Die Zukunft der heterogenen Katalyse im Automobil

Transcription

Die Zukunft der heterogenen Katalyse im Automobil;
„Turbolente“ Katalysatoren
für Otto- und Dieselanwendungen
The Future of Heterogeneous Catalysis
in Automotive Applications;
„Turbulent“ Catalysts
for Spark- and Compression Ignition Engins
Dipl.-Ing. W. Maus, Emitec GmbH
Dipl.-Ing. R. Brück, Emitec GmbH
26. Internationales Wiener Motorensymposium
28. - 29. April 2005
Dipl.-Ing. Wolfgang Maus
Dipl.-Ing. Rolf Brück
EMITEC Gesellschaft für Emissionstechnologie mbH, Lohmar
Die Zukunft der heterogenen Katalyse im Automobil;
„Turbulente“ Katalysatoren für Otto- und Dieselanwendungen
The Future of Heterogeneous Catalysis in Automotive Applications;
“Turbulent” Catalysts for Spark and Compression Ignition Engines
Kurzfassung
Die Entwicklung der Katalysatortechnologie für den automobilen Bereich begann in
den 60er Jahren. Aus der chemischen Industrie wurden zunächst die als hocheffektiv
bekannten Schüttgutkatalysatoren mit sehr gutem, einer turbulenten Strömung
vergleichbarem, Stofftransport übernommen. Mechanische Schwingungen und
Gaspulsationen führten zu Abrasion. Daher wurde dieser Weg aus Gründen der
Dauerhaltbarkeit verlassen und zunächst metallische Wabenkörper, die in
Raffinerieprozessen eingesetzt wurden, verwendet. Diese Folien-Substrate waren
ebenfalls nicht dauerhaltbar, da die Verbindungsverfahren zur Herstellung
monolithischer Strukturen noch nicht zur Verfügung standen. Keramische,
monolithische Wabenkörper wurden später entwickelt, die ebenfalls mit katalytisch
aktiver Beschichtung versehen waren. Der Nachteil monolithischer Wabenkörper
besteht allerdings in der laminaren Kanalströmung, die den Stofftransport und damit
die
volumenspezifische
katalytische
Wirksamkeit
begrenzt.
Neuartige
Katalysatorträger-Entwicklungen für Otto- und Dieselmotoren nutzen den Effekt der
„Turbulenz“ gezielt aus. Die Konvertierung der limitierten Abgaskomponenten basiert
auf Mechanismen des turbulenten Stofftransports. Im Folgenden werden die
zugrunde liegenden Gesetzmäßigkeiten für den Einsatz im automobilen Katalysator
hergeleitet und mit Testergebnissen untermauert.
Abstract
The development of catalyst technology for automotive applications started in the
early sixties with the adoption of highly efficient pellet catalysts that were used in
chemical engineering processes. This catalyst type produced very high mass transfer
rates, which were comparable to those found in turbulent flow. However, catalyst
beds made of this type of pellet failed because mechanical vibrations and gas
pulsations caused abrasion. In order to increase durability, metallic honeycombs as
those used in oil refining processes were seen as a possible solution. Since the
technology for joining foils into a monolithic structure was not available at the time the
durability of the substrates was also limited. Ceramic honeycomb structures with
straight channels coated with the catalytic material were developed later. Their
smooth channels meant that the chemical reaction was limited by a poor mass
transfer rate to the channel wall. New catalyst developments for spark and
compression ignition engines make specific use of “turbulent” effects. The conversion
of regulated pollutants is based on the mechanisms of turbulent mass transfer. The
principles behind their application in automotive catalysts are described, and
substantiated by test results, in the following chapters.
26th International Vienna Engine Symposium 2005
1.
Introduction
In the sixties and seventies the number of vehicles on the road grew rapidly as a
result of increasing wealth and the desire for greater mobility that accompanied it.
Initially in the United States and later also in Europe the negative effects of this trend
were felt in the form of smog. Public and political pressure to build more
environmentally friendly vehicles was set against an increasing demand for mobility
and new vehicles. Pellet fixed-bed reactors as those used in the chemical industry
were seen as an obvious solution (figure 1). This technology had been used
successfully in the chemical industry for a long time. Compared to channels with
laminar flow profiles this technology achieves high Nusselt or Sherwood numbers
similar to those observed in turbulent flow profiles, which helped to achieve a high
level of overall catalytic efficiency [1, 2]. Initial tests with these catalyst systems,
which were still open-loop systems at the time, produced an effective reduction in
gaseous emissions.
