Removal of hydrogen cyanide during exhaust-gas treatment

Abstract

The invention relates to a method for exhaust-gas treatment, comprising the steps of: (a) generating an exhaust gas comprising HCN, NOX and N2O, the HCN content being at least 1.0 ppmv; (b) optionally feeding a reducing agent for NOX and / or for N2O into the exhaust gas; and (c) reducing the HCN content in the exhaust gas by catalytically reacting the HCN on a transition-metal-loaded zeolitic material.

Description

Removal of hydrogen cyanide during exhaust gas aftertreatment

[0001] Priority is claimed for Belgian patent application No. 2024 / 5383 dated June 21, 2024.

[0002] The invention relates to a method for exhaust gas aftertreatment comprising the steps of: (a) generating an exhaust gas comprising HCN, NO X and N2O; wherein the HCN content is at least 1.0 ppmv; (b) optional, addition of reducing agent for NO X and / or for N2O into the exhaust gas; and (c) reducing the content of HCN in the exhaust gas by catalytic conversion of the HCN on a transition metal-loaded zeolitic material.

[0003] Many chemical and industrial processes, such as the combustion of nitrogen-containing organic compounds or the thermal or catalytic combustion of natural gas in the presence of amines or NH3, produce process or exhaust gases that often contain certain amounts or traces of hydrogen cyanide (HCN, prussic acid). HCN is frequently found in the exhaust gases of such combustion processes together with other typical combustion products, such as H2O, CO2, CO, and various nitrogen oxides.

[0004] Even small amounts of HCN in exhaust gases are problematic because HCN is classified as highly toxic due to its inhibitory effect on oxygen uptake in the human body, and correspondingly low limits must be observed for HCN emissions into the environment.

[0005] According to European Commission Implementing Decision (EU) 2022 / 2427 of 6 December 2022 on conclusions on best available techniques (BAT) pursuant to Directive 2010 / 75 / EU of the European Parliament and of the Council on industrial emissions relating to uniform exhaust gas management and treatment systems in the chemical industry (Official Journal of the European Union, L 318 / 157-206, 12.12.2022), hydrogen cyanide BAT 18 must be monitored at least once a year. The mean BAT-associated emission value for hydrogen cyanide is < 0.1–1 mg / Nm³. 3According to BAT 18, the specific techniques for reducing emissions of inorganic compounds into the air for hydrogen cyanide include generally applicable adsorption and the limited applicability of thermal oxidation. For example, the applicability of recuperative and regenerative thermal oxidation may be limited in existing plants due to design and / or operational constraints. Applicability may also be limited if the energy demand is excessively high due to the low concentration of the compound(s) in question in the process exhaust gases.

[0006] In chemical and industrial engineering, HCN-contaminated exhaust gases are purified using various methods. This is done, for example, by alkaline scrubbing, which produces cyanides. However, this has the disadvantage that the separated cyanides, being highly toxic compounds, must be disposed of further.

[0007] Another established method in exhaust gas technology for the purification of HCN-laden exhaust gases is the transfer via special oxidation catalysts with the formation of CO2, H2O and N2 (simplified reaction equation (1)) and various nitrogen oxides as byproducts: 4 HCN + 5 O2^ 4 CO2+ 2 N2+ 2 H2O (1).

[0008] Catalytic oxidation, as a separate step in exhaust gas purification, involves considerable process complexity and costs, particularly because the oxidation catalysts used in technical applications mostly employ precious metals as catalytically active components. Furthermore, the resulting nitrogen oxides must be reduced in a separate step, e.g., by means of SCR (H. Zhao et al., Applied Catalysis B 65 (2006) 282-290).

[0009] The transfer via special catalysts, e.g. based on TiO2, to the hydrolysis of HCN according to The reaction HCN + H₂O → CO + NH₃(2) has been described (O. Kröcher et al., Applied Catalysis B: Environmental, 92(1-2), 2009, 75-89). In this case, a separate step is also necessary: ​​a subsequent further oxidation of the hydrolysis products using appropriate oxidation catalysts. With regard to the purification of HCN-containing engine exhaust gases (N₂ with 10% O₂, 5% H₂O, 50 ppm HCN and alternating mixtures of 200 ppm NO, 100 ppm NO and NO₂, 200 ppm NO₂, or 200 ppm NH₃), it has been shown that HCN can also be hydrolyzed using iron-loaded zeolite catalysts of the Fe-ZSM-5 type. If NO is present simultaneously... X Contained in the exhaust gas, the NH3 formed during the hydrolysis of HCN can react with NO X react on the Fe-ZSM-5 catalyst to form N2 and H2O according to reaction equation (3): 2 NH3+ NO + NO2— >■ 2 N2+ 3 H2O (3).

[0010] The equilibrium of reaction equation (2) shifts to the right as a result. However, no measures are proposed for the removal of the resulting, also toxic, CO. The CO would then also have to be removed in a separate step before being released into the atmosphere, e.g., by being oxidized to CO2 using precious metal-containing oxidation catalysts.

[0011] J. Perez-Ramirez et al., Journal of Catalysis 223 (2004) 13-27 concerns the reduction of N2O with CO on iron-loaded MFI zeolites.

[0012] MN Debbagh et al., Applied Catalysis B 70 (2007) 335-341 concerns the catalytic reduction of N2O on steam-activated, iron-loaded ZSM-5 zeolites.

[0013] M. Osinska, Reac Kinet Meeh Cat (2013) 109:57-65 concerns the decomposition of HCN in the presence of nickel-containing catalysts.

[0014] SG Moussa et al., Atmospheric Environment 131 (2016) 185-195 concerns emissions of hydrogen cyanide from road vehicles powered by gasoline or diesel.

[0015] J. Cheng et al., IOP Conf. Series: Materials Science and Engineering 231 (2017) 1-6 concerns the removal of HCN from manganese-based catalysts at low temperatures.

[0016] N. Liu et al., Catalysis Today, 297, 2017, 201-210 concerns the selective catalytic combustion of hydrogen cyanide on metal-modified zeolite catalysts.

[0017] Y. Li et al., Chemical Engineering Journal, 346, 2018, 621-629 concerns the catalytic decomposition of HCN on copper manganese oxide at low temperatures.

[0018] Y. Hu et al., Applied Surface Science 427 (2018) 843-850 concerns the coupling of the catalytic hydrolysis and oxidation of HCN to HZSM-5, which were modified with metal oxides (Fe, Cu).

[0019] D. Zengel et al., Angew. Chem. Int. Ed. 2020, 59, 14423-14428 concerns the emission of toxic HCN during NO₂ production. x -Removal by ammonia SCR in the exhaust gas of natural gas engines with lean combustion.

[0020] T. Li et al., Atmospheric Environment, 247, 2021, 118218, 1-13 concerns emissions of carboxylic acids, hydrogen cyanide and isocyanic acid from vehicle exhaust.

[0021] K. Cai et al., Energies 2023, 16, 7004, 1-14 concerns combustion behavior and irregular emission characteristics in an ammonia diesel engine.

[0022] EP 0 341 565 A2 relates to a process for the selective removal and destruction of hydrogen cyanide contained in nitrose gases, in which the nitrose-containing gases are washed in an absorption column with nitric acid in countercurrent flow and the nitric acid loaded with hydrogen cyanide is regenerated at elevated temperature with simultaneous hydrolysis and oxidation of the hydrogen cyanide.

[0023] US 5,173,278 A concerns the denitrification of a flue gas containing small amounts of both HCN and NO. X It contains and is produced, for example, by the regeneration of the catalyst during the catalytic fluidized bed cracking of a petroleum product. If the molar ratio of HCN to NO in the flue gas is approximately 1.0, e.g., in the range of approximately 0.8 to 1.2, effective denitrification is achieved without prior alteration of the flue gas composition by bringing it into contact with a catalyst under conversion conditions, including elevated temperature.

[0024] US 5,520,895 A relates to a process for treating exhaust gas in which a specially manufactured catalyst composition is used for the selective catalytic reduction of NO contained in the exhaust gas. X is used. One embodiment of the process includes a catalytic stage for the selective catalytic reduction of NO. Xover a catalyst composition that includes a zeolite catalyst comprises a medium pore size catalyst treated with a water-soluble iron salt or salt precursor in a manner that produces maximum iron dispersion.

[0025] The object of the invention is to provide purification processes for HCN-contaminated exhaust gases that offer advantages over the prior art. The processes should be characterized by simple and cost-effective operation as well as low equipment (cost) requirements. Furthermore, the processes should convert HCN into non-toxic substances that ideally require no further post-treatment.

[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 exhaust aftertreatment comprising the steps: (a) Generating an exhaust gas comprising HCN, NOX and N2O; wherein the HCN content is at least 1.0 ppmv; (b) optional, addition of reducing agent for NO X and / or for N2O in the exhaust gas; and (c) Reducing the HCN content in the exhaust gas by catalytic conversion of the HCN on a transition metal-loaded zeolitic material.

[0028] It was found that HCN can be degraded to N2, H2O and CO2 by passing the exhaust gas over a transition metal-loaded zeolitic catalyst, e.g. a packing of catalyst pellets containing an iron-loaded zeolitic material of structure type BEA, at temperatures of 300-600 °C (preferably 350-550 °C).

[0029] The following reaction equations describe the hydrolysis of HCN (2) according to the invention and a selection of possible subsequent reactions of the hydrolysis products (NH3 and CO) with NO according to the invention. x -Nitrogen oxides (3), (4), (5) and with N2O (6): HCN + H2O NH3 + CO (2) 2 NH3+ NO + NO2— >■ 2 N2+ 3 H2O (Fast SCR) (3) 4 NH3+ 4 NO + O24 N2+ 6 H2O (Standard SCR) (4) 8 NH3+ 6 NO27 N2+ 12 H2O (NO2SCR) (5) CO + N2O N2+ CO2(6).

[0030] In summary, according to the invention, for example, depending on NO x The following gross sales results are based on the degree of oxidation: 4 HCN + 4 NO + 4 N2O + O28 N2+ 2 H2O + 4 CO2(7) 2 HCN + NO + NO2+ 2 N2O 4 N2+ H2O + 2 CO2(8) 8 HCN + 6 NO2+ 8 N2O ^ 15 N2+ 4 H2O + 8 CO2(9).

[0031] In contrast to known methods or scientific investigations, according to the invention, a complete removal of HCN can be achieved in a single-stage process, i.e., in a single process step, without the need for expensive precious metal catalysts (oxidation catalysts). The conversion is achieved into substances that can be safely released into the environment. Preferably, according to the invention, neither a pre-oxidation catalyst for HCN oxidation nor a post-oxidation catalyst for the oxidation of the NH3 and / or CO produced by the HCN hydrolysis is required.

