An Experimental Program to Optimize SCR Catalyst 

Regeneration for Lower Oxidation of SO2 to SO3

Mike Cooper 

SCR-Tech, 11701 Mt. Holly Road, Charlotte, NC 28214 

E-mail:mcooper@CatalyticaEnergy.com Telephone: (650) 940-6383; Fax: (650) 968-5184 

Keith Harrison 

Southern Company Generation, 600 North 18th Street Box 2641, Birmingham, AL 35291 

E-mail: keharris@southernco.com Telephone: (205) 257-6832; Fax: (205) 992-0361 

Chao Lin 

American Electric Power, 1 Riverside Plaza, Columbus, OH 43215 

E-mail: cplin@aep.com Telephone: (614) 716-1274; Fax: (614) 716-3204 

Summary

Introduction 

SCR-Tech has regenerated more than 11,000 m3

 of SCR catalyst in the United States, all completed on currently-used 

catalyst types: corrugated, plate and honeycomb. The SCR catalyst regeneration technology employed by SCR-Tech was 

originally developed in Europe and further optimized in the United States. While these process optimization efforts focused 

on restoring the catalyst’s original activity level and NOx reduction potential, SCR-Tech has observed that the majority of 

catalyst regenerated in accordance with their process demonstrated SO2 conversion rates equal to or lower than those 

originally specified by the manufacturer for the fresh catalyst. SCR-Tech’s observations, combined with industry concern 

with controlling sulfuric acid emissions, prompted the company to initiate an experimental study jointly conducted with 

American Electric Power (AEP) and Southern Company to quantify the primary effects of five carefully selected independent 

factors in SCR-Tech’s catalyst regeneration process on the rate of SO2 oxidation. This presentation will summarize the 

results of this study. 

Catalyst regeneration at SCR-Tech is a multi-step process with each step involving treatment of the deactivated catalyst with 

a unique chemical solution. Following a rigorous mechanical cleaning to remove loose fly ash the overall regeneration 

process can be broken down into three phases: 

• Phase 1 – Remove species known to poison the catalytically active sites and/or block flue gas access to those sites. 

Examples of chemical poisons or pore blocking agents removed in phase 1 include arsenic, phosphorous, silica, 

aluminum oxide, calcium sulfate and alkali metals such as sodium and potassium. 

• Phase 2 – Neutralize and remove the chemicals used in phase one and remove any additional residual, soluble 

catalyst poisons and pore blocking agents leftover from initial treatments. 

• Phase 3 – Re-impregnate active catalytic species onto the support to return the catalyst to a performance level equal 

to or greater than the original specifications offered by the catalyst manufacturer. Phase three includes carefully 

defined heat-treatments that enable the control of both the distribution and dispersion of those active species. 

Experimental Approach 

SCR-Tech selected five treatment chemicals it believes are especially critical to the regeneration process, and then selected 

two concentration ranges for each of those chemical species that bracket the current operating concentration within the 

process. Catalyst type became another factor in the experimental design. Southern Company supplied deactivated honeycomb 

elements from one of its plants and AEP supplied deactivated plate catalyst from one of its facilities. 

 

To define baseline values for Kα, K23, and the concentration of surface and bulk deactivating species against which to 

compare the effects of SCR-Tech’s various regeneration treatment levels, a set of four plate and four honeycomb catalyst 

Proceedings of the

2006 Environmental Controls Conference

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U.S. Department of Energy

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National Energy Technology Laboratory
samples were submitted for testing and analysis. Mean and standard deviation for each of the parameters evaluated in the 

study were established from these baseline data, and the measured variances were used to determine exactly what level of 

difference between the treated samples and the baseline deactivated samples could be defined as statistically significant, and 

at what confidence level. 

Regenerations were run in small-scale equipment designed to treat one honeycomb element or one plate set at a time. These 

small-scale treatment tanks, were designed so that the solution volume to catalytic surface area ratio is equivalent to the ratio 

used in SCR-Tech’s commercial-scale process. Other factors, such as temperatures, flows and iron-to-catalyst surface ratios 

were also controlled to match the commercial-scale process. 

SO2 / SO3 Conversion Results 

Measured K23 on both the deactivated honeycomb and plate type catalyst was high enough to detect statistically significant 

differences for both populations due to the variation in the controlled regeneration process parameters. Baseline SO2

conversion rates on the deactivated honeycomb samples was 0.34 percent under the conditions of the test, while baseline SO2

conversion on the deactivated plate samples was 0.45 percent under the conditions established for those tests. 

After regeneration, the average K23 for all deactivated honeycomb samples was reduced to 0.19 percent, which was 

equivalent to the measured K23 on the fresh samples. There was sufficient resolution in the measured K23 values from the 

regenerated honeycomb samples for the team to develop a linear mathematical model for K23 as a function of all five process 

parameters which could account for approximately 94 percent of the total variability in the data. With this model it is possible 

to calculate the lowest achievable SO2 conversion rate after adjusting all process parameters for minimal K23 within the 

studied range. Based on model results, that value is predicted to be less than 0.1 percent, which is competitive with SO2

conversion rates specified by manufacturers for new low and ultra-low conversion catalysts. 

The average reduction in K23 on the regenerated plate catalyst was less than expected based on SCR-Tech’s experience with 

similar plate type catalyst regenerated in its commercial-scale process. As noted earlier, the baseline SO2 conversion rate in 

the deactivated plate type catalyst was 0.45 percent, while the average regenerated SO2 conversion rate was reduced to 0.38 

percent. Using the K23 optimization model derived from the experimental data one can predict that the lowest achievable 

plate catalyst SO2 conversion rate should be 0.28 percent. Although this is an approximate 38 percent reduction in SO2

conversion relative to the baseline deactivated catalyst, it is significantly less than what SCR-Tech would have predicted 

based on general experience with commercial plate catalyst regenerations. 

 

Conclusions 

This study demonstrated that SCR-Tech’s regeneration process can minimize SO2 conversion while maximizing the 

restoration of NOX reduction activity. The project team developed statistically valid linear models for K23 on both 

honeycomb and plate catalyst as a function of controlled regeneration process parameters. In the case of the honeycomb 

catalyst, the model predicts the ability of regenerated catalyst to achieve SO2 conversion rates that are competitive with new, 

commercially available, low or ultra-low conversion catalyst while still restoring virtually all the original NOx reduction 

potential. For the plate type catalyst samples, the model predicted the ability of regenerated catalyst to achieve a significant 

reduction in SO2 conversion, although not as high a reduction as expected based upon SCR-Tech’s experience in its 

commercial-scale process. 

Proceedings of the

2006 Environmental Controls Conference

49

U.S. Department of Energy

50

National Energy Technology Laboratory