Kinetic studies of ether low temperature combustion mechanisms using laser photolysis and modelling

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.