Utility Experience with SCR in Germany 

Hans-Ulrich Hartenstein 

KWH 

c/o Licata Energy & Environmental Consultants, Inc. 

2150 Central Park Avenue, Suite 207 

Yonkers, N.Y. 10710-1843 

E-mail: 113535.3501@compuserve.com 

Tel.: 914-779-3451 

Fax: 914-779-4234 

Dr. Heinz Gutberlet 

PreussenElektra Engineering GmbH 

Headquarters Analytical Department 

Bergmannsglückstraße 41-43 

D-45896 Gelsenkirchen, Germany 

E-mail: andrea.eickermann-schramm@peeng.preussenelektra.de 

Tel.: 49-209-601-5042 

Fax: 49-209-601-8090 

Anthony Licata 

Licata Energy & Environmental Consultants, Inc. 

2150 Central Park Avenue, Suite 207 

Yonkers, N.Y. 10710-1843 

E-mail: licataener@aol.com 

Tel.: 914-779-3451 

Fax: 914-779-4234 

Sixteenth Annual International Pittsburgh Coal Conference 

Coal – Energy and the Environment 

October 11 – 15, 1999 

Pittsburgh Green Tree Marriott Hotel, Pittsburgh, PA 
1

Abstract

The selective catalytic reduction (SCR) technology was primarily developed by Japanese industry 

for the reduction of NOx concentrations from power plant flue gas emissions. The first commercial 

Japanese DeNOx plant began operation in 1980 while the first German high dust SCR plant started 

up in 1985 and the first German tail end SCR plant started up in 1986. 

Two German environmental resolutions (GFAVO of June, 1983 and UMK of April, 1984) enhanced 

the adaptation of the SCR technique in German power plants. From the end of 1984 to 1987 most 

German utility owners ran pilot plant studies in order to become acquainted with this technology and 

to determine whether there are special deterioration mechanisms. The first full-scale plant started 

operation at the end of 1985. Since then, around 120 SCR plants have been installed on coal, oil and 

gas fired utility power plants. SCR technology for NOx control has also been applied on waste-toenergy plants, wood fired boilers, chemical plants, hazardous waste incinerators, glass smelters, 

refinery crackers, stationary diesel generators and sewage sludge incinerators. In addition, a special 

catalyst was developed to control dioxin/furan emissions in waste-to-energy plants, hazardous waste 

incinerators, sewage sludge incinerators, crematoria, iron ore sintering plants, and other thermal 

processes requiring dioxin/furan control. 

The German NOx emission limit for utility power plants is 200 mg/Nm3 at 6% O2 for dry bottom 

boilers and 5% O2 for wet bottom boilers or about 117 ppmv NOx at 3% O2. This is approximately 

0.12 lbs/MMBtu. Many German SCR units achieve high online availability on an annual basis. 

In the following paper, we will emphasize some aspects of the SCR process: 

a) The chemical and physical/chemical properties of the catalyst 

b) The chemical engineering design of the catalyst volume 

c) The deterioration of catalytic activity with time 

d) The effect of NH3 slip (caused by catalyst deterioration or non-homogeneous NH3

distribution) and of side reactions 

The basis of the SCR technique is described first and each of the above areas is then discussed. 

In extrapolating the German SCR experience to U.S. applications, one of the most significant factors 

will be catalyst deactivation due to the differences in some U.S. coals with high alkaline metals and 

others with high sulfur and SO3 contents. 

The loss of activity potential of SCR reactors, which is related to material activity constant (K-value) 

and the geometrically active surface area, can be caused by: 

• Excessive heat 

• Erosion and pluggage 

• Pore blockage 

• Accumulation of catalyst poisons like arsenic and alkali metals 

• Prevention of gas diffusion by the formation of blinding surface layers 
2

KWH and PreussenElektra have led many developments in the German SCR industry over the past 

15 years. Some of the most relevant advancements include: 

• Development of new derivative titanium based structures 

• Development of new erosive resistant materials 

• Optimization of the catalyst pitch 

• Development of regeneration programs 

• Institution of coal blending programs to extend catalyst life 

• Development of catalyst maintenance programs 

• Development of improved flow-modeling techniques 

These improvements have led to an extended catalyst life for many facilities. This paper will discuss 

the German SCR experience and how it is being extended to the U.S. power industry. 

