|The decision to install an SCR at Edgewater Unit 5 (Edge 5) resulted from the promulgation of increasingly stringent air quality regulations. Photo courtesy of Wisconsin Power & Light|
By Jeffrey Knier and Kendall McFarland, Wisconsin Power and Light, and Ryan Essex and Peter Guletsky, HDR Engineering
Alliant Energy – Wisconsin Power and Light (WPL) recently completed installation and startup of an SCR at its Edgewater Generating Station Unit 5, located in Sheboygan County, WI. The SCR is the last of several NOx emissions reduction projects completed at this site in order to comply with changing environmental regulations. The content of this manuscript focuses solely on the SCR project and is intended to be a project history and “lessons learned” resource for the power generation industry.
This manuscript will highlight the criteria taken into consideration during the design phase of the project that allowed for the maximum amount of operational flexibility while still achieving NOx emission levels that would allow compliance with regulations. The manuscript also discusses operational and construction challenges encountered during the execution phase of this retrofit project. Topics that are addressed in this manuscript include:
- Project summary
- Milestone schedule
- Discussion of design considerations and features that were included in the project scope to aid in assuring preservation of current reliability and low load operation benchmarks
- Highlight specific challenges encountered during SCR design and construction phases
- Review the use of a third party start up and commissioning program
- Provide an operational update on the current performance of the SCR
The decision to install an SCR at Edgewater Unit 5 (Edge 5) resulted from the promulgation of increasingly stringent air quality regulations. In 2004, the EPA designated ten counties in Southeastern Wisconsin, including Sheboygan County where the Edgewater Generating Station resides, as non-attainment areas for the ozone National Ambient Air Quality Standard (NAAQS). As a result, the Wisconsin Department of Natural Resources (WDNR) promulgated NR 428, creating new NOx emissions standards through the adoption of Reasonably Available Control Technology (RACT) requirements for ozone non-attainment areas in Wisconsin. RACT specified that sources comply in two phases, Phase I limits enforced beginning in 2009 and the more stringent Phase II limits enforced beginning in 2013.
In anticipation of lower NOx emissions requirements, WPL began reducing NOx emissions at the Edgewater Generating Station in 1999 through the implementation of the “Combustion Initiative”. Prior to the installation of the SCR, NOx emissions had already been reduced at Edgewater Units 3, 4, and 5 by 58 percent, 84 percent, and 31 percent, respectively, using combustion controls. Selective non-catalytic reduction and rich reagent injection (SNCR/RRI) projects were also installed on Edgewater Units 3 and 4, which further reduced NOx emissions on each unit by approximately 30 to 40 percent. The implementation of the Edge 5 SCR was undertaken to reduce NOx by an additional 60 to 70 percent in order to meet phase II RACT requirements, which went into effect in 2013.
The Edgewater Generating Station is located south of Sheboygan, Wisconsin, along Lake Michigan. Edge 5 began commercial operation in 1985 with a nameplate capacity of 380 MW. The unit currently runs at a gross maximum operating load of 430 MW and burns pulverized low sulfur Powder River Basin (PRB) coal. Edge 5 has a Babcock & Wilcox (B&W) wall-fired boiler retrofitted with Separated Overfire Air (SOFA) technology and Low NOx Burners (LNB) to reduce NOx emissions, followed by a cold-side electro-static precipitator (ESP) for particulate emissions control. Edge 5 also has a unique operating characteristic that allows the unit to be operated across a broad load range which is an attractive attribute that needed to be preserved as best as possible while achieving NOx compliance.
In 2008, WPL, with assistance from their Owner’s Engineer (HDR), began work on the conceptual design and development of the technical and performance specifications for the retrofit of an SCR on Edge 5 for the reduction of NOx. In 2009-2010 separate engineering and construction firms were selected to complete detailed engineering and design, procurement, and construction activities for the Edge 5 SCR.
The Edge 5 SCR was placed into service in December of 2012.
DESIGN FEATURES FOR RELIABILITY & LOW LOAD OPERATION
Edge 5 operates over a wide range of loads, nominally 50 MW – 430 MW, and has a greater unit turn down capability than is typically seen on coal fired boilers. Therefore, the SCR system to be installed on Edge 5 needed to be a system that could maintain Edge 5’s operational flexibility while still reliably providing adequate NOx reduction performance.
With these operational and performance challenges recognized, several design features were identified and investigated early on in the planning and design phase of the project that would ensure the Edge 5 SCR would be successful in maintaining Edge 5’s operational reliability and flexibility as well as the Edgewater Generating Station’s compliance with upcoming 2013 RACT regulations. These design features are discussed in detail in the following paragraphs.
SCR Catalyst Selection
Catalyst modules are used to accelerate the reaction between NOx and Ammonia which primarily reduces NOx into nitrogen (N2) and water (H2O). SCR catalyst modules come in many different types. However the main options in selecting the catalyst are in regard to geometry, physical makeup, and chemical formulation.
