Coal-Fired SCR Operating Experience with High

Removal Efficiency and Low-Nox Firing Systems

by:

Clayton Erickson, Eric Pear

Riley Power Inc., a Subsidiary of Babcock Power Inc.

Scott Straight

LG&E Energy Corp.

Linton Hutcheson

Duke Energy

Riley Power Inc.

5 Neponset Street

Worcester, Massachusetts 01606

www.babcockpower.com

T-188

LG&E Energy Corp.

820 West Broadway

P.O. Box 32020

Louisville, KY 40232

Duke Energy

573 Duke Power Road

P.O. Box 306

Cliffside, NC 28024

TECHNICAL PUBLICATION
© Riley Power Inc., LG&E Energy Corp., Duke Energy 2003

Coal-Fired SCR Operating Experience with High

Removal Efficiency and Low-Nox Firing Systems

by:

Clayton Erickson, Eric Pear

Riley Power Inc.

Scott Straight

LG&E Energy Corp.

Linton Hutcheson

Duke Energy

ABSTRACT

The NOx credit market, system wide emission caps and environment concerns provide an

incentive to design and operate Selective Catalytic Reduction (SCR) systems at the

highest efficiency consistent with avoidance of operational risks, such as ammonium

bisulfate formation. System-wide emission caps offer the incentive of meeting limits with

a lesser number of highly-efficient units while avoiding capital costs and outages for SCR

installation on other units. Operation at NOx removal efficiencies of 90% or more

requires stringent control of inlet gas uniformity, ammonia mixing, and ammonia

injection rates, particularly for low-NOx inlet conditions resulting from state-of-the-art

low NOx burner systems. This paper presents the design considerations for coal-fired,

high-dust SCR designs with low outlet NOx concentrations. Results from recent

operating SCR systems, with low NOx burner configurations, verify that outlet NOx

concentrations from 0.05 lb/MBtu to below 0.03 lb/MBtu can be achieved and sustained

with current SCR technology.

Riley Power Inc.

5 Neponset Street

Worcester, Massachusetts 01606

www.babcockpower.com

LG&E Energy Corp.

820 West Broadway

P.O. Box 32020

Louisville, KY 40232

Duke Energy

573 Duke Power Road

P.O. Box 306

Cliffside, NC 28024
3

INTRODUCTION

The combined SCR technology experience that is globally available has allowed for the development

and continuous improvement of SCR technology. Outlet NOx concentrations once unachievable, are

now achieved with low ammonia slip and minimal plant impacts. The available NOx control

technologies have allowed the US government to tighten the permissible NOx emissions and have

allowed Utility companies to provide energy in an environmentally safe and health conscience manor.

In addition to the regional health benefits to the local communities, power utilities are given incentive

to exceed the goals the US government has set forth through the use of NOx credits.

The application of high removal efficiency, high dust SCR systems on coal fired boilers with low NOx

burner systems requires careful attention to flue gas and ammonia mixing and reagent injection rate

control. Current incentives, both financial and environmental, exist for the installation and operation

of NOx control systems which exceed the 2004 SIP Call of 0.15 lb/MBtu. The financial incentives

include not only the NOx credit trading market but also system wide emission caps allowing for the

installation of fewer high removal systems to meet compliance emission rates. These incentives did

not exist in the European market during the deployment of SCR technology. The European countries

have substantial operating experience and proven SCR designs for the reduction and removal of NOx

emissions. However, due to regulations which required all European plants to be equipped with 

control technology and achieve an outlet NOx emission of 0.19 to 0.08 lb/MBtu, depending on the

country, the incentive for over compliance of NOx emissions did not exist (1,2).

DESIGN PARAMETERS

The design of high removal efficiency SCR system centers on two main issues: 1) the ability to inject

and distribute the reagent, typically ammonia, into the flue gas stream prior to the catalyst inlet

surface and 2) the selection of the correct catalyst activity and deactivation rates. In addition the

operating parameters of the boiler exit gas stream and future modifications to the boiler and fuels

must be fully understood.

NH3/NOx Mixing

The correct mixing of the boiler exit flue gases with the ammonia reagent typically consists of two

distinct mixing regions, before and after the ammonia injection. The combination of both mixing

regions provides a low variation of the ammonia to NOx mole ratio at the catalyst face. The ratio of

