SCR Operating Experience

EPRI SCR Workshop

October 31st, 2006
Agenda

• Regulatory Overview

• Plant Locations 

• SCR Overview

• System Detail

• Lessons Learned

• Summary
Gorgas

Miller

Gaston 

Gadsden

Hammond

Bowen

McDonough

Yates

Wansley Branch

Scherer

Greene County

Watson 

Daniel 

Crist

Smith

Scholz

McIntosh

Mitchell

Kraft

Counties Attaining Standard

Not Monitored

Insufficient Data

Barry

2001 – 2003 data

Class 1 Area Nonattainment (Partial County)

Nonattainment Areas

1-Hour Ozone Nonattainment Areas
Emission Rate Based Control System

• Atlanta Region – Georgia Power 

– 0.13 lb/MMBtu 5-plant average

• Bowen, Hammond, McDonough, Wansley, and Yates

– 0.20 lb/MMBtu 7-plant average

• All plants in 5-plant average, plus Branch and Scherer

• Birmingham Region – Alabama Power

– 0.21 lb/MMBtu 2-plant average

• Gorgas, Miller

• Gulf Coast Region – Gulf Power

– 0.20 lb/MMBtu Plant Crist average
Alabama 

Power 

Georgia 

Power 

Gulf 

Power 

Mississippi 

Power 

Scherer

Branch

Hammond 

Bowen 

McDonough

Yates

Mitchell

Miller 

Gorgas 

Gaston 

Gadsden

Greene Co.

Barry 

Smith

Crist 

Scholz

Daniel

McIntosh

Kraft

Coal Fired Plants 

SCR Installations 

Watson

Wansley 

14 SCR Units 

Currently in 

Operation

One Additional SCR 

Under Construction
Plant Hammond

Plant Bowen

Plant Miller

Plant Gorgas

Plant Wansley

Plant Gaston

Plant Crist
SCR Installations Summary

Plant, Unit MW Startup Date Additional Notes

Bowen 1 700 2001 High-dust, ozone season

Bowen 2 700 2001 High-dust, ozone season

Hammond 4 500 2002 High-dust, ozone season

Gorgas 10 700 2002 High-dust, ozone season

Bowen 3 880 2003 High-dust, ozone season

Bowen 4 880 2003 High-dust, ozone season

Miller 3 660 2003 High-dust, ozone season

Miller 4 660 2003 High-dust, ozone season

Wansley 1 865 2003 High-dust, ozone season

Wansley 2 865 2003 High-dust, ozone season

Miller 1 660 2005 High-dust, ozone season

Miller 2 660 2005 High-dust, ozone season

Crist 7 500 2005 High-dust, year round

Gaston 5 880 2006 Low-dust, ozone season

Barry 5 700 2008 High-dust, ozone season

15 Units 10,810 2009 All Units year round
SCR Systems Overview

Flow from Unit

5. Balance of Plant 

(BOP)

Rectifier

Spare catalyst layer

1. Ammonia 

Injection and 

Static Mixers

2. Flue Gas 

Analysis

3 + 1 Reactor

4. SCR Catalyst

3. LPA Screens
1. Ammonia Systems and Static Mixers

• All plants use Anhydrous 

Ammonia

• Ammonia Injection Grid (AIG) 

systems: 

– Cormetech, Siemens, Sulzer, 

Eco-Diva 

– Usually supplied by the 

original SCR catalyst 

manufacturer

• Cormetech AIG uses a dense 

injection nozzle arrangement

• All other AIG designs are coupled 

with static mixers
1. Ammonia Systems and Static Mixers

 - Lessons Learned

• Replacement of pressure relief valves on ammonia 

storage tanks has proven difficult, with most requiring 

tank emptying and nitrogen purge.

• Ammonia forwarding pumps are unreliable. Southern 

Company is auditing its ammonia delivery systems to 

ensure proper performance in year-round operation.

• For ammonia injection using dilution air, the AIG is 

started up and shut down with ammonia flow turned 

off. The tuning valves are opened 100% to free any 

ash buildup in the injection lines.

