Methods for the aftertreatment of the exhaust gas of an internal combustion engine and exhaust gas aftertreatment system of an internal combustion engine

Abstract

Method for the aftertreatment of the exhaust gas of an internal combustion engine (1) burning a gaseous fuel, namely a gas engine or a dual-fuel engine which can be operated in a gas fuel operating mode, wherein the exhaust gas is passed over a CH4 oxidation catalyst (7) and over an SCR reduction catalyst (8), which are designed as separate catalysts, wherein the CH4 oxidation catalyst (7) for CH4 oxidation and thus as an active compound comprises at least one pyrochlore, wherein the at least one pyrochlore is selected from the following group: Sm2Zr2O7, Sm2Mo2O7, La2Ti2O7, La2Co x Sn 2-x O 7-δ , La2Co x Zr 2-x O 7-δ , Mn2Co x Zr 2-x O 7-δ , Pr2Ru2O7, ZrTiGd2O7, Pr2Co2O7 and Pr2Co x Zr 2-x O 7-δ , where 0 ≤ δ ≤ 2 and where 0 ≤ x ≤ 1, wherein NH3 or an NH3 precursor substance is introduced into the exhaust gas upstream of the CH4 oxidation catalyst (7) as a reducing agent for the SCR reduction catalyst (8).

Description

[0001] The invention relates to a method for the aftertreatment of the exhaust gas of an internal combustion engine burning a gaseous fuel, namely a gas engine or a dual-fuel engine operating in gas fuel mode. The invention further relates to an exhaust gas aftertreatment system of an internal combustion engine, namely a gas engine or dual-fuel engine.

[0002] Gas engines and dual-fuel engines operating in gas fuel mode burn a gaseous fuel, such as natural gas. In these types of internal combustion engines, incomplete combustion can lead to undesirable emissions of CH4 (methane). Since methane is a potent greenhouse gas, CH4 emissions from internal combustion engines burning gaseous fuels must be kept as low as possible.

[0003] It is already known in practice to route the exhaust gas leaving the cylinders of an internal combustion engine burning gaseous fuel through a CH4 oxidation catalyst in order to replace the CH4 within the catalyst. In these known internal combustion engines, platinum group metals, particularly platinum and / or palladium, are used as catalytically active compounds in the CH4 oxidation catalyst for CH4 oxidation. In these known internal combustion engines, the loading of the CH4 oxidation catalyst with a platinum group metal is typically more than 7 grams of platinum and / or palladium per liter of catalyst volume.

[0004] This results in high costs. Furthermore, the operating time of such CH4 oxidation catalysts known from practical applications is relatively short, since sulfur oxides that can enter the CH4 oxidation catalyst can deactivate the catalytically active platinum group compounds. Therefore, the potential for reducing CH4 emissions in conventional internal combustion engines is limited.

[0005] From DE 10 2015 001 495 A1, a method for operating an internal combustion engine in which a gaseous fuel is burned is known. Exhaust gas is passed over a CH4 oxidation catalyst. The CH4 oxidation catalyst preferably comprises cerium and / or cobalt and / or copper and / or iron as active components for CH4 oxidation, which are preferably incorporated into a zeolite matrix of the structures MOR, FER, PER, NFI, LTL, LAU, CHI or CHA.

[0006] From DE 10 2015 001 495 A1 it is further known that an SCR reduction catalyst is arranged downstream of the CH4 oxidation catalyst, such that exhaust gas is passed over a CH4 oxidation catalyst and over an SCR reduction catalyst. A reducing agent for the SCR reduction catalyst, namely NH3 or an NH3 precursor substance, is introduced into the exhaust gas downstream of the CH4 oxidation catalyst and upstream of the SCR reduction catalyst.

[0007] In order to sufficiently decompose the NH3 precursor substance into NH3 and / or to atomize or distribute the reducing agent sufficiently finely and evenly in the exhaust gas, a relatively large distance between the CH4 oxidation catalyst and the SCR reduction catalyst is required according to DE 10 2015 001 495 A1. This leads to significant disadvantages in terms of installation space.

