Exhaust gas aftertreatment system, method for exhaust gas aftertreatment and use of an exhaust gas aftertreatment system

Abstract

Through an exhaust gas aftertreatment system (10) in the exhaust tract of an internal combustion engine (12) which is operated at least partially with an alcohol fuel, such as methanol or ethanol, an exhaust gas flow of the internal combustion engine (12), which contains nitrogen oxides and an alcohol and / or formaldehyde can flow. The exhaust gas aftertreatment system (10) comprises an oxidation catalyst (14) for removing methanol and formaldehyde from the exhaust gas flow, an SCR catalyst (18) and a metering device (16) for metering ammonia into the exhaust gas flow, wherein the oxidation catalyst (14) is arranged in the exhaust tract upstream of the SCR catalyst (18) and is optionally designed as a common monolithic structure. The metering device (16) is arranged in the exhaust tract upstream of the SCR catalyst (18) and upstream or downstream of the oxidation catalyst (14). The oxidation catalyst contains either preferably vanadium and is free of noble metals, or contains a noble metal such as Ag, Au, Pt, Pd, Rh with a metal-oxide-based reaction accelerator W, Mo, K, Mg, Ca, Mn, Fe, on a carrier material such as Al2O3, SiO2, ZrO2, TiO2, CeO2, if the NH3 is added upstream of the oxidation catalyst. The invention further relates to a method for exhaust gas aftertreatment and to the use of the exhaust gas aftertreatment system (10).

Description

[0001] Exhaust aftertreatment system, methods for exhaust aftertreatment and use of an exhaust aftertreatment system

[0002] The invention relates to an exhaust aftertreatment system in the exhaust stream of an internal combustion engine that is at least partially powered by an alcohol fuel, a method for exhaust aftertreatment in such an exhaust aftertreatment system, and the use of such an exhaust aftertreatment system.

[0003] Internal combustion engines that use an alcohol-based fuel represent an alternative to conventional combustion engines that rely exclusively on fossil fuels, such as conventional diesel engines. In particular, the emission of climate-damaging carbon dioxide (CO2) can be reduced by using an alcohol like methanol as a fuel. However, combustion engines that run entirely or at least partially on alcohol produce exhaust streams that differ fundamentally from those of conventional diesel engines. Specifically, the presence of unburned alcohol in the exhaust stream means that, during the reaction in conventional SCR catalysts that use ammonia as a reducing agent, byproducts such as formaldehyde and / or hydrogen cyanide can be formed.However, the use of SCR catalysts is essential to reliably remove nitrogen oxides also contained in the exhaust gas stream.

[0004] The object of the invention is to provide an exhaust aftertreatment system suitable for the exhaust aftertreatment of combustion engines that are at least partially powered by alcohol fuel and that makes it possible to reliably remove nitrogen oxides in the exhaust stream as well as to minimize the content of undesirable by-products during exhaust treatment.

[0005] The object of the invention is achieved by an exhaust aftertreatment system in the exhaust stream of an internal combustion engine that is at least partially powered by an alcohol fuel, wherein the exhaust stream is permeable to an exhaust gas flow from the internal combustion engine containing nitrogen oxides as well as an alcohol and / or formaldehyde. The exhaust aftertreatment system comprises an oxidation catalyst for removing alcohol and formaldehyde from the exhaust gas flow, an SCR catalyst, and a metering device for metering ammonia into the exhaust gas flow, wherein the oxidation catalyst is arranged in the exhaust stream upstream of the SCR catalyst. The metering device is arranged in the exhaust stream upstream of the SCR catalyst and upstream or downstream of the oxidation catalyst.

[0006] An alcohol fuel is a fuel that consists of an alcohol or at least partially consists of an alcohol, for example, containing at least 5% by weight of alcohol, and in particular at least 20% by weight, based on the total weight of the fuel. Other components may include impurities such as water. The alcohol fuel may also be a mixture of the alcohol and a fossil fuel, for example, an alcohol / diesel blend. If the alcohol fuel is a mixture of the alcohol with another component, for example, a mixture of the alcohol and a fossil fuel, the mixture can be prepared upstream of the combustion chamber of the internal combustion engine and fed into the combustion chamber, or the alcohol fuel can be produced within the combustion chamber by mixing the alcohol with the other components.The only crucial factor is that the combustion engine is at least partially powered by alcohol fuel.

[0007] The combustion engine can be operated at least partially or exclusively with alcohol fuel. However, since exhaust gas flows such as those produced during the combustion of alcohol fuel are to be expected at least temporarily, the exhaust aftertreatment system according to the invention ensures that adequate treatment of the exhaust gas flow is guaranteed at all times.

[0008] The invention is based on the fundamental concept of using an oxidation catalyst in addition to the SCR catalyst, which is used to reduce nitrogen oxides in the exhaust stream. This oxidation catalyst reduces unreacted alcohol and undesirable engine byproducts such as formaldehyde, which can be generated when using an internal combustion engine that is at least partially powered by alcohol fuel. This effectively prevents the formation of undesirable compounds such as formaldehyde or hydrogen cyanide in the SCR catalyst, which would necessitate additional post-treatment and reduce the SCR catalyst's activity with respect to the desired reaction, namely the reduction of nitrogen oxides by ammonia (NH3).

[0009] According to the invention, the metering device is arranged in the exhaust gas stream upstream or downstream of the oxidation catalyst, so that the addition of ammonia to the exhaust gas stream can be tailored to the type of oxidation catalyst used. This avoids the need for an additional metering device downstream of the oxidation catalyst and upstream of the SCR catalyst, and / or the risk of overdosing ammonia by the metering device.

