DE102014226669A1 - Exhaust after-treatment system for an internal combustion engine and method - Google Patents

Abstract

In an exhaust aftertreatment system for an internal combustion engine (10), which is provided in particular at least proportionally for the combustion of methane-containing gas, the exhaust aftertreatment system comprises at least one methane oxidation catalyst (13). According to the invention, at least one sulfur adsorption device (12) is furthermore provided in the exhaust gas aftertreatment system.

Description

The present invention relates to an exhaust aftertreatment system for an internal combustion engine, which is provided in particular at least partially for the combustion of methane-containing gas, and a method for operating this exhaust aftertreatment system.

State of the art

Internal combustion engines are known which can be operated both with a methane-containing gas, for example natural gas or methane, and with a mixture of gas and another fuel, for example diesel fuel (dual fuel).

Pure gas engines are often derived from gasoline or diesel engines, where usually a spark ignition to ignite the gas / air mixture is carried out using spark plugs. In diesel / gas engines, the engine is basically based on a diesel engine that allows both pure diesel operation and mixed operation with diesel fuel and gas. Here, part of the diesel heating value is replaced by gas. The ignition of the entire fuel, so the diesel gas / air mixture, via the diesel component. In this case, substitution rates of the diesel fuel by gas of up to 70% are possible.

In all approaches based at least in part on the combustion of methane-containing gas, the problem of high, raw-engine methane emissions occurs. Especially for reasons of climate protection, the methane emissions must be reduced as part of an exhaust aftertreatment. Methane oxidation catalysts (MOCs) are known which oxidize the methane contained in the exhaust gas on the basis of palladium-rich formulations. For this purpose, it is possible to use formulations which have a weight ratio of palladium (Pd) to platinum (Pt) of up to, for example, 7: 1 or even greater. Other methane oxidation catalysts are based on so-called palladium-only formulations, e.g. Pd / alumina. In general, with such formulations, however, only a certain methane conversion can be observed above 400 ° Celsius. For complete oxidation often temperatures of well over 500 ° C are necessary. However, such temperatures are rarely achieved in the energetically efficient gas engine lean mode.

The European patent application EP 1 536 111 A1 describes an apparatus and method for removing by-products from the combustion gases of an internal combustion engine, the apparatus comprising a NOx storage catalyst and a downstream methane reduction catalyst, this downstream catalyst being a palladium catalyst. The German patent application DE 10 2011 005 258 A1 deals with an exhaust system that includes both a NOx storage catalytic converter and a catalytic carbon black filter. The carbon black filter is an oxidation catalyst based on a palladium-rich weight ratio of platinum and palladium.

Disclosure of the invention

Advantages of the invention

The invention provides an exhaust aftertreatment system for an internal combustion engine, which is provided in particular at least proportionally for the combustion of methane-containing gas. Here, the exhaust aftertreatment system comprises at least one methane oxidation catalyst. According to the invention, the exhaust aftertreatment system further comprises at least one sulfur adsorption device. This sulfur adsorption device can be arranged, for example, upstream of the methane oxidation catalyst. Furthermore, it is possible for the sulfur adsorption device to be integrated into the methane oxidation catalyst.

This exhaust aftertreatment system can provide a very robust and efficient system for the sustained and improved oxidation of methane in the exhaust gases of at least partially methane-burning gas engines or diesel / gas engines. The advantages of the exhaust aftertreatment system according to the invention are mainly used in predominantly lean combustion exhaust gases, since the temperatures reached in the lean-burn mode in the exhaust system are often not sufficient for a satisfactory methane oxidation with conventional methane oxidation catalysts. By arranging a sulfur adsorption device in the exhaust aftertreatment system with the methane oxidation catalyst, the efficiency of the methane oxidation can be significantly increased. In particular, the problem of the sulfur sensitivity of methane oxidation catalysts is solved with the exhaust aftertreatment system according to the invention. Conventional methane oxidation catalysts, which for example have a palladium-rich composition, show a dramatic deterioration of their oxidation effect after only a short transit time with sulfur-containing gas and / or diesel fuel. The sulfur adsorption device provided according to the invention in the exhaust aftertreatment system effectively incorporates a sulfur trap into the system for the methane oxidation catalyst. This increases the effectiveness of the methane oxidation catalyst. When in one preferred embodiment provided integration of the sulfur adsorption in the methane oxidation catalyst, the function of the sulfur trap and the function of the methane oxidation catalyst are summarized, so that the number of components in the exhaust aftertreatment system does not need to be increased.