Abbildung 1:
Pellet-Katalysator für den automobilen Einsatz
Figure 1:
Pellet catalyst for automotive applications
In contrast to the static operating conditions of the chemical industry, pellet catalysts
in vehicles were exposed to much higher stresses. Vehicle reactors were subject to
gas pulsation, mechanical vibration and thermo-mechanical stresses under dynamic
operating conditions. Thermal variation in stress causes differential expansion
between the mantle and the ceramic pellets. The vibrations of the vehicle set off
relative motion of the pellets against each other and the mantle leading to catalyst
abrasion. The ceramic pellets simply turned to dust [3]. The catalytic advantage of
this type of catalyst was therefore accompanied by a serious disadvantage with
regard to mechanical, and hence also chemical, durability.
The obvious solution was a metallic honeycomb of the type used in some
applications of the chemical industry, which was made up of smooth and corrugated
steel foils. Initially, these systems also failed because of the stresses inherent in
vehicles and because the technology to join the separate foils into a monolithic
structure was not available at that time. Extruded ceramic honeycombs were
developed as an alternative. The advantage of these catalyst substrates – initially
with 300 cells per square inch (cpsi) and a wall thickness of 0.2mm – was that the
thermo-mechanical stresses were borne by a stable matrix and that the differential
expansion against the mantle was absorbed by flexible “embeddings” [4]. The first of
these consisted of wire mesh; later so-called swelling ceramic fibre mats were used
to compensate for the differential expansion between the metallic mantle and the
ceramic honeycomb. Reports on the various fault hazards of ceramic systems are
being published to this day. The technical reliability of metallic METALIT® substrate
catalysts has been proven since their introduction in 1986.
However, straight smooth channels with laminar flow vectors and profiles, i.e. those
running parallel to the catalyst axis, have a negative catalytic effect. As described
below the pollutants are transported from the centre of the channel to the catalytically
active wall solely by diffusion. The crucial factor, however, was achieving mechanical
durability. Furthermore, the efficiency of the initially open-loop catalysts was more
dependent on fuel mixture generation than on mass transfer.
The continuing growth in vehicle traffic meant that the open-loop catalyst systems
whose efficiency was inherently limited to between 50% and 70% were unable to
prevent a rise in environmental pollution. The United States again led the rest of the
world in adopting gradually tighter emission limits, which today demand over 99%
efficiency. The requirements of exhaust gas after-treatments increased accordingly.
The three-way catalyst was introduced in 1976 [5] as a first step in this direction. The
application of a so-called lambda sensor enabled catalysts to operate in the
stoichiometric range and at the same time ensured an optimum conversion of
hydrocarbons (HC), carbon monoxide (CO) via oxidation and the reduction of
nitrogen oxides (NOx).
The demand for higher conversion rates required a lot of work on both the engines
and the catalysts. New engine generations with reduced raw emissions and
optimised, faster lambda control became the standard as did close-coupled, very
thin-walled catalyst substrates (30m) with a high cell density and high-temperatureresistant coatings, which Emitec and Toyota introduced for the first time in 1997.
With stricter legal requirements with regard to efficiency and durability the technical
complexity and hence cost pressures increased. Catalyst substrates and
configurations (close-coupled, thermal cascade) helped to reduce relative costs
through improved catalytic coatings.
The design of the monoliths with their straight, smooth channels was not changed at
first. Tests that aimed to recreate the positive properties of pellet systems and
introduce “turbulent” catalysts failed because of the inadequate production
engineering available at the time and because of unsuitable coating technology [6, 7].
Catalyst substrates with 900 cpsi were used where even higher volume-specific
efficiency was required. Substrates with as many as 1,200 to 1,600 cpsi were
developed to production stage in order to improve mass transfer [8, 9, 10, 11].
Another advantage of higher cell densities is the increase of the volume-specific
geometric surface, which has a direct effect on catalysis.
Ever since its foundation Emitec had been stressing the importance of the catalytic
surface per unit volume to achieving good conversion rates. The related, lowest
possible, cross-section-related mass that had to be heated was in turn a factor in
catalyst light-off.
Progressively thinner and at the same time more corrosion-resistant metal foils it
have made it possible to increase cell density from 400 cpsi (s.a.) by over a factor of
3 without increasing the proportion of cold-start emissions. A smaller hydraulic
diameter improves heat transfer (improved “light-off”) and increases the conversion
rate by reducing diffusion paths.
The manufacturers of ceramic substrates followed this technological trend after many
years of hesitation but the potential of “turbulent” reactors was never achieved
(figure 2).
