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

Indianapolis 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 been 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 works 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 got 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 various 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 

successfully.

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 smut 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 situation, various research efforts to reduce SO3 emission were conducted 

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

catalyst 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 related 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 improved 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 development 

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 DeNOx 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, decided 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 DeNOx 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 maintaining its durability since the very early stage. Through the efforts, the 

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

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

SO2 to 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 2000th was getting more difficult than early dates.

Usually, changing 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 DeNOx 

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 compared 
with the catalyst applied in 2000th as also shown in Fig. 1.

(a) Investigate the applicability 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 investigation.

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.

DeN

O

x

Activity

SO2 to SO3 Conversion Activity

(a)

(b) Active Components

Increase

Decrease

Developed

Catalyst

Conventional

Technology
0

0.2

0.4

0.6

0.8

1

1.2

1.4

Base A B C D

Material

A

ctivity

R

atio (-)

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 manufactured 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 catalyst 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 ‘80th when DeNOx installation in Europe was started was also indicated. 

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 Type

Activity ratio (-)

DeNOx Efficiency SO2 to SO3 Conversion

Fig. 3 Comparison of Catalyst Activity of Developed Catalyst

The durability of the 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 poisons 

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

Thermal 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 Type

Crystal Diameter Gro

wth

R

ate (-)

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)C

X-T

y

pe/(K/K0)Base (-)
The durability of CX Type catalyst was also tested in the field installing the samples into 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 temperature 

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 (%) Catalyst 

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 for 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 three 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 Station

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 eastern 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% together 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, 

therefore, 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 arrangement 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 the 

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 – 14.40
Table 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

SO2 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 Petersburg 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 ΔP 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 guaranteed 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 inspection 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 experience 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 OperationNote

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 Addition

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 ppm
Because of these differences, operational benefits shown in Table 8 can be achieved 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 SO3 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, utilization 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 DeNOx 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 415oC.

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

various benefits for the plant operation.