Measurements of hydrogen cyanide and its chemical production rate in a laminar methane/air, non-premixed flame using cw cavity ringdown spectroscopy

Cavity ringdown spectroscopy using an external cavity tunable diode laser is coupled with microprobe sampling to quantitatively determine hydrogen cyanide concentrations in a methane/air non-premixed flame. HCN concentration data were combined with concentration, velocity, and temperature data previously collected in this flame system to perform a net production rate analysis from a solution of the species conservation equation. This net rate profile exhibits two dominant features: a production feature in a flame region just rich of the stoichiometric surface and a destruction feature at the stoichiometric surface. A reaction path analysis was performed for a series of HCN elementary reactions. The net rate of HCN formation and destruction calculated from this chemical approach agreed well in peak locations with those calculated using the transport rates described above. Analysis of the individual reaction data suggests that destruction of HCN is dominated by oxygen-atom reactions producing several radical species that are likely to oxidize further to NO. Thus, the magnitude of this destruction feature may be an indirect measure of the local NO formation rate through the prompt mechanism.

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