Premixed ignition behavior of C9 fatty acid esters: A motored engine study
Introduction
Despite the extensive studies on the combustion and emissions characteristics of biodiesel in diesel engines [1], [2], [3], [4], [5], [6], [7], [8], the ignition chemistry of biodiesel is still not well-understood. Since large fuel molecules, such as biodiesel compounds, pose considerable challenges to kinetic modeling, researchers have been selecting smaller methyl esters as surrogates for biodiesel to perform experimental and kinetic modeling studies [9], [10], [11], [12], [13], [14].
Fisher et al. [9] developed a detailed kinetic model for methyl butanoate (C5H10O2), which was considered “large enough to allow fast RO2 isomerization reactions important in the low-temperature chemistry that controls the autoignition under the conditions found in diesel engines.” Gaïl et al. [10] slightly revised the model developed by Fisher et al. and examined it over a wide range of combustion conditions. In their variable pressure flow reactor experiments (equivalence ratio = 0.35, 1, pressure = 1.266 MPa), no evident negative temperature coefficient (NTC) behavior was observed, suggesting that methyl butanoate is not an ideal surrogate for biodiesel in terms of autoignition characteristics since long-chain fatty acid esters that comprise biodiesel are expected to exhibit pronounced low temperature oxidation behavior. Szybist et al. [11] performed an ignition study for methyl decanoate (C11H22O2) in a motored engine. They observed evident low temperature heat release (LTHR) during the oxidation of methyl decanoate, which indicates that the long alkyl chain of the methyl ester experiences the classic paraffin-like low temperature oxidation process [15]. They also reported that CO2 was produced at the very early stage of oxidation. It was then speculated by the authors that the early CO2 production during the low temperature oxidation of methyl decanoate is due to the direct CO2 formation from the ester group, as CO2 formation from CO oxidation is significantly inhibited when there are still sufficient amounts of hydrocarbons present [16]. The phenomenon of early CO2 formation during oxidation of fatty acid methyl esters was also observed by Dagaut et al. [17] in jet-stirred reactor experiments for the oxidation of rapeseed oil methyl esters (RME). They also suggested that the early CO2 production mainly comes from the pyrolysis of the ester functional group. Buchholz et al. [18] studied the molecular structure effects of dibutyl maleate (an oxygenate that contains two ester functional groups per fuel molecule) on soot emissions in a direct injection (DI) diesel engine by using carbon-14 isotope tracing. Their experimental results showed that the carbon atoms in the ester carbonyl groups of dibutyl maleate (DBM) did not contribute to soot formation. Instead, those carbon atoms were found to reside entirely in the exhaust CO2, indicating that the CO bond in the ester functional group of DBM does not break during combustion.
More recently, Dayma et al. [19] studied the oxidation of methyl hexanoate in jet-stirred reactor (JSR) experiments at 10 atm and a constant residence time of 1 s under both fuel-lean and fuel-rich conditions within the temperature range of 500–1000 K. Furthermore, a kinetic model for methyl hexanoate oxidation was developed by the authors, which gives good agreement with the experimental results from the JSR. Reaction path analysis indicates that the oxidation behavior of methyl hexanoate is mainly controlled by the weakness of the CH bond on carbon no. 2 (refer to Fig. 2) over the temperature and pressure range that was studied.
To provide a kinetic model for a methyl ester that can more accurately represent practical biodiesel molecules, Herbinet et al. [20] developed a detailed kinetic mechanism for methyl decanoate oxidation based on the previous n-heptane [15], iso-octane [21] and methyl butanoate [9] oxidation mechanisms. In their mechanism, the primary oxidation reaction pathways for methyl decanoate are shown in Fig. 1.
At high temperature, the main reaction pathway proceeds through β-scission of the alkyl-ester radicals, producing olefins and smaller alkyl-ester radicals. The low temperature oxidation sequence is initiated by the addition of O2 to the alkyl-ester radicals. Subsequently, the peroxy alkyl-ester radicals (RO2) experience isomerization to form hydroperoxy alkyl-ester radicals (QOOH). The hydroperoxy alkyl-ester radicals can then decompose into either unsaturated esters and hydroperoxy radicals or cyclic ether methyl esters and hydroxyl radicals. Alternatively, the hydroperoxy alkyl-ester radicals can undergo another attack by O2, yielding ketohydroperoxides and hydroxyl radicals via isomerization. The decomposition of ketohydroperoxides leads to the formation of carbonyl radicals and additional hydroxyl radicals. As the decomposition of one molecule of stable ketohydroperoxide yields at least two radical species, low temperature chain branching is initiated at this stage.
Since biodiesel is mainly composed of fatty acid esters with different degrees of unsaturation, it is important to understand the impact of the presence of double bonds on the oxidation behavior of fatty acid esters. Gaïl et al. [22] recently studied the oxidation of methyl 2-butenoate (C5H8O2) and methyl butanoate (C5H10O2) in a jet-stirred reactor (JSR) and under counterflow flame conditions. In the JSR experiments (equivalence ratio = 0.375, 0.75, 1.0, pressure = 1 atm and residence time = 0.7 s), the two esters were observed to exhibit similar reactivity and most of the stable products from the oxidation of the two esters were the same. Osmont et al. [23] studied the thermochemistry of CC and CH bond breakings in fatty acid methyl esters by applying density functional theory quantum calculations. According to their calculations, for monounsaturated fatty acid methyl esters, the abstraction of H atoms on the α-carbon of the ethylenic bond (refer to Fig. 2) is preferred among the CH bond scissions and the breakage of the CC bonds β to the ethylenic bond is favored among the CC bond breakings.
The objectives of the current study are to understand the ignition chemistry of fatty acid esters with different degrees of unsaturation and to compare the ignition behavior between fatty acid esters produced from different alcohols under practical engine conditions. Also, it is of interest to get a better understanding of the primary reaction paths responsible for the early CO2 production during low temperature oxidation of fatty acid esters.
Section snippets
Test fuels
In the present study, four C9 fatty acid esters were selected, which are methyl nonanoate, methyl 2-nonenoate, methyl 3-nonenoate and ethyl nonanoate. The molecular structures for the esters are shown in Fig. 2. The boiling points for methyl nonanoate and ethyl nonanoate are 213–214 °C and 227 °C respectively. Full boiling range analysis following the procedure of ASTM D2887 was performed for methyl 2-nonenoate and methyl 3-nonenoate to determine their boiling ranges. The test results showed
Heat release analysis
Ignition behavior of the test fuels can be characterized through heat release analysis. Fig. 3 shows the gradual change of ignition behavior for methyl nonanoate with the increase of compression ratio (CR). As seen in Figs. 3(a) and 3(b), at these compression ratios, only low temperature heat release (LTHR) occurs and the onset of LTHR advances as the compression ratio increases. As previously discussed, low temperature chain branching is initiated by the decomposition of ketohydroperoxides. In
Conclusions
In this study, premixed ignition behavior of four C9 fatty acid esters was investigated in a motored CFR engine experiment. Combustion analysis showed that these fatty acid esters exhibited distinctly different ignition characteristics. At the same compression ratio, ethyl nonanoate had the earliest onset of ignition and the highest magnitude of LTHR, while methyl 3-nonenoate had the latest ignition timing and the lowest magnitude of LTHR. The ignition timing and magnitude of LTHR for methyl
Acknowledgments
The authors wish to express their gratitude to Michael Alessi and Katherine Richard of Infineum USA L.P. and Stuart McTavish of Infineum UK Ltd. for their support of this work.
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