Environmental Catalysis - American Chemical Society

Chapter 17

Family of Versatile Catalyst Technologies for NO Removal in Power Plant Applications Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 29, 2016 | http://pubs.acs.org Publication Date: February 23, 1994 | doi: 10.1021/bk-1994-0552.ch017

x

R. M. Heck, J . M . Chen, Β. Κ. Speronello, and L . Morris Engelhard Corporation, 101 Wood Avenue, South Iselin, N J 08830-0770

Limits on NO emissions from power plant installations often result in the use of catalytic emission control systems. The optimum type and size of the abatement system depends on a variety of performance requirements that are unique to each installation. The main design variable that affects the selection of the NO selective catalytic reduction catalyst is operating temperature. A family of versatile catalyst technologies has been developed by Engelhard to control NO over a wide range of temperature and exhaust gas composition. The technical differences between the catalyst technologies are discussed. Commercial installations are reviewed. x

x

x

Oxides o f nitrogen formed during combustion are caused either by the thermal fixation of atmospheric nitrogen (thermal NO^) or by conversion o f chemically bound nitrogen in the fuel.(7) Selective catalytic reduction ( S C R ) is recognized as the most effective commercial technology to control N O ^ emissions from chemical plants and stationary power sources. The operating environment and process constraints for S C R systems vary greatly from one installation to another. These constraints, which include pressure drop limits, duct dimensions, exhaust gas particulate content, ammonia limits in the exhaust (i.e. N H slip), S 0 oxidation limits, temperature and N O ^ concentration, all impact catalyst and system design. For optimal performance under these varied constraints, catalysts o f different physical and catalytic properties are required. Tailoring for these constraints, Engelhard has developed composite honeycomb S C R catalyst formulations which provide a high degree of flexibility to meet specific system configuration and performance needs. The composite catalyst is manufactured by bonding a layer o f catalytic ingredients onto strong, thin walled ceramic honeycomb supports. This design results i n both substantially reduced catalyst volume, and several unique selectivity characteristics^). 3

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0097-6156/94/0552-0215$08.00/0 © 1994 American Chemical Society In Environmental Catalysis; Armor, John N.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

ENVIRONMENTAL CATALYSIS

216

This paper reviews some o f the more critical factors that affect emissions control systems and describes how the specific requirements o f such systems are better satisfied with a family o f catalyst formulations and gives a summary o f commercial installations.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 29, 2016 | http://pubs.acs.org Publication Date: February 23, 1994 | doi: 10.1021/bk-1994-0552.ch017

Composite Catalyst Features The three classes o f reactions that can be catalyzed by a commercial S C R catalyst are given i n Table I. They include the N O ^ S C R reaction itself, plus ammonia oxidation and S 0 oxidation to S 0 . The S C R reaction between Ν Ο and N H to form N and water is the preferred reaction which removes NO^. from the exhaust stream. A m m o n i a oxidation consumes N H i n competition with the S C R N O ^ reaction, and is undesirable because it lowers Ν Ο removal efficiency and increases N H consumption. These competing reactions are shown schematically as a function o f operating temperature i n Figure 1. S 0 oxidation is undesirable because the resultant S 0 can react with excess N H i n the exhaust stream to form ammonium bisulfate that can plug and corrode downstream equipment. Combining these effects, the S C R N O ^ catalyst is required to possess high S C R activity together with low activities for both ammonia and S 0 oxidation. These properties depend on both catalyst chemical composition and catalyst structure. 2

3

Λ

3

2

3

Λ

3

2

3

3

2

Detailed kinetic analyses have revealed that with honeycomb catalysts, the S C R N O j reaction rate is limited by a combination o f gas phase mass transfer to the honeycomb surface and pore diffusion into the catalyst layer. (2,3) Under these conditions, all N O ^ conversion occurs i n an extremely thin layer o f the catalyst surface. In contrast, the rates o f the S 0 and N H oxidation reactions are mostly controlled by kinetic rates over the catalyst active sites and by the total weight o f catalytic material i n the reactor. Under these conditions, S 0 and N H penetrate deeply into the catalyst layer. Since only a thin layer o f catalyst is needed to achieve maximum N O ^ conversion, and since extra catalyst below the necessary surface layer contributes to the undesirable oxidation reactions, the composite catalyst structure (with its thin catalytic layer) has inherently better selectivity compared to either extruded or plate designs (where the catalyst wall comprises a relatively massive layer o f catalytic material).(7) 2

