SE541544C2 - Process for monitoring a methane oxidation catalyst and device for post-treatment of exhaust gases - Google Patents

Abstract

In a process for monitoring a methane oxidation catalyst (420) in the exhaust system of an internal combustion engine (400), an activity diagnosis of the methane oxidation catalyst (420) is performed with the formation of the NO 1 methane oxidation catalyst (420), and / or the NO / NOx ratio in or after the methane oxidation catalyst, a conclusion is drawn about the methane conversion capacity of the methane oxidation catalyst (420). Based on a reduced formation of NOdras, if any, a conclusion about a reduced methane conversion capacity of the methane oxidation catalyst (420). For the determination of the formation of NO, an SCR catalyst device (460) is preferably used, which is located in a partial flow (450) of the exhaust system.

Description

The present invention relates to a process for monitoring a methane oxidation catalyst in an exhaust gas aftertreatment system of an internal combustion engine, an exhaust aftertreatment apparatus, as well as a computer program, a machine readable memory medium and a readable memory medium. electronic control means, which are designed for carrying out the method.

Prior art Combustion engines are known which can be operated both with a methane-containing gas, for example natural gas or methane, but also with a mixture of gas and another fuel, for example diesel fuel. In all approaches that are at least partly dependent on the combustion of methane-containing gas, the problem of high raw motor methane emissions occurs. Mainly due to climate protection, methane emissions must be reduced within the framework of an after-treatment of exhaust gases. Methane oxidation catalysts (MOCs) are known which, on the basis of palladium-rich preparations, oxidize the methane present in the exhaust gases. For this purpose, preparations which have a weight ratio between palladium (Pd) platinum (Pt) of up to, for example, 7: 1 or higher can be used. Other methane oxidation catalysts are based on so-called palladium-only formulations, such as Pd / alumina. In general, however, in such preparations a certain methane conversion is first observed above 400 ° C. However, a complete oxidation often requires temperatures much higher than 500 ° C.

The behavior of a methane oxidation catalyst and in general the behavior of a catalyst with respect to temperature can be described by the so-called release temperature (light-off temperature), at which a predetermined proportion of a conversion of the harmful substances present in the exhaust gases takes place. In the case of aged catalysts which show a reduced conversion value, the required release temperature is generally elevated. With regard to methane oxidation catalysts, the trigger temperature generally does not show a sharp course even when the catalyst is in a new state. Often the methane conversion extends rather linearly in the range from 400 ° C to 550 ° C.

In many catalysts, on-board-diagnosis (OBD) (vehicle-borne diagnostic system) is monitored for functionality based on temperature values. However, with respect to methane oxidation catalysts, such monitoring is generally troublesome because due to the blurry temperature ratio of the methane oxidation catalyst, a change in the exothermic contributions is not very characteristic of the methane conversion in the methane oxidation catalyst. Another aggravating factor is that palladium-rich methane oxidation catalysts are extremely sensitive to sulfur and even exhibit oscillating behavior (M. Lyubovski, L. Pfefferle, Catalysis Today 47 (1999): 29 -44; R. Schwiedernoch, Dissertation University of Heidelberg, 2005).

Description of the Invention Advantages of the Invention The invention proposes an improved process for monitoring a methane oxidation catalyst (MOC) so that reliable statements about a possible impaired methane conversion capacity of the MOC can be made. In this case, the method is provided for monitoring a MOC in the exhaust gas flow of an internal combustion engine (gas or diesel / gas engine), which is at least partially arranged for the combustion of methane-containing fuel with predominantly lean exhaust gases. In the process according to the invention, an indirect monitoring takes place in a way by performing an activity diagnosis of MOC on the basis of a measurable formation of NO2 (nitrogen dioxide) in MOC. Here, on the basis of the formation of NO2 and in particular from the NO2 content and / or the NO2 / NOx ratio in or after the methane oxidation catalyst, conclusions can be drawn about the methane conversion capacity of MOC. In the case of a comparison with reference values, one can in the event of a reduced reduction of NO2 draw a conclusion about a reduced methane conversion capacity of MOC. A direct measurement of the methane conversion capacity with standard methods can in practice be difficult to carry out. Compared with this, the method according to the invention has the special advantage that through the indirect monitoring reliable statements can be made about the functional capacity of the MOC.

