L ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

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 • 312-269-2000L ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

1 Theodore B. Kurtides Project Associate

2, Fossil Power Technologies

55 East Monroe Street Chicago, Illinois 60603-5780 USA

I NTRODUCTION

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 NO
x 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 NO
x reduction beyond 85% can be a costeffective means of reducing regulatory impact throughout a particular utility system. SCR systems operating at NO
x 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, andL ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

2 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 300 o F 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.L ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

3 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.L ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

4 D ISCUSSION

Emissions

All Sargent & Lundy-designed SCR reactors at coal-fired units, which have been placed into service, have achieved their guaranteed NO
x reduction efficiencies within the specified ammonia slip limits. The minimum design NO
x 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 NO
x 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 SO 3 generation due to SO 2 oxidation that occurs within the SCR reactor. Increased SO 3 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-d

esigned 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 SO 3 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 beenL ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

5 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 SO 3 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 SO 3 related operating problems include:

• Procure catalyst with a low SO 2 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 SO 2 oxidation rate directly addresses the root cause and should be the first consideration. In general, the catalyst SO 2 oxidation rate selected should generate no more than 10 ppm SO 3 at 3% O 2 in wet stacks and 20 ppm SO 3 at 3%
O 2 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 SO 3 concentration and will be readily apparent at 20 ppm. At 30 ppm concentration, the color intensity may reach the point that SO 3 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 SO 3 concentration is increased.

Determining the stack SO 3 concentration requires consideration of the furnace SO 2 oxidation rate, catalyst SO 2 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 SO 2 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.L ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

6 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
SO 3 leaving the SCR reactor. If limestone is used, this is accomplished via reduction of
both SO 2 and SO 3 emissions exiting the boiler. If magnesium hydroxide or magnesium oxide is injected, this is accomplished via selective reduction of the SO 3 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 SO 3 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 SO 3 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 SO 3 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 SO 3 -related emission problems is to minimize the catalyst SO 2 oxidation at the time of procurement. The maximum catalyst SO 2 oxidation rate should be established based on limiting stack SO 3 concentrations to no greater than 10 ppm at 3% O 2 in wet stacks and 20 ppm at 3% O 2 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 SO 3 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.L ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

7 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, NO
x , NH 3 , 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-NO
x 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 momentL ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

8 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 NO X 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 inletL ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

9 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 damp ers 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.L ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

10 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, thatL ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

11 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 onclosed 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.L ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

12 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.L ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

13 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 havebeen 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 platesL ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

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 cyclefrequency. 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.L ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

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 120 o F 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 120 o F 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.L ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

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 300 o F 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 thermalL ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

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 27 o F/min at operating temperature
below 300 o F, and increased to 90 o F/min when operating temperature exceeded 300 o F. For honeycomb and corrugated type catalysts, these limits can be in excess of 100 o F/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 300 o F 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 90 o F /min catalyst temperature ramp limit in effect when initial catalyst temperature exceeded 300 o F. At honeycomb and corrugated type catalyst installations, the catalyst temperature ramp rate limits have beenL ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

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.L ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

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.L ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

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 600 o F. 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 NO
x 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.L ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

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 SO 2 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 lim ited 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 fanL ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

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 afew 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.L ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

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 WK 2 ratings rather than the more expensive, high WK 2 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 ofthe 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 forL ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

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.L ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

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.L ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

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.

C ONCLUSION

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 inL ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

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 SO 3 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 SO 2 oxidation rate between 0.6% and 3.0%, depending on numerous process variables. Ultimately, the selected catalyst SO 2 oxidation rate for a fully charged SCR reactor should generate no more than 10 ppm SO 3 at 3% O 2 in wet stacks and 20 ppm SO 3 at 3%
O 2 in dry stacks. If unacceptable stack emissions occur, then operational adjustments or retrofit of an SO 3 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.L ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

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.L ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

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 SO 2 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 WK 2 ratings as compared with the motors normally used for ID fan service that must have high WK 2 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.L ESSONS L EARNED FROM SCR R EACTOR R ETROFIT

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