Promotion or inhibition of hydrogen–air ignition by iron-containing compounds

Abstract

The ignition of stoichiometric and fuel-lean hydrogen–air mixtures near 1000 K was studied, for varying amounts of iron additives. Depending on the initial conditions, the addition of iron pentacarbonyl promotes or inhibits the ignition of hydrogen/air mixtures. The simulations show that iron compounds at 1 μL/L 1 Note that μL/L is equivalent to ppm by volume. 1 reduce the ignition delay by about 30%, and that 50 μL/L can reduce the ignition delay by a factor or two or three for lean and stoichiometric conditions; at slightly higher volume fraction (>150 μL/L), the ignition is retarded. The effectiveness of the iron compounds is influenced by the form of the added iron, stoichiometry, and initial temperature of the mixture. At low additive volume fraction, the promotion is the result of relatively fast decomposition of iron pentacarbonyl, which provides a source of radicals at the initial reaction stage during radical pool build-up; the ignition promotion is proportional to the amount of iron-containing additive, and mostly does not involve cycling reactions. In contrast, at higher additive volume fraction, the ignition is retarded by the iron compounds, through catalytic radical recombination cycles similar to those acting in flame inhibition. The reduction in chain-carrying radical concentrations occurs at later times in the ignition process when the radicals have reached high concentrations.