# Optimizing SCR Catalyst Design and Performance for Coal-Fired Boilers

Scot Pritchard Chris DiFrancesco

Cormetech, Inc. Durham N.C (919) 620-3000

Shozo Kaneko Norihisa Kobay ashi Mitsubishi Heavy Industries, Ltd. Nagasaki, Japan

(045) 224-9178

Kohei Suy a ma Kozo Iida Mitsubishi Heavy Industries, Ltd. Nagasaki, Japan (958) 28-6401

Presented at EPA/EPRI 1995 Joint Symposium Stationary Combustion NOx Control
May 16-19, 1995

1

### OPTIMIZING SCR CATALYST DESIGN AND PERFORMANCE FOR COAL-FIRED BOILERS

Scot Pritchard Chris DiFrancesco

Cormetech, Inc. 5000 International Drive Durham, NC 27712

Shozo Kaneko Norihisa Kobayashi Mitsubishi Heavy Industries, Ltd. Yokohama, Japan
Kohei Suyama Kozo Lida Mitsubishi Heavy Industries, Ltd. Nagasaki, Japan

Abstract
The design of an SCR system for a coal-fired application is a challenge due to particulate, catalyst poisons, and So 2 in the flue gas. Our ex perience show that coalfired SCRs are successful when the system impact and catalyst deterioration factors are understood and specific countermeasure are implemented in system and catalyst design

There are sev e ral factors that the system and catalyst must consider in order to assure success. They include: type of boiler, required performance, fuel and ash analysis, ash loading, type of SCR, inlet conditions, Catalyst deactiv a tion mechanisms and impact on down stream equipment.
This paper ex plains each system and catalyst deterioration factor that we hav e ex perienced. For each factor, it presents t he specific countermeasure that has resulted in successful SCR systems. Also, it discusses the design of improv ed catalyst deriv ed from our ex perience. Finally, two case studies illustrate the impact of optimized design.



### 2Introduction

Selectiv e Catalyst Reduction (SCR) is recognized worldwide as the most effectiv e NOx Control Technology for utility boilers and combustion turbines when substantial NOx reduction of 50% to 95% is required. In addition to its prov en high performance, It has also become an economically v i able solution, with current installed cost, in the United States, estimated at between $20/kw for natural gas and $50/kw-$70/kw for coal units. The technology has ev en giv en some utilities the capability to achiev e lower heat reduction or omission of flue gas recalculation (FGR).

Mitsubishi Heav y Industries (MHI) pioneered the dev eloped and application of SCR technology staring in the late 70’s in Japan, and installed the worlds’ first High-dust SCR application on a 175 MW coal fired boiler in 1980. MHI transferred their technology through licensing agreement to sev e ral companies in Europe and to Cormetech in the United States. Ov er 319 units including 56 coal fired boilers worldwide operation successfully using this technology.

Figure 1 Catalyst for Coal, Oil, and Gas Applications
Cormetech is a joint equity company of Corning Incorporated, MHI, and Mitsubishi Chemical Company (MCC). Cormetech design and Manufactures homogeneous Titanium-Tungsten-Vanadium (Ti-W-V), Ex truded honeycomb catalyst for fossil-fuelf i r e d a p p l i c a t i o n s

. F i g u r e 1 show a photograph of catalyst for coal, oil, and gas applications. Cormetech draws upon the v a st ex perience database of MHI, the
ex trusion and catalyst know-how of corning, and catalyst technology of MCC to prov ide an optimized product to the market. This paper address the methods successfully implemented to design and optimize catalyst performance in coal fired boiler applications. The basics of deNOx catalysis are



### 3rev i ewed in preparation for the discussion on system impact, catalyst deterioration, mechanism, and countermeasures that follows. Additionally, catalyst design optimization is presented by a discussion of improv ed catalysts for coal applications. Two case studies (pulv e rized coal and cyclone boilers) comparing conv entional and improv ed catalyst are presented.
Background of SCR Reaction

In coal fired boilers in which SOx is presented in the flue gas two chemical reaction that occur in the presence of the SCR catalyst are of most importance to the following discussion
One reaction, of course, is the deNOx reaction:

