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.
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?
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
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
Natural Resources Defense Council Biomass Fact Sheet
Renewable Energy Policy Project Biomass FAQs http://crest.org/articles/static/1/1004994679_6.html
NREL's Biomass Power Project http://www.nrel.gov/programs/biomass.html
Northeast Regional Biomass Program http://www.nrbp.org
American Bioenergy Association http://www.biomass.org
Union of Concerned Scientists - USA: How Biomass Works http://www.ucsusa.org/clean_energy/renewable_energy_basics/offmen-how-biomass-energy-works.html