Arrangement for mixing a reducing agent into an exhaust gas stream, combustion plant with such an arrangement and use of a ceramic baffle plate for mixing a reducing agent

Abstract

The invention relates to an arrangement for mixing a reducing agent into an exhaust gas stream. For this purpose, the arrangement has an exhaust gas guide pipe (04) with a central axis, into which combustion exhaust gases containing nitrogen oxide are introduced via an exhaust gas inlet in order to form an exhaust gas stream. Furthermore, a reducing agent feed (05) is provided, which is located in the exhaust gas guide pipe (04) and comprises a mixing nozzle (06), to which compressed air and reducing agent are supplied via supply lines (07) in order to spray a spray mist into the exhaust gas stream. A ceramic baffle plate (23) is arranged downstream with a fanning distance (l A) is arranged in front of the mixing nozzle (06) so that the spray mist emitted by the mixing nozzle (06) directly impacts the impact plate (23). The impact plate (23) consists of firmly interconnected, irregularly arranged hard bodies. Pores remain between the hard bodies; the firm connection between the hard bodies is created by an added binder. The softening temperature of the binder is selected in relation to the melting temperature of the pore-forming agent such that stable glass bonds are formed between the hard bodies during the ceramic firing process before the pore-forming agent burns.

Description

[0001] The present invention relates to an arrangement for mixing a reducing agent into an exhaust gas stream. Such an arrangement comprises an exhaust gas guide pipe into which nitrogen oxide-containing combustion gases are introduced via an exhaust gas inlet, so that an exhaust gas stream is formed there along a central axis. To achieve the desired reduction of nitrogen oxides in the exhaust gas stream, a reducing agent is supplied and evaporated in the exhaust gas stream. The invention further relates to the use of a ceramic baffle plate in an arrangement for mixing a reducing agent into an exhaust gas stream. Furthermore, the invention relates to a combustion system with such an arrangement.

[0002] Systems are known in the art that serve to reduce particularly environmentally harmful or health-endangering components in combustion exhaust gases. In particular, so-called selective catalytic reduction (SCR) is used to reduce nitrogen oxides in exhaust gases from internal combustion engines, waste incineration plants, gas turbines, or various industrial plants. The chemical reactions taking place in SCR systems reduce the nitrogen oxides, while undesirable reactions such as the oxidation of the introduced reducing agent with oxygen and the oxidation of sulfur dioxide to sulfur trioxide are suppressed as much as possible. In such systems, ammonia (NH3) is regularly added to the exhaust gas as a reducing agent to reduce the nitrogen oxides (NO x ) in the exhaust gas into harmless nitrogen and water.

[0003] In typical SCR units, ammonia is injected directly into the exhaust system via a carrier substance (e.g., urea or ammonia solution in liquid form) using an injection system. The combustion plant to be equipped usually has an exhaust line that carries the hot exhaust gases from the combustion process to the SCR unit. A precise dosing system injects the reducing agent (e.g., 32.5% or 40% aqueous urea solution) into the exhaust stream. The required amount is determined according to the NO x-Concentration in the exhaust gas is regulated. It is important that the reducing agent is mixed with the exhaust gas in order to subject as large a proportion of the nitrogen oxides as possible to the reduction process. For this purpose, mixing units are used, which must be arranged in the exhaust stream. In simple cases, the mixing units cause turbulence in the exhaust gas and the supplied reducing agent, but this requires comparatively long exhaust gas paths. These mixing sections are followed by a catalyst made of specially coated materials that initiate the chemical reaction to convert NO x into nitrogen and water. A control unit is also required that monitors the process and, taking into account numerous recorded exhaust gas parameters (temperature, NO x-concentration, etc.). After catalyst treatment, the purified exhaust gases are discharged to the atmosphere through an outlet, exhaust, or chimney. These catalysts can have different designs and preferably use materials with large effective surfaces.

[0004] Rinie van Helden et al. provide a general overview of SCR systems and their properties in “Optimization of Urea SCR deNOx Systems for HD Diesel Engines,” 2004 SAE International, 2004-01-0154.

