NOx emissions from ammonia-hydrogen fueled SI Engines

Abstract

Ammonia has been suggested as a potential carbon-free fuel for spark ignition reciprocating engines. As ammonia is a nitrogen compound, nitrogen oxide (NOx) emissions are important subjects to study. This paper clarifies features of NOx emissions of an ammonia-hydrogen fueled engine by experiment and NOx formation and extinction process in the cylinder by numerical simulation. Engine test results show the sensitivities of spark timing, load and equivalence ratio are major differences from gasoline engines. Additionally, the use of hydrogen as a combustion promoter tends to promote NOx emissions. Numerical simulation focused on NO, as greater part of NOx was nitric oxide (NO). Three representative chemical kinetic models were applied and compared each other and with experiment. Each model shows the largest NO concentration at flame zone. The values are several times higher than equilibrium although each model predicts quite different value. Two-step calculations are investigated to simulate NO history in the cylinder. First step calculation is to simulate NO formation at the flame zone. Qualitative effects of hydrogen and load on NO corresponded to the experiment, although the effects of spark timing and equivalence ratio slightly differs from the experiment. Multi zone engine simulator was applied in the second step to estimate extinction of NO in burned gas zone by applying the results of first step NO. It was found for NO to decrease considerably in the burned zone and come closer to the experiment at the end of the expansion stroke. Qualitative trends of NO by spark timing and equivalence ratio corresponded to the experiment by two-step calculation although the effect of hydrogen became small. The results provide a possibility to predict NO emissions by this simplified method.

Keywords

• ammonia
• hydrogen
• reciprocating engine
• combustion
• premixed flame propagation
• nitrogen oxides