The chemical influence of flue gas recirculation in NH3-H2 MILD combustion: an experimental and kinetic investigation

Ammonia is a promising hydrogen carrier with potential in carbon-neutral energy systems. However, its combustion faces the challenges of high NO emissions and inadequate stability. Moderate or Intense Low-oxygen Dilution (MILD) combustion preheats NH 3 , promoting its cracking to H 2 and the formation of an NH 3 -H 2 mixture, thereby affecting NO formation and improving stability. This study investigates the kinetic interactions of NH 3 -H 2 MILD combustion, with a focus on the effects of flue gas recirculation. Experiments in a jet-stirred reactor, coupled with kinetic analysis, reveal that H 2 enhances overall reactivity and promotes NH 3 MILD combustion by advancing the transition to an exothermic stage. With increasing reactivity, the high N 2 content in the recirculated flue gas exerts a pronounced chemical influence, reflected in reaction temperature rise, H 2 accumulation or NO formation. Kinetic analysis shows that the HO 2 peak marks a kinetic threshold for rapid recirculation chemistry; its decay via HO 2 + OH coincides with the establishment of the H/OH radicals and rapid heat release. R64 (NH 2 + HO 2 = NH 3 + O 2 ) regulates NH 2 availability and accelerates HO 2 →H/OH. H 2 addition suppresses NH 2 polymerization, concentrating H 2 formation in the NH 3 dehydrogenation chain. In contrast, N 2 retains NH 2 , influencing azide cycling through alternative pathways and shifting the H&OH equilibrium toward OH, thereby affecting both H 2 and NO chemistry during MILD combustion. Finally, the intensity of azide cycling under different conditions is evaluated by quantifying the overall NH 3 recycling ratio. H 2 effectively enhances the azide ratio and amplifies the chemical influence of N 2 by promoting the NH 2 → HNO (Φ < 1) or azide pathways (Φ > 1). These findings provide guidance for regulating flue gas to achieve targeted radical pools in MILD burners, thereby promoting stable NH 3 MILD combustion with low emissions.