Combined Heat and Power and Electricity Generation
Electricity production from woody biomass usually involves combustion of the wood to produce high pressure steam in a boiler; this steam is then used in a steam turbine to generate electricity. The steam turbine is a series of vaned turbine blades which spin when pressurised steam is passed through them. The turbines are connected to a shaft which drives the generator. The used steam enters a condenser and the warm water is returned to the boiler.
The efficiency of modern steam turbines is over 30% for a coal-fired plant, and biomass plants will have a lower efficiency. Because of this, it is more efficient to burn biomass in a CHP plant where some of the heat can be used for space heating rather than being lost.
Gas turbines are more efficient than steam turbines. Operating a gas turbine from wood fuel would, however mean that the wood would first of all have to be used in a gasification or liquefaction plant to make the appropriate fuel. The exhaust gases from a gas turbine are very hot; if they can be passed through a steam generator then a steam turbine can also be run. Such a system is known as a combined cycle gas turbine. These have efficiencies over 40%.
Producing electricity from wood requires a lot of fuel! A biomass-fired power
station is being built near Lockerbie which will produce 44MW of electricity. It is estimated that this will consume around 220,000 oven-dried tons of wood annually, with 45,000 tonnes coming from short-rotation coppice grown locally on 3,700 hectares of land. The plant will provide enough electricity to power 70,000 homes and will displace the emission of 140,000 tonnes of C0
2 annually.
The Lockerbie plant will receive woodchip deliveries daily and will hold a 2-day store of fuel on site. The wood will be burned in a fluidised-bed combustor and the heat used to generate high pressure steam to drive a steam turbine. The flue-gases will be cleaned of dust and contaminants. The overall efficiency of the plant will be around 32.5%. The current economics of the Renewables Obligation Certificates, which do not include energy to produce heat, and the lack of a local market for heat, mean that the Lockerbie plant will produce electricity only.