The Effect of Hydrogen Addition on Flammability Limit and NOx Emission in Lean Counterflow CH4/Air Premixed Flames

The effect of hydrogen addition to lean counterflow CH4/air premixed flames on the extinction limits and the characteristics of NOx emission was investigated by numerical simulation. The detailed chemistry and complex thermal and transport properties were employed. The results show that the addition of hydrogen can significantly enlarge the flammable region and extend the flammability limit to a lower equivalence ratio. If the equivalence ratio is kept constant, the addition of hydrogen increases the emission of NO in a flame, due to the enhancement in the rate of the NNH or N2O intermediate NO formation route. When the equivalence ratio is relatively higher, the addition of hydrogen causes a monotonously decrease in the formation of NO2 and N2O. However for relatively lower equivalence ratio flames, the emission of NO2 and N2O first increases, and then decreases with the increase in the fraction of hydrogen.