A numerical study on the effect of CO addition on extinction limits and NOx formation in lean counterflow CH4/air premixed flames

The effect of CO addition on extinction and NOx formation in lean premixed counterflow CH4/air flames was investigated by numerical simulation. Detailed chemistry and complex thermal and transport properties were employed. A method that gradually switched off the initial reactions of NO formation from different routes was used to analyse the variation of NO formation mechanism. The results indicate that the addition of certain amount of CO increases the strain extinction limits and reduces the radiation extinction limits. As a result, the lean flammability limit of CH4/air premixed flame is extended to leaner side by the addition of CO. The formation of NO in a flame is increased with the addition of CO at a constant equivalence ratio. For an ultra-lean flame, the increase in the formation of NO is mainly because of the increase in the contribution from the NNH intermediate route, while for a near stoichiometric flame, this increase is mainly attributable to the rise in the contribution from the thermal route. With the fraction of added CO being gradually increased, the formation of NO2 in a flame first decreases and then increases at a given equivalence ratio. The addition of CO reduces the formation N2O in an ultra-lean flame, while affects little on the formation of N2O in a near stoichiometric flame.