Numerical Investigation of the Effect of Ammonia Addition on the Performance of a Stoichiometric Operated Natural Gas Spark Ignition Engine

Abstract

Abstract Due to the pressing issue of global warming, there has been a significant focus on zero and low carbon fuels within the international community. Among the hydrocarbon fuels available, methane has gained wide usage in spark ignition engines due to its abundance and its relatively smaller hydrocarbon mass ratio. However, in order to further mitigate carbon emissions, there is growing interest in exploring the potential of using ammonia, a zero-carbon fuel, as a partial replacement for methane. Consequently, it is essential to investigate the impact of ammonia addition on the performance of natural gas spark ignition engines. One of the key challenges in studying ammonia/methane engines is that the introduction of ammonia alters the formation mechanism of nitrogen oxides (NOx) and leads to a coupling of fuel NOx and thermal NOx. Consequently, there has been limited research on the NOx emissions of ammonia/methane engines. This study successfully addresses this issue by differentiating between atmospheric nitrogen and fuel nitrogen elements and effectively decoupling the thermal/fuel NOx. This approach provides valuable insights into the effect of ammonia addition on the NOx characteristics of natural gas engines. The results indicate that the addition of ammonia introduces the presence of N2O species, which is not observed in methane engines. The N2O species primarily originates from the cold wall regions formed during the late oxidation process. Furthermore, although ammonia addition increases the concentration of NO in the burning zone, it appears to slightly reduce the NO concentration at chemical equilibrium at stoichiometric operation. This effect leads to stoichiometric-operated engines fueled with 80% methane and 20% ammonia exhibiting lower NOx emissions compared to those fueled with pure methane. These findings suggest that even a modest addition of 20% ammonia to methane fuel has a significant impact on the formation and evolution of NOx, highlighting the need for further studies to explore the NOx emission characteristics in ammonia/methane spark ignition engines.