Environmental Issues
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Biomass Energy
Biomass energy, one of the oldest energy sources known to man, uses the energy embodied in organic matter. Biomass-based energy systems utilize wood, agricultural products, or landfill gas as fuels. Biomass in all its energy uses currently supplies more than 3% of total U.S. energy needs and provides almost 10,000 MW of electric generating capacity. Wood fuels provide the bulk of this generation (66%), followed by municipal waste (24%), agricultural products (5%), and landfill gas (5%).
A biomass plant for electricity generation is an old and straightforward technology that is still effective today. Agricultural, forest, or other organic products are combusted in a boiler to generate steam, which is used to drive a steam turbine, which in turn drives an electric generator. Such plants have been in use for many years. Such systems are dependent on a continuous supply of fuel.
Wood is the leading biomass energy resource used for power generation, primarily because of its use as boiler fuel in the lumber, pulp and paper industries. The lumber industry satisfies close to 75% of its energy needs through direct wood combustion, while the pulp and paper industry has achieved a 55% aggregate fuel contribution from wood. Many of these companies use cogeneration systems for power generation. The Edison Electric Institute estimates that more than 6,000 MW of non-utility, wood-fired generating capacity was in place at the end of 1991.
Combusting wood as a fuel has environmental advantages in terms of emissions of carbon dioxide (CO2). CO2 emissions are generally considered to be greenhouse-neutral since the source is terrestrial biomass that at some point in its life would oxidize and re-enter the carbon cycle – unlike fossil hydrocarbons that have been sequestered from the atmosphere for millions of years. Another fact worth recognizing is that the burning of a tree releases carbon dioxide, but an equal amount of carbon dioxide is removed from the atmosphere while the tree is growing. Thus, so long as the trees that are burned are replaced by new, growing trees, the net emission of carbon dioxide is zero.
Besides CO2 these systems do generate air emissions including particulates and volatile organics. Emissions levels can be managed but the control equipment adds to the capital cost of a facility.
When this technology is combined with an industrial use in a cogeneration configuration, the facility’s efficiency is dramatically increased. This occurs when the steam already used to generate electricity is sent to the industrial facility for utilization in its process. As this steam condenses into liquid, tremendous heat is released for use by the industrial process like a lumber kiln for drying lumber. Without the cogeneration configuration, all the heat released when the steam condenses (latent heat of vaporization) is lost to the atmosphere through the cooling tower.
A cogeneration configuration with a power generation facility and a mill that includes a dryer or kiln is a natural pairing. This combination provides a highly efficient power generation cycle. Additional synergies are realized when the power plant utilizes the wood products generated by the sawmill operations.
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