Idaho researchers compare biodiesel stability additives

Researchers at the University of Idaho recently released the results of a study that aimed to evaluate the effectiveness of four different oxidative stability additives for biodiesel. The project, sponsored in part by the USDA National Biodiesel Education Program, addressed the impact of antioxidant additives on both new and degraded fuels under U.S. and European specifications.

According to Joe Thompson, research support scientist and manager of the university’s biodiesel lab, his team was especially interested in determining whether it is possible to effectively bring a degraded fuel back into specification using oxidative stability additives. “We were trying to figure out if this was a possibility, and…we were able to actually get a passing score on all of the fuels with at least one [of the four] additives [evaluated],” he said.

To complete the study, Thomspon’s team used four different brands of oxidative stability additive and compared the results. The products featured in the study included Lanxess’s Baynox Plus, Albemarle’s Ethanox 476E, Eastman’s Bioextend and Kemin’s BF 320. Information released by the University of Idaho noted that each additive was tested with fresh fuel at 200 parts per million (ppm) and 500 ppm. Fresh fuels tested included canola methyl ester, canola ethyl ester, mustard methyl ester and soy methyl ester. Each fuel and additive combination was tested under the U.S. ASTM specification for biodiesel and under the European EN specification. According to the study, to meet the ASTM spec, these specifications were tested using a Biodiesel Rancimat instrument, which accelerates the degradation process by heating fuel samples to 110 degrees Celsius and bubbling dry room air through it. To meet the ASTM spec, the fuel sample must remain stable for a minimum of three hours. To meet the more stringent EN specification, the fuel must remain stable for at least six hours.

Results of the study show that all four of the additives, when used at 200 ppm, increased the stability of the fuel enough to meet ASTM specifications. At 500 ppm, three of the samples with additives passed the EN specification. The exception was mustard methyl ester, which passed the European specification with only the addition of Ethanox 4760E.

The study also addresses whether or not these four additives could be used to bring degraded fuel back into spec. To complete that portion of the study, the researchers evaluated their use fuels that had been sitting in storage for approximately two years, including rapeseed ethyl ester, palm methyl ester, tallow methyl ester and mustard methyl ester.

When the antioxidant products were added to the degraded fuels at 200 ppm, three fuel samples were able to meet ASTM specifications with at least one of the additives. No fuel samples were able to meet the EN specification at this level. When the additives were included into fuel samples at 500 ppm, all the fuels met ASTM specifications, and two met EN specifications with the Ethanox additive.

According to the study, most of the antioxidant additive products performed better with new fuel when added to canola ethyl esters when compared to canola methyl esters. However, the additives displayed consistently worse effectiveness on the rapeseed ethyl esters when compared to other aged methyl esters. “Some of the apparent increase in stability for the ethyl ester may be due to their greater molecular weight and thus their lesser number of oxidatively sensitive double bonds per unit of mass,” the researchers state in the study.

Thompson said his team is also working on a follow-up study on distillation. “A lot of companies are now distilling biodiesel, so we tried distilling some biodiesel and then measuring some oxidative stability on the distilled biodiesel,” he said. “It turns out if you distill the biodiesel you have to add an antioxidant because it’s never going to meet the spec without it, because the distilling process destroys all the natural antioxidants that are in the fuel.” According to Thompson, detailed results of the follow-up study should be released near the end of the summer.