Starting up scr systems for an NH 3 combustion process

Abstract

The invention relates to a method for starting up a system for combusting NH3, said system comprises a combustion device and an exhaust gas channel, wherein the method has at least the two following steps: (a) combusting a combustible gas, preferably an NH3-free combustible gas, in the combustion device in order to obtain an exhaust gas and transferring the exhaust gas into the exhaust gas channel; and (e) metering NH3 into the combustible gas as soon as the temperature of the exhaust gas in the combustion device and in the exhaust gas channel is above a threshold temperature TG at which the formation of deposits of NH4NO3 from NH3 and NOX contained in the exhaust gas is prevented.

Description

Starting up SCR systems during NH3 combustion

[0001] The invention relates to a method for starting up a plant for the combustion of NH3, which comprises a combustion device and an exhaust gas channel, wherein the method comprises at least the following two steps: (a) Combustion of a (preferably NH3-free) combustible gas in the combustion device, producing an exhaust gas and transferring the exhaust gas into the exhaust duct; and (e) Metering NH3 into the combustible gas as soon as the temperature of the exhaust gas in the combustion device and in the exhaust gas duct exceeds a limit temperature T G lies in the process where the formation of NH4NO3 deposits occurs from NH3 and NO contained in the exhaust gas. X is prevented.

[0002] To reduce emissions of the climate-damaging greenhouse gas CO2, numerous efforts are underway to replace fossil fuels with alternative fuels that are obtained from renewable sources or processes and whose combustion does not produce CO2. These include, among others, hydrogen (H2) and ammonia (NH3).

[0003] The combustion of NH3, like the combustion of fossil fuels such as natural gas, typically also produces significant amounts of nitrogen oxides, especially NO and NO2 (together referred to as "NOₓ"). X ") but also nitrous oxide (N2O). NO X It is a cause of acid rain and smog; N₂O is a climate-damaging greenhouse gas with a GWP of 278. For reasons of environmental and climate protection, there is therefore an urgent need to reduce potential NOₓ emissions. X - and N2O emissions during the combustion of NH3, whether for the generation of thermal or electrical energy, to avoid or reduce these emissions.

[0004] When natural gas is burned, the nitrogen oxides produced are usually removed using suitable (DeNOx) treatment. x -) Measures reduced. Particularly noteworthy here are processes for the selective catalytic chemical reduction (SCR) of nitrogen oxides, whereby appropriate amounts of reducing agent for the nitrogen oxides are added to the exhaust gases before they enter the catalyst bed, e.g. NH3.

[0005] For safety reasons, the addition of NH3 can typically only occur above a minimum temperature threshold, particularly once the SCR catalyst has reached its minimum required temperature for catalytic chemical reduction with NH3 as the reducing agent, e.g., at least 170°C. Otherwise, ammonium nitrate (NH4NO3) can form, possibly with traces of ammonium nitrite (NH4NO2). These are referred to collectively as "ammonium nitrate" in the following, which forms on the SCR catalyst or downstream in colder system components. The NH4NO3 deposits can potentially decompose spontaneously, i.e., explosively, in an exothermic reaction.

[0006] For the general problem of the formation of ammonium nitrate deposits and its decomposition, e.g. via vanadium-based SCR catalysts, reference is made to the following literature reference: Ottinger, N., Xi, Y., Keturakis, C., and Liu, Z., "Ammonium Nitrate Formation and Decomposition on an Extruded Vanadium-Based SCR Catalyst", SAE Technical Paper 2020-01-1320, 2020.

[0007] When starting up natural gas combustion plants, such as power plants, the supply of NH3 to the flue gas treatment unit is therefore usually only started once the minimum required temperature has been reached. This prevents the formation and deposition of NH4NO3 before the flue gas enters the flue gas treatment unit, or the DeNOx. x -catalyst bed, both in DeNO x -Catalytic converter itself as well as at the outlet from the exhaust gas treatment unit are inherently excluded.

[0008] However, the initial situation is different for plants that burn NH3. During the combustion of NH3, and especially during start-up, i.e., after ignition of the combustion, not only NO is produced. x -formation, but also a slippage of unreacted, i.e., completely combusted NH3 must be expected. This means that NO is present in the exhaust gas at the same time. X and NH3 are present and deposits of NH4NO3 can form, even if no additional external addition of NH3 to the exhaust gas is made as a reducing agent for SCR-Z.

[0009] This is particularly problematic if the plant start-up has to be aborted due to operational malfunctions before the decomposition temperature of NH4NO3 is reached (approximately 170°C at atmospheric pressure), and NH4NO3 thus continues to accumulate with each subsequent start-up. The risk and potential hazard from uncontrolled decomposition of the accumulated NH4NO3 therefore increases significantly.

[0010] Various approaches are being considered to prevent the formation or deposition of NH4NO3 below the limiting temperature.

[0011] The problem could be solved, for example, by separately preheating the exhaust gas stream before igniting the NH3 combustion, e.g., using separate natural gas burners or electric heaters. However, this would mean additional equipment and investment costs.

[0012] Another solution for generally preventing the formation of NH4NO3 deposits in an NH3 cracking plant is described in WO 2023 / 247915 Al. In such plants, there is a risk that NH3, due to leaks in the tubular cracking reactors heated by a combustion chamber, enters the combustion exhaust gas and, after the flue gas cools at temperatures below 170°C, reacts with NO, which is also present in the exhaust gas. X It reacts to form solid NH4NO3. The solution involves dosing water or steam to... Flue gas proposed, so that the water content y H 2o in the exhaust gas is greater than the ratio of the equilibrium vapor pressure of water in an aqueous Nf^NOß solution p* H 2o to the operating pressure of the flue gas P is (y H2o > P* H2o / P)- This means, however, that depending on the flue gas temperature, the water content must be increased to almost 30 mol%, which naturally involves a considerable additional expenditure of equipment and energy.

[0013] It is known from the prior art not to start up combustion devices operated with NH3 using NH3 or pure NH3 as fuel, i.e., not to ignite the burners with pure NH3, but instead to use a separate fuel ("starting fuel"), for example natural gas or H2, to start up the burners.

[0014] Such an operating mode is described, for example, in US 11,702,988 B2. This patent describes an NH3-fired gas turbine whose waste heat is used, among other things, to decompose NH3 into H2 and N2. An NH3 decomposition plant is disclosed, comprising: a heating medium line designed to carry a heating medium heated by the heat generated by a gas turbine; an NH3 supply line designed to carry NH3; an NH3 decomposition device; and an NH3 removal device. The NH3 decomposition device is designed to utilize the heat from the heating medium in the heating medium line to thermally decompose the NH3 from the NH3 supply line and produce a decomposition gas (DG) containing H2, N2, and residual NH3. The NH3 removal device is designed to remove the residual NH3 contained in the decomposition gas (DG) from the NH3 decomposition device.The fuel is gradually switched from a start-up fuel (SF) to the processed gas (PG), i.e. the decomposition gas (DG) from which the remaining NH3 has been removed.

[0015] CN 115 127 112 A describes a device for the mixed combustion of natural gas and NH3 for heating a steam boiler, wherein the combustion is initially started with pure natural gas from a cold boiler state. This serves to initially stabilize the combustion process. Only after the heating load has reached 20% is NH3 gradually added to the fuel gas.

[0016] It is generally known that the combustion of NH3 is associated with some difficulties, due in part to its lower energy density and slower flame velocity compared to natural gas. A summary of the current state of knowledge and technology regarding NH3 combustion is presented in Kobayashi, Hideaki; Hayakawa, Akihiro; Somarathne, KD; Kunkuma A.; Okafor, Ekenechukwu C. (2019): Science and technology of ammonia combustion. In: Proc. Combust. Inst. 37 (1), pp. 109-133. For example, ignition difficulties and flameouts at the burner mouth occur, particularly during cold start-up of NH3 burners. These problems can be eliminated or mitigated by operating the cold burners additionally or exclusively with natural gas or H2 as fuel during start-up.

[0017] This initial operation with natural gas or H2 is only undertaken to ignite the burners, i.e., until a stable flame has formed, meaning until the burner and the adjacent combustion chamber have reached a sufficiently high temperature, which typically takes a few seconds to a few minutes. Afterward, NH3 is gradually added as fuel, or the proportion of NH3 in the fuel is progressively increased.

[0018] The methods known from the prior art for starting up an NH3 combustion plant are not entirely satisfactory, particularly with regard to the formation and deposition of NH4NO3. Therefore, there is a need for improved methods that can be implemented economically on an industrial scale.

[0019] There is also a need for measures that are suitable for this purpose. - Nitrogen oxides (especially N2O and NO) X (i.e. NO and NO2)), - any excess NH3 that may be present, as well as - to at least partially remove any other environmentally harmful components of the exhaust gases (such as CO or HCN) that may be present due to combustion in the exhaust gas of NH3-operated plants, so that the exhaust gas can subsequently be released into the ambient air in compliance with all environmental regulations.

[0020] The preferred systems are those using NH3-fueled combustion plants. NH3-fueled combustion plants can preferably be used generally for the combustion of NH3-containing fuels, for example, for heating purposes, such as heating steam boilers or reactors, for example, for the catalytic cracking of NH3; or also in internal combustion engines to perform mechanical work, for example, in gas turbines or internal combustion engines. According to the invention, preferred combustion plants are an NH3-driven internal combustion engine, an NH3-driven gas turbine, or a furnace for cracking NH3 into N2 and H2.

[0021] In preferred embodiments, the systems are combustion systems comprising a combustion device in which NH3 is burned generating heat of combustion, and an NH3 decomposition device which is in heat exchange with the combustion device and in which NH3 is catalytically decomposed into N2 and H2.

[0022] The specific circumstances arising from the most efficient possible combustion of NH3 for the operation of combustion plants must be taken into account, preferably comprising a combustion unit for burning NH3 and an NH3 decomposition unit for splitting NH3 into N2 and H2. In addition to the different composition of the exhaust gas, key parameters include, in particular, the pressure and temperature of the exhaust gas. These parameters can differ considerably from those of other exhaust gases for which measures for the removal of NO have previously been implemented. X and N2O were developed.

[0023] For example, in the industrial production of nitric acid, NH3 is deliberately reduced to NO. XThe NH3 is oxidized, and nitric acid is subsequently obtained by reacting it with water in an absorption tower. Special catalysts made of precious metals are used for the oxidation, and the reaction often takes place under elevated pressure. The aim of the NH3 combustion is to achieve the highest possible yield of NO. X Typical water contents in the exhaust gas range from approximately 1 to 3 vol.%. Any incompletely combusted NH3 (NH3 slip) from the catalytic NH3 oxidation (combustion) is usually washed out in the absorption tower at the latest and does not enter the exhaust gas.

[0024] In contrast, when NH3 is burned to operate combustion plants, preferably comprising a combustion unit for burning NH3 and an NH3 decomposition unit for splitting NH3 into N2 and H2, preferably only to the N2 stage, this process typically does not require catalysts, and this conversion usually takes place at atmospheric pressure. Typical water contents in the exhaust gas are significantly above 3 vol.%. For example, the combustion of pure NH3 in air with a residual oxygen content of 3 mol.% yields more than 28 mol.% water. The main objective of NH3 combustion is to generate the energy required for the catalytic decomposition reaction of NH3 into N2 and H2.Low levels of nitrogen oxides in the exhaust gas formed during combustion are advantageous because then only a comparatively small exhaust gas treatment unit is required to reduce the nitrogen oxide content in the flue gas and thus meet the regulatory requirements regarding permissible emissions, or because only then can sufficiently low residual concentrations be achieved with known methods for nitrogen oxide reduction.

[0025] In contrast to conventional exhaust gas treatment units, such as those used for exhaust gases from plants for the production of HNO3, the combustion of NH3 according to the invention, preferably in a mixture with H2 and / or CH4, C3H8, or C4H8, combined with the catalytic decomposition of NH3 to H2 and N2, brings with it special features that require special measures.

[0026] Firstly, the pressureless conditions and secondly, the very high water content are crucial. Pressureless means that when using conventional catalyst beds based on packed beds of particulate matter, etc., the pressure losses could potentially be too high. The high water content, due to the hydrothermal stress on the catalysts in the exhaust aftertreatment unit, especially in the case of zeolite material, can lead to progressive deactivation of the catalysts at high temperatures. The maximum temperature should therefore be limited. Apart from aging, the chemical reduction of NO X The high water content hardly affects the degradation of N2O, while the degradation of N2O by decomposition and / or chemical reduction is significantly affected by the high water content.

[0027] Another difference between the exhaust gases to be treated according to the invention and the production of HNO3 is the relatively high NO x-Content, which can amount to several thousand ppmv. The Salary to NO X It depends on the combustion conditions of NH3, in particular the NH3 content, any other combustible gases present (H2 and / or CH4 (natural gas)), and the air-fuel ratio X. Due to the high combustion temperatures of up to 1000°C and more, NO X Furthermore, it initially exists almost exclusively as NO, i.e., with a very high proportion of NO and a very low proportion of NO₂. Due to the slow formation kinetics of NO₂ at high temperatures, only a small portion of the NO is converted to NO₂ by the preferential cooling in the downstream heat exchanger. This means that the oxidation state (β) of the NO X , i.e., the molar fraction of NO2 in the total NO X(β = n(NO2) / (n(NO) + n(NO2))), is small at the point where the exhaust gas enters the exhaust gas treatment unit, typically <5 vol.%. This in turn means that the desired selective catalytic NO reduction is achieved. x -Reduction can only proceed very poorly or slowly, corresponding to the slow so-called normal SCR.

[0028] These are fundamental differences compared to established exhaust gas purification in HNO3 systems, in which the N2O and NO X residual gas contained under an overpressure of mostly 4-10 bar after leaving the absorption tower from a "cold" state (the thermodynamic NO x The equilibrium here is practically entirely on the side of NO2) as it is heated stepwise. Thus, the NO x The degree of oxidation of residual gases during HNO3 production before entering the corresponding exhaust gas treatment unit is typically between 30 and 70 vol.%, i.e., close to the ideal stoichiometric ratio for NO. x-Reduction according to the very fast almost SCR.

[0029] The high NO x -content, associated with a very low NO x The high degree of oxidation and water content at simultaneously low operating pressure (near atmospheric pressure) thus poses particular challenges in the present case to the effectiveness of the exhaust gas treatment unit according to the invention and to preventing the formation of NH4NCÜ deposits, especially due to potentially incompletely combusted NH3 (NH3 slip). In addition, there is the challenge or necessity of removing the N2O also contained in the exhaust gas, which cannot be reduced with conventional SCR processes based on V2O5 / TiO2 catalysts.

[0030] The objectives and reaction products obtained during the combustion of NH3 therefore sometimes differ considerably from one another.

[0031] In conventional plants for the production of nitric acid, the exhaust gas often exhibits a comparatively high pressure. - a comparatively low level of NO X ; - a comparatively high proportion of NO2; - a comparatively high level of N2O; - a comparatively low water content; and - no proportion of unburned NH3 (NH3 slip).

[0032] In contrast, in combustion plants, preferably comprising combustion units and NHβ decomposition units for splitting NH3 into N2 and H2, the exhaust gas often exhibits a comparatively low pressure. - a comparatively high level of NO X ; - a comparatively low proportion of NO2; - a comparatively low N2O content; - a significantly higher water content; - possibly a not insignificant proportion of unburned NH3 (NH3 slip); and - possibly a not insignificant proportion of HCN if NH3 is burned together with CH4 (natural gas); on.

[0033] These special circumstances must be taken into account when removing NO. X and N2O from the exhaust gases, as well as the prevention of the formation and deposition of NH4NO3, which presents a particular challenge.

[0034] Plants for the combustion of NH3 or for the production of H2 from NH3 are typically operated continuously for extended periods without interruption, for example, for several weeks or months. Nevertheless, it is occasionally necessary to shut such plants down to a standby mode, for instance, to carry out safety checks, replace catalyst material, or perform other maintenance work. Unplanned shutdowns can also occur, not least due to operational malfunctions. During standby, the plants cool down, depending on the duration of the interruption, until they reach ambient temperature. To return such a plant to continuous operation from standby, it must first be warmed up to a sufficiently high temperature; NH3 cannot be burned or decomposed at room temperature.

[0035] It is an object of the invention to provide an advantageous method for starting up an NH3 combustion plant, which overcomes the aforementioned disadvantages. Starting up the plant should be possible without the formation of NFUNC^ deposits and should be economical and feasible on an industrial scale.

[0036] This problem is solved by the subject matter of the patent claims.

[0037] It was surprisingly found that NH4NO3 deposits can be avoided if the combustion systems or burners for NH3 combustion are not initially operated with pure NH3 or NH3-containing fuels, i.e., during system start-up, but rather with NH3-free fuels such as H2 or CH4 (natural gas) for a certain period of time. Only when the exhaust gas temperature reaches a certain threshold temperature T GOnce the desired NH3-free fuel has been reached, the NH3-free fuel is gradually replaced by NH3, or the desired NH3 content in the fuel is gradually adjusted. The additional- The next cold exhaust gas can then contain NO. X It contains NH4NO3, but not NH3 at the same time. This prevents the formation of NH4NO3 deposits when starting up the plant.

