Chemistry: analytical and immunological testing – Testing of catalyst
Reexamination Certificate
2000-06-30
2004-12-21
Soderquist, Arlen (Department: 1743)
Chemistry: analytical and immunological testing
Testing of catalyst
C436S147000, C436S149000, C436S151000
Reexamination Certificate
active
06833272
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The invention relates to a method and an apparatus for determining the storage state of an ammonia-storing SCR catalyst.
2. Discussion
The main sources of nitrogen oxide emissions (NOx) in the industrialized states are traffic, fossil-fired power stations and industrial installations. While the power-station and industrial emissions are being increasingly reduced, the proportion accounted for by traffic is coming increasingly to the forefront.
The NOx emissions of petrol-operated spark-ignition engines can be drastically reduced by operating at &lgr;=1 and by post-engine emission control by means of a three-way catalyst. Owing to the principle concerned, this possibility does not exist in the case of a mixture-regulated diesel engine operated with a mixture leaner than stoichiometric. On account of the high oxygen content in the exhaust gas, so far it has not been possible to produce a catalyst which can reduce the NOx emissions without the addition of reducing agents, generally hydrocarbons or ammonia-forming compounds.
To remove the nitrogen from power station emissions, SCR processes (selective catalytic reaction processes)— as described for example in DE 245888 —are used in order to convert nitrogen oxides selectively into water and nitrogen by adding the reducing agent ammonia (NH
3
). Such control has proven suitable given the slow changes over time in the volumetric flow of exhaust gas and NOx concentration occurring in the power station sector.
The complicated processes taking place in the SCR process can be described in a simplified form by equations (1) and (2)
4NO+O
2
+4NH
3
→4N
2
+6H
2
O (1)
NO
2
+NO+2NH
3
→2N
2
+3H
2
O (2)
Such an SCR process can also be used in a modified form for the removal of nitrogen from diesel-engine exhaust gases. For use in a diesel-operated motor vehicle, in particular a commercial vehicle, numerous processes for the reduction of nitrogen oxides in exhaust gases by controlled NH
3
addition are therefore described, for example in; (1) Lepperhoff G., Schommers J.: Verhalten von SCR-Katalysatoren im dieselmotorischen Abgas [Behaviour of SCR catalysts in diesel-engine exhaust gas]. MTZ 49, (1988), 17-21; (2)
Hüthwohl G., Li Q., Lepperhoff G.: Untersuchung der NOx-Reduzierung im Abgas von Dieselmotoren durch SCR-Katalysatoren [Investigation of NOx reduction in the exhaust gas of diesel engines by SCR catalysts]. MTZ 54. (1992), 310-315; and (3) Maurer B., Jacob E., Weisweiler, W.: Modellgasuntersuchungen mit NH3 und Harnstoff als Reduzierungsmittel für die katalytische NOx-Reduktion [Model gas investigations with NH
3
and urea as reducing agents for catalytic NOx reduction]. MTZ 60 (1999), 398-405.
The unknown NH
3
charging state (filling level) of the SCR catalyst in non-steady-state operation proves to be problematical. It is characterized by adsorption and desorption, which occur at different catalyst temperatures. Furthermore, the mass throughput or space velocity of the exhaust-gas flow and the content of NOx or NH
3
in the exhaust gas also affect the charging state. The ageing of the catalyst is also a factor which must not be ignored.
FIG. 1
schematically shows a detail from a cross section of a typical SCR catalyst
10
. Here, the porous catalyst material
12
is permeated by channels
14
through which the exhaust gas flows, also known as ‘cells’. The cell density of such materials may be up to several hundred cells per square inch. With this structure, the porous catalyst material
12
has three tasks: primarily, the constituents of the catalyst permit the desired reaction processes within the available temperature range, furthermore the extruded material provides a mechanically sturdy unit which does not require any additional support components, and finally it permits the adsorption and desorption of NH
3
.
