LESSONS LEARNED FROM SCR REACTOR RETROFIT

Presented at COAL-GEN

August 6-8, 2003

Greater Columbus Convention Center

Columbus, Ohio

Ted Kurtides

Project Associate 2, Fossil Power Technologies

theodore.b.kurtides@sargentlundy.com

55 East Monroe Street • Chicago, IL 60603-5780 USA • 312-269-2000
LESSONS LEARNED FROM SCR REACTOR RETROFIT

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Theodore B. Kurtides

Project Associate 2, Fossil Power Technologies

55 East Monroe Street

Chicago, Illinois 60603-5780 USA

INTRODUCTION

There are numerous considerations in the design and installation of high-dust selective

catalytic reduction (SCR) reactor systems that can be applied to future projects that could

enhance system design and performance, and reduce overall installed cost. This paper

discusses various technical design issues that should be reviewed in order to produce a

low-cost and reliable SCR reactor installation.

A full-scale, high-dust SCR system consists of a stand-alone reactor structure containing

several levels of catalyst located upstream of the flue gas particulate collection

equipment. The primary goal of a full-scale, high-dust SCR system installation is to

reduce NOx emissions by at least 85%, while simultaneously maintaining low ammonia

slip levels so as to avoid detrimental impact to air preheater performance and/or fly ash

handling operations. Maximizing SCR system NOx reduction beyond 85% can be a costeffective means of reducing regulatory impact throughout a particular utility system.

SCR systems operating at NOx reduction levels greater than 90% and ammonia slip levels

less than 2 ppm are currently on-line demonstrating the feasibility of such designs.

Powder River Basin (PRB) coal is unique to the U.S., and its use in SCR systems has

been confined to domestic utilities, accordingly. The high calcium oxide content of PRB

coal initially created uncertainty regarding catalyst deactivation rates and long-term SCR

system operating cost. SCR operation at units with PRB coal-fired boilers has now

confirmed that accelerated catalyst deactivation occurs, but that it is manageable and can

be easily accounted for during initial system design.

SCR reactor system flow modeling is critical to successful SCR system operation.

Ductwork internal structures and components affect system performance and should be

included in the flow model to the fullest extent practical. Experience with reduced scale

flow models has confirmed the validity of this method; however, use of numerical models

may be preferable, depending on project-specific circumstances.

The ability of individual equipment components to perform as intended can have

significant impact on SCR system operation and downstream balance-of-plant

performance. Operating experience with various SCR damper designs, sonic horns, and
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enamel-coated air preheater baskets is presented, and areas that require careful scrutiny

during the design and manufacturing processes are discussed.

Seasonal operation of SCR reactors within the U.S. necessitated the development of a

lay-up system design capable of maintaining suitable environmental conditions within the

reactors as needed to maximize catalyst life and to maintain catalyst warranties in force.

Some of the key parameters needed to establish a successful lay-up system design are

summarized herein.

In addition, the ability to access an SCR reactor with the boiler on-line can be a desirable

feature in utility systems with numerous SCR installations, as it facilitates catalyst

sampling, maintenance, and replacement. Man access conditions can be achieved either

via the reactor isolation dampers or via installation of temporary blanking plates. The

main benefit of using damper isolation is that it can be done on-line, i.e., unit outages are

not required to set and then remove temporary blanking plates. However, the ability of

SCR dampers to provide the desired isolation capability has varied, depending on the

type of damper used and the extent to which thermal effects were addressed in the

damper design.

The ability to preheat the SCR reactor prior to placing it into service provides several

benefits, including elimination of dew-point operation, shortened reactor startup duration,

and reduced stress on reactor structural members. Avoiding reactor operation below

water and acid dew-points will extend catalyst life, particularly at units where the SCR

reactors are started numerous times per year. Catalyst vendors impose startup

temperature ramp rate limits, some of which vary as a function of SCR reactor operating

temperature. During cold starts, more restrictive limits may require the use of

modulating SCR inlet and/or SCR bypass dampers to slowly warm the reactor. For

reactors using a plate-type catalyst, preheating to approximately 300oF prior to

establishing flue gas flow has been found to preclude the need for modulation of the SCR

inlet damper, thereby, expediting reactor startup. Due to the large thermal mass of the

catalyst and reactor structure, severe temperature gradients can exist between the top and

bottom of the reactor during startup. Preheating the SCR reactor prior to startup will

reduce these gradients as well as the associated stresses on the reactor structure.

When a wide range of fuels has been specified for a particular unit, new fuel sources may

be introduced that create operating conditions not previously experienced. Such

conditions may include generation of popcorn ash. Popcorn ash can accumulate within

the SCR reactor, detrimentally affecting system performance. Where stations require

broad fuel purchasing flexibility, inclusion of popcorn ash knock-out devices during the

initial SCR design may be prudent, regardless of whether generation of such ash has been

previously observed.
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A critical component of the overall SCR system is the ammonia storage and forwarding

area. Providing a safe and reliable ammonia system requires consideration of numerous

items ranging from materials selection to simulation of accident scenario impact on

remote populations. Various experiences regarding ammonia system performance that

should facilitate future design efforts are discussed.

Because the installation of an SCR system will increase flue gas system resistance,

modifications to the combustion system fans (forced draft [FD], induced draft [ID], or

booster) will probably be required. Fan modification costs can be substantial, depending

on: the plant condition, the ability to rectify operation and maintenance (O&M) problems

that have increased fan duty over the years, and the SCR system design criteria. In order

to minimize fan modification costs, these issues must be reviewed in detail at project

inception and optimized to the extent practical. In addition, increased fan horsepower

demand can have substantial impact on the auxiliary power system. Inclusion of variable

frequency drive (VFD) ID fan control has been proven to minimize this impact and can

increase station net generation above pre-SCR retrofit levels in certain operating modes.

Further, overall net generation is otherwise increased as compared to using inlet vane

control.

SCR reactors are relatively large pieces of equipment and finding or creating space for

their installation can result in the use of tall, irregular structures that may have limited

footprint area available for foundations and column placement. When building code

seismic design requirements are applied to such structures, steel member sizes can

increase to the point that “jumbo” sizes are required and the use of non-domestic material

sources may need to be considered. Accordingly, significant lead-time may be needed to

obtain the required steel quantities and sizes. These lead-time requirements must be

ascertained and incorporated into the project schedule as early as possible so as to

levelize work load and ensure timely material delivery to the site.

Pile and foundation design offer opportunities to reduce and contain project cost through

testing and surveying at the beginning of the project. Such efforts can reduce pile

quantity and can eliminate foundation installation-related project delays and cost

escalation.

Use of three-dimensional (3-D) modeling should be considered essential when

performing a large retrofit project requiring involved integration of existing structures

and facilities. Modeling facilitates initial design development, layout, detailed

engineering, and erection sequencing. This, in turn, minimizes field problems,

streamlines field fabrication, and reduces total erection cost. Recommended model

content is summarized to help ensure realization of these benefits.
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DISCUSSION

Emissions

All Sargent & Lundy-designed SCR reactors at coal-fired units, which have been placed

into service, have achieved their guaranteed NOx reduction efficiencies within the

specified ammonia slip limits. The minimum design NOx reduction efficiency was 85%

and the maximum reduction efficiency was in excess of 90%. Design ammonia slip

levels ranged between 2 ppm and 3 ppm at the end of catalyst life. Although no SCR

installations have yet operated for the guaranteed catalyst life duration, it is anticipated

that the NOx reduction and ammonia slip performance guarantees will continue to be met

over that period. Operational installations include pulverized coal units burning PRB

coal, Illinois low- to high-sulfur coal, and eastern low- to high-sulfur coal; one cyclone

unit burning PRB coal; and two cyclone units burning Illinois low-sulfur coal. SCR

reactor designs have included 2+1 and 3+1 catalyst level installation sequences and have

used plate, honeycomb, and corrugated type catalysts.

All Sargent & Lundy-designed SCR systems that burn PRB coal were designed for use

with either honeycomb or corrugated type catalysts. Two of the major design concerns

associated with PRB coal are the higher anticipated catalyst deactivation rate associated

with calcium sulfate masking, and catalyst blocking due to the more aggressive ash

accumulation characteristic of this fuel. As expected, catalyst deactivation rates at

facilities burning PRB coal have been estimated to be 10% to 20% higher than at units

burning bituminous coal. Ash bridging and the resultant catalyst blockage have been

avoided by installation of catalysts with relatively large pitch dimensions. The minimum

catalyst pitch that Sargent & Lundy has installed at PRB coal-fired units has been 9.2

mm, or its equivalent.