[0032] The NH3 formed during the hydrolysis of HCN according to reaction equation (2) preferably serves as a reducing agent for NO X according to reaction equations (3), (4) or (5) and is thereby degraded. Additionally, it can also serve as a reducing agent for N₂O according to reaction equations (12), (13) or (14) (see below), but due to its slower kinetics, only to a limited extent.

[0033] The CO formed during the hydrolysis of HCN according to reaction equation (2) preferably serves as a reducing agent for N2O according to reaction equation (6) and is thereby degraded.

[0034] Since the molar fraction of NO X and N2O is preferably larger than the molar fraction of HCN in the exhaust gas ([NO X ] > [HCN], and [N2O] > [HCN]), the molar amount of NH3 and CO formed during the hydrolysis of HCN is sufficient for the chemical reduction of the amount of NO contained in the exhaust gas. X and N2O is not yet removed to the typically desired extent (e.g., completely or almost completely). Therefore, in step (b) of the inventive process, (additional) reducing agent for NO is preferably added. X and / or N2O is added to the exhaust gas. However, the amount added can be reduced by the amount released by the hydrolysis of HCN.

[0035] This patented, preferred design of the process eliminates the need to consider the formation of HCN. HCN is, in fact, a valuable substance that ultimately contributes to reducing NO emissions. Xand contributes N2O. A further advantage is that a downstream catalyst for CO oxidation and / or a pre-stage catalyst for HCN hydrolysis can be omitted.

[0036] Step (b) of the inventive procedure is optional but preferred; steps (a) and (c) are mandatory.

[0037] The inventive process is a method for exhaust gas aftertreatment. Generally, exhaust gas aftertreatment encompasses all processes in which exhaust gases, especially combustion gases, are cleaned mechanically, catalytically, or chemically after leaving a combustion chamber or combustion chamber. According to the invention, the cleaning is chemically catalyzed, using a zeolitic material loaded with transition metals as the catalyst.

[0038] For descriptive purposes, "A and / or B" means (i) only A but not B, (ii) only B but not A, or (iii) both A and B.

[0039] For descriptive purposes, "reducing agent for NOf" means a substance which, under suitable catalytic conditions, reduces NO. X It can be chemically reduced, in which case it is itself oxidized. An example is NH3.

[0040] For descriptive purposes, "N₂O reducing agent" means a substance which, under suitable catalytic conditions, can chemically reduce N₂O, while itself being oxidized. Examples are NH₃ and CH₄.

[0041] For descriptive purposes, "transition metal loaded" means that the zeolitic material has been loaded with at least one transition metal which is responsible for the catalytic activity, i.e., enables the catalytic conversion of HCN.

[0042] For descriptive purposes, "transition metal" also includes elements from the lanthanide group.

[0043] For descriptive purposes, "zeolite" means a zeolitic material which may exhibit various zeolitic structure types. Detailed information on the composition or structure of the zeolitic materials preferably used according to the invention is given in the Atlas of Zeolite Structure Types, Elsevier, 4th revised edition 1996, to which explicit reference is hereby made.

[0044] For descriptive purposes, "NO" includes X "Nitrogen monoxide (NO) and nitrogen dioxide (NO2), but no nitrous oxide (N2O).

[0045] For the purpose of description, the amount of reducing agent for the chemical reduction of NO is given. X e.g. the amount of NH3, expressed in terms of the amount of NO X , which is to be broken down by chemical reduction (reference quantity). Is the NO contained in the exhaust gas to be X Since they are practically completely chemically reduced, the reference amount of NO corresponds to X the concentration of NOX in the exhaust gas provided in step (a). However, if no complete chemical reduction of the NO is achieved, X The aim is, i.e., the exhaust gas treated according to the invention may still contain a residual amount of non-chemically reduced NO. X If contained, the reference quantity corresponds to NO. X the concentration of NO X in the exhaust gas provided in step (a) minus the residual concentration of non-chemically reduced NO X .

[0046] For the purpose of description, the amount of reducing agent for the chemical reduction of N₂O, e.g., the amount of CH₄, is expressed in relation to the amount of N₂O to be reduced by chemical reduction (reference amount). If the N₂O contained in the exhaust gas is to be almost completely chemically reduced, the reference amount of N₂O corresponds to the concentration of N₂O in the exhaust gas provided in step (a). However, if complete chemical reduction of the N₂O is not the goal, i.e., the exhaust gas treated according to the invention may still contain a residual amount of non-chemically reduced N₂O and / or a proportion is to be reduced by catalytic decomposition, the reference amount of N₂O corresponds to the concentration of N₂O in the exhaust gas provided in step (a) minus the residual concentration of non-chemically reduced N₂O.

[0047] In step (a) of the process according to the invention, an exhaust gas is generated which contains HCN, NO Xand N2O, with an HCN content of at least 1.0 ppmv.

[0048] In this context, "generated" means that the aforementioned components of the exhaust gas must be present in the exhaust gas, without necessarily having to be formed during the exhaust gas generation process, e.g., through the combustion of a fuel in a mixture with air. Thus, it is possible that certain components of the generated exhaust gas were already present in a gas mixture from which the exhaust gas is produced, for example, in the fuel and / or in the air. It is also possible that certain components of the exhaust gas are only formed during the exhaust gas generation process. Furthermore, it is possible that certain components of the generated exhaust gas were already present in a gas mixture from which the exhaust gas is produced, but their quantity is reduced or increased because they are partially reacted or formed during the exhaust gas generation process.

[0049] The molar amount of NO in the exhaust gas produced in step (a) is preferred. X greater than the molar amount of N2O.

[0050] The molar amount of NO in the exhaust gas produced in step (a) is preferred. X and N2O, each independently at least as large as the molar amount of HCN (i.e., [NO X ] > [HCN], and [N2O] > [HCN]); preferably at least 1.1 times as large, more preferably at least 1.2 times as large, even more preferably at least 1.3 times as large, most preferably at least 1.4 times as large and in particular at least 1.5 times as large.

[0051] The molar amount of NO in the exhaust gas produced in step (a) is preferred. X and N2O each independently at least 2 times as large as the molar amount of HCN (i.e. [NO X] > [HCN] , and [N2O] > [HCN]); preferably at least 3 times as large, more preferably at least 4 times as large, even more preferably at least 10 times as large, most preferably at least 20 times as large and in particular at least 50 times as large.

[0052] In preferred embodiments, the molar amount of NO in the exhaust gas produced in step (a) is X greater than the molar amount of N2O, and the molar amount of N2O greater than the molar amount of HCN.

[0053] In other preferred embodiments, the molar amount of NO in the exhaust gas produced in step (a) is X greater than the molar amount of N2O, and the molar amount of N2O less than the molar amount of HCN.

[0054] Preferably, the exhaust gas produced in step (a) contains an amount of N2O at least equimolar to the molar amount of HCN, i.e. [A2O] > [HCN] and simultaneously preferentially a relative amount of NO to the molar amount of HCN X , which from the NOX - Oxidation state of the NO contained in the exhaust gas X depends on:

[0055] Provided the NO x - Degree of oxidation a(NO x ) in the range of a(NO x If the value is < 0.5, the exhaust gas contains - in addition to the amount of N2O at least equimolar to the molar amount of HCN - also at least an equimolar amount of NO. X , i.e. [1VO X ] > [HCN].

[0056] Provided the NO x - Degree of oxidation a(NO x ) in the range of 0.5 < a(NO x If the value is less than 1.0, the exhaust gas contains – in addition to the amount of N2O at least equimolar to the molar amount of HCN – also such a molar amount of NO. X , which is greater than or equal to the molar amount of HCN divided by 2 / 3 (1 + a(NO) x )) is, i.e.

[0057] The NO x - Degree of oxidation a(NO x ) is a measure of the relative molar ratio of NO2 to total NO xand 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(NO x ) = [NO2] / ([NO]+[NO2]).

[0058] Preferably, the NO in the exhaust gas produced in step (a) is x - Degree of oxidation a(NO x ) at least 0.025; preferably at least 0.05, more preferably at least 0.075, even more preferably at least 0.1, most preferably at least 0.2 and in particular at least 0.3.

[0059] Preferably, the NO is present in the exhaust gas produced in step (a). x - Degree of oxidation a(NO x ) in the range of 0.3±0.3, 0.4±0.3, 0.5±0.3, 0.6±0.3 or 0.7±0.3; preferably 0.2±0.2, 0.3±0.2, 0.4±0.2, 0.5±0.2, 0.6±0.2, 0.7±0.2, or 0.8±0.2.

[0060] Preferably, the HCN content in the exhaust gas produced in step (a) is at least 2.0 ppmv; preferably at least 4.0 ppmv, more preferably at least 6.0 ppmv, even more preferably at least 8.0 ppmv, most preferably at least 10 ppmv and in particular at least 15 ppmv.

[0061] Preferably, the HCN content in the exhaust gas produced in step (a) is at least 20 ppmv; preferably at least 25 ppmv, more preferably at least 30 ppmv, even more preferably at least 35 ppmv, most preferably at least 40 ppmv and in particular at least 45 ppmv.

[0062] Preferably, the HCN content in the exhaust gas produced in step (a) is at least 50 ppmv; preferably at least 55 ppmv, more preferably at least 60 ppmv, even more preferably at least 65 ppmv, most preferably at least 70 ppmv and in particular at least 75 ppmv.

[0063] Preferably, the HCN content in the exhaust gas produced in step (a) is at most 2000 ppmv; preferably at most 1500 ppmv, more preferably at most 1250 ppmv, even more preferably at most 1000 ppmv, most preferably at most 750 ppmv and in particular at most 500 ppmv.

[0064] Preferably, the NO content in the exhaust gas produced in step (a) is X at least 10 ppmv; preferably at least 25 ppmv, more preferably at least 50 ppmv, even more preferably at least 75 ppmv, most preferably at least 100 ppmv and in particular at least 150 ppmv.

[0065] Preferably, the NO content in the exhaust gas produced in step (a) is X at least 250 ppmv; preferably at least 500 ppmv, more preferably at least 750 ppmv, even more preferably at least 1000 ppmv, most preferably at least 1250 ppmv and in particular at least 1500 ppmv.

[0066] Preferably, the N2O content in the exhaust gas produced in step (a) is at least 4.0 ppmv; preferably at least 5.0 ppmv, more preferably at least 6.0 ppmv, even more preferably at least 7.0 ppmv, most preferably at least 8.0 ppmv and in particular at least 9.0 ppmv.

