Effect of hydrogen and syngas addition on the ignition of iso-octane/air mixtures

There is worldwide interest in using renewable fuels within the existing infrastructure. Hydrogen and syngas have shown significant potential as renewable fuels, which can be produced from a variety of biomass sources, and used in various transportation and power generation systems, especially as blends with hydrocarbon fuels. In the present study, a reduced mechanism containing 38 species and 74 reactions is developed to examine the ignition behavior of iso-octane/H-2 and iso-octane/syngas blends at engine relevant conditions. The mechanism is extensively validated using the shock tube and RCM ignition data, as well as three detailed mechanisms, for iso-octane/air, H-2/air and syngas/air mixtures. Simulations are performed to characterize the effects of H-2 and syngas on the ignition of iso-octane/air mixtures using the closed homogenous reactor model in CHEMKIN software. The effect of H-2 (or syngas) is found to be small for blends containing less than 50% H-2 (or syngas) by volume. However, for H-2 mole fractions above 50%, it increases and decreases the ignition delay at low (T < 900 K) and high temperatures (T > 1000 K), respectively. For H-2 fractions above 80%, the ignition is influenced more strongly by H-2 chemistry rather than by i-C8H18 chemistry, and does not exhibit the NTC behavior. Nevertheless, the addition of a relatively small amount of i-C8H18 (a low cetane number fuel) can significantly enhance the ignitability of H-2-air mixtures at NTC temperatures, which are relevant for HCCI and PCCI dual fuel engines. The CO addition seems to have a negligible effect on the ignition of i-C8H18/H-2/air mixtures, indicating that the ignition of i-C8H18/syngas blends is essentially determined by i-C8H18 and H-2 oxidation chemistries. The sensitivity and reaction path analysis indicates that i-C8H18 oxidation is initiated with the production of alkyl radical by H abstraction through reaction: i-C8H18 + O-2 = C8H17 + HO2. Subsequently, the ignition chemistry in the NTC region is characterized by a competition between two paths represented by reactions R2 (C8H17 + O-2 = C8H17O2) and R8 (C8H17 + O-2 = C8H16 + HO2), with the R8 path dominating, and increasing the ignition delay. As the amount of H-2 in the blend becomes significant, it opens up another path for the consumption of OH through reaction R36 (H-2 + OH = H2O + H), which slows down the ignition process. However, for T > 1100 K, the presence of H-2 decreases ignition delay primarily due to reactions R31 (O-2 + H = OH + O) and R35 (H2O2 + M = OH + OH + M). Copyright (c) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.