Biodiesel More Mutagenic Than Swedish Class 1: Study

Diesel Fuel News, March 5, 2001 by Jack Peckham

Rape methyl ester (RME) biodiesel, a widely used "renewable fuel" in Europe, produces more mutagenic substances in exhaust than the ultra-low-sulfur, low aromatics, ultra-low polyaromatics Swedish Environmental Class 1 (MK1) diesel, according to a recent study.

"The disadvantages of renewable fuels in relation to new conventional fuels have been neglected in research and development," according to a reactor study at prestigious Chalmers University of Technology in Sweden.

"The advantages of renewable energy are advocated strongly by their proponents urging for a quick and subsidized market introduction. Rapeseed oil and RME have a potential for forming high amounts of many toxic organic compounds at comparatively low temperatures," the researchers found.

Asked to explain what prompted the research, Chalmers University associate professor Jim Olsson told Diesel Fuel News that there was a lack of investigations comparing RME and Swedish MK1 in which organic compounds -- alkenes, dienes, aldehydes, and others -- were measured in detail simultaneously.

A key to explaining the high toxic carbonyl emissions of biodiesel exhaust compared to ultra-clean petroleum diesel is biodiesel's thermal instability, he said.

"This instability means that RME has a tendency to produce more deposits in the engines compared to diesel oil," Olsson said. "It also means that especially during cold start and idling conditions, RME will have a tendency for producing harmful and toxic substances such as 1,3 butadiene, benzene, acrolein, as we have found."

This becomes especially important for diesel engine duty cycles where idling is a high portion of total operation. That includes duty cycles typical of city buses, railroad locomotives, delivery trucks and similar operations.

The reactor results at Chalmers mirror those of an engine study on biodiesel in 1997, reported by researchers at VTT Energy in Espoo, Finland (VTT research report ENE724305-97). The Swedish biodiesel company Svenska Ecobransle AB financed the VTT study.

VTT Energy carried out a fairly comprehensive investigation to assess the emissions from two different types of diesel engines. That study showed that exhaust mutagenicity was up to a factor of three times higher using RME compared to Swedish MK1 diesel. The VTT report also shows that emissions of harmful aldehydes such as acrolein are up to a factor of five higher running RME compared to running Swedish MK1 diesel.

For the Chalmers research, investigators oxidized rapeseed oil, RME, and Swedish diesel (MKI) at 550[degrees]C in a special reactor that is analogous to solid fuel pyrolysis reactors.

Compounds produced from oxidation were measured directly by a gas chromatographic/mass spectrometry (GC/MS) method.

"Rapeseed oil and RME produced a factor of 10 more 1-alkenes, dienes, and benzene in comparison with diesel MK1 ," the researchers found. "Rapeseed oil produced high amounts of acrolein and other aldehydes. Oxidation of RME produced significant amounts of methyl acrylate (2-propenoic acid, methyl ester," whereas Swedish diesel MK1 produced markedly lower amounts of these toxics, the researchers reported.

"Our results indicate that rapeseed oil and RME fuel react in a similar way during oxidation regarding formation of organic products. Rapeseed oil and RME both contain the same building block of long unsaturated hydrocarbon chains in the fatty acids or fatty acid methyl ester. The high contents of unsaturated organic compounds in rapeseed oil and RME promote formation of reactive intermediates 1-alkenes, dienes and benzene during oxidation. The inherent carbonyl group in rapeseed oil and RME promote formation of aldehydes.

"Many of the reactive intermediate organic compounds produced during oxidation of rapeseed oil and RME have strong ozone-forming potentials. Some of the rapeseed products such as alkenes, 1,3 butadiene and benzene are also considered to be carcinogenic."

In response to the Chalmers report, Svenska Ecobransle put out a press release saying that typical diesel engine combustion temperatures are higher than the reactor temperatures used in the Chalmers study. Thus the Chalmers conclusions are "not relevant," Ecobransle claims.

However, Ecobransle's own press release cites the same VTT engine study that Chalmers cites, and Ecobransle's own chart shows that RME mutagenicity is almost a factor of three times higher than Swedish MK1 diesel.

Still, the RME blend showed lower PAH emissions and slightly lower formaldehyde emissions, although higher NOx emissions than diesel, Ecobransle's summary of the VTT report shows.

"It is clear that RME as a fuel in diesel engines have both benefits and drawbacks. RME will never be able to completely replace diesel oil simply because of the limiting acreage available for the growing of rapeseed," Ecobransle conceded.

On the other hand, a recent report from the German UBA (equivalent to U.S. EPA) concludes that "from the environmentalist point of view, the use of rapeseed oil and rape oil methyl-ester (RME) in diesel engines has no distinct advantages over the use of diesel fuel made from mineral oil.