NO Formation and Autoignition Dynamics during Combustion of H 2 O-Diluted NH 3 /H 2 O 2 Mixtures with Air

NO formation, which is one of the main disadvantages of ammonia combustion, was studied during the isochoric, adiabatic autoignition of ammonia/air mixtures using the algorithm of Computational Singular Perturbation (CSP). The chemical reactions supporting the action of the mode relating the most to NO were shown to be essentially the ones of the extended Zeldovich mechanism, thus indicating that NO formation is mainly thermal and not due to fuel-bound nitrogen. Because of this, addition of water vapor reduced NO formation, because of its action as a thermal buffer, but increased ignition delay, thus exacerbating the second important caveat of ammonia combustion, which is unrealistically long ignition delay. However, it was also shown that further addition of just 2% molar of H 2 O 2 does not only reduce the ignition delay by a factor of 30, but also reverses the way water vapor affects ignition delay. Specifically, in the ternary mixture NH 3 /H 2 O/H 2 O 2 , addition of water vapor does not prolong but rather shortens ignition delay because it increases OH radicals. At the same time, the presence of H 2 O 2 does not affect the influence of H 2 O in suppressing NO generation. In this manner, we were able to show that NH 3 /H 2 O/H 2 O 2 mixtures offer a way to use ammonia as carbon-less fuel with acceptable NO x emissions and realistic ignition delay.