[0067] Preferably the N2O content in the exhaust gas produced in step (a) is at least 10 ppmv; preferably at least 25 ppmv, more preferably at least 50 ppmv, even more preferably at least 75 ppmv, most preferably at least 100 ppmv and in particular at least 150 ppmv.

[0068] Preferably, the N2O content in the exhaust gas produced in step (a) is at least 250 ppmv; preferably at least 500 ppmv, more preferably at least 750 ppmv, even more preferably at least 1000 ppmv, most preferably at least 1250 ppmv and in particular at least 1500 ppmv.

[0069] Preferably, the exhaust gas produced in step (a) also includes H2O.

[0070] Preferably, the H2O content in the exhaust gas produced in step (a) is at least 2.5 vol.- %; preferably at least 5.0 vol.%, more 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.%.

[0071] Preferably, the exhaust gas produced in step (a) also includes O2.

[0072] Preferably, the O2 content in the exhaust gas produced in step (a) is at least 1.0 vol. %; preferably at least 2.0 vol. %, more preferably at least 3.0 vol. %, even more preferably at least 4.0 vol. %, most preferably at least 5.0 vol. % and in particular at least 6.0 vol. %.

[0073] Preferably, the O2 content in the exhaust gas produced in step (a) is at least 7.0 vol.- %; preferably at least 8.0 vol.%, more preferably at least 9.0 vol.%, even more preferably at least 10.0 vol.%, most preferably at least 11.0 vol.% and in particular at least 12.0 vol.%.

[0074] Preferably, it is an exhaust gas containing water vapor and oxygen.

[0075] Preferably, the exhaust gas produced in step (a) also includes NH3.

[0076] Preferably the NH3 content is at least 10 ppmv; preferably at least 25 ppmv, more preferably at least 50 ppmv, even more preferably at least 75 ppmv, most preferably at least 100 ppmv and particularly at least 150 ppmv.

[0077] Preferably, step (a) is a combustion process wherein the NH3 contained in the generated exhaust gas is residual, unburned NH3.

[0078] Preferably, the exhaust gas produced in step (a) also includes CO.

[0079] Preferably, the exhaust gas produced in step (a) also includes CO2.

[0080] Preferably, the exhaust gas produced in step (a) also includes N2.

[0081] Preferably, step (a) is an industrial process; preferably for generating process heat.

[0082] Preferably, step (a) is a combustion process.

[0083] Preferably, step (a) is not performed in a motor vehicle (passenger car, truck, commercial vehicle, etc.).

[0084] In preferred embodiments, step (a) is integrated into a process for the catalytic decomposition of NH3 into N2 and H2, in particular for the production of H2.

[0085] In other preferred embodiments, step (a) is integrated into a process for burning NH3 to power an internal combustion engine on a ship.

[0086] In other preferred embodiments, step (a) is integrated into a process for the combustion of NH3 to drive a gas turbine.

[0087] Preferably, the combustion process does not take place at a catalyst.

[0088] Preferably step (a) comprises the combustion of a mixture comprising (i) a nitrogen-containing compound, (ii) a carbon-containing compound and (iii) air and / or O2.

[0089] In preferred embodiments, step (a) comprises, during the start-up of the plant, the combustion of a mixture comprising air and / or O2 and a carbon-containing compound, and subsequently, during steady-state operation of the plant, additionally a nitrogen-containing compound, i.e., air and / or O2 and a carbon-containing compound and a nitrogen-containing compound.

[0090] In other preferred embodiments, step (a) comprises, during the start-up of the plant, the combustion of a mixture comprising air and / or O2 and a carbon-containing compound, and subsequently, during steady-state operation of the plant, the combustion of a nitrogen-containing compound instead of the carbon-containing compound, i.e., air and / or O2 and a nitrogen-containing compound.

[0091] Such a start-up procedure may be preferred in order to first heat the exhaust gas treatment unit (catalyst bed) to a sufficiently high temperature and only then begin the combustion of a nitrogen-containing compound, preferably NH3. This procedure is preferred with regard to preventing the formation and deposition of ammonium nitrate (NH4NO3) from NO. X and NH3 in the exhaust gas at temperatures below approximately 170 °C.

[0092] Preferably, step (a) is performed during the start-up or shutdown of a plant, wherein this plant preferably also generates exhaust gas during steady-state operation, which, however, differs in composition from the exhaust gas generated during the start-up or shutdown of the plant. Preferably, the exhaust gas generated during the start-up or shutdown of the plant contains a larger quantity of HCN than the exhaust gas generated during steady-state operation of the plant.

[0093] Preferably, the plant is a plant for the non-catalytic combustion of NH3 to generate heat. Preferably, in step (a), a mixture comprising NH3 and air and / or O2 is combusted, wherein the mixture is supplied with oxygen during the start-up or shutdown of the plant. A hydrocarbon, preferably CH4 and / or C3H8, is added. Preferably, no hydrocarbon is added to the mixture during steady-state operation of the plant.

[0094] If in step (a) a mixture comprising (i) a nitrogen-containing compound and (ii) a carbon-containing compound is burned, the molar amount of nitrogen-containing compound in the mixture is preferably greater than the molar amount of carbon-containing compound, more preferably greater than the total molar amount of carbon atoms of the carbon-containing compound.

[0095] The nitrogen-containing compound NH3 or an amine is preferred; preferably NH3.

[0096] Preferably the carbon-containing compound is a fossil fuel, in particular containing hydrocarbon(s); preferably containing CH4 or C3H8.

[0097] Preferably step (a) comprises the combustion of a mixture comprising (i) NH3, (ii) a fossil fuel, preferably containing hydrocarbon(s), more preferably containing CH4 and / or C3H8, and (iii) air and / or O2.

[0098] In preferred forms, the fossil fuel contains gaseous components, e.g. CH4, C2H6, C3H8, etc.

[0099] In other preferred embodiments, the fossil fuel contains liquid components, e.g. gasoline, diesel, heavy oil, kerosene, petroleum, etc.

[0100] In other preferred embodiments, the fossil fuel contains solid components, e.g. lignite, hard coal, etc.

[0101] In preferred embodiments, the molar amount of the nitrogen-containing compound, in particular NH3, in the mixture is greater than the molar amount of carbon atoms. Preferably, the molar amount of NH3 in the mixture is greater than the molar amount of fossil fuel. Preferably, the molar amount of NH3 in the mixture is greater than the total molar amount of carbon atoms in the fossil fuel.

[0102] In other preferred embodiments, the molar amount of the nitrogen-containing compound, in particular NH3, in the mixture is smaller than the molar amount of carbon atoms. Preferably, the molar amount of NH3 in the mixture is smaller than the molar amount of fossil fuel. Preferably, the molar amount of NH3 in the mixture is smaller than the total molar amount of carbon atoms in the fossil fuel.

[0103] If the fossil fuel contains several components, e.g., different hydrocarbons, then the molar amount of the fossil fuel refers to the sum of the molar amounts of the components. If the fossil fuel contains several components, e.g., different hydrocarbons, then the total molar amount of carbon atoms in the fossil fuel refers to the sum of the molar amounts of carbon atoms in the components.

[0104] According to the invention, the combustion conditions are not particularly restricted, but are fundamentally freely selectable, in particular the mixing ratio of NH3 and fossil fuel, the air-fuel ratio, and the combustion temperature. Preferably, a mixing ratio of NH3 and fossil fuel is set that is optimized with regard to the subsequent combustion.

[0105] Preferably, the proportion of fossil fuel in the mixture is at most 80 mol%, more preferably at most 70 mol%, even more preferably at most 60 mol%, most preferably at most 50 mol%, and in particular at most 40 mol%.

[0106] Preferably the proportion of fossil fuel in the mixture is at least 1.0 mol%, more preferably at least 2.0 mol%, even more preferably at least 3.0 mol%, most preferably at least 4.0 mol%, and in particular at least 5.0 mol%.

[0107] Preferably, the proportion of fossil fuel in the mixture is at least 10 mol%, more preferably at least 20 mol%, even more preferably at least 30 mol%, most preferably at least 40 mol%, and in particular at least 50 mol%.

[0108] In preferred embodiments, the molar ratio of NH3: fossil fuel in the mixture is in the range of 45:55 to 90:10, preferably 50:50 to 85:15, more preferably 55:45 to 80:20, even more preferably 60:40 to 75:25, most preferably 65:35 to 70:30.

[0109] In other preferred embodiments, the molar ratio of fossil fuel : NH3 in the mixture is in the range of 45:55 to 90:10, preferably 50:50 to 85:15, more preferably 55:45 to 80:20, even more preferably 60:40 to 75:25, most preferably 65:35 to 70:30.

[0110] Preferably, the proportion of carbon atoms of the fossil fuel in the mixture is at most 80 mol%, more preferably at most 70 mol%, even more preferably at most 60 mol%, most preferably at most 50 mol%, and in particular at most 40 mol%, based on the total molar amount of carbon atoms of the fossil fuel.

[0111] Preferably, the proportion of carbon atoms of the fossil fuel in the mixture is at least 1.0 mol%, more preferably at least 2.0 mol%, even more preferably at least 3.0 mol%, most preferably at least 4.0 mol%, and in particular at least 5.0 mol%, based on the total molar amount of carbon atoms of the fossil fuel.

[0112] Preferably, the proportion of carbon atoms of the fossil fuel in the mixture is at least 10 mol%, more preferably at least 20 mol%, even more preferably at least 30 mol%, most preferably at least 40 mol%, and in particular at least 50 mol%, based on the total molar amount of carbon atoms of the fossil fuel.

[0113] In preferred embodiments, the molar ratio of carbon atoms of the fossil fuel : NH3 in the mixture is in the range of 45:55 to 90:10, preferably 50:50 to 85:15, preferably preferred ratio 55:45 to 80:20, even more preferred 60:40 to 75:25, most preferred 65:35 to 70:30, based on the total molar amount of carbon atoms of the fossil fuel.

[0114] In other preferred embodiments, the molar ratio of NH3: carbon atoms of the fossil fuel in the mixture is in the range of 45:55 to 90:10, preferably 50:50 to 85:15, more preferably 55:45 to 80:20, even more preferably 60:40 to 75:25, most preferably 65:35 to 70:30, based on the total molar amount of carbon atoms of the fossil fuel.

[0115] The mixture is preferably combusted with air. In preferred embodiments, the air-fuel ratio X for combustion is in the range of 0.8 to 1.8, preferably 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.

