NO Formation and Autoignition Dynamics during Combustion of H2O-Diluted NH3/H2O2 Mixtures with Air

Abstract

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 H2O2 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 NH3/H2O/H2O2, addition of water vapor does not prolong but rather shortens ignition delay because it increases OH radicals. At the same time, the presence of H2O2 does not affect the influence of H2O in suppressing NO generation. In this manner, we were able to show that NH3/H2O/H2O2 mixtures offer a way to use ammonia as carbon-less fuel with acceptable NOx emissions and realistic ignition delay.

Computational investigation of ammonia-hydrogen peroxide blends in HCCI engine mode

“…The latter is simply the result of the much lower inlet temperature used (320 K) which is 31% lower than the one used with the glow plug approach. The notable decrease of the maximum temperature is important because as was reported in the earlier work of Khalil et al 50 the NO production largely depends on the thermal NO mechanism. This justifies the significant decrease of NOx production displayed in Table 3, where it is shown that the addition of hydrogen peroxide leads to a 52% decrease in NOx.…”

“…It is noted that the two-stage ignition exhibited herein is a feature introduced due to the addition of hydrogen peroxide as ammonia as a sole fuel exhibits single stage ignition. 50,51 Consequently, the first stage ignition is instrumental to the control of the whole ignition process as it can lead to qualitatively different outcomes.…”

“…They also reported an increase of NO emissions as a result of the obtained flame temperatures and suggested the use of fuel lean mixtures to counteract the increased NOx emissions. Khalil et al 50 later extended their original computational work to include steam dilution in their ammonia/hydrogen peroxide batch reactor simulations. Using CSP tools, they revealed that NOx formation is mainly due to the thermal NO mechanism, with the fuel-bound nitrogen mechanism having negligible effect on NOx production.…”

Computational analysis of the effect of hydrogen peroxide addition on premixed laminar hydrogen/air flames

“…The contribution of the k-th process (chemical or transport) to the amplitude of the n-th mode can be assessed by the amplitude participation index (API) tool (P n k ) [77][78][79][80]:…”

Asymptotic Analysis of Detonation Development at SI Engine Conditions Using Computational Singular Perturbation

“…The signs of the eigenvectors α i , β i are properly adapted in order to ensure that the amplitudes of all slow (active) modes are positive. The positive API values then indicate that the associated processes enhance the impact of the respective mode while the opposite holds for negative API values [103,104].…”