Sensitivity Analysis of $NOx$ Formation Kinetics in Pilot-Ignited Natural Gas Engines

A sensitivity analysis of $NOx$ formation in pilot-ignited natural gas dual fuel engines is performed based on a phenomenological combustion model. The $NOx$ formation mechanism employed in this study incorporates a super-extended Zel’dovich mechanism (up to 43 reactions). The sensitivity analysis compares the contribution of each major reaction to $NOx$ formation, and identifies the rate-controlling $NOx$ formation reactions in different high-temperature regions—the burning pilot spray, the premixed flame associated with the gaseous fuel-air mixture, and the burned combustion products. The formation rates for reactions involving $NOx$ are also investigated to reveal the primary $NOx$ formation paths. Results show two main $NOx$ formation paths both in the pilot spray (also called the packets zone) and the burned zone. The rate-limiting reactions for the packets zone are $O+N2=NO+N$ and $N2+HO2=NO+HNO$⁠. Rate-limiting reactions for the burned zone are $N2O+M=N2+O+M$ and $N2+HO2=NO+HNO$⁠. Since the aforementioned reactions significantly influence the net $NOx$ prediction, it is important that the corresponding reaction rates be determined accurately. Finally, because the quasi-steady-state assumption is commonly used for certain species in $NOx$ modeling, a transient relative error is estimated to evaluate the validity of the assumption. The relative error in $NOx$ prediction with and without the use of the steady-state assumption is small, of the order of 2%. This work also confirms that sensitivity analysis can provide valuable insight into the possible $NOx$ formation pathways in engines and improve the ability of current prediction tools to obtain more reliable predictions.