WO2007014650A1

WO2007014650A1 - Method for reducing the nitrogen oxide and particle emissions of an internal combustion engine and corresponding exhaust-gas treatment unit

Info

Publication number: WO2007014650A1
Application number: PCT/EP2006/007197
Authority: WO
Inventors: Rolf BRÜCK; Ulf Klein
Original assignee: Emitec Gesellschaft Für Emissionstechnologie Mbh
Priority date: 2005-07-29
Filing date: 2006-07-21
Publication date: 2007-02-08
Also published as: DE102005035555A1

Abstract

The invention relates to an exhaust-gas treatment unit (1), which can be traversed in a flow direction (6). Said unit comprises the following components that are arranged in succession in the flow direction (6): 1.1) an oxidation catalyst (2) which is at least responsible for catalysing nitrogen(II) oxide (NO), 1.2) an SCR catalyst (3) for the selective catalytic reduction of nitrogen oxides (NOX) and 1.3) a particle separator (4). Said exhaust-gas treatment unit (1) and the associated method permit a beneficial reduction of the nitrogen oxide and particle emissions of an internal combustion engine.

Description

Method for reducing the nitrogen oxide and particulate emissions of an internal combustion engine and corresponding exhaust aftertreatment unit

The present invention is an exhaust gas aftertreatment unit for the simultaneous reduction of nitrogen oxide and particulate emissions of an internal combustion engine, and a corresponding method. The exhaust aftertreatment unit and the corresponding method can be used in particular in mobile applications such as in motor vehicles.

In many countries, there are legal limits for the proportions of certain undesirable substances in the exhaust gas of internal combustion engines. Among other things, the unwanted substances are nitrogen oxides (NO _x ) and particulate emissions. Due to the operation of the internal combustion engines with hydrocarbons, these particles contain carbon. Especially with very small and / or average particle diameters, the effect of the particulate matter particle emissions on organisms is unclear, but a harmful effect, in particular of the respirable particles, appears possible. Due to the design of modern internal combustion engines, however, the proportion of nitrogen oxides and particles is regularly coupled together. This means that a reduction in the proportion of nitrogen oxide often results, as a side effect, in an increase in the corresponding particle fraction of the exhaust gas.

Therefore, the present invention is based on the object to propose a method for reducing both the nitrogen oxide and the particulate fraction in the exhaust gas of internal combustion engines and a corresponding exhaust aftertreatment unit. This object is achieved by a Abgasnach- treatment unit with the features of claim 1 and a method with The features of claim 8. Advantageous developments are the subject of the respective dependent claims.

An exhaust gas aftertreatment unit according to the invention can be flowed through in a flow direction and comprises the following components one behind the other in the flow direction:

1.1) an oxidation catalyst at least for the oxidation of nitric oxide (NO),

1.2) an SCR catalyst for the selective catalytic reduction of nitrogen oxides (NO _x ) and

1.3) a particle separator.

The erfmdungsgemäße exhaust gas aftertreatment unit allows advantageously to reduce the content of nitrogen oxides and the content of particles in the exhaust gas at the same time. Each of the three components 1.1), 1.2) and 1.3) may comprise a honeycomb body. Here, for example, ceramic honeycomb body and / or metallic honeycomb body can be used. In this case, honeycomb bodies which comprise at least one at least partially structured metallic layer, which are constructed in such a way that cavities can be formed at least for a fluid, are particularly preferred. The honeycomb bodies can also have, at least partially, at least partially walls permeable to a fluid, which walls are formed, for example, of porous ceramic or a corresponding porous metallic material.

At least the catalysts 1.1) and 1.2) have a corresponding catalytically active coating or a coating comprising a catalytically active substance. In particular, the coating may comprise washcoat.