An undesirable consequence of SCR operation is increased SO3 generation due to SO2

oxidation that occurs within the SCR reactor. Increased SO3 emissions at the stack can

cause coloration of the stack plume and can exacerbate acid aerosol fallout to the point

that remediation is required due to impact on the surrounding community. Plume

coloration problems are primarily a concern at units with wet stacks, but can occur at

units with dry stacks during cold-weather operation. Unfortunately, the ability to predict

the occurrence and magnitude of these problems depends on numerous process variables

and is not readily discernible prior to SCR system operation. This notwithstanding, only

one Sargent & Lundy-designed SCR facility has reported plume coloration of sufficient

magnitude to warrant corrective action, and no Sargent & Lundy-designed facilities have

reported unacceptable acid aerosol emission fallout. It should be noted, however, that all

of the SCR reactors are still operating with only the initial catalyst charge volumes. The

installation of supplemental catalyst volume in the future will increase SO3 generation

and emissions and could possibly alter the conclusions provided herein.

The station reporting plume coloration burns high-sulfur coal, has a high-dust SCR and a

flue gas desulfurization (FGD) system. Although the plume coloration has not been
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severe enough to draw complaints from nearby residents, a remedy is currently being

pursued and will probably involve the addition of limestone to the fuel feed, as this is a

readily available material.

Another potential consequence of elevated SO3 emissions is increased opacity. This can

occur when air preheater gas outlet temperatures are below the acid dew-point, creating

an acid mist within the ductwork. This mist has been observed to increase opacity by

upwards of 20 percentage points. Remediation options include elevating air preheater

gas outlet temperature by use of the combustion air preheat coils or by reducing air

preheater leakage.

In general, means for addressing excess SO3 related operating problems include:

• Procure catalyst with a low SO2 oxidation rate

• Add limestone to the fuel feed

• Inject limestone, hydrated lime, magnesium hydroxide, or similar chemical into

the boiler

• Inject magnesium hydroxide or similar chemical at the economizer gas outlet,

SCR outlet, or air heater gas outlet

• Install wet electrostatic precipitator (WESP)

• Raise air preheater gas outlet temperature

Of these options, minimizing catalyst SO2 oxidation rate directly addresses the root cause

and should be the first consideration. In general, the catalyst SO2 oxidation rate selected

should generate no more than 10 ppm SO3 at 3% O2 in wet stacks and 20 ppm SO3 at 3%

O2 in dry stacks.

Wet stacks are more sensitive to plume coloration problems than are dry stacks. In a wet

stack, a blue plume will generally become visible at 10 ppm SO3 concentration and will

be readily apparent at 20 ppm. At 30 ppm concentration, the color intensity may reach

the point that SO3 emission reduction is required. Plume coloration in dry stacks is

primarily a concern only during winter months and it is difficult to observe significant

color intensity change as SO3 concentration is increased.

Determining the stack SO3 concentration requires consideration of the furnace SO2

oxidation rate, catalyst SO2 oxidation rate, fuel CaO content, fuel sulfur content, air

preheater gas outlet temperature, system in-leakage, and the type of particulate collection

device. As such, the definition of an acceptable catalyst SO2 oxidation rate will vary

from site to site, but should fall within the range of 0.6% to 3% for a fully charged

reactor. The lower end of this range is applicable to units burning high-sulfur coal,

whereas the upper end of this range applies to those units burning low-sulfur bituminous

or PRB coal.
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The addition of limestone to the fuel, or the injection of limestone, magnesium

hydroxide, or similar sorbent above the furnace is intended to reduce the total amount of

SO3 leaving the SCR reactor. If limestone is used, this is accomplished via reduction of

both SO2 and SO3 emissions exiting the boiler. If magnesium hydroxide or magnesium

oxide is injected, this is accomplished via selective reduction of the SO3 emissions.

Alternative injection points for alkalis other than limestone include the economizer gas

outlet, the SCR outlet, and the air heater gas outlet. In order to optimize sorbent injection

utilization, such an installation might include multiple injection points and use of more

than one type of sorbent agent. A limestone fuel feed coupled with magnesium

hydroxide or oxide injection at the SCR outlet exemplifies an arrangement that would

provide efficient chemical usage, results in reduced SO3 concentrations both upstream

and downstream of the SCR reactor and avoids exposure of the catalyst to surplus

chemical additive. Based on industry experience, a 70% to 90% reduction in total SO3

emissions appears feasible. Negative balance-of-plant impacts could include increased

waterwall slagging if limestone fuel supplementation is used, a reduction in precipitator

performance due to increased fly ash resistivity, and an increase in catalyst deactivation if

excess sorbent is injected upstream of the SCR reactor. Regardless of the sorbent type

selected, all options require installation of materials handling equipment.

Wet ESPs represent an established technology, but is only recently being considered for

installation at coal-fired units in response to pending fine particulate control regulations.

Installation of wet ESP equipment is expected to occur predominantly at scrubbed units

due to the inherent reduction in gas temperature and pollutant loading downstream of this

equipment. The wet ESP process is expected to remove >90% of SO3 mists in addition to

other fine particulate. However, in the worst case, flue gas system resistance can increase

by up to 2” WC to 3” WC due to wet ESP operation and, therefore, the impact on ID fan

operation may have to be assessed.

Raising air preheater gas outlet temperature is intended to eliminate acid mist from within

the ductwork in order to rectify mist-related opacity problems, and/or to create greater

dispersion of the plume during cold-weather operation and, thereby, diminish plume

visibility at dry stack units.

In conclusion, the most proactive means of avoiding stack SO3-related emission problems

is to minimize the catalyst SO2 oxidation at the time of procurement. The maximum

catalyst SO2 oxidation rate should be established based on limiting stack SO3

concentrations to no greater than 10 ppm at 3% O2 in wet stacks and 20 ppm at 3% O2 in

dry stacks. Once the SCR reactor is in service, operational remedies involving

manipulation of air preheater gas outlet temperature may be sufficient to alleviate

problematic SO3 emission side effects at low capital cost. Due to the capital and O&M

costs associated with the installation of sorbent injection equipment, this should be

deferred until proven necessary.
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SCR Reactor Flow Modeling

The purposes of the flow model are to determine the optimum type, number, location,

and arrangement of flue gas flow correction and gas mixing devices in the ductwork and

reactor, and to optimize the ammonia injection system in the reactor inlet ductwork.

Such modeling ensures that uniform gas flow conditions (velocity, temperature, NOx,

NH3, fly ash, etc.) exist at the ammonia injection grid (AIG), reactor inlet, and reactor

outlet across the specified load range. Additionally, the flow model is used to determine

and minimize the pressure drop increase imposed by the SCR system on the boiler draft

system.

Internal ductwork trusses and gusset plates can affect flow distribution at the catalyst

inlet. At higher de-NOx efficiencies (>90%), this is a significant concern due to strict gas

distribution limits. Therefore, the flow model should include all internal trusses and

gusset plates located upstream of the catalyst. As an example, truss modification

limitations on one project necessitated adjustment to turning vane design to correct flow

distribution deficiencies induced by the truss; such deficiencies were identified and

corrected with the flow model.

Trusses can obstruct between 5% and 20% of the duct cross-sectional area. Even with

relatively low pressure drop per truss, overall truss quantity can lead to a discernible,

cumulative SCR system pressure drop penalty. Depending on ductwork flue gas

velocities, additional pressure drop on the order of 0.5" WC to 2" WC can develop.

Therefore, the total quantity and extent of all internal trusses should be defined early in

the project. Additionally, the catalyst supplier and flow modeler must ensure that the

total quantity and extent of internal trusses are properly considered in their modeling

efforts. If trusses are not included in the model, then a truss pressure loss based on hand

calculations should be included in the system design.

The impact of trusses both on flow distribution and pressure loss can be minimized as

follows:

• Use temporary shipping braces to facilitate duct erection in lieu of adding

permanent trusses for this purpose

• Minimize the number of bracing members, i.e., eliminate zero-force members

• Minimize the diameter of the bracing members

• Orient the bracing gusset plates parallel to the gas flow, where practical

• Minimize the design size of gusset plates that are perpendicular to the gas flow

• Use external bumpers to transfer horizontal duct loads

Replacement of internal trusses with external moment frames is another option.

However, although clearly feasible on cold-side ductwork, the use of external moment
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frames on hot-side ductwork requires careful consideration. Such consideration includes

engineering cost, material quantity requirements, and thermal gradient effects.

Other issues to consider during flow modeling are as follows:

• Modeled components should be fabricated to scale to the degree practical.

• Field traverse measurements at the economizer gas outlet should be used to

simulate baseline temperature and NOX stratification in the flow model.

• All dampers and other in-duct components should be included in the model, e.g.,

static mixers, flow straightener, AIG, in-duct heat exchanger, turning vanes, guide

vanes, trusses, gusset plates, etc.

