Effects of Alkali Poisons and Sulfationon NOx Reduction Activity of

V2O5/TiO2 SCR Catalysts

Jacob Beutler, Xaioyu Guo, Aaron Nackos, John Ashton, Calvin Bartholomew, William Hecker,

Larry Baxter

ACERC ConferenceFeb 24, 2009

Provo, UT

Outline• Introduction

• Objective and Motivation

• Results and Discussion

• Conclusions

• Acknowledgements

Introduction

• SCR: Selective Catalytic Reduction

• An effective NOx abatement technique

• A commercial post-combustion technique

• Being used on ~¼ of the power generated from coal-fired boilers

• Major problem is catalyst deactivation, especially for low-rank coals and biomass

Example of SCR System

• Modified from http://upload.wikimedia.org/wikipedia/en/5/5c/SCR2.GIF

Ammonia inlet stream

Introduction• SCR: selective catalytic reduction

• 4NH3 + 4NO + O2 4N2 + 6H2O

• 4NH3 + 2NO2 + O2 3N2 + 6H2O• Reactions occur on acid site (Bronsted vs Lewis?)

• SCR catalyst

• Supported vanadia catalyst: 1%V2O5-9%WO3/TiO2

• Currently, in monolith or flat plate structure.

• SCR kinetics: rNO ≈ k CNO

Objectives• Investigate Deactivation mechanisms of Commercial

SCR catalysts• Postulate a NOx Reduction Mechanism on the

molecular level• Explore effects of surface V, W, and SO4 on adsorption of NO

and NH3 and catalysts activity• Determine the impacts of K, Na, Ca, and SO2 on vanadia

catalyst properties and activities of lab-synthesized catalysts• Determine active center sites

• Motivation• Poisoning is a potential mechanism for SCR catalyst

deactivation• SO2 is generally present in boiler flue gas• Previous studies on non-sulfated catalysts

Deactivation Mechanisms• Plugging

• Fouling/Masking

• Poisoning

Channel/Pore Plugging

Fouling/MaskingCalcium sulfate coating on catalyst surface

Poisoning• Catalytic activity linked to

amount/strength of Brønsted acid sites on catalyst surface (Amiridis, Duevel, Wachs, 1999)

• Chemical poisoning results from basicity of metal oxide deposits (Chen et al., 1990)

• Flyash from coal/biomass contains carbonates, sulfates, and oxides of Na, Ca, K, As, and Mg, which are basic materials

Slipstream Reactor• REI operating slipstream

reactor at Rockport, IN• Reactor contains 6 separate

channels for 5 commercial, one BYU -prepared catalyst

• Samples are returned for analysis

Flue Gas

Flue Gas

Duct Wall

Duct Wall

Pneumatic Isolation ValveOne SCR Inlet Heated Sample Line

Six SCR Outlet HeatedSample Lines to Sequencer

Ammonia Injector

SCR Reactor

2.5"

out

5.0"

out

2.5" out 5.0" out

1/8" wall thickness2.25 x 2.25 inner dimension

4.75 x 4.75 inner dimension

Top view of slipstream reactor.(Schematic courtesy of REI)

Fresh and Exposed Catalysts Comparison

Fresh 2063 hourexposure

3800 hourexposure

Catalyst Characterization System

Monolith Test Reactor

Flow out

Plate heater (x4)

Flow in

Flow ThermocoupleCatalyst sample

Monolith Test Reactor Schematic

Flow Rate: 1000 sccmFeed:• 900 ppm NO• 900 ppm NH3

• 3% O2

• ~10% H2O• Balance HeTemperature: 250-325 °C

Fresh and Exposed Catalysts Performance

Monolith 1 kinetic data Monolith 2 kinetic data

15

10

5

0NO re

duct

ion

rate

con

stan

t, k

(cm

3 /m2 •s

)

600580560540520Temperature (K)

M1 Fresh fit M1 Fresh M1 2063 fit M1 2063 M1 Biomass fit M1 Biomass M1 3800 fit M1 3800

30

25

20

15

10

5

0NO re

duct

ion

rate

con

stan

t, k

(cm

3 /m2 s)

600580560540520Temperature (K)

