Although there are numerous benefits associated with the use of biodiesel, the fuel poses some unique challenges. Unlike traditional fossil-based fuels, biodiesel stored under certain conditions is prone to oxidation, which degrades the fuel's quality. Biodiesel Magazine speaks to researchers who are developing innovative and affordable ways to address this issue.

A collaborative research project recently undertaken by two Iowa-based companies has shown that antioxidant additives can be highly effective in maintaining the stability of biodiesel fuels under conditions designed to accelerate the degradation process. The project has been spearheaded by biodiesel producer Renewable Energy Group Inc. and Kemin Industries Inc., a company that specializes in providing shelf-life stability solutions to the food and feed markets.

According to Jennifer Radosevich, Kemin's vice president of research and development, the main objective of the study was to test the effectiveness of a proprietary antioxidant Kemin developed specifically for the biodiesel industry.

"Oxidation is basically the generation of free radicals," Radosevich says. "Free radicals are very, very reactive, especially on fats. What happens is one free radical will react with a fat molecule or, in this case, the biodiesel, and produce another free radical, which produces another free radical." These free radicals continue to propagate, leading to oxidative degradation of the biodiesel.

"What antioxidants do is they react with these initiating molecules, or react with the free radicals, to stop this propagation reaction," she continues. "They quench the reaction. They sacrifice themselves so that you can stop oxidation and stop the degradation of the product."

Under real world conditions, the actual rate of this degradation process depends on a variety of factors, says Dave Slade, REG's quality and technical service manager. The rate of degradation will vary depending on how, where and under what conditions the fuel is stored and handled. Exposure to heat, oxygen and moisture accelerates oxidation. However, even under stressful conditions, it often takes months for this degradation process to significantly impact fuel quality.

"Most biodiesel produced in the U.S. has a stability that is higher than that required by ASTM standards," says Slade. "But occasionally it won't, and occasionally after it's been transferred a few times and come into contact with moisture, heat and air…that stability can start to go down. At that point, it becomes a concern for the producer and supplier of the fuel to meet the [stability] specification at all times." What the antioxidant additives do is improve the stability of the fuel so that it continues to meet specifications for a longer time by reducing the rate of degradation.

In its study, Kemin tested four samples of B100 provided by REG. The proprietary antioxidant was added to two samples of fresh biodiesel. One was treated with 500 parts per million (ppm) of the antioxidant, and the other at a lower rate of 250 ppm.

The two pretreated samples and two untreated samples were then stored at a temperature of 43 degrees Celsius (109 degrees Fahrenheit) for 10 weeks. "The reason this temperature was chosen is that it is a standard temperature used in the fuel industry for an accelerated storage study," Radosevich says. "It represents very warm conditions that might be experienced in the summer."

After six weeks, the researchers attempted to 'rescue' one untreated sample by treating it with 500 ppm antioxidant. At the conclusion of the 10-week study, the samples were tested to determine the effects the antioxidant additives had on the oxidation process.

The researchers found that pretreatment with both 500 ppm and 250 ppm antioxidant was effective at preventing oxidation changes in the fuel. "Although 500 ppm was more effective than 250 ppm, both scored better than untreated," Radosevich says. "Looking at the rescue treatment was very interesting. The rescue treatment of 500 ppm antioxidant added at six weeks did increase the oxidative stability in relation to the never-treated fuel. However, it did not increase the stability to the same point as the time-zero treated material. You can rescue it a little bit, but it's not as good as if you add it to the untreated material earlier. Earlier treatment is better."

Final results showed pretreatment of the biodiesel with 500 ppm antioxidant was most effective in mitigating the effects of oxidation. Pretreatment with 250 ppm was the next effective, followed by the rescue treatment of 500 ppm in week six of the study. Untreated biodiesel degraded the most, meaning it's more effective to pre-treat the fuel with a low level of antioxidant than it is to try to rescue the fuel with a higher level of the additive.

"The study showed that once oxidation begins, you can't reverse it," says Gen Meier, REG's director of technology and feedstock development. "The earlier you treat the fuel, the more effective it is."

According to Slade, REG has offered antioxidant additives to customers for more than a year. However, the company generally doesn't treat the fuel unless requested by the customer. "Most of the fuel REG produces is used within weeks, making it unnecessary to treat all fuels," he says. "It tends to take months for biodiesel to degrade."

One benefit of the antioxidant developed by Kemin is its price. Considering the low inclusion rate of the product that is needed to mitigate the effects of oxidation, it won't be prohibitively expensive for customers to use. "That's one reason we wanted to work with Kemin," says Meier. "So [our customers] could be comfortable with the level of oxidative stability they wanted without pricing themselves out of the market."

