A Presentation for Power-Gen Asia
1-3 September 2015, Bangkok Thailand
Reversing Ash Buildup Problems
in SCR Reactors with Ash Sweepers
Robin Chen
Power Generation Group
Martin Engineering China
ABSTRACT
This presentation will look at how an ash sweeper system can solve many of the problems with
ash accumulation currently experienced in many SCR’s.
Industry experts estimate that ash buildup issues affect as many as 58% of the Selective Catalytic
Reduction (SCR) systems.
The trend in the power generation industry has been to combat these SCR ash piling problems by
replacing the catalyst with larger openings. The idea is simple: the larger the opening the less
likely the ash is to build up. The problem is that by doing this, the SCR loses surface area which
is critical for the removal of NOx. In addition the reduced surface area shortens catalyst life and
increases ammonia usage.
Ash sweepers provide a periodic discharge of compressed air across the top of the catalyst layer,
to prevent buildups and return ash to the gas stream. The development of the ash sweeper system
has allowed power plants to reverse the trend and even install smaller pitch catalyst, while
increasing both NOX removal and catalyst life. In addition this solution offers a lower capital
investment requirement.
This paper will outline the success results in two different installation with ash sweeper systems.
INTRODUCTION: BUILDUPS IN SCRS
Many coal fired power plants now have installed a reactor for a Selective Catalytic Reduction—
an SCR-- as a pollution control device. The SCR uses ammonia and catalyst to convert the
nitrogen oxides –the NOx—in the flue gas into nitrogen and water.
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But inside the SCR, fly ash from the
combustion exhaust drops out of the
exhaust and collects on the layers of
catalyst. This reduces the SCR’s
efficiency, because the NOx must contact
the catalyst to cause the reaction. It also
leads to ammonia passing through the
SCR without reacting, called “ammonia
slip.”
The unmixed ammonia will form ammonia
bisulfate (ABS). ABS can cause a sticky
and very corrosive buildup, which leads to
blockages and corrosion issues in the air
heaters and ESP’s.
Typical ash accumulation on the catalyst inside an SCR.
The effects of the buildup can be significant. Past papers have been presented which indicated
the buildup in the air heater can cost over a million dollars per year in thermal efficiency.
Buildup in the SCR also leads to increased gas velocities through the system, which increases
erosion and thus shortens catalyst life. And this buildup also leads to an increase in pressure
drop, which increases fan power consumption. In addition, the accumulation of this fly ash can
shorten the life of the catalyst, closing the passages, and leading to erosion of those passages that
remain open.
Industry experts estimate that buildup issues affect as many as 58% of the Selective Catalytic
Reduction (SCR) systems. This buildup not only occurs in the SCR but also in the duct work
leading to the SCR. Buildup in duct work is becoming an increasing important safety issue since
in the recent past two years ducts have collapsed in the power industry.
Fighting Buildups with Sonic Horns
To reduce these penalties in efficiency and in operating costs, plants have used several
techniques to reduce and or cleanup (mitigate) the ash accumulations. Sonic horns, which use the
energy of high intensity, low frequency sound to keep dust in suspension, have been accepted as
the best choice for cleaning by the suppliers of SCR systems.
Sonic horns installed on each layer of catalyst in the SCR will reduce fly ash buildup and
improve the plant’s pollution control. The horns generate a burst of sound at frequent intervals to
prevent the buildup of particulates across the catalyst face. They prevent buildup, and should not
be used to try to knock down hardened accumulations.
The typical spacing between horns on an SCR is 9 to 12 feet apart—that is, between two-and-ahalf and four meters. The horns are driven with plant compressed air, typically supplied at 80-90
psi at 60 to 70 cfm.
But the Problem Persists
But the operation of sonic horns does not always eliminate the problems with ash buildup. Often
the cause can be poor gas flow distribution, which allows a large amount of ash to drop out in a
concentrated area, much like a river where a sand bar will starts on an inside bend where the
flow slows. This large concentration overwhelms the sonic horns.
