Field Demonstration of Advanced
Activated Carbon for Mercury
Control in Wet FGD
This paper demonstrates mercury control using ADA Carbon Solutions’ PowerPAC WS™ at
SRP’s Navajo Unit 3’s wet FGD over several months. This project demonstrated that our
product in conjunction with oxidation of the flue gas demonstrated mercury capture sufficient
to maintain consistently below-MATS mercury emissions.
Field Demonstration of Advanced Activated Carbon for
Mercury Control in Wet FGD
Paper # 90
Presented at the
Power Plant Pollutant Control “MEGA” Symposium
August 19-22, 2014
Baltimore, MD
Ariel (Mowen) Li,1 Joe M. Wong,1 Robert Huston,1 Sheila Glesmann,1 William Plona,2;
1
ADA Carbon Solutions, Littleton, CO, 2Salt River Project, Tempe, AZ
ABSTRACT
Wet FGD has shown a co-benefit of capturing mercury in coal-fired power plants. However, the
capture effectiveness depends on factors such as FGD operation, oxidation of the mercury and
removal of the mercury from the scrubber liquor. In some cases, mercury re-emission across the
wet FGD may make it difficult for the plant to achieve MATS compliance mercury levels
consistently. Activated carbon is a cost-effective approach to improve the mercury removal in
the wet FGD. Field testing using ADA Carbon Solutions’ advanced water powdered activated
carbon (PAC) at Salt River Project’s Navajo Station Unit 3 met the MATS mercury compliance
limit and during normal scrubber operation. The testing and results are presented.
INTRODUCTION
Salt River Project operates the Navajo Generating Station near Page, Arizona. The three baseload units have a rated capacity of 2,250 net megawatts. The plant is a zero liquid discharge
(ZLD) facility and operates wet FGD scrubbers. The plant is evaluating several approaches to
MATS compliance and this paper describes the results of an extended field demonstration of one
of those approaches, injection of activated carbon into the wet FGD scrubber, done in
combination with CaBr2 injection onto the coal feed. For the past year the station has injected
ADA Carbon Solutions, LLC’s PowerPAC WS™, a PAC product designed for excellent water
affinity and high dispersibility characteristics in a wet scrubber environment.
Wet scrubbers have been identified as a good option for mercury capture for MATS
compliance1,2. The scrubbers are originally designed for SO2 capture and, in many cases,
gypsum co-production. The mercury capture by limestone forced-oxidation scrubbers is welldocumented as varying from site to site.
Since the scrubber is a large absorber vessel on the order of a million gallons of liquid hold-up, it
is a significant accumulation opportunity for trace species. These trace species include build-up
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A. Li – Paper 90
in mercury and other species such as bromine, if not removed sufficiently in the water treatment /
recycle / bypass process. Navajo’s scrubbers are zero liquid discharge design and so
accumulation of mercury and bromine is an issue that must be addressed by the compliance
technology deployed. The ultimate fate of the mercury, other trace species and additives such as
bromine, and of PAC needs to be determined to dial in mercury control sufficiently to the low
levels needed for MATS compliance.
Additional technologies that the Navajo plant has investigated or continue to test include an
organic sulfide scrubber additive and a solid substrate downstream of the scrubber flue gas
exhaust. Navajo has run various PAC products and has had good success for compliance and
good scrubber operation over several months with PowerPAC WS.
EXPERIMENTAL METHODS
Both laboratory and field approaches were used to develop, optimize, and demonstrate the wet
scrubber mercury control additive PAC.
PAC Development
Besides possessing the proper adsorptive properties, PACs that are used in aqueous applications,
such as wet scrubbers, must provide strong aqueous affinity and dispersibilty and low particleparticle aggregation potential. ADA-CS’s PowerPAC WS incorporates proprietary surface
modifications to enhance aqueous affinity and dispersibility, resulting in high aqueous phase Hg
capture, reduced foaming potential and easy separation from gypsum.
The efficacy of these modified carbons was initially tested in the laboratory with Salt River
Project’s Plant Navajo Unit 1A absorber slurry. Three tests were completed: 1) aqueous affinity
and 2) aqueous dispersibility, and 3) particle size measurements for agglomeration.
