POWER July 2010 - 48

AIR QUALITY MANAGEMENT
lime used in circulating fluid bed (CFB)
systems. After truck or rail unloading of
gravel limestone, the slurry is prepared
using a system typically including a horizontal
ball mill system that includes silos,
feeders, and hydroclones in addition to the
ball mills. Details of FGD technology applied
to recent coal-fired projects are available
online at www.powermag.com using
the keyword " FGD. "
The limestone slurry is next pumped
through banks of spray headers in the absorber
vessel (Figure 2). Flue gas is introduced
at the bottom of the absorber vessel,
beneath the spray nozzles. The slurry droplets
created by the spray nozzles flow countercurrent
to the incoming flue gas, where
intimate and uniform contact of SO2
and the
calcium-rich reagent occurs.
The reactions in the absorber produce
a mixture of calcium sulfite and calcium
sulfate. Air, injected beneath the flue gas,
promotes oxidation of sulfites to sulfates.
The resulting CaSO4
by-product (gypsum)
is precipitated. The oxidized slurry is recirculated
to the spray headers, but a portion
of the slurry is withdrawn to remove
the gypsum in a separate recovery system
that includes hydroclones and drum or belt
filters. The recovered gypsum can then be
3. CFB dry scrubbing system schematic. Courtesy: Nooter/Eriksen
Water
Limestone
Clean gas
Storage
tank
Effluent
Gypsum
Flue gas
Air
Reagent feed tank
Absorber
Belt filter
used in commercial applications such as cement
additives or wallboard.
Because of their extensive limestone
preparation and gypsum recovery systems,
wet FGD processes have high capital, power,
and maintenance costs as compared to
either SDA or CFB dry systems. The limestone
raw material is a lower-cost reagent
than the quicklime or hydrated lime used in
the dry processes, but wet FGD processes
require 30% to 40% more water, which poses
a challenge in arid locations with limited
water availability. Furthermore, absorber
vessels for a wet system require alloy or fiber-reinforced
plastic construction, whereas
for dry processes unlined carbon steel is
adequate. Those more-expensive materials
increase costs for wet FGD systems, especially
for single absorber systems.
Wet FGD is routinely used at large-scale
utilities firing medium- to high-sulfur (≥3%)
coals and result in SO2
to 99% (depending on inlet SO2
removal efficiencies
loading)
with a practical controlled rate of 0.04 lb/
million Btu. To achieve SO3
removal efficiencies
greater than 60% using wet FGD,
additional equipment (adding both cost and
pressure losses) may be required if the inlet
SO3
level exceeds 5% of the total inlet SOx
mass flow. Single wet FGD absorber sizes
in excess of 1,000 MW have been installed.
(Details of wet FGD systems used on many
coal-fired power plants are available online
at www.powermag.com using the keyword
" wet FGD. " )
4. Gaining market share. A CFB dry scrubbing system is operating at the 300-MW
Theiss Power Station located in Austria. There are approximately 17 CFB dry scrubber installations
in Europe operating at coal-fired power plants. Source: Nooter/Eriksen
Many Advantages to Dry FGD
CFB Technology
Although it is relatively new to the U.S.
power industry, CFB technology has been
broadly applied in Europe and China. A
typical 300-MW CFB installation is illustrated
in Figure 3. Today, there are more
than 60 CFB scrubbers in operation in Europe,
with about 34 running on coal-fired
units. Seventeen of those units burning coal
are rated at 300 MW or higher. Also, 14
projects totaling 6,000 MW have been built
in China since 2000.
In North America, the largest CFB
scrubber is under construction at Basin
Electric Power Cooperative's Dry Fork Station
located near Gillette, Wyo. The 420MW
plant is expected to enter commercial
service in 2011. A video tour of the CFB
scrubber under construction is available at
(case-sensitive) www.basinelectric.com/
Projects/Dry_Fork_Station/index.html.
CFB FGD processes are substantially
different than processes using SDA absorbers
(Figure 4). CFB absorbers are upflow
reactors, wherein all reactants (SO2
SO3, lime, and water) are introduced at the
,
48
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POWER July 2010

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