Ignition of C3 oxygenated hydrocarbons and chemical kinetic modeling of propanal oxidation

The relative high temperature ignition behavior of selected C3 oxygenated hydrocarbons, propanal (propionaldehyde, PAL or CH₃CH₂CHO), acetone (propanone or AC), isopropanol (iPOH), and ethyl formate (EF), is studied behind reflected shock waves. An ignition delay time correlation for methyl acetate (MA) from a previous study is also employed in the comparison. This study reveals the influence of different functional groups on the oxidation of the hydrocarbons. Isomer effects are also revealed for the ketone, acetone, and the aldehyde, propanal, with propanal portraying shorter ignition delay times than acetone. In the same manner, using the correlation for methyl acetate, the ester isomers, methyl acetate and ethyl formate, are compared. In this case, ethyl formate shows shorter ignition delay times than methyl acetate. Generally, methyl acetate, isopropanol (iPOH) and acetone (AC) portray comparable ignition behavior. This is thought to be owing to the fact that they are characterized by non-terminally bonded oxygen atoms. They all have terminal methyl groups, though the number of oxygen atoms and the types of carbon-oxygen bonds differ in these three fuels. Propanal and ethyl formate have similar ignition delays that are shorter than those of the other three fuels, due to their ability to form reactive ethyl radicals. The measured ignition delay times are compared to simulated delay times using existing mechanisms for acetone, isopropanol and small alkyl esters. Whereas there is reasonable agreement at high pressures between experiments and modeling results for the small alkyl esters, methyl acetate and ethyl formate, there are deviations for acetone and isopropanol. However, the mechanisms for the latter molecules perform better at lower pressures. The ignition data in this study could be useful for further optimization of the existing models. Furthermore, a chemical kinetic mechanism for propanal oxidation is proposed and good agreement between the proposed model and experiment is observed. However, further validation against a wider set of combustion experiments is recommended. This study contributes towards better understanding of the relative oxidation behavior of C3 oxygenated hydrocarbons which are relevant in combustion processes as fuel components, important intermediate species and, in lower concentrations, as exhaust products.