Introduction 

In April 1984, the Environment Ministers of the German states made a decision that all bituminous 

coal fired power stations in Germany would have to be equipped with SCR-DeNOx reactors (Fig. 1). 

These regulations applied equally to both new and old stations, insofar as the old stations were to 

continue operating indefinitely. A concentration of 200 mg/m3 of NOx corresponding to 117 ppmv 

or . 0.12 lbs/MMBtu must be adhered to as an emission limit for nitrogen oxides for coal fired power 

plants. From 1985 onwards, power plants producing a total of approximately 30,000 MW have been 

equipped with SCR technology. Most of these facilities, in particular the retrofitted ones, went into 

operation about 10 to 15 years ago. 

This decision also affected the German electric utility companies such as PreussenElektra and its 

subsidiary, VEBA Kraftwerke Ruhr AG (VKR) (Fig. 2). PreussenElektra's service area includes 

large areas of the federal states of Schleswig-Holstein, Lower Saxony and Hessen, while VKR, 

solely a power producer, is concentrated in the Ruhr area, the home of most of Germany’s 

underground pit coal mining. PreussenElektra and VKR have a combined generating capacity of 

almost 20,000 MW and have installed SCR-DeNOx reactors in power stations with a capacity of 

almost 10,000 MW (Fig. 3). 

Because of the significance of the SCR technology for VKR, in 1993 it acquired a 49 % share of 

Katalysorenwerke Hüls GmbH (KWH). KWH produces honeycomb catalysts and plays a leading 

role in the European market with a market share of over 40% in Germany. Through this business 

arrangement and its own operating plants, the company has direct access to the entire range of 

DeNOx expertise including production, engineering, and operation and maintenance experience in 

all cases where the SCR technology is applicable. 

In principle, the reduction of NOx emissions with SCR catalysts is a mature technology, which was 

developed many years ago. Nevertheless, experience has shown that in many applications, special 

features were required that necessitate further optimization of the DeNOx system. In particular, 

standard solutions were often not possible or optimal when retrofitting existing power plants. 
3

In the following discussions, the Staudinger power station near Frankfurt will be used as an example 

to show how both supplier and operator expertise have contributed to providing technically 

satisfactory solutions to various problems in the last few years. 

Description of the Staudinger Power Station 

The Staudinger power plant has five units with a net electrical output capacity of nearly 2,000 MW 

(Fig. 4). With the exception of unit 4, which is gas fired, all units fire pulverized coal. The two 

oldest units, 1 and 2, each 250 MW, are equipped with 4 identical cyclone boiler furnaces and have a 

liquid slag outtake. Unit 3 (300 MW) is also slag tap fired, but is overhead or downshot fired. We 

would like to point out at this time that the cyclone units in Germany fire pulverized coal not crushed 

coal like in the U.S. 

Unit 5 (510 MW) went into operation in 1992 and is an example of modern German power plant 

technology. The tower boiler with low NOx opposed firing has dry ash removal, and the cleaned 

flue gases are discharged to the atmosphere via the cooling tower. All coal boilers have high-dust 

SCR reactors, electrostatic precipitators and FGD plants. In the following, we will take a closer look 

at operating experiences in units 1, 2 and 5. 

In the face of deregulation in Germany, the Staudinger power plant burns the lowest priced 

acceptable coal from around the world. The coal burned varies widely in its proximate and ultimate 

analyses. Coals are blended to optimize the lowest generation cost, to extend SCR catalyst life, and 

to minimize FGD operating cost. 

The range of composition for typical coals purchased for the Staudinger power plant is presented in 

Table1. Table 2 presents the operating characteristics for each of the plant‘s SCR reactors. 

Operating Experiences DeNOx Reactors Units 1, 2 

Units 1 and 2, which were constructed in 1965, are single-line units equipped with a high-dust 

DeNOx reactor, regenerative air preheater, electrostatic precipitator and wet limestone-based FGD 

plant (Fig. 5). There are two cyclones per boiler. The coal dust is blown into the cyclones 

tangentially with adjacent openings for the secondary air intake. The tertiary air is fed in behind the 

cyclone exhaust of the combustion chamber. 

