 Coalition for
Responsible Waste Incineration
SULFUR DIOXIDE REMOVAL FROM FLUE GASES
Technical Issue
Sulfur is present as an impurity in fuels, and can also be a
constituent in hazardous waste. Sulfur is oxidized to sulfur dioxide,
with trace amounts of sulfur trioxide in the combustion process.
Emissions of sulfur dioxide and sulfur trioxide to the atmosphere
result in the formation of acid rain. Acid rain harms plant life,
aquatic life, and man-made objects, so it is important to remove SO2 and SO3
from flue gases.
Technical Background
The oxides of sulfur found in flue gases are easily scrubbed.
The most common method of removal is a wet scrubber. Most wet scrubbers
are venturi type devices. Flue gas is accelerated through a nozzle and
deluged with scrubbing liquid. Sodium hydroxide (NaOH) is injected into
the scrubber liquid. When the scrubbing liquor comes in contact with
sulfur dioxide in the flue gas, the sulfur dioxide is converted to
sodium sulfate (Na2S04) which is then removed from the aqueous
stream.
In some cases, calcium hydroxide is used instead of sodium
hydroxide in wet scrubbing systems. When calcium hydroxide is used, an
insoluble calcium sulfate is formed.
Sulfur oxides can also be removed from flue gases by direct
contact with dry sorbents such as lime or limestone. In these cases,
sulfur dioxide in the flue gas reacts with the lime to form solid
calcium sulfate. The spent solids are collected by either a baghouse or
an electrostatic precipitator.
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