Coal - Energy Policy Analysis: A Conceptual Framework - page 31

the 1,579 fossil-fueled electric generating units in the United States used flue gas desulfurization
scrubbers in 2010 (USEIA 2011d, Table 3.10).
Nitrogen oxide emissions from coal-fired boilers are generally reduced by modifying combus-
tion conditions in the boiler to reduce peak temperatures and oxygen available during combustion.
A 20 to 50 percent reduction in nitrogen oxide emissions has been demonstrated using combus-
tion modification techniques, usually by installing more efficient burners and control equipment
(Christman et al. 1980, 225). Controlling NO x emissions is generally considered less demanding
than controlling SO 2 emissions, so when a generating unit is shut down for retrofit with SO 2 control
equipment, NO x emissions controls are often installed at the same time.
Particulate matter and sulfur residues recovered from air pollution control equipment are
often stored in large detention ponds at the power plant site before disposal in sanitary landfills
or returned to the mine site for disposal in mine pits. Ash and particulate storage ponds pose a
significant risk of breaching and contaminating substantial areas of land and surface waters unless
carefully maintained, as was graphically illustrated on December 22, 2008, when an ash pond at
the Tennessee Valley Authority's Kingston Fossil Plant spilled over 1 billion gallons of sludge,
inundating more than 300 acres and many residences along the Emory River in Tennessee (Fausset
2009; Dewan 2008, 1).
Thus, in addition to converting an air pollution health problem into a solid waste management
problem, the use of air pollution control equipment at coal-fired power plants involves convert-
ing environmental costs into dollar costs for equipment, labor, and maintenance that are paid by
consumers of electricity.
Thermal Pollution
Performance standards for power plants prohibit the discharge of heat to natural bodies of water
pursuant to the Clean Water Act (33 U.S. Code § 1316), due to the adverse impact on organisms in
aquatic ecosystems. Conventional cooling systems to control thermal pollution are of three basic
types: once-through cooling systems, cooling ponds or lakes, and cooling towers.
Once-Through Cooling
After superheated steam is used to turn turbines and generate electricity, the steam is cooled by
passing it over a bank of tubes through which cold water is circulated, and heat transferred to the
cooling water is discharged back into its source (e.g., river, lake, ocean). The two sets of pipes, one
containing steam and the other cooling water, are separate except for the heat transfer, so water
from one does not mix with water in the other. Steam water, after cooling, is recirculated to the
boiler for reheat and reuse (Christman et al. 1980, 226). Only the cooling water is discharged to
the environment. Once common due to its low cost, once-through cooling is rarely used on new
generating units due to the need to control thermal pollution.
Cooling Ponds or Lakes
In this method, heated cooling water is discharged to artificial ponds or lakes constructed on the
power plant site for this purpose, with size of the pond determined by capacity of the generating
units using it. A surface area of one to two acres per megawatt of generating capacity is typical
(Christman et al. 1980, 226), and no discharge to natural water bodies is allowed. This method of
cooling is common in areas where land is relatively inexpensive.