Biodiesel Feedstock Oils
By Rick Da Tech
We can make Biodiesel from any plant or animal-derived oil. However, some oils are better than others for making biodiesel. We can partly describe an oil by its source, like peanut oil, canola oil, soy oil, beef tallow, and on and on, but to fully describe an oil we also need to know more about it. We need to know information like, is it hydrogenated, is it a drying oil, or a non-drying oil. Waste oils need more information, like FFA and water content.
"Organic oils are three long strings of fatty acids attached to a glycerin molecule. The Fatty Acids can vary in length and in how they are bonded or put together. The way we describe Fatty Acids is by the number of carbon atoms in it and the number of double bonds. We typically see between 12 and 26 carbon molecules and zero to three double bonds in our oil.
Different oils have different blends of fatty acids as you can see from the chart below.
Percentage Fatty Acid Composition of Various Oils
|Fat or Oil||12:0||14:0||16:0||18:0||18:1||18:2||18:3||20:0||20:1||22:1|
|Yellow Grease (typical)||.||2||23||13||44||7||1||.||.||.|
|Palm kernel oil||48||16||8||.||15||3||.||.||.||.|
|Pongamia pinnata oil||.||.||4-8||3-9||45-71||11-18||.||2-5||10-12||4-5|
If you look at the top row, the numbers indicate how many carbon atoms are in the oil, a colon then the number of double bonds. The numbers in the middle of the chart are the percentage composition of the fatty acid listed on the left.
Each type of fatty acid has a different freezing point, a different tendency to polymerize (dry into a film), and a different energy content.
Nutritional Makup of Various Oils *
|Palm Kernal Oil||81%||11%||2%|
* note- approximated values - actual values vary depending on the individual plant and extraction methods.
Fatty acids without double bonds are called saturated fats. The lower the saturated fat content, the lower the gel point of the biodiesel made from it.
Fatty acids containing double bonds are called unsaturated. The double bonding site is somewhat unstable and can break off or chemically alter in the presence of heat or water. Unsaturated fats tend to spoil faster than saturated fats. If an oil contains too many double bonded sites, the oil becomes a "drying oil."
Drying oils are not good for making biodiesel as they break down quickly. They usually contain three double bonds per fatty acid. Just to give you an idea, they are used in paints and varnish and dry quickly to form a tough film. Drying oils age and turn acidic quickly, sometimes the biodiesel made from drying oils can degrade overnight. Examples of drying oils include Linseed oil, Walnut oil, and Poppy oil.
Cooking oils make some of the best biodiesel. These are unsaturated oils with a single or double bond per fatty acid. Canola is probably the best oil for making biodiesel, since it ages slowly, remains liquid to low temperatures, and has a high energy content. Olive oil is another good oil for making biodiesel. It has a slightly higher gel point, is slightly less stable, and has about the same energy content as Canola oil due to its slightly increased polyunsaturated and saturated content.
Hydrogenated oils are oils that have been chemically altered to remove the double bonding. The purpose behind hydrogenation is to lengthen the shelf life of cooking oil, but they are unhealthy oils for our bodies. Also, it is difficult to make biodiesel from hydrogenated oils. That is not by accident; the best cooking oils make the best biodiesel. There is a persistent myth on the internet that Chinese restaurants have the best oils for biodiesel, that is true if they use unsaturated, non-hydrogenated oils. Look for restaurants that sell healthy foods; their WVO makes the best biodiesel. Not only do they use the better oils, but they are more likely to be lower in FFA than other restaurants due to their cooking practices.
Cooking practices play a vital role in WVO quality, primarily on FFA content. FFA or Free Fatty Acids form when the long carbon chain breaks away from the glycerin molecule. These fatty acids are acidic and turn into soap when using our normal base catalyst processing method. They can only be turned into biodiesel if we use Acid Esterification. The soap made from oils high in FFA can cause problems in processing. To make matters worse, FFA are hygroscopic, meaning they attract water. Oils high in FFA can contain as much as 50% water.
If the restaurant cooks at too high a temperature, the oil breaks down quickly and forms FFA. They need to keep the oil temperature below 300ºF both for healthy food and good biodiesel. There are two important temperatures when cooking in oil, the flashpoint, and the smoke point. Flashpoint is the lowest temperature at which oil can catch on fire. The smoke point is the temperature at which the oil starts to smoke. Typical smoke points for new oils are around 450ºF, however, as the FFA content increases, both the smoke point and flash point go down. Some restaurants wait to change out their oil until the smoke point drops to their cooking temperature. I completely avoid those restaurants.
If the restaurant filters their oil every night before closing, the oil lasts much longer. It is because the breading and stuff that can be filtered out absorbs water from the atmosphere. The water then reacts with the oil to make more FFA. Filtering has been shown to keep FFA levels below 1ml titrations for up to a week of heavy use. Without filtering, the same oil can reach titration levels of 10 or 15 ml in just a few days.
Food Fats and Oils (2006) ISEO 2006 This publication addresses many technical issues associated with fats and oils including their chemical composition and their role in the diet. It also includes information on nutritional aspects, physical factors, refining and reactions of fats and oils.
Typical Smoke, Flash & Fire Points of Commercially Available Fats and Oils ISEO Smoke, Flash and Fire points of various new oils.
NREL Biodiesel Analytical Methods J. Van Gerpen, B. Shanks, and R. Pruszko - Iowa State University
D. Clements - Renewable Products Development Laboratory
G. Knothe - USDA/NCAUR
The NREL publishes this subcontractor report dated July 2004. Covers Biodiesel testing procedures. 100 pages
Preparation and characterization of bio-diesels from various bio-oils
X. Langa, A. K. Dalai, , a, N. N. Bakhshia, M. J. Reaneyb and P. B. Hertzc
Cold Flow Behavior of Biodiesels Derived from Biomass Sources
C. R. Krishna, Kaitlin Thomassen, Christopher Brown, Thomas A. Butcher, Mouzhgun Anjom, and Devinder Mahajan, Oct 2007
Cloud and pour points in fuel blends
J. A. P. Coutinho, , a, F. Mirantea, J. C. Ribeirob, J. M. Sansotc and J. L. Daridon; Jan 2002
Thermodynamic study on cloud point of biodiesel with its fatty acid composition Hiroaki Imahara, Eiji Minami and Shiro Saka; Mar 2006
Fats, Oils, Fatty Acids, Triglycerides by Antonio Zamora. The second page of a three-page article contains a chart showing the fatty acid composition of various oils similar to the chart above.