FUEL HANDLING
Fuel handling is important for an effective biomass power plant. Since most bio- mass fuels are seasonal, there must be facilities to store large quantities if power plants are to be supplied throughout the year. This is particularly significant for grasses that will all be harvested in the autumn. These will normally be formed into briquettes and then stored until needed, but if this is to be the primary source of fuel for a power plant, a store capable of holding one year’s supply will be needed.
Woods can be harvested at different times of the year and this can help with the fuel supply management. Depending on the harvesting techniques the wood will arrive at the power station or storage depot in the form of bundles of coppiced branches, chipped wood, or whole trees. Wood is easier to chip when green so this will normally be carried early in the harvesting process, probably before it reaches the power plant. The latter will generally need to be able to store several weeks’ supply of fuel under conditions where it is protected from the weather so that it does not get wet. Otherwise, the fuel will deteriorate and lose energy content.
The efficiency of a biomass power plant will depend on the moisture content of the fuel. The lower the moisture content, the higher the efficiency. For example, a reduction in moisture content from 50%, as harvested, to 10% after storage can result in boiler efficiency rising from 70% to 83%. It is important, therefore, to allow wood to dry before it is burned.
The form in which fuel is actually supplied to the power station will vary. Most often it will be chipped although the use of pellets is expanding. For plants that use suspension firing (e.g., in co-firing with coal) the chips must be further reduced in size using grinding equipment. There is also a novel approach in which whole trees are supplied to a specially designed power plant. The trees are dried for 30 days and then sections are delivered to the boiler that is designed especially for this form of fuel. Efficiencies of 34% have been predicted for a 150 MW plant of this type.