1

SCR CATALYST PERFORMANCE 

UNDER SEVERE OPERATION CONDITIONS 

Scot G. Pritchard 

Chris E. DiFrancesco 

T. Robert von Alten 

CORMETECH, INC. 

Environmental Technologies 

Treyburn Corporate Park 

5000 International Dr. 

Durham, NC 27712 

Abstract 

Selective Catalytic Reduction (SCR) technology has been applied to a wide variety of 

applications since the late 1970s. Flue gas generated from refinery off gas combustion 

to natural gas-, oil-, and coal-fired units has been treated with SCR. More recent 

applications include reduction of NOx emissions generated from orimulsion-fired boilers, 

diesel engines, process gas streams, i.e., nitric acid plants, calcining ovens, and gas 

turbines firing landfill and/or digester gas. 

At the heart of the SCR system is the catalyst. Each application mentioned above has 

unique design parameters. Therefore, a thorough understanding of catalyst behavior as 

it relates to the operating parameters is necessary, i.e., deactivation mechanisms, effect 

of sulfur content, load swings, ash loading, efficiency requirements, effect of 

maldistribution, etc. 

This paper helps the reader understand the importance of properly defining and 

evaluation design parameters to achieve the most cost-effective design and to assure 

reliable operation. Basic relationships are presented to assess the impact of multiple 

design parameters. In addition, we site a number of specific examples demonstrating 

our experience with design and application of homogeneous honeycomb catalyst. 

Cases include (1) a high dust arrangement SCR designed for a cyclone boiler firing high 

sulfur fuel, and requiring high NOx removal efficiency, and ash re-circulation (2) an dust, 

high flue gas flow velocity, in-duct arrangement, and (5) a high efficiency in-duct utility 

boiler application. 
2

Introduction 

Selective Catalytic 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 is 

also an economically viable solution, with current fully burdened installed costs, in the 

United States, estimated at between $20/kW to $30/kW for natural gas and $40/kW to 

$70/kW for coal unit retrofits. The technology has even given some utilities the 

capability to achieve lower heat rates by allowing optimization of burner operation and 

reduction or omission of flue gas re-circulation, further adding to its cost effectiveness. 

Figure 1 

The capability of SCR to meet and exceed performance expectations economically start 

in the design phase. Cormetech draws upon the vast experience database of 

Mitsubishi Heavy Industries (MHI) and its licensees, the extrusion and materials knowhow of Corning, and the catalyst technology of Mitsubishi Chemical Company (MCC) to 

provide the most effective product to the market. 

This paper presents design techniques used to assure SCR performance, both catalyst 

and system, under severe operating condition. The definition of ìsevereî as it relates to 

this paper is, a condition, or set of conditions, which extend beyond basic performance 

requirements, and significantly impacts SCR catalyst and/or system design. 
3

First, a brief review of the SCR reaction mechanism including undesired side reactions 

in presented. Second, parameters that must be evaluated in order to assure successful 

SCR implementation during the design phase are presented in a tabular form. This is 

followed by a discussion on post implementation, or life analysis tools used to assure 

continued successful operation and provide valuable information on catalyst 

replacement or addition options. Finally, specific case examples are outlined to 

demonstrate the impact of various parameters on the initial SCR system design. 

Background 

The governing chemical reactions that occur in the presence of the SCR catalyst, NOx 

reduction and SO2 oxidation are presented below. The primary NOx reactions are listed 

in equations [1-3]. 

The catalytic reaction can take place over a wide temperature range (3000F ñ 11000F) 

with typical applications between (5000F ñ 8000F). Low operating temperatures are not 

suitable to applications with sulfur or extremely high NOx due to the potential 

formulation changes must be made for high temperature applications to reduce the 

potential for ammonia oxidation and catalyst sintering. 

NO + NO2 + 2NH3 Catalyst 2N2 + 3H2O [1] 

4NO + 4NH3 + O2 Catalyst 4N2 + 6H2O [2] 

6NO2 + 8NH3 Catalyst 7N2 + 12H2O [3] 

Oxidation of sulfur dioxide (SO2) to sulfur trioxide (SO3) also occurs on the catalyst. 

SO2 + 1⁄2 O2 Catalyst SO3 [4] 

The formation of SO3 can lead to problems in downstream equipment due to corrosion 

and/or plugging when combined with excess ammonia slip. Equations [5-6] show the 

reactions for ammonium sulfate and bisulfate respectively. The formation of these slats 

is highly dependent upon the concentration of each constituent; therefore, each 

component is a key design parameter for the system. 