[0116] Preferably, the air-fuel ratio X during combustion is at least 0.75; preferably at least 0.80, more preferably at least 0.85, even more preferably at least 0.90, most preferably at least 0.95, and particularly at least 1.00. Preferably, the air-fuel ratio X during combustion is at least 1.05; preferably at least 1.10, more preferably at least 1.15, even more preferably at least 1.20, most preferably at least 1.25, and particularly at least 1.30. Preferably, the air-fuel ratio X during combustion is at least 1.35; preferably at least 1.40, more preferably at least 1.45, even more preferably at least 1.50, most preferably at least 1.55, and particularly at least 1.60.

[0117] The air-fuel ratio X (i.e., the combustion air-fuel ratio) indicates the mass ratio of 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.

[0118] Preferably, the exhaust gas generated in step (a) has a pressure of at most 5.5 bara; more preferably at most 5.0 bara, even more preferably at most 4.5 bara, most preferably at most 4.0 bara, and in particular at most 3.5 bara. Preferably, the exhaust gas generated in step (a) has a pressure of at most 3.0 bara; more preferably at most 2.5 bara, even more preferably at most 2.0 bara, most preferably at most 1.5 bara, and in particular at most 1.1 bara.

[0119] In a particularly preferred embodiment, the method according to the invention comprises the steps (a) Combustion of a fuel containing NH3 and a carbon-containing compound, preferably a hydrocarbon, more preferably CH4, producing an exhaust gas comprising HCN, NO Xand N2O; wherein the HCN content in the exhaust gas is at least 1.0 ppmv; wherein the exhaust gas preferably contains an amount of N2O at least equimolar to the molar amount of HCN, i.e., [N2O] > [HCN]; and wherein the NO x - Degree of oxidation a(NO) x in the area of (i) a(NO x ) < 0.5 and the exhaust gas contains at least an equimolar amount of NO to the molar amount of HCN x contains, i.e., [IVO^] — [HCN] or (ii) in the range of 0.5 < a(NO x ) < 1.0 and the exhaust gas contains such a molar amount of NO X contains which is greater than or equal to the molar amount of HCN divided by 2 / 3 (1 + a(NO) x )) is, (b) optional, addition of reducing agent for NO X and / or for N2O in the exhaust gas; and (c) Reducing the HCN content in the exhaust gas by passing the exhaust gas over a transition metal-loaded zeolitic material, so that HCN is degraded, preferably to N2, H2O and CO2.

[0120] In preferred embodiments, the zeolitic material loaded with the transition metal is in the form of a honeycomb or is located at least on part of the surface of a honeycomb-shaped support.

[0121] In other preferred embodiments, the zeolitic material loaded with the transition metal is present as a packing of catalyst pellets or is located at least on a part of the surface of support pellets.

[0122] Preferably, the zeolitic material loaded with the transition metal is an iron-loaded zeolitic material, preferably of structure type BEA.

[0123] Preferably, step (c) is carried out at a temperature in the range of 300 to 600 °C, preferably 350 to 550 °C.

[0124] Preferably, the HCN content in the exhaust gas is measured.

[0125] Preferably, the HCN content is measured in the exhaust gas produced in step (a).

[0126] Preferably, the HCN content in the exhaust gas is measured after step (c).

[0127] The NO content is preferred. X measured in the exhaust gas.

[0128] The NO content is preferred. X measured in the exhaust gas produced in step (a).

[0129] The preferred levels of NO and NO2 and / or NO are preferred. x - Degree of oxidation a(NO x ) measured.

[0130] The preferred levels of NO and NO2 and / or NO are preferred. x - Degree of oxidation a(NO x ) measured in the exhaust gas produced in step (a).

[0131] Preferably, the N2O content in the exhaust gas is measured.

[0132] Preferably, the N2O content is measured in the exhaust gas produced in step (a).

[0133] Preferably, the content of HCN, NO, NO2 and N2O in the exhaust gas is measured.

[0134] Preferably, the content of HCN, NO, NO2 and N2O in the exhaust gas produced in step (a) is measured.

[0135] The NO content is preferred. X and N2O measured in the exhaust gas after step (c).

[0136] Preferably, the CO content in the exhaust gas is measured.

[0137] Preferably, the CO content in the exhaust gas produced in step (a) is measured.

[0138] Preferably, the CO content in the exhaust gas is measured after step (c).

[0139] Preferably, the temperature of the exhaust gas is measured.

[0140] In the optional step (b) of the process according to the invention, a reducing agent for NO is added. X and / or N2O is added to the exhaust gas. This is primarily used to remove excess NO levels. Xand / or N2O. NH3 is used as a reducing agent for the chemical reduction of NO. X as well as N2O, whereby the chemical reduction of NO X The chemical reduction of N2O with NH3 is typically significantly faster than the chemical reduction of N2O with NH3. In this process, the NH3 is oxidized to N2. Alternatively or additionally, CO or hydrocarbons such as CH4 or propane can be added to the chemical reduction of N2O.

[0141] The amount of reducing agent for NO X and / or for N2O, in this case, it is preferred according to the amounts of N2O and NO X The amount of reducing agent for NO is measured in the exhaust gas produced in step (a), each reduced by the amount of HCN contained in the exhaust gas produced in step (a). Preferably, the amount of reducing agent for NO is measured. Xand / or for N2O according to the amounts of N2O, NO and NO2 in the exhaust gas produced in step (a), each reduced by the amount of HCN contained in the exhaust gas produced in step (a), i.e. the NO x - Degree of oxidation a(NO x ), or the individual amount of NO and NO2, is also preferably taken into account.

[0142] Determining the required amount of reducing agent for NO X and / or for N2O, taking into account the amount of N2O, NO X and HCN also depends on the oxidation state of the NO X (i.e., the respective amounts of NO and NO2) and the chemical nature of the reducing agent used for NO X and / or for N2O. The NO x - Degree of oxidation a(NO x ) (i.e. the The respective amount of NO and NO2) specifies which reaction equations the reactions of NO and NO2 will mainly take place according to.

[0143] For the chemical reduction of NO X with NH3 according to reaction equation (3) (Fast SCR, reaction of equal parts NO and NO2) the required stoichiometric ratio of [NH3] to {[NO]+[NO2]} is 1:1.

[0144] For the chemical reduction of NO X with NH3 according to reaction equation (4) (standard SCR, conversion of NO) the required stoichiometric ratio of [NH3] to [NO] is 1: 1.

[0145] For the chemical reduction of NO X with NH3 according to reaction equation (5) (NO2SCR, conversion of NO2) the required stoichiometric ratio of [NH3] to [NO2] is 4:3.

[0146] Does NO contain XIn addition to NO, NO2 is also present. Due to its faster reaction kinetics, the NO2 initially reacts almost completely with the NO according to the Fast SCR (reaction equation (3)) in a 1:1 stoichiometric ratio. Only any excess NO2 that may be present subsequently reacts according to the NO2SCR (reaction equation (5)) in a 4:3 stoichiometric ratio. This relationship can be adjusted depending on the oxidation state a(NO). x ) can be expressed quantitatively as follows: [NH3] / [NO X ] = 2 / 3 (1+ a(NO x This applies when there is more NO2 than NO, i.e., when 0.5 < a(NO) x ) < 1.0.

[0147] The chemical nature of the reducing agent determines the number of reducing equivalents present. This then yields the overall stoichiometry, from which the required amount of reducing agent for NO can be calculated. X and / or can be measured for N2O.

[0148] This reduction in the amount of reducing agent for NO Xand / or for N2O is possible according to the invention because, through the preferred hydrolysis of one molar equivalent of HCN, one molar equivalent of NH3 and CO is formed, which then serve as reducing agents for the chemical reduction of NO X or N₂O are available and therefore do not need to be added. Preferably, one molar equivalent of NH₃ is used as a reducing agent for NO, based on the amount of HCN contained in the exhaust gas produced in step (a). X as well as taking into account a molar equivalent of CO as a reducing agent for N2O and the measured amount of added reducing agent for NO X and / or reduced accordingly for N2O.

[0149] In preferred embodiments, the chemical reduction of NO is XWith NH3, it is assumed that an equimolar amount of NH3 is required (stoichiometry 1:1) because the degradation according to Fast SCR and / or Standard SCR dominates. This assumption is particularly preferred when the NO x - Degree of oxidation a(NO x ) in the range of a(NO x ) < 0.5 hegt.

[0150] In other preferred embodiments, the chemical reduction of NO is used X With NH3, it is assumed that a molar amount of NH3 is required, which is equal to the molar amount of NO. X multiplied by 2 / 3 (1+ a(NO) x )) corresponds to, i.e., the ratio of the molar amount of added NH3 to the amount of NO present X should equal 2 / 3 (1+ a(NO) x )) be. This assumption is particularly then preferred if the NO x - Degree of oxidation a(NO x ) in the range of 0.5 < a(NO x ) < 1.0, i.e., when there is more NO2 than NO in the exhaust gas.

[0151] In preferred embodiments, for the chemical reduction of N2O with NH3, it is assumed that an equimolar amount of NH3 is required according to the following reaction equation (stoichiometry 1:1): 4 N2O + 4 NH3+ O2— ► 6 N2+ 6 H2O (10).

[0152] In other preferred embodiments, the chemical reduction of N2O with CH4 is assumed to require a 0.25 times molar amount of NH3 (stoichiometry 1:4).

[0153] Preferred in a NO x - Degree of oxidation a(NO x ) in the range of a(NO x ) < 0.5 the molar amounts of reduction equivalents for N2O and NO X to reduce each by the molar amount of HCN contained in the exhaust gas.

[0154] Preferred in a NO x - Degree of oxidation a(NO x ) in the range of 0.5 < a(NO x) < 1.0 (i) to reduce the molar amount of reduction equivalents for N2O by the molar amount of HCN contained in the exhaust gas, and (ii) the molar amount of reduction equivalents for NO X to reduce by the molar amount of HCN contained in the exhaust gas divided by 2 / 3 (1 + a(NO) x )).

[0155] If excess amounts of N2O are contained in the exhaust gas, which are to be reduced with NH3, CO or hydrocarbons, the NO content is X Preferably reduced by the addition of NH3 so that the N2O in the exhaust gas can also be broken down, preferably by chemical reduction with CO and / or hydrocarbons. Excess amounts of N2O are preferably those amounts of N2O in the exhaust gas for whose chemical reduction the amount of CO released by the hydrolysis of HCN is insufficient.

