EVALUATION OF ADDITIVES FOR IMPROVING
SCRUBBER SLUDGE CHARACTERISTICS FOR DISPOSAL
T. Y. J. Chu, Environmental Engineer
R. J. Ruane, Program Manager
Division of Environmental Planning
Tennessee Valley Authority
Chattanooga, Tennessee 37401
D. Y. G. Chen, Graduate Assistant
D. W. Weeter, Associate Professor
B. D. Marks, Associate Professor
Department of Civil Engineering
The University of Tennessee
Knoxville, Tennessee 37916
INTRODUCTION
To meet federal clean air standards, power industries may need to control sulfur dioxide
emissions from their steam plants. Antipollution measures may include substitution of low-
sulfur fuels, coal cleaning, coal gasification, changes in combustion equipment and installation of flue gas desulfurization (FGD) systems. The choice among these options is based
primarily on cost-effectiveness and marketability, but other considerations such as previous
experience, system reliability and efficiency and space requirements are also important. So
far, the use of either low-sulfur fuels or nonregenerable FGD processes has been favored. A
switch from high-sulfur to low-sulfur coal requires a capital investment of about $5 to $15/
kW [ 1 ] for additional particulate collection and plant conversion. Power plants may experience operational difficulties in burning the different type of coal. Compliance schedules at
power plants have caused demand for eastern low-sulfur coal to exceed demand, and prices
have increased. Wet scrubbing of flue gases is the most developed post-combustion method
for removing sulfur dioxide. However, large amounts of waste materials are produced by
the lime, limestone and double-alkali FGD systems, and certain properties of the sludge produced by these processes may require improvement before the sludge is suitable for disposal.
Therefore FGD processes Can result in problems of solid waste disposal.
The chemical composition of sludges produced by wet scrubbing systems that use slurried
lime or limestone as the absorbent varies, depending upon the scrubber operation, sulfur
dioxide removal efficiency, time of fly ash collection (before or during scrubber operation)
and type of absorbent. The main constituents of the solid waste from the lime and limestone scrubbing processes are CaS04 ■ 2H20, CaS03 ■ &H20, CaC03 and Ca(OH)2.
The engineering properties of this waste material have been investigated  [2-4]. Reports
indicate it has very low strength and strong thixotropic characteristics that make landfill disposal difficult. Thixotropy is the characteristic of some gels that become fluid when subjected to vibration and return to a jelly-like state when vibration ceases. The thixotropic
nature of the sludge would preclude any use of the disposal area after the disposal pond or
landfill had been filled.
In addition to the poor physical characteristics of this waste material, the presence of
many soluble materials such as boron, chloride, mercury, selenium and sulfate in the untreated sludge also results in a potential problem of leaching [5], which may contaminate ground
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