Effects of Hydrogen and Syngas Addition on the Ignition of Iso-octane/Air Mixtures
thesisposted on 21.02.2013 by Sudhanshu Jain
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There is worldwide interest in using renewable fuels within the existing infrastructure. In this context, H2 and syngas have shown significant potential, as these fuels can be produced from a variety of biomass sources, and be used in various transportation and power generation systems, especially as blends with existing 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/H2 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, H2/air and syngas/air mixtures. Simulations are performed to characterize the effects of H2 and syngas on the ignition of iso-octane/air mixtures using the closed homogenous reactor model in Chemkin software. The effect of H2 (or syngas) is found to be small for blends containing less than 50% H2 (or syngas) by volume. However, for H2 mole fractions above 50%, it increases and decreases the ignition delay at low (T<900K), and high temperatures (T>1000K), respectively. For H2 fractions above 80%, the ignition is influenced more strongly by H2 chemistry rather than by iC8H18 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 H2-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 iC8H18/H2/air mixtures, indicating that the ignition of iC8H18/syngas blends is essentially determined by the iC8H18 and H2 oxidation chemistries. The sensitivity and reaction path analysis indicates that the iso-octane oxidation is initiated with the production of alkyl radical by H abstraction through reaction: iC8H18+O2=C8H17+HO2. Subsequently, the ignition chemistry in the NTC region is characterized by a competition between two paths represented by reactions R2 (C8H17+O2=C8H17O2) and R8 (C8H17+O2 = C8H16+HO2), with the R8 path dominating, and increasing the ignition delay. As the amount of H2 in the blend becomes significant, it opens up another path for the consumption of OH through reaction (H2 + OH = H2O + H), which slows down the ignition process. However, for T>1100K, the presence of H2 decreases ignition delay primarily due to reactions O2+H = OH+O and H2O2 + M = OH+OH+M.