REJUVENATION OF THE SCR

CATALYST AT MEHRUM

By

Yoshinori Nagai

Yoshiro Inatsune

Isato Morita

Yasuyoshi Kato

Kouichi Yokoyama

Kazunori Ito

Babcock Hitachi K.K

6-9 Takara-machi, Kure-shi, Hiroshima-ken, 

737-8508, Japan

Paul Otte

Kraftwerke Mehrum GmbH

Triftstrasse 25 Hohenhameln, 31249, 

Germany
SUMMARY

Babcock Hitachi K.K. (BHK) received the contract for the rejuvenation of the SCR catalyst at 

the 750 MW coal-fired Mehrum Power Station in March 2003. The contractual coverage 

was 160m3 of the entire catalyst layer. The catalyst, which had been in operation for sixteen 

years since 1987, was originally supplied by BHK.

The rejuvenation process developed for the Mehrum project consisted of two (2) major steps: 

the first (step) is to dust off the catalyst and remove the catalyst poison, and the second (step) 

is to add active material to enhance the catalyst activity. The catalyst must be dried after 

each washing.

In order to minimize transportation cost and time, the rejuvenation work was done at the 

Mehrum station site. The scope of the rejuvenation work was shared between the owner and 

BHK.

It took about one and a half months to complete the (total) on-site rejuvenation work. The 

performance of the rejuvenated catalyst was superior to show the same level of activity as the 

unused catalyst and maintain the same SO2 conversion rate as the spent catalyst. This paper 

gives the details of the spent coal-fired SCR catalyst rejuvenation work.

BACKGROUND

Mehrum Power Station is owned by Kraftwerke Mehrum GmbH. Shareholders are E.ON 

Kraftwerke GmbH (50%), Stadtwerke Hannover AG (33.3%) and Braunschweiger

Versorgungs AG (16.6%). It is the 750 MW coal-fired boiler located at Hohenhameln in 

Germany. The boiler has one SCR that cleans the entire flue gas stream. The SCR system 

has one reactor, which is one of the largest reactors in the world. BHK designed its 

arrangement and supplied the catalyst in 1987. After 16-year operation, BHK received the 

contract for the rejuvenation work of one layer of the catalyst for Unit #3: 160m3, in March 

2003 through Hitachi Europe Ltd.
CATALYST REJUVENATION EXPERIENCE

BHK, as an SCR supplier, has experienced variously in the catalyst rejuvenation since the 

early stage of the catalyst development. The BHK rejuvenation history is shown in Figure 1. 

As well as laboratory and bench scale tests, BHK has also gained experience in dealing with 

the actual commercial stations: two gas firing plants and one biomass firing plant. The latter 

was in Sweden, where BHK had the first experience of the commercial scale SCR catalyst 

rejuvenation at site. Photographs of the work then are shown in Figure 2. The accumulated 

knowledge gained over a quarter century enabled BHK to execute Mehrum rejuvenation 

work successfully.

Figure 1: History of BHK’s Activity for SCR Catalyst Rejuvenation

Figure 2: BHK’s first rejuvenation work at site in Sweden

Reactivation for gas 

firing plant

(Commercial)

Rejuvenation for Oil 

firing plant

Recycle of TiO2

Activity

Reuse of used SCR 

Catalyst

(Commercial)

Rejuvenation for Wood 

firing plant 

(Commercial)

Rejuvenation for Coal 

firing plant

Durability testing of 

rejuvenated catalyst at 

actual coal / gas firing 

plant

Reactivation for gas 

firing plant

(Commercial)

77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01

Reactivation for gas 

firing plant

(Commercial)

Rejuvenation for Oil 

firing plant

Recycle of TiO2

Activity

Reuse of used SCR 

Catalyst

(Commercial)

Rejuvenation for Wood 

firing plant 

(Commercial)