[0010] The term "SCR catalyst" refers to catalysts used for the selective catalytic reduction of nitrogen oxides (NOx). X ) serve (also known as "selective catalytic reduction"). For selective catalytic reduction, SCR catalysts utilize a nitrogen-containing reducing agent, typically ammonia (NH3) or a urea solution.

[0011] It is understood that the term "metering of ammonia into the exhaust gas stream" also includes configurations in which an ammonia precursor compound is introduced into the exhaust gas stream via the metering device, which decomposes in the exhaust gas stream releasing ammonia.

[0012] Formaldehyde can be present in the exhaust stream due to incomplete combustion of alcohol, such as methanol, in the combustion engine and, due to its hazardous properties, is an undesirable byproduct of alcohol degradation. Furthermore, hydrogen cyanide can form within SCR catalysts if formaldehyde and / or an alcohol and ammonia are present in the exhaust stream at the same time. The oxidation catalyst provided according to the invention ensures that formaldehyde and / or the alcohol do not reach the SCR catalyst, thus preventing the formation of undesirable byproducts such as hydrogen cyanide.

[0013] The alcoholic fuel comprises, in particular, an alcohol with 1 to 4 carbon atoms, or a mixture of alcohols with 1 to 4 carbon atoms, or is such an alcohol. Preferably, the alcohol of the alcoholic fuel is methanol, ethanol, or a mixture of methanol and ethanol. Particularly preferred is the alcohol methanol, so that the alcoholic fuel can be a methanol fuel comprising or consisting of methanol.

[0014] Methanol is particularly suitable for use as a fuel because it significantly reduces CC>2 emissions compared to conventional fossil fuels like diesel and can be produced on an industrial scale as a synthetic fuel. One variant of this alcohol fuel is a methanol / diesel blend.

[0015] In a first variant, the dosing unit is located downstream of the oxidation catalyst, and the oxidation catalyst is free of precious metals. In this context, a "precious metal" is defined as a metal selected from the group consisting of silver, gold, platinum, palladium, rhodium, ruthenium, osmium, and iridium. Because the dosing unit is located downstream of the oxidation catalyst in this first variant, it is possible to use an oxidation catalyst that would also oxidize ammonia contained in the exhaust stream without having to worry about an insufficient supply of ammonia for the operation of the downstream SCR catalyst. In other words, the first variant uses a robust oxidation catalyst that would remove not only alcohol and formaldehyde but also other components such as ammonia from the exhaust stream.

[0016] It has been found that noble metal-free oxidation catalysts are particularly suitable for this purpose, reliably oxidizing the alcohol and any formaldehyde present in the exhaust gas stream. In particular, noble metal-free oxidation catalysts exhibit high selectivity for carbon monoxide (CO), which is formed as a product of the oxidation of alcohol and / or formaldehyde. Consequently, no exhaust gas components are generated downstream of the oxidation catalyst that could be converted into undesirable byproducts such as hydrogen cyanide in the subsequent SCR catalyst.

[0017] Furthermore, precious metal-free oxidation catalysts are more cost-effective, robust, and stable against catalyst poisons such as sulfur, phosphorus, and silicates than oxidation catalysts that rely on precious metals as the catalytically active material. Additionally, precious metal-free oxidation catalysts result in a lower temperature increase in the exhaust gas stream than would be the case with precious metal-based oxidation catalysts, since CO, and not CO2, is formed as the main product, thus reducing the temperature impact on the downstream SCR catalyst.

[0018] In its first variant, the oxidation catalyst is a fully extruded catalyst with a vanadium-based catalytic material. Such fully extruded catalysts are known as SCR catalysts in exhaust aftertreatment systems for conventionally powered combustion engines; they are simple and inexpensive to manufacture and have a long service life. It has now been found that such fully extruded catalysts with a vanadium-based catalytic material are also suitable as oxidation catalysts for use in systems for the aftertreatment of exhaust gas streams from combustion engines that use at least some alcohol fuel, because their chemical composition differs from that of exhaust gas streams from conventionally powered combustion engines.

[0019] The vanadium-based catalytic material can include vandium oxides, tungsten oxide and titanium oxide, for example vanadium pentoxide (V2O5), tungsten trioxide (WO3) and titanium dioxide (TiÜ2).

[0020] The fully extruded catalyst can be a honeycomb structure with a cell density in the range of 25 to 600 cpsi, particularly with a cell density in the range of 50 to 300 cpsi. This allows for both a high surface area to increase catalytic activity and excellent mechanical stability under conditions such as those expected in the exhaust system of a combustion engine powered by methanol fuel. In the first embodiment, the oxidation catalyst can also be a catalyst with a support structure coated with the catalytic material. However, the use of the fully extruded catalyst as the oxidation catalyst in the first embodiment is preferred.

[0021] To achieve particularly effective alcohol and / or formaldehyde oxidation in the oxidation catalyst, the catalyst can have a vanadium content of 1 wt.% or more, based on the total weight of the catalyst and calculated as the corresponding vanadium oxide, particularly vanadium pentoxide (V₂O₅), preferably 3 wt.% or more. A higher vanadium content increases the conversion rate of alcohol in the oxidation catalyst and further reduces the proportion of byproducts formed from the alcohol, such as formaldehyde, which are present in the exhaust gas stream downstream of the catalyst.