2NH3 + SO3 + H2O Catalyst (NH4)2 SO4 [5] 

NH3 + SO3 + H2O Catalyst (NH4) HSO4 [6] 

Figures 2 and 3 illustrate the basic SCR system layouts for gas turbines and fossil fuel 

fired boilers. For reference; NH3 is the location of the ammonia injection grid (AIG); 

SCR is the location of the selective catalytic reduction reactor housing which contains 
4

the catalyst; ESP ñelectro-static precipitator; FGD ñflue gas desulfurization; SH ñ

superheater; HP, IP, LP evap- high, intermediate, and low pressure evaporator. 

Figure 2 

Gas Turbine SCR Arrangements 

Figure 3 

Boiler SCR Arrangements 

Com bined

Cycle

System

Sim ple

Cycle

System

N H 3

SH and

 HP Evap

IP, LP, Evap

and FW H

Gas

Turbine CO Catalyst

SCR STACK

D uct Burner

N H 3

Gas

Turbine CO Catalyst

SCR STACK

H igh

Dust

System

Low

Dust

System

N H 3

Boiler

SCR STACK ESP Air FGD

Heater

N H 3

Boiler

SCR

STACK

ESP FGD

Air

Heater

Heater

G as to G as H eat

Exchanger
5

System Evaluation 

Design Phase- Parameter Assessment

A number of components may cause poor system performance if not properly addressed 

during the designed phase of a project. Components include specific catalyst poisons 

contained in the fuel, reactor and flue design, ammonia distribution/control, and operation 

methods. See reference 1 for more detailed information on catalyst poisoning mechanisms. 

Table 1 below provides each evaluation parameter and a brief description of the potential 

impact on catalyst and/or system components. Many parameters have interrelated impacts 

on design. In some cases, one or a few of these conditions may be severe enough to 

provide a unique challenge and govern the design. Cormetech has, through both its 

internal and external resources, i.e., large customer base with over 120 applications and 

provide an optimized design. 

The information provided in the table is separated into related category topics by the 

reference symbols. The reference symbol is utilized during the case study discussions in 

order to assist the reader. Where the evaluation parameter is specific to a type of 

application, a designator is used, i.e. boiler (Blr), gas turbine (GT). 

Table 1 

Reference 

Symbol 

Evaluation Parameter Potential Impact 

FAN Fuel Analysis, including 

trace elements and firing 

duration: 

♦ Primary 

♦ Secondary 

♦ Duct Burner (GT) 

• Catalyst formulation 

• Catalyst volume 

• Catalyst 

Management 

• Monitoring plan 

FAD Fuel Additives • Catalyst volume 

• Catalyst 

Management 

• Monitoring plan 

FGAN Fuel gas analysis • Catalyst volume 

• Catalyst 

Management 

• Monitoring plan 

AAN Ash Analysis, including 

trace elements 

• Catalyst volume 

• Catalyst 

management 

• Monitoring plan 
6

Table 1 (continued) 

Reference 

Symbol 

Evaluation Parameter Potential Impact 

Al 

AH 

Ash Loading & 

Characteristics 

♦ Handling Method recirculation, disposal, 

recycle 

Catalyst pitch 

Catalyst volume 

Catalyst management 

Ammonia slip 

Downstream equipment design 

Sootblower requirements 

CSOxL SO3 and SO2 vs. Load Catalyst formulation 

Catalyst volume 

Catalyst management 

Economizer bypass (Blr) 

Downstream equipment design 

Ammonia slip 

CNOL NOx vs. Load Catalyst volume 

Economizer bypass (Blr) 

Water or stream injection rate 

(GT) 

TL Temperature vs. Load Catalyst volume 

Economizer bypass (Blr) 

Catalyst management 

ηNOx Removal efficiency Catalyst volume 

Ammonia injection grid (AIG) 

design requirements 

NH3S Ammonia slip Catalyst volume 

APH design (Blr) 

Ash Handling (Blr) 

BF Boiler firing method NOx content 

SO3 content 

Ash characteristics 

∆P Pressure loss Fan or turbine capacity 

Catalyst reactor design 

Boiler and/or ESP 

reinforcement 

(Blr) 

Catalyst pitch 

Catalyst management 
7

Table 1 (continued) 

