Quantitative HCN measurements in CH4/N2O/O2/N2 flames using mid-infrared polarization spectroscopy

Quantitative measurements of hydrogen cyanide (HCN) were nonintrusively performed using mid-infrared polarization spectroscopy (IRPS) in atmospheric pressure flames. The lifted flat, laminar, premixed CH4/N2O/O2/N2 flames were stabilized on a 7 cm diameter home-built McKenna-type burner with variable proportion of N2O and O2. The characteristic spectral structure of HCN molecules was identified in the rotational line-resolved IRPS spectra collected in flames at around 3248 cm−1. The P20 line belonging to the CH stretching band was chosen for quantitative measurements and the line-integrated IRPS signal was recorded in a series of fuel-rich CH4/O2/N2O/N2 flames with equivalence ratios of 1.2, 1.4 and 1.6. Absolute mole fractions of HCN molecules in these flames were obtained through in situ calibration of the optical system with nonreactive gas flow of N2 seeded with known amount of HCN on the same burner. Moreover, the experimental results were compared with calculations performed using the Konnov detailed C/H/N/O mechanism, which implements NCN prompt-NO reactions. Generally good agreement was found with some discrepancies indicating the need for further model improvement.