Optimizing SCR Catalyst Design 

and Performance for Coal-Fired Boilers 

Scot Pritchard 

Chris DiFrancesco 

Cormetech, Inc. 

Durham N.C 

(919) 620-3000 

Shozo Kaneko 

Norihisa Kobayashi 

Mitsubishi Heavy Industries, Ltd. 

Nagasaki, Japan 

(045) 224-9178 

Kohei Suyama 

Kozo Iida 

Mitsubishi Heavy Industries, Ltd. 

Nagasaki, Japan 

(958) 28-6401 

Presented at 

EPA/EPRI 1995 

Joint Symposium 

Stationary Combustion NOx Control 

May 16-19, 1995 

1 
OPTIMIZING SCR CATALYST DESIGN 

AND PERFORMANCE FOR COAL-FIRED BOILERS 

Scot Pritchard 

Chris DiFrancesco 

Cormetech, Inc. 

5000 International Drive 

Durham, NC 27712 

Shozo Kaneko 

Norihisa Kobayashi 

Mitsubishi Heavy Industries, Ltd. 

Yokohama, Japan 

Kohei Suyama 

Kozo Lida 

Mitsubishi Heavy Industries, Ltd. 

Nagasaki, Japan 

Abstract 

The design of an SCR system for a coal-fired application is a challenge due to 

particulate, catalyst poisons, and So2 in the flue gas. Our experience show that coalfired SCRs are successful when the system impact and catalyst deterioration factors are 

understood and specific countermeasure are implemented in system and catalyst 

design 

There are several factors that the system and catalyst must consider in order to assure 

success. They include: type of boiler, required performance, fuel and ash analysis, ash 

loading, type of SCR, inlet conditions, Catalyst deactivation mechanisms and impact on 

down stream equipment. 

This paper explains each system and catalyst deterioration factor that we have 

experienced. For each factor, it presents the specific countermeasure that has resulted 

in successful SCR systems. Also, it discusses the design of improved catalyst derived 

from our experience. Finally, two case studies illustrate the impact of optimized design. 

2 
Introduction 

Selective Catalyst Reduction (SCR) is recognized worldwide as the most effective NOx

Control Technology for utility boilers and combustion turbines when substantial NOx

reduction of 50% to 95% is required. In addition to its proven high performance, It has 

also become an economically viable solution, with current installed cost, in the United 

States, estimated at between $20/kw for natural gas and $50/kw-$70/kw for coal units. 

The technology has even given some utilities the capability to achieve lower heat 

reduction or omission of flue gas recalculation (FGR). 

Mitsubishi Heavy Industries (MHI) pioneered the developed and application of SCR 

technology staring in the late 70’s in Japan, and installed the worlds’ first High-dust SCR 

application on a 175 MW coal fired boiler in 1980. MHI transferred their technology 

through licensing agreement to several companies in Europe and to Cormetech in the 

United States. Over 319 units including 56 coal fired boilers worldwide operation 

successfully using this technology. 

Figure 1 

Catalyst for Coal, Oil, and Gas Applications 

Cormetech is a joint equity company of Corning Incorporated, MHI, and Mitsubishi 

Chemical Company (MCC). Cormetech design and Manufactures homogeneous 

Titanium-Tungsten-Vanadium (Ti-W-V), Extruded honeycomb catalyst for fossil-fuelfired applications. Figure 1 show a photograph of catalyst for coal, oil, and gas 

applications. Cormetech draws upon the vast experience database of MHI, the 

extrusion and catalyst know-how of corning, and catalyst technology of MCC to provide 

an optimized product to the market. 

This paper address the methods successfully implemented to design and optimize 

catalyst performance in coal fired boiler applications. The basics of deNOx catalysis are 

3 
reviewed in preparation for the discussion on system impact, catalyst deterioration, 

mechanism, and countermeasures that follows. Additionally, catalyst design 

optimization is presented by a discussion of improved catalysts for coal applications. 

