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Simulation of Exhaust Gas Aftertreatment Systems -Reaction Engineering in Automotive Applications

Dr. Daniel Chatterjee

Memorial Colloquium Prof. Dr. Dr. h.c. Jürgen Warnatz

31.05.2008

Dr. D. Chatterjee, GR/VPE 2

Outline

� Emission Legislations

� Exhaust Gas Aftertreatment Systems

� Simulation of Exhaust Gas Aftertreatment Systems

� Modeling of SCR-Systems

� Summary


European Emission Legislation

Since the introduction of the emission legislation the standards were reduced drastically

Test procedure: ESCEU 5 not yet defined by law

limits in discussion

Emission legislation for CO, HC, NOx and PM is the driving force for exhaust gas aftertreatment

NOx (g/kWh)

CO

(g

/kW

h)

PM (g/kWh)

5,0

2,1

3,51,5 0,02

2,0

EU 3

EU 4EU 5

0,36

0,15

0,14,0

4,5

8,07,0

EU 2

EU 1

HC+NOx (g/km)

CO

(g

/km

)

PM

(g/km)

0,560,50,3

0,025

0,005

0,23

EU 3

EU 4

0,14

0,08

0,05

0,641,0

2,72

0,970,7

EU 2

EU 1

EU 5

Commercial

Vehicles

Passenger

Cars

3


Development of Emission Limits of Diesel Passenger Cars

Improvement of the engine/combustion process is needed. In addition highly efficient

active aftertreatment systems are required to meet emission limits.

2009-112014-15

USA USA

56% NOx control


Emission Certification Testcycles:New European Driving Cycle (NEDC)

Exhaust System are operated under highly transient operation conditions regarding: mexh., Texh., CO, HC, NOx, PM,…


Outline





� Summary


SCR-Based Exhaust Gas AftertreatmentTechnology: BlueTEC II - GL 320 CDI

Dosing

Valve

BlueTEC II

DPF

Oxi-Catalyst

SCR-

Catalyst

• The exhaust system BlueTEC II is based on the NH3 based SCR process

used already for power plant DeNOx.

• BlueTEC II was introduced for Heavy Duty Trucks with EURO IV and EURO V.

AdBlue/Urea

TankOxidation of CO, HC, NO

Soot Trapping

NOx Reduction

With NH3Urea Injection


NSC Based Exhaust Gas AftertreatmentTechnology: BlueTEC I - E320 CDI

NSC-Catalyst

Oxi-Catalyst

SCR-

Catalyst

BlueTEC I

BlueTEC I

New exhaust gas aftertreatment system based on the combination of

NSC+SCR an low emission combustion. No urea dosing required.

DPF

Oxidation of CO, HC, NO

NOx Trapping/Reduction and

NH3 Formation Soot TrappingNOx Reduction

with NH3


Overview: AT-Components and Systems

AT*-Components (Heavy Duty and Passenger Cars):

AT*-Systems:

DOC+DPF

DOC+DPF+NSC

DOC+DPF+SCR

(BlueTEC II)

DOC+NSC+DPF+SCR

(BlueTEC I)

AdBlue-Injection

Three-Way Catalyst Oxidation Catalyst Diesel Particle Filter NOx Storage Catalyst SCR Catalyst

DOC SCRNSCDPFTWC

TWC only

Diesel Engines

*AT = Aftertreatment

Simulation assisted development required to manage complexity

regarding costs an time.


Outline





� Summary


Exhaust Aftertreatment ModelingExACT:

1D- Simulation of combined

Exhaust Aftertreatment Systems

3D-Simulation:Optimization of Geometry

for Individual Components

Applications:

•Effect of non uniform

inlet conditions

•Temperature/Soot

distribution

•Light-off optimization

Applications:

•Testcycle performance

•System-design

•Operating strategies

0.5s

24s

Focus: SCR-, NSC-, DPF-,

DOC-, TWC-SystemsFocus: Urea Injection, DPF


ExACT Exhaust Aftertreatment System Modeling:Application Examples

0

50

100

150

200

250

300

350

400

450

500

0 200 400 600 800 1000 1200

time [s]

Te

mp

era

ture

[°C

]

T after NSK

T after NSK calc

Tout NSC, exp,

Tout NSC, sim.

