Process for decomposing nitrogen oxides in exhaust gases in multiple reaction phases

Abstract

The invention relates to a process for decomposing nitrogen oxides in exhaust gases. In a non-catalytic reaction phase, reducing agent, e.g. NH3 or urea, is injected into the exhaust gas in a controlled manner at comparatively high temperatures, specifically directly in the combustion chamber in which the exhaust gas is produced or in a collecting device for collecting and transferring the exhaust gas. The decomposition of the nitrogen oxides takes place in said non-catalytic reaction phase by means of a non-catalytic chemical reduction, i.e. in the absence of separate catalysts, preferably as so-called SNCR (selective non-catalytic reduction). In a downstream catalytic reaction phase, the nitrogen oxides are then further decomposed in the presence of a heterogeneous catalyst, specifically by catalytic chemical reduction with reducing agent, preferably as so-called SCR (selective catalytic reduction), and / or by catalytic decomposition.

Description

Method for the reduction of nitrogen oxides in exhaust gases in several reaction phases

[0001] The invention relates to a process for the reduction of nitrogen oxides in exhaust gases. In a non-catalytic reaction phase, a reducing agent, e.g., NH3 or urea, is injected into the exhaust gas in a controlled manner at comparatively high temperatures, either directly in the combustion chamber where the exhaust gas originates or in a collection device for collecting and conveying the exhaust gas. The reduction of the nitrogen oxides in this non-catalytic reaction phase takes place through non-catalytic chemical reduction, i.e., in the absence of separate catalysts, preferably as so-called SNCR (Selective Non-Catalytic Reduction). In a subsequent catalytic reaction phase, further reduction of the nitrogen oxides occurs in the presence of a heterogeneous catalyst, specifically through catalytic chemical reduction with a reducing agent, preferably as so-called SCR (Selective Catalytic Reduction), and / or through catalytic decomposition.

[0002] Nitrogen oxides are formed through (side) reactions in all combustion processes where nitrogen or nitrogen-containing compounds and oxygen are present, i.e., in all combustion processes with air. Therefore, nitrogen oxides are formed in almost all common industrial combustion processes. The amount of nitrogen oxides produced during combustion depends on several factors, primarily the flame temperature and the fuels used.

[0003] The conventional combustion of natural gas, process gases from refineries, or residual gases from chemical plants produces, among other things, NO and NO2 (collectively referred to as "NOx"). For environmental protection reasons, there are strict regulatory requirements that limit permissible emissions. In Germany, for combustion plants with a firing capacity of up to 50 MW, NOx emissions must be limited according to the revised version of the First General Administrative Regulation on the Federal Immission Control Act (Technical Instructions on Air Quality Control, "TA Luft"). For steam reformers, for example, the current maximum limit in Europe is 100 mg / Nm³. 3 This is standard practice. For large combustion plants with a firing capacity of 50 MW or more, the annual average limit according to the thirteenth Ordinance for the Implementation of the Federal Immission Control Act (13th BImSchV) is 80 or 85 mg / Nm³. 3 to be adhered to (approx. 41 ppm).

[0004] Particularly large quantities of NOx and additional amounts of N₂O (nitrous oxide, also known as laughing gas) are produced when the fuel has a high proportion of nitrogen-containing compounds. This is especially problematic when, for example, NH₃ is deliberately added to the fossil fuel or even when pure NH₃ is used as fuel. This is an increasingly discussed and used alternative for avoiding CO₂ emissions from the combustion of fossil fuels (see also, for example, Kobayashi, Hideaki; Hayakawa, Akihiro; Somarathne, KD). Kunkuma A.; Okafor, Ekenechukwu C. (2019): Science and technology of ammonia combustion. In: Proc. Combust. Inst. 37 (1), pp. 109-133).

[0005] CN 113 294 801 A discloses a combustion device that enables efficient, clean combustion of pure ammonia. The combustion device comprises a combustion chamber, an ammonia gas tank, and an air preheater. The ammonia gas tank and the air preheater are connected to the combustion chamber. An ammonia gas vaporizer and an electric ammonia pyrolysis heater are connected between the ammonia gas tank and the combustion chamber in the ammonia gas inlet direction. The air preheater is sequentially connected to an ammonia pyrolysis reactor and an SCR reactor. The hot section of the exhaust duct, at approximately 1000°C, also includes a provision for the injection of NH3 for NOx reduction according to the SNCR principle.

[0006] CN 113 405 116 A discloses a system and a control method for reducing carbon emissions by mixing ammonia gas, comprising an ammonia gas source, a gasification device, a hydrogen processing device and a furnace; an ammonia gas nozzle and ammonia hydrogen are arranged above the exhaust air of the furnace.

[0007] CN 114 963 172 A relates to a carbon-free clean energy system constructed by coupling liquefied natural gas and liquid ammonia fuel. According to a preferred embodiment, flue gas exiting a combustion device is passed through a high-temperature waste heat boiler to reduce its temperature from 900°C to 700°C. The remaining 30-50% of the flue gas is denitrated by a multi-effect SCR denitrification-reduction reactor. The ammonia gas required for denitrification is injected from the rear of the combustion device or from the inlet of the multi-effect SCR denitrification-reduction reactor, and the nitrogen oxide concentration in the flue gas is reduced to 5 mg / m³ under the combined action of the SNCR and SCR devices at the rear of the furnace. 3Reduced. The emission of nitrogen oxides in the flue gas is reduced to almost zero. The molar ratio of the added ammonia gas to the nitrogen oxides in the original flue gas is 1.0-1.2:1.

[0008] CN 116 717 804 A relates to a heating system in the field of thermal engineering and an ammonia combustion heating process based on the heating system. The heating system comprises: a fuel supply device, a combustion furnace, and a flue gas treatment device. The fuel supply device comprises: an ammonia supply device, an ammonia cracker, a natural gas line, a fan, and a buffer tank. The flue gas treatment devices include: heat exchangers and denitrification units. The denitrification unit has a flue gas line connected to the combustion furnace.

[0009] CN 116 857 951 A discloses a purely ammonia-burning roller furnace and an operating method for it and belongs to the technical field of industrial furnaces. A denitrification box has a The catalyst section consists of a flue gas inlet and a flue gas outlet, with the catalyst section located between the flue gas inlet and the flue gas outlet. This reduces the amount of nitrogen oxides in the flue gas discharged from the roller furnace, thus achieving the goal of clean ammonia combustion with low nitrogen oxide emissions.

[0010] US 2012 0009108 Al relates to a process for the combined removal of both NH₄⁺ in an NH₄⁺-containing exhaust gas stream and nitrogen oxides in an additional nitrogen oxide-containing exhaust gas stream in a combined NH₄⁺-ham synthesis plant, after selective non-catalytic reduction in a temperature range of 850°C to 1100°C, achieving nitrogen oxide removal rates of up to 80%, or after selective catalytic reduction in a temperature range of 150°C to 550°C, achieving nitrogen oxide removal rates of up to 99%, wherein the NH₄⁺ and the nitrogen oxides react with each other and are converted to N₂ and H₂O.

[0011] US 2015 0290583 Al relates to a method for treating nitrogen oxide-containing exhaust gases from industrial processes, e.g., flue gases, for the purpose of removing and / or separating the nitrogen oxides and / or reducing the nitrogen oxide content by chemical reduction of the nitrogen oxides, in particular a method for removing nitrogen oxides from exhaust gases of industrial plants, wherein at least two different nitrogen-containing reducing agents are brought into contact with the nitrogen oxide-containing exhaust gases and / or introduced into the stream of the nitrogen oxide-containing exhaust gases.

[0012] US 2015 0362255 Al relates to a process for treating nitrogen oxide-containing exhaust gases from technical processes, such as flue gases, for the purpose of removing and / or separating the nitrogen oxides and / or reducing the nitrogen oxide content, in particular a process for denitrification of exhaust gases from industrial plants, wherein (A) in a first process step a selective cooling of the exhaust gases takes place; and (B) simultaneously and / or subsequently in a second process step the removal and / or separation of the nitrogen oxides from the exhaust gases takes place.

[0013] US Patent 2017 0349542 relates to a process for the production of urea in which the continuous emission of ammonia, which is typically generated in such a process, can be significantly reduced, even to zero. According to a preferred embodiment, the urea-forming reaction of carbon dioxide and ammonia is carried out in a synthesis section that does not require passivation by oxygen. As a result of the absence of oxygen, a hydrogen-rich gas stream is generated in the synthesis section, which can be used as fuel in a combustion plant. In the combustion plant, the ammonia-containing gas streams from urea production are combusted.

[0014] US 2018 0154305 Al relates to a process for treating nitrogen oxide-containing exhaust gases from technical processes, such as flue gases, for the purpose of removal and / or separation. of nitrogen oxides and / or for reducing the nitrogen oxide content, in particular processes for denitrification of exhaust gases from large plants by means of chemical reduction of the nitrogen oxides, wherein at least one nitrogen-containing reducing agent is introduced into the stream of nitrogen oxide-containing exhaust gases, wherein the amount of the reducing agent introduced into the exhaust gas stream is adjusted depending on the exhaust gas velocity, in particular flue gas velocity.

[0015] WO 2022 / 243410 relates to a process for the synthesis of hydrogen by catalytic cracking of ammonia, wherein the heat required for this is advantageously generated by the combustion of ammonia. The cracking gas, containing H₂, N₂, and NH₃, produced during cracking is treated in a purification stage to produce high-purity hydrogen, and the residual gas containing NH₃ is at least partially fed back into combustion to heat the cracking furnace. The NOₓ emissions generated during the combustion of the NH₃-containing fuel gas can optionally be reduced by means of SCR and / or SNCR.

[0016] WO 2023 / 075011 Al relates to a simultaneous process for the treatment of nitrogen oxides in flue gas generated during the use of a large quantity of fuel in a cement manufacturing process, a power plant, an incineration plant or the like, and for the production of high-purity hydrogen, as well as a highly efficient nitrogen oxide treatment process using an ammonia-based reducing agent and a hydrogen-containing gas mixture as a reducing agent for the removal of nitrogen oxides.

[0017] VDI guideline VDI 3927, ICS 13.040.40, November 2015, 1-128 of the Association of German Engineers concerns exhaust gas purification for the reduction of sulfur oxides, nitrogen oxides and halides from exhaust gases of combustion processes (flue gases).

[0018] In H. Lee et al., Energies, 2021, 14, 5604, 1-29, current developments and investigations into the combustion of ammonia for energy production, as a sole fuel or in combination with fossil fuels such as coal or natural gas, with reduced carbon or CO2 emissions are described. The associated generation of NOx emissions and their potential avoidance are also considered.

[0019] Y.-K. Park et al., Chemical Engineering Journal 461 (2023) 141958, 1-13 gives an overview of possible technologies for the catalytic removal of nitrogen oxides (NO, NO2, N2O) from exhaust gases formed during the combustion of ammonia.

[0020] There is a need for processes for the reduction of nitrogen oxides in exhaust gases, preferably in exhaust gases from the combustion of nitrogen-containing fuels, in particular from the combustion of NH3-containing fuels. Specifically, there is a need for processes for the reduction of nitrogen oxides in exhaust gases from the combustion of NH3-containing fuels for firing reactors in which H2 is produced by thermal or, preferably, catalytic cracking of NH3 (NH3 cracking).

[0021] To decompose NFU and thus use it as a source of H₂, NH₃ is catalytically decomposed into H₂ and N₂ at a comparatively high temperature and medium pressure. The decomposition preferably takes place in tubular reactors, which are heated externally, with the heat required for the endothermic decomposition reaction being supplied by a gas-fired system. To generate no or minimal CCL emissions throughout the entire process chain, fuels containing NH₃ or NH₃, preferably mixed with H₂, are preferably used to fire the tubular reactors, either not fossil fuels or not purely fossil fuels such as natural gas. Additionally, the fuel mixture can contain N₂, which is produced alongside H₂ during the catalytic decomposition of NH₃.

[0022] When burning NH3 or NHs-containing fuels, possibly mixed with H2, in the presence of O2, the amounts of NOx (i.e., NO and NO2) produced can be significantly higher than in conventional combustion processes using purely fossil fuels. Depending on the burner type and fuel composition (combustible components), as well as the amount of combustion air or oxygen added, the NOx content can reach several thousand ppm or more (see, e.g., Lee, Hookyung; Lee, Min-Jung (2021): Recent Advances in Ammonia Combustion Technology in Thermal Power Generation System for Carbon Emission Reduction. Energies 14 (18), Article 5604). Furthermore, unlike the combustion of conventional fuels, depending on the combustion conditions, considerable amounts of N2O, up to several hundred ppm, can be formed.

[0023] The emission limits for NOx and, where applicable, N2O that must be met for a plant burning NEL-containing fuels, for example, for heating a reactor for the catalytic decomposition of NH3 into H2 and N2, are subject to the respective licensing procedures. Simply optimizing combustion to comply with these limits (primary measure) is generally insufficient, so separate exhaust gas cleaning is always necessary to reduce the amounts of NOx and N2O produced during combustion below the required limits.

[0024] The processes and systems known from the prior art for the reduction of nitrogen oxides in exhaust gases produced during the combustion of NH3 in the presence of O2 are not entirely satisfactory, i.e., from a technical, economic, or ecological perspective, and improvements are needed. A particular challenge lies in the specific composition and temperatures of the exhaust gases, as well as the integration of the process into the overall process, e.g., into the plant complex for the catalytic NH3 splitting.

[0025] It is an object of the invention to provide an advantageous method and an advantageous system for the reduction of nitrogen oxides in exhaust gases produced during the combustion of NH3 or NHs-containing fuels in the presence of O2 (in particular NOx and N2O). The method according to the invention should be economical and capable of being carried out with reduced effort. The system according to the invention should be cost-effective to manufacture and maintain.

[0026] This problem is solved by the subject matter of the patent claims.

[0027] A first aspect of the invention relates to a method for reducing the nitrogen oxide content in an exhaust gas, wherein the method comprises the steps: (a) Burning NH3, possibly together with one or more other combustible gases such as H2 and / or CH4, in the presence of O2, producing an exhaust gas containing nitrogen oxide; (b) partial reduction of the nitrogen oxide content in the exhaust gas by non-catalytic chemical reduction, preferably selective non-catalytic chemical reduction (SNCR), of nitrogen oxide with a reducing agent to produce a pretreated exhaust gas [non-catalytic reaction phase without heterogeneous gas phase catalysis] ; (c) Cooling the pretreated exhaust gas; and (d) further reducing the nitrogen oxide content in the pretreated exhaust gas by catalytic chemical reduction, preferably selective catalytic chemical reduction (SCR), of nitrogen oxide with a reducing agent and / or catalytic decomposition of nitrogen oxide to produce a post-treated exhaust gas [catalytic reaction phase with heterogeneous gas phase catalysis] .

[0028] According to the invention, the reduction of the nitrogen oxide content takes place in at least two reaction phases, wherein the non-catalytic reaction phase preferably takes place as SNCR (Selective Non-Catalytic Reduction), i.e. preferably in the absence of catalysts for heterogeneous gas phase catalysis, and wherein the catalytic reaction phase, which is downstream of the non-catalytic reaction phase, preferably takes place as SCR (Selective Catalytic Reduction), i.e. in the presence of catalysts for heterogeneous gas phase catalysis.

[0029] The catalytic reaction phase (SCR) preferably takes place in a single stage, i.e., on a single catalyst bed. However, it is also possible for the catalytic reaction phase (SCR) to take place in multiple stages, i.e., on several catalyst beds flowed through sequentially, preferably with multiple additions of reducing agent upstream of the individual catalyst beds.

[0030] Particularly for combustion processes with ammonia-containing gases that produce high concentrations of nitrogen oxides (NOx and N2O) in the untreated exhaust gas, the inventive reduction of nitrogen oxides in at least two reaction phases is advantageous, since by implementing the first, non-catalytic reaction phase in step (b) (SNCR) the amount of catalyst for the second, catalytic reaction phase in step (d) (SCR) can be significantly reduced, with corresponding economic advantages.

[0031] For descriptive purposes, the terms ''reaction phase'', ''stage'', ''catalyst bed'', ''reaction zone'' are used, among others.

[0032] According to the invention, the term "non-catalytic reaction phase" refers to the reactions for the reduction of nitrogen oxides in the exhaust gas that do not occur through heterogeneous gas-phase catalysis. Similarly, according to the invention, the term "catalytic reaction phase" refers to the reactions for the reduction of nitrogen oxides in the exhaust gas that occur (at least also) through heterogeneous gas-phase catalysis. While the reduction of nitrogen oxides in the exhaust gas during the non-catalytic reaction phase occurs essentially through non-catalytic reactions, the reduction of nitrogen oxides in the exhaust gas during the catalytic reaction phase occurs (at least also) under gas-phase catalysis, i.e., through catalytic reactions. It is not excluded that, during the catalytic reaction phase, in addition to the catalyzed reactions, some non-catalyzed reactions also occur.Since non-catalyzed reactions typically require higher temperatures than catalytic reactions, and the catalytic reaction phase preferably occurs at higher temperatures than the non-catalytic reaction phase, catalyzed reactions typically dominate the catalytic reaction phase.

[0033] The catalysis referred here is heterogeneous gas-phase catalysis, i.e., catalytic reactions of exhaust gas components on solid catalysts. If homogeneously catalyzed processes also occur, these are disregarded for the definition of the non-catalytic and catalytic reaction phases.

[0034] The distinction according to the invention between the non-catalytic and catalytic reaction phases therefore serves to fundamentally differentiate between non-heterogeneously catalyzed reactions on the one hand (non-catalytic reaction phase) and heterogeneously catalyzed reactions on the other (catalytic reaction phase). The spatial realization of this division into a non-catalytic and a catalytic reaction phase is typically achieved through the absence of suitable catalysts (non-catalytic reaction phase) or the presence of suitable catalysts (catalytic reaction phase).

[0035] According to the invention, the term "catalyst bed" refers to a spatially enclosed and delimited quantity of catalyst material through which the exhaust gas flows during the execution of the process according to the invention. Several catalyst beds can be arranged in a common pressure vessel. The catalyst material in a catalyst bed can be, for example, a bed of pellets or in the form of a honeycomb. Although it is generally possible according to the invention to mix different catalyst materials together and use them together in one and the same catalyst bed, for example, as a mixture of different pellets, preferably each catalyst bed is based on essentially a single catalyst material.

[0036] The chemical reduction of nitrogen oxides with a reducing agent, as preferred according to the invention, may necessitate the dosing of reducing agents into the exhaust gas. For this purpose, dosing devices are preferably used, each of which may comprise several dosing units for the reducing agent. In preferred embodiments, the dosing is carried out in the direction of exhaust gas flow. Upstream of a catalyst bed, a single metering device with one or more injection units is arranged. In other preferred embodiments, fresh reducing agent is metered into the exhaust gas at various points, i.e., in different sections, along the exhaust gas flow direction via several injection units. As long as the catalyst material forms a single spatially enclosed and delimited unit, such embodiments also constitute a single catalyst bed within the meaning of the invention (see Figures 1B and IC). If, however, the multiple injection units along the exhaust gas flow direction require, for example, a subdivision of the catalyst material into several spatially enclosed and delimited units, then each unit constitutes its own catalyst bed; i.e., the multiple units then form several catalyst beds within the meaning of the invention (see Figures 1D-F).

[0037] Preferably, for step (b), i.e., for the non-catalytic reaction phase (SNCR), the reducing agent is added to the exhaust gas via a metering device, which for the purposes of this description is referred to as "metering device A" and with which the addition of reducing agent to the exhaust gas can be controlled via a "control valve a". Preferably, for step (d), i.e., for the catalytic reaction phase (SCR), the reducing agent is added to the exhaust gas via at least one further metering device. If step (d) is carried out in a single stage, i.e., on a single catalyst bed, this metering device is referred to for the purposes of this description as "metering device B". It is preferably arranged upstream of the single catalyst bed in the direction of exhaust gas flow. The addition of reducing agent to the exhaust gas can then be controlled via a "control valve b". If step (d) is carried out in two stages, i.e.,If the system consists of a first catalyst bed and a second catalyst bed, a metering device can be assigned to each of the two catalyst beds. Metering device B with control valve b is assigned to the first catalyst bed and is preferably arranged upstream of the first catalyst bed in the direction of exhaust gas flow. The metering of reducing agent into the exhaust gas can then be controlled via control valve b. Metering device C with control valve c is assigned to the second catalyst bed and is preferably arranged downstream of the first catalyst bed and upstream of the second catalyst bed in the direction of exhaust gas flow. The metering of reducing agent into the exhaust gas can then be controlled via control valve c.

[0038] In the literature, the term "stage" often refers to a key objective of the catalyzed reactions in a catalyst bed. The literature frequently distinguishes between so-called "DeNOx stages" and so-called "DeN2O stages." The primary objective of DeNOx stages is the catalytic degradation of NOx, typically through catalytic chemical reduction with a reducing agent for NOx. The primary objective of DeN2O stages is the catalytic degradation of N2O, typically through catalytic chemical reduction with a reducing agent for N2O and / or through catalytic decomposition. Since certain reducing agents, such as NH3, are Since reducing agents can be used for both NOx and N2O, and certain catalysts, such as iron-loaded zeolitic materials of a suitable structural type, can catalyze several of these reactions, it can certainly occur in practice that, in addition to the degradation of NOx, the degradation of N2O also takes place at a DeNOx stage, and possibly vice versa at a DeN2O stage, in addition to the degradation of N2O, the degradation of NOx also takes place. For this reason, the term "stage" has a more general meaning according to the invention. According to the invention, a "stage" corresponds to a catalyst bed, independent of the catalyzed reactions taking place thereon. A "single-stage" process according to the invention therefore uses a single catalyst bed (for the catalytic reaction phase, SCR), while a "two-stage" process according to the invention uses two separate and spatially isolated catalyst beds.

