development and validation of multi site kinetic models ...β¬Β¦Β Β· 10/2/2018 11 standard scr rates...
TRANSCRIPT
CHINTAN DESAI*
B PRASHANT BALIGA
KRISHNA NATTI
BRUCE VERNHAM
Development and Validation of Multi Site Kinetic Models for SCR and ASC and Application for SCR Calibration
* Presenter Isuzu Technical Center of America, Inc.
10/2/2018 2
Aftertreatment Simulation
Data Collection (Synthetic Gas Bench)
Model Validation (Dyno and SGB)
Model Application
Maps Generation by Models, SCR FF Cal
GT vs DCU Model Prediction, NH3 Storage
NOx Conversion, %
Cumulative NOx, g
NH3 Storage, g
EXP
MODEL
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Introduction, Objective and Purpose
Cu-CHA small pore SCR catalysts utilized worldwide due to increased range of temperature for high DeNOx, along with good hydrothermal stability
Due to the relatively simple structure of SSZ-13, it has been investigated intensively by researchers (DeNOx DOE Team, Schneider Group at U-Notre Dame, Gounder group at Purdue etc.)
Structure of SSZ-13 [3]
[1] Stewart et al. (2013) Global Kinetic SCR model with Two Ammonia Storage Sites. CLEERS 2013
[2] Olsson, L., Wijayanti, K., Leistner, K., Kumar, A., Joshi, S. Y., Kamasamudram, K., ... & Yezerets, A. (2015).
A multi-site kinetic model for NH 3-SCR over Cu/SSZ-13. Applied Catalysis B: Environmental, 174, 212-224.
[3] Gounder et al. (2016) New insights into the mechanisms and Active Site requirements of Low Temperature
NOx SCR with Ammonia on Cu-SSZ-13 zeolites. CLEERS 2016
Objective
Develop a kinetic model for NH3-SCR over Cu-SSZ-13 in a temperature interval of 150-550Β°C for 2 space velocities and 2 thermal ageing conditions
Purpose
Model-based SCR Calibration, Urea Dosing Calibration and Control
Feasibility studies for hardware modifications
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SCR Model Development
SCR Model Development Results
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NH3 Storage
Fast SCR
NO2 SCR
Standard SCR
NH3 Oxidation
Simply by changing the site densities while keeping the kinetic parameters
constant, predictions were made
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Model Setup Key Conservation Equations (Quasi-Steady) [7]
Gas Phase Continuity :
Gas-Phase Species Mass Balance and Coverage Tracking:
Washcoat Diffusion:
Note: See Appendix for nomenclature
[7] GT-Suite Exhaust Aftertreatment Application Manual v2017 Kinetic Model Setup in GT-Suite v2017
Boundary Conditions:
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Modeling Approach governed by TPD data
Temperature (Β°C) NH3 Concentration (ppm) Catalyst Ageing
and SV (1/h) Low T Site
Desorption Peak High T Site
Desorption Peak Low T Site Desorption
Peak at 150Β°C High T Site Desorption
Peak at 150Β°C Degreened 30k 330 440 230 105
Aged 30k 310 n/a 280 n/a
Low T Site S2 High T Site S1
Low T Site
Ageing led to migration of High T Site to Low T
NH3 Oxidation insignificant below 400Β°C, but interferes with High T site peak
Low T & High T Site named as placeholders
Nature of sites will be discussed post NH3 storage model construction
Desorption Peaks in NH3 TPD (Temperature Programmed Desorption)test
Degreening 650Β°C for 16 hours
Ageing 700Β°C for 100 hours
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NH3 Storage Model β Results at SV: 30k/h and T=150Β°C
The plots represent a NH3 TPD test
Site 1 density reduced by 90%
while Site 2 density increased by
40% upon ageing
Same kinetic constants used for degreened and aged SCR
ππ»3 + π1 ππ»3 β π1
ππ»3 + π2 ππ»3 β π2 S2 Low Temp Site
S1 High Temp Site
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NH3 Oxidation Model β Results with 0.2% O2 at SV: 30k/h
4ππ»3 β π1 + 3π2 β 2π2 + 6π»2π + 4π1