• The catalyst modules should have sealed perimeters and interface boundaries to

eliminate gas bypass.

• The modeled SCR bypass damper should be able to isolate sufficiently so that

SCR bypass flow does not influence ash deposit patterns or reactor flow. It may

be necessary to physically seal the modeled bypass damper with tape or caulk to

achieve gas-tight isolation.

• In order to maintain design accountability and minimize design criteria

communication interface, it is beneficial to minimize the number of parties

involved in the flow model system analysis. To this extent, the catalyst vendor

should take ownership of the entire SCR system model. This includes design of

the guide vanes, turning vanes, AIG, flow straightener, and static mixers. Use of

third parties to design the static mixers and/or the AIG creates additional design

criteria interface and diminishes the responsibility of the catalyst vendor to ensure

acceptable catalyst performance.

Either numerical computer models or 1:12 scale models can be used. Although

numerical flow modeling has gained acceptance, Sargent & Lundy has used 1:12 scale

models due to their inherent accuracy in simulating turbulence downstream of in-duct

obstructions. Our experience has shown good correlation between scale model

predictions and physical plant operation in support of this position. When deciding

which methodology to use, the following major factors should be considered:

• Flow model cost

• Available space

• Ability to investigate various ductwork configurations

• Complexity and variability of temperature and gas species baseline distributions

at the economizer gas outlet

• Required accuracy of gas distribution predictions at the catalyst inlet
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Depending on the project-specific needs, either modeling approach may provide

satisfactory results.

SCR Reactor Dampers

The type of dampers selected for SCR reactor service is a function of many

considerations, including industry experience, cost, application requirements, available

space, and ease of installation. Louver dampers offer a low-cost, compact option, but

may create operational problems that hinder SCR system operation. Guillotine dampers

offer reliable isolation capability but at higher capital cost than louver dampers.

Additionally, they require significant clearance outside the ductwork for installation and

operation and are restricted to isolation duty. Domestic experience with large diverter

and flap dampers for SCR control and isolation was limited at the time of initial SCR

system design efforts, and performance and operational data from foreign utilities was

difficult to obtain. Domestic experience levels have since increased and indicate that

these dampers provide reliable modulating and isolation service.

In Sargent & Lundy’s experience, SCR louver dampers have had more operating

problems than guillotine, diverter, or flap dampers. This can be attributed to two main

factors: 1) the relative complexity of louver damper designs, which involve precision

operation of numerous blades, bearings, and sealing surfaces, and 2) installation of such

equipment in a high-temperature, high-dust environment that subjects the equipment to

high thermal stress. Reliable louver damper design requires thorough analysis of the

various loading combinations such that all frame and blade deflection, distortion, and

deformation are properly assessed.

Guillotine dampers are not suitable for modulating service and, therefore, must be

supplemented with modulating style dampers, where needed. As such, application of

guillotine dampers has been limited to isolation duty; although this experience has

generally been successful.

Diverter and flap dampers have been used successfully both for modulation and isolation

duty. However, these dampers can increase project capital cost as compared with a

louver damper system, depending on the process isolation and control requirements. The

marginal capital cost of a diverter/flap damper type system has ranged between 0% to

50% higher than a louver damper-based installation. On a life-cycle cost basis, however,

these higher capital costs can partially, or even completely, be offset by operating cost

savings associated with reduced seal air consumption and lower flue gas system pressure

loss. Selection of diverter/flap dampers can also have a decided influence on the SCR

reactor ductwork general arrangement and, accordingly, should be incorporated into the

design at the conceptual phase of the project. Further, it should be recognized that

diverter dampers effectively replace a large segment of ductwork. Therefore, the

engineer now has a significant interface effort with the damper vendor in order to

establish the damper/ductwork design criteria and agree upon the load transfer path. This

joint design process requires good cooperation, particularly early in the project.
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All dampers used for SCR inlet, SCR outlet, and economizer bypass isolation are at high

service temperature and should be designed to account for thermal gradient effects across

the damper blades and casing when the dampers are closed. Thermal gradients are

caused by high operating flue gas temperatures on one side of a closed damper

concurrently with relatively cool seal air or lower gas temperatures on the other. This

operating mode occurs in economizer bypass systems when they are in standby mode, as

well as at the economizer bypass and reactor isolation dampers when the SCR reactor is

in lay-up mode. Thermal gradient-induced stress has caused significant damper operating

problems, such as blade binding, casing deformation, seal damage, and excessive seal air

demand.

In all cases, thorough verification of the damper design will help ensure that the

equipment will operate satisfactorily when installed in the field. As a result of experience

gained in the design, installation and operation of SCR reactor dampers, consideration of

the following design verification methods may be warranted, regardless of the style of

damper selected:

• Finite element analysis of the damper design.

• Extensive damper shop testing at simulated operating conditions.

• Verification of the proposed design by comparison with similar equipment at

operating SCR facilities. The benchmark equipment should have been provided

by the damper vendor under consideration and should be similar in size, type,

blade span, frame orientation, blade orientation, frame support scheme, and flue

gas system conditions to that proposed for the new project. Significant deviation

of any of these factors at the proposed installation may invalidate the comparison.

• Any combination of the considerations noted above.

Results of finite element analyses performed on louver dampers used for SCR inlet, SCR

bypass, and SCR outlet isolation and control indicate that thermal gradient effects can

cause severe local stress concentrations within the damper frame. However, because

these are highly localized, secondary thermal stresses, evaluation on a principal stress

intensity basis using the high temperature ASME Boiler & Pressure Code is justified.

This approach permits allowable stresses up to approximately two times the yield stress

to be accepted. Without this latitude, the stress levels created within, for example, a

conventional double-louver damper, would exceed AISC principal stress design limits

and the equipment design would be unacceptable. Clearly, the ability to recognize stress

concentrations, design to applicable code requirements, and provide for stress relief is

essential for proper design of this hot/cold interface boundary.

However, modeling, analysis and evaluation of elevated stress levels is only a theoretical

approach to simulating expected thermal behavior and is primarily used to verify the

structural integrity of the damper design. Actual damper operability is not confirmed

during the numerical analysis and it is for this reason that shop testing is recommended.

As a minimum, shop testing should verify that damper functionality is acceptable, that
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blade deflections are within specifications, and that seal area gaps are within calculated

limits. Advanced shop testing scope can also include simulation of pressure loading on

closed damper blades, seal air demand tests, and testing at elevated temperature in the assupported equipment configuration.

The following are important considerations for shop testing at simulated operating

conditions, if performed:

• Dampers should be thoroughly heat-soaked; however, temperature distribution

should be no worse than expected operating conditions.

• The damper frame should be insulated as required to simulate heat transfer as it

will occur in the as-installed condition.

• The damper frame should be supported as closely as possible to the as-installed

configuration in order to fully assess thermal movements and structural integrity

under both hot and cold operating conditions. Note that dampers may be

cantilevered from the ductwork in the field and will generally have an expansion

joint on one side. As such, shop testing SCR dampers can be a difficult process

involving lifting and support of large and heavy components. Advance

knowledge of the shop's capabilities may affect the final vendor selection.

• Pressure loading across closed damper blades should be simulated; however, it is

acknowledged that simulation of such conditions can be difficult to achieve.

Given a choice, it is more beneficial to simulate thermal gradient effects.

• Completion of shop testing in relation to site delivery dates should be scheduled

such that there is adequate time to address any design problems discovered during

testing. As a result, damper award and design release should occur as early in the

project as possible.

• All shop testing should be witnessed.

Regardless of the design verification methods selected, as a minimum, proper damper

design must account for: thermal and structural support boundary conditions, external

damper loads, connected ductwork deflections and movements transferred to the damper,

and blade and casing deflection. The damper frame is effectively a duct segment and

should be designed accordingly. Failure to design the frame in this manner may result in

deflection, deformation, and/or distortion that may hinder or prevent proper damper

operation, and may shorten the damper expected life. Therefore, the Owner should verify

that the damper structural design will be performed by qualified engineers within, or

contracted by, the damper vendor firm that have extensive experience in thermal design

methods, finite element analysis (if used), and the assessment of metals subjected to

elevated temperature and high thermal gradients.
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There should also be close coordination and communication between the engineer and the

damper vendor to ensure that all design criteria, operating modes, and support and load

transfer path issues between the dampers and the ductwork are clearly understood. Duct

and damper support arrangements should be provided to the damper vendor by the

engineer for each unique damper/duct configuration such that both parties are aware of

the load path and transfer mechanisms from the damper into the surrounding ductwork

and duct support steel.