Abbildung 2:
Stofftransportkoeffizient Beta als Funktion der Strömungsgeschwindigkeit
bei unterschiedlichen Zelldichten und Strömungsformen (T = 600 °C)
Figure 2:
Mass transfer coefficient beta as a function of flow rate at various cell
densities and laminar/turbulent flow (T = 600 °C)
Turbulent flow can be generated by an increase in the flow rate and hence the
Reynolds numbers in the catalyst (reduction of the flow cross-section). Conventional
catalysts are unable to do this for pressure loss reasons. This is only possible in
engines with an exhaust gas turbocharger where the catalyst can be positioned in
front of the turbine since the turbocharger itself causes a very high loss of pressure
whereas the effect of an upstream catalyst is much smaller by contrast. This type of
pre-turbocharger catalyst acts as a pulsation damper and flow straightener for the
flow entering the turbine and has demonstrated its much higher turbulent catalytic
efficiency.
It took the growing pressure of the car industry for cheaper catalysts with smaller
volumes and an even higher level of efficiency to give new impetus to the
development of “turbulent” catalysts. But it was not just the production of these
catalyst substrates but also appropriate catalytic coatings and coating processes that
had to be put into practice. In addition, pressure loss was to be reduced for higher
engine performance and lower CO2 emissions.
2.
Heterogeneous catalysis
The rate of heterogeneous catalytic conversion processes that take place in the
vehicle catalyst depends especially on the mass transfer of the substances involved
as well as the temperature and the generation of an optimum fuel mixture. The term
mass transfer refers to the transport of the gaseous emissions from the gas phase
core to the catalytically active wall of the catalyst channel. Because of the small size
of the channel (small hydraulic diameter), laminar flow profiles with relatively poor
transport conditions develop only a few millimetres behind the gas inlet so that mass
transfer takes place in the form of diffusion. According to Fick’s laws this means that
the transferred mass depends on the mass transfer coefficient on the one hand and
directly on the concentration gradient between the gas phase core and channel wall
on the other. From this it follows that in a catalyst at operating temperature approx.
90% of the emissions have already been converted after approx. 10% of its length
(figure 3).
Abbildung 3:
Abnahme der Emissionen über Katalysatorlänge (exemplarische Berechnung
auf Basis einer stofftransportlimitierten Umsetzung)
Figure 3:
Emission reduction vs. catalyst length (calculated on the basis of mass transfer
limitation)
Figure 3 shows that a large part of the catalyst and hence the precious metals
convert only a relatively small proportion of the emissions because the transport from
the gas phase core (cell channel in the laminar flow regime) to the wall limits the rate
of the overall process. While it is possible to remedy this problem by increasing cell
densities again and thus reducing channel size and diffusion paths this would lead to
a disproportionate rise in pressure loss. Even a reduction in catalyst length made
possible by this, which has a linear effect on pressure loss, cannot compensate for
this disadvantage (cf. figure 4).
Abbildung 4: Druckverlust als Funktion der Zelldichte und der Katalysatorlänge (Berechnung,
Kat-Ø 105 mm, 400 bzw. 800 cpsi, 50 m, 400 kg/h, T = 100 °C, unbeschichtet)
Figure 4:
Pressure loss as a function of cell density and substrate length (calculation,
catalyst Ø 105mm, 400/800 cpsi, 50m, 400kg/h, T = 100°C, uncoated)
There is another trend that requires new methods in catalyst technology: Modern
engines typically produce increasingly lower raw emissions. As mentioned above,
lower concentrations of pollutants also reduce diffusion-based mass transfer. So
measures to increase mass transfer have to be found.
The following offer a potential solution:
A. Radial flow near the wall in the channels (TS design)
B. A reduction of diffusion paths and of the hydraulic diameter and a repeat of the
entrance flow (LS design)
C. Radially open, perforated structures (PE design)
3.
Structures with radial flow components (TS design)
In 1990 radial flow components were successfully realised using the so-called
transversal structure (TS design) [12, 13].
Tests on a flow model, which used a change of colour caused by a chemical reaction
with the wall to qualitatively or semi-quantitatively visualise the amount of a gaseous
substance (ammonia) reaching the substrate wall, demonstrated the advantage of
the TS structure [14], see figure 5.
Abbildung 5: Genutzte Kanalwandung (Abwicklung über Zellumfang) in einem glatten Kanal (links)
und einem Kanal mit transversal strukturierter Folie (TS, rechts), dargestellt als
Schwärzungsgrad in Abhängigkeit von der Position auf der glatten bzw. gewellten
Lage
Figure 5:
Intensity of contact between gas and channel walls (flat and corrugated layer), shown
by adsorption of ammonia; smooth channel (left) compared to a TS-structured channel
(right)
The increase of mass transfer coefficient beta caused by the TS indentations is
shown qualitatively in figure 6.