[0005] In "Marine Technologies for Reduced Emissions," April 2005, Wärtsilä Switzerland Ltd, Heinrich Schmid et al. describe exhaust gas aftertreatment using selective catalytic reduction (SCR) in marine diesel engines.

[0006] In this context, EP 2 891 641 B1 discloses a porous body with an enlarged specific surface area and a method for producing such a porous body. This porous body has a carrier body with numerous channels extending between an inlet and an outlet. The carrier body consists of a plurality of oxidic and / or non-oxidic mineral grains bonded to one another by a ceramic bonding agent, leaving interconnected cavities between the grains. A glass surface coating is applied, at least in sections, to the surface of the channels, from which acid-soluble components have been removed to create a porous glass layer with an enlarged specific surface area.

[0007] In general, systems for mixing a reducing agent into an exhaust stream can be adapted to various systems that generate combustion exhaust gases. For example, they are useful for exhaust gas purification on stationary drives for large industrial plants, in combustion power plants, or on large diesel engines such as ship propulsion systems. However, in many cases, the mixing of the reducing agent requires a significant amount of installation space, namely an exhaust pipe that runs as straight as possible and is more than 1.5 m long to achieve complete evaporation of the reducing agent in the exhaust gas. In many systems, especially when exhaust gas purification is to be retrofitted, the required installation space is not available.

[0008] EP 1 990 513 A1 describes a device for distributing flowable additives in the exhaust system of an internal combustion engine. In particular, the device is intended to be suitable for distributing a water-urea mixture in the exhaust system of a diesel engine. An injection device opens into the exhaust system upstream of an SCR catalyst. A swirl-generating device is arranged in the exhaust system in the area where the additive is introduced. A baffle plate made of ferritic stainless steel can also be arranged.

[0009] US 8,359,832 B2 discloses an exhaust aftertreatment system for engines comprising a mixer arranged in the exhaust stream. The mixer comprises a mesh section with a wire mesh and may include a deflection section with a deflector that redirects the flow direction of the exhaust stream.

[0010] EP 2 929 156 B1 describes a mixer for a device for the selective catalytic reduction of exhaust gases from internal combustion engines. The mixer is intended to achieve the uniform distribution of a reducing agent introduced into the exhaust stream at the catalyst inlet. The mixer has a structure of mixer elements for direct electrical heating, through which the mixture of exhaust gas and reducing agent flows. The mixer elements are arranged in a ring and form a heating circuit.

[0011] EP 1 166 862 B1 discloses a mixer for mixing gases and other Newtonian fluids, comprising a flow channel and a flow-influencing surface arranged therein. The surface is designed as a vortex-generating surface with freely flowing leading edges directed against the flow. An adjustment device enables translational adjustment of the surface relative to the main flow direction.

[0012] DE 10 2014 018 852 A1 discloses an exhaust system for an internal combustion engine, comprising an injection nozzle for injecting a liquid reducing agent into the exhaust gas flowing through the exhaust system. Furthermore, a mixer arranged in the exhaust stream is provided for swirling and distributing the liquid reducing agent in the exhaust stream. An SCR catalyst arranged downstream of the mixer in the exhaust stream serves for the selective catalytic reduction of nitrogen oxides using the reducing agent or its decomposition products. To prevent the formation of a liquid film or deposits on the mixer, the mixer is at least partially coated with a heat-, abrasion-, and corrosion-resistant non-stick coating made of a glass or ceramic material.

[0013] US 2013 / 0 052 095 A1 describes an exhaust gas purification system for an internal combustion engine. The exhaust gas purification system includes a reduction catalyst arranged in an exhaust pipe of the internal combustion engine; and a reducing agent spraying device for spraying a reducing agent within the exhaust pipe, upstream of the reduction catalyst. A dispersing element is arranged at a position upstream of the reduction catalyst within the exhaust pipe, onto which the reducing agent is sprayed by the reducing agent spraying device. The dispersing element consists of a porous element and is mounted in the exhaust pipe so that it runs parallel to the flow direction of the exhaust gas.

