SCR Catalyst Management and Catalyst Regeneration

visual reactor

inspection 

development of 

catalyst management 

strategies (exchange 

frequency, layer)

sampling of independent 

catalyst activity

testing in a benchscale reactor 

 documentation of

deactivation causes

tracking of catalyst

activity and DeNOx 

potential 

independent 

SO2/SO3 conversion 

rate testing in a 

bench-scale reactor 

determination measurement of 

of opportunities 

and limitations for 

catalyst 

catalyst pressure 

drop in a benchscale reactor 

determination of refurbishment 

main deactivation 

causes (chemical and 

physical poisoning) 

from e ach 

 

STEAG LLC STAGLLC 

PO Box 410728 PO Box 410728 

224 Westinghouse Blvd., #607 224 Westinghouse Blvd., #607 

Charlotte, NC 28273-6228 Charlotte, NC 28273-6228 

Phone: +01-704-588-7657 Phone: +01-704-588-7657 

Fax: +01-588-7644 Fax: +01-588-7644 

E-mail: hsobolewski@steagllc.com E-mail: hartenstein@steagllc.com 

7th Sino-American Technology and Engineering Conference 

October 2006 

Beijing, China 

Steag LLC 

PO Box 410728 • 224 Westinghouse Blvd. #607 • Charlotte, NC 28273-6228 

Phone: 704-588-7657 • Fax:704-588-7644 

E-mail: info@steagllc.com • www.steagllc.com 
 

 

Abstract 

Approximately 100,000 MW of coal-fired electric generating capacity in the United States was 

retrofitted during the last 5 years with SCRs in order to comply with the CAAA Title I SIP Call 

aimed for ground level ozone reduction. Most utilities operate their SCR during the ozone 

season (May 1 to September 30), however, some states have already passed legislation 

requiring year round SCR operation and it is expected that most will follow this pattern before 

the end of the decade. SCR operators are now faced with the challenge of long-term operation 

with maximum performance while ensuring minimum long-term operating costs. Germany has 

been faced with this same challenge since 1985 when the first SCRs went into operation. As 

Germany had never operated their SCRs on a seasonal basis, they faced a greater challenge of 

having to develop a year-round plan without the benefit of several years of short-term

operational experience. 

German SCR operators developed tools to help them optimize operation and reduce SCR O&M 

costs. For over 20 years, Steag as well as other utilities have used a perpetual cycle of SCR 

management to help them meet these challenges. The eight step cycle consists of tasks, which

should be undertaken during each outage when the SCR is open. It continually tracks the 

operation and performance of the SCR, and the deactivation of the catalyst. The results of this 

tracking determines the best plan for optimizing the SCR system, tracking and accurately 

forecasting the catalyst deactivation and performance, and developing a catalyst life-cycle 

strategy. 

A major factor in reducing O&M costs was the development of catalyst cleaning and 

regeneration processes, which Steag has done for our own SCRs since 1997. The goal was to 

provide an economically more attractive alternative to buying new catalyst and to significantly 

extend the useful life of the installed catalyst. However, the economic effectiveness of catalyst 

cleaning and regeneration depends on the type and causes of catalyst deactivation as well as 

the regeneration process used, which in return depends on an in depth knowledge of the 

various deactivation causes. 

Introduction 

During the last 5 years, more than 100,000 MW of coal-fired electric generating capacity in the 

United States was retrofitted with mostly high-dust SCRs in order to comply with the Clear Air 

Act Amendments (CAAA) Title I State Implementation Plan (SIP) call aimed for ground level 

ozone reduction. After installation and successful commissioning, most SCR operators are now 

faced with the challenge of long-term SCR operations. As virtually all SCRs will go from a 5 

month ozone seasonal operation between May and September to year-round operation no later 

than 2009, managing an SCR for maximum performance, while ensuring minimum long-term 

operating costs, has become the most recent challenge for SCR operators. 

