POWER July 2010 - 46

AIR QUALITY MANAGEMENT
Circulating Fluid Bed Scrubbers
Bridge the Gap Between Dry and
Wet Scrubbers
Circulating fluid bed (CFB) dry scrubbing technologies provide distinct advantages
over conventional spray dryer absorber scrubbers for removing
SO2 from flue gases. The CFB also competes well against wet limestone
flue gas desulfurization processes typically favored for large boilers firing
high-sulfur coals. With high SO2 removal rates in a dry treatment process,
the CFB scrubber appears to be the best of both technologies: a waterstingy
scrubber with high SO2 removal rates.
By Steven Moss, Nooter/Eriksen Environmental Technologies
T
he typical processes for removing sulfur
dioxide (SO2
) from coal-fired power
plant flue gases include wet flue gas
desulfurization (wFGD) and the more recent
spray dryer absorber (SDA) technology.
Both get the SO2
removal job done, although
both process have a distinct advantages
and disadvantages, such as producing large
amounts of new waste products that require
disposal or consuming large amounts of
water. The circulating fluidized bed (CFB)
process promises high SO2
removal efficiencies,
extremely low water consumption, and
the ability to bridge the size gap between the
SDA and wFGD. To demonstrate why familiar
options may not be the best, we begin by
examining the merits of each system.
SDA Advantages and Limitations
SDA systems are typically used at plants
firing low- to medium-sulfur (2%) coal. Depending
on inlet SO2
SO2
loading, demonstrated
90% to 95% with controlled SO2
rates as low as 0.065 lb SO2
removal efficiencies in the range of
emission
/million Btu
are well documented with SDA systems.
Removal rates in excess of 90% require the
use of a fabric filter to provide additional
contact time between the lime sorbent and
the SO2
Spray
dryer
Particulate collector
sorber vessel and introduced through one
or more high-speed (11,000 rpm) spinning
wheels within rotary atomizers located on the
roof of the vessel (Figure 1). The atomizer(s)
are centrally located within flue gas dispersers.
As flue gas is accelerated and dispersed
evenly around the perimeter of the spinning
wheel, the pollutants are brought into intimate
contact with a spray cloud of atomized
droplets containing the sorbent. The sorbent
reacts with SO2
and SO3
to form calcium
sulfite and calcium sulfate while simultaneous
cooling of the flue gas occurs. The flue
gas then exits the absorber and is directed to
a particulate collection device, either a fabric
filter or electrostatic precipitator.
SDA absorbers are downflow reactors,
with the atomizers and gas dispersers
located on the roof of the vessel.
The absorber design is limited by two
basic parameters: capacity of the rotary
atomizer(s) and dispersion of the flue gas.
As SDA absorbers are applied to increasingly
larger gas flow rates, the energy of
the gas as it is compressed and accelerated
through the disperser will overwhelm and
imbalance the spray cloud of atomized
droplets. One SDA absorber design splits
the gas flow into a roof disperser and a
centrally mounted disperser to balance the
spray cloud from above and below. Another
absorber design incorporates three roof1.
Typical SDA dry scrubbing system schematic. Source: Nooter/Eriksen
Flue gas
ID fan
. The filter cake on the fabric filter
bags acts as a fixed bed of solids and
provides an additional 1 second of contact
time for enhanced mass transfer of SO2
SDA systems, SO3 removal efficiencies are
. For
typically 99%. Details of SDA technology
applied to recent coal-fired projects are
available online at www.powermag.com
using the keyword " SDA. "
In the typical SDA system, the reagent
slurry is pumped to the top of the SDA ab46
Disposal
Water
Lime
Slaker
Water
Disposal
Recycle
silo
Storage
tank
Feed tank
www.powermag.com
POWER | July 2010
http://www.powermag.com http://www.powermag.com
POWER July 2010

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