Temperatures

Rußbeladung & Differenzdruck

0

5

10

15

20

25

30

35

40

45

50

0 300 600 900 1200

Zeit [s]D

iffe

ren

zd

ruc

k [

mb

ar]

Ru

ßb

ela

du

ng

[g

]

backpressure - measured

backpressure - computed

soot mass in filter - computed

Soot Loading/Pressure LossECU-Development

„Virtual Testbench“

Electronic Control Unit (ECU) ModelElectronic Control Unit (ECU) Model

Hardware implementation of

ECU model

Software in the Loop (SiL):

Test of ECU model on engine test bench

Prediction of Soot, NOx, CO, HC Conversion

0 200 400 600 800 1000 1200

0.5

1

1.5

2

2.5

3

3.5

4

Cu

mu

lati

ve

NO

x [

g]/

cy

cle

time [s]

inlet AGN system

outlet T original

outlet T+20°C (whole cycle)


outlet T+20°C (first 600s)

outlet T+40°C (first 600s)

Euro 5

Euro 6

0 200 400 600 800 1000 1200

0.5

1

1.5

2

2.5

3

3.5

4

Cu

mu

lati

ve

NO

x [

g]/

cy

cle

time [s]

inlet AGN system

outlet T original



0 200 400 600 800 1000 12000

50

100

150

200

250

300

time [sec]

NO

x [

pp

m]

NOx inlet EGA-system

NOx outlet EGA-system


Simulation of Exhaust Gas AftertreatmentSystems

Requirements on Simulation Models:

� Scalability:

(e.g. catalyst volume can range from 0.5 L (e.g. PC DOC) to 34L (e.g. HD SCR))

� Valid for a wide range of operating conditions

(e.g. exhaust temperatures from 100°C to 800°C)

� Extrapolation capability

(Not all inlet conditions combinations can be tested/measured.)

Physical and chemical based model approach required


surface reactions:

(chemistry)

z [m]

r[m

]

0 0.025 0.05 0.075 0.10

0.025

0.05

0.075

0.1

monolith:

(solid temperature)

Modeling of all relevant processes required for predictive simulations.

Variation of

coating/ amount

of precious metals.

requires recalibration

Variation of

coating/ amount

of precious metals.

requires recalibration

single channel:

(heat and

mass transfer) Variation of geom.

parameters (e. g. cell

density, length,

structure,...) possible.

Variation of geom.

parameters (e. g. cell

density, length,

structure,...) possible.

Detailed Modeling of Catalytic Converters/DPFs


Outline





� Summary


Exhaust Aftertreatment System Modeling: Example Heavy Duty SCR-System

ECU

SCR Catalyst

N2,

H2O

Urea Injection

NO

NH3

NO

NOx is reduced within the SCR catalyst by stored NH3.


0

100

200

300

400

500

600

0 200 400 600

time [s]

NO

x [

ppm

]

0

10

20

30

40

50

60

70N

H3 [p

pm

]

NOx

NH3

Modeling SCR-Catalytic Converters:

Texh. = 200°C, mexh. = 582 kg/h, α = 0.87, catalyst 25L, 300cpsi

Test Bench Measurement:

Concentrations Behind Catalyst

Buildup of stored NH3

η10ppm

Start of Urea

Injection


Modeling SCR-Catalytic Converters

Chemical Reactions: NO+NH3+

NH3 adsorption: NH3 → NH3*

NH3 desorption: NH3* → NH3

NH3 oxidation: 4NH3*+3O2 → 2N2+6H2O

NO-SCR reaction: 4NH3*+4NO+O2→4N2+6H2O

)1(3 ϑ−=NHadsads

Ckr

ϑγϑ)]1(exp[ −−=°

°

RT

Ekr

des

desdes

β

ϑ

ϑ

ϑ

−+

−= °

02.0

11

)exp( 2O

LH

NONO

NONO

P

K

C

RT

Ekr

•No NH3 adsorption/desorption equilibrium is assumed.

•Eley-Rideal kinetics for NO-SCR reaction.

•Higher O2 concentration increases SCR reaction and NH3 oxidation rates.

+SAE 2005-01-0965

NH3

NH3

NH3NH3

ϑ

β

−= °

02.0)exp( 2Oox

oxox

P

RT

Ekr

β

ϑ

−= °

02.0)exp( 2

3

O

NHNO

NO

NONO

PC

RT

Ekr

NH3

NH3


Modeling SCR-Catalytic Converters:Influence of NH3 Inhibition on NOx Reduction

NH3

NH3**NH3*

NOx

2000 3000 4000 5000 6000

0

200

400

600

800

1000

tempo [s]

Co

ncen

trazio

ni [p

pm

]

2000 3000 4000 5000 6000

0

200

400

600

800

1000

tempo [s]

Exp NH3

Exp N2

Exp NO

Fit NH3

Fit N2

Fit NO

EleyEleyEleyEley――――RidealRidealRidealRideal kinetics Langmuir―Hinshelwood Langmuir―Hinshelwood Langmuir―Hinshelwood Langmuir―Hinshelwood kinetics

time [s] time [s]