4NO + 4NH 3 + O 2 cataly st 4NH 2 + 6H 2 O (1)
Figure 2 illustrates this primary reaction mechanism. Starting in the upper left-hand corner of the figure and proceeding clockwise is:

1. Active catalyst site (Me = metal) 2. Absorption of ammonia molecule 3. Reaction of NO with NH 3 forming 4. Regeneration of activ e site by ox idation

The second reaction is ox idation of sulfur diox ide: (2) SO 2 + 1⁄2 O 2 cataly st SO 3
The NOx conv ersion rate and SO 2 ox idation rate will partly depend upon the rate in which the reactants diffuse through the wall of the porous catalyst to reach activ e sites where reaction takes place. Through our ex perience we hav e determined the rate of chemical reaction is fast reaction for our catalyst. For deNOx conv ersion the rate of chemical reaction is fast relativ e to the rate of diffusion. Therefore the catalyst is effectiv e primarily at the surface of the wall. On the other hand, the SO 2 ox idation chemical reaction rate is slow relativ e to the rate of diffusion so that the reactants diffuse into the entire wall. This conclusion is relev ant to the subsequent discussion on the system impacts, catalyst deterioration mechanisms, and countermeasures.



### 4O 

### N 2 

### H 2 

### O H H N H H
O O
--Me-- O - - M e

- - NH 3 

### Adsorption on Active Site H

O

--Me-- O - - M e

- - O

Active Site NH 3



### H 2 

### O
O 2 

### NO 4NO + 4NH 3 

### + O 2 

### → 4N 2 

### + 6H 2 

### 0

N H H

N H H



### --Me-- O - - M e

- - O O



### React ion of NO with NH 3 

### H H
O O
--Me-- O - - M e

- - Regeneration of Active Site

Figure 2 SCR Surface Reaction Mechanism

High Dust System

Low Dust System NH 3

Boiler SCR S TACK ES P FGD Air He at er

NH 3

Boiler SCR

STACK

ES P FGD Air He at er He at er

Gas to Gas Heat Exchanger

Figure 3 SCR Configurations



### 5Sy stem Design Impact & Countermeasures
Figure 3 shows two typical layout of SCR system applied to coal fired boilers. The LowDust or Tail End option shows the installati on of the SCR reactor after the air preheater (APH), Electrostatic Precipitator (ESP), and flue Gas Desulfurization (FGD) systems. This option requires the flue gas to be reheated to acceptable SCR temperatures, typically 550 0 F-750 0 F. Low-Dust designs hav e primarily been used on wet bottom boilers which hav e ash re-circulation, due to concern ov er catalyst degradation caused by arsenic poisoning. The High-Dust option locates the SCR reactor between the economizer ex it and the AOH inlet. This is more traditional for dry bottom boilers and less costly, and is now being applied to wet bottom boiler based on economics, adv ancements in catalyst resistance to arsenic poisoning, and limestone addition to the fuel, which will be discussed later.

CON T ROL SYSTEM

AQUEOUS
NH3 TANK

PUMP FLOW CON T ROL

H E ATER

FA N VAPORIZER

MIXE R NH3 INJECTION GRID

S O O T BLO WE RS

CA TAL Y S T

LA YERS

AIR
PRE H EATE R ECO N OM IZ ER BYP A S S

STATIC MIXER

TURNING VANES

Figure 4 SCR Components
The SCR system consists of many components in addition to the catalyst. Figure 4 depict a high-dust SCR system which includes the catalyst reactor, economizer bypass used for part-load temperature control, static mix e r used for temperature and/or ammonia mix i ng, turning v anes, ammonia v aporization system, ammonia injection grid (AIG), sootblower, and the control system. Since these components directly influence
ov erall system effectiv eness, Cormetech works closely with system equipment suppliers to ensure proper design.