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To minimize these buildups and the problems they cause, engineers conceived of using blasts of
air to remove these concentrations, suspending the particles in the air so they can move through
the SCR. To supply this blast of air, we proposed to apply ash sweepers. The ash sweeper
systems may be used as a replacement for, or a supplement to, the sonic horns.
So What Is An Ash Sweeper?
Based on air cannon technology, an ash sweeper is a reservoir of plant compressed air, held
behind an actuator valve. When the valve is triggered, the stored air is released out of the tank as
an eruption. The air is discharged through outlets nozzles mounted in the walls of vessels.
Following discharge, the exhaust valve closes, and the air reservoir typically then automatically
refills with air to await the next controlled release.
A number of different air cannon systems are available, including "individual tank” air
cannons, which each serve a single opening into the vessel, and “multiple port” systems,
which feed several outlets alternately from a single air storage tank.
Used in industrial applications since the 1970's, air
cannons are generally installed to improve the
discharge of bulk materials from storage or the flow of
material through a process vessel. Air cannons have a
long history of success in high temperature
applications such as cement kiln preheater towers and
clinker coolers. Consequently, engineers were
confident the ash sweepers could operate in the high
temperature and dusty conditions inside an SCR.
Ash sweepers on an SCR
Ash sweepers are usually
installed on first level of
catalyst–the area that
captures the most ash—
providing overlapping
coverage with sonic horns.
The ash sweepers are placed
four to six feet apart, with the
discharge nozzles placed
directly below the bell of the
sonic horns.
“Multiple port” ash sweeper system
Ash sweeper nozzles are placed to discharge across the top of the
catalyst to sweep ash particle back up into the gas flow.
In a combined cleaning system, the ash sweepers should discharge at the start of the horn cycle,
to break any adhered material loose. The horn continues to sound after the ash sweeper has fired,
to keep material in suspension.
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This combined system of combining ash
sweepers with sonic horns to prevent ash
accumulation and improve efficiency has now
been tried in several coal-fired power plants in
North America. The following are reports on
two of these cases. The first discusses installing
ash sweepers on an SCR; the second covers ash
sweepers installed to clear a ductwork passage.
Combined cleaning system composed of
sonic horns and ash sweepers.
CASE 1: TECO BIG BEND STATION
Tampa Electric, a subsidiary of TECO Energy,
operates five power plants in Florida. The largest
plant is Big Bend Power Station, located across
Tampa Bay in Hillsborough County near the
community of Apollo Beach. Big Bend Power
Station has four coal-fired generating units with a
combined output of more than 1,700 megawatts.
The first unit began service in 1972; the second
and third generating units were added in 1973
and 1976, respectively; with the fourth unit
coming online in 1985. An additional natural gasand fuel oil-fired unit was installed in 2009.
TECO Big Bend Power Station
The units burn a high-sulfur Eastern bituminous coal, which at times is blended with up to 20%
pet coke in the plant’s four boilers. Over the years, pet coke has proven to be troublesome in
SCR, by causing buildup and poisoning the catalyst.
In 2007, TECO began to retrofit SCRs to the four boilers at Big Bend station to ensure
compliance with emission permit limits. This installation was described in an article Big Bends
Multi-Unit SCR Retrofit, published in March 2010 edition of Power Magazine. As that article
pointed out, adding the SCR’s into the existing plant confines provided “particularly complex
challenges.”
Installed in the ‘high dust’ location before the precipitator, the four SCRs were engineered by
Sargent & Lundy and each equipped with roughly one million pounds of 6.9 pitch Cormetech
honeycomb catalyst.
Due to the limited real estate, and the
existing plant configuration, a less-thanideal gas flow path led to problems.
Although sonic horns had been incorporated
in the overall design, the Big Bend SCRs
still had major build-ups of ash on several
of the top layers of the catalyst. Buildups on
the front wall of the SCR required cleaning
four times per year; at times, the buildup
would cover 50% of the catalyst surface.