[0038] This should not involve any additional effort, or only a small one, since in many applications for NH3 combustion, for reasons of better combustion (e.g. higher combustion temperatures), not pure NH3, but NH3-containing mixtures are burned, e.g. with H2, CH4 (natural gas), LPG or similar.

[0039] Furthermore, it was found that not only can the formation of NH4NO3 deposits during start-up be avoided, but also high NO levels. X - and NH3 emissions. The NO xEmissions from the combustion of NH3-free fuels such as H2, CH4 (natural gas), LPG, etc., are typically lower than those from the combustion of NH3 or NH3-containing fuels. After the limiting temperature T G Once the target temperature is reached and NH3 is burned, selective catalytic reduction (i.e., the catalytic conversion of NO) can also occur. X and NH3) are started to increase high NO X - and to avoid NH3 emissions.

[0040] Furthermore, it was found that the exhaust gas temperature is advantageously measured at or after leaving the exhaust gas treatment unit. The exhaust gas should have a certain minimum temperature downstream of the combustion unit, from the combustion of NH3 to the exhaust gas treatment unit, to prevent the formation of NH4NO3 deposits. Within the exhaust gas treatment unit, NH3 and NO contained in the exhaust gas can be removed. Xto be removed. After leaving the exhaust gas treatment unit, the exhaust gas will then cool below the limit temperature T. G This is possible without the formation of NH4NO3 deposits. The exhaust gas should maintain the minimum temperature downstream of the combustion unit up to the exhaust gas treatment unit, especially after optional cooling, e.g., via at least one heat exchanger. Only when the temperature at the outlet of the exhaust gas treatment unit reaches the limit temperature T G Once a temperature of at least 170°C, preferably at least 180°C, and particularly preferably at least 200°C has been reached, the NH3-free fuel can be successively replaced by NH3, or the desired NH3 content in the fuel can be successively adjusted. This prevents the formation of NH4NO3 deposits, especially downstream of the combustion unit up to the exhaust gas treatment unit.

[0041] A first aspect of the invention relates to a method for starting up a plant for the combustion of NH3, which comprises a combustion device and an exhaust gas channel, wherein the method comprises the following steps: (a) Burning a flammable gas in the combustion device, producing an exhaust gas and transferring the exhaust gas into the exhaust duct; (e) Metering NH3 into the combustible gas as soon as the temperature of the exhaust gas in the combustion device and in the exhaust gas duct exceeds a limit temperature T G lies, where the Bil- Removal of NH4NO3 deposits from NH3 and NO contained in the exhaust gas X is prevented.

[0042] The method according to the invention necessarily comprises steps (a) and (e). Preferably, the method according to the invention additionally comprises one or more further steps. The system according to the invention necessarily comprises a combustion device and an exhaust gas duct. Preferably, the system according to the invention additionally comprises one or more further devices.

[0043] Preferably, the exhaust gas channel is arranged downstream of the combustion device in the direction of exhaust gas flow.

[0044] Preferably, in addition to the combustion device and the exhaust gas channel, the system according to the invention comprises an exhaust gas treatment unit downstream of the exhaust gas channel and an outlet downstream of the exhaust gas treatment unit, and the method comprises the additional step: (c) Passing the exhaust gas through the exhaust gas treatment unit and through the outlet; wherein the addition of NH3 to the combustible gas takes place in step (e) as soon as the temperature of the exhaust gas at or after leaving the exhaust gas treatment unit exceeds the limit temperature T G lies.

[0045] For the purposes of description, the "outlet" preferably comprises devices and components arranged downstream of the exhaust gas treatment unit in the direction of exhaust gas flow and ultimately preferably intended to discharge the exhaust gas into the environment. The outlet thus preferably includes devices and components through which the exhaust gas flows after leaving the exhaust gas treatment unit. In a simple embodiment, the outlet comprises an opening with a nozzle. However, it is also possible for the outlet to include components such as valves (e.g., pressure relief valves), pipes, heat exchangers, expansion turbines, scrubbers, filters, etc., up to and including a chimney. The outlet is not part of the exhaust gas treatment unit, but separate from it.

[0046] Preferably, the exhaust gas channel is arranged downstream of the combustion device in the direction of exhaust gas flow, the exhaust gas treatment unit is arranged downstream of the exhaust gas channel and the outlet is arranged downstream of the exhaust gas treatment unit.

[0047] Preferably, the method according to the invention includes the additional step: (b) Addition of a reducing agent for NO X and / or for N2O into the exhaust gas downstream of the combustion device and preferably upstream of the exhaust gas treatment unit.

[0048] Preferably, the method according to the invention includes the additional step: (d) Determining the temperature of the exhaust gas.

[0049] Preferably, in step (d) of the method according to the invention, the temperature T is determined. A of the exhaust gas at or after leaving the exhaust gas treatment unit; and the addition of NH3 to the combustible gas in step (e) takes place as soon as the temperature T Athe limiting temperature T G has exceeded (T Ä > T G ).

[0050] Preferably, in step (d) of the method according to the invention, the determination is carried out by measuring the temperature T. Ä of the exhaust gas at or after leaving the exhaust gas treatment unit.

[0051] The invention preferably relates to a method for starting up a plant for the combustion of NH3, which comprises a combustion device, an exhaust gas channel, optionally an exhaust gas treatment unit and optionally an outlet, wherein the method comprises the following steps: (a) Burning a flammable gas in the combustion device, producing an exhaust gas and transferring the exhaust gas into the exhaust duct; (b) optional addition of a reducing agent for NO X and / or for N2O into the exhaust gas downstream of the combustion device and preferably upstream of the exhaust gas treatment unit; (c) optional, routing the exhaust gas through the exhaust gas treatment unit and through the outlet; (d) optional, determining the temperature of the exhaust gas; and (e) Metering NH3 into the combustible gas as soon as the temperature of the exhaust gas in the combustion device and in the exhaust gas duct exceeds a limit temperature T G lies in the process where the formation of NH4NO3 deposits occurs from NH3 and NO contained in the exhaust gas. X is prevented.

[0052] In preferred embodiments, the inventive method for starting up a plant for the combustion of NH3 comprises the following steps: (a) Burning a flammable gas in a combustion appliance, producing an exhaust gas; (b) optional, addition of a reducing agent for NO X and / or for N2O into the exhaust gas downstream of the combustion device; (c) Passing the exhaust gas through an exhaust gas treatment unit; (d) Measuring the temperature T Ä of the exhaust gas at or after leaving the exhaust gas treatment unit; (e) Adding NH3 to the flammable gas as soon as the measured temperature T Ä a limiting temperature T G has exceeded (T Ä > T G ).

[0053] For descriptive purposes, "and / or" means either "or" or "and", so that, for example, "A and / or B" has the following three meanings: (i) only A but not B, (ii) only B but not A, and (iii) both A and B.

[0054] For descriptive purposes, "NO" includes X "Nitrogen monoxide (NO) and nitrogen dioxide (NO2), but not nitrous oxide (N2O)."

[0055] For the purpose of description, catalysts accelerate certain chemical reactions by lowering their activation energies.

[0056] For descriptive purposes, "exhaust gas" can also be flue gas or hot gas.

[0057] Unless explicitly stated otherwise, all values ​​in ppm are volume-based, i.e., ppm v. Unless explicitly stated otherwise, all percentage values ​​with regard to gas composition are volume-based, i.e., vol.%. Unless explicitly stated otherwise, all other percentage values ​​are weight-based, i.e., wt.%. Unless explicitly stated otherwise, all pressure values ​​in bar mean absolute pressure (not gauge pressure).

[0058] Preferably, steps (a), (c), (d) and (e) of the inventive procedure are carried out in alphabetical order. Step (b) can be performed simultaneously with step (e) or after step (e). Mixed forms of partial simultaneity are also possible.

[0059] Further, unmentioned steps may occur between these steps.

[0060] Step (e) of the inventive procedure takes place as soon as the temperature of the exhaust gas in the combustion device and in the exhaust gas duct is above a limit temperature T G in which the formation of deposits of NH4NO3 from NH3 and NO contained in the exhaust gas X is prevented; preferably as soon as the temperature T Ä the limit temperature T of the exhaust gas at or after leaving the exhaust gas treatment unit G has exceeded (T Ä > T G ).

[0061] Step (b) of the inventive procedure is preferably carried out after the temperature T Ä the limit temperature T of the exhaust gas at or after leaving the exhaust gas treatment unit G has exceeded (T A > T G ), i.e., step (b) is preferably carried out as soon as the temperature T A the limiting temperature T G has exceeded (T Ä > T G ) or at a later date.

[0062] The inventive method is a method for starting up a plant for the combustion of NH3.

[0063] The preferred system is for the combustion of NH3. - in an operating mode (normal operation) at operating temperature T B operable and - in a start-up mode (start-up operation) to bring the NH3 combustion system, preferably the exhaust gas treatment unit, up to operating temperature T B to warm up.

[0064] During the start-up mode, the temperature of the exhaust gas in the combustion unit and in the exhaust duct is adjusted from the standstill state and the corresponding standstill temperature T. o increased, preferably continuously, then passes through the limiting temperature T G , before they finally went into operation- temperature T B reached (T o T G T B ). Upon reaching the operating temperature T BThe start-up mode ends and the operating mode begins.

[0065] The temperature of the exhaust gas at different points in the combustion unit and in the exhaust duct typically does not have the same quantitative value at any given time; rather, each point in the combustion unit and in the exhaust duct goes through an individual temperature profile with individual local temperatures during the start-up phase. If the system according to the invention includes an exhaust gas treatment unit, then the question of whether the limit temperature T G has already been reached or not yet, preferably the temperature of the exhaust gas T A the decisive factor when leaving the exhaust gas treatment unit and the quantitative value for the limit temperature T specified for this point in the system G .

[0066] According to the invention, other temperature levels are relevant which are passed through during the start-up mode and which are above the limit temperature T. G lie or the limit temperature T G can correspond, in particular the temperature T R (NO X ), T R (N2O) and T Z (N2O). These temperatures are particularly relevant for the reactions taking place in the exhaust gas treatment unit to reduce the NO content. X or N2O in the exhaust gas.

[0067] According to the invention, T R (NO X ) the start-up temperature for the catalytic chemical reduction of NO X with reducing agent for NO X under the given conditions, in particular the type of reducing agent for NO X and type of NO x -reduction catalyst. Typically, T o < T G < T R (NO X ) < T B .

[0068] According to the invention, T R (N2O) the start-up temperature for the catalytic chemical reduction of N2O with reducing agent for N2O under the given conditions, in particular the type of reducing agent for N2O and the type of N2O reduction catalyst. Typically, T o < T G < T R (N2O) < T B .

[0069] According to the invention, T Z (N2O) the start-up temperature for the catalytic decomposition of N2O under the given conditions, in particular the type of N2O decomposition catalyst. Typically, T o < T G < T Z (N2O) < T B .

[0070] The NH3 combustion system preferably operates at an operating temperature T B This is only reached when the temperature of the exhaust gas in the combustion unit and in the exhaust gas duct is significantly above the limit temperature T. GThe system for burning NH3 preferably operates at an operating temperature T. B only reached when the temperature of the exhaust gas in the combustion unit, in the exhaust gas duct, and in the exhaust gas treatment unit is significantly above the limit temperature T G nurtures.

[0071] According to the invention, the start-up mode can be divided into a first section and a second section. In the first section of the start-up mode, the temperature of the exhaust gas in the combustion unit and in the exhaust gas duct is still below the limit temperature T. G , in the second section- The cut of the start-up mode means that the temperature of the exhaust gas in the combustion unit and in the exhaust duct has already reached the limit temperature T. G reached or exceeded, but is still below the operating temperature T B , so that the start-up mode is not yet complete: T o first section of the starting mode' T Gsecond section of the starting mode' T B .

[0072] In the first phase of the start-up mode, the exhaust gas temperature preferably has not yet reached the limit temperature T. G The limit is exceeded and preferably no NH3 is added to the combustible gas, i.e., the combustible gas preferably contains no NH3. In the first section of the start-up mode, i.e., preferably before the addition of NH3 to the combustible gas, the combustion of NH3 preferably does not occur.

[0073] In the second phase of the start-up mode and in the subsequent operating mode, the exhaust gas temperature preferably reaches the limit temperature T. G The limit is exceeded and NH3 is preferably added to the combustible gas, i.e., the combustible gas preferably contains NH3. In the second part of the start-up mode and in the subsequent operating mode, i.e., preferably after the addition of NH3 to the combustible gas, the combustion of NH3 preferably takes place.

[0074] The combustion of NH3 means the oxidation of NH3 with O2, whereby this reaction need not be complete according to the invention, so that the exhaust gas may contain residual, unburned (unoxidized, unreacted) NH3 (NH3 slip, NH3 breakthrough). The same applies if NH3 is not burned in pure form, but together with other combustible gases, in particular H2 and / or CH4 (natural gas), C3H8, or C4H8. The O2 used for combustion can be in the form of combustion air, which may optionally be enriched with O2.

[0075] In step (a) of the inventive process, a flammable gas is burned in a combustion device, producing an exhaust gas (hot gas).

[0076] Preferably, the plant for the combustion of NH3 is an NH3-operated combustion plant, preferably an NH3-driven internal combustion engine, an NH3-driven gas turbine, or a furnace for splitting NH3 into N2 and H2. Preferably, the combustion device according to the invention is part of the combustion plant.

[0077] "Combustion plants" within the meaning of the invention generate heat through combustion processes. Preferably, the combustion plants comprise a combustion device for burning NH3 or an NH3-containing combustible gas. Heat is generated by burning combustible gas. "Combustion plants" or the "combustion devices" included therein within the meaning of the invention are any plants in which NH3 or an NH3-containing fuel is oxidized with O2 (preferably from combustion air) with the aim of producing, in particular, N2 and H2O as the main products. Plants in which NH3 is oxidized with O2 with the aim of producing nitrogen compounds with higher oxidation states (e.g., NO3) as the main products are also included. X ), as is the case, for example, with the In the production of nitric acid, neither combustion plants nor combustion devices are required within the meaning of the invention. Catalysts are usually necessary for the production of such nitrogen compounds with high oxidation states as the main products. According to the invention, the combustion plant is preferably not equipped with a catalyst; that is, the combustion of NH3 or an NH3-containing flammable gas and optionally H2 and / or CH4, C3H8, or C4H8 is preferably uncatalyzed.

[0078] In preferred embodiments, a flammable gas is combusted in a combustion system comprising a combustion unit for burning the flammable gas and an NH3 decomposition unit for cracking NH3 into N2 and H2. In the second phase of the start-up mode and in the subsequent operating mode, the combustion of the flammable gas, preferably NH3 or a flammable gas containing NH3, in the combustion unit serves to heat the NH3 decomposition unit, preferably a catalyst-filled reactor, for cracking NH3 into N2 and H2. According to the invention, the cracking of NH3 into N2 and H2 is carried out by the catalytic decomposition of NH3 over an NH3 decomposition catalyst.

[0079] According to the invention, the catalytic decomposition of NH3 means the formation of N2 and H2, occasionally also referred to in the prior art as "cleavage" or "cracking". The terms "catalytic decomposition", "decomposition", "catalytic cleavage", "cleavage", "catalytic cracking", and "cracking" of NH3 are used as interchangeable synonyms for descriptive purposes. According to the invention, the catalytic decomposition of NH3 preferably occurs in the absence of O2.

[0080] In these preferred embodiments, the combustion system according to the invention preferably comprises a combustion unit and an NH3 decomposition unit, which are in heat exchange with each other. In the second phase of the start-up mode and in the subsequent operating mode, the combustible gas, preferably NH3 or NH3-containing combustible gas containing NH3 and preferably H2, is combusted in the combustion unit together with combustion air containing O2, generating heat of combustion. The heat of combustion thus generated is at least partially supplied as a heat flow to the NH3 decomposition unit (or transferred to the NH3 decomposition unit) in order to provide the heat required for the endothermic catalytic decomposition of NH3 to H2 and N2. Furthermore, according to the invention, the heat of combustion is preferably used to preheat the NH3 to be decomposed, the combustible gas, and the combustion air to an elevated temperature.

[0081] Preferably, the combustion device according to the invention is integrated into a system for the thermal and / or catalytic decomposition of NH3 into N2 and H2.

[0082] Preferably, the combustion plant according to the invention (preferably comprising combustion device and NHβ decomposition device) is integrated into a plant for the thermal and / or catalytic decomposition of NH3 into N2 and H2.

[0083] In other preferred embodiments, the combustible gas is burned to power an internal combustion engine.

[0084] "Internal combustion engines" (heat engines) within the meaning of the invention are in particular internal combustion engines, preferably piston heat engines with internal combustion, such as reciprocating piston engines or rotary piston engines.

[0085] In other preferred embodiments, the combustible gas is burned to drive a gas turbine.

[0086] "Gas turbines" within the meaning of the invention are in particular internal combustion engines in which a hot gas flow is generated, which can be used, for example, to generate (mechanical) rotational energy by means of a hot gas expansion turbine.

[0087] Preferably, the combustion of the flammable gas according to the invention takes place in the combustion device to generate heat. The combustion device preferably has one or more burners, preferably at least two burners, more preferably at least three burners.

[0088] Preferably, the combustion of the flammable gas according to the invention, preferably the oxidation of NH3 with O2 (or of the mixture of NH3 with another flammable gas, such as H2, CH4, etc.) is carried out non-catalytically, i.e. the combustion is not carried out in the presence of a heterogeneous catalyst.

