By Rick Da Tech
Biodiesel can go bad in three ways:
- when microbes digest it.
- when water hydrolyzes it.
- when it oxidizes.
When bacteria or fungus attacks biodiesel, it always starts at the place where water and biodiesel meet. There are microbial tests available at most fuel testing labs. There are also a few on-site microbial test kits available on the internet. Boats using biodiesel are the most at risk from infection, to prevent it, keep your biodiesel dry. "Boat Coach Bob" has a good article on Preventing and Removing Diesel Algae that covers biodiesel. To get rid of an infection requires the use of an antibiotic fuel treatment.
Hydrolysis is using water to break the bond between the alcohol and the fatty acid. It is the same process that creates FFA in oil. The only way to prevent it is to keep your oil dry.
Unlike the other methods, oxidation requires oxygen rather than water. When unsaturated fatty acids oxidize, they break down producing foul-smelling acids, like butyric acid. While it may be easy to detect rancid oil, it is a great deal more difficult to predict how long a fuel lasts before it goes rancid. The NREL Biodiesel Handling and Use Guide recommend not storing neat biodiesel longer than six months to prevent oxidation. We can monitor how fuels age and break down with a few tests.
When fats, oils, and biodiesel go bad, there are some measurable effects. The Acid Number (ASTM D-664) increases as the fatty acid breaks down into shorter chain acids. The Kinematic Viscosity (ASTM D-445) increases as the fatty acids polymerize and clump together. The Peroxide Value (ASTM D-3703) increases as free radicals oxidize into peroxide and hydroperoxide. Finally, the sediments (ASTM D-2709) increases as polymerization increases. These tests can tell you how severely the fuel has degraded, but they cannot accurately predict the shelf life of biodiesel.
Predicting shelf life is the essence of measuring stability in biodiesel. The primary test used to determine oxidative stability in Europe is called the Rancimat method. Europe included it in their biodiesel standards (DIN EN 14214). In the US ASTM D6751-07 includes a nearly identical Oil Stability Index (OSI) method (AOCS Cd12b-92 (35)). This test calls for passing a pure sample of air through hot oil. Then it is passed through deionized water. The conductivity of the water is continuously sampled and charted against time. This chart is used to calculate the OSI number. For the math geeks, the OSI number is the maximum of the second derivative of conductivity with respect to time. The practical application of the test is that it predicts how quickly we can expect biodiesel (or oil) to go rancid from oxidation.
When exposed to heat, light, or trace metals, biodiesel forms a free radical or an electron imbalance on the double bond of a fatty acid. It is most likely to happen on a specific type of unsaturated fatty acid with two or more double bonds. This radical is highly reactive and combines with any oxygen present. Once the free radical combines with oxygen, a peroxide radical is formed on the fatty acid.
When this peroxide radical bumps into another unsaturated fatty acid it makes the new fatty acid into a radical and change the peroxide into a hydroperoxide then back into a free radical. This conversion of fatty acids into peroxides continues until the peroxide bumps into a radical rather than into an unconverted fatty acid.
The cycle of rapid radical destruction snowballs. The peroxide radical immediately creates a new free radical from the biodiesel, which in turn binds to oxygen. The destructive radical auto-oxidation cycle begins. In this process, the Peroxide Value increases dramatically. Up to 100 new radicals are created quickly from one single radical, meaning that decomposition occurs at an exponentially rapid rate, and the oil spoils and becomes rancid very quickly.
The auto-oxidation process stops making more radicals when it bumps into another radical rather than an un-radicalized fatty acid. When two radicals bump into each other, the fatty acids are damaged either by splitting the fatty acid at the double bond or by connecting the two fatty acids at the radical. Here we form aldehydes, alcohols, and carbonic acids.
There are three approaches to increase the resistance to the process of auto-oxidation. The first is to starve the biodiesel for oxygen. Not an easy task since biodiesel absorbs four times as much oxygen as water. Displacing the tank headspace with nitrogen helps some. The second approach is to prevent the contact with materials and substances that promote or catalyze the reaction of oxidation such as pro-oxidants, trace metals, higher temperature, and light. The third approach is to use antioxidants.
Vegetable oils have natural anti-oxidants in them. For instance, Soy oil usually has 300-500 ppm of Tocopherol (Vitamin E). The food industry has been adding anti-oxidants to food for years, and there are many very effective anti-oxidants available on the internet, such as tert-butylhydroquinone (TBHQ) or butylated hydroxytoluene (BHT), ascorbyl palmitate, tocopherols, butylated hydroxyanisole (BHA) and propyl gallate (PrG).
The synthetic anti-oxidants are typically more effective than the natural ones at resisting oxidation. The three main ways anti-oxidants work is to attack the radicals to make them less reactive, interfere with the chemical reactions that form the radicals, and chemically remove the pro-oxidants like trace metals.
With the addition of anti-oxidants, storage in a cool place, and kept dry, there is no reason biodiesel cannot last over a year.
Long Term Storage
Most hobbyists use their fuel within a few weeks of making it. For them, long-term storage stability is not an issue. In the rare case where you need to store biodiesel longer than six months, there are some cheap and easy steps to take to ensure it lasts as long as necessary.
- Keep your biodiesel cool.
- Keep your biodiesel dry.
- Keep your biodiesel away from bare metal surfaces.
- Limit the oxygen that can get into your fuel
Monitor your Acid Number and if it starts to go up, add anti-oxidants.
Stability of Biodiesel and Biodiesel Blends: Interm Report
R.L. McCormick, T.L. Alleman, J.A. Waynick and S.R. Westbrook, S. Porter (2006)
Analyzing Biodiesel: Standards and Other Methods
Gerhard Knothe (2006)
Storage stability of mahua oil methyl ester
Dilip Kumar Bora, L M Das, adn MKG Babu (2008)
Empirical Study of the Stability of Biodiesel and Biodiesel Blends: Milestone Report
R.L. McCormick, S.R. Westbrook (2007)
Determining the Influence of Contaminants on Biodiesel Properties
Jon H. Van Gerpen, Earl G. Hammond, Lawrence A. Johnson, Stephen J. Marley, Liangping Yu, Inmok Lee, Abdul Monyem (1996)
The effect of fuel additives on biodiesel (B-100) long term oxidative stability and performance
Dr. Ibrahim Abou-Nemeh (2008)
Comparative Oxidative Stability of Fatty Acid Alkyl Esters by Accelerated Methods
Bryan R. Moser (2009)
Infopop Thread on on using an oxidative stabiliser
Comment posted by JohnO Sept 10 2010
I added an empty GoldenRod filter housing to the bottom of the storage tank to act at a sump. It shows anything that drops out of the biodiesel, such as residual soap, glycerin, water, sediment, rust, dirt, etc. Biodiesel that passed 3/27 when new will still "create" some material to collect in the sump, but it takes months. The 250-gallon tank will collect about a pint of material in 6 months - that's enough to clog a fuel filter, but that's only 0.05%, so it passes specs. It's a good reminder not to use the bottom-most fuel from a tank, and that on-spec fuel can still cause problems.