Steag LLC 

PO Box 410728 • 224 Westinghouse Blvd. #607 • Charlotte, NC 28273-6228 

Phone: 704-588-7657 • Fax:704-588-7644 

E-mail: info@steagllc.com • www.steagllc.com 

Large Particle Ash (LPA) Screen Retrofits 

at Coal-Fired Units in Indiana and Ohio 

2006 Environmental Controls Conference 

Sponsored by 

U.S. Department of Energy’s National Energy Technology Laboratory 

May 16 – 18, 2006 

Pittsburgh, Pennsylvania 
 

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Hans Sobolewski 

STEAG LLC 

224 Westinghouse Blvd., #607, P.O. Box 410728 

Charlotte, NC 28273-6228 

E-mail: hsobolewski@steagllc.com 

Telephone (704) 588-7657; Fax: (704) 588-7644 

Joseph Jancauskas 

Dayton Power & Light, J.M. Stuart Electric Generating Station 

P.O. Box 468, Aberdeen, OH 45101 

E-mail: joseph.jancauskas@dplinc.com 

Telephone: (937) 549-2641 ext. 5735 

Michael Harrell 

Dayton Power & Light, J.M. Stuart Electric Generating Station 

P.O. Box 468, Aberdeen, OH 45101 

E-mail: michael.harrell@dplinc.com 

Telephone: (937) 549-2641 ext. 5842 

Hans Hartenstein 

STEAG LLC 

224 Westinghouse Blvd. #607, P.O. Box 410728, Charlotte, NC 28273-6228 

E-mail: hhartenstein@steagllc.com 

Telephone: (704) 588-7657; Fax: (704) 588-7644 

Marilynn Martin 

STEAG LLC 

224 Westinghouse Blvd. #607, P.O. Box 410728, Charlotte, NC 28273-6228 

E-mail: mmartin@steagllc.com 

Telephone: (704) 588-7657; Fax: (704) 588-7644 

Summary 

The presence of LPA or popcorn ash poses a serious concern for many coal-fired utility 

boiler operators after retrofitting their units with high-dust SCR systems. LPA is formed 

in the boiler and can easily be carried over into the SCR reactor. As LPA is introduced 

to the SCR reactor, it frequently causes: 

• Catalyst pluggage, which leads to 

o Flue gas maldistribution 

o Loss of SCR performance through loss of available DeNOx potential 

o Unacceptable NH3 slip 

o Excessive pressure drop 

• Catalyst erosion damage 

Many unsuccessful attempts were made in the industry experimenting with various types 

of LPA screens, i.e. hard framed, flat screens (straight or angled), and flexible curtain 

type screens (straight). None of them appear to perform satisfactorily in the long term. 
 

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However, a universally proven LPA screen design was developed in 1996 after LPA 

problems at both 760 MW units at Steag’s Voerde Power Station in Germany plugged up 

the installed honeycomb catalyst and caused severe erosion damage. In an attempt to 

solve this problem, extensive CFD and physical modeling was performed to address the 

root cause of the LPA formation and carry over to the SCR reactor. The modeling 

activities addressed specific issues such as geometric scale and the relationship 

between the model and full-scale installation including pressure drop. As a result of the 

modeling, modifications and/or additions to areas such as the boiler’s economizer 

hopper, economizer outlet and the downstream ductwork were made to allow for 

continuous and reliable removal of the LPA and safe operation of the SCR system. 

Unfortunately, none of these efforts resulted in a reliable long-term solution. Flat 

screens either plugged up, eroded away very quickly, or both. 

After several failed attempts, an LPA screen design was developed, which reliably and 

continuously prevented the intrusion of LPA into the SCR reactor by effectively removing 

the LPA from the flue gas stream without the LPA screens plugging up or being eroded 

way. Typically, such LPA screens are installed at the boiler’s economizer outlet utilizing 

the economizer hopper for the continuous extraction of the removed LPA. The special 

design features of these patented LPA screens are: 

• A pleated design, which ensures a most advantageous angle of attack of the LPA 

onto the screen and a low flue gas flow velocity through the LPA screens 

resulting in a low pressure drop. 

