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