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Development of a 2D Model of a SCR Catalyst on a DPF

  • Venkata R. ChundruEmail author
  • Boopathi S. Mahadevan
  • John H. Johnson
  • Gordon G. Parker
  • Mahdi Shahbakhti
Article
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Abstract

A computationally efficient, 2D SCR catalyst on a DPF (SCR-F) model was developed to simulate the pressure drop, filtration efficiency, outlet NO, NO2, and NH3 concentrations, PM, temperature, and NH3 storage distributions. The model extends previous work (Mahadevan et al. :J. Emiss. Control. Sci. Technol.1, 83–202, 2015; Mahadevan et al. :J. Emiss. Control. Sci. Technol.1, 255–283, 2015; Mahadevan et al.:J. Emiss. Control. Sci. Technol. 3, 171–201, 2017) by adding a 2D reaction-diffusion scheme-based chemical species solver with SCR reactions inside the substrate wall (Park et al. :Ind. Eng. Chem. Res. 51, 5582–15592, 2012). The experimental data (Kadam, 2016) used for the model calibration was collected on a Johnson Matthey SCRF® operated with a 2013 Cummins 6.7L ISB engine with tests spanning a wide range of operating conditions including PM loading and oxidation with and without urea injection. The major features in the model consist of (a) inhibition of SCR reactions by wall PM, (b) SCR reaction–based energy release, and (c) diffusion in the channels, forward diffusion between PM cake and substrate wall layers. The model can simulate transient conditions including the prediction of a 60–70% decrease in the NO2-assisted PM oxidation rate during urea injection, 2D temperature distribution within 5 °C of the experimental data, and the resulting PM and NH3 storage distribution by simulating the 10–15 °C temperature rise during urea injection. The maldistribution of NO, NO2, and NH3 concentrations caused by the temperature distribution and the inhibition of SCR reactions by PM in the substrate wall was simulated and the resulting outlet emissions agreed with the experimental data. The model can also be used for 2D CPF analysis by turning off the SCR reactions.

Keywords

Selective catalytic reduction Diesel particulate filter model Selective reduction catalyst on a DPF model Particulate matter oxidation Pressure drop Permeability modeling 

Abbreviation

ANR

Ammonia to NOx ratio

AR

Active regeneration

B10

Diesel blend (ULSD) with 10% biodiesel

B20

Diesel blend (ULSD) with 20% biodiesel

CFD

Computational fluid dynamics

CPF

Catalyzed particulate filter

CO

Carbon monoxide

CO2

Carbon dioxide

CSF

Catalyzed soot filters

CuO

Cupric oxide

CuZe

Copper zeolite

DOC

Diesel oxidation catalyst

DPF

Diesel particulate filter

EPA

Environmental Protection Agency

FTP

Federal test procedure

HC

Hydrocarbons

H2O

Water

MPF

Multi-zone particulate filter

MTU

Michigan Technological University

N2

Nitrogen

N2O

Nitrous oxide

NH3

Ammonia

NH4NO3

Ammonium nitrate

NO2

Nitrogen dioxide

NO

Nitric oxide

O2

Oxygen

OBD

On-board diagnostics

PO

Passive oxidation

PM

Particulate matter

RMS

Root mean square

RU

Ramp up

SCR

Selective catalytic reduction

SCR-F

SCR catalyst on a DPF

SCRF®

Johnson Matthey’s SCR catalyst on a DPF

ULSD

Ultra-low-sulfur diesel fuel

US06

Supplemental federal test procedure

P

Pressure drop

1D

One-dimensional

2D

Two-dimensional

3D

Three-dimensional

Nomenclature

Greek Variables

\( {\alpha}_{NO_2} \)

NO2 oxidation partial factor [-]

\( {\alpha}_{O_2} \)

O2 oxidation partial factor [-]

∆L

Effective zone length [m]

∆r

Effective zone radius [m]

∆t

Solver time step [s]

∆x

Discretization length in axial direction [m]

ε0 ,s

Clean wall porosity [-]

εsi,j

Porosity of the substrate wall [-]

ηcake

PM cake layer filtration efficiency [-]

ηtotal

Total filtration efficiency [-]

ηwall,n

Wall filtration efficiency at each slab [-]

μ

Dynamic viscosity of exhaust gas [Ns m−2]

Ω1

Storage capacity of site 1 [-]

Ω2

Storage capacity of site 2 [-]

ρf

Filter substrate density [kg m−3]

ρs

PM cake density [kg m−3]

ρi,j

Exhaust gas density at each zone [kg m−3]

σ

Stefan-Boltzmann constant [W m−2 K−4]

θ1

Coverage fraction of site 1 [-]

θ2

Coverage fraction of site 2 [-]

