Formation of H-atoms in the Pyrolysis of 1,3-butadiene and 2-butyne: A Shock Tube and Modelling Study

Abstract

Abstract For investigating the pyrolysis of 1,3-butadiene (1,3-C4H6) and 2-butyne (2-C4H6), reactive gas mixtures highly diluted with argon as bath gas were prepared. The experiments were carried out in a high purity shock tube device over a temperature range of about 1500–1800 K at total pressures between 1.2 and 1.9 bar. The time-dependent formation of H-atoms was measured behind reflected shock waves by using the very sensitive method of atomic resonance absorption spectrometry (ARAS). A detailed chemical kinetic reaction mechanism consisting of 33 elementary reactions and 26 species was used to model the experimentally obtained H-atom profiles. From kinetic modelling, with help of sensitivity and reaction flux analysis, it was concluded that reaction R 1 2-C4H6 ↔ 2-C4H5 + H is crucial for the observed formation of H-atoms during the thermal decomposition of both investigated species and within the investigated range of temperatures and pressures. Moreover, at temperatures above about 1650 K, the decay of propargyl radicals (C3H3) turns out to contribute significantly to the amount of produced H-atoms. The following rate expressions were obtained for three reactions (R 1–R 3) – among them the isomerisation from 2-butyne to 1,3-butadiene – important with respect to the formation of H-atoms within the investigated parameter range. The uncertainties are estimated to be ±30%: 2-C4H6 ↔ 2-C4H5 + H (R 1) 2-C4H6 ↔ 1,2-C4H6 (R 2) 1,3-C4H6 ↔ C2H2 + C2H4 (R 3) k 1=3.8⋅1015exp(-44871K/T)s-1 k 2=6.9⋅1013exp(-32496K/T)s-1 k 3=7.0⋅1012exp(-33768K/T)s-1