TECHNICAL PUBLICATION

SCR System Performance at LG&E’S

Trimble County Generating Station

by

Eileen Saunders

Scott Straight

LG&E Energy Corp.

and

Eric Pear

Clayton Erickson

Harry Culberson

Michael Kelly

Riley Power Inc.

(Formerly Babcock Borsig Power, Inc.)

Worcester, Massachusetts

Presented at the

EPRI Workshop on Selective Catalytic Reduction

October 22-23, 2002

Atlanta, Georgia

Riley Power Inc.

5 Neponset Street

Worcester, Massachusetts 01606

www.babcockpower.com

T-177
© Riley Power Inc. and LG&E Energy Corp. 2002

SCR SYSTEM PERFORMANCE AT 

LG&E’S TRIMBLE COUNTY GENERATING STATION

by

Eileen Saunders and Scott Straight

LG&E Energy Corp.

and

Eric Pear, Clayton Erickson, Harry Culberson, and Michael Kelly

Riley Power Inc.

ABSTRACT

An Alliance of LG&E Energy Corp., Riley Power Inc. (formerly Babcock Borsig Power

Inc.), and Duke/Fluor Daniel completed the Engineering, Procurement, and Construction

(EPC) of Selective Catalytic Reduction (SCR) equipment at Louisville Gas and Electric

Company’s Trimble County Generating Station in Bedford, KY. The SCR equipment reduces

the plant’s emission of nitrogen oxides (NOx), by more than 90 percent. The 547-megawatt

unit began commercial operation in 1990 and fires high sulfur coals with a flue gas desulfurization system.

Past utility experience with the introduction of SCR technology has witnessed NOx outlet

emissions as low as 0.040 lbs/MBtu. European SCR experience has generally been limited to

the reduction of NOx outlet levels to > 0.12 lb/MBtu due to compliance laws. This has not

been the case with the Trimble County facility where SCR technology in combination with

modern low NOx burner technology, guaranteed an outlet NOx emission of less than 0.032

lb/MBtu. The SCR NOx removal efficiency was specified at ≥90% at the end of catalyst life.

In order to achieve such an aggressive goal, detailed baseline testing and overall system

understanding was required. The understanding of the overall system was necessary to accurately simulate the boundary conditions of the boiler system for the physical flow models.

Accurately identifying boiler exit distribution and stratification of the flue gas were imperative when developing the design and configuration of the mixing devices in the system.

The challenges of an SCR retrofit in conjunction with Low NOx burner technology have

been met while obtaining more than 90% NOx removal by the SCR. Recent acceptance test

data has shown the SCR at 91.3% NOx removal with an average outlet NOx emission of 0.025

lb/MBtu (19 PPM @ 3% O2) with < 0.1 PPM ammonia slip. The ability to achieve this level

of NOx removal performance is based on the mixing system, which has obtained a NH3/NOx

distribution of ± 3.4% over the catalyst surface. The Trimble County SCR project has successfully matched high performance (>90% NOx removal) with low NOx burner technology.
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INTRODUCTION

Riley Power Inc. (RPI), formerly Babcock Borsig Power, Inc., and Duke Fluor Daniel

(DFD), as part of an SCR Program Alliance with LG&E Energy Corp. (LEC), engineered, procured, and constructed the first of seven SCR projects for LG&E Energy Corp. Trimble

County is a tangentially fired, 547 MW generating plant with 3,960,000 lb/hr main steam.

The plant, built in 1990, was previously equipped with low NOx burners, a cold side ESP and

a flue gas desulfurization system. The addition of the RPI Selective Catalytic Reduction system was designed to reduce the outlet NOx concentration from 0.32 lb/MBtu, by 90%, to

0.032 lb/hr.

The challenges posed at the Trimble County plant include the space restriction of a

power plant retrofit, the complexities of secondary catalytic reactions with the constituents

of the fuels burned (i.e. high sulfur), and the diminishing return of pushing the SCR reactor

to very low outlet NOx concentrations already equipped with low NOx burners. In order to

meet these challenges, extensive base line test data, along with detailed flow and dust model

studies, were used to determine the optimal SCR design.

