What's so great about biofuels? They're made from plants, but how are they carbon neutral?
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Biofuels exploit the fact that crops breathe in carbon dioxide.
Once they are harvested, these crops are either fermented to make ethanol, or their oils - in plants such as coconuts, ground nut and palm - are used directly as simple fuel substitutes (with a few minor chemical additions).
This latter type of biofuel is known as biodiesel because it can be run in standard diesel engines.
Ethanol - or bioethanol as crop-based alcohol is usually known - is either added to standard petrol to reduce fossil fuel consumption or it is used as a fuel on its own.
Car engines can be made to burn pure bioethanol or mixes of bioethanol and petrol. The crucial point is that in burning either type of biofuel, there should be no overall addition of carbon dioxide to the atmosphere.
For example, in the case of bioethanol, carbon dioxide is breathed in by crops; these are then fermented, burned and distilled; and finally, the resulting alcohol is burned in engines, returning the carbon dioxide to the atmosphere.
Experts have argued that by clearing forest for biofuel crops, more carbon dioxide is released into the air than is absorbed by biofuel crops.
However, second generation biofuels that are harvested from crop waste would adhere to the principle of no overall carbon dioxide gain.
By contrast, standard petrol and diesel are extracted from crude oil that is pumped up from reservoirs created millions of years ago.
Burning them adds to carbon dioxide levels in the atmosphere. As a result, amounts of the gas have risen from the pre-industrial 280 parts per million in the atmosphere to their current level of around 380 ppm, a change that is inducing climatic alterations across the planet.