TECHNICAL PUBLICATION

Riley Power Inc.

5 Neponset Street

Worcester, Massachusetts 01606

www.babcockpower.com

T-197

Selective Catalytic Reduction System Performance

at LG&E Energy LLC’s Mill Creek and 

Ghent Generating Stations

by

Eileen Saunders

Scott Straight

LG&E Energy LLC

Thamarai Chelvan

Sr. Staff Engineer

Riley Power Inc.

Clayton Erickson

Director, Environmental Engineering

Riley Power Inc.

Michael Jasinski

Engineer, Process Engineering

Riley Power Inc.

Presented at

EPRI Workshop on Selective Catalytic Reduction

October 19-20, 2004

Pittsburgh, Pennsylvania
© Riley Power Inc. and LG&E Energy LLC. 2004

SELECTIVE CATALYTIC REDUCTION SYSTEM PERFORMANCE

AT LG&E ENERGY LLC’S MILL CREEK AND GHENT

GENERATING STATIONS

by:

Eileen Saunders

Scott Straight

LG&E Energy LLC. 

Thamarai Chelvan

Clayton Erickson

Michael Jasinski

Riley Power Inc.

Presented at:

EPRI Workshop on Selective Catalytic Reduction

October 19-20, 2004

Pittsburgh, Pennsylvania

ABSTRACT

A consortium of LG&E Energy LLC, Riley Power Inc. (a Babcock Power Inc. Company)

and Duke Fluor Daniel has completed the Engineering, Procurement, Construction

(EPC) and acceptance testing for a total of six Selective Catalytic Reduction (SCR)

systems. The SCR systems are located at the LG&E Energy LLC’s Trimble County, Mill

Creek, and Ghent Generating Stations. Each SCR system reduced its respective baseline

nitrogen oxides (NOx) emissions by 90% or greater during the acceptance test periods.

These coal-fired facilities generate upwards of 3600 megawatts while burning a variety

of fuels and fuel blends.

The wide range of design requirements for these SCR projects in the LG&E Energy fleet

included high and low dust loadings, high and low sulfur fuels, bituminous and

bituminous/Powder River Basin (PRB) fuel blends, high and low SCR inlet gas

temperatures, and a variety of inlet NOx loadings. Recent acceptance tests for all of the

LG&E Energy SCR systems showed performance meeting or exceeding design

requirements. The standard deviations of the ammonia to NOx ratios were between 1.7 to

3.9% at design conditions, which indicates that the ammonia and flue gas were

thoroughly mixed on all twelve SCR reactors. The well mixed ammonia and flue gas in

turn produced residual ammonia concentrations at the reactor outlet between 0.10 and

0.49 ppm during the initial test period at design NOx removal rates. In addition to

favorable full load results, excellent ammonia-flue gas mixing was demonstrated over a

range of boiler loads and plant conditions.

The challenges presented by a variety of coal fired boiler systems have been met with SCR

retrofits that have been initially tested to obtain greater than 90% NOx removal. The

publication provides an overview of the SCR system designs, optimization tuning,

acceptance testing, and operating experience.
2

INTRODUCTION

Riley Power Inc. (RPI), a Babcock Power Inc. Company, and Duke Fluor Daniel (DFD), as part of an

SCR Program Alliance with LG&E Energy LLC, engineered, procured, and constructed six SCR

projects within the LG&E Energy LLC System. The first was at the Trimble County Generating

Station’s Unit 1. Trimble County's results have been previously published [1] and will not appear in

this paper. 

The second project was at Mill Creek’s Unit 4 which is an opposed wall fired 520 MW generating

boiler with 3,660,000 lbs/hr of main steam (Arrangement shown in Figure 1). The SCR system was

retrofitted to a unit that contained a cold side ESP and a flue gas desulfurization system. The RPI

Selective Catalytic Reduction system was designed to reduce the outlet NOx concentration from 0.34

lb/MBtu, by 90%, to 0.034 lb/MBtu. Mill Creek 4 is unique in that it has low economizer outlet

temperatures and to allow for a wide SCR operating range, an economizer bypass system was

installed as part of the SCR system.