[0156] In preferred embodiments, NH3 is used as a reducing agent for NO. Xand is also used as a reducing agent for N2O. The exhaust gas produced in step (a) contains amount [N2O] of N2O, amount [NO] of NO, amount [NO2] of NO2, and amount [HCN] of HCN. The required and therefore preferably added amount [NH3] of NH3 is then preferably calculated as follows (with a(NO x ) = [NO2] / ([NO] + [NO2]):

[0157] If a(NO x ) < 0.5: [NH3] = [N2O] + [NO] + [NO2] - 2 [HCN],

[0158] If 0.5 < a(NO x ) < 1.0: [NH3] = [N2O] + 2 / 3 (1 + a(NO x )) • ([NO] + [NO2]) - 2 [HCN],

[0159] The above amounts [NH3] are preferably minimum amounts of added NH3 with a view to a desired chemical reduction of NO. X and N2O. In the case of the preferably targeted complete reduction of NO XThe actual amount of NH3 added is preferably increased by approximately {0.1 to 0.3} • ([NO] + [NO2]) (additive term). The value in curly brackets is intended to define a preferred range of the numerical factor.

[0160] In this context, it should also be noted that any NO present during the chemical reduction of N2O with a reducing agent must be removed. X It almost always has to be completely chemically reduced beforehand, because the chemical reduction of NO x -Reduction is usually much faster than the chemical reduction of N₂O. With very large amounts of NO X And at the same time, with small amounts of N2O, the N2O is sometimes reduced practically en passant, i.e., one needs NO. X not necessarily to reduce completely beforehand.

[0161] In other preferred embodiments, NH3 is used as a reducing agent for NO. Xand CH4 is used as a reducing agent for N2O. The exhaust gas produced in step (a) contains amount [NO] of NO, amount [NO2] of NO2, and amount [HCN] of HCN. The required and therefore preferably added amount [NH3] of NH3 and the required and therefore preferably added amount [CH4] of CH4 are then preferably calculated as follows:

[0162] If a(NO x ) < 0.5: [NH3] = [NO] + [NO2] - [HCN], [CH4] = 0.25 [N2O] - [HCN],

[0163] If 0.5 < a(NO x ) < 1.0: [NH3] = 2 / 3 (1 + a(NO x )) • ([NO] + [NO2]) - [HCN], [CH4] = 0.25 [N2O] - [HCN],

[0164] The above amounts [NH3] and [CH4] are preferably minimum amounts of added NH3 or CH4 with a view to a desired chemical reduction of NO. X or N2O. In the preferably desired complete reduction of NO XHere too, the actual amount of NH3 added is preferentially increased by approximately {0.1 to 0.3} - ([NO] + [NO2]) (additive term). However, since excess NH3 also reacts with N2O, this means that less CH4 needs to be used. Accordingly, the preferentially required amount of CH4 is reduced by -0.25 • {0.1 to 0.3} • ([NO] + [NO2]) (subtractive term).

[0165] The higher the NH3 surplus, the better one can reduce NO. X to be completely chemically reduced. The possible excess of NH3 is limited by the occurrence of undesired NH3 slip, which occurs when excess NH3 has no further reaction partners (NO). X , or also N2O or O2) can no longer be found.

[0166] In order for the CO formed by hydrolysis of HCN to react with N2O as a reducing agent for N2O, it is preferred that the amount of NO contained in the exhaust gas Xis reduced as completely as possible (residual NO content) X preferably at most 50 ppmv), since otherwise the chemical reduction of N2O with CO (or with hydrocarbons) by remaining amounts of NO X would be inhibited. This is particularly problematic if there is more NO in the exhaust gas. X as HCN (which is often the case). Therefore, it is preferred to use an additional reducing agent for NO. X , preferably NH3, the NO X broken down as completely as possible (residual NO content) X preferably at most 50 ppmv). Preferably, NO is broken down as completely as possible. X (Residual NO content X preferably at most 50 ppmv) by additionally adding a reducing agent for NO X regardless of the amount of N2O in the exhaust gas, i.e., for this measure it does not matter whether there is more or less N2O than HCN.

[0167] The NO content is preferred for this purpose. Xin the exhaust gas by the additionally added reducing agent for NO X , preferably NH3, reduced to a maximum of 50 ppmv; preferably a maximum of 40 ppmv, more preferably a maximum of 30 ppmv, even more preferably a maximum of 20 ppmv, most preferably a maximum of 10 ppmv and particularly a maximum of 5.0 ppmv. The NO content is preferably used for this purpose. X reduced to at most 5.0 ppmv; preferably at most 4.0 ppmv, more preferably at most 3.0 ppmv, even more preferably at most 2.0 ppmv, most preferably at most 1.0 ppmv, and particularly approximately 0 ppmv.

[0168] Since the catalytic decomposition of N2O by small amounts of NO X If co-catalyzed, it may be preferable according to the invention to reduce the NO content. X It is not completely broken down in the exhaust gas, but only reduced to a residual amount. This residual amount of NO XIn this case, the concentration is preferably at most 100 ppmv, more preferably at most 50 ppmv, more preferably at most 25 ppmv, and preferably at least 20 ppmv, more preferably at least 10 ppmv, and more preferably at least 5 ppmv.

[0169] The preferred option is the added reducing agent for NO. X in step (b) NH3.

[0170] Preferably, step (c) additionally includes reducing the NO content. X in the exhaust gas by catalytic chemical reduction with a reducing agent for NO X on the zeolitic material loaded with transition metal.

[0171] Preferably, the most selective possible catalytic reduction (SCR) of NO is used. X , i.e., the oxidation of NH3 with NO is given priority. X catalyzed and not, or only secondarily, the oxidation of NH3 with any free oxygen (O2) that may be present in the exhaust gas.

[0172] During the chemical reduction of NO Xare dependent on the reducing agent for NO X Different reaction products are formed. In the case of the reducing agent NH3, which is preferred according to the invention, the chemical reduction of NO results in the following: X especially N2 and H2O are formed, depending on the type of transition metal-laden zeolitic material and the ratio of NO2 to total NO x (i.e. the NO x - Degree of oxidation a(NO x )) e.g. according to reaction equations (3) (Fast SCR), (4) (Standard SCR) or (5) (NO2SCR).

[0173] The combined selective catalytic reduction of NO and NO2 is called so-called fast SCR and generally proceeds much faster than the so-called normal SCR or NO2SCR.

[0174] The transition metal-loaded zeolitic material preferentially catalyzes the chemical reduction of NO. X with the reducing agent for NO XThe catalytic activity of the transition metal-loaded zeolitic material need not therefore be limited exclusively to catalytic conversion- The reaction may be limited to HCN. Furthermore, it is possible, and according to the invention also preferred, that the transition metal-loaded zeolitic material can additionally catalyze further reactions, for example the decomposition of N2O, the chemical reduction of N2O with reducing agents for N2O, the adjustment of NO x -equilibrium, and / or the selective oxidation of excess NH3 with free O2. Whether such further reactions actually take place depends on the conditions of the individual case 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.

[0175] Preferably, the reducing agent added for N2O in step (b) is selected from NH3, CO, and hydrocarbons; preferably NH3, CO, CH4 or C3H8.

[0176] Preferably, step (c) additionally comprises reducing the N2O content in the exhaust gas (i) by catalytic decomposition and / or (ii) by catalytic chemical reduction with a reducing agent for N2O on the transition metal-loaded zeolitic material.

[0177] Reducing the N2O content in the exhaust gas can be achieved in different ways, namely through decomposition and / or chemical reduction with reducing agents for N2O.

[0178] During the decomposition of N2O, N2 and O2 are formed according to the following overall reaction: 2 N2O ^ 2 N2+ O2(11).

[0179] Decomposition of N₂O therefore means decomposition into N₂ and O₂. The transition metal-laden zeolitic material preferably catalyzes the decomposition of N₂O, preferably with comparatively small (residual) amounts of NO. X exert a cocatalytic effect with regard to decomposition.

[0180] The achievable quantitative extent of the reduction in N2O content through catalytic decomposition depends not only on the type, i.e., the chemical nature and physical configuration of the transition metal-laden zeolitic material and the prevailing pressure and temperature conditions, but also, and perhaps most importantly, on the chosen space velocity, i.e., the ratio of exhaust gas volume flow to catalyst volume.

[0181] As mentioned above, the catalytic activity of a transition metal-loaded zeolitic material need not be limited exclusively to the catalytic conversion of HCN and, optionally, the catalytic decomposition of N₂O. It is possible, and indeed preferred according to the invention, that the transition metal-loaded zeolitic material can catalyze not only the catalytic conversion of HCN but also other reactions, for example, the chemical reduction of N₂O with reducing agents for N₂O, or the chemical reduction of NO₂. X with reducing agent for NO X , the setting of the NO x -equilibrium, and / or the selective oxidation of excess NH3 with free O2. Whether such further reactions actually take place depends on the conditions of the individual case, in particular the type of catalyst, and the kinetics of the This may depend on processes occurring in parallel, for example, the presence or quantity of the reducing agent and the presence or quantity of other reactants.

[0182] In the chemical reduction of N2O with reducing agents for N2O, different reaction products are formed depending on the reducing agent.

[0183] 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 NH3— >■ 4 N2+ 3 H2O (12) 4 N2O + 4 NH3 + O2^ 6 N2 + 6 H2O (13) or also in the joint reduction with NO according to 2 NO + N2O + 2 NH3— >■ 3 N2+ 3 H2O (14).

[0184] In the case of hydrocarbons, which are also preferred as reducing agents for N2O according to the invention, CO and H2O are formed in particular during the chemical reduction of N2O, e.g. according to (2n+I) N2O + C n H 2n+2(2n+I) N2+ n CO + (n+I) H2O (15).

[0185] For methane (n=l) as a reducing agent for N2O, the following results: 3 N2O + CH4^ 3 N2+ CO + 2 H2O (16).

[0186] For ethane (n=2) as a reducing agent for N2O, the following results: 5 N2O + C2H„ ^ 5 ​​N2+ 2 CO + 3 H2O (17).

[0187] For propane (n=3) as a reducing agent for N2O, the following results: 7 N2O + C3H8^ 7 N2+ 3 CO + 4 H2O (18).

[0188] The CO formed in this process is also preferred as a reducing agent for N2O according to the invention. It can react further with N2O to form CO2, e.g. according to reaction equation (6).

[0189] The in saturated hydrocarbons C n H 2n+2 The reduction equivalents contained for the chemical reduction of N2O thus also include the subsequent reactions of the CO formed with N2O. According to reaction equation (6), the stoichiometric ratio of [CO] to [N2O] is 1:1.

[0190] Methane therefore contains a total of 4 reducing equivalents for the chemical reduction of N₂O. The gross reaction can thus be formulated according to the following reaction equation: 4 N2O + CH4^ 4 N2+ CO2+ 2 H2O (19)

[0191] Ethane therefore contains a total of 7 reduction equivalents for the chemical reduction of N2O.

[0192] Propane therefore contains a total of 10 reduction equivalents for the chemical reduction of N2O.