To evaluate aqueous affinity, equal amounts of slurry were put into two beakers. The time to
wetting was measured for the baseline PAC and the developmental PAC, using careful addition
with no mixing or stirring. Table 1 shows the wetting time and the comparison of the two
products. A significant improvement was seen with the developmental product. Less wetting
time indicates that the PAC has a higher affinity for the aqueous slurry, which ensures that the
PAC remains in the aqueous phase where the oxidized Hg resides.
To evaluate aqueous dispersibility, an equal amount of each product was added to two beakers
containing utility effluent wastewater. After each PAC sample wetted in the slurry completely, a
jar tester with mixing blade completely dispersed the carbon in the test solution. The blade was
then stopped, and a timer was used to measure the time for the PAC samples to settle completely
on the jar bottom. The two products’ settling times were measured and compared. The
developmental product showed almost twice the time to settling (shown in Table 1), indicating
better dispersive qualities or suspension characteristics in the aqueous environment. Thus, the
developmental sample is more effective in staying in the aqueous phase and being in contact
with the Hg and other contaminant species.
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A. Li – Paper 90
Table 1. Lab PAC Wetting and Settling Tests.
Sample
ADA-CS Gen 1 Baseline
ADA-CS developmental
PowerPAC WS
Wetting Time, minutes
90
Baseline
10
89% Improved
Settling time, minutes
5
Baseline
9
80% Improved
The degree of PAC agglomeration in water was also tested. Less PAC particle-to-particle
agglomeration is desirable, yielding a higher available particle surface area for contact and
capture of contaminants. The particle size distributions (PSD) of comparative baseline and
developmental PACs were measured and compared to illustrate the degree of particle
agglomeration upon aqueous dispersion. In Figure 1, the solid curve illustrates PSD scan results
for the Gen 1 baseline PAC under dry powder conditions, wherein the volume average median
diameter (D50) is measured to be 22 micron. The dashed line illustrates the PSD scan results for
the Gen 1 baseline PAC following dispersion in water. The D50 was observed to increase to 26
micron following aqueous dispersion, and a substantial increase in the number of particles above
100 micron in size was observed. This PSD shift indicates particle agglomeration in an aqueous
medium. Dotted curve illustrates the aqueous phase PSD scan results from the developmental
product. The D50 was measured to be 22 micron following dispersion in water. In addition,
there were very few particles greater than 100 micron in size, indicating that the solid particles of
the treated PowerPAC WS did not agglomerate appreciably when dispersed in water and as such
yielded a more stable and effective dispersion.
Figures 1. Modification Impact on Particle Size Distribution
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A. Li – Paper 90
Plant Description
The Navajo Generating Station includes three 750MW, coal-fired units with a total generating
capability of 2,250MW. The emission control system of the units is configured in the sequence
of economizer, electrostatic precipitator, air preheater, wet scrubber and stack (Figure 2). The
station burns Western bituminous type coal from the Kayenta coal mine in northeast Arizona.
The coal is classified as low-sulfur and high volatile content, with a heating value above 10,000
Btu/lb. Analyses of the coal sample, minerals and trace elements are shown in Appendix 1-3.
Figure 2. Configuration of Navajo Generating Station Units 1-3.
The Navajo Generating Station installed wet flue gas desulfurization (wFGD) scrubbers in the
mid-1990s, and operates two wFGD absorbers per unit. The absorbers are Alstom open spray
tower units. Each absorber module is equipped with one primary dewatering system, which
consists of one bank of hydrocyclones (three each) with associated piping and valves. The
capacity of each scrubber absorber vessel is 881,490 gal, and the operating liquid volume is
approximately 750,000 gal. Slurry from the bleed slurry circuit is pumped to the primary
dewatering hydrocyclone bank, and then the bleed slurry is directed to two dewatering cyclones.
The primary cyclones separate the slurry into two streams: overflow with approximately 5%
solid concentration and underflow with approximately 50% solid concentration. The 5% slurry
stream is routed to the hydrocyclone overflow tank and is pumped back to the absorber reaction
tank. The underflow stream (50% solid concentration) flows through the second dewatering
system and goes to belt filters where gypsum is separated. The plant does not sell gypsum, all
gypsum solids go to a landfill.
