The laser flash photolysis – laser-induced fluorescence technique has been used
to study the reaction kinetics of several potential biofuel ethers under low temperature
combustion conditions, in order to extend the understanding of the reactions that occur in
novel combustion engines. Biofuels offer a potentially carbon-neutral energy source that
could contribute to climate change mitigation, and commercial interest has been given to
the ether family of compounds, which display desirable fuel characteristics such as high
energy densities and favourable ignition properties.
Chapter 3 presents a study of the reaction between the OH radical and trimethyl
orthoformate (TMOF), diethyl- (DEE), di-n-butyl- (DBE), methyl tert-butyl- (MTBE),
and dimethyl ether (DME), from 298 – 744 K in 13 – 190 Torr of nitrogen. This
constitutes the first temperature-dependent study of OH + trimethyl orthoformate, and a
significant extension of the temperature range of previous studies on the OH + di-n-butyl
ether and OH + diethyl ether reactions. The temperature dependences of the rate
coefficients for OH + ether (all in units of cm3 molecule–1 s–1) can be parameterised by:
kOH+TMOF(298–744 K) = (8.0 ± 12.2) × 10–13 [(T/298)(2.6±1.2) + (T/298)(–8.1±4.6)] × e(2.7±3.9)/RT,
kOH+DEE(298–727 K) = (1.28 ± 0.21) × 10–11 × e(–0.11±0.59)/RT,
kOH+DBE(298–732 K) = (3.05 ± 7.13) × 10–12 (T/298)1.3±1.6 × e(6.4±5.8)/RT,
kOH+MTBE(298–680 K) = (9.8 ± 21.6) × 10–13 (T/298)2.7±1.5 × e(2.5±5.6)/RT, and
kOH+DME(298–656 K) = (1.22 ± 2.83) × 10–15 (T/298)6.9±0.5 × e(19.1±3.8)/RT.
Chapter 4 presents a technique for determining R + O2 rate coefficients and OH
yields by the observation of OH regeneration from chemical activation. This technique
was verified using the CH3OCH2 + O2 reaction in the dimethyl ether system via
comparison with previous measurements, and analyses using numerical integration
software determined the optimum experimental conditions for the method. Potentially,
this technique can be used to obtain rate parameters important for the combustion
modelling of a wide range of potential fuel molecules. Rate coefficients for the system
are reported at 291 – 483 K, in 4.1 – 32.6 Torr of nitrogen, and the mean room temperature
rate coefficient was determined to be kCH3OCH2+O2 = (0.94 ± 0.04) × 10–11 cm3 molecule–1 s–1,
across all pressures explored.
Chapter 5 employed the technique described in Chapter 4 to present novel
measurements of the C2H5OC2H4 + O2 reaction rate coefficient integral to the low
temperature combustion of diethyl ether under experimental conditions of 298 – 464 K,
in 5.2 – 28.4 Torr of nitrogen. The mean 298 K rate coefficient was determined to be
kC2H5OC2H4+O2 = (3.10 ± 0.55) × 10–11 cm3 molecule–1 s–1. OH yields and rate coefficients
were compared to ab initio calculations of the diethyl ether low temperature oxidation
surface at the CCSD(T)/Jun-cc-pVTZ//M06-2X/Jun-cc-pVTZ level, using master
equation methods. The transition state barrier to the OH product was required to be
lowered by ~7 kcal mol–1 in order to achieve good agreement between experimental data
and theoretical calculations.
Chapter 6 reports some initial observations of subsequent OH regeneration
following the R + O2 reaction at higher temperatures (~500 K and above), and some
interesting unwanted chemistry occurring under high temperature and high O2 conditions.
The main recommendations for future work are further explorations of the source of this
extraneous chemistry, and development of the data interpretation under such conditions.
The investigation of a wide range of fuels’ R + O2 reactions using the method presented
in Chapter 4 should also be carried out to improve estimated rates in combustion models.
