Routes of formation and destruction of nitrogen oxides in CH4/H2 jet flames in a hot coflow

Abstract

The formation/destruction mechanisms of nitric oxides (NOx) in CH4/H2 jet flames in a hot coflow are systematically investigated by numerical modeling. All calculations use the Eddy Dissipation Concept (EDC) model coupled with GRI-Mech 2.11. The modeling is validated by the measurements of Dally et al. [Proc. Combust. Inst. 29 (2002) 1147–1154]. Dependences of the NOx formation/destruction routes on YO2∗, Tcof∗ and YH2∗ are examined, where YO2∗, Tcof∗ and YH2∗ are the coflow oxygen concentration and temperature, and the hydrogen fraction in the fuel mixture, respectively. The following is obtained.For YO2∗≤3% at Tcof∗=1300K and YH2∗=12%, the N2O-intermediate, prompt and NNH routes are the first (≈50%), second (30%) and third (20%) contributors to the total NO emission; as YO2∗ is increased, the contributions of the three routes all decrease whereas that of the thermal route becomes significant. For YO2∗=3% and YH2∗=12%, the N2O-intermediate route dominates the NO formation (about 90%) at Tcof∗<1000K; as Tcof∗ is increased from 1000 K, the importance of either the prompt, NNH, or thermal routes is enhanced. For YO2∗=3% and Tcof∗=1300K, as YH2∗ is elevated, the contribution of the NNH route to the total NO production increases while those of the N2O-intermediate and prompt routes decrease. Also, in general, the reduction of YO2∗, Tcof∗ or YH2∗, or altogether is found to strengthen the importance of NO2 to the total NOx emission and the NO-reburning.