Mill Creek Unit 3, which is an opposed wall fired 410 MW boiler with 3,144,000 lbs/hr of main steam,

was the next SCR project. The SCR system on the Unit 3 boiler is nearly identical to that one on the

Unit 4 boiler. The SCR system was also designed to remove 90% of the outlet NOx, from 0.35 lb/MBtu

to 0.035 lb/MBtu. Unit 3 like Unit 4 burns a high sulfur coal and also has an economizer bypass

system to allow for a broader SCR operating range.

The fourth and fifth projects are identical systems at Ghent Station. Ghent’s Units 3 and 4 are 520

MW opposed wall fired boilers with 3,800,000 lbs/hr of main steam (Arrangement shown in Figure 2).

These SCR systems were retrofitted downstream of a hot side ESP to reduce 90% of the outlet NOx

from 0.34 lb/MBtu to 0.034 lb/MBtu. Units 3 and 4 unlike all of the other LG&E Energy SCRs, burn

a low sulfur bituminous coal and a blend of low sulfur bituminous and Powder River Basin (PRB)

coals.

Figure 1. Mill Creek 4 Arrangement
3

Figure 2. Ghent 3 & 4 SCR System Arrangement

Ghent Generating Station’s Unit 1 was the final SCR project, which is a tangentially fired 520 MW

boiler with 3,800,000 lbs/hr of main steam (Arrangement shown in Figure 3). Like the Mill Creek and

the Trimble projects, the SCR system was retrofitted to a system with a cold side ESP and a flue gas

desulfurization system. The SCR was designed to remove 90% of the outlet NOx concentration from

0.45 lb/MBtu to 0.045 lb/MBtu.

Figure 3. Ghent 1 SCR System Arrangement
4

With a variety of boilers, operating conditions, and space restrictions, each project required its own

flow model. In the case of Ghent units 3 and 4 the flow and dust models were the same for both units

but additional modeling was necessary for the ESP to determine the optimal position of the mixing

systems. The mixing systems themselves varied between projects as there were either one or two sets

of cross mixing discs to thoroughly mix the gas before ammonia injection. With different operating

flue gas conditions, the number of Delta WingsTM varied project to project. With uniquely different

boiler systems, the modeling of the mixers became an integral part of the design, as the ammonia slip

at the reactor outlet was to be less than 2 ppm (2.7 ppm for Mill Creek 3 & 4) at the end of the 

catalyst life.

The ammonia systems themselves, while similar in design, contain different design aspects such as

support for two, four, or six SCR reactors. In supporting a different number of reactors, the sizes of

the equipment may be different but all systems contain similar components and operate under the

same philosophy.

OBJECTIVES

The most important task in achieving high NOx removal with low ammonia slip is the design of the

mixing system. With an entire fleet of SCR systems to design, modeling the mixing system for each

was the first step. A properly designed mixing system yields many benefits, some of which are:

* Well mixed NOx and O2 profiles at the reactor inlet

* Minor changes in the reactor inlet NOx and O2 profiles over a variety of operating conditions

* Small variations in NH3/NOx ratio with changes in operating conditions

* Few localized areas for potential ammonia slip

* Minimal number of ammonia injectors

* One optimized ammonia injection position for all operating conditions

* Short optimization and acceptance test periods 

The mixing system has many benefits with the goal of achieving a well-mixed ammonia and flue gas

mixture. In evaluating how well mixed the ammonia is in the flue gas, there are several factors that

can be looked at. These factors include the removal efficiency, the ammonia slip, the outlet NOx

variation, and the variation of the NH3/NOx ratio. In optimizing an SCR all of these factors are

reviewed, but from experience RPI has found that the best indication of well-mixed gases is the

standard deviation of the NH3/NOx ratio variation. Since the standard deviation measures how

widely dispersed the NH3/NOx ratio variations are from the mean, using this statistical method will

show how close these variations are. RPI considers deviations less than 5% to be well mixed and

increasingly more uniform as the deviations approach 0%.

