NO Production and Destruction in a Methane/Air Diffusion Flame

Abstract

Concentration profiles have been measured for naturally occurring NO in a laminar CH₄/air diffusion flame burning on a rectilinear Wolfhard-Parker slot burner at atmospheric pressure. Linear laser-induced fluorescence of the A ²σ⁺-X ²π_i (0,0) transition was excited using a frequency doubled tunable dye laser and detected with a dielectric filter/photomultiplier tube combination. The observed fluorescence signals have been corrected for (1) the Boltzmann population in the R₁,(17) rotational level of the ground vibronic state and (2) collisional quenching rates as a function of the local temperature and collider concentrations. The resulting relative concentration profiles have been calibrated using tunable diode laser absorption measurements of Hill and Miller [1994]. Both the overall NO production/destruction rates and the contributions from individual elementary steps have been derived; the latter analysis utilizes previously measured profiles of H, O, OH, CH, and CH₃ as well as an estimated ³CH₂ profile.

The NO profile measurements alone do not distinguish its dominant production pathway in this co-flowing CH₄/air flame, since the contribution of prompt NO production is obscured by compeling CH_i + NO destruction reactions. As a consequence of these reburn reactions, the observed peak NO concentrations are observed to closely track the maximum temperatures. A reaction path analysis and determination of NO fluxes strongly indicate that prompt NO production outweighs the thermal route, but uncertainties in determining the relative contributions to instantaneous NO production are large.

Keywords:

• Prompt NO
• thermal NO
• methane combustion
• reburn chemistry
• laminar diffusion flames
• laser-induced fluorescence
• rate analysis