TECHNICAL PUBLICATION

Babcock Power Environmental Inc.

5 Neponset Street

Worcester, Massachusetts 01606

www.babcockpower.com

T-196

Wet Flue Gas Desulfurization (WFGD) Upgrade at the

Trimble County Generating Station Unit 1

by

Clayton Erickson

Director, Environmental Engineering

Babcock Power Environmental Inc.

Michael Jasinski

Engineer, Process Engineering

Babcock Power Environmental Inc.

Larry VanGansbeke

E.On US

Louisville, Kentucky

Presented at

MEGA Symposium

August 28-31, 2006

Baltimore, Maryland
© Babcock Power Inc. and E.On U.S. 2006

WET FLUE GAS DESULFURIZATION (WFGD) UPGRADE AT THE

TRIMBLE COUNTY GENERATING STATION UNIT 1

by:

Clayton Erickson, Michael Jasinski

Babcock Power Environmental Inc.

Worcester, MA

Larry VanGansbeke

E.On U.S.

Louisville, KY

ABSTRACT

The Trimble Unit 1 WFGD system, originally supplied by Combustion Engineering circa

1980, was originally designed for 90% removal while operating three absorbers (one

spare) each with four of five recycle pumps operating. As of February 2005, the system

was capable of operation close to or at 98% removal but was typically operating around

93-94% SO2 removal while operating all installed equipment. This removal efficiency

was insufficient to meet the 2006 emission requirement of 97% SO2 removal of a 

5.5 lbs/MMBtu inlet (0.17 lbs/MMBtu outlet).

Babcock Power Environmental Inc. (BPEI) was contracted to upgrade the existing WFGD

system to meet the future compliance limit while limiting additional pressure drop. This

upgrade was to be designed such that the WFGD system could consistently maintain a

98% SO2 removal while operating all installed equipment. By using previous experience,

and guidance from E.ON U.S., BPEI utilized multiple tools including CFD and physical

modeling to optimize the inlet gas distribution, slurry spray generation, and gas-liquid

interaction.

The results of the upgraded system exceeded expectations by removing over 99% of a 

6.5 lb/MMBtu SO2 inlet. This removal was not only achieved during the start-up period,

but was also maintained for extended operation. This paper presents the design

development and results of the Trimble WFGD upgrade.
2

INTRODUCTION

The wet flue gas desulfurization (WFGD) upgrade project at E.On U.S.’s Trimble County Generating

Station Unit 1 is an excellent example of the application of current technology to an existing system.

This paper presents an overview of the process that involved the evaluation of the current system and

the design and installation of materials necessary to upgrade the existing WFGD system. In addition,

the performance of the installed enhancements as well as the system without them is reviewed.

Upgrading an existing WFGD system has many areas of potential difficulty, as the entire system

needs to be evaluated to determine the best course to bring the process to an optimal efficiency. This

can be increasingly difficult when the system was originally designed and installed over 20 years ago

and limited design data are available. Additional consideration also needs to be made toward the

current operation and the desired future operation. If the desired performance increase requires the

purchase and installation of numerous pieces of additional equipment (recycle pumps, oxidation air

blowers, etc...), upgrading the existing system may be less cost effective than installing a new WFGD.

From proposal to installation, the Trimble Unit 1 upgrade was accomplished in a 9-month period. All

phases (design, requisition, purchase, fabrication, and installation) of the project schedule were

compressed in order to deliver the material for a planned outage. In addition, the increased removal

efficiency needed to be met by January 1st, 2006 with the release for the project in late February

2005. Each phase of the project contributed to and was instrumental in achieving this goal, and

demonstrated that the solution designed for the system was cost effective. 

Both physical and CFD flow modeling were utilized to optimize the inlet gas distribution to the

absorber and to limit the amount of slurry deposition on the inlet duct. Slurry spray nozzle tests were

conducted to characterize the existing nozzles so that new nozzles could have similar flow and

pressure criteria. Dual direction nozzles were installed with a wider-angle spray cone to ensure

complete coverage of the spray zone, as well as, increased gas-liquid interactions. A header pad or

saddle was designed and installed around the branch pipe connections to the spray header to reduce

erosion and provide additional reinforcement. Wall baffles were designed and installed in each

absorber to direct and better distribute flue gas over the entire cross section of the absorber, as well

as, reduce “gas sneakage” along the absorber walls and corners. The combination of all these items

will improve the gas flow distribution and liquid-to-gas contact and therefore increase the absorbers’

effectiveness to remove SO2 from the flue gas.
3

BACKGROUND

Trimble County Generating Station Unit 1 is a 550 MWG, subcritical, tangential-fired, pulverized

coal boiler designed by Combustion Engineering. The unit is a balanced draft, pulverized coal fired,

drum type boiler with single reheat. The boiler is equipped with six levels of burners located in the

corners of the unit. The original steam side operating conditions at full load are 3,800,000 lbs/hr of

main steam at a design outlet temperature of 1005 °F and a pressure of 2500 psig.