To optimize the SCR reactors, RPI followed a test procedure of initially running a baseline with no

ammonia injected, then to inject ammonia at a low reduction level (50-60% reduction), increase the

reduction to another intermediate level (75-85% reduction), and finally to 90% reduction. Taking NOx

and O2 measurements at these different levels ensures that there is no area for ammonia slip. The

reason this step approach is taken is because the installed valves are usually at a position between

100% and 50% open. 
5

Throughout the optimization and acceptance test periods, the NOx and O2 profiles were measured at

the reactor inlet and outlet grids and during the acceptance tests Ammonia slip was measured as

well. This data allowed RPI to analyze different aspects of the reactor to determine optimum reactor

set points. Measurements were taken by RPI utilizing portable TESTO 350 analyzers and a third

party testing company, E.On Engineering. E.On Engineering has the capability to measure an inlet

and outlet grid (as large as 42-points each in some of these projects) simultaneously and therefore

reducing the total test time, which is useful in capturing data between boiler outlet NOx fluctuations.

RESULTS

The LG&E Energy LLC SCR systems each went through an optimization test period followed by an

acceptance test period. During the optimization test period ammonia injection valves were fixed, some

welded, so that reactor would be in its optimized state for the acceptance test period. Each projects’

results is organized separately for clarity.

MILL CREEK UNIT 4

After the optimization of the Mill Creek 4 SCR system, acceptance testing was performed to

demonstrate the system’s performance. To demonstrate the system NOx cleaning performance, NOx

and O2 were measured before the first layer of catalyst and after the last layer of catalyst. In addition

to this data, the ammonia slip was measured at the reactor outlet. NOx pre-mixers and the Delta

WingTM Mixers, allow the SCR system to operate at any load with any mill configuration without

having to readjust the ammonia injection valves. During the acceptance of the SCR system, the

reactors were tested at full load (500 — 515 MW) and a partial load (290 MW). The resulting

NH3/NOx ratios showed minor changes.

Figure 4. Mill Creek Unit 4 Full Load and Low Load NH3/NOx Distributions

Reactor A — Full Load Reactor A — Low Load

Reactor B — Full Load Reactor B — Part Load
Boiler Load MW 500 289 515 290 508 290

Grid NOx Reduction % 92.3 89.7 92.4 91.4 92.4 90.6

Ammonia Slip ppm 0.36 0.20 0.49 0.30 0.43 0.25

Inlet NOx Variation ppm ± 23.0 ± 9.5 ± 11.7 ± 9.8 ± 17.4± 9.7

Inlet NOx Variation % ± 9.0 ± 4.8 ± 4.5 ± 4.3 ± 6.8± 4.6

Inlet NOx Standard Deviation ppm 10.7 5.7 5.5 4.9 8.1 5.3

Outlet NOx Variation ppm ± 15.8 ± 9.3 ± 14.0 ± 15.8 ± 14.9± 12.6

Outlet NOx Variation % ± 64.8 ± 47.4 ± 65.6 ± 74.1 ± 65.2± 60.8

Outlet NOx Standard Deviation ppm 6.3 5.2 8.3 9.7 7.3 7.5

NH3/NOx Ratio Variation % ± 5.7 ± 5.2 ± 5.8 ± 7.7 ± 5.8± 6.5

NH3/NOx Standard Deviation % 2.7 2.9 3.4 4.7 3.1 3.8

Reactor Reactor Reactor Reactor Full Load Part Load

A A B B Ave Ave

6

Figure 4 shows the NH3/NOx ratio profiles of the Mill Creek Unit 4 reactor at both full load and a

partial load (~ 57% MCR). From the profiles in Figure 4, the majority of the area is in the blue and

green region. The blue and green regions represent a ±5% range for the NH3/NOx ratio variation. In

focusing on certain areas, some variations of the NH3/NOx ratio profile are evident. However,

considering the overall profile, rather than the specific areas of variation, the profile is relatively

uniform. For a more detailed analysis, Table 1 was constructed with acceptance test data.