• Catalyst Geometry
The two most common catalyst geometries in the coal fired power plant market are the honeycomb and plate-type. Corrugated type catalyst is also available, but is less typical, and therefore, this discussion will focus on the comparison of honeycomb vs. plate-type catalysts.
Honeycomb catalysts offer the benefit of greater reactive surface area per unit volume, which can provide NOx reduction performance with a smaller volume of catalyst and an overall smaller SCR reactor size, and typically a lower installed cost of the system vs. an SCR utilizing plate-type catalyst. However, the compactness of the honeycomb catalyst results in a higher pressure drop across the catalyst layers and a greater susceptibility to pluggage when compared to the plate-type; which leads to higher auxiliary power consumption and potentially higher operations and maintenance costs.
• Catalyst Physical Makeup
Plate-type catalysts have a stainless steel carrier with the active ceramic catalyst layer rolled onto the steel plates.
- The steel plates are notched along their width in order to provide a means of maintaining the plate separation for the flue gas flow paths.
- These notched plates provide long open channels for flue gas flow and flyash passage and provide a structure that is more flexible than the rigid honeycomb structure, further accommodating flyash passage.
Honeycomb catalyst is typically either a monolithic extruded ceramic catalyst (homogeneous ceramic carrier and catalyst components) or it is an extruded substrate coated with the ceramic catalyst material.
- The coated substrate type honeycomb catalyst offers the benefit of a lower cost option due to the more expensive catalyst components being located only on the reactive surface of the catalyst. However, the risk with this design is that once the hardened catalyst layer wears away there is no additional reactive layer beneath and the overall catalyst reactivity will decrease. Also, the substrate, in some cases, may not possess an equivalent level of wear resistance as seen in the ceramic coating. Once the ceramic catalyst coating wears away or gets cracked, the substrate can experience accelerated erosion.
- It is the opinion of the Edge 5 SCR project team that homogeneous honeycomb catalysts, while typically more expensive, offer increased durability and reliability of long term performance over the coated substrate type honeycomb catalysts. This is due to the entire thickness of the catalyst having the same physical and chemical characteristics. Therefore, over time as wear occurs on the surface, the layer of catalyst below the initial layer has the same chemical reactivity and wear resistance and will continue to resist wear and continue to reduce NOx.
- Catalyst Chemical Formulation
The chemical formulations of the catalyst will depend on the type of performance that is desired of the catalyst as well as the chemical constituents of the flue gas. Of the numerous operating parameters that can be specified, and to which the catalyst can be formulated, the below listed parameters will be discussed in more detail in this manuscript:
- High temperature service
- Low temperature service
- NOx reduction rate (based on inlet NOx and ammonia injection rate and temperature range)
- Selective oxidation rate (i.e. SO2 to SO3 conversion)
- The flue gas constituent design parameters in which the catalyst will reside, in order to minimize the risk of catalyst poisoning and deactivation.
For Edge 5, in addition to specifying the NOx reduction performance, the other following characteristics were required:
- A low minimum acceptable flue gas temperature to allow Edge 5 to maintain its turn down capability.
- Low SO2 to SO3 conversion rate was specified to minimize ammonium bisulfate (ABS) formation within the catalyst, the air heater and any downstream equipment. In the end this also aided in lowering the acceptable minimum operating temperature of the SCR and maintaining the unit flexibility.
- Note that Edge 5 retrofitted the existing cold end air heater baskets with ceramic coated baskets in an effort to prepare for the potential increase in ABS due to ammonia injection with the SCR. These ceramic coated baskets will provide a surface that is less susceptible to ABS adherence as well as providing a surface that can more easily be cleaned in the event ABS formation occurs. This type of balance of plant project is something that should be evaluated when considering installation of an SCR.
It is important to note that while identifying potential fuels and operating conditions the catalyst may experience is essential to ensure future unit operational flexibility; this flexibility may come with a more complex and likely higher cost catalyst formulation. It is recommended that catalyst formulation options be discussed with the catalyst supplier to ensure the most beneficial catalyst is selected for each specific SCR system.
• Catalyst Selection Summary
The Edge 5 SCR is located upstream of the existing airheater and electrostatic precipitator (ESP) resulting in a high-dust arrangement. Typically for this type of SCR arrangement, a plate type catalyst is viewed as being the more suitable catalyst type due to its ability to resist pluggage. However, due to the tight site constraints and in order to reduce the structural and foundational loading of the elevated SCR, both plate and honeycomb catalyst were evaluated. In the end, the honeycomb type catalyst was evaluated to be a lower life cycle cost option for Edge 5.
Due to Edge 5 burning PRB coal and the inherent ‘sticky’ fly ash produced, it was recognized that this fly ash passing through the SCR would pose an added risk of buildup and catalyst pluggage. To minimize the inherently higher pluggage risk in the honeycomb catalyst, being compounded by the fact that PRB fly ash would be passing through, larger minimum channel pitch dimensions were specified (larger pitch = larger flow channel size). The minimum pitch dimensions specified provided a large enough flow path for ash particles to pass through with minimal pluggage potential, while also maintaining the catalyst volume to a reasonable size to maintain the installed cost advantage of the honeycomb catalyst over the plate type catalyst.
