### Utility Experience with SCR in Germany

Hans-Ulrich Hartenstein

KWH c/o Licata Energy & Environm ental 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 & Environmen tal 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 Inte rnational Pittsburgh Coal Conference

Coal – Energy and the Environment

October 11 – 15, 1999 Pittsburgh Green Tree Marriott Hotel, Pittsburgh, PA1 

### Abstract

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

for the reduction of NO x concentrations from power plant flue gas emissions. The first commercial Japanese DeNO x plant began operation in 1980 while the fi rst German high dust SCR plant started up in 1985 and the first German tail end SCR plant started up in 1986.

Two German environmental resolu tions (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 ra n pilot plant studies in order to become acquainted with this technology and to determine whether there are sp ecial deterioration mechanisms. Th e first full-scale plant started operation at the end of 1985. Since then, around 120 SC R plants have been in stalled 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 emi ssions in waste-to-energy plants, hazardous waste incinerators, sewage sludge incinerators, cremat oria, iron ore sintering pl ants, and other thermal processes requiring dioxin/furan control.

The German NOx emission limit for u tility power plants is 200 mg/Nm 3 at 6% O 2 for dry bottom

boilers and 5% O 2 for wet bottom boilers or about 117 ppmv NOx at 3% O 2 . 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/che mical 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 NH 3 slip (caused by catalyst dete rioration or non-homogeneous NH 3 distribution) and of side reactions

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

In extrapolating the German SCR ex perience to U.S. applications, one of the most sign ificant factors will be catalyst deactivation due to the differences in some U.S. coals with high alkaline metals and others with high sulfur and SO 3 contents.

The loss of activity potential of SCR reactors, which is related to material activity constant (K-value) and the geometrically active su rface 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 fo rmation of blindi ng surface layers2 KWH and PreussenElektra have led many developmen ts 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 pr ograms to extend catalyst life • Development of catalyst maintenance programs • Development of improved flow-modeling techniques

These improvements ha ve led to an extended catalyst life for ma ny facilities. This paper will discuss the German SCR experience and how it is be ing extended to the U.S. power industry.



### Introduction

In April 1984, the Environment Mini sters of the German states made a decision that all bituminous coal fired power stations in Germany would have to be equipped with SCR-De NOx reactors (Fig. 1). These regulations applied equally to both new and old stations, insofa r as the old stations were to continue operating indefinitely. A concentrati on of 200 mg/m3 of NOx corresponding to 117 ppmv

or

. 0.12 lbs/MMBtu must be adhered to as an emis sion 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 on es, 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 R uhr AG (VKR) (Fig. 2). PreussenElektra's service area includes large areas of the federal stat es of Schleswig-Holstein, Lowe r Saxony and Hessen, while VKR, solely a power producer, is concentrated in th e Ruhr area, the home of most of Germany’s underground pit coal mining. Preu ssenElektra and VKR have a combined generating capacity of almost 20,000 MW and have installed SCR-DeNOx r eactors in power stations with a capacity of almost 10,000 MW (Fig. 3).

Because of the significance of the SCR technol ogy 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 sh are of over 40% in Germany. Through this business arrangement and its own operating plants, the comp any has direct access to the entire range of DeNOx expertise including produ ction, 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, experien ce has shown that in many applications, special features were required that necessitate further op timization of the DeNOx system. In particular,

standard solutions were often not possible or optimal when re trofitting existing power plants.3 In the following discussions, the St audinger power station near Frankf urt will be used as an example to show how both supplier and operator expertis e have contributed to providing technically satisfactory solutions to various pr oblems 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 downs hot 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 bo iler with low NOx opposed firing has dry ash removal, and the cleaned flue gases are discharged to the atmosphere via th e 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 St audinger power plant burns the lowest priced acceptable coal from around the world. The coal burne d varies widely in its proximate and ultimate analyses. Coals are blended to optimize the lowest generation cost, to exte nd 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 characte ristics for each of the plant‘s SCR reactors.