Primary and secondary air is fed into the cyclones sub-stoichiometrically (95% theoretical air), thus 

achieving a primary NOx reduction. The NOx concentration at the economizer outlet is 650 ppmv. 

The DeNOx reactors were retrofitted in 1990 and designed for a NOx removal efficiency of 85%. 

All four reactors were originally equipped with catalysts produced by various manufacturers of 

differing geometry. 

All of the fly ash collected in the electrostatic precipitators is returned to the combustion chamber to 

be melted into slag. 
4

Bonding of Arsenic as Catalyst Poison Through the Addition of Limestone

The decision to construct the DeNOx reactors in the "high-dust" configuration was made in 1988, 

after the successful completion of studies into the bonding of arsenic, a catalytic poison contained in 

the coal. With the complete recirculation of the fly ash (a method often practiced with slag tap 

firing), the trace element arsenic (As) is concentrated in the fly ash cycle. This results in gaseous 

arsenic trioxide (As2O3), a potent catalyst poison, which is concentrated in the flue gas(1) of wet 

bottom boilers with fly ash recirculation. 

We have developed a process in the Staudinger power station, which involves the addition of small 

amounts (approximately 1% by weight) of limestone (calcium carbonate, CaCO3) to the coal. 

During the combustion process the limestone is calcined by liberating CO2 and highly reactive lime 

(CaO) is formed. 

Calcium rich fly ash is reactive and bonds the arsenic, thus rendering it harmless to the catalysts. It 

is therefore possible to utilize the economic advantages of a high-dust alternative in contrast to a tail 

end solution. We have learned that there are several important factors that influence arsenic 

poisoning of catalyst. Too often engineers look only at the As content of the coal. We have found 

that in order to evaluate the catalyst life, you also have to consider the following: 

• Type of boiler used 

• The application of fly ash recirculation 

• Concentration of As in the gas phase at the air heater inlet 

• The CaO content of the fly ash in addition to the coal 

Optimization of the Flue Gas Ducts

The limited space available made a complex arrangement of the flue gas ducts necessary (Fig. 6). 

Once in operation, it became clear that the duct geometry was not suitable. This duct geometry 

allowed fly ash deposits to accumulate and potentially drop down “like an avalanche” into the SCR 

reactor and cover the first rows of modules in the upper catalyst layer. The first stage of 

optimization involved the installation of short baffle plates close to the ground, “spoilers” to prevent 

fly ash deposits from building up on the slanted floor of the flue gas duct leading to the reactor inlet 

hood. 

The flue gas velocity in the bottom region was increased but this proved to be insufficient to prevent 

ash buildup. New deposits formed upstream of the spoiler at a distance of about one-meter (3-feet), 

and covered the spoiler. 

The next modification was a redesign of the complete duct geometry with the assistance of the KWH 

flow-modeling laboratory. An optimal solution was found using a scaled flow model. The trials 

were carried out with a two-phase flow stream using original fly ash from the Staudinger station 

proved effective. In 1994, during inspection work carried out on the unit, the duct diameter was 

narrowed by the insertion of an intermediate ceiling. As a result, the flue gas velocity in the ducts of 

the reactor hood was increased from 8 m/s (26 ft/s) to 12 m/s (39 ft/s). In addition, the spoilers and 

baffle plates were removed and replaced by right-angled flow straighteners. These modifications 

have completely and permanently prevented fly ash deposits from building up in the reactor inlet. 
5

Combustion Optimization 

Another special feature was the occurrence of coarse, distended particles of coke, commonly referred 

to as popcorn ash, which accumulated on the catalysts and contributed to their clogging (Fig. 7). 

These particles have a diameter of up to 15 mm and since they cannot pass through the catalyst 

channels they accumulate on the first catalyst layer. 

Under an electron microscope, the pore structure of the coke particles can be identified (Fig. 8). 

They are completely volatilized and are 70 - 90% carbon. 