M2 Fresh fit M2 Fresh M2 2063 fit M2 2063 M2 Biomass fit M2 Biomass M2 3800 fit M2 3800

BET Results

Sample M1 M2

BET surface area, m2/g

Average pore diameter, nm

BET surface area, m2/g

Average pore diameter, nm

Fresh 61.5 16.4 56.6 13.3

2063 53.5 17.5 54.5 13.6

3800 55.6 17.7 50.0 17.7

Biomass 48.2 19.9 43.9 20.0

60

50

40

30

20

10

0

Wei

ght %

O Na Mg Al Si P S K Ca Ti V Fe W Elements

M1 Fresh M1 Bio M1 2000 M1 3800

Flat

Elemental Composition vs. Exposure

Elemental Composition vs. Position

60

50

40

30

20

10

0

Wei

ght %

O Na Mg Al Si P S K Ca Ti V Fe W Elements

M1 Fresh M1 3800 Center M1 3800 Edge M1 3800 Flat M1 3800 Ash

Indications from Slipstream Samples

• Short exposure indicates a slight increase in activity

• Pore plugging and Fouling appear to be more significant initially with poisoning occurring later

• ESEM results indicate poisons deposit more on the outside edge than on the inside or center of the monolith catalyst

• ESEM results show sulfate accumulates on the catalyst outside surface

In situ FTIR - MS Reactor

SCR Mechanism, Ramis

SCR Mechanism, Topsøe

Proposed SCR Mechanism

Objectives• Investigate Deactivation mechanisms of Commercial

SCR catalysts• Postulate a NOx Reduction Mechanism on the

molecular level• Explore effects of surface V, W, and SO4 on adsorption of NH3

and catalysts activity• Determine the impacts of K, Na, Ca, and SO2 on vanadia

catalyst properties and activities of lab-synthesized catalysts• Determine active center sites

• Motivation• Poisoning is a potential mechanism for SCR catalyst

deactivation• SO2 is generally present in boiler flue gas• Previous studies on non-sulfated catalysts

Vanadia Enhances NH3 Adsorption

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Abs

orba

nce

1600 1500 1400 1300 1200 1100Wavenumber / cm-1

5% V2O5/TiO2 2% V2O5/TiO2 1% V2O5/TiO2 TiO2

W Enhances NH3 Adsorption

• W enhances NH3 adsorption on Brønsted acid sites

2.0

1.5

1.0

0.5

0.0

Abs

orba

nce

1800 1600 1400 1200Wavenumber / cm-1

1%V2O5-9%W / TiO2 1%V2O5/ TiO2 9%W / TiO2 TiO2

W and S Enhance NOx Reduction Activity

25

20

15

10

5

NO re

duct

ion

rate

con

stan

t, k

(cm

3 /g•s

)

580560540520500480Temperature (K)

1% V2O5-9%WO3/TiO2 24-hour sulfated 1% V2O5/TiO2 0.5-hour sulfated 1% V2O5/TiO2 Fresh 1% V2O52/TiO2

Comparison of A and Ea

1%V-9%W/TiO2

24-hour sulfated 1%V/TiO2

0.5-hour Sulfated 1%V/TiO2 Fresh 1%

V/TiO2

A6.3×105 ±2.7×104

5.8×105 ±1.1×105

3.0×105 ±1.1×105

1.8×105 ±1.5×105

Ea

4.6×104 ±1.8×103

4.8×104 ±3.5×103

4.6×104 ±3.5×103

4.5×104±3.5×103

W’s Effect on NO Reduction

20

15

10

5

0

NO

redu

ctio

n ra

te c

onst

ant,

k (c

m3 /g

•s)

580560540520500480460Temperature (K)

1%V-9%W/TiO2 1%V/TiO2 9%W/TiO2

Sulfates Enhance NH3 AdsorptionSulfation increases the amount of Brønsted acid sites on the vanadia catalysts surface

4

3

2

1

0

Ab

sorb

ance

4000 3500 3000 2500 2000 1500

Wavenumber

1435 Sulfated Fresh

Peak area: 106

Peak area: 59

Activity Loss vs. Peak Area Loss

80

70

60

50

40

30

20

Activ

ity lo

ss (%

)

807060504030IR peak area loss (%)

