Environmental Engineering Reference
In-Depth Information
treated or disposed of in a secure impoundment. Coal contains the radioactive isotopes
14
C and
40
K, because these elements were inherent in the original biologic tissue of the antecedents of coal.
These radionuclides do not pose an environmental hazard. However, frequently uranium, thorium,
and their radioactive daughter elements are also found in the mineral matter adhering to the coal.
Thus, leachate from coal mines, piles, and slag needs to be monitored for its radioactivity before
entering into the environment and for preventing mine worker exposure.
About 50% of all mined coal in the United States is now “washed” at the mine mouth prior
to shipment to the user (see Section 5.2.9.3). Coal washing increases the heating value per unit
mass of coal by removing the incombustible mineral matter. More importantly, coal washing
removes pyritic sulfur, which can amount up to 50% of the sulfur content of the raw coal. The
most widely used technique for coal washing is mass separation. Mineral matter has a higher
specific gravity than coal. By flushing crushed coal in a stream of water, the mineral matter settles
out, while the lighter coal particles float in the stream. The settled mineral matter contains a high
concentration of acidic, toxic, and possibly radioactive compounds and elements. This slag must
be analyzed for its content. If the content is toxic, the slag must be treated or safely disposed of
in a hazardous waste impoundment. In the United States, discharges from mining activities are
regulated under the Clean Water Act, specifically, the National Pollution Discharge Elimination
System (NPDES).
9.3.2
Solid Waste from Power Plants
Although much of the mineral matter of coal is removed at the mine mouth, coal delivered to a
power plant or other facilities still contains adhering mineral matter, simply called ash. The ash
content can amount to anywhere between 1% and 15% of the coal weight. Even oil contains ash,
amounting to 0.01-0.5% by weight. After combustion of the coal particle or oil droplet, the mineral
matter remains uncombusted, and either falls to the bottom of the boiler or is blown out with the
flue gas as
fly ash
. In modern pulverized-coal-fired boilers, about 90% of the mineral matter forms
fly ash, and 10% bottom ash. Most of the fly ash is collected in electrostatic precipitators (ESP, see
Section 5.2.9.2), except the very small particles (less than 1
m in diameter), which escape into the
air because they are not removed efficiently by the precipitator. The fly ash contains approximately
the same acidic, toxic, and radioactive matter as the original mineral matter in coal. The fly ash is
required to be chemically analyzed and, if found hazardous to human health and the environment,
properly disposed of. Nonhazardous bottom and fly ash is often used as aggregate material in
concrete or asphalt.
As a consequence of acid deposition regulations in the United States and many other coun-
tries, coal-burning power plants and industrial boilers are required to be equipped with
flue gas
desulfurization
devices (see Section 5.2.9.3). For high-sulfur-content coal (
µ
0.6% by weight), a
wet limestone scrubber is necessary. After passing the ESP, the flue gas enters a scrubber tower. A
slurry of limestone is injected from sprinklers at the top of the scrubber. Flue gas containing SO
2
and other sulfur compounds flows countercurrent to the limestone spray. A sludge is collected in
the bottom of the scrubber, consisting of wet calcium sulfate (gypsum) and calcium sulfite along
with unreacted limestone. The sludge is dewatered as much as possible (unfortunately, gypsum is
very difficult to dewater), then disposed of. Because the sludge also may contain toxic elements—
notably arsenic, cadmium, mercury and selenium—its disposal may require a specially designed
secure impoundment.
≥