[0014] In practice, attempts have been made to reduce the length of the exhaust pipe, in which the reducing agent is mixed with the exhaust gas, through various measures. For example, it has been proposed to improve the atomization of the supplied reducing agent by means of baffle plates or similar devices. One problem is that the baffle plates or similar flow elements experience significant wear when the chemically aggressive reducing agent impinges upon them at high flow velocities and at high exhaust gas temperatures. This increases costs and maintenance effort.

[0015] Another measure involves spraying the reducing agent into the exhaust gas using a mixing nozzle with compressed air. This creates a spray containing numerous droplets of the reducing agent. However, these droplets also require a significant distance in the exhaust stream to heat up and evaporate, allowing the desired chemical reactions to occur. This further increases the wear on any baffle plates or other components impacted by the spray. However, all of these measures have only limited effects.

[0016] In "Modeling the Formation of Urea-Water Sprays from an Air-Assisted Nozzle," Appl. Sci. 2020, 10(16), 5723; https: / / doi.org / 10.3390 / app10165723, Jens Frühhaber et al. describe a setup for reducing nitrogen oxides in selective catalytic reduction (SCR) systems. Air-assisted nozzles generate fine sprays that improve ammonia homogenization. In particular, the authors describe a method for modeling spray formation using computational fluid dynamics (CFD) for this type of atomizer.

[0017] An object of the present invention is therefore to provide an arrangement for mixing a reducing agent into an exhaust gas stream that requires significantly less space and exhibits low wear while still demonstrating high efficiency in mixing the reducing agent. A further object is to enable the use of a baffle plate for mixing a reducing agent into an exhaust gas stream without it being subject to significant wear. Finally, an object is to provide a combustion system with such an arrangement.

[0018] These objects are achieved by an arrangement according to the appended claim 1, a use according to the independent claim 10 and an incineration plant according to claim 12. Preferred embodiments are explained in the subclaims and the following description.

[0019] The arrangement according to the invention for mixing a reducing agent into an exhaust gas stream initially comprises, in a conventional manner, a preferably rectilinear exhaust gas guide pipe with a central axis, into which combustion exhaust gases containing nitrogen oxide are introduced via an exhaust gas inlet in order to form an exhaust gas flow in the direction of the central axis in the exhaust gas guide pipe. The exhaust gas guide pipe has an exhaust gas outlet, from which the exhaust gas enriched with the reducing agent is preferably fed to a downstream catalyst.

[0020] The arrangement also has a reducing agent feed located in the exhaust pipe and comprising a mixing nozzle to which compressed air and reducing agent are supplied via supply lines in order to spray a spray of reducing agent and compressed air into the exhaust stream essentially in its flow direction. The purpose of mixing the reducing agent with the compressed air in the mixing nozzle is to create a spray with the smallest possible droplets of the reducing agent. The intention is to spray the spray into the exhaust stream in such a way that the individual droplets move with the exhaust stream, are heated by it, and evaporate before the thus enriched exhaust gas reaches the downstream SCR reactor or catalyst. At the same time, the aim is to prevent any reducing agent from settling on the walls of the exhaust pipe or other components in order to keep corrosion to a minimum.The mixing nozzle is therefore configured so that the spray mist is introduced in the direction of the exhaust gas flow, with a conical spray jet preferably emerging from the mixing nozzle and the spray axis preferably being coaxial with the central axis of the exhaust pipe. It is understandable that the conical spray jet assumes a larger diameter with increasing distance from the injection point (outlet of the mixing nozzle), i.e., the cross-sectional area of ​​the spray jet increases quadratically with increasing distance from the injection point.

