Coal - Energy Policy Analysis: A Conceptual Framework - page 30

Similarly, discharge of pollutants to the nation's waterways without a permit is prohibited
by the Clean Water Act (33 U.S. Code §§ 1251-1387), and permits are required for storage
structures such as impoundments that constitute a potential for discharges (§ 1342). These per-
mits must specify limits for each pollutant or combination of pollutants discharged (§ 1316).
If discharged into publicly owned treatment works, these discharges must meet pretreatment
standards for the applicable industrial category of point source (§ 1317). Solid waste materials
and sludge from air pollution control facilities (42 U.S. Code 6903[26A]-[27]) are regulated
under the Resource Conservation and Recovery Act (42 U.S. Code 6901-6992k) and must be
disposed of in a permitted sanitary landfill designed to isolate waste materials from surface and
ground waters, as distinct from an open dump (42 U.S. Code 6945). Most coal-fired electric
generating plants have on-site solid waste handling and disposal systems, cooling systems, and
water treatment equipment.
Air Pollution Control
Use of air pollution controls at coal-fired bulk electric power generating plants is basically a strategy
of converting a health-impairing air pollution problem into a potentially health-threatening solid
waste disposal problem, which must be managed carefully to prevent harm to the general public.
Coal combustion concentrates lead, mercury, sulfur, and other impurities found in the fuel by a
factor of about ten, producing a contaminated ash that is either retained within the boiler (bottom
ash) or carried out with exhaust gases (fly ash). Depending on the type of furnace used, most of
this is fly ash, which in turn constitutes most of the particulate matter emitted to the atmosphere
in exhaust gases by such facilities. Fly ash is removed from exhaust gases in the form of particu-
late matter by air pollution control equipment such as electrostatic precipitators (dry scrubbers),
venturi or bed scrubbers (wet scrubbers), or fabric filters (baghouses). Electrostatic precipitators
use an electrical charge to ionize particulate matter in exhaust gases and deposit them on metal
plates, which are periodically rapped to dislodge accumulated particulates into a collection area
below. Although they are less effective on units burning low-sulfur coal, electrostatic precipitators
have demonstrated particulate removal efficiencies of 99 to 99.8 percent with moderate power
consumption at units burning medium- to high-sulfur coals.
Venturi scrubbers use high-pressure sprays of atomized water to remove particulates from
exhaust gases. Fixed or moving bed scrubbers percolate flue gases through liquid in a bed of
small spheres, capturing particulates in a scrubbing liquor that is filtered to remove them and
recycled in the scrubber. Baghouses capture particulates by filtering flue gases through long, nar-
row fabric bags woven of synthetic or glass fiber, analogous to household vacuum cleaner bags.
The bags are cleaned periodically by isolating them from the exhaust flow, shaking them or using
a high-pressure pulse of air counter to the direction of gas flow, and particulates are recovered
in a hopper below. Particulate collection efficiencies for all these methods range from 95 to over
99 percent (Christman et al. 1980, 219-222). About 396 (25 percent) of the 1,579 fossil-fueled
electric generating units in the United States continued to operate without collecting particulates
in 2006 (USEIA 2011d, Table 3.10).
Sulfur oxides, principally SO 2 , are removed from exhaust gases using wet or dry scrubbers
containing a chemical sorbent like lime or limestone. The resulting slurry or solid waste, which may
contain trace amounts of inorganic pollutants present in fly ash, is discarded in licensed sanitary
landfills. Wet lime or limestone scrubbers have demonstrated SO 2 removal efficiencies of 80 to
90 percent, but dry scrubbers are effective only where low-sulfur coal is used and SO 2 removal
of less than 80 percent is required (Christman et al. 1980, 223-224). Only 432 (27.4 percent) of