February 15, 2006

Long-term catalyst health care

Scot Pritchard, Cormetech Inc.

Catalyst management refers to 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. Effective catalyst management requires a long-term plan for maximizing performance based on projected plant outage schedules, future emissions regulations, available control technologies, and plant improvements. Catalyst management strategies require the evaluation of numerous issues—boiler/SCR system operations, the plant's and fleet's emissions-reduction strategy, fuel management, the cost of NO_x credits, outage demands, SO₃ emissions levels, mercury oxidation, capital budget considerations, and more—with the goal of achieving an optimal solution (Figure 1).

 

 
 Source: Cormetech Inc.

1. Many considerations. An optimized catalyst management process takes into account more variables than you might expect.

 

The process can be broken down into specific steps, facilitating its management by the plant owner/operator (Figure 2). Each step must be considered and planned for before determining an optimal solution for the site-specific situation.

Source: Cormetech Inc.

2. Steps in catalyst management. Catalyst management involves optimizing a wide range of parameters.

 

Start with an inspection

To optimize the performance of its SCR system, the catalyst's formulation, structure, and volume are designed to take into account key parameters including these:

• Plant performance factors (such as fuel and combustion type, slagging rates, boiler performance, air heater design, and particulate collection devices).
• Operating conditions (such as flue gas flow rate, inlet NO_x levels, temperature, and the impact of O₂ and water content on catalytic potential).
• System scale-up factors (such as non-ideal flow distribution, temperature distribution, ammonia-to-NO_x molar ratio distribution, and catalyst blockage).

If any of the parameters listed above varies from the initial catalyst design conditions, the life of the catalyst may be shortened or extended. The former would be the case if, for example, an SCR system designed to achieve a certain level of NO_x reduction at a given ammonia slip is asked to achieve a higher level of NO_x reduction. Conversely, a lower reduction level or a higher allowable ammonia slip would effectively increase the catalyst's useful life.

SCR system inspections and evaluations—which should be performed at least once a year—should include physical inspections of the catalyst, reactor, and ammonia injection system. Field performance data, including inlet NO_x levels, removal efficiency, ammonia-in-ash levels, etc. (as well as operational data such as the number of system starts/stops, operational load range, total hours, fuel data, and ash data) must be collected and compiled to enable proper evaluation of results. Samples of the catalyst should also be taken to assess its catalytic potential versus both initial design parameter expectations and actual "as-fired" conditions.

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