According to an advantageous development of the exhaust aftertreatment unit according to the invention, a reducing agent feed is formed between the oxidation catalyst and the SCR catalyst. In the selective catalytic reduction nitrogen-containing reducing agents are regularly used. Particularly preferred in this case is the use of ammonia (NH ₃ ) as a reducing agent. The SCR catalytic converter or the exhaust gas aftertreatment unit can be designed such that the so-called "almost SCR reaction" takes place, which is the case in particular when the temperature of the SCR catalytic converter regularly does not exceed about 200.degree a reaction of nitrogen monoxide (NO) and nitrogen dioxide (NO ₂ ) with ammonia (NH ₃ ) to form molecular nitrogen (N ₂ ) and water (H ₂ O):

NO + NO2 + 2 NH3 -> 2 N ₂ + 3 H2O

In addition, side reactions take place in which, for example, nitrogen dioxide is reacted with ammonia to give molecular nitrogen and water and optionally also ammonium nitrate (NH ₄ NO ₃ ):

6 NO ₂ + 8 NH ₃ -> 7 N ₂ + 12 H ₂ O;

2 NO ₂ + 2 NH ₃ -> N ₂ + H ₂ O + NH 4 NO ₃

Therefore, in a particularly advantageous manner, the oxidation catalyst 1.1) is designed so that not a complete conversion of nitrogen monoxide is catalyzed in nitrogen dioxide. Furthermore, it is also possible to direct at least a portion of the exhaust gas flow around the oxidation catalyst in dependence on the applied nitrogen monoxide and nitrogen dioxide concentration, so as to enter the SCR catalyst 1.2) as optimally as possible a mixture of nitrogen monoxide and nitrogen dioxide to carry out the " If, for example, the end temperature of the SCR catalyst 1.2) is above 200 ° C., and thus instead of the "fast SCR reaction", other SCR reactions increasingly take place, for example the conversion of nitrogen monoxide with ammonia to molecular nitrogen and water: 4 NO + 4 NH3 + O2 -> 4N2 + 6 H2O

a bypass of the oxidation catalyst can also take place in order to obtain the best possible mixture of nitrogen monoxide and nitrogen dioxide in the SCR catalyst 1.2). For this purpose, it may be advantageous, for example, to determine the temperature of the SCR catalyst 1.2), and, if appropriate, the nitrogen oxide or nitrogen monoxide or nitrogen dioxide content directly upstream of the SCR catalyst 1.2), ie to calculate or measure, for example. Based on these data, a bypass of the oxidation catalyst can be carried out in an advantageous manner. This regulation of the bypass can be independent of the position of the Partikelabscheiders 1.3) relative to the SCR catalyst 1.2) and also without that a Partikelabscheider 1.3) is formed.

According to a further advantageous embodiment of the exhaust aftertreatment unit according to the invention, the reducing agent supply comprises reducing agent precursor supply means and means for converting the reducing agent precursor into the reducing agent.

The reducing agent comprises in particular a nitrogen-containing compound, more preferably ammonia. The reducing agent precursor is a substance which splits off the reducing agent or which can be converted into the reducing agent. Particularly preferred here is the use of urea as a reducing agent precursor. The urea can be introduced in particular in the form of an aqueous urea solution or as a solid by feed.

In particular, the means for converting the reducing agent precursor into the reducing agent may comprise means for thermolysis and / or hydrolysis of the reducing agent precursor. If urea is used as the reducing agent precursor, in particular a thermolysis of urea ((NH ₂ ) ₂ CO) can be used here. to ammonia (NH ₃ ) and isocyanic acid (HCNO) take place. In this example, in the hydrolysis of the conversion of isocyanic acid (HCNO) and What ^¬ ser to ammonia and carbon dioxide.

(NH ₂ ) 2CO -> NH ₃ + HCNO

HCNO + H ₂ O -> NH ₃ + CO ₂

Thermolysis and hydrolysis can in particular also take place in a single component, for example a honeycomb body provided with a hydrolysis catalyst coating.

According to a further advantageous embodiment of the exhaust gas after-treatment unit according to the invention, means for regeneration of the particle separator are formed.