Ash accumulation inside the SCR at Big Bend
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In many of these cleanings, the plant was
forced to clean each passage through the
“honeycomb catalyst individually by poking
out ash with a welding rod.
The cost and time during outages to remove
the build-up was significant, and the buildups
increase the amount of “ammonia slip”
through the SCR, and so increased the
amount of ammonia consumed and hence the
SCR’s cost of operation.
After a review of the problem and the options
for solution, the plant agreed to try the
installation of ash sweepers on the SCR to
control these buildup problems.
Two eight-port multi-port ash sweeper
systems supplied by Martin Engineering
were installed in November 2011. These
systems serve a total of 16 fan-jet nozzles
spaced across the top layer of one SCR.
In addition, all the sonic horns installed
on the SCR were rebuilt. These horns
suffered from a buildup of ABS and some
had ceased working. Horn sections
suffering from ABS buildup were
replaced, and all sound generators were
rebuilt. Insuring proper operation of the
sonic horns is important to success of the
combined cleaning system.
Multiport ash sweeper system
installed on SCR at Big Bend Station
Fan jet nozzles installed inside the SCR
Big Results at Big Bend
In a shutdown at the end of three months of
run time, the plant evaluated the performance
of the combined cleaning system. An
inspection of the SCR at the end of February
2012 showed the top layer of catalyst had
virtually no build-up of ash.
This led plant officials to investigate and
verify savings derived from the installation of
the ash sweepers systems. Their calculations
included a significant reduction in SCR
cleaning costs, as shown in Table 1.
Reduced buildup inside SCR after installation
of ash sweeper system
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Table 1. SCR Cleaning Costs at TECO Big Bend
Date
of Outage
Days in
Labor
Operation Cost
Prior to
Cleaning
Vacuum
Truck
Cost
BEFORE
March 2011
March 2011
September 2011
October 2011
37
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36
50
$18,701
$17,053
$10,897
$21,849
$14,629
$18,426
$11,146
$27,789
AFTER
March 2012
June 2012
47
10
$9931
$5439
$9463
$5560
Table 2 compares outages categorized as similar, based on the length of operating time leading to
the outage. This Table 2 comparison indicates that cleaning the SCR following the installation of
the ash sweeper systems required less than 50 percent of the labor and truck expense as the
experience “before installation.”
Table 2. Comparing SCR Cleaning Costs based on Length of Operation
Before Installation
October 2011
After Installation
March 2012
Labor
“Run
Cost
Time”
(Days)
Comparison A
50
$21,849
47
$9,931
Vacuum
Truck
Cost
Total
Savings
$27,789
$9,463
$ Saved $11,918
$18,326
$30,244
% Reduction in Cleaning Cost
55%
66%
Comparison B
Before Installation
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$17,053
$18,426
March 2011
After Installation
10
$5,439
$5,560
June 2012
$ Saved $11,614
$12,866
$24,470
% Reduction in Cleaning Cost
68%
70%
Total Savings (two outages ) $54,714
(Four outages per year) Estimated Annual Savings $120,000
Following the two “after installation” outages, the savings over comparable “before installation”
outages” came to $54,714. Estimated annual savings from the plant’s typical four-outage
schedule reach $120,000. (Table 2)
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Reductions in Ammonia Cost
In November and December of 2011—before the ash sweeper installation—plant figures show
the SCR consumed 2.00 pounds of ammonia for ever MW/hour produced. In January and
February 2012, after the installation, the ammonia usage dropped to 1.80 pounds per megawatt
hour. When calculated with the plant’s annual consumption of ammonia, this reduction provides
an annual savings of 209 tons of ammonia.
With the plant’s
budget electricity
production for 2012
for 2093 gigawatts at
the plant’s contracted
ammonia cost for the
past year of $472 per
ton, this provides an
annual savings in
excess of $98,000.
(Table 3.)