To prevent popcorn ash from forming, a check measurement of all cyclones was conducted. This 

check measurement consisted of detailed examination of the pulverization equipment and the coal 

dust distribution at the pulverizers as well as determining the cyclone fuel and air distribution (Fig. 

9). It was determined that the ratio of fuel to air feeding into the region of the cyclone outflow was 

out of balance, which explained the formation of coke particles. By homogenizing the flow of fuel 

and air, the formation of Popcorn coke particles was effectively prevented. Moreover, the content of 

uncombusted carbon in the fly ash was significantly reduced. As an additional measure, low-volatile 

coal with 18 - 20% volatile constituents was preferred, which, among German coal, has a low 

swelling index, thus effectively preventing the Popcorn formation. 

Boiler and burner adjustments were not sufficient, however, to permanently prevent a clogging of 

the catalysts (Fig. 10). As the photographs show, both the built-in plate catalysts (on the left side of 

the photo) and the 7-mm pitch honeycomb catalysts (on the right in the photo) were affected by mass 

clogging of the catalyst channels or slits. We determined that the pressure drop across the catalyst 

layers rapidly increased when we did not use a mixture of German and imported South African coal, 

but instead only burned German Ruhr coal (Fig. 11). In this figure, the correlation between pressure 

drop and fuel used can clearly be seen. We discovered that the fly ash from the German coal formed 

highly adhesive deposits, while the portion of South African coal contributed to the formation of a 

liquefied fly ash. 

The reason for this was determined by a chemical analysis of the fly ash (Fig. 12). It can be seen 

that the content of alkali sulfate, which adheres to the surface of the fly ash particles, favors the 

tendency to agglomerate. A phase analysis of the deposits revealed crystalline sodium aluminum 

sulfate and potassium aluminum sulfate (NaAl(SO4)2; KAl(SO4)2). 

After we determined this, we made South African coal (rich in ash and with a low alkali) a 

permanent ingredient of the coal mixture. We have also learned that we had to analyze not only for 

the sodium (Na) and potassium (K) content of the coal but also we need to know if the Na and K 

compounds are water-soluble. In a water-soluble form, these alkaline metals are highly mobile and 

will migrate throughout the catalyst material, reducing the active sites. The fly ash from some coals 

such as Powder River Basin (PRB) can also have high contents of water-soluble alkaline metals. 

A further problem also arose at the same time as the catalysts were clogging because of a highunburned carbon content in the fly ash. The low NOx burner operation and the use of low-volatile 

coal resulted in the carbon content of the fly ash increasing temporarily to 30%, and not dropping 

below 10%, even after optimizing the boiler. From the energy production point of view, this is not 

an issue, since all of the fly ash is returned to the boiler and melted down into the slag. However, the 

clogging of the catalysts and deposits of ash containing carbon over large areas resulted in local 

combustion and smoldering fires inside the SCR reactor encouraged by the ventilation of the hot 
6

reactor during shut-downs. The results were a continued increase in pressure drop and damage to the 

plate-type catalyst, which made it necessary to prematurely replace the plate-type catalyst material 

with a honeycomb material. 

An analysis of the entire range of problems made it advisable to also optimize the catalyst geometry 

and find a solution, which would prevent the build-up of fly ash deposits. First, test modules were 

outfitted with honeycomb catalysts with different pitches. In addition to the widely used 7 mm-pitch 

honeycomb catalyst, a 10 mm-pitch type was also used. These test modules were installed in the 

reactor at the locations where the risk of deposit accumulation was particularly high. After a long 

period of trial operations, it was determined that a 10-pitch honeycomb in the first layer prevented 

deposit accumulation in a nearly ideal manner (Fig. 13). The larger aperture of the catalyst channels 

in the first layer allowed the ash to fall through also in areas of the reactor with an unfavorable flow 

distribution. As a matter of fact, another advantage of the honeycomb type is that the channels are 

open throughout the entire catalyst layer, in other words, ash clogging in the transition zone between 

stacked catalyst elements is avoided. 

