1 

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# Recent Experience with SCR Catalyst Regeneration

Herwig Maier Ph.D. EnBW Engineering, Stuttgart Germany Jörn Matschke , Ph.D. EnBW Engineering, Stuttgart Germany

1. Introduction

With over two hundred new SCR ́s that are expected to be installed and placed in operation on coal fired power plants in the U.S., the issues related to their efficient and economic operation become of primary importance. Since the chemical utilization in SCR systems is very close to its absolute maximum, the cost of catalyst management is the major component in SCR operating costs. Regenerating the old catalyst instead of disposing it and substituting it with a new one can substantially reduce the cost of operatin g SCR units by increasing the effective life of the individual catalyst layers. Catalyst suppliers and users have worked diligently over the years to develop reliable and economic techniques for catalyst regeneration. Different methods of re-processing, mechanical cleaning, heating with special gases and washing with regenerating solutions were suggested and tested with various degrees of success. Practically all known methods require removing catalyst from the SCR box, regenerating it outside and reinstalling it. Obviously, a regeneration process, which does not require catalyst removal, would be more attractive.2 

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Figure 1: EnBW’s Heilbronn Power Station – 7 coal-fired units equipped with SCRs

2. The ReAct TM Technology

This in situ regeneration method was conceived in 1994 by EnBW, one of the largest utility companies in Germany, operating more than 11,000 MW of generating capacity including numerous large coal-fired station equipped with SCRs, 2 of which are shown in Figures 1 and 2.

Figure 2: Karlsruhe RDK Power Station – 7 coal-fired units equipped with SCRs

Overall EnBW manages over 2,000 m 3 of catalyst supplied by different manufacturers (BASF/CERAM, KWH, MHI, Siemens) within these coal-fired stations. After eight years of following the standard procedure of replacing deactivated catalyst on their numerous SCRs, EnBW commercialized 1996 the catalyst regeneration.

The criteria for the method are:

- all parameters are pre tuned and optimized for each specific case in the laboratory

- cleaning is performed within the SCR reactor with a water based solution containing
minor amounts of non toxic additives

- the regeneration solution is recirculated to improve the efficiency and reduce the amount of spent solution3 

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The technology was developed, refined and extensively tested in the lab and successfully implemented in EnBW power plants. In the meantime it has been proven as a reliable technology in several plants as shown in Table 1 below.

Table 1: ReAct TM reference list

ReAct TM technology is now protected by North American and European patents and is commercially offered in North America by Enra, LLC.

The regeneration process involves the in situ washing of the catalyst with a safe and cost effective regeneration solution and is efficient in catalyst cleaning, dissolving masking materials and removing some poisonous species. It involves several distinct steps:

• In the first step samples are analyzed and tested to find out the reasons of deactivation by chemical analyses, XRD, electronic microscopy, XFA, atomic absorption spectroscopy and the like and to set up the parameters for the procedure. The residual activity is also determined as shown in Figure 3.

• In the second step the reactivation proced ure takes place in a lab scale facility and after that the activity is measured again to see what happened. During the test regeneration some parameters like pH and conductivity are measured on line to control the process, also samples from the suspension are analyzed.

• If the first test results are not satisfying, the procedure is repeated with optimized parameters. After that the parameters for the full scale application are fixed.

• With these optimized regeneration parameters the full scale regeneration takes place with the regeneration solution sprayed over the catalyst modules (Figure 4), 

