Two–stage oxidation of methane with nitric oxide addition by using weak flames in a micro flow reactor with a controlled temperature profile

• CH 4 /O 2 /Ar oxidation shifts to lower T w, max with NO addition. • Increasing NO from 1 % to 10.4 % leads to a less pronounced enhancement in CH 4 consumption. • CH 4 /O 2 /Ar mixture exhibits two-stage oxidation with NO addition. Near flames, the local NO–concentration is likely to be substantially higher in some practical combustion applications, such as re–burning methods and combustion of ammonia or its mixtures with hydrogen/hydrocarbons. Present work investigates the impact of elevated NO–addition to CH 4 /O 2 /Ar mixtures on its oxidation by using weak flames in a micro flow reactor with a controlled temperature profile (MFR). The species concentrations at the MFR outlet are assessed with various wall temperature profiles using a gas chromatograph and a quadrupole mass spectrometer. The NO–concentration in the mixture is set to 0, 594, and 6178 ppm (neat, 1 %, and 10.4 % of CH 4 , respectively). The experimental results exhibit that the oxidation of the CH 4 /O 2 /Ar mixture is promoted with the NO–addition. While CH 4 /O 2 /Ar mixtures exhibit the single–stage oxidation at high–temperatures, the CH 4 /O 2 /Ar mixtures with NO–addition exhibits two–stage oxidation characterized by the first stage oxidation at intermediate–temperatures and followed by the second stage oxidation in the high–temperature region of MFR. The addition of 594 ppm NO results in a significant enhancement of CH 4 oxidation, while an excessive NO–concentration of 6178 ppm does not further improve the fuel oxidation. Rates–of–production analyses reveal that the sequential conversion of CH 4 to CO 2 at intermediate–temperatures is strongly driven by the NO–addition, converting less reactive species into more reactive species at intermediate–temperatures through reactions such as NO + HO 2 = NO 2 + OH and CH 3 + NO 2 = CH 3 O + NO. As NO–concentration further increases, the contribution of the chain–propagating reaction H + NO 2 = NO + OH becomes larger, competing H radicals with the chain–branching reaction H + O 2 = O + OH. Due to this competition of H radicals, the excessive NO–addition does not further enhance fuel oxidation.