Rejuvenation for Coal 

firing plant

Durability testing of 

rejuvenated catalyst at 

actual coal / gas firing 

plant

Reactivation for gas 

firing plant

(Commercial)

77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01
APPROACH

In order to investigate the best available rejuvenation method for Mehrum SCR catalyst, 

several catalyst elements (plates) were taken from the catalyst units and their activity 

(DeNOx efficiency) was checked in laboratory testing. Several methods were tried to reach 

a goal of rejuvenation. Once the process was established, the full-scale results were 

confirmed by utilizing it on the actual full-size catalyst units as shown in Figure 3. Then, 

planning of the actual rejuvenation work began. Figure 4 shows laboratory scale test 

facility and test condition.

Figure.3 Full-size plate type catalyst unit 

Figure.4 Laboratory Scale test facility and test condition

Laboratory Test Condition

Gas Temperature 350?

Inlet Gas Composition

 NO 200ppm

 SO2 500ppm

 O2 3%

 CO2 12%

 H2O 12%
LABORATORY TEST RESULTS

The activity (DeNOx efficiency) of spent catalyst was about two-thirds of the unused catalyst 

activity, while its SO2 conversion rate was 50% higher. The requirements for the rejuvenated 

catalyst were higher performance and at least 90% of the unused (original) catalyst DeNOx 

efficiency in keeping with an SO2 conversion rate and the cost that the owner could accept. 

To fulfill the requirements, BHK firstly conducted the laboratory scale tests using the sample 

catalysts extracted from the actual plant, Mehrum station.

It was relatively easy to recover only activity without considering increase of SO2 conversion 

rate. However, it is quite difficult to fulfill both requirements, because they compete each 

other. At first, a simple process was considered. But it was not acceptable, because it 

could not meet all the technical goals. Some multiple processes were then considered. 

The best (process) that could meet all the goals was selected after the elaborate screening 

tests. It is Case F in Figure 5.

Figure 5: Laboratory test results

0

1

2

3

4

5

6

7

8

Unused Spent A B C D E F G

SO2 to SO3 Conversion (%)

0

10

20

30

40

50

60

70

Unused Spent A B C D E F G

DeNOx Efficiency at 350C (%)

Target Performance

Target Efficiency
PROCESS

The process (Case F in Figure 5) consists of two (2) major steps. Refer to Figure 6. The 

first step is to dust off the catalyst and remove catalyst poisons. The second is to add active 

material to enhance the catalyst activity. Catalyst must be dried after each washing. 

Selecting an appropriate method, the results were examined by utilizing that methodical

process on the actual catalyst units. When the process was established through the

examination, the actual rejuvenation work at site had to be arranged. In the entire process, 

temperature, chemical solution concentrations, and the time required affect the rejuvenation 

performance. Therefore the detailed process data was carefully checked and controlled 

during site work to secure product quality.

Figure 6: Rejuvenation Process

Dedust

Primary

Washing Drying Washing Drying

Storage

Secondly
SCOPE OF WORK

In order to minimize the cost, BHK proposed to do the rejuvenation work at the site sharing 

the scope of work. The owner accepted it. The owner’s scope was removal of the dust from 

the catalyst and treating and disposal of both the dust and the wastewater generated by the 

process. This approach is effective to minimize the cost and save any catalyst transportation 

costs, because the owner utilizes the existing equipment and the systems. The cost and the 

schedule for the process would have been considerably enormous if BHK had to establish 

waste treatment facilities outside and ship the catalyst back and forth between the generating 

station and the work place. The owner also supplied the general utilities such as steam, 

water and electricity for the rejuvenation at site. BHK’s scope was supplying mobile 

rejuvenation equipment with the labor to the site and overall project management

responsibilities in addition to establishing and confirming the rejuvenation process. Table 1 

shows the summary of “scope of work”.