4ππ»3 β π1 + 5π2 β 4ππ + 6π»2π + 4π1
4ππ»3 β π2 + 5π2 β 4ππ + 6π»2π + 4π2
Aging leads to reduced oxidation (as would be expected if catalyzed by ZCuOH sites)
NH3 storage data at 0.2% O2, along with oxidation data at 10% O2, allows for determination of O2 reaction order
Calibrated value ~ 0.5
Further ageing beyond 750Β°C will increase oxidation due to cluster formation
2ππ»3 β π2 + 2π2 β π2π + 3π»2π + 2π2
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Standard SCR Model β Results at SV: 30k/h
4ππ»3 β π1 + 4ππ + π2 β 4π2 + 6π»2π + 4π1
4ππ»3 β π2 + 4ππ + π2 β 4π2 + 6π»2π + 4π2
4 global reactions used
Identical rate order w.r.t. O2 as the NH3 oxidation reaction
In general, reactions proceed on both sites, with different activation energies
2ππ»3 β π1 + 2ππ + π2 β π2 + π2π + 6π»2π + 4π1
2ππ»3 β π2 + 2ππ + π2 β π2 + π2π + 6π»2π + 4π2
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Standard SCR Rates at SV: 60k/h
4ππ»3 β π1 + 4ππ + π2 β 4π2 + 6π»2π + 4π1
4ππ»3 β π2 + 4ππ + π2 β 4π2 + 6π»2π + 4π2
S1 was the most active site for standard SCR above 250 Β°C, in line with its ability to store NH3 at high temperatures
Hydrothermal ageing led to a reduction of the SCR rate on the S1 site, along with a concomitant increase on the S2 site
This site-specific behavior of the global model follows the expected trends based on the deduced site definitions in the βNH3 Adsorption/Desorption Reactionβ section
2ππ»3 β π1 + 2ππ + π2 β π2 + π2π + 6π»2π + 4π1
2ππ»3 β π2 + 2ππ + π2 β π2 + π2π + 6π»2π + 4π2
S2
High Temp Site
Low Temp Site
S1
Site 1 density reduced by 90%
while Site 2 density increased by
40% upon ageing
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Fast SCR Model β Results at SV: 30k/h
2 global reactions used
In general, reactions proceed on both sites, with different activation energies
Low temperature N2O from nitrate formation and decomposition (discussed in later slides)
High temperature N2O from direct NH3 oxidation (shown earlier)
2ππ»3 β π1 + ππ + ππ2 β 2π2 + 3π»2π + 2π1
2ππ»3 β π2 + ππ + ππ2 β 2π2 + 3π»2π + 2π2
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NO2 SCR Model β Results at SV: 30k/h
8ππ»3 β π1 + 6ππ2 β 7π2 + 12π»2π + 8π1
8ππ»3 β π2 + 6ππ2 β 7π2 + 12π»2π + 8π2
Nitrate formation and decomposition accounted for explicitly in reaction model
Addition of enhanced SCR reaction on S2
In general, reactions proceed on both sites with different activation energies
N2O slip model captures overall trend, but needs improvement
2ππ»3 β π1 + 2ππ2 β π2 + π΄π β π1 + π»2π + π1
2ππ»3 β π2 + 2ππ2 β π2 + π΄π β π2 + π»2π + π2
π΄π β π1 β π2π + 2π»2π + π1
π΄π β π2 β π2π + 2π»2π + π2
π΄π β π2 + ππ β ππ2 + π2 + 2π»2π + π2
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Nitrate formation inhibits low temperature SCR reaction
Low Temperature NH4NO3 Inhibition of Fast SCR
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SCR Model Validation on Engine (Steady State & Transient)
Full-Scale SCR Model Setup
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SCR Model setup in GT with appropriate initial and boundary conditions
Aged kinetics utilized for validation purpose (further Degreened data required for additional validation)
Provision to add ASC kinetics and validate tailpipe NOx
Key Assumptions:
1. Urea-NH3 Conversion = 100%
2. No significant difference between dyno part (hydrothermally aged at 650Β°C for 100 hours with minor engine ageing) and reactor part (hydrothermally aged at 700Β°C for 100 hours)
Steady-State Validation Results
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Inlet Conditions Parameter Units Value
Temperature Β°C 353 NO2/NOx Ratio 0.41
Standard Space Velocity 1/h 14785
Transient SCR Model β Simulation Sequence