Table 1

Mill Creek Unit 4 SCR System Acceptance Test Data

In looking at the NH3/NOx standard deviation, the partial load tests do show a minor change in the

mixing quality, which is expected at non-design conditions. However, even at the partial load, the

standard deviation of the NH3/NOx ratio variation is still below 5% indicating a homogeneous mixing

of gases.
7

MILL CREEK UNIT 3

The optimization of Mill Creek’s Unit 3 SCR system was done in a slightly different manner than its

Unit 4 SCR system. An ID Fan upgrade and design rating on the ductwork and ESP, limited the load

and the amount of optimization that could be performed. However, preliminary optimization revealed

a mal-distribution of NH3/NOx with the standard deviation of the NH3/NOx ratio variation between

7 — 13 %. RPI utilized the upgrade period to modify the design and optimize the flow model and SCR

system. To do this, additional ammonia injection valve influence tests were performed in the field. The

results of these tests were to be compared to identical tests performed on the revised flow model,

which was scaled precisely to the as-built reactor. The conclusion of the testing was that the NOx premixers and the Delta WingsTM needed minor modifications. 

To verify that the changes to the mixing system, a series of field influence test were conducted during

the optimization period. These tests were done to mimic identical tests done in the flow model where

a constant ammonia flow was fed through each of the four injection valves while the remaining three

remained closed. The results of the field test after the modifications (Figure 5) matched the flow model

results exactly.

Figure 5. Field Influence Testing of Mill Creek Unit 3 SCR System

After the above testing was conducted, the system was optimized. The improvements to the NH3/NOx

ratio variation can be seen in Figure 6.
8

With the mixing system modified, the optimization the SCR system was conducted and was followed

by the acceptance test period. The result of the acceptance testing (Table 2) was that the standard

deviation of the NH3/NOx ratio variation decreased from 7 — 10 % to 1.7 — 2.3 %. 

Figure 6. Mill Creek Unit 3 SCR System NH3/NOx Ratio Before and After Modifications

Reactor A — Before Reactor B — Before

Reactor A — After Reactor B — After

Boiler Load MW 422 430 426

Grid NOx Reduction % 92.3 91.7 92.0

Ammonia Slip ppm 0.22 0.11 0.17

Inlet NOx Variation ppm ± 14.0 ± 12.9 ± 13.5

Inlet NOx Variation % ± 5.3 ± 4.7 ± 5.0

Inlet NOx Standard Deviation ppm 7.3 6.8 7.1

Outlet NOx Variation ppm ± 10.5 ± 7.8 ± 9.2

Outlet NOx Variation % ± 45.9 ± 35.8 ± 40.9

Outlet NOx Standard Deviation ppm 5.5 4.3 4.9

NH3/NOx Ratio Variation % ± 4.3 ± 3.1 ± 3.7

NH3/NOx Standard Deviation % 2.3 1.7 2.0

Reactor A Reactor B Average

Table 2

Mill Creek Unit 3 SCR System Acceptance Test Results
9

GHENT 3

The Ghent Unit 3 SCR system had an element, which the Mill Creek SCR systems did not. Ghent

Unit 3 regularly burns either a low sulfur bituminous coal or a blend of the low sulfur bituminous

coal and PRB coal. With the plant frequently alternating fuels, the optimization and acceptance test

periods incorporated testing with the two fuels. With the installed mixing system, the ammonia

injection valves did not have to be adjusted while switching fuels. The NOx pre-mixers produce

relatively the same distributions regardless of plant operating conditions. Furthermore, the NH3/NOx

ratio will not change if the boiler operates under different loads, mill configurations, or fuels. Figure

7 shows the similarity in the NH3/NOx profiles with the two different fuels without a change in the

ammonia injection valves.

