Device for the aftertreatment of exhaust gases from an internal combustion engine

Abstract

Device (1) for the aftertreatment of exhaust gases from an internal combustion engine (2), in particular for NOx reduction in NOx-containing exhaust gases, comprising an exhaust gas line (3), an injection device (4) for injecting an additive (5) into the exhaust gas line (3), and a catalyst (6) arranged downstream of the injection device (4) in a section (3a) of the exhaust gas line (3) in the direction of flow (x) of the exhaust gases, characterized in that at least two sections (3c, 3d) of the exhaust gas line (3) open into the section (3b) of the exhaust gas line (3) containing the catalyst (6) in such a way that the introduced exhaust gas moves at least approximately in the form of a spiral (7) along an inner wall (8) of the exhaust gas line (3) in the direction of the catalyst (6), and that the injection device (4) is arranged on the section (3a) of the exhaust gas line (3) containing the catalyst (6) in such a way thatthat the additive (5) injected by the injection device (4) is injected downstream of the junction of the at least two sections (3c, 3d) and substantially centrally into the spiral (7) formed by the exhaust gas in the exhaust pipe (3), that the injection device (4) projects into the section (3a) of the exhaust pipe (3) containing the catalyst (6) in such a way that an annular space (9) is formed between the inner wall (8) of the section (3a) of the exhaust pipe (3) containing the catalyst (6) and the injection device (4), and that an outlet opening (4a) of the injection device (4) opens downstream of the annular space (9) into the section (3a) of the exhaust pipe (3) containing the catalyst (6).

Description

The invention relates to a device for the aftertreatment of exhaust gases from an internal combustion engine, in particular for NOx reduction in NOx-containing exhaust gases, comprising an exhaust gas line, an injection device for injecting an additive into the exhaust gas line, and a catalyst arranged downstream of the injection device in a section of the exhaust gas line in the direction of flow of the exhaust gases. Due to increasingly stringent emissions regulations, it is already largely necessary to treat the exhaust gases of combustion engine vehicles to remove nitrogen oxides. The term nitrogen oxides encompasses various oxidation states of nitrogen, such as nitric oxide (NO) and nitrogen dioxide (NO2). These gases are highly toxic to humans. Along with sulfur dioxide, nitrogen oxides contribute to acid rain, as the reaction of nitrogen dioxide with water produces nitric acid and nitrous acid. This acid rain is partly responsible for forest dieback caused by the leaching of nutrients from the soil and for damage to buildings constructed with acid-sensitive materials. Furthermore, nitrogen oxides play a key role in the formation of ozone in the atmosphere.Ozone can cause irritation and inflammation of the respiratory tract in humans and acts as a cell toxin in plants. In the stratosphere, however, ozone is destroyed by nitric oxide, which contributes to the formation of the ozone hole. Nitrogen oxide emissions can be of both natural origin, for example from microbial processes and wildfires, and anthropogenic origin, such as from power plants, industry, small combustion plants in households, and traffic. Catalytic converters have therefore been used for some time to treat exhaust gases. A method for removing nitrogen oxides from an exhaust gas stream is described in EP 0 666 099 B1. The catalyst used in this process adsorbs the nitrogen oxides present in the exhaust gas, after which a gas containing a specific concentration of a reducing agent is supplied to the catalyst at predetermined intervals and for specific durations. However, such storage catalysts, which use basic components such as lithium oxide, potassium oxide, sodium oxide, barium oxide, or similar oxides, require relatively complex control and usually have a high regeneration requirement. In these NOx storage catalysts, the predominantly emitted NO is oxidized to NO2 on a platinum-based catalyst, which is then adsorbed onto special storage media, such as BaCO3. When the storage capacity of this catalyst is exhausted, an engine-induced regeneration of the catalyst is initiated, during which the introduced nitrogen oxides are converted into nitrogen. Another disadvantage of the known NOx storage catalysts, which operate according to the so-called NSK process, is the risk of poisoning the NOx sorbents by the sulfur oxides SO2 