Trees will be used to fire power station

The construction of Britain's first wood burning power station will start this summer, after the project won planning permission this week.

The eight megawatt power station at Eggborough, North Yorkshire, will generate enough power for 18,000 homes. The pounds 20m scheme will be fuelled partly from commercial forests in the region, using the small branches and trunks produced when young woodlands are thinned out.

Eventually the bulk of its fuel wood will come from fast growing willow tree plantations grown on nearby farmland. To date 34 hectares have been planted, and a further 125 will be planted this spring. The willows will be fertilised with the sludge from sewage works, and wood ash.

Government scientists have calculated that if a quarter of Britain's total area of farmland was given over to such "energy" woodlands this would provide sufficient fuel to continuously generate nearly two-thirds of current electricity demand.

It is regarded as an environment-friendly form of electricity generation because unlike coal, oil and gas burning power stations, those burning wood do not increase the amount of global-warming carbon dioxide gas in the atmosphere.

Like fossil fuels, burning wood does give off carbon dioxide. But an equal quantity of the gas is absorbed by the growing trees which provide fresh wood for the power station.

The willow shoots will be harvested by cutting them off at ground level. They quickly grow new shoots which reach a height of three metres or more before the cutting machine comes round again, three years later.

The harvested shoots will be cut into chips and then "cooked" in the power station at a temperature of 800C with a limited supply of oxygen. This "gasification" process gives off large amounts of heat, and hydrogen, methane, carbon monoxide and dioxide gases.

The mixture of gases is cleaned by chemical scrubbers and filters hen used as fuel for a gas turbine engine which powers an electricity generator.

The hot exhaust gases from the turbine then flow through a boiler, which raises steam for another turbine-generator. When the exhaust gases finally emerge they are still hot enough for one more task - drying the incoming wood chips.

Gasifying the wood reduces air pollution and allows much more of the chemical energy to be converted into electricity.

Three separate projects for wood burning power stations have been approved under a government scheme which boosts electricity generated from renewable energy sources. Eggborough, backed by an international consortium led by a subsidiary of Yorkshire Water, is the most advanced.