For the Edge 5 SCR, a homogeneous ceramic catalyst was specified and installed. In addition to utilizing the more wear resistant homogenous catalyst design, the Edge 5 SCR team obtained an extended mechanical warranty, such that the mechanical characteristics of the catalyst would not adversely affect its NOx reduction performance for a period of 60,000 hours (typical mechanical warranties range from 20,000-40,000 hrs.). This guarantee was obtained to ensure one or more cleaning/ rejuvenation/ regeneration cycles of the catalyst modules (which would restore the catalyst chemical reactivity after its typical 24,000 hour performance guarantee period) would be mechanically possible.
Economizer Flue Gas Bypass
Due to the frequent turn down experienced at Edge 5 it was vital that the SCR be designed so as not to limit the unit’s cycling capability. To accommodate the extent that Edge 5 is turned down on a routine basis, the Edge 5 SCR was equipped with an economizer flue gas bypass system (bypass duct and damper) that is used to introduce higher temperature flue gas into the SCR inlet ductwork flue gas stream at lower loads.
At Edge 5, a low temperature catalyst was selected. Therefore, the SCR system could operate without use of the bypass damper from the maximum unit load of 430 MW down to approximately 185 MW. At this point the flue gas temperature reaches a point at which the bypass damper begins to open. The bypass damper and a downstream back pressure damper are modulated to bypass flue gas flow around sections of the economizer. This flue gas temperature control is maintained down to approximately 125 MW at which point the flue gas temperature approaches the technical limitation of the minimum catalyst operating temperature. The pros and cons (installed cost, minimum operating load and its associated economics, and boiler efficiency) of several bypass locations were considered and utilized in selecting the final bypass design. This evaluation is recommended during design of the SCR to determine whether a bypass system is needed, and if so, the final location and design of such a system.
SCR Catalyst Cleaning Systems
The Edge 5 SCR was equipped with several cleaning systems to prevent and eliminate buildup of fly ash on the catalyst surface and pluggage within the catalyst.
• Large Particle Ash Screen System
Large particle ash (LPA) also known as ‘popcorn’ ash is a source of catalyst pluggage as the ash particles are too large to pass through the catalyst channels. These LPA particles can get lodged in the catalyst channels and can also serve as a buildup point for smaller ash particles; thus increasing the plugging risk of the catalyst. Therefore, LPA screens were installed at the economizer outlet as well as at the inlet to the economizer bypass duct to filter the flue gas stream of any LPA material. The LPA screens are installed in a manner that allows ash build up on the screens to fall off due to gravity once a certain level of buildup is reached. In areas where the screen could not be oriented such that ash would be removed by gravity, screen rappers were installed to promote the removal of ash build up from the LPA screens.
• Steam Sootblowers
Steam sootblowers were installed at the three existing layers of catalyst along with a fourth level to accommodate the future layer of catalyst to be added. The steam sootblowers dislodge ash from the surface of the catalyst by injecting superheated steam onto the catalyst surface. The ash is then carried away from the catalyst and out of the reactor by the flue gas stream.
When installing a steam sootblower system it is important to know the catalyst supplier’s requirements for steam temperature and pressure when considering the steam source.
The steam source must be selected such that it is providing a sufficient amount of superheat to ensure dry steam is being blown on to the catalyst surface (typically a minimum of 50 deg. F of superheat is required). If moisture is blown onto the catalyst surface there is a high risk for ash build up and fly ash set-up on the catalyst surface; such that a unit shut down and mechanical means of cleaning the catalyst surface would be required.
Designing the steam system to provide steam at the appropriate temperature is important to ensure the catalyst surface does not incur thermal shock due to a large differential temperature between the catalyst surface and the cleaning steam. Typically steam temperature differentials within the 50-100 deg. F range are acceptable, but each catalyst supplier may require different criteria for steam temperatures.
The catalyst supplier will also recommend an acceptable cleaning pressure to prevent premature wear of the catalyst surface.
• Sonic Horn System
|2 Straight and Curved Sonic Horns|
|The sonic horns at Edge 5 were insulated to minimize condensation risk within the horns.|
Sonic horns were installed at the three existing layers of catalyst along with a fourth level which could accommodate the addition of a future layer of catalyst to be added. Sonic horns fluff ash from the surface of the catalyst by causing particles to resonate at a frequency of about 75 Hz at an output power level of 147 dBA. The ash is then carried away from the catalyst and out of the reactor by the flue gas stream.
When designing the sonic horn system it is important to ensure the horn layout design is such that adequate cleaning power is provided to all areas of catalyst surface, specifically areas that may be prone to lower flue gas velocities. The SCR reactor’s inside turn (nearest the inlet duct) at the top catalyst layer may be of particular concern.
During a reference site visit of a similar SCR it was communicated to the Edge 5 project team that the particular sonic horn layout provided at the site was not optimal, as the sonic horns were all located on a single wall opposite from a low flue gas velocity region. Therefore, the cleaning energy of the sonic horns being dissipated to a degree on the opposite wall of the SCR reactor, combined with low flue gas velocities to carry away the ash build up, resulted in an area of constant ash build up issues for this site.