#  Avedorevaerket, Denmark (full scale) 

#  Combined heat and power plant Heilbronn (full scale, several times) 

#  Combined heat and power plant Karlsruhe (full scale, several times) 

#  Power plant Walheim (full scale, several times) 

#  Power plant of Altbach (full scale) 

#  Power plant Aschaffenburg (test scale) 

#  Power plant Weser (test scale) 

#  Power plant Franken (test scale) 

#  Power plant Bergkamen (test scale) 

#  Combined heat and power plant Reuter West, Berlin (test scale) 

#  Combined heat and power plant Rudow, Berlin (test scale) 

#  Combined heat and power plant München (test scale) 

#  Combined heat and power plant Schwedt (test scale) 

#  Power Plant Münster (full scale)4 

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collected at the bottom of the layer an d recirculated (Figures 5 and 6). The equipment is simple, sectional and adaptable to any type of SCR plant. A team of 5 to 7 people can regenerate over 35 cubic feet of catalyst per hour. The process runs at ambient temperature and moderate pressure, so energy consumption and danger of corrosion is negligible. Furthermore, the process has been developed to handle also difficult catalyst by intermitted washing or reconditioning. The wastewater, approximately 1 m 3 /m 3 of catalyst reactivated, can be easily handled containing about 5% solid and being nothing else as the air heater or boiler washing water. It can be treated in the wastewater treatment plant downstream a wet FGD, if there is one, or be used in the wet or dry FGD as process water or be "burned" in the boiler via the start up fuel burners or spread on the coal or be used for ash moistening. If none of these options are po ssible, the wastewater has to be treated externally before discharged.

• The final step is drying the catalyst after regeneration, which does not require much more time than the normal start up of the SCR reactor.

Figure 3: EnBW’s certified catalyst testing lab – here the bench scale reactor

But is it save to wet the catalyst?

When using ReAct TM it is, because5 

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• the SCR has been cleaned before implementing ReAct TM , so no ash plugging of the equipment should occur

• a continuously controlled flow flushes out residuals of fly ash

• the duration of flushing and composition of the suspension is carefully adopted to the catalysts need fixed previously in extensive lab tests to avoid any danger of damaging or chemical attack

• after regeneration the drying process is smooth and avoids any thermal destruction

Figure 4: Spray headers on top of the catalyst modules6 

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Figure 5: Liquid collection pan below the catalyst module7 

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Figure 6: Regeneration solution recirculation tank

3. Results and Experience

So far, 13 successful full scale applications with regeneration of over 1600 m 3 of catalyst and a lot of lab scale tests also with catalysts from US power plants have proven

ReAct TM as a powerful technology. Different types of catalysts have been regenerated

and ReAct TM was found to be equally efficient with plate type and honeycomb catalyst.

The effect of ReAct TM is more pronounced when the degradation of the catalyst is already substantial:

• Lower catalyst activity (k/k 0 << 100%) leaves more room for improvement

• Exponential nature of deacti vation process yields more additional operating time per percent of increased activity.

Two parameters are most instrumental in evaluating the efficiency of regeneration

process:

• Gain of lost activity, which is measured as a fraction of lost activity reversed by regeneration. Typically this parameter is in the 35 to 70 % range as can be seen in

Table 2.

• Additional operating time without catalyst replacement resulting from the regeneration process. Typically one to three years per regeneration.



## P o wer P lant Type o f

Furnace Type of

catal y s t G ain o f lo st

activit y H e ilb ro n n D ry b o tto m H o n e y c o m b 6 7 % W e se r S la g ta p H o n e y c o m b 3 8 % A sc h a ffe n b u rg S la g ta p P la te ty p e 6 3 % F ra n k e n D ry b o tto m P la te ty p e 1 7 % W a lh e im D ry b o tto m H o n e y c o m b 3 8 % R euter D ry bottom H oneycom b 45% A ltb a c h D ry b o tto m H o n e y c o m b 4 5 % K a rlsru h e D ry b o tto m H o n e y c o m b 4 4 % B ergkam en D ry bottom H oneycom b 40% M ünster D ry bottom H oneycom b 34% W e se r S la g ta p P la te ty p e 2 7 %8 

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Table 2: Typical regeneration results using EnBW’s ReAct TM process

Both these parameters are highly site specific and can be predicted based on the lab regeneration of representative samples and us ing the historic trend of catalyst activity at the power plant. The achievable improvement differs for various mechanisms of catalyst degradation and depends on such factors as furnace type, ash composition, flue gas parameters and SCR reactor operating practices. It is predicted on the basis of the lab test results tuned from prior field experiences.

The ReAct TM process has some limitations:

• Complete recovery of the initial activity (k/k 0 = 100%) is not achievable, however the recovery of lost activity is usually high enough to yield a significant increase in operating time without catalysts replacement.

• If the catalyst is mechanically at the end of its lifetime, no regeneration procedure, however applied, can help.

• ReAct TM results for heavily poisoned catalysts at times may be measurably less than normal. Recently ReAct TM was applied to a plate type catalyst with unusually high
concentrations of arsenic. As shown in Tabl e 3, the gain of lost activity was rather
moderate. However, with a modified ReAct TM process a similar catalyst from a US power plant around 54% of lost activity could be recovered.