In the context of this invention, the regeneration of the particle separator is understood in particular to mean the reaction of the carbon-containing particles into carbon monoxide (CO) and / or carbon dioxide (CO ₂ ). The means for regenerating the particle separator can comprise, for example, an oxidation catalyst, before which hydrocarbons are introduced into the exhaust gas flow, for example by a superstoichiometric filling of at least one cylinder of the internal combustion engine. In or on the oxidation catalyst, which may also be applied in the form of a correspondingly formed coating on a honeycomb body, the exothermic reaction and oxidation of the hydrocarbons takes place. As a result, the exhaust gas heats up, so that the downstream particle separator 1.3) is heated. From a certain limit temperature occurs in the presence of oxygen oxidation of the carbon in the particles and thus a regeneration of the particulate filter. Another possibility of the regeneration is to form as a means of regeneration heating means that make the particulate filter heatable. Thus, in If the regeneration intervals of the particulate filters were to be raised above the aforementioned limit temperature, in order to initiate the conversion of the carbon. Furthermore, the particle separator 1.3) may comprise means for regeneration, by means of which a surface sliding discharge for promoting the oxidation of the carbon particles can be formed. In addition to all of the above-mentioned possibilities of regeneration, the particle separator can have a corresponding coating which lowers the temperature from which oxidation of the carbon takes place. The above-mentioned different means for regeneration of the particle separator can also be advantageously combined with one another.

According to a further advantageous embodiment of the exhaust gas aftertreatment unit according to the invention, the means for regenerating the particle separator comprise means for generating a plasma.

Preference is given to the formation of a non-thermal plasma, in particular a non-thermal surface sliding discharge. This is understood to mean, in particular, an electrical gas discharge burning in contact with a generally electrically insulating or only weakly conductive surface for the purpose of producing a non-thermal plasma while largely avoiding gas heating. This surface sliding discharge can be operated continuously or discontinuously, in particular depending on the loading state of the particle separator. With regard to the formation of the particle separator, the electrodes and / or the operation of the plasma, reference is made to DE 100 57 862 C1, the disclosure content of which is hereby included in the disclosure content of the present invention.

According to a further advantageous embodiment of the exhaust aftertreatment unit according to the invention, the particle separator comprises a closed particle filter. Under a closed particulate filter, a par- understood in which a plurality of channels are formed and in which the exhaust gas must flow through at least one wall between these channels.

According to a further advantageous embodiment of an exhaust gas aftertreatment unit according to the invention, the particle separator comprises means for the electrostatic precipitation of particles.

In particular, in this case electrodes may be formed which have a DC voltage or a low-frequency AC voltage, preferably in the range of frequencies of less than 120 Hz, preferably less than 90 Hz, more preferably even less than 10 Hz. By means of these means, particles comprising the coal can be charged and deposited on the positively charged electrode. This can lead to an agglomeration of the particles at the same time, in which several particles adhere to one another.

According to a further aspect of the invention, a method for reducing the nitrogen oxide and particulate emissions of an internal combustion engine is proposed, comprising the following steps: 8.1) oxidation of at least nitrogen monoxide (NO), 8.2) selective catalytic reduction of nitrogen oxides (NO _x ) and subsequently 8.3 ) Deposition of at least a portion of the particles in the exhaust gas.

Preference is given here to a process procedure in which after the oxidation 8.1) and before the selective catalytic reduction 8.2) reducing agent is supplied and / or generated. Preference is given here a nitrogen-containing reducing agent is supplied and / or generated, in particular ammonia. It is particularly preferred in this case that a reducing agent precursor is fed and converted into reducing agent. A reducing agent precursor is understood here to mean a compound which can split off reducing agents and / or which can be converted into reducing agents. A possible reducing agent precursor for the re reducing agent ammonia is, for example, urea. It is particularly preferred here that the particle separator comprises a closed particle filter.

