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TP1189EN.docx Jun-11

Water Technologies & Solutions technical paper

slag control treatment program at a southeastern utility Authors:

M. Domingo Tubio, Product Applications Engineer

Rick Higginbotham, Account Executive

abstract Coal-fired power plants supply over half the electricity to the US grid. Currently, utilities are facing a range of challenges including decreasing industrial demand for electricity, competition from low cost natural gas, and rising coal prices. High quality Eastern Bituminous Central Appalachian (CAPP) coal costs are increasing due to rising exports, increasing transportation, and environmental costs, and decreasing production, (Buchsbaum 2008; Metzroth 2008). To stay competitive, some utilities are investigating burning lower-cost, lower-quality “opportunity” coals such as Northern Appalachian (NAPP) and Illinois Basin. The most efficient plants can be dispatched for longer periods for improved financial performance. The change to lower rank coal and increased operation can result in increased slag deposits in the furnace and super heater areas, (Gabriel 2011).

A Southeastern utility desired to blend lower-cost low ash fusion temperature Northern Appalachian (NAPP) coal with their typical CAPP coal in their 745 MW pulverized-coal boiler. Soot blower cleaning alone is not effective when slag deposits are a liquid or pseudo-plastic state which deforms under pressure. A proprietary mixture of chemical additives was recommended to elevate ash fusion temperature and modify the deposit to make it more easily removable by soot blowers.

The blend is a unique combination of water-soluble magnesium hydroxide and copper oxide slurries which has a synergistic effect when used together to mitigate slag formation and impact. During the fourth quarter of 2010, the utility consumed over 44,000 tons of NAPP opportunity coal treated with this combination of proprietary fireside chemical additives over a four-week period. SUEZ’s approach allowed the customer to minimize the detrimental effects of burning slag prone coal while reducing fuel costs. This paper summarizes the trial and performance results.

slag and fouling formation and cost There are numerous non-combustible inorganic impurities in coal besides hydrocarbons. Depending on the ratio of these minerals and compounds, slagging, and convective pass fouling can occur in boilers. Slag formation accelerates when the furnace exit gas temperature (FEGT) exceeds the fusion temperature of the ash. Indices such as the basicity ratio can help predict slag viscosity and ash fusion temperature (Babcock & Wilcox 1978).

As slag density increases with time and temperature, a deposit is formed that is difficult to remove with soot blowing. Deposits can “grow” as particles accumulate; it is not uncommon to observe large deposits on the leading edge of platen superheat tubes and secondary super heater tubes above the bull nose of the boiler. When the slag eventually falls it can damage tube banks lower in the boiler, resulting in unscheduled outages and lower availability.

CAPP coal typically has a high ash fusion temperature and less tendency to create excessive

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Figure 1: Fuel Comparison and Basicity Ratio (Babcock & Wilcox, 1978)

slagging. NAPP coal is becoming more economically attractive for several reasons, including availability at lower delivered costs than CAPP coal (Pusateri 2009). Figure 1 illustrates the challenge of using NAPP coal with a lower ash fusion temperature. Slag deposits are expected to be in liquid state at furnace temperatures with noncombustible mineral content present. Soot blower cleaning alone is not effective when the slag is a liquid or pseudo-plastic state which deforms under pressure.

Fouling, which is closely related to slagging, usually occurs in the boiler’s cooler convective back-pass section as gaseous ash components (such as sodium and potassium) condense. It typically occurs in the vertical and horizontal re-heaters and primary super heater. Fouling deposits can “bridge” across tubes and restrict gas flow.

That increases induced fan horsepower, which raises the plant heat rate and, therefore, lowers plant efficiency. Slagging and fouling can result in derating (shedding load) and costly unscheduled outages and repairs from damaging slag falls. But these problems can be eased by combining chemical additives for fireside applications with mechanical removal (soot blowers).

