POWER July 2010 - 47

AIR QUALITY MANAGEMENT
mounted gas dispersers to avoid any need
to balance the spray cloud.
Ultimately, the application of SDA
technology is limited by the motor, gear
box, and wheel capacities of the rotary atomizer.
SDA systems are in operation at
numerous generating plants ranging in size
from 10 MW to over 600 MW. However,
multiple absorber vessels have historically
been required for plants greater than 300
MW. Practically speaking, SDA absorbers
with a single large 1,000 hp atomizer or
three 250 hp atomizers designs are limited
to facilities generating approximately
350 MW. For capacities in excess of 350
MW, a second absorber vessel will result
in more complex system layout and additional
capital costs.
Operating labor and maintenance costs
for any FGD system are functions of the
system complexity and the number of
rotating pieces of equipment within the
envelope of the system. All systems with
atomizers must be taken off-line periodically
for inspection, cleaning, and parts replacement.
The high-speed atomizers will
develop scaling on the wheels, causing a
vibration in the output shaft of the gear
box or motor. Wear components, including
wheel inserts, gear box bearings, and motor
bearings, require monthly inspections
and cleaning and periodic replacement
(typically annually).
Plants will experience a reduction in SO2
removal when a single large rotating atomizer
must be removed and replaced with a
spare. For SDA systems with multiple atomizers,
there is less SO2
removal degradation
during these maintenance periods, but there
will be a higher frequency of interruptions
due to a greater number of machines. Also,
the slakers, screens, mix tanks, agitators,
pumps, piping, and valves associated with
the lime preparation system represent additional
maintenance items.
All dry FGD processes incorporate an
aqueous phase reaction between a lime sorbent
and gaseous SO2
and SO3
. With FGD
systems incorporating SDA technology, the
lime sorbent is usually (pebble) quicklime
(CaO), which must be prepared as a slurry
reagent, with water added during slaking of
the quicklime. The complete lime-handling
and preparation system consists of truck or
rail car unloading, solids-handling equipment,
storage silos, detention or ball mill
slakers, and slurry storage tanks. The resultant
lime slurry also contains dust from
the system particulate collection device to
increase the total solids content of the mixture.
The final ratio of solids (slaked lime
plus particulate dust) to water in the slurry
is always less than 1:1.
July 2010 | POWER
2. Typical wet FGD system schematic. Source: Nooter/Eriksen
Air
Fabric filter
Water
Water
storage
Absorber
Recirculation
ID fan
Byproduct discharge
Boiler
Byproduct
silo
Hydrated
lime silo
Dry lime
hydration
Lime silo
Stack
Lime
Recovery
Wet Limestone Scrubbing
Pros and Cons
Wet limestone FGD systems are well recognized
as the predominant FGD technology
for large power generators around the
world. The process uses a slurry of ground
limestone (CaCO3
) instead of the slaked
lime used in SDA processes or hydrated
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POWER July 2010

Table of Contents for the Digital Edition of POWER July 2010
