DE10156191A1 - Diesel exhaust pre-filter has electrically heated surface bringing exhaust gases to particle self-ignition temperature - Google Patents

Abstract

In a process to regenerate an automotive diesel engine exhaust filter, a soot particle filter is located before a ceramic honeycomb exhaust gas filter. The soot particle filter has especially a heating element which is activated during the regeneration phase and heats the particles to their ignition point causing exothermic release of heat from the particles to the exhaust gases. The heated exhaust gases then reach or exceed the soot particle ignition point. An Independent claim is also included for an exhaust assembly with a soot particle collector which separates less than 50% of the soot particles. The soot particle collector and filter medium have a catalytic coating or themselves have catalytic properties. A number of soot particle filters are arranged in series. Each filter has a series of heating elements which are activated sequentially and maintain the downstream exhaust gases at the ignition temperature. The soot collected in the pre-filter is of higher density than that collected in the main ceramic filter.

Description

Over the past decades, a large number of Aftertreatment systems to reduce the particle emission to their application potential for Vehicle diesel engines examined. For passenger car diesel engines they have Filter processes with discontinuous regeneration receive the greatest attention experience.

The filtration takes place in a porous storage medium. Taking into account Degree of separation, storage capacity, exhaust gas back pressure and durability the ceramic honeycomb filter (wallflow filter) the most favorable behavior. depth filter made of fibers, sintered metal or foam ceramics have the same size significantly lower separation efficiency and are therefore not currently used.

With increasing soot loading, the flow resistance in the filter increases, which increases Performance loss and fuel consumption leads. That is why it is necessary to Oxidize the soot contained in the filter from time to time (filter regeneration). In the Soot oxidation releases a large amount of heat within one short time. The amount of heat released is proportional to the amount collected in the filter Soot. To damage the filter material during regeneration a certain amount of soot must not be exceeded.

The soot oxidation starts when the exhaust gas temperature is the ignition temperature of the Soot exceeds and sufficient oxygen is contained in the exhaust gas. The The ignition temperature of the soot is between 470, depending on the soot composition and 600 ° C. By adding catalytically active additives to the fuel the ignition temperature can be reduced by about 100 ° C.

Modern diesel engines achieve what is required for soot combustion Temperature only close to full load. Also the use of catalytic coating or catalytic additives is not sufficient to regenerate the to ensure the entire engine map.

In order to enable regeneration even in the lower partial load range, the exhaust gas temperature can be taken by internal engine measures, e.g. B. by post-injection, or other supply of secondary energy, for. B. by means of burners. However, this process is associated with high energy consumption. The required power requirement is between 9 kW / dm ³ displacement (WR Wade including Diesel Particulate Trap Regeneration Techniques SAE 810 118) and 14 kW / dm ³ displacement (WR Wade including Thermal and Catalytic Regeneration of Diesel Particulate Traps SAE 830 083).

By reducing the exhaust gas mass flow through the filter, the need for secondary energy to about 1.7 kW / dm ³ displacement (E. Pauli et al. The Calculation of Regeneration Limits of Diesel Particulate Traps for Different Regeneration Methods SAE 840 075) or 1, 3 kW / dm ³ cubic capacity (VD Rao including Advanced Techniques for Thermal and Catalytic Diesel Particulate Trap Regeneration SAE 850 014). In a further solution it is proposed to achieve the required increase in the exhaust gas temperature by temporarily reducing the flow cross section.

Internal engine measures also have the disadvantage that they are just in engine map near idle are not able to maintain a sufficient temperature to reach. Solutions with additional burners do not have this disadvantage, however they are very expensive compared to other technical solutions.

• - vor dem Filter: US 5101095, US 4,404,795, US 4744216, DE 382 42 478 C2, Versuchsaufbau von G. Hüthwohl und G. Kroon, DE 195 30 749, DE 38 90 556, DE 197 48 561,
• - auf der Filteroberfläche: US 5,144,798, US 4516993 und Versuchsaufbau G. Hüthwohl und G. Kroon
• - zwischen Filterplatten US 6,024,927
• - im Filterkanal: EP 0857862 und
• - im Filtermaterial eingebettet EP 275 372, DE 198 21 869.

There are a number of technical designs that differ in the arrangement of a heating element in relation to the filter body:

• - in front of the filter: US 5101095, US 4,404,795, US 4744216, DE 382 42 478 C2, experimental setup by G. Hüthwohl and G. Kroon, DE 195 30 749, DE 38 90 556, DE 197 48 561,
• - On the filter surface: US 5,144,798, US 4516993 and experimental setup G. Hüthwohl and G. Kroon
• - between filter plates US 6,024,927
• - in the filter channel: EP 0857862 and
• embedded in the filter material EP 275 372, DE 198 21 869.

Technical concepts based on heating the entire filter require approximately as much energy as concepts with an increase in the Exhaust gas temperature. This is due to the very good heat coupling between porous Filter material and exhaust gas.