The SCR reactor consisted of 674.6 cubic meters of plate-type catalyst using anhydrous

ammonia as a reducing reagent. Two sets of Delta WingTM mixing devices were implementSCR

Reactor

Precipitator

Air Heater

Economizer

Hopper

Figure 1 Trimble County SCR retrofit arrangement
3

ed for the Trimble County reactors. A cross mixing set of discs were used to mix the flue

gases laterally, and a second set of mixing devices, located at the point of ammonia injection,

was used to mix the flue gases vertically and mix in the ammonia stream. Four ammonia

injection points were used and later optimized to produce the best distribution of NH3 and

NOx possible. Figure 2 shows the four valves used to tune the SCR reactor and obtain the

best possible outlet NOx distribution.

Figure 2 Dilution air and ammonia injection to SCR reactor

The Process Target values set at the start of the project are described below:

• Pressure Drop: The pressure total drop of catalyst, mixing devices, internal structures, and changes in ductwork was to be limited to 6.8 IWC at a dirty condition. The

pressure drop was measured from the economizer outlet to the air heater inlet.

• NOx removal Efficiency: The inlet NOx concentration was to be reduced by 90%; limited to an inlet NOx loading of ≥0.30 lb/MBtu.

• Ammonia Slip: The injection of ammonia was to be mixed effectively to limit the concentration of ammonia at the outlet of the reactor to less than 2 PPM at end of catalyst life.

• SO2 to SO3 Conversion: The catalyst used to reduce NOx will also oxidize sulfur dioxide to sulfur trioxide. This reaction was to be limited to 1.5%.

As a design restriction, the Trimble County SCR system was to be built to minimize the

pressure loss experienced so that no fan modifications would be necessary. In order to

accomplish this, flow modeling became an increasingly critical part of the design.

OBJECTIVES 

Key to designing the Trimble County SCR system was optimizing the mixing of NOx,

ammonia, ash, and temperature distribution, while simultaneously minimizing the pressure

drop required to achieve optimal mixing. For the Trimble County SCR system, four mixing

devices (the patented Delta WingsTM), were used to facilitate the optimal distribution of flue

gas. With sufficient mixing, limited tuning of the four injection points would be required and

an accurate amount of ammonia could be injected to each of the four nozzles. As designed,
4

the NH3/NOx distribution required to maximize the NOx reduction reaction would have to

be below ±5%, as calculated in equation 1-1.

[(InletAvg - OutletAct) / (InletAvg - OutletAvg) - 1] * 100% (Eq. 1-1)

Variables in Equation 1-1 are as follows: InletAvg is the average of all measurements

made at the SCR inlet grid, OutletAct is one of 42 actual measurements made at the outlet

grid and OutletAvg is the average of all measurements made at the SCR outlet grid.

To test the SCR reactor at Trimble County, an independent third party, E.ON

Engineering, was brought in to test and verify that all design guarantees were met. The procedures used to test the guarantee design values are listed below.

The testing of the SCR reactor followed the following standard test methods:

• Pressure Drop: Manometer readings at the outlet of the economizer, inlet to the SCR,

outlet of the SCR and inlet to the air heater where taken to give a pressure profile of

the SCR system. U-tube water manometers were used for this application.

• NOx Efficiency: NOx concentrations were measured at the inlet and outlet of the

SCR using an extractive dilution method. At any one time, the testing equipment

was capable of reading five NOx measurements: one inlet measurement and four outlet measurements. The outlet NOx concentration was measured continuously

throughout the duration of each test. The changes in the NOx concentration at this

point were used to observe any changes in the overall NOx flow rate through the SCR

(i.e. fuel changes, boiler upsets, etc.). An average of the inlet NOx concentrations and

outlet NOx concentrations were taken and the overall removal efficiency was calculated as shown in equation 1-2.

[1 – (Outlet NOx AVG) / (Inlet NOx AVG)]*100 = Removal Efficiency (Eq. 1-2)

• Ammonia Slip: Once the SCR NOx grids were taken, ammonia slip measurements

were made at three points along the outlet grid. The points along the grid that gave

the highest outlet NOx concentration, lowest NOx concentration and a moderate outlet NOx concentration where measured for ammonia slip. Ammonia is dissolved in a

sulfuric acid solution and measured by titration. EPA Method 350.3 was used for

ammonia slip measurements.

