Publications

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

Welz, Oliver W.; Savee, John D.; Eskola, Arkke J.; Sheps, Leonid S.; Osborn, David L.; Taatjes, Craig A.

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 (4-5.1 Torr) experiments at 550 and 700 K. 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 a-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 b-hydroxyalkyl radicals derived from both tert-butanol and isobutanol. In the tert-butanol case, direct HO2 elimination is not possible in the b-hydroxyalkyl + O2 reaction because of the absence of a beta C-H bond; this channel is available in the b-hydroxyalkyl + O2 reaction for isobutanol, but we find that it is strongly suppressed. Observed evolution of the main products from 550 to 700 K can be qualitatively explained by an increasing role of hydroxyalkyl radical decomposition at 700 K. © 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.