VEGETABLE OIL-BASED FUELS, ADDITIVES AND COPRODUCTS
Location: Bio-oils Research Unit
Title: Fuel properties of highly polyunsaturated fatty acid methyl esters: Prediction of fuel properties of algal biodiesel
Submitted to: Energy and Fuels
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 28, 2012
Publication Date: August 20, 2012
Citation: Knothe, G.H. 2012. Fuel properties of highly polyunsaturated fatty acid methyl esters: Prediction of fuel properties of algal biodiesel. Energy and Fuels. 26(8):5265-5273.
Interpretive Summary: Biodiesel is a renewable alternative to diesel fuel obtained from petroleum. It has usually been produced from vegetable oils such as soybean, sunflower, rapeseed, or palm oils as well as animal fats and used cooking oils. To increase the supply of biodiesel, algae which have the potential to produce significant amounts of oil, have found increasing interest as potential biodiesel sources. However, due to issues such as growth and harvesting, little to no biodiesel has yet been produced from algal oils. Accordingly, virtually no fuel properties have been determined experimentally. This work investigates essential fuel properties of biodiesel from algae by studying some pure compounds as they would be expected to occur in algal biodiesel and predicting the properties of others that are not available. In turn, these experimental and predicted properties are used to predict the properties of algal biodiesel fuels which are mixtures of the individual compounds. It is shown that many algal biodiesel fuels will likely have problems regarding oxidative stability, cold flow, and a combustion-related property called the cetane number. Other specifications in biodiesel standards may also present problems. This information is important because it shows that production of algal oils should also address the issue of quality of biodiesel fuels produced from them.
Biodiesel, defined as the mono-alkyl esters of vegetable oils and animal fats, can be derived from other triacylglycerol-containing feedstocks. Especially algae are being considered for this purpose due to their claimed high production potential. However, there are no comprehensive reports regarding the fuel properties of biodiesel obtained from algal oils. Algal oils, with examples of some exceptions also mentioned here, often contain significant amounts of saturated and highly polyunsaturated (greater than or equal to four double bonds) fatty acid chains which influence fuel properties of the resulting biodiesel. In this connection, the relevant fuel properties of biodiesel from algal oils and the important fuel properties of highly polyunsaturated fatty acid methyl esters as they would occur in many biodiesel fuels obtained from algal oils, have not yet been reported. To fill this gap, in the present work, for the first time, two neat highly polyunsaturated fatty acid methyl esters with more than three double bonds, methyl 5(Z),8(Z),11(Z),14(Z)-eicosatetraenoate (C20:4) and methyl 4(Z),7(Z),10(Z),13(Z),16(Z),19(Z)-docosahexaenoate (C22:6), were investigated. The cetane numbers were determined as 29.6 for C20.4 and 24.4 for C22:6. Kinematic viscosity values were observed as 3.11 mm2/s for C20:4 and 2.97 mm2/s for C22:6 while oxidative stability values were below 0.1 h for both by the Rancimat test and densities were above 0.9 g/cm3. Two polyunsaturated C20 methyl esters, methyl 11(Z),14(Z)-eicosadienoate and 11(Z),14(Z),17(Z)-eicosatrienoate, were also studied for kinematic viscosity, density, and oxidative stability to expand the database on these properties of C20 compounds and provide additional data to predict the properties of other highly polyunsaturated fatty acid methyl esters. Properties of biodiesel from algal oils are predicted with cetane numbers expected in the low to upper 40's and kinematic viscosity between 3 and 4 mm2/s for most algal biodiesel.