Fuel Type Typical Opportunity

** Basicity Ratio = (Fe2O3+CaO+MgO+Na2O+K2O)

(SiO2+Al2O3+TiO2)

Source Central Appalachian

Northern Appalachian

Cost per ton, $US (2010)

$70 – $75

$58 – $70

HHV, Btu/lb ~12,000 ~13,000

SO2, lb/MMBtu

1.1 – 1.5 4.5 – 5.0

Ash, wt% 11 – 12 7 – 8

Moisture, wt% 6.7 – 7.0 6.0 – 7.0

Ash Softening Temp, deg F

2,700 2,250

Basicity Ratio **

0.12 – 0.14 0.45 – 0.55

Ash, wt%

SiO2

Al2O3

Fe2O3

K2O

TiO2

MgO

CaO

Na2O

53 – 56

28 – 30

5 – 6

3.3 – 3.6

1.3 – 1.5

0.9 – 1.0

0.7 – 1.3

0.2 – 0.3

39 – 40

20 – 21

22 – 24

1.3 – 1.4

0.85 – 0.95

1.05 – 1.15

5 – 6

0.95 – 1.05

TP1189EN.docx Page 3

boiler and trial design The 745-MW pulverized coal-fired boiler is a Riley Stoker Corporation front-wall fired boiler with 2,500,000 lbs./hr steam productions at 2610 psig and 1,005 deg F at super heater terminal outlet.

The boiler fires 250 tons pulverized coal per hour at maximum load, and the boiler train is equipped with SCR, cold-side electrostatic precipitators and a wet flue gas desulfurization (Wet FGD) scrubber system.

Trial results using the same opportunity fuel- NAPP coal- at a sister station indicated it could not be burned untreated, as the resulting slag was severe enough to slag the boiler, and block the gas path. Operating experience indicated boiler conditions could deteriorate within days of introducing opportunity fuel. To minimize the risks of boiler outage during trial, the utility blended its typical fuel with a small proportion of opportunity fuel treated with a mix of proprietary chemical additives to reduce severity of fireside slagging. Product dosages were optimized as the percentage of opportunity coal was increased until it reached the target level of 50 percent.

chemical additives for slag control A range of chemical additives were considered before the two products were selected based on ultimate analyses of the fuels. The proprietary mix of additives selected for this trial included a magnesium based compound and a metal oxide. The magnesium is known in the industry to elevate ash fusion temperatures due to the high melting point of magnesium oxide. This treatment keeps the slag in a solid state instead of liquid-phase deposit. The metal oxide-based slurry contains copper which has been used in the industry as a combustion catalyst. Less well known is that copper can reduce the cohesive strength of the ash via a nucleating effect with iron species. Gradual thermal decomposition of the metal oxide product also makes the slag porous, and therefore, weaker. These mechanisms complement the magnesium effect for certain types of coals or coal blends, depending on the ratio of minerals and other non-combustible species. Together, the proprietary additives create fracture planes in the solidified slag, weakening the deposits so that they can be more easily removed by soot blowers.

treatment application The chemical additives were transferred from agitated trailer-mounted base totes to the coal belts via peristaltic pumps, where the chemicals were the dosed at predetermined amounts via a manifold mounted above the coal conveyor (Figure 2).

Dosing occurred when the coal belts conveyed NAPP coal. Aqueous magnesium-based slurry dosages were reduced from 3 lbs. of product per ton of NAPP coal to optimum of 1.0-1.5 lbs. Aqueous metal oxide slurry was introduced to determine its impact on slag mitigation in conjunction with the magnesium-based product. It was determined that the optimum product feed rate was 0.25 lbs. product per ton of NAPP coal. The NAPP coal quantity was ramped up from 16 percent to the target of 50 percent, where it was maintained for a week until the end of the trial. The dynamic test environment confronted the trial team with challenges that included outages, inclement weather, and real-time adjustments to the dosage based on visual observations of furnace slag conditions.