[0021] The arrangement according to the invention has a ceramic baffle plate arranged downstream of the mixing nozzle at a fan-out distance. The fan-out distance is selected such that the spray emitted by the mixing nozzle impinges directly and essentially completely on the baffle plate. Knowing the angle of the spray cone emitted by the mixing nozzle and taking into account the diameter of the baffle plate, the fan-out distance between the mixing nozzle and the baffle plate can be suitably selected so that the spray impinges completely on the baffle plate. If the reducing agent mixed with compressed air leaves the mixing nozzle with an opening angle of 15°, for example, and the diameter of the baffle plate is 10 cm, a suitable fan-out distance of approximately 40 cm results if the baffle plate area is to be fully utilized.In a test example, to maintain a predetermined pipe length, the distance between the mixing nozzle and the baffle plate was selected at approximately 16 cm, with an available baffle plate diameter of 8.5 cm. The opening angle of the mixing nozzle was then adjusted so that the fan-out was always smaller than the diameter of the baffle plate, ensuring that all droplets hit the baffle plate without damaging the suspension.

[0022] The ceramic impact plate of the assembly consists of firmly interconnected, irregularly arranged ceramic hard bodies, with pores remaining between the hard bodies, which are formed during the ceramic manufacturing process by adding inorganic pore-forming agents. The hard bodies preferably have a grain size of 3 - 12 mm and preferably have an irregular, polygonal shape. The firm connection between the hard bodies is created during the ceramic manufacturing process with the help of a binder added to the raw ceramic mass. To ensure that the distances between the hard bodies defined by the pore-forming agent in the raw ceramic mass do not become gaps orTo convert pores into solids, the softening temperature of the binder in relation to the melting temperature of the pore-forming agent is selected in such a way that stable glass bonds are formed between the hard bodies during the ceramic firing process, even before the pore-forming agent is completely burned or decomposed upon further temperature increase.

[0023] According to a preferred embodiment, a magnesium aluminum silicate is used as the base material for producing the ceramic impact plate. Cordierite is particularly preferred, in particular a synthetically produced, dense cordierite chamotte with a grain size in the range of 3-12 mm.

[0024] Dense cordierite occurs naturally and can also be produced synthetically. This material exhibits low thermal conductivity, low thermal expansion, high heat resistance, low dielectric losses, and corrosion resistance, particularly against molten metals. These properties make the material well-suited for use as a ceramic baffle plate for the reducing agent in the exhaust stream. The injected reducing agent is both chemically and mechanically aggressive. The ambient temperature in the exhaust gas area can reach up to 500°C. The continuous temperature strength of dense cordierite is approximately 1,000°C.

[0025] Unlike many other ceramic materials, which are regularly sintered or fired to a dense finish, cordierite hard bodies exhibit low thermal expansion. This, combined with their irregular arrangement and the resulting pores, results in the impact plate's high resistance to thermal shock. This allows the reducing agent, which is colder than the exhaust gas, to be sprayed onto the ceramic impact plate at high pressure. This ensures that thermal shocks or material damage to the impact plate are avoided in the long term.

[0026] For the production of the ceramic impact plate, it is important that the hard bodies are firmly bonded to one another, while at the same time maintaining pores or cavities between them. This requires a procedure that is different from conventional ceramic manufacturing steps. Pouring the irregularly or randomly shaped hard bodies into a mold alone would result in undesirable compaction. Voids between the hard bodies would be minimized or eliminated. This bulk compaction is undesirable here, as it would bring the physical properties of the bulk body closer to those of a single, solid body.Compared to a porous body, where the impact of physical forces, particularly those resulting from thermal expansion, leads to chaotic deformation forces that are easier to compensate for in a porous body, a solid or partially solid body experiences linear deformations during large temperature fluctuations, which can then lead to high material stresses and potentially to the destruction of the body. Therefore, the porous ceramic impact plate proposed here is particularly advantageous in the case of thermal expansion or thermal shock. Furthermore, the porosity reduces the mass of the impact plate, which has a beneficial effect on handling and processing. Furthermore, the use of less material leads to lower material and firing costs.