### 6Operational impact of an SCR system on coal fired boilers included drafts loss, SO 3 formation, and ammonia emission, Pressure drop is caused by the installation of the catalyst material in the flue gas and is typically minimized by ex panding the flue gas path. Typical pressure v a lue for conv entional high dust applications range from 0.5 to 1inch of water per catalyst layer, with 2-3 layers installed with an additional 1-2 inches for the components of the system.
As shown in equation (2) the SCR catalyst will conv ert a small amount of SO 2 to SO 3
. This conv ersion rate is typically less than 0.55 per layer depending upon the catalyst formulation, deNOx requirements, and operating temperature. If the SO 2 ox idation rate is too high, corrosion and plugging will occur in the (APH) due to the formation of H 2 SO 4 and NH 4 HSO 4. As a countermeasure, our catalyst is designed to achiev e high deNOx activ i ty while keeping SO 2 ox idation activ i ty low. When considering the catalyst formulation, a high concentration of v anadium will result in a high deNOx activ i ty, but will also result in a high SO 2 ox idation activ i ty. One countermeasure to this is a v anadium concentration is selected such that there is sufficient deNOx activ i ty with an acceptable lev e l of SO 2 ox idation.
As a system countermeasure, the cold end surface of regenerativ e-type APH may be enameled and/or sootblowing capabilities may be enhanced.
Ammonia slip may contaminate flyash, and combination with S0 3 caused (APH) plugging. Designers can combat the effect by considering:

• 2ppm to 5ppm NH 3 slip at end-of-life

• ammonia injection grid with adequate repres entativ e NOx input v a lue to control system

• multi-point outlet NOx sampling control gr id to assure representativ e NOx input
v a lue to control system.

• permanent measuring grid for AIG tuning to assure proper distribution of ammonia

• Flyash ammonia concentration monitoring.
Cataly st Deterioration Mechanisms & Countermeasures
When designing for a giv en application, designers must carefully rev i ew fuel And ash constituents ( see Table A ). Under ideal conditions the catalyst will reduce NOx for an unlimited period of time. ( Table B ) lists the primary deterioration mechanisms
v e rsus the type of coal fired units, wet or dry bottom.



### 7Ash Moisture, % 6-33 Total Sulfur in Coal, % 0.6-1.6* Trace Material, ppm N i 3

- 4 0 C r 7

- 4 6 A s 1

- 2 5 C l 4 1

- 1
, 9 0 0 Ash analysis, % Sl O2 41-71 As2 O3 2-33 Fe2 O3 2.5-10 CaO 2.4-26 M g O 0

. 7

- 4 9 Ti0 2 0.1-1.8 M n O 0

. 0 2

- 0

. 2 V2 05 0.01-0.1 Na2 O 0.05-1.6 K 2 O 0

. 1

- 4

. 0 P2 O6 0.06-1.3 S O 3 1

. 6

- 1 6

. 5 *Application Experience on Oil up 5.4% Sulfur

Table A Typical Fuel/Ash Composition

Boiler Type Wet Bottom Dry Bottom S i n t e r i n g N e g l i g i b l e N e g l i g i b l e Accumulation of Alkaline Metal Small Small
Accumulation Earth Metal M a sking L a r g e L a r g e

Accumulation of Arsenic Oxide Large with Ash Re-circulation Moderate Flyash Deposition* Small Small
E r o s i o n * S m a l l S m a l l *Highly dependent on proper design of SCR system

Table B Main Causes of Catalyst Deterioration



### 8Based on a sound understanding of the deterioration mechanisms, specific countermeasures are implemented, such as:

◆ system design ev aluation. ◆ catalyst formulation. ◆ catalyst v o lume selection. ◆ catalyst geometry. ◆ catalyst macro and micro pore design. ◆ catalyst characterization testing and predictiv e tools.
Thermal Sintering
Thermal sintering is the growth of primary catalyst particle resulting in a reduction of catalyst surface area, which reduces catalyst performance. Figure 5 illustrates this mechanism. Thermal stability is max i mized with the corporation of Tungsten in the catalyst formulation. As a result, sintering is negligible at normal SCR operating temperatures.