The process according to the invention is based on the fact that Pd-rich catalysts and thus also methane oxidation catalysts show a certain formation of NO2. Here, there is a relationship between the NO2 formation capacity of a MOC and its methane conversion capacity. Almost no NO (nitrogen monoxide) is oxidized to NO2 as long as the light-off temperature for methane oxidation is not significantly exceeded. It is only above the light-off temperature, and thus depending on the methane conversion capacity, that a certain conversion from NO to NO2 takes place at MOC, or it is only above the light-off temperature that the equilibrium concentration of NO2 according to the following formula is achieved: NO 1/2 O2 <=> NO2 The ability for NO conversion or for the formation of NO2 is utilized according to the invention in order to draw conclusions about the methane conversion ability based on the measurable NO2 formation. Preferably, the process is then carried out within a temperature range of MOC within which there is a clear correlation between the methane conversion and the NO2 content and / or the NO2 / NOx ratio after a methane oxidation catalyst. Depending on the catalyst, this particularly suitable temperature range may be, for example, between about 450 ° C to 500 ° C, preferably in a range between 470 ° C and 490 ° C. Studies on a catalyst that serves as an example have shown that within this temperature range, the correlation between NO2 formation and methane conversion is strongest.

According to the invention, the NO2 formation capacity of a MOC is used for the monitoring of MOC. For this purpose, the NO2 / NOx ratio or NO2 content can in principle be measured directly after the MOC. However, it is particularly preferred to use an SCR catalyst device located downstream of the methane oxidation catalyst for the measurement of the NO2 formation capacity of the MOC. This is particularly advantageous compared to a direct measurement of the NO2 / NOx ratio or NO2 content after MOC, as sensors suitable for this purpose are generally not provided in ordinary motor vehicles.

For the use of an SCR catalyst device for the detection of the conversion from NO to NO2, NO2-sensitive SCR catalysts are particularly suitable, the NOx conversion ratio of which depends on the NO2 content and / or on the NO2 / NOx ratio of the gas (feed gas) which flows into the SCR catalyst device. NO2-sensitive SCR catalyst devices of this kind can to some extent be used as sensors for the process according to the invention. Some SCR catalysts are known, for example, which, especially at temperatures below about 250 ° C, show for the NOx conversion a pronounced dependence on the NO2 / NOx ratio in the feed gas. Examples of this are ferrous beta-zeolite or VWT catalysts (VWT-Vanadium-Tungsten-Titanium Oxide).

In a particularly preferred embodiment of the process according to the invention, a NO 2 -sensitive SCR catalyst device is used in a partial flow of the exhaust gas flow, this SCR catalyst device then being preferably an auxiliary SCR catalyst device. This auxiliary SCR catalyst device preferably has a smaller volume than a main SCR catalyst device in the main flow of the exhaust system. Since NO2-sensitive SCR catalyst devices suitable for this purpose show this dependence, especially at temperatures below 250 ° C, the auxiliary SCR catalyst device is preferably operated at a lower temperature than, for example, the main SCR catalyst device in the main stream, for example with a temperature in the range between 170 ° C and 250 ° C. For the tempering, regardless of the mounting position, for example an electric heating can be arranged, or in case of an excessively high temperature level, an active or passive cooling possibility can be arranged in the partial flow path.

Upstream and downstream of the auxiliary SCR catalyst device, a measurement or calculation of NOx takes place for determining the NO2 formation in MOC, for this purpose NOx sensors are preferably used, which are arranged upstream and / or downstream of the auxiliary SCR catalyst device.

The invention further comprises a device for post-treatment of exhaust gases for the exhaust system of an internal combustion engine comprising at least one MOC in a main flow line of the exhaust system. According to the invention there is an auxiliary SCR catalyst device in a partial flow line of the exhaust system, this being a NO2-sensitive SCR catalyst device whose NOx conversion ratio is dependent on the NO2 content and / or on the NO2 / NOx ratio of the gas which flows into the auxiliary SCR catalyst device. In the manner described above, with the aid of this NO2-sensitive SCR catalyst device, the NO2 content of the exhaust gases which have left the MOC can be determined. Based on this NO2 content, one can conclude about the formation of NO2i in MOC, whereby from the NO2 formation in MOC one can indirectly draw conclusions about the methane conversion capacity of MOC. For the measurement of the NOx conversion at the auxiliary SCR catalyst device, one or more NOx sensors may be arranged upstream of / or upstream of the auxiliary SCR catalyst device. In the case of the auxiliary SCR catalyst device, it is preferably an SCR catalyst device which has a smaller volume than a main SCR catalyst device in a main flow line of the exhaust system. The auxiliary SCR catalyst device is preferably operated at a lower temperature level than the main SCR catalyst device. For the setting of the lower temperature level, especially in a range between 170 ° C and 250 ° C, for example heating and / or coolant are arranged in the range of the auxiliary SCR catalyst device.

