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 containing nitrogen oxides and an alcohol and / or formaldehyde can flow. The exhaust gas aftertreatment system (10) comprises a metering device (14) for metering ammonia into the exhaust gas flow, a first SCR catalyst (16) and a second SCR catalyst (18), wherein the first SCR catalyst (16) is arranged in the exhaust tract downstream of the metering device (14) and is designed to generate a treated exhaust gas flow from the exhaust gas flow of the internal combustion engine (12), which treated exhaust gas flow contains the nitrogen oxides and at least one nitrogen-containing hydrocarbon compound, such as hydrogen cyanide, due to e.g. zeolite acting as a catalyst. The second SCR catalyst (18) is arranged in the exhaust tract downstream of the first SCR catalyst (16) and is designed to degrade the nitrogen oxides contained in the generated treated exhaust gas flow while consuming the nitrogen-containing carbon water compound contained in the generated treated exhaust gas flow. The invention further relates to a method for exhaust gas aftertreatment and to the use of an 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 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 a metering device for metering ammonia into the exhaust gas flow, a first SCR catalyst and a second SCR catalyst, wherein the first SCR catalyst is arranged in the exhaust stream downstream of the metering device and is configured to generate a treated exhaust gas flow from the exhaust gas flow of the internal combustion engine, containing nitrogen oxides and at least one nitrogen-containing hydrocarbon compound.The second SCR catalyst is located in the exhaust stream downstream of the first SCR catalyst and is designed to reduce the nitrogen oxides contained in the generated treated exhaust stream by consuming the nitrogenous carbon-water compound contained in the generated treated exhaust stream.

[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. The invention is based on the fundamental concept of providing a two-stage exhaust gas purification system with SCR catalysts connected one after the other in the direction of flow, wherein the nitrogen-containing hydrocarbon compound, which is formed as a byproduct in the first SCR catalyst from alcohol and / or formaldehyde contained in the exhaust gas flow, is used as a nitrogen source in the downstream second SCR catalyst.In this way, the reduced conversion rate of nitrogen oxides in the first SCR catalyst, compared to alcohol- and formaldehyde-free exhaust gas streams, which is caused by the formation of the nitrogen-containing hydrocarbon compound through the consumption of the added ammonia, can be compensated for by the second SCR catalyst. At the same time, the nitrogen-containing hydrocarbon compound is reliably removed from the exhaust gas stream. Thus, according to the invention, the otherwise occurring disruptive byproduct in the form of the nitrogen-containing hydrocarbon compound is specifically taken into account as an intermediate product in the design of the exhaust aftertreatment system and consumed in the second SCR catalyst.

[0008] In particular, the use of additional dosing devices for ammonia upstream of the second SCR catalyst can be dispensed with, since sufficient nitrogen-containing hydrocarbon compound can be generated in the first SCR catalyst to achieve the desired efficiency in the reduction of nitrogen oxides.

[0009] The term "SCR catalyst" refers to catalysts used for the selective catalytic reduction of nitrogen oxides (NOx) (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.

[0010] According to the invention, ammonia supplied via the metering device serves as a nitrogen-containing reducing agent in the first SCR catalyst, and the nitrogen-containing hydrocarbon compound produced in the first SCR catalyst serves as a nitrogen-containing reducing agent in the second SCR catalyst. Naturally, it is also possible that any ammonia not converted in the first SCR catalyst is available for selective catalytic reduction in the second SCR catalyst. Generally, however, the amount of ammonia supplied to the exhaust gas stream upstream of the first SCR catalyst by means of the metering device is dimensioned such that the supplied ammonia is completely or substantially completely converted in the first SCR catalyst, in order to keep the amount of ammonia consumed as low as possible.

[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] 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.

[0013] 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.

[0014] The type of nitrogen-containing hydrocarbon compound produced in the first SCR catalyst depends on the type of alcohol fuel used. Methanol present in the exhaust stream is primarily converted to hydrogen cyanide, while alcohols with more than one carbon atom can also be converted to alternative nitrogen-containing hydrocarbon compounds such as amines, amides, nitriles, or others.

