## New Life for Old Catalyst

Technology developed in Europe is now being optimized to make an attractive catalyst rejuvenation option even better



### By Mike Cooper, SCR-Tech

For the past 10 to 15 years, much of the U.S. coal-fired power generation industry has successfully deployed selective catalytic reduction (SCR) as the preferred technology for the mitigation of nitrogen oxide (NO x ) emissions on a seasonal basis. With increasingly stringent environmental regulations scheduled to take effe ct within the next few years, most of these plants will soon be required to begin operating their SCR reactors on a year-round basis. While year-round operation will serve to further reduce NO x emissions in line with impending air quality standards, the industry is now becoming increasingly concerned with the conversion of sulfur dioxide (SO 2 ) to sulfur trioxide (SO 3 ) as a byproduct of operating SCR systems.

SO 3 is formed within the SCR reactor bed when the SO 2 present in the flue gas of coal-fired boilers oxidizes within the catalyst. The fundamental problem with SO 3 is that it readily reacts with the water in the flue gas stream forming sulfuric acid (H 2 SO 4 ). This reaction begins to occur at the relatively high temperature of about 410 F, primarily in the air pre-heater. As a result, the sulfuric acid remains in the gas phase until it condenses to droplets when it reaches its dew point. If the sulfuric acid vapor is cooled to its dew point very rapidly, for example in a conventional wet flue gas desulfurization unit (FGD), the acid mist forms sub-micron sized droplets that are not readily trapped by the scrubber. Without further mitigation, approximately 50 percent of the acid mist formed in a wet FGD will be emitted from the stack as a dense, bluish plume.

Since its formation in 2000, SCR-Tech has successfully 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. Catalyst regeneration has become an attractive alternative to purchasing new catalyst. A typical 500 MW plant might have 650 m 3 of SCR catalyst arranged in three layers of 150 modules each. The cost of purchasing new catalyst normally varies between $3,500 and $4,500 per m 3 depending on catalyst type and manufacturer, whereas the cost to regenerate is about 60 percent of that price. With a 500 MW SCR unit operating year round, assuming a normal three- year catalyst life, regeneration will offerapproximately $300,000 to $400,000 per layer in annual savings for the plant. For a complete 5,000 MW utility system, over the course of 20 years the estimated cost of new replacement catalyst will be on the order of $90 million, while the total cost for regeneration would be on the order $50 million. That equates to a net savings of about $40 million.

Click here to enlarge image R-Tech Director of Technology Howard anklin (left) records data as Senior Process

om

ent

-

In addition, since regenerati ates the need for disposal of used catalyst, it can significantly reduce the costs associated with that disposal process. Whether spent catalyst is nty

ogy employed by SCR-Tech was originally developed in Europe and further optimized in the United St ates. While these process optimization efforts

s ons,



### Regeneration Process Overview

ti-step process with each step involving treatment of the deactivated catalyst with a unique chemical solution. Following a rigorous mechanical SC Fr Chemist Nagesh Patel removes a sample fr the sub-scale plate processing tank as part of the catalyst regeneration test. The ultrasound processing tank behind Franklin is a critical element in SCR-Tech’s proprietary catalyst regeneration technology. The honeycomb processing tank behind Patel was sized to handle one standard size honeycomb elem along with its holder. Photo courtesy of SCR Tech.

on obvi

deposited in a normal landfill or treated as hazardous waste depends on the state/cou where the landfill is located, the type of cataly st, the type of coals the plant has fired and the poisons present on the catalyst. Experience has shown disposal cost vary between $50/ton to more than $2,000/ton. For one layer of catalyst in a 500 MW unit, this amounts to $20,000 to $500,000 in disposal cost.

The SCR catalyst regeneration technol

had primarily been focused on restoring the catalyst’s original activity level and NO x reduction potential, SCR-Tech observed over the years that the majority of catalyst regenerated in accordance with their proprietary process demonstrated SO 2 conversion rates equal to or lower than those originally specified by the manufacturer for the fresh catalyst. SCR-Tech’ observations, combined with growing industry concern with controlling sulfuric acid emissi prompted the company to initiate an experimental study jointly conducted with American Electric Power (AEP) and Southern Company. Th e study’s purpose was toquantify the primary effects of five carefully selected independent factors in SCR-Tech’s catalyst regeneration process on the rate of SO 2 oxidation. This article summarizes the results of this study.