For the Edge 5 SCR the sonic horns were arranged such that a majority of sonic horns were installed on a single reactor wall; however, the horns were arranged such that their cleaning energy was directed from side to side across the reactor. This arrangement provided a horn dedicated to the low flow area of the Edge 5 SCR catalyst surface. In addition to the modified arrangement of the main wall of sonic horns, the Edge 5 project team requested two horns be installed opposite and perpendicular to the main row of horns such that they were dedicated to cleaning along the wall opposite the main row of horns. To date, this arrangement has worked efficiently in cleaning the entire surface area of the catalyst layers.
Another lesson learned from the reference site visit was to use straight sonic horns where space permits. The reference site had curved sonic horns installed and they had experienced fly ash pluggage and set-up within these horns. The curved shape and orientation of the horns provided a low spot for the ash to settle and build up. Compounding this issue was the fact that these horns were exposed to ambient conditions in a northern climate. This created a cold spot and promoted condensation within the horns which in turn promoted fly ash setup within the horns.
For the Edge 5 SCR the access platforms were extended, where needed, to allow for the installation of straight sonic horns to eliminate low spots within the horns. The sonic horns at Edge 5 were insulated to minimize condensation risk within the horns. In addition to insulating the sonic horns, dried instrument air vs. regular compressed air was used as the motive sounding force of the horns to further prevent condensation risk within the horns.
• Cleaning System Summary
The use of the LPA screen system, to prevent LPA from migrating to the SCR catalyst, along with the combined sonic horn and sootblower cleaning system provided a ‘belt and suspenders’ type approach to the cleaning system for the Edge 5 SCR. However, due to recent pluggage experiences at another Alliant Energy SCR site, this conservative approach was considered prudent to ensure the operational reliability of Edge 5.
SCR Flue Gas and Reagent Distribution and Mixing
Selecting the proper catalyst type and formulation, maintaining adequate flue gas temperatures, and designing and installing the appropriate cleaning system are all vital aspects of the flexible and reliable performance of the SCR system. However, despite successful implementation of all these design features, improper and inadequate flue gas mixing could still greatly reduce the performance of an SCR system. Therefore, it is vital to ensure the flue gas mixing and flow distribution devices incorporated into the design of the SCR are evaluated very carefully.
For the Edge 5 SCR an SCR OEM with a proven reagent injection and flue gas mixing and distribution system was selected. During the design phase the SCR OEM was required to create a physical model of the SCR system to prove the design would meet the stringent flue gas constituent, temperature, velocity, and injected reagent distribution requirements and variance restrictions. The physical model was tested at points that spanned the entire expected SCR system operating range to ensure that reliable performance could be achieved at all unit loads.
In addition to the proven static mixing system the SCR OEM provided, they also possessed a proven reagent injection system which included adequate turn down capability to operate reliably through the entire operating range of the SCR, which the Edge 5 project team knew was an area of concern in other OEM aqueous ammonia injection systems.
In summary, the project team ensured that the SCR technology selected had proven operating experience similar to the operational conditions for each specific SCR installation. It is recommended to ‘prove’ the specific SCR design with physical modeling c ications identified in the modeling stage will be significantly less expensive than those identified after a system is in commercial operation.
Over the lifecycle of the project, the SCR project team expended a lot of effort to keep the plant operations and maintenance staff involved and informed of the SCR design criteria being selected. The plant was encouraged to provide input into the SCR design focusing on safe operation and ease of maintenance of the overall system. Methods of engaging the plant in the project included weekly internal project team meetings and having plant staff participate in key drawing reviews, design basis updates and operability reviews. Monthly project meetings were also held at the generating station, and often included a second day of technical working design meetings that plant staff attended.
Approximately halfway through the SCR project, WPL dedicated a full-time plant resource to the project. The “project plant liaison” was designated as the single point of coordination between construction and ongoing plant operations and maintenance.
The plant liaison was a valuable position that not only interfaced daily between construction and the plant, but also ensured new assets were entered into WPL’s maintenance management software system, made sure lock out/tag out isolation points were developed and clearly labeled, and assisted with organizing and completing final “punch list” items on the project.
WPL also hired a third party to conduct supplemental training for operations and maintenance staff.
The plant liaison was instrumental in developing the training criteria, and directly supervised all project training.
The Edge 5 SCR project brought many challenges to the project team. Through careful planning and a collaborative effort of all parties involved these challenges were identified, evaluated and resolved.
The major challenges of the Edge 5 SCR project are discussed in the following sections.
Challenge: Brown Field Retrofit
As with all large retrofit projects, the Edgewater SCR required careful planning and advanced engineering in order to design and install the SCR within an already-congested back end flue gas stream. The SCR was installed upstream of the existing rotary air heater and cold-side ESP, which located the SCR in the area above the combined forced draft and primary air fan room. WPL and HDR worked with the OEM engineer to design the SCR to fit into the constrained site. Due to the SCR location, the OEM specified an innovative substructure foundation design. In order to minimize construction issues associated with the SCR retrofit design, WPL solicited the constructor’s feedback on the constructability of the design by incorporating their feedback into design reviews prior to mobilization to the site.