[0015] The nitrogen-containing hydrocarbon compound includes, or is in particular, hydrogen cyanide. Hydrogen cyanide can be formed by the reaction of formaldehyde and / or alcohol contained in the exhaust gas stream with ammonia in the first SCR catalyst and represents a toxic byproduct that must be removed from the exhaust gas stream as completely as possible before it leaves the exhaust aftertreatment system.

[0016] Formaldehyde or alcohol contained in the exhaust stream can either originate from the combustion engine, for example due to incomplete combustion of alcohol in the combustion engine, or be produced by at least partially incomplete conversion of the alcohol in the first SCR catalyst.

[0017] It was found that hydrogen cyanide in particular can serve as a suitable nitrogen source in the downstream second SCR catalyst and can thus be reliably removed from the exhaust gas stream by reacting with remaining nitrogen oxides, as well as enabling sufficient reductive nitrogen oxide degradation to nitrogen in the exhaust gas stream.

[0018] A conventional SCR catalyst, as known in the prior art, can be used as the first SCR catalyst.

[0019] 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).

[0020] It is also possible that the first SCR catalyst will be a fully extruded catalyst, especially one based on titanium oxide, vanadium oxide and tungsten oxide (also known as a "VWT" catalyst).

[0021] The second SCR catalyst can also be a support catalyst onto which a catalytic material is applied, for example, using a washcoat coating process. However, it is also possible for the support and the catalytic material to be produced from a single extruded material using an extrusion process followed by sintering. In particular, the second SCR catalyst uses a zeolite as its catalytic material. Zeolites are well-established for use as catalytically active materials in SCR catalysts.It has been shown that corresponding zeolites are not only suitable for selective catalytic reduction based on ammonia as a reducing agent, but also as catalytically active materials for converting nitrogen oxides using nitrogen-containing hydrocarbons, in particular hydrogen cyanide, as reducing agents in exhaust gas streams such as those produced in combustion engines that are at least partially operated with an alcohol fuel.

[0022] The zeolite can be a zeolite of a structural type selected from the group BEA (Beta polymorph A-type), MFI (also known as ZSM-5-type), SAPO (Siicoaluminophosphate), Faujasite, Ferrierite and Chabazite, or a combination of zeolites of these structural types.

[0023] To further increase the catalytic activity and / or selectivity of the zeolite in the second SCR catalyst, the zeolite can be a metal-ion-exchanged zeolite, in particular an iron- and / or copper-exchanged zeolite. In a metal-ion-exchanged zeolite, some of the ions present in the zeolite framework are replaced by other metal ions. In this way, the catalytic activity and selectivity of the zeolite-based catalytic material can be precisely tailored to the respective components and / or concentrations expected in the exhaust gas stream. In particular, the metal ions exchanged into the zeolites support the adsorption of formaldehyde onto the catalytic material.

[0024] The content of metal ions used as exchange agents in the metal ion-exchanged zeolite is, in particular, at least 1 wt.%, for example 3 wt.% or more, in each case based on the total mass of the metal ion-exchanged zeolite, the content being based on the corresponding oxide of the exchanged metal ion. For example, the stated metal ion content in an iron-exchanged zeolite refers to Fe₂Ü₃ and in the case of a copper-exchanged zeolite to CuO.

[0025] The specific BET surface area of ​​the second SCR catalyst is, in particular, at least 250 m². 2 / g, for example more than 400 m 2 / g. The BET surface can be determined according to DIN ISO 9277, using a pressure range p / po of 0 to 0.125.

[0026] The second SCR catalyst can be 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. While precious metals such as palladium, rhodium, or platinum exhibit high catalytic activity for the oxidation of nitrogenous components in the exhaust stream, such as ammonia and nitrogenous hydrocarbons like HON, they lead to increased nitrogen oxide formation through their degradation, lack SCR activity, and are expensive. Furthermore, alternative catalyst materials, such as the previously described zeolites, can be more resistant to catalyst poisons and water contained in the exhaust stream.

[0027] The second SCR catalyst includes, in particular, a shaped body onto which the catalytic material is applied. This shaped body is, for example, a honeycomb structure based on cordierite. The shaped body can have a cell density in the range of 46 to 600 cpsi, particularly in the range of 100 to 400 cpsi.

[0028] The catalytic material can be applied to the molded body using a so-called washcoat. In this case, a washcoat loading in the range of 30 to 200 g / L is used, for example, 50 to 150 g / L.

