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Accelerated Aging of Fe-Zeolite SCR Catalysts: Engine and Bench-Flow Approaches
Prof. Ke Nguyen
Mechanical, Aerospace and Biomedical Engineering
University of Tennessee at Knoxville, USA
Presented
at
HCM City University of Technology
May 27, 2008
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Acknowledgements
Research is supported by the U.S. Department of Energy (DOE), Office of FreedomCAR and Vehicle Technologies, Fuels Technology Program.
Fe-Zeolite SCR catalysts used were provided by Catalytic Solutions
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Outline
Engine-based aging and performance evaluation
Bench flow reactor-based performance evaluation of fresh and engine-aged Fe-Zeolite SCR catalysts
Results of surface characterization studies of fresh and engine-aged Fe-Zeolite SCR catalysts
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
High-Temperature Thermal Aging of Fe-Zeolite SCR Catalysts
Key concern for Fe-Zeolite SCR catalysts durability High temperatures periodically required to
regenerate DPFs
Expected deactivation mechanisms Lube-oil poisoning Contaminations from DOC Changes in zeolite structure Washcoat breakdown, i.e., crackings
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Background of SCR Catalysts Metal oxide SCR catalysts: V2O5 supported on either TiO2 or Al2O3
- High NOx reduction ( ~90%) - Low temperature (350 - 450°C) - Lack of high-temperature durability - Emitting volatile vanadyl species - High catalyzing potential to oxidize SO2 to SO3
SO3 reacts with NH3 to form ammonium sulfate and bisulfate: catalyst poisoning
Zeolite SCR catalysts: most common Cu and Fe-Zeolite - High NOx reduction (~90%) over large temperature range(300-600°C) - High temperature heavy-duty diesel applications - Low potential to oxidize SO2 to SO3
- Cu-Zeolite produces more N2O than Fe-Zeolite
Focus only on Fe-Zeolite SCR catalysts
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
SCR Reactions Between NH3 and NOx
Major reactions in the SCR process :
4NH3 + 4NO + O2 4N2 + 6H20 (1)
4NH3 + 2NO + 2NO2 4N2 + 6H20 (2)
4NH3 + 3NO2 3.5N2 + 6H20 (3)
Reaction 2 is the fastest reaction Reaction 1 is the intermediate reaction Reaction 3 is the slowest reaction
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
SCR of NOx by NH3
Alpha (α) is defined as the ratio of NH3 to NOx
Exhaust gases consist mainly of NO (~90%), an α of 1 is required to reduce NOx (reaction 1)
In most systems some NO is oxidized to NO2 with a DOC to increase the ratio NO2:NO in exhaust gases
SCR generally proceeds according to reaction 2 NO2/NO must not be greater than 1, since
reaction 3 is slowest NH3 can be solely introduced to the exhaust gases
without prior oxidation of NO to NO2
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Engine-Based Aging
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Schematic of Engine Aftertreatment
Fe-Zeolite SCR catalysts evaluated as system with DOC and DPF
Drive motor
T1 P1 T2
T3
T4 T5 P2
T6 T7 T8
T9
517 cc diesel engine
Fuel pump
Temperature controller
Sample line
Sample line
NH3 line
DOC NH3 mixer SCR DPF Supplemental air
T1 Inlet Air
T2 Oil Sump
T3 Exhaust Manifold
T4 DOC Inlet gas
T5 DOC Exit gas
T6 SCR Inlet gas
T7 SCR Exit/DPF Inlet gas
T8 DPF Exit gas
T9 DPF Internal
P1 Oil Pressure
P2 Exhaust Pressure
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Specifications of Aftertreatment System
DOC Mixer SCR DPF
DOC Inlet gas
DOC Exit gas
SCR Inlet gas
SCR Exit Gas
DPF T1
DPF Exit gas
Cat In Cat Out
DOC SCR DPF Substrate material
cordierite cordierite cordierite
cpsi 400 400 200
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Engine Bench Setup Located at NTRC
SCR Catalysts
Syringe Pump
Drive Motor
500cc HatzEngine
ThrottleController
ExhaustSampling
Exhaust FuelInjection
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Engine-Based Aging and Evaluation Engine operates steady state at 1500 rpm
~800 ppm NOx, exhaust temperature ~350°C, GHSV = 60000 h-1
SCR catalysts are aged by raising exhaust gas temperature during active DPF regeneration Supplemental fuel is injected into the exhaust upstream of the DOC
Aging cycles Up to 50 aging cycles of 25 minutes each Atomized fuel is injected in five-minute increments until exhaust
