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 (NOx) emissions on a seasonal basis. With increasingly stringent 

environmental regulations scheduled to take effect 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 NOx emissions in line with impending air 

quality standards, the industry is now becoming increasingly concerned with the conversion of 

sulfur dioxide (SO2) to sulfur trioxide (SO3) as a byproduct of operating SCR systems. 

SO3 is formed within the SCR reactor 

bed when the SO2 present in the flue 

gas of coal-fired boilers oxidizes within 

the catalyst. The fundamental problem 

with SO3 is that it readily reacts with 

the water in the flue gas stream 

forming sulfuric acid (H2SO4). 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 m3

 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 m3

 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 m3

 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 offer 
approximately $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 States. 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 catalyst, 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 NOx reduction 

potential, SCR-Tech observed over the years that the majority of catalyst regenerated in 

accordance with their proprietary process demonstrated SO2 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. The 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 SO2 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 SO2 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 that 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

C
were 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 SO2 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 was significantly deactivated with regard to NOx

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 NOx reduction activity (K0) and initial SO2 conversion (K23) values using bo

the same test facility and the same testing conditions used for the entire study. 

To define baseline values for Kac, 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 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. Analytical 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. 

NOx 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/K0 seen in the baseline deactivated test 

relatively active catalyst for a study of this type is that the absolute increase in Kac 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 Kac 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/K0 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 of 
regeneration chemicals used in this study restored fresh activity. Of course, some of the 

regenerated honeycomb catalyst activities were higher than the measured K0, 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/K0 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 Kac as a 

set. 

on 

 m e mb and plate type catalyst was high 

differences for both populations due to the variation in 

the controlled regeneration process parameters. Baseline SO2 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 K23 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 SO2 conversion rate in the deactivated plate type 

le plate catalyst SO2 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/K0 of 0.91, while K/K0 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 Kac as a function of regeneration 

process parameters, the team demonstrated, in principle, the ability to tune the regenerati

for maximum achievable NOx reduction activity. 

SO2 / SO3 Conversion Results 

The easur d K23 on both the deactivated honeyco

enough to detect statistically significant 

deactivated honeycomb samples were 0.34 percent under the test conditions, while base

SO2 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, and in at least one significant aspect: flue gas 

SO2 content. This ultimately impacted how the test results were analyzed. 

The difference in test conditions led to the results, particularly K23 values, falling into two 

distinct categories which could not be easily combined. Ultimately the result

a result of having to work with somewhat reduced data sets, the effect of random process 

testing variation tended to be amplified. 

After regeneration, the average K23 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 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 mode

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

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 achievab

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. 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 SO2

 NOx reduction activity. The project team was 

able to develop statistically valid linear models for K23 on both honeycomb and plate catalyst 

e 

l 

SO2 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 SO2

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 related 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 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 th

plate type catalyst samples, the model also 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 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 deactivated 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 NOx 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 NOx reduction products. 

Power Engineering March, 2006 

Author(s) : Mike Cooper