[0193] The reduction equivalents for other hydrocarbons can be easily determined using analogous considerations. For saturated hydrocarbons, for example, the following overall reaction results: (3n+l) N2O + C n H 2n+2 (3n+l) N2+ n CO2+ (n+1) H2O (20).

[0194] Preferably, in step (b) the amount of added reducing agent for NO is Xand / or regulated for N2O depending on the composition of the exhaust gas produced in step (a) (feed-forward control).

[0195] In preferred embodiments, the amount of added reducing agent for NO is X chosen such that, under the given conditions, a practically complete chemical reduction of the total amount of NO is achieved. X this occurs, which is contained in the gas provided in step (a).

[0196] In other preferred embodiments, the amount of added reducing agent for NO is X chosen such that, under the given conditions, an incomplete chemical reduction of the amount of NO occurs. X This occurs, which is contained in the gas provided in step (a), so that after carrying out the inventive process a residual amount of NO is present in the exhaust gas obtained. X remains.

[0197] Thus, it is quite possible that in the inventive procedure NOX or N2O should not be completely degraded independently of each other. The invention's embodiments and definitions regarding the stoichiometry of reducing agent and NO X N2O may therefore only refer to the chemically reduced portion of NO. X or N2O. This applies analogously to the residual NO content. X and independently of this for the residual N2O content in connection with the control strategy for dosing reducing agent for NO X or for reducing agents for N₂O. In particular, the stoichiometric ratios do not necessarily refer to the inlet concentration of NO as a control or adjustment ratio. X in the exhaust gas provided in step (a). Rather, the stoichiometric ratios may only apply to a portion of the NO. Xor N2O, which is derived independently of each other from the respective difference in concentration in the exhaust gas provided in step (a) and the residual concentration tolerated after carrying out the inventive procedure.

[0198] Preferably, in step (b) the amount of added reducing agent for NO is X and / or measured for N2O depending on the NO content X , N2O and HCN in the exhaust gas produced in step (a).

[0199] The preferred amount of added reducing agent for NO X and / or for N2O additionally measured depending on the content of NH3 in the exhaust gas produced in step (a).

[0200] The preferred amount of added reducing agent for NO X and / or for N2O additionally measured depending on the CO content in the exhaust gas produced in step (a).

[0201] In another preferred embodiment, NH3 is added in step (b) as an additional common reducing agent for NO. X and N2O is used. The dosage is preferably controlled depending on the composition of the exhaust gas produced in step (a) (feed-forward control) or alternatively preferably depending on the NO x -Concentration in the exhaust gas after step (c) feedback control). In the feedback control, a specific value for the concentration of NO is used. X The target value (setpoint) is specified at the exit of the catalyst bed, and the actual concentration of NO X The actual value is measured in the exhaust gas after step (c), and in the event of a difference between the setpoint and the actual value (control deviation), the actuation degree of the control valve for NH3 dosing is changed to minimize the difference. The setpoint of the NO is preferably used. x The concentration in the exhaust gas after step (c) and thus the amount of additional NH3 is chosen so that the residual concentration of NOx The NO₂ level in the exhaust gas after step (c) is at most 50 ppmv, preferably at most 40 ppmv, more preferably at most 20 ppmv, and particularly at most 10 ppmv. The target value of the NO₂ is preferably x The concentration in the exhaust gas after step (c) and thus the amount of additional NH3 is chosen so that the residual concentration of NO x The NO in the exhaust gas after step (c) is at least 5 ppmv, preferably at least 10 ppmv, more preferably at least 20 ppmv, and particularly at least 40 ppmv. The feedback control described above is particularly preferred when the NO x -Content of the exhaust gas after step (a) is greater than the N2O content after step (a), preferably at least 2 times as much, more preferably at least 3 times as much, most preferably at least 5 times as much and in particular at least 10 times as much.

[0202] In step (c) of the inventive process, the content of HCN in the exhaust gas is reduced by catalytic conversion of the HCN on a zeolitic material loaded with transition metal (zeolite catalyst).

[0203] This means that the zeolitic material 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 the sake of simplicity, the elements of the transition metals and lanthanides are collectively referred to as "transition metals." The transition metals can be present in the zeolitic material in various bonding forms, for example, as ions bound at ion exchange sites of the respective zeolite structures or as oxide compounds embedded within the zeolitic material.

[0204] Preferred transition metals include iron (Fe zeolites), copper (Cu zeolites), and cobalt (Co zeolites). Iron-loaded zeolitic materials (i.e., Fe zeolites) are particularly preferred and may also be loaded with or contain other transition metals besides iron, such as manganese, vanadium, chromium, nickel, or mixtures thereof.

[0205] The preferred zeolitic material is of the structure type MFI, BEA, FER, MOR, FAU and / or MEL; preferably MFI, BEA and / or FER; preferably BEA.

[0206] The transition metal-loaded zeolitic material can comprise different materials. According to the invention, iron-loaded zeolitic materials particularly preferred contain essentially, preferably > 50 wt.%, and in particular > 70 wt.%, one or more iron-loaded zeolitic materials. For example, the transition metal-loaded zeolitic material used according to the invention can contain, in addition to an iron-loaded zeolitic material of the ZSM-5 type (Fe-ZSM-5), another iron-loaded zeolitic material, such as an iron-containing zeolitic material of the FER type (Fe-FER).

[0207] Furthermore, the zeolitic material loaded with transition metal, i.e. the zeolite catalyst, may contain other additives known to those skilled in the art, such as binders.

[0208] The iron content of the transition metal-loaded zeolitic material can be up to 25 wt.% based on the total weight, preferably 0.1 to 10 wt.%.

[0209] The inventive process also includes the use of transition metal-loaded zeolitic materials in which the lattice aluminum is partially isomorphically substituted by one or more elements, for example, by one or more elements selected from B, Be, Ga, Fe, Cr, V, As, Sb, and Bi. Also included is the use of transition metal-loaded zeolitic materials in which the lattice silicon is isomorphically substituted by one or more elements, for example, by one or more elements selected from Ge, Ti, Zr, and Hf.

[0210] Particularly preferred in the process according to the invention are zeolitic materials that have been treated with steam ("steamed" catalysts). Such treatment causes the lattice of the zeolitic material to become dealuminized; this treatment is known per se to those skilled in the art. These hydrothermally treated zeolitic materials are characterized by particularly high activity in the process according to the invention. Preferably, hydrothermally treated zeolitic materials are used that have been loaded with iron and in which the ratio of extra-lattice aluminum to lattice aluminum is at least 1:2, preferably 1:2 to 20:1.

[0211] Preferably, in step (b) the space velocity is in the range of 2,000 to 200,000 h⁻¹. 1 preferably 5,000 to 100,000 h 1 .

[0212] The zeolitic material is preferably in pelletized form as a packing or bulk of catalyst pellets.

[0213] Preferably, the zeolitic material is formed as a monolithic honeycomb structure (honeycomb catalyst), with the individual honeycomb structures optionally arranged or packed above and / or next to each other to form modules.

[0214] Preferably, no (further) oxidation catalyst for the oxidation of HCN is placed upstream of the zeolitic material loaded with transition metal in the direction of exhaust gas flow.

[0215] Preferably, no (further) catalyst for the oxidation of the NH3 and / or CO produced by the hydrolysis of HCN is connected downstream of the zeolitic material loaded with transition metal in the direction of exhaust gas flow.

[0216] Preferably, N2 is formed in step (c).

[0217] Preferably, H2O is formed in step (c).

[0218] Preferably, CO is formed in step (c).

[0219] Preferably, NH3 is formed in step (c).

[0220] Preferably, CO2 is formed in step (c).

[0221] Preferably, step (c) comprises the sub-steps (cj) reaction of HCN and H2O; and (c2) oxidation of at least one intermediate obtained in the reaction in sub-step (c) with an oxidizing agent.

[0222] Preferably, step (cj) includes the formation of NH3 and / or CO. Preferably, HCN is hydrolyzed with H2O, initially forming formamide and subsequently ammonium formate, which then decomposes into ammonia and formic acid. The formic acid decomposes by thermolysis into CO and H2O, resulting in the overall reaction according to reaction equation (2) for step (c).

[0223] Preferably, step (c2) includes the oxidation of CO with N2O.

[0224] Preferably, step (c2) comprises the oxidation of NH3 with NO. X and / or N2O (SCR).

[0225] Both the NH3 formed and the CO formed preferentially react in partial step (c2).

[0226] Preferably, step (c) includes at most a small amount, preferably essentially no oxidation of HCN with O2 according to reaction equation (1).

[0227] Preferably, the exhaust gas has a temperature in the range of 300 to 600 °C immediately before the catalytic conversion in step (c); preferably 350 to 550 °C.

[0228] Preferably, in step (c) the amount of NO is X , which is contained in the exhaust gas produced in step (a), a reduction in the NO content X by at least 80%; preferably at least 85%, more preferably at least 90%, even more preferably at least 95%, most preferably at least 97% and in particular at least 99%.

[0229] Preferably, immediately after step (c) the NO content is X The amount of volatile organic compound (VOC) in the exhaust gas is at most 50 ppmv; preferably at most 40 ppmv, more preferably at most 30 ppmv, even more preferably at most 20 ppmv, most preferably at most 10 ppmv, and particularly at most 5.0 ppmv. Preferably, the Salary to NO X at most 5.0 ppmv; preferably at most 4.0 ppmv, more preferably at most 3.0 ppmv, even more preferably at most 2.0 ppmv, most preferably at most 1.0 ppmv, and in particular approximately 0 ppmv.

[0230] Since the catalytic decomposition of N2O by small amounts of NO X In the case of co-catalysis, it may be preferable according to the invention to increase the NO content X It is not completely broken down in the exhaust gas, but only reduced to a residual amount. This residual amount of NO XIn this case, the concentration is preferably at most 100 ppmv, more preferably at most 50 ppmv, more preferably at most 25 ppmv, and preferably at least 20 ppmv, more preferably at least 10 ppmv, and more preferably at least 5 ppmv.

[0231] Preferably, in step (c) the amount of N2O contained in the exhaust gas produced in step (a) is reduced by at least 80%; preferably at least 85%, more preferably at least 90%, even more preferably at least 95%, most preferably at least 97% and in particular at least 99%.

[0232] Preferably, immediately after step (c) the N2O content in the exhaust gas is at most 50 ppmv; preferably at most 40 ppmv, more preferably at most 30 ppmv, even more preferably at most 20 ppmv, most preferably at most 10 ppmv and in particular at most 5.0 ppmv.

[0233] Preferably, in step (c) the amount of HCN contained in the exhaust gas produced in step (a) is reduced by at least 80%; preferably at least 85%, more preferably at least 90%, even more preferably at least 95%, most preferably at least 97% and in particular at least 99%.