In a preferred embodiment of the exhaust gas aftertreatment system according to the invention, the sulfur adsorption device is an SOx storage device (SOx = sulfur oxides). As a material for the SOx storage device, for example, a material is suitable, as in the European patent application EP 0 945 165 A2 which is incorporated herein by reference. The material for the SOx storage device may for example comprise a magnesium aluminate spinel, wherein the magnesium oxide is preferably present in a stoichiometric excess. The molar ratio of magnesium oxide to aluminum oxide is in particular in a range of more than 1.1: 1. In a further preferred embodiment, the SOx storage device is equipped with a dominant palladium mass fraction. In addition, the formulation for the sulfur adsorption device according to the invention preferably has a subordinate platinum and / or rhodium mass fraction. In this embodiment, the sulfur adsorption so palladium forms the largest mass fraction in comparison to platinum and / or rhodium. Alternatively, it is also possible, for example, for palladium to form the sole noble metal of the sulfur adsorption device.

The formulation of the sulfur adsorption device can furthermore be based, for example, on a conventional NOx storage catalyst (NSC) which, due to its storage material, for example barium carbonate and cerium oxides, and its noble metal composition, for example platinum / palladium / rhodium, also for the quantitative uptake of sulfur oxides, especially in NOx-containing , lean exhaust gases is suitable. Starting from such a customary formulation for a NOx storage catalyst, the sulfur adsorption device according to the invention can be provided, wherein the formulation for the sulfur adsorption device can in particular have a dominant palladium mass fraction and optionally a subordinate platinum and / or rhodium component. On the other hand, it is also possible to choose a material for the sulfur adsorption device which is suitable for storing SOx but not for storing NOx. A suitable material for example, from the above-mentioned European application document EP 0 945 165 A2 out. Furthermore, it is possible to fulfill the SOx storage function of the sulfur adsorption by a corresponding operating strategy in which, for example, a NOx-saturated NOx storage catalyst is used, which no longer has storage capacity for NOx, but which is still capable of storing SOx ,

Suitable formulations for the sulfur adsorption device are based, for example, on SOx storage materials based on barium carbonates and cerium oxides. Other examples of SOx adsorber formulations are in particular magnesium / alumina based and palladium dominated, yet platinum containing, materials.

For preferably reducing regeneration of the sulfur adsorption device, high temperatures between, for example, about 600 ° C. to 750 ° C. are generally required. These high temperatures prevent desorbing sulfur, for example in the form of sulfur oxides or hydrogen sulfide, from being deposited on the components of the methane oxidation catalyst or other downstream equipment in the exhaust aftertreatment system. This is also true for the integrated solution in which the sulfur adsorption device is integrated into the methane oxidation catalyst.

When the internal combustion engine is driven in the slightly rich range (λ <1 but close to 1) or in λ = 1 operation, especially a rhodium-containing sulfur adsorption device also assumes the function of a 3-way catalytic converter, in particular in dynamic operation Carbon monoxide, unburned hydrocarbons and nitrogen oxides can be converted to nitrogen, carbon dioxide and water at the same time. The use of a sulfur adsorption device with a rhodium mass fraction thus has the advantage that under certain circumstances no separate 3-way catalyst is required. The high palladium mass fraction of the sulfur adsorption device according to the invention also has the advantage that this device can also assume the functions of a 3-way catalytic converter, at least during steady-state operation.

