Page 176 - Wastewater Solids Incineration Systems

Emission Control and Monitoring 143

Thermal NOx is generated from the thermal conversion of the nitrogen and oxygen
in the combustion air to NOx at high temperatures. The rate of thermal NOx forma-
tion is highly dependent on local flame temperatures and, to a lesser extent, on
oxygen concentrations. Because thermal NOx becomes significant at temperatures
higher than 1093°C (2000°F), thermal NOx is not as important as fuel NOx in munic-
ipal WWTP incinerators. However, auxiliary fuel burners on an MHF can be a signif-
icant source of thermal NOx.
Multiple-hearth furnaces can have significant NOx emissions, particularly if
combustion temperatures exceed 899°C (1650°F). Nitrogen oxide concentrations from
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an MHF will typically range from 250 to 587 mg/Nm dv (150 to 400 ppm dv ).
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Niessen (1990) analyzed 154 sets of MHF data and found that NOx concentrations
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from an MHF averaged 417 mg/Nm dv (284 ppm dv ), with a standard deviation
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of 240 mg/Nm dv (164 ppm dv ).
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In contrast, because of its lower combustion temperatures of lower than 871°C
(1600°F) and lower excess air levels (lower percent oxygen), a fluid bed incinerator
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typically has NOx emissions of less than 147 mg/Nm dv (100 ppm dv ). A fluid bed
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incinerator can have higher NOx emissions if the feed cake solids are high (greater
than 28%) and the volatile content is also high (greater than 80%) (Dangtran and Butt,
2004; Sapienza et al., 1998). Particularly at plants with high-solids centrifuges, it is
important to appropriately size the combustion air preheated (heat exchanger) to
achieve a combustion air preheat temperature that will result in combustion temper-
atures of approximately 843°C (1550°F) in the freeboard of the fluid bed incinerator.
If the combustion air preheat temperature is too high, the freeboard temperature can
climb to higher than 871°C (1600°F) and NOx emissions will increase. Preferably, the
fluid bed incinerator system will be designed for as wide a range of percent solids
and percent volatile solids as is practically possible. This can be done by providing a
bypass duct (with flow control valve) around the bottom portion of the combustion
air preheater which will allow the temperature of the preheated combustion air to be
adjusted depending on the temperature in the fluid bed incinerator.
In general, NOx emissions from a fluid bed incinerator can be controlled by lim-
iting peak temperatures to lower than 871°C (1600°F) and minimizing excess air
levels while maintaining adequate combustion efficiency. If feed cake with high
solids and high volatile solids content is anticipated, flexibility should be incorpo-
rated to the system design as discussed above. Achieving low NOx emissions with
an MHF is more difficult and may preclude its use in sensitive air quality areas.