The Ignition of C₁-C₇ Natural Gas Blends and the Effect of Hydrogen Addition in the Low and High Temperature Regimes

New ignition delay time (IDT) measurements for two natural gas (NG) blends composed of C₁-C₇ n-alkanes, NG6 (C₁:60.625%, C₂:20%, C₃:10%, C₄:5%, nC₅:2.5%, nC₆:1.25%, nC₇:0.625%) and NG7 (C₁:72.635%, C₂:10%, C₃:6.667%, C₄:4.444%, nC₅:2.965%, nC₆:1.976%, nC₇:1.317%) by volume with methane as the major component are presented. The measurements were recorded using a high-pressure shock tube (HPST) for stoichiometric fuel in air mixtures at reflected shock pressures (p₅) of 20-30 bar and at temperatures (T₅) of 987-1420 K. The current results together with rapid compression machine (RCM) measurements in the literature show that higher concentrations of the higher n-alkanes (C₄-C₇) ~1.327% in the NG7 blend compared to the NG6 blend result in the ignition times for NG7 being almost a factor of two faster than those for NG6 at compressed temperatures of (T_C) ≤ 1000 K. This is due to the low temperature chain branching reactions that occur for higher alkane oxidation kinetics in this temperature range. On the contrary, at T_C > 1000 K, NG6 exhibits ~20% faster ignition than NG7, primarily because about 12% of the methane in the NG7 blend is primarily replaced by ethane (~10%) in NG6, which is significantly more reactive than methane at these higher temperatures. The performance of NUIGMech1.2 in simulating these data is assessed, and it can reproduce the experiments within 20% for all the conditions considered in the study. We also investigate the effect of hydrogen addition to the auto-ignition of these NG blends using NUIGMech1.2, which has been validated against the existing literature for natural gas/hydrogen blends. The results demonstrate that hydrogen addition has both an inhibiting and a promoting effect in the low- and high-temperature regimes, respectively. Sensitivity analyses of the hydrogen/NG mixtures are performed to understand the underlying kinetics controlling these opposite ignition effects. At low temperatures, H-atom abstraction byOH ^̇ radicals from C₃ and larger fuels are the key chain-branching reactions consuming the fuel and providing the necessary fuel radicals, which undergo low temperature chemistry (LTC) leading to ignition. However, with the addition of hydrogen to the fuel mixture, the competition by H₂ for OH ^̇ radicals via the reaction H₂ + OH ^̇ ↔ H^̇ + H₂O reduces the progress of the LTC of the higher hydrocarbon fuels thereby inhibiting ignition. At higher temperatures, since H^̇ + O₂ ↔ Ö + OH ^̇ is the most sensitive reaction promoting reactivity, the higher concentrations of H₂ in the fuel mixture lead to higher H^̇ atom concentrations leading to faster ignition due to an enhanced rate of the H^̇ + O₂ ↔ Ö + OH ^̇ reaction.