Power Engineering - Comprehensive Catalyst Management

The U.S. power industry is fast approaching the time when significant catalyst management decisions must be made to maintain performance and comply with new regulatory challenges. And the key to successfully operating a selective catalytic reduction (SCR) system is proper management of the catalyst itself.

Many SCR systems in the United States are now approaching their sixth or seventh year of operation. Furthermore, starting in January 2009 SCR systems will be operated year-round to meet the new Clean Air Interstate Rule (CAIR) standards. As a result, a number of power plants that were historically operating their SCR systems for five months a year will now be operational for a full 12 months. This means they will operate more than 8,000 hours a year instead of about 4,000 hours as they have historically.

This change in SCR operations means that power plants have to consider a comprehensive catalyst management strategy that accounts for a wide range of old and new issues to achieve peak performance. The issues include:

Catalyst life management is a comprehensive methodology for predicting when catalyst layers should be replaced or regenerated, or a new layer added, based on catalyst deactivation rates, performance requirements and system capabilities (see Figure 1).

The key to managing catalysts effectively is to determine an optimum plan for catalyst replacement or addition. Traditional tools are available to the customer for developing an effective catalyst management strategy. They include performance audits that analyze the remaining potential of the catalyst with plant operating history, projected use of the SCR, fuels used, the position of catalyst layers, outage schedules, economic/financial factors and analysis of recent catalyst technology advancements. Also becoming critical when developing a comprehensive catalyst management program are considerations such as low-load operation flexibility, mercury (Hg) oxidation and lower SO3 emissions.

SCR system inspections and evaluation are essential to a catalyst management strategy and should be performed at least once a year. This typically includes a physical inspection of the catalyst, reactor and NH3 injection system. During physical inspection of the catalyst, performance and operational data such as NOX levels, removal efficiencies and ammonia (NH3) in ash levels are analyzed. Also analyzed are operational load, fuel and ash data and samples of the catalyst. These performance tests audit the catalytic potential of an SCR catalyst by accurately comparing the catalytic potential of the field sample to a fresh catalyst in a laboratory environment. Such pilot sample tests determine the catalyst activity under specific conditions utilizing representative size catalyst samples for SO2/SO3 conversion rate, pressure drop (∆p), initial activity (K0) and actual activity (K). Physical tests evaluate physical properties such as catalyst surface area and porosity. The deactivation rate is determined by comparing the change in catalytic potential versus operating hours of the sample.

Variations in fuel used can significantly affect catalyst life. The nature of fuel-Powder River Basin, Eastern bituminous, petcoke or other blends-can dictate the rate of deactivation and impact on SO2 conversion over time. Physical and chemical properties of the coal can impact both the selection of pitch (cell opening) and the materials engineering of the catalyst. For high sulfur coals, catalyst SO2 oxidation should be minimized to less than 1 percent to minimize the impact of elevated levels of SO3 on both the air heater and stack opacity. Depending on the boiler exit, the content of SO3 and sensitivity of the particulate collection device to ash resistively, the catalyst can be designed to have an SO2 oxidation rate of more than 1 percent for low sulfur coals without negative impact on plant operations.

Arsenic and calcium oxide (CaO) are the predominant SCR catalyst deactivation agents in U.S. coals. Their levels vary quite extensively among various coals. It is therefore important to understand the range of these constituents in a specific boiler because they determine the volume of catalyst needed. Tools such as Cormetech’s FIELD Guide can be used to quantify the effect on catalyst life under various fuel-firing scenarios and allow the evaluation of fuel savings versus total catalyst lifecycle costs.

SCR effectiveness depends on satisfying the design criteria of uniformity for velocity and NH3/NOX distribution at the catalyst inlet. Uniformity of the NH3 /NOX ratio can profoundly affect SCR performance for high efficiency systems, with misdistribution of NH3/NOX affecting not only NOX reduction, but also the extent of NH3 slip. Tuning of the ammonia injection grid (AIG) minimizes local high spots of ammonia slip, which can lead to potential airpreheater (APH) fouling. Proper tuning can also allow better system efficiency and/or improved catalyst life. Figure 2 shows the effect of a NH3/NOX distribution on NOX removal efficiency and ammonia slip. Wider distribution will make it more difficult to achieve the desired level of performance within the acceptable average and local ammonia slip.

Whether using a dense grid tunable AIG, or a low density grid with bulk mixing, specific tests are recommended for evaluating the NH3/NOX distribution over time to maximize catalyst performance.