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Management Tools 

Steag, who owns and operates a fleet of 24 SCRs in their own power plants, has been faced 

with these challenges since 1986 when the first SCR went into operation on a year-round basis. 

As Germany had never operated their SCRs on a seasonal basis, Steag was forced to

immediately develop all the tools and procedures needed to manage our SCR system for 

minimum operating cost while ensuring year-round compliance without the benefit of short-term 

operating experience. The tools we developed can be categorized into two fundamentally 

different types, namely: 

• SCR equipment management, which addresses properly tuning, maintaining and keeping up 

critical SCR equipment components including the ammonia injection grid (AIG), dampers, 

expansion joints, catalyst seals, and the SCR reactor(s) and 

• Catalyst management, which addresses properly testing, analyzing, determining 

deactivation causes, DeNOx potential tracking and forecasting, exchanging, replacing, 

purchasing and/or regenerating the catalyst. 

SCR Management Cycle 

Steag, as well as other SCR users, developed and have used for two decades what we refer to 

as the perpetual cycle of SCR management. The cycle describes all the tasks and 

responsibilities to be undertaken during each outage when the SCR reactor is accessible. 

Performing all the tasks as outlined in the SCR management cycle diligently on a perpetual 

basis allows for achievement of the lowest overall SCR operating cost while ensuring the most 

sought after SCR efficient performance. Particularly in the early years of SCR operation, the 

operator should take advantage of every opportunity to access the system and quickly establish 

an accurate data history of the reactor and the installed catalyst. The perpetual SCR 

management cycle includes the following steps: 

• Visual reactor inspection and sampling of catalyst from each catalyst layer 

• Independent catalyst activity testing in a bench scale reactor

• Independent SO2/SO3 conversion rate testing in a bench scale reactor 

• Measurement of catalyst pressure drop in a bench scale reactor 

• Determination of the main deactivation causes (chemical and physical) 

• Determination of opportunities and limitations for catalyst refurbishment 

• Documentation of deactivation causes & tracking of catalyst activity and DeNOx potential 

• Development of catalyst management strategies (exchange frequency, layer, etc.) 

As soon as a reasonably long operating history has been documented which typically takes 

three to four cycles, the plant operator can start consolidating the data and begin developing an 

optimized catalyst management strategy. Such a strategy can either be developed for a single 

unit or for an entire fleet of SCRs. As a user of 24 SCRs, Steag quickly realized the enormous 

economic benefit of managing the entire inventory of catalyst on a fleet-wide basis with a pool of 

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catalyst flowing freely among the various units. Such a strategy includes the catalyst exchange 

frequency, catalyst rejuvenation or regeneration, and the purchase of new catalyst layers as 

necessary. 

SCR Reactor Inspection and Catalyst Sampling

Based on Steag’s 20 years of SCR operating experience at least one (1) annual SCR inspection

takes place and at least one (1) catalyst sample from each layer in service must be taken, 

tested and analyzed for deactivation and DeNOx potential tracking purposes. This is the same 

frequency we routinely apply to our own fleet of 24 SCR systems. We also recommend

additional SCR reactor inspections be done at any time when an individual SCR reactor is 

offline and accessible whether it is due to a scheduled or a forced outage. 

Especially during the first years of SCR operation, more frequent than annual inspections and 

catalyst sampling has been proven to be very advantageous by providing as many data points 

related to the catalyst deactivation as possible early on. As the experience with each SCR 

reactor and the predictability of its catalyst deactivation rate increases, the inspection and 

catalyst testing frequency can be reduced to a long-term frequency of once per year. SCR 

reactor inspection should include at a minimum:

• Inspection of SCR reactor and all associated equipment 

• Inspection of the reactor insulation and lagging 

• Inspection of all catalyst layers in each reactor including: 