[0089] In preferred embodiments, in the first stage of the start-up mode, H₂ and / or hydrocarbons, preferably CH₄, C₃H₈, or C₄H₈, are burned as the sole fuel; that is, in addition to H₂ and / or hydrocarbons, preferably CH₄, C₃H₈, or C₄H₈, preferably no NH₃ is burned. Besides H₂ and / or hydrocarbons, preferably CH₄, C₃H₈, or C₄H₈, the combustible gas may optionally contain other components, e.g., N₂.

[0090] In the first section of the start-up mode, i.e. preferably before the addition of NH3 to the combustible gas, in step (a) the combustion of a combustible gas which does not contain NH3 preferably takes place.

[0091] Preferably the flammable gas in step (a) in the first section of the start-up mode', i.e. preferably before the addition of NH3 to the flammable gas, contains H2 and / or hydrocarbon; preferably H2 and / or CH4 (natural gas), C3H8, or C4H8.

[0092] In preferred embodiments, the flammable gas in step (a) in the first section of the start-up mode, i.e. preferably before the addition of NH3, contains at least 50 mol% H2 and / or hydrocarbon; preferably at least 60 mol%, more preferably at least 70 mol%, even more preferably at least 80 mol%, most preferably at least 90 mol%, and in particular at least 95 mol%.

[0093] In preferred embodiments, the flammable gas in step (a) in the first section of the start-up mode, i.e. preferably before the addition of NH3 to the flammable gas, consists essentially of H2 and / or hydrocarbons; preferably H2 and / or CH4, C3H8, or C4H8.

[0094] In preferred embodiments, the flammable gas in step (a) in the first section of the start-up mode, i.e. preferably before the addition of NH3 to the flammable gas, contains at least 50 mol% H2; preferably at least 60 mol%, more preferably at least 70 mol%, more preferably at least 80 mol%, most preferably at least 90 mol%, and in particular at least 95 mol%.

[0095] In preferred embodiments, the flammable gas in step (a) in the first section of the start-up mode, i.e. preferably before the addition of NH3 to the flammable gas, consists essentially of H2.

[0096] In preferred embodiments, the flammable gas in step (a) in the first section of the start-up mode, i.e. preferably before the addition of NH3 to the flammable gas, contains at least 50 mol% hydrocarbon, preferably CH4, C3H8, or C4H8; preferably at least 60 mol%, more preferably at least 70 mol%, even more preferably at least 80 mol%, most preferably at least 90 mol%, and in particular at least 95 mol%.

[0097] In preferred embodiments, the flammable gas in step (a) in the first section of the start-up mode, i.e. preferably before the addition of NH3 to the flammable gas, consists essentially of hydrocarbons, preferably CH4, C3H8, or C4H8.

[0098] In preferred embodiments, the flammable gas in step (a) in the first section of the start-up mode, i.e. preferably before the addition of NH3 to the flammable gas, contains at most 10 mol% NH3; preferably at most 3.0 mol%, more preferably at most 1.0 mol%, even more preferably at most 0.1 mol%, most preferably at most 0.01 mol%, and in particular at most 0.001 mol%.

[0099] In preferred embodiments, the flammable gas in step (a) in the first section of the start-up mode, i.e. preferably before the addition of NH3 to the flammable gas, contains at most 0.08 mol% NH3; preferably at most 0.06 mol%, more preferably at most 0.04 mol%, even more preferably at most 0.02 mol%, most preferably at most 0.01 mol%, and in particular at most 0.005 mol%.

[0100] In preferred embodiments, the combustible gas in step (a) in the first section of the start-up mode', i.e. preferably before the addition of NH3, contains no NH3.

[0101] When the mixture of combustible gas and combustion air is burned, the air-fuel ratio X for combustion is preferably in the range of 0.9 to 3.3, more preferably 1.0 to 2.8, even more preferably 1.1 to 2.3, and most preferably 1.2 to 1.5.

[0102] When the mixture of combustible gas and combustion air is burned, the air-fuel ratio X for combustion is preferably in the range of 0.9 to 1.7, more preferably 1.0 to 1.6, even more preferably 1.1 to 1.5, and most preferably 1.2 to 1.4. Another highly preferred range for the air-fuel ratio X lies between 1.0 and 1.2.

[0103] In particularly preferred embodiments, the air ratio X is in the range of 1.06±0.06, preferably 1.06±0.05, more preferably 1.06±0.04, even more preferably 1.06±0.03, most preferably 1.06±0.02, and in particular 1.06±0.01.

[0104] The air-fuel ratio X (i.e., the combustion air ratio) indicates the mass ratio of combustion air to combustible gas 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.

[0105] The combustion of the combustible gas in the combustion device in step (a) of the inventive process produces an exhaust gas. The composition of the exhaust gas formed during combustion depends on the combustible gas used.

[0106] In preferred embodiments, the exhaust gas in the first section of the start-up mode, i.e., preferably before the addition of NH3, comprises the combustible gas, NO. X , N2O, N2and H2O.

[0107] Preferably, the exhaust gas in step (a) in the first section of the start-up mode, i.e. preferably before the addition of NH3 to the combustible gas, contains at most 10 ppmv NH3; preferably at most 5 ppmv, more preferably at most 1 ppmv, even more preferably at most 0.5, most preferably at most 0.1 ppmv, and in particular at most 0.05 ppmv.

[0108] Preferably the exhaust gas contains no NH3 in the first section of the start-up mode, i.e. preferably before the addition of NH3 to the combustible gas.

[0109] Preferably, the exhaust gas does not contain both NH3 and NO simultaneously. X in the first section of the start-up mode, i.e. preferably before the addition of NH3 to the combustible gas; preferably before the exhaust gas temperature exceeds the limit temperature T G lies.

[0110] In preferred embodiments, the exhaust gas comprises further gaseous components; preferably selected from the group consisting of O2, CO, CO2, CH4 and mixtures thereof.

[0111] In preferred embodiments, the exhaust gas has an H2O content of more than 4.0 vol.%; preferably at least 5.0 vol.%, more preferably at least 6.0 vol.%, even more preferably at least 7.0 vol.%, most preferably at least 8.0 vol.%, and in particular at least 9.0 vol.%.

[0112] In further preferred embodiments, the exhaust gas has an H2O content of at least 10 vol.%; preferably at least 12 vol.%, more preferably at least 14 vol.%, even more preferably at least 16 vol.%, most preferably at least 18 vol.%, and in particular at least 20 vol.%.

[0113] Preferably, the exhaust gas has a pressure of at most 9.0 bar upon leaving the combustion unit; more preferably at most 7.0 bar, more preferably at most 5.0 bar, even more preferably at most 3.0 bar, and most preferably at most 1.5 bar; preferably atmospheric pressure. In further particularly preferred embodiments, the exhaust gas has a negative pressure of up to -20 mbar g upon leaving the combustion unit, more preferably up to -10 mbar g, and particularly down to -5 mbar g.

[0114] Preferably, step (a) of the method according to the invention comprises the additional sub-step: (a2) Cooling of the exhaust gas.

[0115] In preferred embodiments, the exhaust gas is cooled in at least one cooling device, preferably a heat exchanger, which is arranged downstream of the combustion device and preferably upstream of the exhaust gas treatment unit in the direction of exhaust gas flow.

[0116] In other preferred embodiments, the cooling of the exhaust gas may be carried out additionally by a heat exchanger by performing or converting thermal energy into mechanical work.

[0117] Invented heat exchangers serve to transfer heat from one medium to another medium without the media being mixed together.

[0118] The exhaust gas leaves the combustion unit at a temperature Ti and is preferably cooled to a temperature T2 in step (a2), at which the exhaust gas is then subsequently fed into the exhaust gas treatment system. The unit of action is transferred. Preferably, at least one cooling device, and more preferably at least one heat exchanger, is arranged in the exhaust gas duct.

[0119] In preferred embodiments, a single heat exchanger is arranged downstream of the combustion device in the direction of exhaust gas flow, in which the exhaust gas is cooled.

[0120] In other preferred embodiments, at least two heat exchangers are arranged downstream of the combustion device in the direction of exhaust gas flow, in which the exhaust gas is cooled successively.

[0121] In further preferred embodiments, at least three heat exchangers are arranged downstream of the combustion device in the direction of exhaust gas flow, in which the exhaust gas is cooled successively.

[0122] The cooling of the exhaust gas in the at least one heat exchanger is achieved by transferring heat from the exhaust gas to a heat transfer medium.

[0123] According to the invention, NH3 preferably serves as the heat transfer medium, which is then subsequently fed to the catalytic decomposition in an NH3 decomposition device at an NH3 decomposition catalyst.

[0124] In step (a2) the exhaust gas is preferably cooled to a temperature T2, at which the exhaust gas is then subsequently transferred to the exhaust gas treatment unit.

[0125] In step (b) of the inventive method, a reducing agent for NOx and / or for N2O is preferably added to the exhaust gas downstream of the combustion device and preferably upstream of the exhaust gas treatment unit.

[0126] Preferably, step (b) is only carried out after the temperature of the exhaust gas has exceeded the limit temperature T. Ghas exceeded. In preferred embodiments, the reducing agent for NOx and / or for N2O is added to the exhaust gas downstream of the combustion unit simultaneously with the addition of NH3 to the combustible gas as soon as the temperature of the exhaust gas exceeds the limit temperature T. G has exceeded; preferably in a time-dependent manner with respect to the addition of NH3 to the combustible gas, i.e., in close temporal proximity (promptly), i.e., shortly before or after the addition of NH3 to the combustible gas, preferably ±10 min, ±5 min, ±2 min. In other preferred embodiments, the reducing agent for NO is added X and / or for N2O into the exhaust gas downstream of the combustion device at a later time and not simultaneously with the addition of NH3 into the combustible gas, as soon as the temperature of the exhaust gas reaches the limit temperature T G has exceeded.

[0127] Preferably the reducing agent in step (b) is selected from NH3, hydrocarbons, CO, H2 and mixtures thereof; preferably NH3, CO, H2 and mixtures thereof; more preferably NH3.

[0128] Preferably, the reducing agent in step (b) is NH3, which is used in an amount of 0.9 to 2.5 molar parts, preferably 1.0 to 1.4 molar parts, more preferably 1.0 to 1.2 molar parts, based on a molar part of NO to be chemically reduced. X .

[0129] In step (c) of the method according to the invention, the exhaust gas is preferably passed through an exhaust gas treatment unit and an outlet.

[0130] The exhaust gas treatment unit is preferably arranged downstream of the combustion device and, if applicable, of the at least one heat exchanger in the direction of exhaust gas flow.

[0131] The outlet is preferably arranged downstream of the combustion device, possibly the at least one heat exchanger and the exhaust gas treatment unit, in the direction of exhaust gas flow.

[0132] The exhaust gas is preferably released into the atmosphere through the outlet after leaving the exhaust gas treatment unit.

[0133] Preferably in step (c) of the inventive method, the reduction of the N2O and / or NO content takes place preferably in the second section of the start-up mode and in the subsequent operating mode, i.e. preferably after the addition of NH3 to the flammable gas. X in the exhaust gas.

[0134] Preferably, step (c) of the method according to the invention comprises one or more of the following sub-steps: (ci) Reducing the N2O content in the exhaust gas by (ci. a ) Decomposition of N2O on an N2O decomposition catalyst and / or (ci.b) chemical reduction of N2O with reducing agent at an N2O reduction catalyst; (c2) Reducing the NO content X in the exhaust gas through chemical reduction of NO X with reducing agent at an NO x -Reduction catalyst.

[0135] In preferred embodiments, the exhaust gas treatment unit according to the invention comprises - an N2O reduction catalyst; - an N2O decomposition catalyst; and - a NO x -Reduction catalyst; which may be the same or different depending on the given functionality or multiple functionality and may be present in common or separate reaction zones (preferably catalyst beds).

[0136] In preferred embodiments, the exhaust gas treatment unit according to the invention additionally comprises at least one further catalyst or fulfills one of the aforementioned N2O reduction, N2O decomposition or NO processes.x -Reduction catalyst at least one further functionality selected from NHβ oxidation catalyst; HCN degradation catalyst; and CO oxidation catalyst.

[0137] The NH3 oxidation catalyst is preferably used when the proportion of unburned NH3 in the exhaust gas (NH3 slip) in the second part of the start-up mode and in the subsequent operating mode, i.e. preferably after the addition of NH3 to the combustible gas, is greater than the requirement for NH3 as a reducing agent for NO. X and / or for N2O in the exhaust gas treatment unit, so that the exhaust gas after passing through the steps (ci. a ) and / or (ci. b ) and (c2) still contain residual amounts of NH3, which should not or must not be released into the environment. These residual amounts of NH3 can then be reduced by oxidation of NH3 using the downstream NH3 oxidation catalyst.

[0138] If the exhaust gas treatment unit according to the invention additionally comprises at least one further catalyst or if one of the aforementioned N2O reduction, N2O decomposition or NOx reduction catalysts fulfills at least one further functionality, the exhaust gas treatment unit according to the invention preferably additionally performs at least one of the following steps (f4) to (f4): (fi) Cooling the exhaust gas in at least one heat exchanger, which is preferably arranged within the exhaust gas treatment unit; preferably in the direction of exhaust gas flow upstream of the NH3 oxidation catalyst; (f2) Reducing the NH3 content in the exhaust gas by oxidation with an oxidizing agent on an NH3 oxidation catalyst; wherein the oxidizing agent preferably comprises O2; (f3) Reducing the HCN content in the exhaust gas by hydrolysis and oxidation of the hydrolysates with an oxidizing agent on an HCN degradation catalyst; wherein the oxidizing agent preferably comprises NOx and / or N2O; and (f4) Reducing the CO content in the exhaust gas by chemical oxidation with an oxidizing agent on a CO oxidation catalyst; wherein the oxidizing agent preferably comprises O2.

[0139] Preferably in step (c) of the inventive method, preferably in the second section of the start-up mode and in the subsequent operating mode, i.e. preferably after the addition of NH3 to the combustible gas, the reduction of the N2O content in the exhaust gas takes place.

[0140] This can be done by (ci.g. a ) Decomposition of N2O on an N2O decomposition catalyst and / or (ci. b ) Chemical reduction of N2O with reducing agent at an N2O reduction catalyst.

[0141] Preferably, in the second phase of the start-up mode, one waits until the exhaust gas temperature reaches temperature T. R (N2O) has reached, which is typically above the limit temperature- temperature T G The catalytic chemical reduction of N2O may require the addition of a reducing agent for N2O, SO, which, according to the invention, is preferably only added to the exhaust gas after the temperature of the exhaust gas has reached the temperature T. R (N2O). For example, the chemical reduction of N2O with hydrocarbons on iron-loaded zeolites typically requires a minimum temperature of approximately 300°C.

[0142] For the potentially parallel catalytic decomposition of N2O, in the second part of the start-up mode, it is preferably waited until the temperature of the exhaust gas reaches the temperature T. Z (N2O) has reached, which is typically above the limiting temperature T GThe catalytic decomposition of N2O, unlike the catalytic chemical reduction of N2O, requires no further measures, in particular no addition of reducing agent for N2O, but begins or becomes noticeable as soon as the temperature of the exhaust gas reaches the temperature T. Z (N2O) has been reached.

[0143] During the decomposition of N2O, N2 and O2 are formed according to the following overall reaction: 2 N2O 2 N2+ O2.

[0144] Decomposition of N₂O therefore means decomposition into N₂ and O₂. An "N₂O decomposition catalyst" according to the invention catalyzes the decomposition of N₂O. The achievable reduction of N₂O through catalytic decomposition depends not only on the type, i.e., the chemical nature and physical design of the N₂O decomposition catalyst and the prevailing pressure and temperature conditions, but also, and perhaps most importantly, on the selected space velocity, i.e., the ratio of exhaust gas volume flow to catalyst volume. However, the catalytic activity of an N₂O decomposition catalyst need not be limited exclusively to this reaction. It is quite possible, and indeed preferred according to the invention, for the N₂O decomposition catalyst to also catalyze other reactions, for example, the chemical reduction of N₂O and / or the chemical reduction of NO. XWhether such further conversions actually take place depends on the conditions of the individual case, in particular the type of catalyst, and the kinetics of any parallel processes, for example the presence or quantity of the reducing agent and the presence or quantity of other reactants.

[0145] In the chemical reduction of N2O with a reducing agent, different reaction products are formed depending on the reducing agent.

[0146] In the case of the reducing agent NH3, which is preferred according to the invention, N2 and H2O are formed in particular during the chemical reduction of N2O, e.g. according to: 3 N2O + 2 NH34 N2 + 3 H2O or 4 N2O + 4 NH3 + O2 6 N2 + 6 H2O or also in the joint reduction with NO according to 2NO + N2O + 2 NH33 N2+ 3 H2O.

[0147] In the case of hydrocarbons, which are also preferred as reducing agents according to the invention, CO and H2O are formed in particular during the chemical reduction of N2O, e.g. according to (2n+l) N2O + C n H 2n+2 (2n+l) N2+ n CO + (n+1) H2O or also CO2 and H2O according to 4n N2O + C n H 2n+2 - 4n N2+ n CO2+ 2n H2O.

