38    CLEAN AIR, CLEAR WATER-THE TREATMENT OF
WASTEWATERS FROM
FLUE GAS DESULFURIZATION PLANT
Marek K. Mierzejewski, Senior Process Engineer
Infilco Degremont Inc.
Richmond, Virginia 23229
BACKGROUND
The passing of the Clean Air Act Amendments into law in November 1990 has produced a massive
growth of interest in flue gas desulfurization (FGD) technology. The part of the new amendments
dealing with sulfur dioxide emissions from fossil fuel-fired utilities, Title IV, the Acid Deposition
Program, specify that by the year 2000, U.S. power stations will have had to have reduced the sulfur
input to the atmosphere by 10 million tons annually from 1980 levels. The U.S. Environmental
Protection Agency (USEPA) has identified which utilities and which particular power stations will
have to reduce their emissions. The legislation will be enforced in two phases: Phase I power stations
having to comply by January 1, 1995 and those in Phase II by January 1, 2000.
Since the sulfur dioxide in flue gas comes from coal, some power stations may achieve compliance
by "fuel switching", i.e., changing to coal with a lower sulfur content. Geographical and cost considerations do not, however, make this a realistic solution for most utilities. Extensive research in
alternative means of reducing sulfur emissions, e.g., boiler design, etc., continues in the U.S. under
the Department of Energy's Clean Coal Technology Program.1
It is likely that most utilities will adopt some form of FGD in order to comply with the new
regulations. Although FGD technology is not new in the U.S., having been established here over the
past two decades, it is likely to change in two interesting aspects. Firstly, the cost of disposal of the
resultant FGD sludge is increasing as landfill sites become full and more carefully controlled; secondly, the EPA and National Pollutant Discharge Elimination System (NPDES) are imposing progressively stricter limits on liquid wastes that can be discharged into the environment. Both these
factors, one applicable to solids, the other to liquids, will require new technology being applied to the
solution of these problems.
The situation in the United States today is analogous to that faced in West Germany with the
enactment in 1983 of that country's own clean air legislation, namely the Federal TA Luft and GFAVO
programs. Much as a result of these programs, German FGD technology is highly advanced and the
question of the treatment of the associated wastewater has been satisfactorily addressed.2,3 Japan is
regarded as the other world leader, likewise using advanced FGD technology.
The treatment of FGD wastewaters, then, has been developed largely in those two countries. One of
the main Japanese processes, known as the Magnesium Circulation method, has been addressed at an
earlier conference.4 The present paper discusses the European approach, as developed by companies
in the Degremont Group.
THE SOURCE OF FGD WASTEWATER
Flue gas is formed during the combustion of coal in the boiler; prior to discharge into the atmosphere, via a stack, it can be treated to remove flyash particles by electrostatic precipitation (ESP) and
to remove acid gas constituents, namely nitrogen oxides (NOx) and sulfur oxides, principally sulfur
dioxide (S02). Desulfurization of the flue gas is usually achieved by reaction of S02 with lime (CaO)
or limestone (CaC03). Many different technologies are available5,6 but in the most commonly-used
process, a limestone slurry is recycled in a wet scrubber where it reacts with the flue gas, yielding
calcium sulfite, CaS03. In a further development of this process, the sulfide can be oxidized either in
situ (in the scrubber reservoir) or ex situ, yielding the sulfate which, in its dihydrate form
(CaS04.2H20), is known as "gypsum". This latter is the basis of the Limestone Forced Oxidation
(LSFO) process as shown in Figure 1.
46th Purdue Industrial Waste Conference Proceedings, 1992 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.