Optimising the biodiesel fuel process

In these times of rising fuel prices and changing climate, the search for alternatives to fossil fuels has gained high priority. Franhofer researchers have developed a new process for converting rapeseed oil into biodiesel. Enzymes make it possible.

Advertisers are fond of marketing biodiesel by displaying fields of blooming rapeseed, accompanied by the slogan 'CO2 neutral'. In actual fact, we are seeing the 'renewable' plants from which most of these types of motor fuel are derived.

The proportion of such fuels being used in the EU is currently around 2%; that figure is estimated to grow to 5.75% by 2010.

Of course, even using biodiesel as an alternative does not entirely eliminate the emission of greenhouse gases. Mowers, threshers, oil mills and tanker trucks all need energy, as does the process of transesterification necessary before rapeseed oil can be burned in unmodified diesel engines.

Composed primarily of glycerin and fatty acids, the rapeseed oil is mixed with sodium or potassium hydroxide and methanol, then heated. The alkaline solution separates the glycerin from the fatty acids, which bond with the methanol to form rapeseed methyl ester.

One disadvantage of the conventional process is that the end product contains about 10% glycerin by weight. Rather than dissolving into the diesel fuel, the glycerin has to be separated out just like other by-products. This represents yet another energy-consuming step in the process.

Researchers at the Franhofer Institute for Process Engineering and Packaging IVV in Freising are investigating a completely different method in which enzymes attached to a substrate catalyse the transesterification process. The key difference between this and the conventional method is that the rapeseed oil is not completely converted. Instead, some of the fatty acids remain chemically bonded to the glycerin, which renders the glycerin soluble in diesel fuel - thus eliminating the 'glycerin problem' and increasing yield by 10%.

Additional advantages of the process are that it needs about a third less alcohol, it can use ethanol instead of toxic, synthetically-produced methanol, it requires no acids or lyes - eliminating soap build-up - and it generates less wastewater contaminated with oil residues. "Our preliminary experiments showed that the process is suitable for partial transesterification of plant oils," comments Dr Peter Eisner of the IVV.

"A preliminary analysis of lab products indicates that the fuel has excellent properties." In partnership with the department of Energy and Environmental Technology in the Food Industry at Munich Technical University and several mid-sized companies, the IVV researchers are applying to the BMBF for a follow-up project to spur on the development and optimisation of the new fuel.