[0027] The pore-forming agent used in the production of the ceramic impact plate prevents the undesired compaction of the hard bodies and permanently establishes a defined porosity of the impact plate. The pore-forming agent must therefore remain in place during the firing process until a stable bond has been established between the hard bodies. The pore-forming agent must then dissolve to enable porosity. If the material dissolves or burns too early, such as wood chips or Styrofoam, this would result in compaction during the ceramic firing. The hard bodies would "slide" together. Especially with heavy, large hard bodies such as those used here, the pore-forming material must remain stable for long enough. At the same time, the material mixture (raw mass), consisting of hard bodies and pore-forming agent, must be malleable in order to be able to shape the impact plates to be produced.

[0028] According to a preferred embodiment, perlite is used as the pore-forming agent. This is a rock that occurs as volcanic glass. At temperatures between 800°C and 1,000°C, it expands to 15-20 times its volume. It then disintegrates and bonds with the hard particles (especially cordierite) and the additional binder.

[0029] In an alternative embodiment, diatomaceous earth is used as a pore-forming agent. This is also a SiO2 with properties similar to perlite.

[0030] To permanently bond the hard bodies, a binder is added during the ceramic manufacturing process. This is preferably glass powder or glass frit. Glass frit consists primarily of quartz, aluminum oxide, and calcium oxide. The melting temperature of the binder is matched to the pore-forming agent so that, upon dissolution of the pore-forming agent, stabilizing glass bonds already exist to keep the hard bodies stable in their irregular or chaotic arrangement. The glass frit is also selected to produce a durable, mechanically stable glass. The melting point of the selected glass is particularly preferably between 1,220 and 1,260°C.

[0031] As already mentioned above, a key advantage of the impact plate produced in the described manner results from the irregular or chaotic arrangement of the hard bodies and the pores formed between them. Occurring thermal expansion is distributed in all directions by this design of the impact plate. The risk of expansion cracks, which is common with almost all other ceramics, is thus eliminated. Furthermore, the material protects itself from mechanical wear, which threatens when the reducing agent is injected at high pressure. The glass bond across all hard bodies provides additional protection for the cordierite, which is preferably used as the hard body. Furthermore, the hard bodies conceal the contact points between each other. This ensures that the bond between the individual hard bodies is maintained even under the continuous, eroding impact of the reducing agent.

[0032] The positioning of the ceramic baffle plate in front of the mixing nozzle leads to a rapid conversion of the liquid reducing agent (as droplets in the spray) into its gas phase. The disordered, porous surface of the baffle plate is advantageous because the impinging liquid molecular chains are effectively atomized.

[0033] According to a further embodiment, the arrangement comprises a microwave source that directs microwave radiation onto the impact plate. Cordierite, which forms the hard bodies, absorbs the microwave radiation and heats up as a result. This allows the impact plate to be heated in a targeted manner, particularly when the exhaust gas temperature is too low (start-up phase / low load / very cold reducing agent), to accelerate the conversion of the reducing agent into the gas phase or even enable it. At temperatures that are too low, there is a risk of irreversible byproducts forming.

[0034] A preferred embodiment is characterized in that the outlet opening of the mixing nozzle and the ceramic baffle plate are arranged coaxially to the central axis in the exhaust gas guide pipe.

[0035] Particularly preferably, the ceramic baffle plate has a surface facing the mixing nozzle that is at least slightly larger than the cross-sectional area of ​​the spray mist upon impact with the baffle plate. This ensures that the spray mist, injected under high pressure, completely impacts the baffle plate and does not cause erosion on other components.

[0036] In an advantageous embodiment, the ceramic baffle plate is attached to a suspension which comprises a holding element located behind the baffle plate in the direction of flow, so that the spray mist does not hit the holding element.

[0037] The invention is also realized by using a ceramic baffle plate consisting of rigidly connected, randomly arranged ceramic hard bodies, between which pores are formed and which are connected to one another by glass material, for mixing a reducing agent into an exhaust gas stream. For this purpose, the ceramic baffle plate is arranged downstream of a mixing nozzle at a fan-out distance, so that the spray emitted by the mixing nozzle directly and preferably completely impinges on the baffle plate. All previously described embodiments of the arrangement can be used—also in combination with one another—in the aforementioned application.