Abbildung 6:
Aufbau der TS-Struktur und Vergleich des Stofftransportkoeffizienten Beta eines
glatten Kanals mit einem Kanal mit transversaler Struktur (berechnet, T = 700 °C,
w = 50 m/s, Zelldichte = 200 cpsi)
Figure 6:
Design of TS structure, comparison of mass transfer coefficient beta between a
smooth channel and a TS-structured channel (calculated, T = 700°C, w = 50m/s, cell
density = 200 cpsi)
Improved utilisation of the catalyst wall, as shown in figures 5 and 6, was used to
reduce cell density and thus backpressure while maintaining the same level of
efficiency and also to reduce the dimensions of the catalyst. Since it was possible to
coat this substrate by standard processes it was widely used in mass-produced
catalysts with low cell densities from 200 to 500 cpsi. The products achieved
significant market penetration.
4.
Structures to reduce diffusion paths (LS design)
Papers on developments dealing with unfavourable mass transfer conditions and
measures to increase mass transfer were published as early as the seventies. The
fact that, for instance, the turbulent and hence more effective entrance flow could be
repeated if the honeycomb was divided into suitable, short circular discs with axial
clearances (cf. figure 7a) was described as a very effective solution. Results
published at a later date highlighted the possibility of reducing the catalyst volume by
increasing efficiency through this disc catalyst design [15]. These positive findings did
not find their way into mass production for cost and technical reasons. However, tests
clearly showed the potential of “turbulent” flow control.
Substrate structures, whose conventional smooth channel walls had been replaced
by walls that had regular gaps and were positioned along the flow, were introduced
by Behr as early as 1989 and a short time later by Emitec. The so-called Behr-SQ
and Emitec-LS technologies are described in [16] and [17] (cf. figures 7b and 7c).
The effect of the substrate designs was to repeat the entrance flow and reduce the
diffusion paths (s.a.).
LS- Design
Abbildung 7:
a) Scheiben-Kat-Technologie (links)
b) SQ-Technologie (rechts)
c) LS-Technologie (Mitte)
Figure 7:
a) Disc catalyst technology (left)
b) SQ technology (right)
c) LS technology (middle)
SQ- Design
The first hurdle was to find suitable production methods for the metal substrate since
apart from corrugating, the foil profiles also required the structure to be embossed
and cut. Since, as mentioned above, an adequate coating process was not available
even reproducible research projects were doomed to fail. The early theories can now
be put into practice because new production technologies, process technologies and
materials have become available. New coating processes – adapted to structured
metal substrate technologies – are now being mass-produced.
A common feature of the SQ and the LS designs is that the pollutant particles do not
have to rely solely on diffusion to travel from the channel core flow to the wall and
that a new wall projecting into the channel is positioned directly in the middle of the
flow core. An LS blade is, in fact, pushed into the channel before laminar flow and a
resulting concentration profile are able to develop (figure 8).
Abbildung 8:
Radiale Strömungen und Konzentrationsprofile in einem LS-Kanal
Figure 8:
Radial flow profiles and concentration profiles in an LS channel
This effect is further explained by calculations. The alternating arrangement of a
normal flow channel and an LS blade is effectively comparable to a packed column of
Raschig rings (figure 8) where flow passes through hollow catalyst cylinders as the
bulk material. This effect of a multiple entrance flow is also similar to the multiple
alternating arrangements of short catalyst discs and corresponding clearances as
shown in figure 7a. The LS design therefore represents the first “turbulent” catalyst
system for automotive applications based on systems found in the chemical industry.
Figure 9 shows the effect on the mass transfer along an LS channel compared to a
standard channel.
The designation of the LS-structure requires a new nomenclature. The base structure
(main corrugation), which defines the amount of material used, forms the first part of
the designation. The effective part of the structure (main corrugation cell density plus
LS fins), including the LS fins that are visible in when looking through the catalyst,
forms the second part of the designation. The effectiveness of the LS structure is
dependent on an optimum layout of the LS fins.
