Low-temperature combustion chemistry of biofuels: Pathways in the low-temperature (550–700 K) oxidation chemistry of isobutanol and tert-butanol

Abstract

Butanol isomers are promising next-generation biofuels. Their use in internal combustion applications, especially those relying on low-temperature autoignition, requires an understanding of their low-temperature combustion chemistry. Whereas the high-temperature oxidation chemistry of all four butanol isomers has been the subject of substantial experimental and theoretical efforts, their low-temperature oxidation chemistry remains underexplored. In this work we report an experimental study on the fundamental low-temperature oxidation chemistry of two butanol isomers, tert-butanol and isobutanol, in low-pressure (4–5.1Torr) experiments at 550 and 700K. We use pulsed-photolytic chlorine atom initiation to generate hydroxyalkyl radicals derived from tert-butanol and isobutanol, and probe the chemistry of these radicals in the presence of an excess of O2 by multiplexed time-resolved tunable synchrotron photoionization mass spectrometry. Isomer-resolved yields of stable products are determined, providing insight into the chemistry of the different hydroxyalkyl radicals. In isobutanol oxidation, we find that the reaction of the α-hydroxyalkyl radical with O2 is predominantly linked to chain-terminating formation of HO2. The Waddington mechanism, associated with chain-propagating formation of OH, is the main product channel in the reactions of O2 with β-hydroxyalkyl radicals derived from both tert-butanol and isobutanol. In the tert-butanol case, direct HO2 elimination is not possible in the β-hydroxyalkyl+O2 reaction because of the absence of a beta C–H bond; this channel is available in the β-hydroxyalkyl+O2 reaction for isobutanol, but we find that it is strongly suppressed. Observed evolution of the main products from 550 to 700K can be qualitatively explained by an increasing role of hydroxyalkyl radical decomposition at 700K.