[0148] According to the invention, CO is also preferred as a reducing agent. It can react further with N2O to form CO2, e.g. according to: N2O + CO N2+ co2.

[0149] An "N2O reduction catalyst" according to the invention catalyzes the chemical reduction of N2O with a reducing agent. However, the catalytic activity of an N2O reduction catalyst need not be limited exclusively to this reaction. It is quite possible, and indeed preferred according to the invention, that the N2O reduction catalyst can also catalyze other reactions, for example, the decomposition of N2O and / or the chemical reduction of NO. X Whether such further conversions actually take place depends on the conditions of the individual case, in particular the type of catalyst, and the kinetics of any parallel processes, for example the presence or quantity of the reducing agent and the presence or quantity of other reactants.

[0150] Preferably in step (c2) of the inventive process, the reduction of the NO content preferably takes place in the second section of the start-up mode and in the subsequent operating mode, i.e. preferably after the addition of NH3 to the flammable gas. X in the exhaust gas through chemical reduction of NO X with reducing agent at an NO x -Reduction catalyst.

[0151] Preferably, such NOs are used. x -Reduction catalysts, which preferably enable the selective catalytic reduction (SCR) of the nitrogen oxides contained in the exhaust gas, especially NO X enable, i.e. the NO x -Reduction catalysts primarily catalyze the oxidation of NH3 with NO X and not, or only secondarily, the oxidation of NH3 with any free oxygen (O2) that may be present in the exhaust gas.

[0152] During the chemical reduction of NO XDifferent reaction products are formed with reducing agents, depending on the reducing agent. In the case of the preferably used reducing agent NH3, the chemical reduction of NO yields different products. X especially N2 and H2O are formed, depending on the type of NO x -reduction catalyst and the ratio of NO to NO2, e.g. according to: 4 NH3+ 2 NO + 2 NO2N2+ 6 H2O (so-called almost SCR) 4 NH3+ 4 NO + O24 N2+ 6 H2O (so-called normal SCR) 8 NH3+ 6 NO27 N2+ 12 H2O (so-called NO2SCR).

[0153] The combined selective catalytic reduction is called fast SCR and generally proceeds much faster than the so-called normal SCR or NO2SCR.

[0154] A "NOx reduction catalyst" according to the invention catalyzes the chemical reduction of NO X with reducing agent. The catalytic activity of NO xHowever, the reduction catalyst need not be limited exclusively to this conversion. It is quite possible, and indeed preferred according to the invention, that the NO x -Reduction catalyst can also catalyze other reactions, for example the decomposition of N2O, the chemical reduction of N2O and / or the adjustment of NO x -Equilibrium 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.

[0155] The exhaust gas treatment unit according to the invention preferably comprises at least one catalyst; preferably an N2O decomposition catalyst and / or an N2O reduction catalyst and / or an NO x-Reduction catalyst.

[0156] The N2O decomposition catalysts, N2O reduction catalysts, and NO according to the invention x Reduction catalysts preferably contain, independently of one another, zeolitic materials (for descriptive purposes also referred to as "zeolites") loaded with at least one transition metal (atomic numbers 21-30, 39-48, 57-80, 89-112) and / or with at least one lanthanide (also called "lanthanide"; atomic numbers 57-71). For descriptive purposes, transition metals and lanthanides are collectively referred to as "transition metals." Iron ("Fe zeolites"), copper ("Cu zeolites"), and cobalt ("Co zeolites") are preferred transition metals. Iron-loaded zeolitic materials (i.e., Fe zeolites) are particularly preferred and may also be loaded with or contain other transition metals in addition to iron, for example, manganese, vanadium, chromium, nickel, or mixtures thereof.

[0157] The zeolitic materials according to the invention preferably exhibit high hydrothermal resistance. Particularly preferred are SiO2-rich zeolites, so-called "high-silica zeolites", which have a molar ratio of [SiO2] to [AlO2] units, and thus a molar Si / Al ratio, of at least 8, preferably at least 9, more preferably at least 10, even more preferably at least 11, most preferably at least 12, and particularly at least 13.

[0158] According to the invention, preferred zeolitic materials essentially have a zeolite structure of structure type BEA, MFI, MOR, MEL, or FER, preferably of structure type MFI and BEA, and even more preferably of structure type BEA. Of structure type MFI, type ZSM-5 is particularly preferred. Further details regarding the designation of the structure types of zeolitic materials and their The structure can be found in the Atlas of Zeolite Structure Types, Elsevier, 4th revised Edition 1996.

[0159] According to the invention, particularly preferred N2O decomposition, N2O reduction or NO x Reduction catalysts contain, independently of one another, at least 50 wt% Fe-zeolite by weight of the total weight of the zeolitic material, preferably at least 70 wt% Fe-zeolite, wherein a single structure type or several structure types may be present. In preferred embodiments, in addition to Fe-BEA zeolite, another Fe-zeolite of a different structure type is present, preferably Fe-MOR zeolite.

[0160] The loading (doping) of the zeolitic materials with transition metals can be carried out according to relevant methods for loading or doping zeolites with transition metals, which are known to those skilled in the art. Preferably, the loading is carried out starting from the commercially available H-form or, more preferably, the NH4-form of the zeolitic materials by ion exchange with corresponding salts of the transition metals, either in aqueous phase or by solid-state reaction. The loaded zeolitic materials thus obtained are then calcined, preferably in air in an oven at temperatures in the range of 400 to 650°C. After calcination, the loaded zeolitic materials are washed intensively in distilled water, and the filtered loaded zeolitic materials are then dried.Preferably, the loaded zeolitic materials thus obtained are mixed with suitable binders, such as aluminosilicates, boehmite, or silica sol, and optionally with additives for plasticization or for the production of slurries. In preferred embodiments, the mixtures thus obtained are extruded into catalyst bodies (complete catalysts) and subsequently calcined. In other preferred embodiments, the mixtures thus obtained are mounted onto catalyst supports (supported catalysts) and subsequently calcined. These methods are also known to those skilled in the art and are established in many technical applications.

[0161] The N2O decomposition, N2O reduction, NO reactions according to the invention xReduction, NH3 oxidation, HCN degradation, and CO oxidation catalysts can be independently available as shaped bodies of any size and geometry, preferably in geometries that have a large surface area to volume ratio and through which the pressure drop is minimized. Typical geometries include all those known in catalysis, such as cylinders, hollow cylinders, multi-hole cylinders, rings, trilobes, or star-shaped strands. Particularly preferred are monolithic catalyst elements with parallel channels, e.g., monolithic honeycomb structures, so-called "catalyst honeycombs," as known, for example, from the purification or denitrification of power plant or automotive exhaust gases.

[0162] Preferably, the exhaust gas treatment unit according to the invention or the catalyst beds comprised therein have monolithic honeycomb bodies, so-called catalyst honeycombs, preferably several catalyst honeycombs that are parallel to each other with each oriented longitudinally to the flow direction of the exhaust gas. The catalyst honeycomb channels are arranged within the exhaust gas treatment unit. The geometry of the cross-sectional area of ​​the catalyst honeycomb (perpendicular to the exhaust gas flow direction) is, in principle, freely selectable. Preferably, the catalyst honeycomb has a rectangular or, in particular, square cross-sectional area; however, other cross-sectional areas are also possible, especially hexagonal, triangular, trapezoidal, etc. Suitable geometries are known to those skilled in the art. Therefore, according to the invention, the term "honeycomb" is not limited to a rectangular or square cross-sectional area.

[0163] If the exhaust gas treatment unit according to the invention comprises a first reaction zone (preferably a first catalyst bed) and a spatially separated second reaction zone (preferably a second catalyst bed) downstream in the direction of exhaust gas flow, which is preferred according to the invention, the first and second reaction zones (preferably the first and second catalyst beds) preferably each have several catalyst honeycombs arranged parallel to each other with honeycomb channels in the channel of the exhaust gas treatment unit, each aligned longitudinally to the direction of exhaust gas flow.

[0164] In preferred embodiments, several catalyst honeycombs, i.e., several monolithic honeycomb bodies, are combined into a honeycomb module, preferably by a metal frame that is open in the direction of exhaust gas flow. Preferably, two, four, or six honeycomb bodies, preferably monolithic honeycomb bodies, are combined into a honeycomb module. This modular design allows for efficient utilization of the available cross-sectional area of ​​the exhaust gas treatment unit's channel and facilitates the easy replacement of defective or deactivated honeycomb bodies.

[0165] The honeycomb structure preferably has a rectangular cross-section. Preferably, the rectangular cross-section has a first edge length (perpendicular to the exhaust gas flow direction) in the range of 5 to 20 cm, more preferably 10 to 15 cm, and a second edge length (also perpendicular to the exhaust gas flow direction) in the range of 5 to 20 cm, more preferably 10 to 15 cm. The height of a honeycomb structure (in the direction of exhaust gas flow) is preferably in the range of 5 to 25 cm, more preferably in the range of 7.5 to 15 cm.

[0166] The so-called cell density, i.e., the density of channels in a single catalyst honeycomb, is preferably 150 to 500 cpsi, more preferably 180 to 450 cpsi (cells per square inch). 100 cpsi, i.e., 100 cells or honeycomb channels per square inch, corresponds to approximately 15.5 catalyst channels per cm². 2 .

[0167] Preferably, the individual honeycomb modules are stacked one above the other and next to each other in the direction of flow and fixed by suitable holding devices in such a way as to achieve the best possible utilization of the inflow area, i.e., the cross-sectional area of ​​the exhaust gas treatment unit's channel. Bypass flows between the individual honeycomb modules or in the outer edge region between the outer edge of the honeycomb modules and the inner wall of the exhaust gas treatment unit's channel should be avoided. Suitable sealing devices are preferably used for this purpose. Materials are applied between the individual honeycomb modules and between the outer honeycomb modules and the inner wall. For larger wall distances, cover plates are used, which are attached to the inner wall of the exhaust gas treatment unit's channel in the flow direction, upstream and / or downstream of the honeycomb module packing. Preferably, the cover plates are fitted with seals at the contact points with the honeycomb modules. Preferably, the honeycomb modules are arranged and sized such that the usable flow area at the catalyst is preferably at least 60% of the inner cross-sectional area of ​​the exhaust gas treatment unit's channel, more preferably at least 70%, and even more preferably at least 80%.

[0168] In circular exhaust ducts or exhaust pipes, the gaps formed at the edges of the honeycomb module packing are preferably not filled with specially cut honeycomb modules, but rather sealed with blanking plates, unless these gaps can be easily filled with rectangular honeycomb modules. This has the advantage that when replacing worn honeycomb modules, only standardized modules need to be exchanged and no special modifications are required.

[0169] When using exhaust gas pipelines, it is preferable to use individual, larger honeycomb bodies adapted to the pipeline cross-section with a circular inflow cross-section, of which several can be arranged one behind the other in the flow direction in a preferred embodiment. In this case, it is not necessary to combine several honeycomb bodies parallel to each other to form honeycomb modules. The honeycomb bodies adapted to larger pipeline cross-sections preferably have a metallic support structure (metallic substrate).

[0170] In preferred embodiments, the honeycomb bodies or honeycomb body modules are arranged in several layers offset along the longitudinal axis in the direction of exhaust gas flow. Preferably, the honeycomb bodies or honeycomb body modules are arranged in 2 to 5 layers, particularly preferably in 2 to 3 layers. A gap is preferably provided between the layers, i.e., between the end faces of the honeycomb bodies or honeycomb body modules, preferably in the range of 3 to 30 mm, more preferably 4 to 20 mm. This gap allows for intermediate, particularly radial, mixing of the gas stream exiting a first layer of the honeycomb bodies or honeycomb body modules. Furthermore, it prevents any potential slippage of unreacted reducing agent and / or its not yet fully oxidized reaction products from the first layer of honeycomb bodies into a subsequent, second layer of honeycomb bodies.

[0171] The supply and distribution of the reducing agents for NO X and, if applicable, N2O is preferably introduced via a multiply branched piping system equipped with a multitude of openings or nozzles, which is arranged in the channel of the exhaust gas treatment unit or in the exhaust gas line in the direction of flow upstream of the respective catalyst bed, preferably the packing of the catalyst honeycombs as honeycomb bodies or honeycomb body modules.

[0172] The distribution pipes are preferably designed in the form of grids or in the form of concentrically connected circles, which extend as far as possible over the cross-sectional area of ​​the channel of the exhaust gas treatment unit or the flow area of ​​the catalyst bed.

[0173] The specific design and dimensioning of these distributors, including suitable outlet nozzles, is part of the expertise in catalytic exhaust gas purification technology and is widely used, for example, in the exhaust gas purification of coal-fired power plants.

[0174] Preferably, the exhaust gas treatment unit additionally comprises an oxidation catalyst for the catalytic oxidation of unused reducing agent and / or optionally of its not fully oxidized reaction products.

[0175] Oxidation catalysts are well-known to experts and typically contain as catalytically active components noble metals, such as Pt, Pd, and / or Rh, supported and dispersed on surface-rich oxides and / or ceramic substrates, transition metal oxides (e.g., of Fe, Mn, Cu, Cr, Co, Ni, etc.), or metal-loaded zeolites. Such catalysts are described, for example, in "Handbook of Heterogeneous Catalysis," Wiley-VCH, edited by Ertl, Knötzinger, Schüth, and Weitkamp, ​​2nd edition, 2008, Volume 5, Chapter 11.5, "Solid Catalysts for the Oxidation of Volatile Organic Compounds."

[0176] Preferably, the oxidation catalyst is an NH3 oxidation catalyst or one of the aforementioned N2O reduction, N2O decomposition or NO catalysts. xThe reduction catalyst fulfills the functionality of an NH3 oxidation catalyst. Preferably, the NH3 oxidation catalyst essentially comprises an iron- or copper-loaded zeolite; more preferably, an iron- or copper-loaded zeolite of the structural type MFI, BEA, FER, MOR, FAU, AEI and / or MEL. In particular, the NH3 oxidation catalyst essentially comprises an iron-loaded zeolite of the structural type BEA.

[0177] In preferred embodiments, the NH3 oxidation catalyst and the N2O decomposition catalyst and / or the N2O reduction catalyst and / or the NO x -Reduction catalyst made of the same material.

[0178] In preferred embodiments, steps (ci. a ) and / or (ci. b) and / or (c2) of the inventive method preferably in the second section of the start-up mode' and in the subsequent operating mode, i.e. preferably after the addition of NH3 to the combustible gas, at different temperatures, i.e. at different temperature levels.

[0179] In principle, it is possible that an earlier step, or one carried out upstream in the direction of exhaust gas flow, occurs at a higher temperature than a later step. gender or downstream step in the direction of exhaust gas flow. This can be achieved, if necessary, with the aid of one or more heat exchangers.

[0180] Preferably, an earlier step, or an upstream step in the direction of exhaust gas flow, takes place at a lower temperature than a subsequent step, or a downstream step in the direction of exhaust gas flow, which preferably corresponds to the heat of the catalyzed reactions.

[0181] Depending on the type of catalyst used, the steps may not be completely separable, either spatially or temporally. If a single catalyst is used to catalyze several of the steps simultaneously (ci_ a ), (ci. b) and (c2) are suitable, these steps may occur simultaneously and / or sequentially. In the direction of exhaust gas flow, individual segments of the same catalyst can be considered, through which the exhaust gas flows sequentially and on which different reactions may dominate. Which reaction dominates in which segment depends in particular on the respective reaction kinetics, the local temperature, and the local concentrations of the reactants, possibly including the concentration of reducing agent and possibly including the concentration of cocatalytically active species.

[0182] The exhaust gas treatment unit according to the invention serves in particular to carry out steps (ci) and (c2) of the inventive process. However, it is also possible that further steps and chemical reactions are carried out within the exhaust gas treatment unit in addition to steps (ci) and (c2).

[0183] This preferably relates to the installation of a catalyst bed arranged downstream in the direction of exhaust gas flow for the oxidation of incompletely converted reducing agents or their incompletely oxidized reaction products, e.g., for the oxidation of NH3 (NH3 oxidation catalyst) or CO (CO oxidation catalyst; when using hydrocarbons as reducing agents). In such embodiments, the exhaust gas is preferably cooled before being introduced into the downstream catalyst bed, i.e., the oxidation of NH3 and / or CO preferably takes place at a lower temperature than steps (ci) and (c2). This is particularly the case when the exhaust gas temperature is already significantly above the limit temperature T. G so that the cooling does not lead to the temperature falling below the limit T. G leads.

[0184] In carrying out steps (ci) and (c2) of the inventive process, there are, according to the invention, various preferred variants of the process, which may differ from one another with regard to the sequence of the reactions taking place, the catalysts used, the reducing agents used, the space velocities and other reaction conditions.

[0185] In preferred embodiments, these reactions are carried out in a common reaction zone (preferably a catalyst bed), which is equipped upstream with a device for dosing reducing agent into the exhaust gas.

[0186] In other preferred embodiments, these reactions are carried out in two successively arranged, separate reaction zones (preferably catalyst beds), of which preferably at least one, and preferably both, reaction zones are equipped independently of each other upstream with a device for dosing reducing agent into the exhaust gas. The exhaust gas then flows first through the first reaction zone and subsequently through the second reaction zone.