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trial details To be considered successful, the trial had to meet several criteria, including:

1. Demonstrating that the magnesium content increases the ash fusion temperature and, therefore, makes the deposit more friable and easily removable.

2. Demonstrating the metal oxide slurry synergistically assists in slag mitigation.

3. Determining the optimum product feed rates for the CAPP/NAPP blends while monitoring (with an infrared (IR) camera) real-time slagging phenomena along with boiler parameters such as load, pressure drop, and exit temperature. Customer’s fuel blends included 84 percent CAPP/16 percent NAPP, 77 percent CAPP/33 percent NAPP, and 50 percent CAPP/50 percent NAPP as the “highest stress test.” (the untreated “baseline” NAPP blend slag indices were not available, as they presented unacceptably high operational risks to the customer.)

4. Demonstrating that the use of magnesium and/or metal oxide products has no adverse effects on boiler operations- i.e., it does not exacerbate slagging or fouling or emissions. Obtained Flue Gas Desulfurization (FGD) wastewater grab samples and ash pond samples, analyzed for copper, and other components.

Data was recorded during the “baseline” (100% CAPP coal) and chemical treatment trial periods, assuming equipment parameters such as tube cleanliness, soot blower availability, and thermocouple calibrations. Note that during the chemical trial the unit was not derated overnight for deslagging. Overnight load shedding allows the slag to contract in the cooler flue gas, and this uneven contraction in the matrix causes cracks and gravity-assisted removal of the accumulated slag. This beneficial procedure was not conducted during most of the chemical trial (Figure 3).

Here is the key chronology of the trial

• November 15- the chemical trial began as 16 percent NAPP coal was dosed at 3.0 lbs./ton magnesium-based product on the coal belt en route to bunkering silos #5 and #6.

• November 24-29- the trial was suspended over Thanksgiving

• December 1 and 2- duct testing occurred. The flue gas sampling successfully obtained a baseline for the CAPP/NAPP blend while the treatment consisted only of magnesium-based product. The magnesium-based product dosage was decreased as the metal oxide product was introduced.

• Mid-December- following a number of weather-related outages, inclement weather compelled the suspension of the trial.

• After a weather-related equipment outage, NAPP coal treatment was resumed. Dosage was increased by 10 percent after a “gooey slag” was seen in the furnace. The slag subsequently returned to its semi-solid state.

• December 20- the trial concluded.

DCS data The trial team collected data every 5 minutes from the customer’s “Pi” distributed control system (DCS), including load (MW), heat rate (MMBtu/hr), furnace exit gas temperature (FEGT, deg F), soot blower, and coal mill operation including feeder flow (kpph). For the analysis, the team removed all data points below 700 MW so that only “full load” data was considered and outliers could not impact calculations.

The most obvious change over time was soot blower activity. If at least one of the 60 IR (radiant) or IK (convective) soot blowers was active when the data was recorded, the event was logged in Pi. In the DCS, soot blowers were either “on” or “off”; it was not possible to record which soot blowers were active. The trial team decided to record the average number of soot blowers per day to understand how this rate was changing over time. The frequency seemed to have doubled as the NAPP was increased from 0 (baseline) to 33 percent and apparently tripled by the time the trial achieved 50 percent NAPP. It is important for a utility to keep this parameter in mind given operations concerns such as steam consumption and tube wear over time.

Furnace Exit Gas Temperature (FEGT) data is valuable for a slagging study since it can be a proxy indictor of slag conditions. FEGT values should rise as slag increases, since slag is an excellent insulator and because increasing slag conditions will push the fireball farther back into the convective pass of the furnace, this analysis indicated that furnace operation vis-à-vis FEGT was approximately

TP1189EN.docx Page 3 Figure 3: Key Dates of Trial and Results Summary (for Gross Load >700 MW)

equivalent during the baseline and chemical treatment periods. The team observed decreasing temperature variances between east and west side thermocouples across the boiler.