Abbildung 9: Stofftransportkoeff. Beta in einem Standard- und einem LS-Kanal (berechnet, T =700
°C, w = 50 m/s, Zelldichte = 200/400 cpsi, Schaufellänge = 7 mm, Abstand = 17 mm)
Figure 9:
Mass transfer coefficient beta in a standard channel and an LS channel (calculated,
T = 700°C, w = 50m/s, cell density = 200/400 cpsi, blade length = 7mm, distance
between blades = 17mm)
Figures 10a, 10b and 10c show the effects of different blade lengths, blade numbers
or blade depths for an LS cell density of 200/400 cpsi
Abbildung 10a: Einfluss Einprägungsanzahl (Schaufellänge 2mm, Einprestiefe 50 %): Effizienzen /
Druckverluste div. LS-Geometrien (Messung, Eff.: T = 600 °C, w = 10 m/s, Propen
7 mM/m3, Lambda = 1, Vkat = 3,2 cm3, PM = 30g/ft, Pd:Rh=5:1; Gegendruck:
m = 10 kg/h, T = 100 °C)
Figure 10a:
Effect of blade number (blade length 2mm, blade depth 50%): efficiency/pressure loss
of various LS designs (measurement, eff.: T = 600°C, w = 10m/s, propene 7mM/m3,
lambda = 1, Vkat = 3.2cm3, PM = 30g/ft, Pd:Rh=5:1; pressure loss: m = 10kg/h,
T = 100°C)
Abbildung 10b: Einfluss Einprägungslänge (Schaufelanzahl 2 + 1, Einpresstiefe 50 %, 200/400 LS):
Effizienzen und Druckverluste div. LS-Geometrien (Messung, Eff.: T = 600 °C,
w = 10 m/s, Propen 7 mM/m3, Lambda = 1, Vkat = 3,2 cm3, PM = 30g/ft, Pd:Rh=5:1;
Gegendruck: m = 10 kg/h, T = 100 °C)
Figure 10b:
Effect of blade length (blade number 2 + 1, blade depth 50%): efficiency and
pressure loss of various LS designs (measurement, eff.: T = 600°C, w = 10m/s,
propene 7mM/m3, lambda = 1, Vkat = 3.2cm3, PM = 30g/ft, Pd:Rh=5:1; pressure
loss: m = 10kg/h, T = 100°C)
Abbildung 10c: Einfluss Einprägungstiefe ET (Schaufelanzahl 2 + 1, Schaufellänge 7 mm):
Effizienzen und Druckverluste div. LS-Geometrien (Messung, Eff.: T = 600 °C, w =
10 m/s, Propen 7 mM/m3, Lambda = 1, Vkat = 3,2 cm3, PM = 30g/ft, Pd:Rh=5:1;
Gegendruck: m = 10 kg/h, T = 100 °C)
Figure 10c:
Effect of blade depth ET (blade number 2 + 1, blade length 7mm): efficiency and
pressure loss of various LS designs (measurement, eff.: T = 600°C, w = 10m/s,
propene 7mM/m3, lambda = 1, Vkat = 3.2cm3, PM = 30g/ft, Pd:Rh=5:1; pressure
loss: m = 10kg/h, T = 100°C)
The blade length of the substrate designs shown above was varied irrespective of the
number of blades (LS structures); the channel length was identical in each case.
Figure 10a shows that an excessive number of blades causes disproportionately high
pressure loss. Therefore the length and the depth of the blades was changed in the
next steps (Figures 10b and 10c). The resulting design, marked with arrows, was
chosen for series production (7mm length at 70% ET). The pressure loss of this
design, which is used with a cell density of 200/400 cpsi LS, is equivalent to that of a
conventional substrate with 400 cpsi. Generally the effects of the blade design can
also be applied to higher cell densities.
Due to the usually poor flow distribution in close-coupled locations with notional
cylinder exhaust entry points, radial equalisation between the individual exhaust entry
points of the catalyst face is desirable. On the one hand this improves the utilisation
factor of the catalyst while on the other the lambda values for each cylinder, which
differ slightly during actual operating conditions, are combined in a stoichiometric
mixture that is ideal for catalytic efficiency.
The unstructured flat layer in the LS structure limits the extent of radial flow
equalisation in the catalyst in individual layers of the LS structure.
5.
Radially open structures (PE design)
It is possible to achieve effective radial flow equalisation inside a METALIT® catalyst
substrate through the appropriate perforation of the foils.
The improvement of mass transfer and flow equalisation to compensate for a
reduced geometric surface area is a design criterion for the size of the cut and the
density of the holes. The catalytic coating is of particular interest in this case since
loss of surface area at a constant washcoat quantity per volume results in a thicker
washcoat, which in turn impairs the pore diffusion properties of the reactants inside
the washcoat and causes higher pressure loss. The coating manufacturers have
successfully worked on a solution and developed new washcoats with a lower
washcoat amount but with the same oxygen storage capacity.
Perforated foils have a dual effect: The first relates to the holes of one layer
“permitting” the flow to change from one channel to another because of pressure loss
differences, which facilitates flow equalisation (figure 11).
The second effect is based on the geometric structure of the winding form with
perforated foils. The perforations are arranged so that they are positioned on
successive cross-sections along the direction of the axis. The winding method
produces cavities in an offset successive arrangement along the axis (caves, see
figure 12).