[0029] 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, consisting of the first and second SCR catalysts, also provides effective exhaust gas treatment in this case. The first and second SCR catalysts can be implemented as separate or integrated components within the exhaust aftertreatment system.

[0030] In one variant, the first and second SCR catalysts are arranged in a shared catalyst housing. This means that the first and second SCR catalysts follow each other directly in the exhaust gas stream and form a single unit. This reduces the installation space required for the exhaust aftertreatment system. Furthermore, it simplifies the installation of the shared catalyst housing as a retrofit unit in existing exhaust aftertreatment systems, thus enabling at least partial or partial operation with alcohol-based fuel.

[0031] The exhaust aftertreatment system can also include further components that serve to convert additional components in the exhaust stream. For example, an N₂O 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 first SCR catalyst and / or the second SCR catalyst in which, at least under certain conditions, nitrous oxide can be produced as a byproduct during the degradation of nitrogen oxides, methanol, formaldehyde, and / or other exhaust gas components.

[0032] 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.

[0033] Furthermore, an oxidation catalyst may be provided, which is designed to oxidize 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.

[0034] For example, the exhaust aftertreatment system comprises, in the direction of flow, a sequence of diesel particulate filter, metering device, first SCR catalyst, second SCR catalyst, and oxidation catalyst. Naturally, different sequences are also possible, as long as the sequence according to the invention, consisting of a metering device, first SCR catalyst, and second SCR catalyst, is ensured.

[0035] 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 a nitrogen-containing hydrocarbon compound is produced in the first SCR catalyst, which is used as a nitrogen source for a selective catalytic reduction for the degradation of nitrogen oxides in the second SCR catalyst.

[0036] 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.

[0037] In particular, the exhaust gas stream is passed through the first SCR catalyst and / or the second SCR catalyst at a temperature in the range of 250 to 500 °C, preferably at a temperature in the range of 350 to 500 °C. In this way, the conversion rate of nitrogen oxides in the exhaust gas stream can be optimized after treatment with the exhaust aftertreatment system, and the content of formaldehyde, alcohol, and nitrogen-containing hydrocarbons such as hydrogen cyanide can be minimized.

[0038] Furthermore, the object of the invention is solved by the use of an exhaust aftertreatment system as described above in the exhaust stream of an internal combustion engine that is at least partially powered by an alcohol fuel.

[0039] 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.

[0040] 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.

[0041] Further features and characteristics of the invention are clarified by the following description of an exemplary embodiment and experimental examples, which are not to be understood in a limiting sense, as well as by the figures. The figures show:

[0042] Fig. 1 shows a first embodiment of a device according to the invention.

[0043] Exhaust aftertreatment system,

[0044] - Fig. 2 a second embodiment of an invention

[0045] Exhaust aftertreatment system,

[0046] - Fig. 3 a diagram of the conversion rate of methanol in a first SCR catalyst according to the exhaust aftertreatment system from Fig. 1 ,

[0047] - Fig. 4 a diagram of the conversion rate of nitrogen oxides in the first SCR catalyst according to the exhaust aftertreatment system from Fig. 1 , and

[0048] - Fig. 5 shows a diagram of the conversion rate of nitrogen oxides in a second SCR catalyst according to the exhaust aftertreatment system from Fig. 1.

[0049] 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 operated at least partially or proportionally with an alcohol fuel. The alcohol fuel comprises in particular an alcohol with 1 to 4 carbon atoms or a mixture of such alcohols.

[0050] In the embodiment shown, the alcohol fuel consists of methanol (MeOH) or at least partly of methanol.

[0051] 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.

[0052] 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.

[0053] The exhaust aftertreatment system 10 according to Fig. 1 comprises, along the flow direction of the exhaust gas stream, a metering device 14 for metering ammonia into the exhaust gas stream, a first SCR catalyst 16 and a second SCR catalyst 18, which are each flow-wise connected to each other and each form a component of the exhaust aftertreatment system 10.

[0054] 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.

[0055] During the combustion of the alcohol fuel in the internal combustion engine 12, an exhaust stream is produced containing nitrogen oxides (NOx) as well as unburned methanol (MeOH) and / or byproducts such as formaldehyde (HCHO). Naturally, the exhaust 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 stream depends on the internal 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 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 according to the invention.