[0039] The term "reaction zone" refers, by design, to a specific section of one and the same catalyst bed through which the exhaust gas flows. Different reaction zones of one and the same catalyst bed are traversed sequentially by the exhaust gas. In different reaction zones, i.e., different sections of one and the same catalyst bed, different catalyzed reactions predominate, particularly due to the different reaction kinetics of potential competing reactions. For example, it is known that the chemical reduction of NOx with NH3 as the reducing agent for NOx on Fe-zeolites proceeds significantly faster than the chemical reduction of N2O with NH3 as the reducing agent for N2O. If an exhaust gas containing NH3, NOx, and N2O flows through a catalyst bed based on Fe-zeolite, the reaction in the front section of the catalyst bed, through which the exhaust gas initially flows, i.e.,In a "first reaction zone," typically NOx is initially produced predominantly through chemical reduction with NH3. Only when the NOx is practically completely degraded or the NH3 is practically completely consumed can the slower reactions take effect and predominate, i.e., in a "second reaction zone," for example, the chemical reduction of N2O with NH3 or the catalytic decomposition of N2O.

[0040] Figure 1 schematically illustrates preferred sequences of non-catalytic reaction phase (SNCR) and catalytic reaction phase (SCR) according to the invention. NH3 is used as an example of a reducing agent, which is metered into the exhaust gas via metering device A, B, or C.

[0041] The process shown in Figures 1A, 1B, and IC is "single-stage" with respect to the catalytic reaction phase (SCR), which takes place on a single catalyst bed. The additional injection of NH3 as an exemplary reducing agent, as shown in Figure 1B (in contrast to Figure 1A), or the injection of NH3 as an exemplary reducing agent at a different location, as shown in Figure IC (in contrast to Figure 1A), does not result in a spatial separation of the catalyst bed into separate units. Therefore, both of these process configurations are also "single-stage" within the meaning of the invention. Different preferred embodiments of the injection of NH3 as an exemplary reducing agent are implemented.

[0042] In contrast, the processes shown in Figures ID, IE, and 1F are each "two-stage" with regard to the catalytic reaction phase (SCR), which takes place sequentially on two delimited and spatially separated catalyst beds. Different preferred embodiments of the addition of NH3 as an exemplary reducing agent are implemented.

[0043] For descriptive purposes, all percentages are weight percentages unless explicitly stated otherwise.

[0044] For descriptive purposes, all values ​​in ppm are given in ppm, unless explicitly stated otherwise, and refer to volume, i.e., ppmv.

[0045] The inventive process serves to reduce the content of nitrogen oxide (nitrogen oxide) in an exhaust gas.

[0046] The nitrogen oxide whose concentration in the exhaust gas is reduced can, in principle, be any oxide of nitrogen, in particular NO, NO₂ (collectively referred to as "NOx" for descriptive purposes), N₂O, N₂O₄, etc. According to the invention, NOx is preferred as a nitrogen oxide. For descriptive purposes, the term "nitrogen oxide" includes not only individual oxides of nitrogen but also any mixture of several oxides of nitrogen, in particular NOx, in any relative proportion. In this context, reducing the nitrogen oxide concentration means that at least the concentration of one of the oxides of nitrogen, which may be present in a mixture with other oxides of nitrogen, is reduced in the exhaust gas. However, it is also possible, and according to the invention preferred, that the concentration of several oxides of nitrogen is reduced, to the same or different extents, simultaneously or sequentially.In particular, the N2O (nitrous oxide) content is to be reduced simultaneously with other nitrogen oxides, since N2O is a greenhouse gas that, according to the current classification by the UN and the IPCC, is approximately 273 times more harmful than CO2. This is another significant advantage of the invented process.

[0047] The inventive procedure comprises steps (a), (b), (c) and (d). Preferably, the steps are carried out in alphabetical order.

[0048] The inventive process may, in principle, include any further steps not mentioned. These further steps may be carried out independently of one another before, during, or after any of the steps (a), (b), (c), and (d) mentioned.

[0049] In preferred embodiments, the method according to the invention comprises the additional step (e) Production of H2 and N2 by catalytic decomposition of NH3, preferably with the supply of heat, which in step (a) is generated by burning NH3 in the presence of O2 and / or in step (b) is released during the non-catalytic chemical reduction of nitrogen oxide with reducing agent. Step (a)

[0050] In step (a) of the inventive process, NH3 is burned in the presence of O2, producing an exhaust gas containing nitrogen oxide. The exhaust gas containing the nitrogen oxide, the content of which is reduced, is thus produced by burning NH3 in the presence of O2, with the NH3 preferably being burned in a mixture with H2.

[0051] Preferably, the combustion of NH3 is non-catalyzed and yields N2 and H2O as the main products of the combustion reaction. The nitrogen oxide contained in the exhaust gas is thus an undesired byproduct formed during the combustion reaction, but not the desired main product. The non-catalytic combustion of NH3 preferred according to the invention therefore differs fundamentally from the catalytic combustion of NH3, as carried out, for example, in plants for the production of nitric acid with the aim of generating large quantities of NO and NO2.

[0052] In preferred embodiments, the nitrogen oxide contained in the exhaust gas comprises NOx, i.e. NO and NO2.

[0053] Preferably, the exhaust gas produced in step (a) has a NOx content of at least 500 ppm, preferably at least 1000 ppm, more preferably at least 2000 ppm, even more preferably at least 3000 ppm, most preferably at least 4000 ppm, and in particular at least 5000 ppm, or at least 6000 ppm.

[0054] In preferred embodiments, the NOx in the exhaust gas produced in step (a) has an oxidation level a(NOx) of at least 0.6, preferably at least 0.7, more preferably at least 0.8, even more preferably at least 0.85, most preferably at least 0.9, and in particular at least 0.95.

[0055] In preferred embodiments, the NOx in the exhaust gas produced in step (a) has an oxidation level a(NOx) of at most 0.9, preferably at most 0.7, more preferably at most 0.5, even more preferably at most 0.3, most preferably at most 0.2, and in particular at most 0.1.

[0056] The oxidation state a(NOx) is a measure of the relative molar ratio of NO2 to total NOx and is defined as the ratio of the molar concentration of NO2 to the sum of the molar concentrations of NO and NO2, i.e. a(NOx) = [NO2] / ([NO]+[NO2]).

[0057] In preferred embodiments, the nitrogen oxide contained in the exhaust gas comprises N2O.

[0058] Preferably, the exhaust gas produced in step (a) has an N2O content of at least 5 ppm, more preferably at least 10 ppm, more preferably at least 25 ppm, even more preferably at least 50 ppm, most preferably at least 100 ppm, and in particular at least 200 ppm. Preferably The exhaust gas produced in step (a) has an N2O content of at least 500 ppm, preferably at least 1000 ppm, more preferably at least 2000 ppm, more preferably at least 3000 ppm, most preferably at least 4000 ppm, and in particular at least 5000 ppm, or at least 6000 ppm.

[0059] Preferably, the exhaust gas produced in step (a) has a pressure of at most 1.5 bar abs; more preferably atmospheric pressure; even more preferably a pressure below atmospheric pressure, preferably at least -50 mbar below atmospheric pressure, more preferably at least -100 mbar. Preferably, the exhaust gas produced in step (a) has a pressure of at most 0.95 bar abs, more preferably at most 0.9 bar abs.

[0060] Preferably in step (a) the combustion of NH3 in the presence of O2 does not take place over a catalyst.

[0061] Preferably in step (a) for the combustion of NH3 in the presence of O2, the NH3, which may be present in a mixture with other combustible gases, e.g. H2 and / or CH4 (fuel), is mixed with O2 from the air (combustion air).

[0062] During the combustion of the fuel-air mixture, the air-fuel ratio X for combustion is preferably in the range of 0.9 to 1.7, more preferably 1.0 to 1.6, even more preferably 1.1 to 1.5, and most preferably 1.2 to 1.4. Another highly preferred range for the air-fuel ratio X lies between 1.0 and 1.2.

[0063] In particularly preferred embodiments, the air ratio is in the range of 1.06±0.06, preferably 1.06±0.05, more preferably 1.06±0.04, even more preferably 1.06±0.03, most preferably 1.06±0.02, and in particular 1.06±0.01.

[0064] The air-fuel ratio X (i.e., the combustion air ratio) indicates the mass ratio of combustion air to fuel relative to the stoichiometrically ideal ratio for a theoretically complete combustion process. It is defined as the air-fuel ratio that contains a sufficient mass of oxygen to achieve complete combustion of a given mass of fuel (see, e.g., K. Soman, Thermal Engineering, PHI, 2011, p. 224, no. 5.4.2). In principle, the ratio can be expressed in terms of mass or amount of substance (see, e.g., P. Majumdar, Design of Thermal Energy Systems, Wiley 2021, p. 66, no. 2.13.5.2). For the purposes of description, the ratio is expressed in terms of mass. If a combustion process uses a gas other than air containing oxygen, then strictly speaking, "air" should be replaced by "oxygen carrier." However, the parameter X is still used in the definition above.

[0065] The formation of nitrogen oxides is related, among other things, to the residual oxygen content in the exhaust gas, which can be, for example, 3 mol% or 1 mol%. To achieve a residual oxygen content of 1 mol%, for instance, an air-fuel ratio of approximately 1.06 is required.

[0066] Preferably, in step (a) NH3 is burned in a mixture with H2 and in the presence of O2.

[0067] In preferred embodiments, the proportion of H2 in the mixture with NH3 is at most 60 mol%, preferably at most 50 mol%, more preferably at most 40 mol%, even more preferably at most 30 mol%, most preferably at most 20 mol%, and in particular at most 10 mol%, based on the total amount of H2 and NH3.

[0068] In preferred embodiments, the proportion of H2 in the mixture with NH3 is at least 2.0 mol%, preferably at least 5.0 mol%, more preferably at least 10 mol%, even more preferably at least 20 mol%, most preferably at least 30 mol%, and in particular at least 40 mol%, based on the total amount of H2 and NH3.

[0069] In preferred embodiments, the exhaust gas produced in step (a) has an H2O content of more than 4.0 vol.%; preferably at least 5.0 vol.%, more preferably at least 6.0 vol.%, even more preferably at least 7.0 vol.%, most preferably at least 8.0 vol.%, and in particular at least 9.0 vol.%.

[0070] In preferred embodiments, the exhaust gas produced in step (a) has a H2O content of at least 10 vol.%; preferably at least 12 vol.%, more preferably at least 14 vol.%, even more preferably at least 16 vol.%, most preferably at least 18 vol.%, and in particular at least 20 vol.%.

[0071] In preferred embodiments, the exhaust gas produced in step (a) contains N2.

[0072] Preferably, the NH3 combusted in step (a) originates from unreacted reactant of a product gas stream from a plant for the catalytic decomposition of NH3 into N2 and H2. Preferably, the product H2 is removed from the product gas stream, which contains not only H2 and N2 but also undecomposed NH3, by pressure swing adsorption, and the remaining NEE-containing residual gas is fed to the combustion process according to step (a). Since, under economical operating conditions, the separation of H2 from the decomposition product is often incomplete, the remaining gas mixture contains not only undecomposed NH3 and N2 but also some unremoved H2. According to the invention, such a residual gas mixture is preferably fed to step (a).

[0073] The O2 in whose presence the NH3 is burned in step (a), possibly in a mixture with H2, is preferably air. However, it is also possible to use a gas mixture with a higher O2 content compared to air. Step (b)

[0074] In step (b) of the process according to the invention, the nitrogen oxide content in the exhaust gas is partially reduced by a non-catalytic chemical reduction, preferably by a Selective non-catalytic chemical reduction of nitrogen oxide with a reducing agent to produce a pretreated exhaust gas [non-catalytic reaction phase, SNCR].

[0075] Preferably, heat of reaction is released by the chemical reduction of nitrogen oxide in step (b).

[0076] The reduction of nitrogen oxide is partial, meaning that the pretreated exhaust gas obtained in step (b) still contains residual nitrogen oxide. Only the nitrogen oxide content is lower than before step (b).

[0077] The partial reduction of the nitrogen oxide content is achieved by non-catalytic chemical reduction, preferably as "selective non-catalytic reduction" (SNCR). In this process, a nitrogen-containing reducing agent, preferably NH₃ or urea, optionally as an aqueous solution, reacts with nitrogen oxide, preferably NOx, by thermolysis to form H₂O and N₂.

[0078] Preferably, in step (b), the reducing agent is selected from NH3 and urea, preferably NH3, wherein the reducing agent may optionally be in aqueous solution. It need not be a pure substance. Rather, according to the invention, it is possible, for example, to use NH3 or NEL. + to use mixtures containing -, for example waste mixtures.

[0079] Particularly preferred reducing agents for step (b) are listed in the table below:

[0080] Preferably, for step (b) the reducing agent is provided in a mixture with a carrier medium.

[0081] Preferably, the carrier medium is selected from steam, water and compressed air; preferably steam; preferably steam with a pressure of at most 5 bar a (low-pressure steam).

[0082] Preferably, the carrier medium and the reducing agent are sprayed with a nozzle. Preferably, the carrier medium and the reducing agent are fed separately to a two-component injection lance, at the end of which a nozzle is arranged in which the carrier medium and the reducing agent are mixed together and then sprayed. This has the advantage that, as a result of a greater penetration depth, better mixing of the reducing agent with the exhaust gas occurs.

[0083] Preferably, the carrier medium and the reducing agent are mixed together via a plurality of two-component injection lances and sprayed in the direction of the exhaust gas flow; preferably via at least 2 separate two-component injection lances.

[0084] Preferably, the amount of reducing agent added is determined by the amount of NOx nitrogen oxides to be reduced. Preferably, the amount of N2O present in the exhaust gas is also taken into account, which can preferably also be reduced, at least partially, by the reducing agent. The molar ratio of NH3, expressed as molar NH3 reduction equivalents, to NOx, based on the NOx reacted, is preferably in the range of 1.0 to 3.0, more preferably 1.5 to 3.0, even more preferably 1.5 to 2.5, and most preferably 1.75 to 2.75. The NOx reacted is the amount of NOx in the exhaust gas before step (b) minus the amount of NOx in the exhaust gas after step (b).

[0085] Preferably, the molar ratio of reducing agent, expressed in NHs reduction equivalents, to NOx, based on the NOx content in the exhaust gas after step (a) and before step (b), is in the range of 1.0 to 3.0, more preferably 1.5 to 3.0, even more preferably 1.5 to 2.5, and most preferably 1.75 to 2.75.

[0086] The reducing agent is expressed in NH3 reduction equivalents, i.e., as the equivalent amount of NH3. In the case of urea as the reducing agent, the equivalent amount of NH3 is adjusted accordingly, as 1 mole of urea releases 2 moles of NH3. When NH3 is used as the reducing agent, the factor is 1, meaning there is no change in the equivalent amount of NH3.

[0087] Preferably in step (b) the exhaust gas has a temperature in the range of 800 to 1075°C before the reduction of the nitrogen oxide content, more preferably 820 to 870°C, and even more preferably 850°C to 870°C.

[0088] Figure 2 illustrates, in a highly simplified manner, the approximate relationship between the reaction temperature and the reactions occurring during the non-catalytic chemical reduction of NOx with NH3. The non-catalytic chemical reduction of NOx with NH3 proceeds particularly efficiently at a temperature of approximately 975 °C. At higher temperatures, competing reactions increase, while at lower temperatures, the slip of NH3 increases. The preferred temperature range (A) of the inventive process differs from the optimal temperature range (B) of an SNCR as well as the typical temperature range (C) according to VDI 3927, Part 1.

[0089] Contrary to the usual operating mode of SNCR processes, step (b) according to the invention is preferably carried out at temperatures to the left of the temperature at which the non-catalytic chemical reduction of NOx has its maximum, i.e., at a lower temperature than the temperature optimal for the non-catalytic chemical reduction of NOx. The associated slip Breakthrough of the reducing agent for NOx is accepted according to the invention, because this reducing agent, which is not converted during the non-catalytic chemical reduction in step (b), can advantageously be utilized in the subsequent catalytic chemical reduction of NOx in step (d) without any significant slippage (breakthrough) of the reducing agent for NOx occurring in step (d). In this respect, the conditions preferred according to the invention for step (b) are outside the conditions that are conventionally typical for a non-catalytic chemical reduction of NOx according to VDI guideline VDI 3927 and have previously been considered necessary.

[0090] In preferred embodiments, in step (b) the exhaust gas has a temperature of at least 800°C before the reduction of the nitrogen oxide content, preferably at least 810°C, more preferably at least 820°C, even more preferably at least 830°C, most preferably at least 840°C, and in particular at least 850°C.

[0091] In preferred embodiments, in step (b) the exhaust gas has a temperature of at least 860°C before the reduction of the nitrogen oxide content, preferably at least 870°C, more preferably at least 880°C, even more preferably at least 890°C, most preferably at least 900°C, and in particular at least 910°C.

[0092] In preferred embodiments, in step (b) the exhaust gas has a temperature of at most 960°C before the reduction of the nitrogen oxide content, preferably at most 950°C, more preferably at most 940°C, even more preferably at most 930°C, most preferably at most 920°C, and in particular at most 910°C.

[0093] In preferred embodiments, in step (b) the exhaust gas has a temperature of at most 900°C before the reduction of the nitrogen oxide content, preferably at most 890°C, more preferably at most 880°C, even more preferably at most 870°C, most preferably at most 860°C, and in particular at most 850°C.

[0094] In preferred embodiments, in step (b) the exhaust gas has a temperature in the range of 800 to 950°C before the reduction of the nitrogen oxide content, preferably 800 to 900°C, more preferably 825 to 875°C.

[0095] In preferred embodiments, in step (b) the exhaust gas has a temperature in the range of 840±70°C before the reduction of the nitrogen oxide content, preferably 840±60°C, more preferably 840±50°C, even more preferably 840±40°C, most preferably 840±30°C, and in particular 840±20°C.

[0096] In preferred embodiments, in step (b) the exhaust gas has a temperature in the range of 850±70°C before the reduction of the nitrogen oxide content, preferably 850±60°C, more preferably 850±50°C, even more preferably 850±40°C, most preferably 850±30°C, and in particular 850±20°C.

[0097] In preferred embodiments, in step (b) the exhaust gas has a temperature in the range of 860±70°C before the reduction of the nitrogen oxide content, preferably 860±60°C, more preferably 860±50°C, even more preferably 860±40°C, most preferably 860±30°C, and in particular 860±20°C.

[0098] In preferred embodiments, in step (b) the exhaust gas has a temperature in the range of 870±70°C before the reduction of the nitrogen oxide content, preferably 870±60°C, more preferably 870±50°C, even more preferably 870±40°C, most preferably 870±30°C, and in particular 870±20°C.

[0099] In preferred embodiments, in step (b) the exhaust gas has a temperature in the range of 880±70°C before the reduction of the nitrogen oxide content, preferably 880±60°C, more preferably 880±50°C, even more preferably 880±40°C, most preferably 880±30°C, and in particular 880±20°C.

[0100] In preferred embodiments, in step (b) the exhaust gas has a temperature in the range of 890±70°C before the reduction of the nitrogen oxide content, preferably 890±60°C, more preferably 890±50°C, even more preferably 890±40°C, most preferably 890±30°C, and in particular 890±20°C.

[0101] In preferred embodiments, in step (b) the exhaust gas has a temperature in the range of 900±70°C before the reduction of the nitrogen oxide content, preferably 900±60°C, more preferably 900±50°C, even more preferably 900±40°C, most preferably 900±30°C, and in particular 900±20°C.

[0102] Even if, at this temperature, the efficiency of the non-catalytic chemical reduction of nitrogen oxide with reducing agent is possibly less than at its maximum, i.e., at the optimal temperature with maximum efficiency, the conversion rates achieved according to the invention are advantageous for various reasons: (1) If the exhaust gas contains a comparatively high concentration of nitrogen oxides, especially NOx, the catalyst volume required for further reduction by catalytic chemical reduction and / or decomposition in step (d) can be reduced accordingly by the upstream partial reduction by non-catalytic chemical reduction in step (b). It may also be possible to forgo a multi-stage design of the catalytic reaction phase (SCR), i.e., with multiple catalyst beds, in step (d). This reduces investment and maintenance costs for the reactor and the catalyst; (2) the additional investment costs for metering devices for adding reducing agent to the exhaust gas for step (b) are only minor; and (3) Due to the comparatively low temperature, a pronounced slip of reducing agent, in particular NH3, is expected in step (b), resulting in the pretreated exhaust gas, which has been fed to the downstream step (d), already containing a significant amount of reducing agent. This amount of reducing agent is advantageously consumed and thereby removed in step (d) for further reduction by catalytic chemical reduction. (4) The heat of reaction released in step (b) during the non-catalytic (selective) reduction of nitrogen oxides occurs at a significantly higher temperature level of 800°C to 1075°C compared to the selective catalytic reduction in step (d). This means that heat can be used more effectively in related processes, particularly in a plant complex for the catalytic decomposition of NH3, e.g. for preheating the NFh-Fccd-Gastromcs entering the reactors for the catalytic decomposition of NH3 or for generating high-pressure steam.

