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Aftertreatment: SCR Modeling using STAR-CCM+ and STAR-CD
Richard JohnsCD-adapco
Contents
• SCR System Operation
• Modeling Considerations
• Validation: Spray and Catalyst Chemistry
• Simple and Detailed Catalyst Chemistry
• Application to off-road SCR system
• Summary
SCR System Operation
Urea(NH2)2CO
+ H2O
MP = 130°C
(NH2)2CO HNCO + NH3 thermolysisurea iso-cyanic acid ammonia
HNCO + H2O CO2 + NH3 hydrolysis
Slow Fast
NH3+ NO +1/4O2 N2+3/2H2ONO Reduction
NONONH3
Modelling Considerations
• Unsteady:
Engine exhaust flow and pulsed (typically 4 Hz) injection
• Multi-component
Urea + H2O - H2O evaporates and molten urea decomposes (thermolysis) to ammonia and isocyanic acid
• Impingement & mixing
Complex process involving impingement dynamics, wall-film and turbulent mixing
• Chemistry:
Gas-phase and catalyst surface reactions
• Objective:
To provide minimum dosing and achieve total NO reduction without either NO or NH3 slip
Validation Test Case upstream of SCR Catalyst
• Experimental Set up of Kim et al. (Proc. 2004 Fall Tech Conf ASME ICE Div.) to study the conversion of Urea-Water Solution (UWS) into Ammonia
• UWS (40% Urea) is injected at the axis
• Inlet gas Temperatures of 573, 623, 673 K were used at different average velocities from 6.0 – 10.8 m/s thus yielding different residence times
• Rosin Rammler droplet distribution with average size of 44 microns and injection velocity of 10.6 m/s, mass flow rate of 3.3e-4 kg/s, and injection temperature of 20 C
Thermolysis Reaction(NH2)2CO HNCO + NH3 ; Rate = 4.9e3 exp (-2.303e7/RT) units in J, kmol, m, s
Hydrolysis Reaction Upstream of SCRHNCO + H2O NH3 + CO2 ; Rate = 1.25e5 exp (-6.22e7/RT) units in J, kmol, m, s
Results – Mass Fractions & Temperature
Results – Uniformity Calculations
• Flow Direction is from left to right
• Solid Cone Spray with 70o, not much turbulent dispersion
• Thermolysis consumes Urea quite rapidly
• Conversion Efficiency & Uniformity Index of NH3 and H2O
can be deduced from this analysis.
Results – Comparison with Experiments (350 C)
Gas Velocity = 10.8 m/s
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Residence time (s)
NH3
Conv
ersi
on
Expt, Kim et al. Numerical ModelGas Velocity = 6.4 m/s
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.2 0.4 0.6 0.8 1
Residence time (s)
NH3
Conv
ersi
on
Expt, Kim et al. Numerical Model
Gas Velocity = 9.1 m/s
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Residence time (s)
NH3
Conv
ersi
on
Expt, Kim et al. Numerical Model
All Residence Times
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.2 0.4 0.6 0.8 1
Residence time (s)
NH3
Conv
ersi
on
Expt, Kim et al. Numerical Model
Modeling of the SCR Catalyst
The structured mesh in the axial direction represents multiple channels of the honeycomb structure in SCR.
Select the following Physics Models– 3D– Steady– Multi-Component Gas– Reacting– Non-Premixed Combustion– Homogeneous Reactor with
Surface Chemistry– Chemistry ADI (with Surface
Reactions)– Turbulent k-Epsilon
Complex Reaction Mechanisms - DARS
After import, STAR-CCM+ has the gas and surface species definitions, and reaction details.
Ref: Dumesic et al., Journal of Catalysis,
163, 409-417 (1996)
Results – NO, NH3, V3+(s) fractions
• Flow Direction is from left to right• Mass Fractions at the inlet are uniform
[O2, H2O, NH3, NO, CO2, N2] = [0.11, 0.09, 0.01, 0.001, 0.073, 0.716]
• Standard post-processing quantities can all be setup using reports in STAR-CCM+ and automated
- Conversion Efficiency- Trapping Efficiency - Uniformity Index - NH3 Slip
Part (3.3) Reduced Chemistry Approach For SCR
• A two-step Global Kinetics Model* has been adopted for implementing surface reactions in SCR region
• The reaction kinetics was developed for a V2O5-WO3/TiO2 catalyst
• The honeycomb porous structure could directly employ the proposed kinetic parameters obtained from the kinetic study over a packed-bedflow reactor
• In STAR-CCM+, the reaction rates from the paper are modeled through species source/sink terms provided directly in the SCR porous regions.
* Ref: “ Direct Use of Kinetic Parameters for Modeling and Simulation of a Selective Catalytic Reduction Process” Chae et al., Ind. Eng. Chem. Res., 2000, 39
Two-Step SCR Model Kinetics Parameters
Results – NOx Reduction Comparison
Two-Step Model
Detailed Surface Chemistry
Summary
STAR-CCM+ and STAR-CD have been developed to model SCR aftertreatment systems in detail
All key phenomena – spray dynamics, impingement and wall film behaviour, multicomponent liquid, gas phase and surface chemistry are included
Validation and testing against experimental data has demonstrated that accurate solutions can be obtained
Application to aftertreatment system development is identifying areas for design changes and helping to develop optimized designs
Thank You