The term "biomass" encompasses diverse fuels derived from timber, agriculture and food processing wastes or from fuel crops that are specifically grown or reserved for electricity generation. Biomass fuel can also include sewage sludge and animal manure. Some biomass fuels are derived from trees. Given the capacity of trees to regenerate, these fuels are considered renewable. Burning crop residues, sewage or manure - all wastes that are continually generated by society -- to generate electricity may offer environmental benefits in the form of preserving precious landfill space OR may be grown and harvested in ways that cause environmental harm.
At present, most biomass power plants burn lumber, agricultural or construction/demolition wood wastes. Direct Combustion power plants burn the biomass fuel directly in boilers that supply steam for the same kind of steam-electric generators used to burn fossil fuels. With biomass gasification, biomass is converted into a gas - methane - that can then fuel steam generators, combustion turbines, combined cycle technologies or fuel cells. The primary benefit of biomass gasification, compared to direct combustion, is that extracted gasses can be used in a variety of power plant configurations. In terms of capacity, biomass power plants represent the second largest amount of renewable energy in the nation.
Because biomass technologies use combustion processes to produce electricity, they can generate electricity at any time, unlike wind and most solar technologies, which only produce when the wind is blowing or sun is shining. Biomass power plants currently represent 11,000 MW - the second largest amount of renewable energy in the nation.What are the environmental impacts?
Whether combusting directly or engaged in gasification, biomass resources do generate air emissions. These emissions vary depending upon the precise fuel and technology used. If wood is the primary biomass resource, very little SO2 comes out of the stack. NOx emissions vary significantly among combustion facilities depending on their design and controls. Some biomass power plants show a relatively high NOx emission rate per kilowatt hour generated if compared to other combustion technologies. This high NOx rate, an effect of the high nitrogen content of many biomass fuels, is one of the top air quality concerns associated with biomass.
Carbon monoxide (CO) is also emitted - sometimes at levels higher than those for coal plants.
Biomass plants also release carbon dioxide (CO2), the primary greenhouse gas. However, the cycle of growing, processing and burning biomass recycles CO2 from the atmosphere. If this cycle is sustained, there is little or no net gain in atmospheric CO2. Given that short rotation woody crops (i.e., fast growing woody plant types) can be planted, matured and harvested in shorter periods of time than natural growth forests, the managed production of biomass fuels may recycle CO2 in one-third less time than natural processes.
Biomass power plants also divert wood waste from landfills, which reduces the productions and atmospheric release of methane, another potent greenhouse gas.
Another air quality concern associated with biomass plants is particulates. These emissions can be readily controlled through conventional technologies. To date, no biomass facilities have installed advanced particulate emission controls. Still, most particulate emissions are relatively large in size. Their impacts upon human health remain unclear.
The collection of biomass fuels can have significant environmental impacts. Harvesting timber and growing agricultural products for fuel requires large volumes to be collected, transported, processed and stored. Biomass fuels may be obtained from supplies of clean, uncontaminated wood that otherwise would be landfilled or from sustainable harvests. In both of these fuel collection examples, the net environmental plusses of biomass are significant when compared to fossil fuel collection alternatives. On the other hand, the collection, processing and combustion of biomass fuels may cause environmental problems if, for example, the fuel source contains toxic contaminants, agricultural waste handling pollutes local water resources, or burning biomass deprives local ecosystems of nutrients that forest or agricultural waste may otherwise provide.