Skeletal mechanism construction for heavy saturated methyl esters in real biodiesel fuels

Abstract

Methyl decanoate, methyl-5-decenoate and methyl-9-decenoate have been widely adopted as biodiesel surrogates for engine simulation. However, these surrogates have relatively short chain length and very different physical properties compared with biodiesel molecules. Additionally, it is difficult to distinguish the unsaturation degree of different biodiesel fuels by using these surrogates. The direct use of the heavy methyl esters in real biodiesel fuels as surrogates is essential to accurately simulate biodiesel fuels. In this work, a four-part scheme to formulate skeletal mechanism for heavy saturated methyl esters has been proposed. Within the scheme, the oxidation mechanism is divided into four parts: low temperature oxidation, high temperature decomposition, ester group reactions and detailed C4-C0 chemistry. A skeletal mechanism for two of the five main methyl esters in real biodiesel fuels, i.e. saturated methyl palmitate and methyl stearate, has been constructed. The obtained skeletal mechanism contains only 6 fuel-dependent species and 13 fuel-dependent reactions for each heavy saturated methyl ester. Extensive validations were performed against shock tube experimental data for ignition delay timing under different initial pressure, temperature and equivalence ratio. The ignition delay behavior at high temperature has been well captured by the developed skeletal mechanism. As for ignition at low to medium temperature (from 650 K to 900 K), there is no experimental data due to the low vapor pressure and high melting point of heavy methyl esters. The comparison of ignition behavior at low temperature has been made between several models. Furthermore, the oxidation of n-decane/methyl palmitate and benzene/methyl stearate in a jet-stirred reactor has been utilized to validate the important species concentration. Good agreements have been observed through the validations. The results indicate that the developed skeletal mechanism is capable of predicting the combustion characteristics of methyl palmitate and methyl stearate.