On the use of foam monoliths in the NH3-SCR process
for the aftertreatment of Diesel exhausts
Isabella Nova, Enrico Tronconi, Antonio Grossale
Dipartimento di Energia, Laboratorio di Catalisi e Processi Catalitici, Politecnico di Milano,
Piazza Leonardo da Vinci 32, 20133 Milano, Italy
Daniel Chatterjee, Michel Weibel
Daimler AG, Abteilung GR/VPE, D-70546 Stuttgart, Germany
Topic: 5. Structured catalysts and reactors for innovative environmental, automotive and energy
applications
increasing pore density and with decreasing
foam porosity. However, an optimal foam
design is required that minimizes the
washcoat thickness and pressure losses.
In the case of transient operation, as shown
in Fig.1, the use of foams grants better
performances both in terms of NO
abatement and ammonia slip due to a faster
heat up of the catalyst, that is associated
with improved gas-solid heat transfer rates.
References
Concentration (ppm) or Temperature (°C)
NH3/urea SCR is considered one of the most promising technologies for the abatement of NOx
emissions from Diesel vehicles [1]. Commercial catalysts include Fe- and Cu-promoted zeolite
based systems used in the form of washcoated cordierite monoliths. However, intra- and inter-phase
diffusional limitations are present in the NO-NO2/NH3 main SCR reactions, which can limit the
deNOx conversion in the high T-range.
In this work the performances in the SCR reactions of Cu-zeolite based foam monolith catalysts
(with different pore density, porosity and washcoat thickness) are simulated by means of a 1D+1D
dynamic model of SCR converters: results are then compared to those obtained over the same Cuzeolite based catalyst in the form of washcoated honeycomb monolith.
A transient, two-phase, single channel, 1D+1D generalized model of SCR structured reactors was
used, which takes into account inter and intra-phase diffusional limitations and thermal effects [2],
requiring the input of physical, geometrical and morphological parameters of the catalyst. Intrinsic
kinetic parameters of the SCR reactions were estimated in a parallel work by regression of transient
data collected over the same Cu-zeolite catalyst in the form of powders. Gas-solid mass and heat
transfer coefficients in honeycombs and foams were estimated from [3] and [4], respectively. In the
case of other honeycomb SCR catalysts, the model was validated against full-scale NOx conversion
data from engine test bench runs [2].
The SCR reactivity over foams and
500
honeycomb monoliths with identical
400 CPSI honeycomb
30 PPI foam (OFA=0.83)
NO
out
washcoat loads (120 g/L) was first
400
investigated at steady state. It was found that
T gas out
for the NO-NH3 reacting system higher
300
T gas in
NOx conversions can be achieved on foams
at T>250°C due to enhanced gas/solid mass
200
transfer rates. NOx conversions increased with
100
NH3 slip
0
0
10
20
30
40
50
60
70
80
Time, s
Fig. 1. Temperature, NO and NH3 outlet concentration
profiles during a fast startup transient (T=100  300 °C in
10 s). Feed: NO=NH3=500 ppm, O2= H2O = 10% v/v, GHSV
= 175000 h-1
[1] Johnson T., Platinum Metals Rev. 52, 23-37 (2008)
[2] Chatterjee et al., SAE papers 2005-01-965, 2006-01-0468, 2007-01-1136
[3] Tronconi & Forzatti, AIChE J. 38, 201-210 (1992)
[4] Groppi, Giani, Tronconi, IEC Res. 46, 3955 – 3958 (2007)
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