Flame structure studies of rich ethylene-oxygen-argon mixtures doped with CO2, or with NH3, or with H2O

Structures of several premixed ethylene-oxygen-argon rich flat flames burning at 50 mbar have been established by using molecular beam mass spectrometry in order to,investigate the effect of CO2, or NH3, or H2O addition on species concentration profiles. The aim of this study is to examine the eventual changes of profiles of detected hydrocarbon intermediates which could be considered as soot precursors (C2H2, C4H2, C5H4, C5H6, C6H2, C6H4, C6H6, C7H8, C6H6O, C8H6, C8H8, C9H8 and C10H8). The comparative study has been achieved on four flames with an equivalence ratio (f) of 2.50: one without any additive (F2.50), one with 15% of CO2 replacing the same quantity of argon (F2.50C), one with 3.3% of NH3 in partial replacement of argon (F2.50N) and one with 13% of H2O in replacement of the same quantity of argon (F2.50H). The four flat flames have similar final flame temperatures (1800 K). CO2, or NH3, or H2O addition to the fresh gas inlet causes a shift downstream of the flame front and thus flame inhibition. Endothermic processes CO2 + H = CO + OH and H2O + H = H-2 + OH are responsible of the reduction of the hydrocarbon intermediates in the CO2 and H2O added flames through the supplementary formation of hydroxyl radicals. It has been demonstrated that such processes begin to play at the end of the flame front and becomes more efficient in the burnt gases region. The replacement of some Ar by NH3 is responsible only for a slight decrease of the maximum mole fraction of C2H2, but NH3 becomes much more efficient for C4H2 and C-5 to C-10 species. Moreover, the efficiency of NH3 as a reducing agent of C-5 to C-10 intermediates is larger than that of CO2 and H2O for equal quantities added. (c) 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.