[0008] US 2017 / 0 341 022 A1 reveals further state of the art.

[0009] Based on this, the invention aims to create a novel method for the aftertreatment of the exhaust gas of an internal combustion engine burning a gaseous fuel and a corresponding exhaust gas aftertreatment system for an internal combustion engine.

[0010] This problem is solved by a method according to claim 1.

[0011] The exhaust gas is passed through a CH4 oxidation catalyst and an SCR reduction catalyst, which are designed as separate catalysts.

[0012] The CH4 oxidation catalyst comprises at least one pyrochlorine as the active compound for CH4 oxidation, wherein the at least one pyrochlorine is selected from the following group: Sm2Zr2O7, Sm2Mo2O7, La2Ti2O7, La2Co x Sn2- x O 7-δ , La2Co x Zr 2-x O 7-δ , Mn2Co x Zr 2-x O 7-δ , Pr2Ru2O7, ZrTiGd2O7, Pr2Co2O7 and Pr2Co xZr 2-x O 7-δ , where 0 ≤ δ ≤ 2 and where 0 ≤ x ≤ 1.

[0013] As a reducing agent for the SCR reduction catalyst (8), NH3 or an NH3 precursor substance is introduced into the exhaust gas upstream of the CH4 oxidation catalyst.

[0014] The oxidation of CH4 proceeds via NO2 CH4+2NO2 → CO2 + 2H2O (Equation 1)

[0015] As a reducing agent for the SCR reduction catalyst, NH3 or an NH3 precursor substance is introduced into the exhaust gas upstream of the CH4 oxidation catalyst. The reduction of nitrogen oxides at the SCR catalyst occurs according to the following equations. 2NO + 2NH3 + 0.5O2 → N2 + 3H2O (Equation 2) NO + NO2 + 2NH3 → N2 + H2O (Equation 3)

[0016] It is therefore proposed that, in the exhaust aftertreatment of the exhaust gas from a gaseous fuel-burning internal combustion engine whose exhaust gas is passed through both a CH4 oxidation catalyst and an SCR reduction catalyst, the NH3 or the NH3 precursor substance should be introduced into the exhaust gas not downstream of the CH4 oxidation catalyst, but rather upstream of it. This eliminates the need for a long mixing section between the CH4 oxidation catalyst and the SCR reduction catalyst, thus achieving space savings. An exhaust aftertreatment system implementing this process can be designed to be significantly more compact.

[0017] According to an advantageous embodiment of the invention, the exhaust gas is first passed over the CH4 oxidation catalyst and then over a separate SCR reduction catalyst, with the NH3 or the NH3 precursor substance being introduced into the exhaust gas upstream of the CH4 oxidation catalyst. In this first embodiment, the CH4 oxidation catalyst and the SCR reduction catalyst are designed as separate catalysts. In this case, the CH4 oxidation catalyst is preferably arranged upstream of the SCR reduction catalyst, and the CH4 oxidation catalyst exhibits negligible NH3 oxidation activity by oxygen. In particular, it is provided that the CH4 oxidation catalyst does not contain any platinum group metal elements in order to prevent the oxidation of NH3 to N2, NO, NO2, or N2O by means of O2.

[0018] The exhaust aftertreatment system of the internal combustion engine according to the invention is defined in claim 8.

[0019] Preferred embodiments of the invention are described in the dependent claims and the following description. Exemplary embodiments of the invention are explained in more detail with reference to the drawing, without being limited thereto. The drawing shows: Fig. 1 a highly schematic view of an internal combustion engine to illustrate the exhaust gas aftertreatment method according to the invention, Fig. 2 a highly schematic view of a non-further internal combustion engine.

[0020] The invention relates to an exhaust aftertreatment system for an internal combustion engine in which a gaseous fuel is burned. Furthermore, the invention relates to a method for aftertreating the exhaust gas of the internal combustion engine burning the gaseous fuel.

