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

Abstract New ignition delay time (IDT) measurements for two natural gas (NG) blends, NG6 and NG7, composed of C1-C7 n-alkanes with methane as 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 of 20-30 bar and at temperatures 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 for NG7 being almost factor of two faster than NG6 at compressed temperatures = 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 temperatures > 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 reproduces the experiments within 20%. 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 promoting effect in the low- and high-temperatures regime, respectively.