[0022] In the first variant, a conventional SCR catalyst, as known in the prior art, can be used as the SCR catalyst.

[0023] For example, this involves an SCR catalyst with a support body onto which a catalytic material is applied. The support body is, for example, a porous extruded honeycomb structure. The catalytically active material of the first SCR catalyst is, in particular, a material based on titanium oxide, vanadium oxide, for example, vanadium pentoxide (V₂O₅), and / or tungsten oxide, for example, tungsten trioxide (WO₃).

[0024] Thus, in the first embodiment, the oxidation catalyst and the SCR catalyst can have the same catalytically active material, but depending on their position in the exhaust stream relative to the metering device, they can perform different functions.

[0025] It is also possible that the SCR catalyst is a fully extruded catalyst, especially based on titanium oxide, vanadium oxide and tungsten oxide (also known as a "VWT" catalyst).

[0026] Furthermore, the SCR catalyst can incorporate a zeolite as the catalytic material. Zeolites have been proven effective for use as catalytically active materials in SCR catalysts. In a second variant, the metering device is arranged upstream of the oxidation catalyst, and the oxidation catalyst is configured to selectively degrade alcohol and formaldehyde in the presence of ammonia in the exhaust gas stream. The oxidation catalyst comprises a catalytic material containing an active component selected from the group consisting of silver, gold, platinum, palladium, rhodium, ruthenium, iridium, and combinations thereof.Because in the second variant the oxidation catalyst removes the undesirable components methanol and formaldehyde from the exhaust gas stream, but does not break down ammonia or only in negligible quantities, the dosing device upstream of the oxidation catalyst can already add ammonia to the exhaust gas stream without having to fear that it will be broken down in the oxidation catalyst and thus would no longer be available for the conversion of nitrogen oxides in the SCR catalyst.

[0027] In other words, the second variant uses a selective oxidation catalyst that only breaks down selected components of the exhaust gas stream. This eliminates the need for ammonia overdosing and an additional dosing device between the oxidation catalyst and the SCR catalyst.

[0028] To achieve the selectivity of the oxidation catalyst in the second variant, a catalytic material is used that employs an active component based on a precious metal, specifically one selected from the group consisting of silver, gold, platinum, palladium, rhodium, ruthenium, iridium, and combinations thereof. This active component ensures the reliable conversion of alcohol and / or formaldehyde to carbon dioxide and, within the temperature range relevant to the exhaust aftertreatment system, only a negligible amount of ammonia oxidation. This behavior is surprising because conventional oxidation catalysts based on other active materials, which exhibit a high conversion rate for alcohols such as methanol, oxidatively degrade all components of the exhaust stream, including ammonia, and do not possess the desired selectivity.In this context, selective degradation means that at most 25 vol% of the ammonia added by the metering device is degraded at the operating temperature of the oxidation catalyst, based on the total volume of ammonia added by the metering device, preferably at most 10 vol%.

[0029] The proportion of active material in the catalytic material can be in the range of 0.1 to 15 wt.%, based on the total weight of the catalytic material, preferably in the range of 3 to 8 wt.%

[0030] The active component of the catalytic material is specifically selected from the group consisting of silver, gold, rhodium, ruthenium, iridium and combinations thereof.

[0031] Preferably, the active component contains or is silver. It has been shown that silver is particularly suitable as an active component for the selective degradation of alcohol and / or formaldehyde in the treatment of exhaust gas streams in the exhaust system of an internal combustion engine that is at least partially powered by an alcohol fuel, achieving particularly high conversion rates. This is especially surprising because platinum or palladium are usually considered particularly active catalytic materials for similar applications. However, it has been shown that when alcohol fuel is used, silver enables even better selectivity for carbon dioxide and a high conversion rate of the alcohol, while at the same time ammonia is not oxidized or only minimally oxidized.

[0032] The catalytic material can include a reaction accelerator selected from the group consisting of oxides of tungsten, molybdenum, potassium, magnesium, calcium, manganese, or iron, and combinations thereof. The reaction accelerator ensures faster conversion of alcohol, thus guaranteeing the most complete possible alcohol degradation even at higher exhaust gas flow rates. Simultaneously, the reaction accelerator further suppresses ammonia oxidation and increases the selectivity of the catalyst.

[0033] In particular, the catalytic material comprises a support material selected from the group consisting of the oxides of aluminum, silicon, zirconium, cerium, or titanium, and combinations thereof. This includes structural classes formed from combinations of the oxides, for example, zeolites based on a network of aluminum and silicon oxides. The support material enables, in particular, a high specific surface area of ​​the catalytic material, which in turn has a beneficial effect on the catalytic activity of the oxidation catalyst and contributes to the long-term stability of the catalytic material.

[0034] A preferred combination is silver as the active material and titanium dioxide as the support material for the catalytic material. This allows for the creation of an oxidation catalyst that exhibits both a high conversion rate of alcohol and formaldehyde and high selectivity, resulting in particularly low ammonia degradation.

[0035] The catalytic material can be applied as a coating to a molded body. This makes it possible to combine the high and selective activity of the catalytic material for the degradation of methanol and / or formaldehyde with the properties of the respective molded body, such as high mechanical stability. The molded body is, in particular, a type used in oxidation catalysts of exhaust aftertreatment systems for conventional combustion engines.

[0036] The shaped body can have a honeycomb structure, i.e., be a honeycomb body.