Because the SCR needed to operate across a temperature range of approximately 550°F to 800°F, the SCR reactor was installed upstream of the existing rotary air heaters. The existing flue gas ductwork selected for SCR tie-in was the economizer outlet duct. This ductwork was located approximately 100 feet in the air, and directed flue gas from the economizer outlet down to the air heater inlet (see Figure 3).
|3 Retrofit Location of SCR|
|Because the SCR needed to operate across a temperature range of approximately 550°F to 800°F, the SCR reactor was installed upstream of the existing rotary air heaters.|
This location was selected because the SCR operating temperature could be achieved without flue gas re-heaters, the length of duct runs were minimized (reducing system pressure drop and minimizing required ID fan power) and open space to the north of the stack was preserved for future AQCS projects. A design emphasis was placed on minimizing the pressure drop of the SCR, both to minimize auxiliary power use and because the ID fan footprints were limited in size increase by the configuration of their existing fan housings. Two access locations to the SCR reactor were provided from the existing boiler room. One access point included an access path between the SCR reactor and an existing elevator door to allow tool cart access to the SCR.
The OEM engineer specified innovative use of micropiles to support the SCR in an extremely congested area. The SCR is located directly above the forced draft and primary air fan room. The OEM engineer used a micropile design in order to keep the SCR foundation support independent from the existing boiler concrete slab foundation, and to allow installation in the tight working areas of the fan room. The SCR is supported on 81 steel-casing micropiles. Each 9-5/8″ diameter pile is installed in bedrock, approximately 90 feet below grade. The micropiles are located in 14 groups of 3 to 11 micropiles, with each group supporting a steel pile cap pedestal, which in turn supports a structural column. The pedestals are large structural supports, weighing up to 27.9 tons.
A specialty contractor was utilized by the constructor to perform detail design and installation of the micropiles. Installation of the micropiles was a difficult process due to the nature of the work, performing the installation in winter, and the obstructions of plant operating equipment in the fan room. Some micropiles failed their structural proof tests, so WPL hired a third party expert to review the detail design, make remediation recommendations, and review the proof test results. WPL learned that when dealing with specialty engineering and installation, hiring an expert to review key design points and critical installation methods can be a worthwhile investment.
As previously mentioned WPL brought the constructor on to the project approximately four months prior to site mobilization. At that time, the constructor performed constructability reviews of the SCR and gave feedback to the OEM engineer. This was a critical time to bring the constructor to the project because engineering was not yet complete, and the input received from the constructor reduced installation time and costs. The constructor contributed worthwhile feedback in three notable areas: the modification of existing air heater outlet duct, the load transfer of the existing economizer outlet duct and suggestions for additional laydown area.
The first example of constructability review involved the air heater outlet duct, which needed to be moved in order to make room for new SCR structural columns above the fan room. The constructor reviewed the drawings and worked with the OEM engineer to develop a plan to modify and move the ductwork, shown in Figure 4, during a unit outage. The actual movement of the existing duct went without difficulty due to proper planning beforehand.
|4 Air Heater Outlet Duct|
|The air heater outlet duct needed to be moved in order to make room for new SCR structural columns above the fan room.|
The next example of significant constructability review occurred mid-project while planning a significant structural load transfer. The boiler economizer outlet duct was supported by existing steel cantilevered off the boiler room structural steel. The support for this duct needed to be transferred to the new SCR steel, so the existing cantilever steel could be removed. WPL and HDR facilitated several working meetings between the constructor and OEM engineer in order to develop a load transfer plan. The original load transfer plan required a boiler outage, but because the constructor and OEM engineer worked together, a safe plan to perform the load transfer online was developed.
The constructor also provided value by suggesting WPL develop additional laydown space for the project. The Edgewater plant is a constrained site, as it is located in town and borders Lake Michigan. As SCR ductwork was being fabricated, it was apparent that the new ductwork could not all be stored on site. A WPL-owned piece of undeveloped land existed across the street from the plant, and the project team suggested that developing that space into a laydown yard would allow all ductwork to be delivered to site, and would enable crews to inspect and prep the pieces for erection. WPL realized that opening up as much space as was practical for the constructor allowed them to more efficiently execute fabrication and erection. After proper permitting and site preparation the project team was able to open this additional area up and use it for the duration of construction.
Challenge: Burner Management System Upgrade
NFPA 85 requires that the SCR ammonia isolation/block valves be forced closed upon a master fuel trip (MFT), which necessitates interface with modifications to the plant’s Burner Management System (BMS). The existing BMS was equipment installed when the plant was originally placed in-service in 1985 and included an outdated Bailey 762 system, which had become obsolete and for which the plant had difficulty finding replacement parts.
As a result, the project team, in conjunction with the plant, decided to take the opportunity presented by the extended SCR outage required for the project to update the plant’s BMS. To simplify maintenance and spare parts stocking, minimize training on new equipment, and simplify the interface requirements between the existing plant DCS and the new BMS, an Emerson Process Controls (Emerson) Ovation system was selected.