The pH in the wet scrubber was routinely measured by using Orion 3 STAR portable pH meter
from Thermo Electron Corporation. The ORP was measured occasionally by using an Orion
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A. Li – Paper 90
9180BNMD probe with Internal reference: Ag/AgCl on the Orion 3 STAR pH meter described
above.
Field Test Program
The purpose of the field tests at the SRP Navajo Units 1 and 3 was to demonstrate options for
MATS compliance with the 1.2 lb/Tbtu limit (30-day rolling average basis). The technology
discussed in this paper entails oxidation of the mercury in the boiler using a CaBr2 coal additive
in combination with PAC injection into the wet FGD scrubber. The PAC was slurried and added
into the scrubber recycle loop. Two series of tests were conducted: several months of
continuous injection on Unit 3 in 2013 followed by a few months of injection on Unit 1 in 2014.
In both tests, calcium bromide was applied continuously to the coal belt in a solution form. PAC
was introduced to the wet FGD scrubber at concentrations of 0 to 500 ppm based on total
water/solid absorber slurry by weight.
900-lb Supersacks of PAC were delivered to the site for use in the slurry system. PAC was
added into the scrubber absorber vessel after slurrying in recycle liquid using equipment leased
from STEAG Energy Services, LLC. The PAC was introduced intermittently into the wet FGD
scrubber to maintain the desired PAC ppm dosage. This dosage occurred manually, typically
once or twice a day, for a timed period of injection.
Mercury levels at the gas exit of the scrubber or entrance to the stack were monitored using
Method 30 B non-speciated traps. Traps were typically changed out by plant personnel twice
weekly, and sometimes more frequently depending on operational events.
RESULTS AND DISCUSSION
Flow Balance and PAC Concentration
In order to establish injection rates into the scrubber for both initial dosing and maintenance of a
consistent PAC level, a rough flow and mass rate balance was calculated around the scrubber.
This is described in Figure 3. Levels of 250-500 ppm PAC were targeted in the scrubber for an
initial dosage of PAC. PAC addition was then done in a daily dose that was timed by a local
operator of the injection system. The estimated amount in the daily dose was 175 lb or 28 ppm
addition daily. With the scrubber balance calculation shown below, this should result in a
maximum ppm in the scrubber of 600 ppm. The manual addition of PAC was timed rather than
weighed, so the concentration levels are approximate.
The operation of the wFGD included a continuous blowdown of slurry liquid and solids, recycle
of liquid overflow, PAC addition intervals and continuous make-up water. The level of PAC
was initially dosed to 600 ppm in the absorber, but was depleted and made-up through the daily
operation to reach a steady state. Full PAC and Hg mass balances were not obtained, but
characterization of key streams was achieved.
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A. Li – Paper 90
Figure 3. Scrubber Flow and Mass Rate Balance Calculation.
Mercury Transport and Partitioning
PAC removes mercury from the scrubber by adsorption. This requires oxidation of the mercury,
which drives the mercury into the liquid phase so that it can be captured by the PAC in the
slurry. Since PAC has a high affinity for mercury and captures it in stable form in the pore
structure, it is an ideal adsorbent.
Figure 4 shows a graphic of a proposed mercury adsorption process at the molecular level. Flue
gas mercury is a combination of elemental, oxidized and particulate phase mercury, i.e.
elemental and/or oxidized mercury associated with a solid particle entrained in the flue gas.
Because the flue gas at Navajo has low native oxidation, the predominant form is naturally
elemental, gaseous mercury. Only small amounts are probably oxidized or in particulate form
with no mercury control reagents. The figure depicts a mixture of mercury species in the
gas/liquid interface, and then in the liquid/solid sorbent interface. The mercury molecules
transport through the phases within the scrubber. The reagents used such as the oxidant calcium
bromide and the adsorbent PAC effectively drive the mercury through these phases, ultimately
capturing the mercury in the PAC pores for removal from the system with the slurry solids.
Elemental mercury formed in the slurry liquid will diffuse back into the gas phase and ultimately
will be re-emitted back into the flue gas.
