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

Abstract

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.