An experimental and kinetic modeling study of combustion of isomers of butanol

A kinetic model is developed to describe combustion of isomers of butanol—n-butanol (n-C4H9OH), sec-butanol (sec-C4H9OH), iso-butanol (iso-C4H9OH), and tert-butanol (tert-C4H9OH). A hierarchical approach is employed here. This approach was previously found to be useful for developing detailed and semi-detailed mechanism of oxidation of various hydrocarbon fuels. This method starts from lower molecular weight compounds of a family of species and proceeds to higher molecular weight compounds. The pyrolysis and oxidation mechanisms of butanol isomers are similar to those for hydrocarbon fuels. Here, the development of the complete set of the primary propagation reactions for butanol isomers proceeds from the extension of the kinetic parameters for similar reactions already studied and recently revised for ethanol, n-propanol and iso-propanol. A detailed description leading to evaluation of rate constants for initiation reactions, metathesis reactions, decomposition reactions of alkoxy radicals, isomerization reactions, and four-center molecular dehydration reactions are given. Decomposition and oxidation of primary intermediate products are described using a previously developed semi-detailed kinetic model for hydrocarbon fuels. The kinetic mechanism is made up of more than 7000 reactions among 300 species. The model is validated by comparing predictions made using this kinetic model with previous and new experimental data on counterflow non-premixed flames of n-butanol and iso-butanol. The structures of these flames were measured by removing gas samples from the flame and analyzing them using a gas chromatograph. Temperature profiles were measured using coated thermocouples. The flame structures were measured under similar conditions for both fuels to elucidate the similarities and differences in combustion characteristics of the two isomers. The profiles measured include those of butanol, oxygen, carbon dioxide, water vapor, carbon monoxide, hydrogen, formaldehyde, acetaldehyde, and a number of C1–C4 hydrocarbon compounds. The predictions of the kinetic model of flame structures of the two isomers were satisfactory. Validation of the kinetic model was also performed by comparing predictions with experimental data reported in the literature. These data were obtained in batch reactors, flow reactors, jet-stirred reactors, and shock tubes. In these configurations, combustion is not influenced by molecular transport. The agreement between the kinetic model and experimental data was satisfactory.