and SO3 contained in the exhaust gas. To circumvent this problem, complex engine management strategies are usually required. From EP 0 960 649 B1, an exhaust gas purification catalyst is known in which the materials used include cerium oxide and / or zirconium dioxide, which form mixed oxides that serve to remove saturated hydrocarbons from the exhaust gas. Ammonia is used as a reducing agent for the nitrogen oxides contained in the exhaust gas. However, the active component V₂O₅, frequently used in such SCR catalysts, is toxicologically problematic and can also melt or evaporate at very high exhaust gas temperatures. Another limiting factor regarding the activity of the SCR process at low temperatures lies in the on-board production of the reducing agent NH₃ from urea, which is currently only technically feasible above approximately 220 °C. EP 0 763 380 A1 describes shell catalysts which consist of a core and at least one outer shell or of a support, at least one inner and at least one outer shell, wherein the outer shells contain oxides of certain elements. In most cases, a disadvantage of the known solutions for removing NOx from oxygen-rich exhaust gases is that the nitrogen oxides are only converted above 200°C, and thus at temperatures such as those that occur particularly in diesel or hydrogen engines. Because the continuous optimization of combustion engine efficiency constantly reduces exhaust gas temperature, existing solutions face a significant problem regarding their effectiveness. For example, in modern diesel-powered combustion engines for passenger cars, the exhaust gas temperature is below 150°C for approximately 60% of the time and below 200°C for approximately 75% of the time during the relevant EU certification cycle. In NOx catalyst technology, activity at low temperatures is limited by the required generation of NO2 in the exhaust stream, as platinum or palladium-based oxidation catalysts only show sufficient activity from about 220 °C. The SCR and NSK processes therefore do not fulfill the central requirement of selectively converting NOx to nitrogen and water at low temperatures. Another catalyst, as well as a device and a method for selective NOx reduction in NOx-containing exhaust gases, are known from DE 10 2010 040 808 A1. Hydrogen is used as a reducing agent, resulting in improved low-temperature activity of the catalyst. German patent DE 11 2006 003 078 T5 discloses an emission control system for reducing nitrogen oxides in diesel engine exhaust gases under lean combustion conditions, in which, for example, palladium is used as the active component and γ-aluminum oxide as the support layer. This system, or rather the resulting catalyst, also does not exhibit sufficient activity at low temperatures. In this case, hydrogen and carbon monoxide are used as reducing agents. WO 2007 / 020035 A1 also discloses such a catalyst, a manufacturing process for the same, as well as a device and a method for selective NOx reduction in NOx-containing exhaust gases. However, the catalysts described therein do not achieve a sufficiently selective conversion of nitrogen oxides for practical purposes. In particular, the process described in WO 2007 / 020035 A1 produces a relatively large proportion of nitrous oxide, which is very disadvantageous since nitrous oxide contributes more than 300 times more to the greenhouse effect than, for example, CO2. Another disadvantage of the catalyst described therein is that only the tetragonal crystal modification can be used for the zirconium oxide support material, which requires a very high level of manufacturing effort. From EP 1 475 149 A1, another catalyst for the reduction of NOx to N2 with hydrogen under O2-rich conditions is known. This well-known catalyst is based on platinum, which is distributed in an amount of between 0.1 and 2 percent by weight on a support material consisting of magnesium or cerium oxide or a precursor thereof. While this catalyst already achieves quite good results in NOx reduction, it too will reach its limits when future emissions limits are met. German patent DE 102 16 748 A1 describes modified catalysts for the selective hydrogenation of aromatic hydrocarbons with bulky or branched substituents. However, NOx reduction in NOx-containing exhaust gases is not feasible with these catalysts. German patent DE 44 36 890 A1 describes a process for the simultaneous reduction of hydrocarbons, carbon monoxide, and nitrogen oxides contained in the exhaust gas of an internal combustion engine. The catalyst used in this process has an