Environmental Engineering FOURTH EDITION

Air Pollution Control 403

SO2 may also be removed from flue gas by dissolution in an aqueous solution of

sodium citrate by the reaction

SOz(g) + H20(1) + HSO; + H+.

Citrate does not itself enter into the reaction but buffers the solution at about pH 4.5,

which the reaction requires. The citrate buffer is readily regenerated. Removal effi-

ciency of the citrate process is between 80 and 99%: much better than the 75%

achievable by alkali scrubbing.

Control of Nitrogen Oxides

Wet scrubbers absorb NO2 as well as S02, but are usually not installed primarily for
NO2 control. An effective method often used on fossil fuel burning power plants is
off-stoichiometric burning. This method controls NO formation by limiting the amount
of air (or oxygen) in the combustion process to just a bit more than is needed to bum
the hydrocarbon fuel in question. For example, the reaction for burning natural gas

CH.4 + 02 -b 202 + co2

competes favorably with the high-temperature combination of nitrogen in the air with

oxygen in the air to form NO (which eventually is oxidized to NOz)

N2 + 02 + 2NO.

The stoichiometric ratio of oxygen needed in natural gas combustion is

32 g of O2:16 g of CH4.

A slight excess of oxygen in the combustion air will cause virtually all of the oxygen to
combine with fuel rather than with nitrogen. In practice, off-stoichiometric combustion
is achieved by adjusting the air flow to the combustion chamber until any visible plume
disappears.

Control of Volatile Organic Compounds and Odors

Volatile organic compounds and odors are controlled by thorough destructive oxidation,
either by incineration or by catalytic combustion, since they are only slightly soluble
in aqueous scrubbing media. “Catalytic incinerators,” which use a metal oxide or
mixed metal oxide catalyst and operate at 45OOC and sometimes higher temperatures,
can achieve 95 to 99% destruction of volatile organic compounds, like chlorinated
hydrocarbons. These very efficient incinerators depend on catalysts that can with-
stand and function at high temperatures. Such catalysts were generally not available
before 1996.