• SO2 to SO3 Conversion: Measurement of SO2 and SO3 were made both before and

after the SCR catalyst. The Controlled Condensation Method (CCM) is used with

special extractive probes required for these measurements, because the stainless

steel test grid interferes with the measurements. Additionally, the sample line must

be heated so that no SO2 or SO3 falls out of the gas stream and condenses in and on

the test equipment. Once a sample of the gas steam is taken, EPA Method 6 (barium perchlorate titration) was used to quantify the amount of sulfur oxides. The SO2

to SO3 conversion was calculated using equation 1-3.

(Outlet SO3 – Inlet SO3) / Inlet SO2 = SO2 to SO3 Conversion (Eq. 1-3)

E.ON Engineering provided a state-of-the-art, mobile testing truck (MARA) equipped

with 60 sample lines. Each line was connected to one of 60 points on two test grids: an 18-

point inlet test grid and a 42-point outlet test grid.
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RESULTS

The acceptance testing of the Trimble County SCR reactor took place the week of

September 23, 2002. The results of each test gave the following results:

Note: Testing was conducted after one ozone season of operation ~30% of catalyst life.

Table 1-1 Trimble County acceptance test results

Reactor Pressure Drop

In all test cases the SCR system pressure drop measured was within the guarantee

value. The baseline conditions under which the SCR system was designed were slightly

different than those of the test conditions. To correct for the increased boiler load (from

525 MW to 547 MW) and the changes in fuel (from 10,000 Btu/lb to 12,000 Btu/lb) the

pressure drop values were adjusted, and remained well below the guarantee values.

NOx Reduction Efficiency

Of particular importance is the confirmation of NOx removal capability of >90% during the acceptance tests. Trimble County’s SCR reactor is one of the first to accomplish

such high removal efficiency with the combination of low NOx burners. The RPI design

of mixing devices allows for increasingly high removal efficiencies while limiting the

ammonia slip concentrations.

SO2 Conversion

The conversion of SO2 to SO3 across the SCR reactor was measured to determine the

rate of conversion. Tests were made of SO2 to SO3 conversion at full and part loads. In

both conditions, the SO2 conversion was well below the 1.5% guarantee target.

Ammonia Slip Measurements

The measurement of ammonia slip was made at the SCR outlet and found to be well

less than 2.0 PPM in the flue gas. Although the guarantee value of 2.0 PPM is representative of the end of catalyst life condition, no ammonia slip was found at concentrations above 0.1 PPM. Based on the current test data of 0.1 PPM over the first four

months of operation, RPI estimates ammonia slip will not exceed 0.5 PPM when extrapolated out over 10,000 hours. This performance estimate is based on using correction

curves provided by the catalyst manufacturer.
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CONCLUSIONS

The Trimble County acceptance testing confirmed the operation of the SCR reactor

design. All design criteria were met and exceeded target conditions. The resulting pressure drop has prevented the complications of existing fan modifications and will give

Trimble County higher net electricity generation. Further, the plant will be able to

achieve longer durations between air heater washings due to preserved fan margins. The

operation of the SCR reactor to NOx reduction of greater than 90% will allow for more

flexibility during ozone control periods (May-September). The application of low NOx

burners in concert with a high efficiency SCR system has proven to result in NOx reduction beyond guaranteed values. Ammonia slip measurements were well below the 2.0

PPM (end of life) design value and therefore, will allow ash sales to continue or be landfilled without the concern of chemical treatment. The low ammonia slip with high NOx

removal efficiency is attributed to the effective mixing of NH3/NOx distribution and performance of the Hitachi plate-type catalyst.

To put the outlet concentrations of the Trimble County SCR system in perspective, the

following tables contain the national average outlet NOx concentration for over 2,548 individual power plants burning natural gas, oil, and coal in comparison to that of the Trimble

County station (Table 1-2), as well as national and state regulations for coal and gas fired

power plants (Table 1-3).