aluminum silicate as a high-surface-area support material. The injection of urea is now established in cars, buses, and commercial vehicles. Due to stricter emission limits, it is expected that this technology will also be used in other means of transport, such as tractors, ships, or other vehicles or equipment that use diesel-powered combustion engines or lean-burn gasoline engines, such as construction machinery or similar equipment. In known devices, urea is injected into the exhaust pipe. Current injection systems are designed to ensure a minimum time interval of approximately 0.1 seconds between the injection of the urea solution and its impact on the catalyst. This ensures sufficient vaporization of the urea solution and its conversion to NH3 and CO2. Depending on the manufacturer, the metering systems used for injection are based on single- or dual-fluid nozzles, with the latter operating with air assistance. In both cases, ideal droplet sizes of less than 0.1 mm are achieved. With the aid of on-board sensors and electronics, the amount of reducing agent required is adjusted to the raw nitrogen oxide emissions. Depending on the application, injection only occurs at exhaust gas temperatures above 200 to 230 °C. Due to insufficient release of the actual reducing agent NH3, injecting urea below this temperature range is currently not practical, as the slow evaporation and subsequent condensation of the urea on the inner wall of the exhaust pipe can lead to deposit formation. Since these deposits, which can result from conversion products of the injected urea, reduce the free cross-section of the exhaust pipes or at least prevent the generation of NH3 and can clog and thus deactivate the catalyst, the urea is only injected into the exhaust pipe above a certain temperature, as mentioned above.As a result, nitrogen oxides cannot be converted at lower exhaust gas temperatures, which makes compliance with emissions regulations considerably more difficult. According to the device described in DE 10 2014 214 093 A1, the additive is fed radially or tangentially to a mixing cylinder located upstream of the catalyst. An air guide element is located inside the mixing cylinder, creating an annular flow within the cylinder. However, this solution merely shifts the problem of deposits described above to the air guide element and therefore does not eliminate it. From DE 10 2016 101 191 A1, it is known to arrange an exhaust gas line tangentially to the housing of an exhaust gas purification device. The additive is injected into the exhaust gas line upstream of the inlet to the exhaust gas purification device. This results in the same problem as described above, namely that the additive evaporates at excessively low temperatures on the inner wall of the exhaust gas line, leading to deposits on the same wall. Despite some innovative concepts suitable for specific situations or engines, there is no universal solution for the trouble-free dosing of the additive for all diesel engines or lean-burn gasoline and hydrogen engines. This is primarily because different engine configurations and performance profiles necessitate covering different exhaust gas composition and temperature maps, meaning that the additive injection system configurations are usually adapted to the specific conditions. As an alternative solution, the substitution of urea with other NH3-releasing substances, such as guanidinium salts or cobalt hexamine chloride, for on-board ammonia production was discussed. However, the substitution or even abandonment of the recently established infrastructure for supplying urea to auto repair shops and gas stations is generally considered very problematic. A device of this type is known from US 2011 / 0308234 A1. DE 10 2008 048 428 A1 describes a device and a method for cleaning an exhaust gas stream of an internal combustion engine, which has a coating of titanium dioxide. It is therefore an object of the present invention to create a device for the aftertreatment of exhaust gases from an internal combustion engine, with which deposits of the injected additive on the exhaust pipe can be prevented. According to the invention, this problem is solved by the features mentioned in claim 1. According to the invention, two sections of the exhaust pipe open essentially tangentially into the section of the exhaust pipe containing the catalyst. This causes the exhaust gases to flow along the inner wall of the exhaust pipe in a spiral pattern towards the catalyst. In other words, the tangential introduction of the exhaust gas into the