Catalyst regeneration at SCR-Tech is a mul

cleaning to remove loose fly ash, the overall regeneration process can be broken down into three phases:Phase 1 - Removing species known to poison the catalytically active sites and/or block flue g access to those as 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

atalyst poisons and pore blocking agents left over from initial treatments.

o a vel equal to, or greater than, the original catalyst manufacturer’s original specifications. Phase 3 includes carefully defined heat-treatments that help control distribution



### tions

esigning an experimental matrix to identify and quantify variables within a multi-step res some compromises. Where one might prefer to evaluate the effect of all independent factors within the process at two or three ith

st

. A two-level full factorial design requires 2 to be raised to a power equal to the number of experimental e number

d. as sodium and potassium.

Phase 2 - Neutralizing and removing the chemicals used in phase 1 and removing any additional residual, soluble c

Phase 3 - Re-impregnating active catalytic species onto the support to return the catalyst t performance le

and dispersion of those active species.



### Experimental Design Considera

D regeneration process that influence SO 2 oxidation requi

levels, experimental logistics and cost considerations make this an impractical option. W this in mind, 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. Cataly 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. Their contribution of these two different types of catalyst for this study enabled a sixth factor in the two-level experimental design.

To quantify both primary effects and the effects of all possible interactions between independent factors, a full factorial experimental design is typically used

factors’ separate experiments. In this case th at means 64 separate trials. Such a larg of separate trials would have been logistically impractical. SCR-Tech determined that a reasonable compromise would be to use a partial factorial design. And to increase the statistical confidence in any process models built from the experimental results and show evidence of non-linearity where it might exist, two center points to the design were adde

Click here to enlarge image This contour plot shows the relationship be

a tween the concentration of chemical C and

related value on 23

igger changes are better in this case). oncentrations of chemical B and chemical D (but independently controllable) pH the magnitude of change in K (b Cwere held at their lower values.

The honeycomb catalyst ele ely 12,000 hours in a high dust application at the Sout s were removed after the 2003 ozone season because of la te type catalysts were exposed for approximately plication at the AEP plant. They were replaced with low SO 2 conversion catalyst in the spring of 2005 as a result of high sulfuric acid st

th

d study were established from these baseline data, and the measured variances were used to determine

ducted for both catalyst surface and catalyst bulk at the inlet, and the outlet of the catalyst samples via

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

o

sed

for

ine deactivated honeycomb and plate type catalyst es demonstrated that neither population wa s significantly deactivated with regard to NO x c ts and the relatively small results, SCR-Tech would not typically recommend that either of these catalysts be regenerated. The primary issue with using

e ments were exposed for approximat hern Company plant. These module rge particle ash (LPA) plugging. The pla 7,000 hours in a high dust ap

emissions at the plant. Southern Company and AEP also provided samples of unused cataly representative of each of the deactivated populations. These fresh samples were used to establish initial NO x reduction activity (K 0 ) and initial SO 2 conversion (K 23 ) values using bo the same test facility and the same testing conditions used for the entire study.

To define baseline values for K ac , K 23 , 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 samples were submitted for testing an analysis. Mean and standard deviation for each of the parameters evaluated in the

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.

All activity and conversion testing was conducted by a certified, independent lab. Specific catalyst test conditions were determined by AEP and Southern Company in accordance with operating conditions at their respective plants, and carried out in accordance with the VGB Guideline second revised edition VGB-R 302 He. An alytical determinations were con

X-ray fluorescence spectroscopy. These analytical measurements were carried out by an independent third party lab.

All test regenerations were run in small-scale equipment specifically designed to treat one honeycomb element or one plate set at a time. These small-scale treatment tanks, shown in the photo accompanying this article, have been designed so that the solution volume to catalytic surface area ratio is e

also controlled to match the commercial-scale process. In general, every effort was made t ensure that results gathered from the test-scale equipment would scale to SCR-Tech’s commercial regeneration process. These sub-scale catalyst regeneration tanks are also u by SCR-Tech to evaluate the effectiveness of various regeneration schemes in preparation full scale commercial regeneration jobs.



### NO x 

### Reduction Activity Recovery Results

Test and analytical results from the basel sampl redu tion potential. In fact, based on fresh catalyst test resul reduction in K/K 0 seen in the baseline deactivated test

relatively active catalyst for a study of this type is that the absolute increase in K ac for regenerated samples is not expected to be large, so it becomes more difficult to isolate the effects of control variables. This proved to be the case in this study becayse any statistically significant effects on restoration of K ac as a result of variation in regeneration parameters wer not detached for the honeycomb sample.

Those honeycomb samples were the least deactivated of the two populations, having a K/K 0 of only 0.96. After regeneration, the mean K/K0 for all the honeycomb samples was 1.03, which demonstrates that within the measurement error, both the low and high concentrations ofregeneration chemicals used in this study restored fresh activity. Of course, some of the regenerated honeycomb catalyst activities were higher than the measured K 0 , but it is impossible to clearly define the controlling factors for this enhanced restoration of activity level.