 ▪ Catalyst modules as well as the catalyst itself 

 ▪ Flue gas seals between the catalyst modules 

 ▪ Flue gas seals between the catalyst modules and reactor wall 

 ▪ Soot blowers and/or sonic horns 

 ▪ Reactor support structure 

• Catalyst sampling (at least one sample per layer) 

• Inspection of the flue gas guide vane system upstream of the SCR reactor and evaluation of 

possible fly ash deposits 

• Inspection of the gas inlet distribution system and rectifier 

• Inspection of the flue gas and economizer bypasses including bypass dampers and 

expansion joints 

• Inspection of the AIG system, particularly the nozzles and tuning valves 

• Inspection of key ancillary equipment, e.g., the fixed installed sampling grid downstream

of the last catalyst layer, seal air system including dampers and expansion joints, etc.

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Ammonia System Inspection and Evaluation 

As a result of our own 20 year operating experience of ammonia (NH3 and NH4OH) systems as 

well as the extensive experience gathered in the U.S. over the past 5 years, Steag 

recommends a yearly inspection of the ammonia storage and supply system, and an 

assessment of the NH3- and NH4OH-systems’ performance once a year. The ammonia 

storage and supply system inspection should include at a minimum: 

• Inspection of ammonia unloading station (tank truck and/or rail car) 

• Inspection of the transfer compressors to the NH3 storage tanks 

• Inspection of all ammonia storage tanks 

• Inspection of the ammonia transfer pump station to the vaporizers 

• Inspection of the ammonia vaporizers 

• Inspection of the dilution air systems 

AIG Tuning 

As part of our SCR management program, Steag performs a yearly optimization of the flue gas 

distribution (NH3 and NOx profiles) and tuning of the (AIG) to: 

• Optimize the operation of the AIG, 

• Minimize ammonia (NH3) slip, and 

• Reduce O&M costs. 

This strategy directly applies what Steag has learned and developed during the past 20 years 

while tuning the AIGs of our own 24 SCR units for maximum SCR performance and minimum 

NH3 consumption. This is achieved by maximizing the NOx removal efficiency while ensuring 

as little as possible NH3 slip. In order to ideally overlay the NH3 profile over the NOx profile for 

combining maximum removal efficiency with minimum NH3 slip, the fully O2- corrected three 

dimensional NO profile after the last catalyst layer must be measured utilizing up to seven 

NO/O2 analyzers in parallel. Subsequently, the NH3 injection system should be adjusted in 

such a way that the NO profile after the last catalyst layer is as uniform as possible at the 

lowest possible level. This ensures a minimum ammonia slip at a maximum NOx removal 

efficiency. 

Steag’s tests each unit’s AIG under actual full load operating conditions at maximum NOx 

removal efficiency using the fixed installed sampling grid after the last catalyst layer. We then 

conduct a series of tests during the tuning of the AIG and adjustment the AIG tuning valves until 

the SCR is operating at peak performance. During actual AIG tuning the unit must be operated 

at full load on a continuous basis, thus ensuring the maximum flue gas volume flow rate and 

maximum NOx mass flow rate through the SCR. It is required that each unit’s SCR reactor be 

equipped with a fixed installed sampling grid after the last catalyst layer. Steag uses a multiple 

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analyzer system of up to seven NO/O2 analyzers in parallel. The use of hand-held units yields 

numerous significant disadvantageous and is considered inadequate for the task. 