In an embodiment of the exhaust gas after-treatment device according to the invention, the partial flow line in which the auxiliary SCR catalyst device is arranged extends as a bypass to the main flow line of the exhaust system within the area of the main SCR catalyst device. In a further particularly advantageous embodiment, the partial flow line with the auxiliary SCR catalyst device relates to an exhaust line for exhaust gases, in particular an exhaust line for exhaust gases, in particular a return line for low-pressure exhaust gases, which branches downstream of the methane oxidation catalyst and opens before or in the internal combustion engine. The invention further relates to the use of a device of this kind for after-treatment of exhaust gases in a process for monitoring a methane oxidation catalyst. In this regard, reference is made to the above description.

Furthermore, the invention comprises a computer program which is designed for carrying out the steps in the method according to the invention. Finally, the invention comprises a machine-readable memory medium on which a computer program of this kind is stored, as well as an electronic control device which is designed for carrying out the steps in the method according to the invention. The implementation of the process according to the invention as a computer program or as a machine-readable memory medium or electronic control device has the special advantage that in this way the advantages of the process according to the invention can also be used on existing motor vehicles to monitor the methane conversion capacity of the methane oxidation catalyst.

Further features and advantages of the invention will become apparent from the following description of exemplary embodiments taken in conjunction with the figures. In this case, the individual and respective characteristics can be realized individually or in combination with each other.

In the figures: Figure 1 shows the relationship between the methane conversion and the NO2 formation capacity of a methane oxidation catalyst as a function of temperature; Figure 2 shows the NOx conversion of a NO2-sensitive SCR catalyst as a function of the NO2 / NOx ratio in the exhaust gases; Figure 3 shows a schematic representation of a possible arrangement of components in an exhaust aftertreatment system; Figure 4 shows a schematic representation of a further possible arrangement of components in an exhaust aftertreatment system; and Figure 5 shows a schematic representation of a further possible arrangement of components in an exhaust after-treatment system.

Description of exemplary embodiments Figure 1 illustrates the relationship between the methane conversion and the NO2 formation capacity of a MOC in lean exhaust gases containing NO and methane. Here, the methane conversion 10, 20, 30 for three different catalyst states and three different MOCs, respectively, are shown as a function of the temperature between about 250 ° C and 550 ° C. Processes 10, 20, 30 represent, for example, different strongly aged and / or catalysts which are differently strongly inhibited by oxygen, or catalysts rift to stand at different speeds in space. Processes 11, 21, 31 show the NO2 / NOx ratio in the exhaust gases leaving the respective MOC. It follows that MOCs of this kind (Pd-rich catalysts) to some extent convert NO to NO2. However, this NO2 formation first takes place above the light-off temperature of the respective catalyst, ie above the temperature at which the methane is converted to the necessary extent. As long as the light-off temperature of the methane oxidation is not significantly exceeded, almost no NO is oxidized to NO2. The NO2 formation is thereby dependent on the methane conversion capacity. This correlation is most pronounced in the temperature range between about 450 ° C and 500 ° C, especially between 470 ° C and 490 ° C (reference numeral 40). This temperature range is particularly suitable for carrying out the process according to the invention. However, this ratio does not occur with catalysts which have a significantly higher platinum content than a MOC (Pt / Pd catalysts), such as diesel oxidation catalysts, which means that in general the process according to the invention is particularly suitable for Pd-rich catalysts which have no platinum. or less platinum than palladium.

Figure 2 schematically shows the NOx conversion ratio of a NO2-sensitive SCR catalyst as a function of the NO2 / NOx ratio upstream of the SCR catalyst for temperatures below 250 ° C. Typically, the NO2 / NOx ratio upstream of the SCR catalyst according to a MOC according to Figure 1 is below 0.5. Thereafter, in practice, the NOx conversion of a NO2-sensitive SCR catalyst with increasing NO2 content in the exhaust gases increases approximately linearly, which means that based on the NOx conversion of the SCR catalyst, conclusions can be drawn about the NO2 formation in the upstream MOC.

For the purposes of the present invention, a NO 2 -sensitive SCR catalyst of the kind provided in an auxiliary partial flow to the main exhaust system is preferably used. Such an auxiliary SCR catalyst device preferably has a smaller volume than a main SCR catalyst device in the main flow of the exhaust system. In this case, the auxiliary SCR catalyst is expediently operated at a temperature level where the dependence of the NOx conversion on the NO2 content of the feed gas occurs in a particularly pronounced form, especially at temperatures below 250 ° C.