exhaust pipe according to the invention creates a swirl of the exhaust gases along the inner wall of the exhaust pipe. This prevents recirculation of the exhaust gas towards the coaxially located injection system. The coaxial injection of the additive results in a symmetrical spray pattern. Due to the spiral movement, which the additive also undergoes, it has sufficient time to transition into the gas phase and convert to NH3 on its way to the catalyst. The device according to the invention thus prevents the formation of deposits on the inner wall of the exhaust pipe, enabling the additive to be injected into the exhaust pipe with greater efficiency even at lower exhaust gas temperatures. Ultimately, this allows nitrogen oxides to be converted at lower exhaust gas temperatures, which contributes to a significant reduction in nitrogen oxide emissions from combustion engines by effectively reducing the nitrogen oxides emitted by the combustion engine at the catalyst using the additive. This significantly shifts the operating range of the known and already practically used SCR process to lower exhaust gas temperatures, thereby substantially improving its efficiency. Because the injection device projects into the section of the exhaust pipe containing the catalyst in such a way that an annular space is created between the wall of the section of the exhaust pipe containing the catalyst and the injection device, and because an outlet opening of the injection device opens downstream of the annular space into the section of the exhaust pipe containing the catalyst, the exhaust gases introduced into the section of the exhaust pipe containing the catalyst from the at least two sections are held even more effectively against the inner wall of the exhaust pipe, which supports the formation of the swirl or spiral of the exhaust gases described above. In a highly advantageous embodiment of the invention, the at least two sections can originate from a section of the exhaust pipe connected to the combustion engine. This significantly simplifies the design of the exhaust pipe, allowing it to be used without difficulty in a wide variety of vehicles and machinery. Such an exhaust pipe thus divides into the two sections or branches out and then merges back into a single, one-piece exhaust pipe containing the catalyst. In order to support the flow of exhaust gases towards the catalyst and thus towards the outlet of the exhaust pipe, it may also be provided that the at least two sections of the exhaust pipe are inclined in the direction of exhaust gas flow in their area which enters the section of the exhaust pipe containing the catalyst. A further advantageous embodiment of the invention can consist in coating at least the portion of the inner wall of the exhaust pipe adjacent to the injection device in the direction of exhaust gas flow with a titanium dioxide-based catalyst material. In this way, any droplets of the additive that may nevertheless reach the inner wall of the exhaust pipe can contribute to the conversion of nitrogen oxides. Furthermore, it can be provided that at least the portion of the exhaust gas line adjacent to the injection unit in the direction of exhaust gas flow can be heated by a heating device. Heating the exhaust gas line ensures a safe and reliable conversion of the additive into the desired substance, in this case, a conversion of urea and nitrogen oxides into NH3 and CO2. This allows the additive to be injected into the exhaust gas line and the conversion of the nitrogen oxides to be achieved even at exhaust gas temperatures that are inherently too low or that are typical of known processes. The following are exemplary embodiments of the invention illustrated in principle with reference to the drawing. It shows: Fig. 1 an embodiment of the device according to the invention; Fig. 2 an enlarged section through the device from Fig. 1, in which the flow of the exhaust gases within the exhaust pipe is shown; Fig. 3 a section along line III-III from Fig. 2; Fig. 4 an embodiment of the device not belonging to the invention; and Fig. 5 an embodiment of the device not belonging to the invention. Fig. 1 shows a device 1 for the aftertreatment of exhaust gases from an internal combustion engine 2. The internal combustion engine 2 is in particular a diesel engine or a lean-burn gasoline or hydrogen engine. The device 1 is therefore used in particular for NOx reduction in NOx-containing exhaust gases emitted by the internal combustion engine 2. The device 1 comprises an exhaust pipe 3, an injection device 4 for