On combustion the carbon from biomass
power plant is released into the atmosphere as carbon dioxide (CO2).
The amount of carbon stored in dry wood is approximately 50% by weight. When
from agricultural sources, plant matter used as a fuel can be replaced by
planting for new growth. When the biomass is from forests, the time to
recapture the carbon stored is generally longer, and the carbon storage
capacity of the forest may be reduced overall if destructive forestry
techniques are employed.
The existing biomass power
generating industry in the United States, which consists of
approximately 11,000 MW of summer operating capacity actively supplying power
to the grid, produces about 1.4 percent of the U.S. electricity supply.
Currently, the New Hope Power Partnership
is the largest biomass power plant in North America. The 140
MW facility uses sugar cane fiber (bagasse)
and recycled urban wood as fuel to generate enough power for its large milling
and refining operations as well as to supply renewable electricity for nearly
60,000 homes. The facility reduces dependence on oil by more than one million
barrels per year, and by recycling sugar cane and wood waste, preserves
landfill space in urban communities in Florida.
Biomass power plant size is often driven by biomass availability in
close proximity as transport costs of the (bulky) fuel play a key factor in the
plant's economics. It has to be noted, however, that rail and especially
shipping on waterways can reduce transport costs significantly, which has led
to a global biomass market. To make small plants of 1 MWel economically
profitable those power plants have need to be equipped with technology that is
able to convert biomass to useful electricity with high efficiency such as ORC
technology, a cycle similar to the water steam power process just with an
organic working medium. Such small power plants can be found in Europe.
Despite harvesting, biomass crops may
sequester carbon. So for example soil organic carbon has been observed to be
greater in switchgrass stands than in cultivated cropland soil, especially at
depths below 12 inches. The grass sequesters the carbon in its increased root
biomass. Typically, perennial crops sequester much more carbon than annual
crops due to much greater non-harvested living biomass, both living and dead,
built up over years, and much less soil disruption in cultivation.
The biomass-is-carbon-neutral proposal put
forward in the early 1990s has been superseded by more recent science that
recognizes that mature, intact forests sequester carbon more effectively than
cut-over areas. When a tree’s carbon is released into the atmosphere in a
single pulse, it contributes to climate change much more than woodland timber
rotting slowly over decades. Current studies indicate that recapturing carbon
released by burning will take minimally hundreds of years.
Using biomass as a fuel produces the same
air-pollution challenges as other fuels. Black carbon - a pollutant created by
incomplete combustion of fossil fuels, biofuels, and biomass - is possibly the
second largest contributor to global warming. In 2009 a Swedish study of the
giant brown haze that periodically covers large areas in South Asia determined
that it had been principally produced by biomass burning, and to a lesser
extent by fossil-fuel burning. Researchers measured a significant concentration
of 14C, which is associated with recent plant life rather than with fossil
fuels.