Since the NO2 formation at MOC is also dependent on the methane conversion capacity of MOC and in addition both quantities are known as a function of catalyst load (velocity in space) and temperature, which can be deposited or modeled, for example as a characteristic diagram, the measured NO2-dependent NOx conversion at the auxiliary SCR catalyst device under per se known boundary conditions (eg temperature, velocity in space, NOx content in the partial flow, etc.) according to the invention, conclusions are drawn about the methane conversion capacity of MOC.

Figure 3 schematically illustrates an exhaust system of an internal combustion engine 300 with a turbocharger 310. A methane oxidation catalyst (MOC) 320 is provided in the exhaust system. Downstream of MOC 320, there is a main SCR catalyst device (SCR) 340 in the main flow line 330, for example: a standard SCR catalyst or a particulate filter with SCR coating. A bypass line 350 with an auxiliary SCR catalyst device (mini-SCR) 360 is provided as a bypass to the main SCR catalyst device 340. Upstream of the main SCR catalyst device 340 and simultaneously upstream of the branch of the by-stream 350 there is a metering point 370 for the necessary reducing agent solution for the SCR catalyst devices 340, 360. Between the MOC 320 and the main SCR catalyst device 340 there is a first NOx sensor 380, which for example can also be designed as a combined? - / ?? x sensor. An additional NOx sensor 390 is located upstream of the main SCR catalyst device 340 and at the same time downstream of the inlet mouth of the partial flow 350 in the main flow 330. Additional sensors known per se and required in the system such as for temperature,? Value etc. are not shown. in detail. With sufficient sensitivity and precision of the two NOx sensors 380 and 390, one can directly calculate the NOx conversion through the partial flow 350 and the Mini-SCR path at known total exhaust volume flow and known partial flow volume flow through Mini-SCR 360, based on the signals from the two NOx sensors 380, 390. under conditions in which preferably a complete NOx conversion is expected at the main SCR catalyst device 340. In this case, a residual content of NOx in the main exhaust system then originates exclusively from the partial flow path 350. Thereby the NOx conversion in the auxiliary SCR catalyst device can be calculated exactly - the mass flows respectively.

Figure 4 shows a comparable exhaust after-treatment system, the various components of the exhaust after-treatment system being denoted by the same reference numerals as in Figure 3. In contrast to Figure 3, there is another NOx sensor 391 in the partial flow in the layout according to Figure 4. 350 downstream of the auxiliary SCR catalyst device 360. This embodiment is particularly suitable for cases where the sensitivity and accuracy of the NOx sensor 390 arranged downstream of the main SCR catalyst device 340 is not high enough.

Figure 5 illustrates a further particularly preferred embodiment of a device for after-treatment of exhaust gases according to the invention. In the exhaust system of the internal combustion engine 400 comprising a turbocharger 410, a MOC 420 is provided. In the main flow 430 of the exhaust system there is a main SCR catalyst device 440, a dosing device 470 for the necessary reducing agent being connected before the same. A NOx sensor 480 and 490, respectively, are located upstream and downstream of the main SCR catalyst device 440, respectively. the catalyst device 440. The partial flow 450 is a low pressure return line for exhaust gases comprising an exhaust valve 453 for the internal combustion engine 400. The device with the auxiliary SCR catalyst device 460 in the low pressure return line for exhaust gases is particularly advantageous because the temperature level can be set very precisely in that the return cooler 451 for exhaust gases and possibly 452 (optional) as arranged in the return line 450 for exhaust gases can set the optimum temperature level between approx. 170 ° C and 250 ° C. In addition, the exhaust volume flow or exhaust mass flow in the exhaust gas recirculation line 450 is very accurately known for other reasons, allowing the NOx conversion in the auxiliary SCR catalyst device 460 to be calculated very accurately using the NOx sensor 491 downstream of the auxiliary SCR catalyst device 460. The auxiliary SCR catalyst device 460 and the exhaust gas recooler or coolers 451, 452 can be assembled or integrated into one component.

For carrying out the monitoring method according to the invention and for a MOC diagnosis, the following steps can be performed: First, the prevailing exhaust temperature at the MOC for the diagnosis is checked and an operating point suitable for the engine is set (eg T = 460 ° C, etc.). This suitable operating point can be derived from a reference ratio (expected value). Furthermore, the prevailing temperature is checked at the auxiliary SCR catalyst device and suitable conditions for the diagnosis are set (eg = 180 ° C by the corresponding cooling effect; setting of the partial flow mass flow, etc.). Then the NOx conversion is determined by the auxiliary SCR catalyst device and possibly the corresponding NO2 / NOx ratio is calculated and compared with a reference value (expected value), for example 60% NOx conversion at a corresponding NO2 / NOx ratio of 0.25 . When the NOx conversion is sufficiently high in comparison with a predetermined threshold value, the methane conversion effect of the methane oxidation catalyst is still sufficient. If an insufficient methane conversion effect is determined, various reactivating measures known per se may be taken for MOC.

