# Development and Operating Results of Low

SO2 to SO3 Conversion Rate Catalyst for

DeNOx Application

By
Isato Morita Yoshinori Nagai Dr. Yasuyoshi Kato
Babcock

- Hitachi K.K., Japan

Dr.Howard N. Franklin Hitachi America Ltd., U.S.A.

John Cooper Indianap olis Power & Light Company,

U.S.A.Abstract

DeNOx process, using Selective Catalytic Reduction of NOx by ammonia, is now widely

applied to reduce NOx emissions from stationary sources. And, nowadays, hundreds of

DeNOx plants for coal fired flue gas have be en operating successfully in Japan, Europe,

U.S.A., Canada and Eastern Asian counties. Though, through the operation with very high

Sulfur content coal, the development of low SO2 to SO3 conversion rate catalyst became one

of the most important research wo rks for the catalyst manufacturer. After extensive research

work, we, Babcock

- Hitachi developed the low SO2 to SO3 conversion rate catalyst keeping

the DeNOx activity high, and applied it for commercial application, and two DeNOx plants

were successfully

g ot into commercial operation in 2004.

1. Introduction

About 30 years have passed since the first catalytic DeNOx system for large utility boilers

was started operation in 1977 in Japan. Since then, the system was getting more and more

widely applied for vari ous fuels such as gas, oil and coal, and the installation was expanded to

various industrialized countries, Japan, Europe, U.S.A., Canada and Eastern Asian countries.

And nowadays, hundreds of DeNOx plants for coal fired flue gas have been operated

success fully.

Though recently, through the increasing commercial plants ’ operation in U.S.A. firing Eastern

Bituminous coal which contains high Sulfur(3

– 5%), several plants got difficulties in

operation, such as severer blue plum generation and Sulfuric acid sm ut emission. The

increased SO3 concentration also causes (1) acceleration of corrosion on downstream

equipments, (2) increased possibility of Ammonium Bisulfate formation within air preheater

elements and (3) increased PM emission, etc.

To overcome this s ituation, various research efforts to reduce SO3 emission were conducted

from various aspects such as additives injection to combine SO3 into less troublesome formsand so on. Though, as a fact of matter, the development of low SO2 to SO3 conversion rate

c atalyst can be the most effective solution to minimize the production of SO3 through catalyst,

and therefore it became the most important research target amongst others for the catalyst

manufacturer.

However, the SO2 to SO3 conversion rate was closely rela ted with DeNOx activity. And

when the SO2 to SO3 conversion activity was reduced, larger volume of catalyst became

usually required to maintain the required DeNOx performance. Therefore, the development

has to be focused to find out the catalyst having imp roved DeNOx activity with low SO2 to

SO3 conversion activity. Through diligent research work, not only for the catalyst

manufacturing process but for the screening of the new materials, we, Babcock

- Hitachi(BHK),

has developed

low SO2 to SO3 conversion rate catalyst, keeping the DeNOx activity high,

and applied it for commercial application, and two DeNOx plants were already got into

commercial operation in 2004 successfully.

This paper presents this new SO2 to SO3 conversion rate catalyst, including the dev elopment

efforts and the results of commercial operation experience.

2. Development of Low SO2 to SO3 Conversion Rate Catalyst

(1) Target Performance for Developing Low SO2 to SO3 Conversion Catalyst

Table 1 gives the representative design gas conditions for DeN Ox catalyst for high Sulfur coal

fired flue gas in U.S.A. To dates, the DeNOx plants were usually designed with SO2 to SO3

conversion rate of less than 1.0 to 1.5 %. Though, because of the high NOx removal

requirement (more than 90%) with less than 2 ppm NH3 slip, difficulty appeared to achieve

the requirements at elevated temperature such as 400 to 420 ° C. Furthermore, operational

difficulties such as blue plum generation and acid smut emission became appeared in several

DeNOx plants. Then, we, BHK, decid ed to develop the low SO2 to SO3 conversion rate

catalyst that can achieve less than 0.5 % SO2 to SO3 conversion even at elevated temperature.Thus, the developed catalyst should have approximately 1/4 of SO2 to SO3 conversion rate

maintaining comparable D eNOx activity as of the catalyst used at the timeframe.

