SCR Performance Optimization Through Advancements in Aftertreatment PackagingJared Blaisdell, Andrew Gilb, Phebe Preethi, Joe Sweeney, Karthik Viswanathan, Paul Way, Nathan Zambon

DEER 2008

Outline Optimization Through Aftertreatment Packaging

•

Urea Doser Integration–

Urea deposit formation & chemistry

–

Eliminating deposit formation

•

Urea Preparation–

Mixer design & simulation

–

Urea solution vaporization

–

Flow distribution

–

Urea distribution & mixing

•

Single & Dual Wall Packaging–

NOX

reduction impact

–

Skin temperature

•

Summary

Control Strategy

Urea Dosing System

Optimized Packaging

Catalysts

Doser Integration What Are Deposits?

232

322 )(CONHOHHNCOHNCONHNHCO

+→++⎯→⎯Δ

Step 1 (Vaporization): Water evaporates from spray dropletStep 2 (Decomposition): Urea thermolysis & hydrolysis reaction

Incomplete decomposition results in deposit formation

High Performance Liquid Chromatography

Confirms composition of collected deposits:• Urea• Melamine• Cyanuric acid• Biuret

AU

-0.10

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

1.10

1.20

1.30

1.40

1.50

1.60

1.70

Minutes2.00 2.20 2.40 2.60 2.80 3.00 3.20 3.40 3.60 3.80 4.00 4.20 4.40 4.60 4.80 5.00 5.20 5.40 5.60 5.80 6.00 6.20 6.40 6.60 6.80 7.00 7.20 7.40 7.60 7.80 8.00 8.20 8.40

Cyanuric Acid

UreaBiuret

Melamine

Thermogravimetric Analysis

•

Deposit decomposition requires significant energy & time

•Prevention through proper doser integration & urea preparation

Doser Integration Understanding Deposit Chemistry

0

20

40

60

80

100

0 100 200 300 400 500 600 700Temperature [C]

Wei

ght C

hang

e [%

]

UreaBiuretMelamineCyanuric AcidDeposits with DPF

Urea Doser Integration Deposit Formation & Root Cause

•

Deposits form in three areas:

–

Injector location (spray tip/boss)

•

Cool tip/boss temperatures, direct spray impingement, spray recirculation

–

Interior wetted walls

•

Direct spray impingement on cool wall surface

–

Catalyst surface

•

“Wet”

urea contacting catalyst surface (poor urea mixing/preparation)

Injector Tip/Boss

Wetted Body/Pipe Walls

Urea Doser Integration Eliminating Deposits

•

Eliminated wall wetting at injection location and pipe walls (CFD)

•

Eliminated flow recirculation zone (CFD)

•

Incorporate Wire-mesh “accumulator”–

Direct spray impingement–

Eliminates wall wetting–

Re-directs urea spray

Recirculation

Straight-line Flow

Urea Preparation Mixer Design and Simulation

•

What is the “best”

mixer type?–

In-pipe mixers

–

In-body mixers

–

Multiple injection locations

–

Multiple mixer variations

•

Water injection used to simulate urea

•

Model Outputs:

–

% Vaporization

–

H2

O vapor uniformity (γH2O

)

–

Velocity uniformity (γVel

)

–

Backpressure penalty

• Simulation/Test Modes:

–

332g/s @ 500˚C exh

(high flow)–

151g/s @ 290˚C exh

(low flow)

A

B

C

D

E

Urea Preparation Mixer Design and Simulation

•

Ideal mixer determined to be in-body design–

Low backpressure penalty

–

Maximum mixing/vaporization

–

Maximum injection location flexibility

BP (kPa)

γH2OHigh Flow

γH2OLow Flow

γVelHigh Flow

γVelLow Flow

A –

Continuous Body & On-Mixer

Doser 2.2 0.77 0.90 0.95 0.98B –

DPF Outlet Mixer & On-Mixer

Doser 2.0 0.97 0.98 0.96 0.99C –

SCR Inlet Mixer & On-Pipe

Doser 2.6 0.87 0.92 0.94 0.97D –

SCR Inlet Mixer & On-Mixer

Doser 1.6 0.82 0.93 0.94 0.98

E –

In Pipe Vortex & On-Pipe Doser 1.9 0.96 0.87 0.97 0.99

Incomplete vaporization at SCR face

In-Body

In-Pipe

Urea Preparation Simulation Validation

•

In-body mixer refined via simulation

•

Simulation results compared against engine test:–

15 liter, Cummins ISX 500

–

342 g/s

@ ~477 C 1,616 ml/hr injection

–

190 g/s

@ ~429 C 638 ml/hr injection

–

224 g/s

@ ~204 C 1,741 ml/hr injection

•

Baseline (no mixer) compared to with-mixer case

Gas Sensing ArrayMixer

Empty Inlet(normally DOC + DPF)