Reference 

Symbols 

Evaluation Parameter Potential Impact 

Dα

DF 

DT 

Distribution Criteria 

♦ NH3:NOx 

♦ Flow 

♦ Temperature 

• Catalyst volume 

• AIG Design 

• Reactor and flue design 

OLS 

OCF 

OBF 

Systems Operation 

♦ Load swing 

♦ Capacity factor 

♦ % Time on Backup 

fuel 

• Catalyst volume 

• Catalyst pitch 

• Life management plan 

• Control logic 

• Catalyst management systems 

inspection 

RS 

RT 

Regulations 

♦ Seasonal reduction 

requirements 

♦ ERC or allowance 

trading environment 

• NOx reduction requirements 

• Catalyst volume 

• Systems capacity 

• SCR bypass (BIr) 

SC Site Conditions (1) 

♦ Multiple boiler 

♦ Back-end 

arrangement 

♦ Foundation 

♦ Electrical 

• NOx reduction requirements 

• Catalyst volume 

• Reactors geometry 

• Type of reagent 

• Reagent vaporization methods 

(1) Site conditions can impact many segments of the design. The items is shown in this 

table to make the reader aware to make the reader aware of some of the potential 

impact and is mot meant to be all inclusive. 

Operations Phase ñ Performance Assessment Tools

Once the catalyst and systems have been designed and installed, the next phase of 

assuring continued satisfactory performance through system and catalyst monitoring. 

This includes analysis of field data and catalyst sample analysis. 
8

The table provided in attachment 1 shows the field data necessary to assist in the 

evaluation of the SCR system performance. Data must be measured at regular intervals 

and under consistent and repeatable operating conditions. The purpose of measuring the 

field data is to understand the interaction between the system and the catalyst. When 

utilizing field data to assess performance potential, a thorough understanding of the relative 

accuracy for each of the measured points and their respective impact is extremely 

important. 

Since the catalyst activity decreases overtime due to factor such as poisoning, surface 

masking, or thermal degradation, catalyst testing should be performed. Testing catalyst 

sample provides specific information on the condition of the catalyst relative to expectations 

set during design. Specific catalyst testing plans, including physical and chemical property 

test, are especially important where expected deactivation was the governing factor of the 

design. The information is also used in conjunction with the field data to assess overall 

system performance. For example, if catalyst testing shows a low degree of degradation 

but overall system performance is poor, further in the case of high conversion efficiency 

design where ammonia distribution is a governing factor. 

In addition to system assessment, catalyst testing provides vital in formation to the owner 

regarding the remaining useful life of the catalyst. This information can be used to devise 

the most efficient catalyst management plan as it relates to catalyst volume and know plant 

outage schedules. 

Ultimately, all catalyst deactivation data obtained is correlated to various parameters such 

as fuel type, operation hour, temperature, etc. and applied to new project during the design 

phase. 

Case Studies

The following case studies are presented to show the procedure or method of analysis 

performed during the proposal, design, and operation phase of the project. Case study one 

(1) details all aspect of the analysis, while cases two (2) through five (5) concentrate on 

specific areas. Cases two (2) and three (3) relate the importance of evaluating and 

understanding fuel constituent and impact on catalyst and downstream equipment design. 

Case four (4) details the impact of high velocity dust laden environment, included catalyst 

and fan requirements. Case (5) illustrates the impact of flow, ammonia, and temperature 

distribution on the effective catalyst life. 

Case study 1: Coal Fired Cyclone Boiler 

This case study utilizes a high arrangement SCR designed for cyclone boiler with high 

sulfur fuel, high NOx removal efficiency, and re-circulation to demonstrate the use of the

design tools on parameters described above. Data for each evaluation parameter is 

provided in Table 2. 
9

Designed Data Table 2 

Reference 

symbol 

Evaluation Parameter Value 

FAN Fuel Analysis 

 Sulfur , % wt. 2.8 - 3.3 

 As, ppm 5 

 Ni, ppm 20 

 Cr, ppm 30 

 Cl, ppm 500 

FAN Fuel Additive limestone (2% by wt. of fuel) 