For this reason, there are currently concepts with local inflammation of the soot particles in the foreground. Here, energy is localized at one or more points introduced into the deposited soot and this brought to ignition. Under Under certain conditions, the combustion thus initiated is planted in the filter continued. This method has the advantage that only very little power is required become. As a rule, they are below one kilowatt and are therefore electrical Take on-board electrical system.

The basic requirement for this procedure is the direct contact between Heating element and deposited soot. Therefore it would be advantageous to use the heater in the To be placed inside the filter. This is difficult to achieve with honeycomb filters. To the a heating zone inside the filter leads to thermomechanical stresses, on the other hand, it is difficult to insert a heating element into the filter without sustainably damage the honeycomb body. US 4,662,911 proposes a two-part Filter, the two parts of which are stored in direct contact and between them a heating element is attached.

As a rule, the heating elements are placed on the upstream side of the filter. However, there are two disadvantages. First, soot only settles on the face if a lot of soot is collected in the filter. So that increases at the same time the exothermic energy stored in the filter, which during the Regeneration as heat is released. In addition, at high Exhaust gas flow the soot is removed from the front into the filter interior, so that front ignition is not possible.

Experiments have shown that the propagation initiated by the ignition Combustion only takes place if the fuel contains catalytic additives if there is a very large amount of soot in the filter and if the Exhaust mass flow is small, i. H. only in the lower engine map. At higher Mass flow, the regeneration is incomplete or does not occur. This The process has therefore proven to be unreliable.

The use of additives also has the disadvantage that they are used in Regeneration leads to ash residues that clog the filter over time.

The known technical methods and devices are not suitable without Excessively high energy consumption in the entire engine map with for that Filter material of safe soot concentration a reliable regeneration initiate.

The invention has for its object a method and an apparatus for Aftertreatment of exhaust gases from internal combustion engines, in particular for Removal of particles with continuous filtration and discontinuous To develop regeneration with which it is possible, on the one hand, with a low Get along with energy and still regenerate the filter safely without closing it to destroy. The regeneration should therefore also with low soot concentration in the Filters may be possible. Regeneration should be possible in the entire engine map. The process should do without additives.

According to the invention, the object is achieved by a method for regeneration a soot filter with features according to claim 1 and by a device with the Features according to claim 6.

The process for the aftertreatment of exhaust gases from internal combustion engines, especially for the removal of soot particles, provides that in front of the main filter Soot depot is located in the collection phase. The regeneration will triggered according to the invention in that the soot depot has so much activation energy is supplied that the soot is burned in the depot. The depot and the Heating devices are to be selected so that the heat of reaction liberates the Exhaust gas temperature is increased so that the main filter also regenerates. The process can be combined with catalytic filter coating.

The device for regenerating a soot filter consists of at least one Housing, with exhaust gas inlet and exhaust gas outlet, a first filter body with a Degree of separation <50% with at least one heating device and a second one Filter body. The second filter is larger than the first.

The first filter body can be made of porous ceramic or metallic material or consist of fibers. It can also be designed as an electrical separator. Of the second filter is advantageously a ceramic honeycomb body.

The heating device can be designed in different ways. It can the filter material itself be partially conductive, it can heating element in the Protrude filter body or be placed on the end face. Even external ones Energy sources such as microwave generators are suitable.

Both filter bodies can be coated catalytically.

The operation of the invention is as follows:
In normal collection operation, a certain proportion of the particles contained in the exhaust gas are collected in a pre-filter. The remaining particles are collected by a downstream main filter. The degree of separation of the pre-filter is smaller than that of the main filter, so that the majority of the soot particles are collected in the main filter. The degree of separation and the size of the pre-filter are adjusted so that there is a greater soot density in the pre-filter than in the main filter.

If the main filter is so full that regeneration has to be initiated, the pre-filter is activated Energy supplied until the soot burns. It is crucial that the prefilter is to be interpreted so that the energy released during combustion is sufficient Exhaust gas flowing through to heat the soot ignition temperature. this happens through a suitable choice of the geometry and the degree of separation. That warmed up Exhaust gas then flows into the main filter and triggers regeneration there.

embodiments

The invention is to be explained in more detail on the basis of exemplary embodiments.

Fig. 1 device for regenerating a soot filter

Fig. 2 pre-filter with heating element inside

Fig. 3 point burner

Fig. 4 bypass arrangement

Fig. 5 cascade arrangement

Fig. 1 shows the side view of an inventive device for regenerating a soot filter.

The device consists of a housing 3 , with an inlet 2 and an outlet 4 for the exhaust gas 1 , a filter body 5 made of porous material as a prefilter with an electric heater 6 and a honeycomb filter 7 as the main filter.

Fig. 2 shows a possible integration of the heating element in the prefilter. Here, a glow plug 8 is inserted into a bore 9 of the porous filter material.

Fig. 3 shows an embodiment with a hot gas heater 9 , which is designed so that only a part of the end face of the pre-filter 5 is coated with hot gas.