Table 1 Representative Design Conditions for DeNOx Catalyst in U.S. High Sulfur Plants

Items Condition
Inlet NOx concentration 300 ppm
SO2 concentration 1,800

– 3,500 ppm DeNOx Effeiciency More than 90 % Slip NH3 concentration Less than 2 ppm
Life time More than 24,000 hours

(2) Concept for development of low SO2 to SO3 conversion rate catalyst

Continuous efforts were paid to improve the catalyst properties for high dust coal fired

application ma intaining its durability since the very early stage. Through the efforts, the

DeNOx catalyst applied for commercial use in early 2000 th gave us approximately 20%

higher DeNOx activity compared with the one used in late 1980 th , maintaining the similar

SO2 t o SO3 conversion activity. Such improvement was become available through the

modification of catalyst manufacturing process and the utilization of new materials. Thus,

further improvement in 2000 th was getting more difficult than early dates.

Usually, chan ging the active component’s composition made the control of catalyst activity

. However, with this technology, the SO2 to SO3 conversion activity was closely dependent on

catalyst active component ’ s composition. If we add the active components to increase D eNOx

activity, the SO2 to SO3 conversion activity was also increased as shown in Fig. 1. Thus, to

achieve low SO2 to SO3 conversion rate in design, considerable increase in catalyst volume

could not be avoided with the conventional technology.

At this time , we decided to look for the combination of the following process to achieve the

low SO2 to SO3 conversion rate keeping similar or rather higher DeNOx activity comparedwith the catalyst applied in 2000 th as also shown in Fig. 1.

(a) Investigate the applic ability of the new catalyst materials, including the modification

of catalyst manufacturing process suitable for the materials

(b) To prepare the catalyst with thinner thickness maintaining the durability of catalyst

against catalyst poisons

High

Low

Low High

Fig. 1 Concept for Development of Low SO2 to SO3 Conversion Rate Catalyst

(3) Results of Development

Apart from the catalyst manufacturer ’ s efforts to improve the catalyst performance, material

suppliers also made continuous efforts to improve the material characteristics so that the users

could create better final products. By maintaining contacts with material suppliers, several

materials were selected for inv estigation.

As an example, Fig. 2 shows one of the investigation results to find suitable Titanium Dioxide

material. As shown in Fig. 2, several Titanium Dioxide materials make it possible to reduce

the SO2 to SO3 conversion rate. Similar investigation was also conducted for other catalyst

materials.DeNOxActivity

SO2 to SO3 Conversion Activity (a) (b) Active Components

Increase

Decrease Developed

Catalyst

Conventional Technology0 0.2 0.4 0.6 0.8 1 1.2 1.4

Base A B C D



### MaterialActivityRatio(-) DeNOx Eff. SO2 to SO3 Conversion

Fig. 2 TiO2 Material Screening Test Results(Example)

The other issue required for developing new catalyst is, of course, the optimization for the

manufacturing process. As the new materials are usually manu factured with different process,

the physical and chemical characteristics of the materials became different from the

previously

used materials. Such change used to cause different mixing characteristics.

Furthermore, by changing the mixing process, the ca talyst characteristics are also controlled

to a certain degree.

Through extensive research work, as stated above, we, BHK, developed a new series of

DeNOx catalyst (called CX Type) applied for coal fired flue gases with high dust loading. Fig.

3 shows the catalyst performance of the CX Type catalysts at 350 ° C compared with the most

commonly used catalyst (called Base) at the time. In the figure, the performance for the

catalyst used in late ‘ 80 th

when DeNOx installation in Europe was started was also indic ated.

As observed in Fig. 3, CX and CXL catalyst achieved considerably lower SO2 to SO3

conversion rate maintaining similar DeNOx activity compared with Base catalyst.0 0.2 0.4 0.6 0.8 1 1.2 1.4

'80th Base CXL CX CXM 

### Catalyst TypeActivityratio(-) DeNOx Efficiency SO2 to SO3 Conversion

Fig. 3 Comparison of Catalyst Activity of Developed Catalyst

The durability of th e developed catalyst was also tested prior to the commercial application.

The tests were conducted not only in laboratory but also in field. In laboratory, the catalyst

was tested with severer conditions than commercially applied conditions for catalyst po isons

such as Alkaline materials and Arsenic, and also for thermal stability.

Fig. 4 shows the Arsenic resistance test results of the developed catalyst. As U.S. Eastern

Bituminous coal used to contain rather high Arsenic in coal, Arsenic resistance is one of the

most important characteristics required for the catalyst. As shown in Fig. 4, the developed

CX

- Type catalyst gave better durability against Arsenic oxide compared with the Base

catalyst.Fig. 4 Arsenic Resistance Test Results

Therm al sintering also could decreased the catalyst activity . Therefore, the sintering

characteristic of the CX Type catalyst was tested by exposing the catalyst at 600 ° C for the

period of 2,000 hours. Fig. 5 shows the crystal diameter growth rate after 2,000 hours.

Compared with the Base catalyst, the CX Type catalyst gives considerably stable

characteristic for thermal sintering.

