Reinhold Environmental 

2008 NOx-Combustion RoTable & Expo Presentation

February 4-5, 2008 in Richmond, VA
SCR Catalyst Selection & Design

Reinhold Environmental

NO

x Round table & Expo

WPCA SCR Training

Richmond, VA

February 5, 2008
Quick Review of SCR Basics

z SCR = Selective Catalytic Reductionz Purpose is to reduce NOx (NO & NO2) fr

combustion exhaust

z Ammonia (NH3) is injected into flue gas 

agent. Flue gas passes through catalysinstalled in a reactor

z NH3 reacts with NOx on the catalyst surf

nitrogen and water vapor

CatalystCatalyst

4NO + 4NH3 + O2 4N2 

2NO2 + 4NH3 + O2 3N2 

Flue Gas: NOx, O2, SO2

N2, H2O, O2, SO2(SO3)

NH3
Catalyst

Catalystsurface

Gas phase

4

1

3 25

7

6

1 External diffusion of

NH3 to catalyst surfa

Internal diffusion of N

into the catalyst poreNH3 adsorption on th

active centersNO diffusion from gaphase to adsorbed N

Reaction of NO and to N2 and H2O

Desorption from N2 a

H2O to catalyst surfa

Diffusion of N2 and H

into gas phase

234567

How SCR Works

Catalyst has porous structure = lots of surface area
Heavy Duty

Vehicles

Wood-Fired Boilers

Gas Turbines

Marine Engines

Power Plants

Waste Incineration

Plants

Greenhouses

SCR Applications

NOx

Control
SCR Configuration in Power Plants

High Dustz SCR upstream of air preheater and z Catalyst with higher pitch required

ESP

FGD

SCR Reactorwith Catalyst

Air

Preheater

Boiler

Air

Preheater

Hot-Side ESP

Low Dustz SCR between ESP and air preheaterz Catalyst with lower pitch can be suitable

Tail Endz Mostly in Europe, none in z SCR at the end after air prz Flue gas reheating require

Flue Gas Reheat
Geometry Selection: Plate-Type Catalyst

Compositionz Stainless steel carrier, ceram

rolled on

z TiO

2, V-oxide/W-oxide/ Mo-

the active catalytic material

z Variable plate-spacing: 60 t

per element box

z Variable plate height: 450 to 

Advantages

z Most suitable for high dust cz Thermal and mechanical resz Low pressure lossz Low SO

2/SO

3 conversion ra
Geometry Selection: Honeycomb Catalyst

Compositionz Homogeneously extruded ceramz TiO

2, V-oxide and W-oxide as

the active catalytic material

z Variable monolith height: 250 mm z 10 to 100 cells per square inch (

Advantages

z Most suitable for low/no dust appz Operating temperature range: 34z High active surface area per unit 
pitch pitch

Pitch = center to center distance from one plate/wall to the next

Geometry: Structure & Pitch

Plate-Type Structurez Flexible platesz Rectangular Openingsz Pitch: 5 mm to 7 mm

Honeycomb Structure

z Rigid structurez Square openingsz Pitch: 3 mm to 8 mm
Pitch Selection vs Dust Load

Honeycomb 8 mm Plate 5 mm – 7 mm 

Catalyst Type High Dust 

4567

0 10 20 30 40 50

Dust Load, g/m

3

Pitch, mm
Geometry Selection: Module Structure

z Catalyst elements arranged 

modules

z Standardized cross-sectionz Module height can vary as rz Possible to interchange cata

within reactor

z Modules can be customized 

requirements
Catalyst Design Approach

z Starting Point

z New SCR reactor vs existing SCR reactorz Initial catalyst charge or reloadz SCR reactor layout - # of layers, # of modules per layerz SCR configuration – high dust / low dust

z Desired operating period, 16000/24000 hrsz Understand performance requirements

z DeNOx, amount of NOx to reduce, 70 - 90%z SO2 oxidation allowed, <0.5% per catalyst layerz NH3 slip allowed, 2 ppm standard (5 ppm for low S fuel)z Pressure drop limitations, 0.5 - 1.0” WC

z Determine catalyst activity required at end of operating period (EOL)z Specify appropriate catalyst to do the job: Volume, element height, pitch 
Details of the Design Process

z

ηNOx = 

ƒ {kNOx (EOL), area velocity (AV), NH

3/NOx ratio (α)}

z Calculate maximum AV (gas flow per total active surface area)z Select pitch 

Î sets the catalyst specific area, Aspec = area/volume

z Determine required volumez Adjust volume as necessary to fit the applicationz Determine SO

