Nitrogen-based emissions from Ammonia Combustion in a Heavy-Duty Spark Ignition Engine

Ammonia (NH3) is increasingly recognized as one of the paths toward decarbonizing power generation and transportation. Its carbon-free molecular structure and ease of storage compared to hydrogen put NH3 at the forefront of alternative fuels. However, transitioning from hydrocarbon fuels to NH3 in internal combustion engines significantly alters the profiles of nitrogen oxide (NOx) and nitrous oxide (N2O) emissions. In traditional hydrocarbon combustion, NOx is formed mainly via the thermal Zeldovich mechanism, where atmospheric nitrogen reacts with oxygen at high temperatures. In contrast, ammonia combustion introduces "fuel-NOx," where the nitrogen atom within the NH3 molecule itself is oxidized. This study used experiments and numerical simulations to evaluate the effect of engine geometry and operating condition on these differences. In addition, the study observed the differences in NOx and N2O formation and destruction with respect to the engine cycle timeline. While ammonia's lower adiabatic flame temperature reduced thermal NOx for a similar engine power output, the abundance of fuel-bound nitrogen led to higher total levels under lean-burn conditions. The more critical divergence was in the N2O emissions. In hydrocarbon engines, N2O is typically emitted in negligible quantities (often <1 ppm). However, the ammonia-fueled engine produced N2O concentrations several orders of magnitude higher, particularly during incomplete combustion or in low-temperature zones. This was driven by reaction pathways involving specific radicals and intermediates. Because N2O has a global warming potential approximately 300 times that of CO2, these emissions can potentially offset the carbon-reduction benefits of ammonia. However, this was not the case for the experimental platform used in this study.