[0187] Particularly preferred variants / designs include [a] (c2) the chemical reduction of NO X with NH3 [b] (c2) the chemical reduction of NO X with NH3 and (ci. b ) the chemical reduction of N2O with NH3, preferably together in a reaction zone; [c] (c2) the chemical reduction of NO X with NH3 and (ci. b) the chemical reduction of N2O with hydrocarbons (CH4, natural gas, etc.), preferably together in a reaction zone; [d] (c2) the chemical reduction of NO X with NH3 and (ci. b ) the chemical reduction of N2O with NH3 and hydrocarbon (CH4, natural gas, etc.), preferably together in a reaction zone; [e] (c2) the chemical reduction of NO X with NH3 and (ci. a ) the decomposition of N2O, preferably together in a reaction zone; [f] (c2) the chemical reduction of NO X with NH3 and (ci. b ) the chemical reduction of N2O with NH3 and (ci.a) the decomposition of N2O, preferably together in a reaction zone; [g] (c2) the chemical reduction of NO X with NH3 and (ci. b ) the chemical reduction of N2O with hydrocarbons (CH4, natural gas, etc.) and (ci. a ) the decomposition of N2O, preferably together in a reaction zone; [h] (ci. a ) the decomposition of N2O, preferably in a first reaction zone; and subsequently (c2) the chemical reduction of NO X with NH3, preferably in a second reaction zone; [i] (ci. a ) the incomplete decomposition of N2O, preferably in a first reaction zone; and subsequently (c2) the chemical reduction of NO X with NH3 and (ci. b ) the chemical reduction of residual N2O with NH3, preferably in a second reaction zone; [j] (ci. a ) the incomplete decomposition of N2O, preferably in a first reaction zone; and subsequently (c2) the chemical reduction of NO X with NH3 and (ci. b ) the chemical reduction of residual N2O with hydrocarbon (CH4, natural gas, etc.), preferably in a second reaction zone; [k] (ci_a) the incomplete decomposition of N2O, preferably in a first reaction zone; and subsequently (c2) the chemical reduction of NO X with NH3 and (ci. b ) the chemical reduction of residual N2O with NH3 and hydrocarbon (CH4, natural gas, etc.), preferably in a second reaction zone; [l] (ci.a) the incomplete decomposition of N2O, preferably in a first reaction zone; and subsequently (c2) the chemical reduction of NO X with NH3 and (ci. a *) the decomposition of remaining N2O, preferably in a second reaction zone; [m] (ci.a) the incomplete decomposition of N2O, preferably in a first reaction zone; and subsequently (c2) and the chemical reduction of NO X withNH3 and (ci. b ) the chemical reduction of residual N2O with NH3 and (ci. a *) the decomposition of remaining N2O, preferably in a second reaction zone; [n] (ci.a) the incomplete decomposition of N2O, preferably in a first reaction zone; and subsequently (c2) the chemical reduction of NO X with NH3 and (ci. b ) the chemical reduction of residual N2O with hydrocarbon (CH4, natural gas, etc.) and (ci. a *) the decomposition of remaining N2O, preferably in a second reaction zone; [o] (c2) the incomplete chemical reduction of NO X , preferably in a first reaction zone; and subsequently (c2*) the chemical reduction of remaining NO X with NH3 and (ci. a ) the decomposition of N2O, preferably in a second reaction zone; [p] (c2) the incomplete chemical reduction of NO X , preferably in a first reaction zone; and subsequently (c2*) the chemical reduction of remaining NO X with NH3 and (ci. b ) the chemical reduction of N2O with NH3 and (ci. a) the decomposition of N2O, preferably in a second reaction zone; [q] (c2) the incomplete chemical reduction of NO X , preferably in a first reaction zone; and subsequently (c2*) the chemical reduction of remaining NO X with NH3 and (ci. b ) the chemical reduction of N2O with hydrocarbons (CH4, natural gas, etc.) and (ci. a ) the decomposition of N2O, preferably in a second reaction zone; [r] (c2) the incomplete chemical reduction of NO X , preferably in a first reaction zone; and subsequently (c2*) the chemical reduction of remaining NO X with NH3 and (ci. b ) the chemical reduction of N2O with NH3 and hydrocarbon (CH4, natural gas, etc.), preferably in a second reaction zone; [s] (c2) the incomplete chemical reduction of NO X and (ci. a) the incomplete decomposition of N2O, preferably in a first reaction zone; and subsequently (c2*) the chemical reduction of remaining NO X with NH3 and (ci. b *) the chemical reduction of residual N2O with NH3, preferably in a second reaction zone; [t] (c2) the incomplete chemical reduction of NO X and (ci. a ) the incomplete decomposition of N2O, preferably in a first reaction zone; and subsequently (c2*) the chemical reduction of remaining NO X with NH3 and (ci. b *) the chemical reduction of residual N2O with hydrocarbon (CH4, natural gas, etc.), preferably in a second reaction zone; or [u] (c2) the incomplete chemical reduction of NO X and (ci. a ) the incomplete decomposition of N2O, preferably in a first reaction zone; and subsequently (c2*) the chemical reduction of remaining NO Xwith NH3 and (ci. b *) the chemical reduction of residual N2O with NH3 and hydrocarbon (CH4, natural gas, etc.), preferably in a second reaction zone.

[0188] This does not mean, however, that the explicitly mentioned reactions must be the only reactions that take place in the respective reaction zone (preferably the catalyst bed). Depending on the catalyst used, it is, in fact, preferred according to the invention that additional reactions also take place simultaneously, which are not explicitly mentioned but can proceed in parallel. The explicitly mentioned reactions are therefore only those reactions that take place at a minimum in the respective variant / embodiment.

[0189] If NO X , N2O and NH3 are present in a mixture and the catalyst used facilitates both the chemical reduction of NO XThe chemical reduction of N2O with NH3 is also catalyzed, as is 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. If the catalyst used also catalyzes the decomposition of N2O, the decomposition of N2O is typically superimposed on the chemical reduction of N2O with NH3, whereby the extent of the chemical reduction of N2O can be increased by increasing the amount of NH3 added.

[0190] For descriptive purposes, "*" denotes a process step that was previously only partially carried out in a similar process step, whereby the process step marked with "*" then continues the previously only partially carried out process step, possibly, however, in a different reaction zone or catalyst bed. As with all other process steps, unless explicitly stated otherwise, the result achieved at the end of all process steps is not quantitatively defined. Thus, for example, if NO is produced in a first process step (c2), X If the chemical reduction is incomplete, the fact that subsequent process step (c2*) is carried out does not necessarily imply that the total amount of NO must be reduced by the chemical reduction. XIt must have been completely chemically reduced, i.e., down to 0.0 ppmv. Rather, it is quite possible that a residual amount of NO remains at the end of process step (c2*). X is available.

[0191] The exhaust gas treatment unit includes at least one injection point for reducing agents. The exhaust gas treatment unit may include multiple injection points for reducing agents.

[0192] The method of introducing the reducing agent into the exhaust gas stream to be treated is freely configurable according to the invention, as long as this occurs upstream of the N₂O reduction catalyst or NOx reduction catalyst. The reducing agent can be introduced in the form of a gas, a liquid, or an aqueous solution, which evaporates in the exhaust gas stream to be treated. The injection is effected by a suitable device, such as a corresponding pressure valve or appropriately designed nozzles, which open into a mixer for the exhaust gas stream to be treated and the supplied reducing agent. When using different reducing agents for NOₓ X The supply and introduction of N2O into the exhaust gas can be done separately or together.

[0193] When the catalyst beds are designed as a packing of catalyst honeycomb or honeycomb body modules, the supply and distribution of the reducing agents for NO takes place. Xand, if necessary, N2O to one or more reaction zones (preferably catalyst beds) preferably via a multiply branched piping system provided with a multitude of openings or nozzles, which is arranged upstream of the respective reaction zone (preferably catalyst bed), i.e. the packing of the catalyst honeycomb or honeycomb body modules, in the direction of flow of the exhaust gas.

[0194] The distributors are preferably designed in the form of grids or concentrically connected circles, which extend as far as possible over the cross-sectional area of ​​the channel of the exhaust gas treatment unit or the flow area of ​​the reaction zone (preferably catalyst bed).

[0195] The specific design and dimensioning of these distributors, including suitable outlet nozzles, is part of the expertise in catalytic exhaust gas purification technology and is widely used, for example, in the exhaust gas purification of coal-fired power plants.

[0196] The exhaust gas treatment unit according to the invention can comprise a single reaction zone (preferably a catalyst bed). In this case, the catalyst used in this single reaction zone serves as an N₂O decomposition catalyst and / or N₂O reduction catalyst as well as an NOₓ catalyst. x -reduction catalyst. In this case, steps (ci) and (c2) of the process according to the invention take place essentially simultaneously within this reaction zone. However, it should be noted that the kinetics of the individual reactions can be quite different. For example, depending on the catalyst material used, the chemical reduction of NO can X Using NH3 as a reducing agent, for example, the reduction of N2O with NH3 is significantly faster than the chemical reduction of N2O with NH3. Therefore, if NO XIf NO₂ and N₂O are present in the mixture and NH₃ is introduced as a reducing agent, different reactions take place in the front section of the single reaction zone than in the rear section. Due to its faster kinetics, the chemical reduction of NO₂ predominates in the front section. X and only in the rear section, when the majority of the NO X Once the N2O has broken down, the chemical reduction of N2O takes place.

[0197] Alternatively, the exhaust gas treatment unit can comprise several reaction zones (preferably catalyst beds), which is preferred according to the invention. If several reaction zones are included, they are preferably arranged one after the other, i.e., the exhaust gas flows through them successively, first the first reaction zone, then the second reaction zone, and optionally then the third reaction zone.

[0198] In preferred embodiments, the reaction zones are each spatially separated catalyst beds.

[0199] In preferred embodiments, the exhaust gas undergoes the steps of the invention-based process in one of the following sequences: (i) (a) (a2) (ci_a) (c2); where preferred step (ci_ a ) takes place in a first reaction zone; and step (c2) takes place in a second reaction zone; (ii) (a) (a2) (c2) (ci_ b ); wherein step (c2) preferably takes place in a first reaction zone; and step (ci. b ) takes place in a second reaction zone; (iii) (a) (a2) (c2) (ci_ b ) (ci_ a ); where step (c2) preferably takes place in a first reaction zone; step (ci_ b ) takes place in a second reaction zone; and step (ci. a ) takes place in a third reaction zone; (iv) (a) (a2) (c2) (ci. a)+(ci. b ); wherein step (c2) preferably takes place in a first reaction zone; and step (ci. a ) as well as step (ci_ b ) take place in a second reaction zone; (v) (a) (a2) (c2) (ci. a ); wherein step (c2) preferably takes place in a first reaction zone; and step (ci. a ) takes place in a second reaction zone; (vi) (a) (a2) (ci. a )+(c2) (c2*); where preferred step (ci_ a ) and incomplete step (c2) in a first reaction zone; and the remaining step (c2*) in a second reaction zone; (vii) (a) (a2) (ci. a )+(c2) (c2*)+(ci. b ); where preferred step (ci. a ) and incomplete step (c2) occur in a first reaction zone; and step (ci_ b ) and the remaining step (c2*) take place in a second reaction zone; (viii) (a) (a2) (ci. a )+(ci. b)+(c2) (ci. a *)+(ci. b *)+(c2*); where preferably incomplete step (ci. a) and incomplete step (ci_ ) b ) and incomplete step (c2) in a first reaction zone, which preferably does not contain zeolitic material as a catalyst; and remaining step (ci_ a *) as well as remaining step (ci. b *) and the remaining step (c2*) take place in a second reaction zone, which preferably contains zeolitic material as a catalyst; (ix) (a) (a2) (ci. a )+(ci. b )+(c2) (ci. a *)+(ci. b *)+(c2*); where preferably incomplete step (ci. a) and incomplete step (ci_ ) b ) and incomplete step (c2) take place in a first reaction zone which preferably contains zeolitic material as a catalyst; and remaining step (ci. a *) as well as remaining step (ci. b*) and remaining step (c2*) in a second reaction zone proceed, which preferably uses NO as a catalyst x contains a -sensitive N2O decomposition catalyst; (x) (a) (a2) (ci_a) (ci-a*)+(ci. b ) + (c2); wherein preferably incomplete step (ci. a ) takes place in a first reaction zone, which preferably contains zeolitic material as a catalyst; and remaining step (ci. a *) as well as step (ci. b ) and step (c2) take place in a second reaction zone, which preferably contains zeolitic material as a catalyst; (xi) (a) (a2) (ci. a ) (ci. a *)+(ci. b )+(c2); where preferably incomplete step (ci. a ) takes place in a first reaction zone, which preferably uses NO as a catalyst x -sensitive N2O decomposition catalyst; and remaining step (ci. a *) as well as step (ci. b) and step (c2) take place in a second reaction zone, which preferably contains zeolitic material as a catalyst.

[0200] However, it is also possible for multiple reaction zones to be realized by a single catalyst bed. Two reaction zones on a common catalyst bed can be formed, in particular, by introducing reducing agent in the middle (or at another position along the longitudinal extent) of the catalyst bed. Upstream of the injection point, no reducing agent is then present, so that steps (ci_ b ) and (c2) of the inventive process cannot take place due to the lack of a reducing agent. Upstream, the decomposition of N2O then essentially occurs according to step (ci. a ) (first reaction zone). Reducing agent is present downstream of the injection point, so steps (ci. b) and (c2) of the inventive procedure may take place, possibly superimposed by step (ci. a ) of the inventive process (second reaction zone). In this case too, due to the different reaction kinetics, different reactions can occur in the front section of each reaction zone than in the rear section of each reaction zone; however, the first reaction zone and the second reaction zone differ from each other in that, in the first reaction zone, due to the lack of a reducing agent, no chemical reduction of N₂O and no chemical reduction of NO can take place. X This has been done.

[0201] In particularly preferred embodiments, the exhaust gas treatment unit comprises a first reaction zone and a second reaction zone. It is possible that further reaction zones are present.

[0202] In preferred embodiments, the first and second reaction zones are spatially separated. In this case, they are preferably separate catalyst beds. With spatial separation of the catalyst beds, it is possible to adjust the temperature of the second catalyst bed, or of the gas stream entering it, by removing or adding heat so that it is lower or higher than that of the first catalyst bed. The temperature of a single catalyst bed can advantageously be determined as the arithmetic mean of the gas stream temperature at the inlet and outlet of the catalyst bed.

[0203] In preferred embodiments, the temperature in the first reaction zone (in the first catalyst bed) is higher than the temperature in the second reaction zone (in the second catalyst bed).

[0204] The reducing agent is preferably used in step (ci.). b) selected from NH3, hydrocarbons, CO, H2 and mixtures thereof; preferably NH3.

[0205] In preferred embodiments, the reducing agent is in step (ci. b ) NH3, which is preferably used in an amount of 0.5 to 3.0 molar proportions, more preferably in an amount of 0.7 to 2.5 molar proportions, and even more preferably in an amount of 0.8 to 2.0 molar proportions, based on a molar proportion of chemically reduced N2O, i.e., based on the amount of N2O at the entrance to the catalyst bed of the N2O reduction catalyst.

[0206] In preferred embodiments, the reducing agent is in step (ci. b) NH3, which is preferably used in an amount of 0.5 to 3.0 molar proportions, more preferably in an amount of 0.7 to 2.5 molar proportions, and even more preferably in an amount of 0.8 to 2.0 molar proportions, based on the molar amount of N2O in the exhaust gas at the inlet to the catalyst bed of the N2O reduction catalyst. This amount is additive to any amount of NH3 required for NO x -Reduction, provided that step (c2) also takes place in the catalyst bed of the N2O reduction catalyst.

[0207] In other preferred embodiments, the reducing agent is a hydrocarbon or a mixture of several hydrocarbons, preferably in an amount of 0.2 to 1.0 molar proportions, more preferably in an amount of 0.2 to 0.7 molar proportions, based on the molar amount of N₂O in the exhaust gas at the inlet to the catalyst bed of the N₂O reduction catalyst. This amount is also additive to any amount of NH₃ required for NO₂ reduction. x-Reduction, provided that step (c2) also takes place in the catalyst bed of the N2O reduction catalyst.

[0208] The reducing agent can also already be present in the exhaust gas, e.g., as residual combustion gases and / or their oxidation products. The inventive process then not only reduces the nitrogen oxide (NOx) content. X and N2O), but also the content of these impurities (remaining combustion gases and / or their oxidation products).

[0209] Preferably the reducing agent in step (c2) is selected from NH3, hydrocarbons, CO, H2 and mixtures thereof; preferably NH3.

[0210] Preferably, the reducing agent in step (c2) is NH3, which is used in an amount of 0.9 to 2.5 molar parts, preferably 1.0 to 1.6 molar parts, more preferably 1.0 to 1.4 molar parts, and even more preferably 1.0 to 1.2 molar parts, based on a molar part of NO to be chemically reduced. X .

[0211] In preferred embodiments, the reducing agent is in step (ci. b ) the same as the reducing agent in step (c2); preferably NH3.

