Comparative Study of the Autoignition of Methyl Decenoates

Aug 17, 2013 - methyl ester (FAME) fuels, has been studied using the shock tube technique. ... that double bonds within FAME fuel components will have...
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Comparative Study of the Autoignition of Methyl Decenoates, Unsaturated Biodiesel Fuel Surrogates Weijing Wang, Sandeep Gowdagiri, and Matthew A. Oehlschlaeger* Department of Mechanical, Aerospace, and Nuclear Engineering, Rensselaer Polytechnic Institute, 110 8th Street, JEC 2049, Troy, New York 12180, United States S Supporting Information *

ABSTRACT: The autoignition of methyl 9-decenoate and a mixture of methyl 5-decenoate and methyl 6-decenoate [methyl 5(6)-decenoate], biodiesel fuel surrogate compounds structurally representative of unsaturated compounds found in fatty acid methyl ester (FAME) fuels, has been studied using the shock tube technique. Measurements of ignition delay times were made in reflected shock-heated gases at pressures around 20 atm for fuel/air mixtures at equivalence ratios of 0.5, 1.0, and 1.5 and at a wide range of temperatures from 700 to 1300 K, spanning a range of temperatures relevant to autoignition in compression ignition engines. A comparison of ignition delay times for the methyl decenoates and methyl decanoate illustrates the influence of the presence and location of a double bond within the methyl ester carbon chain on low- and high-temperature reactivities. At high temperatures (>900 K), the three surrogate components have indistinguishable ignition delay. While in the negative temperature coefficient (NTC) and low-temperature regimes ( methyl 3-nonenoate. They hypothesized the lower low-temperature reactivity of unsaturated esters occurs through a reduction in the number of energetically preferred six- and seven-membered transitionstate rings, because of the presence of the double bonds within the carbon chain, necessary for the rate-limiting isomerization reactions that result in H atom transfers within the carbon chain and lead to low-temperature chain branching: R + O2 ↔ RO2 ↔ QOOH (+O2) ↔ OOQOOH → products + 2OH. 5530

dx.doi.org/10.1021/ef4012593 | Energy Fuels 2013, 27, 5527−5532

Energy & Fuels

Article

the surrogate representation and prediction of biodiesel fuel reactivity.

increased rates of chain propagation by the reduced stability of the ROO adduct, increased HO2 elimination from ROO species, and addition of the Waddington mechanism (see Figure 7).

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ASSOCIATED CONTENT

S Supporting Information *

Experimental data in tabular form (Tables S1−S3). This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation under Grant CBET-1032453.

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REFERENCES

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Figure 7. Low-temperature mechanism for methyl decenoates, adapted from refs 7, 24, and 37−39.

4. SUMMARY Ignition delay time measurements made using a heated shock tube technique are reported for methyl decenoates, potential unsaturated FAME fuel surrogate components, and compared to previous measurements for saturated methyl decanoate. The measurements show that the presence and location of a double bond in the long carbon chain of a FAME have no discernible influence on high-temperature ignition (T > 900 K); however, at low temperatures (T < 900 K), differences of around a factor of 2 were observed for methyl decenoates, depending upon the location of a single double bond. Comparisons to the recent kinetic model by Herbinet et al.24 show that the model captures all of the experimental trends and the high-temperature reactivity very well but deviates from the experimental ignition delay times in the low-temperature region. However, the model does accurately capture the difference in reactivity between methyl 5-decenoate and methyl 9-decenaote. Previous work on n-alkenes suggests that the dependence of the low-temperature reactivity of unsaturated methyl esters upon the location of the double bond within the carbon chain is governed by competition between chain branching through the conventional mechanism involving two molecular oxygen additions and the inhibitive direct elimination of HO 2 from ROO and Waddington mechanism, both of which increase in rate with a reduction in saturated alkyl chain length. The ignition delay times will be valuable for the future development of kinetic models for unsaturated alkyl ester species. The temperature dependence of the reactivity differences observed for the different compounds are of significance for the development of biodiesel fuel surrogates and modeling strategies for diesel engine simulations, because they suggest that, at high temperatures, the degree of saturation for surrogate compounds has no influence on biodiesel fuel reactivity, while at low temperatures, saturation considerations are more important for 5531

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