CD-adapco Contents SCR System Operation Modeling Considerations - - PowerPoint PPT Presentation

Aftertreat eatment ent: S SCR M Model elin ing g usin ing S g STAR-CCM CCM+ a and S STAR-CD CD

Richard Johns

CD-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 thermolysis urea iso-cyanic acid ammonia HNCO + H2O CO2 + NH3 hydrolysis Slow Fast NH3+ NO +1/4O2 N2+3/2H2O NO Reduction NO NO NH3

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
• f 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 SCR HNCO + 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 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Residence time (s) NH3 Conversion Expt, Kim et al. Numerical Model Gas Velocity = 6.4 m/s 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.2 0.4 0.6 0.8 1 Residence time (s) NH3 Conversion Expt, Kim et al. Numerical Model Gas Velocity = 9.1 m/s 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Residence time (s) NH3 Conversion Expt, Kim et al. Numerical Model All Residence Times 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.2 0.4 0.6 0.8 1 Residence time (s) NH3 Conversion 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) Reduce educed C d Chem hemistry Approac pproach F For

• r SCR
• A two-step Global Kinetics Model* has been adopted for

implementing surface reactions in SCR region

• The

he rea eaction kinet netics was as dev developed for

• r a

a V2O 2O5-WO3/ 3/TiO2 2 catalyst

• The

e honey eycomb b porous

• us struc

uctur ure e could d directly employ

• y the propos
• posed

ed kinet netic par parameters obt

• btai

ained from

• m the

he kinet netic stud udy ov

• ver a

a pac packed-bed bed flow reac actor

• r
• In ST

STAR AR-CCM+, the he rea eaction rat ates from

• m the

he pape paper ar are mode

• deled thr

hrou

• ugh

spec ecies es sour urce/sink nk terms prov

• vided

ded direc ectly in the SCR porous

• us regions
• ns.

* 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

Than hank Y You