Table 3: Arsenic removal using EnBW’s ReAct process

Monitoring the catalyst activity before and after regeneration shows no change in the degradation pattern, it only brings the activity to a new higher level. So far, experiences have shown, that the process is repeatable several times without changing the degradation pattern of the SCR plant.

4. Economics and Savings 

### Power

Plant As in catalyst

as %w Type of catalyst Gain of lost activity

Heilbronn 0.01 Honeycomb 67%

Franken 1.70 Plate type 17%

US catalyst 1.56 Plate type 54%9 

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Assuming that the price of new catalyst is about $ 6,000 per m 3 honeycomb and a typical catalyst management and exchange strategy without ReAct TM requires new catalyst at least every three to six years, the average annual costs will result in at least $ 1,000 but up to $ 2,000 per m 3

.

ReAct TM used as a repeatable process extends significantly the catalyst life time, at least it will be doubled. Consequently by applying the ReAct TM technology the average yearly costs for catalyst will drop down to half of the above figures or normally even less. The example of power plant Heilbronn , Unit 7 (760 MW PC-fired), where ReAct TM was implemented first commercially shows (Figure 7), how this technology reduces drastically SCR operating costs:

• the exchange cycle of catalyst before ReAct TM was about 3 years, that means every three years one layer had to be changed

• since the implementation of the ReAct TM process no change of catalyst has been necessary the past 7 years, which means savings of two exchange cycles or 2 x $ 6,000 per m 3

• during that time the ReAct TM regeneration has been performed successfully 3 times

• the value of one regeneration done is $ 4,000 per m 3

Figure 7: Additional gain in catalyst life using EnBW’s ReAct TM technology 

### Operating time Catalyst activity Regeneration Additional operating time10 

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With regular implementation of the ReAct TM technology in 1996 the savings of the power plant Heilbronn Unit 7 concerning SCR operating costs exceed several million dollars till the end of its expected technical life time.

Of course the life expectancy of catalyst and the time of necessary changes of layers depends on various parameters including:

- coal quality

- mode of operation

- boiler characteristics

- ash composition

- ash usage or disposal method (pond, cement industry, etc.)

Thus, the value of the ReAct TM regeneration has to be determined for each specific

case.

5. Recommendations

To take full advantage of the potential benefits of ReAct TM and to plan the optimum catalyst management strategy power plants should follow some simple
recommendations:

During the design phase

• sufficient space around the SCR to place the regeneration equipment (10x30 ft)

• man holes (2-3 ft diameter) to bring in the spray grid, collecting trough, and of
course staff

• sufficient space between catalyst layers (about 6 ft)

• the position of soot blowers and the steel construction should be known exactly to adopt the construction of the regeneration equipment within the SCR duct.

During the operating phase:

• keep information on activity (or even a sample) of the fresh catalyst

• provide regular activity measurements of installed catalyst samples to establish a degradation pattern

• proceed with frequent measurements of ammonia in the fly ash as an indicator of current catalyst condition11 

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• extract representative samples of catalyst in advance and arrange for their test regeneration

• use the test regeneration results and the established degradation curve to make a conscious economic choice between regeneration and replacement of the worst catalyst layer.

6. Conclusion

ReAct TM is an excellent , proven tool to affect significantly the operational costs of a SCR plant. In the power plant Heilbronn (Figure 8), where the technology has been developed and first applied, savings of several million dollars are a severe argument in the deregulated European energy market.

ReAct TM is

• simple

- in-situ regeneration without removing the catalyst from the SCR reactor

- minimal man-hours to accomplish regeneration during short outages

- no special start up requirements

• safe

- regeneration procedure is designed for each specific case

- no harm to catalyst or SCR reactor

- chemicals are benign requiring no special permits

• proven

- many successful full layer regenerations

- repeatable process with no change in the deactivation pattern

- protected by North American and European patents

• economical

- catalyst disposal volume and cost reduced

- significant reduction of catalyst replacement cost

• available

- ReAct TM is commercially available in North America from Enra, LLC

Figure 8: High-dust SCR system at
EnBW’s Heilbronn Power
Station unit 7

- 760 MW,

- bituminous coal,

- wall fired, dry bottom
boiler,

- two 4 layer reactors,12 

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- 825 m 3 of honeycomb catalyst installed in 3 layers,

- 80% NOx reduction,

- 2 ppm NH 3 slip,

- SCR start-up: 1986