Fig. 4 shows an arrangement with a bypass 10 and control flap 11. Here, the pre-filter 5 can be bypassed if enough soot is collected in the pre-filter 5 , but no regeneration is to be triggered. This arrangement has the advantage that unnecessary back pressure is avoided.

Fig. 5 shows a cascade arrangement. Several filter elements 5 a, 5 b,. , , are attached one behind the other, some of the elements can be ignited by means of heating element 6 a, 6 b. This arrangement makes it possible to regenerate the rear elements first and then the front elements in stages. This prevents excessive heating of the exhaust gas.

The advantages of the invention are as follows:
Regeneration is initiated outside the main filter in a pre-filter. This has the advantage that the pre-filter can be optimally matched to the regeneration triggering. In particular, it has no exhaust gas cleaning task.

The pre-filter is designed so that it collects enough soot to provide the required Raising the exhaust gas temperature. Since it is smaller than the main filter the soot density can be higher when loaded. The during the Regeneration temperatures are due to the small size of the Prefilter nonetheless critical. Due to the high soot density, it is easier to Ignite soot using a heating element. It is also unproblematic to have a Place the heating element in a depth filter. So you can Simply drill the foam ceramic filter and insert the element into the hole in a fiber filter, the element can even be integrated without drilling. Of the the low degree of separation of the depth filters is not a disadvantage in this application, but even wanted.

The pre-filter is designed so that a larger soot density is accumulated in it. This ensures that the pre-filter is always full when regeneration must be triggered. The regeneration can thus be triggered in a controlled manner.

The fact that the main filter is regenerated by the hot exhaust gas is the Burn completely and evenly. Local overload and Temperature stresses of the filter material are prevented.

By using the soot particles collected in the pre-filter as an energy source the system uses less energy than burner systems, which also cause an increase in the exhaust gas temperature.

This is compared to known systems with electrical ignition of the soot Process much more reliable and gentle, since the regeneration in the Main filter is triggered by raising the exhaust gas temperature.

Catalytic coating of both filters is advantageous. This will make the Ignition temperature reduced by about 100 ° C. There is less external energy needed to ignite the pre-filter. In addition, the exhaust gas does not have to be so much be heated. In this case, the pre-filter can be smaller.

A variation of the system described is the cascade arrangement. she allows the soot to be burned gradually so that the exhaust gas temperature does not rise too much.

Claims (13)

1. A method for regenerating a soot filter with a arranged in the exhaust gas main filter body, preferably a ceramic honeycomb filter in which soot is collected, characterized in that the main filter is preceded by a soot collecting device and that a heating device is present, which at the time when the Regeneration is to be triggered, the soot collected in the soot collecting device is at least locally heated to at least soot ignition temperature and the soot collecting device is designed in such a way that the soot burned in it releases so much heat that the exhaust gas flowing through it is heated to at least soot ignition temperature. 2. A method for regenerating a soot filter according to claim 1, characterized characterized in that the degree of separation of the soot collecting device is less than Is 50%. 3. A method for regenerating a soot filter according to claims 1, characterized characterized in that the soot collecting device and filter medium are catalytic coated or even catalytically active. 4. A method for regenerating a soot filter according to claims 1, characterized characterized in that the soot collecting device by means of a bypass line is avoided if it has stored enough soot and no regeneration should be triggered. 5. Method for regenerating a soot filter with several in the exhaust gas stream filter bodies arranged one behind the other, in which soot is collected, characterized in that the filter body partially by heaters can be heated and the soot contained burns in that this Heaters are triggered at different times and that the filter body so are swept that the exhaust gas temperature behind these filter bodies as a result of Regeneration has reached at least the soot ignition temperature. 6. Device for regenerating a soot filter with one in the exhaust gas stream arranged main filter body, preferably a ceramic honeycomb filter, in the soot is collected, characterized in that the main filter Pre-filter is upstream and that a heater is present, which in the point in time at which the regeneration is to be triggered, in the Soot collecting device collects soot at least locally on at least Soot ignition temperature is heated and that the soot collecting device is designed in this way is that the soot burned in it releases so much heat that the flowing exhaust gas is heated to at least soot ignition temperature. 7. Device for regenerating a soot filter according to claim 6, characterized characterized in that the degree of separation and volume of the pre-filter is selected so that the soot collected in the pre-filter has a greater density than that in Main filter collected. 8. A device for regenerating a soot filter according to claim 6, characterized characterized in that the pre-filter made of porous material such as sintered metal, fibers or foam ceramic. 9. Device for regenerating a soot filter according to claim 6, characterized characterized in that the heating element is an electrical resistance element. 10. Device for regenerating a soot filter according to claim 6, characterized characterized in that the heating element is integrated in the prefilter. 11. Device for regenerating a soot filter according to claim 6, characterized characterized in that the heating element is a microwave generator. 12. Device for regenerating a soot filter according to claim 6, characterized characterized in that the heating element is a hot gas source. 13. Device for regenerating a soot filter according to claim 6, characterized characterized in that the pre-filter is an electrostatic filter.