Liquid fuels

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Biomass is already used extensively to produce liquid fuels. The most important of these are ethanol, made from the fermentation of grain or sugar cane and biodiesel produced from oil-rich crops such as sunflower and oil-seed rape.

Biodiesel is produced extensively in Europe where the total production was over 850,000 tonnes in 2001. Production is now on target to reach 2% of the liquid fuel market by 2005 though 5.75% by 2010 may be harder to attain.

Europe also produces ethanol but the largest producers are the USA and Brazil where the product of fermentation is added to petrol. US production is around 4 billion litres each year. Up to 10% ethanol can be blended with petrol, a product sold in the USA as gashol.

Both ethanol and biodiesel are important as substitutes for fossil-oil-derived products and their production is likely to increase both for environmental and for security reasons. Much of this fuel will be used for transportation but it is quite feasible to operate stationary engines designed to generate electricity on these fuels too. However this is likely to make only a very small contribution to global electricity generation in the foreseeable future.

Environmental considerations

Biomass can be viewed as a direct replacement for fossil fuels, particularly coal. In power generation applications it will be burnt or gasified in an entirely analogous manner to coal and like coal it will produce atmospheric emissions, principally carbon dioxide. Why, then, is biomass considered renewable?

The difference lies in the fact that biomass is a replaceable fuel. Fossil fuels such as coal and oil were originally biomass, biomass that as a result of age and geological changes had become trapped within the earth's crust. When these materials are extracted and then burned they release their carbon (their principle component) which was previously sequestered within the earth into the atmosphere. Since the industrial revolution this has led to a steady but accelerating increase in the concentration of carbon dioxide within the earth's atmosphere.

When biomass is burned it too releases carbon in the form of carbon dioxide into the atmosphere. However when replacement fuel is grown it takes carbon dioxide from the atmosphere during photosynthesis. Thus over a complete cycle of growth, harvesting and combustion there is no net addition or subtraction of carbon from the atmosphere. So burning biomass instead of coal can help stabilise carbon dioxide concentrations in the atmosphere.

Aside from carbon dioxide, combustion of biomass releases some organic compounds, carbon monoxide, particular material and nitrogen oxide into the atmosphere in exactly the same way that combustion of coal produces these materials. Depending on emission regulations these may have to be removed from a power plants's flue gas.

There is one major difference between biomass and coal. Biomass contains virtually no sulphur. Thus whereas most coal-fired power plants require an expensive flue gas scrubbing system to remove sulphur dioxide before it is released to the atmosphere, the flue gas from a biomass plant does not require this treatment.

Biomass contains virtually no toxic metals either, so the release of these into the atmosphere is reduced where biomass is burned instead of coal. Biomass also produces significantly less ash than coal, and the ash which is produced can often be returned to the soil as a fertiliser.

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