[0037] To achieve particularly high mechanical stability under operating conditions, the shaped body can be a ceramic structure or a metallic support, for example a ceramic structure based on cordierite, mullite, aluminum titanate or silicon carbide (SiC).

[0038] The molded body can be a honeycomb body with a cell density in the range of 25 to 600 cpsi, in particular with a cell density in the range of 50 to 300 cpsi.

[0039] It is also possible that the catalyst body is a fully extruded catalyst comprising vanadium oxides, tungsten oxide, and / or titanium oxide, for example, vanadium pentoxide (V₂O₃), tungsten oxide, tungsten trioxide (WO₃), and titanium dioxide (TiO₂). Such fully extruded catalysts are particularly cost-effective to produce. The fully extruded catalyst can also be a honeycomb structure with a cell density in the range of 25 to 600 cpsi, particularly with a cell density in the range of 50 to 300 cpsi.

[0040] To minimize the space required for the exhaust aftertreatment system, in the second variant the oxidation catalyst and the SCR catalyst can be formed as a single monolithic structure. This means that the oxidation catalyst and the SCR catalyst share a common mold body, with the oxidation catalyst being formed by a first mold body section and the SCR catalyst by a second mold body section of the common mold body, and with the catalytic material of the oxidation catalyst being applied to the common mold body in the first mold body section.

[0041] The common shaped body corresponds in particular to the SCR catalyst as previously described, wherein the catalytic material is additionally applied in the first shaped body section to form the oxidation catalyst.

[0042] Such a common monolithic structure can be produced by immersing the common mold body in a washcoat, where the immersion depth defines the proportion of the first mold body section to the overall mold body. The washcoat contains the catalytic material of the oxidation catalyst or one or more precursor compounds of the oxidation catalyst, which can be converted into the catalytic material by a sintering or calcining process. A washcoat loading in the range of 10 to 200 g / L can be used, for example, 50 to 150 g / L. This provides a simple and cost-effective manufacturing process for producing the common monolithic structure.

[0043] The oxidation catalyst and the SCR catalyst can be arranged in a common catalyst housing, particularly if the oxidation catalyst and the SCR catalyst are monolithic structures. This allows for a particularly compact exhaust aftertreatment system. The exhaust aftertreatment system can also include additional components for converting other components in the exhaust stream. For example, an IX^O catalyst can be provided, which converts nitrous oxide (N₂O) contained in the exhaust stream through oxidative or reductive degradation using a nitrogen-containing reducing agent. In this way, materials can be used in the oxidation catalyst and / or the SCR catalyst in which, at least under certain conditions, nitrous oxide can be produced as a byproduct during the degradation of nitrogen oxides, alcohol, formaldehyde, and / or other exhaust components.

[0044] The exhaust aftertreatment system can include a diesel particulate filter with active or passive regeneration to reduce the particle mass and / or number in the exhaust stream.

[0045] Furthermore, a second oxidation catalyst may be provided, which is designed to oxidize any remaining carbon monoxide (CO) or other hydrocarbons contained in the exhaust stream, such as those known from conventional combustion engines. The oxidation catalyst may be precious metal-based.

[0046] The object of the invention is further solved by a method for the exhaust aftertreatment of an exhaust gas stream generated by an internal combustion engine in an exhaust aftertreatment system as described above, wherein the internal combustion engine is at least partially operated with an alcohol fuel, and wherein alcohol contained in the exhaust gas stream is broken down by means of the oxidation catalyst.

[0047] The features and properties of the method according to the invention apply accordingly to the exhaust aftertreatment system according to the invention and vice versa.

[0048] The object of the invention is further achieved by using an exhaust aftertreatment system as described above in the exhaust stream of a vehicle that is at least partially powered by an alcohol fuel.

[0049] Internal combustion engine. The features and properties of the use according to the invention apply accordingly to the exhaust aftertreatment system according to the invention as well as to the method according to the invention and vice versa.

[0050] In particular, the exhaust gas stream is passed through the oxidation catalyst and / or the SC R catalyst at a temperature in the range of 250 to 500 °C. This maximizes the methanol conversion rate and, if ammonia was added upstream of the oxidation catalyst, minimizes the oxidative degradation of ammonia.

[0051] In the second variant of the exhaust aftertreatment system, the exhaust gas flow is passed through the oxidation catalyst and / or the SCR catalyst, particularly at temperatures in the range of 250 to 450 °C.

[0052] The temperature at which the exhaust gas stream passes through the SCR catalyst is, in particular, higher than the temperature at which the exhaust gas stream passes through the oxidation catalyst, with the temperature difference being primarily due to the reactions taking place in the oxidation catalyst. Thus, temperature control of the SCR catalyst is possible directly downstream of the oxidation catalyst via the respective oxidation catalyst used and the expected reaction products.

[0053] An internal combustion engine can be the combustion engine of a mobile system, such as a vehicle (e.g., a land or water vehicle), or of a stationary system, such as a power plant. In particular, an internal combustion engine is the combustion engine of a watercraft, such as a ship.

[0054] The internal combustion engine is specifically designed to operate on a lean mixture of alcohol fuel and air. In other words, a combustion air-fuel ratio (A) greater than 1 is used during the combustion of the alcohol fuel, resulting in particularly oxygen-rich exhaust gas. It has been found that the exhaust aftertreatment system also provides effective exhaust gas treatment in this case.