[0021] Fig. Figure 1 shows a highly schematic diagram of an internal combustion engine 1. The internal combustion engine 1 has at least one cylinder block 2 with cylinders 3. A gaseous fuel, such as natural gas, is burned in the cylinders 3 of the internal combustion engine 1. The internal combustion engine 1 is either a gas engine or a dual-fuel engine that can be operated in a gas fuel mode.

[0022] Fig. Figure 1 visualizes with a feed 4 that gaseous fuel, in particular a mixture of charge air and gas, is supplied to the cylinders 3 of the internal combustion engine. Figure 5 visualizes that exhaust gas produced during combustion is carried away from the cylinders 3 and routed through an exhaust aftertreatment system 6 of the internal combustion engine 1.

[0023] The exhaust aftertreatment system 6 includes a CH4 oxidation catalyst 7. The exhaust aftertreatment system 6 also includes an SCR reduction catalyst 8.

[0024] In the exemplary embodiment of the Fig. In the exemplary embodiment of the CH4 oxidation catalyst 7 and the SCR reduction catalyst 8, the CH4 oxidation catalyst 7 and the SCR reduction catalyst 8 are designed as separate catalysts. Fig. 1 is arranged downstream of the CH4 oxidation catalyst 7. Exhaust gas leaving the CH4 oxidation catalyst 7 is routed via the SCR reduction catalyst 8 to reduce the nitrogen oxide content in the exhaust gas.

[0025] In the direction of exhaust gas flow, upstream of the CH4 oxidation catalyst 7, an injection device 9 for introducing NH3 or NH3 precursor substance into the exhaust gas is arranged in order to effectively break down or reduce nitrogen oxides in the exhaust gas in the area of ​​the SCR reduction catalyst 8.

[0026] Because the injection device 9 introduces the NH3 or the NH3 precursor substance into the exhaust gas upstream of the CH4 oxidation catalyst 7, no mixing section is required between the CH4 oxidation catalyst 7 and the SCR reduction catalyst 8. These can then be arranged with minimal spacing, preferably directly one behind the other, and particularly advantageously in a common housing 11.

[0027] In the exemplary embodiment of the Fig. 1 The CH4 oxidation catalyst 7 for CH4 oxidation contains at least one pyrochlore as a catalytically active compound and optionally at least one zeolite.

[0028] At least one pyrochlore is selected from the following group: Sm2Zr2O7 Sm2Mo2O7 La2Ti2O7 La2Co x Sn 2-x O 7-δ La2Co x Zr 2-x O 7-δ' Mn2Co x Zr2-x O 7-δ Pr2Ru2O7 ZrTiGd2O7 Pr2Co2O7 Pr2Co x Zr 2-x O 7-δ where 0 ≤ δ ≤ 2 and 0 ≤ x ≤ 1

[0029] The CH4 oxidation catalyst 7 may contain one, more, or all of the above pyrochloros.

[0030] Then, if CH4 oxidation catalyst 7 additionally includes a zeolite for CH4 oxidation, the at least one zeolite can be selected from the following group: MOR Zeolites FER Zeolites PER Zeolites NFI Zeolites LTL Zeolites LAU Zeolites CHI Zeolites Beta zeolites, especially BEA zeolites CHA zeolites FAU Zeolites LSX Zeolites

[0031] The CH4 oxidation catalyst 7 can contain one, more, or all of the above zeolites.

[0032] Preferably, elements of pyrochlore and / or zeolite are substituted with rare earth metals and / or with iron and / or with cobalt and / or with copper and / or with manganese.

[0033] Furthermore, the pyrochlore and / or the zeolite can be enriched with Rh, Ru, Ir, Os, Bi, Cn, Ed by incorporating these elements into the pyrochlore and / or the zeolite.

