Direct combustion

Direct combustion represents the greatest use of biomass for fuel worldwide. Sweden and Austria generate a significant proportion of their electricity by burning the residue from timber processing. The direct burning of municipal waste is becoming increasingly popular. However, the presence of heavy metals in such waste poses a danger from toxic emissions including, it is claimed, dioxins. In the UK there is a major plant in Lewisham in southeast London, capable of generating 30 MW of electricity (DTI Renewable Energy Case Study: 'Energy from Municipal Solid Waste', SELCHP, Lewisham). Sheffield has one of the most extensive systems using Finnish technology and providing the city centre with heat and supplying power to the grid.

The direct burning of rapid rotation crops is a technology which is said to be CO2 efficient since the carbon emissions balance the carbon fixed during growth. However, a paper published in 1980 by Michael Allaby and James Lovelock drew attention to the risks to health associated with wood burning ('Wood stoves: the trendy pollutant', New Scientist, 13 November 1980). The authors identified nine compounds found in wood smoke that are known or suspected carcinogens.

The first UK commercial biomass electricity generating plant fuelled by poultry litter (a mixture of straw, wood and poultry droppings) was built at Eye, Suffolk. It has a capacity of 12.5 MW and uses about half the total of litter from broiler farms in the county. It is claimed to reduce greenhouse gas emissions by 70 per cent compared with coal-fired plants. It also eliminates the production of the powerful greenhouse gas methane and nitrates which enter the water supply. A much larger biomass plant is in operation in Thetford which consumes 450 000 tonnes per year of poultry litter to deliver 38.5 MW of power. Its environmental benefit is that it reduces net CO2 emission by recycling carbon rather than producing new CO2. It also eliminates methane emissions from stored poultry litter.

In the UK about 1.8 million tonnes of poultry waste and 12 million tonnes of livestock slurry are produced annually. This offers substantial biomass-to-energy conversion opportunities either as direct combustion or by using anaerobic digestion technologies.

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