65    SLUDGE REDUCTION IN COAL-FIRED POWER PLANT
FLUE-GAS DESULFURIZATION WASTEWATER
TREATMENT
Yoshihiro Etoh, Senior Researcher
Tadashi Takadoi, Manager, Technical Section
Ikuo Itoh, Vice-Manager, Technical Section
Kurita Water Ind., Ltd.
Atsugi 243-01, JAPAN
INTRODUCTION
The most important problem facing coal-fired power plants is environmental pollution. The flue
gas from coal-fired plants contains larger quantities of SOx, NOx, and dust than that from oil-fired
plants. These pollutants are one cause of air pollution and acid rain. Flue-gas desulfurization in coal-
fired power plants is mostly carried out by the wet lime and limestone process. The process removes
the pollutants to satisfactorily low levels, but the water circulating in the system must be drawn off
mainly to prevent accumulation of chloride ions coming from the coal and make-up water, which
cause corrosion. The blow-down water is the so-called flue-gas desulfurization wastewater. The
wastewater contains fluorides, heavy metals, organic and inorganic COD components, and other
pollutants. Since 1974 we have been studying removal of those pollutants and have developed a
treatment process consisting of coagulation (mainly for fluorides and heavy metals removal) and
adsorption (mainly for COD removal). The process has already been adopted at many power plants.
However, this process (hereafter designated as the conventional process) has the problem that large
amounts of chemicals have to be added, and, therefore, a large amount of sludge forms in the
coagulation stage. Since 1980 we have been trying to improve the process and have succeeded in
reducing the amount of chemicals and sludge by half through the effective use of the aluminum and
magnesium already contained in the wastewater.
This paper discusses the treatment of wastewater from the flue-gas desulfurization process in coal-
fired power plants, focusing on the improved method.
WASTEWATER CHARACTERISTICS
The wastewater quality varies depending on the type of desulfurization equipment and the kind of
coal. Table I shows how the quality of wastewater from desulfurization plants varies with the kind of
coal. As can be seen in Table I, the boron concentrations vary by a factor of 10 with South African
coal having the lowest and Australian coal the highest. This is important because fluoride removal
depends on the concentration of boron in the wastewater. The amount of fluorides in the form of
fluoroborate ions (BF4T increases as the boron concentration increases, which makes fluoride
removal difficult. However, the presence of a certain amount of aluminum ions in the wastewater
precludes interference of boron. Therefore, the addition of aluminum may be required when there are
extremely high concentrations of boron or low concentrations of aluminum.
Table II shows the typical quality of the wastewater from the desulfurization plant of the wet lime
and limestone process including dust separation.
CONVENTIONAL PROCESS
A basic flow diagram for the conventional process is illustrated in Figure I though there may be
some variations for different plants. A settling basin often precedes the raw water tank, and fly ash in
the wastewater is settled by a polymer coagulant yielding raw water with a SS content of less than 500
mg/L.
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