Quantitative LIF measurements and kinetics assessment of NO formation in H2/CO syngas–air counterflow diffusion flames

Abstract

Quantitative non-intrusive measurements of NO species using the Laser Induced Fluorescence technique are reported in laminar counterflow syngas–air diffusion flames for a wide range of strain rates and as a function of fuel composition (i.e. H2/CO ratio). The local strain rate of the flames was varied from 35s−1 to 750s−1 for three compositions of syngas (1:4, 1:1 and 4:1 H2/CO ratio). Numerical simulations corresponding to the experimental conditions were conducted using OPPDIF with six H2/CO chemical kinetic mechanisms from the literature. The peak [NO] predictions by mechanism M3 (GRIMech. 3.0) were found to be closest to measurements at low strain rates, while predictions using M1 (partial C1 mechanism by Li et al.) and M4 (full C1 mechanism by Li et al.) mechanisms improved at higher strain rates. A Quantitative Reaction Path Diagram (QRPD) analysis showed that mechanism M3 (GRIMech 3.0) predicted a shift from the NNH (dinitrogen monohydride) and prompt NO pathways towards NNH pathway contributing to NO formation with increasing strain rates. Mechanism M6 (Konnov 0.6) predicted significant prompt-NO contributions via an alternate CNN pathway in addition to the NNH pathway at low as well as high strain rates. Unlike M3 and M6, the M1 mechanism consistently predicted the NNH pathway to be the sole major route for NO formation across all strain rates. The N2O intermediate pathway was found to be insignificant for most of the flames. The sensitivity analysis concerning the effect of CH4 on NO formation routes indicated the need to include CH chemistry in the mechanisms, so as to model the NO formation accurately for H2/CO mixtures containing even trace amounts of hydrocarbons. Lastly, the NO emission indices (EINO) are reported as a function of strain rates for the three compositions of syngas showing 32%H2:8%CO case resulting in the lowest NO emissions and fuel consumption per unit power output.