Carbon Balance
The synthetic route uses oil as the source of ethene feedstock. This means that combustion will add carbon dioxide to the atmosphere, and is one reason that interest in ethanol-powered transport is lower in the UK than in the US.

In contrast, fermenting and burning ethanol has the potential to be carbon neutral, as carbon dioxide is fixed through photosynthesis by the plants used as feedstock for fermentation. Energy is needed, however, for distillation and if this is from carbon-based fuel it will generate some additional carbon dioxide.

Some of the issues surrounding ethanol are concerned with its use as a fuel additive or petrol/diesel substitute. The choice of synthetic or fermentation route has implications for the impact on atmospheric carbon dioxide levels and on the use of agricultural land. This is of particular interest in the US where there is strong movement to promote ethanol-powered vehicles.

Land Balance
In order to increase the output of fuel ethanol (as a replacement for petrol), large areas of agricultural land would have to be given over to growing crops such as corn and sugar cane. For many countries this land area may not be available, or may be better used in growing food supplies rather than industrial feed crops.

Large-scale production of biomass for fermentation may also require the use of large amounts of fertilisers, pesticides and herbicides, and the drive for new agricultural land could promote the clearance of forests, important carbon reservoirs.

diagram: carbon dioxide

Biotechnology research is opening the way for using waste biomass, rather than purpose-grown crops, making use of modified bacteria. This may help overcome some of the problems of land use, and retain the advantages of the fermentation route.

photo: sugar cane for ethanol production

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