In conclusion, installation of dampers in an SCR system subjects the equipment to

elevated temperatures and severe thermal gradients. Both of these effects impact the

structural integrity of the frame and blades and must be accounted for in the damper

design. Failure to do so can lead to casing and blade deformation, distortion, and/or

deflection severe enough to hinder equipment operation and cause seal air demand to

exceed guaranteed limits. This can further cause significant project cost escalation and

can undermine the ability of the SCR lay-up and seal air systems to function properly. In

all cases, the selected damper vendor must demonstrate experience and knowledge in the

design of high temperature dampers subjected to high thermal gradients. Inclusion of

design verification methods such as finite element analysis and extensive shop testing

should be considered depending on the operating history and similarity of dampers

previously installed by the damper vendor. Although the capital cost of a diverter or flap

damper installation may be higher than a louver damper-based system, these costs can be

offset by reduced pressure drop and seal air demand associated with the diverter/flap

damper designs. Use of diverter, flap, or guillotine dampers should be strongly

considered if the project budget allows.

Sonic Horns

Sonic horns have been installed at numerous SCR reactor installations with successful

operating results. Due to the uncertainty regarding sonic horn effectiveness at the

beginning of many of Sargent & Lundy SCR projects, provision for future retrofit of

conventional steam or air sootblowers was included in the system designs. Reserved

space on the reactor walls, provision of wide galleries enabling sootblower installation

and access, and even increased spacing between the SCR reactors to permit future

inboard sootblower installation were part of SCR system construction scope. However,

based on the success to date with sonic horn performance, such allowances may never be

used and should not be considered mandatory in future SCR retrofit projects.

Additionally, there had been concern regarding the projected depth that a single row of

sonic horns could effectively clean. It can now be reported that satisfactory sonic horn

performance has been achieved in SCR reactors just over 40 feet wide using a single row

of sonic horns located on one side of the SCR reactor.
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On this basis, overall SCR project cost can be reduced on several counts:

• Sonic horns can be installed in lieu of steam or air sootblowers with no concern

over their ability to maintain catalyst cleanliness. The capital cost of a sonic horn

is approximately one-fourth that of a steam sootblower, excluding process piping.

Additionally, the cost to engineer and install sootblower steam supply piping is

higher than the cost to field-route sonic horn compressed air piping, further

increasing the cost advantage of sonic horns over steam sootblowers.

• There is no need to include provision for future retrofit of sootblowers.

• Sonic horns have demonstrated effective cleaning ability across SCR reactors

over 40 feet wide. This effectiveness can reduce the quantity of sonic horns

needed at each reactor level because it permits the horns to be installed on only

one side of the reactor rather than in an opposed arrangement.

Air Preheater Modifications

A common balance-of-plant modification associated with SCR reactor retrofit involves

replacement of the air preheater cold-end baskets with enamel-coated elements. Such

modifications generally includes conversion of three-layer air preheaters to two-layer

designs in order to eliminate the split between the intermediate-and cold-end layers.

These modifications facilitate removal of ammonium bisulfate and acid fouling deposits

by the air preheater sootblowers and water washing equipment.

To date, two of the major problems associated with air preheater basket replacement have

been heat transfer surface plate cracking and subsequent ejection from the basket subframes, and plate edge damage from excessive sootblowing steam force. Inadequate

plate packing pressure and excessive steam sootblower force were found to be the root

causes of these failures.

When fabricating enamel-coated air preheater baskets, the heat transfer surface plates are

compressed within basket sub-frames using externally applied pressure, such as that from

a hydraulic ram. However, this process must be carefully controlled so that packing

pressure is uniformly applied over the entire plate area and is below the level that could

damage the plates. Excessive or non-uniform application of packing pressure can lead to

loss of the enamel coating either in the shop, during handling, or after initial operation.

Loss of the coating thickness between plates, if extensive enough, can relieve the plate

compression force and may result in loose plates within the basket sub-frame.

Loss of plate compression can result in premature plate failure due to two mechanisms.

First, loosely packed plates tend to vibrate both during normal gas flow as well as during

sootblowing operations. This may result in eventual fatigue failure cracking along

contact points or at plate bend lines. These cracks can eventually run the full depth of the

plate, thereby, forming plate fragments that may be ejected from the basket due to

aerodynamic forces, further compounding the phenomena. Second, loosely packed plates
LESSONS LEARNED FROM SCR REACTOR RETROFIT

14

can be damaged by sootblowing steam if the steam pressure force is high enough and the

plate face is impacted by the steam flow. This type of damage is seen as blunted,

flattened plate edges over the entire area of a basket sub-frame. It has been observed in

both hot-end and cold-end baskets.

These problems were addressed via improved control of the basket fabrication process

and a reduction in sootblowing steam pressure. The basket manufacturing method was

reviewed and modified to provide a uniform, controlled force during the plate packing

process. This minimized enamel coating failure, thereby, maintaining plate compression.

It also improved packing tightness within the uncoated basket sub-frames, thereby,

enhancing reliability of the hot-end baskets. Sootblowing steam pressure force should be

consistent with the air preheater basket vendor's design criteria, yet be compatible with

the sootblower equipment supplier's requirements. The air preheater basket vendor

should review the force delivered by the sootblower nozzle against that required to clean

the baskets. Sootblowing steam pressure may have to be adjusted to provide the proper

cleaning force, particularly is new nozzle designs are being used. For example, steam

pressure was reduced from 175 psig at the poppet valve outlet to 110 psig at one SCR

installation. This reduced pressure setting has been found to eliminate plate edge damage

and still adequately clean the baskets, albeit with a potentially increased sootblower cycle

frequency. Whereas, two sootblowing cycles per day are expected at units burning lowto medium-sulfur coal, and up to four cycles may be required at units burning high-sulfur

coal. Consequently, plate life at the high-sulfur coal units may be somewhat shorter than

at the low- to medium-sulfur coal units due to the higher number of sootblowing cycles.

In conclusion, both hot and cold air preheater basket life is a function of plate

compression within the basket sub-frames. Lack or loss of compression can lead to

fatigue failure cracking of heat transfer surface plates and/or severe plate edge damage

due to sootblowing steam force. The air preheater basket vendors have demonstrated an

understanding of these problems and have made adjustments to the manufacturing

process to prevent future occurrences. However, it is recommended that purchasers of air

preheater baskets review the manufacturing process and perform shop surveillance during

basket construction to ensure that plate compression is adequate and that enamel coating

damage, if any, is minimal. Additionally, the sootblowing steam force should be verified

as being consistent with both the air preheater basket vendor’s and sootblower equipment

supplier’s requirements.

Reactor Lay-Up System Design

Seasonal SCR reactor operation in the U.S. has necessitated the development and

implementation of SCR lay-up systems designed to maintain warm, dry conditions within

reactors when they are off line. In general, the recommended minimum reactor

temperature for bituminous coals is that needed to maintain relative humidity within the

reactor less than 40%. A somewhat lower limit may be required for PRB coals with high

alkali content. In all cases, the catalyst vendor should be consulted to define the

minimum lay-up conditions for a given application.
LESSONS LEARNED FROM SCR REACTOR RETROFIT

15

Achieving and maintaining the required lay-up conditions is most easily accomplished by

injecting heated ambient air into the reactor. For example, if the maximum allowable

relative humidity is 40%, a minimum reactor lay-up temperature of 120oF will create a

sufficiently dry environment all year. Although lower temperatures are theoretically

possible during the winter, this can create uncertainty as to whether the humidity criterion

was maintained during ambient transients. Installation of a reactor humidity data

recorder can be considered, but these have been found to be relatively expensive and

sensitive pieces of equipment not suitable for permanent installation in an SCR reactor

environment. Simply maintaining minimum reactor temperature above 120oF at all times

during lay-up mode ensures that minimum reactor humidity levels are maintained

regardless of fluctuations in ambient wet bulb and dry bulb temperatures.

Either a recirculation or a once-through type lay-up system can be designed; however, in

either case, a heat source must be provided in order to maintain elevated reactor

temperatures. Noteworthy of mention has been the development and use of the in-duct

heat exchanger (IDHX) to heat both dilution and lay-up air. An IDHX is a tubular heat

exchanger located in the SCR outlet ductwork and is part of the SCR bypass loop. This

location permits the IDHX to remain in service when the SCR reactor is in lay-up mode.

The main advantage of using an IDHX is that it passively recovers energy from the flue

gas stream to heat the dilution and lay-up air, thereby, avoiding parasitic electrical

demand or steam usage. The main lesson learned with respect to IDHX design concerns

tube erosion. High-dust installations using carbon steel IDHX tubes have experienced

accelerated tube wastage. Therefore, the use of stainless steel tubes should be considered

in conjunction with tube shielding at the first tube row, depending on flue gas velocities.

Other reactor lay-up heat sources include: steam coils, heated combustion air, and electric

heaters.

