Combustion kinetics of C3-C5 ketones
Session chaired by Pr. Christine Rousselle
Most of the energy consumed in the world is produced by the combustion of fossil fuels. The combustion of these fossil fuels releases massive amounts of CO2, which was captured by forests and buried millions of years ago. The extraction and combustion of fossil fuels since the 1850s such as coal, oil and natural gas have led to a rapid increase in the concentration of greenhouse gases in the atmosphere with CO2 as the main contributor, threatening the humanity as global warming changes the climate. However, the population and the standard of living are increasing rapidly and the demand for energy is constantly increasing. In the field of transport fuel, solutions exist to reduce CO2 emissions. Biomass grows by capturing CO2 thanks to the photosynthesis process, harvested, transformed by catalytic process into biofuel to be finally used as fuel. This short cycle stabilizes the period during which CO2 remains in the atmosphere. A second solution is to reduce the fuel consumption of the engines by increasing the efficiency. In the case of spark-ignition engines, the quality of gasoline is the key to efficiency. Gasoline must be knock resistant, allowing higher operating pressure, leading to superior engine efficiency. Usually, knock resistance is estimated via the research octane number (RON). Ketones meet both of these requirements. Ketones can be produced from biomass, as shown for butanone. In addition, the RONs measured for acetone (RON=110) and butanone (RON=117) are high compared to conventional gasoline (RON = 95) and can lead to an improved knocking behavior during engine operation. However, to ensure use of this family of compounds, their combustion behavior must be studied. In the literature, numerous studies are found on the kinetics of the oxidation of acetone or 2-butanone. The literature on longer carbon chain ketones is sparse. However, the length of the carbon chain influences the heating value of a fuel. The longer the chain, the higher the energy delivered by volume. However, increasing the length of the carbon chain is known to increase the knocking propensity of the fuel through the low temperature chemistry reaction pathway. To investigate the reactivity of acetone, butanone, 2-pentanone, 3-pentanone and 3-methyl-2-butanone under engine-relevant conditions, experiments where conducted in a rapid compression machine and a shock tube. In this regard ignition delay time (IDT) measurements of ketone/air mixtures for pressures between 20 and 40 bar have been performed. The ignition delay time is a good indicator of knock propensity of a fuel and serves as validation target for kinetic models. Furthermore, the stable intermediates during the oxidation of butanone, 2- and 3-pentanone have been measured in laminar flow reactors in previous studies. These experimental data were used in this study to validate a detailed kinetic model for acetone, butanone, 2-pentanone, 3-pentanone and 3-methyl-2-butanone. The important aspect of this detailed kinetic model is the systematic use of consistant reaction rates constants, allowing the establishment of rate rules for kinetic modeling of longer carbon chain ketones. In addition, the thermochemical data for the fuels, their primary radicals and associated peroxyalkyl radicals were calculated (G4 compound method), and are sensitive on the IDTs in the low temperature range. The predictive ability of the model is very good for the ignition delay time the mole fractions of the intermediates of oxidation. Thanks to this well validated kinetic model, the influence of carbonyl on the combustion behavior of ketones is examined and reveals the kinetics behind the octane booster propensity of ketones. For temperatures higher than 850 K, ketones are consumed by H-atom abstraction reactions followed by beta-scission reactions, with the exception of acetone which rapidly undergoes unimolecular reactions. At temperatures below 850 K, the low-temperature chemistry pathway is sensitive on the IDTs. However, the reactivity reduction of ketones compared to alkanes is explained by an equilibrium shifted to the reactants for the ketonyl stabilized by resonance addition to O2. Moreover, the carbonyl group in ketones prohibits the H-atom migration reactions peroxyalkyl radical = hydroperoxyalkyl radical. These two classes of reaction are kinetically limiting steps in the low temperature chain branching sequence.
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