August 1, 2010

Advanced SCR Catalysts Tune Oxidized Mercury Removal

By Scot Pritchard, Cormetech Inc.; Masashi Kiyosawa and Katsumi Nochi, Mitsubishi Heavy Industries, Hiroshima, Japan

Performance Modeling

Information gathered through a combination of significant laboratory testing and field data analysis over the past decade has resulted in the development of highly predictive performance models. These models provide the opportunity to prepare a parametric performance assessment to determine the best methods to achieve desired performance levels from the SCR. In order to set the desired performance target from the SCR, a full system evaluation must be considered. Typical questions that make up the model input include these:

• What is the overall system goal?
• What are the fuels and fuel blends to be considered?
• What are the operating conditions and other key performance requirements of the SCR, such as NO_x reduction and SO₂ conversion?
• What level of mercury oxidation should be expected through the air preheater?
• Will the dust collection system contribute to mercury reduction performance?
• Will fuel or postcombustion additives be utilized?
• What collection efficiency should be expected from the FGD system, including consideration of re-emission control (conversion back to elemental mercury)?
• What are the economic considerations, including, but not limited to, cost of additives, cost of catalyst, pressure loss, impact on ash sales, boiler corrosion, and material-handling equipment?

Once requirements are understood, a series of “what-if” scenarios can be run and supplemented with additional testing as needed. The net result will allow the user to assess the best method(s) to achieve the desired performance requirements at the lowest total ownership cost.

Catalyst Advancements for Increased Mercury Oxidation

We have presented many of the factors that affect the rate and amount of mercury oxidation in an SCR catalyst. As discussed, two of the larger influences are halogen content and temperature.

Mitsubishi Heavy Industries and Cormetech have developed an advanced catalyst tailored to oxidize mercury to high levels under challenging conditions involving high temperatures and low HCl concentrations, both of which negatively impact mercury oxidation across conventional SCR catalysts.

Let’s now examine the performance differences between advanced Hg oxidation SCR catalysts and standard SCR catalyst products with respect to halogen content and temperatures.

Aged Catalyst Performance. Figure 3 shows an example of actual field performance versus predictive models for an aged SCR catalyst at varying levels of HCl in the flue gas. At a temperature of approximately 690F, the percentage of oxidized mercury at the SCR outlet measured during actual tests on aged catalyst ranged from 75% to 90% over a range in flue gas HCl concentration of 10 to 25 ppmvd. The predicted levels of oxidized mercury increase to >95% levels at higher levels of HCl.

3. Mercury rising. Oxidized mercury levels increase with rising HCl concentrations in the flue gas. Data were taken from a large utility boiler operating at full load with 12,000 operating hours on the SCR. Source: Cormetech Inc.

Advanced Catalyst Performance. Figure 4 illustrates the performance of the advanced Hg oxidation catalyst versus standard catalyst at extremely low HCl concentrations and moderate flue gas temperatures (5 ppmvd and 700F). The advanced catalyst exhibited high levels of oxidized mercury for both fresh and aged catalyst and shows a substantial increase over an existing standard catalyst.

4. Low HCl operation. Advanced SCR catalysts demonstrate better mercury oxidizing levels over time. Data were taken at 5 ppm HCl and 700F. Source: Cormetech Inc.

In Figure 5, the percentage of oxidized mercury is shown, as expected, to be greater due to the higher HCl concentration, despite a higher temperature (65 ppmvd and 757F).

5. High HCl operation. Advanced SCR catalysts, at high levels of HCl (65 ppm) in the flue gas, also exhibit improved mercury oxidizing potential over time. Data were taken at 757F. Source: Cormetech Inc.

Planning for the Future

It’s reasonable to assume that the performance of mercury reduction technologies and other co-benefits, such as we’ve discussed here, will be used to set future maximum achieveable control technology regulations. Regardless of future limits, we suggest that you adopt a holistic approach for optimizing mercury reduction that considers all the components in your air quality control system, from the coal pile through the stack. Only then will you meet your goal of maximizing mercury removal while minimizing overall system and operating costs.