[0034] The CH4 oxidation catalyst 7 for CH4 oxidation can additionally contain at least one of the following compounds: CoO-Yb2O3 NiO LiNiLaO LiCoLaO LiFeLaO NaNiLaO KNiLaO LiNiCeO LiNiYO LiNiSmOLaNiO Cu-Co-O Cu-Co-O La2O3 SrO HfO2 YO3 ZrO2 MnO CuO TiO2 Fe2O3 MoO3

[0035] The proportion of platinum and palladium in the catalytically active components used for the decomposition of CH4 is each less than 1%, preferably less than 0.5%, most preferably less than 500ppm.

[0036] After advantageous further processing, the proportion of the sum of platinum and palladium in the active components used for CH4 decomposition is less than 1%, advantageously less than 0.5%, and most advantageously less than 1000ppm.

[0037] The CH4 oxidation catalyst free of platinum and palladium is particularly preferred.

[0038] Al2O3, TiO2, SiO2 and WO3, either individually or in combination, are preferably used as carriers for the above-mentioned catalytically active components.

[0039] In Fig. 1 The exhaust gas to be passed over the CH4 oxidation catalyst 7 has an NO2 content, based on the total content of nitrogen oxides in the exhaust gas, of at least 15%, preferably at least 30%, and particularly preferably at least 50%. Fig. 1 The exhaust aftertreatment system 6 comprises an NO oxidation catalyst 10 upstream of the CH4 oxidation catalyst 7 in order to first pass the exhaust gas leaving the cylinders 3 over an NO oxidation catalyst 10 and, with the aid of the NO oxidation catalyst 10, to adjust the proportion of NO2 in the exhaust gas, based on the total proportion of nitrogen oxides in the exhaust gas, to at least 15%, preferably to at least 30%, and particularly preferably to at least 50%. Alternatively or additionally to the NO oxidation catalyst 10, the NO2 proportion in the exhaust gas can also be increased via a combustion parameter of the internal combustion engine 1 that burns the gaseous fuel.

[0040] It is also possible that, if, as in the example above, the Fig. As shown in Figure 1, the CH4 oxidation catalyst 7 and the SCR reduction catalyst 8 are designed as separate catalysts, in contrast to Fig. 1. The CH4 oxidation catalyst 7 is arranged downstream of the SCR reduction catalyst 8. In this case, the exhaust gas from the internal combustion engine 1 is first passed over the SCR reduction catalyst 8 and then over a separate CH4 oxidation catalyst 7, whereby the exhaust gas to be passed over the SCR reduction catalyst 8 then has an NO2 content, based on a total nitrogen oxide content, of at least 55%, preferably at least 60%, and particularly preferably at least 70%. Furthermore, it has proven advantageous in this case if the NO2 content is at least 0.55 times the NO content. x - Turnover, advantageously at least 0.65 times the NO x - Sales, extremely advantageous at least 0.75 times the NOx - The conversion rate is high because this ensures that sufficient NO2 is still available for methane oxidation at the CH4 oxidation catalyst downstream of the SCR catalyst. For further details, please refer to the explanations above. Fig. 1 will be referred.

[0041] While in the exemplary embodiment of the Fig. Figure 1 shows that the CH4 oxidation catalyst 7 and the SCR reduction catalyst 8 are designed as separate catalysts. Fig. 2. A non-inventive embodiment in which the CH4 oxidation catalyst 7 and the SCR reduction catalyst 8 are provided by a combined or integrated CH4 oxidation and SCR reduction catalyst 12. This allows the exhaust aftertreatment system 6 to be designed even more compactly with minimal installation space.

[0042] In Fig. 2. It is provided that the proportion of NO2 in the exhaust gas, relative to the total proportion of nitrogen oxides in the exhaust gas upstream of the combined CH4 oxidation and SCR reduction catalyst 12, is adjusted to at least 25%, preferably at least 35%, and particularly preferably at least 55%. This can be achieved, as described in Fig. Figure 2 shows that this can again be achieved via a NO oxidation catalyst 10, or alternatively or additionally by adjusting a combustion parameter of the internal combustion engine burning the gaseous fuel. Furthermore, it has proven advantageous if the NO2 content is at least 0.1 times the NO x - Turnover, preferably at least 0.2 times the NO x - Turnover, extremely advantageous at least 0.3 times the NO x - Sales amount to.