FGAN Fuel Gas Analysis 

 Flow Rate, lb/hr 5,400,000 

 NOx, ppm 1500 

 O2 , % vol. 2.5 

 H2O, % vol. 5.0 

 SO2 ppm (max) 1800 

 SO3 ppm (max) 36 

AAN Ash analysis, % wt 

 SiO2 50 

 As2O3 20 

 Fe2O3 3 

 CaO (Free / Amorphous) 1.5 / 1.5 

 MgO 1 

 TiO2 0.5 

 MnO 0.1 

 V2O5 0.03 

 Na2O 0.05 

AAN Ash analysis, % wt ( continued) 

 K2O 1 

 P2O6 0.3 

AH Ash recirculation, % 100 

AL Ash loading , mg/Nm3 10,000 

CSOXL SO2 and So3 vs. load flat 

C NOXL NOx vs. load linear (min =1000 ppm) 
10

Design Data Table 2 (continued) 

Reference 

Symbol

Evaluation Parameter Value 

TL Temperature vs. load linear (min = 6000F) 

ηNOx NOx removal efficiency 90% 

NH3S Ammonia slip, ppm 2 

BF Boiler Firing Method Cyclone 

∆P Pressure loss 5î w.g. for system 

 Distribution Criteria 

Dα NH3:NOX ± 5% RMS 

DF Flow ± 15% RMS 

DT Temperature ± 0F absolute 

 System Operation 

OLS Load Swings Base loaded 

OLS Min. operating load 50% 

OCF Capacity Factor 0.75 

R Regulations Year round reduction is 

required 

SC Site Condition 

 Air preheater type Ljungstrom 

 Retrofit difficulty Moderate 

Catalyst Design 

The required base catalyst surface area is determined as a function of gas constituents 

(FGAN), design efficiency (ηNOx), and operating temperature (TL) . 

Design ammonia slip is set at 2 ppm based of the SO2 content (FGAN) and ash handling 

method (AH). The initial cost impact of designing with 2ppm vs. 5 ppm ammonia slip is 

estimated at 5-10 % of the total capital and includes catalyst and reactor alterations. 

Adjustments to the required base surface area performed based on the design distribution 

criteria (Dα DF, DT ). The impact of flow maldistribution on a high dust design are twofold. 

Poorly distributed flow increased the potential for catalyst erosion and plugging through 

proper system design and catalyst erosion and plugged through proper system design and 

catalyst erosion and plugging through proper system design and catalyst edge hardening 

(figure 4). The second area of impact concern meeting the required performance and 

achieving the longest possible catalyst life . Further discussion regarding this item is 

detailed under case study 5. 
11

Figure 4

Erosion Resistant Edge Hardened Catalyst 

Once the base catalyst surface area is set, the next step in determining the required 

catalyst volume for an application can be taken. The fuel analysis (FAN ), fuel additives 

(FAD), ash analysis ( AAN), ash load (AL), and evaluation of the effect of ash recirculation 

are performed. The designer utilizes historical database information, laboratory and field 

test results to determine expected catalyst deactivation rates. 

Based on high sulfur oil experience, as well as results of the DOE clean coal demonstration 

project performed at Gulf Powerís Plant Crist, the effect of high fuel sulfur was addressed. 

The Plant Crist application burned a high sulfur coal and tested our catalyst in a high and 

low dust arrangement for over 10,000 and 6,000 hours, respectively. The measure results 

showed that the deactivation rate was well within expected limits and in fact showed that the 

deactivation was well within expected limits and in fact surpassed expectation (see figure 

5). Figure 6 shows a typical catalyst management plan based on prediction degradation 

data for a cyclone boiler with 100% ash recirculation. 

Hardened Edge

Flyash

Catalyst
12

Figure 5

Plant Crist High Sulfur Coal Demonstration 

Catalyst Degradation Data 

Figure 6

Typical Catalyst Management for Cyclone Boiler 100% ash Recircualtion 

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 2 0

1

2

3

4

5

70% 0

75%

80%

85%

90%

95%

100%

Year

N

O

x

R

e

m

o

v

al E

fficie

n

c

y, %

N

H3 slip, p

p

m

Add

Layer 4

R eplace 

Layer 1 R eplace 

Layer 2

R eplace

Layer 3

R eplace 

Layer 4

N O x R em oval Efficiency - - - - N H 3 slip lim it

Catalyst Activity

M easured 

Catalyst Activity

Expected Deactivation Range

Time (hrs)

0 5000 10000 15000 20000

+

+ x

x

x
13

The design sensitivity to the use of ash recirculation and fuel additive can be significant. In 

this case 100% ash recirculation was implemented and a limestone fuel additive was used. 

