A decoupled modeling approach and experimental measurements for pyrolysis of C6-C10 saturated fatty acid methyl esters (FAMEs)

Biodiesels are promising renewable fuels that can aid in the transition to carbon neutrality. The high molecular weight and complex composition of real biodiesel fuels complicate development of compact kinetic models needed for engine simulations. Our group previously proposed the functional group approach (FGMech) to model real-fuel combustion based on the identification of intrinsic relationships between fuel molecular structure and model parameters. Establishing these relationships requires a database consisting of the model parameters of pure fuels for training. In this work, we selected five fatty acid methyl esters (FAMEs) as target fuels, including methyl pentanoate (MPE), methyl hexanoate (MHX), methyl heptanoate (MHP), methyl octanoate (MO) and methyl nonanoate (MN). To facilitate development of an FGMech reaction scheme, a decoupling model approach is adopted here for model construction. Lumped reaction mechanisms are developed to describe the (oxidative) pyrolysis of fuels while a detailed model is used for describing the conversion of pyrolysis intermediates. To validate the present model, pyrolysis experiments for these FAMEs are conducted in a jet-stirred reactor (JSR) at 1 atm and over 790–1120 K. Both the synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) and gas chromatography (GC)/GC–MS are applied for measuring pyrolysis intermediates. The fuels, primary hydrocarbon and oxygenated products, secondary products including various aromatic compounds, are identified and quantified for model validation. The present model well-predicts the temperature window of fuel decomposition, and reasonably predicts the yields of most pyrolysis products under both present atmospheric conditions and high pressure conditions in literature. The agreement between the measured and predicted results indicates that the present decoupling methodology can accurately describe fuel decomposition and the evolution of intermediates under pyrolysis conditions. In addition, it is found that increasing alkyl CH2 groups in C6 to C10 FAMEs has little influence on the yields of primary oxygenated products; however, increasing yields of hydrocarbon products with increasing alkyl CH2 groups indicates that alkane chemistry becomes more important moving from MPE to MN.