Further preferably, the deposited particles are at least partially set. An at least partial conversion is understood to mean, in particular, an at least partial oxidation of the carbon contained in the particles. Further preferred is a regenerable particle separator. In this case, the particles deposited on the particle separator are reacted, for example as explained above.

Particularly preferred in this case is a process control, in which the reaction of the .Partikel and thus also the regeneration of the Partikelabscheiders is plasma assisted. In particular, the reaction of the particles or the regeneration of the particle separator can take place by way of a non-thermal surface lubricant discharge as explained above.

Also particularly preferred is a method in which the particle deposition is at least partially supported by an electric field.

Such an electrostatic deposition or even a deposition based on a low-frequency AC voltage can be combined in a particularly advantageous manner with a so-called open particle filter or particle, which is designed so that the exhaust gas does not have to flow through a wall between two channels, but Rather, if appropriate, without being able to flow through a wall through the particle separator.

The application possibilities, advantages and details disclosed here in connection with the exhaust gas aftertreatment unit according to the invention can be transferred and applied in the same way to the method according to the invention and vice versa. The invention will be explained in more detail below with reference to the appended FIGURE, without the invention being restricted to the exemplary embodiment shown there as well as the advantages and details disclosed therein. The only figure 1 shows specific ^¬ matically an exhaust gas treatment unit of the invention 1. This comprises an oxidation catalyst 2, an SCR catalyst 3, and a particle separator 4. The exhaust after-treatment unit 1 can be flowed through by the exhaust gas 5 of an internal combustion engine not shown in a flow direction. 6 According to the invention, the oxidation catalytic converter 2, the SCR catalytic converter 3 and the particle separator 4 are formed one behind the other in the flow direction 6.

Between oxidation catalyst 2 and SCR catalyst 3, a reducing agent supply 7 is formed. This comprises feed means 8 for supplying a reducing agent precursor and means 9 for converting the reducing agent precursor into the reducing agent. The means 9 for converting the reducing agent precursor to reducing agent comprise in particular a hydrolysis catalyst on which a reducing agent precursor urea is thermally and / or hydrolyzed to ammonia as a reducing agent. Downstream of the SCR catalyst 3, a barrier catalyst 10 is formed. In this, possibly by the SCR catalyst 3 penetrating reducing agent is reacted. In particular, the barrier catalyst 10 has an oxidation catalyst coating that effects oxidation of the reductant.

The oxidation catalyst 2, the SCR catalyst 3, the means for converting the reducing agent precursor to the reducing agent 9, the barrier catalyst 10 and / or the particle separator 4 may advantageously comprise honeycomb bodies which have walls separated by walls for an exhaust gas flow through channels. The honeycomb bodies can in particular be constructed of metallic layers which are at least partially structured so that the layers limit channels. Furthermore, a bypass 11 is formed, by means of which the exhaust gas 5 can at least partially ^flow around the oxidation catalyst 2. There are flow ^¬ conducting means 12 are formed by means of controlling the amount of exhaust gas which flows through the by pass ^¬ 11, and / or can be regulated. For example, the flow-guiding means 12 may be a movable flap. Here, the proportion of the exhaust gas 5 flowing through the bypass is controlled depending on the temperature of the SCR catalyst 3. The bypass flow is adjusted in each case so that an optimal ratio of the content of nitrogen monoxide (NO) and nitrogen dioxide (NO ₂ ) is present before the SCR catalyst, so that the SCR reactions taking place at this SCR catalyst temperature as described above are as optimal as possible - Have ratio, so that as complete as possible implementation of the nitrogen oxides in the exhaust gas 5 takes place at the SCR catalyst 3.