In addition to temperature control, the lay-up system must be designed to either maintain

reactor pressure greater than flue gas system pressure or, alternatively, isolate the reactor

sufficiently to prevent flue gas migration into the reactor. Additionally, in order to

prevent uncontrolled ambient tramp air entry into the reactor, the ability of the lay-up

system to maintain a slightly positive reactor pressure is desirable.

The reactor isolation system must prevent unintentional circulation of flue gas through

the reactor when in lay-up mode. Low-velocity gas circulation can create large ash

deposits that can overload the structure. In addition, exposure of the catalyst to flue gas,

whether or not ammonia injection is in service, consumes catalyst life. Gas circulation

via the SCR inlet damper is driven by the differential pressure between the SCR inlet and

SCR outlet dampers. Gas circulation via the economizer bypass damper is driven by the

differential pressure between the convection pass and the SCR outlet damper. Therefore,

the dampers used to isolate the SCR inlet and economizer bypass must provide gas-tight

isolation. This is usually accomplished with seal air, regardless of the type of damper

installed, but has also been achieved with removable blanking plates.
LESSONS LEARNED FROM SCR REACTOR RETROFIT

16

With the SCR inlet and economizer bypass dampers normally provided with either seal

air or mechanical isolation in order to prevent gas circulation through the reactor, the

SCR outlet damper is the remaining source of potential flue gas entry. To prevent flue

gas migration into the SCR reactor via backflow through the SCR outlet damper during

load swings, this damper should either seal tightly enough that the reactor pressurizes due

to lay-up air injection, or the damper should be sealed with seal air.

The ability to hold reactor pressure above ambient pressure has been successful at a

balanced draft unit that used guillotine isolation dampers with pressurized seal air at the

SCR inlet and outlet. At installations where double-louver dampers were used, thermal

gradient effects caused excessive damper blade and frame distortion that increased seal

air demand several times higher than expected. Therefore, positive seal air pressure

could not be maintained and reactor pressure fell negative. These units were

subsequently modified to use blanking plates for man access but have yet to use the

plates to isolate the reactors. However, it is anticipated that operation of the blanking

plates will permit positive pressure lay-up to occur.

If negative pressure lay-up is used, the lay-up temperature should be maintained high

enough that all reactor zones are below the maximum humidity levels and/or above

minimum temperatures as established by the catalyst vendor. Controllable tramp air

entry points, such as sootblower lances with ambient draft purge air connections, should

be isolated. Beyond this, reactor temperature should be monitored to identify abnormal

cold spots indicating uncontrolled air ingress. Due to the uncertainty with which a

reactor at a negative pressure is protected from uncontrolled tramp air entry, maintaining

lay-up air temperature in excess of the minimum value is advisable. Maintaining lay-up

temperatures on the order of 300oF has proven satisfactory to date, but it must be

cautioned that significant amounts of air ingress can occur when negative pressure lay-up

is used. This air ingress may be sufficient to overcome the relatively low heat input of

the lay-up system and may cause local catalyst damage. Use of heated lay-up air

provides other potential benefits, as discussed below under SCR Reactor Preheat.

Reactor Man Access

The ability to access the SCR reactor while the boiler is in-service is desirable in order to

perform online catalyst maintenance, inspection, and replacement. This is of particular

value at utilities with numerous SCR reactors because of the scheduling flexibility

afforded by online reactor access capability.

Man access capability was provided at several balanced draft boiler SCR installations.

This was achieved by providing positive pressure seal air within the seal air chambers of

the SCR inlet, SCR outlet, and economizer bypass dampers while the boiler was on line.

However, although positive seal air chamber pressures could be generated at a station that

used guillotine reactor isolation dampers, such pressures could not be reliably attained at

stations that used double-louver isolation dampers. As previously discussed, the root

cause of this problem was directly attributed to excessive seal air demand due to thermal
LESSONS LEARNED FROM SCR REACTOR RETROFIT

17

gradient induced damper distortion. To remedy the situation, the utility retrofit

removable blanking plates at the SCR inlet and SCR outlet louver damper locations.

These plates are to be bolted and gasketed tight to achieve man access isolation. The

remaining SCR installations still under construction that require man access capability

have been re-designed to use a combination of blanking plates and improved damper

designs in order to provide a variable degree of reactor isolation, as needed.

No installations that used diverter or flap dampers included reactor man access as part of

the SCR reactor design criteria. Therefore, there is no direct experience to report.

It should be noted that installation of a gas-tight barrier at the SCR reactor outlet may

require the installation of a lay-up and purge jumper around the SCR outlet damper in

order to vent reactor contents to the downstream ductwork. The lay-up and purge jumper

is normally closed during reactor operation, but contains a control valve that modulates

open during lay-up and purge operations as needed to regulate reactor pressure.

SCR Reactor Preheat

Installation of a lay-up system provides the means to preheat the SCR reactor prior to

placing it into service. SCR reactor preheat may provide several benefits, depending on

the degree of preheat implemented, as follows:

• Simplifies startup by avoiding low catalyst temperature ramp rate limit operating

ranges.

• Reduces thermal gradient induced stress on reactor structural components during

a cold start.

• Extends catalyst life at units where the reactors are started multiple times per year.

Catalyst vendors may impose temperature ramp rate limits that must be adhered to during

reactor startup. The ramp rate limits are a function of the catalyst type and, in some

cases, SCR reactor operating temperature. Plate type catalyst can have more restrictive

ramp rate limits than honeycomb or corrugated type catalyst. For the plate catalysts that

Sargent & Lundy has installed, the ramp rate limit was 27oF/min at operating temperature

below 300oF, and increased to 90oF/min when operating temperature exceeded 300oF.

For honeycomb and corrugated type catalysts, these limits can be in excess of 100oF/min

and are not necessarily a function of operating temperature. Adherence to lower range

limits during a cold reactor start may require modulation of the SCR inlet, and possibly

the SCR bypass dampers, in order to control the flue gas admission rate to the reactor.

However, it has been found that maintaining an elevated lay-up temperature within the

SCR reactor of approximately 300oF precluded the need to modulate these dampers when

damper stroke times of approximately 2 minutes were used. This occurred at a plate

catalyst installation and was due to the less stringent 90oF/min catalyst temperature ramp

limit in effect when initial catalyst temperature exceeded 300oF. At honeycomb and

corrugated type catalyst installations, the catalyst temperature ramp rate limits have been
LESSONS LEARNED FROM SCR REACTOR RETROFIT

18

sufficiently high, such that reactor preheat has not been a determining factor in startup

duration.

It should be noted that due to concern over the severe thermal gradient that can exist

between the top and bottom of the SCR reactor during a cold start, and the resultant

stresses that this gradient can cause in the SCR structure, it is possible that the SCR

reactor structural design may impose a more restrictive temperature ramp rate limit than

the catalyst. However, the reactor designs developed by Sargent & Lundy have

deliberately addressed and alleviated this concern, thereby, rendering the catalyst

temperature ramp rate as the governing criteria.

During startup, exposure of the catalyst to gas temperature below acid dew-point can

result in a small amount of catalyst deactivation. In fact, the catalyst performance

warranty may limit the number of reactor cold starts due to this effect. Therefore,

preheating the reactor above acid dew-point, as may otherwise be performed to avoid a

temperature ramp rate limit, alleviates this concern as well.

Popcorn Ash

At project inception, it must be determined whether there is any history of popcorn ash

and whether such ash is currently present in the economizer outlet flue gas stream. This

can be accomplished by ash sampling at the economizer outlet, the economizer hopper,

and in the first row of precipitator hoppers. Popcorn ash can also be trapped within the

air preheater baskets. Further insight can be gained by interviewing plant operating

personnel to determine whether popcorn ash has been previously observed when burning

either the current coal or coal from other sources, and if the occurrence of popcorn ash is

boiler operation dependent. Any indication of the presence of popcorn ash, however

small, past or present, should be included in the project design criteria.

At issue is the size of the popcorn ash particles relative to the size of the catalyst screen

openings. Popcorn ash larger than these openings will accumulate on top of the catalyst

modules, thereby, promoting fly ash deposits that will eventually block flue gas flow.

These ash accumulations, if over a significant reactor area, can detrimentally affect

catalyst performance and can impart potentially significant concentrated ash loads onto

the top catalyst level and reactor structure. Ash deposits several feet deep and blocking

25% of the reactor cross-section have been reported as a consequence of popcorn ash

accumulations on the catalyst inlet screens.

