Krishna Kamasamudram Cary Henry, Aleksey Yezerets

Cummins Corporate Research and Technology

N2O Emissions From 2010 SCR Systems

Nitrous oxide (N2O) is produced in limited quantities by combustion processes and by all NOx reduction catalysts

N2O selectivity from TWC declined with years, due to improved catalyst formulations and operation strategies

In three-way catalysts (TWC), N2O forms mostly under cold-start conditions

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0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

1990 2000 2005 2006 2007 2008 2009

N2O

, % o

f tot

al G

HG

em

issi

ons

N2O fraction in total GHG emissions from the transportation sector

Based on Table 3.8 in US EPA Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2009

Durbin et al. CRC E-60 Final Report. 2003

ULEV LD Truck

Background

In SCR system, N2O forms in different parts of the cycle, not just the cold start Various catalysts can

contribute to N2O formation and mitigation

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0

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0 200 400 600 800 1000 1200

T, °C

N2O

, ppm

Time, sec

3

ISX-2010

N2O formation on

N2O mitigation on

Cu- & Fe- zeolite SCR 4 ASC 5

SCR 2 ASC 3 DPF DOC 1

N2O Emissions From SCR System

N2O formation on

DPF DOC 1

4

Oxidation catalysts

5 NOx = 200 ppm, C3H6 = 450 ppm, SV = 60kh-1

Under certain conditions, NOx can be reduced over oxidation catalysts primarily to N2O and N2 (lean-NOx catalysis or HC-SCR)

N2O Formation on Oxidation Catalysts

N2O Formation on Oxidation Catalysts N2O yield is affected by exhaust conditions, including

temperature, concentration and nature of hydrocarbon species

C1 [ppm] 1700 1150 650 350

C1 [ppm] 1350

550 280

40

C12H26 C3H6

NOx = 200 ppm, SV = 60kh-1

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N2O Formation on Oxidation Catalysts N2O selectivity is also affected by catalyst formulation details

Different development DOC formulations for SCR systems

NOx = 200 ppm, C3H6 = 450 ppm, SV = 60kh-1 7

N2O formation on

SCR 2

Vanadia-, Fe-zeolite and Cu-zeolite catalysts can all produce N2O Even within a given class of

catalysts (e.g. Cu-zeolites), N2O selectivity depends on catalyst formulation

Cu-zeolite-2 Cu-zeolite-3

Cu-zeolite-1

Cu-zeolite-3

SCR

NH3 oxidation

Dual mode N2O formation mechanism: At higher T – mostly due to

parasitic NH3 oxidation

At lower T – primarily through the NH4NO3 mechanism

Small amounts of N2O can be produced by all classes of SCR catalysts

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N2O Formation on SCR

NOx = 200 ppm, NH3 = 200 ppm, SV = 20kh-1

Selectivity to N2O is strongly affected by the NO2/NOx ratio, especially above 0.5 Upstream DOC/DPF influence N2O production

over SCR catalyst

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N2O formation on

ASC 3

Conventional oxidation catalysts have poor selectivity in the NH3 oxidation process

NH3 = 100 ppm, SV = 100 kh-1

Precious metal-based oxidation catalysts are highly active in NH3 oxidation, but produce high yields of NOx, N2O

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Use of dedicated ASC reduces N2O formation in the NH3 oxidation process

NH3 = 100 ppm, SV = 100 kh-1

ASC can be formulated to maximize selectivity to N2, while maintaining high NH3 conversion

N2O NOx

NH3

13

N2O formation depends on reaction conditions and ASC formulation

Decrease in PGM content of ASC decreases selectivity to N2O

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1.0x g/ft3

0.5x g/ft3

0.1x g/ft3

Presence of NOx along with NH3 leads to an increase in selectivity to N2O

Impact of feed gas composition

Impact of PGM content

N2O mitigation on

Cu- & Fe- zeolite SCR 4 ASC 5

N2O conversion on SCR and ASC is very limited under practically relevant conditions

Each color corresponds to a particular set of feed gas composition

Symbols – steady-state points

Dotted curves – continuous temperature-programmed experiments

Fe-zeolite SCR

Very little N2O reduction activity below ~400 C ─ Fe-zeolites are known to be among the most active catalysts

N2O does not store on the catalyst ─Similar steady-state and transient profiles indicate this

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Temperature [ C]

N2O

form

atio

n [a

rb.]

DOC

ASC SCR

N2O formation and reduction/decomposition temperature windows have limited overlap

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rem

oval

]

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Focus: Minimize N2O formation

Conclusions N2O is a common byproduct of fossil fuel combustion, with

or without NOx aftertreatment ─ No direct health impact, hence unregulated until the recent advent

of GHG regulations

Various elements of SCR system can contribute to N2O emissions, depending on the conditions

Minimization of N2O involves catalyst formulations, system architecture, operation strategy and controls

With proper understanding of the underlying chemical processes, new EPA standards for N2O emissions can be met by SCR systems ─Test data indicate that Cummins 2010 SCR systems are

below the adopted N2O threshold

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Acknowledgements

Randy Jines Neal Currier Shirish Shimpi

For assistance with data collection For valuable discussions For advice and examples of engine data