In addition, Trimble 1 also has electrostatic precipitators (ESP) and in 2002 was retrofitted with a

selective catalytic reduction (SCR) system. While the ESP was originally installed with the boiler, the

SCR was supplied by Riley Power (A Babcock Power Inc. company) and was designed to remove 90%

of the inlet NOx.

Combustion Engineering also supplied the existing WFGD system, which was originally designed in

the 1980’s. The discharge from the two induced draft fans is fed to a common plenum, which then

feeds four absorber modules. The discharge of the absorbers is again fed to a common duct before

exiting through the stack. There are two reaction tanks supplied for the plant, each having two of the

four absorber modules located above such that the absorbed SO2 is directed into the tanks. Each

absorber has five installed recycle pumps, each feeding a dedicated spray level. The original design

condition for the WFGD system was to remove 90% of a 7.0 lbs/MMBtu SO2 inlet while operating

three of the four absorbers and four of the five installed recycle pumps per absorber.

Figure 1. Trimble 1 WFGD Arrangement

Because of operational issues with the absorber inlet dampers and a desire for higher SO2 removal

efficiencies, the WFGD system has been and is currently operating with all four absorbers in service

and with all five recycle pumps per absorber in service. This arrangement was the design basis for

the WFGD upgrade.
4

WFGD UPGRADE DESIGN

During the initial design of the Trimble 1 WFGD upgrade, it was important to understand the

dynamics of the installed WFGD to determine a suitable solution. The original request for quote had

suggested many potential areas of improvement including the addition of wall baffles, modifying the

inlet ducts to eliminate solids buildup, and spray header repair. However, it was necessary to learn

the operational history of the WFGD system so that additional design issues and potential solutions

could be identified. Through interactions with the plant, it was discovered that removal efficiencies

greater than 97% up to 98% were achievable for short periods of time. However, items such as

equipment maintenance outages would result in a significant drop in efficiency. In addition, the

scrubber had experienced quite extensive solids deposition on the inlet duct, and during outage

inspections, it was noticed that there were a considerable number of plugged spray nozzles and spray

headers. All of these items were viewed to be linked to unstable removal efficiency. Therefore, they

were considered in the design and scope provided for the upgrade.

Operating History

In characterizing the existing WFGD the first step was to document the operating history. To do so,

the hourly emission data on the Environmental Protection Agency’s website was used for the

characterization.1 By using the data directly from the EPA, all graphs and results are reproducible

from data that is publicly available. 

Figure 2. Trimble County 1 Rolling SO2 Emissions Data (2003 & 2004)

Rather than plotting the raw data from 2003 and 2004, Figure 2 shows the 24-hour and 30-day rolling

averages of the SO2 emissions data. These rolling averages were used to better illustrate the overall

operation of the scrubber instead of the hourly operation that may be misleading when a process

variable changes for a short period of time. In addition to the cumulative graphs for 2003 and 2004,

rolling averages for the individual years were also made.
5

Figure 3. Trimble 1 SO2 Emissions Data (2003)

Figure 4. Trimble 1 SO2 Emissions Data (2004)

Figures 3 and 4 show the rolling averages (24-hour and 30-day) for the individual years of 2003 and

2004. While 2004 had a lower overall average emission rate than 2003, 0.240 and 0.357 lbs/MMBtu

respectively, maintaining a 24-hour average or 30-day average emission below 0.200 lbs/MMBtu was

inconsistent. While there are a few periods below 0.200 lbs/MMBtu and even 0.100 lbs/MMBtu over

these two years, consistent operation at these levels with the existing absorber configuration seems

improbable. 
6

Boiler Performance and Baseline Test

After the system history had been established, it was necessary to evaluate the boiler performance

and perform a baseline test for the WFGD system. The baseline test would verify that the data

provided during the initial design were valid, or it would establish a new set of operating conditions

to compare to the EPA emission data. This operating data could then be used to fine-tune the

performance calculations to establish a better picture of the existing system’s performance.