Figure 7. Ghent Unit 3 SCR System NH3/NOx Ratio Profiles for the Two Fuels

Reactor A — 100% Bituminous Reactor A — PRB Blend

Reactor B — 100% Bituminous Reactor B — PRB Blend

Figure 6 shows that the majority of the area is blue and green. Using the color scale at the right, this

would translate into a range for the NH3/NOx ratio to be approximately ± 5 % over 100 % of the

catalyst area. A more detailed comparison of the acceptance test data is contained in Table 3.
10

Boiler Load MW 530 530 530 530 530 530 530

Grid NOx Reduction % 91.9 91.4 90.4 90.5 91.2 91.0 91.1

Ammonia Slip ppm 0.17 0.24 0.10 NA 0.14 0.24 0.17

Inlet NOx Variation ppm ± 16.3 ± 10.3 ± 7.0 ± 6.5 ± 11.7 ± 8.4± 10.0

Inlet NOx Variation % ± 6.6 ± 4.8 ± 3.2 ± 3.3 ± 4.9 ± 4.1± 4.5

Inlet NOx Standard Deviation ppm 8.3 4.6 3.3 3.5 5.8 4.1 4.9

Outlet NOx Variation ppm ± 12.5 ± 11.3 ± 11.8 ± 9.8 ± 12.2 ± 10.6± 11.4

Outlet NOx Variation % ± 69.5 ± 71.4 ± 48.5 ± 46.9 ± 59.0 ± 59.2± 59.1

Outlet NOx Standard Deviation ppm 5.5 4.6 5.4 4.5 5.5 4.6 5.1

NH3/NOx Ratio Variation % ± 5.5 ± 5.7 ± 5.8 ± 5.4 ± 5.7 ± 5.6± 5.6

NH3/NOx Standard Deviation % 2.4 2.3 2.7 2.5 2.6 2.4 2.5

Reactor Reactor Reactor Reactor Bit Bit Overall

A Bit A Blend B Bit B Blend Ave Blend Ave

Table 3

Ghent Unit 3 SCR System Acceptance Test Results

Figure 6 shows that there are some variations between fuels. However, when the data is compared,

the magnitude of these changes can be seen. Table 3 illustrates that when fuels are switched the effect

to the NH3/NOx ratio is less than or equal to 0.2%.

GHENT 4

Ghent’s Unit 4 SCR system is a mirror image of Unit 3’s. In addition, unit 4 also frequently switches

between the same two fuels as Unit 3 so the optimization and acceptance tests were very similar. One

element that was added to the Unit 4 acceptance test period was testing the SCR with both fuels at

full and part loads. The resulting acceptance test data is presented in Tables 4 and 5. The NH3/NOx

mixing results are also presented graphically in Figure 8.
11

Figure 8. NH3/NOx Mixing Results for the Ghent Unit 4 SCR System

Boiler Load MW 518 523 524 529 521 526 524

Grid NOx Reduction % 89.9 91.6 92.2 93.2 91.1 92.4 91.7

Ammonia Slip ppm 0.26 0.12 0.10 0.16 0.18 0.14 0.16

Inlet NOx Variation ppm ± 20.5 ± 22.5 ± 33.8 ± 15.0 ± 27.2 ± 18.8± 23.0

Inlet NOx Variation % ± 8.3 ± 8.5 ± 10.3 ± 5.5 ± 9.3 ± 7.0± 8.2

Inlet NOx Standard Deviation ppm 8.0 10.0 16.0 7.3 12.0 8.7 10.3

Outlet NOx Variation ppm ± 17.8 ± 25.3 ± 18.8 ± 15.5 ± 18.3 ± 20.4± 19.4

Outlet NOx Variation % ± 70.3 ± 93.4 ± 70.0 ± 79.5 ± 70.2 ± 86.5± 78.4

Outlet NOx Standard Deviation ppm 7.8 8.3 8.2 6.6 8.0 7.5 7.7

NH3/NOx Ratio Variation % ± 7.8 ± 8.8 ± 6.2 ± 5.7 ± 7.0 ± 7.3± 7.2

NH3/NOx Standard Deviation % 3.4 3.4 2.7 2.7 3.1 3.1 3.1

Reactor Reactor Reactor Reactor Bit Blend Full Load

A Bit A Blend B Bit B Blend Ave Ave Ave

Table 4

Ghent Unit 4 SCR System Full Load Acceptance Test Results
12

As seen in Mill Creek Unit 4 and Ghent Unit 3, changing the fuel or the load only affects the mixing

on a minor scale. By comparing the data in Table 4 and Table 5, the same conclusion can be reached.

