Catalyst technology can make clean coal plants look as clean as they are.
By Scot Pritchard, Cormetech, Inc.
Over the past 30 years, emissions technology has vastly improved coal-fired power plant emissions in terms of their impact on air quality. Now, it can help improve stack emission visibility as well. Although such improvements are primarily visual, their significance is immense.
Controlling sulfur trioxide (SO3) produced and emitted from coal-fired boilers represents a growing area of interest for the North American power industry. In addition to a significant increase in the use of flue gas desulfurization (FGD) systems on coal-fired boilers, the use of eastern bituminous coal with high sulfur content (three percent or greater) is also expected to increase, as will blending coal with petcoke. When SO3 condenses to form sulfuric acid (H2SO4), the resulting power plant stack emissions can affect plume opacity and acid deposition. Typically, significant portions of the H2SO4 will be removed from the flue gas in the air pre-heater (APH), electrostatic precipitator (ESP) and FGD. However, the portion that remains in an aerosol form will not be captured, potentially creating plume opacity problems.
In addition to affecting stack plume opacity, higher levels of SO3 in the flue gas can lead to corrosion of downstream equipment, an increase in particulate matter emissions due to the condensable component and an increased sensitivity to ammonium bisulfate formation in the APH. For all these reasons, SO3 mitigation is a growing need among power plant owners and operators. This article examines the need and available methods for SO3 control with a specific focus on a catalyst technology developed by Cormetech. The article also presents the results of commercial operating experience.
High NOx Performance and Low SO2 Conversion
Higher SO3 emissions are typically observed at plants firing high sulfur coal or coal/petcoke blends and equipped with selective catalytic reduction (SCR) systems and wet scrubbers. Typically, a fraction of sulfur dioxide in the flue gas (around one percent) is converted to SO3 in the furnace. With the addition of petcoke this level may increase due to the influence of vanadium. The deployment of SCR systems for NOx emission control increases the concentration of SO3 ahead of the APH through an undesired side reaction that oxidizes SO2. The extent of this oxidation depends on SCR design conditions, operating parameters catalyst formulation and structure.
A major challenge facing power plant operators today is maintaining high NOx reduction levels and low ammonia slip while yielding low conversion of SO2 to SO3. In the past, minimizing SO2 conversion while maintaining high NOx reduction levels were in conflict, and in some cases could not be achieved. Therefore, catalyst manufacturers such as Cormetech focused on developing additional catalyst product features to minimize SO2 conversion while maintaining high catalyst activity to yield high NOx reduction with low ammonia slip.
The product developed by Cormetech can, in some cases, achieve less than 0.1 percent SO2 oxidation while maintaining all other key product performance and durability features. Use of the advanced SCR catalyst product may be exclusive, or combined with other SO3 mitigation techniques including fuel switching, in-furnace mitigation with reagent, and pre/post APH mitigation with reagent.
Background and Theory
Most SCR systems in the United States were designed with an SO2 conversion rate in the range of 0.75 percent to two percent, or higher for some PRB applications. The target objective for catalyst advancements was determined based on the need for effective SO3 mitigation (conversion rates of 0.1 – 0.5 percent) while maintaining high NOx removal (90 percent and above) and low ammonia slip (less than two parts per million-ppm). As a result, any new catalyst design had to be optimized to reduce volume, pressure drop and SO2 oxidation rate. Based on extensive experience and understanding of various cell structure products, reaction mechanisms, materials and extrusion techniques, Cormetech undertook a series of due-diligence and product qualification initiatives in the laboratory and the field. The goal was to extend and develop catalyst performance features that can be incorporated into new or layered addition/replacement designs. The unique capability to extrude high open area products allows Cormetech to take advantage of the inherent reaction mechanisms that achieve low SO2 conversion and excellent NOx activity while maintaining the hardness, mechanical strength, durability and poison resistance of a conventional catalyst.
