and NO mole fraction profiles (ranging, respectively, from 0.7 to 10 ppm and from 2.6 to 24 ppm) has been obtained and will serve for further improvement of chemical mechanisms of NO formation in natural gas flames. It is shown that, for a given equivalence ratio, NO quantity remains unchanged when substituting methane by ethane and/or propane, while CH mole fraction exhibits a slight decrease. Predominance of prompt-NO mechanism is shown to be sensitive to the equivalence ratio. These new results have been modelled by using the GRI3.0 mechanism, and a reaction path analysis has been performed in the stoichiometric methane flame to highlight the different routes of NO formation in the different zones of the flame, i.e., the preheat zone, the reaction zone, and the burned gases.