Synergistic effects of nanosecond plasma discharge and hydrogen on ammonia combustion.

• Using plasma to assist ammonia reactivity results in fewer NOx emissions than those in the flames assisted by hydrogen. • Plasma-assisted ammonia combustion is less prone to strain rate than that of hydrogen-assisted ammonia combustion. • High energy plasma discharges broaden the reaction zones by generating radical pools in the flame preheating zone, being more intense under fuel-rich conditions. • High energy plasma discharges activate the DeNOx mechanism in preheating and post-flame zones. • Increasing the mixture pressure deteriorates the impact of plasma on combustion. Such effects are weakly controlled by the reduced electric field. Synergistic effects of nanosecond plasma discharge and hydrogen on the combustion characteristics of ammonia/air are numerically studied under conditions relevant to gas turbine combustion chambers. It is shown that increasing the plasma contribution in assisting the flame results in lower NO X emissions by up to 27% than those in flames assisted by hydrogen for the range of operating conditions considered in this study. Plasma makes the consumption speed of the reactants less prone to the strain rate than that in flames assisted by hydrogen. It is found that discharging plasma with the pulse energy density of 9 mJ/cm3 alongside using 12% hydrogen by volume in the fuel increases the flame speed of ammonia/air to those of conventional fossil fuels such as methane—an improvement that is not achievable by just using hydrogen, even at a high concentration of 30%. Furthermore, raising the pulse energy density beyond a specific value broadens the reaction zones by generating radical pools in the flame preheating zone, which is expedited in fuel-rich conditions with high H 2 fuel fractions. Investigations show that the simultaneous utilization of high-energy plasma and hydrogen reduces the NO X emissions by activating the mechanisms of nitrogen oxide denitrification (DeNO X) in preheating and post-flame zones, being more significant under the lean condition as compared with rich and stoichiometric cases. It is shown that increasing mixture pressure significantly deteriorates the impacts of plasma on combustion. Such unfavorable effects are weakly controlled by changes in the reduced electric field caused by pressure augmentations. [ABSTRACT FROM AUTHOR]