PR IMA R Y PA R T IC L E O F TIO2 SEC O N D A R Y PA R T IC L E O F TIO2



### FR ESH C A T A L YST HE AT S I NTE R E D CATALY S T

Figure 5 Thermal Sintering
Alkaline Metal (Na, K)
Alkaline metal may directly react with activ e sites and render them inert as shown in Figure 6 . Since the deNox reaction takes place primarily on the surface, the degree of deactiv a tion depends on the surface concentration of the alkaline metals. In a watersoluble form, these alkaline metals are highly mobile and will migrate throughout the catalyst material. Since the wall of our catal yst consists entirely of catalyst material, the surface concentration of alkaline metal is diluted by this migration, minimizing the deactiv a tion rate.



### 9

## H

O O

--Me-- O - - M e--
FRESH CATA LYST Na (K)

O O

--Me-- O - - M e--

A KALINE DETERIO RATIO N Na + 

## (K = 

## )

Figure 6 Alkaline Deterioration Mechanism
For the subject catalyst in coal-fired applications, it is our ex perience that this type of poisoning has a small impact, since most of the alkaline metals in the coal ash are not water soluble. The impact is larger in oil-fired applications where the majority of the alkaline metals in the ash are water-soluble or when the thickness of the catalyst material is low. For ex ample, giv en the same concentration of alkaline metals in the flue gas, this catalyst will hav e a lower surface poison concentration than a coated product and, thus hav e a lower deactiv a tion rate and longer useful life as shown in Figure 7

.NOx Removal Efficiency, (%)



### 10 20 30 40 0 60 80 100

Coated Type Solid Ho neycomb Type

Operat ion Period (‘ 000 hrs) Figure 7 Comparison of Durability (Oil Firing)



### 10Arsenic (As)
Arsenic poisoning is caused by gaseous arsenic As 2 O 3 in the flue gas. The As 2 O 3 diffuses into the catalyst and solidifies on both activ e and non-activ e sites, as shown in Figure8

. As with alkaline metals, arsenic poison is best abated by use of homogenous catalyst compositions, which effectiv ely reduce the surface poison concentration. Since the mechanism of arsenic poisoning is more diffusion-limited and subject to capillary condensation, optimized catalyst pore struct ure also lessens its effects. Further, engineers use accumulation rates determined by both lab and field tests to ensure a proper catalyst v o lume is prov ided for each specific application.

FRE S H GAS E O US ARS E N I C DE T E RI OR AT UI O N --Me-- O --Me- -O

- - M e

--

O O O H

--Me-- O --Me- -O

- - M e

-- O O O --A s--O--A s--O -- As--

O O O As 2 O 3 O 2

H o neyco m b C a ta ly s t

                        As 2 O 5

A c t i ve M a t e rial   S u p port ( T i

- W) Pr imar y Par t icl e Of

C a t a l y st

Figure 8 Model of Arsenic Compound Accumulation
A system countermeasure incorporates the use of a fuel additiv e . As mentioned prev iously, wet bottom boiler with 100% ash recalculation present a worst case scenario for catalyst deterioration caused by arsenic poisoning. In order to combat the high lev e ls of gaseous arsenic (As 2 O 3 ) in cyclone boilers, limestone may be added to the fuel. A typical limestone to fuel ratio is approx imately 1:50. Figures 9a, 9b, and 10 show how the addition of limestone effectiv ely reduces gaseous arsenic at the entrance to the catalyst. The free CaO in the limestone reacts with arsenic, forming a solid, Ca (AsO 4 ) that will not poison the catalyst. The impact of adding limestone will be addressed further in the wet bottom boiler case study.



