Synergistic effects of nanosecond plasma discharge and hydrogen on ammonia combustion

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 NOX 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 H2 fuel fractions. Investigations show that the simultaneous utilization of high-energy plasma and hydrogen reduces the NOX emissions by activating the mechanisms of nitrogen oxide denitrification (DeNOX) 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.