Analysis of field-aged and artificially aged SCR catalysts for model development
Heavy-Duty-, On- und Off-Highway-Motoren 2016
11. Internationale MTZ-Fachtagung
Referent(in): Robert Bank, Uwe Etzien, Bert Buchholz, Georg Töpfer, Adrian Troeger, Horst Harndorf
Today the drive train consists of a combustion engine as well as of an exhaust gas aftertreatment
(EAT) system. Only efficient EAT concepts allow highest conversion rates
and fulfillment of emission limits. To guarantee compliance over the engines life time
cycle knowledge of EAT ageing is important and has to be considered while concept
development. Due to exposition to high temperatures, e.g. by high loads or DPF regeneration,
or to deposition and chemical binding of single elements from fuel, lube oil or
engine wear the EAT components will age over operation time. Those effects are partially
In order to guarantee a certain emission limit over life time a consolidated knowledge
base about ageing and poisoning processes is necessary. Using this knowledge developing
model based approaches to describe EAT components and the change of their characteristics
over lifetime becomes possible. Furthermore such an approach allows development
of diagnosis functions to determine the actual state of the EAT system.
The results in this article are derived from the BMWi funded project BlueExSys (AdBlue®
Exhaust System). From different applications field-aged SCR catalysts as well as
unaged and artificially aged samples were examined and compared using a synthetic gas
test bench and chemical analyses. Using the collected data a model based approach to
describe the ageing phenomena was developed.
Comparing real field-aged SCR catalyst with unaged and artificially aged samples the
first point is that an artificial ageing, regarding hydro-thermal effects only, does not reflect
reality. In order to generate an equivalent to a field-aged catalyst sample the deposition
and chemical binding of additional species has to be considered. Those two effects,
deposition and chemical binding of elements from fuel, lube oil and engine wear,
show an axial distribution from inlet to outlet of the catalyst monolith.
The single effects of ageing on the characteristics of the catalyst can be considered in
models. The loss of storage capacity for the reducing agent ammonia can be adapted by
using a factor. This factor uses the ratio of stored mass of ammonia of the actual sample
to the mass stored on an unaged sample. Furthermore the loss of activity can be modelled
using another factor for the kinetic coefficient within the Arrhenius approach of
the single reactions. Here good results were achieved using a single factor for all SCR
reactions considered in the model.
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