3

2

Selective C a t a l y t i c R e d u c t i o n of Ν Ο

3

χ

Selective catalytic reduction ( S C R ) o f NO^. with ammonia was first discovered and patented by Dr. Gunther Cohn at Engelhard. (4) This initial work was targeted at nitric acid tail-gas exhausts, and used precious metal catalysts. It was not until the m i d 1970's, however, that S C R entered widespread commercial use i n Japan using base metal catalysts. The key performance criteria for S C R are analogous to complete oxidation systems: N O ^ conversion, pressure drop, catalyst/system life, cost, and minimum S 0 oxidation to S 0 (for systems firing liquid fuels). Temperature is the single most important variable i n N O ^ S C R . N o S C R catalyst can operate economically over the whole temperature range possible for power plant applications. A s a result, three general classes o f catalysts have evolved into commercial use: 2

3

In Environmental Catalysis; Armor, John N.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

17. HECK ET AL.

Versatile Catalyst Technologies for NQ


Table I.

Removal

Important Chemical Reactions Over Ν 0

Λ

217

S C R Catalyst

Preferred Activity Level NO SCR x

4NH + 4NO + 0 — • 4N + 6H 0

High

4NH + 2N0 + 0 — • 3N + 6H 0

High

3

2

3

2

2

2

2

2

2

S0 Oxidation 2

Low

2S0 + 0 — • 2S0 2

2

3

NH Oxidation 3

4NH + 50

2

• 4NO + 6H 0

Low

4NH + 30

2

• 2N + 6H 0

Low

3

3

2

2

Selective NOx Removal: Reactants: NH3, NOx, 02

Temperature Figure 1.

2

NH3 Oxidation And Decomposition: Reactants: NH3, 02

Temperature Competing Reactions i n S C R N O ^ .



218

precious metals for operation at temperatures between 177 and 290°C, base metals for operation at temperatures between 260 and 450°C, and zeolites for operation at higher temperatures. The important catalyst characteristics for each operating temperature range are discussed below.


L o w Temperature O p e r a t i o n L o w temperature, precious metal S C R catalysts have been installed on a small number of power plant turbines firing natural gas. Sensitivity to poisons makes precious metal S C R catalyst unsuitable for most co-generation applications, since variations i n exhaust sulfur levels o f as little as 0.4 ppm can shift the catalyst temperature window completely out o f a system's operating temperature range.(5) In addition, operation on liquid fuels is further complicated by the potential for deposition o f ammoniumsulfate salts within the pores o f the catalyst. However, once these limitations are realized and the proper design models developed, l o w temperature S C R systems provide the S C R system designer with an additional degree o f design freedom. Because the Ν Ο concentration i n a combustion turbine exhaust is generally very low, the characteristic o f l o w temperature S C R systems to manufacture nitrous oxide ( N 0 ) is not usually an issue. In other types o f systems operating with higher concentrations o f N O ^ and ammonia, a low temperature S C R system w i l l generate proportionally more nitrous oxide. Since nitrous oxide has been linked to both the destruction o f stratospheric ozone and contribution to the greenhouse effect, there are reasonable odds that its emission w i l l be controlled i n the future. The operating temperature range for low temperature S C R catalysts is determined by the balance between the S C R and ammonia oxidation reaction listed below and shown schematically i n Figure 1.: χ

2

SCR:

4NO + 4 N H

Oxidation:

4NH 4NH

3

3

3

+ 0

2

— • 4N + 6H 0 2

2

+ 5 0 — • 4NO + 6 H 0 + 30 — • 2N + 6H 0 2

2

2

2

2

A t l o w temperatures the S C R reaction dominates and N O ^ conversion increases with increasing temperature. But as temperature increases, the oxidation reactions become relatively more important. Eventually as temperature increases further, the destruction of ammonia and generation o f N O ^ v i a the oxidation reactions causes overall NO^. conversion to reach a plateau and finally decrease with increasing temperature. The general form o f this curve is shown i n Figure 2. For l o w temperature S C R catalysts, the peak i n N O ^ conversion (>90%) typically occurs i n a 40°C window between 177 and 316°C. M e d i u m Temperature O p e r a t i o n The most popular S C R catalyst formulations are those comprising V 0 supported on T i 0 ( V / T i ) . Ingredients such as tungsten and molybdenum may be added to diminish S 0 oxidation activity and improve operation above 425°C but the basic formulation remains generally the same. These catalysts operate best i n a temperature range 2

5

2

2


Versatile Catalyst Technologies for NQ

Removal


17. HECK ET AL.


219



220

between 288 and 450°C. They are generally active and durable catalysts that have functioned well i n power plant systems since the mid-1980's. M e d i u m temperature V / T i catalysts have curves o f N O ^ conversion vs. temperature that are similar to low temperature catalysts (see Figures 1 and 2). N O ^ conversion rises with increasing temperature to a plateau and then falls as ammonia oxidation begins to dominate the S C R reaction. However, for these medium temperature catalysts, the peak conversion occurs over a broad temperature range and the response is usually more gradual than with low temperature catalysts. M e d i u m temperature V / T i S C R catalysts have one clear weakness compared to both l o w temperature precious metal and high temperature zeolite based catalysts; V / T i catalysts are irreversibly deactivated by extended exposure to temperatures above 4 5 0 ° C . This is caused by the transformation o f the high surface area anatase form o f titania to the lower surface area rutile form. In contrast, typical precious metal S C R catalyst formulations may be heated to temperatures well above 538°C without excessive deactivation, and high temperature catalysts w i l l operate at temperatures as high as 510 to 593°C depending on the formulation. H i g h Temperature O p e r a t i o n The suitability o f zeolite catalysts for S C R above 450°C has been known since the 1970's.(6) They have received limited acceptance for power plant applications, however, at least i n part because their upper operating temperature limit (approximately 510°C) was lower than the exhaust temperature o f the most popular turbine designs. Since the heat recovery boiler had to be split to accommodate either V / T i or zeolite catalysts, most systems used the more active medium temperature V / T i formulations. Recently Engelhard commercialized N O x C a t Z N X zeolite S C R catalyst with the capability to operate at temperatures as high as about 593°C.(2) When N O ^ is present this catalyst does not oxidize ammonia, so its N O ^ conversion continually increases with increasing temperature. This catalyst is designed to operate i n the direct exhaust o f combustion turbines; either ahead o f the heat recovery boiler or i n simple cycle systems without heat recovery boilers. Overall S C R Operation Figure 3 summarizes the operating temperature ranges for the different S C R catalyst formulations. For temperatures between 177 and 219°C N O x C a t L T - 1 catalyst provides optimum N O ^ removal. N O x C a t L T - 2 catalyst w i l l provide the best performance for systems that operate between 249 and 321°C. For medium temperatures i n the range o f 300 to 425°C, N O x C a t V N X V / T i catalyst yields the optimum N O ^ removal efficiency. Finally, for temperatures between about 400 and 593°C N O x C a t Z N X catalyst maximizes N O ^ conversion. This family o f catalyst as designated i n Table II, provides coverage over essentially the full temperature range possible for power plant applications.


HECK ET AL.

Versatile Catalyst Technologies for NQ Removal


17.


221

222


Table II.


Effective Catalysts

Operating

Temperature

Ranges

for Commercial

Product

Temperature Range, ° C

NOxCat L T - 1

177-219

NOxCat L T - 2

249 - 321

NOxCat V N X

300 - 425

NOxCat Z N X

400 - 593

Table III.