Due to the continuous nature of SCR operation, even small concentrations of popcorn ash

can accumulate and lead to eventual blockage of catalyst modules. Where popcorn ash

has been identified to exist, the following steps should be implemented:

• Determine whether the problematic fuel source(s) or combustion operating

mode(s) can be permanently changed so as to eliminate popcorn ash generation.
LESSONS LEARNED FROM SCR REACTOR RETROFIT

19

• If the source of the popcorn ash cannot be eliminated, then include provision of

knock-out screens at the economizer gas outlet to force the ash to drop out into

the economizer hopper.

• Routinely inspect the top catalyst elevation, particularly near the front SCR

reactor wall. Monitor the popcorn ash accumulation rate and remove any ash

deposits before they become problematic.

Because the occurrence of popcorn ash can be fuel-dependent, some consideration should

be given to inclusion of ash knock-out screens in the initial SCR reactor design regardless

of whether popcorn ash has been previously observed. Such a design would permit

greater flexibility in the purchase of spot market fuels whose combustion characteristics

in this regard may be unknown. It also provides assurance that low popcorn ash

concentrations that may have gone undetected in the past will not cause reactor

operational problems in the future. If deferred, note that retrofit of knock-out screens

requires a unit outage, will add unanticipated pressure rise duty to the ID fans and will

reduce system operating pressure lower than originally anticipated. One can expect ash

knock-out screens to add approximately 0.5” WC pressure loss to the flue gas system.

Ammonia System

Anhydrous, 19% aqueous and 29% aqueous ammonia systems have been installed at the

various Sargent & Lundy SCR installations. However, the predominant choice has been

anhydrous ammonia due to its relatively low capital and operating costs as compared

with an aqueous installation. Design considerations concerning ammonia system design

are related to material selection, pumping system design, equipment fabrication, siting,

and excess flow control.

Material selections must be compatible with exposure to ammonia. In anhydrous

systems, stainless steel instrumentation and valve trim have performed without incident,

to date. Yellow metals, such as brass and copper, galvanized surfaces, and certain

elastomers should be avoided. Even Buna N, which is characterized as a good choice for

static seals, has failed and caused system leaks. Material choices that should perform

well in this application include stainless steel, Teflon, Butyl rubber, and neoprene.

Pumping of a volatile liquid, such as anhydrous ammonia, requires close attention to

pump suction design, especially ammonia forwarding pump NPSHR vs. NPSHA.

Suction piping should be sized to provide 2.5 fps to 6 fps liquid velocities and should be

restricted in length with a minimum number of fittings. Full-port isolation valves should

be used. The piping should be designed with no high points that could collect vapor, and

the pump centerline should be a minimum of 4 feet below the bottom of the tank in order

to ensure that the pump suction is suitably flooded. Failure to comply with these

guidelines can create either inadequate NPSHA, resulting in forwarding pump cavitation,

or suction capacity problems so severe that the pump fails to deliver the required flow.
LESSONS LEARNED FROM SCR REACTOR RETROFIT

20

Selection of dilution air blower equipment should reflect the heavy-duty nature of power

plant operation. In one instance, the inlet vane drives provided were not suitable for the

installation resulting in repeated control card failures. Replacement drive electronics

designed to better withstand heat and vibration exposure have proven successful to date.

Dilution air blower shaft seal leakage can also be an issue, depending on the dilution air

temperature at the blower inlet. In systems where hot combustion air is used for dilution

air, the dilution air blower operating temperatures can exceed 600oF. Blower shaft seal

leakage can then detrimentally affect the adjacent shaft bearings unless the blower

manufacturer has provided both high temperature seals and bearings.

The offsite consequence analysis (OCA) can impact tank storage volumes and

containment design. For example, a utility had planned on installing four 90,000-gallon

anhydrous ammonia storage tanks. However, the OCA indicated that toxic end-point

limits were exceeded in a nearby town. As a result, tank volume was reduced to 70,000

gallons per tank and the quantity of tanks was increased from four to five. Additionally,

the containment design was modified to provide individual bermed areas for each tank in

order to reduce the exposed surface area of the ammonia pool subsequent to tank rupture.

Therefore, the OCA, if required, should be performed early in the project such that the

impact to the ammonia storage area arrangement is identified before detailed

specifications and designs are prepared.

As a safety feature, ammonia systems are provided with excess flow valves in the

forwarding pump suction and discharge piping that automatically close in the event of a

pipe break. However, if undersized, these valves may inadvertently close during system

transients, thereby, forcing cessation of SCR operation until the valves can be re-opened.

Re-opening an excess flow valve requires pressure equalization upstream and

downstream of the valve. This is normally accomplished by ammonia leakage through a

weep hole or passage in the valve until downstream pressure equalizes with upstream

pressure. This process can take hours to complete in large ammonia systems.

Meanwhile, the SCR reactor system has been taken off line and NOx emissions have

increased to baseline levels. To avoid this situation, the ammonia system supplier should

ensure that excess flow valves are properly sized in order to prevent nuisance closures.

Generally, valves sized for approximately 1.5 to 2 times the normal process demand

should suffice. It is important not to oversize the valves or they will not function when

needed during a true emergency situation.

Induced Draft Fans

ID fan design duty is impacted by SCR retrofit due to the additional draft loss across the

SCR reactor system. Generally, rotor replacement has been necessary to provide the

additional fan capacity. However, the scope of these modifications has varied. In some

cases, no fan or motor modifications were required. In other cases, complete fan and

motor replacement was needed.
LESSONS LEARNED FROM SCR REACTOR RETROFIT

21

Means of minimizing ID fan modifications and operating costs are as follows:

• Review current operating conditions to determine if deviation from the original

plant design basis has caused significant reduction in ID fan margin, and establish

whether these operating problems can be rectified. Common examples include

excessive air preheater leakage, boiler infiltration, precipitator infiltration,

elevated air preheater gas outlet temperature, poor fuel quality, elevated excess

air, and other heat rate related inefficiencies. Frequently, however, many of these

deficiencies cannot be repaired or cannot be maintained in a repaired state over

the long-term. In such cases, some or all of this additional fan duty may have to

be included in the new fan design criteria.

• Size the reactor supply and return ductwork for flue gas velocities in the 3600-

fpm range.

• Minimize head losses across ductwork internal obstructions by considering lowpressure loss damper designs and utilization of minimalist truss design criteria.

• Consider a 2+1 reactor design in lieu of a more conventional 3+1 arrangement.

This eliminates one catalyst level and can reduce system draft loss by 0.5” WC to

0.75” WC.

• Limit maximum allowable catalyst SO2 oxidation rate such that the existing air

preheater differential pressure water wash setpoint and gas outlet temperature can

be maintained.

• Optimize ductwork arrangements to minimize draft loss.

• Install variable frequency drives (VFD) for ID fan capacity control.

Once these various design decisions are made and the fan impact is established, the

feasibility of blade tipping should be evaluated first. Although blade tipping has been

uncommon in Sargent & Lundy’s SCR retrofit projects due to the limited improvement in

flow and pressure rise provided, generally less than 10% in each case, blade tipping can

reduce fan modification cost substantially. Tipped fan rotors can usually be installed for

less than 10% of the cost to install new fans. However, blade tipping carries a fan

efficiency penalty of between 1% and 3.5%, which will further increase auxiliary power

demand. Additionally, the existing rotors have to be structurally sound and able to

withstand the increased operating stress. All of these factors must be considered when

evaluating the feasibility of blade tipping.

If blade tipping is not feasible, rotor replacement is required. Replacement rotors are

generally designed to operate at higher speed than the existing wheels in order to permit

salvage of the existing fan housings. Although the fan motors are generally replaced due

to increased power demand, in instances where the motors are otherwise adequate, it

should be noted that increasing design rotor speed will force motor replacement in order

to match fan and motor speed. This consideration should be integrated into the fan
LESSONS LEARNED FROM SCR REACTOR RETROFIT

22

assessment to determine the optimum solution. ID fan rotors designed for speeds up to

1200 rpm are now in service and operating satisfactorily to date.

Electrical load impacts can be substantial in an SCR retrofit project. Significant

percentage changes in ID fan horsepower can necessitate improvement to the plant

medium-voltage auxiliary power system (4 kV to 13.8 kV). The thermal rating (kW) of

the existing reserve auxiliary as well as unit auxiliary transformers can be exceeded,

thereby, necessitating replacement. In addition, the switchgear must be capable of

handling across-the-line start capability of the new, larger motors and must have the

necessary short circuit interrupting capacity. If these abilities are exceeded, then the

switchgear must be replaced. Higher ampacity cable may also be required to handle the

higher current draw of the larger ID fan motors. If existing cable trays are at capacity,

new raceways will have to be installed in order to route the power cables to the ID fan

motors. This work will generally be performed pre-outage in order to minimize outage

duration and scope. As a result, this design must be completed early in the project.

Retrofit of VFDs for ID fan control should be considered as an option to reduce short

circuit interrupting requirements and to eliminate motor in-rush current considerations.