The data below (Figure 3) presents average parameter values from the trial, implies that chemical treatment does not adversely impact the boiler. A longer trial could indicate if boiler operation significantly improvers with chemical treatment while burning the opportunity coal.

infrared photography The trial team used port inspection and photography to monitor boiler slagging conditions in the furnace. The team developed a standardized naming convention for referencing boiler ports at various elevations. For example, port “3A4E” refers to Unit 3, Alpha side, Level 4, Port E. This ensured that slagging conditions were properly recorded, since more than a dozen trial team members viewed, photographed, and commented on the visual conditions during the trial.

Visual and infrared (IR) photos of conditions at the various boiler elevation ports (Figure 4) were made with an Olympus brand “point-and-click” digital camera and a Mikron Lumasense brand infrared camera.

Slag conditions inside the boiler were recorded with a portable, battery-operated Lumasense Mikron-brand model 7604F infrared camera.

The camera’s integral flame filter and high temperature range covered the boiler’s operating range throughout the trial and its multi-spot

temperature measurement capability enabled final images to include reference temperature profiles, (Mikron 2011). Thermal photos of the boiler slag conditions were recorded at the boiler ports throughout the trial.

Not only were visual observations of these conditions important, they were, arguably, the best way to measure the impact of chemical treatment on boiler slag, since instantaneous changes in many variables (such as soot blower activity, number of pulverizes in operation, load changes, etc.) make it difficult to compare with other parameters in isolation (such as FEGT).

Several photos of the slagging conditions in the boiler on November 23 and 30 (Figures 5 and 6) clearly show the bottom of the superheater (SH) pendant and no or minimal slag accumulation. There was some initial concern about the underside of the tubes at Level 3; however, the slag was removed with a firm push of a spade. Overall slagging was considered minor at this point, and noticeably worsened as the inspection continued down the boiler. (The worst slag was near the burners.)

Plant monitoring and the attached pictures from the early phase of the trial revealed that the slag formation was minimal, with little day-to-day change, and was being removed by the soot blowers where possible. Several locations were identified for possible future installations of IK sootblowers to minimize slag formation. Initial indications showed the slag to be self-limiting and still friable.

PARAMETERS

(start – end dates)

10/18 – 11/14

11/15 – 11/23

11/30 – 12/3

12/4 – 12/8

12/10 – 12/12

12/17 12/18 – 12/20

NAPP Coal, % 0 16 33 33 50 50 50

Magnesium product, lbs./ton 0 3.0 2.0 1.5 1.5 1.0 1.1

Metal oxide product, lbs./ton 0 0 0 0.25 0.25 0.25 0.275

# of Pi data points 4,118 342 634 391 699 123 771

Avg # of soot blows/day 36 57 63 82 104 94 157

Avg Heat Input, MMBtu/hr 7,003 7,060 6,278 6,216 6,892 6,953 6,986

Avg Coal to Boiler, ton/hr 336 321 325 327 327 332 331

U3A Avg Max FEGT, oF 2,318 2,683 2,602 2,496 2,468 2,561 2,627

U3B Avg Max FEGT, oF 2,497 2,536 2,464 2,486 2,521 2,479 2,511

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Figure 4: Unit 3 Ports (Elevation Drawing #69085B5, revised 2-19-99)

Operations and SUEZ continued to monitor and report on slag formation, and SUEZ continued to photograph the Unit at these ports and report the finding after each inspection.