Two case studies (pulverized coal and cyclone boilers) comparing conventional and 

improved catalyst are presented. 

Background of SCR Reaction 

In coal fired boilers in which SOx is presented in the flue gas two chemical reaction that 

occur in the presence of the SCR catalyst are of most importance to the following 

discussion 

One reaction, of course, is the deNOx reaction: 

4NO + 4NH3 + O2 catalyst 4NH2 + 6H2O 

 (1) 

Figure 2 illustrates this primary reaction mechanism. Starting in the upper left-hand 

corner of the figure and proceeding clockwise is: 

1. Active catalyst site (Me = metal) 

2. Absorption of ammonia molecule 

3. Reaction of NO with NH3 forming 

4. Regeneration of active site by oxidation 

The second reaction is oxidation of sulfur dioxide: 

 (2) 

SO2 + 1⁄2 O2 

catalyst SO3

The NOx conversion rate and SO2 oxidation rate will partly depend upon the rate in 

which the reactants diffuse through the wall of the porous catalyst to reach active sites 

where reaction takes place. Through our experience we have determined the rate of 

chemical reaction is fast reaction for our catalyst. For deNOx conversion the rate of 

chemical reaction is fast relative to the rate of diffusion. Therefore the catalyst is 

effective primarily at the surface of the wall. On the other hand, the SO2 oxidation 

chemical reaction rate is slow relative to the rate of diffusion so that the reactants diffuse 

into the entire wall. This conclusion is relevant to the subsequent discussion on the 

system impacts, catalyst deterioration mechanisms, and countermeasures. 

4 
O

N2 H2O

H H

N

H H

O O

--Me--O- -Me--

NH3 Adsorption on Active Site

H

O

--Me--O- -Me--

O

Active Site

NH3

H2O

O2

NO

4NO + 4NH3 + O2 → 4N2 + 6H20

N H H

N

H H

--Me--O- -Me--

O O

Reaction of NO with NH3

H H

O O

--Me--O- -Me--

Regeneration of Active Site

Figure 2 

SCR Surface Reaction Mechanism 

High

Dust

System

Low

Dust

System

NH3

Boiler

SCR STACK ESP Air FGD

Heater

NH3

Boiler

SCR

STACK

ESP FGD

Air

Heater

Heater

Gas to Gas Heat

Exchanger

Figure 3 

SCR Configurations 

5 
System Design Impact & Countermeasures 

Figure 3 shows two typical layout of SCR system applied to coal fired boilers. The LowDust or Tail End option shows the installation of the SCR reactor after the air preheater 

(APH), Electrostatic Precipitator (ESP), and flue Gas Desulfurization (FGD) systems. 

This option requires the flue gas to be reheated to acceptable SCR temperatures, 

typically 5500

F-7500F. Low-Dust designs have primarily been used on wet bottom 

boilers which have ash re-circulation, due to concern over catalyst degradation caused 

by arsenic poisoning. The High-Dust option locates the SCR reactor between the 

economizer exit and the AOH inlet. This is more traditional for dry bottom boilers and 

less costly, and is now being applied to wet bottom boiler based on economics, 

advancements in catalyst resistance to arsenic poisoning, and limestone addition to the 

fuel, which will be discussed later. 

CONTROL SYSTEM

AQUEOUS

NH3 TANK

PUMP

FLOW

CONTROL

HEATER

FAN

VAPORIZER

MIXER

NH3 INJECTION GRID

SOOTBLOWERS

CATALYST

LAYERS

AIR 

PREHEATER

ECONOMIZER BYPASS

STATIC MIXER

TURNING VANES

Figure 4 

SCR Components 

The SCR system consists of many components in addition to the catalyst. Figure 4

depict a high-dust SCR system which includes the catalyst reactor, economizer bypass 

used for part-load temperature control, static mixer used for temperature and/or 

ammonia mixing, turning vanes, ammonia vaporization system, ammonia injection grid 

(AIG), sootblower, and the control system. Since these components directly influence 

overall system effectiveness, Cormetech works closely with system equipment suppliers 

to ensure proper design. 