### 11A r se n i c C o m p o u n d

ES P A P H A s 2O 3 ( G ) + 3C aO + O 2 Î C a3( A s O 4 ) 2 ( S ) A s 2O 3 ( G ) + O 2 Î A s 2O 5 ( S )

2F eA s S + 402 Î
A s 2O 3 ( G ) + F e O 3 + SO 2 A s 2O 3 ( G ) Ø

De N O x Re a c t o r So l i d St a t e G ase o u s S t at e

Co a l

Fe A s A

Figure 9a Arsenic Compound in Flue Gas

As2 O 3 (G) + 3C a O + O2 Î Ca 3(AsO 4 )2 (S) Ar seni c Com pou nd



### ES P A P H As2 O 3 (G) + O 2

Î

A s 2O 5 ( S )

2F e A sS + 40 2 Î As2 O 3 (G) + FeO 3 + S O 2 A s 2O 3 (G) 

### Ø

De NO x R eac tor So lid S t a t e Gas e ous S t ate

Co al

Fe As A Ca Co3 Î 3C a C o + C O 2

Figure 9b Effect of Limestone Injection



### 12

### 1 10 100 1000

3 1 0 3 0 : Plan t A : Plan t B : Plan t C Gaseous arsenic @ 1st Cat a l y st

L a yer (micro grams/Nm 3 ) Arsenic Recycle Ratio = 100%



### E f fect of L imestone

In

je ctio n

Arsenic in Coal (m g/kg)

Figure 10 Relationship Between Arsenic in Coal and Gaseous Arsenic
Alkaline Earth Metals (Ca)
Alkaline earth metal poisoning primarily occurs when free CaO in the flyash reacts with SO 3 adsorbed on the catalyst surface forming CaSO 4 . The CaSO 4 may cause catalyst surface masking, prev enting the reactants from diffusing in to catalyst surface masking, prev enting the reactants from diffusing into catalyst. Figures 11 and 12 illustrate this mechanism.

FRE S H CALCIU M DETERIORATION Ca SO 4

Ca SO 4

Figure 11 Mechanism of Calcium Deterioration



### 13Level of Ca

Catalyst Wall C a talyst Surface
Figure 12 Accumulation of calcium Composition in Catalyst
In order to address masking, particularly in dry bottom boilers where the free CaO content is nearly double that wet bottom boiler ( s e e F i g u r e 1 3 ), characterization of the mechanism and accurate prediction of deactiv a tion rates is employed. Ex perience shows that quantity of free CaO in the fly ash gov erns the amounts of deactiv a tion. Ex tensiv e modeling of deactiv a tion from full scale and laboratory data has been dev elopment prov iding high confidence in design to counter this phenomenon.

0 20 40 60 80 100

0 20 40 60 80 100

A B C D E G H I F avera g e avera g e

Plants

Fr ee C a O A m o r ph ous P h ase Fr ee C a O A m o r ph ous P h ase

Wet Bott om B o il er A s h D ry Bottom B o ile r A s h Figure 13 Free CaO Ratio in Fly Ash



### 14Cataly st Plugging
Catalyst plugging has two primary causes: (a) ammonia salt deposition and (b) flight deposition. Proper selection of catalyst pitch and cell opening size minimizes plugging. Good system design is also a primary prev ention tool.
Ammonia salt formation/deposition will not occur prov ided the SCR inlet temperature maintained abov e the salt formation lev e l. Typically temperature maintenance is only a problem during part load operation and is elev ated by installing a water- or gas-side economizer bypass in the system. If a gas bypass is used, careful attention must be paid to prov ide adequate temperature mix i ng upstream of AIG. This av oids low temperature streamline, which can cause localized salt formation at the AIG and catalyst.
Flight deposition is minimized through proper flow distribution across the catalyst face. This accomplished through engineering of SCR system flue layout, Computational Fluid Dynamics (CFD) codes, and/or cold flow modeling.
The design limit the number of areas where dust accumulation may occur, i.e.,

• Av oiding flat surface as long leading edge to turning v anes.

• Installing as deflection dev ices on support beam flanges and sootblower pipes.

• Av oiding flue dead between dampers where ash accumulation may occur in closed position and ex haust onto catalyst when opened, etc.

Erosion
Catalyst erosion is caused by the impingement of flyash on the catalyst face. Catalyst erosion is a function of gas v e locity, ash character, angle of impingement, and catalyst properties. Cormetech has ex perience with flyash loading as high as 30g/Nm 3 and flue gas v e locities up to 6.2 m/s.