• Static mixers can cut down on frequency of AIG tuning, 

but correlating the injection grid to the measurement 

grid can prove difficult.
2. FGAS – Flue Gas Analysis System

• Circulating flue gas sampling system 

– double extraction

• Continuous average NOx concentration

– SCR inlet (8 locations per reactor)

– SCR outlet (7 locations per reactor)

• Consists of ducting, fans, probes, sample 

lines, and analyzers
2. FGAS

 - Lessons Learned

• FGAS analyzers were bought as duplicate 

systems to the plant’s CEMs.

– Familiarity of maintenance and calibration 

procedures

• High particulate loading in sampling system is 

problematic.

– Fan housing and blade wear, sample line 

plugging, probe contamination
3. Large Particle Ash (LPA), Screens

• Bowen 1 & 2 SCRs were taken offline 

in their first season of operation due to 

catalyst plugging.

• Since then, all existing and new SCR 

retrofit designs include LPA screens 

with accompanying ash-fallout 

hoppers.
3. LPA, Screens 

- Lessons Learned

• Screens are a high O&M-cost solution for LPA 

mitigation (DP and wear)

• Ongoing iterative process of optimum screen 

design (material, coating, geometry, location, 

and flow modeling)

• LPA best solved though duct design, lowvelocity fallout before SCR

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

5.50

6.00

0 48 96 144 192 240 288 336 384 432 480 528 576 624 672 720 768 816 864 912 960 1008 1056 1104 1152

Time (hr) First Catalyst Layer Pressure Drop (in WG)

0

100

200

300

400

500

600

700

800 Unit Load (MW)

June 7 June 15 June 23 July 1 July 8 July 16 July 23

MW

Catalyst Layer 1 dP 

Operating time
4. SCR Catalyst

• Coal-fired SCRs built to receive any 

manufacturers catalyst design

• Standard 1m x 2m geometry

• 9 Units have honeycomb catalyst 

– (7.1, 8.2, and 9.2 mm pitch)

• 3 Units have plate catalyst 

– (5.7 mm pitch)

• 1 Unit has mixed plate and honeycomb 

layers

• 1 Unit has corrugated-fiber catalyst
4. SCR Catalyst

 - Lessons Learned

• Still relatively young in catalyst age 

• Oldest unit began operation in 2001 

– < 20K operation hours

• Preliminary test results:

– E. Bit. units indicate arsenic poisoning 

– PRB units show masking via 

gypsum/calcium sulfate formation

– Some evidence of phosphorus 

poisoning on PRB units Wansley Catalyst Management Plan

Initial Loading 1 + 1 + 1 Layers

0

20

40

60

80

100

120

0 20000 40000 60000 80000 100000 120000

Operating Time [h]

Activity Potential K(t)/K(0) [%]

0

1

2

3

4

5

6

7

8

9

10

Gaseous Ammonia Slip [ppmv]

K(t)/K(0)

NH3-Slip

Add 4th Layer Exch 1st Layer Exch 2nd Layer Exch 3rd Layer 

As

Pb

Na

K

CaSO4

P

As F Cl
5. Balance of Plant

• Ammonia slip

– ammonia-in-ash

– air-heater plugging

• Unit turndown

– Minimum Operating Temperature

• Draft loss

– catalyst plugging
5. Balance of Plant

 - Lessons Learned

• SCR systems were installed on 

largest generating units

– Base-loaded in Summer Peak

• Loss of plant equipment 

– Boiler-feed pumps , mills, etc.

• Ammonia-in-ash can vary 

dramatically

– Best samples are collected in-situ

– Alternative is cumulative sample 

on bulk-ash transfer line

– Data from hopper samples is very 

noisy
Summary

• Preventative maintenance on ammonia systems is key for uninterrupted 

operation. Ammonia forwarding systems need improvement.

• FGAS is a main contributor for uninterrupted SCR operation. High-particulate 

gas streams can cause excess equipment wear.

• LPA is best mitigated by duct design. Never assume that you do not have it.

• Catalyst is still young, but early indicators are that bituminous coals are 

depositing arsenic and PRB coals are forming gypsum.

• Ammonia slip has not been a problem. 

• Concerns over boiler equipment failures and MOT.
Future Challenges

• CAMR – Clean Air Mercury Rule

– SCR-FGD co-benefit for mercury 

capture

– Impacts of low-oxidation catalyst

• CAIR – Clean Air Interstate Rule

– Year-round SCR operation

Ozone & Particulate

Ozone Only

Particulate Only