POWER May 2013 - 92

EMISSIONS
CFB Scrubbing: A Flexible
Multipollutant Technology
The number of regulated air emission constituents is increasing while the
acceptable amounts for release are decreasing. In the long run, picking
the most flexible multipollutant technology is surely the least cost
option.
By Robert Giglio, Foster Wheeler Global Power Group
E
mission limits for conventional pollutants
emitted from power plants-particularly
SO2
, NOx, and particulate matter-continue
to tighten around the globe. In the U.S., the
Cross-State Air Pollution Rule, although vacated
in August 2012, will likely be reworked and
eventually promulgated, mandating additional
SO2
A Better Way to Clean Flue Gases
In the past, due to its proven large scale and
high ability to capture SO2
over a wide range
reductions. Consequently, plant owners
must continue to evaluate the costs and benefits
of adding a back-end air quality control system
(AQCS) against shutting down noncompliant
units. The timing of rule changes makes this a
very difficult evaluation.
The North American Electric Reliability
Corp., responsible for the bulk power system
reliability, has spoken often about the impact
of increasingly restrictive emissions regulations
on the reliability of the U.S. power
delivery infrastructure. In effect, the debate
centers on balancing lower emissions limits
with the effect on system reliability of excessive
coal plant retirements.
This discussion is not limited to the U.S.
Europe's new Industrial Emission Directive
(IED) has tightened both SOx
sion limits by 50 mg/Nm3
limits by 10 mg/Nm3
and NOx
emisand
particulate
compared to the prior
large combustion plant directive. And last
year, China lowered its SOx
, NOx, and particulate
limits for power plants to levels even
lower than those in Europe's IED.
New constituents are also being added to the
U.S. regulatory milieu, such as metals, acid gases,
and organic compounds rolled under Mercury
and Air Toxics Standards rules. Europe is
not far behind, with the IED now also requiring
a best available technology standard for these
constituents. These constituents have always
been regulated for certain waste fuel applications,
such as waste-to-energy plants and incinerators,
but now regulators have set limits for all
boilers, including large utility coal boilers.
Plant owners are being asked to make billion-dollar
AQCS purchase decisions when
the regulations are in flux and court oversight
is uncertain. In this era of regulatory ambiguity,
selecting the most flexible AQCS design
is surely the prudent decision.
92
of fuel sulfur levels, wet flue gas desulfurization
(WFGD) scrubbing technology was
the most popular choice for removing sulfur
from boiler flue gases in large power plants
and industrial facilities. WFGD technology
has a low operating cost because it utilizes
low-cost limestone as the reagent and can
produce gypsum for sale to wallboard manufacturers,
if there is a local market. However,
on the downside, a WFGD system is expensive
to build, uses the most water, occupies
the largest amount of real estate, and can
keep a full crew busy maintaining its large
number of pumps, pipes, valves, and vessels.
But more importantly, due to its chemistry
and process, a WFGD system is only marginal
for capturing metals, including mercury, or
acid gases such as SO3
, HCl, or HF.
Now with U.S. regulations requiring capture
of mercury, acid gases, dioxins, and
furans, in addition to SO2
and particulates,
other FGD technologies are becoming more
popular due to their ability to capture this expanded
set of pollutants. There are different
types of technologies, ranging from simple
injection of a sorbent into the boiler flue gas
(direct sorbent injection) to the more established
spray dryer absorber (SDA) technology
(which sprays a fine dry mist of lime into
the flue gas), to newer circulating fluidized
bed (CFB) technology, which circulates the
boiler ash and lime between an absorber reactor
and fabric filter.
These different FGD technologies have
their pros and cons, but for many midsize
to large power and industrial facilities, CFB
scrubbers are growing in popularity. This is
most evident in the pipeline of U.S. retrofit
scrubber projects, where more and more
projects are selecting CFB technology.
In the past, these alternative scrubbing
technologies were typically chosen over wet
FGD technology for their much lower capital
cost and water usage, provided that the boiler
size was not too large and the fuel sulfur
www.powermag.com
Capability/requirement
SO2 capture capable
Low water consumption
Fuel flexibility, sulfur content
Fine particlate capture
High SO3 capture
Compact system footprint
Low maintenance
requirements
Includes mercury capture
Reduces CO2 emissions
Includes wastewater
treatment
Uses low-quality water
Uses limestone reagent
Large scale (>350 MW)
Necessary for retrofit: ESP
improvements
Necessary for retrofit: Stack
improvements
Necessary for retrofit: Flue
gas reheater
POWER | May 2013
level was not too high. Today, CFB scrubber
technology has broken through these limitations
with single-unit designs up to 700 MWe
backed by operating references on coal power
plants of over 500 MWe and on fuels with
sulfur levels above 4%. CFB scrubber technology
has now stepped out in front of other
technologies due to five key advantages:
■ High multipollutant capture capability
■ Low installed cost
■ Low water use
1. Many scrubbing options. Foster
Wheeler proposes that circulating fluidized
bed (CFB) scrubbing is superior to wet flue
gas desulfurization (FGD) and spray dryer absorber
(SDA) FGD technologies. Source: Foster
Wheeler Global Power Group
Advantage
Neutral
Disadvantage
Wet
FGD
SDA
FGD
CFB
FGD
http://www.powermag.com
POWER May 2013

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