Large Particle Ash Screens 

The presence of large particle ash (LPA) or popcorn ash poses a serious concern for many 

coal-fired utility boiler operators after retrofitting their units with high-dust SCR systems. LPA is 

formed in the boiler and can easily be carried over into the SCR reactor. As LPA is introduced 

to the SCR reactor, it frequently causes: 

• Catalyst pluggage, which leads to: 

• Flue gas maldistribution

• Loss of SCR performance through loss of available DeNOx potential 

• Unacceptable NH3 slip 

• Excessive pressure drop

• Catalyst erosion damage 

After experiencing problems with LPA in their Voerde 4 plant, Steag developed a proprietary 

and patented LPA screen design which prevents the intrusion of LPA into the SCR reactor by 

effectively removing the LPA from the flue gas stream. Typically, such LPA screens are 

installed at the boiler’s economizer outlet utilizing the economizer hopper for the continuous 

extraction of the removed LPA. The special design features of Steag’s LPA screens which are 

patent protected, are: 

• A pleated design, which ensures a most advantageous angle of attack of the LPA onto the 

screen and a low flue gas flow velocity through the LPA screens resulting in a low pressure 

drop. 

• A hinged design (typically at the top of the economizer outlet duct), which allows for the LPA 

screens to be rapped frequently in order to remove LPA that gets caught in the screens. 

• A modular design, which allows for the use of numerous different types of screen materials 

depending on the flue gas velocity. Screen materials used include: 

o Uncoated or coated stainless wire mesh screens 

o Uncoated or coated stainless steel wedge wire screens. 

Steag uses coating materials which include various commercially available types of metallic 

coatings such as e.g. ConformaClad, MetalSpray, etc. as well as tungsten carbide coatings. 

This allows for the selection of the correct screen design and material needs depending on the 

particular application and the actually present flue gas velocity through the LPA screens. 

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SCR Catalyst Management 

SCR catalyst testing is a critical part of successful overall SCR management. Unbiased and 

independent catalyst testing and chemical analysis is needed to determine the: 

• Initial catalyst performance parameters and chemical composition; 

• Actual catalyst activity, SO2/SO3 conversion and catalyst deactivation rate; 

• Change in chemical composition and identification of catalyst deactivation causes; 

• Determination of initially installed, actual residual, & minimum required DeNOx potentials; 

• Development tracking and forecast of SCR system performance. 

It is a well known fact that typically all test results heavily depend on the test protocol and 

equipment used. The same is true for catalyst testing which is the reason why a standardized, 

publicly available catalyst testing protocol is mandatory. In order to obtain the necessary data, a 

standardized catalyst testing procedure was developed in Germany in 1987/1988. It has 

become the internationally accepted standard for catalyst testing and analysis. Steag was one 

of the two large scale SCR operators selected by the German Association of Large Power Plant 

Operators (VGB – Vereinigung der Großkraftwerksbetreiber) for participating in the 

development of this standard protocol for testing of DeNOx catalyst, “VGB Guideline VGBR302He, 2nd Edition”[1]

. Steag has also developed a supplement to VGB-R302He, which 

eliminates some remaining ambiguities and defines best practices for bench scale testing based 

on our long-term bench scale testing experience [2]. 

Catalyst testing and chemical analysis is required in order to determine the exact catalyst 

performance, chemical composition and catalyst deactivation causes and to provide long-term 

tracking data. All bench scale catalyst testing should be performed in an independent catalyst 

testing laboratory, which frequently qualifies its accuracy and precision through extensive 

QA/QC round robin testing to be conducted every 3 to 5 years through VGB. Steag always 

participates in these round robin tests and successfully passed these “VGB-certifications” [3]

. 

A bench scale reactor, in accordance with VGB-R302He such as Steag’s, is designed 

specifically for the sole purpose of evaluating SCR catalyst under simulated, controlled plant 

conditions. It provides the only practical way of examining SCR DeNOx catalyst in a nondestructive manner while generating absolute values for catalyst activity, SO2/SO3 conversion 

rate and pressure drop. The most important feature of a bench scale reactor is its particular 

suitability for exactly replicating the actual operating conditions of a full scale SCR reactor while 

still ensuring useful data. The only two deviations from the actual operation conditions of the full

scale SCR reactor are the lack of any fly ash in the bench scale and the molar ratio of ammonia 

to NOx, which is commonly referred to as α set equal to 1. 

