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Overview of

SCR’s

Clayton EricksonDirector

Process Engineering

AgendaOverview of SCR Systems

• NOx Formation• Chemistry & Catalyst• SCR Reactor and Ductwork• Ammonia Storage and Supply• Ammonia Injection and Flue Gas Mixing• Controls and Instrumentation• Testing and Commissioning

Overview of an SCR System������������� �������������� �

NOX Formation

τ

BAHR

Pulverizer

Feeder

LNB

OFA

Coal Fineness

Fuel Characteristics:� Fixed Carbon� Volatile Matter� Ash Content� Reactivity

NOX

SRB

O2

NOX = ƒ (BAHR, SRB, XSA, FC, VM, N, LNB)LOI = ƒ (τ, BAHR, FC, VM, Ash, Size, NOX)

Chemistry & Catalyst

NOX NH3

N2 H2O

4 NO + 4 NH3 + O2 4 N2 + 6 H2O7 N2 + 12 H2O6 NO2 + 8 NH3

Undesirable side reactionsSO2 + 1/2 O2

NH3 + SO3 + H2OSO3

NH4 HSO4

Basic reaction equations

Typical coal flue gas95% NO & 5% NO2

Chemistry & Catalyst

Catalyst surface

DesorptionAdsorption

ReactionDiffusionDiffusion

4 NOO2

4 NH3 4 N2

6 H2O

NOx reactor occurs on catalyst surface

SCR Catalyst Surface Reactions

4NO + 4NH3 + O2 4N2 + 6H2O

Active Site

H2OO2

NO

Reaction of NO with NH3Me--O--Me

O O

HHN

HN O

Me--O--Me

O O

H

N2 H2O

Regeneration of Active Site

Me--O--Me

O O

HH

NH3

NH3 Adsorption on Active SiteMe--O--Me

O O

HHN

HH

Chemistry & Catalyst

• SO2 to SO3 Conversion– Reaction occurs inside catalyst wall– Furnace conversion rates 0.1 to 1.5%– System component removal varies– Stack rule of thumb

• 10 ppm H2SO4 Dry stack• 5 ppm H2SO4 Wet Stack

Minimum Catalyst Operating Temperature

• SCR inlet SO3, NH3 and H2O

• Varies with fuel• Catalyst pore size

effects

SCR Catalyst Types

• Honeycomb

• Corrugated

• Plate

Chemistry & Catalyst

• Major Catalyst Deactivation Issue– Arsenic– CaO– Ash Content– Ammonia Bisulfate

Chemistry & Catalyst

0

20

40

60

80

100

1 2 3 4

CaO content in Precipitator Fly Ash

Per

cent

Gas

eous

Ars

enic

R

educ

tion

in F

lue

Gas

Control of Flue Gas Gaseous Arsenic

Catalyst Management PlanOPPD Nebraska Power

2 Initial Layers + 1 Spare Layer

0.00.51.01.52.02.53.03.54.04.55.0

0 20000 40000 60000 80000 100000 120000 140000 160000

Operating Hours

NH

3 S

lip, p

pmvd

c

NH3 Slip, ppmvdc Catalyst Potential

Catalyst Management Plan

Catalyst Loading and Unloading

SCR Reactor and Ductwork

• SCR System Configurations• Damper and SCR Bypass Configurations• Low Load Temperature Control• Catalyst Cleaning • Large Particle Ash (Popcorn ash)

High Dust Arrangement

SCR Reactor Between

Boiler Exit and Air Heater

SCR Reactor Between

Hot ESP and Air Heater

Low Dust Arrangement

Tail End Arrangement

In Duct SCR

SCR Reactor and Ductwork

• Full SCR bypass– Able to isolate reactor during operation and startup– No catalyst deactivation during non-ozone season

• Partial SCR bypass for startup– Able to isolate during startup only

• No SCR bypass or dampers

SCR Reactor and Ductwork

• Low Load Temperature Control– Flue gas economizer bypass– Economizer water side bypass– Split economizer– Feed water heater pegging

Ammonia Systems

• Anhydrous Ammonia– Hazardous chemical

governed by codes

• Aqueous Ammonia– Concentration based

codes, maybe changed in future

• Urea Based Ammonia– Safe storage, more

equipment and complex

Ammonia Injection• Anhydrous

– Vaporizers– Direct Injection– Dilution air, 5% by Volume

• Aqueous– Vaporizers– Direct Injection– Dilution air, 5% by Volume

• Urea– Direct Injection– Dilution air, 5% by Volume

0.0

3.0

6.0

9.0

12.0

15.0

01.5

34.5520

570

620

670

Tem

pera

ture

Duct Width

Duct Depth

0.0

3.0

6.0

9.0

12.0

15.0

01.5

34.5520

570

620

670

Tem

pera

ture

Duct Width

Duct Depth

NH3 Injection

Ammonia Injection & Flue Gas Mixing

Econ Outlet Catalyst Inlet

SCR Flow Models

Ammonia Injection & Flue Gas Mixing

PLANT 3Burner NOx Test 1

300

350

400

450

500

550

600

0 20 40 60 80 100

Width, ft.

NO

x, p

pm

Series1

Series2

Series3

Series4

Series5

Series6

• Inlet variations of flue gas composition

• Load and burner group dependent

• Mix prior to ammonia injection

Mixing Prior to Ammonia Injection

Gas Flow from Boiler

Static Mixers – Delta Wings

Delta Wing Ammonia Injection

Static Mixer

Static Mixer – Injection Grid

Ammonia Injection Control

• Adjusted based on tesing

Commissioning Results

100% BITUMINOUS

NH3/NOx Std Dev = 2.1% NH3/NOx Std Dev = 2.6%

-20-18.5-17-15.5-14-12.5-11-9.5-8-6.5-5-3.5-2-0.512.545.578.51011.51314.51617.519

PRB BLEND

NH3/NOx Std Dev = 2.2% NH3/NOx Std Dev = 2.5%

Reactor

“A” Reactor

“B”

0

1

2

3

4

5

6

7

8

9

10

250 275 300 325 350 375 400 425 450 475 500 525 550

LOAD (MWG)

NH

3/N

Ox

Sta

nd

ard

Dev

iati

on

(%

)

Reactor A 100% Bit. Reactor A PRB Blend Reactor B 100% Bit. Reactor B PRB Blend

Commissioning Results

SCR System Performance

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

0.900

1.000

1.100

1/1/

03

1/29

/03

2/26

/03

3/26

/03

4/23

/03

5/21

/03

6/18

/03

7/16

/03

8/13

/03

9/10

/03

10/8

/03

11/5

/03

12/3

/03

12/3

1/03

% L

oad

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

Stack Gross Load (MWg) 24Hr RollingNOx Emission Rate (lb/mmBTU) 24Hr RollingNOx Emission Rate (lb/mmBTU) 30 Day Rolling

NO

x E

mis

sio

n R

ate

(lb/m

mB

TU

)

Questions

Thanks You