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Written by Rickdatech
Biodiesel can go bad when microbes digest it, when water hydrolyzes it, or when it oxidizes. When bacteria or fungus attacks biodiesel it is always 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. To prevent infection keep your biodiesel dry. 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 will 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 will last before it goes rancid. The NREL suggests that biodiesel be stored no longer than six months, since the older a fuel is, the more likely it is to break down. We can monitor how fuels age and break down with a few simple tests.
Measuring Stability
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) will increase as the fatty acids polymerize and clump together. The Peroxide Value (ASTM D-3703) will increase as free radicals oxidize into peroxide and hydroperoxide. Finally the sediments (ASTM D-2709) will increase as polymerization increases. These tests can tell you how badly the fuel has degraded, but they can’t 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 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. In basic terms the test predicts how quickly we can expect biodiesel (or oil) to go rancid from oxidation.
Auto-Oxidation
When exposed to heat, light and stress, 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 will combine 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 will make 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 rapid radical destruction cycle snowballs. The peroxide radical immediately creates a new free radical from the biodiesel, which in turn binds with 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 unradicalized 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 together 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 will absorb four times as much oxygen as water. Displacing the tank headspace with nitrogen will help 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, light etc. The third approach is to use antioxidants.
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 can't last over a year.
Long Term Storage
Most hobbyists will 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 biodiesel needs to be stored over six months, there are some cheap and easy steps to take to ensure it will last as long as needed.
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Keep your biiodiesel cool.
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Keep your biodiesel dry.
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Keep your biodiesel away from bare metal surfaces.
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Limit the oxygen that can get into your fuel
Monitor your Acid Number and if it starts to go up, add anti-oxidants.
Related Links:
Effect of antioxidants on the oxidative stability of methyl soyate (biodiesel) 
Robert O. Dunn (2005)
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)
Review of Recent Advances in Lipid Oxidation
Edwin N Frankel (1990)
Springboard Biodiesel Oxidative Stabilizer Performance Data
Effect of Oxidation Under Accelerated Conditions on Fuel Properties of Methyl Soyate (biodiesel)
Robert O. Dunn (2002)
Determination of the oxidative stability of biodiesel (fatty acid methyl esters, FAME)
U. Loyall, B. Zumbrägel, M. Kalcher
Stability of BioDiesel and the Iodine Value
Brevard Biodiesel
Infopop Thread on on using an oxidative stabiliser
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Comments
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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 passesd 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.
asking about auto-oxidation and this was his reply:
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I will try and explain what I know about auto-oxidation
(autoxidation) in layman’s terms.
Autoxidation is defined as the direct reaction of molecular oxygen
with organic compounds under mild conditions. Oxidation of lipids
proceeds by a chain reaction mechanism, which is generally described
in terms of three steps: 1) initiation, 2) propagation and 3) termination.
Initiation: In the presence of initiators, an unsaturated lipid is attacked
to form a free radical. Initiators include the presence of heat or light,
metal surfaces, pro-oxidants or oxygen itself. It is important to
remember that autoxidation will eventually occur if the unsaturated
lipid (such as biodiesel) is left exposed to ambient air.
Propagation: The free radical then reacts with molecular oxygen (O2)
in such way as to form hyderperoxides and more free radicals that
propagate the chain reaction. The hydroperoxides are considered
primary degradation products since their decomposition in later
stages of the process leads to formation of secondary non-radical
products. During this step there are several “types” of free radicals
drifting about in the mixture.
Part of this step is reversible and under mild conditions some
time may be required before hydroperoxide concentration will
be sufficient to allow their decomposition to form secondary
products. This is known as the induction period. The presence
of heat, light or metal surfaces can accelerate this process.
Termination: In the latter stages of oxidation there is an abundance
of free radicals that react with each other to form non-radical products.
During this step, hydroperoxides decompose to form epoxides (from
interaction with double bonds) or smaller molecules such as low-
molecular weight (small) aldehydes, ketones, alcohols and hydrocarbons.
Other radicals may combine to form larger polymeric products.
The nature and concentration of the decomposition and combination
products depends on the nature of the original lipid “substrate” and
conditions (heat, light, metals etc.) of the oxidation reaction.
Finally, I followed your link given below and I reviewed your article
on biodiesel stability. The article is written reasonably well considering
you explaining things in layman’s terms. A few notes:
1. Polymers formed from degradation of biodiesel are soluble and
may not respond when tested by ASTM D 2709. This is not a huge
concern since the presence of polymers usually signals some very
“out of spec” biodiesel that has significant degradation.
2. Peroxide value (D 3703) may be useful unless biodiesel is close
to or past its induction period. At that point in the oxidation
reaction peroxide concentrations reach a maximum and begin
to decrease as the peroxides decompose into secondary products.
3. Antioxidants – Some research has shown that binary antioxidants
may be more effective than single ones. Some antioxidants act as
“synergists” that refortify “spent” antioxidant molecules enabling
them to quench other free radicals.
I hope to have provided information that will be useful to you.
Regards,
Robert O. Dunn, Jr.
Robert O. Dunn, Ph.D.
Chemical Engineer
Bio-Oils Research
USDA-ARS-NCAUR
Peoria, IL