All of the changes to the NH3/NOx standard deviation by switching fuels or changing loads are less

than or equal to 0.5%.

Boiler Load MW 392 303 395 306 394 305 349

Grid NOx Reduction % 91.3 90.9 93.7 93.3 92.5 92.1 92.3

Ammonia Slip ppm NA NA NA NA NA NA NA

Inlet NOx Variation ppm ± 11.8 ± 17.0 ± 13.5 ± 12.0 ± 12.7 ± 14.5± 13.6

Inlet NOx Variation % ± 5.4 ± 7.7 ± 5.2 ± 6.0 ± 5.3 ± 6.9± 6.1

Inlet NOx Standard Deviation ppm 5.2 6.6 6.2 5.4 5.7 6.0 5.9

Outlet NOx Variation ppm ± 16.3 ± 15.3 ± 15.8 ± 12.3 ± 16.1 ± 13.8± 14.9

Outlet NOx Variation % ± 91.7 ± 83.6 ± 79.7 ± 77.6 ± 85.7 ± 80.6± 83.2

Outlet NOx Standard Deviation ppm 6.4 5.9 7.0 5.5 6.7 5.7 6.2

NH3/NOx Ratio Variation % ± 8.1 ± 7.5 ± 6.5 ± 6.6 ± 7.3 ± 7.1± 7.2

NH3/NOx Standard Deviation % 3.2 2.9 2.9 3.0 3.1 3.0 3.0

Reactor Reactor Reactor Reactor Bit Blend Part Load

A Bit A Blend B Bit B Blend Ave Ave Ave

Table 5

Ghent Unit 4 SCR System Part Load Acceptance Test Results
13

Boiler Load MW 534 454 528 454 531 454

Grid NOx Reduction % 92.7 91.3 91.2 91.0 92.0 91.2

Ammonia Slip ppm 0.22 0.15 0.24 0.13 0.23 0.14

Inlet NOx Variation ppm ± 34.3 ± 15.8 ± 22.5 ± 11.8 ± 28.4 ± 13.8

Inlet NOx Variation % ± 11.8 ± 6.0 ± 10.1 ± 5.8 ± 11.0 ± 5.9

Inlet NOx Standard Deviation ppm 11.7 6.5 10.4 5.1 11.1 5.8

Outlet NOx Variation ppm ± 13.8 ± 9.3 ± 15.5 ± 8.5 ± 14.7 ± 8.9

Outlet NOx Variation % ± 56.0 ± 38.7 ± 63.5 ± 39.6 ± 60.0 ± 39.2

Outlet NOx Standard Deviation ppm 6.9 4.3 8.2 4.8 7.6 4.6

NH3/NOx Ratio Variation % ± 4.8 ± 3.8 ± 7.4 ± 4.5 ± 6.1 ± 4.2

NH3/NOx Standard Deviation % 2.4 1.8 3.9 2.5 3.2 2.2

Reactor Reactor Reactor Reactor Full Load Part Load

A A B B Ave Ave

Table 6

Ghent Unit 1 SCR System Acceptance Test Results

GHENT 1

The Ghent Unit 1 SCR system is quite different than the Unit 3 and 4 SCR systems at Ghent. Unit

1 burns a high sulfur coal instead of the low sulfur coal or coal blend. Even the boilers themselves are

different. However, the approach to the optimization and acceptance were similar. The results from

the acceptance test can be seen in Table 6.

CONCLUSIONS

The results from all of the LG&E Energy LLC SCR systems confirm that they have met or exceeded

the design performance. The additional parameters such as partial loads and different fuels have also

met the design criteria and also show the effectiveness of the mixing systems. The static mixing

system simplifies the SCR system by having one ammonia injection valve setting for all operating

conditions. By using the static mixing system in their SCRs, LG&E Energy LLC has obtained systems

with low ammonia slip and the potential for > 90% removal efficiency.

REFERENCES

(1). Pear, Eric, et al. SCR System Performance at LG&E’s Trimble County Generating Station. 

Atlanta: EPRI Workshop on Selective Catalytic Reduction (T-177), 2002