[0043] The above-mentioned proportion of NO2 in the exhaust gas prevents the SCR reduction reaction from consuming all the NO2, leaving insufficient NO2 available for CH4 oxidation. Therefore, with the above-mentioned NO2 proportion in the exhaust gas, effective CH4 oxidation and NO reduction can be achieved simultaneously. X -Reduction will be ensured.

[0044] Such a combined or integrated CH4 oxidation and SCR reduction catalyst 12 has for CH4 oxidation and NO X -Reduction as a catalytically active compound is a beta-zeolite, in particular a beta-polymorph A-type (BEA) zeolite and / or a chabazite (CHA) zeolite and / or a pentasil zeolite (MFI / ZSM-5) and / or a mordernite (MOR) zeolite and / or a ferrierite (.FER) zeolite. These zeolites are suitable for both CH4 oxidation and simultaneous NOx reduction.

[0045] A particularly preferred CH4 oxidation and SCR reduction in the combined CH4 oxidation and SCR reduction catalyst 12 is possible when elements of the respective beta-zeolite, in particular BEA-zeolite, and / or CHA-zeolite and / or MFI-zeolite and / or MOR-zeolite and / or FER-zeolite are exchanged or substituted with rare earth metals and / or iron and / or cobalt and / or copper and / or manganese. This combined catalyst is further characterized by the fact that, at temperatures below 450°C, the nitrogen selectivity of the SCR reaction according to equations 2 and 3 is at least 70%, preferably at least 80%, and most preferably at least 85%. This means that the oxidation of ammonia, which does not proceed according to the equations mentioned above but via oxygen, can be neglected.The total amount of oxidized ammonia according to the following equations is less than 30%, preferably less than 20%, most preferably less than 15%, based on the amount of ammonia supplied upstream of the combination catalyst. 4NH3 + 3O2 → 2N2 + 6H2O (Equation 4) 4NH3 + 5O2 → 4NO + 6H2O (Equation 5) 2NH3 + 2O2 → N2O + 3H2O (Equation 6)

[0046] The combined catalyst 12 for CH4 oxidation and NO x - The reduction may also contain at least one of the following compounds: V2O5 V2O4 CoO-Yb2O3 NiO LiNiLaO LiCoLaO LiFeLaO NaNiLaO KNiLaO LiNiCeO LiNiYO LiNiSmOLaNiO Cu-Co-O Cu-Co-O La2O3 SrO HfO2 YO3 ZrO2 MnO CuO TiO2 Fe2O3 MoO3

[0047] The internal combustion engine 1 can be either a gas engine or a dual-fuel engine that can be operated in a gas fuel operating mode.

[0048] Then, when the arrangement of Fig. 2. When used in a dual-fuel engine, in which both gaseous and liquid fuel are burned in a gaseous fuel operating mode, it can be designed to reduce the amount of NH3 or NH3 precursor substance introduced into the exhaust gas as the proportion of gaseous fuel increases and thus the proportion of liquid fuel burned decreases. This reduces the NO that is converted at the catalyst with SCR activity. x—and thus reduces the amount of NO2 converted. This ensures that a sufficient amount of NO2 is available in the combined CH4 oxidation and SCR reduction catalyst 12 to effectively oxidize the CH4. However, if a higher proportion of liquid fuel is burned with a decreasing proportion of gaseous fuel in the dual-fuel engine, then fewer CH4 emissions are produced, and the amount of NO2 required for CH4 oxidation decreases, resulting in more NH3 or more of the NH3 precursor to NO. X -reduction is introduced into the exhaust gas, so that the converted NO x - Quantity increases. Reference symbol list 1 internal combustion engine 2-cylinder block 3 cylinders 4 Feed 5 Discharge 6 Exhaust aftertreatment system 7 CH4 oxidation catalyst 8 SCR catalyst 9 Insertion device 10 NO oxidation catalyst 11 cases 12 CH4 oxidation and SCR reduction catalyst