In addition to the installation of the Ovation-based BMS-proper, the upgrade scope consisted of installing new redundant instrumentation, reworking some hardwired trips within the auxiliary relay cabinet, wiring additional signals from switchgear and the bench board into the BMS. Plant drawings, including schematic diagrams and wiring diagrams, also required updating. The work was completed through collaboration between plant staff, HDR, and Emerson. Wiring and instrumentation changes were performed by the plant during the outage.
When completing major Air Quality Control System (AQCS) projects, it is essential that potential impacts to other existing plant systems be considered. While changes to the existing DCS were anticipated, the most significant impacts upon the DCS were expected to be in the boiler control system and any motor controls that were involved. From early conceptual engineering it was identified that the logic for the ID fan would be brought from auxiliary relays and the bench board into the DCS to allow the incorporation of SCR logic impacts upon the ID fan logic. In addition to the substantial changes that were required with the BMS upgrade, the plant requested an update to the hard-wired window-annunciator to reflect the new equipment, instead of relying on the DCS alone for alarming as is often done. Ultimately, when preparing for a major AQCS retrofit project, it is essential to have dialogue with the plant personnel early in the project regarding preferences and expectations on equipment to update that may not have been considered when initially scoping the job. This dialog also allows the team to consider the spare capacity of existing equipment, even if the impact to those systems is only expected to be minimal, as was the case with the BMS at Edge 5.
Challenge: ID Fan Upgrade
|5 ID Fan Rotor Retrofit|
|A fan foundation strength test and constructability review confirmed that the existing fan and motor foundations could be reused with modifications for added mass for the proposed new, larger and heavier centrifugal fans and motors.|
With the addition of the new SCR System, WPL and HDR performed a fan study during the conceptual engineering phase to evaluate the capability of the existing ID fans and motors to be upgraded for the new draft system modifications. In addition, the fan study evaluated alternative new fan configurations and the impact of future AQCS retrofits for sulfur dioxide and particulate matter control, on the fan and draft system.
The pre-SCR fan configuration at Edge 5 included two 50 percent capacity centrifugal ID fans with speed reducing fluid couplings to control fan speed and boiler draft. The first phase of the study included an evaluation of operating data to determine if the fans would be adequate as-is or could be upgraded for the intended service with the SCR system. In addition to the owner’s team evaluating the available margins in the existing fans with plant test data, the fan OEM performed an independent investigation to validate the findings and propose, if needed, potential housing or rotor modifications (e.g., blade tipping) to add pressure margin. This first phase concluded that the existing fans and motors were significantly pressure and flow limited for the expected SCR pressure drop increase and proposed test block conditions. Further, the OEM confirmed that possible modifications to the housing and rotor were insufficient for the expected post-SCR operation. Therefore, WPL decided that the existing ID fans and motors would be retired and new fans and motors were installed for the SCR Project.
For the selection of the new fans, motors and drives, the owner’s team evaluated multiple options in terms of feasibility, evaluated costs (net present value of capital and life cycle costs, including auxiliary power and maintenance), space requirements, outage impacts, flexibility for low load operation, boiler transient impacts and constructability. The fan options that were evaluated included:
- Centrifugal vs. axial fans
- Existing fluid coupling drive vs. variable frequency drives for the centrifugal fan option
- The addition of two new 50 percent capacity fans sized only for the new SCR system (with two future booster fans installed at a later date) vs. two new 50 percent capacity fans sized for both the SCR system and future AQCS
Using preliminary fan sizing data, performance data and costs provided by the fan OEM, the owner’s team determined that the option for two 50 percent capacity centrifugal ID fans with variable frequency drives (VFDs) had the lowest evaluated price and the greatest certainty of meeting the RACT compliance goals. The addition of VFDs also met the station’s goals for efficient fan operation at all loads, improved soft start capability for the motors and the added protection against boiler transients with the SCR addition. A fan foundation strength test and constructability review confirmed that the existing fan and motor foundations could be reused with modifications for added mass for the proposed new, larger and heavier centrifugal fans and motors. Further, the team confirmed that the construction sequence of removing the existing fans and motors, modifying the existing foundations, assembling and commissioning the new fans, motors and associated tie-in ductwork could be completed during the allotted time for the fall 2012 outage.
Based on the study findings, the centrifugal fan and VFD configuration was procured, designed, installed and commissioned for the Edge 5 SCR Project. Since the fall 2012 outage, the new fans, motor and VFDs have operated reliably and the VFDs have allowed the Edge 5 Unit to maximize the efficiency of its fan operation despite the flow rates through the fans being reduced significantly from their design rating; due to air heater seal repair and redesign and other balance of plant modifications and repairs.
