Decomposition and hydrocarbon growth processes for esters in non-premixed flames

Biomass fuels are a promising renewable energy source, and so, the mechanisms that may produce toxic oxygenated byproducts and aromatic hydrocarbons from oxygenated hydrocarbons are of interest. Esters have the form R-(C=O)-O-R' and are components of biodiesel fuels. The five specific esters studied here are isomers of C5H10O2. The experiments were performed in atmospheric pressure coflowing methane/air non-premixed flames. A series of flames were generated by separately doping the fuel mixture with 5 000 ppm of each ester. This concentration is sufficiently large to produce measurable changes in intermediate hydrocarbon concentrations, yet small enough to not disturb the overall flame structure. Since the overall structure is not perturbed, the measured changes in the intermediate hydrocarbons can be directly attributed to the reactions of the esters. Analysis of these changes reveals that unimolecular six-centered dissociation is the primary decomposition pathway for the three esters with molecular arrangements capable of undergoing that mechanism. The remaining two esters exhibited decomposition rates and products that are consistent with simple fission as the dominant decomposition mechanism, though we do not exclude other pathways from playing a significant role in their decomposition. All of the esters produce aromatic hydrocarbons at higher rates than the undoped fuel, and the molecular arrangement of the ester isomers plays a role in the degree of aromatic formation. Isomer variations also influence the type and quantity of toxic oxygenates that are produced in the flames.