Spray dryers followed by fabric filters have become the control option of choice for municipal
waste incineration facilities. A survey conducted in 1990 by the Institute of Resource Recovery
(IRR) reported that of 158 municipal waste combustion facilities, 47 used fabric filters for
particulate control. Almost all of these were preceded by a spray dryer. In fact spray dryers followed
by fabric filters are typically considered best available control technology for municipal
waste incinerators since this equipment is effective in removing acid gases, particulate matter,
and a number of hazardous air pollutants.
Modern municipal waste incinerators recover waste heat by using boilers to generate steam
and electricity. After passing through the heat recovery equipment, the flue gas typically enters
the air pollution control system at 350 to 400°F (177 to 204°C). Emission controls typically
consist of a spray dryer absorber to remove acid gases followed by a fabric filter to remove
particulate matter, which includes acid gas reaction products, unreacted reagent, fly ash, and
trace metals. A survey of spray dryer applications on municipal waste incinerators in the U.S.
shows that lime is used exclusively as the reagent. Onsite lime slaking systems are typically
used to prepare the lime slurry.
A calcium hydroxide [Ca(OH)2] slurry, frequently referred to as lime slurry, is injected into the
spray dryer reaction vessel as a finely atomized spray. Acid gases (mainly HCl and SO2) are
absorbed into the atomized lime slurry. The hot flue gas causes the water in the droplets to
evaporate and leave behind dry reaction products (calcium salts).
Spray dryers must be operated at flue gas temperatures adequate to produce a dry reactant
product. Spray dryers are typically designed to operate with an inlet (flue gas) temperature of
approximately 350 to 400°F (177 to 204°C) and outlet temperature of 260 to 300°F (127 to
149°C). Some major benefits can be realized when operating at these temperatures, including
increased boiler efficiency, lime utilization, and trace metal and organic removal efficiency.
Potential operating problems can occur when handling the reaction products that contain calcium
chloride (CaCl2). This material is hygroscopic, and can cause caked deposits on reactor
walls, bag plugging or blinding problems in the baghouse, and/or caking and plugging problems
in the fly ash removal equipment. The spray dryer and fabric filter must be operated
within the above specified design temperature limits, be well insulated, and be designed to
minimize air inleakage to prevent these potential problems from occurring.
A fabric filter is used downstream from the spray dryer to collect reactant products, unreacted
sorbent, and fly ash. Fabric filters applied to incinerators often use woven fiberglass bags to
remove particulate matter from the flue gas stream. Fabric filters can act as secondary acid gas
collectors because the dust cake that builds on the bags contains some unreacted sorbent that
provides a surface to neutralize some of the acid gases passing through the cake. Many recent
fabric filter designs applied to municipal waste incinerators use pulse-jet cleaning and have
easily achieved the NSPS of 0.015 gr/dscf corrected to 7% O2 (Pompelia and Beachler 1991).
Use of fabric filters on municipal waste incinerators is also effective in removing heavy metals
and organics (Brna and Kilgroe 1990).
Performance of this equipment has been studied in depth since the mid 1980s in support of
revising the NSPS for Municipal Waste Combustors (58 FR 5488). Typically, use of a spray
dryer followed by a fabric filter has shown to remove 75 to 85% of SO2 and 90 to 95%ofHCl.
Higher removal efficiencies have been achieved when calculating removal efficiencies over
long term time periods (i.e. long term averages) (EPA 1989; Beachler and Joseph 1992).
waste incineration facilities. A survey conducted in 1990 by the Institute of Resource Recovery
(IRR) reported that of 158 municipal waste combustion facilities, 47 used fabric filters for
particulate control. Almost all of these were preceded by a spray dryer. In fact spray dryers followed
by fabric filters are typically considered best available control technology for municipal
waste incinerators since this equipment is effective in removing acid gases, particulate matter,
and a number of hazardous air pollutants.
Modern municipal waste incinerators recover waste heat by using boilers to generate steam
and electricity. After passing through the heat recovery equipment, the flue gas typically enters
the air pollution control system at 350 to 400°F (177 to 204°C). Emission controls typically
consist of a spray dryer absorber to remove acid gases followed by a fabric filter to remove
particulate matter, which includes acid gas reaction products, unreacted reagent, fly ash, and
trace metals. A survey of spray dryer applications on municipal waste incinerators in the U.S.
shows that lime is used exclusively as the reagent. Onsite lime slaking systems are typically
used to prepare the lime slurry.
A calcium hydroxide [Ca(OH)2] slurry, frequently referred to as lime slurry, is injected into the
spray dryer reaction vessel as a finely atomized spray. Acid gases (mainly HCl and SO2) are
absorbed into the atomized lime slurry. The hot flue gas causes the water in the droplets to
evaporate and leave behind dry reaction products (calcium salts).
Spray dryers must be operated at flue gas temperatures adequate to produce a dry reactant
product. Spray dryers are typically designed to operate with an inlet (flue gas) temperature of
approximately 350 to 400°F (177 to 204°C) and outlet temperature of 260 to 300°F (127 to
149°C). Some major benefits can be realized when operating at these temperatures, including
increased boiler efficiency, lime utilization, and trace metal and organic removal efficiency.
Potential operating problems can occur when handling the reaction products that contain calcium
chloride (CaCl2). This material is hygroscopic, and can cause caked deposits on reactor
walls, bag plugging or blinding problems in the baghouse, and/or caking and plugging problems
in the fly ash removal equipment. The spray dryer and fabric filter must be operated
within the above specified design temperature limits, be well insulated, and be designed to
minimize air inleakage to prevent these potential problems from occurring.
A fabric filter is used downstream from the spray dryer to collect reactant products, unreacted
sorbent, and fly ash. Fabric filters applied to incinerators often use woven fiberglass bags to
remove particulate matter from the flue gas stream. Fabric filters can act as secondary acid gas
collectors because the dust cake that builds on the bags contains some unreacted sorbent that
provides a surface to neutralize some of the acid gases passing through the cake. Many recent
fabric filter designs applied to municipal waste incinerators use pulse-jet cleaning and have
easily achieved the NSPS of 0.015 gr/dscf corrected to 7% O2 (Pompelia and Beachler 1991).
Use of fabric filters on municipal waste incinerators is also effective in removing heavy metals
and organics (Brna and Kilgroe 1990).
Performance of this equipment has been studied in depth since the mid 1980s in support of
revising the NSPS for Municipal Waste Combustors (58 FR 5488). Typically, use of a spray
dryer followed by a fabric filter has shown to remove 75 to 85% of SO2 and 90 to 95%ofHCl.
Higher removal efficiencies have been achieved when calculating removal efficiencies over
long term time periods (i.e. long term averages) (EPA 1989; Beachler and Joseph 1992).
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