Challenge: ESP Reinforcement
|6 ESP Reinforcement|
|The majority of the reinforcement work consisted of adding 69,000 lbs. of reinforcing steel to the external stiffeners on the casing, hoppers and nozzles.|
With the addition of the SCR system to Unit 5, the operating and transient pressure conditions through the flue gas draft system were significantly altered, and the potential for implosion and structural damage to some existing equipment increased unless modifications were made. For the Edge 5 Project, the pressure drop across the SCR system was in the range of 8 to 9 inches WC at the maximum unit capacity conditions. As a result, the equipment downstream of the SCR system would be exposed to greater negative steady-state operating pressures that had the potential to exceed the original equipment design pressures.
To prudently reduce these operating risks, WPL implemented a boiler implosion mitigation program consisting of the following steps:
- NFPA 85 compliance study;
- Boiler implosion and controllability study;
- Review of the structural design ratings of the existing draft components and ductwork and comparison to the post-SCR steady state operating pressure profile through the system;
- Engineering analysis of equipment and ductwork structural framings requiring reinforcement;
- Final design and installation of structural reinforcement modifications.
An engineering firm specializing in furnace draft analysis performed a review of the station’s compliance to the NFPA 85, the Boiler and Combustions Systems Hazard Code for the post-SCR operating condition. To ensure that the planned post-SCR draft system modifications did not threaten the structural integrity of the system, a dynamic simulation of the draft system was performed. The boiler implosion and controllability study involved modeling the entire draft system, in its existing state and a post-retrofit state, for multiple postulated operating scenarios. The scenarios evaluated for the Edge5 SCR retrofit included multiple credible cases, including master fuel trips (MFTs), forced draft and ID fan trips with and without a MFT, and runaway ID fans, from minimum load to maximum load. In parallel with these activities, the original plant records and OEM data for the air preheater, electrostatic precipitator (ESP) and the interconnecting ductwork were reviewed to determine the equipment minimum structural pressure ratings at design and yield. These existing structural ratings were compared to the new steady state draft system pressure profile and pressure transients for the post-SCR configuration. Based on the findings of these studies and investigations, the following mitigation steps were implemented:
- Three additional pressure transmitters were installed ahead of each upgraded ID fan along with an override control loop to limit pressure at that location.
- With the exception of the ESP, all of the existing draft system was designed adequately to withstand the lower negative steady state and transient pressure conditions for the post-SCR configuration. The ESP was originally designed for a negative pressure rating of -30 inches WC With the SCR system in operation, the operating pressure through the ESP was predicted to be -37 inches WC at the maximum unit capacity condition. Thus, structural reinforcement of the ESP casing, hoppers and outlet nozzles was required.
To complete the structural reinforcement, the ESP OEM was retained to perform the engineering analysis of the structural framing, prepare construction drawings and supply all reinforcement materials. The majority of the reinforcement work consisted of adding 69,000 lbs. of reinforcing steel to the external stiffeners on the casing, hoppers and nozzles.
Although the structural reinforcement and other control system modifications were successfully completed such that future potential implosion risks were avoided, this activity highlighted some important lessons for future similar work for WPL projects. The boiler implosion mitigation program was completed over a two-year period with the final installation during the Fall 2012 outage. Ideally, the installation should have been completed earlier to minimize schedule risk and avoid a congested work site during the final Edge 5 SCR tie-in outage. Based on these experiences, it is recommended that boiler implosion and controllability studies be started as early as possible after a project’s Final Notice to Proceed and that workaround plans be considered to complete as much of the modeling work as possible in parallel with final AQCS engineering and procurement activities. This approach will allow greater flexibility and efficiency in scheduling the structural reinforcement work with a utility’s outage schedules. Also, when performing draft system reinforcements required by AQCS retrofits, the conceptual and detailed engineering should be performed by qualified contractors, and preferably OEMs, with experience in these types of modifications.
Challenge: Startup & Commissioning
The Edgewater SCR project included some unique challenges while planning and executing the startup and commissioning phase of work. Challenges included the initial commissioning scope planning, defining roles and responsibilities, integration of the startup and commissioning schedule into the project schedule, coordination of the final tie-in outage, and the boiler startup with the SCR and new ID fans.
The SCR project was executed with two main contractors: an OEM with the scope of engineering and equipment procurement, and a constructor with scope of SCR erection. The startup and commissioning scope of work was defined during the bid phase to be collaborative between the OEM, constructor and owner. Because of this arrangement, the owner took the lead in organizing the startup and commissioning execution.
A third party was utilized to coordinate and execute the startup and commissioning phase of the project. WPL evaluated several companies with commissioning capabilities, and selected a reputable company based upon previous project experience and a strong reference project with a similar AQCS retrofit startup and commissioning scope. The start up and commissioning company was selected in late 2011, when engineering was approximately 90 percent complete and construction was approximately 35 percent complete. Their role was defined as having complete responsibility for startup and commissioning, including schedule development, definition of commissioning packages (the entire SCR project was divided into 14 commissioning packages), coordination of system turnovers (both from construction to commissioning, and from commissioning to owner), coordination with OEM of vendor technical startup assistance, execution of system checkout, documentation of all commissioning activities, creation of commissioning turnover books, and startup support.