Ca

240 °C 250 °C1421 cm-1 1200 cm-1

Na

K

Ca

Na

K

parity line

Brønsted acid sites correlate with activity. Lewis acid sites do not

Sulfates Enhance NO Reduction Reactivity

Sulfation enhances NO reduction activity by increasing A, while Ea remains approximately constant before and after sulfation

35

30

25

20

15

10

5

K, c

m3 /g

*s

570560550540530520Temperature, K

24-hour sulfated 1% V2O5/TiO2 Lightly sulfated 1% V2O5/TiO2 Fresh 1% V2O5/TiO2

A = 1716400 +/- 468000 Ea = 52566 +/- 1240 A = 300888 +/- 76400 Ea = 45758 +/- 1160 A = 182003 +/- 56700 Ea = 44921 +/- 1430

Ca, Na, and K Effects on NH3 Adsorption

0.4

0.3

0.2

0.1

0.0

Abso

rban

ce

1600 1500 1400 1300 1200 1100Wavenumber

1%V-9%W/TiO2 0.5Ca 1%V-9%W/TiO2 0.5Na 1%V-9%W/TiO2 0.5K 1%V-9%W/TiO2

NH3 adsorption ↓ with ↑ basicity of metals

K, Na, and Ca Impacts on SCR Activity

20

18

16

14

12

10

8

6

4

2

k, c

m3 /g

•s

560540520500480460Temperature, K

1V-9W/TiO2 0.5 Ca 1V-9W/TiO2 0.5Na1V-9W/TiO2 0.5k1V-9W/TiO2

Conclusion

• Major deactivation mechanisms for vanadiacatalysts during low-rank coal combustion:• Plugging by popcorn ash• Fouling and masking by fly ash• Chemical poisoning increases with time

• Impacts of sulfation• Enhances NOx reduction activity • Increases ammonia adsorption• Influences rate, but not mechanism

Conclusion cont.• Impact of V

• Increases NH3 absorption on Brønsted acid sites

• Increases NOx reduction activity

• Impact of Tungsten• Increase the amount of Brønsted acid sites

• Does not provide activity but assists vanadia species significantly in the NOx reduction

Proposed SCR Mechanism

Current work

• Affect of in situ SO2 on catalysts activity• Appears to significantly enhance NOx reduction

activity in preliminary experiment• More data is needed to validate effect

• Problems• Forms sticky ammonium salts when mixed with NH3

in stream• Must heat all lines and MS above 300°C to prevent

plugging

Acknowledgments

• EPRI

• DOE/NETL

• REI

• University of Utah

Conclusion cont.• Impact of K, Na, Ca

• Significant poisons

• K>Na>Ca, proportional to basicity

• Biomass potentially is worse than coal

• Impact of Tungsten• Increase the amount of Brønsted acid sites

• Does not provide activity but assists vanadia species significantly in the NOx reduction

Statistical Experimental Design

Runs Composition Factor Runs Composition Factor

K Na Ca SO4 K Na Ca SO4

1 0 0 0 0 9 0 0 0 1

2 0.5 0 0 0 10 0.5 0 0 1

3 0 0.5 0 0 11 0 0.5 0 1

4 0 0 0.5 0 12 0 0 0.5 1

5 0.5 0.5 0 0 13 0.5 0.5 0 1

6 0.5 0 0.5 0 14 0.5 0 0.5 1

7 0 0.5 0.5 0 15 0 0.5 0.5 1

8 0.5 0.5 0.5 0 16 0.5 0.5 0.5 1

Statistical Experimental Design

Runs Composition Factor Runs Composition Factor

K Na Ca SO4 K Na Ca SO4

1 0 0 0 0 9 0 0 0 1

2 0.5 0 0 0 10 0.5 0 0 1

3 0 0.5 0 0 11 0 0.5 0 1

4 0 0 0.5 0 12 0 0 0.5 1

Empirical rate constant expression

⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜

⎝

⎛

⎟⎟⎠

⎞⎜⎜⎝

⎛−−⎟⎟

⎠

⎞⎜⎜⎝

⎛−+

+++−−−

=

000

000

1112.01127.0

55.038.017.03.076.016.15.2

exp

TTSS

TT

SS

VNa

SS

VK

SS

VCa

VNa

VK

k