Therefore, the station operators decided to equip the reactors progressively with 10-pitch honeycomb 

catalyst in the first layer of the reactor. In order to maintain a high reactive surface area for NOx 

reduction, an 8.2-mm pitch KWH honeycomb catalyst was installed in the second and a 7.1-mm 

pitch KWH honeycomb catalyst in the third layer. This progressive arrangement continues to 

operate without plugging while meeting the NOx reduction requirement. Consequently, all the platetype catalyst was replaced by this progressive arrangement with the first 10-mm pitch catalyst layer 

acting partially as a flow straightener for all the following layers. 

Ammonia in the Fly Ash

In addition to noting the pressure drop across the catalyst layers, observing the NH3 content of the 

fly ash is another simple and certain method of monitoring the operating performance of a high-dust 

DeNOx reactor (Fig. 14). The improvement to the fly ash NH3 content of Unit 1 from 1993 to 1998 

is depicted in Fig. 14 and provides a clear overview of the chronological development of the NH3

slip of the DeNOx reactor. 

The results of flue gas cleaning endeavors and changes to the flue gas ducts can clearly been seen. 

Also, the dramatic decrease in reactor performance is obvious due to the loss of active catalyst 

surface caused by clogging and local combustion of the fly ash. 

After implementing the above-described technical improvements in regard to combustion and flow 

dynamics, as well as carrying out the two-staged catalyst replacement with 10-mm pitch honeycomb 

catalyst in the first layer, the NH3 concentration in the fly ash was permanently reduced. 

A similar situation could be observed with the NH3 content in the wastewater of the FGD plants 

(Fig. 15). Shown in Fig. 15 are the monthly average values. Here one should note that the 

Staudinger power station has a central wastewater treatment plant for the FGD plants of all the units 

and the values shown also include data from units 3 and 5. Here too, one can see that since 1996, 

changing the catalyst geometry to 10-pitch honeycombs in the first layer has permanently lowered 

the NH3 slip. Because ammonia is highly toxic to fish, ammonium nitrate is stringently monitored 

in Germany with a limit of 10 mg/l in the wastewater. Exceeding the limit results in very high 

wastewater fees and penalties. Using the measures described, the official limits could be maintained, 
7

without having to resort to a costly technical solution for removing ammonia from the FGD 

wastewater treatment plant.

Operating Experiences with the DeNOx Reactor in Unit 5

Unit 5 of the Staudinger power station commenced operation in 1992 (Fig. 16). The boiler was 

constructed with low-NOx burners as a tower boiler. The NOx-concentration in the flue gas is 250 

ppmv (.0.3 lbs./MMBtu). The high-dust DeNOx reactor is located directly above the air preheater. 

Fly ash is removed from the cooled flue gases in a 5-field electrostatic precipitator and the gas is 

then desulfurized with a limestone-based FGD. The fly ash concentrations at the SCR inlet varied 

from 10 - 15 g/m3 (4 - 6 gr/acfm). 

As a special feature, it should be mentioned that the flue gas is not reheated after passing through the 

FGD. The purified waste gases are discharged to the atmosphere through the cooling tower of the 

power station at a temperature of 50°C (122ΕF). 

Unit 5 is fired with a wide range of imported coals from around the world including the U.S. After 

approximately 8,000 operating hours, fly ash deposits were observed on the 1st catalyst layer. At the 

same time, an increase in the pressure drop of this catalyst layer was noted (Fig. 17). Here too, the 

heavy accumulation of deposits made it necessary to replace the catalysts after only 24,000 hours of 

operation. 

During an inspection of the reactor, ash samples were taken from the deposits and examined under a 

microscope (Fig. 18). These coarse ash particles, in contrast to the ones in units 1 and 2, were not 

coke, but inorganic ash substance. It was be pointed out that the formation of blistered ash particles 

was only observed during the combustion of coal or coal mixtures with an ash melting point 

<1,300°C (2,370ΕF). This problem was especially pronounced when using American coal from the 

Appalachians. The blistered popcorn ash particles were up to 6 mm in size and irreversibly clogged 

the catalyst channels. Here too, switching to a catalyst with a bigger pitch, starting in the first layer 

provided a permanent solution to the problem. 

Conclusions 

The example of the DeNOx reactors of the Staudinger power station has shown that in order to arrive 

at an optimal design, it is necessary to look at the entire power plant, from the fuel to the boiler and 

the entire path of the flue gas up to the DeNOx reactors (Fig. 19). 