Port Name Port Qty Elevation Notes

Level 1

Ports A, B

2 ~580’ Pendant Super heater

Level 2

Ports A, B, C, D

4 ~566’ Cold Reheat Piping

Level 2.5

Port A

1 ~563’ Above bullnose

Level 3

Ports A, B

2 ~550’ Below bullnose

Level 4 Ports A, B, C, D, E, F

6 ~538’ Bullnose

Level 5

Ports A, B, C, D

4 ~520’ Wall tubes

Level 6

Ports A, B, C, D

4 ~506’ Wall tubes

Level 7

Ports A, B, C, D

4 ~496’ Wall tubes

Level 8

Ports A, B

2 ~488’ Over fire Air

Level 9

Ports A, B, C, D

4 ~483’ Burners

Level 10

Ports A, B

2 ~473’ Below burners

TP1189EN.docx Page 3

3A Level 1 Port A, 11-30-10

3A Level 1 Port A, 11-30-10

3A Level 3 Port B, 11-30-10

3A Level 3 Port C, 11-23-10

3B Level 4 Port E, 11-30-10

3A Level 4 Port A, 11-23-10

Figure 5: Visual and IR Photography, 11-23 and 11-30

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By December 17, the trial team began burning 50 percent NAPP, dosed with 1.0 lbs. magnesium-based product per ton and 0.25 lbs. metal oxide-based product per ton. Since the slag appeared gooey and there was concern over the sub-minimum amount of feed required for operation, both feeds were increased 10 percent (Figure 6). By December 20, the boiler recovered and slagging conditions had improved.

wastewater and ash pond analyses Two FGD water discharge samples were collected in order to establish a baseline. No measureable copper was detected and no significant change in operation was noted. In future trials, additional samples should be taken to provide a greater understanding of the normal state of the FGD composite analysis.

Ash pond water samples were collected on November 19 and 20 (prior to the metal oxide addition) and on December 12 (after the metal oxide feed) to determine the effects of copper in the ash pond. The trial team wanted to rule out any negative impacts from copper carryover from the bottom ash sluice into the ash pond and subsequent NPDES outfall. No measurable copper was detected in the ash pond on all three sample dates.

SO3 testing The trial team also conducted two duct tests, sampling flue gas to determine the sulfur trioxide (SO3) concentration during varying operating conditions and when chemicals were added to the coal feed. Samples were collected from a single point approximately seven feet from the duct wall at a test port downstream of the SCR- the only test port available at the furnace. Measurements were conducted to observe the treatment regimen’s effect on “blue plume”. (There have been studies on magnesium oxide’s effect on SO3 in fossil fuel-fired furnaces (Schmidtchen 2002)). Ammonia injection was off during the testing to avoid free ammonia interference with the analysis. A third-party stack testing firm was contracted to collect samples using the Controlled Condensate Method (US EPA Method 8A) and analyze them by High Performance Liquid Chromatography (HPLC).

TP1189EN.docx Page 3

Figure 6: Visual and IR Photography, contrasting 11-30-10 with 12-17-10

3A Level 5 Port D, 11-30-10

(note the minor slag)

3A Level 5 Port D, 12-17-10

(note the large formation on the wall)

3A Level 3 Port B, 11-30-10

(note the minor blinding)

3A Level 3 Port A, 12-17-10

(note the blinding and hot runny formation)

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The untreated opportunity fuel SO3 baseline was unavailable, and the SCR is expected to convert some SO2 to SO3. The testing indicated that the apparent SO3 removal was within sampling error and that there was no observable difference in the SO3 mitigation when MgO addition rates were varied. Thus, the results did not indicate that the MgO chemical treatment significantly mitigated SO3 in the furnace. However, it is also important to note that there was no apparent increase in SO3 formation overall on December 11, when the copper-based metal oxide slurry was added to the system (Figure 7).

trial results and conclusions The magnesium-based product alone showed an improvement in slag removal until the NAPP blend increased above 33%. Magnesium alone could not elevate ash fusion temperature to avoid sticky deposits in the upper regions of the furnace, Tenacious, viscous deposits throughout the boiler became firmer and were easily removed by IR and IK soot blowers during the trial once the magnesium and metal oxide products were combined.