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Base CX Type 

### Catalyst TypeCrystalDiameterGrowthRate(-)

Fig. 5 Thermal Stability test Results 0 0.2 0.4 0.6 0.8 1 1.2

Base CXL CX CXM Catalyst Type(K/K0)CX-Type/(K/K0)Base(-)The durability of CX Type catalyst was also tested in the field installing the samples i nto the

flue gas stream from coal

- fired plant. Through the field tests, the durability of CX Type

catalyst was also confirmed to be as same as Base catalyst.

All other durability test results for CX Type catalyst gave us better or similar tendency over

the Base catalyst.

After completion of the development, feasibility study was conducted using the conditions

described in Table 1. The results were summarized in Table 2.

As obvious from Table 2, the CX Type catalysts can be used even at an elevated temperat ure

like 415 ° C with less than 0.5 % SO2 to SO3 conversion rate. Thus, the CX Type catalyst can

be used more flexibly

, and can minimize the impact of SO3 towards the downstream

equipments.

Table 2 Applicable Temperature Ranges for New Installation

SO2 to SO3 Conversion Rate (%) Catalys t Type 0.5 1.0 1.5

Base

- < 370 ° C < 390 ° C

CXM < 340 ° C < 380 ° C < 395 ° C

CX < 380 ° C < 415 ° C < 435 ° C

CXL < 415 ° C < 445 ° C

-

3. Operational results of Low SO2 to SO3 Conversion Rate Catalyst

(1) Design conditions fo r operating plants with low SO2 to SO3 conversion rate catalyst

The first application of the developed low SO2 to SO3 conversion rate catalyst (CXL) was in

the Indianapolis Power & Light Company ’ s Petersburg DeNOx units. In Petersburg station,

there are th ree coal

- fired boilers, and the boilers for Units 2 and 3 were retrofitted with

DeNOx plants to reduce NOx emissions from the plants.Fig. 6 shows the photograph of Petersburg station.

Fig. 6 Indianapolis Power & Light Company / Petersburg St ation

A major technical concern for the installation of DeNOx plants was the formation of SO3 by

DeNOx catalyst because of the reports stating the problems on SO3 emission during the

planning stage.

Petersburg station utilizes several local high sulfur ea stern bituminous coals. These coal

analysis data was summarized as shown in Table 3.

Table 4 shows the design basis of DeNOx catalyst for Petersburg station.

As shown in Table 4, the application also required higher DeNOx efficiency of more than

90% togeth er with lower slip NH3 of less than 2 ppm compared with the European plants.

Because of these requirements, the DeNOx plants have to be designed to achieve very good

distribution for NH3/NOx, Temperature and gas flow rate. The DeNOx system supplier,

theref ore, conducted physical flow model testing to achieve the following inlet variations.NH3/NOx Less than ± 5 % RMS

Temperature Less than ± 11 ° C

Gas flow rate Less than ± 15 % RMS

The final arra ngement of Petersburg station Unit No.2 & 3 DeNOx plants were shown in Fig.

7, and Fig. 8. As shown in Figures, two static mixers were installed at the downstream of

ammonia injection grid to achieve the required NH3/NOx distribution criteria for both of t he

plants.

Table 3 Coal Analysis Data for Petersburg Boilers

Ultimate Analysis (Dry) Ash Mineral Analysis

Items Typical (%) Items Range (%; as ash)
Moisture

-- SiO2 34.89

– 56.0
Ash 9.0 Al2O3 16.13

– 26.21
Carbon 73.6 TiO2 0.68

– 0.95 Hydrogen 5.0 Fe2O3 10.92

– 15.60 Oxygen 7.6 CaO 1.47

– 2.48 Nitrogen 1.3 MgO 0.26

– 1.35
Sulfur 3.5 Na2O 0.22

– 1.27
K2O 0.75

– 3.24
SO3 0.2

– 5.25
P2O5 0.02

– 0.78 As in coal (ppm) 5.74

– 14.40Table 4 Design Basis of DeNOx Catalyst for Petersburg Station

Items Units Petersburg #2 Petersburg #3
No of Reactor Number/Boiler 2 2 Generation Capacity MW 463 563
Flue Gas Flow m3/h(Normal) 1,552,065 1,878,943 Flue Gas Temp. ° C 414.5 394.1
Inlet O2 %, Dry 4.59 4.65
H2O % (Volume) 8. 78 8.96
SO2 ppmvd @ 3% O2 2,645 2,225
SO3 ppmvd @ 3% O2 25 21
NOx ppmvd @ 3% O2 303.1 335.6

