Transport and reactivity effects of H2 additive on NO formation of NH3-H2 counterflow diffusion flames

The reactivity and transport effects of added H-2 on the NO formation of NH3 diffusion flames were investigated quantitatively. The Creck Stagni mechanism was modified properly to block the interaction between the added H-2 and the flame-generated H-2, ensuring that only the effects of added H-2 were determined and evaluated in the study. Results show that, with the increased H-2 additive, the NO mole fraction decreases primarily due to the substitute effects of added H-2 and suffers a further drop due to suppression of the reactivity effects on the fuel- type NO. Additionally, the transport effects of added H-2 can also decrease the NO mole fraction moderately by varying the local H/OH ratio in the flame. The NH3 flame with a small H-2 additive still has a high NO concentration because a small H-2 additive inhibits the NO reduction routes in the NH3 oxidation network considerably and improves the thermal NO production simultaneously. Besides, with the increased H-2 additive, the improved Soret diffusion of active radicals results in a fuel-side shift of NO mole fraction. For the NH3 diffusion flame with the H-2 additive, the H-2 reactivity dominates changes of major NO-related reactions. Apart from HNO + HG=>H-2 and NH+NO<=>N2O+H, N+OH<=>NO+H and N+NO<=>O+N-2 become increasingly important to the production and consumption of NO, respectively, with the H-2 additive. Although the H-2 transport seems to have a moderate influence on the flame temperature and NO mole fraction of the NH3- H-2 counterflow diffusion flame, it is confirmed that the flame structure, H-2 oxidation and NO production can be influenced considerably by H-2 transport based on the analysis of rate-of-production (ROP) and convection-diffusion-reaction (CDR) budget.