[0212] Besides NH3, in steps (ci. b) and / or (c2) of the process according to the invention may also include other nitrogen-containing reducing agents, for example, hydrogen compounds of nitrogen, such as azanes, hydroxyl derivatives of azanes, as well as amines, oximes, carbamates, urea, or urea derivatives. Examples of azanes are hydrazine and, in particular, ammonia. Examples of hydroxyl derivatives of azanes are hydroxylamine. Examples of amines are primary aliphatic amines, such as methylamine. An example of carbamates is ammonium carbamate. Examples of urea derivatives are N,N'-substituted ureas, such as N,N'-dimethylurea. Ureas and urea derivatives are preferably used in the form of aqueous solutions. Ammonia or substances that release ammonia upon introduction, such as urea or ammonium carbamate, are particularly preferred.

[0213] In particularly preferred embodiments, the exhaust gas treatment unit according to the invention comprises a first catalyst bed and a spatially separate second catalyst bed; wherein the first catalyst bed is arranged upstream of the second catalyst bed in the direction of exhaust gas flow; wherein, optionally and preferably, a first device with a first control valve for metering NH3 into the exhaust gas is arranged upstream of the first catalyst bed; wherein, downstream of the first catalyst bed and upstream of the second catalyst bed, a second device with a second control valve for metering NH3 into the exhaust gas is arranged, preferably in the second phase of the start-up mode and in the subsequent operating mode, i.e.,preferably, after the addition of NH3, the combustible gas is added, and further NH3 is added to the exhaust gas; wherein both the first catalyst and the second catalyst each contain an iron-loaded zeolite catalyst; wherein (i) in the first catalyst (ci_. a ) N2O is decomposed; and (c2) NO X incompletely reduced with NH3, wherein optionally and preferably at least part of the NH3 originates from an incomplete combustion of NH3 in step (a) (NH3 slip); and (ii) in the second catalyst (ci. b ) remaining N2O is chemically reduced with NH3 and optionally (ci_a*) remaining N2O is decomposed; and (c2*) remaining NO X is chemically reduced with NH3.

[0214] Preferably, in the first catalyst stage, the catalytic decomposition of N2O takes place co-catalyzed by NO present in the exhaust gas. X .

[0215] The incomplete chemical reduction of NO preferably leads to Xwith NH3 in the first catalyst up to a predetermined residual NO content X , which is sufficient to cause a cocatalytic effect on the decomposition of N₂O in the first catalyst bed. Since the chemical reduction of NO in the first catalyst bed X The reduction of N2O with NH3 is typically significantly faster than the chemical reduction of N2O with NH3, and not the entire amount of NO is produced in the first catalyst. X Since the chemical reduction of N2O with NH3 in the first catalyst is chemically reduced, the extent of any parallel chemical reduction of N2O with NH3 is typically negligible.

[0216] Preferably, additional NH3 is added to the NO via the first device. x -Reduction added to the exhaust gas; preferably under fee <a&ac -Regelung, d.h. es wird ein bestimmter Wert für die Konzentration The NOx concentration at the outlet of the first catalyst bed is specified as a target value (setpoint) and the actual concentration of NO XThe actual value is measured at the outlet of the first catalyst bed, and in the event of a difference between the setpoint and the actual value (control deviation), the actuation degree of the first control valve is changed to minimize the difference. The setpoint of the NO is preferably used. x The concentration at the outlet of the first catalyst bed, and thus the amount of additional NH3, was chosen so that the residual concentration of NO x The NOx concentration at the outlet of the first catalyst bed is at most 1000 ppmv, preferably at most 500 ppmv, more preferably at most 100 ppmv. Preferably, the target value of the NOx concentration at the outlet of the first catalyst bed, and thus the amount of additional NH3, is chosen such that the residual NO concentration is at least 1000 ppmv. x The expected specific consumption of NH3 for the chemical reduction of NO is at least 10 ppmv at the exit point of the first catalyst bed, preferably at least 20 ppmv, more preferably at least 40 ppmv. XThe concentration in the first catalyst is typically in the range of 0.9 to 1.1 mol of NH3 per mol of reduced NO. X and is therefore significantly smaller than the expected specific (mol / mol) consumption of NH3 in the second catalyst.

[0217] Preferably, the temperature of the exhaust gas at the exit from the first catalyst bed in operating mode, i.e. preferably after the addition of NH3 to the combustible gas, is in the range of 400 to 550°C.

[0218] Preferably, the exhaust gas at the outlet of the first catalyst bed has a pressure which is greater than atmospheric pressure, i.e. > 1.0 bar, but at most 1.2 bar, more preferably at most 1.1 bar.

[0219] Preferably, the exhaust gas at the outlet of the first catalyst bed has an oxidation level of NO₂. X from at least 5.0%, preferably at least 7.5%, more preferably at least 10%, even more preferably at least 12.5%, most preferably at least 15%, and in particular at least 17.5%.

[0220] In preferred embodiments, the exhaust gas at the outlet of the first catalyst bed has an oxidation level of NO. X in the range of 30 to 50%.

[0221] In other preferred embodiments, the exhaust gas at the outlet of the first catalyst bed exhibits an oxidation level of NO. X in the range of 15 to 35%, preferably 15 to 30%.

[0222] In further preferred embodiments, the exhaust gas at the outlet of the first catalyst bed has an oxidation level of NO. X in the range of 10 to 20%.

[0223] In other preferred embodiments, the exhaust gas at the outlet of the first catalyst bed has an oxidation level of NO. X in the range of 5 to 15%.

[0224] Preferably, residual N2O is degraded in the second catalyst bed to a residual concentration of N2O at the outlet of the second catalyst bed of at most 20 ppmv, more preferably at most 10 ppmv, even more preferably at most 5 ppmv, most preferably at most 2 ppmv.

[0225] The remaining NO is preferentially deposited in the second catalyst bed. X up to a residual concentration of NO X at the exit of the second catalyst bed, the amount of oxidation is reduced by at most 20 ppmv, preferably at most 10 ppmv, even more preferably at most 5 ppmv, most preferably at most 2 ppmv.

[0226] Preferably, the additional NH3 is added via the second device under feedforward control, i.e., the NO concentrations are increased. Xand optionally preferably measured at N2O at the outlet of the first catalyst bed or optionally at the inlet to the second catalyst bed; taking into account the amount of exhaust gas entering the second catalyst bed, the NO X - Reduction and optionally preferred the sum of the NO x -Reduction and the amount of NH3 required for N2O reduction using stored ratios, i.e., molar ratios (mol / mol) of NH3 / NO X and optionally preferably calculated from NH3 / N2O or factors derived therefrom; and with the calculated result (control variable) the control degree of the second control valve is changed in order to meter the required amount of NH3.

[0227] The molar NH3 concentration [NH3] of the exhaust gas at the inlet to the second catalyst bed is preferably in the range of the sum of 0.7 x [N2O] and 1.0 x [NO] according to the invention. X ] to the sum of 4.0 x [N2O] and 2.0 x [NOX ], preferably in the range of the sum of 1.0 x [N2O] and 1.1 x [NO X ] to the sum of 3.0 x [N2O] and 1.6 x [NO X ], even more preferably in the range of the sum of 1.5 x [N2O] and 1.2 x [NO X ] to the sum of 2.5 x [N2O] and 1.4 x [NO X ], where [N2O] is the molar concentration of N2O and [NO X the molar concentration of NO X each is located in the exhaust gas at the inlet to the second catalyst bed.

[0228] For fee-farwar control of the NH3 dosage into the second catalyst bed, the preferred method is to use NO. x -Reduction a molar ratio of NH3 / NO X in the range of 1.0 to 2.0; preferably 1.1 to 1.6; more preferably 1.2 to 1.4.

[0229] For fee farwar control of the NH3 dosage into the second catalyst bed with respect to N2O reduction, a molar ratio of NH3 / N2O in the range of 0.7 to 4.0 is preferably chosen; preferably 1.0 to 3.0; more preferably 1.5 to 2.5.

[0230] Preferably, the additional NH3 is not dosed with the second device under feet / oil control, since a chemical reduction of NO that is as complete as possible is desired. X in the second catalyst bed, i.e., no or only very low residual concentrations of NO. X and N2O result, which would be poorly suited for use as control variables.

[0231] Preferably, the amount of catalyst, i.e., the space velocity (= ratio of exhaust gas volume flow under standard conditions to catalyst volume), is chosen such that a reduction of N2O of at least 50%, preferably at least 70%, and even more preferably at least 80%, occurs in the first catalyst bed, based on the concentration of N2O at the inlet to the first catalyst bed.

[0232] Preferably, the amount of catalyst and the amount of additional NH3 are chosen such that the molar ratio of NO at the exit of the first catalyst bed X / N2O is at least 5, preferably at least 10, even more preferably at least 20.

[0233] Preferably, the space velocity of the first catalyst bed is in the range of 5,000 h⁻¹. -1 up to 100,000 h 1 , preferably 10,000 h -1 up to 50,000 h 1 , even more preferred 15,000 h -1 up to 45,000 h 1 .

[0234] In the context of the invention, "space velocity" is the quotient of the volume flow rate of the gas mixture passing through the catalyst bed (measured at 0 °C and 1.014 bar and usually expressed in Nm). 3 h -1 ) is to be understood as being relative to the volume of the catalyst or catalyst bed. The space velocity can therefore be adjusted via the volume flow rate of the gas and / or the amount of catalyst.

[0235] Is the molar ratio of NO X If the concentration of N2O at the outlet of the first catalyst bed is at least 10, then the addition of NH3 to the second catalyst bed via the second device can preferably be carried out solely in relation to the amount of NO entering the device. X take place.

[0236] Preferably, the temperature of the exhaust gas in operating mode, i.e., preferably after the addition of NH3 to the combustible gas, at the inlet to the first catalyst bed is at least 400°C, more preferably at least 425°C, and even more preferably at least 450°C. Preferably, the temperature of the exhaust gas at the inlet to the first catalyst bed is at most 550°C, more preferably at most 525°C, and even more preferably at most 500°C. The temperature can be adjusted by measures known to those skilled in the art, in particular the design of heat exchangers and the conditions of NH3 combustion.

[0237] Depending on the heat of reaction of the chemical reactions taking place in the first catalyst bed and in the second catalyst bed, the inlet temperature of the exhaust gas into the first catalyst bed is preferably chosen such that the temperature of the exhaust gas in operating mode, i.e. preferably after the addition of NH3 to the combustible gas, at the outlet of the second catalyst bed is at most 600°C, more preferably at most 550°C, and even more preferably at most 520°C.

[0238] Preferably, the space velocity of the second catalyst bed is in the range of 5,000 h⁻¹. -1 up to 100,000 h 1 , preferably 10,000 h -1 up to 50,000 h 1 , even more preferred 15,000 h -1 up to 45,000 h 1 .

[0239] The preferred ratio of the catalyst volumes (Vl) kat / V2 kat ) from the first catalyst bed Vlkat to the second catalyst bed V2 katin the range of 1 / 2 to 20 / 1, preferably 1 / 2 to 10 / 1, even more preferably 1 / 1 to 4 / 1.

[0240] In preferred embodiments, at least one, several or all of the following conditions are met: The exhaust gas pressure at entry into the first catalyst bed is at most 5 bar, preferably at most 4 bar, even more preferably at most 1.3 bar, most preferably at most 1.2 bar, and particularly at most 1.1 bar; the H₂O content in the exhaust gas at entry into the first catalyst bed is at least 5 vol%, preferably at least 10 vol%, even more preferably at least 15 vol%, most preferably at least 20 vol%, and particularly at least 25 vol%; the NO content XThe N₂O content in the exhaust gas upon entry into the first catalyst bed is at least 500 ppmv, preferably at least 1000 ppmv, more preferably at least 1500 ppmv, most preferably at least 2000 ppmv, and particularly at least 2500 ppmv; the N₂O content in the exhaust gas upon entry into the first catalyst bed is at most 500 ppmv, more preferably at most 200 ppmv, more preferably at most 100 ppmv, but at least 5 ppmv, preferably at least 10 ppmv, more preferably at least 50 ppmv; the exhaust gas contains, upon entry into the first catalyst bed in the second phase of the start-up mode and in the subsequent operating mode, i.e.preferably after the addition of NH3 to the combustible gas, unburned residues of NH3 from the combustion of NH3; the N2O decomposition catalyst and / or the N2O reduction catalyst is in the form of a honeycomb structure; the NOx reduction catalyst is in the form of a honeycomb structure; the first catalyst bed contains Fe-zeolite; the second catalyst bed contains Fe-zeolite; the exhaust gas flows through a heat exchanger before entering the first catalyst bed and is heated therein; the NO content. XThe N₂O content at the outlet of the first catalyst bed is at most 1000 ppmv, preferably at most 500 ppmv, even more preferably at most 300 ppmv, most preferably at most 100 ppmv; however, preferably at least 10 ppmv, more preferably at least 20 ppmv, even more preferably at least 40 ppmv, most preferably at least 100 ppmv, and in particular at least 250 ppmv; the N₂O content at the outlet of the first catalyst bed is at most 20 ppmv, more preferably at most 15 ppmv, even more preferably at most 10 ppmv, most preferably at most 5 ppmv, and in particular at most 2 ppmv; after leaving the first catalyst bed and until entering the second catalyst bed, no intermediate cooling of the exhaust gas takes place; • the molar ratio of N2O : NO X at the entry into the first catalyst bed, the value is at most 0.5, preferably at most 0.2, and even more preferably at most 0.1; • the molar ratio of N2O : NO Xat the exit from the first catalyst bed is at most 0.20, preferably at most 0.1, even more preferably at most 0.05; • The injection of NH3 into the exhaust gas upstream of the first catalyst bed in the direction of exhaust gas flow takes place in the second section of the start-up mode and in the subsequent operating mode, i.e., preferably after the addition of NH3 to the combustible gas, optionally; if injection takes place, it is preferably substoichiometric with regard to the NO content X at the entrance to the first catalyst bed; • The injection of NH3 into the exhaust gas in the direction of flow of the exhaust gas downstream of the first catalyst bed and upstream of the second catalyst bed takes place in the second section of the start-up mode and in the subsequent operating mode, i.e. preferably after the addition of NH3 to the combustible gas, necessarily, preferably superstoichiometrically with regard to the total NO contentX and N2O at the entrance to the second catalyst bed.

[0241] The above-described process using Fe-zeolite catalysts in two catalyst beds enables, in comparison to classical DeNO₂ x -Process using V2O5 / TiO2 catalysts - the complete or near-complete reduction of large quantities of NO X , without the risk of NH3 slippage; and - the simultaneous, complete or almost complete degradation of N2O at comparatively small catalyst volumes, i.e. at comparatively high space velocities.

[0242] This is achieved not only through the operating mode according to the invention but also through the oxidative properties of the Fe-zeolite catalysts used according to the invention. In the first catalyst bed, the molar ratio of NO to NO₂ is brought as close as possible to the thermodynamic equilibrium position according to the invention. Thus, the NO xThe degree of oxidation (molar ratio of NO₂ / (NO + NO₂)) before entering the first catalyst bed is expected to be less than 5% due to the upstream NH₃ combustion in the second phase of the start-up mode and in the subsequent operating mode, i.e., preferably after the addition of NH₃ to the combustible gas, at very high temperatures and the slow establishment of equilibrium in the gas phase during the cooling of the exhaust gas in the heat exchanger(s) that may be downstream. This is significantly below the thermodynamic equilibrium valid for the inlet temperature to the first catalyst bed. However, this is very disadvantageous for the efficient chemical reduction of NO. X , because this only results in a small portion of the NO present in the exhaust gas X corresponding to an almost SCR reduced can be and a large part of the N0 x or the remaining NO must be broken down according to the significantly slower normal SCR process.

[0243] Due to the chosen operating mode of limited NH3 dosing in the first catalyst bed and the ability of the Fe-zeolite catalysts to oxidize NO or to catalytically accelerate the equilibrium establishment, a significantly faster, i.e., more efficient, chemical reduction of NO occurs in the first catalyst bed in the second part of the start-up mode and in the subsequent operating mode, i.e., preferably after the addition of NH3 to the combustible gas. X achieved and simultaneously the maximum possible NO x - Degree of oxidation of the escaping residual NO X This also ensures efficient chemical reduction of NO in the second catalyst bed. X made possible right from the start.

[0244] It was found that large quantities of NH3, such as those used for the complete chemical reduction of high concentrations of NO, are necessary. X necessary, similar to water, is the adjustment of NO.x -Equilibrium on the Fe-zeolite catalyst inhibit.

[0245] Furthermore, the chemical reduction of NO X As such, it is also inhibited by NH3 itself at sufficiently high doses. This results in a situation where the reaction depends on temperature, catalyst quantity, and NO content. X With increasing addition of NH3, from a certain amount of NH3 onwards, no further increase in NO x -Degradation occurs less frequently. With a further increase in NH3 addition, a decrease in NO may even occur. x -Degradation is observed when NH3 slip occurs simultaneously.

[0246] Through chemical reduction of NO X Previously in the first catalyst bed, the chemical reduction of NO is achieved X The required amount of NH3 in the second catalyst bed is significantly reduced.

[0247] In this way, together with the above-described setting or permanent tracking of the NO x-equilibrium, also in the second catalyst bed a very efficient chemical reduction of NO X even possible with the superstoichiometric dosage of NH3 as per the invention.

