New NIST Method Accelerates Stability Testing of Soy-Based Biofuel; Validates Performance of Three Additives for High-Temp Stabilization

14 January 2009

Researchers at the National Institute of Standards and Technology (NIST) adapted a new technique to accelerate stability testing of biodiesel fuel and used the method to assess the performance of three additives that enhance soybean biodiesel stability at high temperatures. The results are described in a paper published online 2 January in the ACS journal Energy & Fuels.

Both oxidation and heating can cause biodiesel to break down, adversely affecting performance. These two effects usually are analyzed separately, but NIST chemists developed an advanced distillation curve method to approximate both effects at the same time while also analyzing fluid composition. NIST’s advanced distillation curve (ADC) metrology could accelerate and simplify testing of biodiesels, according to lead author Tom Bruno.

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 unstable fluids such as biodiesels, 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. These data 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.

Antioxidants often are added to vegetable oils to retard oxidation during storage. The study focused on three compounds that help neutralize highly reactive free radicals formed at temperatures above 300 °C.

Hydrogen donor fluids are fluids or solvents that are capable of providing hydrogen to a process such as coal liquefaction. In such a process, they enable the conversion of heavier residuals into distillable fractions, and have also been used to prevent the formation of coke deposits by heavier residuals. The hydrogen donor capability of some of these fluids has been used to stabilize aviation fuels.

Hydrogen donors act to cap aliphatic radicals formed at temperatures in excess of 300 °C (the limit discussed earlier [thermal decomposition of a fuel begins between 250 °C and 350 °C in the absence of oxygen]. In so doing, C₂ and C₃ alkyl aromatic compounds are typically formed.

—Bruno et al. (2009)

Bruno and his colleagues used the ADC technique to show that three hydrogen donor molecules (1,2,3,4-tetrahydroquinoline (THQ), 1,2,3,4-tetrahydronaphthalene (tetralin), and trans-decahydronaphthalene (t-decalin)) stabilize biodiesel fuel at these elevated temperatures.

Test results showed that all three compounds stabilized biodiesel. As expected from studies of aviation fuels, THQ and t-decalin perform similarly and outperform tetralin. For solutions containing 1% additive, THQ performed best overall.

The NIST work may be the first to enhance stability of biofuel at high temperatures, Bruno said.

We recognize that our work here on a particular B100 fluid is limited to this one fluid, and in this respect this report must be considered preliminary. We further recognize that there are many variations of B100; indeed we have produced and tested our own such fluid made from olive oil. Here, we have chosen as a demonstration of the technique a commercially available soy based B100 that is considered typical.

—Bruno et al. (2009)

In the near future, Bruno plans to incorporate a specific oxygen analysis into the ADC metrology to further characterize distillate fractions.

Resources

• Thomas J. Bruno, Arron Wolk and Alexander Naydich (2009) Stabilization of Biodiesel Fuel at Elevated Temperature with Hydrogen Donors: Evaluation with the Advanced Distillation Curve Method. Energy Fuels, Article ASAP doi: 10.1021/ef800740d