[0056] 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.

[0057] First, the exhaust gas stream, which contains at least nitrogen oxides as well as methanol and / or formaldehyde, encounters the metering device 14, in which ammonia (NH3) is added to the exhaust gas stream, which mixes with the exhaust gas stream in the metering device 14 and / or in the flow path towards the first SCR catalyst 16 and is distributed in the exhaust gas stream.

[0058] The added ammonia serves as a nitrogen-containing reducing agent in the first SCR catalyst 16 to break down the nitrogen oxides contained in the exhaust gas stream.

[0059] The type of the first SCR catalyst 16 is not further restricted. In particular, the first SCR catalyst 16 can be a conventional SCR catalyst as described in the prior art for

[0060] exhaust aftertreatment systems are known to be used in conjunction with

[0061] Internal combustion engines are used that are operated exclusively on the basis of a fossil fuel, for example using diesel or natural gas.

[0062] For example, the selective catalytic reduction carried out in the first SCR catalyst 16 is based on the use of a catalytic material based on titanium oxide, vanadium oxide and / or tungsten oxide.

[0063] In addition to the desired reduction of nitrogen oxides in the exhaust gas stream, the first SCR catalyst 16 also converts any methanol contained in the exhaust gas stream, so that the exhaust gas stream downstream of the first SCR catalyst 16 is preferably free of methanol or methanol is only present in insignificant trace amounts.

[0064] Because the exhaust gas stream contains not only nitrogen oxides but also unreacted methanol and / or formaldehyde from the combustion engine 10, with formaldehyde originating from the incomplete conversion of methanol in the combustion engine 10 and / or being generated in the first SCR catalyst 16, nitrogen-containing hydrocarbon compounds such as hydrogen cyanide are formed in addition to the desired reduction of nitrogen oxides in the first SCR catalyst 16. This means that the nitrogen present in the exhaust gas stream due to the ammonia added in the metering device 14 is not fully available for the selective catalytic reduction of nitrogen oxides in the first SCR catalyst 16 and is partially consumed by the formation of the nitrogen-containing hydrocarbon compound.

[0065] This leads to an insufficient conversion rate of nitrogen oxides in the first SCR catalyst 16, so that immediately downstream of the first SCR catalyst 16 there is a treated exhaust gas stream which has a proportion of nitrogen oxides that necessitates further treatment of the exhaust gas stream.

[0066] Furthermore, the treated exhaust gas stream includes the nitrogen-containing hydrocarbon compound formed in the first SCR catalyst 16, which is also usually produced in an amount that should not be released by the exhaust aftertreatment system 10, especially if the nitrogen-containing hydrocarbon compound is an acutely toxic compound such as hydrogen cyanide.

[0067] Other possible components in the treated exhaust gas stream are formaldehyde, other unburned hydrocarbons and carbon monoxide, these components should be removed from the exhaust gas stream as completely as possible before the exhaust gas stream leaves the exhaust aftertreatment system 10.

[0068] For these reasons, according to the invention, the second SCR catalyst 18 is arranged downstream of the first SCR catalyst 16, in particular immediately following the first SCR catalyst 16.

[0069] The second SCR catalyst 18 comprises a zeolite as the catalytic material, in particular a zeolite of a structural type selected from the group BEA, MFI, SAPO, faujasite, ferrierite and chabazite or a combination of zeolites of these structural types.

[0070] Thus, the second SCR catalyst 18 is free of precious metals.

[0071] The second SCR catalyst 18 performs a selective catalytic reduction, in which the nitrogen-containing hydrocarbon compound formed in the first SCR catalyst 16 serves as a nitrogen-containing reducing agent. Thus, in the second SCR catalyst 18, any remaining nitrogen oxides in the exhaust gas stream are converted to nitrogen, consuming the nitrogen-containing hydrocarbon compound in the process. In this way, the nitrogen oxide content of the exhaust gas stream can be minimized, and at the same time, the nitrogen-containing hydrocarbon compound can be broken down as completely as possible.

