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United States Department of Agriculture

Agricultural Research Service


item Knothe, Gerhard - Gary
item Sharp, Christopher
item Ryan, iii, Thomas

Submitted to: Energy and Fuels
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/10/2005
Publication Date: 11/30/2005
Citation: Knothe, G.H., Sharp, C.A., Ryan, III, T.W. 2005. Exhaust emissions of biodiesel, petrodiesel, neat methyl esters, and alkanes in a new technology engine. Energy and Fuels. 20(1):403-408.

Interpretive Summary: Biodiesel is an alternative diesel fuel derived from vegetable oils such as soybean oil or other sources such as animal fats and waste frying oils. It consists mainly of derivatives termed methyl esters of fatty acids, which in turn are found in triacylglycerols (triglycerides), the major components of fats and oils. These methyl esters of fatty acids have some features in common with those components of petroleum-based diesel fuel that make petrodiesel suitable as fuel. In this work, biodiesel and petrodiesel fuels as well as some of their components in neat form were examined as to their effect on exhaust emissions. For the first time, a newer technology engine employing a technology known as exhaust gas recirculation was used. This is significant because of future regulations calling for reduced exhaust emissions, which require new technologies. Most significantly, biodiesel and its components were found to reduce an emissions type known as particulate matter (PM) well below the levels of petrodiesel and its components. The levels of PM with biodiesel and its components were only slightly above regulations taking effect in 2007. This may result in further research if an emissions-reducing technology known as particulate traps is even necessary when operating a modern engine on biodiesel. The results may also have significance for designing biodiesel fuels optimized for their components.

Technical Abstract: Biodiesel is a renewable, alternative diesel fuel of domestic origin derived from a variety of fats and oils by a transesterification reaction, thus consisting of the alkyl esters, usually methyl esters, of the fatty acids comprising the parent oil or fat. An advantage of biodiesel is its potential to significantly reduce most regulated exhaust emissions, including particulate matter (PM), with the exception of nitrogen oxides (NOx). In this work, three neat fatty acid methyl esters, methyl laurate, methyl palmitate, and technical grade methyl oleate, were selected for exhaust emissions testing in a heavy-duty 2003 six-cylinder, 14L diesel engine with exhaust gas recirculation. These fuels were compared with neat dodecane and hexadecane as well as commercial samples of biodiesel and low-sulfur petrodiesel, thus establishing for the first time a baseline of the exhaust emissions of neat hydrocarbon (alkane) fuels vs. neat methyl esters. PM emissions were significantly reduced with biodiesel and methyl oleate (about 77 and 73%, respectively), while reductions with methyl palmitate and methyl laurate were even greater at around 82-83% compared to commercial petrodiesel as base fuel. PM emissions with biodiesel were only slightly above upcoming emissions standards, which raises the possibility that biodiesel may meet these standards by using only a diesel oxidation catalyst without employing a particulate trap. NOx emissions increased with biodiesel (about 12%) and technical grade methyl oleate (about 6%) but decreased with methyl palmitate and methyl laurate in the range of 4-5% relative to the base fuel. PM emissions decreased with both dodecane and hexadecane in the range of 45-50%. NOx emissions were also reduced by dodecane and hexadecane in the range of 15.5-16% compared to the petrodiesel base fuel. Also, the methyl ester moiety influences exhaust emissions by reducing particulate matter considerably more than neat straight-chain hydrocarbons, which are enriched in "clean" petrodiesel fuels, while NOx exhaust emissions, which showed little chain-length dependence, are less reduced. Thus no future "clean" petrodiesel fuel should be able to achieve the low PM exhaust emissions levels of biodiesel without some kind of additional additive treatment or support by engine technology. Unsaturated fatty esters show slightly increased NOx and particulate emissions compared to their saturated counterparts. The soluble organic fraction of the PM emissions was higher for the ester fuels. Hydrocarbon (HC) and CO exhaust emissions were also determined. Although HC emissions were low, a strong effect of chain length was observed.

Last Modified: 8/24/2016
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