[0248] The fact that this occurs according to the invention without or with only a negligible NH3 slip of preferably at most 10 ppmv, more preferably at most 5 ppmv, and even more preferably at most 3 ppmv, is also due to the oxidative properties of the Fe-zeolite catalysts used according to the invention. If the inlet temperature of the exhaust gas into the second catalyst bed in operating mode, i.e., preferably after the addition of NH3 to the combustible gas, is preferably at least 400°C, more preferably at least 425°C, and even more preferably at least 450°C, the excess NH3 added within the limits specified in the invention is selectively oxidized to N2 and H2O by the residual oxygen content of the exhaust gas.

[0249] All these advantages cannot be achieved when using conventional V₂O₅ / TiO₂-based SCR catalysts, such as those typically used for the denitrification of exhaust gases from natural gas-fired reformers, in either a single-stage or multi-stage configuration. For example, these conventional SCR catalysts typically cannot be operated at temperatures above 400°C due to stability limitations, which restricts the achievable degradation rates. Furthermore, conventional SCR catalysts exhibit very limited oxidation activity, making it impossible to adjust or continuously replenish the NOₓ concentration. x Neither is an equilibrium possible, nor do these catalysts enable effective and N2-selective oxidation of excess NH3. In fact, there is even a risk of the undesired formation of N2O.

[0250] In a preferred embodiment of the above-described configurations, the first catalyst bed and the second catalyst bed contain the same catalyst. In preferred embodiments, the second device with a second control valve for metering NH3 into the exhaust gas is omitted, and preferably the spatial separation of the first catalyst bed from the second catalyst bed is eliminated – there is then effectively only one common catalyst bed, with a first device with a first control valve for metering NH3 into the exhaust gas preferably arranged upstream of this common catalyst bed. Preferably, in the second part of the start-up mode and in the subsequent operating mode, i.e., preferably after the metering of NH3 into the combustible gas, additional NH3 is metered into the exhaust gas via the first device; preferably under feedforward control, i.e., the concentration of NO XThe concentrations of N₂O and NH₃ in the exhaust gas upstream of the common catalyst bed are measured; taking into account the amount of exhaust gas entering the common catalyst bed, the additional amount of NH₃ required is calculated; and the control input of the first control valve is adjusted using the calculated result (control variable) to meter the additional amount of NH₃ required. Preferably, in such embodiments, an NH₃ oxidation catalyst is arranged downstream of the common catalyst bed to reduce potential NH₃ slip.

[0251] In step (d) of the method according to the invention, the temperature of the exhaust gas is preferably determined.

[0252] Preferably, in step (d) of the method according to the invention, the temperature T is determined. A of the exhaust gas at or after leaving the exhaust gas treatment unit; and the addition of NH3 to the combustible gas in step (e) takes place as soon as the temperature TA the limiting temperature T G has exceeded (T Ä > T G ).

[0253] Preferably, in step (d) of the method according to the invention, the determination is carried out by measuring the temperature T. Ä of the exhaust gas at or after leaving the exhaust gas treatment unit.

[0254] Suitable methods for determining the temperature of the exhaust gas are known to those skilled in the art, preferably the temperature T. Ä of the exhaust gas during or after leaving the exhaust gas treatment lung unit. In particular, it is known to those skilled in the art that the temperature T A The temperature of the exhaust gas can be deduced or calculated from various other process and measurement data at or after it leaves the exhaust gas treatment unit, even if the temperature of the exhaust gas is not measured directly at or after it leaves the exhaust gas treatment unit.

[0255] In step (e) of the inventive process, NH3 is added to the combustible gas as soon as the temperature of the exhaust gas in the combustion device and in the exhaust gas duct exceeds a limit temperature T. G in which the formation of NH4NO3 deposits from NH3 and NO contained in the exhaust gas X is prevented.

[0256] The limiting temperature T G is the temperature above which the formation of NH4NO3 deposits from NH3 and NO contained in the exhaust gas begins. X This is prevented. The exhaust gas contains NH3 and NO. X , so below the limit temperature T G Form and deposit NH4NO3.

[0257] Preferably, no NH3 is added to the combustible gas as long as the exhaust gas temperature does not exceed the limit temperature T. G has not exceeded; preferably as long as temperature T A the limit temperature T of the exhaust gas at or after leaving the exhaust gas treatment unit Ghas not exceeded.

[0258] Preferably, the exhaust gas treatment unit comprises at least one catalyst for the selective catalytic reduction of NO. X (NO x -reduction catalyst) and the limiting temperature T G corresponds to the catalyst's start-up temperature to NO x -Reduction of the exhaust gas treatment unit (T R (NO X )).

[0259] Preferably, the exhaust gas treatment unit comprises at least two or more catalysts made of different materials and the limit temperature T G corresponds to the start-up temperature of the catalyst with the lowest start-up temperature.

[0260] For descriptive purposes, the "start-up temperature" is the temperature at which at least 5% conversion, preferably at least 10% conversion, more preferably at least 20% conversion, even more preferably at least 30% conversion, and in particular at least 50% conversion is achieved on the given catalyst, relative to the maximum achievable conversion (=100%) at the optimal temperature under otherwise identical conditions. The temperature is preferably the temperature of the exhaust gas as it leaves the exhaust gas treatment unit. Preferably, the conversion is always relative to the functionality of the catalyst. If the exhaust gas treatment unit, for example, uses NOₓ x -Reduction catalyst- includes the "start-up temperature" which is the temperature at which 5% NO is produced on the given catalyst. x -Reduction, preferably at least 10% NO x -Reduction, preferably at least 20% NO X - Degradation, preferably at least 30% NO x -Reduction, in particular at least 50% NOx -Degradation is achieved based on the maximum achievable conversion (=100%) at optimal temperature under otherwise identical conditions. The minimum NO x -Degradation is the value obtained through the catalytic reduction of NO. X with a reducing agent, preferably NH3, under the given process and exhaust gas conditions, without a measurable NH3 slip, preferably >0.5 ppmv NH3, occurring at the outlet of the catalyst or the exhaust gas treatment unit. That means that to the NO x The added NH3 reduction is completely converted on the catalyst or in the exhaust gas treatment unit. The defined start-up temperature depends on the given process and exhaust gas conditions, in particular, for example, on the exhaust gas flow rate or the space velocity in the catalyst bed and on the amount and oxidation state of the NO. X .

[0261] In preferred embodiments, the exhaust gas treatment unit comprises a NOₓ x -Reduction catalyst and the limiting temperature T G corresponds to the start-up temperature of the NO x -Reduction catalyst (T R (NO X In these preferred embodiments, the exhaust gas is preferably cooled after leaving the exhaust gas treatment unit and before being released into the atmosphere, preferably below the limit temperature T. G ; preferably to a temperature of less than 170°C; preferably to a maximum of 150°C, more preferably to a maximum of 120°C, even more preferably to a maximum of 100°C, and most preferably to a maximum of 80°C.

[0262] In particularly preferred embodiments, the exhaust gas treatment unit comprises an iron-loaded zeolite catalyst and the limiting temperature T G corresponds to the start-up temperature of the iron-loaded zeolite catalyst (T R (NO X )).

[0263] In preferred embodiments, the limiting temperature T is G at least 150°C; preferably at least 160°C, more preferably at least 170°C, even more preferably at least 180°C, most preferably at least 190°C, and in particular at least 200°C.

[0264] In preferred embodiments, the limiting temperature T is G at most 450°C; preferably at most 400°C, more preferably at most 350°C, even more preferably at most 300°C, most preferably at most 250°C, and in particular at most 200°C.

[0265] In preferred embodiments, the limiting temperature T G in the range of 185±40°C; preferably in the range of 185±35°C, more preferably in the range of 185±30°C, even more preferably in the range of 185±25°C, most preferably in the range of 185±20°C, and particularly in the range of 185±15°C.

[0266] Preferably, NH3 is added to the combustible gas at least 30 seconds after the start of combustion of the combustible gas in step (a); preferably at least 60 seconds, more preferably at least 90 seconds, even more preferably at least 120 seconds, most preferably at least 150 seconds, and in particular at least 180 seconds.

[0267] Preferably, NH3 is added to the flammable gas at least 5 minutes after the start of combustion of the flammable gas in step (a); preferably at least 10 minutes, more preferably at least 15 minutes, even more preferably at least 20 minutes, most preferably at least 25 minutes, and in particular at least 30 minutes.

[0268] Preferably, the flammable gas exhibits the following properties in the second section of the start-up mode and in the subsequent operating mode, i.e., preferably after the addition of NH3 to the flammable gas. a different composition than in step (a) in the first section of the start-up mode', i.e. preferably before the addition of NH3 to the combustible gas.

[0269] In preferred embodiments, in the second phase of the start-up mode and in the subsequent operating mode, NH3 is burned as the sole fuel; that is, preferably no other gas is burned besides NH3. In other preferred embodiments, in the second phase of the start-up mode and in the subsequent operating mode, NH3 is burned in a mixture with H2. In further preferred embodiments, in the second phase of the start-up mode and in the subsequent operating mode, NH3 is burned in a mixture with a hydrocarbon, preferably CH4 (natural gas), C3H8, or C4H8. These gases or mixtures are also referred to as "combustible gas" for descriptive purposes. In addition to NH3 and optionally H2 and / or hydrocarbon, preferably CIE, C3H8, or C4H8, the combustible gas may optionally contain further components, e.g., N2.

[0270] In the second section of the start-up mode and in the subsequent operating mode, i.e. preferably after the addition of NH3 to the flammable gas, the combustion of a flammable gas containing NH3 preferably takes place.

[0271] Preferably, in the second section of the start-up mode and in the subsequent operating mode, i.e., preferably after the addition of NH3 to the combustible gas, the flammable gas contains a mixture of NH3 with H2 and / or hydrocarbons; preferably a mixture of NH3 with H2 and / or CIE (natural gas), C3H8, or C4H8.

[0272] In preferred embodiments, the proportion of NH3 in the flammable gas in the second section of the start-up mode and in the subsequent operating mode, i.e. preferably after the addition of NH3 to the flammable gas, is at least 0.5 mol%; preferably at least 1.0 mol%, more preferably at least 2.0 mol%, even more preferably at least 4.0 mol%, most preferably at least 6.0 mol%, and in particular at least 8.0 mol%.

[0273] In preferred embodiments, the proportion of NH3 in the flammable gas in the second section of the start-up mode and in the subsequent operating mode, i.e. preferably after the addition of NH3 to the flammable gas, is at least 10 mol%; preferably at least 20 mol%, more preferably at least 30 mol%, even more preferably at least 50 mol%, most preferably at least 75 mol%, and in particular at least 90 mol%.

[0274] In preferred embodiments, the proportion of NH3 in the flammable gas in the second section of the start-up mode and in the subsequent operating mode, i.e. preferably after the addition of NH3 to the flammable gas, is at most 95 mol% NH3; preferably 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%.

[0275] In preferred embodiments, the flammable gas in the second section of the start-up mode and in the subsequent operating mode, i.e. preferably after the addition of NH3 to the flammable gas, comprises a mixture of NH3 with H2.

[0276] Preferably, the proportion of H₂ in the mixture is at most 80 mol%; preferably at most 70 mol%, more preferably at most 60 mol%, even more preferably at most 50 mol%, most preferably at most 40 mol%, and particularly at most 30 mol%. Preferably, the proportion of H₂ in the mixture is at most 30 mol%; preferably at most 25 mol%; more preferably at most 20 mol%, even more preferably at most 15 mol%, most preferably at most 10 mol%, and particularly at most 5.0 mol%.

[0277] Preferably, the proportion of H₂ in the mixture is at least 1.0 mol%; preferably at least 2.0 mol%, more preferably at least 3.0 mol%, even more preferably at least 4.0 mol%, most preferably at least 5.0 mol%, and particularly at least 6.0 mol%. Preferably, the proportion of H₂ in the mixture is at least 5.0 mol%; preferably at least 10 mol%, more preferably at least 20 mol%, even more preferably at least 30 mol%, most preferably at least 40 mol%, and particularly at least 50 mol%.

[0278] Preferably, the molar ratio of H2: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 or 70:30 to 75:25.

[0279] In preferred embodiments, the flammable gas in the second section of the start-up mode and in the subsequent operating mode, i.e. preferably after the addition of NH3 to the flammable gas, comprises a mixture of NH3 with hydrocarbon, preferably CH4, C3H8, or C4H8.

[0280] Preferably, the proportion of hydrocarbon, preferably CH4, C3H8, or C4H8, in the mixture is at most 80 mol%; more preferably at most 70 mol%, more preferably at most 60 mol%, even more preferably at most 50 mol%, most preferably at most 40 mol%, and particularly at most 30 mol%. Preferably, the proportion of H2 in the mixture is at most 30 mol%, more preferably at most 25 mol%, more preferably at most 20 mol%, even more preferably at most 15 mol%, most preferably at most 10 mol%, and particularly at most 5.0 mol%.

[0281] Preferably, the proportion of hydrocarbon, preferably CH4, C3H8, or C4H8, in the mixture is at least 1.0 mol%; preferably at least 2.0 mol%, more preferably at least 3.0 mol%, even more preferably at least 4.0 mol%, most preferably at least 5.0 mol%, and particularly at least 6.0 mol%. Preferably, the proportion of H2 in the mixture is at least 5.0 mol%, more preferably at least 10 mol%, more preferably at least 20 mol%, even more preferably at least 30 mol%, most preferably at least 40 mol%, and particularly at least 50 mol%.

[0282] Preferably, the molar ratio of hydrocarbon, preferably CH4, C3H8, or C^Hs : 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 or 70:30 to 75:25.

[0283] Preferably, the exhaust gas in the second section of the start-up mode and in the subsequent operating mode, i.e. preferably after the addition of NH3 to the combustible gas, has a different composition than in the start-up mode, i.e. preferably before the addition of NH3 to the combustible gas.

[0284] In preferred embodiments, the exhaust gas in the second section of the start-up mode and in the subsequent operating mode, i.e. preferably after the addition of NH3, comprises the combustible gas, NH3, NO X , N2O, N2and H2O.

[0285] Preferably the exhaust gas contains NH3 in the second section of the start-up mode and in the subsequent operating mode, i.e. preferably after the addition of NH3 to the combustible gas.

[0286] Preferably, the exhaust gas in the second section of the start-up mode and in the subsequent operating mode, i.e., preferably after the addition of NH3 to the combustible gas, contains at least 0.005 mol% NH3; preferably at least 0.01 mol%, more preferably at least 0.02 mol%, even more preferably at least 0.04 mol%, most preferably at least 0.06 mol%, and in particular at least 0.08 mol%.

[0287] Preferably, the exhaust gas contains both NH3 and NO. X in the second phase of the start-up mode and in the subsequent operating mode, i.e., preferably after the addition of NH3 to the combustible gas; preferably after the exhaust gas temperature reaches the limit temperature T G has exceeded.

[0288] In preferred embodiments, the exhaust gas comprises further gaseous components; preferably selected from the group consisting of O2, CO, CO2, CH4 and mixtures thereof.

[0289] In preferred embodiments, the exhaust gas has an H2O content of more than 4.0 vol.%; preferably at least 5.0 vol.%, more preferably at least 6.0 vol.%, even more preferably at least 7.0 vol.%, most preferably at least 8.0 vol.%, and in particular at least 9.0 vol.%.

[0290] In further preferred embodiments, the exhaust gas has an H2O content of at least 10 vol.%; preferably at least 12 vol.%, more preferably at least 14 vol.%, even more preferably at least 16 vol.%, most preferably at least 18 vol.%, and in particular at least 20 vol.%.

[0291] Preferably, the exhaust gas has a pressure of at most 9.0 bar when leaving the combustion device; preferably at most 7.0 bar, more preferably at most 5.0 bar, even more preferably at most 3.0 bar, most preferably at most 1.5 bar; preferably atmospheric pressure.

[0292] Preferably, the exhaust gas upon leaving the combustion device in operating mode, i.e., preferably after the addition of NH3 to the combustible gas, has a temperature Ti of at least 500°C, more preferably at least 600°C, even more preferably at least 700°C, most preferably at least 800°C, and in particular at least 900°C.

[0293] Preferably, the exhaust gas upon leaving the combustion device in operating mode, i.e., preferably after the addition of NH3 to the combustible gas, has a temperature Ti of at most 1100°C, more preferably at most 1000°C, even more preferably at most 900°C, most preferably at most 800°C, and in particular at most 700°C.

[0294] Preferably, the temperature T2 in operating mode, i.e., preferably after the addition of NH3 to the flammable gas, is at least 180°C, preferably at least 200°C, more preferably at least 250°C, even more preferably at least 300°C, most preferably at least 320°C, and in particular at least 350°C.

[0295] Preferably, the temperature T2 in operating mode, i.e., preferably after the addition of NH3 to the flammable gas, is at most 550°C, preferably at most 500°C, more preferably at most 480°C, even more preferably at most 460°C, most preferably at most 440°C, and in particular at most 420°C.

[0296] The method according to the invention is preferably used to prevent the formation of NH4NO3 deposits when starting up a plant for the combustion of NH3.

[0297] Another aspect of the invention relates to a method for shutting down an NH3 combustion plant comprising a combustion device and an exhaust gas duct, wherein the method comprises the following steps: (a) Burning a flammable gas in the combustion device, producing an exhaust gas and transferring the exhaust gas into the exhaust duct; (e) Stopping the metering of NH3 into the combustible gas as soon as the temperature of the exhaust gas in the combustion device and in the exhaust gas duct falls below a limit temperature T G lies in the process where the formation of NH4NO3 deposits occurs from NH3 and NO contained in the exhaust gas. X is prevented.