Claims

[1] Method for the aftertreatment of the exhaust gas of an internal combustion engine (1) burning a gaseous fuel, namely a gas engine or a dual-fuel engine which can be operated in a gas fuel operating mode, wherein the exhaust gas is passed over a CH4 oxidation catalyst (7) and over an SCR reduction catalyst (8) which are designed as separate catalysts, wherein the CH4 oxidation catalyst (7) for CH4 oxidation and thus as an active compound comprises at least one pyrochlore, wherein the at least one pyrochlore is selected from the following group: Sm2Zr2O7, Sm2Mo2O7, La2Ti2O7, La2Co x Sn 2-x O 7-δ , La2Co x Zr 2-x O 7-δ , Mn2Co x Zr 2-x O 7-δ , Pr2Ru2O7, ZrTiGd2O7, Pr2Co2O7 and Pr2Co x Zr 2-x O 7-δ , where 0 ≤ δ ≤ 2 and where 0 ≤ x ≤ 1, wherein NH3 or an NH3 precursor substance is introduced into the exhaust gas upstream of the CH4 oxidation catalyst (7) as a reducing agent for the SCR reduction catalyst (8). [2] Method according to claim 1, characterized by , that the exhaust gas of the internal combustion engine (1) is first passed over the CH4 oxidation catalyst (7) and then over the separate SCR reduction catalyst (8). [3] Method according to claim 2, characterized by , that the exhaust gas to be passed through the CH4 oxidation catalyst (7) has a NO2 content, based on a total content of nitrogen oxides, of at least 15%, preferably at least 30%, particularly preferably at least 50%. [4] Method according to claim 1, characterized by , that the exhaust gas of the internal combustion engine (1) is first passed over the SCR reduction catalyst (8) and then over the separate CH4 oxidation catalyst (7). [5] Method according to claim 4, characterized by , that the exhaust gas to be routed via the SCR reduction catalyst (8) has an NO2 content, based on a total content of nitrogen oxides, of at least 55%, preferably at least 60%, particularly preferably at least 70%. [6] Method according to any one of claims 1 to 5, characterized by , that elements of pyrochlore are exchanged or substituted with rare earth metals and / or iron and / or cobalt and / or copper and / or manganese. [7] Method according to claim 3 or 5, characterized by , that the NO2 content in the exhaust gas is adjusted via at least one combustion parameter of the internal combustion engine (1) burning the gaseous fuel, and / or the NO2 content in the exhaust gas is adjusted via a NO oxidation catalyst (10). [8] Exhaust aftertreatment system (6) of an internal combustion engine (1), namely a gas engine or a dual-fuel engine with cylinders (3) in which a gaseous fuel can be combusted wherein the exhaust aftertreatment system (6) comprises a CH4 oxidation catalyst (7) and an SCR reduction catalyst (8) which are designed as separate catalysts, wherein the CH4 oxidation catalyst (7) for CH4 oxidation and thus as an active compound comprises at least one pyrochlore, wherein the at least one pyrochlore is selected from the following group: Sm2Zr2O7, Sm2Mo2O7, La2Ti2O7, La2Co x Sn 2-x O 7-δ , La2Co x Zr 2-x O 7-δ , Mn2Co x Zr 2-x O 7-δ , Pr2Ru2O7, ZrTiGd2O7, Pr2Co2O7 and Pr2Co x Zr 2-x O 7-δ , where 0 ≤ δ ≤ 2 and where 0 ≤ x ≤ 1, wherein the exhaust aftertreatment system (6) has an injection device (9) arranged upstream of the CH4 oxidation catalyst (7) for introducing NH3 or an NH3 precursor substance into the exhaust gas as a reducing agent for the SCR reduction catalyst (8). [9] Exhaust aftertreatment system according to claim 8, characterized by , that the CH4 oxidation catalyst (7) is arranged upstream of the SCR reduction catalyst (8). [10] Exhaust aftertreatment system according to claim 8, characterized by , that the CH4 oxidation catalyst (7) is located downstream of the SCR reduction catalyst (8).

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