Since the presence or absence of fly ash in the flue gas has no influence on the actual activity 

of the catalyst, it is more practical and less interfering to operate a bench scale reactor without 

fly ash in the flue gas. Also, as the availability of NH3 is the limiting factor of the DeNOx reaction 

in a full scale SCR system, which is not desired in a bench scale test for determining the 

catalyst’s activity, the molar ratio of NH3 to NOx must be set to 1. However, to enable correct 

measurements of the catalytic activity (K) and the SO2/SO3 conversion rate under the exact 

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same conditions that the catalyst experiences in an actual utility SCR system, catalyst samples 

must be exposed to synthetic flue gas of the exact same composition, velocity and temperature 

as present in the full scale system. 

SCR performance and catalyst deactivation are generally tracked as potential of each individual 

layer as well as potential of the entire SCR reactor versus operating time. Each data point 

generated should be entered into the SCR Performance and Catalyst Potential Tracking Sheet 

to follow the history of the unit and assist in predicting the need for catalyst cleaning, 

regeneration, or replacement with new catalyst. 

Catalyst Testing 

Steag typically performs a series of catalyst testing procedures including: 

• Bench scale reactor testing for full elements including 

 ▪ - SO2/SO3 conversion rate, 

 ▪ - Pressure drop ∆p, 

 ▪ - Initial activity K0, 

 ▪ - Actual activity Kact, 

• Micro scale reactor testing for small size catalyst coupons, 

• XRF analysis for catalyst surface and bulk composition , 

• SEM analysis for catalyst surface scans (optional), 

• XRD analysis for mineralogical e.g. coal 

Initial Catalyst Baseline Testing and Analysis

Bench scale testing and chemical composition analysis of new, unused catalyst, which is 

necessary in order to determine the necessary baseline for all future catalyst management and 

deactivation tracking should be done to establish the initial chemical composition of the catalyst

before flue gas exposure with which deactivated catalyst will be compared. During bench scale 

testing the following initial catalyst parameters will be determined for the one (1) catalyst 

sample comprising either of one honeycomb log or one plate catalyst sample (the preparation 

of each plate catalyst sample requires twelve individual plates) or one corrugated catalyst 

sample: 

• Initial catalyst activity K0, 

• Catalyst SO2/SO3 conversion rate, 

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• Catalyst pressure drop ∆p, 

• Chemical composition analysis of the catalyst substrate (bulk and surface), 

• Determination of the initial catalyst potential of each layer, 

• Determination of the initial DeNOx potential, which was installed in the unit’s SCR reactor 

with all new, unused catalyst layers before initial start up of the unit’s SCR systems. 

The test results will serve as a baseline for the future development of a catalyst deactivation 

tracking chart for the particular unit. Additionally, the chemical composition analysis is crucial in 

order to quantify the actual uptake and accumulation of catalyst poisons. The data is 

necessary for the purpose of comparison in order to identify the cause and degree of catalyst 

deactivation. 

Fleet-wide Strategy

Steag has found through the operation of our 24 SCRs that operators with two or more SCR 

systems can optimize their overall SCR operating cost by implementing a fleet wide SCR 

management strategy. All data generated for each of the individual SCR systems should be 

evaluated and interpreted and form the basis for developing an optimum catalyst management 

strategy for the SCR fleet as a whole thereby optimizing operations while keeping operations 

and maintenance (O&M) costs at a minimum. 

Catalyst Regeneration 

Steag found that the flue gas from burning a variety of bituminous coals, and certain secondary 

fuels such as bone meal, gradually deactivated the installed catalyst due to fly ash and large 

particle ash plugging and/or the formation of blinding layers on the catalyst surface. Chemical 

deactivation results from catalyst poisons such as arsenic, phosphorus and earth alkali metals, 

which chemically react with the catalytically active ingredients of the catalyst, thus rendering 

them ineffective. 