Dust mg/m3(Normal) 9,800 10,200 DeNOx Efficiency % More than 90 More than 90 Slip NH3 ppmvd @ 3% O2 Less than 2 Less than

2
S O2 to SO3 conv. % Less than 0.5 Less than

0.5 Catalyst Life hours 24,000 24,000

Ash cleaning devices were usually installed for the DeNOx plants with high dust coal fired

applications. For DeNOx plants at Petersburg station, sonic horns were applied on top of

each catalyst layer instead of the conventional steam or air soot blowers.Fig. 7 Arrangement of DeNOx Plant for Petersburg Station Unit 2

Fig. 8 Arrangement of DeNOx plant for Petersburg Station Unit 3(2) Operating results of P etersburg station Unit 2 & 3 DeNOx plants

In the spring of the year 2004, the DeNOx plants for both units were got into operation. After

the initial operation adjustments, the performance tests were performed in June 2004.

The performance test results were summarized in Table 5.

Table 5 Performance Test Results

Performance Test Results Parameter Units Guarantees Unit 2 Unit 3 DeNOx Efficiency % 90 90 90
NH3 Slip ppmvd @ 3% O2 Less than

2 << 2 << 2
Catalyst Δ P inch W.G. 1.4(Unit 2) 1.8(Unit 3) 1.1 1.5

SO2 to SO3 Conversion Rate % Less than 0.5 < 0.3 < 0.3

As shown in Table 5, the catalyst installed in DeNOx plants at Petersburg station met all of

the guarantees. In particular, measured SO2 to SO3 conversion rate was considerably lower

than the gu aranteed 0.5% conversion rate.

The DeNOx plants at Petersburg station have been operated successfully during the year 2004

Ozone season without any problem for both DeNOx and boiler systems.

During the annual inspection period of boilers, inside inspecti on of DeNOx plants were

conducted. Representative observation results were shown in Fig. 9 and 10. As shown in the

Figures, both ash accumulation and erosion of catalyst were not observed.Fig. 9 Inside Inspection Result

– General View

-

Fig. 10 Inside Inspection Results

– Close

- up Observation of Catalyst

-(3) Other applications

The newly developed CX Type catalyst was supplied to various DeNOx plants since the first

installation at Petersburg station. Table 6 shows the exper ience list of CX Type catalyst that

was supplied by Spring of 2005.

Table 6 Experience List of CX Type Catalyst

No. Plant Name Catalyst Type Start Operation Note

1 Petersburg 2 CXL 2004 New Installation

2 Petersburg 3 CXL 2004 New Installation

3 Farge CXM 2004 Replacement

4 Datteln CXM 2004 Replacement

5 Muskingumriver CX 2005 New Installation

6 Gavin 1 CXL 2005 Replacement

7 Keystone 1 CXM 2005 Addition

8 Trimble County 1 CXM 2005 Addition

9 Logan CXM 2005 Replacement

10 Lambton 4 CXL 2005 Addi tion

11 Duernrohr 1 CXM 2005 Replacement

4. Benefits for Application of CX Type Catalyst

The design requirements for European DeNOx plants are much moderate compared with U.S.

applications as shown in Table 7.

Table 7 Comparison of Design Requirements of DeNOx Plants Requirements U.S.A. Europe DeNOx Efficiency More than 90 % 60

- 80 % Slip NH3 Less than 2 ppm 3

- 5 ppm SO2 concentration 2,500

- 4,000 ppm 1,000

- 1,500 ppmBecause of these differences , operational benefits shown in Table 8 can be ach ieved by

utilizing the superior characteristics of CX Type catalyst as described in Paragraph 3.

Table 8 Benefits of Application of CX Type Catalyst
Existing DeNOx Plants New Installation

- Extension of catalyst maintenance
interval

- Reduction of SO2 to S O3 conversion rate

- Reduction of pressure drop(Application of wider pitch catalyst) - Low SO2 to SO3 conversion rate

- Smaller reactor

- Lower pressure drop (Application of wider pitch catalyst)

Apart from above benefits for the existing DeNOx plants, utilizati on of CX Type catalyst can

also achieve higher NOx removal efficiency which may contribute in reducing total NOx

emission from the nation.

5. Conclusion

Babcock

- Hitachi developed new CX Type catalyst, having low SO2 to SO3 conversion rate

maintaining high De NOx potential.

The catalyst was first applied to the DeNOx plants at Indianapolis Power & Light/Petersburg

station Unit 2 & 2, and achieving less than 0.3% SO2 to SO3 conversion rate even at the

elevated temperature as 415 o C.

By application of CX Type cat alyst in European DeNOx plants, the catalyst can achieve

various benefits for the plant operation.