NH3 shut off NH3 shut off

NH3

NH3*

NOx

Powder Microreactor: GHSV = 90000 h-1, O2=2%, H2O = 1 %, NO = NH3 = 1000 ppm, T = 200 °C

NO

NH3

N2

NO

NH3

N2

0

20

40

60

80

100

120

140

160

180

200

0 500 1000 1500

time [s]

NO

x [

pp

m]

NOx, exp

NOx, sim

0

20

40

60

80

100

120

140

160

180

200

0 500 1000 1500

time [s]N

Ox [

pp

m]

NOx, exp

NOx, sim

Engine test bench: Texh.=297°C, mexh.1061 kg/h, SCR Vol.=32L, α=1.01


Modeling SCR-Catalytic Converters

Chemical Reactions: NO+NH3+

NH3 adsorption: NH3 → NH3*

NH3 desorption: NH3* → NH3

NH3 oxidation: 4NH3*+3O2 → 2N2+6H2O

NO-SCR reaction: 4NH3*+4NO+O2→4N2+6H2O

)1(3 ϑ−=NHadsads

Ckr

ϑγϑ)]1(exp[ −−=°

°

RT

Ekr

des

desdes

β

ϑ

ϑ

ϑ

−+

−= °

02.0

11

)exp( 2O

LH

NONO

NONO

P

K

C

RT

Ekr

•No NH3 adsorption/desorption equilibrium is assumed.

•Two-sites L.-H. expression accounts for NH3 inhibition of the SCR reaction.

•Higher O2 concentration increases SCR reaction and NH3 oxidation rates.

+SAE 2005-01-0965

NH3

NH3

NH3NH3

ϑ

β

−= °

02.0)exp( 2Oox

oxox

P

RT

Ekr


0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 100 200 300 400 500

time [s]N

H3

, in

[-]

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

0.020

NH

3,

ou

t [-

]

SCR-Model Validation: Testcycle Simulation (ETC)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 100 200 300 400 500

time [s]

NO

x [-

]

NOx in

NOx out, exp.

NOx out, sim.

NH3 in

NH3 out, exp.

NH3 out, sim.

The overall NOx efficiency within an testcycle

is predicted with an typical accuracy of 3-4%

OM906, SCR : Argillon9999 18L, 300cpsi


SCR-Model Application: Identification of RequiredCatalyst Volume - Steady State NOx Conversion

η10ppm, α = optimized ηmax, α = 1

Catalyst: 18L, 300cpsi

ESC

operating points

90

80

70

60

50

40

30

20

10

90

80

70

60

50

40

30

20

10

• Low Texh. : NOx conversion limited by slow SCR kinetics.

kinetic limitation

• High mexh.: NOx conversion limited by diffusion effects � high NH3 slip.

diffusion effects

• High Texh. : NOx conversion limited by low NH3 storage capacity and NH3 oxidation.

low NH3 storage,

NH3 oxidation


90

80

70

60

50

40

30

20

10

90

80

70

60

50

40

30

20

10

90

80

70

60

50

40

30

20

10

η10ppm η10ppm

η10ppm

14L 18L

25L

908825

898118

816114

ηETC [%]ηESC [%]catalyst

volume [L]

*10% deduction on the NOx conversion if α > 1

18L catalyst volume is sufficient to fulfill the

application target of 80% NOx conversion.

SCR-Model Application: Identification of RequiredCatalyst Volume - Steady State NOx Conversion

OM906LA, α=optimized, SCR : Argillon9999, 300cpsi


SCR Model Application:Transient behavior within an ETC tescycle

α=1, Catalyst: 18L, 300cpsi

NOx in/out NH3 in/out

Ts along monolith axis Stored NH3* along monolith axis Stored NH3* within monolith wall


Exhaust Aftertreatment System Modeling of Advanced SCR Systems: DOC+DPF+SCR

ECU

SCR Catalyst

N2,

H2O

Urea Injection

NO

NH3

NO

DOC DPF

,NO2

DOC upstream of the SCR oxidizes NO to NO2, which improves SCR performance.



Chemical Reactions: NO+NO2+NH3

� NO2 disporoportion: 2NO2 +H2O ⇔ HNO2 + HNO3

� HNO2 reaction with NH3: HNO2 + NH3* → N2 + 2H2O

� NH4NO3 adsorption-desorption: NH3*+ HNO3 ⇔ NH4NO3*

� HNO3 reaction with NO: HNO3 + NO ⇔ HNO2 + NO2

� N2O formation: NH3*+ HNO3 → N2O + 2H2O

� NO2- SCR reaction: 4NH3*+ 3NO2→ 3.5N2 + 6H2O

•HNOx and NH4NOx are considered as intermediate species in the mechanism.

