Slow-Combustion of n-Heptane, iso-Octane and a Toluene/n-Heptane Mixture

Abstract

The oxidation of n-heptane, iso-octane and a toluene/n-heptane mixture was studied in a jet-stirred flow reactor operating under pressure at stoichiometric feed ratio in the low-temperature range. In particular, the transition from slow-combustion to a higher reactivity condition was examined by analyzing the composition of the reaction products as the residence time was increased.

It was found that high-octane-number fuels, such as iso-octane and toluene, have a high tendency to add O₂ keeping intact the original fuel structure forming 2,2,4,4-tetramethyltetrahydrofuran and benzaldehyde, respectively. Conversely, n-heptane has a high tendency to fragment in olefins and light oxygenated species already in slow-combustion. The different tendency to form oxygenated fuel-skeleton retaining species could be responsible for their different reactivity and consequently autoignition tendency.

In the oxidation of the toluene/n-heptane mixture the slow-combustion is essentially characterized by benzaldehyde formation indicating that, between the two fuel components, mainly toluene contributes to the fuel reactivity. As the residence time increases n-heptane begins to react forming olefins and light oxygenated compounds causing the passage of the system from slow-combustion to cool flames. In this phase toluene reactivity is reduced as testified by the benzaldehyde disappearance. This is due to the reaction temperature which increasing overcomes the very low ceiling temperature of aromatic radicals and produces, before the backward shift of the heptylperoxy equilibrium, the backward shift of the benzylperoxy equilibrium toward benzyl radicals.

Keywords:

• low-temperature oxidation
• autoignition