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A. Li – Paper 90
Figure 4. Proposed Mercury Transport Model for Wet FGD
Samples of the absorber slurry on Unit 3 (absorbers 3A and 3B) were taken during various
conditions in 2012 and 2013. The slurry samples were analyzed for mercury content by URS
Corporation (Austin, TX) and the results are charted here. Figure 4 below shows both the liquidphase mercury and the scrubber solids mercury content. In all cases the liquid mercury content
was very low, indicating that all mercury was either in the solid or gaseous phase. The increase
in mercury content of the scrubber solids from baseline to CaBr2 addition alone and then with
PAC addition to the scrubber is a reflection of mercury transporting from the flue gas to the
liquid phase and then to the solids as seen in Figure 3. The solid phase mercury indicates the
capture by the system. At baseline conditions with no reagents added to the absorbers, solid
phase mercury concentration measured approximately 5-10 ug Hg/100 g slurry. Once the flue
gas mercury was oxidized, enabling improved transport into the liquid phase, this level increased
to about 12-20 ugHg/100 g slurry, indicating an improvement in mercury control (the balance of
mercury will go out with the flue gas). With the further addition of PAC to the scrubber, the
solids mercury level increased further (about 22-32 ugHg/100 g slurry). This progression
indicates that more of the mercury is being adsorbed onto the solids with each change. The
improvement seen when PAC was added indicates the need for use of a scrubber additive, such
as PAC, that securely removes the mercury from the system.
One outlier data point on these charts is on September 27, 2012, shown as the yellow square on
each of Figure 5a and 5b. The data appear to be outliers because the 26th and 28th are consistent
with each other and the other data available. But on the 27th both adsorber vessels showed high
solid mercury levels and slightly higher-than-usual ORP, and absorber A exhibited a pH over 6.
It is likely that there was a unit upset, possibly a trip or change in oxidation air, on this date.
However it seemed to recover quickly as the data on the following day is more consistent with
the other data points.
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A. Li – Paper 90
Figures 5a and 5b. Impact of PAC on Mercury Content of Scrubber Solids.
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A. Li – Paper 90
In 2013 Unit 3 was operated on the combination of calcium bromide feed onto the coal and
PowerPAC WS into one absorber vessel for several months. The mercury flue gas results are
depicted on Figure 6. Individual trap samples are shown in addition to the 30-day rolling
average of total stack mercury. Over time the trend is that the rolling average drops. This may
be due to accumulation of PAC in multiple scrubber absorbers, because the liquor processing is
common at the absorber outlets. Initially only one absorber vessel is treated, and this is then
gradually affecting the entire scrubber water system for all six absorbers on the three units. Once
PAC levels increase in both Unit 3 absorbers, mercury levels drop.
A short period at the beginning of the test shows the stack mercury with controls off. The level
on July 11-15 is 3.6 lb/TBtu, the baseline level for this period. Once controls are implemented,
the 30-day rolling average mercury level at the stack remains below or at 1.2 lb/TBtu for the next
4.5 months. The test program ended on December 1 when the unit went offline. No unusual
foaming issues were observed during the testing period.
Figure 6. 2013 Unit 3 Stack Hg Emissions
SUMMARY
Salt River Project’s Navajo Unit 3 was tested for mercury control options in planning for future
compliance with MATS. From 2012 to 2014, SRP has tested various technologies. This paper
presents and discusses control using oxidation of the flue gas mercury followed by PAC injection
into the wet scrubber absorber vessel. ADA Carbon Solutions’ PowerPAC WS was an effective
PAC, with good wetting and dispersion characteristics leading to consistent mercury capture. No
problems were experienced in handling the material in the field, and no scrubber foaming issues
arose. Other conclusions and observations:
PAC loading requirements to achieve compliance on the Navajo units are fairly low when
oxidation is adequate, with daily injection of about 175 lb of PAC into an absorber
vessel.
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A. Li – Paper 90
An increase in solid-phase mercury in the scrubber solids was measured in comparison
with baseline, when flue gas mercury was oxidized. Further increase was measured when
PAC was added to the scrubber. This supports the need for oxidation of the flue gas and
for use of a scrubber additive to securely remove the mercury in a solid-contained form.
Flue gas mercury levels were below 1.2 lb/TBtu (30-day rolling average measured by
sorbent traps) for 4.5 months using oxidation and PowerPAC WS in combination.
The scrubbers were operated normally and no adverse impacts on operations were
observed.
