PRIORITY POLLUTANTS DM WASTEWATER DISCHARGES
FROM A WET LIMESTONE SCRUBBER SYSTEM
Harvey Olem, Environmental Engineer
John W. Shipp, Environmental Engineer
Tien-Yung J. Chu, Environmental Engineer
Richard J. Ruane, Program Manager
Division of Water Resources
Tennessee VaUey Authority
Chattanooga, Tennessee 37401
INTRODUCTION
Coal contains many forms and types of contaminants, such as trace metals, trace organics,
and radionuclides. When coal is burned in a power plant, a high percentage of less volatde
forms of the contaminants is generally retained in the fly ash and bottom ash, whereas, a
high percentage of the more volatde forms is generaUy emitted to the air environment
through the stack. To meet federal clean-air standards, most coal-fired power plants need to
control their sulfur dioxide (S02) emissions. Antipollution measures may include substitution of low-sulfur fuels, coal cleaning, coal gasification, changes in combustion equipment,
and installation of gas desulfurization (FGD) systems. So far, the use of either low-sulfur
fuels or nonregenerable FGD processes has been favored. Wet scrubbing of the flue gases is
the most developed post-combustion method for removing S02. Scrubbing these stack gases
for control of S02 removes some of the more volatile contaminants from the flue gas.
Several of these volatUe constituents are included in the list of 129 toxic (priority) poUu-
tants estabUshed by the U. S. Environmental Protection Agency [1]. The fate of the contaminants contained in the liquid and solid scrubber wastes is of concern because of their
potential adverse effect on the water environment.
This study evaluated the priority pollutants in wastewater discharges from the first full-
scale limestone system at a Tennessee Valley Authority coal-fired power plant. The Widows
Creek unit 8 limestone scrubber was originally designed and installed as a full-scale research
and demonstration facility, but it has now become an integral part of the overall plan to
achieve and maintain compliance with Alabama SC^ and emission regulations for the plant. All
aspects of this scrubber process are of interest because the installation of 2350 MW of
additional FGD equipment is being planned in the TVA system of coal-fired power plants. In
addition to evaluation of aU organic and inorganic priority poUutants except asbestos and
cyanide, wastewaters were characterized for conventional and nonconventional pollutants
and radionucUdes. The hazardous waste potential of scrubber sludge was also evaluated.
The slurry from the scrubber is discharged to a pond for settling of solids. The supernatant is returned to the plant for slurrying of limestone and other uses. The system is
not a totaUy closed cycle, however, because the water used to eUminate the mist in the
stack gas foUowing wet scrubbing is pumped to the plant ash pond which discharges to the
Tennessee River. In addition, the mist eUminator system may use either river water or
scrubber pond return water. Figure 1 includes an aerial photograph of the scrubber sludge
Pond, ash pond, and associated waste streams at the Widows Creek power plant.
PROCESS DESCRIPTION
The removal of S02 gas from coal-fired power plant stack emissions by wet limestone
(CaC03) scrubbing is a chemical process involving a reaction between acid gases and alkaline liquids. The reaction creates soUds crystals (CaS04#nH20 and CaS03*'/2H20) that can
be physically or mechanicaUy removed. The relationship between the ash coUection system,
limestone scrubber, mist eliminator, and resultant liquid waste streams is shown schematically
in Figure 2. Flue gas passes through electrostatic precipitators which currently remove about
293