The particle separator 4 has means 13 for the electrostatic or low-frequency separation of particles from the exhaust gas 5. In the figure, these are symbolized by corresponding voltage connections. By means of an electrostatic precipitation, which is initiated by the means 13 for the electrostatic deposition of particles, particles can be deposited and agglomerated. A low-frequency deposition is to be understood here in particular as a deposition, which is based on a low-frequency AC voltage. For example, here channel walls may be formed in the particle separator, which have different electrical potentials on opposite walls. Furthermore, the particle separator has means 14 for generating a plasma, in particular a non-thermal surface lubricant discharge. In particular, means for regenerating the particle separator, in which regeneration of the particle separator 4, that is to say conversion of the carbon in the separated particles, due to the non-thermal surface sliding discharges, can be realized by means of this plasma. In principle, the regeneration of the particle separator 4, that is to say the at least partial reaction of the particles which are deposited on the particle separator 4, can take place continuously or discontinuously. Particularly advantageous is a discontinuous regeneration, which operates depending on the loading state or degree of separation of the Partikelabscheiders 4.

The exhaust aftertreatment unit 1 according to the invention as well as the method according to the invention advantageously make it possible to reduce the nitrogen oxide and particle emissions of an internal combustion engine.

LIST OF REFERENCE NUMBERS

I exhaust aftertreatment unit 2 oxidation catalyst

3 SCR catalyst

4 particle separators

5 exhaust

6 flow direction 7 Reduktionsmitteluhr

8 feed

9 means for converting the reducing agent precursor into reducing agent

10 barrier catalyst

I 1 bypass 12 flow guide

13 means for the electrostatic precipitation of particles

14 means for generating a plasma

Claims

claims

1. exhaust gas aftertreatment unit (1), which can be flowed through in a flow direction (6), comprising in flow direction (6) one after the other the following components:

1.1) an oxidation catalyst (2) at least for the oxidation of nitrogen monoxide (NO),

1.2) an SCR catalyst (3) for the selective catalytic reduction of nitrogen oxides (NO _x ) and

1.3) a particle separator (4).

2. exhaust gas aftertreatment unit (1) according to claim 1, wherein between the oxidation catalyst (2) and SCR catalyst (3) a reducing agent supply (7) is formed.

3. exhaust gas aftertreatment unit (1) according to claim 2, wherein the reducing agent supply (7) supply means (8) for a reducing agent precursor and means (9) for converting the reducing agent precursor are formed in the reducing agent.

4. exhaust gas aftertreatment unit (1) according to one of the preceding claims, wherein the means (14) for the regeneration of the Partikelabscheiders (4) are formed.

5. exhaust aftertreatment unit (1) according to claim 4, wherein the means (14) for regeneration of the Partikelabscheiders (4) comprise means for generating a plasma.

6. exhaust gas aftertreatment unit (1) according to any one of the preceding claims, wherein the particle separator (4) comprises a closed particle filter.

7. exhaust gas aftertreatment unit (1) according to any one of the preceding claims, wherein the particle separator (4) comprises means (13) for the electrostatic deposition of particles.

8. A method for reducing the nitrogen oxide and particulate emissions of an internal combustion engine, comprising the following steps:

8.1) oxidation of at least nitrogen monoxide (NO),

8.2) selective catalytic reduction of nitrogen oxides (NOx) and subsequently

8.3) deposition of at least a portion of the particles in the exhaust gas.

9. The method of claim 8, wherein after the oxidation and before the selective catalytic reduction reducing agent is supplied and / or generated.

A process according to claim 9, wherein a reducing agent precursor is fed and converted to reducing agent.

11. The method according to any one of claims 8 to 10, wherein the step 8.3) in a particle separator (4) is performed.

12. The method of claim 11, wherein the particle separator (4) comprises a closed particle filter.

13. The method according to any one of claims 8 to 12, wherein the deposited particles are reacted at least temporarily.

14. The method according to any one of claims 11 to 13, wherein the particle separator (4) is regenerable.

15. The method according to any one of claims 13 or 14, wherein the implementation of the particles takes place plasma assisted.

16. The method according to any one of claims 8 to 15, wherein the Partikelabscheidimg is at least partially supported by an electric field at least.