Effects of mixture composition and air preheating on extinction limits of non-premixed ammonia/hydrogen/nitrogen counterflow flames

The extinction behaviours of ammonia (NH 3 )/hydrogen (H 2 ) non-premixed counterflow flames are examined for a range of high NH 3 heat fractions ( E N H 3 , 0.75–0.95) and air temperatures ( T Air , 300–1000 K) using a high temperature counterflow burner. The experimental and simulated results show that the extinction limits ( a E ) of NH 3 /H 2 non-premixed flames narrow significantly as E N H 3 increases from 0.75 to 0.95, while they are greatly extended as T Air increases from 300 to 1000 K. Chemical reaction analysis using a detailed reaction model reveals that the major factor for the large decreases in a E with the increase of E N H 3 is the nature of NH 3 consuming ȮH radicals and not reproducing Ḣ atoms, which breaks the cycle between heat generation and radical production through the high-temperature chain-branching reaction Ḣ + O 2 = Ö + ȮH. Increasing T Air strengthens the coupling of radical production via Ḣ + O 2 = Ö + ȮH and heat release from H 2 + ȮH = Ḣ + H 2 O, leading to a significant increase in a E . These findings from the present study contribute to the design of more stable and efficient operations of practical furnace applications utilizing high-temperature air combustion technology fueled with NH 3 /H 2 blends.