The effect of hydrogen addition on flammability limit and NO_x emission in ultra lean coterflow CH₄/air premixed flames

The effect of hydrogen addition to ultra lean counterflow CH₄/air premixed flames on the extinction limits and the characteristics of NO_x emission was studied by numerical simulation. Detailed chemistry and complex thermal and transport properties were employed. Hydrogen addition could significantly enlarge the flammable region and extend the flammability limit to lower equivalence ratios. If the equivalence ratio was kept constant, the addition of hydrogen increased the emission of NO in a flame, due to the enhancement in the rate of the NNH or N₂O intermediate NO formation routes. The addition of hydrogen caused a monotonous decrease in the formation of NO₂ and N₂O, except flames near the extinction limits, where the emission of NO₂ and N₂O first increased, and then decreased with the increase in the fraction of hydrogen. Hydrogen enrichment technology allows stable combustion under ultra lean conditions, resulting in significant CO₂ and NO emission reduction.