Table 1: Summary of "Scope of Work "

Owner 

Dust removal & disposal

Utility supply & waste water treatment

BHK

Rejuvenation work at site

Laboratory test
REJUVENATION WORK

Figure 7 shows the tent erected at the site to do the rejuvenation work inside. It was located 

near the boiler house. All the required equipment was installed inside the tent and the 

rejuvenation process were conducted there. This setting was conducive to efficient work 

under the clean conditions. 

Figure 7: Working tent located by boiler house

The photographs in Figure 8 show the equipment and the working procedure. The dust was 

removed by vacuum cleaner in combination with the de-dusting tools (left). The open

storage tank is designed to mix the chemical solution uniformly and to remove wastewater 

from inside the units easily (right). The washing water was neutralized, and then

transported to the nearest point of the existing wastewater treatment system in the power 

station through wastewater pipe line.

Figure.8 Rejuvenation equipment 
Through the work, BHK learned that quick and uniform drying was quite important and 

conducive to both the efficient work and the excellent product performance. For this 

purpose, the drying chamber (Figure 9) is designed to make uniform distribution of drying air. 

This is demonstrated in Figure 10, which shows CFD (Computational Fluid Dynamics) study 

of the drying chamber.

Figure.9 Drying Chamber

Figure 10: CFD study of drying air flow
SCHEDULE

The work started in May 2003. Before starting the actual rejuvenation of Mehrum catalyst, 

half a month was needed for equipment preparation. Preparation included equipment set-up 

and adjustments, and performance examination. The actual catalyst rejuvenation work took 

about one and a half months for the 160 m3 catalyst layer, although work was suspended 

according to the owner’s convenience. The rejuvenation facility was operated on a single 

shift basis.

Figure 11: Rejuvenation schedule

Month ‘03/5 6 7 8 Remarks

Dust removal

PreTest &

preparation

Rejuvenation

Effort 21

Completion

Work Interruption
Approx.460mm

Approx.100mm

Approx.

150mm

PERFORMANCE

The performance test was conducted at BHK’s bench scale test facility to examine DeNOx 

activity and SO2 to SO3 conversion rate of the rejuvenated catalyst (to be) used at Mehrum 

site. Figure 12 shows the photograph of the bench scale test facility and the test condition. 

The catalyst elements were cut into small plates to make test sample catalyst units.

Figure.12 Bench Scale test facility and test condition

Test condition for rejuvenated SCR catalyst

 Flue gas flow rate : 98.7 m3(normal)/h

 Flue gas temperature : 380 deg.C

 Flue gas composition : NOx = 390 ppm

 SOx = 1,074 ppm

 O2 = 4.4 %
Figure 13 shows the test results for the rejuvenated catalyst at the bench scale test facility. 

The rejuvenated catalyst showed superior performance that is the same level activity (DeNOx 

efficiency) as the unused catalyst, maintaining the same SO2 conversion rate as the spent 

catalyst. The bench scale tests showed that the expected performance was achieved in spite 

of the large-scale work at the site. BHK concluded that the strict quality management could 

achieve these goals.

Figure.13 Obtained Performance of Product

0

0.2

0.4

0.6

0.8

1

1.2

Unused Un treated Product

Sample Name

DeNOx Activity ratio(-)

0

0.2

0.4

0.6

0.8

1

1.2

Un treated Product

Sample Name

SO2 to SO3 Conversion Ratio (-)
CONCLUSIONS

The catalyst rejuvenated at Mehrum Power station showed superior performance with the 

same level activity as the unused catalyst, keeping the same level of SO2 conversion rate as 

the spent catalyst, and all the rejuvenation performance goals were achieved. This work 

also proved that the on-site rejuvenation process would be able to minimize the cost to pay by 

utilizing the owner’s existing facility to dispose the dust and wastewater generated in the 

process as well as saving the catalyst transportation costs. Thus BHK has added the catalyst 

rejuvenation work at the site to their overall services of the catalyst management, such as the 

catalyst lifetime estimation, supply of replacement catalyst, and the rejuvenation services.