The addition of limestone to the fuel effectively mitigates much of the potential catalyst 

deactivation caused by arsenic poisoning. Free CaO in the limestone reacts with gaseous 

arsenic to form a solid, Ca, (AsO4)2 which does not poison the catalyst. Figure 7 shows the 

impact of Limestone injection on the gaseous arsenic content for multiple boilers. The 

decrease in the relative rate of catalyst deactivation results in a cost saving directly for the 

reactor. The total catalyst cost savings, of course, must be measured against the cost for 

the limestone addition. In this case, limestone addition was a viable countermeasure. 

Figure 7

Effect of limestone Addition 

The ash loading (AL) and characteristics dictate the catalyst pitch. For this application a 

fairly standard 7.1mm pitch product was selected. Potential cost saving through use of a 

reduced catalyst pitch which provides higher surfaces area per unit volume is currently 

being investigated. 

1

10

100

1000

3 10 30

: Plant A

: Plant B

: Plant C

G

a

s

e

o

u

s

a

r

s

e

nic

@ 1st C

ataly

st

L

ay

e

r (mic

r

o

g

r

a

m

s

/N

m3)

Arsenic Recycle Ratio = 100%

Effect of 

Limestone 

Injection

Arsenic in Coal (mg/kg)
14

System Design 

Impact to the system design are evaluated based on the distributed criteria established ( DA 

, DF , DT ) ash loading (AL), and SO2 and SO3 concentration (COSOXL), Pressure loss criteria 

(∆P), operating temperature (TL), regulations (R), and conditions (SC). 

The ammonia injection grid design is based on the distribution criteria. For this application a 

thirty- six zone (36) adjustable grid was selected. A cold flow model and or a computer 

CFD model test will be performed prior to fabrication to assure the adequacy of the initial 

design. 

The reactor and flue design is dictated by the site conditions, pressure loss criteria, 

operating temperature, distribution criteria, regulations, and ash loading. The site conditions 

allowed for an in-line reactor between the economizer outlet and air-preheater inlet. An 

economizer bypass and static mixer was required in order to maintain sufficient temperature 

at the catalyst to achieve the required NOx reduction and avoid salt formation. Due to the 

year round NOx reduction requirement a SCR bypass was not necessary. 

Sufficient test ports were designed into the system to assure the capability for proper 

system tuning during plant start-up. In addition removable catalyst sample were designed 

into the system to allow for laboratory performance audits. The combination of field data 

and catalyst laboratory testing throughout the life of the plant will yield valuable information 

for scheduling catalyst addition and/or replacements, especially in this case where the 

catalyst management plan is a governing factor. 

Case Study 2: Orimulsion Fired Boiler 

The primary area of concern for applying SCR to a boiler fired with orimulsion fuel is 

associated with the high levels of two components, namely SO3 and vanadium. Since 

orimulsion is relatively new fuel, there is somewhat limited full scale operating experience. 

Therefore the method of analysis relies heavily on the related experience of heavy oil. 

The system must be designed to with stand high SO3 concentrations. The catalyst will be 

designed to cost effectively manage the increase in SO2 to SO3 conversion that will be 

designed over time due to vanadium deposition and deactivation. Catalyst management 

may de dictated by, either a decrease in NOx reduction performed or an increased in SO2 t to 

SO3 conversion. 

As previously mentioned, if proper operating temperatures are maintained SO3 does not 

have any detrimental effects on catalyst performance, however downstream equipment 

must be considered. Measure such as enameling of cold end layers may be taken in order 

to limit air preheater corrosion and plugging. In addition, the ash particle size distribution of 

orimulsion enhances the potential for ash agglomeration. The designer must take this into 

account when considering the catalyst formulation and cleaning method i.e. sootblower 

designer and frequency of operation. 
15

Case Study 3: Gas Turbine landfill and digester gas cofiring with natural gas 

This case is similar to the orimulsion case the potential focuses on the effect of fuel constituents, 

however in this case the potential damage to catalyst is somewhat different and to a larger 

degree. 

Cormetech currently has three gas turbine unites which co-fire either land fill gas or digester 

gas with natural gas. Catalyst has been evaluated in both turbines and duct firing of these 

waste fuels. The primary catalyst concern when firing landfill of digester gas is a 

component in the fuel which can caused severe catalyst deactivation, namely siloxanes. 