In the case of the supported catalyst
20
shown in
FIG. 2
, the actual catalyst material is applied as a coating
22
to a substrate
26
, for example cordierite. The substrate
26
likewise has channels
24
through which the exhaust gas flows.
A schematic overall view
30
of a catalyst is represented in FIG.
3
.
The exhaust gas flows in the z direction.
As
FIG. 4
reveals, a typical catalyst material, shown here by way of example, consists of the semiconductor metal oxides titanium oxide (TiO
2
), vanadium oxide (V
2
O
5
) and tungsten oxide (WO
3
). These semiconductor metal oxides can change their physical properties, in particular their electrical properties such as conductivity and permittivity, with the chemical composition or by the adsorption of NH
3
surface charges.
The amount of NH
3
supplied to the catalyst is partly converted directly on the surface with NOx and the remainder is adsorbed in the porous catalyst layer. If more ammonia than can be converted by the reaction with NOx is supplied, adsorption of the excess ammonia occurs in a way corresponding to the profile sketched in
FIG. 5
a
.
FIG. 5
a
shows in case A a catalyst saturated with NH
3
at the inlet of the catalytic converter. Assumed by way of example is a maximum adsorption capacity of 4 g of NH
3
/kg of catalyst mass. The NH
3
mass not reacting with NOx can no longer continue to be adsorbed at the inlet of the catalytic converter and, in the example represented, only finds adsorption possible again after about 200 mm of the length of the catalyst. An ‘NH
3
’ front is formed, descending over the length of the catalyst from the saturated state to 0 g/kg. If the excess supply of NH
3
continues, this NH
3
front moves in the direction of the outlet of the catalytic converter. In case A* represented, part of the excess NH
3
is already emitted (NH
3
leakage) even though the catalyst is not yet saturated over the entire length.
The adsorption capacity of the catalyst is dependent on the catalyst temperature. Case B in
FIG. 5
a
shows the amount of adsorption over the length of the catalyst for an increased temperature. With approximately equal NH
3
leakage, the integrally stored amount of NH
3
in case B is significantly reduced, see also
FIG. 5
b.
With a controlled addition of NH
3
, determination of the NH
3
filling level is performed by computation and so far it has not been possible for this to be verified by measurement. To prevent NH
3
breakthrough, the adsorption capacity of the catalyst must not be used up completely on account of the relatively inaccurate computation of the filling level; as a safety measure, additional storage volume must be kept in reserve, taking up additional installation space.
In the event of malfunctions, so far it has not been possible for an increased filling level to be detected. Changes in the NOx emission of the engine—for example due to changed ambient conditions (atmospheric humidity, air temperature), engine ageing, production variations, etc.—, or changes in the catalyst properties (for example ageing, reduction in the adsorption capacity) influence the mass of NH
3
to be adsorbed in the catalyst and are not covered by the filling level calculation.
To ensure a correct metered amount of the reducing agent ammonia or an ammonia-forming compound, such as urea for example, the literature proposes use of one or more exhaust-gas sensors for regulating the amount of the metered agent. Thus, EP 0 554 766 A 1 presents a method which requires one or two NOx sensors. DE 41 17 143 A1 proposes a method which requires one NH
3
sensor and DE 42 17 552 C1 proposes a method in which two NH
3
sensors prove to be necessary. For a further method, proposed in DE 195 36 571 A1, an NH
3
sensor is likewise indispensable.
All the methods mentioned employ control methods which are very complex and scarcely cover all eventualities, since, as stated above, the charging state of the SCR catalyst is dependent on very many operating parameters, which also to a great extent involve the prehistory, i.e. earlier operating states.
If it were possible to detect the charging st
Binder Klaus
Braun Tillmann
Busch Michael-Rainer
Knezevic Aleksandar
Marquardt Klaus-Jürgen
Crowell & Moring LLP
Daimler-Chrysler AG
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