Table 3. Ammonia Usage at TECO Big Bend
Ammonia Used
(Pounds per MW/hour)
November/December 2011 Before Installation
January/February 2012 After Installation
Reduction in Ammonia Consumed per MW/hour
Yearly Reduction in ammonia consumption (lbs)
Converted to tons
Ammonia price (delivered)/ton
Savings in ammonia consumed
2.00
1.80
0.20
418,600
209.3
$472
$98,789.60
At the plants where the ammonia cost is closer to the industry average of $750 per ton, the
annual savings would be more than $150,000.
Reductions in Pressure Drop
Buildups in the SCR create an increase in pressure drop, which in turn causes the ID fan to work
harder, consuming more electricity as it tries to maintain gas flow. The pressure drop caused by
poor SCR performance then results in additional ash buildups in the SCR, air preheater,
ductwork, and precipitator, magnifying and accelerating the problem. In some cases, this
pressure drop (DP) became so severe it required stopping the boiler to allow the SCR to be
cleaned.
The improved flow through the catalyst resulting from the addition of the ash sweeper system
provides other improvements and other savings to the plant. One benefit was a reduction in
system pressure drop of 2.5 inches water column. At the unit load of 380 MW, this resulted in a
reduction in the power consumption by the plant’s ID fan of 1.93 MW/hour. Assuming an
operating factor of 85%, this turns into an annual savings of $356,418.10. This means less
electricity used inside the plant, and more electricity to sell.
Improved Catalyst Performance and Life
In addition to the operating savings, the key benefit of the ash sweeper system is the improved
performance of the SCR in helping the plant achieve its regulated emission levels.
When ash builds up, it blocks usable catalyst and so increases wear on the catalyst sections that
are still exposed. Erosion is a problem but more importantly, the activity of the catalyst—its
ability to stimulate the reaction of ammonia and flue gas inside the SCR—is reduced much
faster.
For one thing, the plant now receives the benefit of all the catalyst it paid for. If catalyst costs a
million dollars a layer, and a layer is 40 percent covered with ash (and so NOT interacting with
the flue gas), the plant has lost $400,000 of its investment.
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In addition, the flow rate through the catalyst that remains open is increased, and as a
consequence, receives more erosive wear. In essence, it wears out sooner, because it has more
flue gas with ash moving through it at an increased speed.
And this effect is not only seen on the first layer--the layer with the buildup—but is also seen on
the following second and third layers of catalyst, because of the restrictions in the first layer have
narrowed the flow path through the lower layers.
And one final note: if the buildup deflects
the flow of gas at a 30 degree angle—so
that the particles in that gas strike the
catalyst at an angle--this angled assault
increases the erosive wear on the catalyst
at three times the previous rate. In short,
the catalyst adjacent to the pile wears three
times faster than normal because it must
cope with the “run off” from the pile of
ash.
Improvement in Plant Safety
Ash accumulations change gas flow,
increasing errosive wear on catalyst.
A reduction in SCR cleaning improves plant safety, as any personnel trip into the SCR
exposes workers to the hazards from hot ash, as well as back, shoulder, wrist and arm injuries
from repetitive physical work in these conditions.
In summary: Prompt Payback for Big Bend
The result of the installation of the ash sweeper systems to remove the buildups from the SCR
catalyst provides a significant opportunity for savings and for operational improvements at
TECO Big Bend.
The plant’s calculations
showed an operational
savings of more than
$574,000 annually. This
figure represents nearly
four times the cost of the
installed ash sweeper system.
Table 4. Combined Savings at Big Bend in 1 Year (Rounded)
Reduction in Cleaning Costs
$120,000
Reduction in Ammonia Costs
$ 98,000
Reduction in ID Fan Energy Consumption
$356,000
Total Annual Savings
$574,000
CASE 2: OPPD NEBRASKA CITY
Located about 50 miles south of the city of Omaha
along the Missouri River, Omaha Public Power
District (OPPD) operates the Nebraska City Power
Station. This is a two-unit facility, with the 646-MW
Unit 1 opened in 1979 and the 720-MW Unit 2
started-up thirty years later in 2009. Both units are
fueled with PRB coal. While Unit 2 features state-of
the art emission control systems, the operation of
these systems was not without problems.