injecting an additive 5, in particular a urea solution, into the exhaust pipe 3, and a catalyst 6, which is arranged downstream of the injection device 4 in a section 3a of the exhaust pipe 3 in the direction of exhaust gas flow designated by "x". In the direction of flow x after the catalyst 6, the exhaust gases can exit the exhaust pipe 3. The exhaust pipe 3 further comprises a section 3b connected to the internal combustion engine 2, which also includes an exhaust manifold (not specified in detail). Of course, the section 3b of the exhaust pipe 3 extending from the internal combustion engine 2 could also be configured differently. Section 3b of the exhaust pipe 3, connected to the internal combustion engine 2, divides into two sections 3c and 3d, which together form the exhaust pipe 3 over a certain length and thus connect section 3b, connected to the internal combustion engine 2, with section 3a, in which the catalyst 6 is located. Sections 3c and 3d therefore originate from section 3b of the exhaust pipe 3 and, as shown in Fig. 2, terminate in section 3a of the same. As can be seen in particular from the illustration in Fig. 3, the two sections 3c and 3d of the exhaust pipe 3 open at least approximately tangentially into the section 3a of the exhaust pipe 3 which contains the catalyst 6. This ensures that the exhaust gas introduced into the exhaust pipe 3 via sections 3c and 3d moves at least approximately in the form of a spiral or swirl, designated by reference numeral 7 in Fig. 2, along an inner wall 8 of the exhaust pipe 3, or in this case of section 3a, in the direction of the catalyst 6, i.e., in the flow direction designated by “x”. The spiral motion of the exhaust gases is also generated by the fact that the two sections 3c and 3d of the exhaust pipe 3 are inclined in the direction x of the exhaust gas flow in the area where they enter section 3a of the exhaust pipe 3, which contains the catalyst 6. This creates a superimposed, direction x-directed motion on the circular motion of the exhaust gases resulting from the tangential entry of sections 3c and 3d into section 3a, leading to the spiral 7. However, this inclination of sections 3c and 3d could potentially be omitted, since the exhaust gas flow, due to the pressure gradient and the resulting velocity of the exhaust gases, is already directed towards the catalyst 6 or towards the outlet of the exhaust pipe 3 located downstream of the catalyst 6 in the direction x of flow. Figure 1 further shows that the injection device 4, which serves to inject the additive 5, is arranged on section 3a of the exhaust pipe 3, which contains the catalyst 6, such that the additive 5 injected by the injection device 4 is injected downstream of the junction of the at least two sections 3c, 3d and substantially centrally into the spiral 7 formed by the exhaust gas in the exhaust pipe 3. In other words, the additive 5 is injected into the free space created by the exhaust gas moving spirally along the inner wall 8 of the exhaust pipe 3, as described above. The injection device 4 is arranged coaxially with section 3a of the exhaust pipe 3, which contains the catalyst 6, resulting in coaxial injection of the additive 5 into section 3a and thus a symmetrical spray pattern. The injection device 4 projects into section 3a of the exhaust pipe 3, creating an annular space 9 between the inner wall 8 of section 3a and the injection device 4. This annular space 9 facilitates the formation of the exhaust gas spiral 7, as the exhaust gas can only flow in the region of the inner wall 8. The injection device 4 thus forms the end of section 3a of the exhaust pipe 3, facing the combustion engine 2. The injection device 4 has an outlet opening 4a that opens downstream of the annular space 9 into the section 3a of the exhaust pipe 3 containing the catalyst 6. As can be seen in Fig. 2, this helps to keep the additive 5 within the free space created by the spiral 7 formed by the exhaust gas moving along the inner wall 8 of the exhaust pipe 3, thus preventing, or at least largely preventing, the additive 5 from reaching the inner wall 8 of the exhaust pipe 3. Due to the spiral movement of the exhaust gases described above, which the additive 5 also follows as it is carried along by the exhaust gases, it has sufficient time to transition into the gas phase on its way to the catalyst 6 and to react to form NH3.The spiral movement of the additive 5 prevents it from settling on the inner wall 8 of the exhaust pipe 3, even if individual droplets of the additive 5 should come into contact