An experimental and numerical investigation of fundamental combustion properties of biofuels

Presently, there is broad world interest in advancing the characterization, understanding, and use of biofuels, which are produced from a variety of cellulosic sources or organic matter, as they constitute one of the most viable and affordable partial solutions to reduce dependence on conventional petroleum. Additionally, they are more environmentally friendly as their use produces significant savings in net CO2 emissions. Biofuels encompass many distinct classes of chemical species, such as alcohols, ethers, and esters. In this study, the oxidation of several selected biofuels, namely the two isomers dimethyl ether (DME) and ethanol, as well as a group of alkyl esters, ranging from C2 to C11, that can act as surrogates of actual biodiesels, were investigated in both premixed and non-premixed flames. The experiments were conducted in the counterflow configuration at ambient pressure and elevated temperatures generally over a wide range of fuel/oxidizer ratios. Fundamental flame properties such as laminar flame speeds and local extinction limits were determined by measuring flow velocities using digital particle image velocimetry. The alkyl ester data were compared against those derived for flames of n-alkanes of similar carbon number, in order to assess the effects of saturation, the length of carbon-chain, and the presence of the ester group. A number of detailed chemical kinetic models were tested against the experimental data, and insight was provided into the high-temperature oxidation kinetics and controlling physico-chemical processes in flame environments. Several pertinent issues, including the accuracy of the Lennard-Jones potential parameters assigned to fuel species in the transport databases and the effect of discrepancies in the rate constants of several pertinent reactions, among different models, on the predictions of flame propagation and extinction, were addressed. Overall, this study constitutes part of a larger effort to study the fundamental combustion and emission characteristics of biofuels and to understand the chemical differences that are at the root of their different combustion performance.