Effect of NO2 addition on the oxidation kinetics of n-pentane and natural gas blends with C1-C5 n-alkanes

This study presents new ignition delay time (IDT) data for NO2 blended multi-component natural gas mixtures with methane as the major component including C-2-C-5 primary n-alkanes in different concentrations. The data are measured using a high-pressure shock tube and a rapid compression machine at compression pressures of 20 and 30 bar in the temperature range of 715-1480 K for stoichiometric mixtures. This paper builds upon our previous work on the development of a C-1-C-3/NOx kinetic mechanism and focusses on understanding the interaction chemistry of heavier n-alkane/NOx mixtures by determining the IDT characteristics of various NO2 blended n-C5H12/air combinations with 200, 400 and 1000 ppm concentrations of NO2 present at compression pressures of 10, 20 and 30 bar. The newly measured IDT results and existing experimental data from the literature are used to improve model predictions for the oxidation of n-C4H10/NOx, n-C5H12/NOx, NG2/NOx, and NG3/NOx by updating NUIGMech1.2. The improvement in the performance of GalwayMech1.0 is attributed to updated rate constants for the n-C5H12 + NO2 <-> (C) over dot(5)H(11)-1, -2, -3 + HONO reactions as well as the inclusion of (R) over dot + NO2 <-> RONO reaction channels. The IDT decreased slightly when 1000 ppm of NO2 is added to n-pentane, which contrasts with propane (C3H8), where there was a significant decrease in the mixture reactivity as reported previously (A. A. E. S. Mohamed, A. B. Sahu, S. Panigrahy, M. Baigmohammadi, G. Bourque, H.J. Curran, The effect of the addition of nitrogen oxides on the oxidation of propane: An experimental and modeling study. Combust. Flame 245 (2022) 112306). This is because the competition between NO2 and O-2 for 1-propyl radicals formed by H-atom abstraction from C3H8 decreases mixture reactivity, while in the case of n-C5H12, although NO2 and O-2 compete for 1-pentyl radicals, the increase in reactivity is facilitated by the availability of 2-pentyl radicals in the system which also leads to chain branching when they add to O-2, unlike 2-propyl (iso-propyl) radicals which ultimately do not lead to chain branching in the oxidation of propane.