6 
Operational impact of an SCR system on coal fired boilers included drafts loss, SO3 

formation, and ammonia emission, Pressure drop is caused by the installation of the 

catalyst material in the flue gas and is typically minimized by expanding the flue gas 

path. Typical pressure value for conventional high dust applications range from 0.5 to 

1inch of water per catalyst layer, with 2-3 layers installed with an additional 1-2 inches 

for the components of the system. 

As shown in equation (2) the SCR catalyst will convert a small amount of SO2 to SO3. 

This conversion rate is typically less than 0.55 per layer depending upon the catalyst 

formulation, deNOx requirements, and operating temperature. If the SO2 oxidation rate 

is too high, corrosion and plugging will occur in the (APH) due to the formation of H2SO4

and NH4HSO4. 

As a countermeasure, our catalyst is designed to achieve high deNOx activity while 

keeping SO2 oxidation activity low. When considering the catalyst formulation, a high 

concentration of vanadium will result in a high deNOx activity, but will also result in a 

high SO2 oxidation activity. One countermeasure to this is a vanadium concentration is 

selected such that there is sufficient deNOx activity with an acceptable level of SO2 

oxidation. 

As a system countermeasure, the cold end surface of regenerative-type APH may be 

enameled and/or sootblowing capabilities may be enhanced. 

Ammonia slip may contaminate flyash, and combination with S03 caused (APH) 

plugging. Designers can combat the effect by considering: 

• 2ppm to 5ppm NH3 slip at end-of-life 

• ammonia injection grid with adequate representative NOx input value to control 

system 

• multi-point outlet NOx sampling control grid to assure representative NOx input 

value to control system. 

• permanent measuring grid for AIG tuning to assure proper distribution of ammonia 

• Flyash ammonia concentration monitoring. 

Catalyst Deterioration Mechanisms & Countermeasures 

When designing for a given application, designers must carefully review fuel 

And ash constituents (see Table A). Under ideal conditions the catalyst will reduce NOx

for an unlimited period of time. (Table B) lists the primary deterioration mechanisms 

versus the type of coal fired units, wet or dry bottom. 

7 
Ash Moisture, % 6-33 

Total Sulfur in Coal, % 0.6-1.6* 

Trace Material, ppm 

Ni 3-40

Cr 7-46

As 1-25

Cl 41- 1,900

Ash analysis, % 

Sl O2 41-71 

As2 O3 2-33 

Fe2 O3 2.5-10 

CaO 2.4-26 

MgO 0.7-49

Ti02 0.1-1.8 

MnO 0.02-0.2

V2 05 0.01-0.1 

Na2 O 0.05-1.6 

K2O 0.1-4.0

P2 O6 0.06-1.3 

SO3 1.6-16.5

*Application Experience on Oil up 5.4% Sulfur 

 Table A 

 Typical Fuel/Ash Composition 

Boiler Type Wet Bottom Dry Bottom 

Sintering Negligible Negligible

Accumulation of Alkaline Metal Small Small 

Accumulation Earth Metal 

Masking 

Large Large

Accumulation of Arsenic Oxide Large with Ash Re-circulation Moderate 

Flyash Deposition* Small Small 

Erosion* Small Small

*Highly dependent on proper design of SCR system 

Table B 

Main Causes of Catalyst Deterioration 

8 
Based on a sound understanding of the deterioration mechanisms, specific 

countermeasures are implemented, such as: 

◆ system design evaluation. 

◆ catalyst formulation. 

◆ catalyst volume selection. 

◆ catalyst geometry. 

◆ catalyst macro and micro pore design. 

◆ catalyst characterization testing and predictive tools. 

Thermal Sintering 

Thermal sintering is the growth of primary catalyst particle resulting in a reduction of 

catalyst surface area, which reduces catalyst performance. Figure 5 illustrates this 

mechanism. Thermal stability is maximized with the corporation of Tungsten in the 

catalyst formulation. As a result, sintering is negligible at normal SCR operating 

temperatures. 

