Comparing the low-temperature oxidation chemistry of butane isomers with ozone addition: An experimental and modeling study

Abstract

Butane is the simplest alkane with isomers of linear and branched structures. The low-temperature oxidation kinetics of the butane isomers is essential in constructing a comprehensive combustion model for hydrocarbon and oxygenated fuels. This paper studies the low-temperature oxidation of n-butane and isobutane in an atmospheric pressure jet-stirred reactor (JSR) with ozone addition. The experiments were conducted within a temperature range of 350 to 800 K, maintaining a consistent initial molar fraction, equivalence ratio, and residence time. Over thirty species were measured and quantified using the synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) and gas chromatography (GC). The NUIGMech1.3 and the Princeton ozone submechanism were modified to predict the reactivity of the two butane isomers from 350 to 750 K, with particular emphasis on bimolecular reactions of peroxy radicals, alkyl radical-ozone reactions, and hydrogen peroxide thermal decomposition. The experimental results suggest that while butane isomers exhibit similar reactivity from 350 to 575 K, significant differences emerge from 575 to 750 K. The experiments show that the low-temperature oxidation of n-butane primarily yields C₂ products (C₂H₄, CH₂CO, CH₃CHO, C₂H₃OH, C₂H₅OH, CH₃COOH, and C₂H₅O₂H), whereas isobutane favors the production of C₃ products, particularly CH₃COCH₃ and C₃H₆. A comprehensive analysis of experimental data and model simulations reveals that these differences can be attributed to the distinct reaction pathways of butyl peroxy radicals and the thermal decomposition reactions of C₄-ketohydroperoxides and C_1–4 alkyl hydroperoxides. Compared to n-butane, ozone significantly promotes the low-temperature reactivity of isobutane. Furthermore, ozone strongly promotes the peak mole fraction of C₂H₅OH, C₂H₅O₂H, PC₄H₉OH, NC₃H₇CHO, PC₄H₉O₂H and C₄-KHP during the low-temperature oxidation of n-butane. These promotions highlight the role of hydroperoxides and peroxy radicals in the ozone-assisted combustion system.