Ex perience has prov en that no significant erosion will occur with proper system design, catalyst material durability, and catalyst edge hardening. Poor flow and ash distribution at the inlet to the catalyst has been the source of most problems, and has improv ed significantly ov er the entire SCR ex perience hi story. Careful attention must be paid to utilize flow modeling (CFD and cold flow rectifier grid should be installed to straighten flow into the catalyst bed. Since most cata lyst erosion occurs at the catalyst leading edge, which is ex posed, to the direct impingement of dust particle the catalyst may also be hardened at the entrance region to prov ide further protection as shown in Figure 14

.



### 15FL YA SH

HARDENED EDGE

CATALYST

Figure 14 Honeycomb Catalyst with Hardened Edge
Although in most cases In-Duct SCR is not attractiv e due to ex tremely high system draft loss and difficulty in control of gas flow and ammonia distribution, some utility site are considering the concept due to site plan limitations. Therefore, we will pilot test our hardened edge catalyst at v e locities > 12m/s in 1995.
Catastrophic Failure of SCR Cataly st
Catastrophic Failure is defined as sudden and permanent loss of catalyst performance based on our ex perience, catastrophic failure are ex tremely rare. The primary cause is associated with the ignition of ash buildup. T he intense heat of a fire can irrev e rsibly damage any SCR catalyst. Cormetech’s ceramic honeycomb catalyst, unlike plate catalyst with stainless steel mesh substrates , will not promote ox idation; therefore fires are less likely to spread and are more easily contained.
Coal Cataly st Developments
Based on our understanding of reaction and deactiv a tion mechanisms we are improv ing our catalyst. The objectiv e is to increase the deNOx reaction rate without increasing the SO 2 ox idation rate. One method is to modify the pore structure of the catalyst wall in order to reduce diffusion resistance. The other is nov el method and requires further
ex planation.

Since the deNOx reaction is only effectiv e at the surface, we hav e dev eloped an improv ed catalyst of the same composition as our conv entional catalyst with the
ex ception that the v anadium is preferentially distributed to the surface if the catalyst wall. In this manner, we can max i mize the concentration of v anadium in the effectiv e fraction of the wall, max i mizing deNOx activ i ty without incurring high SO 2 ox idation. Figure 15 shows the performance of this improv ed catalyst compared to that of
conv entional catalyst. Since the product is still made entirely of catalytic material, resistance to poisons in maintained.



### 16Figure 15 Comparison of Catalyst Performance
Development and Qualification of Improved Coal Cataly sts

We are dev eloping the improv ed coal catalyst described according to the schedule shown in Figure 16

.

199 3 199 4 199 5 199 6

Sc re ening of Improved Coal Cata lysts

Co nfi r mati on Test

En du rance Test

Pi lo t Pl an t Te st

Figure 16 Improv ed Coal Catalyst R&D Schedule

We hav e already completed laboratory performance confirmation and durability tests. The performance of the improv ed catalyst is approx imately 15% higher that our
conv entional catalyst, without impact on SO 2 ox idation.



### 17We hav e tested durability to gaseous arsenic poisoning by an accelerated method using the apparatus in Figure 17 . The test results are shown in Figure 18 . The improv ed catalyst maintains its performance adv antage ov er conv entional catalyst at any arsenic poisoning lev e l.

A s 2O 3 P o w d e r

S CR CA T A L Y S T T H ER M O C O U P LE T H ER M O C O U P LE

H 2 O S uppl y

G a s F l o w C ont r o l l e r

G a s S uppl y U n i t F U R NAC E E XHAUS T

A r s e n i c R e m ova l

U n i t TI TI

Figure 17 Schematic of Arsenic Testing Apparatus

Activity



### Arsenic Loa d, ppm*hr Conve ntional Improve d

Figure 18 Arsenic Durability



### 18Since the degree of Deactiv a tion due to surface masking by CaSO 4 is dependent only on the fraction of surface masked we ex pected that the rate is deactiv a tion is the same for both conv entional and improv ed catalyst. Therefore, giv en the same installed catalyst v o lume, the improv ed catalyst will maintain field performance much longer due to the higher initial activ i ty, as shown in Figure 19

.