[0055] Further features and characteristics of the invention are described by the following

[0056] The description of exemplary embodiments and experimental examples, which should not be understood in a restrictive sense, is illustrated by the figures. The figures show:

[0057] Fig. 1 shows a first embodiment of an exhaust aftertreatment system according to the invention,

[0058] - Fig. 2 a diagram of the conversion rate of methanol in an oxidation catalyst according to the exhaust aftertreatment system from Fig. 1 ,

[0059] - Fig. 3 a diagram of the selectivity of carbon monoxide of the oxidation catalyst according to the exhaust aftertreatment system from Fig. 1 ,

[0060] - Fig. 4 a diagram of the formaldehyde selectivity of the oxidation catalyst according to the exhaust aftertreatment system from Fig. 1 ,

[0061] Fig. 5 shows a second embodiment of the exhaust aftertreatment system according to the invention,

[0062] - Fig. 6 shows a diagram of the conversion rate of methanol in an oxidation catalyst according to the exhaust aftertreatment system from Fig. 5, and

[0063] - Fig. 7 shows a diagram of the conversion rate of ammonia in an oxidation catalyst according to the exhaust aftertreatment system from Fig. 5.

[0064] Fig. 1 schematically shows a first embodiment of an exhaust aftertreatment system 10 according to the invention, which is arranged in the exhaust stream of an internal combustion engine 12 that is at least partially operated with an alcohol fuel. The alcohol fuel comprises in particular an alcohol with 1 to 4 carbon atoms or a mixture of such alcohols.

[0065] In the embodiment shown, the alcohol fuel consists of methanol (MeOH) or at least partly of methanol.

[0066] The combustion engine 12 thus uses methanol at least partially as fuel and converts it by generating an exhaust gas stream, which is indicated in Fig. 1 as arrow P.

[0067] The type of internal combustion engine 12 is not further restricted. Thus, the internal combustion engine 12 can be used in a mobile or stationary system such as a vehicle or a power plant. The internal combustion engine 12 is fluidically connected to the exhaust aftertreatment system 10, with the exhaust gas flow having a direction of flow as indicated by arrow P in Fig. 1. In other words, components of the exhaust aftertreatment system 10 shown to the right of a respective reference point in Fig. 1 are located downstream of that reference point, and components of the exhaust aftertreatment system 10 shown to the left of a respective reference point in Fig. 1 are located upstream of that reference point.

[0068] The exhaust aftertreatment system 10 in the first embodiment according to Fig. 1 comprises, along the flow direction of the exhaust gas stream, an oxidation catalyst 14, a metering device 16 for metering ammonia into the exhaust gas stream and an SCR catalyst 18, each of which is flow-wise connected to the others and each forms a component of the exhaust aftertreatment system 10.

[0069] The oxidation catalyst 14 and the SCR catalyst 18 are provided as separate components in the exhaust aftertreatment system 10, each with its own catalyst housing (not shown in detail). This allows for flexible arrangement within the exhaust aftertreatment system 10.

[0070] The exhaust aftertreatment system 10 serves to clean the exhaust gas produced by the combustion engine 12 by chemically converting unwanted components of the exhaust gas stream.

[0071] During the combustion of the alcohol fuel in the internal combustion engine 12, an exhaust stream is produced which contains nitrogen oxides (NOx). X ) as well as unburned methanol (MeOH) and / or byproducts such as formaldehyde (HCHO). Naturally, the exhaust gas stream may contain other components, for example, carbon monoxide (CO), carbon dioxide (CO2), other hydrocarbons, and / or water (H2O). It is further understood that the exact composition of the exhaust gas stream depends on the combustion engine 12 used, the type of alcohol fuel, and the prevailing load condition. For example, it is to be expected that a different alcohol or alcohols will be present in the exhaust gas stream if methanol is not used as the alcohol in the alcohol fuel. Where methanol is mentioned below, the same statements apply analogously to alternative alcohols provided for according to the invention.

[0072] The aim is to achieve the most complete possible conversion of nitrogen oxides, methanol, and byproducts such as formaldehyde contained in the exhaust gas stream within the exhaust gas aftertreatment system 10 to nitrogen (N2), water, and carbon monoxide or carbon dioxide. This is made possible by the coordinated sequence of components of the exhaust gas aftertreatment system 10 according to the invention, which will be described in more detail below.

[0073] In the first embodiment, the exhaust gas stream, which contains at least nitrogen oxides as well as methanol and / or formaldehyde, encounters the oxidation catalyst 14, which is designed as a robust oxidation catalyst and serves to break down the methanol contained in the exhaust gas stream as completely as possible, in particular to carbon monoxide. At the same time, formaldehyde contained in the exhaust gas stream is also broken down to carbon monoxide.

[0074] In other words, under the operating conditions of the exhaust aftertreatment system 10, the oxidation catalyst 14 exhibits a high conversion rate of methanol and high selectivity for carbon monoxide. In this way, the exhaust gas stream immediately downstream of the oxidation catalyst 14 contains no or only negligible amounts of methanol or methanol degradation byproducts that could react in the downstream SCR catalyst 18 to form undesirable byproducts such as hydrogen cyanide.

[0075] The oxidation catalyst 14 is free of precious metals and has a vanadium-based catalytic material comprising in particular vanadium oxides, tungsten oxide and titanium dioxide.