NH3/NOx is the essential parameter for high removal efficiency SCR systems. The standard deviation

of the NH3/NOx at the catalyst face is a typical measure of the mixing system's performance; for 90%

removal systems a standard deviation of NH3/NOx mole ratio of 5% is typically chosen. The mixing

of the flue gas stream from the boiler outlet before the ammonia injection is required to ensure the

uniform distribution of NOx to the catalyst face for all operating loads and conditions. Mixing systems

before the ammonia injection location need to cross mix the boiler exit gases on a large scale, i.e. from

side to side of the unit, to ensure uniform distribution during all operating conditions. It is important

to note that if the NOx concentration at the ammonia injection location varies with load or operating

equipment selection (mill groups, etc.) then it is not possible to have a single ammonia injection

distribution profile that achieves NH3 to NOx mole ratios at the catalyst face for all operating

conditions. The inlet stream mixing requirement is determined based on experience and boiler outlet

boundary conditions determined from the baseline test program. With the inlet NOx mixing achieved

prior to ammonia injection, the next requirement is to mix the ammonia into the flue gas stream prior

to the catalyst face. The ammonia injection profile required to achieve the desired mixing is evenly

distributed with proper upstream NOx mixing. This even distribution of ammonia injection

significantly aids the optimization period of the SCR system resulting in reduced commissioning and

optimization periods. The mixing system is also designed to provide the temperature, flow angle and

velocity distribution required at the catalyst face.
4

Catalyst Selection

Catalyst selection requires knowledge of both the current coals and boiler systems and future plans

for fuels and boiler systems. The current and future coals of the plant are analyzed to determine the

need for deactivation rate control such as limestone addition to minimize arsenic poisoning. If arsenic

levels in the coal are not balanced with sufficient CaO in the fly ash undesirable deactivation may

result. In order to control this balance, limestone addition systems are installed that add limestone

to the coal during bunker loading. These systems can have variable speed addition rates to ensure

catalyst protection for a wide range of fuels while minimizing limestone addition to protect against

furnace and convective back pass slagging and fouling.

Additionally the conversion rate of SO2 to SO3 must be carefully specified to avoid undesirable plant

and emission impacts. With the current understanding of SO3 generation and capture, a prediction

of the impact the SCR system will have on the plant and emissions can be made based on experience.

Based on recent testing and correlations, most plants can be analyzed without extensive testing (3).

If the SO3 generation and capture characteristics of a unit are in question an SO3 test program can

be used to determine specific plant values. With the understanding of the specific plant SO3

characteristics, the SO2 to SO3 conversion of the catalyst can be specified for the initial and final

catalyst charges.

The boiler design parameters, flue gas flow rates, economizer outlet gas compositions and

distributions, etc. are typically determined from detailed baseline test programs implemented prior

to the design of the SCR system. These test programs typically include test matrices that determine

a wide range of boiler operating conditions. While the baseline test program provides detailed insight

on the characteristics of the unit, a measure of experience must be applied to the results. Using the

data collected from the baseline test program and experience with similar applications, boundary

conditions for the mixing system and physical flow model study are derived. These boundary

conditions include low and part load conditions and plant conditions such as mill groups out 

of service. Consideration of plant operating conditions is essential for high efficiency across a 

load range.

DUKE POWER CLIFFSIDE STATION #5 OPERATING EXPERIENCE

Riley Power Inc. and Duke Fluor Daniel (DFD), as part of an SCR Program Alliance with Duke

Energy, engineered, procured and constructed the first of three SCR projects for Duke Energy.

Cliffside Unit 5 is a tangentially fired, 562 MW generating plant with 4,080,000 lb/hr main steam.

The plant, built in 1971 and has been equipped with a low NOx burner system during the SCR retrofit

project. The cold side ESP was also upgraded and rebuilt during the SCR retrofit project. The

addition of the Riley Power Inc. Selective Catalytic Reduction system was designed to reduce the

outlet NOx concentration from 0.45 lb/MBtu, by 85%, to 0.0675 lb/MBtu. Figure 1. shows an isometric

view of the Cliffside Unit 5 SCR system project. Based on the current and future coals expected for

the plant, a limestone addition system was part of the project scope to control gaseous arsenic and

prevent catalyst deactivation.

Acceptance testing of the Cliffside Unit 5 SCR system confirmed the operation of the SCR system

design. All design criteria were met or exceeded during the acceptance test. The operation of the SCR

system for NOx reductions of greater than 85% will allow for increased NOx credit accumulation

during ozone control periods (May-September) if desired by the plant. The plants current ability to

obtain lower than design inlet NOx concentrations from the new low NOx burner system has been

operationally tested with the SCR system at full removal efficiency. The NH3/NOx standard deviation

at the catalyst face was measured by the acceptance test to be < 3% for both reactor systems. During

acceptance testing of the SCR system ammonia slip measurements were well below the 2.0 PPM (end

of life) design value as expected at 85% NOx removal. The measured ammonia slip values of < 0.1 ppm

@ 3% O2 are below the values expected even at initial catalyst life. The low ammonia slip has resulted
in no impact on ash sales, which continue as normal without the need for land filling or chemical

treatment. The catalyst deactivation rate has been measured at the end of the first operational season

and agrees with predicted values based on the use of a limestone addition system (4). The low

ammonia slip, high NOx removal efficiency and low catalyst deactivation rate are attributed to the

effective mixing of NH3/NOx distribution, the performance of the Cormetech honeycomb-type catalyst

and the limestone addition system. A comparison of Cliffside Unit 5 operating data to national and

state regulations for coal and gas fired power plants is provided in Table 1.