Siloxanes are a family of polymers commonly found in health and beauty products which 

find their way into general waste streams. They have been the subject of much study and 

concern due to their detrimental affect on both CO and NOx catalyst deactivation. Siloxanes 

deposit on the surface and prevent the reactants from reaching active sites for conversion. 

Siloxane poisoning cannot be practically reversed. There has been some limited success in 

rejuvenation trails on CO catalyst. 

Unlike the coal or orimulsion fired applications discussed above, the solution to fired this 

fuel does not lie within the catalyst or SCR system design. Instead, fuel treatment system 

must be employed which removes the siloxanes components. Selective elimination is not 

practical and successful treatment systems are in operation strip both siloxanes and other 

components, including VOCís. Activated carbon is needed to reach the removal levels 

necessary and can be either regenerated or disposed of, whichever is most suited to the 

specific site demands. Once cleaned, these waste fuel can be fired with little or no 

detriment to catalyst. The oldest units have been successfully operating approximately two 

and one-half years with out difficulty. 

A catalyst testing plan has been developed to monitor catalyst performance and silicon 

levels at both sites. Testing assure that the fuel treatment system is effective and provides 

useful information for evaluating catalyst life potential. 

Case Study 4: High velocity dust laden application 

Although traditional or stand-alone SCRs are the most effective means of reducing significant 

quantities of NOx, some focus to ìhigh velocity SCRsî for coal and oil fired boilers. a number of 

issue arise when considering this type of application; 1) pressure drop, 2) NOx removal potential, 

and 3) erosion potential. 

Typical high velocity SCR pressure drop values range from 8-10 inches water versus typical 

values of 4 to 5 inches of water. Due to the increased system pressure caused by the addition 

of the high velocity SCR, electrostatic precipitators and furnace structure must be re-evaluated to 

assure structural integrity. 

NOx removal potential for high velocity SCRs can vary greatly from unit to unit. Typical reduction 

efficiency is approximately 30% to 40% with a maximum of 50%. 

For coal fired applications where ash is present catalyst erosion must be considered. Cormetech 

has completed a short term high velocity test ( approximately 2 month duration) and is currently 
16

participating in another (approximately 18 months). Results thus far, show that catalyst erosion 

has been minimized through proper system design and catalyst leading edge hardening. 

Case Study 5: High efficiency in-duct utility boiler application 

This case addresses the impact of maldistribution on a high performance SCR system installed 

on a large gas fired utility boiler. Three (3) components controlled much of the design for this 

application; 1) site condition 2) removal efficiency, and 3) distribution criteria. 

The site condition presented a very tight back-end arrangement with little room for expansion. 

The space between the economizer exit and the stack was nearly completely occupied by the 

APH. The resulting SCR design required severe transition both to and from the reactor. The 

ammonia injection grid was placed in an area which allowed very little residence time for mixing, 

but did provide substantial coverage of the flue. 

The removal efficiency of the units is 92.6% with a maximum ammonia slip of 10 ppmvd @ 3% 

O2 after four (4) years. 

The distribution criteria was set based on an iterative process which involved both Cormetech 

and the system supplier. A cold flow model was built and tested. Specific volume based on the 

performance results. Limiting factors included cost, pressure loss and space restrictions. 

A simplified example of the iterative analysis and the associated impact on catalyst performance 

is provided graphically in figures 8 (before) and 9 (after). The figures depict the flue cross-section 

divided into distinct areas representing the extent of the flow, ammonia, and temperature 

distribution before and after modifications. 
17

Input Output 

Flow: +30% 

Temperature: +500

F 

NH3: -20% 

Outlet NOx: 14.6 ppmvdc 

NH3 slip: 7.1 ppmvdc 

Flow: +15% 

Temperature: +250F 

NH3: -10% 

Outlet NOx: 10.0 ppmvdc 

NH3 slip: 6.8 ppmvdc 

Flow: -15% 

Temperature: +250F 

NH3: -10% 

Outlet NOx: 4.5 ppmvdc 

NH3 slip: 10.0 ppmvdc 

Flow: -30% 

Temperature: -500

F 

NH3: +20% 

Outlet NOx: 2.6 ppmvdc 

NH3 slip: 12.4 ppmvdc 

Results

• Catalyst able to achieve NOx reduction from 122 to 9 ppmvdc 

• However, NH3 slip is 10 ppmvdc 

Figure 8

Effect of Maldistribution on High Performance SCR (Before Modification) 