In addition or as an alternative, it is possible to quantify the degree of deactivation of MOC by repeating the procedure at a different temperature at MOC. For this purpose, the exhaust gas temperature at MOC can be raised to another, for example higher temperature level by setting a different engine operating mode (eg T = 500 ° C). The temperature at the auxiliary SCR catalyst device is then checked again and suitable conditions for the diagnosis (eg T = 180 ° C) are set. The NOx conversion by the auxiliary SCR catalyst device is determined and, if necessary, the corresponding NO2 / NOx ratio is calculated and compared with a new expected value (threshold value). If the NOx conversion is not high enough even at the elevated MOC temperature, ie is below a threshold value, the methane conversion effect is also no longer sufficient and reactivating measures can be initiated for MOC, if applicable.

This embodiment of the process according to the invention is particularly meaningful if a sufficiently good methane oxidizing MOC has been chosen as the reference state. This means that NO2 formation can also be significantly reduced even with a slightly reduced methane conversion (here, for example, 90% at 470 ° C instead of 98% at 500 ° C as a reference condition). It may also happen that the diagnosis is carried out at, for example, 470 ° C, whereby, however, MOC in real operation and in reference state operation, respectively, is operated at 500 ° C. In this case, at 470 ° C of MOC due to a very low NO2 content, MOC would be incorrectly classified as too bad for methane oxidation. Therefore, it is advantageous to perform the NO2 formation diagnosis again at one or more higher temperatures at which a better activity of the MOC can be expected and to correlate the methane conversion to at least one further NO2 threshold. If even at the higher temperature the NO2 formation is too low, the MOC can with particular high certainty be diagnosed as insufficient or as "deactivated". By such a temperature increase in the diagnosis according to the invention, it is also possible to draw up (additional point determination) the light-off curve for the methane conversion and thereby accurately or more accurately quantify the degree of methane oxidation deactivation, since the NO2 ratio is detected over a larger temperature range.

Claims (10)

Patent claims Process for monitoring a methane oxidation catalyst (320; 420) in the exhaust system of an internal combustion engine (300; 400), characterized in that an activity diagnosis of the methane oxidation catalyst (320) is carried out on the basis of a formation of NO 2 in the methane oxidation catalyst (320; 420); , whereby from the formation of NO2 and in particular from the NO2 content and / or the NO2 / NOx ratio in or after the methane oxidation catalyst a conclusion is drawn about the methane conversion capacity of the methane oxidation catalyst (320; 420), whereby from a reduced conclusion of a degraded methane conversion capacity of the methane oxidation catalyst (320; 420). Process according to Claim 1, characterized in that the process is carried out within a temperature range of the methane oxidation catalyst (320; 420), within which there is a correlation between the methane conversion and the NO 2 content and / or the NO 2 / NOx ratio after a methane oxidation catalyst. Process according to Claim 1 or 2, characterized in that at least one SCR catalyst device (360; 460) is arranged downstream of the methane oxidation catalyst (320; 420), the formation of NO 2 being determined by means of the SCR catalyst device (360; 460). Process according to Claim 3, characterized in that the SCR catalyst device (360; 460) is a NO2-sensitive SCR catalyst device whose NOx conversion ratio depends on the NO2 content and / or on the NO2 / NOx ratio of the gas flowing. into the SCR catalyst device (360; 460). Process according to Claim 3 or Claim 4, characterized in that the SCR catalyst device (360; 460) is an auxiliary SCR catalyst device which is arranged in a partial flow (350; 450) of the exhaust system. Process according to Claim 5, characterized in that the auxiliary catalyst device (360; 460) is operated with a smaller volume than a main SCR catalyst device (340; 440) in the main stream (330; 430) of the exhaust system and with a lower temperature than the main SCR catalyst device (340; 440), in particular with a temperature in the range between 170 ° C and 250 ° C. Process according to one of Claims 3 to 6, characterized in that a measurement of NOx takes place upstream and / or downstream of the SCR catalyst device (360; 460), in particular with the aid of NOx sensors (380, 390, 391; 480). 491), for determining the formation of NO2. Computer program, which is designed to perform the steps of a method according to any one of claims 1 - 7. A machine-readable memory medium on which a computer program according to claim 8 has been stored. Electronic control device, which is designed to perform the steps in a method according to any one of claims 1 - 7.