Application and Operating Results of Low SO2 to SO3

Conversion Rate Catalyst for DeNOx Application at AEP 

Gavin Unit 1 

Anthony C. Favale, P.E. Presenter 

Hitachi Power Systems America Ltd., 645 Martinsville Rd., Basking Ridge, NJ 07920 

E-mail: Anthony.favale@hal.hitachi.com Telephone: (908) 605-2758; FAX: (908) 604- 6211 

 Chao Lin Co-author 

American Electric Power, 1 Riverside Plaza, Columbus, OH 43215 

E-mail: cplin@aep.com Telephone: (614) 716-1274 FAX (614) 716 3204 

Isato Morita Co-author 

Babcock-Hitachi K.K., 6-9 Takara-machi, Kure-shi, Hiroshima-ken, 737-8508 Japan 

E-mail: morita-i@Kure.bhk.co.jp Telephone: +81(823)21-1125 FAX +81(823)21-6198 

Summary 

The impact of increased SO2 to SO3 oxidation due to the installation of an SCR on many units in the United States that 

burn high sulfur coal was more significant than expected. Soon after the SCR start up on Gavin Units 1 and 2, AEP 

recognized this issue and began the evaluation of several mitigation techniques and technologies to reduce SO3

emissions. At the same time, the development of low SO2 to SO3 conversion rate catalyst became one of the most 

important research projects for catalyst manufacturers. After extensive research work, Babcock-Hitachi developed low 

SO2 to SO3 conversion rate catalyst while maintaining high DeNOx activity 

Identification of the Necessity for Low SO2 to SO3 Conversion Rate Catalyst 

The first catalytic DeNOx system for large utility boilers was commissioned for operation in 1977 in Japan. Since then, 

SCR applications have been applied for various fuels such as gas, oil and coal, and the installations have expanded to 

many industrialized countries around the world. 

In the past several years, the SCR units in the US that burn high sulfur (3-7%) Eastern Bituminous coal experienced 

operational difficulties such as severe blue plume generation and acid mist emission. The increased SO3 concentration 

also causes: acceleration of corrosion of downstream equipments, increased possibility of ammonium bisulfate (ABS) 

formation within air preheater elements and increased PM emission. 

To overcome this problem, several research efforts to reduce SO3 emissions were conducted such as injecting different 

additives. The development of low SO2 to SO3 conversion rate catalyst can be a very cost effective solution to minimize 

the production of SO3. 

Development of Low SO2 to SO3 Conversion Rate Catalyst 

The development of low SO2 to SO3 conversion catalyst focused on the SCR units in the US firing high sulfur Eastern 

Bituminous coals with high DeNOx efficiency, low ammonia slip and a gas temperature range from 6900

 F to 7900 F. It 

was decided that the target SO2 to SO3 conversion rate of less than 0.5% for initial installation would be optimal. 

A discussion of this development was addressed in a paper at the ICAC Clean Air Technologies and Strategies in March, 

2005 titled,” Development and Operating Results of Low SO2 to SO3 Conversion Rate Catalyst for DeNOx Application”. 

The results of this research were the development of a new series of low SO2 to SO3 conversion rate catalysts called the 

CX series. 

Proceedings of the

2006 Environmental Controls Conference

50

U.S. Department of Energy

National Energy Technology Laboratory
SCR Start Up and Initial Observation of Increased Levels of SO3 at Gavin Unit 1 

The Gavin Plant is located in Cheshire, Ohio. It has two 1300 MWnet sister Units. Each Unit has B&W pulverized coal, 

dry bottom boiler. The Gavin Plant has been burning high sulfur (6.5 lb SO2/mmBtu) coal since mid 1980’s. 

The initial start up of the SCR at Gavin Unit 1 was on May 1, 2001. This is the largest SCR installed on a US boiler. The 

Gavin Units has always burned a high sulfur coal and when the initial catalyst design parameters were specified, the 

SO2 to SO3 conversion rate was 1.5% which was the industry standard. The SCR arrangement for each Gavin unit was 

divided into three equally sized parallel reactors to match the three air preheaters. The reactors are a 3 + 1 ( three initial 

layers and one spare) layer design and the catalyst volume for the initial three layers was 2124 m3. The initial design 

parameters were typical for a retrofit SCR with a life guarantee of 16,000 hours and a SO2 Conversion rate of 1.6%. 

Results of the AEP Study to Mitigate SO3 at Gavin Units 1 and 2 

Following the start up of the SCR at Gavin unit 1 in 2001, AEP recognized the need to lower the SO3 emission and began 

a study to determine the best means to mitigate the increase of SO3 over the life cycle of the plant. Some of the 

mitigation techniques evaluated by AEP were: WESP, Magnesium Hydroxide, Low Conversion SCR Catalyst, Hydrated 

Lime, Trona Ammonia and several combinations of these techniques. 

The AEP evaluation determined that the best life cycle cost for this unit was to replace all three layers of existing 

catalyst with low SO2 to SO3 conversion rate catalyst prior to the end of the normal life cycle and to use Trona injection 

at air preheater outlet. 

Operational Results of Low SO2 to SO3 Conversion Rate Catalyst at Gavin Unit 1 

The application also required DeNOx efficiency of 90% with ammonia slip less than 2 ppm. These conditions require 

the DeNOx plant to be designed to achieve very good distributions for NH3/NOx, temperature and gas velocity. The 

DeNOx system supplier had initially conducted physical flow model testing to achieve the following inlet 

variations:NH3/Nox of less than ±5 % RMS, temperature of less than ±20 °F, and Gas velocity of less than ±15 % RMS. 

Following the removal of the old catalyst and the installation of the new Hitachi catalyst in the spring of 2005, the SCR 

went into operation. After the initial adjustments, performance tests were performed in June 2005. 

The catalyst installed in DeNOx plants at Gavin Unit 1 met all of the guarantees. In particular, the measured SO2 to SO3

conversion rate was measured at less than 0.1% which was considerably lower than the guaranteed 0.38% conversion 

rate. 

A comparison of Trona injection rates between Unit 1 and Unit 2 indicates that the Trona consumption of Unit 1 which 

has Hitachi low conversion rate catalyst is approximately 35% of that of Unit 2. 

The SCR at Gavin operated successfully during the 2005 Ozone season without any problem for both DeNOx and boiler 

systems. 

Conclusion 

Babcock-Hitachi developed new CX Series Type catalyst having low SO2 to SO3 conversion rate while maintaining high 

DeNOx potential. Since its first application, it has consistently achieved low SO2 to SO3 conversion rates as low as 

0.1% even at elevated temperature above 740oF. 

By application of CX Series Type catalyst in DeNOx plants, the catalyst can achieve various benefits for a wide range of 

plant applications, operating conditions and fuels. 

REFERENCES 

J. Cooper, W. J. Gretta, A.C. Favale, H.N. Franklin, First Application of Babcock-Hitachi K.K. Low SO2 to SO3 

Oxidation Catalyst at the IPL Petersburg Generation Station. ICAC Clean Air Technologies and Strategies, Baltimore, 

MD, March 2005. 

Proceedings of the

2006 Environmental Controls Conference

51

U.S. Department of Energy

National Energy Technology Laboratory