Page 175 - Wastewater Solids Incineration Systems

142 Wastewater Solids Incineration Systems

without an afterburner. In many cases, maintaining a top hearth temperature of
593°C (1100°F) or greater is important for achieving the standard (Waltz, 1990; Bat-
uray, 1990).
In contrast, because of its turbulent high-temperature conditions, a fluid bed
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incinerator has low total hydrocarbon emissions, typically less than 14 mg/Nm dv
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(10 ppm dv ) as propane.
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1.2.4 Polycyclic Organic Matter
Polycyclic organic matter (POM) is a subset of VOCs that are of particular concern to
the regulatory community because of their potentially high health effect risks. Some
of the primary POMs of interest are polychlorinated biphenols (PCBs), polychlori-
nated dibenzo-p-dioxin (PCDD), and dibenzo furan (PCDF). U.S. EPA does not have
specific emission limits for these pollutants. However, some state regulatory agencies
have included emission criteria for a few of these compounds in air permits of new
incinerators. Methods to achieve low POM emissions are to maximize the combus-
tion efficiency of the incinerator. For MHFs, if additional control of POMs is neces-
sary, a high temperature afterburner may be required. This would not be necessary
for a fluid bed incinerator. Emission factors from the U.S. EPA Compilation of Emis-
sion Factors (AP-42) indicate that emissions of the above pollutants are low for both
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–9
MHFs and fluid bed incinerators, with ranges as follows: 0.5 10 to 0.5 10 g/kg
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–9
(1 10 to 1 10 ) (U.S. EPA, 1998).
1.2.5 Nitrogen Oxide
Nitrogen oxide is an important pollutant because it is an ozone precursor that is acti-
vated by UV light in the upper atmosphere to produce ozone. Because many of the
large metropolitan areas in the United States are designated as ozone nonattainment
areas, NOx emissions are carefully scrutinized by air quality control boards.
Nitrogen oxides can take many forms, with the nitrogen atom combining with
one or more oxygen atoms. Nitrogen oxide is a product of all air oxidized combus-
tion processes, including incineration. The two mechanisms that generate NOx emis-
sions during the combustion process are fuel NOx and, to a lesser extent, thermal
NOx. Fuel NOx is produced from the oxidation of the organically bound nitrogen in
the fuel. Its formation rate is strongly affected by the rate of mixing of fuel and air
and the local oxygen concentrations and combustion temperatures. Residuals from
WWTPs have ample quantities of nitrogen (typically 3 to 6%) to generate NOx, but
fortunately only a small fraction of the nitrogen in the feed cake is converted to NOx.