Steag LLC

PO Box 4107 28 • 224 We stingh ou se Blvd. #607 • Charlotte, NC 2827 3-6 228

Phone: 704 -5 88-7 657 • Fax:704-5 8 8

- 7 644 E-mail: info@steagll c .com • www.ste agl lc.com 

# Large Particle Ash (LPA) Screen Retrofits

at Coal-Fired Units in Indiana and Ohio

2006 Env i ronmental Controls Conference Sponsored by U.S. Department of Energy’s Nati onal Energy Technology Laboratory May 16 – 18, 2006 Pittsburgh, Pennsylv a nia

### 2 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 NH 3 slip

o Excessive pressure drop

• Catalyst erosion damage

Many unsuccessful attempts were made in t he 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.

### 3 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 phy sical 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 installe d 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.

### 4 Steag’s patented LPA screen design

Since insta lling the Steag designed LPA screens in 1996, th e Voerde Power Station has not had any problems with LPA intrusion into the i r SCR syste m s. Subsequently,

numerous other coal-fire d units equipped with SCR systems in Europe, as well as th e United States, were sup p lied with St eag LPA screens.

In spring 20 05 prior to t he 2005 ozo ne season, t w o bituminous coal-fired units, one in northern Indiana and Unit 3 of Dayton Power & L i ght’s J. M. Stuart Gene ration Statio n in southern Ohio, were successfully equipped with this patente d LPA screen design.

In the unit in Northern Indiana, unco a ted stainle ss steel wire mesh scree n inlays were used, sin c e the average flue gas velocity in the economizer outlet duct was found to be

no more than about 60 ft /sec. The u n it has been in operation since May 2005 without any problems. During a recent inspe c tion, no sig n s of erosio n or screen damage could

be detected. Thus, the same materia l was used f o r 2 coal-fire d units in we stern Pennsylvani a, which also have less than 60 ft/se c flue gas v e locity and f o r which Steag’s patented LPA screens w e re supplied in April 200 6.

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

around 75 ft /sec. Theref ore, wire me sh screen in lays with a highly erosio n resistant metal spray coating wer e employed. The LPA screens in D P&L’s Stuart Unit 3 have

also been in operation since May 20 05 and have not experienced any significant pluggage or operational problems.

Hinge point at Steags’s LPA scree n allo w s for free sw ingi ng motion

Even thoug h this hinge d and pleate d LPA screen design is mostly self-cleaning, fully

automated electric dr ive rapper systems were installed on the s e base loa ded units,

which ensur e that no LPA remains on the LPA screens durin g operation.

Based on th e succe ssful operating e x perience of DP&L’s Stuart Unit 3, DP&L decided to also equip U n its 1 and 4 with the sa me LPA screen design. However, rather than employing coated wire mesh as a screen inlay material, DP&L opted for uncoated

stainless ste e l wedge wire (see pictur e below; scr een on right ). This material is much

more erosion resistant t han wire me sh but sign ificantly less e x pensive. Tungsten

### 6 carbide coat ed wedge wire screen inlays (see picture below; screen on le ft) were also

installed in Unit 3 in so me selected areas of pa rticularly hig h flow velocities known t o have caused erosion da mage.

Different sc reen inla y s : uncoated stainless steel w edge w i re on right and tungsten carbide coated wedge w i re screen inla y on th e left.

The same materials wer e also used for the LPA screen inlay s at a coal-f ired power plant

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

was used in some select ed areas while uncoated stainless st eel wedge wire was use d in

most areas. In all three cases – Stu a rt 1 and 3 as well as th e plant in N o rthern Alabama

– the uncoat ed and/or co ated wedge wire screen inlays will g o into operat ion beginnin g with the 2006 ozone sea s on. Stuart 4 will go into operation in fall 2006 using the same type of screens as used in Unit 1.

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