Submitted to: Energy and Fuels
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 28, 2004
Publication Date: March 20, 2005
Citation: Knothe, G.H., Steidley, K.R. 2005. Lubricity of components of biodiesel and petrodiesel. The origin of biodiesel lubricity. Energy and Fuels. 19(3):1192-1200. 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 is largely a mixture of compounds called fatty acid methyl esters. Fatty acids in the form of triacylglycerols (triglycerides) are the major components of fats and oils. To obtain biodiesel, the oil or fat is subjected to a reaction called transesterification with a chemical class of compounds known as alcohols. Because no chemical reaction such as transesterification is ever 100% complete and usually proceeds through intermediate steps, biodiesel consists not only of the desired material but also contains contaminants in very small amounts. These contaminants are limited in biodiesel standards. It has been assumed that biodiesel imparts lubricity to low-sulfur petroleum-derived diesel fuel when blended with the latter at approximately 1-2% level. However, the present results show that some of the contaminants (free fatty acids, monoacylglycerols = monoglycerides) in biodiesel are the material that cause the lubricity of 1-2% biodiesel blends with low-sulfur petroleum-derived diesel fuel. In pure form, these materials also have better lubricity than the methyl esters comprising biodiesel. Biodiesel needs to be added at 1-2% levels to the petroleum-derived fuel in order for the contaminants to be effective in imparting lubricity. This research impacts anyone who intends to use biodiesel in low-level blends with petroleum-based diesel fuel or who is looking for lubricity-imparting material for petroleum-based diesel fuel by showing which materials in biodiesel are the most effective in causing lubricity.
Technical Abstract: An alternative diesel fuel that is steadily gaining attention and significance is biodiesel which is defined as the mono-alkyl esters of vegetable oils and animal fats. Previous literature states that biodiesel can, at low blend levels, restore lubricity to (ultra-) low-sulfur petroleum-derived diesel (petrodiesel) fuels, which have poor lubricity. This feature has been discussed as a major technical advantage of biodiesel. In this work, the lubricity of numerous fatty compounds is studied and compared to hydrocarbon compounds as they can be found in petroleum-derived diesel fuels. Lubricity was determined by means of the HFRR (high-frequency reciprocating rig; ASTM D 6079, ISO 12156) test. Effects of blending some compounds found in biodiesel and petrodiesel on lubricity were also studied. Dibenzothiophene, a sulfur-containing compound in non-desulfurized petrodiesel, does not enhance lubricity of petrodiesel. Fatty compounds possess better lubricity than hydrocarbons, which is attributed to the polarity-imparting oxygen atoms. Neat free fatty acids, monoacylglycerols and glycerol possess better lubricity than fatty esters due to their free OH groups. Lubricity improves somewhat with chain length and presence of double bonds. To determine which structural features enhance lubricity, beyond the fatty compounds and hydrocarbons several oxygenated C10 compounds were studied by HFRR. The following order of oxygenated moieties enhancing lubricity was established: COOH > CHO > OH > COOCH3 > C=O > C-O-C. Oxygen is more effective than nitrogen and sulfur in enhancing lubricity as results on neat C3 compounds with OH, NH2, and SH groups show. Adding commercial biodiesel improves lubricity of low-sulfur petrodiesel more than neat fatty esters, which indicates that other biodiesel components are responsible for its lubricity enhancement at low blend levels. Adding glycerol to a neat ester, which in turn is added at low blend levels to low-lubricity petrodiesel, did not improve lubricity of petrodiesel. However, adding polar compounds such as free fatty acids or monoacylglycerols increases lubricity of a low-level blend of an ester in low-lubricity petrodiesel. Therefore, some materials (free fatty acids, monoacylglycerols) considered contaminants resulting from production of biodiesel are the species responsible for the lubricity of biodiesel at low blend levels with (ultra-)low sulfur diesel fuels. Commercial biodiesel is required at levels of 1-2% in low-lubricity petrodiesel, which exceeds typical additive levels, in order to achieve lubricity-imparting additive level of the biodiesel contaminants in petrodiesel.