Engelhard's S C R Catalyst Experience Flow

Application

Catalyst

(lb/sec)

Fuel

Start-up

Gas Turbine (1) BBC (50MW)

VNX

650

NG

11/90

(2) Westinghouse 251

VNX

422

NG

10/92

(1) Kawasaki 1

ZNX

23

NG

2/91

(1) Allison 3.5

VNX

38

NG

9/91

(1) Rolls Royce 25 MW

VNX

259

NG

8/92

(1) GE Frame 7

VNX

669

NG

10/93

(1) LM-2500

VNX

157

NG

(1) LM-6000

VNX

283

NG

5/93 4QJ93

Reciprocating Engine (1) 800 Hp

VNX

8

NG

1993

(1) 2000 Hp

VNX

97

NG

1985

(2) 4000 Hp

VNX

23

NG/OG

1986

(3) 1500 Hp

ZNX

9

NG

5/91

Industrial Heater/Boiler (1) Refinery Heater

VNX

84

NG

10/90

(1) Refinery Heater

ZNX/VNX

46

NG

10/90

(1) Boiler

VNX

26

NG

10/90

(1) Annealing Furnace

VNX

29

NG

1991

(5) Refinery Heater

VNX

16-31

NG

3/91

(1) Refinery Heater

VNX

39

NG

6/91

Chemical Plant (1) Process Off-Gas

ZNX

4

CP

8/90

(1) Nitric Acid Plant

VNX

99

NG

3/91

Field Tests VNX/ZNX

NG

1987

(1) Coal Boner · LD

VNX

Coal

1985

(1) Coal Boiler · LD

VNX/ZNX

Coal

1987

(1) Coal Boiler · HDW

VNX/ZNX

Coal

1989

(1) Coal Boiler - HDD

VNX/ZNX

Coal

1989

(8) Gas Turbines


NO^

17. HECK ET AL.

Versatile Catalyst Technologies for NQ Removal

223


Commercial Experience

A number of SCR NO^ systems containing the vanadium/titania (VNX) and zeolite (ZNX) catalysts are now installed in commercial installation for gas turbines, reciprocating engines, industrial heaters and boilers and chemical plants. These installations are summarized in Table III. Note that the commercial installations include gas turbines, reciprocating engines, industrial heaters/boilers and chemical plants. A number of installations have been in operation for over five years. All of the commercial installations are meeting performance for NO^ conversion, N H slip and pressure drop. Also a number of field tests have been successfully conducted on coal fired boilers. 3

Conclusion

Operating requirements for NO^ control varies substantially with each application. The dominant factor for design of an SCR NO^. system is the operating temperature. A number of catalyst formulations are needed to meet the requirements of the wide range of exhaust temperatures. Engelhard has developed a family of catalysts for selective catalytic reduction of NO^ designated NOxCat SCR. These different catalyst have been optimized to function best in each different power plant environment thus covering the complete range of operating temperatures. Acknowledgement

The information included in this paper was generated in collaboration with several Engelhard colleagues, including Dr. John Byrne, Joseph Hansell and Marvin Tiller. Literature Cited 1. 2. 3. 4. 5.

6.

Durilla, M.; Chen, J.M.; Speronello, B.K.; Heck, R.M. Presented at IGCI Meeting, Baltimore, MD, March, 1990. Byrne, J.W.; Chen, J.M.; Speronello, B.K. Catalysis Today, 1992, 13, 33-42 (1992). Chen, J.M.; Speronello, B.K.; Byrne, J.W.; Heck, R.M. Presented at AIChE National Meeting, San Diego, CA, August, 1990. Cohn, J.G.; Steele, D.R. and Andersen, H.C. U.S. Patent 2 975 025, 1961. Pereira, C.J.; Plumlee, K.W. and Evans, M. 2nd International Symposium on Turbomachinery, Combined-Cycle Technologies and Cogeneration; IGTI, 1988; Vol. 3, pp. 131-136. Pence, D.T.; Thomas, T.R. U.S.Patent 4 220 632, 1980.

RECEIVED September 30, 1993


Environmental Catalysis - American Chemical Society

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