temperatures of 600, 750 and 850°C are achieved then cut off fuel Desired exhaust temperatures are obtained at fuel cut-off
Evaluation cycles Evaluation is carried out after a given number of aging cycles at an
exhaust temperature of ~350°C Inject 5% of NH3 in N2 mixture such that α = NH3/NO = 0.6 to minimize
NH3 slip
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
SCR Catalysts Used in Engine Bench Fe-Zeolite SCR catalysts of 400 cpsi and cordierite substrate mainly used for heavy-duty diesel applications are provided by Catalytic
Solutions
7.6 cm 15.2 cm
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
SEM Micrographs of Fresh Fe-Zeolite SCR Catalyst
• Channels shown are uniform • Washcoat thickness varies
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Energy Dispersive Spectroscopy (EDS) of Fresh Fe-Zeolite SCR Catalyst
•EDS performed on washcoat corner of few atomic layer
•Al, Si, O, Fe, Ce, Zr and Au peaks detected
•Al, Si, O and Fe are components of zeolite
•Ce and Zr are components of oxygen storage materials
•Au peak is from gold coating applied to the sample before performing SEM
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Typical temperature profiles during SCR catalyst aging on engine bench at 650°C(α = 0.6)
•Repeatable temperature profile during aging
• Axial temperature variation in SCR catalyst
•SCR catalyst inlet experiences higher temperature than exit (~90°C)
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
NOx emissions during SCR catalyst aging on engine bench at 650°C (α = 0.6)
•Cycle to cycle variation of NOx emissions during aging
•Half of NO is oxidized to NO2 by DOC and DPF
•During engine evaluation less than half of NOx out is NO
•NOx conversion of ~50% is obtained with α = 0.6
Cat In NO and NOx (no NH3)
Cat Out NOx (with NH3)
Cat Out NO (no NH3)
Cat Out NO (with NH3)
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Typical temperature profiles during SCR catalyst aging on engine bench at 750°C (α = 0.6)
• Temperature profile is repeatable
• Axial temperature variation in SCR catalyst
• Front section of SCR catalyst experiences higher temperature than rear section (~100°C)
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
NOx emissions during SCR catalyst aging on the engine bench at 750°C
•Cycle to cycle variation of NOx emissions during aging
•NOx conversion evaluated with α = 0.6
Cat In NO and NOx (no NH3)
Cat Out NOx (with NH3)
Cat Out NO (no NH3)
Cat Out NO (with NH3)
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Typical temperature profiles during SCR catalyst aging on engine bench at 850ºC (α = 0.6)
•Repeatable temperature profile
•Axial temperature variation along SCR catalyst
•Smaller temperature difference between SCR inlet and exit (~ 40°C)
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
NOx emissions during SCR catalyst aging on engine bench at 850°C
•Cycle to cycle variation of NOx emissions during aging
•NOx conversion evaluated with α = 0.6
Cat Out NO (no NH3)
Cat In NO and NOx (no NH3)
Cat Out NO (with NH3)
Cat Out NOx (with NH3)
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Bench-Flow Reactor Performance Evaluation
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Bench reactor evaluation of fresh and engine-aged Fe-Zeolite SCR catalysts Temperature varied between 200 and 600°C α varied between 0.5 and 1.2
Surface characterization studies SEM-EDS, EPMA, XRD and BET
Bench-Flow Reactor Evaluation
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Performance Evaluation of Fresh and Engine-Aged SCR Catalysts is Carried Out on Bench-Flow Reactor
SCR Catalyst Reactor
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Fe-Zeolite SCR Catalysts
- Fresh Fe-Zeolite SCR catalysts
- Engine-aged Fe-Zeolite catalysts • Two cored samples are obtained from engine-aged catalysts: front half and rear half • Evaluation is performed separately on both engine-age samples
22 mm 75 mm
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Thermocouple locations in SCR catalyst
5mm5mm
19mm
38mm
57mm
Flow
75.5mm
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Composition of Simulated Exhaust Gases
Gas Species Concentration
H2O (g) 5%
O2 14%
CO2 5%
NO 350ppm
NH3 175-350ppm
N2 balance
NOx performance evaluations are performed atGHSV of 30,000 h-1