The results from the baseline test indicated that the flue gas flow rates provided from the original

WFGD design were rather conservative. The design gas flow provided was approximately 7.5

MMlbs/hr where as the tested flows were only about 6.0 MMlbs/hr. In addition, the test confirmed

that the removal of the WFGD was operating less than its best efficiency of 97 — 98% and was

approximately 90%.

CFD and Physical Model Study

With the WFGD system experiencing solids deposition on the inlet duct and concern about ID fan

capacity, a physical model and a CFD model were constructed in an attempt to optimize the gas side

of the WFGD system. The goals of the CFD and physical model studies are defined below:

1. Evaluate absorber inlet duct modifications to move the wet/dry interface away from the inlet

duct. These modifications include adding a “false roof” and sidewalls to the flared inlet duct 

as well as an inlet hood over the inlet duct penetration.

Figure 5. BPEI Proposed Inlet Duct Modifications

2. Evaluate the existing and new flow controls for the inlet plenum to improve flow distributions

to the absorbers and reduce the overall gas side pressure drop. This is to include an evaluation

of the installed perforated plates to see if they can be removed or replaced with a more open

plate.
7

3. Model the BPEI designed wall baffles for pressure loss and flow distribution.

After the completion of the models and nine model modifications between the two, the study produced

the following results.

1. The proposed inlet redesign by BPEI with a false roof and an inlet hood reduces the amount

of flow bouncing off of the floor and recirculating back to the inlet duct.

2. There still exists a small backflow zone on the floor of the absorber inlet duct, most likely due

to the unequal flooring which creates a low pressure zone above the drain into the reaction 

tank.

3. A flow bias exists within the absorber with higher flows on the outboard and rear walls. This

is most likely due to the unequal floor, which is shallow along the outboard wall and deep 

along the inboard wall.

4. The installed perforated plates provided the best distribution of gas flow between the four 

absorbers as well as the best rms % for velocity distribution at the absorber inlet.

5. The installed wall baffles were successful in diverting gas from the walls to the center. The 

test cases with baffles also showed an increase in flow along the front wall when compared to

the test cases without baffles. In addition, the measured pressure loss of the baffles was 

0.06 iwg.

Figure 6. Physical and CFD Models
8

Spray Nozzle Characterization and Selection

One of the major components was to install new dual direction (up and down on the bottom 4 levels

and a double orifice down on the top level) wider-angle spray nozzles in all of the absorbers. By

providing a nozzle with the same flow and pressure loss characteristics, but with twice the orifices,

the number of droplets increases creating more surface area and, therefore, increased overall mass

transfer. To ensure complete coverage of droplets within the vessel, the new interior nozzles would

also have a 120° spray angle instead of a 90° angle.

To design a new spray nozzle that does not impact the recycle pump operation, a spare existing nozzle

was independently tested. The results produced the required pressure and droplet size for the

appropriate flow. These values could then be provided to design the new replacement nozzles so that

the recycle pump operation would remain the same. This was particularly important as it was not

desirable to increase the velocity in the recycle lines above the design velocity of 9 ft/s and introduce

additional sprayed liquid that would lead to higher gas-side pressure drop through the absorber.

Wall Baffle Design

All open spray towers are designed such that there is some degree of spray overlap to optimize the

liquid-gas interaction. However, due to the absorber geometry and spray nozzle layout, this level of

overlap is not achievable along the walls. This lack of overlap leads to a less dense spray zone at the

wall, which in turn leads to less resistance and higher gas flows along the perimeter of the absorber.

The increased flow causes the spray along the wall to be less efficient and, therefore, lead to a

decrease in overall removal efficiency. This effect is known by multiple names, which include “gas

sneakage” and “wall effect”.

Based on BPEI’s previous experience and success with the Culley WFGD upgrade2, wall baffles were

designed and installed in Trimble Unit 1’s four absorbers. These baffles were designed such that the gas

from the walls and the corners of the square absorbers would be deflected back to the denser spray

coverage of the interior spray zone. This design would ensure that 100% of the flue gas would be treated.

Process Audit and Chemistry Evaluation

One of the last items to be completed in the upgrade of the Trimble 1 WFGD was an evaluation of the

absorber chemistry. With all installed equipment operating, a small interruption in operation or an

equipment failure can be devastating to the system removal efficiency. In an effort to prevent such

events from happening, BPEI performed an audit of the existing system, which included a process

chemistry evaluation.

