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Biomass, also known as biofuels or bioenergy, comes from organic matter, either directly from plants or indirectly from industrial, commercial, domestic or agricultural products.
The use of biomass is generally classed as a ‘carbon-neutral’ process. This is because the carbon dioxide released when biomass generates energy is offset by that absorbed by plants during their growth. However, other energy inputs may affect this carbon-neutral balance, for example if fertiliser is used or energy consumed by vehicles harvesting or transporting the biomass to its point of use.
Biomass falls into three main groups:
Biomass can be converted into heat and electricity in a number of ways. Depending on its source, these processes include burning, pyrolysis (the decomposition or transformation of a compound caused by heat), gasification, anaerobic digestion or fermentation.
In 2003, biomass accounted for 87 per cent of renewable energy sources in the UK – this includes biomass used for both heat and electricity generation. Most of this came from landfill gas (33 per cent) and waste combustion (14 per cent) (source: DTI, ‘UK Energy in Brief’, July 2004). Smaller amounts also came from sewage gas, domestic wood and industrial wood. Electricity produced from biomass accounted for 1.55 per cent of total electricity supply in the same year.
Biomass has the potential to make a significant contribution to UK heat and energy generation in the future. Limited resources (e.g. landfill gas) may restrict the development of some forms of biomass. Although energy crop-based solutions are difficult to import economically, many biomass feedstocks can be grown in the UK. In some cases, crop yields will need to be improved before the process becomes economic.
Likely areas for development are smaller-scale regional projects, and the promotion of energy crops such as short-rotation coppice.
Europe’s biggest power station, Drax, is testing the use of thousands of tonnes of wood pulped into biomass. If the tests are successful, willow-based biomass could provide 5 per cent of the power station's fuel by 2009 and cut 700,000 tonnes of carbon dioxide emissions.
As biomass is carbon-neutral it is good for the environment. Anaerobic digestion schemes can contain the decay process without which, under current disposal practice, methane is released into the atmosphere. The carbon dioxide produced when methane is burned under an anaerobic digestion scheme is absorbed from the atmosphere by growing plants.
A properly-managed anaerobic digestion scheme will contain the intense nutrients found in animal slurries and food residues. These can otherwise leach out in high concentrations and pollute soil and water courses. The by-products from anaerobic digesters can be used as a fertiliser and soil improver. Incombustible materials, such as ash from incineration schemes, also have to be removed, although ash from burning wood fuels can also be used as a fertiliser.
Transporting materials by lorry for biomass plants can cause pollution and disruption, although this can be reduced by transporting them at less disruptive periods.
Combustion chambers used for the burning of biomass need to be well managed to ensure that by-products such as particulates and polyaromatic hydrocarbons cannot escape into the atmosphere.