This generates a “zigzag” flow from the channels into the caves and back into the
channels at the end of the caves. The direction of the flow in the caves depends on
the pressure difference and the pressure loss of adjacent channels. The changeover
from cave to channel generates the same “turbulent” effect as the entrance flow of a
catalyst.
Abbildung 11: Schematisch dargerstellter Strömungsausgleich und Strömungsverteilungsergebnisse
Figure 11:
Diversion of channel flow (schematic) and results of flow distribution measurements
The fact that diffusion equalisation
takes place as a positive effect in
addition to flow equalisation has to
be taken into consideration.
Figure 13 shows the course of the
mass transfer coefficient along the
channel length of a PE structure
compared to a conventional
substrate and an LS substrate. A
significant increase in the mass
transfer
coefficient
can
be
observed in the LS structure.
Abbildung 12:
PE-Struktur mit Kavernenbildung
Figure 12:
PE structure with “caves”
This is due to the constant repetition of the entrance flow and the reduced hydraulic
diameter in the area of the indentation.
The pressure loss of the PE structure is dramatically reduced in the caves because of
a significantly greater hydraulic diameter and the resulting reduction of side friction so
that the repeated entry loss in the formation of the flow from the caves into the cell
channels is overcompensated for.
Figure 14 shows pressure loss as a function of hole size on which this function
depends. In series production, the hole diameter was specified as 8mm and porosity
as ~40%.
Abbildung 13: Stofftransportkoeffizient Beta über Kanallänge eines Standardträgers / LS-Trägers /
PE-Trägers (berechnet, T= 700 °C, w = 50 m/s, Zelldichte = 200/400 cpsi,
Einprägungslänge = 7 mm, Abstand = 17 mm, Lochdurchmesser = 8 mm,
Lochabstand = 3 mm)
Figure 13:
Mass transfer coefficient beta along channel length of a standard substrate / LS
substrate / PE substrate (calculated, T = 700°C, w = 50m/s, cell density = 200/400
cpsi, blade length = 7mm, distance between blades = 17mm, hole diameter = 8mm,
distance between holes = 3mm)
Abbildung 14: Druckverluste von Substraten mit diversen PE-Lochgrößen (Messung, T = 100 °C,
m = 300 kg/h, Träger Ø 118 x 74,5 mm/ 400 cpsi / 50 m, unbesch., Porosität = 37 %)
Figure 14:
Pressure loss of substrates with various PE hole sizes (measurement, T = 100°C, m =
300kg/h, substrate Ø 118 x 74.5mm/ 400 cpsi / 50m, uncoated, porosity = 37%)
As mentioned above, apart from flow distribution, the equalisation of the cylinderdependent gas concentration plays is particularly important. Depending on the
position of the lambda sensors, different measurement results for the pre-catalyst
and post-catalyst lambda sensor may be obtained, especially in close-coupled
catalyst systems that consist of only one substrate. In extreme cases the pre-catalyst
sensor (e.g. integrated in the manifold) detects an ideal mixture in all cylinders while
the post-catalyst sensor is primarily exposed to the exhaust gas of only one cylinder.
Clogging of the fuel injectors can result in incorrect fuel mixture generation or even
the activation of the OBD lamp. Figure 15 shows the effects of the PE structure on
the equalisation of the lambda signals in front of and behind the catalyst with a
cylinder-selective leaning of the mixture by 20% even at relatively short substrate
lengths of 50.8mm.
Abbildung 15:
Vergleichmäßigung der Lambdasignale vor und hinter Kat durch die PE-Struktur bei
schrittweiser Abmagerung eines Zylinders (Messung am Motorprüfstand,
Katalysator Ø 110 x 50,8 mm/600 cpsi/40 m), delta Lambda = Lambda nach Kat
– Lambda vor Kat
Figure 15:
Equalisation of lambda signals in front of and behind the catalyst by using a PEstructured substrate with gradual leaning of the mixture of one cylinder
(measurement on roller test bench, catalyst Ø 110 x 50.8mm/600 cpsi/40m), delta
lambda = lambda behind catalyst – lambda in front of catalyst
These research results fully validated earlier considerations [18] to insert a perforated
metal substrate structure at the front of the substrate to improve light-off behaviour
(by reducing the mass that had to be heated). The fluidic and mass transfer-related
advantages could not be determined theoretically, let alone practically, at that time.
The results presented here clearly demonstrate that only an axially completely
structured matrix is capable of releasing the full fluidic, thermodynamic and chemical
potential [19].
