Ing. Industriao

Wood Biomass for Energy

Wood fuel has several environmental advantages over fossil fuel. The main advantage is that wood is a renewable resource, offering a sustainable, dependable supply. Other advantages include the fact that the amount of carbon dioxide (CO2) emitted during the burning process is typically 90% less than when burning fossil fuel. Wood fuel contains minimal amounts of sulfurand heavy metals. It is not a threat to acid rain pollution, and particulate emissions are controllable. The principle economic advantage of wood biomass energy is that wood is usually significantly less expensive than competing fossil fuels. Public institutions, such as schools, hospitals, prisons, and municipality-owned district heating projects, are prime targets for using wood biomass energy.Before building or remodeling a facility to utilize wood biomass for energy, potential users should evaluate the local market for the available supply of wood. Transportation costs may limit the benefits of burning wood fuel—hauling wood biomass from outside a 50-mile radius is usually not economical. This should be followed by a rigorous life-cycle analysis for the energy system. Initial costs ofa wood biomass energy system are generally 50% greater than that of a fossil fuel system due to the fuel handling and storage system requirements. Today, the installed cost of a 1 to 5 million Btu/h (0.3 to 1.5 MW) wood fuel burner/boiler system is estimated at $50,000 to $75,000 per million Btu/h (0.3 MW) of heat input. New and existing technology for using wood fuel

effectively can be acombination of wood combustion, wood gasification, cogeneration, and cofiring, depending on the fuel application.

Wood Combustion
Instead of paying disposal costs, wood combustion for electricity and heat is one way in which forest products companies can utilize their wood residues. Typically, wood in a variety of forms, particularly green chips (45% to 50% moisture content on a wet basis), isshipped and maintained at a holding site by the energy plant. Augers or belt conveyors transport the wood chips to the combustor, where they are burned, and the heat of combustion is transferred to a steam or hot water boiler. Steam is converted to electrical power by steam turbines. Excess steam can be used in other plant processes—for example, in a dry kiln. Hot water boilers can provide heat to abuilding through a piping distribution network.

Wood Gasification
Wood gasification is the process of heating wood in an oxygen-starved environment until volatile pyrolysis gases (carbon monoxide and hydrogen) are released from the wood. Depending on the final use of the typically lowenergy wood (producer) gas (~150 Btu/ft³ (5.6 MJ/m³)), the gases can be mixed with air or pure oxygen forcomplete combustion and the heat produced transferred to a boiler for energy distribution. Otherwise, the gases can be cooled, filtered, and purified to remove tars (a major concern for any wood gasification process) and particulates and used as fuel for internal combustion engines, microturbines, and gas turbines.

Forest Products Laboratory ● State & Private Forestry Technology Marketing Unit OneGifford Pinchot Dr. ● Madison, WI 53726–2398 ● 608–231–9200 ● 608–231–9592 (fax) WOE–1 mailroom_forest_products_laboratory@fs.fed.us ● www.fpl.fs.fed.us Updated 04/04

Cogeneration
Cogeneration is the simultaneous production of heat and electricity, commonly called combined heat and power (CHP), from a single fuel. Traditionally, a steam turbine is used to produce electricity, although a woodgasification/ internal combustion unit can also be a cogeneration unit. Several factors affect the economic feasibility of a CHP unit including wood waste disposal problems, high electricity costs, and year-round steam use. Two common mistakes when installing a CHP system are buying a steam boiler that is designed for less than 100 lbf/in² (689 kPa) or oversizing the system. Buying a steam boiler...