[0103] In addition to a suitable temperature, which according to the invention is preferably below the temperature optimal for non-catalytic chemical reduction, a homogeneous mixture of reducing agent and nitrogen oxides in the exhaust gas is desirable. Preferably, the reducing agent is added to the exhaust gas from various directions. For this purpose, the reducing agent is preferably added not only from below, but also preferably from above and / or from the side, and optionally also from below. The arrangement, position, orientation, and type of the metering devices or the addition units for the reducing agent, as well as the choice of propellant for the reducing agent, can be important factors in achieving the most homogeneous mixture possible.

[0104] Preferably, for step (b), the reducing agent is added to the exhaust gas via a metering device, which preferably comprises at least one dosing unit, more preferably at least one injection lance, and more preferably at least one two-component injection lance. For the purposes of description, this metering device for the reducing agent, which is added to the exhaust gas for step (b) for non-catalytic chemical reduction, is also referred to as "metering device A," which preferably comprises several dosing units and with which the dosing of reducing agent into the exhaust gas can preferably be controlled via a "control valve a." The entirety of these dosing units is then part of metering device A.

[0105] Preferably, for step (b) the reducing agent is added to the exhaust gas via a first addition unit of the metering device A, preferably a first injection lance, more preferably a first two-component injection lance.

[0106] Preferably, step (a) takes place in a combustion chamber and the reducing agent for step (b) is introduced into the combustion chamber.

[0107] In particularly preferred embodiments, step (a) takes place in a combustion chamber, the generated exhaust gas is then fed into a collecting device for collecting and conveying the exhaust gas is directed, and the reducing agent for step (b) is introduced into the collecting device for collecting and conveying the exhaust gas.

[0108] Preferably, the reducing agent for step (b) is introduced continuously.

[0109] Preferably, the collecting device for collecting and conveying the exhaust gas along its longitudinal extent can be conceptually divided into a first section and a directly adjoining, preferably equally long, second section; wherein, for step (b), the reducing agent is metered into the exhaust gas within the first section via a first addition unit of the metering device A, preferably a first injection lance, more preferably a first two-component injection lance, and in the second section via a second addition unit of the metering device A, more preferably a second injection lance, more preferably a second two-component injection lance.

[0110] In other preferred embodiments, the reducing agent for step (b) is subsequently added to the exhaust gas via a second addition unit of the metering device A, preferably a second injection lance, preferably a second two-component injection lance, after the collecting device for collecting and conveying the exhaust gas.

[0111] Preferably, the same reducing agent is added via the first addition unit and via the second addition unit for step (b).

[0112] Preferably, a larger quantity of reducing agent for step (b) is added via the second addition unit of the dosing device A than via the first addition unit of the dosing device A. Preferably, the ratio of the quantity for the first addition unit to the quantity for the second addition unit is approximately 3:4.

[0113] Preferably, the angle between the direction of flow of the exhaust gas and the direction of the addition of the reducing agent for step (b) for the first addition unit and the second addition unit is independently in the range of 30° to 70°.

[0114] Preferably, the reducing agent for step (b) is metered into the exhaust gas stream via the first metering unit of the metering device A at a different angle than via the second metering unit of the metering device A. Preferably, this angle is smaller for the first metering unit than for the second. Preferably, this angle α for the first metering unit is in the range of 20° to 40°, more preferably 25° to 35°; and the angle β for the second metering unit is in the range of 35° to 55°, more preferably 40° to 50° (see Figure 4C). The angle α and the angle β are enclosed by the metering direction (in the case of a nozzle, by the primary spray direction) and the flow direction of the exhaust gas from the first metering unit towards the second metering unit.

[0115] In preferred embodiments, the pretreated exhaust gas produced in step (b) has a NOx content reduced by at least 5%, preferably at least 10%, more preferably at least 15%, more preferably at least 20%, most preferably at least 25%, and in particular at least 30% lower than the NOx content in the exhaust gas produced in step (a).

[0116] In preferred embodiments, the pretreated exhaust gas produced in step (b) has a NOx content that is at least 35%, preferably at least 40%, more preferably at least 45%, more preferably at least 50%, most preferably at least 55%, and in particular at least 60% lower than the NOx content in the exhaust gas produced in step (a).

[0117] In preferred embodiments, the pretreated exhaust gas produced in step (b) has a NOx content that is at most 90%, preferably at most 80%, more preferably at most 70%, more preferably at most 60%, most preferably at most 50%, and in particular at most 40% lower than the NOx content in the exhaust gas produced in step (a).

[0118] In preferred embodiments, the pretreated exhaust gas produced in step (b) has a NOx content in the range of 30 to 80%, preferably 40 to 75%, more preferably 50 to 70%, based on the NOx content in the exhaust gas produced in step (a), i.e. the reduction of NOx in the non-catalytic reaction phase (SNCR) is preferably 20 to 70%, more preferably 25 to 60%, and more preferably 30 to 50%.

[0119] In preferred embodiments, the pretreated exhaust gas produced in step (b) has a NOx content of at least 500 ppm, preferably at least 1000 ppm, more preferably at least 2000 ppm, more preferably at least 3000 ppm, most preferably at least 4000 ppm, and in particular at least 5000 ppm, or at least 6000 ppm.

[0120] Preferably, the NOx has an oxidation degree a(NOx) of at least 0.05, preferably at least 0.1, more preferably at least 0.2, even more preferably at least 0.3, most preferably at least 0.4, and in particular at least 0.5.

[0121] Preferably, the NOx has an oxidation degree a(NOx) of at least 0.6, preferably at least 0.7, more preferably at least 0.8, even more preferably at least 0.85, most preferably at least 0.9, and in particular at least 0.95.

[0122] Preferably, the NOx has an oxidation degree a(NOx) of at most 0.9, preferably at most 0.7, more preferably at most 0.5, even more preferably at most 0.3, most preferably at most 0.2, and in particular at most 0.1.

[0123] In preferred embodiments, the pretreated exhaust gas produced in step (b) comprises N2O.

[0124] In preferred embodiments, the pretreated exhaust gas produced in step (b) has an N2O content that is at least 5%, preferably at least 10%, more preferably at least 15%, more preferably at least 20%, most preferably at least 25%, and in particular at least 30% lower than the N2O content in the exhaust gas produced in step (a).

[0125] In preferred embodiments, the pretreated exhaust gas produced in step (b) has an N2O content which is at most 30%, preferably at most 25%, more preferably at most 20%, more preferably at most 15%, most preferably at most 10%, and in particular at most 5% lower than the N2O content in the exhaust gas produced in step (a).

[0126] In preferred embodiments, the pretreated exhaust gas produced in step (b) has an N2O content of at least 5 ppm, preferably at least 10 ppm, more preferably at least 25 ppm, more preferably at least 50 ppm, most preferably at least 100 ppm, and in particular at least 200 ppm.

[0127] In preferred embodiments, the pretreated exhaust gas produced in step (b) has an N2O content of at least 500 ppm, preferably at least 1000 ppm, more preferably at least 2000 ppm, even more preferably at least 3000 ppm, most preferably at least 4000 ppm, and in particular at least 5000 ppm, or at least 6000 ppm.

[0128] In preferred embodiments, the pretreated exhaust gas produced in step (b) has a residual content of unused reducing agent.

[0129] In preferred embodiments, the unused reducing agent is in the same form as it was added to the exhaust gas for step (b), preferably as NH3.

[0130] In other preferred embodiments, the unused reducing agent is in a different form, preferably NH3, than that which was added to the exhaust gas for step (b), preferably urea.

[0131] The non-catalytic chemical reduction of NOx with NH3 or urea is based on many partial reactions, the equilibrium of which depends on the reaction temperature and the initial concentration of the compounds involved. Even with a superstoichiometric ratio of reducing agent to NOx, NO cannot be completely removed. Likewise, some of the reducing agent re-emerges as NH3 from the reaction. Thus, even at the maximum reduction rate for NO around 950°C, there is a "slippage" of NH3. This "slippage" increases with lower temperatures. At higher temperatures, more NO is re-emerged from the overall reaction.

[0132] Depending on the process temperature, many different partial and side reactions take place. The situation is complex. The following explanations are greatly simplified.

[0133] For NH3 and NHraq, the chemical reduction reactions proceed, in simplified terms, as follows: 4 NO + 4 NH3+ O2^ 4 N2+ 6 H2O |-1627 kJ / mol| (1) 6 NO2+ 8 NH37 N2+ 12 H2O 1-2732 kJ / mol| (2).

[0134] For urea aq., the chemical reduction reactions proceed in simplified terms as follows: 4 NO + 2 NH2CONH2 + O24 N2+ 4 H2O + 2 CO2(3) 2 N02+ 4 NH2CONH2 +4 025 N2+ 8 H20 + 4 C02(4).

[0135] The reaction begins with the formation of NH2* radicals through the reaction of O2* radicals or OH* radicals with NH3, whereas urea is thermally decomposed after injection into the exhaust gas: 4 NH3 + 2 O2* 4 W + 4 OH* (5) NH3 + OH* - NH2* + H2O (6) NH2CONH2 [NH3+ HNCO] 2 NH2* + CO (7).

[0136] Finally, the NH3* radicals react with nitrogen oxides to form N2 and H2O, e.g.: NH2* + NO N2+ H2O (8) NH2* + NO2N2+ H2O + 'N O2(9).

[0137] In theory, with NH3 as the reducing agent, the stoichiometry of the reduction reaction is 1% for pure NO and 1% for pure NO2, assuming no other side reactions and an infinite residence time. In practice, however, significant amounts of the reducing agent, often 40% or more, are lost through the oxidation of NH3 to N2 and H2O and through other side reactions.

[0138] In preferred embodiments, the pretreated exhaust gas produced in step (b) has a residual content of unused reducing agent, preferably NH3, which is at least 5%, more preferably at least 10%, more preferably at least 15%, more preferably at least 20%, most preferably at least 25%, and in particular at least 30%, in each case based on the amount of reducing agent used in step (b).

[0139] In preferred embodiments, the pretreated exhaust gas produced in step (b) has a residual content of unused reducing agent, preferably NH3, which is at most 30%, more preferably at most 25%, more preferably at most 20%, more preferably at most 15%, most preferably at most 10%, and in particular at most 5%, in each case based on the amount of reducing agent used in step (b).

[0140] In preferred embodiments, the pretreated exhaust gas produced in step (b) has a residual content of unconsumed reducing agent, preferably NH3, of at least 5 ppm, preferably at least 10 ppm, more preferably at least 15 ppm, more preferably at least 20 ppm, most preferably at least 25 ppm, and in particular at least 30 ppm.

[0141] In preferred embodiments, the pretreated exhaust gas produced in step (b) has a residual content of unconsumed reducing agent, preferably NH3, of at least 35 ppm, preferably at least 40 ppm, more preferably at least 45 ppm, more preferably at least 50 ppm, most preferably at least 55 ppm, and in particular at least 60 ppm.

[0142] In preferred embodiments, the pretreated exhaust gas produced in step (b) has a residual content of unconsumed reducing agent, preferably NH3, of at least 65 ppm, preferably at least 70 ppm, more preferably at least 75 ppm, more preferably at least 80 ppm, most preferably at least 85 ppm, and in particular at least 90 ppm.

[0143] In preferred embodiments, the pretreated exhaust gas produced in step (b) has a residual content of unconsumed reducing agent, preferably NH3, of at most 600 ppm, preferably at most 500 ppm, more preferably at most 400 ppm, even more preferably at most 300 ppm, most preferably at most 200 ppm, and in particular at most 100 ppm.

[0144] In other preferred embodiments, the pretreated exhaust gas produced in step (b) has a residual content of unconsumed reducing agent, preferably NH3, of at most 90 ppm, preferably at most 80 ppm, more preferably at most 70 ppm, more preferably at most 60 ppm, most preferably at most 50 ppm, and in particular at most 40 ppm.

[0145] In other preferred embodiments, the pretreated exhaust gas produced in step (b) has a residual content of unconsumed reducing agent, preferably NH3, of at most 30 ppm, preferably at most 25 ppm, more preferably at most 20 ppm, more preferably at most 15 ppm, most preferably at most 10 ppm, and in particular at most 5 ppm.

[0146] In preferred embodiments, the pretreated exhaust gas produced in step (b) has a residual content of unused reducing agent, preferably NH3, of at least 20 mg / m³ 3 preferably at least 40 mg / m³ 3 , preferably at least 60 mg / m³ 3 , preferably at least 80 mg / m² 3 , preferably at least 100 mg / m² 3 , and in particular at least 120 mg / m² 3 .

[0147] In particularly preferred embodiments, this residual content of unused reducing agent, preferably NH3, is used at least partially as a reducing agent in step (d).

[0148] In preferred embodiments, the reducing agent is added in step (b) in such an amount that the residual reducing agent content of the pretreated exhaust gas after step (b), expressed as molar NEE reduction equivalents, is less than or equal to the molar residual NOx content. In a particularly preferred embodiment, the molar ratio of the residual NHs reduction equivalents content to the residual NOx content is 0.1 to 1.0, more preferably 0.2 to 0.9, and even more preferably 0.3 to 0.8. These ratios according to the invention enable, on the one hand, the achievement of the desired NOx reduction rates in step (b) with a well-controlled and adjustable dosage of reducing agent, and on the other hand, also an effective and well-controlled dosage of reducing agent for the catalytic reduction of the remaining nitrogen oxides in step (d). Step (c)

[0149] In step (c) of the inventive process, the pretreated exhaust gas is cooled. Preferably, heat obtained from the exhaust gas for cooling is used for step (e) of the The process according to the invention is used, i.e., to generate H2 and N2 by catalytic decomposition of NH3 by supplying heat, which is released in step (a) when burning NH3 in the presence of O2 and / or in step (b) when non-catalytic chemical reduction of nitrogen oxide with reducing agent.

[0150] Since heat of reaction is preferably released during the non-catalytic chemical reduction of nitrogen oxide with a reducing agent in step (b), at least some of the heat used for step (e) of the process according to the invention is preferably this heat of reaction. Therefore, the heat of reaction released in step (b) of the process according to the invention, as specified, for example, for reaction equations (1) and (2), is preferably used for step (e) of the process according to the invention.

[0151] Preferably in step (c) the pretreated exhaust gas is cooled by at least 50°C, preferably at least 100°C, more preferably at least 150°C, even more preferably at least 200°C, most preferably at least 250°C, and in particular at least 300°C.

[0152] For cooling, and preferably also for utilizing the heat of reaction released in step (b) of the process according to the invention, heat exchangers known to those skilled in the art can be used. The heat can be transferred to various heat transfer media, e.g. NH3, water or air. Step (d)

[0153] In step (d) of the process according to the invention, the nitrogen oxide content in the pretreated exhaust gas is further reduced. - by catalytic chemical reduction of nitrogen oxide, preferably by selective catalytic chemical reduction (SCR), with a reducing agent; and / or - by catalytic decomposition of nitrogen oxide to produce a post-treated exhaust gas [catalytic reaction phase, SCR].

[0154] Nitrogen oxides that can be reduced by catalytic chemical reduction, preferably by selective catalytic chemical reduction (SCR), with a reducing agent are NOx (NO and NO2) and N2O. Catalysts suitable for catalyzing these chemical reductions are also referred to as "NOx reduction catalyst" and "N2O reduction catalyst" for descriptive purposes. Catalytic chemical reduction of NOx on a NOx reduction catalyst

[0155] Preferred are NOx reduction catalysts which enable the most selective catalytic reduction (SCR) of the NOx contained in the exhaust gas, i.e. the NOx reduction catalysts primarily catalyze the oxidation of NH3 with NOx and not, or secondarily, the oxidation of NH3 with any free oxygen (O2) that may be present in the exhaust gas.

[0156] The chemical reduction of NOx with a reducing agent yields different reaction products depending on the reducing agent. In the case of the reducing agent NH3, which is preferred according to the invention, the chemical reduction of NOx primarily yields N2 and H2O, depending on the type of NOx reduction catalyst and the NO to NO2 ratio, e.g., according to: 4 NH3+ 2 NO + 2 NO24 N2+ 6 H2O (so-called almost SCR) (10) 4 NH3 + 4 NO + O24 N2+ 6 H2O (so-called normal SCR) (11) 8 NH3+ 6 NO27 N2+ 12 H2O (so-called NO2SCR) (12)

[0157] The combined selective catalytic reduction is called fast SCR and generally proceeds much faster than the so-called normal SCR or NO2 SCR.

[0158] A "NOx reduction catalyst" according to the invention catalyzes the chemical reduction of NOx with a reducing agent. However, the catalytic activity of a NOx reduction catalyst need not be limited exclusively to this reaction. It is quite possible, and indeed preferred according to the invention, that the NOx reduction catalyst can additionally catalyze further reactions, for example, the decomposition of N₂O, the chemical reduction of N₂O, and / or the establishment of NOx equilibrium, or even the selective oxidation of excess NH₃ with free O₂. Whether such further reactions actually occur depends on the specific conditions, i.e., the type of catalyst and the kinetics of any parallel processes, for example, the presence or amount of the reducing agent and the presence or amount of other reactants.

[0159] NOx reduction catalysts are known per se, and a wide variety of material classes can be used as catalytically active components, as listed, for example, as SCR catalysts in Han, L., Cai, S., Gao, M., Hasegawa, J., Wang, P., Zhang, J., Shi, L., Zhang, D., (2019) Selective Catalytic Reduction of NOx with NH3 by Using Novel Catalysts: State of the Art and Future Prospects. Chem. Rev., 119 (19), 10916-10976, or in Janssen, F. Handbook of heterogeneous Catalysis, Vol. 4, chap. 1.2 Environmental Catalysis - Stationary Sources, p.1633-1668, Wiley-VCH, Weinheim, 1997. Preferred examples of NOx reduction catalysts according to the invention are metal-loaded zeolite catalysts, precious metal catalysts or transition metal oxide catalysts.

[0160] The NOx reduction catalyst preferably comprises zeolite catalysts loaded with transition metals (including the lanthanides). Preferably selected as transition metals are cobalt, particularly copper, and most preferably iron. Other possible transition metals, which preferably occur together with cobalt, copper, and / or iron in the zeolite, are manganese, vanadium, chromium, or nickel. The zeolites are preferably high-silica zeolites exhibiting high hydrothermal resistance. Preferably, the zeolites are selected from the group of types MFI, BEA, FER, MOR, and MEL, or mixtures thereof, preferably from the group consisting of... Type BEA or MFI, preferably a BEA zeolite. Further particularly preferred catalysts are the classical A^CE-TiCh-based SCR catalysts widely used in NOx-SCR, preferably doped with WO3.

[0161] Supported catalysts, in which the catalytically active compounds are applied to a support material, are generally preferred. The support materials are preferably refractory oxides such as SiO₂, TiO₂, ZrO₂, MgO, or Al₂O₃, or mixtures of two or more of these, or derived ceramic materials such as hydrotalcite or cordierite. In classic V₂O₃-TiO₂-based SCR catalysts, TiO₂ itself is preferably used as the support material, particularly glass fiber-impregnated. Catalytic chemical reduction of N2O using an N2O reduction catalyst

[0162] N2O reduction catalysts are preferred which enable the selective catalytic reduction (SCR) of the N2O contained in the exhaust gas, i.e. the N2O reduction catalysts catalyze the oxidation of NH3 with N2O, possibly in parallel or secondarily to the oxidation of NH3 with NOx and not or secondarily the oxidation of NH3 with free oxygen (O2) that may be present in the exhaust gas.

[0163] In the chemical reduction of N2O with a reducing agent, different reaction products are formed depending on the reducing agent.

[0164] In the case of the reducing agent NH3, which is preferred according to the invention, N2 and H2O are formed in particular during the chemical reduction of N2O, e.g. according to: 3 N2O + 2 NH34 N2 + 3 H2O or 4 N2O + 4 NH3 + O2 6 N2 + 6 H2O or also in the joint reduction with NO according to 2NO + N2O + 2 NH33 N2+ 3 H2O.

[0165] In the case of hydrocarbons, which are also preferred as reducing agents according to the invention, CO and H2O are formed in particular during the chemical reduction of N2O, e.g. according to (2n+I) N2O + CnH 2n +2 (2n+I) N2+ n CO + (n+I) H2O or also CO2 and H2O according to 4n N2O + C n H2n+2 — * 4n N2 + n CO2 + 2n H2O.

[0166] According to the invention, CO is also suitable as a reducing agent. It can react further with N2O to form CO2, e.g. according to: N2O + CO N2+ CO2.

[0167] An "N₂O reduction catalyst" according to the invention catalyzes the chemical reduction of N₂O with a reducing agent. However, the catalytic activity of an N₂O reduction catalyst need not be limited exclusively to this reaction. It is quite possible, and indeed preferred according to the invention, that the N₂O reduction catalyst can also catalyze other reactions, for example, the decomposition of N₂O and / or the chemical reduction of NOx. Whether such other reactions actually occur depends on the conditions of the individual case, in particular the type of catalyst, and the kinetics of any parallel processes, for example, the presence or amount of the reducing agent and the presence or amount of other reactants.

