Scot Pritchard, Cormetech Inc, USA
The key to successfully operating any selective catalytic reduction (SCR) system is the proper management of the catalyst itself. Power plants have to consider a comprehensive catalyst management strategy that takes into account a wide range of both traditional and new considerations to achieve peak catalyst performance.
A typical multilayer SCR reactor for a coal fired applicationSCR Catalyst Management
The issues include: projected plant outage schedules and demands, boiler/SCR operations assessment, system inspections and field sample data analysis, fuel management and flexibility, nitrogen oxides (NOx) performance and sulphur dioxide (SO2) conversion objectives, catalyst technology advancements, and integration with multi-pollutant control programmes.
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 (Figure 1).
Figure 1: Catalyst managemment plan – two initial plus one spare layer
The key to managing catalysts effectively is to determine an optimum plan for catalyst replacement or addition. Traditional tools are available to SCR operators 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 and financial factors, and analysis of recent catalyst technology advancements. In addition, new variables such as low-load operation flexibility, mercury (Hg) oxidation and lower sulphur trioxide (SO3) emissions are becoming critical considerations.
SCR system inspections and evaluations 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 ammonia (NH3) injection system. During a physical inspection of the catalyst, performance and operational data such as NOx levels, removal efficiencies and 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 a 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 the 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 impact catalyst life. The nature of fuel 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 sulphur coals, catalyst SO2 oxidation should be reduced to less than one per cent to minimize the impact of elevated levels of SO3 on both the air heater and stack opacity. Depending on the boiler exit SO3 content and sensitivity of the particulate collection device to ash resistivity, SCR catalysts for low-sulphur coal applications can be designed for an SO2 oxidation rate of less than one per cent without negatively impacting on plant operations.
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 Design Criteria
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 maldistribution 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 air preheater fouling.
Proper tuning can also provide better system efficiency and/or improved catalyst life. Figure 2 shows the impact of NH3/NOx distribution on NOx removal efficiency and ammonia slip. A wider distribution will make it more difficult to achieve the desired level of NOx reduction within the acceptable range of ammonia slip.
Figure 2: NH3/NOx distribution analysis versus performance
Source: Cormetech Inc
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 can be considered for each application, and 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 the centre-line of one wall to the centre-line of an adjacent wall. In addition to pitch, catalyst total open area should also be considered when assessing a given product for an application.
In general, a 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 tends to be dictated by limitations in upstream solutions, such as excessive screen erosion because of the design of existing ductwork.
New Versus Regenerated Catalyst
One of the key considerations when choosing 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 can 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 in a catalyst management strategy generally results in the most economic option for utilities because of 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. Regeneration/cleaning however, should be part of normal catalyst management planning.
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: consideration of warranties related to the level of activity recovery (physical and/or chemical; impact on SO2 conversion versus prior actual levels; impact on mercury oxidation performance; direct cost; and outage planning (duration and frequency).
During the decision process phase of a catalyst management programme it is also vital to examine advancements in available catalyst technology 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 per cent 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 in the US, which represented a unique challenge for catalyst management. The units were built without an SCR bypass, which 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. With potential future use and resulting catalyst management cost savings in mind, 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.
A number of options were investigated ranging from in-kind replacement to in-situ washing, external washing and off-site regeneration. The final solution 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 the 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 a honeycomb catalyst, and can be extended to the replacement of low surface area plate or corrugated SCR catalysts. Further, this 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 turnaround, there are construction considerations or limited availability of cranes and other services needed for a traditional extract and install plan, there is limited access to the reactor or the utility wants to use existing entry doors.
Outage Alignment & Economic Impacts
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 is likely to 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 workspace, 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, considerations will include whether the assessment is for the short-term or long-term, the value of credits, cost of NH3, cost of SO3 mitigation and the unit’s capacity factor.
Recent changes in operating practices, fuel management and regulations have also created additional 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 is 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, and therefore 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 that 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 in order to maintain temperature after analyzing the associated capital, operating and maintenance costs. Both of these however, are sub-optimal solutions.
Based on an identified market need, Cormetech has focused its research on a phenomenon that allows the formation of ABS in the catalyst, and has 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 some 12 units operated by several utilities and using various fuels all resulting in direct cost savings to plant owners.
SCR catalyst will oxidize Hg to varying degrees based on a wide variety of inputs including catalyst type, NH3 concentration, hydrochloric acid concentration and temperature. The basic reaction process is outlined in Figure 3. Using the SCR in combination with a wet flue gas desulfurization system can be an effective way to meet Hg emission limits.
Figure 3: General SCR chemistry schematic depicting Hg oxidation
Ongoing research at Cormetech with industry partners 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 4 presents an example of actual field performance versus predictive models for aged SCR catalyst.
Figure 4: Predicted versus actual SCR Hg emissions large utility unit at full load after several seasons
One challenge in developing a catalyst management programme is how 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 NH3 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 per cent 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.
The superior performance of the low SO2 conversion catalyst relative to a conventional catalyst is because of a greater open area and improved composition and geometry for strength. This design is optimized to reduce volume, pressure drop and SO2 oxidation.
Table 1 outlines some of the performance characteristics of the advanced product in comparison to the conventional catalyst in terms of relative catalyst volume, pressure drop and SO2 oxidation. Two cases are shown to illustrate the alternative methods for utilizing the features of the high open area product. This advanced product has been used in more than 10 GW of SCR applications as either the initial charge or as part of an optimized catalyst management approach.
Catalyst management planning can be described as simple yet complicated. The landscape of the decision process is complicated by 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.
Scot Pritchard is vice president, Sales and Marketing, Cormetech Inc, USA, which produces titania-based ceramic honeycomb catalyst for SCR systems. For more information visit www.cormetech.com.