### Operating Experiences DeNOx Reactors Units 1, 2

Units 1 and 2, which were constr ucted in 1965, are si ngle-line units equipped with a high-dust DeNOx reactor, regenerative air preheater, electrostatic precipita tor and wet limestone-based FGD plant (Fig. 5). There are two cyclones per boiler. The coal dust is blown in to the cyclones tangentially with adjacent openings for the secondary ai r intake. The tertiary air is fed in behind the cyclone exhaust of the combustion chamber.

Primary and secondary air is fed into the cyclones s ub-stoichiometrically (95% theoretical air), thus achieving a primary NOx reduction. The NOx concen tration at the economizer outlet is 650 ppmv.

The DeNOx reactors were retrofitted in 1990 and de signed for a NOx removal efficiency of 85%. All four reactors were originally equipped w ith catalysts produced by various manufacturers of differing geometry.

All of the fly ash collected in the electrostatic precipitators is retu rned 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 th e bonding of arsenic, a catalytic poison contained in the coal. With the comp lete 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 (As 2 O 3 ), 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 pow er station, which involves the addition of small amounts (approximately 1% by weight) of limestone (calcium carbonate, CaCO 3 ) to the coal. During the combustion process the li mestone is calcined by liberating CO 2 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 advantag es of a high-dust alternativ e 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 th e catalyst life, you also have to consider the following:

• Type of boiler used

• The application of fly ash recirculation

• Concentration of As in the ga s 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 co mplex 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 potentia lly 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 increas ed but this proved to be insufficient to prevent ash buildup. New deposits formed upstream of the s poiler at a distance of a bout one-meter (3-feet), and covered the spoiler.

The next modification was a redesign of the comple te 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 us ing 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 re placed by right-angled flow straighteners. These modifications have completely and permanently prevented fly as h deposits from building up in the reactor inlet.5 Combustion Optimization Another special feature was the occurrence of coar se, 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 volati lized and are 70 - 90% carbon.

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

check measurement consisted of detailed examina tion of the pulverization equipment and the coal dust distribution at the pulverizers as well as dete rmining 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 c oke particles. By homogeni zing the flow of fuel and air, the formation of Popcorn coke particles wa s effectively prevented. Moreover, the content of uncombusted carbon in the fly ash was significantly re duced. As an additional measure, low-volatile

coal with 18 - 20% volatile constituents wa s preferred, which, among German coal, has a low swelling index, thus effectively preventing the Popcorn formation.

Boiler and burner adjustments were not sufficien t, 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 honeyc omb catalysts (on the right in the photo) were affected by mass clogging of the catalyst channels or slits. We determined that the pressure drop acro ss the catalyst layers rapidly increased when we did not use a mi xture of German and impo rted South African coal, but instead only burned German Ruhr coal (Fig. 11). In this figure, the corr elation between pressure drop and fuel used can clearly be seen. We discove red that the fly ash from the German coal formed highly adhesive deposits, while the portion of South African coal co ntributed 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 th e deposits revealed crystalline sodium aluminum sulfate and potassium aluminum sulfate (NaAl(SO 4 ) 2 ; KAl(SO 4 ) 2 ).

After we determined this, we made South African coal (rich in ash and with a low alkali) a permanent ingredient of the coal mi xture. 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 wate r-soluble alkaline metals.

A further problem also arose at the same time as the catalysts were clog ging 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 conten t of the fly ash increasing tem porarily to 30%, and not dropping below 10%, even after optimizing th e boiler. From the energy produc tion 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 cont aining carbon over large ar eas resulted in local combustion and smoldering fires inside the SCR r eactor encouraged by th e ventilation of the hot6 reactor during shut-downs. The results were a conti nued increase in pressure drop and damage to the plate-type catalyst, which made it necessary to pr ematurely 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 pitc hes. In addition to the widely used 7 mm-pitch honeycomb catalyst, a 10 mm-pitch t ype was also used. These test modules were installed in the reactor at the locations where the risk of depos it 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 w ith 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 re duction requirement. Consequently, all the platetype catalyst was replaced by this progressive arra ngement 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 laye rs, observing the NH 3 content of the fly ash is another simple and certain method of monitoring the operating pe rformance of a high-dust DeNOx reactor (Fig. 14). The improvement to the fly ash NH 3 content of Unit 1 from 1993 to 1998 is depicted in Fig. 14 and provides a clear overv iew of the chronological development of the NH 3 slip of the DeNOx reactor.