This may permit the existing transformers and switchgear to remain in-place, and will

result in reduced auxiliary power consumption, thereby, generating potentially significant

operating cost savings, particularly if the unit load follows or cycles. However,

installation of VFDs will require additional space to house the drives, controls,

transformers and coolers. Additionally, VFD installation may also result in upgrade of

the ID fan control system that, in turn, will cause upgrade of the FD fan controls.

Nonetheless, reduced power consumption can justify the choice to install VFD within a

few years. The following discussion from a 660-MW coal fired plant that retrofit VFDcontrolled ID fans illustrates this point:

• Station auxiliary power demand when in SCR bypass mode at part load has been

reduced by as much as 7% below auxiliary power demand prior to SCR system

retrofit.

• Station auxiliary power demand when in SCR mode at part load is the same as

auxiliary power demand prior to SCR system retrofit.

• At full load, station auxiliary power demand is higher than prior to SCR system

retrofit but is estimated to be 2% to 6% lower than if inlet vane controlled fans

been used.

The benefits of VFD fan control over maintaining existing inlet vane control, are

summarized as follows:

• VFD fan control permits selection of the optimum fan design operating speed by

the fan vendor.
LESSONS LEARNED FROM SCR REACTOR RETROFIT

23

• Fan efficiency is improved across the load range and, in particular, at low loads as

compared to inlet vane or fluid coupling control. This minimizes auxiliary power

consumption and increases net generation.

• VFD eliminates motor starting in-rush current considerations and reduces short

circuit interrupting requirements on the auxiliary power system. This may reduce

the modification scope to the existing auxiliary power system.

• VFD allows the use of less expensive fan motors with standard WK2 ratings

rather than the more expensive, high WK2 motors that are usually required for ID

fan operation.

Structural Steel

The design of tall steel superstructures that span over existing back-end plant equipment,

and which support the SCR-related reactors and flue gas ducts, are significantly affected

by wind and seismic loading, especially those structures that are building code classified

as “irregular.” Irregular structures require a more rigorous seismic analysis than

“regular” structures. In addition, building codes are continually revised and upgraded

with respect to seismic design requirements. Newer codes such as the International

Building Code (IBC) place an even greater emphasis on seismic events in select areas of

the country. When designing steel members and connections to newer or upgraded codeimposed requirements, the results may require heavier and deeper members (known as

jumbo sections), and more massive and complex non-standard connections than

previously used.

Procurement of jumbo steel can require significant lead-time, depending on the required

sizes, tonnage, supplier inventories, economic conditions, and whether foreign suppliers

and fabricators are used. Total lead-time from initial mill order to site delivery can be up

to 28 weeks. This is a significant schedule requirement that must be defined and verified

early in the project so as to levelize the work loads of all parties involved, and ensure

timely material delivery to the site.

Piles

A well-conceived pile installation plan involves a coordinated effort between the

engineer and the installation contractor(s) to establish pile design locations that consider

installation access and pile driving/drilling equipment positioning. For example, in-place

steel framing can be temporarily removed, shored, braced, and eventually reinstalled such

that H-pile segments and auger flights can be maximized in length to reduce construction

efforts and cost.

However, in certain situations, significant existing overhead obstructions such as the air

preheater support structure, other back-end structures, and various piping systems,

conduits and cable trays cannot be easily removed or temporarily relocated. At one site,

such obstructions resulted in segmented pile installations in lengths as short as 5 feet for
LESSONS LEARNED FROM SCR REACTOR RETROFIT

24

piles that would reach eventual refusal at 100 feet. These overhead restrictions required

additional H-pile splice welding and augercast pile flights. In addition, specialized low

headroom drilling/driving equipment was required.

Pile type is influenced by the available space both above and below grade. Heavily

congested areas at or near grade where building foundations and buried underground

components exist can necessitate investigation into various pile type options. These

options should be reviewed with close attention to construction cost and installation

schedule. Pile options include rolled steel shapes, augercast, step-tapered, caissons, and

pipe piles, including small diameter mini-piles. The investigation should also include a

review of pile capacities at various soil embedment depths down to refusal at bedrock in

order to select the optimum design.

Performing a pile test prior to design and installation should be considered in order to

determine the maximum usable axial compression, lateral and uplift capacities. This will

confirm whether greater capacities can be achieved, thereby, reducing the number of piles

needed in the design. Otherwise, pile designs may be limited to capacities imposed by

the governing building code, which may be considerably lower than the capacities as

calculated for the project-specific soil conditions and as verified through load tests. Prior

to performing the test, however, the cost of pile testing should be weighed against the

potential cost benefit afforded by the anticipated test results as compared to code

capacities.

The evaluation of pile type options should consider the potential impact of the pile

installation method on existing nearby foundations and buried underground components.

The engineer and installation contractor should be prepared to have in-place an

alternative pile type selection as a substitute and/or replacement pile should the original

choice generate unfavorable results during installation. For example, driving or drilling

of piles may cause settlement of existing foundations not founded on piles or supported

on piles that do not reach bedrock. Pile driving vibrations may cause soil densification,

and drilled piles may cause decompression, thereby creating a potential for settlement of

existing structures. As such, misalignment, additional stress, and loss of support for

operational equipment could occur. Such outcomes could lead to higher construction

costs and schedule delays due to installation of temporary shoring, rigging, and bracing

as may be needed to stabilize these components and structures. Depending upon the

number of nearby affected foundations and buried components, the added temporary

construction support could create significant unexpected cost escalation.

Conducting a survey to document elevations of structures and in-place components prior

to pile installation, and subsequently monitoring the change in conditions during pile

installation, is a prudent means to reduce the risk of disrupting ongoing plant operations.

Limiting the hammer energy of pile driving equipment or determining a safe separation

distance for new pile installations from existing structures and exposed underground

components can help mitigate settlement.
LESSONS LEARNED FROM SCR REACTOR RETROFIT

25

Existing and New Foundations

Early in the design stage of the project, exploratory soil excavations encompassing the

area of new SCR-related foundations should be performed so as to establish the location,

depth, size, type, and function of the exposed underground components. Existing buried

components may have been installed in shapes and locations that deviate from original

plant designs, may be inactive or abandoned in-place, or may be currently active but have

no supporting design documentation. New pile and foundation designs must account for

these revised and/or unaccounted for installations. Advance information on buried

components as well as nearby existing foundations will help to reduce engineering efforts

and better define needed construction support. Such information will help mitigate

design and construction delays.

There are several methods for performing these exploratory excavations. However, one

method that is particularly quick and non-destructive is the hydrovac system. This

system uses water, pressure, and vacuum to wash and remove soil to expose the utilities.

Generally, a 4-inch to 6-inch wide slot trench is excavated to a depth of 5 feet around the

perimeter of the proposed foundation. Deeper and wider excavations could be made as

needed. Specialty contractors perform such excavations.

Modeling

SCR retrofit projects should consider the development of 3-D computer models in

heavily congested areas and spaces where new structures and components will be placed

in proximity to existing structures and components. Such models are an integral part of

conceptual planning, general arrangement preparation, and final detailed design, and

contribute to successful construction scheduling and cost control. The basic model scope

should include, but is not limited to, the following:

• All new designs.

• Existing plant equipment, piping, cable tray, conduit, piles, foundations, and

structures in the vicinity of the new design.

• O&M space, including access doors, removal paths, pull-spaces and lay-down

areas for new and existing equipment.

• Gallery headroom clearances.

• Construction crane positions with defined hook limits.

• Buried components in the areas of the foundation work, as well as in the areas of

crane travel.

• Temporary staging areas for construction support.

• Stairwells.

• Construction elevator locations.
LESSONS LEARNED FROM SCR REACTOR RETROFIT

26

• Plant roads in the vicinity of construction in order to maintain current plant truck

access and to define the space available for structural steel and ductwork module

delivery.

• Reserved space for future plant additions.

• Reserved construction space.

• Conservative preliminary steel and gusset plate sizes.

• Actual steel and gusset plate sizes as detailed designs are developed.

Inclusion of additional detail in the model should be considered such that all potential

interferences are identified. This level of detail should include, but is not limited to, the

following:

• Ductwork and large bore piping insulation system thicknesses and thermal

movement allowances.

• Utility chases for small bore piping and conduit.

• SCR reactor, ductwork, and piping supports.

• As-built arrangements of previous plant modifications that were not documented

in the original design documents.

• “Other” project scope in the vicinity of the new design.

• Test probe pull-spaces.

In addition to facilitating design efforts, 3-D models have been proven to expedite

construction and minimize erection cost when reviewed by the installation contractor,

who can use the model to optimize the sequence of steel erection and duct module lifts.