[0038] Finally, the invention is realized by an incineration plant equipped with exhaust gas aftertreatment by selective catalytic reduction (SCR) and comprising a system for mixing a reducing agent into an exhaust gas stream according to one of the previously described embodiments. It is irrelevant whether the incineration plant serves as a drive and burns fuel, burns other materials, such as waste, releasing nitrogen oxide-containing exhaust gases, or whether nitrogen oxides are produced in an industrial process through a chemical reaction, such as in nitric acid production.

[0039] Further details and advantages of the invention will become apparent from the following description of a preferred embodiment, with reference to the drawings. They show: Fig. 1 an overview of a combustion plant with SCR exhaust gas aftertreatment according to the state of the art; Fig. 2 a schematic representation of an arrangement according to the invention for mixing a reducing agent into an exhaust gas stream.

[0040] Fig. Figure 1 shows a simplified overview of the general structure of a combustion plant with SCR exhaust aftertreatment, as used in the state of the art. In the example shown, an internal combustion engine 01, for example, a marine diesel engine, drives a generator 02.

[0041] The combustion engine 01 discharges exhaust gas into an exhaust line 03. One section of the exhaust line 03 is formed by an exhaust pipe 04, into which a reducing agent is sprayed via a reducing agent feed 05. For this purpose, the reducing agent feed 05 uses a mixing nozzle 06, to which compressed air and reducing agent, in particular urea, are supplied via supply lines 07a, 07b. The reducing agent is supplied, for example, from a tank 08. In the exhaust pipe 04, the reducing agent is mixed with the exhaust gas flow conducted there and evaporated by heating. The mixture of exhaust gas and evaporated reducing agent is subsequently fed to an SCR reactor 09 or catalytic converter. The exhaust gas, from which the nitrogen oxides have been largely removed, is discharged via an outlet 10.

[0042] Fig. Figure 2 shows an arrangement according to the invention for mixing a reducing agent into an exhaust gas stream. For a better overview, only the section of the arrangement modified in accordance with the invention is shown. Fig. 1. The exhaust guide pipe 04 is divided into a front section 04a and a rear section 04b in the embodiment shown, which are connected via a flange 21. This facilitates maintenance and replacement work. In the front section, in the embodiment shown, there is a mixing unit 22, which is located in the exhaust gas flow upstream of the mixing nozzle 06 and serves to swirl the exhaust gas. In the exhaust gas flow downstream of the mixing nozzle 06, which sprays the spray of compressed air and reducing agent into the exhaust gas flow, there is a ceramic baffle plate 23. The baffle plate 23 is spaced apart from the mixing nozzle 06 at a fan-out distance l A The fan-out distance l Ais preferably selected so that the spray mist leaving the mixing nozzle 06 completely impinges on the baffle plate, so that the droplets of the spray mist are further atomized and subsequently mix optimally with the exhaust gas. For this purpose, the mixing nozzle 06 and baffle plate 23 are preferably located on a spray axis 20, which particularly preferably runs coaxially with the central axis of the exhaust gas guide pipe 04.

[0043] The baffle plate 23 preferably has a circular cross-section whose center lies on the central axis of the exhaust pipe 04. The baffle plate 23 is held by a retaining element 24, which is concealed behind the baffle plate 23 in the direction of exhaust flow. This prevents the spray mist from directly impinging on the retaining element. The retaining element 24 is part of a suspension 25 that extends to the wall of the exhaust pipe. Reference symbol 01 Combustion engine 02 Generator 03 Exhaust pipe 04 Exhaust pipe 05 Reducing agent feed 06 Mixing nozzle 07 Supply lines 08 Tank for reducing agent 09 SCR reactor 10 Outlet 20 spray axis 21 Flange 22 Mixing unit 23 ceramic baffle plate 24 Holding element 25 Suspension l A Fan-out distance