[0168] N₂O reduction catalysts are also known per se, and a wide variety of material classes can be used. Examples include zeolite catalysts loaded with transition metals (including the lanthanides). Preferably selected as transition metals are cobalt, particularly copper, and most preferably iron. Other possible transition metals, which preferably occur together with cobalt, copper, and / or iron in the zeolite, are manganese, vanadium, chromium, or nickel. The zeolites are preferably SiO₂-rich zeolites, so-called "high silica" zeolites, which exhibit high hydrothermal resistance.Particularly preferred are “high-silica zeolites” which have a molar ratio of [SiC>2] to [AICU] units, and thus a molar Si / Al ratio, of at least 8, preferably at least 9, more preferably at least 10, even more preferably at least 11, most preferably at least 12, and in particular at least 13.

[0169] Preferably the zeolites are selected from the group of types MFI, BEA, FER, MOR and MEL or mixtures thereof, preferably of type BEA or MFI, particularly preferably a BEA zeolite. Catalytic decomposition of N2O on an N2O decomposition catalyst

[0170] A nitrogen oxide that can be broken down by catalytic decomposition is N₂O. Catalysts suitable for catalyzing this decomposition are also referred to as "N₂O decomposition catalysts" for descriptive purposes.

[0171] The decomposition of N2O produces N2 and O2 according to the following overall reaction: 2 N2O 2 N2+ O2.

[0172] Decomposition of N₂O therefore means decomposition into N₂ and O₂. An "N₂O decomposition catalyst" according to the invention catalyzes the decomposition of N₂O. The achievable reduction of N₂O through catalytic decomposition depends not only on the type, i.e., the chemical nature and physical design of the N₂O decomposition catalyst and the prevailing pressure and temperature conditions, but also, and perhaps most importantly, on the selected space velocity, i.e., the ratio of exhaust gas volume flow to catalyst volume. However, the catalytic activity of an N₂O decomposition catalyst must The N2O decomposition catalyst is not limited exclusively to this conversion. It is quite possible, and indeed preferred according to the invention, that the N2O decomposition catalyst can also catalyze further conversions, for example, the chemical reduction of N2O and / or the chemical reduction of NOx. Whether such further conversions actually take place depends on the conditions of the individual case, in particular the type of catalyst, and the kinetics of any parallel processes, for example, the presence or amount of the reducing agent and the presence or amount of other reactants.

[0173] N₂O decomposition catalysts are known per se, and a wide variety of material classes can be used. N₂O decomposition catalysts are preferred that exhibit, for example, high catalytic activity for the decomposition of N₂O into N₂ and O₂ in the temperature range of 350 to 600°C.

[0174] Preferred examples of N₂O decomposition catalysts according to the invention are metal-loaded zeolite catalysts, for example, zeolite catalysts loaded with copper or cobalt, or especially with iron, precious metal catalysts, or transition metal oxide catalysts, such as catalysts containing cobalt oxide. Examples of suitable catalysts are described, among others, by Kapteijn et al. in Appl. Cat. B: Environmental 9 (1996), 25-64, in US-A-5, 171, 553, in Actes du 2ieme Congres International sur la Catalyse, Technip, Paris 1961, 1937-1953, and in WO-A-01 / 58, 570. When using iron-loaded zeolite catalysts, the NOx still present in the gas accelerates the desired N2O decomposition as expected through an activating effect (co-catalytic effect), as described for different N2O / NOx ratios by Kögel et al. in Catal. Comm. 2 (2001) 273-276. Variants of the procedure in step (d)

[0175] In carrying out step (d) of the process according to the invention, there are various preferred variants of the process, which may differ from one another with regard to the reactions taking place, the sequence of the reactions taking place, the number of catalyst beds (stages), the catalysts used, the reducing agents used, the space velocities and other reaction conditions.

[0176] In step (d) of the process according to the invention, the post-treated exhaust gas can be produced by further reducing the content of the following nitrogen oxides in the pre-treated exhaust gas by (di) Catalytic chemical reduction of NOx, preferably by selective catalytic chemical reduction (SCR), with a reducing agent at a NOx reduction catalyst; and / or (d2) catalytic chemical reduction of N2O, preferably by selective catalytic chemical reduction (SCR), with a reducing agent on an N2O reduction catalyst; and / or (ds) catalytic decomposition of N2O at an NsO decomposition catalyst.

[0177] In preferred embodiments, these reactions are carried out in a common catalyst bed, which is equipped upstream with a metering device B for adding reducing agent to the exhaust gas [single-stage catalytic reaction phase, SCR].

[0178] In other preferred embodiments, these reactions are carried out in two successively arranged, separate catalyst beds, of which preferably at least one, and preferably both, catalyst beds are equipped independently of each other upstream in the direction of exhaust gas flow with a metering device B and optionally with a metering device C for adding reducing agent to the exhaust gas. The exhaust gas then flows first through the first catalyst bed and subsequently through the second catalyst bed [two-stage catalytic reaction phase, SCR].

[0179] In the case of a single-stage process for step (d), at least two metering devices are preferably used to add reducing agent to the exhaust gas (see Figure 1A), namely - the dosing device A for step (b) [non-catalytic reaction phase, SNCR] as well as - the dosing device B for step (d) [single-stage catalytic reaction phase, SCR].

[0180] Depending on the design of the single catalyst bed, the reducing agent can be added at different locations via metering device B (see Figures 1A, 1B and IC).

[0181] In the case of a two-stage process of step (d), different embodiments can also be distinguished (see Figures 1D-F).

[0182] In preferred embodiments, in the case of a two-stage process, at least two metering devices are used to add reducing agent to the exhaust gas (see Figure IE), namely - the dosing device A for step (b) [non-catalytic reaction phase, SNCR] as well as - the metering device B for the first stage of step (d) [first catalyst bed of the two-stage catalytic reaction phase, SCR].

[0183] In further preferred embodiments, in the case of a two-stage process, at least two metering devices are also used for adding reducing agent to the exhaust gas (see Figure 1F), namely - the dosing device A for step (b) [non-catalytic reaction phase, SNCR], - the metering device C for the second stage of step (d) [second catalyst bed of the two-stage catalytic reaction phase, SCR].

[0184] In other preferred embodiments, in the case of a two-stage process, a total of at least three metering devices are used for adding reducing agent to the exhaust gas (see Figure ID), namely - the dosing device A for step (b) [non-catalytic reaction phase, SNCR], - the metering device B for the first stage of step (d) [first catalyst bed of the two-stage catalytic reaction phase, SCR] as well as - the metering device C for the second stage of step (d) [second catalyst bed of the two-stage catalytic reaction phase, SCR].

[0185] For descriptive purposes, "*" denotes a process step that was previously only partially carried out in a similar process step, whereby the process step marked with "*" then continues the previously only partially carried out process step, possibly, however, in a different catalyst bed. As with all other process steps, unless explicitly stated otherwise, the result achieved at the end of all process steps is not quantitatively defined. Thus, for example, if NOx is chemically reduced incompletely in a first process step (di), the fact that process step (di *) is subsequently carried out does not necessarily imply that the total amount of NOx must be completely chemically reduced at the end of process step (di *), i.e., down to 0.0 ppmv.Rather, it is quite possible that a residual amount of NOx is still present at the end of process step (di*).

[0186] This does not mean, however, that the explicitly mentioned reactions are necessarily the only reactions that take place in the respective catalyst bed. Depending on the catalyst used, it is, according to the invention, preferable for additional reactions to occur simultaneously, which are not explicitly mentioned but can proceed in parallel. The explicitly mentioned reactions are therefore only those reactions that take place at a minimum in the respective variant / implementation.

[0187] When NOx, N₂O, and NH₃ are present in a mixture, and the catalyst used catalyzes both the chemical reduction of NOx with NH₃ and the chemical reduction of N₂O with NH₃, the chemical reduction of NOx with NH₃ is typically significantly faster than the chemical reduction of N₂O with NH₃. If the catalyst also catalyzes the decomposition of N₂O, the decomposition of N₂O is typically superimposed on the chemical reduction of N₂O with NH₃, and the extent of the chemical reduction of N₂O can be increased by increasing the amount of NH₃ added.

[0188] In particularly preferred embodiments, the reactions for reducing nitrogen oxides, or steps (di), (d2) and / or (d,), can optionally take place together in a single catalyst bed. The catalytic reaction phase (SCR) then proceeds in a single stage and comprises the following steps on the single catalyst bed: [a] (di) the chemical reduction of NOx with NH3; [b] (di) the chemical reduction of NOx with NH3 and (d2) the chemical reduction of N2O with NH3; [c] (di) the chemical reduction of NOx with NH3, and (d2) the chemical reduction of N2O with hydrocarbons (CH4, natural gas, etc.); [d] (di) the chemical reduction of NOx with NH3, and (d2) the chemical reduction of N2O with NH3 and hydrocarbon (CH4, natural gas, etc.); [e] (di) the chemical reduction of NOx with NH3 and (d,) the decomposition of N2O; [f] (di) the chemical reduction of NOx with NH3 and (d2) the chemical reduction of N2O with NH3 and (d, ) the decomposition of N2O; [g] (di) the chemical reduction of NOx with NH3 and (d2) the chemical reduction of N2O with hydrocarbons (CH4, natural gas, etc.) and (d, ) the decomposition of N2O; or [h] (di) the chemical reduction of NOx with NH3 and (d2) the chemical reduction of N2O with NH3 and hydrocarbon (CH4, natural gas, etc.) and (d,) the decomposition of N2O.

[0189] These single-stage variants [a], [b], [c], [d], [e], [f], [g] and [h] are particularly preferred according to the invention.

[0190] In other particularly preferred variants / embodiments, the reactions for reducing nitrogen oxides, or steps (di), (d2) and / or (d,), can optionally take place sequentially in a first reaction zone and a second reaction zone arranged downstream in the direction of exhaust gas flow within one and the same catalyst bed. The catalytic reaction phase (SCR) then also occurs in a single stage and comprises the reaction zones on the single catalyst bed. [i] (di) the incomplete chemical reduction of NOx, and (d,) the decomposition of N2O preferably in a first reaction zone of the single catalyst bed; and subsequently and (di*) the chemical reduction of further NOx with NH3, preferably in a second reaction zone of the single catalyst bed; [j] (di) the incomplete chemical reduction of NOx and (d2) the incomplete decomposition of N2O, preferably in a first reaction zone of the single catalyst bed; and subsequently (di*) the chemical reduction of further NOx with NH3 and (d2*) the chemical reduction of further N2O with NH3, preferably in a second reaction zone of the single catalyst bed; [k] (di) the incomplete chemical reduction of NOx and (d2) the incomplete decomposition of N2O, preferably in a first reaction zone of the single catalyst bed; and subsequently (di *) the chemical reduction of further NOx with NH3 and (d2*) the chemical reduction of further N2O with hydrocarbon (CH4, natural gas, etc.), preferably in a second reaction zone of the single catalyst bed; [l] (di) the incomplete chemical reduction of NOx and (d2) the incomplete decomposition of N2O, preferably in a first reaction zone of the single catalyst bed; and subsequently (di*) the chemical reduction of further NOx with NH3 and (d2*) the chemical reduction of further N2O with NH3 and hydrocarbon (CH4, natural gas, etc.), preferably in a second reaction zone of the single catalyst bed; or [m] (di) the complete chemical reduction of NOx and (d,) the incomplete reduction of N2O with NH3, preferably in a first reaction zone of the single catalyst bed; and subsequently (di* ) the chemical reduction of further N2O with hydrocarbon (CH4, natural gas, etc.), preferably in a second reaction zone of the single catalyst bed.

[0191] These single-stage variants [i], [j], [k], [1] and [m] are also particularly preferred according to the invention.

[0192] In further particularly preferred variants / embodiments, the reactions for reducing nitrogen oxides, or steps (di), (d2) and / or (d,), can optionally take place sequentially in a first catalyst bed and a spatially separated second catalyst bed arranged downstream in the direction of exhaust gas flow. The catalytic reaction phase (SCR) then proceeds in (at least) two stages and comprises [i 1] (di) the incomplete chemical reduction of NOx, and (d,) the decomposition of N2O preferably in a first catalyst bed; and subsequently and (di*) the chemical reduction of further NOx with NH3, preferably in a second catalyst bed; [j 1 ] (di) the incomplete chemical reduction of NOx and (d2) the incomplete decomposition of N2O, preferably in a first catalyst bed; and subsequently (di*) the chemical reduction of further NOx with NH3 and (d2*) the chemical reduction of further N2O with NH3, preferably in a second catalyst bed; [k 1] (di) the incomplete chemical reduction of NOx and (d2) the incomplete decomposition of N2O, preferably in a first catalyst bed; and subsequently (di*) the chemical reduction of further NOx with NH3 and (d2*) the chemical reduction of further N2O with hydrocarbon (CH4, natural gas, etc.), preferably in a second catalyst bed; [I 1 ] (di) the incomplete chemical reduction of NOx and (d2) the incomplete decomposition of N2O, preferably in a first catalyst bed; and subsequently (di*) the chemical reduction of further NOx with NH3 and (d2*) the chemical reduction of further N2O with NH3 and hydrocarbon (CH4, natural gas, etc.), preferably in a second catalyst bed; or [m 1] (di) the complete chemical reduction of NOx and (d,) the incomplete reduction of N2O with NH3, preferably in a first catalyst bed; and subsequently (ds*) the chemical reduction of further N2O with hydrocarbon (CH4, natural gas, etc.), preferably in a second catalyst bed.

[0193] These (at least) two-stage variants [i 1 ], [j 1 ], [k 1 ], [I 1 ] and [m 1 ] are also particularly preferred according to the invention.

[0194] Step (d) of the process according to the invention can proceed in a single stage within a single catalyst bed. Such a process configuration is particularly preferred according to the invention. In this case, the catalyst used in this single catalyst bed preferably serves as an N₂O decomposition catalyst and / or an N₂O reduction catalyst, as well as a NOx reduction catalyst. In this case, the catalyzed reactions preferably take place substantially simultaneously within this single catalyst bed. However, it should be noted that the kinetics of the individual reactions can differ considerably. For example, depending on the catalyst material used, the chemical reduction of NOx with NH₃ as a reducing agent will usually proceed significantly faster than the chemical reduction of N₂O with NH₃.If NOx and N2O are present in the mixture and NH3 is introduced as a reducing agent, different reactions take place in the front section (first reaction zone), the single catalyst bed, than in the rear section (second reaction zone). Due to its faster kinetics, the chemical reduction of NOx predominates in the front section (first reaction zone), while the chemical reduction of N2O only occurs in the rear section (second reaction zone) once most of the NOx has been reduced.

[0195] Alternatively, step (d) of the inventive process can proceed in (at least) two stages: in a first catalyst bed and in a second catalyst bed arranged downstream in the direction of exhaust gas flow. It is also possible for further catalyst beds to be arranged downstream in the direction of exhaust gas flow, on which further reactions according to step (d) take place, so that the process is then multi-stage, e.g., three-stage. If several catalyst beds are included, they are preferably arranged one after the other, meaning that the exhaust gas flows through them sequentially: first the first catalyst bed, then the second, and possibly subsequently the third.

[0196] As explained above, it is possible, according to the invention, for one and the same catalyst bed to comprise several reaction zones. Two reaction zones on a common catalyst bed can be formed, for example, by an intermediate complete or almost complete conversion of the reducing agent initially entering / introduced into the catalyst bed. In the first reaction zone, chemical reductions then take place with the consumption of the reducing agent, while in the second reaction zone, due to a lack of sufficient amounts of reducing agent, no chemical reductions occur, but instead other reactions take place, such as the catalytic decomposition of N₂O.

[0197] In particular, two reaction zones on a common catalyst bed can also be formed by introducing the reducing agent only in the middle (or at another position downstream along the longitudinal extent) of the catalyst bed (see Figure IC). Upstream of the injection point (metering device) in the catalyst bed, there may then be no reducing agent present at all, so that steps (di) and (d2) of the inventive process cannot take place due to the lack of reducing agent. Upstream of the injection point (metering device), the decomposition of N₂O essentially occurs according to step (d,) (first reaction zone). Downstream of the injection point (metering device), reducing agent is present, so that steps (di) and (d2) of the inventive process can take place, possibly superimposed by step (d,) of the inventive process.

[0198] If the reducing agent, e.g., NH3, is added to the exhaust gas before the exhaust gas enters a catalyst bed, for example, via a metering device B or as excess reducing agent, e.g., NH3, from a metering device A that was not reacted in step (b), a second reaction zone can form in this catalyst bed after the NH3 has been completely converted by NOx reduction according to step (d) in a first reaction zone. In this second zone, the decomposition of N2O then takes place according to step (d), possibly catalyzed by a residual amount of NOx. In this case, too, different reactions can occur in the front section (i.e., in the first reaction zone of the catalyst bed) than in the rear section (i.e., in the second reaction zone) due to the different reaction kinetics.in the second reaction zone of the catalyst bed; however, the first reaction zone and the second reaction zone differ from each other in that in the first reaction zone (with intermediate injection of NH3) or in the second reaction zone (with initial injection of NH3) no chemical reduction of N2O and no chemical reduction of NOx takes place due to the lack of reducing agent.

[0199] In preferred embodiments, step (d) of the inventive process essentially comprises (di) the catalytic chemical reduction of NOx with a reducing agent on an N Ox reduction catalyst.

[0200] In preferred embodiments, step (d) of the inventive process comprises, in addition to step (di), the catalytic chemical reduction of N2O with a reducing agent on an N2O reduction catalyst. Preferably, the NOx reduction catalyst and the N2O reduction catalyst are made of the same material.

[0201] In preferred embodiments, step (d) of the inventive process comprises, in addition to step (di), the catalytic decomposition of nitrogen oxide on an N2O decomposition catalyst. Preferably, the NOx reduction catalyst and the N2O decomposition catalyst are made of the same material.

[0202] In preferred embodiments, step (d) of the process according to the invention comprises, in addition to step (di), both (d2) the catalytic chemical reduction of N2O with a reducing agent on an N2O reduction catalyst and (ds) the catalytic decomposition of nitrogen oxide on an N2O decomposition catalyst. Preferably, the NOx reduction catalyst, the N2O reduction catalyst, and the N2O decomposition catalyst are made of the same material. Zeolite catalysts

[0203] Preferably, the NOx reduction catalyst, the N2O reduction catalyst or the N2O decomposition catalyst contains a zeolitic material.

[0204] The zeolitic material (also referred to as "zeolite" for descriptive purposes) is loaded with at least one element from the group of transition metals (atomic numbers 21-30, 39-48, 57-80, 89-112) and / or with at least one element from the group of lanthanides (also called "lanthanide"; atomic numbers 57-71). For descriptive purposes, the elements of the transition metals and lanthanides are referred to collectively as "transition metals" for the sake of simplicity. The transition metals iron ("Fe-zeo / zt / ze"), copper ("Cu-zeolites"), and cobalt ("Co-zeolites") are preferred. Iron-loaded zeolitic materials (i.e., Fe-zeolites) are particularly preferred and may also be loaded with or contain other transition metals in addition to iron, for example, manganese, vanadium, chromium, nickel, or mixtures thereof.The transition metals can be present in the zeolitic material in various bonding forms, for example as ions bound to ion exchange sites of the respective zeolite structures or as oxide compounds embedded in the zeolitic material.

[0205] The zeolitic materials according to the invention preferably exhibit high hydrothermal resistance. Particularly preferred are SiO2-rich zeolites, so-called "high-silica zeolites", which have a molar ratio of [SiC>2] to [AlIO2] units, and thus a molar Si / Al ratio, of at least 8, preferably at least 9, more preferably at least 10, even more preferably at least 11, most preferably at least 12, and particularly at least 13.

[0206] Preferred zeolitic materials according to the invention essentially contain zeolites having a zeolite structure of type BEA, MFI, MOR, MEL, or FER, preferably of structure type MFI and BEA, and even more preferably of structure type BEA. Of structure type MFI, type ZSM-5 is particularly preferred. Further details regarding the designation of the structure types of zeolitic materials and their structure can be found in the Atlas of Zeolite Structure Types, Elsevier, 4th revised edition 1996.

[0207] According to the invention, particularly preferred NOx reduction catalysts, N2O reduction catalysts or N2O decomposition catalysts contain at least 50 wt% Fe-zeolite with respect to the total weight of the zeolitic material, preferably at least 70 wt% Fe-zeolite, wherein a single structure type or several structure types may be present. In preferred In embodiments, in addition to Fe-BEA zeolite, another Fe-zeolite of a different structure type is present, preferably Fe-MOR zeolite.

[0208] The loading (doping) of the zeolitic materials with the transition metals / lanthanides can be carried out according to relevant methods for loading or doping zeolites with transition metals / lanthanides, which are known to those skilled in the art. Preferably, the loading is carried out starting from the commercially available H-form or, more preferably, the NFE-form of the zeolitic materials by ion exchange with corresponding salts of the transition metals, either in aqueous phase or by solid-state reaction. The loaded zeolitic materials thus obtained are then calcined, preferably in air in an oven at temperatures in the range of 400 to 650°C. After calcination, the loaded zeolitic materials are washed intensively in distilled water, and the filtered loaded zeolitic materials are then dried.Preferably, the loaded zeolitic materials thus obtained are mixed with suitable binders, such as aluminosilicates, boehmite, or silica sol, and optionally with additives for plasticization or for the production of slurries. In preferred embodiments, the mixtures thus obtained are extruded into catalyst bodies (complete catalysts) and subsequently calcined. In other preferred embodiments, the mixtures thus obtained are applied to catalyst supports (supported catalysts) and subsequently calcined. These methods are also known to those skilled in the art and are established in many technical applications.

