Patent Application
20070009401
The invention relates to a device for removing soot particles from an exhaust gas stream of a motor vehicle operated with diesel, comprising a catalyst element for the at least partial oxidation of nitrogen monoxide (NO) to nitrogen dioxide (NO2) and a particulate filter arranged downstream of the catalyst element and means to guide the exhaust gas stream through the catalyst and the particulate filter.
Such a device has become known, e.g., from EP 0 341 832 A1, and is used in the exhaust train of diesel-operated automobiles in order to remove and to combust soot particles present in the exhaust gas as completely as possible. With this device, nitrogen monoxide (NO) present in the exhaust gas is oxidized on a catalyst element provided at least partially to nitrogen dioxide (NO2) and/or other nitrogen oxides, such as N2O5, all combined in a simplified manner below under the term nitrogen dioxide. The exhaust gas thus enriched with nitrogen dioxide subsequently arrives at a particulate filter arranged downstream of the catalyst element, at which particulate filter the nitrogen dioxide reacts with soot particles that are retained by the particulate filter or at the particulate filter. Reactions can thereby run according to the reaction equations given below:
The catalyst element of a known device preferably comprises a ceramic monolith that is coated with a noble metal, such as platinum, in order to achieve catalytically supported a high NO2 formation in the exhaust gas flowing through and to optimize a reaction conversion. A downstream particulate filter comprises a wire mesh filter with a coating of lanthanium, cesium and vanadium pentoxide (La/Cs/V2O5), which can reduce a combustion temperature of soot particles so that they can be oxidized or combusted on the particulate filter below the so-called particulate ignition temperature (approx. 550° C.) at temperatures of 250 to 400° C.
Although it is possible with a known device to remove soot particles ideally virtually completely from a diesel exhaust gas even at low exhaust gas temperatures, this is also associated with drawbacks.
Thus it has turned out that, due to high NO2 contents, an increased formation of sulfates from the sulfur contained in the fuel can also occur, which, alone or together with ash particles likewise present in the exhaust gas due to motor oil, can settle on the catalyst and/or particulate filter and there form larger agglomerates, since particles of this type cannot be combusted. This causes clogging in the exhaust gas treatment system, as a consequence of which a back pressure buildup occurs in the exhaust train of the motor vehicle, which leads to a drop in the engine performance.
Another drawback is that with a cold start of an engine a known device requires a long warm-up time in order to reach operating temperature and its full effect.
The invention takes up at this point, the objective of the invention being to disclose a device of the type mentioned at the outset, which can be operated essentially free of clogging even with high sulfate and ash contents in the exhaust gas and which quickly reaches an operating temperature with a cold start.
This object is achieved with a generic device in that the catalyst element is a coated open-porous metal foam body and the particulate filter is an open-porous ceramic foam body.
One advantage achieved with the invention is that on the basis of an open-porous metal foam body provided and a ceramic foam body of the same type, the entire device has a three-dimensionally multiply branched pore structure. Thus a plurality of possible flow paths are available for an exhaust gas flowing through, and potential clogs due to unoxidizable particles, in particular ash and/or sulfate particles, are effectively and continuously counteracted.
Another advantage of the invention is that exhaust gas heat can be effectively utilized. The metal foam body provided heats up very quickly during operation, which is to be attributed on the one hand to a good thermal conductivity of metals per se and on the other hand to an open-porous foam structure essentially formed by metal links freely accessible on all sides. The consequence of this is that, after a cold start of the engine, the entire surface of the metal foam body and the coating thereof within a short time already takes on a temperature of the exhaust gas or an operating temperature is reached, and an adequate generation of NO2 according to equation (1) can occur on the coating in a catalytically supported manner
Although, compared to the upstream metal foam body, the ceramic foam body heats up more slowly, even with a cold start it can immediately perform its temperature-independent function as a particulate filter and stores heat well once absorbed due to a high heat capacity. The latter has advantages if an exhaust gas temperature drops for a short time during the operation of a motor vehicle, e.g., when the engine is operated for a certain period in the lower rpm range. Soot particles deposited on the ceramic foam body are then kept at or brought to temperature through the ceramic foam body and can thus also be combusted, although an exhaust gas temperature would not be sufficient therefor.
Another advantage of a device according to the invention is that exhaust gas flowing in in a laminar manner is agitated in the metal foam body. This formation of a turbulent flow in the metal foam body causes on the one hand an improved contact of the exhaust gas with the catalytically active coating of the metal foam body, which leads to a higher catalytic efficiency. On the other hand, the exhaust gas flows in a turbulent manner to the downstream particulate filter, and it is thus acted on with nitrogen dioxide over its cross-sectional surface in a uniform manner.
Any desired coating of the metal foam body can be given per se, as long as a nitrogen dioxide formation is catalyzed. However, a particularly effective conversion of nitrogen monoxide to nitrogen dioxide is achieved if the metal foam body is coated with a noble metal or several noble metals selected from the group comprising ruthenium, rhodium, palladium, osmium, iridium and platinum.
In principle, the ceramic foam body can be made from any desired ceramic material. As far as the highest possible mechanical loadability and thermal capacity are concerned, it is preferred for the ceramic foam body to essentially comprise an aluminum oxide, cordierite or silicon carbide.
The ceramic foam body can additionally be coated with cerium orthovanadate, Ce(III)VO4. Cerium orthovanadate is a compound stable up to temperatures of over 2200° C., which with direct contact with soot particles can easily reduce the combustion temperature thereof to below approx. 350° C. Moreover, cerium orthovanadate is an oxygen-storing compound, so that if there is an excess of oxygen in the exhaust gas, this is temporarily stored and subsequently can be released again when there is a lack of oxidizing agents for combusting soot particles.
