POWER July 2010 - 50

AIR QUALITY MANAGEMENT
5. Comparing stats. This chart shows capacity ranges for dry and wet FGD processes
compared to the CFB dry scrubber when using a single absorber vessel. The technology bridge
conditions afforded by CFB technologies reflect capacity limits for single absorber vessels, capital
costs, labor and maintenance costs, unit availability, and SO2 removal efficiencies. Maximum
CFB single scrubber limit will vary by supplier. Source: Nooter/Eriksen
sorber and above the introduction point of
the recirculated and sorbent solids, spray
an atomized cloud of water droplets into
the bed of solids fluidized by the incoming
flue gas.
CFB technology affords an enhanced heat
SDA
CFB
Wet FGD
100 200 300 400 500 600 700 800 900 1000 1100
Applicable MW range
Notes: SDA=spray dryer absorber, CFB=circulating fluidized bed dry scrubbing, wet FGD=wet flue gas desulfurization
bottom of the absorber vessel along with
a large portion of particulate solids collected
from the downstream particulate
collection device. They need no external
lime preparation system requiring slakers,
grit screens, pumps, and associated slurry
system piping. Rather, dry hydrated lime,
Ca(OH)2
abt.power1-2 . 7 .10:abt.power1/2 .09 6/7/10 3:18 PM Page 6
, is injected into the CFB absorber
independently of the water and recirculated
particulate.
Through multiple venturis, flue gas is
introduced beneath the bed of sorbent and
particulate solids and distributed across the
full diameter of the CFB absorber vessel.
Single-phase mechanical water injection
nozzles, located on the perimeter of the abtransfer
rate within the bed by spreading the
water spray over a much larger surface area
of solids. The ratio of solids to spray water
is in the range of 20:1, translating into
an approximate 20-fold increase in surface
area as compared to an SDA absorber.
Rather than relying on the filter cake in the
downstream fabric filter for added residence
time, gases entering the CFB absorber
spend about 5 seconds passing through a
fluidized bed some 75 feet in depth, churning
within the confines of the vessel walls.
The added residence time afforded by a tall,
narrow CFB absorber vessel translates into
improved SO2
removal efficiencies within a
small FGD system footprint.
Unlike SDA absorber designs, the CFB
design is not limited by the capacities of
rotary atomizers. The only practical limit
for a CFB absorber is its ability to disperse
the flue gas throughout the vessel diameter
to provide adequate suspension of the
bed of solids. This limitation is readily ad " 18k
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POWER July 2010

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