Various SCR catalyst types may be considered for each application. They include honeycomb, plate and corrugated geometries. Generally honeycomb geometries provide the highest level of performance per unit volume.

Catalyst pitch is a description of the catalyst cell dimension defined from centerline to centerline of one wall to an adjacent wall. In addition to pitch, catalyst total open area should also be considered when assessing a given product for an application. Generally, larger pitch product is associated with ultra-high ash loading or as a means to help mitigate the impact of large particle ash (LPA). The decision to use catalyst pitch as a method for LPA mitigation is case-specific and generally dictated by limitations of upstream solutions, such as excessive screen erosion due to existing ductwork design.

One of the key considerations in determining an effective catalyst management strategy is to determine when a catalyst should be replaced or new catalyst added. As stated earlier, catalyst performance audits are integral to this determination since they provide useful information about the performance potential of the catalyst. Because catalyst layers will deactivate at different rates depending upon the fuel fired, the performance audit can also identify which layer should be addressed first. This information might also be potentially applied to other units as part of a fleet-wide catalyst management approach.

In most cases, SCR catalyst reactors are built to accommodate at least one spare layer of catalyst. Using this layer as the first step of catalyst management generally results in the best economic option for utilities due to the significant life benefit provided versus removing and regenerating a layer. Therefore, for most utilities, catalyst additions may be the first option in improving catalyst performance, followed by replacements or regeneration. In all cases, the regeneration process should be pre-qualified on samples. Ideally this process should occur before the evaluated cost assessment of various catalyst management plan options. The evaluated cost assessment may include the following aspects to varying degrees:

During the decision process phase of a catalyst management program, it is also vital to examine advancements in catalyst technology available because it might be possible to improve performance substantially. Such a case might be where units either have a large pitch (low geometric surface area) or a short catalyst length. Moreover, advancements in catalyst technology may provide additional benefits. They could include generating NOX credits, and in some cases, meeting a mercury emissions reduction goal that may compensate for the cost associated with an SCR catalyst action during a planned outage.

Making the transition to an advanced alternate product can be done in a staged and qualified manner to minimize any risk of a sudden change in the catalyst. Moreover, the performance benefit of the advanced catalyst may actually be more cost-effective. In either case, evaluating catalyst advancements should be an integral part of the SCR catalyst management planning process. In some cases, advancements may provide a better than 50 percent performance enhancement or cost reduction compared to installing a new layer of the original catalyst.

A case in point is the Tennessee Valley Authority (TVA) Allen plant, which represented a unique challenge for catalyst management. The units were built without an SCR bypass. That has forced Allen to operate, in effect, on a year-round basis for many years. Additional challenges stemmed from its two-layer reactor limitation.

The units started with an 8.2 mm pitch Cormetech honeycomb catalyst. Keeping in mind potential future use and resulting catalyst management cost savings, a 7 mm product was also qualified during the initial design and installation. In the second year of service, after approximately 16,000 hours of operation, catalyst life cycle planning was needed. Numerous options were investigated ranging from in-kind replacement to in-situ washing, external washing and off-site regeneration. The solution ultimately selected involved a patent-pending in-situ replacement of the 8.2 mm pitch product with an extension of the pre-qualified 7 mm pitch product, another patent-pending process. The same procedure has been successfully carried out on all three units over the past two years.

The in-situ catalyst replacement process consists of removing catalysts from modularized sections of catalyst layers within a reactor. It is performed without removing the steel frame modules that support the catalyst. The result is reduced time and costs compared to conventional replacement methods.

Prior to catalyst removal, a visual inspection or spot sampling is conducted in conjunction with chemical and physical performance tests to pre-qualify the process for a given unit. The in-situ method has been used to replace honeycomb and can be extended to perform replacement of low surface area plate or corrugated SCR catalysts. The in-situ method can be performed on an entire layer or partial layer of catalyst. On a case-specific basis the in-situ method can be a more economical and efficient approach compared to conventional methods of catalyst replacement. This would be the case when a facility’s key requirements are performance improvement by installing the latest catalyst technology; there is a compressed outage turn-around; there are construction considerations or limited availability of cranes and other services needed for a traditional extract and install plan; and there is limited access to the reactor or the utility wants to use existing entry doors.

