Page 171 - Wastewater Solids Incineration Systems

138 Wastewater Solids Incineration Systems

1.2 Gaseous Pollutants

Gaseous pollutants result from the oxidation and volatilization of sulfur, nitrogen,
and chlorine in the feed cake and from incomplete combustion of hydrocarbons and
other organic compounds. The gaseous pollutants of primary concern are the acid
gases, sulfur dioxide (SO ), and hydrogen chloride or hydrochloric acid (HCl);
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carbon monoxide (CO); volatile organic compounds (VOCs); polycyclic organic
matter (POM); nitrogen oxides (NOx); and greenhouse gases (GHGs).

1.2.1 Acid Gases
Incineration of the feed cake results in two acid gases (SO and HCl). Sulfur can
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exist in three forms: sulfate sulfur (SO ), pyritic sulfur (S ), and organic sulfur. All
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three forms are essentially oxidized to SO during the combustion process. The SO
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then combines with moisture, either in the wet scrubbing system or as moisture in
the atmosphere, to form sulfurous or sulfuric acids. Sulfur content of the feed cake
typically varies from approximately 0.2 to 1.0%. To estimate uncontrolled SO emis-
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sions, it is reasonable to assume that all the sulfur in the feed cake is oxidized and
released as SO .
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Chlorine in the feed cake can be in the form of organically bonded chlorine such
as in chlorinated hydrocarbons or in inorganic salts such as NaCl. Chlorine content
typically is relatively low, approximately 0.1 to 0.4%. However, road salting can cause
higher chlorine levels of 1.0 to 2.0%. The use of ferrous or ferric chloride at the
WWTP and, for coastal plants, intrusion of saltwater to the sewer system can also
increase chlorine levels. In the incinerator, chlorine is released and combines with
hydrogen to form hydrogen chloride. In addition to the severe health effects of SO
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and HCl, both acid gases are significant because of their highly corrosive effect on
downstream equipment. It is customary to fabricate all components of the incinerator
wet scrubbing system and downstream ductwork and stack of 316L stainless steel.
Wastewater treatment plant solids typically contain sufficient sulfur and chlorine
to result in uncontrolled SO emissions of approximately 612 to 1630 mg/Nm 3
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(normal cubic meters) on a dry volume basis corrected to 11% oxygen (mg/Nm dv )
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or 300 to 800 ppm on a dry volume basis corrected to 7% oxygen (ppm dv ) and
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uncontrolled HCl emissions of approximately 116 to 348 mg/Nm dv (100 to 300
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ppm dv ). Fortunately, the conventional wet scrubbing system (Venturi and impinge-
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ment tray scrubbers using plant effluent as the scrubbing liquid) can control SO
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emissions by approximately 80 to 85% and HCl emissions to an even greater degree.
It should be noted that the use of plant effluent improves acid gas control because of