Understanding the reverse effects of NO2 addition on the auto-ignition behavior of n-dodecane/methane mixtures from low to intermediate temperature

To investigate the impact of NO2 addition on the auto-ignition characteristic of n-dodecane/methane mixtures, the ignition delay times of a n-dodecane/methane binary mixture with varying NO2 concentrations (0–800 ppm) were measured utilizing a rapid compression machine at temperatures ranging from 650 to 1100 K and pressures of 5–10 bar. Experimental results show that NO2 addition can promote auto-ignition, which notably depends on both NO2 concentration and temperature. This promoting effect is in positive correlation with the amount of NO2 addition at higher temperatures. While the impact of NO2 content on the promoting effect is negligible at lower temperatures around 650 K. A chemical kinetic model for the n-dodecane/NOx system was proposed in this study with reasonably good performance in predicting the newly measured ignition delay times. The kinetic model was further adopted for kinetic analyses. A reversal effect of NO2 addition on the auto-ignition behavior is identified when the temperature passes 650 K. At low temperatures, the reaction C12H25Ȯ2+NOC12H25Ȯ+NO2 competes with the isomerization of the C12H25Ȯ2 radicals and inhibits the low-temperature chain-branching reactions. This inhibition effect intensifies with an increasing concentration of NO2. As temperature increases over 650 K, concerted HO2 elimination gradually dominates and the NO produced from NO2 transforms HO2 to ȮH through the reaction of NO+HȮ2NO2+ȮH, thereby enhancing the overall reactivity. This promotion effect is also strengthened with higher NO2 concentration. The transformation of key factors affecting reactivity leads to the reverse effect of NO2 addition on the auto-ignition behavior of the dual-fuel strategy.