Science - February 17, 2010

Eco-biofuels from the tropics

Palm oil from South-East Asia, sugarcane from Brazil and sweet sorghum in China are the most sustainable energy crops at present. Maize from the US and wheat in Europe have a much more negative environmental impact. This conclusion was drawn by the Plant Production Systems chair group, after testing nine energy crops against nine sustainability criteria. The findings will be published soon in Biomass and Bioenergy.

Spider diagrams that show the production ecology score of various energy crops
Oil palms, sugar cane and sorghum make the most efficient use of land, water, nitrogen and pesticides in proportion to the amount of energy they produce. If no forest is cut down to make way for their production, they produce far smaller quantities of greenhouse gases than fossil fuels, say the researchers. Major energy crops such as maize in the US and wheat in Europe score much lower on nearly all the sustainability criteria. Sugar beet and oil seed rape (Europe), cassava (Thailand) and soya (Brazil) got an average score.
Crop residues
'We only assessed the production ecology of the crops, and not the socio-economic consequences or the impact on biodiversity', explains author Sander de Vries. 'If you have a tract of virgin land for energy production, your best option is to grow oil palms. Sugar cane is very good too.' An advantage of these crops is that their residues produce enough energy to extract the oil and ethanol. In order to process maize into ethanol, the US currently has to use supplementary fossil fuels, as the crop residues are not used to generate energy. It is also important that enough crop residues are left behind in the fields to keep up the level of organic matter in the soils.
Elephant grass
We are now waiting for a second generation of energy crops, in which the cellulose in the crop residues can be turned into ethanol. Then Europe and the US will also be able to produce much more energy per hectare with crops such as elephant grass, says De Vries. Until then, the best bet is to mix energy from certified sustainable sugar cane and oil palms, he says, as these crops get the most energy out of each hectare, and do the least environmental damage per litre of fuel.
De Vries conducted a literature study and compiled an overview of earlier research on the environmental impact of various energy crops. 'My way of comparing and ranking is original, he says. He points to his spider diagrams which show the production ecology scores of the energy crops at a glance.
The article in Biomass and Bioenergy.