In a preferred embodiment of the exhaust aftertreatment system according to the invention, the exhaust aftertreatment system further comprises at least one NOx SCR catalyst (Selective Catalytic Reduction). This embodiment has the advantage that nitrogen oxides are also selectively reduced in the exhaust gas.

Furthermore, the exhaust aftertreatment system according to the invention preferably comprises a further catalyst device which has a higher platinum mass fraction than the methane oxidation catalyst. This further, platinum-enriched Catalyst device has the effect that nitrogen monoxide, which is almost exclusively present after the palladium-rich methane oxidation catalyst, is oxidized again at least proportionally to nitrogen dioxide. As a result, the efficiency of the NOx-SCR catalyst is significantly increased, especially at temperatures <300 ° C.

The platinum-enriched catalyst device may be integrated into the methane oxidation catalyst. In another embodiment, this further catalyst device may be part of an optionally present, catalytic filter coating. For example, a particle filter with a platinum-containing coating may be provided upstream of the optionally present SCR catalyst, wherein this further catalyst device is realized by adapting the coating formulation.

The invention further includes a sulfur adsorption device configured for placement in an exhaust aftertreatment system with a methane oxidation catalyst. The sulfur adsorption acts as a sulfur trap to improve the efficiency of the methane oxidation catalyst. The sulfur adsorption means may be provided for placement upstream of the methane oxidation catalyst. Alternatively or additionally, the sulfur adsorption device may be integrated into the methane oxidation catalyst. The sulfur adsorption device is preferably based on a formulation optimized for SOx storage. Suitable formulations for this purpose are, for example, the already mentioned European patent application EP 0 945 165 A2 refer to. For example, the formulation for the sulfur adsorption device can also be derived from a conventional formulation for a NOx storage catalyst. In this case, a dominant palladium mass fraction is preferably provided, wherein the formulation may in particular have a subordinate platinum and / or rhodium mass fraction. With regard to further features of the sulfur adsorption device according to the invention, reference is made to the above description.

The invention further comprises a method for operating the described exhaust gas aftertreatment system, wherein desulfurization, for example periodic desulfurization, of the sulfur adsorption device occurs upon reaching a predefinable loading threshold of the sulfur adsorption device. For the desulfurization, the internal combustion engine is operated for a limited period of time so that a reducing exhaust gas is supplied to the sulfur adsorption device. This can be achieved by operating the gas or diesel / gas engine substoichiometrically, that is, "rich" for a time or for a limited period of time. λ as the value for the combustion air ratio is <1. Furthermore, it is possible that a net substoichiometric mixture is achieved for example by post-injection into the engine. Under these conditions, stored sulfur compounds are expelled as predominantly SO ₂ and in traces as hydrogen sulfide. The desulphurization of the sulfur adsorption device can take place at specific times or after certain periods or as needed.

The invention further comprises a computer program which is set up to carry out the described method. Furthermore, the invention comprises a machine-readable storage medium, on which the computer program described is stored, and an electronic control unit, which is set up to carry out the method according to the invention.

The exhaust aftertreatment system according to the invention and the sulfur adsorption device according to the invention can be used with particular advantage for lean-running gas or diesel / gas engines. In principle, it is also possible for the exhaust aftertreatment system according to the invention and the sulfur adsorption device according to the invention also to be used for other internal combustion engines, for example for a conventional diesel engine for exhaust aftertreatment. In general, the sulfur adsorption device according to the invention and the exhaust gas aftertreatment system according to the invention are suitable above all for the exhaust aftertreatment of internal combustion engines, which are operated at least temporarily with excess air, that is to say with λ> 1.

Further features and advantages of the invention will become apparent from the following description of embodiments in conjunction with the drawings. In this case, the individual features can be implemented individually or in combination with each other.