5

Table 1

Outlet NOx Concentration National and State Regulations

Cliffside Uint 5 Power

Plant

2004 SIP Call 2010 North Carolina Clear

Skies for Coal Plants (5)

2010 North Carolina Clear

Skies for Gas Plants (5)

0.0675 lb/MBtu 0.15 lb/MBtu 0.16 lb/MBtu 0.10 lb/MBtu

Figure 1. Duke Energy Cliffside Unit 5 SCR Retrofit Arrangement
6

LG&E ENERGY TRIMBLE COUNTY OPERATING EXPERIENCE

Riley 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 Riley Power Inc. Selective Catalytic

Reduction system was designed to reduce the outlet NOx concentration from 0.32 lb/MBtu, by 90%,

to 0.032 lb/MBtu. Figure 2. shows an isometric view of the Trimble County SCR system project.

Acceptance testing of the Trimble County SCR system confirmed the operation of the SCR system

design. All design criteria were met or exceeded during the acceptance test. The low SCR system

pressure drop has prevented the need for complicated fan modifications at Trimble County. The

operation of the SCR system for NOx reductions of greater than 90% will allow for increased NOx

credit accumulation during ozone control periods (May-September), if desired by the plant. The

application of low NOx burners in concert with a high efficiency SCR system has proven to result in

NOx reduction beyond guaranteed and target values. The NH3/NOx standard deviation was

measured to be < 3 % by the acceptance test for both SCR reactors. Ammonia slip measurements were

well below the 2.0 PPM (end of life) design value as expected. After approximately 30% catalyst life,

the measured ammonia slip values of 0.1 ppm @ 3% O2 are below the expected values. The low

ammonia slip has resulted in no impact on ash sales, which continue as normal without the need for

land filling or 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. A comparison of Trimble County operating data to national and state regulations for coal

and gas fired power plants is provided in Table 2.

Table 2

Outlet NOx Concentration National and State Regulations

Trimble County Power

Plant

2004 SIP Call 2010 Kentucky Clear Skies

for Coal Plants (6)

2010 Kentucky Clear Skies

for Gas Plants (6)

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

Riley Power Inc has installed SCR systems through out the United States for units ranging in fuels,

boiler types and required outlet emission concentrations. Two such SCR systems: Cliffside Unit 5 of

Duke Energy and Trimble County of LG&E Energy Corp., are examples of successful installations of

high removal efficiency, high dust SCR systems in combination with state-of-the-art low NOx burner

technology. Both units employ patented Delta Wing static mixing technology that has met and

exceeded the project goals resulting in higher NOx removal efficiencies with low ammonia slip. The

initial catalyst charge has maintained the expected activity levels based on fuels fired.

CONCLUSIONS

The design, installation, and successful operation of high efficiency SCR systems, in conjunction with

state-of-the-art low NOx burner systems, have been achieved. The measured standard deviation of

NH3/NOx mole ratio at the catalyst face for both systems was < 3%. Based on Riley Power Inc.

analysis and industry experience (2,7) mixing systems with < 3% standard deviation can obtain ultra

high NOx removal efficiencies of > 93% with the correct catalyst volume. The Delta Wing static mixing

system has successfully achieved high mixing quality with low reagent concentrations and, to date,

shows no sign of reaching its full potential.

7

Figure 2. LG&E Energy Corp. Trimble SCR Retrofit Arrangement
REFERENCES

(1). Performance of Selective Catalytic Reduction on Coal-Fired Steam Generating Units - Final 

Report. EPA Office of Air and Radiation, Report 6204-J, June 25, 1997.

(2). Twenty-Five Years of SCR Evolution: Implications For US Application and Operation, J. E.

Cichanowicz, and L. J. Muzio, Presented at EPA, DOE, EPRI and AWMA Combined Power 

Plant Air Pollution Symposium/Mega Symposium, Chicago, IL, August 21-23, 2001

(3). Emissions of Sulfur Trioxide from Coal-fired Power Plants. Srivastava, R.K., Miller, Erickson, C.

and Jambhekar, R., Presented at Power Gen 2002, Orlando, FL, December 10-12, 2002.

(4). Limestone Injection for Protection of SCR Catalyst Report 6204-J, June 25, 1997.

(5). U.S.EnvironmentalProtection Agency. Clean Skies in North Carolina.

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

(6). U.S. Environmental Protection Agency. Clean Skies in Kentucky.

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

(7). How Low Can We Go? Controlling Emissions in New Coal-Fired Power Plants. Bielawski, G.T.,

Rogan, J.B., McDonald, D.K., The U.S. EPA/DOE/EPRI Combined Power Plant Air Pollutant 

Control Symposium: "The Mega Symposium," August 20-23, 2001.

© Riley Power Inc., LG&E Energy Corp., Duke Energy 2003