[0076] The metering device 16 is located downstream of the oxidation catalyst 14. The metering device 16 serves to dose ammonia (NH3) into the exhaust gas stream, which mixes with the exhaust gas stream in the metering device 16 and / or in the flow path towards the SCR catalyst 18 and is distributed within the exhaust gas stream. The dosed ammonia acts as a nitrogen-containing reducing agent in the SCR catalyst 16 to break down the nitrogen oxides contained in the exhaust gas stream. Because the exhaust gas stream downstream of the oxidation catalyst 14 is free or at least substantially free of methanol, undesirable byproducts such as hydrogen cyanide are not formed in the SCR catalyst 16, or only in negligible amounts, despite the presence of ammonia in the exhaust gas stream.

[0077] The type of SCR catalyst 18 is not further restricted. In particular, the SCR catalyst 18 can be a conventional SCR catalyst as known in the prior art for exhaust aftertreatment systems used in conjunction with internal combustion engines that are operated exclusively on the basis of a fossil fuel, for example using diesel or natural gas.

[0078] For example, the selective catalytic reduction carried out in the SCR catalyst 18 is based on the use of a catalytic material based on titanium oxide, vanadium oxides and / or tungsten oxide.

[0079] The following section further explains the functioning of the exhaust aftertreatment system 10 of the first embodiment according to the invention by means of examples that were carried out on a first model system of an exhaust aftertreatment system 10 according to the invention.

[0080] The first model system comprised an oxidation catalyst, a metering device and an SCR catalyst, which were arranged in series in terms of flow technology and were fed with a synthetic exhaust gas mixture that simulates an exhaust gas flow as expected from an internal combustion engine running on an alcohol fuel.

[0081] The synthetic exhaust gas mixture consisted of 700 ppm NO as nitrogen oxide, 3000 ppm methanol as an example of "methanol slip". 1 from the combustion engine, 5 vol% water and the remainder ambient air, which acted as a carrier gas.

[0082] The pollutant components were metered from a gas cylinder and mixed with the carrier gas stream, consisting of ambient air with 5 vol% water, which was heated to a specific test temperature T in the range of 250 to 500 °C. A fully extruded catalyst made of a vanadium-based material was used as the oxidation catalyst. This material comprised vanadium pentoxide (V₂O₅), tungsten trioxide (WO₃), and titanium dioxide (TiÜ₂) and had a cell density of 103 cpsi. In the first test series, the vanadium content of the oxidation catalyst was 1 wt%, and in the second test series, 3 wt%, both based on the total mass of the oxidation catalyst.

[0083] In the model system, a commercially available vanadium, tungsten, titanium oxide catalyst (also known as a "VWT" catalyst), such as the RFV version from HUG Engineering AG, was used as the SCR catalyst 18.

[0084] The composition of the exhaust gas stream was determined by sampling downstream of the oxidation catalyst, and the conversion rate of methanol and the selectivity of selected components were calculated based on the initial composition of the synthetic exhaust gas mixture and the composition of the respective sample.

[0085] Tables 1 and 2 show the achieved conversion rates x of methanol in percent and the selectivities S with respect to formaldehyde, carbon monoxide and carbon dioxide in percent, each based on methanol.

[0086] Table 1: Properties of the exhaust gas stream after treatment with the oxidation catalyst of the first embodiment at a vanadium content of 1 wt.%.

[0087] Table 2: Properties of the exhaust gas stream after treatment with the oxidation catalyst of the first embodiment at a vanadium content of 3 wt.%.

[0088] Figures 2 to 4 show diagrams representing the conversion rate of methanol and the selectivities of CO and formaldehyde according to the data from Tables 1 and 2, where a solid line represents the first series of experiments with a vanadium content of 1 wt% and the dashed line represents the second series of experiments with a vanadium content of 3 wt%.

[0089] As can be seen, the oxidation catalyst enables the complete or near-complete degradation of methanol in the exhaust gas stream, with higher exhaust gas temperatures and higher vanadium content promoting methanol degradation. However, in particular, virtually complete methanol degradation can already be achieved at an exhaust gas temperature of 350 °C with a vanadium content of 1 wt% and at 300 °C with a vanadium content of 3 wt%.

[0090] Furthermore, the selectivity of carbon monoxide increases with rising exhaust gas temperature and higher vanadium content, while that of formaldehyde decreases, with a higher vanadium content being even more advantageous in this respect. This behavior is attributed to the fact that formaldehyde, as an intermediate in the methanol degradation to carbon monoxide, is further converted.

[0091] The noble metal-free oxidation catalyst of the first embodiment does not produce carbon dioxide during the degradation of methanol. This means that the carbon monoxide formed is not further oxidized to carbon dioxide. Consequently, the temperature rise of the exhaust gas stream as it passes through the oxidation catalyst is lower than would be expected compared to noble metal-containing oxidation catalysts, which do carry out a conversion to carbon dioxide.

[0092] Overall, in the first embodiment of the exhaust aftertreatment system 10 according to the invention, the components used are designed in a coordinated manner, wherein the oxidation catalyst 14 ensures, by removing methanol and formaldehyde from the exhaust gas stream, that no undesirable byproducts such as hydrogen cyanide can form in the subsequent SCR catalyst despite the added ammonia.

[0093] Fig. 5 schematically shows a second embodiment of an exhaust aftertreatment system 10 according to the invention.

[0094] The second embodiment is essentially identical to the first, so only the differences will be discussed below. Identical reference numerals denote identical or functionally equivalent components, and reference is made to the explanations above.

