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.

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A. Li – Paper 90

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;

1ADA 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 base-

load 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 well-

documented 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 particle-

particle 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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Table 1. Lab PAC Wetting and Settling Tests.

Sample Wetting Time, minutes Settling time, minutes

ADA-CS Gen 1 Baseline 90 Baseline 5 Baseline

ADA-CS developmental

PowerPAC WS 10 89% Improved 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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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 liquid-

phase 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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Figures 5a and 5b. Impact of PAC on Mercury Content of Scrubber Solids.

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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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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-to-

mercury-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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APPENDIX

Appendix 1. Typical Coal Analysis.

Method As Received/Ave. Dry/Ave. MAF/Ave.

wt% Moisture, Total ASTM D 3302 12.9

wt% Ash ASTM D 7582 10.4 11.9

wt%Volatile Matter ASTM D 758] 35.9 41.1 46.6

wt% Fixed Carbon ASTM D 3172 40.9 47.1 53.4

Gross Calorific Value (Btu/lb) ASTM D 5865 10,571 12,130 13,764

wt% Sulfur ASTM D 4239 0.6 0.7

wt% Carbon ASTM D 5373 59.5 68.4

wt% Hydrogen ASTM D 5373 4.3 4.9

wt% Nitrogen ASTM D 5373 1.0 1.1

wt% Oxygen (Calculated) ASTM D 3176 11.5 12.9

Result Unit

Pounds of Ash/mm Btu 9.8 Lb

Pounds of Sulfur/mm Btu 0.6 Lb

Pounds of SO2/mm Btu 1.2 Lb

Appendix 3. Typical Mineral Analysis.

Tests Results/Ave. Unit Method

Ash Analysis Basics Ignited ASTM D 4326

Silicon Dioxide SiO2 57.1 wt% ASTM D 4326

Aluminum Oxide Al2O3 22.6 wt % ASTM D 4326

Titanium Dioxide TiO2 1.2 wt % ASTM D 4326

Iron Oxide Fe2O3 4.4 wt % ASTM D 4326

Calcium Oxide CaO 5.1 wt % ASTM D 4326

Magnesium Oxide MgO 1.2 wt % ASTM D 4326

Potassium Oxide K2O 0.9 wt % ASTM D 4326

Sodium Oxide Na2O 2.1 wt % ASTM D 4326

Sulfur Trioxide SO3 4.9 wt % ASTM D 4326

Phosphorous Pentoxide P2O5 0.13 wt % ASTM D 4326

Strontium Oxide SrO 0.21 wt % ASTM D 4326

Barium Oxide BaO 0.32 wt % ASTM D 4326

Manganese Oxide MnO2 0.02 wt % ASTM D 4326

Sum 100.00 wt %

Alkalies 2.6 wt %

Silica Value 84.2 wt %

Base:Acid Ratio 0.17

T250 Teperature 2,873 oF

Type of Ash Lignitic

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Appendix 3. Trace Elements Analysis.

Tests Results Unit Method

Bromine, Dry < 20 µg/g ASTM D 4208-Br

Chlorine, Dry 15 – 23 µg/g ASTM D 6721

Arsenic 1 – 5 µg/g ASTM D 3684/6357

Selenium 1 – 2 µg/g ASTM D 3684/6357

Mercury, Dry 0.02 – 0.09 µg/g ASTM D 6722