ξ

Stoichiometric coefficient of species l in reaction m [-]

Symbols

a

Width of the clean inlet and outlet channel [m]

A

Heat transfer area normal to the gas flow [m2]

a

Effective width of the clean inlet channel loaded with PM [m]

Aamb

Surface area of outer surface [m2]

cf

Specific heat of filter material [J kg−1 K−1]

cp

Constant pressure specific heat of exhaust gas [J kg−1 K−1]

cs

Specific heat of PM cake [J kg−1 K−1]

DI

Effective diffusivity of species [-]

\( {d}_{C,{s}_{i,j}} \)

Instantaneous wall collector diameter at each zone [m]

dC0 ,s

Initial wall collector diameter [m]

Dkn,l

Knudsen diffusivity of species [-]

Dmol,l

Molecular diffusivity of species [-]

dpore,wall

Diameter of pore in the substrate wall [m]

D

Overall diameter of the SCR-F [m]

d

Side length of square channels [m]

F

Friction factor in the inlet and outlet channel of the particulate filter [-]

hamb

Ambient convective heat transfer coefficient [W m-2 K-1]

hg

Convective heat transfer coefficient [W m-2K-1]

J1

Radiosity of channel inlet surface [W m-2]

J2

Radiosity of filter wall surface [W m-2]

J3

Radiosity of channel outlet surface [W m-2]

kg

Thermal conductivity of channel gas [W m-1K-1]

L

Axial length [m]

Lt

Total length of CPF or SCR-F [m]

M

Number of radial zones [-]

[mwi,j]n

Mass of PM in each zone for slab n [kg]

\( \dot{m} \)

Instantaneous exhaust mass flow rate [kg s−1]

\( {\dot{m}}_{i,j} \)

Mass flow rate at each zone [kg s−1]

\( {\dot{m}}_{total} \)

Total mass flow rate into CPF/SCR-F[kg s−1]

nmax

Maximum number of wall slabs [-]

N

Number of axial zones [-]

n

Wall slab index [-]

Pin

CPF or SCR-F inlet gas pressure [kPa]

\( {\dot{Q}}_{cond, axial} \)

Axial energy conduction [W]

\( {\dot{Q}}_{cond, radial} \)

Radial energy conduction [W]

\( {\dot{Q}}_{conv} \)

Convection between channels gases and filter wall [W]

\( {\dot{Q}}_{rad} \)

Radiation energy between channel surfaces [W]

\( {\dot{Q}}_{reac, HC} \)

Energy released during oxidation of HC in the inlet gas [W]

\( {\dot{Q}}_{reac, PM} \)

Energy released during oxidation of PM [W]

\( {\dot{Q}}_{reac, SCR} \)

Energy released during SCR reactions [W]

R

Universal gas constant [J mol-1 K-1]

Sp

Specific surface area of PM (5.5*107) [m-1]

Smax

Number of ways of calculating the inlet pressure at each radial section [-]

Tamb

Ambient temperature [K]

Tf

Temperature of filter and PM cake [K]

\( {T}_{inlet_{i,j}} \)

Temperature of the gas at the inlet channel at each zone [K]

tins+ can

Thickness of insulation and can [-]

T fi,j

Temperature of combined filter and PM cake at each zone [K]

\( {t}_{s_{i,j}} \)

PM cake thickness at each zone [m]

t

Time [s]

Vt

Total volume of CPF or SCR-F [m3]

v1

Inlet channel velocity at each zone [m s-1]

v2

Outlet channel velocity at each zone [m s-1]

Vi,j

Total volume of a zone [m3]

Vinlet

Volume of inlet channels [m3]

Voutlet

Volume of outlet channels [m3]

\( {V}_{s_{i,j}} \)

PM cake volume in each zone [m3]

VFi

Volume fraction at each axial section of the CPF or SCR-F [-]

ws

Substrate wall thickness [m]

\( {w}_{p_{i,j}} \)

PM cake layer thickness at each zone [m]

W

Exhaust gas molecular weight [kg kmol-1]

Yl

Concentration of species l [mg L-1]

Subscripts and Superscripts

i

Radial Direction

j

Axial Direction

l

Species index

m

Reactions index

p

Wall slab index

s

Stream line index

Notes

Acknowledgements

The authors would like to thank the MTU Diesel Engine Aftertreatment Consortium which supported this research work. The partners in the consortium are the following: Cummins, John Deere, Diamler-Detroit Diesel, Tenneco, Corning, Johnson Matthey, and Isuzu. The authors would like to thank the reviewers whose comments and feedback have helped us to improve the paper and the SCR-F model.

Authors have no competing interests to declare.

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Copyright information

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Authors and Affiliations

  1. 1.Michigan Technological UniversityHoughtonUSA

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