Investigation of the kinetics of conjugated diolefins using UV absorption spectroscopy

Abstract

Conjugated diolefins are not only crucial intermediates in larger hydrocarbon pyrolysis and oxidation, but also key species in the formation and growth of polycyclic aromatic hydrocarbons (PAHs). In this work, we employed a sensitive UV laser diagnostic to measure absorption cross-sections and decomposition rates of three conjugated diolefins, namely 1,3-butadiene, 2-methyl-1,3-butadiene (isoprene), and 2,3-dimethyl-1,3-butadiene. The single-pass UV absorption diagnostic achieved a ppm-level detection limit between the wavelengths of 212.5 and 220.5 nm. The use of dilute conditions (119 - 500 ppm fuel in argon) enabled nearly isothermal measurements despite reaction enthalpy. Temperature-dependent absorption cross-sections were measured from room temperature to 1850 K and pressures ranging 0.75 - 1.50 bar in a shock tube. Decomposition of the molecules was observed at temperatures above - 1350 K, and all three molecules exhibited similar activation energy. Around 1800 K, 2,3-dimethyl-1,3-butadiene decomposed twice as fast as isoprene and 4 times faster than 1,3-butadiene. Our measured overall decomposition rate coefficients are given as (unit of s - 1, ± 20% uncertainty): 1,3-Butadiene decomposition rate coefficients agree well with previous measurement at similar pressures. To our knowledge, this work reports first measurements of the decomposition rate coefficients of isoprene and 2,3-dimethyl-1,3-butadiene. As an additional application of the current UV diagnostic, we measured 1,3-butadiene decay time-histories during fuel-lean oxidation and compared our data with the predictions of AramcoMech 3.0. We updated the model with our measured 1,3-butadiene decomposition rate coefficients, which significantly improved the model prediction of fuel oxidation.