Combination of PE and LS structures
In this design an LS structure is added to the corrugated layer of the metallite, the flat
layer is perforated according to the specifications of the PE design (figure 16).
Abbildung 16:
Kombination der LS- und PE-Strukturen in einem Träger als LS/PE-Struktur
Figure 16:
Combination of LS and PE structures in one substrate to form an LS/PE structure
The potential applications of “turbulent” catalysts are exemplified in the following
chapter.
6.
Application examples
The structured foils described above can be used to generate completely new
application-dependent advantages:
I.
II.
III.
IV.
Reduction of pressure loss
Equalisation function of the concentrations
Flow distribution
Improved mass transfer
The PE structure is already being used in sports application with high requirements
regarding pressure loss. In the course of the development of catalysts for EU4, ULEV
and SULEV spark ignition engines, substrates with high cell densities (s.a.) have
become the standard because of their efficiency, size and low system costs. Highperformance sports cars were accordingly equipped with substrates with up to 600
cpsi and corresponding pressure loss disadvantages. The PE design now makes it
possible, for example, to replace a standard 600-cpsi substrate with a 600-cpsi PE
substrate of the same volume and the same level of efficiency but with a pressure
loss of a 400-cpsi substrate. Last year, the Audi RS6 was the first vehicle with this
substrate to go into series production [20]. Other vehicles, such as the Maserati V8,
followed. Ever since catalysts have first been used in car racing, metal substrates
have been the preferred choice because of their pressure loss advantage.
In 2004, Volvo tested the PE structure on the racing circuit in an extremely successful
S60 vehicle. In 2005, most racing cars are being converted to PE. This METALIT®
type is therefore rebuilding the reputation of metal substrates for sports cars.
6.1. Applications for spark injection engines
The exhaust gas after-treatment of spark injection engines has been incorrectly
described as having reached the end of its development since existing technologies
had already made it possible to build cars that cleaned the environment. Because
modern standard catalysts are already able to achieve the required level of efficiency
the aim of “turbulent” substrates is to reduce catalyst and system costs. This must
include above all the functions relating to lambda control and catalyst diagnostics.
A modern standard catalyst system usually consists of two substrates that are
arranged behind each other inside a mantle. In order to improve cold starts the first
substrate has a smaller diameter than the second; this is known as a so-called
cascade system [21]. The gap between the two substrates is generally used to
accommodate the second lambda sensor. The gap also causes flow mixing and flow
equalisation.
In these very effective systems, which are in common use today, both the LS design
and the PE design produce significant functional and cost advantages. Both
structural types have a lower thermal mass, which makes it possible to dispense with
the first, smaller substrate in the cascade.
By adding another innovation to this structure, that is, the so-called lambda sensor
catalyst [22], the gap between the substrate becomes superfluous, because this type
of substrate allows one or more lambda sensors to be positioned directly inside the
substrate. The radial “permeability” in the substrate now also allows diffusion
equalisation of the pollutant concentration, which has a positive effect on the control
accuracy of fuel mixture generation.
The production method of metallic catalyst substrates makes it possible to punch a
hole for the lambda sensor in almost any axial position during production through a
simple die cutting process. The holes punched into the metal foils combine to form a
lambda sensor hole when the substrate is assembled. In combination with the PE
structure a single substrate system with the functions of a double substrate system is
created.
Another advantage of the LS/PE structure is the reduced thermal mass, which
improves cold start behaviour and renders the cascading of the substrates obsolete.
Figure 17 contains a comparison between a traditional cascade and a single brick
system with an integrated lambda sensor and a PE structure.
This shows that system costs can be significantly reduced by combining the LS/PE
structure with an integrated lambda sensor.
Abbildung 17:
Vergleich von Kaskadensystem und Einzelträgersystem mit integrierter Sonde
und PE-Design
Figure 17:
Comparison of a catalyst cascade design and a single brick catalyst with integrated
lambda sensor and PE design
Figure 18 shows the gaseous emission of a catalyst system using LS technology
compared to a standard substrate in a spark ignition engine. A 600-cpsi system can
therefore evidently be replaced by a 300/600-cpsi LS substrate.
Abbildung 18:
Emissionen eines Katalysators Ø105x135mm (70 g/ft3; 1:7:1) im FTP-75 Test
gemessen an einem1,8l-Turbo-Motor
Figure 18:
Emissions of a substrate Ø105x135mm (70g/ft3; 1:7:1) in an FTP-75 test cycle using
a turbocharged 1.8-litre engine
6.2. Applications for compression ignition engines
6.2.1. Oxidation catalysts; application of LS structure:
A benchmark test was carried out on a medium-sized vehicle with a 2.2-litre
turbodiesel engine. A 200/400-cpsi LS substrate and a 400/800-cpsi substrate with a
smaller volume were compared to a 400-cpsi standard substrate. Each catalyst was
subjected to an NEFZ cycle.