• A hinged design (typically at the top of the economizer outlet duct), which allows 

for the LPA screens to be rapped frequently in order to remove LPA that gets 

caught in the screens. 

• A modular design, which allows for the use of numerous different types of screen 

materials depending on the flue gas velocity. Screen materials typically used 

include: 

o Uncoated or coated stainless steel wire mesh 

o Uncoated or coated stainless steel perforated plates 

o Uncoated or coated stainless steel wedge wire 

Coating materials used include various commercially available types of metallic coatings 

such as flame spray and tungsten carbide coatings. The selection of the correct screen 

design and material must be made depending on the particular application and the 

actual flue gas velocity through the LPA screens. 
 

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Steag’s patented LPA screen design 

Since installing the Steag designed LPA screens in 1996, the Voerde Power Station has 

not had any problems with LPA intrusion into their SCR systems. Subsequently, 

numerous other coal-fired units equipped with SCR systems in Europe, as well as the 

United States, were supplied with Steag LPA screens. 

In spring 2005 prior to the 2005 ozone season, two bituminous coal-fired units, one in 

northern Indiana and Unit 3 of Dayton Power & Light’s J. M. Stuart Generation Station in 

southern Ohio, were successfully equipped with this patented LPA screen design. 

In the unit in Northern Indiana, uncoated stainless steel wire mesh screen inlays were 

used, since the average flue gas velocity in the economizer outlet duct was found to be 

no more than about 60 ft/sec. The unit has been in operation since May 2005 without 

any problems. During a recent inspection, no signs of erosion or screen damage could 

be detected. Thus, the same material was used for 2 coal-fired units in western 

Pennsylvania, which also have less than 60 ft/sec flue gas velocity and for which Steag’s 

patented LPA screens were supplied in April 2006. 
 

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DP&L’s Stuart Unit 3 has an average flue gas velocity in the economizer outlet duct of 

around 75 ft/sec. Therefore, wire mesh screen inlays with a highly erosion resistant 

metal spray coating were employed. The LPA screens in DP&L’s Stuart Unit 3 have 

also been in operation since May 2005 and have not experienced any significant 

pluggage or operational problems. 

Hinge point at Steags’s LPA screen allows for free swinging motion 

Even though this hinged and pleated LPA screen design is mostly self-cleaning, fully 

automated electric drive rapper systems were installed on these base loaded units, 

which ensure that no LPA remains on the LPA screens during operation. 

Based on the successful operating experience of DP&L’s Stuart Unit 3, DP&L decided to 

also equip Units 1 and 4 with the same LPA screen design. However, rather than 

employing coated wire mesh as a screen inlay material, DP&L opted for uncoated 

stainless steel wedge wire (see picture below; screen on right). This material is much 

more erosion resistant than wire mesh but significantly less expensive. Tungsten 
 

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carbide coated wedge wire screen inlays (see picture below; screen on left) were also

installed in Unit 3 in some selected areas of particularly high flow velocities known to

have caused erosion damage. 

Different screen inlays: uncoated stainless steel wedge wire on right and tungsten 

carbide coated wedge wire screen inlay on the left. 

The same materials were also used for the LPA screen inlays at a coal-fired power plant 

in northern Alabama. As in DP&L’s Stuart Unit 3, tungsten carbide coated wedge wire 

was used in some selected areas while uncoated stainless steel wedge wire was used in 

most areas. In all three cases – Stuart 1 and 3 as well as the plant in Northern Alabama 

– the uncoated and/or coated wedge wire screen inlays will go into operation beginning

with the 2006 ozone season. Stuart 4 will go into operation in fall 2006 using the same 

type of screens as used in Unit 1. 

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