Ignition of the butene isomers: An experimental and kinetics study

There is considerable interest to understand the influence of the molecular structure on the ignition of alkenes, particularly that of the position of the double bond. The present study selected the smallest set of the alkene isomers, namely the butene isomers, and determined their ignition temperatures under identical experimental conditions. Specifically, experiments were conducted in a counterflow with heated air impinging onto the fuel flow consisting of nitrogen-diluted butene isomers for various strain rates and system pressures up to 5 atmospheres. The relative order of ignitability is that 1- butene is the easiest, cis- and trans-2-butenes are similar, and isobutene is the hardest. To further understand the observed trend, kinetic analysis on three reaction paths important for ignition was performed using enthalpies calculated by the ab initio CBS-QB3 method. One is the chain initiation by O₂ abstraction, i.e., C₄H₈ + O₂= C₄H₇ + HO₂, and the calculated reaction rate constants for different isomers exhibit a similar trend as that of the experiment. This trend is consistent with the bond strength of the allylic C-H bond and the number of equivalent H atoms in the different isomers. The calculated rate constants for the second reaction path, HO₂ addition to the double bond, show the opposite trend with the experiment. This could be ascribed to the formation of oxiranes from the butene-OH adduct, which converts fuel to stable species and therefore inhibits ignition. The third reaction path is the OH addition and subsequent O₂ addition followed by intramolecular H transfer. Four butenes isomers were found to be similar in both the well depth and the rate constants for the isomerization.