From a unit operational perspective, it appears that to achieve MATS compliance reliably:


PowerPAC WS at both 250 and 500 ppm work
The limiting mechanism for compliant mercury capture for these units appears to be the
extent of mercury oxidation.
ACKNOWLEDGEMENTS
The authors would like to thank Navajo Generating Station management and staff for the support
of the test program and paper. In addition we would like to thank the researchers at URS
Corporation (Austin, Texas) for providing the data to SRP and review of the paper.
REFERENCES
1. Sankey, M., M. Golden, D. Koza, “Challenges to Mercury Emissions Compliance at New
and Existing Coal Fired Power Plants,” Power Engineering Magazine August 2013.
http://www.power-eng.com/articles/print/volume-117/issue-8/features/challenges-tomercury-emissions-compliance-at-new-and-existing-coal-fired-power-plants.html
2. Looney, B., N. Irvin, C. Acharya, J. Wong, S. Glesmann, “The Role of Activated Carbon in a
Comprehensive MATS Strategy,” POWER Magazine, March 2014.
http://www.powermag.com/the-role-of-activated-carbon-in-a-comprehensive-mats-strategy/
KEYWORDS
Wet FGD, scrubber, re-emission, mercury, MATS, PAC
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A. Li – Paper 90
APPENDIX
Appendix 1. Typical Coal Analysis.
wt% Moisture, Total
wt% Ash
wt%Volatile Matter
wt% Fixed Carbon
Gross Calorific Value (Btu/lb)
wt% Sulfur
wt% Carbon
wt% Hydrogen
wt% Nitrogen
wt% Oxygen (Calculated)
Pounds of Ash/mm Btu
Pounds of Sulfur/mm Btu
Pounds of SO2/mm Btu
Method
ASTM D 3302
ASTM D 7582
ASTM D 758]
ASTM D 3172
ASTM D 5865
ASTM D 4239
ASTM D 5373
ASTM D 5373
ASTM D 5373
ASTM D 3176
As Received/Ave.
12.9
10.4
35.9
40.9
10,571
0.6
59.5
4.3
1.0
11.5
Result
9.8
0.6
1.2
Unit
Lb
Lb
Lb
Appendix 3. Typical Mineral Analysis.
Tests
Ash Analysis Basics
Silicon Dioxide SiO2
Aluminum Oxide Al2O3
Titanium Dioxide TiO2
Iron Oxide Fe2O3
Calcium Oxide CaO
Magnesium Oxide MgO
Potassium Oxide K2O
Sodium Oxide Na2O
Sulfur Trioxide SO3
Phosphorous Pentoxide P2O5
Strontium Oxide SrO
Barium Oxide BaO
Manganese Oxide MnO2
Sum
Alkalies
Silica Value
Base:Acid Ratio
T250 Teperature
Type of Ash
Results/Ave.
Ignited
57.1
22.6
1.2
4.4
5.1
1.2
0.9
2.1
4.9
0.13
0.21
0.32
0.02
100.00
2.6
84.2
0.17
2,873
Lignitic
Unit
wt%
wt %
wt %
wt %
wt %
wt %
wt %
wt %
wt %
wt %
wt %
wt %
wt %
wt %
wt %
wt %
Dry/Ave.
11.9
41.1
47.1
12,130
0.7
68.4
4.9
1.1
12.9
MAF/Ave.
46.6
53.4
13,764
Method
ASTM D 4326
ASTM D 4326
ASTM D 4326
ASTM D 4326
ASTM D 4326
ASTM D 4326
ASTM D 4326
ASTM D 4326
ASTM D 4326
ASTM D 4326
ASTM D 4326
ASTM D 4326
ASTM D 4326
ASTM D 4326
o
F
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A. Li – Paper 90
Appendix 3. Trace Elements Analysis.
Tests
Results
Bromine, Dry
< 20
Chlorine, Dry
15 – 23
Arsenic
1–5
Selenium
1–2
Mercury, Dry
0.02 – 0.09
Unit
µg/g
µg/g
µg/g
µg/g
µg/g
Method
ASTM D 4208-Br
ASTM D 6721
ASTM D 3684/6357
ASTM D 3684/6357
ASTM D 6722
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A. Li – Paper 90