PRIMARY

PARTICLE OF

TIO2

SECONDARY

PARTICLE OF

TIO2

FR ESH CATALYST HE AT SINTERED CATALYST

Figure 5 

Thermal Sintering 

Alkaline Metal (Na, K) 

Alkaline metal may directly react with active sites and render them inert as shown in 

Figure 6. Since the deNox reaction takes place primarily on the surface, the degree of 

deactivation depends on the surface concentration of the alkaline metals. In a watersoluble form, these alkaline metals are highly mobile and will migrate throughout the 

catalyst material. Since the wall of our catalyst consists entirely of catalyst material, the 

surface concentration of alkaline metal is diluted by this migration, minimizing the 

deactivation rate. 

9 
H

O O

--Me--O- -Me--

FRESH CATALYST

Na (K)

O O

--Me--O- -Me--

AKALINE DETERIORATION

Na+ (K=)

Figure 6 

Alkaline Deterioration Mechanism 

For the subject catalyst in coal-fired applications, it is our experience that this type of 

poisoning has a small impact, since most of the alkaline metals in the coal ash are not 

water soluble. The impact is larger in oil-fired applications where the majority of the 

alkaline metals in the ash are water-soluble or when the thickness of the catalyst 

material is low. For example, given the same concentration of alkaline metals in the flue 

gas, this catalyst will have a lower surface poison concentration than a coated product 

and, thus have a lower deactivation rate and longer useful life as shown in Figure 7. NOx Removal Efficiency, (%)

0 10 20 30 40

60

80

100

Coated Type

Solid Honeycomb Type

Operation Period (‘000 hrs)

Figure 7 

Comparison of Durability (Oil Firing) 

10 
Arsenic (As) 

Arsenic poisoning is caused by gaseous arsenic As2O3 in the flue gas. The As2O3

diffuses into the catalyst and solidifies on both active and non-active sites, as shown in 

Figure8. 

As with alkaline metals, arsenic poison is best abated by use of homogenous catalyst 

compositions, which effectively reduce the surface poison concentration. Since the 

mechanism of arsenic poisoning is more diffusion-limited and subject to capillary 

condensation, optimized catalyst pore structure also lessens its effects. Further, 

engineers use accumulation rates determined by both lab and field tests to ensure a 

proper catalyst volume is provided for each specific application. 

FRESH GASEOUS ARSENIC DETERIORATUION

--Me--O--Me--O--Me--

O O O

H

--Me--O--Me--O--Me--

O O O

--As--O--As--O -- As--

O O O

As2O3 O2

Honeycomb

Catalyst







 













 























As2O5

Active Material



 Support (Ti-W)

Primary

Particle

Of Catalyst

Figure 8 

Model of Arsenic Compound Accumulation 

A system countermeasure incorporates the use of a fuel additive. As mentioned 

previously, wet bottom boiler with 100% ash recalculation present a worst case scenario 

for catalyst deterioration caused by arsenic poisoning. In order to combat the high levels 

of gaseous arsenic (As2O3) in cyclone boilers, limestone may be added to the fuel. A 

typical limestone to fuel ratio is approximately 1:50. Figures 9a, 9b, and 10 show how 

the addition of limestone effectively reduces gaseous arsenic at the entrance to the 

catalyst. The free CaO in the limestone reacts with arsenic, forming a solid, Ca (AsO4) 

that will not poison the catalyst. The impact of adding limestone will be addressed 

further in the wet bottom boiler case study. 