[0095] The exhaust aftertreatment system 10 in the second embodiment according to Fig. 5 comprises, along the flow direction of the exhaust gas stream, the metering device 16 for metering ammonia into the exhaust gas stream, an oxidation catalyst 20 and the SCR catalyst 18, which are each flow-wise connected to each other and each form a component of the exhaust aftertreatment system 10.

[0096] In the second embodiment shown, the oxidation catalyst 20 and the SCR catalyst 18 are arranged as a single monolithic structure housed in a common catalyst housing 22. In other words, the oxidation catalyst 20 and the SCR catalyst 18 are arranged directly adjacent to one another along the flow direction P within the common catalyst housing 20. This allows the exhaust aftertreatment system 10 to be implemented with an even smaller footprint.

[0097] It is understood that the oxidation catalyst 20 and the SCR catalyst 18 can also be present as separate components within the exhaust aftertreatment system 10, wherein the oxidation catalyst 20 and the SCR catalyst 18 can be present as separate components in a common catalyst housing or in separate catalyst housings.

[0098] The metering device 16 and the SCR catalyst 18 are otherwise essentially analogous to the first embodiment. However, in the second embodiment, the oxidation catalyst 20 is used instead of the oxidation catalyst 14 according to Fig. 1 in order to take account of the modified arrangement of the metering device 16 in relation to the oxidation catalyst 20.

[0099] In the second embodiment, ammonia is first added to the exhaust gas stream originating from the combustion engine 12 by means of the metering device 16, so that the exhaust gas stream downstream of the metering device 16 comprises nitrogen oxides, methanol and / or formaldehyde as well as ammonia.

[0100] To avoid excessive oxidative degradation of the added ammonia before it reaches the SCR catalyst 18 and can be used to reduce nitrogen oxides, the second embodiment uses the selective oxidation catalyst 20, which is designed to selectively degrade methanol and formaldehyde in the exhaust gas stream, while ammonia is not oxidized or only minimally oxidized.

[0101] The oxidation catalyst 20 has a catalytic material comprising an active component selected from the group consisting of silver, gold, platinum, palladium, rhodium, ruthenium, iridium and combinations thereof.

[0102] The active component is applied to a support material selected from the group consisting of the oxides of aluminum, silicon, zirconium, cerium, or titanium, and combinations thereof. Optionally, the catalytic material can include a reaction accelerator selected from the group consisting of the oxides of tungsten, molybdenum, potassium, magnesium, calcium, manganese, or iron, and combinations thereof.

[0103] The catalytic material is applied to a molded body, which in the illustrated embodiment is a fully extruded catalyst comprising vanadium oxides, tungsten oxide and / or titanium oxide, for example vanadium pentoxide (V₂O₅), tungsten trioxide (WO₃) and titanium dioxide (TiÜ₂). In this way, the part of the monolithic structure that is not provided with the catalytic material can serve as an SCR catalyst 18, which is thus a corresponding fully extruded catalyst.

[0104] It is understood that the shape of the oxidation catalyst 20 may also differ from that of the SCR catalyst 18, particularly if the oxidation catalyst 20 and the SCR catalyst 18 are not formed as a common monolithic structure. For example, the oxidation catalyst 20 and the SCR catalyst 18 may also be located in separate catalyst housings.

[0105] The following section further explains the functioning of the exhaust aftertreatment system 10 of the second embodiment according to the invention by means of examples that were carried out on a second model system of an exhaust aftertreatment system 10 according to the invention.

[0106] The second model system comprised an oxidation catalyst and an SCR catalyst, which were arranged in series in terms of flow technology and were fed with a synthetic exhaust gas mixture that simulates an exhaust gas flow as expected from an internal combustion engine running on an alcohol fuel after ammonia has been added to the exhaust gas stream.

[0107] The synthetic exhaust gas mixture consisted of 700 ppm NO as nitrogen oxide, 630 ppm ammonia as a nitrogen-containing reducing agent, and 1000 ppm methanol as an example of "methanol slip". 1 The mixture consisted of a combustion engine, 5% water by volume, and the remainder ambient air, which acted as a carrier gas. The pollutant components were metered from a gas cylinder and mixed into the carrier gas stream, which was heated to a specific test temperature T in the range of 250 to 500 °C.

[0108] A selective oxidation catalyst 20 was used, consisting of a cordierite body with a honeycomb structure and a cell density of 230 cpsi. A catalytic material was applied to the body, comprising silver as the active component and titanium dioxide as the support material.

[0109] The proportion of silver in the oxidation catalyst 20 was 5 wt.%, based on the total weight of the oxidation catalyst 20.

[0110] The catalytic material was applied to the molded body via a washcoat coating with a loading in the range of 10 to 25 g / L.

[0111] In the model system, a commercially available vanadium, tungsten, titanium oxide catalyst (also known as a "VWT" catalyst), such as the RFV available from HUG Engineering AG, was used as the SCR catalyst 18.

[0112] The composition of the exhaust gas stream was determined by sampling downstream of the oxidation catalyst 20 and, based on the initial composition of the synthetic exhaust gas mixture and the composition of the respective sample, the conversion rate of methanol and the selectivity of selected components were calculated.

[0113] Tables 3 and 4 show the achieved conversion rates x of methanol and ammonia in percent, as well as the selectivities S with respect to formaldehyde, hydrogen cyanide, carbon monoxide and carbon dioxide, each based on methanol, and the selectivities S of nitrogen oxides and nitrous oxide, each based on ammonia, in percent of the oxidation catalyst.

[0114] Table 3: Properties of the exhaust gas stream after treatment with the oxidation catalyst of the second embodiment.

