Biomass power plants use biomass sources to generate steam or heat.
A biomass power plant most often produces steam through combustion. The steam can be used for industrial processes or to produce electricity. Alternatively, the heat from combustion could be directly used in an industrial process. Another variation is a co-fired power plant fueled by coal or natural gas combined with a biomass source.
Biomass Power Plant Components
A biomass power plant includes the following major components:
Wood waste fuel would typically be stored in an outdoor pile in quantities adequate to fuel the power plant anywhere from 60 days to several years. Agricultural waste fuel would be stored in silos. Agricultural products such as switchgrass, hybrid poplars, or cottonwood trees would be stored in barns or storage domes on site. Depending upon the type of boiler, the fuel is either transported directly to the powerhouse via a belt conveyor, or first processed in a chipper/grinder to produce a finer texture. Municipal solid waste is deposited into pits where cranes mix the refuse and remove any large, non-combustible items; sometimes, it is further processed to remove ferrous materials, glass, and other non-combustible materials. In a fluidized bed boiler, the fuel is suspended on high-pressure jets of air during the combustion process. This not only allows combustion at relatively low temperatures and higher efficiency, but it also decreases the production of nitrogen oxide, an air pollutant.
Combustion of the fuel produces steam in the boiler that powers a turbine/generator to generate electricity. The steam is then returned to the liquid state in a condenser by circulating cooling water around the condenser tubes containing the steam. The condensed water is then recirculated to the boiler to again be heated into steam.
The heated cooling water from the condenser is either discharged directly to a receiving body of water (a "once-through" cooling system) or, more commonly, directed to a "closed loop" cooling system where it can be cooled and recirculated back to the condenser. A number of different closed loop cooling systems are used in power plants, either individually, or in combination. These include evaporative wet cooling systems (also known as recirculating wet cooling systems) and dry cooling systems. As the name implies, evaporative wet cooling systems remove heat from the cooling water by allowing some small portion of that water to evaporate to the atmosphere. Thus, evaporative wet cooling systems require a continuous supply of additional cooling water to replace the water lost to evaporation as well as an additional amount of water that is periodically removed from the system (blowdown) to maintain dissolved solids levels at suitable levels. Under certain ambient air temperature and humidity conditions, evaporative wet cooling systems can also create vapor plumes as the evaporated water quickly condenses to a liquid as it leaves the cooling system. Dry cooling systems are closed-loop systems that cool the water through convection. Dry cooling systems are efficient in the winter months but least efficient in hotter weather when electricity demand tends to be the highest. Because they are less thermally efficient than wet cooling systems, dry cooling systems affect the overall performance of the plant, imposing a penalty of as much as a 25% reduction in power production during the hottest hour, and an annualized power production reduction of as much as 10%. Nevertheless, dry cooling systems may be the most appropriate choice in arid regions where water resources are limited. Although they require up to two and a half times more land area than evaporative wet cooling systems, dry cooling systems are also often preferred in areas where the viewshed is particularly sensitive to the effects of vapor plumes. Finally, wet/dry hybrid cooling systems are also used and are especially effective in areas with low relative humidity.
Variations in these cooling systems are described below:
Once-through cooling systems withdraw water from a water body such as a river or lake, and the heated water from the condenser is returned to the river or lake. Water requirements, water availability, distance from the water source, and ecological effects of water withdrawal and heated effluent return are considerations when choosing this system.
A mechanical-draft wet cooling tower is a closed-cycle system that cools the heated water by evaporation as the water falls from the top of the tower and passes through moving air produced by large fans.
A natural-draft wet cooling tower uses the buoyancy of the exhaust air rising in a tall chimney to provide the draft that cools the water. This type of cooling system is used primarily at very large power plants.
Dry cooling towers operate by convective heat transfer through a surface that separates the cooling water from ambient air, such as in a heat exchanger or automobile radiator. They do not use evaporation and are primarily used when water availability is limited. Dry cooling towers can be mechanical-draft or natural-draft.
Wet/dry hybrid cooling systems spray a fine mist of water into the ambient air flowing over a dry condenser or spray water onto the surface of the condenser. The water flash evaporates, removing heat energy from the air or cooling the surface of the condenser, dramatically increasing cooling capacity. Although some water is lost to evaporation, the amount is substantially less than evaporative losses from equivalently sized wet cooling systems.
A number of pollution control devices are used at power plants:
In a cyclone separator, the particulate-laden combustion gas is forced to change direction. As the gas changes direction, the inertia of the particles causes them to continue in the original direction and be separated from the gas stream. Cyclones are efficient in removing large particles and therefore are often used in combination with other devices.
A baghouse is a device that utilizes fabric filter tubes or a cartridge to capture fly ash and other particulates from the combustion gases before these gases are released to the environment through a smoke stack.
An electrostatic precipitator removes these same particulates by inducing a negative charge to the particles and collecting them on a positively charged or grounded plate. These are very efficient, usually more than 99%.
A wet scrubber uses a high-energy liquid spray to remove particulate and gaseous pollutants from the flue gas.
For an industrial process that requires steam, other industrial equipment replaces the turbine/generator and condenser. For an industrial process that requires only a heat source, the condenser is still required.
Biomass Power Plant Size
A biomass power plant typically requires 15 to 100 acres for all the facilities. The total area of the site is usually larger, landscaped, and serves as an exclusion area to the public.