(130a) Modeling and Control of the Heavy Duty Automotive SCR Catalyst

Modeling and Control of the
Heavy Duty Automotive SCR Catalyst

Andreas Åberg*, Anders
Widd**, Jens Abildskov*, Jakob Kjøbsted Huusom*^

* CAPEC-PROCESS,
Department of Chemical and Biochemical
Engineering, Technical University of Denmark, Søltofts Plads, Building 229,
DK-2800 Kgs. Lyngby, Denmark

** Haldor
Topsøe A/S, Nymøllevej 55, 2800 Kgs. Lyngby, Denmark

^
Corresponding author: jkh@kt.dtu.dk

Diesel engine exhaust gases
contain amongst other things nitrous gases such as NO and NO₂ (together
NOx). Reducing the amount of these gases is of great importance due to new
legislations, because of their effect on urban air quality [1, 2]. A promising
and widely used technology for this is based on Selective Catalytic Reduction
(SCR) of the gases, with ammonia as a reducing agent [3, 4]. Challenges with
this technology include dosing the right amount of urea for sufficient NOx
conversion, while keeping the ammonia slip below the legislative limit. This
requires efficient dosing algorithms. To this end, model-based control is a
promising strategy. A model based on first principles that can predict the
complicated dynamics of the SCR catalyst will be useful both for system
understanding, and as a benchmark for new control algorithms, to reduce the
required experimental work.

A first principles single channel
model of high complexity has been developed and coupled with a kinetic model. A
second model of slightly lower complexity has also been developed and coupled
with the same kinetic model. The kinetic parameters have been calibrated using steady-state
data from reactor tests with bench-scale equipment under isothermal conditions.
The model has been validated with transient data from a full-scale monolith and
a real engine following the European Transient Cycle (ETC) [5], which shows
satisfactory results. Results from the validation can be seen in Fig. 1.

To standardize evaluation of control
algorithms for the automotive exhaust gas cleaning system an objective function
based on current legislations for the automotive system has been developed. A
simplified form of the objective function can be used for on-line optimization
in for example a model predictive controller. Three different controllers, a
feedforward, a feedback and a combined feedforward/feedback have been
implemented and tested with the derived model. The controllers have been
evaluated using the objective function.

Figure
1.
NOx outlet variations during full scale validation. Full line represents data
and dotted line represents model predictions.

References:

[1]
A. Fritz, V. Pitchon, The current state of
research on automotive lean NOx catalysts, Appl. Catal. B 13 (1997).

[2]
R. M. Heck, R. J. Farrauto, Automotive exhaust
catalysis, Appl. Catal. A 221 (2001).

[3]
M. Koebel, M. Elsener, M. Kleeman, Urea-SCR: a promising techniquie to reduce
NOx emissions from automotive diesel engines, Catal. Today 59 (2000).

[4]
P. Gabrielsson, Urea-SCR in automotive applications, Top. Catal. 28 1-4 (2004).

[5]
DieselNet hompage. [Online]. Available:
http://www.dieselnet.com/standards/cycles/etc.php