[0115] Table 4: Properties of the exhaust gas stream after treatment with the

[0116] Oxidation catalyst of the second embodiment. Figures 6 and 7 show diagrams illustrating the conversion rates of methanol and ammonia in the selective oxidation catalyst 20 of the second embodiment, according to the data in Table 3. It can be seen that the oxidation catalyst 20 achieves almost complete or complete methanol degradation even at relatively low exhaust gas temperatures of 300 °C, and particularly at 350 °C. At the same time, only comparatively small amounts of ammonia are degraded, so that it remains available as a reducing agent for the subsequent SCR catalyst 18. In the range of 350 to 450 °C, a particularly favorable ratio of effective methanol degradation to a low amount of oxidized ammonia is achieved. Furthermore, regardless of the exhaust gas temperature, neither hydrogen cyanide nor nitrous oxide, which could represent undesirable byproducts of methanol degradation in the presence of ammonia, are formed in the selective oxidation catalyst 20.Furthermore, the amount of formaldehyde produced is also extremely low.

[0117] At higher temperatures, increasingly small amounts of nitrogen oxides form in the oxidation catalyst 20, but these can be reliably broken down by the subsequent SCR catalyst 18.

[0118] The increased rate of ammonia degradation at higher temperatures is attributed to the formation of nitrogen (N2) in the oxidation catalyst 20. However, the experimental data show that selective methanol oxidation is ensured at all temperatures investigated.

[0119] The selective oxidation catalyst primarily produces carbon dioxide as the end product of methanol conversion, rather than carbon monoxide. Only at very high exhaust gas temperatures of 450 °C or higher are small amounts of carbon monoxide produced. Therefore, no further treatment of the exhaust gas stream is necessary to convert carbon monoxide to carbon dioxide.

[0120] Overall, the exhaust aftertreatment system 10 of the second embodiment enables ammonia to be added to the exhaust stream upstream of the oxidation catalyst 20 without it being significantly degraded in the oxidation catalyst 20, while at the same time methanol and formaldehyde can be reliably removed from the exhaust stream in the oxidation catalyst 20.

Claims

Patent claims 1. Exhaust aftertreatment system (10) in the exhaust stream of an internal combustion engine (12) that is at least partially powered by an alcohol fuel, wherein the exhaust stream is accessible to an exhaust stream of the internal combustion engine (12) containing nitrogen oxides as well as alcohol and / or formaldehyde, wherein the exhaust aftertreatment system (10) comprises an oxidation catalyst (14, 20) for removing alcohol and formaldehyde from the exhaust stream, an SCR catalyst (18) and a metering device (16) for metering ammonia into the exhaust stream, wherein the oxidation catalyst (14, 20) is arranged in the exhaust stream upstream of the SCR catalyst (18), and wherein the metering device (16) is arranged in the exhaust stream upstream of the SCR catalyst (18) as well as upstream or downstream of the oxidation catalyst (14, 20).

2. Exhaust aftertreatment system according to claim 1, wherein the alcohol fuel comprises an alcohol having 1 to 4 carbon atoms or a mixture of alcohols having 1 to 4 carbon atoms, preferably methanol, ethanol or a mixture of methanol and ethanol, particularly preferably methanol.

3. Exhaust aftertreatment system (10) according to claim 1 or 2, wherein the metering device (16) is arranged downstream of the oxidation catalyst (14) and the oxidation catalyst (14) is free of precious metals.

4. Exhaust aftertreatment system (10) according to claim 3, wherein the oxidation catalyst (14) is a fully extruded catalyst with a vanadium-based catalytic material.

5. Exhaust aftertreatment system (10) according to claim 3 or 4, wherein the oxidation catalyst (14) has a vanadium content of 1 wt.% or more, based on the total weight of the oxidation catalyst (14).

6. Exhaust aftertreatment system (10) according to claim 1, wherein the metering device (16) is arranged upstream of the oxidation catalyst (20). is and the oxidation catalyst (20) is configured to selectively degrade alcohol and formaldehyde in the presence of ammonia in the exhaust gas stream, and wherein the oxidation catalyst (20) comprises a catalytic material having an active component selected from the group consisting of silver, gold, platinum, palladium, rhodium, ruthenium, iridium and combinations thereof.

7. Exhaust aftertreatment system (10) according to claim 6, wherein the catalytic material comprises a reaction accelerator selected from the group consisting of the oxides of tungsten, molybdenum, potassium, magnesium, calcium, manganese or iron and combinations thereof.

8. Exhaust aftertreatment system (10) according to claim 6 or 7, wherein the catalytic material comprises a support material selected from the group consisting of the oxides of aluminium, silicon, zirconium, cerium or titanium and combinations thereof.

9. Exhaust aftertreatment system (10) according to one of claims 6 to 8, wherein the oxidation catalyst (20) and the SCR catalyst (18) are formed as a common monolithic structure.

10. Method for exhaust aftertreatment of an exhaust gas stream generated by an internal combustion engine (12) in an exhaust aftertreatment system (10) according to one of the preceding claims, wherein the internal combustion engine (12) is operated at least partially with an alcohol fuel, and wherein alcohol contained in the exhaust gas stream is broken down by means of the oxidation catalyst (14, 20).

11. Use of an exhaust aftertreatment system (10) according to one of claims 1 to 9 in the exhaust stream of an internal combustion engine (12) that is at least partially powered by an alcohol fuel.

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