Alternatively, the ceramic foam body can be coated with a noble metal or several noble metals selected from the group comprising ruthenium, rhodium, palladium, osmium, iridium and platinum, in order to keep an NO2 concentration high in the ceramic foam body.
In a particularly preferred variant, the ceramic foam body is coated with cerium orthovanadate, Ce(III)VO4, and at least one noble metal selected from the group comprising ruthenium, rhodium, palladium, osmium, iridium and platinum. A coating of this type can be produced, for example, in that first a cerium orthovanadate coating is applied to the ceramic foam body through a wash coat or sol-gel method and this is subsequently calcined. The cerium orthovanadate coating thus applied is subsequently impregnated with a noble metal solution and optionally calcined again. Overall a cerium orthovanadate coating is thus formed in which catalytically active noble metal is present in a finely distributed manner. This means that not only is a combustion of soot particles on the ceramic foam body catalytically supported by cerium orthovanadate, but also a generation of NO2 can occur directly on or in the coating of the ceramic foam body and sufficient oxidizing agent is available for the complete combustion of soot particles.
For analogous reasons it can also be provided for the coating of the metal foam body to contain cerium orthovanadate, Ce(III)VO4.
The metal foam body expediently has a pore count of 3 to 50 ppi (pores per inch). If the metal foam body is embodied with a pore count of 3 to approx. 20 ppi, it is essentially permeable for soot particles and the metal foam body acts exclusively as a catalyst. If the metal foam body is embodied with a pore count of 20 to 50 ppi, soot particles can in part also be deposited thereon and combusted, so that a downstream particulate filter is relieved. This represents a particular advantage because such high temperature peaks in the particulate filter due to intensive combustion of soot particles can be avoided.
The ceramic foam body, which primarily serves to filter soot particles, has a pore count of 30 to 80 ppi in accordance with its primary function as a filter for soot particles. If a porosity of the metal foam body and the ceramic foam body is between 70 and 98% this has a favorable effect with respect to an avoidance of clogs and a rapid heating of the foam body.
In principle, a volume of the metal foam body and the ceramic foam body can be variably selected. It is favorable if the metal foam body, which should have no filter effect or a lower filter effect than the ceramic foam body, is constructed shorter and its volume is 5 to 45% of the volume of the ceramic foam body.
In order to avoid a deposit of sulfates and/or ash between the metal foam body and the ceramic foam body, it can be provided for the metal foam body to be directly connected to the ceramic foam body.
Further advantages and effects of the invention result from the context of the specification and the exemplary embodiments.
The invention is shown in further detail below on the basis of exemplary embodiments and one figure.
It shows:
Apart from the connection areas necessary for a connection to the housing 5, the metal foam body 1, like the ceramic foam body 2, is adapted in its dimensions essentially to the free inner cross section of the housing 5 embodied, e.g., cylindrically in order to counteract a pressure build-up with high cleaning force as well as possible.
The housing 5, as shown in
A metal foam body according to
Devices of the type described above were tested together with comparison devices with respect to their properties during operation; detailed data on the devices tested are provided in Table 1.
Exhaust gas of a diesel engine was fed to all the devices listed in Table 1 for the same test duration (50 hours) and under the same conditions. The diesel engine was thereby loaded in all the tests according to a specific standardized driving cycle, and devices 1 through 6 were tested during and after operation with respect to their operating performance, whereby the following results were obtained:
During operation, a soot trapping rate or removal rate for soot particles of at least 70% by weight and a conversion of soot constituents of the exhaust gas in the range of at least 95% were detected for all the devices, so that corresponding minimum requirements for a soot particle removal can be met by all devices.
In contrast to the devices 4 through 6, an improved operating performance was given with a cold start with the devices 1 through 3 according to the invention. Thus directly after a cold start a lower soot particle discharge and lower NO concentrations on the outlet side were detected with the devices 1 through 3, which already indicates an effective conversion according to equation 1 and 2 only shortly after the engine startup. In this connection it also seems to be decisive that catalysts of the devices 1 through 3, as temperature measurements at the same have shown, quickly heat up to exhaust gas temperature, whereas with catalysts of the devices 4 through 6 this heating was delayed in comparison.
After the test duration of 50 hours, the devices were tested for unburnt residue, in particular sulfate and ash residue, with the aid of light-microscopical and analytical techniques. It was thereby shown that perceptible proportions of ash and sulfate residue were present in the devices 4 through 6, whereas the devices 1 through 3 were virtually free of such residue. This shows that devices 1 through 3 according to the invention are suitable for the continuous removal and combustion of soot particles from exhaust gas in lengthy operation without there being any danger of clogging by ash and/or sulfate particles.
In the following table 1, structure of devices 1 through 6 and the test results obtained with these devices have been qualitatively combined.
*DOC ... commercial diesel oxidation catalyst,
**cpsi ... cells per square inch
Scale A through E:
A . . . excellent, B . . . good, C . . . satisfactory, D . . . still suitable, E . . . not acceptable.
Naturally, within the scope of the invention it is also possible to connect several devices according to the invention in parallel or in series, in order to optimize an exhaust gas cleaning. It is also possible to utilize the electric conductivity of the metal foam body and to heat it with low energy expenditure.
| Number | Date | Country | Kind |
|---|---|---|---|
| A 771/2005 | May 2005 | AT | national |