Note that because this sequencing can impact superstructure design, such as the need to

define leave-out steel, it is recommended that this review be performed prior to design

completion.

CONCLUSION

There are numerous considerations associated with the retrofit of an SCR reactor system.

Addressing the following issues will facilitate the design of future coal-fired SCR

installations:

• Design of SCR reactors for removal efficiencies greater than 90% at ammonia slip

levels less than 2 ppm to 3 ppm has been demonstrated and should be considered

as a feasible design criterion.

• PRB coal may cause approximately 10% to 20% faster catalyst deactivation due

to calcium sulfate masking as compared with bituminous coals. This loss in

catalyst service life is easily compensated for by a corresponding increase in
LESSONS LEARNED FROM SCR REACTOR RETROFIT

27

catalyst surface area. Where a honeycomb or corrugated catalyst has been

installed, catalyst blockage caused by bridging of the more aggressive PRB coal

ash deposits has so far been avoided by use of 9.2 mm pitch catalyst, or its

equivalent.

• Increased SO3 emissions due to SCR reactor operation may create stack plume

coloration and/or acid aerosol fallout problems. The probability of such problems

occurring can be reduced via procurement of a catalyst with an SO2 oxidation rate

between 0.6% and 3.0%, depending on numerous process variables. Ultimately,

the selected catalyst SO2 oxidation rate for a fully charged SCR reactor should

generate no more than 10 ppm SO3 at 3% O2 in wet stacks and 20 ppm SO3 at 3%

O2 in dry stacks. If unacceptable stack emissions occur, then operational

adjustments or retrofit of an SO3 remediation technology such as sorbent injection

to the boiler and/or downstream of the SCR reactor may be required.

• The number and extent of internal trusses should be defined early in the project.

• Catalyst suppliers and flow modelers must ensure that the number and extent of

trusses are properly considered in their modeling efforts.

• All other reactor and ductwork internal structures and equipment should be

included in the flow model such that gas distribution and pressure drop effects are

documented.

• Either reduced 1:12 scale or numerical flow modeling should be performed to

optimize SCR system design, document compliance with distribution limits, and

determine the expected draft loss.

• SCR systems require installation of large dampers in a high-temperature

environment that subjects the equipment to severe thermal stress. These stresses

can result in distortion, deflection, and/or deformation of such degree that the

damper equipment fails to operate as intended. Such failures can compromise the

ability of the lay-up and damper seal air systems to perform as designed. Use of

guillotine, diverter, and flap type dampers should be strongly considered if the

project budget allows. Use of louver dampers, whether for modulation or

isolation, requires detailed engineering and analysis by the damper vendor.

However, in all cases, consideration should be given to verification of damper

structural integrity through numerical modeling, and damper functionality through

shop testing at simulated operating conditions. Evaluation of comparable damper

equipment at existing facilities can minimize modeling and shop testing scope.

• To date, sonic horn operating experience has been positive. Cleaning capability

across approximately 40-foot reactor spans has been demonstrated. Accordingly,

provision for retrofit of steam or air sootblowers in the SCR reactor design and

layout appears unnecessary.
LESSONS LEARNED FROM SCR REACTOR RETROFIT

28

• Heat transfer surface packing pressure within the air preheater baskets must be

sufficient to prevent unwanted vibration and flutter, but must be uniformly

applied during fabrication so as to minimize plate and enamel coating damage.

Loss of enamel coating at plate contact points can relieve packing pressure,

thereby, causing eventual fatigue failure cracking of individual plates and loss of

plate surface area. Inadequate packing pressure can also increase sensitivity to

sootblowing steam force and may result in plate edge damage. Air preheater

basket fabrication should be witnessed by the Owner to verify that adequate and

uniform packing pressure is applied during fabrication and that enamel coating

damage during the plate packing process is minimized.

• Reactor lay-up systems should be designed to maintain a minimum 120°F

temperature within the reactor, or as otherwise specified by the catalyst vendor,

and should prevent flue gas circulation through, or back flow into, the SCR

reactor. Tramp air entry can be eliminated by designing the lay-up system to

maintain a slightly positive reactor pressure. Negative pressure lay-up is also

permissible but the reactor temperature should be monitored to identify whether

excessive tramp air entry is occurring.

• Reactor man access requires either selection of proven SCR damper designs with

low leakage rates that can be successfully isolated with pressurized seal air, or the

installation of blanking plates at the SCR reactor inlet, outlet, and economizer

bypass locations.

• The ability to preheat the SCR reactor can preclude the need to modulate the SCR

inlet damper in order to comply with catalyst temperature ramp rate limits, can

extend catalyst life by avoiding reactor operation below acid dew-point, and can

reduce thermal gradient induced stress on the reactor structure. The optimal

preheat temperature at plate catalyst installations is on the order of 300°F. Lower

preheat temperatures may be possible at units using a honeycomb or corrugated

type catalyst, or that burn low-sulfur coal.

• Any history of popcorn ash should be identified and included in the design criteria

at project inception. This type of ash can accumulate on the top catalyst level

screens, block gas flow, and create significant ash loading. Because fuel and unit

operational changes can create popcorn ash where none previously existed,

projects should consider inclusion of popcorn ash knock-out devices as part of the

initial SCR system design in order to maintain fuel purchasing and unit operating

flexibility.

• Improper material selection for ammonia system components can lead to chemical

attack and equipment failure. Yellow metals, such as brass and copper, and

galvanized surfaces should be avoided. Use of stainless steel, Teflon, Butyl

rubber, and neoprene are suitable choices for valve and instrumentation trim.
LESSONS LEARNED FROM SCR REACTOR RETROFIT

29

• Ammonia forwarding pump suction piping should be designed for liquid

velocities less than 6 fps with no high points that could collect ammonia vapor.

Suction line resistance should be minimized by use of full port valves and

minimal pipe length and fitting quantities. The forwarding pump centerline

should be at least 4 feet below the bottom of the storage tank elevation in order to

ensure the pump suction is suitably flooded.

• The OCA will impact the design of the ammonia storage facility and should be

performed prior to establishing final ammonia system specifications and

arrangements.

• Proper sizing of the ammonia system excess flow valves will prevent spurious

closure and temporary loss of SCR system operation. Excess flow valves should

be sized to close at 1.5 to 2 times the normal ammonia system demand.

• Impact to the ID fans can be minimized through judicious selection of various

SCR system design criteria and conducting plant maintenance to restore the

original unit operating conditions. SCR system draft losses can be minimized by

selecting a 2+1 reactor design instead of a 3+1 design, maintaining design flue

gas velocities less than 3,600 fpm, minimizing duct truss cross sectional area,

using low pressure drop damper designs, and specifying low SO2 oxidation rate

catalyst in order to minimize air preheater pressure drop penalties. Repair of

excessive air preheater leakage and precipitator infiltration can recover ID fan

capacity and minimize the size of the new ID fans.

• If fan modifications are required, blade tipping should be the first consideration

due to the low capital cost of this option. When new rotors are required,

increasing fan operating speed will minimize rotor size and may permit salvage of

the existing fan housings. However, installation of higher speed fan motors will

become necessary.

• Installation of VFD fan control can minimize the impact to the medium-voltage

auxiliary power system transformers and switchgear, and reduce auxiliary power

consumption. Where VFD-controlled ID fans have been installed, station

auxiliary power demand has been demonstrated to be the same or lower at part

load as compared to pre-SCR retrofit operation. At full load, station auxiliary

power demand is estimated to be up to 6% lower than if inlet vane-controlled fans

had been used. Use of VFD control also reduces fan motor cost by permitting use

of motors with standard WK2 ratings as compared with the motors normally used

for ID fan service that must have high WK2 ratings for fan starting purposes.

• SCR reactor support structures may require use of jumbo steel. The availability

of such steel in the required sizes and tonnages may result in a 7-month lead-time

between steel mill order and site delivery.
LESSONS LEARNED FROM SCR REACTOR RETROFIT

30

• Careful pile layout, type selection, soil testing, and excavation surveys can

optimize the pile design so as to minimize engineering and construction costs, and

eliminate schedule delays. Use of alternative pile designs or installation methods

may be required to rectify installation problems. Such contingencies should be

planned for prior to installation in order to minimize detrimental impact to

installation cost and schedule.

• Subsurface exploration is critical to identification of underground components

and foundations in order to define their impact on new SCR pile and foundation

design and installation. Use of a hydrovac system is relatively quick and nondestructive means to perform such excavations.

• Creating a 3-D computer model of the new and existing foundations, structures,

equipment, and systems will minimize interference and construction problems.

The model can also be used by the installation contractor(s) to optimize erection

sequence and to reserve space in which to perform the work.

COAL-GEN 2003 SCR Lessons Learned.doc