Matching catalyst management activities to plant outage schedules will be challenging. It will be necessary to understand system capabilities of each unit or plant. Regional requirements will dictate the flexibility of the utility to optimize a catalyst management plan within a plant or system. For example, a unit that must meet a rate-based emission level will likely have less flexibility than a unit with a total period emission limit. Catalyst life extension through modified performance goals, and coordination with units within a utility fleet should be considered in each decision related to coordinating a catalyst action to a planned outage.

Each unit should be evaluated and considered in terms of its readiness to perform a catalyst action, be it layer addition or replacement. Items that must be considered include site readiness, site storage, reactor access, crane or hoist access and/or maintenance, handling tools and reactor structure evaluation. Additional considerations, for either on-site or off-site regeneration processes, include work space, permits, storage and shipping logistics.

No single catalyst management plan will fit all situations. Economic considerations will be utility and/or site-specific. They will include how the equipment is valued and its impact on other related cost/revenue operations. In addition to the basic catalyst capability assessment of catalyst potential (activity times surface area), considerations will include whether the assessment is for short or long term, the value of credits, cost of ammonia, cost of SO3 mitigation and the unit’s capacity factor.

Recent changes in operating practices, fuel management and regulations have also created new variables that must be considered in a catalyst management process. Topics of particular significance are the expansion of the allowable operating temperature range, Hg oxidation, and SO3 emission limits.

One of the primary limitations of SCR is operation under low load/low temperature scenarios due to the formation of ammonia bisulfate (ABS). The temperature at which ABS forms depends on the concentration of water vapor, SO3 and NH3. Under certain conditions, ABS will condense in the catalyst pores, masking active catalytic sites, thus limiting SCR catalyst potential to reduce NOX. The common techniques for limiting the low temperature operation of SCR catalysts include restricting the unit operation to a load which maintains SCR inlet temperature above ABS formation and installing a flue gas temperature control such as a burner, gas-side bypass or water-side bypass.

Until now, many units have installed economizer bypass systems to maintain SCR inlet temperature. Other units have increased the minimum load specification to maintain temperature after analyzing the associated capital, operating and maintenance costs. Both of these are sub-optimal solutions. Based on this market need, Cormetech research has focused on a phenomenon that allowed formation of ABS in the catalyst (see Figure 3) and assessed the resulting performance impacts and risks. The basic process includes an understanding of the catalyst performance as a function of its available catalyst pore structure, principles of pore condensation and boiler operational goals in terms of NOX reduction and time, as well as the limitations, including SO3 emissions, recovery temperature, ramp rate and used control logic options.

This advanced catalyst know-how has been successfully employed on about a dozen units operated by several utilities and using various fuels - all resulting in direct cost savings to plant owners.

SCR catalyst will oxidize mercury to varying degrees based on a wide variety of inputs including catalyst type, NH3 concentration, HCl concentration and temperature. The basic reaction process is described in Figure 4. Using the SCR in combination with a wet flue gas desulfurization (FGD) system can be an effective way to meet Hg emission limits. Ongoing research at Cormetech with industry partners, the Environmental Protection Agency and utilities over the last few years has demonstrated considerable advancements in predicting and guaranteeing the level of Hg oxidation through its SCR catalyst. Figure 5 shows an example of actual field performance versus predictive models for aged SCR catalyst.

One challenge in developing a catalyst management program is to achieve high levels of NOX reduction with low SO2 conversion. Historically, minimizing SO2 conversion while maintaining high levels of NOX reduction seemed to be conflicting goals and in some cases could not be achieved. Therefore, Cormetech focused on developing additional catalyst product features to minimize SO2 conversion while maintaining high catalyst activity and yielding high NOX reduction capability with low ammonia slip. The product extension utilizes advanced extrusion, product and materials know-how in combination with a well-proven product base. The performance enhancement can, in some cases, achieve less than 0.1 percent SO2 oxidation while maintaining all other key product performance and durability features. The advanced SCR product may be used exclusively or combined with other SO3 mitigation techniques including fuel switching, in-furnace mitigation with reagent, and pre/post APH mitigation with reagent.

Catalyst management planning is simple yet complicated. The landscape of the decision process is complicated with many nuanced challenges that are ever changing including outage planning, regulatory environment, fuels, performance goals, credit/trading markets, advanced product considerations, regeneration and plant load/cycling. It is critical to consider all these issues when deciding how and when to perform an effective catalyst management activity.

Author: Scot Pritchard is Vice President of Sales and Marketing of Cormetech, which produces titanium-based ceramic honeycomb catalyst for NOX emission control used in selective catalytic reduction systems.

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