[0072] Formaldehyde contained in the treated exhaust gas stream is also broken down to carbon monoxide and carbon dioxide in the second SCR catalyst 18, if necessary first by conversion to the nitrogen-containing hydrocarbon compound, provided that ammonia is still present in the exhaust gas stream.

[0073] The sequence of first SCR catalyst 16 and second SCR catalyst 18 according to the invention thus makes it possible to dispense with the need to add ammonia again as a reducing agent between the first SCR catalyst 16 and the second SCR catalyst 18, and to keep the amount of ammonia that must be supplied via the metering device 14 upstream of the first SCR catalyst 16 as low as possible. In this way, an overdose of ammonia to compensate for the additional components of the exhaust gas stream expected when using alcohol fuel is not necessary.

[0074] It is understood that the amount of ammonia supplied to the exhaust gas stream by means of the metering device 14 can also be adjusted on the basis of the current load state of the combustion engine 10 and / or sensor data obtained from one or more (not shown) sensors that are arranged along the exhaust gas stream.

[0075] Fig. 2 shows a second embodiment of the exhaust aftertreatment system according to the invention.

[0076] 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.

[0077] In the second embodiment, the first SCR catalyst 16 and the second SCR catalyst 18 are housed in a common catalyst housing 20. In other words, the first SCR catalyst 16 and the second SCR catalyst 18 are arranged immediately one after the other along the flow direction P within the common catalyst housing 20.

[0078] In this way, the exhaust aftertreatment system 10 can be implemented with an even smaller space requirement.

[0079] The following section further explains the functioning of the exhaust aftertreatment system according to the invention using examples from a model system of an inventive system.

[0080] The exhaust aftertreatment system was examined.

[0081] The model system comprised a first SCR catalyst and a second 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.

[0082] The synthetic exhaust gas mixture consisted of 700 ppm NO as nitrogen oxide, 700 ppm ammonia as a nitrogen-containing reducing agent, and 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.

[0083] The pollutant components and ammonia were dosed from a gas cylinder and mixed into the carrier gas stream consisting of ambient air with 5 vol% water, which was heated to a respective test temperature T in the range of 250 to 500 °C.

[0084] The first SCR catalyst 14 used in the model system was a commercially available vanadium, tungsten, titanium oxide catalyst (also known as a "VWT" catalyst), as it is available under the name RFV from HUG Engineering AG.

[0085] The second SCR catalyst 16 in the model system used an iron-exchanged zeolite as its catalytic material. The zeolite exhibited a specific BET surface area of ​​more than 500 m². 2The iron content in the zeolite was more than 3 wt%, calculated as Fe₂Ü₃ and based on the exchanged zeolite. The catalytic material was applied to a cordierite honeycomb substrate with a cell density of 230 cpsi by means of a washcoat coating.

[0086] The composition of the exhaust gas stream was determined by sampling after the first SCR catalyst 16 and the second SCR catalyst 18 and, based on the initial composition of the synthetic exhaust gas mixture and the composition of the respective sample, the conversion rate of nitrogen oxide and methanol as well as the selectivity of selected components were calculated.

[0087] Tables 1 and 2 show the achieved conversion rates x of nitrogen oxides (NOx) and methanol in percent, as well as the selectivities S of the SCR catalysts with respect to formaldehyde and hydrogen cyanide in percent, each based on methanol and the total system of first SCR catalyst and second SCR catalyst as a function of the test temperature.

[0088] Table 1: Properties of the exhaust gas flow after treatment with the first SCR catalyst.

[0089] Figure 3 shows a diagram illustrating the methanol conversion rate in the first SCR catalyst as a function of the test temperature. As can be seen, even at a relatively low exhaust gas temperature of 250 °C, almost complete methanol degradation is observed, and at a test temperature of 300 °C, complete degradation is achieved. Accordingly, the presence of methanol need not be taken into account in the design of the second SCR catalyst 16 if the exhaust gas aftertreatment is carried out at an exhaust gas temperature of at least 300 °C.

[0090] Figure 4 shows a diagram illustrating the NOx conversion rate after the first SCR catalyst as a function of the test temperature. It can be seen that the conversion rate reaches a maximum of only 76% at a test temperature of 350 °C. The selectivities of hydrogen cyanide, which is formed as a nitrogen-containing compound in the first SCR catalyst, as shown in Table 1, indicate that the NOx conversion rate is essentially correlated with the selectivity of hydrogen cyanide, with an increase in the amount of hydrogen cyanide formed being associated with a decrease in the NOx conversion rate. In contrast, the selectivity of formaldehyde decreases with increasing test temperature.

