CCl, CH, and NO LIF measurements in methane-air flames seeded with chlorinated species: Influence of CH3Cl and CH2Cl2 on CCl and NO formation

In this work, the laser-induced fluorescence (LIF) technique is used to detect minor species (CCl, NO, and CH) in premixed stoichiometric methane-air flames seeded with monochloromethane or dichloromethane. Quenching data are extracted from time-resolved fluorescence lifetime measurements for all the excited species. First quenching measurements of CCl under flame conditions are reported. It is shown that LIF measurements are strongly perturbed by the presence of background emissions issued from the radiative relaxation of photolytic fragments (HCl^*, CCl^*, CH^*, and C₂^*) formed upon laser excitation. The parent molecules that are partly responsible for these emissions are C₂H₃Cl (for HCl^*, CH^*) and CHCl₂ (for CCl^*).

Profiles of both photolytic fragments and species directly measured by LIF are used to study the influence of CH₃Cl and CH₂Cl₂ addition on CCl and NO formation in methane-air flames. CCl radical is found to be formed in the reaction zone of the flames. The reaction path leading to CCl appears to be dependent on the nature of the chlorinated hydrocarbon (CHC) seeded in the flame. The suggested reaction paths may preferentially involve the contribution of CHCl₂ in case of CH₂Cl₂ degradation and CH₂Cl in case of CH₃Cl degradation. An important increase of NO in presence of CHC is pointed out for the first time. The NO formation in flames containing CHC appears to occur in the reaction zone of the flames, and [NO] is found to be constant in the burned gases: This suggests a predominance of the prompt-NO mechanism in this kind of flame as confirmed experimentally by the observed [CH] increase. Reaction paths involving the degradation of CHCs, particularly CHCl₂, should largely contribute to the formation of CH in flames seeded with CHCs.