[0209] The NOx reduction catalysts, N2O reduction catalysts, and N2O decomposition catalysts according to the invention can be independently available as shaped bodies of any size and geometry, preferably in geometries that have a large surface area to volume ratio and through which the pressure drop is minimized. Typical geometries include all those known in catalysis, such as cylinders, hollow cylinders, multi-hole cylinders, rings, trilobes, or star-shaped extrudates. Particularly preferred are monolithic catalyst elements permeated by parallel channels, e.g., monolithic honeycomb structures, so-called "catalyst honeycombs," as known, for example, from the purification or denitrification of power plant or automotive exhaust gases. Catalyst honeycomb

[0210] Preferably, the NOx reduction catalysts, N2O reduction catalysts, or N2O decomposition catalysts are in the form of catalyst honeycombs, preferably in the form of several catalyst honeycombs arranged parallel to each other in the exhaust gas duct, each with honeycomb channels oriented longitudinally to the exhaust gas flow direction. The geometry of the cross-sectional area of ​​the catalyst honeycombs (perpendicular to the exhaust gas flow direction) is generally freely selectable. Preferably, the catalyst honeycombs have a rectangular or, in particular, square cross-sectional area; however, other cross-sectional areas are also possible, especially hexagonal, triangular, or trapezoidal. etc. Suitable geometries are known to experts. The term "honeycomb" is therefore not, by its very nature, limited to a rectangular or square cross-sectional area.

[0211] Provided that the reactions in step (d) of the invention take place in a first catalyst bed and downstream in a second catalyst bed in the direction of exhaust gas flow, the first and second catalyst beds preferably each have several catalyst honeycombs arranged parallel to each other with honeycomb channels in the exhaust gas channel oriented longitudinally to the direction of exhaust gas flow.

[0212] In preferred embodiments, several catalyst honeycombs, i.e., several monolithic honeycomb bodies, are combined into a honeycomb module, preferably by a metal frame that is open in the direction of exhaust gas flow. Preferably, two, four, or six honeycomb bodies, preferably monolithic honeycomb bodies, are combined into a honeycomb module. This modular design allows for good utilization of the available cross-sectional area of ​​the exhaust duct and easy replacement of defective or deactivated honeycomb bodies.

[0213] The honeycomb structure preferably has a rectangular cross-section. Preferably, the rectangular cross-section has a first edge length (perpendicular to the exhaust gas flow direction) in the range of 5 to 20 cm, more preferably 10 to 15 cm, and a second edge length (also perpendicular to the exhaust gas flow direction) in the range of 5 to 20 cm, more preferably 10 to 15 cm. The height of a honeycomb structure (in the direction of exhaust gas flow) is preferably in the range of 5 to 25 cm, more preferably in the range of 7.5 to 15 cm.

[0214] The so-called cell density, i.e., the density of channels in a single catalyst honeycomb, is preferably 150 to 500 cpsi, more preferably 180 to 450 cpsi (cells per square inch). 100 cpsi, i.e., 100 cells or honeycomb channels per square inch, corresponds to approximately 15.5 catalyst channels per cm². 2 .

[0215] Preferably, the individual honeycomb modules are stacked one above the other and side by side in the direction of flow and fixed by suitable holding devices to achieve the best possible utilization of the inflow area, i.e., the cross-sectional area of ​​the exhaust duct. Bypass flows between the individual honeycomb modules or in the outer edge region between the outer edge of the honeycomb modules and the inner wall of the exhaust duct should be avoided. For this purpose, suitable sealing materials are preferably applied between the individual honeycomb modules and between the outer honeycomb modules and the inner wall. In the case of larger wall distances, cover plates are used, which are attached to the inner wall of the exhaust duct in the direction of flow upstream and / or downstream of the honeycomb module packing. Preferably, the cover plates are fitted with gaskets at the contact points with the honeycomb modules.Preferably, the honeycomb body modules are arranged and selected in such a way that the usable The flow area at the catalyst is preferably at least 60% of the inner cross-sectional area of ​​the exhaust gas channel, more preferably at least 70%, and even more preferably at least 80%.

[0216] In circular exhaust ducts or exhaust pipes, the gaps formed at the edges of the honeycomb module packing are preferably not filled with specially cut honeycomb modules, but rather sealed with blanking plates, unless these gaps can be easily filled with rectangular honeycomb modules. This has the advantage that when replacing worn honeycomb modules, only standardized modules need to be exchanged and no special modifications are required.

[0217] When using exhaust gas pipelines, it is preferable to use individual, larger honeycomb bodies adapted to the pipeline cross-section with a circular inflow cross-section, of which several can be arranged one behind the other in the flow direction in a preferred embodiment. In this case, it is then not necessary to combine several honeycomb bodies parallel to each other into honeycomb body modules.

[0218] In preferred embodiments, the honeycomb bodies or honeycomb body modules are arranged in several layers offset along the longitudinal axis in the direction of exhaust gas flow. Preferably, the honeycomb bodies or honeycomb body modules are arranged in 2 to 5 layers, particularly preferably in 2 to 3 layers. A gap is preferably provided between the layers, i.e., between the end faces of the honeycomb bodies or honeycomb body modules, preferably in the range of 3 to 30 mm, more preferably 4 to 20 mm. This gap allows for intermediate, particularly radial, mixing of the gas stream exiting a first layer of the honeycomb bodies or honeycomb body modules. Furthermore, it prevents any potential slippage of unreacted reducing agent and / or its not yet fully oxidized reaction products from the first layer of honeycomb bodies into a subsequent, second layer of honeycomb bodies. Reducing agent for step (d)

[0219] In preferred embodiments, step (d) comprises the chemical reduction of NOx with a reducing agent on a NOx reduction catalyst and optionally additionally the chemical reduction of N2O with a reducing agent on an N2O reduction catalyst. Reducing agent is required for both reactions. Although, according to the invention, a significant amount of unreacted reducing agent preferably remains in the exhaust gas in step (b) due to the preferably comparatively low temperature and is thus supplied to the subsequent step (d), this amount of unreacted reducing agent in the exhaust gas is preferably not yet sufficient to effect further chemical reduction to the desired extent in step (d). Therefore, it is preferred according to the invention that further reducing agent is added to the exhaust gas for step (d). This can be whether it is the same reducing agent that was added for step (b) or a different reducing agent.

[0220] Preferably, for step (d), the reducing agent is added to the exhaust gas via a further metering device, which preferably comprises at least one metering unit. For the purposes of description, this further metering device for the reducing agent, which is added to the exhaust gas for catalytic chemical reduction in step (d), is also referred to as "metering device B", which preferably comprises several metering units and with which the metering of reducing agent into the exhaust gas can preferably be controlled via a "control valve b". The entirety of these metering units is then part of metering device B.

[0221] In the metering device B, the supply and distribution of the reducing agent preferably takes place via addition units, preferably via a multiply branched pipe system provided with a multitude of openings or nozzles, which is arranged in the exhaust gas duct or in the exhaust gas line in the direction of flow upstream of the NHs reduction catalyst.

[0222] In preferred embodiments, the metering device is arranged upstream of a first catalyst bed for NOx reduction.

[0223] In other preferred embodiments, the metering device is arranged downstream of a first catalyst bed and upstream of a second catalyst bed for NOx reduction.

[0224] In further preferred embodiments, the catalyst bed comprises a first reaction zone and a second reaction zone for NOx reduction, and the metering device is arranged downstream of the first reaction zone and upstream of the second reaction zone for NOx reduction. The metering device is then positioned at a location within the catalyst bed (along its longitudinal extent in the direction of exhaust gas flow).

[0225] The distribution pipes are preferably designed in the form of grids or in the form of concentrically connected circles, which extend as far as possible over the cross-sectional area of ​​the exhaust gas channel or the flow area of ​​the NHs reduction catalyst.

[0226] The specific design and dimensioning of these distributors, including suitable outlet nozzles, is part of the expertise in catalytic exhaust gas purification technology and is widely used, for example, in the exhaust gas purification of coal-fired power plants.

[0227] The reducing agent for step (di) NIL is preferred. It is preferably supplied to the exhaust gas in gaseous form in pure form or together with water vapor or, for example, as an aqueous solution.

[0228] The NFL is preferably used for step (di) in an amount of 0.9 to 2.5 molar proportions, preferably 1.0 to 1.4 molar proportions, more preferably 1.0 to 1.2 molar proportions, based on a molar fraction of NOx to be chemically reduced. The content of unreacted reducing agent from step (b) is preferably taken into account, if necessary in a suitable manner, i.e., preferably on the basis of an analytical online measurement of the NHs slip from step (b).

[0229] Preferably the reducing agent for step (d2) is selected from NH3, CH4 or other hydrocarbons, CO, H2 and mixtures thereof; preferably NH3.

[0230] In preferred embodiments, the reducing agent in step (d2) is NH3, which is preferably used in an amount of 0.5 to 3.0 molar fractions, more preferably in an amount of 0.7 to 2.5 molar fractions, and even more preferably in an amount of 0.8 to 2.0 molar fractions, based on a molar fraction of N2O to be chemically reduced, i.e., based on the amount of N2O at the inlet to the catalyst bed or the corresponding reaction zone of the N2O reduction catalyst. The content of unreacted reducing agent from step (b) is preferably taken into account, if applicable.

[0231] In preferred embodiments, the reducing agent in step (d2) is NH3, which is preferably used in an amount of 0.5 to 3.0 molar fractions, more preferably in an amount of 0.7 to 2.5 molar fractions, and even more preferably in an amount of 0.8 to 2.0 molar fractions, based on the molar amount of N2O in the exhaust gas at the inlet to the catalyst bed or the corresponding reaction zone of the N2O reduction catalyst. This amount is additive to any amount of NH3 required for NOx reduction, provided that step (di) also takes place in the catalyst bed of the N2O reduction catalyst. The amount of unreacted reducing agent from step (b) is preferably taken into account.

[0232] In other preferred embodiments, the reducing agent for the N₂O reduction according to step (d2) is a hydrocarbon or a mixture of several hydrocarbons, preferably in an amount of 0.2 to 1.0 molar proportions, more preferably in an amount of 0.2 to 0.7 molar proportions, based on the molar amount of N₂O in the exhaust gas at the inlet to the catalyst bed or the corresponding reaction zone of the N₂O reduction catalyst. This amount is also additive to any amount of NH₃ required for NOx reduction, provided that step (di) also takes place in the catalyst bed or the reaction zone of the N₂O reduction catalyst.

[0233] Besides NH3, other nitrogen-containing reducing agents are also suitable in steps (di) and / or (d2) of the inventive process, for example, hydrogen compounds of nitrogen such as azanes, hydroxyl derivatives of azanes, as well as amines, oximes, carbamates, urea, or urea derivatives. Examples of azanes are hydrazine and, in particular, ammonia. Examples of hydroxyl derivatives of azanes are hydroxylamine. Examples of amines are primary aliphatic amines, such as methylamine. An example of a carbamate is ammonium carbamate. Examples of urea derivatives are N,N'-substituted ureas, such as N,N'-dimethyl urea. Ureas and urea derivatives are preferably used in the form of aqueous solutions. Ammonia or substances that release ammonia when introduced, such as urea or ammonium carbamate, are preferred. Preferred single-stage embodiments of the catalytic reaction phase (SCR) of step (d)

[0234] In particularly preferred embodiments, step (d) is carried out on a single catalyst bed (i.e., single-stage), which preferably comprises a catalyst comprising an iron-loaded zeolitic material (Fe-zeolite), preferably of the structural type BEA, MFI, MOR, MEL, or FER, more preferably of the structural type MFI and BEA, and even more preferably of the structural type BEA. The single catalyst bed may comprise one or more reaction zones. Upstream of the single catalyst bed, in the direction of exhaust gas flow, a metering device B for metering NH3 into the exhaust gas is arranged.

[0235] In a preferred embodiment (single-stage variant 1), in a first reaction zone of the single catalyst bed, the NOx content is incompletely reduced by chemical reduction with NH3. This NH3 originates partly from the non-catalytic reaction phase (SNCR) of metering device A as a reducing agent that did not react in step (b) and partly from the metering device B. The NOx content is reduced by chemical reduction with NH3 to a specific residual NOx level. Preferably, the catalytic decomposition of N2O, co-catalyzed by residual NOx present in the exhaust gas, takes place simultaneously in the first reaction zone of the single catalyst bed and / or subsequently in a second reaction zone of the single catalyst bed.Since the chemical reduction of NOx with NH3 typically proceeds significantly faster than the chemical reduction of N2O with NH3, and not the entire amount of NOx is chemically reduced in the first reaction zone of the single catalyst bed, the extent of the chemical reduction of N2O with NH3 that may occur simultaneously in the first reaction zone of the single catalyst bed and / or subsequently in a second reaction zone of the single catalyst bed is typically negligible.

[0236] In this preferred embodiment (single-stage variant 1), the addition of NH3 via the metering device B preferably takes place under feet / oil control, i.e., a specific value for the NOx concentration at the outlet of the single catalyst bed is specified as a target value (setpoint), and the actual NOx concentration at the outlet of the single catalyst bed is measured (actual value). In the event of a difference between the setpoint and the actual value (control deviation), the control valve b of the metering device B is adjusted to minimize the difference. Preferably, the setpoint for the NOx concentration at the outlet of the single catalyst bed, and thus the amount of additional NH3, is selected such that the residual NOx concentration at the outlet of the single catalyst bed is at most 100 ppmv, more preferably at most 60 ppmv, even more preferably at most 30 ppmv, and most preferably at most 15 ppmv.Preferably, the target value of the NOx concentration at the outlet of the single catalyst bed, and thus the amount of additional NH3, is chosen such that the. The residual concentration of NOx at the exit of the single catalyst bed is at least 5 ppmv, preferably at least 10 ppmv, even more preferably at least 20 ppmv, and most preferably at least 40 ppmv. The expected specific consumption of NH₄ for the chemical reduction of NOx is typically in the range of 0.9 to 1.1 mol of NH₄ per mol of reduced NOx.

[0237] In another preferred embodiment (single-stage variant 2), the NOx content is practically completely reduced in a first reaction zone of the single catalyst bed by chemical reduction with NH₃. This NH₃ originates partly from the non-catalytic reaction phase (SNCR) via metering device A as a reducing agent that remained unreacted in step (b) and partly from the metering device B. Preferably, the catalytic reduction of N₂O takes place simultaneously in the first reaction zone of the single catalyst bed and / or subsequently in a second reaction zone of the single catalyst bed. Since the residual NOx content in the second reaction zone of the single catalyst bed is zero or nearly zero, the extent of any simultaneous catalytic decomposition of N₂O in the second reaction zone of the single catalyst bed is typically negligible.

[0238] In this preferred embodiment (single-stage variant 2), the addition of NH3 via the metering device B preferably takes place under feed-through control, i.e., the concentration of NOx and preferably the concentration of N2O are each measured at the inlet of the exhaust gas to the single catalyst bed. Taking into account the amount of exhaust gas entering the single catalyst bed and considering, i.e., subtracting, any amount of unreacted NH3 from the non-catalytic reaction phase (SNCR) originating from the metering device A, the amount of NH3 required for the chemical reduction of NOx is calculated. Preferably, the sum of the amounts of NH3 required for the chemical reduction of NOx and for the chemical reduction of N2O is calculated. The calculation is preferably performed using stored ratios, i.e., molar ratios (mol / mol) of NH3 / NOx and, optionally, preferably of NH3 / N2O.The factors derived from this are calculated. Preferably, the calculated result (control variable) is used to change the control degree of the control valve b of the metering device B in order to meter the required amount of NH3.

[0239] The molar concentration of NH3 in the exhaust gas at the inlet to the single catalyst bed, i.e. [NH3], is preferably in the range of {0.7- [N2O] + 1.0- [NOx]} to {4.0- [N2O] + 2.0- [NOx]}, more preferably from {1.0 [N2O] + 1.1 [NOx]} to {3.0 [N2O] + 1.6- [NOx]}, and even more preferably from {1.5 [N2O] + 1.2 [NOx]} to {2.5 [N2O] + 1.4- [NOx]}, where [N2O] is the molar concentration of N2O and [NOx] is the molar concentration of NOx, respectively, in the exhaust gas at the inlet to the single catalyst bed.

[0240] Regarding the chemical reduction of NOx, for the fee / örwar control of the dosage of NH3 into the single catalyst bed, a molar ratio of NFE / NOx in the range of 1.0 to 2.0 is preferably chosen; preferably 1.1 to 1.6; more preferably 1.2 to 1.4.

[0241] Regarding the chemical reduction of N2O, for fee / bnvar control of the dosage of NH3 into the single catalyst bed, a molar ratio of NH3 / N2O in the range of 0.7 to 4.0 is preferably chosen; preferably 1.0 to 3.0; more preferably 1.5 to 2.5.

[0242] Preferably, the additional NH3 is not added using the dosing device B under Feet / öacF control, since in this preferred embodiment (variant 2) the aim is to achieve the most complete possible chemical reduction of NOx, i.e., no or only very low residual concentrations of NOx and N2O result, which would be difficult to use as control variables under Feet / öacF control.

[0243] The single-stage reaction procedure preferred according to the invention for step (d) is possible because the upstream non-catalytic chemical reduction in step (b) [non-catalytic reaction phase, SNCR] significantly reduces the NOx content in the pretreated exhaust gas. The pretreated exhaust gas entering the single catalyst bed for step (d) therefore already has a significantly lower NOx content than the exhaust gas formed in step (a).

[0244] Thus, the combustion of NH3 or NFF-containing fuels with O2, in contrast to the combustion of CH4 with O2, may produce significantly larger quantities of NOx (up to several thousand ppm) and, depending on the conditions of the NFF combustion, may also produce considerable quantities of N2O (up to several hundred ppm).

[0245] Without the upstream non-catalytic chemical reduction [non-catalytic reaction phase, SNCR] according to the invention, a satisfactory reduction in the nitrogen oxide content in a single-stage catalytic reaction phase (SCR) would be difficult to achieve. For example, the chemical reduction of large quantities of NOx, such as those produced during the combustion of NH3 with O2, would require the addition of correspondingly large quantities of NH3 as a reducing agent, as well as correspondingly large quantities of catalyst, resulting in low space velocities for the single-stage catalytic reaction phase (SCR). However, in such large quantities, NH3 inhibits the chemical reduction of NOx; this is particularly true when high NOx reduction rates are desired and when temperatures are comparatively low, as this enhances the inhibitory adsorption of NH3.

[0246] The non-catalytic chemical reduction (non-catalytic reaction phase, SNCR) performed upstream according to the invention allows for the degradation of significant amounts of nitrogen oxides, particularly NOx, by chemical reduction. This mitigates the conditions for the subsequent chemical reduction of NOx according to the invention, so that the catalytic reaction phase (SCR) in step (d) can preferably be designed as a single stage according to the invention. With the same amount of catalyst, higher degradation rates are possible, and the risk of NfU slip in the treated exhaust gas after step (d) is low. The single catalyst bed in step (d) can therefore be smaller. In contrast to processes known from the prior art, not only NOx can be reduced. and NO2, i.e. NOx, are broken down, but according to the invention, N2O is also broken down. Preferred two-stage embodiments of the catalytic reaction phase (SCR) of step (d)

[0247] In other particularly preferred embodiments, step (d) is carried out on two spatially separated catalyst beds (i.e., two-stage with a first catalyst bed and a second catalyst bed), each preferably independently comprising a catalyst comprising an iron-loaded zeolitic material (Fe-zeolite), preferably of the structural type BEA, MFI, MOR, MEL, or FER, more preferably of the structural type MFI and BEA, and even more preferably of the structural type BEA. The two catalyst beds can each independently comprise one or more reaction zones. The first catalyst bed is arranged upstream of the second catalyst bed in the direction of exhaust gas flow.

[0248] Preferably (but not necessarily) a metering device B for metering NH3 into the exhaust gas is arranged upstream of the first catalyst bed in the direction of exhaust gas flow, with which the metering of reducing agent into the exhaust gas can preferably be controlled via a "control valve b".

[0249] Preferably (but not necessarily), for step (d), the reducing agent is added to the exhaust gas via an additional metering device, which preferably comprises at least one metering unit. This additional metering device is arranged downstream of the first catalyst bed and upstream of the second catalyst bed in the direction of exhaust gas flow. For the purposes of description, this additional metering device for the reducing agent, which is added to the exhaust gas for step (d) for catalytic chemical reduction, is also referred to as "metering device C," which preferably comprises several metering units and with which the addition of reducing agent to the exhaust gas can preferably be controlled via a "control valve c." The entirety of these metering units is then part of metering device C.

[0250] In a preferred embodiment (two-stage variant 1), step (d) of the inventive method takes place in a first catalyst bed and a spatially separate second catalyst bed; wherein the first catalyst bed is arranged upstream of the second catalyst bed in the direction of exhaust gas flow; wherein the addition of reducing agent to the exhaust gas is carried out via the metering device B arranged upstream of the first catalyst bed with the control valve b for metering NH3.