[0234] Preferably, immediately after step (c) the HCN content in the exhaust gas is at most 10 ppmv; preferably at most 5.0 ppmv, more preferably at most 2.0 ppmv, even more preferably at most 1.0 ppmv, most preferably at most 0.5 ppmv and in particular at most 0.1 ppmv.

[0235] Preferably, immediately after step (c), the NH3 content in the exhaust gas is at most 40 ppmv; preferably at most 20 ppmv, more preferably at most 10 ppmv, even more preferably at most 5.0 ppmv, most preferably at most 3.0 ppmv and in particular at most 1.0 ppmv.

[0236] Preferably, immediately after step (c) the CO content in the exhaust gas is at most 80 ppmv; preferably at most 50 ppmv, more preferably at most 30 ppmv, even more preferably at most 20 ppmv, most preferably at most 10 ppmv and in particular at most 5.0 ppmv.

[0237] Preferably, the zeolitic material loaded with transition metal is the only catalyst used.

[0238] Preferably, the process according to the invention does not include a catalytic reaction on a precious metal catalyst.

[0239] The inventive method is preferably used to reduce the HCN content in the exhaust gas by catalytic conversion on the transition metal-loaded zeolitic material.

[0240] Another aspect of the invention relates to an exhaust aftertreatment system which is configured to carry out the inventive method described above.

[0241] Another aspect of the invention relates to the use of a transition metal-loaded zeolitic material to reduce the HCN content in an exhaust gas in the inventive process described above.

Claims

Patent claims:

1. A process for exhaust aftertreatment comprising the following steps: (a) Generating an exhaust gas comprising HCN, NO X and N20; wherein the HCN content is at least 1.0 ppmv; (b) optional, addition of reducing agent for NO X and / or for N2O in the exhaust gas; and (c) Reducing the HCN content in the exhaust gas by catalytic conversion of the HCN on a transition metal-loaded zeolitic material.

2. The method according to claim 1, wherein the molar amount of NO in the exhaust gas produced in step (a) X is greater than the molar amount of N2O.

3. The method according to claim 1 or 2, wherein the molar amount of NO in the exhaust gas produced in step (a) Xand N2O is each independently at least as large as the molar amount of HCN; preferably at least 1.1 times as large, more preferably at least 1.2 times as large, even more preferably at least 1.3 times as large, most preferably at least 1.4 times as large and in particular at least 1.5 times as large.

4. The method according to one of the preceding claims, wherein the molar amount of NO in the exhaust gas produced in step (a) X and N2O is each independently at least 2 times as large as the molar amount of HCN; preferably at least 3 times as large, more preferably at least 4 times as large, even more preferably at least 10 times as large, most preferably at least 20 times as large and in particular at least 50 times as large.

5. The method according to one of the preceding claims, wherein the exhaust gas produced in step (a) contains an amount of N2O at least equimolar to the molar amount of HCN.

6. The method according to one of the preceding claims, wherein the exhaust gas NO produced in step (a) X contains, whose NO x - Degree of oxidation a(NO x ) (i) in the area of ​​a(NO x ) < 0.5, whereby the exhaust gas contains an amount of NO at least equimolar to the molar amount of HCN. X contains, i.e. [ / VO X ] > [HCN] or (ii) in the range of 0.5 < a(NO x ) < 1.0, where the exhaust gas contains such a molar amount of NO X contains a value greater than or equal to the molar amount of HCN divided by 2 / 3 (1 + a(NO x )) is, 7. The method according to one of the preceding claims, wherein the HCN content in the exhaust gas produced in step (a) is at least 2.0 ppmv; preferably at least 4.0 ppmv, more preferably at least 6.0 ppmv, more preferably at least 8.0 ppmv, most preferably at least 10 ppmv and in particular at least 15 ppmv.

8. The method according to one of the preceding claims, wherein the HCN content in the exhaust gas produced in step (a) is at least 20 ppmv; preferably at least 25 ppmv, more preferably at least 30 ppmv, more preferably at least 35 ppmv, most preferably at least 40 ppmv and in particular at least 45 ppmv.

9. The method according to one of the preceding claims, wherein the HCN content in the exhaust gas produced in step (a) is at least 50 ppmv; preferably at least 55 ppmv, more preferably at least 60 ppmv, more preferably at least 65 ppmv, most preferably at least 70 ppmv and in particular at least 75 ppmv.

10. The method according to one of the preceding claims, wherein the HCN content in the exhaust gas produced in step (a) is at most 2000 ppmv; preferably at most 1500 ppmv, more preferably at most 1250 ppmv, more preferably at most 1000 ppmv, most preferably at most 750 ppmv and in particular at most 500 ppmv.

11. The method according to one of the preceding claims, wherein the NO content in the exhaust gas produced in step (a) is reduced. X at least 10 ppmv; preferably at least 25 ppmv, more preferably at least 50 ppmv, even more preferably at least 75 ppmv, most preferably at least 100 ppmv and in particular at least 150 ppmv.

12. The method according to one of the preceding claims, wherein the NO content in the exhaust gas produced in step (a) is reduced. X at least 250 ppmv; preferably at least 500 ppmv, more preferably at least 750 ppmv, even more preferably at least 1000 ppmv, most preferably at least 1250 ppmv and in particular at least 1500 ppmv.

13. The method according to one of the preceding claims, wherein the NO in the exhaust gas produced in step (a) x - Degree of oxidation a(NO x) at least 0.025; preferably at least 0.05, more preferably at least 0.075, even more preferably at least 0.1, most preferably at least 0.2 and in particular at least 0.

3.

14. The method according to one of the preceding claims, wherein the N2O content in the exhaust gas produced in step (a) is at least 4.0 ppmv; preferably at least 5.0 ppmv, more preferably at least 6.0 ppmv, more preferably at least 7.0 ppmv, most preferably at least 8.0 ppmv and in particular at least 9.0 ppmv.

15. The method according to one of the preceding claims, wherein the N2O content in the exhaust gas produced in step (a) is at least 10 ppmv; preferably at least 25 ppmv, more preferably at least 50 ppmv, more preferably at least 75 ppmv, most preferably at least 100 ppmv and in particular at least 150 ppmv.

16. The method according to one of the preceding claims, wherein the N2O content in the exhaust gas produced in step (a) is at least 250 ppmv; preferably at least 500 ppmv, before- at least 750 ppmv, more preferably at least 1000 ppmv, most preferably at least 1250 ppmv and in particular at least 1500 ppmv.

17. The method according to one of the preceding claims, wherein the exhaust gas produced in step (a) additionally comprises H2O.

18. The process according to claim 17, wherein the H2O content in the exhaust gas produced in step (a) is at least 2.5 vol%; preferably at least 5.0 vol%, more preferably at least 10 vol%, more preferably at least 15 vol%, most preferably at least 20 vol% and in particular at least 25 vol%.

19. The method according to one of the preceding claims, wherein the exhaust gas produced in step (a) additionally comprises O2.

20. The method according to claim 19, wherein the O2 content in the exhaust gas produced in step (a) is at least 1.0 vol%; preferably at least 2.0 vol%, more preferably at least 3.0 vol%, more preferably at least 4.0 vol%, most preferably at least 5.0 vol% and in particular at least 6.0 vol%.

21. The method according to claim 19 or 20, wherein the O2 content in the exhaust gas produced in step (a) is at least 7.0 vol.%; preferably at least 8.0 vol.%, more preferably at least 9.0 vol.%, more preferably at least 10.0 vol.%, most preferably at least 11.0 vol.% and in particular at least 12.0 vol.%.

22. The method according to one of the preceding claims, wherein the exhaust gas produced in step (a) additionally comprises NH3.

23. The method according to claim 22, wherein the NH3 content is at least 10 ppmv; preferably at least 25 ppmv, more preferably at least 50 ppmv, more preferably at least 75 ppmv, most preferably at least 100 ppmv and in particular at least 150 ppmv.

24. The method according to claim 22 or 23, wherein step (a) is a combustion process and wherein the NH3 contained in the generated exhaust gas is residual, unburned NH3.

25. The method according to one of the preceding claims, wherein the exhaust gas produced in step (a) additionally comprises CO.

26. The method according to one of the preceding claims, wherein the exhaust gas produced in step (a) additionally comprises CO2.

27. The method according to one of the preceding claims, wherein the exhaust gas produced in step (a) additionally comprises N2.

28. The method according to any of the preceding claims, wherein step (a) is an industrial process; preferably for generating process heat.

29. The method according to any of the preceding claims, wherein step (a) is a combustion process.

30. The method according to claim 29, wherein the combustion process does not take place on a catalyst.

31. The method according to any of the preceding claims, wherein step (a) comprises the combustion of a mixture comprising a nitrogen-containing compound, a carbon-containing compound and O2.

32. The method according to claim 31, wherein the molar amount of nitrogen-containing compound in the mixture is greater than the molar amount of carbon-containing compound.

33. The method according to claim 31 or 32, wherein the molar amount of nitrogen-containing compound in the mixture is greater than the total molar amount of carbon atoms in the carbon-containing compound.

34. The method according to claim 31, wherein the molar amount of nitrogen-containing compound in the mixture is smaller than the molar amount of carbon-containing compound.

35. The method according to claim 31 or 34, wherein the molar amount of nitrogen-containing compound in the mixture is less than the total molar amount of carbon atoms in the carbon-containing compound.

36. The method according to any one of claims 31 to 35, wherein the nitrogen-containing compound is NH3 or an amine; preferably NH3.

37. The method according to any one of claims 31 to 36, wherein the carbon-containing compound is a fossil fuel; preferably hydrocarbon(s); more preferably CH4 and / or C3H8.

38. The method according to any of the preceding claims, wherein step (a) comprises the combustion of a mixture comprising (i) NH3, (ii) a fossil fuel; preferably hydrocarbon(s), more preferably CH4 and / or C3H8, and (iii) air and / or O2.

39. The method according to claim 38, wherein the molar amount of NH3 in the mixture is greater than the molar amount of fossil fuel; preferably wherein the molar amount of NH3 in the mixture is greater than the total molar amount of carbon atoms of the fossil fuel.

40. The method according to claim 38, wherein the molar amount of NH3 in the mixture is smaller than the molar amount of fossil fuel; preferably wherein the molar amount of NH3 in the mixture is smaller than the total molar amount of carbon atoms of the fossil fuel.