Single 10.5”

x 6”

SCRpromotes ammonia slip for distribution measurement

Flow Distributor

Urea Injection/Mixer

Model SCR InletγH2O

= 0.66Model SCR Inlet

γH2O

= 0.69Model SCR Inlet

γH2O

= 0.67

•

Linear flow with some turbulence (limited vaporization time)

•

> 75% of urea impacts core face as liquid

•

Poor reactant distribution at the SCR core face

Mode 1 Mode 2 Mode 3

Baseline Simulation Results

Urea Preparation Simulation Validation

Model SCR InletγH2O

= 0.97Model SCR Inlet

γH2O

= 0.97Model SCR Inlet

γH2O

= 0.96

Mode 1 Mode 2 Mode 3

•

Strong vortex = increase vaporization time

•

No liquid urea escapes

•

Uniform distribution

Mixer Simulation Results

Urea Preparation Simulation Validation

Urea Preparation Measured Ammonia Slip Distribution

γNH3

= 0.81 γNH3

= 0.90 γNH3

= 0.97

γNH3

= 0.55 γNH3

= 0.55 γNH3

= 0.56

Mode 1 Mode 2 Mode 3

Bas

elin

eW

ith M

ixer

Significant Urea Pooling

No Pooling = Increased SlipCompared to Baseline

Urea Preparation On-Engine Performance Evaluation

•

2004 CAT C7

•

600 HP eddy-current dynamometer

•

Pseudo FTP cycle–

Cold start transient (duration: 20 min)

–

20 min hot soak

–

Hot start transient (duration: 20 min)

•

3 steady-state SET modes •

1901 RPM, 175 ft-lb (B25)

•

1901 RPM, 525 ft-lb (B75)

•

2240 RPM, 611 ft-lb (C100)

•

Alpha = NH3

:NOX

= 1.0 while exhaust > 200˚C

Empty

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

1 2 3Mode

% N

ox R

educ

tion

With Mesh No MixerNo Mesh With MixerWith Mesh With Mixer

SCR Inlet Temp: 300 C

SCR Inlet Temp: 450 C

SCR Inlet Temp: 483 C

Urea Preparation On-Engine Performance

Evaluation

FTP NOx Red No Mixer With Mixer

No Accumulator

47.2 % 442ppm NH3

74.9 % 15ppm NH3

With Accumulator

62.5 % 132ppm NH3

81.9 % 15ppm NH3

Cycle Peak Slip

With Accumulator, No Mixer

No Accumulator, With Mixer

With Accumulator, With Mixer

Packaging Style Dual or Single Wall?

•

Single or dual wall drivers:

-

Skin temperature: dual wall has lower skin temperature

-

Heat retention: improved cold start NOx reduction

-

Cost: dual wall is higher cost

•

Performance tested

-

Cold start + hot start FTP

-

Steady state, ~ 480˚C exhaust gas temperature

Test System

050

100150200250300350400450

Skin

Tem

pera

ture

(C)

Dual WallSingle Wall

Test Cell Evaluation Impact of Dual Walled Packaging –

NOX

ReductionFTP %NOx Reduction

Dual Wall (System) 77.2%

Single Wall (SCR only) 75.7%

Post-D

PF

Post-M

ixer

SCR

Post-SCR

Exhaust Temp ~ 480˚C

Summary•

Doser integration priorities–

Zero wall wetting–

Zero recirculation–

In-pipe accumulator reduces deposits

•

Urea preparation significantly impacts system performance–

In-pipe mixers require long pipe lengths for full vaporization–

In-body swirl mixer provides the best performance:•

100% vaporization •

excellent flow distribution •

6”

package space, doser flexibility–

Combination of in-body mixer with in-pipe accumulator offers performance and deposit advantages

•

Single or dual wall packaging? –

Little to no impact on NOx reduction over cold + hot FTP–

Skin temperature, cost -

primary drivers–

Impact on deposit formation needs to be evaluated

Control Strategy

Urea Dosing

Optimized Packaging

Catalysts