Kinetic analysis of the role of selective NOx recirculation in reducing NOx emissions from a hydrogen engine

The kinetics of NO formation is used to study the effect of NO injected into a hydrogen engine on its emission characteristics. Considering the engine to be a variable-volume reactor, it is shown that the injection of NO reduces its further formation. In contrast to the earlier studies, it is highlighted that a reduction in the amount of NO can be achieved even if the reactions between the fuel and NO do not take place. Additionally, the predicted reduction in NO is not due to the lowering of temperature, which is the focus of most of the exhaust gas recirculation studies. The NO conversion efficiency is predicted to be low for equivalence ratios in the range of 0.4-0.5. However, for higher equivalence ratios between 0.65 and 0.9, the model predicts NO conversion efficiencies above 90% for various amounts of NO injected. Consistent with the data reported in the literature, the model predicts that the NO reduction is a weak function of the engine speed and that the NO conversion efficiency is almost independent of the amount of NO injected for equivalence ratios greater than 0.6. A significant reduction in the NO emissions can be achieved, provided that the temperature generated is high enough for the NO formation reactions to approach equilibrium. It is deduced that conditions such as high equivalence ratio, high load, and high compression ratio will result in a high reduction of NOx. These results can be used to design selective recirculation systems for an enhanced removal of NOx. Even though H-2 is used as a fuel in the present work, the insights obtained are equally applicable to other fuels as well.