41. The method according to any of the preceding claims, wherein the exhaust gas produced in step (a) has a pressure of at most 5.5 bara; preferably at most 5.0 bara, more preferably at most 4.5 bara, most preferably at most 4.0 bara and in particular at most 3.5 bara; preferably at most 3.0 bara; more preferably at most 2.5 bara, more preferably at most 2.0 bara, most preferably at most 1.5 bara and in particular at most 1.1 bara.

42. The method according to one of the preceding claims, wherein the HCN content in the exhaust gas is measured.

43. The V method according to claim 42, wherein the HCN content in the exhaust gas produced in step (a) is measured.

44. The method according to claim 42 or 43, wherein the HCN content in the exhaust gas is measured after step (c).

45. The method according to one of the preceding claims, wherein the CO content in the exhaust gas is measured.

46. ​​The method according to claim 45, wherein the CO content in the exhaust gas produced in step (a) is measured.

47. The method according to claim 45 or 46, wherein the CO content in the exhaust gas is measured after step (c).

48. The method according to any one of the preceding claims, wherein the NO content X measured in the exhaust gas.

49. The method according to claim 48, wherein the NO content X in the exhaust gas produced in step (a).

50. The method according to any one of the preceding claims, wherein the content of NO and NO2 and / or the NO x - Degree of oxidation a(NO x ) are measured.

51. The method according to claim 50, wherein the content of NO and NO2 and / or the NO X - Oxidation state a(NO x ) in the exhaust gas produced in step (a).

52. The method according to one of the preceding claims, wherein the N2O content in the exhaust gas is measured.

53. The method according to claim 52, wherein the N2O content in the exhaust gas produced in step (a) is measured.

54. The method according to any one of the preceding claims, wherein the NO content X and N2O is measured in the exhaust gas after step (c).

55. The method according to one of the preceding claims, wherein the added reducing agent for NOx in step (b) is NH3.

56. The method according to any of the preceding claims, wherein step (c) additionally includes reducing the NO content X in the exhaust gas by catalytic chemical reduction with a reducing agent for NOx on the transition metal-laden zeolitic material.

57. The method according to any of the preceding claims, wherein the reducing agent added for N2O in step (b) is selected from NH3, CO, and hydrocarbons; preferably NH3, CO, CH4 or C3H8.

58. The method according to one of the preceding claims, wherein step (c) additionally comprises reducing the N2O content in the exhaust gas by catalytic decomposition and / or by catalytic chemical reduction with a reducing agent for N2O on the transition metal-loaded zeolitic material.

59. The method according to any one of the preceding claims, wherein in step (b) the amount of added reducing agent for NO X and / or for N2O is regulated (i) depending on the composition of the exhaust gas produced in step (a) feed-forward),- and / or (ii) depending on the NOx concentration in the exhaust gas after step (c) (feedback control).

60. The method according to any one of the preceding claims, wherein in step (b) the amount of added reducing agent for NO X and / or is measured for N2O depending on the NO content X , N2O and HCN in the exhaust gas; preferably depending on the content of NO, NO2, N2O and HCN in the exhaust gas.

61. The method according to claim 59 or 60, wherein the amount of added reducing agent for NO X and / or for N2O according to the amounts of NO X and N2O in the exhaust gas is measured, preferably according to the amounts of NO, NO2 and N2O in the exhaust gas, each reduced by the amount of HCN contained in the exhaust gas produced in step (a).

62. The method according to any one of claims 59 to 61, wherein, based on the amount of HCN contained in the exhaust gas, one molar equivalent of NH3 is used as a reducing agent for NO. Xas well as taking into account a molar equivalent of CO as a reducing agent for N2O and the measured amount of added reducing agent for NO X and / or is reduced accordingly for N2O.

63. The method according to any one of claims 59 to 62, wherein - in the case of a NO x - Degree of oxidation a(NO x ) in the range of a(NO x ) < 0.5 the molar amounts of reduction equivalents for N2O and NO X each reduced by the molar amount of HCN contained in the exhaust gas; and - in the case of a NO x - Degree of oxidation a(NO x ) in the range of 0.5 < a(NO x ) < 1.0 (i) the molar amount of reduction equivalents for N2O by the molar amount of HCN contained in the exhaust gas, and (ii) the molar amount of reduction equivalents for NO X the molar amount of HCN contained in the exhaust gas divided by 2 / 3 (1 + a(NO) x )) be reduced.

64. The method according to any one of claims 59 to 63, wherein any excess amounts of N2O contained in the exhaust gas are reduced with NH3, CO or hydrocarbon as a reducing agent for N2O, preferably by chemical reduction with CO and / or hydrocarbon.

65. The method according to claim 64, wherein the NO content X is reduced by the additional addition of NH3, preferably immediately after step (c) the NO content X in the exhaust gas is at most 5.0 ppmv; preferably at most 4.0 ppmv, more preferably at most 3.0 ppmv, even more preferably at most 2.0 ppmv, most preferably at most 1.0 ppmv, and in particular approximately 0 ppmv.

66. The method according to any one of claims 59 to 65, wherein the amount of added reducing agent for NO X and / or is additionally measured for N2O depending on the content of NH3 in the exhaust gas produced in step (a).

67. The method according to any one of claims 59 to 66, wherein the amount of added reducing agent for NO X and / or is additionally measured for N2O depending on the CO content in the exhaust gas produced in step (a).

68. The method according to any of the preceding claims, wherein the transition metal comprises or consists substantially of iron.

69. The method according to claim 68, wherein the zeolitic material is of the structural type MFI, BEA, FER, MOR, FAU and / or MEL; preferably MFI, BEA and / or FER; more preferably BEA.

70. The method according to one of the preceding claims, wherein in step (b) the space velocity is in the range of 2,000 to 200,000 h⁻¹ -1 nurtures, preferably 5,000 to 100,000 h 1 .

71. The method according to one of the preceding claims, wherein the zeolitic material is present as a packing of catalyst pellets.

72. The method according to any one of claims 1 to 70, wherein the zeolitic material is present as a honeycomb catalyst.

73. The method according to one of the preceding claims, wherein in step (c) N2 is formed.

74. The method according to one of the preceding claims, wherein in step (c) H2O is formed.

75. The method according to one of the preceding claims, wherein CO2 is formed in step (c).

76. The method according to one of the preceding claims, wherein CO is formed in step (c).

77. The method according to any one of the preceding claims, wherein in step (c) NH3 is formed 78. The method according to any one of the preceding claims, wherein step (c) comprises the sub-steps (ci) reaction of HCN and H2O; and (c2) oxidation of at least one intermediate obtained in the reaction in sub-step (c) with an oxidizing agent.

79. The method according to claim 78, wherein partial step (c) comprises the formation of NH3 and / or CO.

80. The method according to claim 78 or 79, wherein partial step (c2) comprises the oxidation of CO with N2O.

81. The method according to any one of claims 78 to 80, wherein step (c2) is the oxidation of NH3 with NO X and / or includes N2O.

82. The method according to one of the preceding claims, wherein the exhaust gas has a temperature in the range of 300 to 600 °C immediately before the catalytic reaction in step (c); preferably 350 to 550 °C.

83. The method according to one of the preceding claims, wherein in step (c) with respect to the amount of NO X , which is contained in the exhaust gas produced in step (a), a reduction in the NO content Xby at least 80%; preferably at least 85%, more preferably at least 90%, even more preferably at least 95%, most preferably at least 97% and in particular at least 99%.

84. The method according to one of the preceding claims, wherein immediately after step (c) the NO content X in the exhaust gas is at most 50 ppmv; preferably at most 40 ppmv, more preferably at most 30 ppmv, even more preferably at most 20 ppmv, most preferably at most 10 ppmv and in particular at most 5.0 ppmv.

85. The method according to any one of the preceding claims, wherein immediately after step (c) the NO content X in the exhaust gas is at most 5.0 ppmv; preferably at most 4.0 ppmv, more preferably at most 3.0 ppmv, even more preferably at most 2.0 ppmv, most preferably at most 1.0 ppmv, and in particular approximately 0 ppmv.

86. The method according to one of the preceding claims, wherein in step (c) the N2O content is reduced by at least 80% with respect to the amount of N2O contained in the exhaust gas produced in step (a); preferably at least 85%, more preferably at least 90%, more preferably at least 95%, most preferably at least 97% and in particular at least 99%.

87. The method according to one of the preceding claims, wherein immediately after step (c) the N2O content in the exhaust gas is at most 50 ppmv; preferably at most 40 ppmv, more preferably at most 30 ppmv, more preferably at most 20 ppmv, most preferably at most 10 ppmv and in particular at most 5.0 ppmv.

88. The method according to one of the preceding claims, wherein in step (c) the HCN content is reduced by at least 80% with respect to the amount of HCN contained in the exhaust gas produced in step (a); preferably at least 85%, more preferably at least 90%, more preferably at least 95%, most preferably at least 97% and in particular at least 99%.

89. The method according to any of the preceding claims, wherein immediately after step (c) the HCN content in the exhaust gas is at most 10 ppmv; preferably at most 5.0 ppmv, more preferably at most 2.0 ppmv, more preferably at most 1.0 ppmv, most preferably at most 0.5 ppmv and in particular at most 0.1 ppmv.

90. The method according to one of the preceding claims, wherein immediately after step (c) the NH3 content in the exhaust gas is at most 40 ppmv; preferably at most 20 ppmv, more preferably at most 10 ppmv, more preferably at most 5.0 ppmv, most preferably at most 3.0 ppmv and in particular at most 1.0 ppmv.

91. The method according to one of the preceding claims, wherein immediately after step (c) the CO content in the exhaust gas is at most 80 ppmv; preferably at most 50 ppmv, more preferably at most 30 ppmv, more preferably at most 20 ppmv, most preferably at most 10 ppmv and in particular at most 5.0 ppmv.

92. The method according to any of the preceding claims, wherein the transition metal-loaded zeolitic material is the only catalyst used.

93. The method according to any of the preceding claims, which does not include a catalytic reaction on a precious metal catalyst.

94. The method according to one of the preceding claims, wherein, in the direction of flow of the exhaust gas, no (further) oxidation catalyst for the oxidation of HCN is placed upstream of the zeolitic material loaded with transition metal.

95. The method according to one of the preceding claims, wherein, in the direction of flow of the exhaust gas, no (further) catalyst for the oxidation of the NH3 and / or CO produced by the hydrolysis of HCN is downstream of the zeolitic material loaded with transition metal.

96. The method according to one of the preceding claims for reducing the HCN content in the exhaust gas by catalytic reaction on the transition metal-loaded zeolitic material.

97. An exhaust aftertreatment system configured to carry out the method according to any of the preceding claims.

98. Use of a transition metal-loaded zeolitic material to reduce the HCN content in an exhaust gas in the method according to any one of claims 1 to 96.

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