[0251] In another preferred embodiment (two-stage variant 2), step (d) of the inventive process takes place in a first catalyst bed and a spatially separate second catalyst bed; wherein the first catalyst bed is arranged upstream of the second catalyst bed in the direction of exhaust gas flow; wherein the addition of reducing agent to the exhaust gas is carried out exclusively via the slip generated by the metering device A in step (b). Reducing agent, preferably of NH3, i.e., by means of excess NH3 that was not previously consumed in the non-catalytic reaction phase (SNCR). In this two-stage variant, therefore, no reducing agent is added via a dosing device B upstream of the first catalyst bed or via a dosing device C upstream of the second catalyst bed.

[0252] In a preferred embodiment (two-stage variant 3), step (d) of the inventive process takes place in a first catalyst bed and a spatially separate second catalyst bed; wherein the first catalyst bed is arranged upstream of the second catalyst bed in the direction of exhaust gas flow; wherein the addition of reducing agent to the exhaust gas is effected both by the slip of reducing agent, preferably NH3, generated by the metering device A in step (b), i.e., by excess NH3 that was not previously consumed in the non-catalytic reaction phase (SNCR), and by the metering device B arranged upstream of the first catalyst bed.

[0253] In another preferred embodiment (two-stage variant 4), step (d) of the inventive process takes place in a first catalyst bed and a spatially separate second catalyst bed; wherein the first catalyst bed is arranged upstream of the second catalyst bed in the direction of exhaust gas flow; wherein the addition of reducing agent to the exhaust gas is effected both by the slip of reducing agent, preferably NH3, generated by the metering device A in step (b), i.e., by excess NH3 which was not previously consumed in the non-catalytic reaction phase (SNCR), and by the metering device C arranged upstream of the second catalyst bed.

[0254] In the two-stage variants 1, 2, 3 and 4, the preferred option is in each case (i) in the first catalyst NOx is incompletely chemically reduced with NH3 according to step (di) and N2O is decomposed according to step (d); and (ii) in the second catalyst, further NOx is chemically reduced with NH3 according to step (di *); and further N2O, if still present, is chemically reduced with NH3 according to step (d2), and optionally decomposed according to step (di * ).

[0255] Preferably, in the first catalyst stage, the catalytic decomposition of N2O is co-catalyzed by NOx present in the exhaust gas.

[0256] Preferably, the incomplete chemical reduction of NOx with NH3 in the first catalyst bed leads to a predetermined residual NOx content sufficient to exert a cocatalytic effect on the decomposition of N2O in the first catalyst bed. Since the chemical reduction of NOx with NH3 in the first catalyst bed typically proceeds significantly faster than the chemical reduction of N2O with NH3, and since not the entire amount of NOx is chemically reduced in the first catalyst bed, the extent of any parallel chemical reduction of N2O with NH3 in the first catalyst bed is typically negligible.

[0257] In preferred embodiments, additional NH3 is metered into the exhaust gas upstream of the first catalyst bed via the metering device B for the chemical reduction of NOx; preferably under fee <a&ac -Regelung, d.h. es wird ein bestimmter Wert für die Konzentration an NOx am Austritt des ersten Katalysatorbetts als Zielwert (Sollwert) vorgegeben und die tatsächliche Konzentration an NOx am Austritt des ersten Katalysatorbetts gemessen (Istwert) und im Falle einer Differenz aus Sollwert und Istwert (Regeldifferenz) der Stellgrad des Regelventils b des Dosiervorrichtung B stromaufwärts des ersten Katalysatorbetts verändert, um die Differenz zu minimieren.Preferably, the target value for the NOx concentration at the exhaust gas outlet from the first catalyst bed, and thus the amount of additional NH3, is selected such that the residual NOx concentration at the exhaust gas outlet from the first catalyst bed is at most 1000 ppmv, preferably at most 500 ppmv, more preferably at most 100 ppmv. Preferably, the target value for the NOx concentration at the exhaust gas outlet from the first catalyst bed, and thus the amount of additional NH3, is selected such that the residual NOx concentration at the exhaust gas outlet from the first catalyst bed is at least 10 ppmv, preferably at least 20 ppmv, more preferably at least 40 ppmv. The expected specific consumption of NH3 for the chemical reduction of NOx in the first catalyst bed is typically in the range of 0.9 to 1.1 mol NH3 per mol reduced NOx and is therefore preferably significantly smaller than the expected specific (mol / mol) consumption of NH3 in the second catalyst bed.

[0258] Preferably, the temperature of the exhaust gas at the exit from the first catalyst bed is in the range of 400 to 550°C.

[0259] Preferably, the exhaust gas at the exit from the first catalyst bed has a pressure greater than atmospheric pressure, i.e., > 1.0 bara, but at most 1.5 bara, more preferably at most 1.2 bara, and even more preferably at most 1.1 bara.

[0260] Preferably, the NOx in the exhaust gas at the exit from the first catalyst bed has an oxidation degree a(NOx) of at least 0.05, preferably at least 0.075, more preferably at least 0.1, even more preferably at least 0.125%, most preferably at least 0.15%, and in particular at least 0.175%.

[0261] In preferred embodiments, the NOx in the exhaust gas at the exit from the first catalyst bed has an oxidation degree a(NOx) in the range of 0.3 to 0.5.

[0262] In other preferred embodiments, the NOx in the exhaust gas at the exit from the first catalyst bed has an oxidation degree a(NOx) in the range of 0.15 to 0.35, preferably 0.15 to 0.3.

[0263] In further preferred embodiments, the NOx in the exhaust gas at the exit from the first catalyst bed has an oxidation degree a(NOx) in the range of 0.1 to 0.2.

[0264] In other preferred embodiments, the NOx in the exhaust gas at the exit from the first catalyst bed has an oxidation level a(NOx) in the range of 0.05 to 0.15.

[0265] Preferably, N2O is reduced in the first catalyst bed to a residual concentration of N2O at the outlet of the exhaust gas from the first catalyst bed of at most 50 ppmv, more preferably at most 25 ppmv, even more preferably at most 10 ppmv, most preferably at most 5 ppmv.

[0266] Preferably, residual N2O is broken down in the second catalyst bed to a residual concentration of N2O at the outlet of the exhaust gas from the second catalyst bed of at most 10 ppmv, more preferably at most 5 ppmv, even more preferably at most 2 ppmv, most preferably at most 1 ppmv.

[0267] Preferably, residual NOx is reduced in the second catalyst bed to a residual NOx concentration at the exhaust gas exit from the second catalyst bed of at most 20 ppmv, more preferably at most 10 ppmv, even more preferably at most 5 ppmv, most preferably at most 2 ppmv.

[0268] In preferred embodiments, additional NH3 is metered into the exhaust gas downstream of the first catalyst bed and upstream of the second catalyst bed via the metering device C for the chemical reduction of NOx and / or N2O; preferably under Feerf / bnvarrf control, i.e., the NOx concentration and preferably the N2O concentration are each measured at the exhaust gas inlet to the second catalyst bed. Taking into account the amount of exhaust gas entering the second catalyst bed and taking into account, i.e., subtracting any amount of unreacted NH3 from the non-catalytic reaction phase (SNCR) originating from the metering device A, the amount of NH3 required for the chemical reduction of NOx is calculated. Preferably, the sum of the amounts of NH3 required for the chemical reduction of NOx and for the chemical reduction of N2O is calculated. The calculation is preferably performed using stored ratios, i.e.,The molar ratios (mol / mol) of NHJNOx and, optionally preferably, of NH3 / / N2O or factors derived therefrom are used. Preferably, the calculated result (control variable) is used to change the control valve c of the metering device C in order to meter the required amount of NH3.

[0269] According to the invention, the molar concentration of NH3 in the exhaust gas at the inlet to the second catalyst bed, i.e., [NH3], is preferably in the range of {0.7- [N2O] + 1.0- [NOx]} to {4.0- [N2O] + 2.0- [NOx]}, more preferably from {1.0 [N2O] + 1.1 [NOx]} to {3.0 [N2O] + 1.6- [NOx]}, and even more preferably from {1.5 [N2O] + 1.2 [NOx]} to {2.5 [N2O] + 1.4- [NOx]}, where [N2O] is the molar concentration of N2O and [NOx] is the molar concentration of NOx in the exhaust gas at the inlet to the second catalyst bed.

[0270] Regarding the chemical reduction of NOx, for the fee / örwarrf control of the dosage of NH3 into the second catalyst bed, a molar ratio of NFE / NOx in the range of 1.0 to 2.0 is preferably chosen; preferably 1.1 to 1.6; more preferably 1.2 to 1.4.

[0271] Regarding the chemical reduction of N2O, for fee / bnvar control of the dosage of NH3 into the second catalyst bed, a molar ratio of NH3 / N2O in the range of 0.7 to 4.0 is preferably chosen; preferably 1.0 to 3.0; more preferably 1.5 to 2.5.

[0272] Preferably, the additional NH3 is not added via the dosing device C below / T / NzcF Rege hing, since the aim is to achieve the most complete possible chemical reduction of NOx in the second catalyst bed, i.e., no or only very low residual concentrations of NOx and N2O result, which would be difficult to use as control variables.

[0273] Preferably, the amount of catalyst, i.e., the space velocity (= ratio of exhaust gas volume flow under standard conditions to catalyst volume), is chosen such that a reduction of N2O of at least 50%, preferably at least 70%, and even more preferably at least 80%, occurs in the first catalyst bed, based on the concentration of N2O at the inlet to the first catalyst bed.

[0274] Preferably, the amount of catalyst and the amount of additional NH3 are chosen such that the molar ratio of NOX / N2O at the exit of the first catalyst bed is at least 5, more preferably at least 10, and even more preferably at least 20.

[0275] Preferably, the space velocity of the first catalyst bed is in the range of 5,000 h⁻¹. 1 up to 100,000 h 1 , preferably 10,000 h 1 up to 50,000 h 1 , even more preferred 15,000 h 1 up to 45,000 h 1 .

[0276] If the molar ratio of NOX / N2O at the outlet of the first catalyst bed is at least 10, then the addition of NH3 to the exhaust gas upstream of the second catalyst bed via the metering device C can preferably be carried out solely in relation to the amount of NOx entering the system.

[0277] Preferably, the exhaust gas temperature at the inlet to the first catalyst bed is at least 400°C, more preferably at least 425°C, and even more preferably at least 450°C. Preferably, the exhaust gas temperature at the inlet to the first catalyst bed is at most 550°C, more preferably at most 525°C, and even more preferably at most 500°C. The temperature can be adjusted by measures known to those skilled in the art, in particular the design of heat exchangers and the combustion conditions of NH3.

[0278] Depending on the heat of reaction of the chemical reactions taking place in the first catalyst bed and in the second catalyst bed, the inlet temperature of the exhaust gas into the first catalyst bed is preferably chosen such that the temperature of the exhaust gas at the outlet of the second catalyst bed is at most 600°C, more preferably at most 550°C, and even more preferably at most 520°C.

[0279] Preferably, the space velocity of the second catalyst bed is in the range of 5,000 h⁻¹. 1 up to 100,000 h 1 , preferably 10,000 h 1 up to 50,000 h 1 , even more preferred 15,000 h 1 up to 45,000 h 1 .

[0280] The preferred ratio of the catalyst volumes (VI i) ilt / V2i ilt ) from the first catalyst bed Vlkat to the second catalyst bed V2kat in the range of 1 / 2 to 20 / 1, preferably 1 / 2 to 10 / 1, even more preferably 1 / 1 to 4 / 1.

[0281] In preferred embodiments, at least one, several or all of the following conditions are met: • the pressure of the exhaust gas at the entrance to the first catalyst bed is at most 5 bar abs, preferably at most 4 bar abs, even more preferably at most 1.3 bara, most preferably at most 1.2 bara and in particular at most 1.1 bar abs; • the H2O content in the exhaust gas at the entry into the first catalyst bed is at least 5 vol.%, preferably at least 10 vol.%, even more preferably at least 15 vol.%, most preferably at least 20 vol.% and in particular at least 25 vol.%; • The NOx content in the exhaust gas at the entrance to the first catalyst bed is at least 500 ppmv, preferably at least 1000 ppmv, even more preferably at least 1500 ppmv, most preferably at least 2000 ppmv, and in particular at least 2500 ppmv; • The N2O content in the exhaust gas at the entrance to the first catalyst bed is at most < 500 ppmv, preferably at most 200 ppmv, even more preferably at most 100 ppmv, but at least 5 ppmv, preferably at least 10 ppmv, more preferably at least 50 ppmv; • The exhaust gas contains unreacted residues (slippage) of NH3 from step (d) upon entering the first catalyst bed and possibly unburned residues of NH3 from the combustion of NH3; • the N2O decomposition catalyst and / or the N2O reduction catalyst is in the form of a honeycomb structure; • The NOx reduction catalyst is in the form of a honeycomb structure; • the first catalyst bed contains a catalyst containing Fe-zeolite; • the second catalyst bed contains a catalyst containing Fe-zeolite; • Before entering the first catalyst bed, the exhaust gas flows through a heat exchanger and is heated there; • The NOx content at the outlet of the first catalyst bed is at most 1000 ppmv, preferably at most 500 ppmv, even more preferably at most 300 ppmv, most preferably at most 100 ppmv; however, preferably at least 10 ppmv, more preferably at least 20 ppmv, even more preferably at least 40 ppmv, most preferably at least 100 ppmv, and in particular at least 250 ppmv; the N2O content at the outlet of the first catalyst bed is at most 20 ppmv, more preferably at most 15 ppmv, even more preferably at most 10 ppmv, most preferably at most 5 ppmv, and in particular at most 2 ppmv; • After leaving the first catalyst bed and until entering the second catalyst bed, no intermediate cooling of the exhaust gas takes place; • the molar ratio of N2O : NOx at the entrance to the first catalyst bed is at most 0.5, preferably at most 0.2, even more preferably at most 0.1; • the molar ratio of N2O : NOx at the exit from the first catalyst bed is at most 0.20, preferably at most 0.1, even more preferably at most 0.05; • The injection of NH3 into the exhaust gas upstream of the first catalyst bed in the direction of exhaust gas flow is optional; if injection takes place, it is preferably substoichiometric with regard to the NOx content at the inlet to the first catalyst bed; • The injection of NH3 into the exhaust gas in the direction of flow of the exhaust gas downstream of the first catalyst bed and upstream of the second catalyst bed is mandatory, preferably superstoichiometric with regard to the total content of NOx and N2O at the inlet to the second catalyst bed.

[0282] The invention's preferred use of Fe-zeolite catalysts in two catalyst beds enables, compared to the use of classical V2O5 / TiO2-based DeNOx catalysts, - the complete or near-complete reduction of large quantities of NOx without the risk of NFF slippage; as well as - the simultaneous, complete or almost complete degradation of N2O at comparatively small catalyst volumes, i.e. at comparatively high space velocities.

[0283] This is achieved, in addition to the two-stage operation of step (d) [catalytic reaction phase, SCR] preferred according to the invention, by the pre-reduction of NOx in step (b) [non-catalytic reaction phase, SNCR] and the associated reduced addition of NH3 in step (d). This is made possible by the oxidative properties of the Fe-zeolite catalysts preferably used in step (d) according to the invention and the associated influence on the oxidation state a(NOx). Thus, in the first catalyst bed, the molar ratio of NO to NO2 is brought as close as possible to the thermodynamic equilibrium position according to the invention. Therefore, the oxidation state a(NOx) before entering the first catalyst bed is, due to the upstream NFE combustion at very high temperatures and the slow establishment of equilibrium in the gas phase during the cooling of the exhaust gas in the (if applicable)The temperature in the subsequent heat exchangers is expected to be less than 0.05, and thus significantly below the thermodynamic equilibrium for the inlet temperature to the first catalyst bed. This is, however, very disadvantageous for efficient chemical reduction of NOx, as it results in only a small fraction of the NOx being removed from the exhaust gas. The existing NOx can be reduced according to an almost SCR (see reaction equation (10)) and a large part of the NOx or the remaining NO must be reduced according to the significantly slower normal SCR (see reaction equation (11)).

[0284] The chosen operating mode of limited NH₃ dosing in the first catalyst bed, combined with the ability of the Fe-zeolite catalysts to oxidize NO and accelerate the catalytic establishment of equilibrium, results in a significantly faster, and therefore more efficient, chemical reduction of NOx in the first catalyst bed. Simultaneously, the maximum possible degree of NOx oxidation a(NOx) of the remaining NOx exiting the catalyst bed is achieved. This also enables efficient chemical reduction of NOx in the second catalyst bed from the outset.

[0285] It was found that large amounts of NH₄⁺, such as those necessary for the complete chemical reduction of high concentrations of NOx, inhibit the establishment of the NOx equilibrium on the Fe-zeolite catalyst, similar to water.

[0286] Furthermore, the chemical reduction of NOx itself is also inhibited by NH₄ at sufficiently high doses. This means that, depending on temperature, catalyst quantity, and NOx concentration, no further increase in NOx degradation occurs with increasing NH₄ addition beyond a certain amount. With further increases in NH₄ addition, a decrease in NOx degradation may even be observed, accompanied by NH₄ slippage.

[0287] By prior non-catalytic reduction of part of the NOx in step (b) of the process according to the invention [non-catalytic reaction phase, SNCR] and the further catalytic reduction of the NOx in the first catalyst bed of step (d) [catalytic reaction phase, SCR], the amount of NH required for the chemical reduction of NOx in the second catalyst bed of step (d) is significantly reduced.

[0288] In this way, together with the above-described adjustment or permanent maintenance of the NOx equilibrium, i.e. the oxidation state a(NOx), a very efficient chemical reduction of NOx is also possible in the second catalyst bed, even with the required superstoichiometric dosage of NH.

[0289] The fact that this occurs according to the invention without or with only a negligible NFfi slip of preferably at most 10 ppmv, more preferably at most 5 ppmv, and even more preferably at most 3 ppmv, is also due to the oxidative properties of the Fe-zeolite catalysts used according to the invention. If the inlet temperature of the exhaust gas into the second catalyst bed is preferably at least 400°C, more preferably at least 425°C, and even more preferably at least 450°C, the excess NHs dosed within the limits specified in the invention is selectively oxidized to N2 and H2O by the residual oxygen content of the exhaust gas.

[0290] All these advantages cannot be realized to the same extent when using conventional V₂C>5 / TiO₂-based SCR catalysts, such as those typically used for the denitrification of exhaust gases from natural gas-fired reformers, in single-stage or multi-stage configurations. For example, these conventional SCR catalysts typically cannot be operated at temperatures above 400°C for stability reasons, which limits the achievable degradation rates. Furthermore, conventional SCR catalysts exhibit only very limited oxidation activity, making it impossible to adjust or continuously maintain the NOx equilibrium, i.e., the oxidation state a(NOx), nor do they enable effective and N₂-selective oxidation of excess NH₃. In fact, there is even a risk of the undesired formation of N₂O. Exhaust gas composition after step (d)

[0291] Preferably, the post-treated exhaust gas produced in step (d) has a NOx content that is at least 90%, preferably at least 93%, more preferably at least 96%, more preferably at least 97%, most preferably at least 98%, and in particular at least 99% lower than the NOx content in the exhaust gas produced in step (a).

[0292] Preferably, the post-treated exhaust gas produced in step (d) has a NOx content of at most 100 ppm, preferably at most 50 ppm, more preferably at most 25 ppm, even more preferably at most 10 ppm, most preferably at most 5 ppm, and in particular at most 1 ppm.

[0293] In the chemical reduction of nitrogen oxides in step (d), the reducing agent used is oxidized. Depending on the type of reducing agent used and the reaction conditions, i.e., temperature or residence time in the catalyst bed, incompletely oxidized reaction products may be formed. For example, as already described above, when hydrocarbons are used as reducing agents for N₂O according to the invention, CO is produced in addition to CO₂. The incompletely oxidized reaction products of the reducing agents may be found in the post-treated exhaust gas from step (d), possibly together with a residual amount, i.e., slip, of unconsumed reducing agent. This is particularly the case when (high) molar excesses of reducing agent or reducing equivalents are dosed, which is particularly preferred according to the invention for achieving high reduction rates of NOx and / or N₂O.

[0294] According to the invention, process flows are possible in which (i) only a slip of reducing agent is present (e.g. NH3), (ii) only an incompletely oxidized reaction product is present (e.g. CO formed from CsFE), or (iii) both are present (e.g. CH4 and CO).

[0295] Preferably, the post-treated exhaust gas produced in step (d) has a residual content of unused reducing agent and / or optionally of its incompletely oxidized reaction products, and wherein the process comprises the additional step (f) Reducing the residual content of unconsumed reducing agent and / or optionally of its incompletely oxidized reaction products in the post-treated exhaust gas by catalytic oxidation with an oxidizing agent, preferably molecular oxygen contained in the post-treated exhaust gas.

[0296] If step (f) is included in the process, the exhaust gas exiting the catalyst bed(s) of step (d) exhibits - in the use of NH3 as a reducing agent for NOx and / or N2O according to the invention in step (d) an NH3 concentration of preferably at least 3 ppmv, more preferably at least 5 ppmv, more preferably at least 10 ppmv but preferably at most 20 ppmv, more preferably at most 10 ppmv, more preferably at most 5 ppmv; and - when hydrocarbons are used as a reducing agent for N2O in step (d) according to the invention, a CO concentration of preferably at least 10 ppmv, more preferably at least 20 ppmv, more preferably at least 40 ppmv but preferably at most 100 ppmv, more preferably at most 150 ppm, more preferably at most 200 ppmv.