11 
Arsenic Compound

ESP

A

P

H

As2O3 (G) + 3CaO + O2

Î Ca3(AsO4)2 (S)

As2O3 (G) + O2

Î As2O5 (S)

2FeAsS + 402 Î

As2O3 (G) + FeO3

+ SO2

As2O3 (G)

Ø

DeNOx

Reactor

Solid State

Gaseous State

Coal

FeAsA

Figure 9a 

Arsenic Compound in Flue Gas 

As2O3 (G) +

3CaO + O2 Î

Ca3(AsO4)2 (S)

Arsenic Compound

ESP

A

P

H

As2O3 (G) + O2

Î As2O5 (S)

2FeAsS + 402 Î

As2O3 (G) + FeO3

+ SO2

As2O3 (G)

Ø

DeNOx

Reactor

Solid State

Gaseous State

Coal

FeAsA

CaCo3 Î

3CaCo + CO2

Figure 9b 

Effect of Limestone Injection 

12 
1

10

100

1000

3 10 30

: Plant A

: Plant B

: Plant C

Gaseous arsenic @ 1st Cat

alyst

L

ayer (micro grams/Nm3)

Arsenic Recycle Ratio = 100%

Effect of 

Limestone 

Injection

Arsenic in Coal (mg/kg)

Figure 10 

Relationship Between Arsenic in Coal and Gaseous Arsenic 

Alkaline Earth Metals (Ca) 

Alkaline earth metal poisoning primarily occurs when free CaO in the flyash reacts with 

SO3 adsorbed on the catalyst surface forming CaSO4. The CaSO4 may cause catalyst 

surface masking, preventing the reactants from diffusing in to catalyst surface masking, 

preventing the reactants from diffusing into catalyst. Figures 11 and 12 illustrate this 

mechanism. 

FRESH CALCIUM DETERIORATION

CaSO4

CaSO4

Figure 11 

Mechanism of Calcium Deterioration 

13 
Level of Ca

Catalyst Wall Catalyst Surface

Figure 12 

Accumulation of calcium Composition in Catalyst 

In order to address masking, particularly in dry bottom boilers where the free CaO 

content is nearly double that wet bottom boiler (see Figure 13), characterization of the 

mechanism and accurate prediction of deactivation rates is employed. Experience 

shows that quantity of free CaO in the fly ash governs the amounts of deactivation. 

Extensive modeling of deactivation from full scale and laboratory data has been 

development providing high confidence in design to counter this phenomenon. 

0

20

40

60

80

100

0

20

40

60

80

100

A B C D E F G H I

average

average

Plants

FreeCaO Amorphous Phase FreeCaO Amorphous Phase

Wet Bottom Boiler Ash Dry Bottom Boiler Ash

Figure 13 

Free CaO Ratio in Fly Ash 

14 
Catalyst Plugging 

Catalyst plugging has two primary causes: (a) ammonia salt deposition and (b) flight 

deposition. Proper selection of catalyst pitch and cell opening size minimizes plugging. 

Good system design is also a primary prevention tool. 

Ammonia salt formation/deposition will not occur provided the SCR inlet temperature 

maintained above the salt formation level. Typically temperature maintenance is only a 

problem during part load operation and is elevated by installing a water- or gas-side 

economizer bypass in the system. If a gas bypass is used, careful attention must be 

paid to provide adequate temperature mixing upstream of AIG. This avoids low 

temperature streamline, which can cause localized salt formation at the AIG and 

catalyst. 

Flight deposition is minimized through proper flow distribution across the catalyst face. 

This accomplished through engineering of SCR system flue layout, Computational Fluid 

Dynamics (CFD) codes, and/or cold flow modeling. 

The design limit the number of areas where dust accumulation may occur, i.e., 

• Avoiding flat surface as long leading edge to turning vanes. 

• Installing as deflection devices on support beam flanges and sootblower pipes. 

• Avoiding flue dead between dampers where ash accumulation may occur in closed 

position and exhaust onto catalyst when opened, etc. 

Erosion 

Catalyst erosion is caused by the impingement of flyash on the catalyst face. 

Catalyst erosion is a function of gas velocity, ash character, angle of impingement, and 

catalyst properties. Cormetech has experience with flyash loading as high as 30g/Nm3

and flue gas velocities up to 6.2 m/s. 