OPPD Nebraska City Station
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Buildups in the ductwork
Nebraska City Station was experiencing ash buildup in
the ductwork leading from the plant’s dual SO2
scrubbers to the baghouse. The outlet ductwork from the
two scrubbers joined together in a horizontal run and
then as a combined flow the duct passage turned up to
the baghouse inlet plenum. A number of vertical vanes
are located just before the juncture of the two horizontal
ducts, followed by a number of horizontal turning vanes
located where the duct sloped up to the baghouse. With
the combining of gas paths and the upward slope of the
duct a substantial amount of fly-ash would drop out of
the gas flow, building on the slope plate and burying the
horizontal turning vanes.
Ash buildup inside the ductwork
The ash buildup in this 18-foot tall duct had reached a height as much as six feet. Plant and
pollution control OEM engineers were concerned that the weight of this buildup would exceed
the structural capacity of the ductwork, creating a safety issue and threatening compliance with
environmental permits.
The ash sweeper system
To remedy the problem with the material buildups in the ductwork, Nebraska City plant agreed
to install ash sweeper systems from Martin Engineering.
Installation consisted of two independent sets of multi-port ash sweeper systems, one set for each
scrubber, with eight ports on each duct outlet generally positioned between or upstream of the
vertical turning vanes.
The number of vertical turning vanes played a major role in determining the number of ash
sweeper discharge nozzles used.
Fanjet nozzles were used on the ash sweeper discharge ports in the ductwork, to provide a
wider (if somewhat shorter) cleaning action. At the Nebraska City plant, the fan jet nozzles
proved effective in moving ash 15 to 20 feet.
After installation and operation of the multiport ash
sweeper system on the ductwork, the overall ash
depth has been reduced and the area where the
system was installed has been swept clean.
Success at Nebraska City
During the Nebraska City plant’s last outage cycle, the
PLC controlling the ash sweeper cycle on the “A” set of
ash sweepers had failed, and so the ash sweeper system
was not functional.
Inspection of the “A side” ductwork showed an ash level
estimated at 54 inches deep along the horizontal duct
with about 48 inches of buildup at the vertical turning
After installation of the ash sweeper,, ash
buildups at the turning vanes was reduced.
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vanes where the ash sweeper discharge nozzles are located. The slope plate was covered up to
the bottom horizontal turning vane about 30 inches above the duct floor
On the “B” side where the ash sweeper system remained in operation showed a significant
reduction in ash buildup. The ductwork at the nozzle installation between the vertical turning
vanes was cleared to the bare metal and the ash drop out downstream on the upward slope plate
was estimated at one half the depth of the “A” side. The horizontal portion of the outlet duct that
is located upstream of the ash sweeper had a deposit of ash 28 inches deep.
The reduced level of ash in the horizontal duct is believed to be a result of the ash sweeper
installation combined with process changes in scrubber operations. Future operation with
consistent PLC functionality will be needed to determine upstream impact.
As the ash sweeper systems in the Nebraska City Station reduces the depth of the ash
accumulations inside the ductwork, the plant anticipates it will reduce the number of hours (and
resulting costs) required to clean the ductwork during plant outages.
In Conclusion
Ash sweeper systems have now been installed in coal-fired power plants in China and eliminates
some significant ash buildups.
The installation of ash sweepers on selective catalytic reactor systems provides significant
improvements for coal-fired power plants.
These benefits include extending life of catalyst, reducing ammonia consumption and cost,
Reducing cleanup cost and frequency, and improving plant efficiency by extending the length of
time available for operation before cleanup requires an outage.
Additional installations will provide further opportunities for research and verification of the
results discussed above.
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