Kinetic Studies of Bio-butanol Oxidation under Low Temperature Combustion Conditions

The work presented in this thesis utilises the laser flash photolysis-laser induced fluorescence method to study the reaction kinetics of the four isomers of butanol, potential biofuels, under conditions relevant to low temperature combustion. Prior to the widespread use of these potential alternative fuels, their mechanism of low temperature combustion must be well understood, to ensure the best conditions for these fuels are employed in novel engines. Chapter 3 describes a detailed experimental study for the reaction of each of the four isomers of butanol (n-, i-, s- and t-butanol) with the hydroxyl radical at temperatures between 298 and 714 K, and at pressures between 30 and 130 Torr of nitrogen. This work presents the first temperature dependent study of these reactions at temperatures above 400 K and below 800 K, and has thus assisted in reducing the uncertainty of the bimolecular rate coefficient in this temperature region, which is crucial to low temperature combustion of these alternative fuels in an engine. Temperature dependent modified Arrhenius parameterisations were provided from this work, and also from concatenate fits to this work and high temperature shock tube data from the literature. The temperature dependence for the reaction of each isomer with OH obtained in this work can be described by modified Arrhenius parameterisations (all in units of cm3 molecule-1 s-1): kn-butanol+OH (298 – 715 K) = (1.15 ± 2.62) × 10–19 × T (2.64 ± 0.31) × exp(7800 ± 1100 R×T) ki-butanol+OH (298 – 607 K) = (2.05 ± 6.79) × 10–18 × T (2.20 ± 0.45) × exp (6800 ± 1600 R×T) ks-butanol+OH (298 – 690 K) = (1.38 ± 2.58) × 10–21 × T (3.22 ± 0.25) × exp (10330 ± 970 R×T) kt-butanol+OH (298 – 614 K) = (4.50 ± 23.7) × 10-21 × T (2.99 ± 0.71) × exp(5200 ± 2600 R×T) These parameterisations are crucial to the improvement of structure-activity relationships, for calculating branching ratios for the reaction of the butanol isomers with OH at temperatures relevant to low temperature combustion. Accurate total bimolecular rate coefficients and branching ratios for the reaction of butanol with OH are vital for modelling of combustion parameters such as ignition delay times, as demonstrated later in Chapter 5. Chapter 4 presents experimental observations and kinetic measurements of the b-scission decomposition of b-hydroxybutyl radicals above 480 K. The b-site radicals formed through hydrogen abstraction from butanol by OH decompose at these sufficiently high temperatures to produce a butene molecule and an OH radical. The ab initio energetic barriers v to decomposition for these radicals have been calculated at the CCSD/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ level. The barriers calculated have also been optimised using master equation solving for b-site radicals derived from all isomers of butanol with the exception of s-butanol. The optimised barriers to decomposition of the radical are 103.4 ± 1.3, 93.0 ± 1.0 and 100.0 ± 2.6 kJ mol-1 for t-, i- and n-butanol respectively. The OH yield obtained in these experiments has also been used as a proxy for abstraction at the beta site of each butanol isomer, and equated to the percentage of OH reaction abstractions occurring at this site. Chapter 5 presents some unusual observations in which a growth of OH signal was observed at temperatures above 600 K in the presence of oxygen following the photolysis pulse, and some experimental tests were conducted to deduce the source of this behaviour. The bimolecular rate coefficient for the reaction of O (3P) atoms with n-butanol was measured at temperatures of 490 – 730 K, and pressures of 40 – 55 Torr, described by the Arrhenius parameterisation: kO(3P)+n-butanol (490 – 730 K) = (1.64 ± 0.55) × 10-10 × exp(-18700 ± 1700/RT) cm3 molecule–1 s–1.