What are biofuels?
Biofuels are any kind of fuel made from living things, or from the waste they produce.
This is a very long and diverse list, including:
- wood, wood chippings and straw
- pellets or liquids made from wood
- biogas (methane) from animals' excrement
- ethanol, diesel or other liquid fuels made from processing plant material or waste oil
In recent years, the term "biofuel" has come to mean the last category - ethanol and diesel, made from crops including corn, sugarcane and rapeseed.
Bio-ethanol, an alcohol, is usually mixed with petrol, while biodiesel is either used on its own or in a mixture.
Rudolph Diesel: biofuel pioneer
Pioneers such as Henry Ford and Rudolph Diesel designed cars and engines to run on biofuels. Before World War II, the UK and Germany both sold biofuels mixed with petrol or diesel made from crude oil; the availability of cheap oil later ensured market dominance.
Ethanol for fuel is made through fermentation, the same process which produces it in wine and beer. Biodiesel is made through a variety of chemical processes.
There is interest in trying biobutanol, another alcohol, in aviation fuel.
Are biofuels climate-friendly?
In principle, biofuels are a way of reducing greenhouse gas emissions compared to conventional transport fuels.
Burning the fuels releases carbon dioxide; but growing the plants absorbs a comparable amount of the gas from the atmosphere.
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However, energy is used in farming and processing the crops, and this can make biofuels as polluting as petroleum-based fuels, depending on what is grown and how it is treated.
A recent UK government publication declared that biofuels reduced emissions "by 50-60% compared to fossil fuels".
Where are biofuels used?
Production of ethanol doubled globally between 2000 and 2005, with biodiesel output quadrupling.
Brazil leads the world in production and use, making about 16 billion litres per year of ethanol from its sugarcane industry.
Sixty percent of new cars can run on a fuel mix which includes 85% ethanol.
The European Union has a target for 2010 that 5.75% of transport fuels should come from biological sources, but the target is unlikely to be met.
The British government's Renewable Transport Fuel Obligation requires 5% of the fuel sold at the pump by 2010 to be biofuel.
In the US, the Renewable Fuels Standard aims to double the use of biofuels in transport by 2012.
What are the downsides?
From the environmental point of the view, the big issue is biodiversity.
With much of the western world's farmland already consisting of identikit fields of monocultured crops, the fear is that a major adoption of biofuels will reduce habitat for animals and wild plants still further.
Asian countries may be tempted to replace rainforest with more palm oil plantations, critics say.
If increased proportions of food crops such as corn or soy are used for fuel, that may push prices up, affecting food supplies for less prosperous citizens.
The mixed picture regarding the climate benefit of biofuels leads some observers to say that the priority should be reducing energy use; initiatives on biofuels detract attention from this, they say, and are more of a financial help to politically important farming lobbies than a serious attempt to cut greenhouse gas emissions.
There are few problems technically; engines can generally cope with the new fuels.
But current technologies limit production, because only certain parts of specific plants can be used.
The big hope is the so-called second-generation of biofuels, which will process the cellulose found in many plants. This should lead to far more efficient production using a much greater range of plants and plant waste.