The results of flue gas cleaning e ndeavors and changes to the flue gas ducts can clearly been seen. Also, the dramatic decrease in reactor performan ce is obvious due to the loss of active catalyst surface caused by clogging and local combustion of the fly ash.

After implementing the above-descr ibed 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 NH 3 concentration in the fly ash was permanently reduced.

A similar situation could be observed with the NH 3 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 includ e data from units 3 and 5. Here too, one can see that since 1996, changing the catalyst geometry to 10-pitch honeycom bs 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 lim its could be maintained,7 without having to resort to a costly techni cal 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 boile r. 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 pr ecipitator and the gas is

then desulfurized with a limestone-based FGD. Th e fly ash concentrations at the SCR inlet varied from 10 - 15 g/m 3 (4 - 6 gr/acfm).

As a special feature, it should be mentioned that th e 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 (122 Ε F).

Unit 5 is fired with a wide range of imported co als 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 pres sure drop of this catal yst layer was noted (Fi g. 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 we re taken from the depos its 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 fo rmation of blistered ash particles was only observed during the combustion of coal or coal mixtures with an ash melting point <1,300°C (2,370 Ε F). 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, sw itching 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 th e 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 applic ation as a high-dust SCR syst em downstream of a wet bottom cyclone fired boiler. The number of iterative steps to an optima l station design can be noticeably reduced, however, if one takes adva ntage 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 pl ants 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 Ge rmany. As such its catalysts are used in the majority of German power stations . Thus, it can refer to th e 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 T ype on Poisoning of DENOX Catalysts by Arsenic", VGB-Kraftwerkstechnik 68 (3), 264, March 1988.

2. Schönrok, M., Hein, M., "7 Jahre Betriebser fahrung mit High-Dust-D ENOX-Anlagen hinter Schmelzkammerfeuerungen-Auswirkungen auf nac hgeschaltete 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

Fe 2 O 3 % 11- 13 2.5 – 20

CaO % 4 0.54 – 12 MgO % 2 0.2 – 5.5

K 2 O % 3 0.3 – 5.0

Na 2 O % 1 0.1 – 2.2

SiO 2 % 43 – 45 40 – 55

TiO 2 % 1 0.6 – 3.1

Al 2 O 3 % 28 – 29 14 – 37.5

SO 3 % 3 0.3 – 8

P 2 O 5 % 1 0.01 – 1.5



# Table 2 Individual SCR Reactor Operating Parameters

Flue Gas Flow Nm 3 /h 200,000 – 440,000 scfm 127,000 – 280,000 NOx Reactor Inlet mg/Nm 3 1,000 – 1,300 ppmdv 487 – 633 NOx Reactor Outlet mg/Nm 3 185 ppmdv 90 Ammonia Slip ppmv 2

H 2 O % 7.2 – 9

O 2 % dry 1.8 – 3.5

SO 2 mg/Nm 3 1,400 – 3,500 ppmdv 490 – 1225

SO 3 mg/Nm 3 18 – 70 ppmdv 5 – 20 Fly ash w/o sootblowing Fly ash with sootblowing g/Nm 3
g/Nm 3 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% O 2

.10 

## Fi g ure 1

Fi g ure 211 

## Fi g ure 3

Fi g ure 412 

## Fi g ure 5

Fi g ure 613 

## Fi g ure 7

Fi g ure 814 

## Fi g ure 9

Fi g ure 1015 

## Fi g ure 12 Fi g ure 1116 

## Fi g ure 14 Fi g ure 1317 

## Fi g ure 16 Fi g ure 1518 

## Fi g ure 18 Fi g ure 1719 

## Fi g ure 19