2 oxidation rate, pressure drop

Volcat

=

AV

How to get k
kNOx of Fresh Catalyst, k0

z k0 depends on

z Temperaturez Composition of active, catalytic material Î V2O5z H2O, O2, flue gas flow rate

z k0 data and correction factors are obtained from controlled experiments
Catalyst Activity vs Temperature

450 500 550 600 650 700 750 800 

Temperature, F

kNOx, m/h

High V2O5 Med V2O5 Low V2O5

kNOx – Influence of Temperature and V2O5

NH3 + O2 Î NOx

(Side reaction at higher T)
kNOx – Influence of Water and O2

0 5 10 15 20 25

H2O, Vol %

Catalyst Activity

0 5 10 15 O2, Vol %

Catalyst Activity
T, V2O5, H2O, O2, flow rate Î k0

Catalyst deactivates over time Î k
Masking:Macroscopic blockage of catalyst surface by cemented fly ash

Plugging:Microsopic blockage of pore systemby small fly ash particles

Poisoning:Deactivation of sites by chemic

Catalyst Surface

Pore System

Small Fly Ash Particles

Catalyst Surface

Pore System

Cemented Fly Ash

Catalyst Surfa

Pore System

Catalyst Deactivation Mechanisms 
Catalyst Poisons

z Alkali metals – Na, Kz Alkaline earth metals – Caz Arsenic – vapor phase As2O

3

z Phosphorus – P

2O

5
kNOx Deactivation Model – Relative Activity, kt/k0

kt/k0 decreases exponential

Proportionality constant, λ

z Characteristic of SCR co

fuel, deactivation mecha

z Obtained from catalyst a

1.0

Operating Time, hr

Relative Activity

kt/k0

λ increasing

kt/k0 = e -λt

Lignite firing 16PRB firing 16Bituminous firing 16Low dust config 16

Application time, h
T, V2O5, H2O, O2, flow rate Î k0

k0 Î Deactivation Model Î kt=EOL
SO2 to SO3 Oxidation, kSOx

z Undesired side reaction in SCR catalyst

2SO2 + O2 2SO3

z Occurs in the bulk of catalyst material (in the walls)z kSOx depends on V2O5 and Temperaturez “Blue Plume” is a result of SO3/H2SO4z SO3 reacts with NH3 to form ammonium bisulfate (ABS) and ammonium s

NH3 + SO3 + H2O NH4HSO4 (ABS)

NH4HSO4 + NH3 (NH4)2SO4 (AS)2NH3 + SO3 + H2O (NH4)2SO4 (AS)

z AS is dry, powdery compound that can pass through SCR catalystz ABS is sticky, viscous compound that can plug catalyst and corrode dow

equipment – example: air preheater
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

RelativeActivity

V2O5 content [wt-%]

012345

Relative

Oxidation Rate

kSOx

kNOx

SO

2 Oxidation – Influence of V

2O

5
SO2 Oxidation – Influence of Temperature

500 600 700 8

Temperature, F

kSO

x

V2O5 = 1.2%V2O5 = 0.6%
Minimum Operating Temperature, MOT (Tmin)

z Minimum temperature for continuous 

SCR operation

z Of particular concern for low load 

operation and firing fuels containing sulfur

z Operating above Tmin prevents AS/ABS 

formation

z Tmin depends on SO

2/SO

3, NH

3, H

2O in 

flue gas 

z Must stop NH

3 when operating below 

Tmin

Tmin vs SO3 inlet

NOx in 250 ppm, 88% DeNOx, NH3 

525550575600625

0 5 10 15 20 

SO3 inlet (ppm wet) 

Tmin (F) 
Catalyst Design is an Iterative Process

Design Inputs•DeNOx criteria - % of NOx to remove•Operating period – 16,000/24,000 hr•Flue gas flow conditions

•Flow rate•Temperature•Pressure•Composition (NOx inlet, O2, H2O, SO2/SO3)

•Allowable NH

3 slip

•Catalyst geometry/composition

Considerations•Reactor configuration/space, flow distribution•Coal type/specs, ash load•Catalyst deactivation•SO

2 to SO

3 oxidation

•Allowable pressure drop

Design Out•Catalyst vol•Catalyst act•SO

2 to SO

3 

•Pressure dr•Design mar
20406080

100120140

0 20000 40000 60000 80000 100000 

Operating Time [h]

Activity Potential kt/k

0 [%]

Catalyst Management Plan (CMP)

Minimum potential to meet DeNOx, NH3 slip requirements

Add L3 Replace L1 Replace L2 Replace L3

2+1 layer example: initial load 2 layers, L3 empty
Thank You

Reinhold Environmental

NO

x Round table & Expo

WPCA SCR Training

Richmond, VA

February 5, 2008