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Typical SCR catalyst temperature at different locations during evaluation at 500ºC, GHSV = 30,000 h-1
• Steady state temperatures obtained during evaluation
• ∆Tmax=19°C
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
NOx conversion of fresh Fe-Zeolite SCR catalysts with 350 ppm NO at GHSV of 30,000 h-1
•Maximum NOx conversion of 94% occurs at 400ºC at α = 1.2
•NOx conversion increases with α
•Lower NOx conversion at T>500°C due to oxidation of NH3
•Low NOx conversion at 200°C due to kinetics
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
NOx Conversion of Engine-Aged Fe-Zeolite SCR Catalysts (α=1)
•Front section of both engine-aged catalysts degraded severely
•Reduction in NOx performance in front-half is possibly due to lube-oil poisoning, contaminations from DOC and cracking of washcoat
•Catalyst aged at 650°C is being evaluated
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
XRD patterns of fresh and aged Fe-Zeolite SCR Catalysts
•Significantly less zeolite detected in aged samples
•Fe2O3 and Al2O3 peaks are discernable in aged samples
•Zeolite structure begins to breakdown around 800°C for many high-silica zeolites
□ ●
□
∆
●
○∆
∆∆
◊
∆
† †
††
† †
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64 hrs on BFR (28ppm SO2)
13 cycles at 850°C (Front)
13 cycles at 850°C (Rear)
Fresh Fe-SCR
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Expanded XRD patterns of fresh and engine-aged Fe-Zeolite SCR catalysts
Zeolite
-------- Fresh
-------- 800°C
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
BET surface area measurements of fresh and engine-aged Fe-Zeolite SCR catalysts
•Surface area includes zeolite and cordierite substrate
•Front sections of engine-aged SCR catalysts experience severe loss of surface area
•Surface area is higher if cordierite substrate is excluded
•Surface area reduction correlates with aging temperature
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
EPMA elemental maps of fresh Fe-Zeolite SCR catalyst
• Fe is well-dispersed in washcoat
• ZrO2 and CeO2 are the oxygen storage materials
• Si is one of constituent of zeolite
• Trace of P is detected
Zn P
S Ca Fe
Si Zr Ce
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
EPMA elemental maps of Fe-SCR catalyst aged for 31 cycles at 650°C (front)
• Fe is still well dispersed in washcoat
• Agglomeration of ZrO2
and CeO2
• More P is detected in front due to lube-oil contamination
• Trace of Pt is detected
Pt P
S Ca Fe
Si Zr Ce
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
EPMA elemental maps of Fe-Zeolite SCR catalyst engine-aged for 31 cycles at 650°C (rear)
•Fe is well-dispersed in washcoat
•Trace of Pt is detected
•Agglomeration of ZrO2 and CeO2
• Less P deposited in rear section
Pt P
S Ca Fe
Si Zr Ce
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
EPMA elemental maps of Fe-Zeolite SCR catalyst engine-aged for 50 cycles at 750°C (front)
• Cracking of washcoat is visible in front section due to higher temperature
• Contamination from lube oil, i.e., P
Pt P
S Ca Fe
Si Zr Ce
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
EPMA elemental maps of Fe-Zeolite SCR catalyst engine- aged for 50 cycles at 750°C (rear)
•Temperature in rear-half is much less than 750C
•No cracking of washcoat is observed in rear-half, which explains higher NOx conversion
•Only trace of Pt is detected
Pt P
S Ca Fe
Si Zr Ce
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
EPMA elemental maps of Fe-ZeoliteSCR catalyst engine-aged for 13 cycles at 850°C (front)
• Severe crackings of washcoat
Zn P
S Ca Fe
Si Zr Ce
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
EPMA elemental maps of Fe-SCR engine-aged for 13 cycles at 850°C (rear)
•Severe cracking of washcoat
Zn P
S Ca Fe
Si Zr Ce
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OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Conclusions Loss of NOx performance of engine-
aged Fe-Zeolite SCR catalysts is mainly due to:
- Loss of surface area (BET)
- Breakdown of zeolite structure at temperatures above 800°C (XRD)
- Severe cracking of washcoat at temperatures above 750°C (EPMA)
- Agglomeration of CeO2 and ZrO2
- Contamination from lube-oil
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