A comparative study on the autoignition characteristics of cyclopropane and propane at high temperatures

Abstract

An understanding of the autoignition characteristics of cyclopropane, the smallest cycloalkane, would have potential implications for constructing combustion and pyrolysis models for polycyoalkane fuels. However, there are limited studies on the combustion chemistry of cyclopropane in the literature, and the ignition difference between cyclopropane and propane is also unclear. In this work, we explored the reaction pathways of cyclopropane and propane at high temperatures by quantum-chemical calculation and kinetic modeling, and a combustion mechanism for cyclopropane was constructed based on the public core mechanism, UCSD. The computed results show that cyclopropane has a ring-opening reaction, H-abstraction and addition-ring-opening reactions with active radicals and oxygen molecules in the chain initiation process, while propane only has the usual H-abstraction and bond dissociation reactions. Moreover, the kinetic modeling results show that cyclopropane (air mixture) has a shorter ignition delay time (IDT) than propane at a high equivalence ratio (ϕ = 3, p = 5.5 atm), but a longer IDT than that of propane when the equivalence ratio decreases to 0.333, in accordance with available experimental data. Sensitivity analyses demonstrate that reactions of H and CH3 radicals with oxygen molecules accelerate both the ignition of cyclopropane and propane under the conditions considered in this study. Reaction pathway analyses indicate that cyclopropane is consumed through conversion to propylene at high temperatures, whereas oxidation of propane is mainly initiated by H-abstraction reactions. In contrast with propane, the IDTs of cyclopropane increase with decreasing equivalence ratio. The reason is that ring strain favors the ring-opening reaction of cyclopropane to propylene both kinetically and thermodynamically. At high equivalence ratios, more energy is released, leading to an increase in the system temperature which promotes the ignition of cyclopropane.