Brief description of the figures

In the drawings show:

1 schematic representation of components of an exhaust aftertreatment system according to the invention for an internal combustion engine;

2 schematic representation of components of another embodiment of an exhaust aftertreatment system according to the invention;

3 schematic representation of components of another embodiment of a exhaust gas aftertreatment system according to the invention and

4 schematic representation of components of another embodiment of an exhaust aftertreatment system according to the invention.

Description of exemplary embodiments

1 shows in a schematic way the arrangement of components of an exhaust aftertreatment system according to the invention, in the exhaust system of an internal combustion engine 10 are provided. In the internal combustion engine 10 it is in particular a lean-running gas engine or diesel / gas engine, which can be operated with a mixture of gas and diesel. To increase the performance of the internal combustion engine 10 is the internal combustion engine 10 a turbocharger 11 assigned. The exhaust gases of the internal combustion engine 10 be in the exhaust system first by a sulfur adsorption according to the invention 12 guided. In a sense, this function is a sulfur trap, whereby sulfur-containing components, in particular sulfur oxides, which are contained in the exhaust gas are adsorbed. Downstream of the sulfur adsorption device 12 is a methane oxidation catalyst 13 arranged in the methane oxidation catalyst 13 an oxidation of the methane contained in the exhaust gas takes place. By the upstream sulfur adsorption 12 it is achieved that the function of the methane oxidation is not affected or worsened by sulfur-containing components. In the sulfur adsorption device 12 in particular, it is an SOx storage device which, for example, has a dominant palladium mass fraction. Furthermore, the formulation of the SOx storage device may have a subordinate platinum and / or rhodium mass fraction. In the in 1 In the illustrated embodiment of the exhaust aftertreatment system, the system further includes an SCR catalyst 16 to reduce the mass fraction of the nitrogen oxides contained in the exhaust gas. That for the catalytic reaction within the SCR catalyst 16 Required reactants, such as liquid urea water solution, are passed upstream of the SCR catalyst 16 arranged metering point 15 sprayed into the exhaust system. This example of an exhaust aftertreatment system further includes a catalytic particulate filter 14 on the upstream of the SCR catalyst 16 is arranged.

2 shows schematically a similar system as 1 In this embodiment, the sulfur adsorption device into the methane oxidation catalyst 23 is integrated. In detail, the internal combustion engine 20 a turbocharger 21 assigned. The exhaust gases of the internal combustion engine 20 be in the exhaust system by a catalyst device 23 guided, this catalyst device 23 represents a methane oxidation catalyst with integrated sulfur adsorption. In the methane oxidation catalyst 23 In a sense, a sulfur trap is installed. After methane oxidation, the exhaust gas passes through a catalytic particulate filter 24 before there is an SCR catalyst 26 is supplied.

Upstream of the SCR catalyst 26 is a metering point 25 provided for the reagent solution, which for the catalytic processes in the SCR catalyst 26 is required.

In another embodiment of the exhaust aftertreatment system, a coated particulate filter and an SCR catalyst can be combined in one component as so-called "SCR on filter" (SCRoF). Such embodiments are in the 3 and 4 illustrated. This shows 3 a system with a methane oxidation catalyst 33 to which a sulfur adsorption device 32 upstream. 4 shows a system in which the sulfur adsorption in the methane oxidation catalyst 43 is integrated. Comparable with the systems 1 and 2 is the internal combustion engine 30 respectively. 40 , So in particular a lean-running gas engine or a lean-running diesel / gas engine, a turbocharger 31 respectively. 41 assigned. Referring to 3 go through the exhaust gases of the internal combustion engine 30 the sulfur adsorption device 32 before entering the methane oxidation catalyst 33 reach. Downstream of the methane oxidation catalyst 33 is the part ("SCR on filter") 36 arranged, which integrates an SCR catalyst on a filter. Upstream of the "SCR on filter" 36 there is a metering point 35 for the liquid reagent used for the catalytic reaction in the "SCR on filter" 36 is required. Referring to 4 go through the exhaust gases of the internal combustion engine 40 the methane oxidation catalyst 43 with integrated sulfur adsorption device. The exhaust gases then pass through the "SCR on filter" 46 where upstream of the "SCR on filter" 46 a metering point 45 for the liquid reagent used for the catalytic reaction in the "SCR on filter" 46 is required, is provided.

