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United States Department of Agriculture

Agricultural Research Service

Title: Cetane Numbers of Branched and Straight-Chain Fatty Esters Determined in An Ignition Quality Tester

item Knothe, Gerhard
item Matheaus, Andrew - SWRI,SAN ANTONIO,TX
item Ryan, Thomas - SWRI,SAN ANTONIO,TX

Submitted to: Fuel
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 20, 2002
Publication Date: May 20, 2003

Interpretive Summary: Biodiesel is an alternative diesel fuel derived from vegetable oils such as soybean oil. It can be used both neat and in blends with conventional, petroleum-derived diesel fuel since biodiesel and conventional diesel fuel are miscible at all blend levels. Most exhaust emissions are reduced when using biodiesel compared to conventional diesel fuel derived from petroleum. An exception are nitrogen oxides, also known as NOx. These exhaust emissions are important because they contribute to the formation of ozone, which in turn is a major component of urban smog. It is known that a diesel fuel quality index termed the cetane number (which is conceptually similar to the well-known octane number used for gasoline and is related to the combustion of the fuel) can, within certain limits, be related to such exhaust emissions. Every diesel fuel or component thereof has a cetane number. The same holds for biodiesel. Therefore it is important to determine the cetane numbers of components of biodiesel. Here the cetane numbers of biodiesel components were determined for the first time with a newly developed apparatus termed the Ignition Quality TesterTM (IQTTM). This is important because the IQTTM, which is being commercialized, uses only a small amount of sample and offers rapid, reliable testing compared to previous methods. Also, components of a biodiesel that potentially has better properties in cold weather than the currently used biodiesel were tested. It was shown that these components have cetane numbers comparable to the components of current biodiesel.

Technical Abstract: The cetane number is a widely used diesel fuel quality parameter related to the ignition delay time (and combustion quality) of a fuel. It has been applied to alternative diesel fuels such as biodiesel and its components. In this work, the cetane numbers of numerous straight-chain and branched fatty acid esters were determined. Specifically, 29 samples of esters of methyl, ethyl, n-propyl, n-butyl, iso-propyl, iso-butyl, 2-butyl, and 2-ethylhexyl esters of palmitic, stearic, oleic and linoleic acid were investigated. It was found that branching in the ester moiety does not significantly affect cetane number compared to the straight-chain esters. Therefore, branched esters, which have been suggested as a possible improvement for the cold-flow properties of biodiesel, can be employed without significantly affecting ignition properties compared to the more common methyl esters. Unsaturation in the fatty acid chain was again observed to be the most significant factor causing lower cetane numbers. Cetane numbers were determined in an Ignition Quality Tester (IQT) which is a newly developed, automated rapid method using only small amounts of material for determining cetane numbers. The IQT is as applicable to biodiesel and its components as previous cetane-testing methods.

Last Modified: 4/22/2015
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