[0297] The catalytic oxidation of unused reducing agent and / or optionally of its not completely oxidized reaction products, with an oxidizing agent, is preferably carried out using an oxidation catalyst, according to the invention. Oxidation catalyst

[0298] Oxidation catalysts, e.g., for NH3 and / or CO oxidation, are known to those skilled in the art and typically contain as catalytically active components noble metals, such as Pt, Pd, and / or Rh, supported and dispersed on surface-rich oxides and / or ceramic substrates, transition metal oxides (e.g., of Fe, Mn, Cu, Cr, Co, Ni, etc.), or metal-loaded zeolites. Such catalysts are described, for example, in "Handbook of Heterogeneous Catalysis", Wiley-VCH, edited by Ertl, Knötzinger, Schüth, Weitkamp, ​​2nd ed. 2008, Volume 5, Chapter 11.5 "Solid Catalysts for the Oxidation of Volatile Organic Compounds". According to the invention, the oxidation catalysts used in step (f) are preferably platinum group metal-free, and more preferably noble metal-free.

[0299] For descriptive purposes, "platinum group metal-free" means that essentially no platinum group metals (i.e., Ru, Rh, Pd, Os, fr, Pt) are present. However, minute traces of platinum group metals that can be detected analytically are possible. For descriptive purposes, "precious metal-free" means that essentially no precious metals are present. However, minute traces of precious metals that can be detected analytically are possible.

[0300] Preferred platinum group metal-free oxidation catalysts are selected from transition metal oxides (e.g., of Fe, Mn, Cu, Cr, Co, Ni, etc.), metal-loaded zeolites, e.g., as described in Handbook of Heterogeneous Catalysis, Wiley-VCH, Edited by Ertl, Knötzinger, Schüth, Weitkamp, ​​2nd Ed. 2008, Volume 5, Chapter 11.5 "Solid Catalysts for the Oxidation of Volatile Organic ('ompoiinds",

[0301] Preferred oxidation catalysts include - cobalt-containing catalysts; in particular CO3O4; mixed oxides derived from CO3O4 (CO3- y M y O4), which preferably crystallize in the spinel structure like CO3O4, wherein M is preferably selected from Zn, Cu, Fe, Mn and V; cobalt-loaded zeolites, preferably of the structure type MFI, BEA, FER, Mor, FAU, CHA, or AFI; - manganese-containing catalysts; in particular MnOx with x = 1-2; mixed oxides derived from MnOx (Mn x.y M y 0x), wherein M is preferably selected from Zn, Cu, Fe and Mn; manganese-loaded zeolites, preferably of the structure type MFI, BEA, FER, Mor, FAU, CHA or AFI; - copper-containing catalysts; in particular CuOx with x = 0.5-1; mixed oxides derived from CuOx (Cu x-y M y Ox), wherein M is preferably selected from Zn, Co, Fe and Mn; copper-loaded zeolites, preferably of the structure type MFI, BEA, FER, Mor, FAU, CHA, AFI; - silver-containing catalysts; in particular supported, preferably supported on Al2O3, TiCE, or Si O2- preferably e.g. X% Ag / TiCE, X% Ag / AECE, or X% Ag / SiCE each with X=l-10.

[0302] Preferably, the oxidation catalyst is an NFF oxidation catalyst and comprises an iron- or copper-loaded zeolite; preferably an iron- or copper-loaded zeolite of the structure type MFI, BEA, FER, MOR, FAU, AEI and / or MEL.

[0303] In the use of NH3 as a reducing agent for NOx and / or N2O according to the invention in step (d), the exhaust gas treated after step (f) has an NFF concentration of preferably at most 5.0 ppmv, more preferably at most 2.0 ppmv, even more preferably at most 1.0 ppmv and particularly preferably at most 0.5 ppmv.

[0304] When hydrocarbons are used as a reducing agent for N2O according to the invention, the exhaust gas treated after step (f) has a CO concentration of preferably at most 80 ppmv, more preferably at most 50 ppmv, even more preferably at most 20 ppmv and particularly preferably at most 10 ppmv. Attachment

[0305] Another aspect of the invention relates to a system configured to carry out the method described above. All preferred embodiments of the method described above also apply analogously to the system according to the invention.

[0306] The system according to the invention comprises: - a combustion chamber configured to burn NH3 in the presence of O2, producing an exhaust gas that includes nitrogen oxide; - optionally, downstream of the combustion chamber in the direction of exhaust gas flow, a collecting device configured to collect and convey the exhaust gas; - a metering device A, which is configured to add reducing agent to the exhaust gas to partially reduce the nitrogen oxide content in the exhaust gas by non-catalytic chemical reduction, preferably selective non-catalytic reduction (SNCR), of nitrogen oxide with the reducing agent, producing a pretreated exhaust gas; - optionally, downstream of the metering device A in the direction of exhaust gas flow, at least one heat exchanger configured to cool the exhaust gas; and - downstream of the metering device A in the direction of exhaust gas flow, an exhaust gas treatment device configured to further reduce the nitrogen oxide content in the pretreated exhaust gas by catalytic chemical reduction, preferably selective catalytic chemical reduction (SCR), of nitrogen oxide with a reducing agent and / or catalytic decomposition of nitrogen oxide to produce a post-treated exhaust gas.

[0307] Preferably, this exhaust gas treatment device comprises a metering device configured to add reducing agent to the exhaust gas to further reduce the nitrogen oxide content in the exhaust gas by catalytic chemical reduction, preferably selective catalytic reduction (SCR), of nitrogen oxide with the reducing agent.

[0308] In preferred embodiments, the exhaust gas treatment device according to the invention is designed as a single stage, i.e., it comprises a single catalyst bed, preferably comprising a catalyst containing an iron-loaded zeolitic material (Fe-zeolite), preferably of the structural type BEA, MFI, MOR, MEL or FER, more preferably of the structural type MFI and BEA, for the catalyzed reactions of this single stage, i.e., on this single catalyst bed.

[0309] In other particularly preferred embodiments, the exhaust gas treatment device according to the invention is (at least) designed in two stages, i.e., it comprises two spatially separated catalyst beds (first catalyst bed and second catalyst bed), each preferably independently comprising a catalyst containing an iron-loaded zeolitic material (Fe-zeolite), preferably of the structural type BEA, MFI, MOR, MEL or FER, more preferably of the structural type MFI and BEA, and even more preferably of the structural type BEA. The first catalyst bed is preferably arranged upstream of the second catalyst bed in the direction of exhaust gas flow.

[0310] A further metering device (metering device C) is preferably arranged downstream of the first catalyst bed and upstream of the second catalyst bed, which is configured to meter reducing agent into the exhaust gas, preferably NH3.

[0311] Upstream of the first catalyst bed, an additional metering device (metering device B) is preferably arranged, which is configured to meter reducing agent into the exhaust gas, preferably NH3 and optionally preferably CH4.

[0312] In preferred embodiments, the system according to the invention additionally comprises a reactor which is configured to produce H2 and N2 by catalytic decomposition of NH3 with the supply of heat which is released in the combustion chamber when NH3 is burned in the presence of O2.

[0313] Preferably, the system according to the invention is configured such that heat removed during the cooling of the exhaust gas with the aid of the heat exchanger is supplied to the reactor.

[0314] Preferred embodiments of the invention are listed below as sentences: Sentence 1: A method for reducing the nitrogen oxide content in an exhaust gas, the method comprising the steps of: (a) burning NH3 in the presence of O2 to produce an exhaust gas containing nitrogen oxide; (b) partially reducing the nitrogen oxide content in the exhaust gas by non-catalytic chemical reduction, preferably selective non-catalytic chemical reduction (SNCR), of nitrogen oxide with a reducing agent to produce a pretreated exhaust gas; (c) cooling the pretreated exhaust gas; and (d) further reducing the nitrogen oxide content in the pretreated exhaust gas by catalytic chemical reduction, preferably selective catalytic chemical reduction (SCR), of nitrogen oxide with a reducing agent and / or catalytic decomposition of nitrogen oxide to produce a post-treated exhaust gas. Sentence 2: The process according to Sentence 1, comprising the additional step (e) generating H2 and N2 by catalytic decomposition of NH3 by supplying heat which is released in step (a) during the combustion of NH3 in the presence of O2 and / or in step (b) during the non-catalytic chemical reduction of nitrogen oxide with reducing agent. Sentence 3: The process according to sentence 1 or 2, wherein in step (b) the reducing agent is selected from NH3 and urea, preferably NH3, wherein the reducing agent may optionally be in aqueous solution. Sentence 4: The method according to one of the preceding sentences, wherein for step (b) the reducing agent is provided in a mixture with a carrier medium. Sentence 5: The method according to Sentence 4, wherein the carrier medium is selected from steam, water and compressed air; preferably steam; preferably steam with a pressure of at most 5 bar a (low-pressure steam). Sentence 6: The method according to sentence 4 or 5, wherein the carrier medium and the reducing agent are supplied separately to a two-component injection lance, at the end of which a nozzle is arranged in which the carrier medium and the reducing agent are mixed together and then sprayed. Sentence 7: The method according to one of sentences 4 to 6, wherein the carrier medium and the reducing agent are mixed and sprayed together via a plurality of two-component injection lances; preferably via at least 2 separate two-component injection lances. Sentence 8: The method according to any of the preceding sentences, wherein the nitrogen oxide comprises NOx. Sentence 9: The method according to any of the preceding sentences, wherein the exhaust gas produced in step (a) has a NOx content of at least 500 ppm, preferably at least 1000 ppm, more preferably at least 2000 ppm, more preferably at least 3000 ppm, most preferably at least 4000 ppm, and in particular at least 5000 ppm, or at least 6000 ppm. Sentence 10: The process according to Sentence 9, wherein the NOx has an oxidation degree a(NOx) of at least 0.05, preferably at least 0.1, more preferably at least 0.2, even more preferably at least 0.3, most preferably at least 0.4, and in particular at least 0.5. Sentence 11: The process according to sentence 9 or 10, wherein the NOx has an oxidation degree a(NOx) of at least 0.6, preferably at least 0.7, more preferably at least 0.8, even more preferably at least 0.85, most preferably at least 0.9, and in particular at least 0.95. Sentence 12: The process according to one of sentences 9 to 11, wherein the NOx has an oxidation degree a(NOx) of at most 0.9, preferably at most 0.7, more preferably at most 0.5, even more preferably at most 0.3, most preferably at most 0.2, and in particular at most 0.1. Sentence 13: The method according to any of the preceding sentences, wherein the nitrogen oxide comprises N2O. Sentence 14: The method according to any of the preceding sentences, wherein the exhaust gas produced in step (a) has an N2O content of at least 5 ppm, preferably at least 10 ppm, more preferably at least 25 ppm, more preferably at least 50 ppm, most preferably at least 100 ppm, and in particular at least 200 ppm. Sentence 15: The method according to any of the preceding sentences, wherein the exhaust gas produced in step (a) has an N2O content of at least 500 ppm, preferably at least 1000 ppm, more preferably at least 2000 ppm, more preferably at least 3000 ppm, most preferably at least 4000 ppm, and in particular at least 5000 ppm, or at least 6000 ppm. Sentence 16: The method according to one of the preceding sentences, wherein the exhaust gas produced in step (a) has a pressure of at most 1.5 bar; preferably atmospheric pressure; more preferably a pressure below atmospheric pressure; particularly preferably a pressure of at most 0.95 bar, more preferably of at most 0.9 bar. Sentence 17: The method according to one of the preceding sentences, wherein in step (a) the burning of NH3 in the presence of O2 does not take place on a catalyst. Sentence 18: The method according to one of the preceding sentences, wherein in step (a) the combustion of NH3 in a mixture with H2 and in the presence of O2 takes place. Sentence 19: The process according to Sentence 18, wherein the proportion of H2 in the mixture with NH3 is at most 60 mol. - %, preferably at most 50 mol %, more preferably at most 40 mol %, more preferably at most 30 mol %, most preferably at most 20 mol %, and in particular at most 10 mol %, based on the total amount of H2 and NH3. Sentence 20: The process according to sentence 18 or 19, wherein the proportion of H2 in the mixture with NH3 is at least 2.0 mol%, preferably at least 5.0 mol%, more preferably at least 10 mol%, more preferably at least 20 mol%, most preferably at least 30 mol%, and in particular at least 40 mol%, based on the total amount of H2 and NH3. Sentence 21: The method according to one of the preceding sentences, wherein the exhaust gas produced in step (a) has an H2O content of more than 4.0 vol.%; preferably at least 5.0 vol.%, more preferably at least 6.0 vol.%, more preferably at least 7.0 vol.%, most preferably at least 8.0 vol.%, and in particular at least 9.0 vol.%. Sentence 22: The method according to one of the preceding sentences, wherein the exhaust gas produced in step (a) has a H2O content of at least 10 vol.%; preferably at least 12 vol.%, more preferably at least 14 vol.%, more preferably at least 16 vol.%, most preferably at least 18 vol.%, and in particular at least 20 vol.%. Sentence 23: The method according to one of the preceding sentences, wherein the exhaust gas produced in step (a) additionally includes N2. Sentence 24: The method according to one of the preceding sentences, wherein in step (b) the exhaust gas has a temperature in the range of 800 to 1075°C, preferably more than 850°C and less than 900°C, before the reduction of the nitrogen oxide content. Sentence 25: The method according to one of the preceding sentences, wherein in step (b) the exhaust gas has a temperature of at least 800°C before the reduction of the nitrogen oxide content, preferably at least 810°C, more preferably at least 820°C, more preferably at least 830°C, most preferably at least 840°C, and in particular at least 850°C. Sentence 26: The method according to one of the preceding sentences, wherein in step (b) the exhaust gas has a temperature of at least 860°C before the reduction of the nitrogen oxide content, preferably at least 870°C, more preferably at least 880°C, more preferably at least 890°C, most preferably at least 900°C, and in particular at least 910°C. Sentence 27: The method according to one of the preceding sentences, wherein in step (b) the exhaust gas has a temperature of at most 960°C before the reduction of the nitrogen oxide content, preferably at most 950°C, more preferably at most 940°C, more preferably at most 930°C, most preferably at most 920°C, and in particular at most 910°C. Sentence 28: The method according to one of the preceding sentences, wherein in step (b) the exhaust gas has a temperature of at most 900°C before the reduction of the nitrogen oxide content, preferably at most 890°C, preferably at most 880°C, even more preferably at most 870°C, most preferably at most 860°C, and in particular at most 850°C. Sentence 29: The method according to one of the preceding sentences, wherein in step (b) the exhaust gas has a temperature in the range of 840±70°C before reducing the nitrogen oxide content, preferably 840±60°C, more preferably 840±50°C, more preferably 840±40°C, most preferably 840±30°C, and in particular 840±20°C. Sentence 30: The method according to one of the preceding sentences, wherein in step (b) the exhaust gas has a temperature in the range of 850±70°C before reducing the nitrogen oxide content, preferably 850±60°C, more preferably 850±50°C, more preferably 850±40°C, most preferably 850±30°C, and in particular 850±20°C. Sentence 31: The method according to one of the preceding sentences, wherein in step (b) the exhaust gas has a temperature in the range of 860±70°C before reducing the nitrogen oxide content, preferably 860±60°C, more preferably 860±50°C, more preferably 860±40°C, most preferably 860±30°C, and in particular 860±20°C. Sentence 32: The method according to one of the preceding sentences, wherein in step (b) the exhaust gas has a temperature in the range of 870±70°C before reducing the nitrogen oxide content, preferably 870±60°C, more preferably 870±50°C, more preferably 870±40°C, most preferably 870±30°C, and in particular 870±20°C. Sentence 33: The method according to one of the preceding sentences, wherein in step (b) the exhaust gas has a temperature in the range of 880±70°C before reducing the nitrogen oxide content, preferably 880±60°C, more preferably 880±50°C, more preferably 880±40°C, most preferably 880±30°C, and in particular 880±20°C. Sentence 34: The method according to one of the preceding sentences, wherein in step (b) the exhaust gas has a temperature in the range of 890±70°C before reducing the nitrogen oxide content, preferably 890±60°C, more preferably 890±50°C, more preferably 890±40°C, most preferably 890±30°C, and in particular 890±20°C. Sentence 35: The method according to one of the preceding sentences, wherein in step (b) the exhaust gas has a temperature in the range of 900±70°C before reducing the nitrogen oxide content, preferably 900±60°C, more preferably 900±50°C, more preferably 900±40°C, most preferably 900±30°C, and in particular 900±20°C. Sentence 36: The method according to one of the preceding sentences, wherein in step (b) the molar ratio of reducing agent expressed in NHs reduction equivalents to NOx based on NOx reacted is in the range of 1.0 to 3.0, preferably 1.5 to 3.0, more preferably 1.5 to 2.5, most preferably 1.75 to 2.75. Sentence 37: The method according to one of the preceding sentences, wherein in step (b) the molar ratio of reducing agent expressed in NHs reduction equivalents to NOx is related to the content of NOx in the exhaust gas according to step (a) is in the range of 1.0 to 3.0, preferably 1.5 to 3.0, even more preferably 1.5 to 2.5, most preferably 1.75 to 2.75. Sentence 38: The method according to one of the preceding sentences, wherein for step (b) the reducing agent is metered into the exhaust gas via at least one metering device, which preferably comprises at least one addition unit, preferably at least one injection lance, more preferably a two-component injection lance. Sentence 39: The method according to one of the preceding sentences, wherein for step (b) the reducing agent is metered into the exhaust gas via a first addition unit of a metering device A, preferably via a first injection lance, more preferably a first two-component injection lance. Sentence 40: The method according to any of the preceding sentences, wherein step (a) is carried out in a firing chamber and wherein the reducing agent for step (b) is introduced into the firing chamber. Sentence 41: The method according to one of the preceding sentences, wherein step (a) is carried out in a combustion chamber, the generated exhaust gas is subsequently directed into a collecting device for collecting and conveying the exhaust gas, and the reducing agent for step (b) is introduced into the collecting device for collecting and conveying the exhaust gas. Sentence 42: The method according to Sentence 41, wherein the collecting device for collecting and conveying the exhaust gas along its longitudinal extent can be conceptually divided into a first section and a directly adjoining, preferably equal-length, second section; and wherein for step (b) the reducing agent is metered into the exhaust gas within the first section via a first addition unit, preferably a first injection lance, more preferably a first two-component injection lance, and in the second section via a second addition unit, more preferably a second injection lance, more preferably a second two-component injection lance. Sentence 43: The procedure according to Sentence 42, wherein a larger quantity of reducing agent is added via the second addition unit than via the first addition unit. Sentence 44: The method according to sentence 42 or 43, wherein the angle between the flow direction of the exhaust gas and the direction of the metering of the reducing agent for the first addition unit and the second addition unit is independently in the range of 30° to 70°. Sentence 45: The method according to Sentence 44, wherein the reducing agent is added to the exhaust gas stream at a different angle via the first addition unit than via the second addition unit. Sentence 46: The method according to one of the preceding sentences, wherein the pretreated exhaust gas produced in step (b) has a NOx content which is at least 5%, preferably at least 10%, more preferably at least 15%, more preferably at least 20%, most preferably at least 25% and in particular at least 30% lower than the NOx content in the exhaust gas produced in step (a). Sentence 47: The method according to any of the preceding sentences, wherein the pretreated exhaust gas produced in step (b) has a NOx content which is at least 35%, preferably at least 40%, more preferably at least 45%, more preferably at least 50%, most preferably at least 55% and in particular at least 60% lower than the NOx content in the exhaust gas produced in step (a). Sentence 48: The method according to one of the preceding sentences, wherein the pretreated exhaust gas produced in step (b) has a NOx content which is at most 90%, preferably at most 80%, more preferably at most 70%, more preferably at most 60%, most preferably at most 50% and in particular at most 40% lower than the NOx content in the exhaust gas produced in step (a). Sentence 49: The method according to any of the preceding sentences, wherein the pretreated exhaust gas produced in step (b) has a NOx content of at least 500 ppm, preferably at least 1000 ppm, more preferably at least 2000 ppm, more preferably at least 3000 ppm, most preferably at least 4000 ppm, and in particular at least 5000 ppm, or at least 6000 ppm. Sentence 50: The process according to sentence 49, wherein the NOx has an oxidation degree a(NOx) of at least 0.05, preferably at least 0.1, more preferably at least 0.2, even more preferably at least 0.3, most preferably at least 0.4, and in particular at least 0.5. Sentence 51: The process according to sentence 49 or 50, wherein the NOx has an oxidation degree a(NOx) of at least 0.6, preferably at least 0.7, more preferably at least 0.8, even more preferably at least 0.85, most preferably at least 0.9, and in particular at least 0.95. Sentence 52: The process according to one of sentences 49 to 51, wherein the NOx has an oxidation degree a(NOx) of at most 0.9, preferably at most 0.7, more preferably at most 0.5, even more preferably at most 0.3, most preferably at most 0.2, and in particular at most 0.1. Sentence 53: The method according to any of the preceding sentences, wherein the pretreated exhaust gas produced in step (b) comprises N2O. Sentence 54: The method according to any of the preceding sentences, wherein the pretreated exhaust gas produced in step (b) has an N2O content of at least 5 ppm, preferably at least 10 ppm, more preferably at least 25 ppm, more preferably at least 50 ppm, most preferably at least 100 ppm, and in particular at least 200 ppm. Sentence 55: The method according to any of the preceding sentences, wherein the pretreated exhaust gas produced in step (b) has an N2O content of at least 500 ppm, preferably at least 1000 ppm, more preferably at least 2000 ppm, more preferably at least 3000 ppm, most preferably at least 4000 ppm, and in particular at least 5000 ppm, or at least 6000 ppm. Sentence 56: The method according to one of the preceding sentences, wherein the pretreated exhaust gas produced in step (b) has a residual content of unconsumed reducing agent. Sentence 57: The method according to any of the preceding sentences, wherein the pretreated exhaust gas produced in step (b) has a residual content of unused reducing agent of at least 5%, preferably at least 10%, more preferably at least 15%, more preferably at least 20%, most preferably at least 25%, and in particular at least 30%, in each case based on the amount of reducing agent used in step (b). Sentence 58: The method according to one of the preceding sentences, wherein the pretreated exhaust gas produced in step (b) has a residual content of unconsumed reducing agent of at most 30%, preferably at most 25%, more preferably at most 20%, more preferably at most 15%, most preferably at most 10%, and in particular at most 5%, in each case based on the amount of reducing agent used in step (b). Sentence 59: The method according to any of the preceding sentences, wherein the pretreated exhaust gas produced in step (b) has a residual content of unconsumed reducing agent of at least 5 ppm, preferably at least 10 ppm, more preferably at least 15 ppm, more preferably at least 20 ppm, most preferably at least 25 ppm, and in particular at least 30 ppm. Sentence 60: The method according to any of the preceding sentences, wherein the pretreated exhaust gas produced in step (b) has a residual content of unconsumed reducing agent of at least 35 ppm, preferably at least 40 ppm, more preferably at least 45 ppm, more preferably at least 50 ppm, most preferably at least 55 ppm, and in particular at least 60 ppm. Sentence 61: The method according to any of the preceding sentences, wherein the pretreated exhaust gas produced in step (b) has a residual content of unconsumed reducing agent of at least 65 ppm, preferably at least 70 ppm, more preferably at least 75 ppm, more preferably at least 80 ppm, most preferably at least 85 ppm, and in particular at least 90 ppm. Sentence 62: The method according to one of the preceding sentences, wherein the pretreated exhaust gas produced in step (b) has a residual content of unconsumed reducing agent of at most 600 ppm, preferably at most 500 ppm, more preferably at most 400 ppm, more preferably at most 300 ppm, most preferably at most 200 ppm, and in particular at most 100 ppm. Sentence 63: The method according to any of the preceding sentences, wherein the pretreated exhaust gas produced in step (b) has a residual content of unconsumed reducing agent of at most 90 ppm, preferably at most 80 ppm, more preferably at most 70 ppm, more preferably at most 60 ppm, most preferably at most 50 ppm, and in particular at most 40 ppm. Sentence 64: The method according to one of the preceding sentences, wherein the pretreated exhaust gas produced in step (b) has a residual content of unconsumed reducing agent of at most 30 ppm, preferably at most 25 ppm, more preferably at most 20 ppm, more preferably at most 15 ppm, most preferably at most 10 ppm, and in particular at most 5 ppm. Sentence 65: The method according to one of sentences 67 to 64, wherein the residual content of unused reducing agent is at least partly used as reducing agent in step (d). Sentence 66: The method according to one of the preceding sentences, wherein in step (c) the pretreated exhaust gas is cooled by at least 50°C relative, preferably at least 100°C, more preferably at least 150°C, more preferably at least 200°C, most preferably at least 250°C, and in particular at least 300°C. Sentence 67: The method according to any of the preceding sentences, wherein step (d) comprises the chemical reduction of NOx with reducing agent at a NOx reduction catalyst. Sentence 68: The method according to one of the preceding sentences, wherein for step (d) the reducing agent is metered into the exhaust gas via a metering device B. Sentence 69: The procedure according to sentence 67 or 68, wherein the reducing agent is NFF. Sentence 70: The process according to one of sentences 67 to 69, wherein the NOx reduction catalyst is selected from metal-loaded zeolite catalysts, precious metal catalysts or transition metal oxide catalysts; preferably metal-loaded zeolite catalysts or transition metal oxide catalysts. Sentence 71: The process according to one of sentences 67 to 70, wherein the NOx reduction catalyst is a TiO2-based catalyst; preferably a TiO2-supported catalyst, particularly preferably glass fiber-impregnated. Sentence 72: The process according to one of sentences 67 to 71, wherein the NOx reduction catalyst comprises a catalytically active compound of a transition metal, preferably vanadium or tungsten. Sentence 73: The process according to one of sentences 67 to 72, wherein the NOx reduction catalyst is a TiO2 support, preferably glass fiber impregnated, with a catalytically active compound of a transition metal, particularly preferably vanadium and tungsten. Sentence 74: The process according to any one of sentences 67 to 73, wherein the NOx reduction catalyst comprises a zeolitic material; preferably a zeolite loaded with transition metal (including lanthanide), in particular with iron, cobalt or copper; more preferably an iron-loaded zeolite; even more preferably an iron-loaded zeolite of the type MFI, BEA, FER, MOR, FAU and / or MEL. Sentence 75: The method according to one of sentences 67 to 74, wherein the NOx reduction catalyst is in the form of catalyst honeycombs, preferably in the form of several catalyst honeycombs arranged parallel to each other with honeycomb channels oriented longitudinally to the direction of flow of the exhaust gas. Sentence 76: The process according to one of the preceding sentences, wherein in step (d) the nitrogen oxide content is further reduced by (di) catalytic chemical reduction of NOx, preferably by selective catalytic chemical reduction (SCR), with a reducing agent on a NOx reduction catalyst; and / or (d2) catalytic chemical reduction of N2O, preferably by Selective catalytic chemical reduction (SCR) with a reducing agent on an N2O reduction catalyst; and / or (ds) catalytic decomposition of nitrogen oxide on an N2O decomposition catalyst. Sentence 77: The method according to any of the preceding sentences, wherein step (d) takes place in a single catalyst bed. Sentence 78: The process according to Sentence 77, wherein the catalyst used in the single catalyst bed serves as an N2O decomposition catalyst and / or N2O reduction catalyst as well as an NOx reduction catalyst. Sentence 79: The process according to Sentence 78, wherein the catalyst is iron-loaded zeolitic material (Fe-zeolite), preferably of the structure type BEA, MFI, MOR, MEL or FER, more preferably of the structure type MFI and BEA, and even more preferably of the structure type BEA. Sentence 80: The method according to one of sentences 77 to 79, wherein the reducing agent required for the chemical reduction of NOx and, if applicable, the additional reducing agent required for the chemical reduction of N2O are metered into the exhaust gas via a metering device B; preferably wherein the metering device B is arranged upstream of the single catalyst bed in the direction of flow of the exhaust gas. Sentence 81: The process according to one of sentences 77 to 80, wherein the chemical reduction of NOx with a reducing agent, preferably NH3, on a NOx reduction catalyst, preferably an Fe-zeolite, is carried out to reduce the content of NOx in the exhaust gas. Sentence 82: The process according to Sentence 81, wherein the reduction of the N2O content in the exhaust gas is additionally carried out (i) by decomposition on an N2O decomposition catalyst, preferably on the same material which is also used as a NOx reduction catalyst, preferably an Fe zeolite; and / or (ii) by chemical reduction with a reducing agent, preferably NH3 or CH4, more preferably NH3, on an N2O reduction catalyst, preferably on the same material which is also used as a NOx reduction catalyst and optionally as an N2O decomposition catalyst, preferably an Fe zeolite. Sentence 83: The process according to one of sentences 77 to 82, wherein the reduction of NOx by chemical reduction with a reducing agent, preferably NH3, takes place on the NOx reduction catalyst, preferably to a residual content of NOx in the exhaust gas, and subsequently or in parallel the decomposition of N2O and / or the chemical reduction of N2O with a reducing agent, preferably NH3 or CH4, more preferably NH3, takes place in the single catalyst bed, which is preferably based on a single catalyst material, preferably an Fe-zeolite. Sentence 84: The method according to sentence 83, wherein the amount of NH3 is dosed such that a residual NOx content of at most 50 ppm remains in the exhaust gas, preferably at most 40 ppm, more preferably at most 30 ppm, most preferably at most 20 ppm, and in particular at most 10 ppm. Sentence 85: The process according to one of sentences 77 to 84, wherein the chemical reduction of NOx with NHs takes place jointly in the single catalyst bed (di). Sentence 86: The process according to one of sentences 77 to 84, wherein the chemical reduction of NOx with NHs and the chemical reduction of N2O with NH3 take place together in the single catalyst bed (di). Sentence 87: The process according to one of sentences 77 to 84, wherein the chemical reduction of NOx with NH3 and the chemical reduction of N2O with hydrocarbon (CH4, natural gas, etc.) take place together in the single catalyst bed (di). Sentence 88: The process according to one of sentences 77 to 84, wherein the chemical reduction of NOx with NH3 and the chemical reduction of N2O with NH3 and hydrocarbon (CH4, natural gas, etc.) take place together in the single catalyst bed (di). Sentence 89: The process according to one of sentences 77 to 84, wherein the chemical reduction of NOx with NH3 and the decomposition of N2O take place together in the single catalyst bed (di). Sentence 90: The process according to one of sentences 77 to 84, wherein the chemical reduction of NOx with NH3 and (d2) the chemical reduction of N2O with NH3 and (d, ) the decomposition of N2O take place together in the single catalyst bed (di). Sentence 91: The process according to one of sentences 77 to 84, wherein (di) the chemical reduction of NOx with NH3 and (d2) the chemical reduction of N2O with hydrocarbon (CH4, natural gas, etc.) and (d, ) the decomposition of N2O take place together in the single catalyst bed. Sentence 92: The process according to one of sentences 77 to 84, wherein (di) the chemical reduction of NOx with NH3 and (d2) the chemical reduction of N2O with NH3 and hydrocarbon (CH4, natural gas, etc.) and (d,) the decomposition of N2O take place together in the single catalyst bed. Sentence 93: The method according to any of the preceding sentences, wherein the post-treated exhaust gas produced in step (d) has a NOx content which is at least 90%, preferably at least 93%, more preferably at least 96%, more preferably at least 97%, most preferably at least 98% and in particular at least 99% lower than the NOx content in the exhaust gas produced in step (a). Sentence 94: The method according to any of the preceding sentences, wherein the post-treated exhaust gas produced in step (d) has a NOx content of at most 100 ppm, preferably at most 50 ppm, more preferably at most 25 ppm, more preferably at most 10 ppm, most preferably at most 5 ppm, and in particular at most 1 ppm. Sentence 95: The method according to one of the preceding sentences, wherein the post-treated exhaust gas produced in step (d) has a residual content of unconsumed reducing agent and wherein the method comprises the additional step (f) Reducing the residual content of unconsumed reducing agent in the post-treated exhaust gas by catalytic decomposition of reducing agent and / or catalytic oxidation with an oxidizing agent. Paragraph 96: An installation configured to carry out the procedure according to one of the preceding paragraphs and comprising: - a combustion chamber configured to burn NH3 in the presence of O2, producing an exhaust gas comprising nitrogen oxide; - optionally, downstream of the combustion chamber in the direction of exhaust gas flow, a collecting device configured to collect and convey the exhaust gas; - a metering device A configured to add reducing agent to the exhaust gas to partially reduce the nitrogen oxide content in the exhaust gas by non-catalytic chemical reduction of nitrogen oxide with the reducing agent, producing a pretreated exhaust gas;and - in the direction of exhaust gas flow downstream of the metering device A, an exhaust gas treatment device configured to further reduce the nitrogen oxide content in the pretreated exhaust gas by catalytic chemical reduction of nitrogen oxide with a reducing agent and / or catalytic decomposition of nitrogen oxide, producing a post-treated exhaust gas. Sentence 97: The plant according to sentence 96, which additionally includes a reactor configured to produce H2 and N2 by catalytic decomposition of NH3 with the input of heat released in the combustion chamber during the combustion of NH3 in the presence of O2.