with the inner wall 8. Figure 4 shows an embodiment of the device 1 not belonging to the invention. In this embodiment, at least the portion of the inner wall 8 of the exhaust gas line 3 adjoining the injection device 4 in the direction x of exhaust gas flow, in this case section 3a thereof, is coated with a titanium dioxide-based catalyst material 10. The catalyst material 10 decomposes the injected additive 5, in particular the urea solution. This ensures that any droplets of the additive 5 that may reach the inner wall 8 of the exhaust gas line 3 are used to contribute to the conversion of nitrogen oxides within the exhaust gas. The titanium dioxide forming the catalyst material 10 is preferably in the anatase modification. Preferably, the specific surface area of ​​the catalyst material 10 is 20 to 125 m² / g. The embodiment of device 1 shown in Fig. 4, which does not belong to the invention, can be combined with that shown in Figs. 1, 2 and 3. However, it is also possible to use the embodiment of Fig. 4 without the specific embodiment shown in Figs. 1, 2 and 3. Fig. 5 shows another embodiment of the device 1, not belonging to the invention. In this embodiment, at least the part of the exhaust gas line 3 adjoining the injection device 4 in the direction x of exhaust gas flow can be heated by means of a heating device 11. In the present case, the heating device 11 is designed as a heating mat enclosing the exhaust gas line 3. The heating device 11, or in this case the heating mat, can be electrically heated, for example. The heating device 11 can be powered, for example, by a battery of a vehicle, machine, or device equipped with the device 1. The embodiment of device 1 shown in Fig. 5, which does not belong to the invention, can be combined with the embodiment shown in Figs. 1, 2 and 3 and / or 4. However, it is also possible to use the embodiment of Fig. 5 without the specific embodiment shown in Figs. 1-4.

Claims

Device (1) for the aftertreatment of exhaust gases from an internal combustion engine (2), in particular for NOx reduction in NOx-containing exhaust gases, comprising an exhaust gas line (3), an injection device (4) for injecting an additive (5) into the exhaust gas line (3), and a catalyst (6) arranged downstream of the injection device (4) in a section (3a) of the exhaust gas line (3) in the direction of flow (x) of the exhaust gases, characterized in that at least two sections (3c, 3d) of the exhaust gas line (3) open into the section (3b) of the exhaust gas line (3) containing the catalyst (6) in such a way that the introduced exhaust gas moves at least approximately in the form of a spiral (7) along an inner wall (8) of the exhaust gas line (3) in the direction of the catalyst (6), and that the injection device (4) is arranged on the section (3a) of the exhaust gas line (3) containing the catalyst (6) in such a way thatthat the additive (5) injected by the injection device (4) is injected downstream of the junction of the at least two sections (3c, 3d) and substantially centrally into the spiral (7) formed by the exhaust gas in the exhaust pipe (3), that the injection device (4) projects into the section (3a) of the exhaust pipe (3) containing the catalyst (6) in such a way that an annular space (9) is formed between the inner wall (8) of the section (3a) of the exhaust pipe (3) containing the catalyst (6) and the injection device (4), and that an outlet opening (4a) of the injection device (4) opens downstream of the annular space (9) into the section (3a) of the exhaust pipe (3) containing the catalyst (6). Device according to claim 1, characterized in that the at least two sections (3c, 3d) originate from a section (3b) of the exhaust pipe (4) connected to the internal combustion engine (2). Device according to claim 1 or 2, characterized in that the at least two sections (3c, 3d) of the exhaust pipe (3) are inclined in the direction of flow (x) of the exhaust gases in their area which opens into the section (3a) of the exhaust pipe (3) which has the catalyst (6). Device according to one of claims 1 to 3, characterized in that at least the part of the inner wall (8) of the exhaust gas line (3) adjoining the injection device (4) in the direction of flow (x) of the exhaust gas is coated with a catalyst material (10) based on titanium dioxide. Device according to one of claims 1 to 4, characterized in that at least the part of the exhaust gas line (3) adjoining the injection device (4) in the direction of flow (x) of the exhaust gas can be heated by means of a heating device (11).

Images