TI M E , ye ars Im prove d

Convent i onal AC T I VI T Y

2 468 Figure 19 CaO Durability

Before Commercialization, the endurance of the catalyst will be confirmed by operation in actual plant. Initial endurance tests will start by charging catalyst samples into commercial SCR plants in Japan. During 1995 catalyst samples will be charged into a coal unit in the United States and three coal units in Germany. The performance of the catalyst sample after aging in the commercial plants will be compared to fresh performance in order to ev aluate endurance.
Impact of Improved Coal Cataly st on SCR Design for Coal Fired

Whether achiev ing an increase in performance by optimizing the pore structure of the catalyst on the SCR design, two SCR case studies are described below comparing improv ed with conv entional catalyst.



### 19Case Study 1: Dry Bottom Boiler

A 250 MW unit is selected as an ex ample unit. Table C shows the design data for this unit. Figure20 shows the 10-year catalyst v o lume of the improv ed catalyst is higher by 15%; the catalyst v o lume of the improv ed catalyst is decreased by approx imately 15% from the conv entional catalyst v o lume.

Design Condition; F u e l C o a l Plant Output, MW 250 Flue Gas FlowRate,Nm3/hr 784,000 NOx inlet, ppmvd @ 3% O2 154 NOx outlet, ppmvd @ 3% O2 31 N O x R e moval Effic i enc y, % 80 Ammonia Slip, ppmvd @ 3% O2 5 S p e c i f i c a t i o n : Type of System SCR Type of Reactor Vertical Flow-Fixed Bed Type catalyst Honeycomb Number of reactor 1

Table C Dry Bottom Boiler

Cat a ly st V o lu me Rat i o

1.0 I m pr oved
Conv entio nal Add 4 t h Layer

Add 4 t h Layer

Operatin g Y ears 0 1 2 345 6 7 8 9 1 0

Figure 20 Impact of Improv ed Catalyst Dry Bottom Boiler



### 20Case Study 2: Wet Bottom w i th Ash Recalculation

A 320 MW unit is selected as an ex ample unit. Table D shows data for this unit. Figure 21 shows the 10 years catalyst management plan. The total catalyst v o lume of the improv ed catalyst for ten years of operation is two-thirds that of the conv entional catalyst. The impact on the catalyst management plan is shown in Figure22 . With Limestone Injection, the need for addition and repl acements of catalyst are significantly delayed.

Design Condition Coal F u e l 3 2 0 Plant Output, MW 1,125,000 Flue Gas Flow Rate,Nm 3 / h r 1
, 6 5 2 NOx Inlet, ppmvd @ 3% O2 530 NOx removal efficiency, % 68 Ammonia Slip, ppmvd @ 3% O2 5 Flash recalculation Rate, % 100 S p e c i f i c a t i o n : Type of System SCR Type of Reactor vertical flow-fixed bed Type of Catalyst honeycomb Number Of Reactors 1

Table D Wet Bottom Boiler



### 21Improved
Conventional

Ope r ating Years 0 1234 5 6 7 8 9 1 0 1 2 3 4 Ca talyst Volume Ratio

Ad d 3rd Layer Ad d 3rd Layer Ad d 4th Lay e r Exchange 1st Layer Exchange 2nd Layer

Ad d 4th Lay e r

Figure 21 Impact of Improv ed Catalyst Wet Bottom Boiler

Im p r o v e d w / L i m e In j.
C o n v e n t i ona l w / o L i m e I n j

.

O p erat i n g Y e a r s 0 123 4 5 678 9 1 1 2 3 4 Ca t a l y s t Vo l u m e Ra t i o

A d d 3r d L a yer A d d 4t h L a y e r

A d d 3r d L a yer

0

Figure 22 Impact Of Limestone

Wet Bottom Boiler



### 22Conclusions

Our ex perience shows that coal-fired SCRs are successful when proper attention is paid to system and catalyst design.
Proper system design is crucial to Max i mizing Catalyst performance potential and achiev ing lowest annualized cost.
By ex ploiting the ex perience and know-how gained on ex isting facilities it is possible to further optimize catalyst design.



### 23