An integrated team approach to commissioning was encouraged by WPL in order to draw upon the strengths of all parties involved. Examples of the team approach include the development of the startup and commissioning schedule with significant input from both the OEM and constructor, WPL assigning a dedicated plant operator to the project for commissioning support, integrated commissioning meetings, and the use of dedicated craft from the constructor to assist with commissioning activities.
Because the commissioning team was added mid-project, defining commissioning packages and integrating the commissioning schedule with the entire project schedule was a challenge. It would have been easier to integrate the commissioning scope if it were done at the same time the construction schedule was developed. The OEM had already developed a high-level commissioning schedule, which was used by the start up and commissioning company as a guideline for sequencing activities. The constructor was flexible in working with the project team to commit to turnover dates for each of the 14 commissioning packages. In general, a project team objective was to commission as many of the 14 commissioning packages as possible prior to the start of the tie-in outage. Approximately 8 of the 14 systems were completely commissioned prior to the tie-in outage. The remaining packages were partially commissioned to the greatest extent possible. The commissioning team coordinated critical checkout activities to occur when construction craft was not in the area – this included some commissioning during weekends and other off-shift hours.
The final tie-in outage required careful coordination by WPL. Major scopes of work included duct demolition and installation for the SCR tie in, ID fan rotor, motor and lube oil system replacement, precipitator reinforcement, and plant maintenance activities. The tie-in outage was an 8 week outage that occurred in September-November 2012. The constructor initially expressed concerns to WPL about the ability to attract enough labor to complete all work within the 8 week outage period but had little problem fulfilling the positions necessary to complete the work during this prescribed outage window. The SCR project reached a peak labor force of approximately 250 people. In order to coordinate outage work, WPL dedicated an outage planning resource to monitor the entire outage, and integrated the SCR construction planning with the plant’s outage planning. Multiple planning meetings were held prior to the outage in order to define and “freeze” scope, and integrate schedules. Outage coordination meetings were held 3 times per week to review each scope of work and update the outage schedule.
Receiving accurate outage schedule updates from the contractor was challenging in the first half of the outage. It was clear the constructor was ahead of schedule, but the constructor would not show this progress and early finish dates due to fear that the finish date may move back out if progress slowed. Understanding the constructor’s completion date was critical to WPL in order to schedule ID fan startup activities and coordinate with generation dispatch for timing to bring the unit back online. WPL used prior outage experience with the constructor and direct observation of the constructor’s progress to accurately estimate the outage completion date.
Startup and commissioning of the SCR system concluded with the tuning of the ID fans and injection of ammonia. The upgraded ID fans with VFD control required remarkably little tuning during boiler first-fire. The VFD control of the fans was smooth, requiring only minor adjustment to control loop response timing. Boiler draft control logic remained similar to pre-VFD and SCR installation, which allowed for an easy transition to operations. The startup and commissioning team remained on-site for the first few days of fan operation and were then released. First ammonia injection occurred in the second week of post-outage operation. Tuning and balancing of the ammonia injection system was accomplished by the OEM. Minor control loop tuning for the ammonia control valves also occurred at this time. WPL learned that it would have been beneficial to have kept some of the startup team onsite for the first few weeks of ammonia injection, because some startup knowledge/continuity was lost when letting the OEM take the lead in tuning the ammonia system. Startup representatives from the OEM remained onsite after SCR tuning until the performance testing was completed.
POST INsTALLATION OPERATION
After successful startup and tuning of the SCR system, performance tests were conducted to verify the actual performance of the SCR met or exceeded the guarantee values. A third-party test company was hired to conduct the following performance tests: Outlet NOx emission; Average ammonia slip; SO2 to SO3 conversion; Ammonia consumption; Power consumption
Each parameter was tested with the boiler at full load, and three tests for each parameter were conducted. The results were averaged and compared to the performance guarantees using contractual correction curves to account for actual boiler operation. In all cases, actual test data were below the guarantee values.
After performance testing was complete, the SCR was subjected to a 60-day system availability test. During this test, the boiler was allowed to dispatch as needed, and the SCR system was left in “auto” to follow boiler load. In order to pass the 60-day test, the SCR system had to meet or exceed the guaranteed NOx reduction over a 30-day rolling average. Data was collected from the Edge 5 Continuous Emissions Monitoring System (CEMS) to verify SCR operation. As with the prior performance tests, the SCR system passed the 60-day availability test with no issues.
The SCR system has been operated continuously since it was placed in service in December, 2012. After testing was completed, WPL has maintained operation of the SCR in order to reduce NOx below levels required by the phase II RACT rules. Figure 7 shows NOx levels pre-SCR and post-SCR.
The Edgewater SCR project team faced significant challenges in designing an SCR system that would preserve the operational flexibility of the existing unit after the SCR was installed. By identifying the criteria that would preserve this flexibility during the design phase and carrying it over to the construction phase, the project team was successful in achieving this objective. Involvement of plant operations during the early design phases of the project, integrating WPL’s project team members with team members from the OEM, Constructor and startup and commissioning team along with the support of the Owner’s Engineer were key elements in the successful achievement of the project scope in time for compliance and within budget.
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