•No equilibrium is assumed for disproportion, adsorption/desoprtion etc.

•Two-sites L.-H. expression accounts for NH3 inhibition of the HNO3+NO reaction.

Fast SCR:

2NH3+NO2+NO

→ 2N2+3H2O

(NH4NO2)

(NH4NO3*)

� Fast-SCR reaction: 2NH3*+ NO2 + NO→ 2N2 + 3H2O

• The global reaction between NH3+NO+NO2 is very fast.

• However, the direct reaction between NH3 and NO2 is slow.


Low temperature� 2NH3 + 2NO2 � N2 + H2O + NH4NO3 (ammonium nitrate formation)

High Temperature

� NH4NO3 � N2O + 2H2O (ammonium nitrate decomposition)

� 6NO2 + 8NH3 � 7N2 + 12H2O (NO2 SCR)

150 175 200 225 250 275 300 325 350 375 400 425 450

0

100

200

300

400

500

600

700

800

900

1000

0

200

400

600

800

1000

1200

1400

1600

1800

2000

2200

2400

Co

ncn

etr

atio

n (

pp

m)

Temperature (°C)

N2ONO

2

NO

NH3 N

ba

lan

ce

(pp

m)N

2

N balance

Modeling SCR-Catalytic Converters:NO2+NH3 Experiment


Modeling SCR-Catalytic Converters:Proposed Reaction Mechanism for NO2+NO+NH3on V-Based and Fe-Zeolites SCR Catalysts:

Data analysis and literature supports the assumption of a similar reaction

mechanism on V-based and Fe-Zeolites SCR catalysts.

C. Ciardelli, I. Nova, E. Tronconi, D. Chatterjee, B. Bandl-Konrad, Chem. Commun. 23 (2004), 2718,

D. Chatterjee, T. Burkhardt, M. Weibel, E. Tronconi, I. Nova, C. Ciradelli, SAE 2006-01-0468,

I. Nova, C. Ciardelli, E. Tronconi, D. Chatterjee, B. Bandl-Konrad, Catal. Today, 114 (2006), 3,

A.Gossale, I. Nova, E. Tronconi, D. Chatterjee, M. Weibel, J. of Catalysis (submitted)

O. Kröcher, 1st Conference MinNOx, Feb. 2007 Berlin (Germany).

2NO2

↔↔↔↔ N2O

4+ H

2O ↔↔↔↔ HONO + HNO

3

NH3ads

[NH4NO

2] →→→→ N

2+ 2H

2O

NH4NO

3

NH3ads

2NO2 + 2NH3 →→→→ NH4NO3 + N2 + H2O2NO2

↔↔↔↔ N2O

4+ H

2O ↔↔↔↔ HONO + HNO

3

NH3ads

[NH4NO

2] →→→→ N

2+ 2H

2O

NH4NO

3

NH3ads

2NO2 + 2NH3 →→→→ NH4NO3 + N2 + H2O2NO2 + 2NH3 →→→→ NH4NO3 + N2 + H2O

NO

HONO + NO2 NH4NO3 + NO →→→→ N

2+ NO

2+ 2H

2ONH4NO3 + NO →→→→ N

2+ NO

2+ 2H

2O

2 NH3 + NO2 + NO →→→→ 2 N2 + 3 H2O

[NH4NO

2] →→→→ N

2+ 2H

2O

NH3ads

NO-NO2/NH

3

NO2/NH

3

NH4NO

3↔↔↔↔ NH

3+ HNO

3NH

4NO

3↔↔↔↔ NH

3+ HNO

3NH

4NO

3↔↔↔↔ NH

3+ HNO

3



Chemical Reactions: NO+NO2+NH3

� NO2 disporoportion: 2NO2 +H2O ⇔ HNO2 + HNO3

� HNO2 reaction with NH3: HNO2 + NH3* → N2 + 2H2O

� NH4NO3 adsorption-desorption: NH3*+ HNO3 ⇔ NH4NO3*

� HNO3 reaction with NO: HNO3 + NO ⇔ HNO2 + NO2

� N2O formation: NH3*+ HNO3 → N2O + 2H2O

� NO2- SCR reaction: 4NH3*+ 3NO2→ 3.5N2 + 6H2O

•HNOx and NH4NOx are considered as intermediate species in the mechanism.

•No equilibrium is assumed for disproportion, adsorption/desoprtion etc.