[0091] Table 2: Properties of the exhaust gas stream after treatment with the second

[0092] SCR catalyst.

[0093] Figure 5 shows a diagram illustrating the conversion rate of NO after the second SCR catalyst as a function of the test temperature. The conversion rates of NO can be significantly increased by using the second SCR catalyst compared to the conversion rates in Table 1, achieving complete or nearly complete conversion at test temperatures in the range of 350 to 500 °C. At the same time, the selectivity of hydrogen cyanide decreases with increasing test temperature. This is primarily attributed to the fact that hydrogen cyanide, as a nitrogen-containing reducing agent, serves to convert the nitrogen oxide and is therefore degraded. In this context, the second SCR catalyst can thus also be referred to as an "HCN-SCR catalyst," i.e., a catalyst for selective catalytic reduction in which HCN is used as the nitrogen-containing reducing agent.

[0094] Methanol could no longer be detected in the exhaust gas stream after the second SCR catalyst at any of the test temperatures.

[0095] Overall, the tests show that with the sequence of first SCR catalyst and second SCR catalyst provided according to the invention, an exhaust gas stream such as can be expected in an internal combustion engine that is at least partially operated with alcohol fuel can be successfully cleaned of nitrogen oxides without large quantities of undesirable byproducts such as formaldehyde or hydrogen cyanide being produced after treatment with the exhaust gas aftertreatment system.

Claims

Patent claims 1. Exhaust aftertreatment system (10) in the exhaust stream of an internal combustion engine (12) operating at least partially on an alcohol fuel, wherein the exhaust stream is accessible to an exhaust gas flow from the internal combustion engine (12) containing nitrogen oxides and an alcohol and / or formaldehyde, wherein the exhaust aftertreatment system (10) comprises a metering device (14) for metering ammonia into the exhaust gas flow, a first SCR catalyst (16) and a second SCR catalyst (18), wherein the first SCR catalyst (16) is arranged in the exhaust stream downstream of the metering device (14) and is configured to produce a treated exhaust gas flow from the exhaust gas flow of the internal combustion engine (12) containing nitrogen oxides and at least one nitrogen-containing hydrocarbon compound, and wherein the second SCR catalyst (18) is arranged in the exhaust stream downstream of the first SCR catalyst (16) and is configured toto break down the nitrogen oxides contained in the generated treated exhaust gas stream by consuming the nitrogen-containing carbon-water compound contained in the generated treated exhaust gas stream.

2. Exhaust aftertreatment system (10) according to claim 1, wherein the nitrogen-containing hydrocarbon compound comprises or is hydrogen cyanide.

3. Exhaust aftertreatment system (10) according to claim 1 or 2, 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.

4. Exhaust aftertreatment system (10) according to one of the preceding claims, wherein the second SCR catalyst (18) comprises a zeolite as the catalytic material.

5. Exhaust aftertreatment system (10) according to claim 4, wherein the zeolite is a zeolite of a structural type selected from the group BEA, MFI, BETA, SAPO, Faujasite, ferrierite and chabazite or a combination of zeolites of these structure types.

6. Exhaust aftertreatment system (10) according to claim 4 or 5, wherein the zeolite is a metal ion exchanged zeolite, in particular an iron and / or copper exchanged zeolite.

7. Exhaust aftertreatment system (10) according to one of the preceding claims, wherein the second SCR catalyst is free of precious metals.

8. Exhaust aftertreatment system (10) according to one of the preceding claims, wherein the first SCR catalyst (16) and the second SCR catalyst (18) are arranged in a common catalyst housing (20).

9. 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 a nitrogen-containing hydrocarbon compound is generated in the first SCR catalyst (16) which is used as a nitrogen source for a selective catalytic reduction for the degradation of nitrogen oxides in the second SCR catalyst (18).

10. Use of an exhaust aftertreatment system (10) according to one of claims 1 to 8 in the exhaust stream of an internal combustion engine (12) that is at least partially powered by an alcohol fuel.

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