Page 173 - Wastewater Solids Incineration Systems

140 Wastewater Solids Incineration Systems

will effectively remove SO . At temperatures higher than approximately 149°C
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(300°F), SO is a vapor. Once the vapor cools below its acid dewpoint, it condenses
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into sulfuric acid mist (H SO ) which can cause a visible plume (National Lime Asso-
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ciation, 2002). Some coal-fired power plants with wet scrubbers have had to install
wet ESPs to capture the sulfuric acid mist emitted from their wet scrubbers.

1.2.2 Carbon Monoxide
Carbon monoxide is a product of incomplete combustion (PIC) from partial oxida-
tion of carbon in the feed cake with the oxygen in the combustion air. The formation
of CO is caused by one or more of the following deficiencies with the combustion
system: inadequate temperatures, inadequate residence time of the combustion
gases, or inadequate mixing or turbulence which is necessary to bring the combus-
tion gases in dynamic contact with the oxygen in the air supply. In general, the com-
bustion environments of an MHF and fluid bed incinerator are markedly different
and each will be discussed separately.
In an MHF, the feed cake first dries on the upper hearths and then is combusted
on the middle hearths. Although this arrangement is efficient in terms of using the
heating value in the dried feed cake to dry the incoming wet feed cake, it results in
the release of partially oxidized combustion gases and PICs from the upper hearths
of the furnace where the feed cake is drying and just beginning to burn. The slow,
stratified flow of combustion gases (i.e., lack of turbulence) in this part of the furnace
results in increased emissions of CO and PICs. Carbon monoxide emissions from an
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MHF without an afterburner can range from 900 to 2700 mg/Nm dv (1000 to 3000
11
ppm dv ); a typical CO mass emission rate is 15.5 g/kg (31 lb/dry ton) of solids incin-
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erated. High CO and VOC emissions from an MHF are one of the primary reasons
that their use has declined steadily over the last several decades. High CO emissions
from an MHF can be controlled by use of a high-temperature afterburner. Several
MHFs have been retrofitted with top hearth (or “zero hearth”) afterburners, which
have significantly reduced their CO and VOC emissions. However, the additional
fuel use required to operate the afterburner is a significant drawback.
In contrast to an MHF, a fluid bed incinerator is a completely mixed, highly tur-
bulent system in which drying and combustion take place concurrently and rapidly,
within a matter of seconds. The turbulent fluid bed of a fluid bed incinerator pro-
vides complete and intimate contacting of the feed cake, the volatilized gases, and
the oxygen in the fluidizing air. The hot combustion gases rising from the bed then
enter the freeboard of the fluid bed incinerator, which provides a long gas residence