In the early days of SCR operation, the only choice an SCR operator had was to add new 

catalyst and/or replace the existing spent catalyst in order to meet the desired NOx removal 

efficiency dictated by permits and regulations and at a minimum NH3 slip. In order to provide an 

economical alternative, technologies were developed that would clean and restore original 

catalyst activity levels without damaging the physical or structural integrity of the catalyst. 

In 1997 Steag developed a catalyst cleaning and regeneration process which has been a major 

factor in reducing the O&M costs for our own fleet of SCRs. Steag utilizes a patented deionized 

(DI) water-based process to remove the physical materials which plug or blind the catalyst 

layers. In the event of chemical deactivation, after the removal of the catalyst poisons or 

deactivation causes, Steag adds chemicals used in the original manufacturing of the catalyst to 

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the regeneration process thereby restoring the catalyst it to its original activity level. Steag has 

cleaned and regenerated several layers of catalyst a number of times and has found that there 

is no changed in the deactivation rate between new catalyst and cleaned and/or regenerated 

catalyst. 

However, the economic effectiveness of catalyst regeneration strongly depends on the 

efficiency of the regeneration process used, which in return depends on in depth knowledge of 

the specific deactivation causes which bring us back to the need for using a perpetual cycle of 

SCR management to optimize SCR operations and reduce O&M costs. 

Fuel Testing and Chemical Analysis 

Fuel (i.e. coal), fuel ash (i.e. coal ash), fly ash, catalyst pluggage and/or SCR deposit samples 

should also be analyzed in order to provide a complete overview of the operation and impact of 

the SCR and its associated equipment. A complete analysis should be performed regularly for 

coal, coal ash and fly ash samples including: 

• Proximate and ultimate laboratory analysis of the fuel sample including fuel ash 

composition, ash fusion temperature, mineralogy (XRD-analysis), trace elements (As, halogens, 

etc.), water soluble compounds (Na, K, P, etc.) 

• Trace element analysis (XRF-analysis) of the fly ash or catalyst pluggage or SCR 

deposit samples including determination of critical, water soluble compounds and catalyst 

poisons such Na2O, K2O, P2O5, CaO, MgO, SO3, As, etc. 

• Interpretation and evaluation of the results on the basis of our extensive data base 

Steag developed a proprietary software program SPICE (Steag Program for Import Coal 

Evaluation), which serves not only as database for all coals and secondary fuels we burn but 

also provides an accurate estimate of the true life cycle cost effects of any specific fuel including 

its short-term and long-term deactivation effects on the SCR catalyst. 

Conclusion 

Whether developing a SCR management program for a single unit or a fleet of SCRs, the SCR 

catalyst management strategy should include a perpetual cycle of inspections, tunings, testing 

and evaluations. The outcome of these tasks should be a plan tailored to ensuring the least 

overall long-term cost while providing the maximum possible lifetime for the installed catalyst 

with optimal SCR system performance. Outage schedules, SCR operating schedules (seasonal 

or year round), power demand curves, unit capacity loading, availability of the specific residual 

DeNOx potential of each SCR system and the fleet wide NOx reduction requirements should be 

taken into account in order to maximize the utilization of the SCR catalyst installed. 

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References

1. Guideline for the Testing of DeNOx Catalytic Converters, VGB R-302He, 2nd revised version, 

published by VGB Kraftwerkstechnik, Klinkestrasse 27-31, 45136 Essen, Germany, 1998. 

2. Supplement to VGB-R302He 2nd Edition - Common Best Practices for Bench Scale Testing 

and Chemical Analysis of SCR DeNOx Catalyst; Steag encotec Meßtechnik – Zentrallabor, 

Herne, Germany, 2003.

3. VGB Bench Scale Catalyst Testing Round Robin Results 2005 – VGB Bereich 

Umweltschutz und Querschnittsaufgaben, Auswertung nach DIN 48402-42, Ausgabe 1984-05, 

Essen, Germany, 2005.

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