Prior to direct focus on reduced SO2 conversion, the inherent catalyst features were conventionally applied to achieve higher catalyst activity with moderate SO2 conversion. The SO2 conversion process for coal application catalysts is reaction rate-limited, while the DeNOx reaction is diffusion-limited. In other words, the DeNox reaction is fast and occurs mostly on the surface while the SO2 oxidation reaction is slow and utilizes the catalyst’s entire body. By understanding these reaction mechanisms, principles of catalyst deactivation, product application and manufacturing principles, the new catalyst product features increases and/or maintains the inherent DeNox activity while reducing SO2 conversion by optimizing the amount of catalytic material available for both reactions. These products have been successfully applied to more than 40,000 MW of coal-fired applications and continue to be extended to achieve the lower SO2 conversion.
Product Qualification & Results
The product evaluation and qualification process for the new product features included laboratory and field testing for:
- Mechanical durability (that is, abrasion resistance and hardness, strength and washability).
- Chemical durability (that is, poison resistance and operating life).
- Overall performance comparison with other lower open area products.
As with the basic principles governing the SO2 conversion and DeNox activity, the principles of abrasion are also critical to understand. The abrasion phenomenon is governed by the following equation and is inversely related to material hardness:
Key to the successful development of the new product features was to assess the potential abrasion risk. Catalyst abrasion analysis validated the theory that no increased risk of abrasion existed with the low SO2 conversion catalysts.
During normal operations observation showed that primary particle impingement occurs at the face of the catalyst, not on the catalyst walls. This is primarily because “entrance effects” cause more abrasion at the face due to flow contraction into the cells. Following the “entrance effect,” the laminar boundary layer starts to form immediately. This results in low gas velocity and particle mass flow at the cell wall. This is also demonstrated by historical experience that shows effectively no change in wall thickness within the catalyst channel from front-to-rear of the catalyst element (Figure 1).
The risk of abrasion along the catalyst wall depends on the catalyst material’s hardness. The abrasion rate is inversely related to the hardness. At high velocities risk of abrasion along the catalyst wall is less for harder products. Cormetech’s low SO2 conversion catalyst products have incorporated materials technology to ensure abrasion resistance and washability without sacrificing poison resistance and represent one of the most durable products in the industry. Additionally, a reinforced cellular structure, material additives and leading edge hardening are all used to achieve required strength and abrasion resistance properties.
The assessment that the new catalyst shows no abrasion issues has been confirmed over more than five years of operational experience. In addition, even at units with large particle ash (LPA) plugging problems that were used to represent an accelerated abrasion test, results were equal to, or better than, low open area honeycomb products, corrugated or plate type products.
The lower SO2 conversion achieved by the new catalyst compared with a conventional catalyst is due to greater open area, thinner wall thickness and improved composition and geometry for strength. The new design is optimized to reduce volume, pressure drop and SO2 oxidation. Table 1 outlines some of the performance characteristics of the new product in comparison to the conventional catalyst in terms of relative catalyst volume, pressure drop and SO2 oxidation. The two cases illustrate the alternative methods for utilizing the features of the high open area product.
Initial installations of the low SO2 conversion catalysts at full reactors with less than 0.5 percent SO2 conversion were designed in 2002 and installed in 2003. They continue to operate successfully. Subsequent individual layer additions of the low SO2 conversion catalyst have been designed and installed with conversion rates of less than 0.1 percent. Table 2 compares the design considerations and performance test results for the low SO2 conversion catalyst at two recent installations.
The need for cost effective SO3 mitigation is clear and immediate. Advanced high performance catalysts with low SO2 conversion and high NOx reduction are now available. They can be used in combination with other SO3 mitigation strategies. They can progressively achieve less than 0.1 percent conversion, are applicable to new and existing SCRs and provide advanced catalyst management solutions as layer additions and replacements. Additional beneficial features include reduced pressure loss and lower weight. Future advancements for more advanced catalyst products for further reduction of the SO3 impact and improved NOx reduction are in development.
Scot Pritchard is vice president of sales and marketing at Cormetech, Inc., a supplier of SCR catalysts and catalyst management services.