The Effect of Carbon-Carbon Double Bonds on the Combustion Chemistry of Small Fatty Acid Esters

Environmentally friendly biodiesel is a mixture of saturated and unsaturated methyl (or ethyl) esters of long-chain fatty acids. To experimentally examine the effect of C=C double bonds on the combustion chemistry of fatty acid esters, low-pressure premixed laminar flames of four small esters have been studied using flame-sampling molecular-beam mass spectrometry. Mole fraction profiles of reactants, products, and well-identified stable and reactive combustion intermediates in flames of the saturated species methyl propanoate (CH3CH3COOCH3) and its isomer ethyl acetate (CH3COOCH2CH3) have been compared with results from flames of the unsaturated fuels methyl propenoate (CH2CHCOOCH3) and vinyl acetate (CH3COOCHCH2) flames. A total of eight flames have been studied, with two fuel-rich flame conditions investigated (fuel-equivalence ratios phi = 1.2 and 1.56) for each fuel. In addition, the underlying oxidation chemistry at these premixed flame conditions has been investigated using a detailed chemical kinetic reaction mechanism, which is largely based on a previously proposed model for saturated esters [B. Yang et al., Phys. Chem. Chem. Phys. 13 (2011) 7205]. The combined results provide a detailed understanding of the similarities and differences between the combustion of saturated vs. unsaturated esters. Meanwhile, the isomeric and stoichiometric effects on their combustion chemistry are also addressed. In this paper, experimental and modeling details are discussed with a special focus on the different reaction pathways.