Experience has proven that no significant erosion will occur with proper system design, 

catalyst material durability, and catalyst edge hardening. Poor flow and ash distribution 

at the inlet to the catalyst has been the source of most problems, and has improved 

significantly over the entire SCR experience history. Careful attention must be paid to 

utilize flow modeling (CFD and cold flow rectifier grid should be installed to straighten 

flow into the catalyst bed. Since most catalyst erosion occurs at the catalyst leading 

edge, which is exposed, to the direct impingement of dust particle the catalyst may also 

be hardened at the entrance region to provide further protection as shown in Figure 14. 

15 
FLYASH

HARDENED EDGE

CATALYST

Figure 14 

Honeycomb Catalyst with Hardened Edge 

Although in most cases In-Duct SCR is not attractive due to extremely high system draft 

loss and difficulty in control of gas flow and ammonia distribution, some utility site are 

considering the concept due to site plan limitations. Therefore, we will pilot test our 

hardened edge catalyst at velocities > 12m/s in 1995. 

Catastrophic Failure of SCR Catalyst 

Catastrophic Failure is defined as sudden and permanent loss of catalyst performance 

based on our experience, catastrophic failure are extremely rare. The primary cause is 

associated with the ignition of ash buildup. The intense heat of a fire can irreversibly 

damage any SCR catalyst. Cormetech’s ceramic honeycomb catalyst, unlike plate 

catalyst with stainless steel mesh substrates, will not promote oxidation; therefore fires 

are less likely to spread and are more easily contained. 

Coal Catalyst Developments

Based on our understanding of reaction and deactivation mechanisms we are improving 

our catalyst. The objective is to increase the deNOx reaction rate without increasing the 

SO2 oxidation rate. One method is to modify the pore structure of the catalyst wall in 

order to reduce diffusion resistance. The other is novel method and requires further 

explanation. 

Since the deNOx reaction is only effective at the surface, we have developed an 

improved catalyst of the same composition as our conventional catalyst with the 

exception that the vanadium is preferentially distributed to the surface if the catalyst 

wall. In this manner, we can maximize the concentration of vanadium in the effective 

fraction of the wall, maximizing deNOx activity without incurring high SO2 oxidation. 

Figure 15 shows the performance of this improved catalyst compared to that of 

conventional catalyst. Since the product is still made entirely of catalytic material, 

resistance to poisons in maintained. 

16 
Figure 15 

Comparison of Catalyst Performance 

Development and Qualification of Improved Coal Catalysts 

We are developing the improved coal catalyst described according to the schedule 

shown in Figure 16. 

1993 1994 1995 1996

Screening of Improved Coal Catalysts

Confirmation Test

Endurance Test

Pilot Plant Test

Figure 16 

Improved Coal Catalyst R&D Schedule 

We have already completed laboratory performance confirmation and durability tests. 

The performance of the improved catalyst is approximately 15% higher that our 

conventional catalyst, without impact on SO2 oxidation. 

17 
We have tested durability to gaseous arsenic poisoning by an accelerated method using 

the apparatus in Figure 17. The test results are shown in Figure 18. The improved 

catalyst maintains its performance advantage over conventional catalyst at any arsenic 

poisoning level. 

As2O3 Powder

SCR CATALYST THERM OCOUPLE

THERM OCOUPLE

H2O Supply

G as Flow

Controller

Gas Supply 

U nit

FURNACE

EXHAUST

A rsenic

Removal

U nit

TI

TI

Figure 17 

Schematic of Arsenic Testing Apparatus 

Activity

Arsenic Load, ppm*hr

Conventional

Improved

Figure 18 

Arsenic Durability 

18 
Since the degree of Deactivation due to surface masking by CaSO4 is dependent only 

on the fraction of surface masked we expected that the rate is deactivation is the same 

for both conventional and improved catalyst. Therefore, given the same installed 

catalyst volume, the improved catalyst will maintain field performance much longer due 

to the higher initial activity, as shown in Figure 19. 

