Reactivity of CO/H2/CH4/Air Mixtures Derived from In-Cylinder Fuel Reformation Examined by a Micro Flow Reactor with a Controlled Temperature Profile

The effects of CO/H-2/CH4 mixture compositions on the reactivity are investigated based on weak flame responses in a micro flow reactor with a controlled temperature profile (MFR). The ratios of H-2 and CH4 are varied widely at a constant fraction of CO and the effects of variations in the compositions on the reactivity of the stoichiometric CO/H-2/CH4 mixtures are evaluated both experimentally and computationally at atmospheric pressure. The results show that a weak flame is stabilized at the higher temperature region in MFR as the CH4 mole fraction increases, which indicates that the reactivity of the CO/H-2/CH4 mixtures decreases with the increase of the CH4 mole fraction. Ignition delay times of the CO/H-2/CH4 mixtures are also computed using a detailed chemical reaction model. The computed ignition delay times of the CO/H-2/CH4 mixtures largely increase as the CH4 mole fraction increases, which is consistent with the variation in the reactivity shown by weak flames in MFR. Rate-of-production analyses using a detailed chemical reaction model show that the OH radical consumption by CH4 increases and that by H-2 decreases as the CH4 fraction increases. Consequently, the production rate of CH3 radicals, which are less reactive than H radicals, increases and the production rate of H radicals decreases. This can suppress the OH formation from H radicals. Therefore, the primary factor of the significant reduction of the reactivity of the CO/H-2/CH4 mixtures seems to be the increase of the OH radical consumption by CH4 and the CH3 radical production due to the increase in the CH4 fraction.