In a diesel / gas engine, even a partially sulfurized sulfur adsorption device according to the invention can make a considerable and possibly quantitative contribution to the oxidation of the hydrocarbons originating from the fuel which are not methane in the exhaust gas. Therefore, the methane-oxidizing component of the sulfur adsorption device of the present invention can be fully designed for its methane-oxidizing action.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 1536111 A1 [0005]
• DE 102011005258 A1 [0005]
• EP 0945165 A2 [0008, 0009, 0016]

Claims (15)

Exhaust after-treatment system for an internal combustion engine ( 10 ; 20 ; 30 ; 40 ), wherein the exhaust aftertreatment system at least one methane oxidation catalyst ( 13 ; 23 ; 33 ; 43 ), Characterized in that the exhaust after-treatment system further comprises (at least one Schwefeladsorptionseinrichtung 12 ; 32 ). The exhaust aftertreatment system according to claim 1, characterized in that the Schwefeladsorptionseinrichtung ( 12 ; 32 ) upstream of the methane oxidation catalyst ( 13 ; 33 ) or that the sulfur adsorption device into the methane oxidation catalyst ( 23 ; 43 ) is integrated. Exhaust gas after-treatment system according to claim 1 or claim 2, characterized in that the sulfur adsorption device ( 12 ; 32 ) is a SOx storage device. Exhaust after-treatment system according to one of the preceding claims, characterized in that the sulfur adsorption device ( 12 ; 32 ) has a dominant palladium mass fraction and preferably a minor platinum and / or rhodium mass fraction. Exhaust after-treatment system according to one of the preceding claims, characterized in that the sulfur adsorption device ( 12 ; 32 ) based on the formulation of a NOx storage catalyst. An exhaust after-treatment system according to any one of the preceding claims, characterized in that the exhaust aftertreatment system comprises at least one further catalyst device having a higher platinum mass fraction than the methane oxidation catalyst ( 13 ; 23 ; 33 ; 43 ) having. Sulfur adsorption device ( 12 ; 32 ) for an exhaust aftertreatment system of an internal combustion engine ( 10 ; 30 ) with a methane oxidation catalyst ( 13 ; 33 ), characterized in that the sulfur adsorption device ( 12 ; 32 ) for the upstream arrangement of the methane oxidation catalyst ( 13 ; 33 ) is set up. Sulfur adsorption device for an exhaust aftertreatment system of an internal combustion engine ( 20 ; 40 ) with a methane oxidation catalyst ( 23 ; 43 ), characterized in that the sulfur adsorption device into the methane oxidation catalyst ( 23 . 43 ) is integrated. Sulfur adsorption device according to claim 7 or claim 8, characterized in that the sulfur adsorption device ( 12 ; 32 ) is a SOx storage device. Sulfur adsorption device according to one of claims 7 to 9, characterized in that the sulfur adsorption device ( 12 ; 32 ) has a dominant palladium mass fraction and preferably a minor platinum and / or rhodium mass fraction. Method for operating an exhaust gas aftertreatment system according to one of claims 1 to 6, characterized in that upon reaching a predefinable loading threshold of the sulfur adsorption device ( 12 ; 32 ) desulfurization of the sulfur adsorption takes place. A method according to claim 11, characterized in that for the desulfurization of the sulfur adsorption ( 12 ; 32 ) the internal combustion engine ( 10 ; 20 ; 30 ; 40 ) is operated for a limited period of time so that a reducing exhaust gas is supplied to the sulfur adsorption. A computer program adapted to perform the steps of a method according to claim 11 or claim 12. Machine-readable storage medium on which a computer program according to claim 13 is stored. An electronic control device configured to perform the steps of a method according to claim 11 or claim 12.

Images