Experimental and Modeling Study of Trends in the High-Temperature Ignition of Methyl and Ethyl Esters

Methyl and ethyl esters of carboxylic acids can be obtained from the trans-esterification of fatty acid triglycerides using methanol and ethanol, respectively. In this work, the relative ignition of methyl and ethyl esters is reported, based on new high-temperature shock tube ignition data of ethyl acetate (EA) and ethyl propanoate (EP). These are compared with literature data for other methyl and ethyl esters, and isomer and alkyl group effects are also investigated. It is found that ethyl esters are generally characterized by shorter ignition delay times than those of methyl esters of the corresponding alkanoic acid. Increased reactivity of ethyl esters is also observed for isomeric ethyl and methyl esters. A combined high-temperature chemical kinetic model is proposed for methyl and ethyl acetates as well as for ethyl formate in order to shed light on the link between chemical structure and the observed reactivity trends. The proposed model reflects the experimental trends and generally predicts ignition delay times in close agreement with measured data. The reduced methyl acetate reactivity is attributed both to the absence of more reactive secondary C H sites and the ethyl group which facilitates complex unimolecular decomposition reactions in ethyl esters such as acid and ethylene elimination. The model and the new experimental data contribute toward improved understanding and modeling of the combustion properties of biodiesel surrogates.