# An Experimental Program to Optimize SCR Catalyst Regeneration for Lower Oxidation of SO 2 

# to SO 3

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 18 th 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 Rive rside 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 m 3 of SCR catalyst in the United States, all completed on currently-used catalyst types: corrugated, plate and honeycomb. The SCR catalyst regeneratio n 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 reduc tion potential, SCR-Tech has observed that the majority of catalyst regenerated in accordance with their process demonstrated SO 2 conversion rates equal to or lower than those originally specified by the manufacturer for the fresh catalyst. SCR-Tech’s observ ations, 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 regene ration process on the rate of SO 2 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 th e catalytically active sites and/or bl ock 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 ph ase 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 orig inal specifications offered by the catalyst manufacturer. Phase thre e 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 chemical s it believes are especially critical to th e regeneration process, and then selected two concentration ranges for each of those chemical species th at bracket the current opera ting concentration within the process. Catalyst type became another f actor in the experimental design. Southern Company supplied deactivated honeycomb elements from one of its plants and AEP supplied d eactivated plate catalyst from one of its facilities.

To define baseline values for K α , K 23 , and the concentration of surface and bulk deactivating species against which to compare the effects of SCR-Tech’s variou s regeneration treatment levels, a set of four plate and four honeycomb catalystsamples were submitted for testing and anal ysis. Mean and standard deviation for eac h 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 rat io used in SCR-Tech’s commercial-scale process. Other factors, such as temperatures, flows an d iron-to-catalyst surface ratios

were also controlled to match the commercial-scale process.

SO 2 / SO 3 Conversion Results

Measured K 23 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 SO 2 conversion rates on the deactivated honeycomb samples was 0.34 percent under the conditions of the test, while baseline SO 2 conversion on the deactivated plate samples was 0.45 percent under the co nditions established for those tests. After regeneration, the average K 23 for all deactivated honeycomb samples was reduced to 0.19 percent, which was equivalent to the measured K 23 on the fresh samples. There was su fficient resolution in the measured K 23 values from the regenerated honeycomb samples for the team to develop a linear mathematical model for K 23 as a function of all five process parameters which could account for approximately 94 percent of th e total variability in the data. With this model it is possibl e to calculate the lowest achievable SO 2 conversion rate after adjusting a ll process parameters for minimal K 23 within the studied range. Based on model results, that value is predicte d to be less than 0.1 percent, which is competitive with SO 2 conversion rates specified by manufacturers fo r new low and ultra-low conversion catalysts.

The average reduction in K 23 on the regenerated plate catalyst was less than expected based on SCR-Tech’s experience with similar plate type catalyst regenerated in its commerc ial-scale process. As noted earlier, the baseline SO 2 conversion rate in the deactivated plate type catalyst was 0.45 percent, while the average regenerated SO 2 conversion rate was reduced to 0.38 percent. Using the K 23 optimization model derived from the experimental data one can predict that the lowest achievable plate catalyst SO 2 conversion rate should be 0.28 percent. Although this is an approximate 38 percent reduction in SO 2 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 SO 2 conversion while maximizing the restoration of NOX reduction activity. The project team developed statistically valid linear models for K 23 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 SO 2 conversion rates that are competitive with new, commercially available, low or ultra-low conversion catalyst wh ile still restoring virtually all the original NOx reduction potential. For the plate type catalyst samples, the model pred icted the ability of regenerated catalyst to achieve a significan t
reduction in SO 2 conversion, although not as high a reduction as expected based upon SCR-Tech’s experience in its commercial-scale process.