Another benefit of using antioxidants is that they allow customer to pick fuels based on qualifications other than stability, Meier continues. "REG makes biodiesel from a variety of feedstocks," he says. "Some of these feedstocks have more natural antioxidants than others." By using antioxidant additives, REG can take a fuel containing fewer natural antioxidants, and make it equivalent to a fuel that contains more. This allows customers to pick fuel based on other qualifications, such as cetane levels or cloud point without worrying about oxidative stability. "Regardless of the feedstock, we can meet whatever oxidative stability specification they were looking for," Meier says.

Addressing Thermal Decomposition
For transportation biodiesel, the issues of heat and oxidation are generally associated with storage. However, those researching the use of biodiesel fuel in aviation applications face a different set of challenges. This is because aviation fuels are also subjected to high temperatures during use. In order to use biodiesel-based fuels in the aviation industry, researchers must be able to find a way to mitigate the effects of thermal decomposition.

Researchers at the National Institute of Standards and Technology discovered a way to overcome this problem while developing methods to overcome biodiesel's instability in distillation tests. In late 2008 researchers led by Thomas J. Bruno, NIST's leader of Properties for Process Separations Group, along with students Arron Wolk and Alex Naydich, published a paper in the Feb. 19, 2009 issue of Energy & Fuels titled, "Stabilization of Biodiesel Fuel at Elevated Temperature with Hydrogen Donors: Evaluation with the Advanced Distillation Curve Method". The paper describes the team's findings.

"We found two things," says Bruno. "We found that we can use the advanced distillation curve method to assess the stability at high temperatures of B100, and we found we can use it to determine that certain kinds of additive species are effective at high temperature."

Traditional distillation curves work by providing a measurement of the percentage of a liquid that evaporates as the sample is heated. Different components in a fuel mixture have different boiling points, which allow researchers to determine how much of each component each fraction of the liquid contains. The advanced distillation curve method developed by NIST researchers expands on the test's capabilities by providing low uncertainties and allowing for analysis of the chemical composition of each fraction. The curve itself is represented visually by plots on a graph. When researchers can replicate these data points with multiple samples of a fuel, they know the distillation curve is accurate. This is represented by a low standard deviation, typically about 0.4 degrees.

When the research team attempted to analyze B100 using this method, they experienced a high rate of variability even when several tests were run on the same biodiesel sample lot. The standard deviation was up to 8 degrees.

"What we theorized was happening was that [the B100] was oxidatively decomposing because of the high temperature," says Bruno. To overcome this thermal decomposition effect, the researchers effectively removed the oxygen from the testing environment by blanketing the test fluid with argon gas. "Low and behold, what happened was all of a sudden we were able to measure distillation curves that were right on top of each other again," he continues. This means that the standard deviation was once again within an acceptable range. The researchers had mitigated the effects of thermal decomposition and were able to accurately plot the distillation curve of the B100.

The researchers further theorized that adding hydrogen donors to the B100 would increase the stability of the biodiesel and allow them to produce distillation curves with similar standard deviations, even when the samples were tested without the blanket of argon gas.

"Hydrogen donors are species that when added to fuel at high temperature will decrease free radical reactions by scavenging oxygen," says Bruno. "They do this by donating hydrogen. What we decided to do was determine whether or not you could stabilize B100 with a hydrogen donor, and we decided to use the distillation curve approach to actually assess the stability."

The team tested three hydrogen donors; tetrahydroquinoline, tetrahydronaphthalene and trans-decahydronaphthalene. While all three were effective to a degree, tetrahydroquinoline and trans-decahydronaphthalene were most effective in reducing the standard deviation to levels similar to those of the B100 blanketed in argon gas. This is of practical importance because aviation fuel is often re-circulated and used as a coolant in the engine before it is burned, according to Bruno.

"If you were to ever use something like B100 as an aviation fuel, this would be of importance," Bruno says. "You could not use straight B100 unless you deoxygenate it somehow as an aviation fuel, because it is just not stable enough…You have to preserve its chemical integrity for a bit longer and at a bit higher temperature than simple B100 will provide you without an additive."

Bruno says he hopes to complete additional research on commercially produced bio-based jet fuel in the future. "The difficulty has been in getting samples of it," he says. He is currently looking to collaborate with companies producing bio-based aviation fuels.

Erin Voegele is an associate editor for Ethanol Producer Magazine. Reach her at evoegele@bbiinternational.com or (701) 373-8040.