To properly evaluate the absorber chemistry, a compilation of the plants’ chemical analyses for the

absorber slurry, make-up water, and limestone slurry for the previous five years was reviewed. In

addition, an Oxidation-Reduction Potential (ORP) meter was used so that as future absorber slurry

samples were taken; the ORP of the absorber slurry could be measured. This measurement is useful

in qualitatively verifying sufficient oxidation air flow as well as good distribution throughout the

reaction tank. While these items were done in parallel to the absorber and duct modifications, they

produced results that would benefit future operation.

1. Overall the installed equipment is sufficient to meet the removal requirements. However, as

indicated in the model study results, the layout and design of the floor of the absorber may 

cause some continued solids deposition in the inlets. Solids deposition could cause problems,

as the drains from the absorbers are located directly above the recycle pump suctions. With 

this relative location, any deposited solids that are blown into the absorber drains are likely

to be taken in by the recycle pumps. This could lead to excessive wear or even spray nozzle 

pluggage.
9

2. The reaction tank chemistry is in good standing. Both chemical analysis and ORP 

measurements indicate that the absorber slurry is well oxidized and that both the oxidation

air and agitation systems are sufficient for future operation.

3. While the absorber chemistry is good, it was discovered that the water used for washing the

mist eliminators was of poor quality and had a high potential for scaling. The water used was

saturated with many dissolved salts some of which were predicted to plate out during the 

wash cycle. To remedy this problem, an existing fresh water connection on the mist eliminator

wash tank was used to provide better quality wash water.

UPGRADE RESULTS

In November of 2005, the Trimble 1 WFGD system started up with the new modifications for the first

time. After the system completed its start-up, the performance of the upgrades was to be tested.

However, since the reaction tanks were empty and the system started up from a tank with only water

with small concentrations of limestone, the reaction tanks had to come to equilibrium first. To verify

the system was in equilibrium, extensive amounts of control room as well as local data were collected.

After it was determined that the system had reached equilibrium, the system was tested so that the

performance of the upgrades could be verified.

Initial Performance Testing

The initial performance testing of the upgraded unit was done to verify that the installed

modifications met the guaranteed performance when operating under the design conditions. To do so,

BPEI conducted multiple tests that met the plants’ requirements and were in compliance with EPA

40 CFR 60 Test Method 193 to determine the removal efficiency of the upgraded WFGD.

Table 1

Initial Performance Testing Results of the Trimble 1 WFGD Upgrade

Test Description Fuel Bound SO2 CEMs Emission WFGD SO2

Removal Efficiency

5 Level Operation 6.61 lbs/MMBtu 0.049 lbs/MMBtu 99.3%

(All Levels on All Absorbers)

4 Level Operation 6.42 lbs/MMBtu 0.190 lbs/MMBtu 97.0%

(Bottom 4 Levels on Each Absorber)

4 Level Operation 6.64 lbs/MMBtu 0.135 lbs/MMbtu 98.0%

(Top 4 Levels on Each Absorber)

Unbalanced 6.64 lbs/MMBtu 0.071 lbs/MMBtu 98.9%

(1 Bottom Level Out on 1 Absorber)

Unbalanced 6.64 lbs/MMBtu 0.086 lbs/MMBtu 98.7%

(1 Top Level Out on 1 Absorber)
10

As Table 1 illustrates, when operating with all installed equipment (the upgrade design basis), the

WFGD not only met the required 98% removal, it exceeded it while operating with a higher than

design inlet SO2. In addition to the performance case, multiple other tests were conducted to verify

the operation should there be recycle pump failures. Since the design of the system is to operate with

all installed equipment, any loss of recycle pumps will cause the emissions to increase. These tests

will be essential in predicting emissions for such periods. As the data show, only one test case, the

lower four levels on each absorber operating, is below 98% reduction. 

90-Day Performance Testing

The next step in verifying the performance of the upgrade was to once again test the system after 90

days of operation. This test would verify that the initial operation data was not biased with little

operating history. The 90-day performance test was a more concise test but was conducted in a similar

fashion to the initial performance testing.

Table 2

90-Day Performance Test Results for the Trimble 1 WFGD Upgrade

Test Description Fuel Bound SO2 CEMs Emission WFGD SO2

Removal Efficiency

5 Level Operation 6.61 lbs/MMBtu 0.052 lbs/MMBtu 99.2%

(All Levels & All Absorbers)

Like the initial performance test results, the 90-Day performance test results showed a performance

of the WFGD upgrade exceeding expectations. While the performance decreased slightly, the overall

removal efficiency well exceeded the required 98% reduction of a 5.5 lbs/MMBtu SO2 inlet.

