February 15, 2006

Long-term catalyst health care

Scot Pritchard, Cormetech Inc.

Analyze the data

Variations in fuel characteristics can have a considerable impact on catalyst life. The nature of the fuel fired (be it Powder River Basin or eastern bituminous coal, petcoke, or blends of both) will dictate in large part the rate of catalyst deactivation. Historical data and know-how are key to an accurate assessment of what to expect in the future. Analytic tools such as the one shown in Figure 3 are invaluable in assessing a fuel's impact on a catalyst's long-term effectiveness.

Ways to measure catalytic activity/potential

Source: Cormetech Inc.

3. FIELD guide. This is an example of a fuel impact evaluation and life determination (FIELD) guide for SCR catalysts.

 

Once catalyst testing and data evaluation are complete, the catalyst's activity potential is assessed in combination with field data and the future operating considerations described in the previous section; the goal here is to determine the available catalyst-management options. Figure 4 is an example of a catalyst management plan that can take into account a wide range of influences on catalyst management decisions.

4. Have a plan. This catalyst management plan shows governing parameters.Source: Cormetec Inc

Generally, there are three catalyst-management options that may be used alone or in combination: system optimization, catalyst addition/replacement, and catalyst rejuvenation.

System optimization

The purpose of ammonia injection grid (AIG) tuning is to ensure an even ammonia-NO_x molar ratio distribution to the catalyst. Contrary to popular belief, the goal of doing so is not necessarily to get the outlet NO_x profile flat; this is possible only when the inlet profile is flat. If the inlet profile is skewed, there are generally two options:

• Tune the AIG to match the inlet distribution over the load range. In this case, the outlet NO_x profile will not necessarily be flat.
• Attempt to flatten the distribution through mixing technology.

The level of AIG tuning will depend upon the system design and operational methodology. Proper AIG tuning/NH₃:NO_x distribution ensures maximum system efficiency and reduced ammonia slip and can effectively extend catalyst life. The process also eliminates localized regions of high ammonia slip, which can lead to air preheater fouling when combined with SO₃.

Specific tests can be conducted to facilitate tuning the AIG until it is operating at peak performance. Periodic checking and optimization of the ammonia-to-NO_x distribution are essential for ensuring the long-term optimal performance of an SCR system. The frequency of tuning is dictated by unit-specific conditions, including NO_x distribution from the boiler and the style of AIG/mixer system in use. In general, it is worthwhile to check the distribution annually. Figure 5 shows a multipoint sampling system capable of efficiently and accurately performing AIG tuning.

5. Tune-up tool. This multipoint automated sampling system (MASS) is used for ammonia injection grid tuning. Source: Clean Air Engineering

Flow modeling and catalyst operating limitations are also important aspects in the initial design of an SCR system. Over time, these considerations may influence the performance life, range of operation, and effective cost of the system. Specific areas where additional testing and/or evaluation may be warranted for system optimization include:

• Testing for expansion of operating temperature range to allow use of the SCR at lower loads, thus avoiding the cost associated with "lost" NO_x reduction.
• Flow modeling for evaluation of LPA (large-particle ash)-mitigation methods.
• Flow modeling for AIG/mixing optimization.