Trimble County

Power Plant

National Avg. for

Coal Fired Plants*

National Avg. for

Oil Fired Plants*

National Avg. for

Gas Fired Plant*

0.025 lb/MBtu 0.424 lb/MBtu 0.318 lb/MBtu 0.133 lb/MBtu

Table 1-2 Outlet NOx concentration comparisons corrected to 3% O2

*Data averaged from http://www.epa.gov/airmarkets/emissions/score01/index.html Table B1 of the EPA

Emissions Scorecard 2001, Appendix B.

Table 1-3 Outlet NOx concentration national and state regulations

Trimble County

Power Plant

2004 SIP Call 2010 Kentucky Clear

Skies for Coal Plants*

2010 Kentucky Clear

Skies for Gas Plants*

0.025 lb/MBtu 0.15 lb/MBtu 0.12 lb/MBtu 0.04 lb/MBtu

*Data taken from http://www.epa.gov/clearskies/pdfs/KY-summary-9-16.PDF Table 1. Projected Emission

Rates in 2010 and 2020 in Kentucky from Power Generators.

Startup Issues

As in most large-scale operations, there were a number of difficulties encountered during startup and commissioning of the SCR system. These included the operation of the insitu NOx analyzers, canned motor pumps, and thermal stress issues associated with the

reactor dampers and expansion joints.

Due to the temperature gradient across the guillotine damper and resultant thermal

expansion, the guillotine dampers would hang-up partway through the stroking process. To

resolve this problem, the damper vendor reinforced and retrofitted their design. These

design changes have been incorporated into the design and fabrication of future dampers for

the LEC SCR Alliance.
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The Trimble County power plant burns high sulfur coal (~4% by weight), resulting in

high SO2 concentrations in the flue gas. In-situ NOx analyzers were initially installed to

measure SCR inlet and outlet NOx concentrations and control ammonia injection into the

system. These analyzers utilize Differential Optical Absorption Spectroscopy (DOAS) with

optimized evaluation algorithms. This technology was found to be limited in its ability to

measure low SCR NOx outlet concentrations in the presence of high SO2 concentrations.

The higher levels of SO2 masked the readings of NOx and increased the error in measurement. As a solution to this problem, a dilutive-extractive NOx analyzer was purchased to

operate in its place. This system extracts a sample of flue gas and measures the sample in

a remote analyzer building.

The last major issue that arose during the commissioning of the SCR system was that of

the anhydrous ammonia pumps. Since ammonia is a pressurized liquid, the opportunity for

ammonia to change phases within the process is present. Pumping two-phase flow is

extremely difficult. As such, it is critical to the ammonia design that the process fluid

remained liquid. To accomplish this, the storage tanks were elevated well above grade and

the process lines were insulated to protect the liquid from external radiant heat. This was

sufficient for continuous operation. However, when the pump stopped, it heated the liquid

and created a vapor lock around the pump. This prevented the pump from restarting. The

solution to this problem was to make a change in the control logic that allowed ammonia

vapor to vent back to the storage tank.

Figure 3 Trimble County completed SCR retrofit
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REFERENCES

1. U.S. Environmental Protection Agency. Clean Skies in Kentucky.

http://www.epa.gov/clearskies/pdfs/KY-summary-9-16.PDF. 2002.

2. U.S. Environmental Protection Agency. EPA Emissions Scorecard 2001.

http://www.epa.gov/airmarkets/emissions/score01/index.html. 2002.

3. The SCR Retrofit at the Montour Steam Electric Station. Worcester, MA.: Babcock

Borsig Power, Inc. October 2000. Electric Power Generation Association Conference.

RST-169.

The data contained herein is solely for your information and is not offered,

or to be construed, as a warranty or contractual responsibility.
8

REFERENCES

1. U.S. Environmental Protection Agency. Clean Skies in Kentucky.

http://www.epa.gov/clearskies/pdfs/KY-summary-9-16.PDF. 2002.

2. U.S. Environmental Protection Agency. EPA Emissions Scorecard 2001.

http://www.epa.gov/airmarkets/emissions/score01/index.html. 2002.

3. The SCR Retrofit at the Montour Steam Electric Station. Worcester, MA.: Babcock

Borsig Power, Inc. October 2000. Electric Power Generation Association Conference.

RST-169.

The data contained herein is solely for your information and is not offered,

or to be construed, as a warranty or contractual responsibility.