Understanding Trends in Autoignition of Biofuels: Homologous Series of Oxygenated C5 Molecules
Abstract
Oxygenated biofuels provide a renewable, domestic source of energy that can enable adoption of advanced, high-efficiency internal combustion engines, such as those based on homogeneously charged compression ignition (HCCI). Of key importance to such engines is the cetane number (CN) of the fuel, which is determined by the autoignition of the fuel under compression at relatively low temperatures (550-800 K). For the plethora of oxygenated biofuels possible, it is desirable to know the ignition delay times and the CN of these fuels to help guide conversion strategies so as to focus efforts on the most desirable fuels. For alkanes, the chemical pathways leading to radical chain-branching reactions giving rise to low-temperature autoignition are well-known and are highly coincident with the buildup of reactive radicals such as OH. Key in the mechanisms leading to chain branching are the addition of molecular oxygen to alkyl radicals and the rearrangement and dissociation of the resulting peroxy radials. Prediction of the temperature and pressure dependence of reactions that lead to the buildup of reactive radicals requires a detailed understanding of the potential energy surfaces (PESs) of these reactions. In this study, we used quantum mechanical modeling to systematically compare the effects of oxygen functionalitiesmore »
- Authors:
-
[1];
[1];
[1];
[1];
[1];
- National Renewable Energy Lab. (NREL), Golden, CO (United States)
- National Renewable Energy Lab. (NREL), Golden, CO (United States)s
- Publication Date:
- Research Org.:
- National Renewable Energy Lab. (NREL), Golden, CO (United States)
- Sponsoring Org.:
- USDOE Office of Energy Efficiency and Renewable Energy (EERE)
- OSTI Identifier:
- 1373677
- Report Number(s):
- NREL/JA-5100-68971
Journal ID: ISSN 1089-5639
- Grant/Contract Number:
- AC36-08GO28308
- Resource Type:
- Accepted Manuscript
- Journal Name:
- Journal of Physical Chemistry. A, Molecules, Spectroscopy, Kinetics, Environment, and General Theory
- Additional Journal Information:
- Journal Volume: 121; Journal Issue: 29; Journal ID: ISSN 1089-5639
- Publisher:
- American Chemical Society
- Country of Publication:
- United States
- Language:
- English
- Subject:
- 09 BIOMASS FUELS; biofuels; autoignnition; oxygenation
Citation Formats
Ciesielski, Peter N., Robichaud, David J., Kim, Seonah, McCormick, Robert L., Foust, Thomas D., Nimlos, Mark R., and Bu, Lintao. Understanding Trends in Autoignition of Biofuels: Homologous Series of Oxygenated C5 Molecules. United States: N. p., 2017.
Web. doi:10.1021/acs.jpca.7b04000.
Ciesielski, Peter N., Robichaud, David J., Kim, Seonah, McCormick, Robert L., Foust, Thomas D., Nimlos, Mark R., & Bu, Lintao. Understanding Trends in Autoignition of Biofuels: Homologous Series of Oxygenated C5 Molecules. United States. https://doi.org/10.1021/acs.jpca.7b04000
Ciesielski, Peter N., Robichaud, David J., Kim, Seonah, McCormick, Robert L., Foust, Thomas D., Nimlos, Mark R., and Bu, Lintao. Wed .
"Understanding Trends in Autoignition of Biofuels: Homologous Series of Oxygenated C5 Molecules". United States. https://doi.org/10.1021/acs.jpca.7b04000. https://www.osti.gov/servlets/purl/1373677.
@article{osti_1373677,
title = {Understanding Trends in Autoignition of Biofuels: Homologous Series of Oxygenated C5 Molecules},
author = {Ciesielski, Peter N. and Robichaud, David J. and Kim, Seonah and McCormick, Robert L. and Foust, Thomas D. and Nimlos, Mark R. and Bu, Lintao},
abstractNote = {Oxygenated biofuels provide a renewable, domestic source of energy that can enable adoption of advanced, high-efficiency internal combustion engines, such as those based on homogeneously charged compression ignition (HCCI). Of key importance to such engines is the cetane number (CN) of the fuel, which is determined by the autoignition of the fuel under compression at relatively low temperatures (550-800 K). For the plethora of oxygenated biofuels possible, it is desirable to know the ignition delay times and the CN of these fuels to help guide conversion strategies so as to focus efforts on the most desirable fuels. For alkanes, the chemical pathways leading to radical chain-branching reactions giving rise to low-temperature autoignition are well-known and are highly coincident with the buildup of reactive radicals such as OH. Key in the mechanisms leading to chain branching are the addition of molecular oxygen to alkyl radicals and the rearrangement and dissociation of the resulting peroxy radials. Prediction of the temperature and pressure dependence of reactions that lead to the buildup of reactive radicals requires a detailed understanding of the potential energy surfaces (PESs) of these reactions. In this study, we used quantum mechanical modeling to systematically compare the effects of oxygen functionalities on these PESs and associated kinetics so as to understand how they affect experimental trends in autoignition and CN. The molecules studied here include pentane, pentanol, pentanal, 2-heptanone, methylpentyl ether, methyl hexanoate, and pentyl acetate. All have a saturated five-carbon alkyl chain with an oxygen functional group attached to the terminal carbon atom. The results of our systematic comparison may be summarized as follows: (1) Oxygen functionalities activate C-H bonds by lowering the bond dissociation energy (BDE) relative to alkanes. (2) The R-OO bonds in peroxy radicals adjacent to carbonyl groups are weaker than corresponding alkyl systems, leading to dissociation of ROO radicals and reducing reactivity and hence CN. (3) Hydrogen atom transfer in peroxy radicals is important in autoignition, and low barriers for ethers and aldehydes lead to high CN. (4) Peroxy radicals formed from alcohols have low barriers to form aldehydes, which reduce the reactivity of the alkyl radical. In conclusion, these findings for the formation and reaction of alkyl radicals with molecular oxygen explain the trend in CN for these common biofuel functional groups.},
doi = {10.1021/acs.jpca.7b04000},
journal = {Journal of Physical Chemistry. A, Molecules, Spectroscopy, Kinetics, Environment, and General Theory},
number = 29,
volume = 121,
place = {United States},
year = {Wed Jul 05 00:00:00 EDT 2017},
month = {Wed Jul 05 00:00:00 EDT 2017}
}
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