Under these conditions, the SCR technology can be mastered, both technologically and 

economically, including its application as a high-dust SCR system downstream of a wet bottom 

cyclone fired boiler. The number of iterative steps to an optimal station design can be noticeably 

reduced, however, if one takes advantage of the expert advice of experienced companies right from 

the start. 

In Germany, KWH and PreussenElektra have gained more than 15 years of full scale SCR operating 

experience for coal and oil fired utility power plants with a cumulative capacity of over 30,000 MW. 
8

The PreussenElektra group owns approximately 30% of these stations. All process-engineering 

variations and all types of catalyst available in Germany are represented here. KWH is the largest 

producer of honeycomb catalysts with a market share of over 40% in Germany. As such its catalysts 

are used in the majority of German power stations. Thus, it can refer to the operating experiences of 

many power stations. The knowledge and expertise derived from this experience is available to the 

U.S. utility market. 

References

1. Gutberlet, H., "Influence of Furnace Type on Poisoning of DENOX Catalysts by Arsenic", 

VGB-Kraftwerkstechnik 68 (3), 264, March 1988. 

2. Schönrok, M., Hein, M., "7 Jahre Betriebserfahrung mit High-Dust-DENOX-Anlagen hinter 

Schmelzkammerfeuerungen-Auswirkungen auf nachgeschaltete Anlagen-", VGB-Konferenz" 

Kraftwerk und Umwelt 1997".
9

Table 1 

Typical Coal & Ash Analysis 

Typical Coal Analysis Ruhr Valley Coals Imported Coals 

Lower Heating Value MJ/kg 27.6 – 29.8 24.3 – 27.3 

Lower Heating Value Btu/lb 11,900 – 12,800 10,400 – 11,700 

Ash % as received 6 – 8 6 – 16.5 

Water % as received 8 – 10 6 – 13.0 

Volatile Matter % dry basis 19 - 38 21 – 41.0 

Chlorine % as received 0.2 0.01 – 0.35 

Sulfur % as received 0.8 – 1.4 0.3 – 1.5 

Nitrogen % dry basis 1.4 – 1.5 1.2 – 2.0 

 

Typical Coal Ash Analysis 

Fe2O3 % 11- 13 2.5 – 20 

CaO % 4 0.54 – 12 

MgO % 2 0.2 – 5.5 

K2O % 3 0.3 – 5.0 

Na2O % 1 0.1 – 2.2 

SiO2 % 43 – 45 40 – 55 

TiO2 % 1 0.6 – 3.1 

Al2O3 % 28 – 29 14 – 37.5 

SO3 % 3 0.3 – 8 

P2O5 % 1 0.01 – 1.5 

Table 2 

Individual SCR Reactor Operating Parameters 

Flue Gas Flow Nm3/h 200,000 – 440,000 scfm 127,000 – 280,000

NOx Reactor Inlet mg/Nm3 1,000 – 1,300 ppmdv 487 – 633 

NOx Reactor Outlet mg/Nm3 185 ppmdv 90 

Ammonia Slip ppmv 2 

H2O % 7.2 – 9 

O2 % dry 1.8 – 3.5 

SO2 mg/Nm3 1,400 – 3,500 ppmdv 490 – 1225 

SO3 mg/Nm3 18 – 70 ppmdv 5 – 20 

Fly ash w/o sootblowing 

Fly ash with 

sootblowing 

g/Nm3 

g/Nm3

4 – 6 

9.2 – 25 

gr/dscf 

gr/dscf 

1.6 – 2.4 

3.7 – 10.1 

Flue Gas Temperature °C 315 – 440 °F 599 - 824 

 * Flue gas emissions corrected to 5% O2. 
10

Figure 1 

Figure 2 
11

Figure 3 

Figure 4 
12

Figure 5 

Figure 6 
13

Figure 7 

Figure 8 
14

Figure 9 

Figure 10 
15

Figure 12 

Figure 11 
16

Figure 14 

Figure 13 
17

Figure 16 

Figure 15 
18

Figure 18 

Figure 17 
19

Figure 19