Abbildung 19: CO-Emissionsergebnisse bei einem Trägervergleich an einem 2,2 l Dieselmotor
(Messung am Rollenprüfstand, Träger Ø 118mm, Länge 150 mm (V=1,64 l) bei 400
cpsi und 200/400 LS bzw. Länge 110 mm (V=1,20l) bei 400/800LS, alle 50 m, PM =
90 g/ft3 Pt:Pd:Rh = 1:0:0) ECE, EUDC und Gesamttest
Figure 19:
CO emission results for a 2.2-litre diesel engine (roller test bench, substrate Ø
118mm, length 150mm (V=1.64l) at 400 cpsi and 200/400 LS or 110mm (V=1.20l) at
3
400/800 LS respectively, all 50m, PM = 90g/ft Pt:Pd:Rh = 1:0:0) ECE, EUDC and
overall test result
In this special application, the advantage of the 200/400 LS structure in the ECE part
of the EU test cycle (cf. figure 19) is due to lower thermal capacity and hence faster
heating behaviour (figure 20), while the catalyst volume is significant at low
temperatures (diesel) because of the limited reaction rate compared to the 400/800
LS substrate. The mass transfer advantage of LS structures, which is able to
overcompensate for the cold start disadvantage, becomes clear at operating
temperature (EUDC part) (figure 19).
Abbildung 20:
Strukturtemperaturen der einzelnen Katalysatoren 50 mm hinter der
Gaseintrittsseite
Figure 20:
Structure temperatures of individual catalysts 50mm downstream from the gas
inlet side
The 400/800 LS catalyst has the same level of efficiency as the 200/400 LS substrate
despite its 27% smaller volume (1.2 instead of 1.64l). Overall the efficiency of the LS
structures in the EUDC part of the cycle is a good 30% above that of the normal
structure.
The overall test result reflects the individual parts of the test. In this case the cold
start advantage of the 200/400 LS substrate had the greatest impact. The HC test
results of the individual designs correspond to the CO results.
The LS structure either opens up the possibility of saving material by using a 200/400
LS instead of a 400-cpsi standard substrate with identical volume, which reduces
costs and weight, and/or the possibility of reducing the catalyst volume at the same
cell density, which saves on precious metals.
In case of poor flow distribution caused by problematic locations a combination of LS
and PE can produce an optimum result (s.a.).
In September 2004 the LS/PE structure had its world premiere as an oxidation
catalyst used in front of a particle filter.
6.2.2. Reduction catalysts; application of LS/PE structure:
Other areas of application for the LS/PE structure include substrates for so-called
SCR catalysts, where, apart from mass transfer, it is above all the distribution of urea
or ammonia inside the substrate that is an important factor to ensure optimum
metering over the entire substrate without breakthrough. Figure 21 shows the NOx
reduction rates of different substrate types (full extrudate, standard metal substrate
with the same cell density, LS/PE substrate) at different temperatures. The space
velocity was 100,000h-1 for each catalyst. Ammonia feed was controlled by ammonia
slip set at 5ppm. The rising maximum conversion, which is explained by an optimum
ammonia distribution and improved mass transfer, is clearly evident.
Abbildung 21: NOx-Reduktion mit SCR-Katalysatoren als Funktion der Temperatur auf Basis
verschiedener Strukturen (Raumgeschwindigkeit 100 000 1/h)
Figure 21:
7.
NOx reduction with SCR catalysts as a function of temperature based on various
substrate structures (GHSV 100,000 1/h)
Summary
The new “turbulent” catalyst substrates represent the logical further development of
the advantages of “turbulent” catalyst systems that were previously utilised in static
applications. Ever since catalyst technology was first used in vehicles over three
decades ago it had not been possible to meet the requirements of mobile
applications with these substrate systems. As a result honeycombs with laminar flow
regimes of the type commonly used today became established at the time. Because
of their mass transfer, flow distribution and pressure loss advantages they can be
used in a wide range of applications for spark and compression ignition engines.
The main development objective, which was to produce more compact and cheaper
catalyst systems for spark ignition engines, has therefore been achieved.
For diesel catalysts still under development the “turbulent” METALIT® types offer
functionally specialised solutions for oxidation, hydrolysis, mixer and reduction
catalysts.
New control strategies, which ensure improved catalytic efficiency and more stable
long-term behaviour, can be developed in combination with an integrated lambda
sensor or integrated gas sensors, for example, in NOx adsorbers.
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