•Two-sites L.-H. expression accounts for NH3 inhibition of the HNO3+NO reaction.

Fast SCR:

2NH3+NO2+NO

→ 2N2+3H2O

(NH4NO2)

(NH4NO3*)

• The global reaction between NH3+NO+NO2 is very fast.

• However, the direct reaction between NH3 and NO2 is slow.


SCR-Modeling: Influence of NO2 on SCR Conversion Efficency

Steady state NOx conversionSteady state outlet concentrations

Good prediction quality of NO2 influence on product selectivity and NOx conversion.

0.0 0.2 0.4 0.6 0.8 1.0

0

200

400

600

800

1000 experimental

calculated

N2O

NO

NO2

NH3

N2

ppm

NO2/NO

x feed ratio

0.0 0.2 0.4 0.6 0.8 1.00

20

40

60

80

100

experimental

calculated

275°C

225°C

200°C

NO2/NO

x feed ratio

NO

x c

onve

rsio

n (

%)

1000

800

600

400

200

0

concentration [ppm]

100

80

60

40

20

0

NOxconversion [%]

0.0 0.2 0.4 0.6 0.8 1.0

NO2/NOx feed ratio

0.0 0.2 0.4 0.6 0.8 1.0

NO2/NOx feed ratioMicroreactor sim./exp.:

Feed 1000ppm NH3, 1000ppm NOx, GHSV = 210000 h-1 at 225°C

Microreactor sim./exp.:

Feed 1000ppm NH3, 1000ppm NOx, GHSV = 210000 h-1


1D-Exhaust Aftertreatment System Modeling:DOC+SCR Simulation (ESC)

• Model predicts transient temperatures and species conversions within testcycle

• Presence of NO2 in the SCR inlet exhaust increases NOx conversion in

the lower temperature parts of the ESC.

NO2/NOx=DOCout at SCR inletNO2/NOx=50% at SCR inlet

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 500 1000 1500

time [s]

NO

x [-

]

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 500 1000 1500

time [s]

NO

x [-

]

(NO2/NOx)in≈ 40%-50%

NO2/NOx behind DOC

0

10

20

30

40

50

60

70

80

90

100

0 500 1000 1500

time [s]

NO

2/N

Ox

[%]

100

150

200

250

300

350

400

450

tem

pe

ratu

re [

°C]

OM906, α=1, DOC: JM DF87 (20g/ft3), SCR : Argillon9999 18L, 300cpsi

Baseline 0% NO2

NOx inlet

Baseline 0% NO2

NO2/NOx=DOCout


DOC aged

30

40

50

60

70

NO

x-c

on

v.

[%]

0 % NO2

50 % NO2

20g/ft3

40g/ft3

55g/ft3

DOC aged

30

40

50

60

70

NO

x-c

on

v.

[%]

0 % NO2

50 % NO2

20g/ft3

40g/ft3

55g/ft3

DOC aged

30

40

50

60

70

NO

x-c

on

v.

[%]

0 % NO2

50 % NO2

20g/ft3

40g/ft3

55g/ft3

20g/ft3

40g/ft3

55g/ft3

DOC degreened

30

40

50

60

70

NO

x-c

on

v.

[%]

20g/ft3

40g/ft3

55g/ft30 % NO2

50 % NO2

DOC degreened

30

40

50

60

70

NO

x-c

on

v.

[%]

20g/ft3

40g/ft3

55g/ft30 % NO2

50 % NO2

DOC degreened

30

40

50

60

70

NO

x-c

on

v.

[%]

20g/ft3

40g/ft3

55g/ft3

20g/ft3

40g/ft3

55g/ft30 % NO2

50 % NO2

Exhaust Aftertreatment System Modeling:DOC Optimization (Cold Start FTP)

0 0.5 1 1.5 2 0 0.5 1 1.5 2

relative DOC volume relative DOC volume

• Noble metal loading higher than 40 g/ft3 required

• Significant volume reduction possible if degreened state could be stabilized.


Outline





� Summary


Simulation of Complex Exhaust Gas Aftertreatment Systems: Summary

• Exhaust aftertreatment simulation is needed due to increasing system

complexity and required system performance.

• Exhaust aftertreatment simulation has become a very efficient tool for

system design and operation strategies.

• Usage of global chemistry offers fast calculation times. However, a detailed

understanding of the chemical processes is required.

• Refined (reaction kinetic) models needed to improve prediction quality

and to reduce recalibration effort.

• Ongoing development needed for new types of catalyst, e.g. NH3-Slip

Catalyst.

• Aging and noble metal variations have to be included in future models.

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