[0315] The invention is explained in more detail below with reference to Figures 2 to 5. However, these figures are not to be interpreted as limiting.

[0316] Figure 2 shows, as mentioned above, for illustrative purposes and in a highly simplified manner, the approximate relationship between the reaction temperature and the reactions taking place during the non-catalytic chemical reduction of NOx with NH3.

[0317] Figure 3 schematically shows a preferred apparatus according to the invention for burning NH3 in the presence of O2 (step (a)) and for partially reducing nitrogen oxide by non-catalytic chemical reduction with a reducing agent (step (b)). The apparatus comprises a combustion chamber (1) and several tubular reactors (2), which are preferably loaded with an NH3 decomposition catalyst and preferably through which NH3 flows from top to bottom. In the tubular reactors (2), the catalytic decomposition of NH3 into N2 and H2 takes place. To provide the heat required for this endothermic decomposition of NH3, the combustion chamber (1) additionally comprises several burners (3) with which step (a) of the process according to the invention is carried out, i.e., with which NH3 is burned, optionally in a mixture with H2 and in the presence of O2, producing exhaust gas containing nitrogen oxide. The O2 is contained in combustion air (5).The exhaust gas is separated by the walls of the tubular reactors (2) from the NH3 to be catalytically decomposed and from the N2 and H2 formed during decomposition; that is, no mass exchange takes place. Heat generated during the combustion of NH3, possibly in a mixture with H2 and in the presence of O2, can However, the heat flows through the walls of the tubular reactors (2) to the NH3 decomposition catalyst, thus supplying the heat required for the endothermic decomposition reaction. Each burner (3) is associated with a collection device for collecting and conveying the exhaust gas (4), so that the exhaust gas flows from the burner through an exhaust gas duct to the collection device (4).

[0318] Figure 4 schematically shows an enlarged representation of the collection device for collecting and conveying the exhaust gas (4) according to Figure 3. Figure 4A shows a front view, Figure 4B a perspective view, and Figure 4C a side view. The exhaust gas flows from above through openings (6) into the collection device (4), is then deflected, and flows out to the rear (see Figures 3A and 3B). A first injection unit (7a) and a second injection unit (7b) are arranged along the longitudinal dimension of the collection device (4), through which the reducing agent can be metered into the exhaust gas in a controlled manner. The longitudinal dimension of the collection device (4) corresponds to the direction of exhaust gas flow from the first injection unit (7a) to the second injection unit (7b) (indicated by the three arrows in Figure 4C).Preferably, the reducing agent is metered into the exhaust gas stream via the first injection unit (7a) at a different angle than via the second injection unit (7b). Preferably, this angle is smaller for the first injection unit (7a) than for the second injection unit (7b). The angle α and the angle β are each defined by the injection unit (in the case of a nozzle, by the primary spray direction) and the direction of exhaust gas flow from the first injection unit (7a) towards the second injection unit (7b).

[0319] Figure 5 schematically shows a preferred apparatus according to the invention for burning NH₃ in the presence of O₂ (step (a)), for partially reducing nitrogen oxide by non-catalytic chemical reduction with a reducing agent to produce a pretreated exhaust gas (step (b)), for cooling the pretreated exhaust gas (step (c)), and for reducing the nitrogen oxide content in the pretreated exhaust gas by catalytic chemical reduction of nitrogen oxide with a reducing agent (step (d)). The apparatus comprises a combustion chamber (1) and a tubular reactor (2), which is preferably loaded with an NH₃ decomposition catalyst and preferably through which NH₃ flows from top to bottom. In the tubular reactor (2), the catalytic decomposition of NH₃ into N₂ and H₂ takes place.To provide the heat required for this endothermic decomposition of NH3, the combustion chamber (1) additionally includes several burners (3) with which step (a) of the inventive process is carried out, i.e., with which NH3, optionally in a mixture with H2 and in the presence of O2, is combusted, producing exhaust gas containing nitrogen oxide. The O2 is contained in the combustion air (5). The exhaust gas is separated by the walls of the tubular reactor (2) from the NH3 to be catalytically decomposed and from the N2 and H2 formed during the decomposition; i.e., no mass transfer takes place. During the combustion of NH3, optionally in a mixture. However, the heat generated with H2 and in the presence of O2 can flow through the walls of the tubular reactors (2) as a heat flow to the NH3 decomposition catalyst, thus supplying the heat required for the endothermic decomposition reaction. The exhaust gas flows from above into the collection device (4) for collecting and conveying the exhaust gas, is then deflected and flows off to the right. A metering device A (7) introduces a reducing agent, preferably NH3, into the collection device for collecting and conveying the exhaust gas. Step (b) of the process according to the invention is carried out with the reducing agent, i.e., the nitrogen oxide content in the exhaust gas is partially reduced by non-catalytic chemical reduction of nitrogen oxide. This produces a pretreated exhaust gas, which flows to a heat exchanger (10), in which step (c) of the process according to the invention is carried out, i.e., in which the pretreated exhaust gas is cooled.The pretreated exhaust gas flows to a NOx reduction catalyst (9), at which step (d) of the inventive process is carried out, i.e., the nitrogen oxide content in the pretreated exhaust gas is reduced by catalytic chemical reduction of nitrogen oxide with a reducing agent. The reducing agent, preferably NH3, for step (d) is added to the exhaust gas via a metering device B (8). Reference symbol list: 1 firing chamber 2-tube reactor 3 burners 4. Collecting device for collecting and conveying the exhaust gas 5 Combustion air 6 Opening 7 Dosing device A 7a first additional unit 7b second additional unit 8 Dosing device B 9 NOx reduction catalyst 10 heat exchangers

Claims

Patent claims:

1. A method for reducing the nitrogen oxide content in an exhaust gas, the method comprising the steps of: (a) Burning NH3 in the presence of O2 producing an exhaust gas which includes nitrogen oxide; (b) partial reduction of the nitrogen oxide content in the exhaust gas by non-catalytic chemical reduction of nitrogen oxide with a reducing agent to produce a pretreated exhaust gas; (c) Cooling the pretreated exhaust gas; and (d) further reducing the nitrogen oxide content in the pretreated exhaust gas by catalytic chemical reduction of nitrogen oxide with a reducing agent and / or catalytic decomposition of nitrogen oxide to produce a posttreated exhaust gas.

2. The method according to claim 1, comprising the additional step (e) Production of H2 and N2 by catalytic decomposition of NH3 by supplying heat, which is released in step (a) by burning NH3 in the presence of O2 and / or in step (b) by non-catalytic chemical reduction of nitrogen oxide with reducing agent.

3. The method according to claim 1 or 2, wherein in step (b) the reducing agent is selected from NH3 and urea, preferably NH3, wherein the reducing agent may optionally be in aqueous solution.

4. The method according to any of the preceding claims, wherein the nitrogen oxide comprises NOx.

5. The method according to one of the preceding claims, wherein the exhaust gas produced in step (a) has a NOx content of at least 500 ppm, preferably at least 1000 ppm, more preferably at least 2000 ppm, more preferably at least 3000 ppm, most preferably at least 4000 ppm, and in particular at least 5000 ppm, or at least 6000 ppm.

6. The method according to any one of the preceding claims, wherein the exhaust gas produced in step (a) has a pressure of at most 1.5 bar; preferably atmospheric pressure; more preferably a pressure below atmospheric pressure; particularly preferably a pressure of at most 0.95 bar, more preferably of at most 0.9 bar.

7. The method according to one of the preceding claims, wherein in step (a) the burning of NH; in the presence of O2 is not carried out on a catalyst.

8. The method according to one of the preceding claims, wherein in step (a) NH3 is burned in a mixture with H2 and in the presence of O2.

9. The method according to one of the preceding claims, wherein in step (b) the exhaust gas has a temperature in the range of 800 to 1075°C before reducing the nitrogen oxide content; preferably more than 850°C and less than 900°C.

10. The method according to one of the preceding claims, wherein for step (b) the reducing agent is metered into the exhaust gas via a metering device A, preferably at least one injection lance, preferably a two-component injection lance.

11. The method according to one of the preceding claims, wherein the pretreated exhaust gas produced in step (b) has a residual content of unused reducing agent.

12. The method according to claim 11, wherein the residual content of unused reducing agent is at least partially used as a reducing agent in step (d).

13. The method according to one of the preceding claims, wherein step (d) comprises the chemical reduction of NOx with reducing agent on a NOx reduction catalyst.

14. The method according to one of the preceding claims, wherein the post-treated exhaust gas produced in step (d) has a NOx content which is at least 90%, preferably at least 93%, more preferably at least 96%, more preferably at least 97%, most preferably at least 98%, and in particular at least 99% lower than the NOx content in the exhaust gas produced in step (a).

15. A system configured to perform the method according to any of the preceding claims and comprising: - a combustion chamber configured to burn NH3 in the presence of O2, producing an exhaust gas that includes nitrogen oxide; - optionally, downstream of the combustion chamber in the direction of exhaust gas flow, a collecting device configured to collect and convey the exhaust gas; - a metering device A, which is configured to add reducing agent to the exhaust gas to partially reduce the nitrogen oxide content in the exhaust gas by non-catalytic chemical reduction of nitrogen oxide with the reducing agent, producing a pretreated exhaust gas; and - downstream of the metering device A in the direction of exhaust gas flow, an exhaust gas treatment device configured to further reduce the nitrogen oxide content in the pretreated exhaust gas by catalytic chemical reduction of nitrogen oxide with a reducing agent and / or catalytic decomposition of nitrogen oxide, producing a post-treated exhaust gas.

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