NIST develops faster biodiesel test method

Web exclusive posted Jan. 15, 2009, at 2:43 p.m. CST

The National Institute of Standards and Technology (NIST) has developed a method to accelerate stability testing of biodiesel fuel and identified additives that enhance stability at high temperatures.

The results of the NIST work are described in the paper titled "Stabilization of biodiesel fuel at elevated temperature with hydrogen donors," and authored by T.J. Bruno, A. Wolk and A. Naydich. The paper is available online.

Both oxidation and heating can cause biodiesel to break down, adversely affecting performance. These two effects usually are analyzed separately, but NIST chemists developed a method to approximate both effects at the same time while also analyzing fluid composition. NIST's advanced distillation curve method could accelerate and simplify testing of biodiesels, according to lead author Tom Bruno. NIST researchers used the new method to demonstrate the effectiveness of three additives in reducing oxidation of biodiesel at high temperatures, as would occur in aviation fuels.

Antioxidants often are added to vegetable oils to retard oxidation during storage. The NIST work may be the first to enhance stability of biofuel at high temperatures, Bruno said. The study focused on three compounds, THQ, t-decalin and tetralin, which help neutralize highly reactive free radicals formed at temperatures above 300 degrees Centigrade (570 degrees Fahrenheit). Test results showed that all three compounds stabilized biodiesel. As expected from studies of aviation fuels, Bruno said, THQ and t-decalin performed similarly and outperformed tetralin. For solutions containing 1 percent additive, THQ performed best overall.

The researchers demonstrated the testing method using a typical soy-based biodiesel. A distillation curve charts the percentage of a mixture that evaporates as a sample is slowly heated. Because the different components of a complex mixture typically have different boiling points, a distillation curve gives a good measure of the relative amount of each component. NIST chemists enhanced the traditional technique by improving precision and control of temperature measurements and adding the capability to analyze the chemical composition of each boiling fraction.

To adapt the method for biodiesel, the authors made repeated distillation curves of samples and quantified the variation in parameters, such as temperature, for each distillate fraction across the different runs of the experiment. The results were averaged over the entire distillation curve to identify the range of variations that might occur. This range was extended to theoretically model the potential oxidative and thermal decomposition of the samples.