Algorithmic Analysis of Chemical Dynamics of Ammonia Autoignition and NO formation

Abstract

Ammonia is a hydrogen-containing molecule on the management and transportation of which substantial know-how and infrastructure have developed. The feasibility of internal combustion engines operating on ammonia fuel has been demonstrated in mid-1940s. However, ammonia combustion, is caveated by two major disadvantages, namely unrealistically long ignition delays and elevated NOx emissions. These caveats are addressed in this study through the use of various additives that had an effect on the dynamics of ammonia ignition. The dynamics of a homogeneous adiabatic autoignition of an ammonia/air mixture at constant volume was studied, using the algorithmic tools of CSP. The time frame of action of the modes that are responsible for ignition was analyzed by calculating the developing time scales throughout the process and by studying their possible relation to NOx emissions. The reactions that support or oppose the explosive time scale were identified, along with the variables that are related the most to the dynamics that drive the system to an explosion. It is shown that reaction H2O2 (+M) ! OH + OH (+M) is the one contributing the most to the time scale that characterizes ignition and that its reactant H2O2 is the species related the most to this time scale. These findings suggested that addition of H2O2 in the initial mixture will influence strongly the evolution of the process. The ignition delay could be reduced by more than two orders of magnitude through H2O2 addition, which causes only a minor increase in NOx emissions. NO formation was studied during the isochoric, adiabatic autoignition of ammonia/air mixtures using the algorithm of Computational Singular Perturbation. 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, the addition of water vapor as a thermal buffer reduced NO formation, 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.

Date of Award	May 2021
Original language	American English