Claims

[1] Arrangement for mixing a reducing agent into an exhaust gas stream comprising: - an exhaust gas guide pipe (04) with a central axis, into which combustion exhaust gases containing nitrogen oxide are introduced via an exhaust gas inlet in order to form an exhaust gas flow in the direction of the central axis in the exhaust gas guide pipe (04); - a reducing agent feed (05) which is located in the exhaust gas guide pipe (04) and comprises a mixing nozzle (06) to which compressed air and reducing agent are supplied via supply lines (07) in order to spray a spray mist of reducing agent and compressed air into the exhaust gas stream in its flow direction; characterized by that a ceramic baffle plate (23) is arranged downstream with a fanning distance (l A) is arranged in front of the mixing nozzle (06) so that the spray mist emitted by the mixing nozzle (06) strikes the impact plate (23) directly, wherein the ceramic impact plate (23) consists of firmly connected, irregularly arranged hard bodies, wherein pores formed by an inorganic pore former added in the ceramic manufacturing process remain between the hard bodies, wherein the firm connection between the hard bodies is produced by a binder added in the ceramic manufacturing process, and wherein the softening temperature of the binder in relation to the melting temperature of the pore former is selected such that stable glass bonds are formed between the hard bodies in the ceramic firing process before the pore former burns. [2] Arrangement according to claim 1, characterized bythat the outlet opening of the mixing nozzle (06) and the ceramic baffle plate (23) lie on a common spray axis (20) which runs coaxially to the central axis in the exhaust gas guide pipe (04), and that the baffle plate (23) has a surface facing the mixing nozzle (06) which is larger than the cross-sectional area of ​​the spray mist when it strikes the baffle plate (23). [3] Arrangement according to claim 1 or 2, characterized by that the mixing nozzle (06) is configured to allow the spray mist of compressed air and reducing agent to emerge with an opening angle not greater than 20°. [4] Arrangement according to one of claims 1 to 3, characterized by that it comprises a microwave source which emits microwaves onto the ceramic baffle plate (23), and that the hard bodies of the ceramic baffle plate (23) contain a material which absorbs the microwaves in order to be heated thereby. [5] Arrangement according to claim 4, characterized bythat the ceramic baffle plate (23) is supported by a suspension (25) which comprises a holding element (24) located behind the baffle plate (23) in the direction of flow, so that the spray mist does not strike the holding element (24) directly. [6] Arrangement according to one of claims 1 to 5, characterized by that the exhaust pipe (04) has a total length of less than 2 m. [7] Arrangement according to one of claims 1 to 6, characterized by that the hard bodies consist predominantly of magnesium aluminum silicate. [8] Arrangement according to one of claims 1 to 7, characterized by that in the ceramic manufacturing process for the impact plate (23) perlite and / or diatomaceous earth are added as inorganic pore formers. [9] Arrangement according to one of claims 1 to 8, characterized bythat in the ceramic manufacturing process for the impact plate (23) a glass frit is added as a binder, the melting temperature of which is lower than the combustion temperature of the pore former. [10] Use of a ceramic baffle plate (23) consisting of rigidly connected, randomly arranged hard bodies, between which pores are formed and which are connected to each other by glass material, for mixing a reducing agent into an exhaust gas flow, wherein the baffle plate (23) is arranged downstream with a fan-out distance (l A ) is arranged in front of a mixing nozzle (06) so that the spray mist of the reducing agent and compressed air emitted by the mixing nozzle (06) strikes the impact plate (23) directly. [11] Use according to claim 10, characterized bythat the arrangement for mixing in a reducing agent is designed according to one of claims 1 to 9 and is a component of the exhaust system of a stationary internal combustion engine, a marine diesel engine, a gas engine, a biomass combustion plant, an industrial plant emitting exhaust gases containing nitrogen oxide or a waste incineration plant. [12] Combustion plant for the combustion of fuel or waste with exhaust gas aftertreatment by selective catalytic reduction (SCR), characterized by that the exhaust gas aftertreatment comprises an arrangement for mixing a reducing agent into an exhaust gas stream according to one of claims 1 to 9.

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