The plate catalyst was somewhat more deactivated than the honeycomb, having a deactiva K/K 0 of ted 0.81. Because the total magnitude of recovery was greater for these plate catalysts after regeneration, the team was able to develop a linear model for maximizing K ac as a

set.

on

m e mb and plate type catalyst was high differenc es for both populations due to the variation in the controlled regeneration process parameters. Baseline SO 2 conversion rates on the line

s were analyzed as coming from two separate populations of nine rather than one contiguous population of 18. As and

measured K 23 on the fresh samples. There was sufficient resolution in the measured K values from the regenerated honeycomb samples for

l

process. As noted earlier, the baseline SO 2 conversion rate in the deactivated plate type

le plate catalyst SO 2 conversion rate, after adjusting all appropriate process parameters, should be 0.28 percent. Although this is an approximate 38 percent reduction in function of three of the five studied control parameters. On average, the regenerated plate samples had a K/K 0 of 0.91, while K/K 0 for the more effective regenerations was 1.0. The optimization model accounted for approximately 81 percent of the variability in the data In this case, using SCR-Tech’s new optimization model for K ac as a function of regeneration process parameters, the team demonstrated, in pr inciple, the ability to tune the regenerati for maximum achievable NO x reduction activity.



### SO 2 

### / SO 3 

### Conversion Results

The easur d K 23 on both the deactivated honeyco enough to detect statistically significant

deactivated honeycomb samples were 0.34 percen t under the test conditions, while base SO 2 conversion on the deactivated plate samples was 0.45 percent under the conditions established for those tests. It should be noted that test conditions for the two catalyst populations differed in several minor aspects, an d in at least one significant aspect: flue gas SO 2 content. This ultimately impacted how the test results were analyzed.

The difference in test conditions led to the results, particularly K 23 values, falling into two distinct categories which could not be easily combined. Ultimately the result

a result of having to work with somewhat redu ced data sets, the effect of random process testing variation tended to be amplified.

After regeneration, the average K 23 for all deactivated honeycomb samples was reduced to 0.19 percent, which was equivalent to the

23 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 the total variability in the data. With this model it is possible to calculate the lowest achievable SO 2 conversion rate after adjusting all process parameters for minimal K 23 within the studied range. Based on mode results, that value is predicted to be less than 0.1 percent, which is competitive with SO 2 conversion rates specified by manufacturers for 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 commercial-scale

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 achievab

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. SCR-Tech is currently working to determine if there are unique aspects of this particular population of plate catalyst samples tested that might explain the anomaly.

### Conclusions, Issues and Follow-up

T conversion while maximizing the restoration of

his study demonstrated the capability of SCR-Tech’s regeneration process to minimize SO 2 NO x reduction activity. The project team was able to develop statistically valid linear models for K 23 on both honeycomb and plate catalyst

e

l SO 2 conversion rates that can be achieved in commercial-scale regeneration of all honeycomb further

st

ain a better understanding of the physical and chemical properties of these regenerated catalysts that lead to a simultaneous decrease in SO 2

ike Cooper is manager of process development for SCR-Tech, a division of Catalytica tems Company. He has more than 20 years experience in catalyst development and optimization. He holds several patents relate d to solvent-based regeneration processes for uring as a function of controlled regeneration process parameters. In the case of the honeycomb catalyst, the model developed 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 while still restoring virtually all the original NO x reduction potential. For th plate type catalyst samples, the model also predicted the ability of regenerated catalyst to achieve a significant reduction in SO 2 conversion, although not as high a reduction as expected based upon SCR-Tech’s real-world experience in its commercial-scale process.

While these preliminary results are very encouraging, it is too early to draw definitive conclusions from a study of two unique catalyst populations and limited data sets on actua

or all plate type catalyst. SCR-Tech plans to conduct multiple follow-on experiments to validate the observations and predictions drawn from this study and help understand how generally applicable these results may be to honeycomb and plate catalyst types from different manufacturers that have been deactiva ted under different conditions of operation. Effects on changes in the catalyst’s ability to oxidize mercury as well as on long-term cataly deactivation rates are also being considered.

SCR-Tech, AEP and Southern Company are conducting more rigorous analytical evaluations of the samples used in this study in an effort to g

conversion levels and an increase in NO x reduction activity.



### Author

M Energy Sys

super-acid gasoline alkylation catalysts, and is currently involved in developing manufact processes for Catalytica Energy Systems’ proprietary diesel NO x reduction products.

Power Engineering March, 2006 Author (s) : Mike Cooper