Long-Term Performance (EPA Reported SO2 Emission Data)

As was done previously in determining the system’s operating history, SO2 emission data from the

EPA website was compiled. This time, the data were collected for a comparison of the “before and

after” WFGD performance and so that the long-term performance could be viewed.
11

Figure 7. Trimble 1 SO2 Emissions Data (2003 — 2005)

Figure 8. Trimble 1 SO2 Emissions Prior to Upgrade (2005)
12

Figures 7 and 8 both show the operating history from 2003 up to the installation of the upgrade, as

well as, the 2005 operating history prior to the upgrade. Figure 9 shows the SO2 emission history of

the upgraded WFGD system. As it can be seen in Figure 9, the start-up of the WFGD system was

unstable as is typical after an outage. This instability is primarily due to additional required

maintenance in one reaction tank. Because of this, the WFGD was started up with only two absorbers

and one reaction tank in operation instead of four absorbers and two reaction tanks. After

approximately one month of operation and all absorbers operating, the system settled out and with

the exception of two excursions, the emissions remained between 0.05 and 0.1 lbs/MMBtu. It should

be noted that after the system stabilization, neither the 24-hour average nor the 30-day rolling

average exceeded the 0.17 lbs/MMBtu limit with the overall average for the upgraded scrubber being

0.074 lbs/MMBtu.

Figure 9. Trimble 1 SO2 Emissions Post Upgrade
As illustrated in Table 3, the WFGD performance has been maintained at a consistent high efficiency

level. Prior to the upgrade, small periods of exceptional performance could be seen as the emissions

at times were below 0.1 lbs/MMBtu. However, this performance was not maintainable as the averages

for the analyzed periods all exceed 0.2 lbs/MMBtu. The post-upgrade data shows the opposite effect.

The majority of the data is well below 0.1 lbs/MMBtu with only brief periods of less efficient

performance.

CONCLUSIONS

The Trimble 1 WFGD upgrade project was successful in meeting its performance goals. Not only did

it achieve sustainable high removal efficiency, but it also allows for a high efficiency when one or more

spray levels are inactive. While only five months of operating data were available at the time of this

paper, it is expected that with the current maintenance and continuous improvement efforts, good

results will continue to be achieved. It should be noted that this testing period occurred immediately

following a planned maintenance outage. The unit will remain in continuous operation until the next

planned outage. Although some performance degradation may occur during this long period of run, it

appears that the unit should be able to achieve a long-term removal efficiency of 98% or better. The

achievements of this project were made possible by a careful evaluation of a problem with a foreign

system and applying current technology and knowledge in developing a solution.

Future Recommendations

Throughout this paper, multiple areas of improvement have been identified. As the Trimble 1 WFGD

continues to operate, ensuring recycle pump operation and maintenance, as well as nozzle and spray

header cleanliness are vital to continued peak performance. With potential problem areas in the

design of the absorber inlet floors and relative location to the recycle pump this area could cause the

performance of the system to decrease. Investigating solutions such as pump suction screens, or drain

relocation will help to maintain the pumps and spray levels operating at optimum operating

efficiency. 

ACKNOWLEDGEMENTS

The authors wish to acknowledge the contribution of the staff of Alden Labs in the design of the inlet

duct modifications and performing the physical and CFD modeling under tight schedule restraints.

In addition, we would like to acknowledge the important contribution of the E.On U.S./BPEI team

whose efforts were essential to this projects success.

13

Table 3

Trimble 1 SO2 Emissions Data

Period Average SO2 Emissions

2003 0.357 lbs/MMBtu

2004 0.240 lbs/MMBtu

2005 — Prior to Upgrade 0.343 lbs/MMBtu

2003 to 2005 — Prior to Upgrade 0.308 lbs/MMBtu

Post Upgrade 0.074 lbs/MMBtu
REFERENCES

(1). Clean Air Markets — Progress and Results. U.S. Environmental Protection Agency. 7 July 

2006 <http://www.epa.gov/airmarkets/emissions/raw/index.html>.

(2). Quitadamo, Matthew, et al. SO2 Removal Enhancement to the Vectren, Culley Generating 

Station Units 2 & 3 Wet Flue Gas Desulfurization System. Baltimore: ICAC Forum, 2005.

(3). CFR Promulgated Test Methods. U.S. Environmental Protection Agency. 7 July 2006 

<http://www.epa.gov/ttn/emc/promgate.html>.

© Babcock Power Inc. and E.On U.S. 2006