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1. Pre-conditioning HDT
2. Overnight Soak
1 and 2 provide NH3 storage distribution on both sites for start of cold HDT
The absolute NH3 storage in g is scaled to match the SCRMod_mEstNH3Ld variable at t=0 for Cold HDT
3. Cold HDT
4. Intermediated Hot Soak
5. Hot HDT
Initial NH3 Coverage (g/L)
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Prior to cold HDT cycle, the
preconditioning cycle was
simulated to obtain the pre stored
NH3 and axial distribution
Most of the pre stored NH3 is near
the front of the catalyst
Cold HDT Results β Mid-Bed Temperature and SV
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H2O Storage Exotherm
H2O storage can be
modeled with a
reversible H2O
adsorption/desorption
reaction
Cold HDT Species Concentration
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There is NO storage at the
beginning of the cycle indicating low
temperature storage followed by a
large peak
NO storage model required to
predict this behavior (This can be added to the mechanism later)
Later peak locations and
magnitudes predicted reasonably
well
N2O trend also predicted well
Cold HDT Cumulative Mass & NH3 Storage
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Mass based NOx conversion
predicted within 3% for both cold
and hot HDT
N2O slip also predicted reasonably
well with minor underestimation
NH3 storage prediction indirectly
tells about the NH3 slip at around
720 s where there is a sharp rise in
the temperature
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ASC Model Development & Validation
ASC Dual Layer S.V. 70K
10/2/2018 24
Degreened
Aged
The results shown are of dual layer
(PGM+SCR layers)
Intra-porous diffusion in the dual layer
ASC was accounted by the asymptotic
approach by GT
No significant increase in
computational time
The porosity/tortuosity ratios was
calibrated using the dual layer data
These are the NOx yields for NH3 O2
oxidation and NH3 NO O2 interaction
Significant improvement seen in the model
by using the parallel pore diffusion model
compared to constant effective diffusivity
as per the following equation
ASC Validation on Engine
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NOx slip generated in
ASC
Cold and hot HDT cycle normalized
instantaneous NOx concentrations
from 600 to 1200 s
After 700 s unselective conversion of
NH3 to NOx predicted by the model
Development of ASC model is
important to predict the NOx
generation across it
Model Applications
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SCR Calibration
MODEL
ENGINE
This data is normalized
This is the NOx conversion
map against temperature
and NH3 load
Remaining maps were also
generated by model
Good correlation observed
Manual calibration time
and effort would be
reduced
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GT- DCU Co-Simulation
β’ Simulink (Control Model) β Master, GT (Plant Model) as Slave
β’ GT model gives current NH3 load to control model β’ Control model gives urea dosing to GT model (plant
model)
GT (Plant) Model
Input from Control model (test data)
Urea Dosing Calculated by Control model using NH3 storage from GT (Plant) model
Current NH3 load to control Model
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Urea Dosing
Control Model Improvement required
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NH3 Storage
Figure shows reasonable correlation between test data and GT-DCU simulation
GT model could be used to generate maps simultaneously minimizing the dyno tests
NH3 Storage on different engine
Test 1 Test 2 Test 3
Plots shown here are for 2 different
engines and aftertreatment systems
Thank You
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