TP1189EN.docx Page 3

Run No. Sample Time (EST)

SO3 Concentration (ppm, dry basis) Start End

11/30 and 12/1 – 33% NAPP Coal, 2.0 lbs./ton magnesium-based product

1 10:40 11:40 11.3

2 13:05 13:50 12.3

3 14:45 15:30 13.0

4 * 16:25 17:10 6.0 *

5 08:37 09:22 12.4

6 12:10 12:55 9.5

7 14:05 14:50 11.6

8 16:05 16:50 15.3

Average 12.2

12/11 – 50% NAPP Coal, 1.5 lb/ton magnesium-based product

and 0.25 lb/ton metal oxide-based product

9 08:30 09:15 13.6

10 10:05 10:50 12.1

11 11:30 12:15 9.5

12 12:45 13:30 10.4

Average 11.4

Run 4 is suspected as an outlier when compared to other runs under the same operating condition and is not included in the calculated average.

Figure 7: SO3 Testing Summary

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The visual observations and operator feedback indicated that the synergistic combination of the 1.0-lb to 1.5-lb magnesium-based product range per ton of NAPP coal in conjunction with 0.25-lb metal oxide-based product per ton of NAPP coal was very effective in mitigating slag of the blended fuel. Different coals or coal blends may require customized product ratios to appropriately address potential slagging issues.

In addition, several locations were identified as possible sites for future IK soot blowers and observation ports to control and monitor slag formation. Lessons learned include the importance of freeze protection on the lines and providing options for continuous mixing with water-based slurries to reduce material handling difficulties. Future trial goals include quantifying loss on ignition (LOI) with the metal oxide slurry and evaluation of different additives for SO3 control in the furnace.

Estimates for fuel cost savings are projected to range from $5,000,000 to $13,000,000 per year by switching to a 50 percent blend of NAPP opportunity coal. Even greater savings could be achieved by increasing the blend to include Southern Illinois Basin (ILB) coal, (Figure 8).

Figure 8: Customer’s Coal Price Ranges

references • Babcock & Wilcox. “Steam”. 39th Edition, 1978.

• Buchsbaum, Lee. “New Coal Economics”. Energybiz, November / December 2008. Retrieved from http://energycentral.fileburst.com/EnergyBizOnline/2008-6-nov-dec/Financial_Front_New_Coal.pdf

• Gabriel, Mark, Manager of Business Development, SUEZ Water Technologies & Solutions. “Slag Control Treatment Program at EKPC Spurlock Station.” Presented at Electric Power Conference, Chicago, IL. May 2011.

• Hatt, Rod. “Correlating the Slagging of a Utility Boiler with Coal Characteristics”. Retrieved from http://www.coalcombustion.com/PDF%20Files/CorreSlagefc03.pdf April 2011

• Metzroth, Lawrence, VP Analysis & Strategy, Arch Coal Inc. “Regulatory and Other Constraints on CAPP Coal Supply.” Presented at 7th Annual Coal Trading Conference, New York City, NY. December 2008. Retrieved from http://www.coaltrade.org/wp-content/uploads/2011/02/Metzroth.pdf

• Mikron camera information retrieved April 2011 from http://www.mikroninfrared.com/EN/products/thermal-imagers-detectors-and-cores/portable-thermal-imagers/portable-special/m7604f.html

• Pusateri, Robert, President Consol Energy Sales. “NAPP and the Marketplace.” Presented at McCloskey Coal USA 2009 Conference, New York City, NY. June 2009. Retrieved from http://www.consolenergy.com/Newsroom/Speeches/McCloskeyNewYork2009.pdf

• Schmidtchen, Paul. “High Activity Magnesia Use for SCR Related SO3 Problems.” Presented at NETL 2002 Conference. Retrieved from http://www.netl.doe.gov/publications/proceedings/02/scr-sncr/schmidtchensummary.pdf

$60

$70

$80

$90

$100

$110

CAPP12,000

MMBtu/lb

NAPP13,000

MMBtu/lb

ILB11,700

MMBtu/lb

Customer Coal Price Ranges (per short ton; delivered)