Input Output 

Flow: +10% 

Temperature: +250F 

NH3: -5% 

Outlet NOx: 10.8 ppmvdc 

NH3 slip: 5.2 ppmvdc 

Flow: -10% 

Temperature: -250

F 

NH3: +5% 

Outlet NOx: 6.8 ppmvdc 

NH3 slip: 5.0 ppmvdc 

Results

• Catalyst achieve NOx reduction from 122 to 9ppmvdcH3 

• NH3 Slip is 2.5 ppmvdc 

Figure 9

Effect of Maldistribution on High Performance SCR (After Modification) 
18

Figure 10 shows the significance of proper distribution in terms of effective catalyst life. After 

only two years of operation, an improperly designed system would not be able to meet the 

required performance. The maldistribution effectively makes the system operate as if the 

catalyst was between 5 and 6 years old. 

Figure 10

Effect of Maldistribution on High Performance SCR 

(Effective Catalyst Life) 

If this system had been a coal or oil fired boiler with significant quantities of sulfur, poor 

distribution would not only have caused poor NOx reduction performance, but may have also 

caused significant air heater plugging. Therefore, it is important to understand and account for 

maldistributions in order to assure a successful and reliable system. 

Conclusion 

Severe operating condition can be defined over a wide range of parameter including fuel types, 

performance requirements, and systems operating. It is important to understand which design 

parameters to assess and the proper evaluation techniques. Once a design is implemented it is 

important to retrieve and analyze data as well as perform laboratory tests on catalyst field 

samples. Information gained from the field and catalyst testing can be utilized to optimize future 

catalyst replacement or additions and provide valuable information for future designs. 

Time, years

60

80

100

40

20

0

0 12 3 4 5 6

2.5

5.0

7.5

10.0 Poor

Good

NOx Removal Efficiency, (%)

N

H3 Slip, p

p

m

v

d

c
19

Reference:

1. Optimizing SCR Catalyst Design and Performance for Coal-Fired Boilers by Pritchard et 

al presented at EPA/EPRI 1995 Symposium on Stationary Combination NOx Control. 
20

ATTACHMENT 1 

Purpose: As a tool for SR catalyst system performance monitoring over time 

Notes: 

• Use this form to record SCR operational parameter at start-up and monthly thereafter. 

• It is very important to achieve consistent operating condition, preferably at or near design point, before 

recording data each month. 

• Use this form as a master. Make copies for recording data. 

Date 

Gas flow (lb./hr) 

SCR gas Flow Temperature (0F) 

Inlet NOx (ppmvd) @ 15% O2 

Outlet NOx (ppmvd) @ 15% )O2 

O2 (Vol. %, dry) 

H2O (Vol. %) 

NH3 Flow (lb./hr)1 

NH3 Slip (ppmvd) @ 15% O2 

Date of last Equipment Calibration ( Analyzers, NH3 metering pump, 

etc.)

Date of last Relative Accuracy Test 

Operating Hours on Catalyst (total) 

Operating Hours on primary Fuel 

Operating Hours on Back-up Fuel 

Total Number of Stops and Starts since Catalyst Installation 

AIG Balancing Valve Positions (gauge ∆P, Zone 1,Zone 2,...) 

Catalyst ∆p, in. wg 

1

If aqueous ammonia, % solution should be recorded. 

2

Method of measurement and accuracy should be noted for all measured values, e.g., flow (boiler load signal or stack 

measurement ±___%,) NOx (dilution method chemiluminescence ±___%) NH3 (chemiluminesence subtraction method or 

calculated ±__%, ammonia flow lb./hr ±__%), etc. 

3

 Impact of instrument accuracy and repeatability must be evaluated on a case basis. 

4

 This form should be completed at start ñup, during all relative accuracy tests, and on a monthly basis. 

5 Month to month data correction; NH3 m x (Flowm / Flow r ) x ( ∆NOxm /NOxR ) 

 Where: NH3 C =Corrected NH3 flow 

 NH3 M =Measure NH3 flow 

 FlowM =Measure flue gas flow 

 FlowR =Reference flue gas flow 

 ∆NOxM =(Measure Inlet NOx)- (Measure Outlet NOx) 

 ∆NOxR =(Reference Inlet NOx) ñ (Reference Inlet NOx) 

If measurements for temperature, oxygen, and water content vary greatly, additional corrections must be performed 

6 Corrected ammonia flow data should be trend charted to provide indication of performance capability.