TIME, years

Improved

Conventional

ACTIVITY

2 468

Figure 19 

CaO Durability 

Before Commercialization, the endurance of the catalyst will be confirmed by operation 

in actual plant. Initial endurance tests will start by charging catalyst samples into 

commercial SCR plants in Japan. During 1995 catalyst samples will be charged into a 

coal unit in the United States and three coal units in Germany. The performance of the 

catalyst sample after aging in the commercial plants will be compared to fresh 

performance in order to evaluate endurance. 

Impact of Improved Coal Catalyst on SCR Design for Coal Fired 

Whether achieving an increase in performance by optimizing the pore structure of the 

catalyst on the SCR design, two SCR case studies are described below comparing 

improved with conventional catalyst. 

19 
Case Study 1: Dry Bottom Boiler 

A 250 MW unit is selected as an example unit. Table C shows the design data for this 

unit. Figure20 shows the 10-year catalyst volume of the improved catalyst is higher by 

15%; the catalyst volume of the improved catalyst is decreased by approximately 15% 

from the conventional catalyst volume. 

 

Design Condition; 

Fuel Coal

Plant Output, MW 250 

Flue Gas FlowRate,Nm3/hr 784,000 

NOx inlet, ppmvd @ 3% O2 154 

NOx outlet, ppmvd @ 3% O2 31 

NOx Removal Efficiency, % 80 

Ammonia Slip, ppmvd @ 3% O2 5 

Specification:

Type of System SCR 

Type of Reactor Vertical Flow-Fixed Bed 

Type catalyst Honeycomb 

Number of reactor 1 

Table C 

Dry Bottom Boiler 

Catalyst Volume Ratio

1.0

Improved

Conventional Add 4th Layer

Add 4th Layer

Operating Years

0 1 2 345 6 7 8 9 10

Figure 20 

Impact of Improved Catalyst 

Dry Bottom Boiler 

20 
Case Study 2: Wet Bottom with Ash Recalculation 

A 320 MW unit is selected as an example unit. Table D shows data for this unit. Figure 

21 shows the 10 years catalyst management plan. The total catalyst volume of the 

improved catalyst for ten years of operation is two-thirds that of the conventional 

catalyst. The impact on the catalyst management plan is shown in Figure22. With 

Limestone Injection, the need for addition and replacements of catalyst are significantly 

delayed. 

Design Condition Coal 

Fuel 320

Plant Output, MW 1,125,000 

Flue Gas Flow Rate,Nm3/hr 1,652

NOx Inlet, ppmvd @ 3% O2 530 

NOx removal efficiency, % 68 

Ammonia Slip, ppmvd @ 3% O2 5 

Flash recalculation Rate, % 100 

Specification:

Type of System SCR 

Type of Reactor vertical flow-fixed bed 

Type of Catalyst honeycomb 

Number Of Reactors 1 

Table D 

Wet Bottom Boiler 

21 
Improved

Conventional

Operating Years

0 1234 5 6 7 8 9 10

1

2

3

4

Catalyst Volume Ratio

Add 3rd Layer

Add 3rd Layer

Add 4th Layer

Exchange 1st Layer

Exchange 2nd Layer

Add 4th Layer

Figure 21 

Impact of Improved Catalyst 

Wet Bottom Boiler 

Improved w/ Lime Inj.

Conventional w/o Lime Inj.

Operating Years

0 123 4 5 678 9 1

1

2

3

4

Catalyst Volume Ratio

Add 3rd Layer

Add 4th Layer

Add 3rd Layer

0

Figure 22 

Impact Of Limestone 

Wet Bottom Boiler 

22 
Conclusions 

Our experience shows that coal-fired SCRs are successful when proper attention is paid 

to system and catalyst design. 

Proper system design is crucial to Maximizing Catalyst performance potential and 

achieving lowest annualized cost. 

By exploiting the experience and know-how gained on existing facilities it is possible to 

further optimize catalyst design. 

23