[0298] Preferably, the procedure for shutting down an NH3 combustion plant comprises the following steps: (a) Burning a flammable gas in a combustion appliance, producing an exhaust gas; (b) optional, addition of a reducing agent for NO X and / or for N2O into the exhaust gas downstream of the combustion device; (c) Passing the exhaust gas through an exhaust gas treatment unit; (d) Measuring the temperature T A of the exhaust gas at or after leaving the exhaust gas treatment unit; (e) Stopping the addition of NH3 to the flammable gas as soon as the measured temperature T A the limiting temperature T G has fallen below (T Ä < T G ).

[0299] Preferred embodiments of the invention are summarized below as sentences: Sentence 1: A method for starting up an NH3 combustion plant comprising a combustion unit and an exhaust gas duct, the method comprising the following steps: (a) burning a combustible gas in the combustion unit to obtain exhaust gas and transferring the exhaust gas to the exhaust gas duct; (e) metering NH3 into the combustible gas as soon as the temperature of the exhaust gas in the combustion unit and in the exhaust gas duct exceeds a limit temperature T G in which the formation of deposits of NH4NO3 from NH3 and NOx contained in the exhaust gas is prevented. Sentence 2: The method according to Sentence 1, wherein the system, in addition to the combustion device and the exhaust duct, further comprises an exhaust gas treatment unit downstream of the exhaust gas duct and an outlet downstream of the exhaust gas treatment unit, and wherein the method further comprises: (c) passing the exhaust gas through the exhaust gas treatment unit and through the outlet; wherein the metering of NH3 into the combustible gas takes place in step (e) as soon as the temperature of the exhaust gas at or after leaving the exhaust gas treatment unit is above the limit temperature T G lies. Sentence 3: The procedure according to sentence 1 or 2, comprising the additional step of: (b) adding a reducing agent for NO X and / or for N2O into the exhaust gas downstream of the combustion device and preferably upstream of the exhaust gas treatment unit. Sentence 4: The procedure according to any of the preceding sentences, comprising the additional step: (d) Determining the temperature of the exhaust gas. Sentence 5: The procedure according to Sentence 4, wherein in step (d) the determination of the temperature T Ä of the exhaust gas at or after leaving the exhaust gas treatment unit; and wherein the addition of NH3 to the combustible gas takes place in step (e) as soon as the temperature T Ä the limiting temperature T G has exceeded (T A > T G ). Sentence 6: The method according to Sentence 4 or 5, wherein in step (d) the determination by measuring the temperature T A of the exhaust gas at or after leaving the exhaust gas treatment unit. Sentence 7: The method according to any of the preceding sentences, comprising the following steps: (a) burning a flammable gas in a combustion device to obtain an exhaust gas; (b) optionally, adding a reducing agent for NO Xand / or for N2O into the exhaust gas downstream of the combustion unit; (c) directing the exhaust gas through an exhaust gas treatment unit; (d) measuring the temperature T Ä of the exhaust gas at or after leaving the exhaust gas treatment unit; (e) Dosing- REN of NH3 into the flammable gas as soon as the measured temperature T A a limiting temperature T G has exceeded (T A > T G ). Sentence 8: The method according to one of the preceding sentences, wherein the combustion of the flammable gas is carried out to generate heat. Sentence 9: The method according to one of the preceding sentences, wherein the combustion of the flammable gas is non-catalytic. Sentence 10: The method according to one of the preceding sentences, wherein the combustion device is integrated into a plant for the thermal and / or catalytic decomposition of NH3 into N2 and H2. Sentence 11: The method according to one of the preceding sentences, wherein the plant for the combustion of NH3- in an operating mode (normal operation) at operating temperature T B is operable and - in a start-up mode (start-up operation) to bring the NH3 combustion system, preferably the exhaust gas treatment unit, up to operating temperature T B to warm up. Sentence 12: The method according to Sentence 11, wherein the temperature of the exhaust gas in a first section of the start-up mode preferably does not yet reach the limit temperature T G has exceeded the limit and preferably no NH3 is added to the flammable gas, i.e. the flammable gas preferably contains no NH3. Sentence 13: The method according to sentence 11 or 12, wherein the temperature of the exhaust gas in a second section of the start-up mode and in the subsequent operating mode preferably the limit temperature T Ghas exceeded and NH3 is preferably added to the flammable gas, i.e. the flammable gas preferably contains NH3. Sentence 14: The method according to any of the preceding sentences, wherein in step (a) the combustion of the flammable gas to power an internal combustion engine takes place. Sentence 15: The method according to one of the preceding sentences, wherein in step (a) the combustion of the combustible gas takes place to drive a gas turbine. Sentence 16: The method according to one of the preceding sentences, wherein the flammable gas after the addition of NH3 has a different composition than the flammable gas before the addition of NH3. Sentence 17: The method according to one of the preceding sentences, wherein the flammable gas in step (a) preferably contains H2 and / or hydrocarbon prior to the addition of NH3; preferably H2 and / or CH4, C3H8, or C4H8. Sentence 18: The method according to one of the preceding sentences, wherein the flammable gas in step (a) preferably contains at least 50 mol% H2 and / or hydrocarbons before the addition of NH3 to the flammable gas; preferably at least 60 mol%, more preferably at least 70 mol%, more preferably at least 80 mol%, most preferably at least 90 mol%, and in particular at least 95 mol%. Sentence 19: The method according to one of the preceding sentences, wherein the flammable gas in step (a) preferably consists of H2 and / or hydrocarbons prior to the addition of NH3 to the flammable gas; preferably H2 and / or CH4, C3H8, or C4H8. Sentence 20: The method according to one of the preceding sentences, wherein the flammable gas in step (a) preferably contains at least 50 mol% H2 before the addition of NH3 to the flammable gas; preferably at least 60 mol%, more preferably at least 70 mol%, more preferably at least 80 mol%, most preferably at least 90 mol%, and in particular at least 95 mol%. Sentence 21: The method according to one of the preceding sentences, wherein the flammable gas in step (a) preferably consists of H2 before the addition of NH3 to the flammable gas. Sentence 22: The method according to one of the preceding sentences, wherein the flammable gas in step (a) preferably contains at least 50 mol% hydrocarbon, preferably CH4, C3H8, or C4H8: preferably at least 60 mol%, more preferably at least 70 mol%, even more preferably at least 80 mol%, most preferably at least 90 mol%, and in particular at least 95 mol%. Sentence 23: The method according to one of the preceding sentences, wherein the flammable gas in step (a) preferably consists of hydrocarbons, preferably CH4, C3H8, or C4H8, prior to the addition of NH3. Sentence 24: The method according to one of the preceding sentences, wherein the flammable gas in step (a) preferably contains at most 10 mol% NH3 before the addition of NH3 to the flammable gas; preferably at most 3.0 mol%, more preferably at most 1.0 mol%, more preferably at most 0.1 mol%, most preferably at most 0.01 mol%, and in particular at most 0.001 mol%. Sentence 25: The method according to one of the preceding sentences, wherein the flammable gas in step (a) preferably contains at most 0.08 mol% NH3 before the addition of NH3 to the flammable gas; preferably at most 0.06 mol%, more preferably at most 0.04 mol%, even more preferably at most 0.02 mol%, most preferably at most 0.01 mol%, and in particular at most 0.005 mol%. Sentence 26: The method according to one of the preceding sentences, wherein the flammable gas in step (a) preferably does not contain NH3 before the addition of NH3 to the flammable gas. Sentence 27: The method according to one of the preceding sentences, wherein the air ratio X is in the range of 0.9 to 3.3, preferably 1.0 to 2.8, more preferably 1.1 to 2.3, most preferably 1.2 to 1.5. Sentence 28: The method according to one of the preceding sentences, wherein the air ratio X is in the range of 0.9 to 1.7, preferably 1.0 to 1.6, more preferably 1.1 to 1.5, most preferably 1.2 to 1.4. Sentence 29: The procedure according to one of the preceding sentences, wherein the exhaust gas NO X , N2O, N2 and H2O comprises, preferably before the addition of NH3 to the flammable gas. Sentence 30: The method according to one of the preceding sentences, wherein the flammable gas in step (a) preferably contains at most 10 ppmv NH3 before the addition of NH3 to the flammable gas; before- at most 5 ppmv, preferably at most 1 ppmv, even more preferably at most 0.5, most preferably at most 0.1 ppmv, and in particular at most 0.05 ppmv. Sentence 31: The method according to one of the preceding sentences, wherein the exhaust gas does not contain NH3, preferably before the addition of NH3 to the combustible gas. Sentence 32: The method according to one of the preceding sentences, wherein the exhaust gas comprises further gaseous components; preferably selected from the group consisting of O2, CO, CO2, CH4 and mixtures thereof. Sentence 33: The method according to one of the preceding sentences, wherein the exhaust gas has an H2O content of more than 4.0 vol.%; preferably at least 5.0 vol.%, more preferably at least 6.0 vol.%, even more preferably at least 7.0 vol.%, most preferably at least 8.0 vol.%, and in particular at least 9.0 vol.%. Sentence 34: The method according to one of the preceding sentences, wherein the exhaust gas has a content of H2O of at least 10 vol.%; preferably at least 12 vol.%, more preferably at least 14 vol.%, even more preferably at least 16 vol.%, most preferably at least 18 vol.%, and in particular at least 20 vol.%. Sentence 35: The method according to one of the preceding sentences, wherein the exhaust gas has a pressure of at most 9.0 bar a when leaving the combustion device; preferably at most 7.0 bar a, more preferably at most 5.0 bar a, even more preferably at most 3.0 bar a, most preferably at most 1.5 bar a; preferably atmospheric pressure. Sentence 36: The method according to one of the preceding sentences, wherein step (a) of the inventive method comprises the additional sub-step: (a2) Cooling the exhaust gas; preferably in at least one cooling device, preferably a heat exchanger, which is arranged downstream of the combustion device and preferably upstream of the exhaust gas treatment unit in the direction of flow of the exhaust gas. Sentence 37: The procedure according to one of the preceding sentences, wherein step (b) is only carried out after the temperature of the exhaust gas has reached the limit temperature T G has exceeded. Sentence 38: The process according to any of the preceding sentences, wherein the reducing agent in step (b) is selected from NH3, hydrocarbons, CO, H2 and mixtures thereof; preferably NH3. Sentence 39: The process according to one of the preceding sentences, wherein the reducing agent in step (b) is NH3, which is used in an amount of 0.9 to 2.5 molar parts, preferably 1.0 to 1.6 molar parts, more preferably 1.0 to 1.4, more preferably 1.0 to 1.2 molar parts, based on a molar part of NO to be chemically reduced X . Sentence 40: The procedure according to one of the preceding sentences, wherein in step (c) the content of N2O and / or NO X The amount of NH3 in the exhaust gas is reduced, preferably after adding NH3 to the combustible gas. Sentence 41: The procedure according to one of the preceding sentences, wherein step (c) comprises one or more of the following sub-steps: (c) reducing the content of N2O in the exhaust gas by (ci. a ) Decomposition of N2O on an N2O decomposition catalyst and / or (ci. b(c) Chemical reduction of N2O with a reducing agent over an N2O reduction catalyst; (c2) Reduction of the NO content X in the exhaust gas through chemical reduction of NO X with reducing agent at an NO x -Reduction catalyst. Sentence 42: The method according to one of the preceding sentences, wherein the exhaust gas treatment unit comprises at least one catalyst; preferably an N2O decomposition catalyst and / or an N2O reduction catalyst and / or an NO x -Reduction catalyst. Sentence 43: The process according to sentence 42, wherein the N2O decomposition catalyst and / or the N2O reduction catalyst and / or the NO x-Reduction catalyst independently comprises a mesoporous, preferably zeolitic material; preferably a zeolite loaded with transition metal (including lanthanide), in particular with iron, cobalt or copper; more preferably an iron-loaded zeolite; even more preferably an iron-loaded zeolite of type MFI, BEA, FER, MOR, FAU and / or MEL. Sentence 44: The method according to sentence 42 or 43, wherein the exhaust gas treatment unit additionally comprises a further catalyst or one of the aforementioned N2O reduction, N2O decomposition or NO x -Reduction catalysts fulfill at least one further functionality; preferably an NHβ oxidation catalyst. Sentence 45: The method according to sentence 44, wherein in the exhaust gas treatment unit, preferably after the addition of NH3 to the combustible gas, the following additional step is carried out: (f2) Reducing the content of NH3 in the exhaust gas by oxidation with an oxidizing agent on an NH3 oxidation catalyst; wherein the oxidizing agent preferably comprises O2. Sentence 46: The method according to one of the preceding sentences, wherein the exhaust gas treatment unit includes at least one catalyst for the selective catalytic reduction of NO X includes and the limit temperature T G the catalyst's start-up temperature relative to NO x -Reduction of the exhaust gas treatment unit corresponds to (T G = T R (NO X )). Sentence 47: The method according to one of the preceding sentences, wherein the exhaust gas treatment unit comprises at least two or more catalysts made of different materials and the limit temperature T Gcorresponds to the starting temperature of the catalyst with the lowest starting temperature. Sentence 48: The method according to one of the preceding sentences, wherein the exhaust gas treatment unit has a NO x -Reduction catalyst includes and the limiting temperature T G the NO start-up temperature x -Reduction catalyst corresponds to (T G = T R (NO X )). Sentence 49: The method according to one of the preceding sentences, wherein the exhaust gas treatment unit comprises an iron-loaded zeolite catalyst and the limiting temperature T G the start-up temperature of the iron-loaded zeolite catalyst corresponds to (T G = T R (NO X )). Sentence 50: The procedure according to one of the preceding sentences, wherein the limiting temperature T Gat least 150°C; preferably at least 160°C, more preferably at least 170°C, even more preferably at least 180°C, most preferably at least 190°C, and in particular at least 200°C. Sentence 51: The procedure according to one of the preceding sentences, wherein the limiting temperature T G at most 450°C; preferably at most 400°C, more preferably at most 350°C, even more preferably at most 300°C, most preferably at most 250°C, and in particular at most 200°C. Sentence 52: The procedure according to one of the preceding sentences, wherein the limiting temperature T G in the range of 185±40°C; preferably in the range of 185±35°C, more preferably in the range of 185±30°C, even more preferably in the range of 185±25°C, most preferably in the range of 185±20°C, and particularly in the range of 185±15°C. Sentence 53: The method according to any of the preceding sentences, wherein NH3 is added to the flammable gas at least 30 seconds after the commencement of combustion of the flammable gas in step (a); preferably at least 60 seconds, more preferably at least 90 seconds, even more preferably at least 120 seconds, most preferably at least 150 seconds, and in particular at least 180 seconds. Sentence 54: The method according to one of the preceding sentences, wherein NH3 is added to the flammable gas at least 5 minutes after the start of combustion of the flammable gas in step (a); preferably at least 10 minutes, more preferably at least 15 minutes, more preferably at least 20 minutes, most preferably at least 25 minutes, and in particular at least 30 minutes. Sentence 55: The method according to one of the preceding sentences, wherein the flammable gas preferably contains, after the addition of NH3, a mixture of NH3 with H2 and / or hydrocarbons; preferably a mixture of NH3 with H2 and / or CH4 (natural gas), C3H8, or C4H8. Sentence 56: The method according to one of the preceding sentences, wherein the proportion of NH3 in the flammable gas is preferably at least 0.5 mol% after the addition of NH3 to the flammable gas; preferably at least 1.0 mol%, more preferably at least 2.0 mol%, even more preferably at least 4.0 mol%, most preferably at least 6.0 mol%, and in particular at least 8.0 mol%. Sentence 57: The method according to one of the preceding sentences, wherein the proportion of NH3 in the flammable gas is preferably at least 10 mol% after the addition of NH3 to the flammable gas; preferably at least 20 mol%, more preferably at least 30 mol%, even more preferably at least 50 mol%, most preferably at least 75 mol%, and in particular at least 90 mol%. Sentence 58: The method according to one of the preceding sentences, wherein the proportion of NH3 in the flammable gas is preferably at most 95 mol% NH3 after the addition of NH3 to the flammable gas; preferably 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%. Sentence 59: The method according to one of the preceding sentences, wherein the flammable gas preferably comprises a mixture of NH3 with H2 after the addition of NH3 to the flammable gas. Sentence 60: The method according to Sentence 59, wherein the proportion of H2 in the mixture is at most 80 mol%; preferably at most 70 mol%, more preferably at most 60 mol%, even more preferably at most 50 mol%, most preferably at most 40 mol%, and in particular at most 30 mol%. Sentence 61: The process according to sentence 59 or 60, wherein the proportion of H2 in the mixture is at most 30 mol%, preferably at most 25 mol%; more preferably at most 20 mol%, still more preferably at most 15 mol%, most preferably at most 10 mol%, and in particular at most 5.0 mol. Sentence 62: The method according to one of sentences 59 to 61, wherein the proportion of H2 in the mixture is at least 1.0 mol%; preferably at least 2.0 mol%, more preferably at least 3.0 mol%, more preferably at least 4.0 mol%, most preferably at least 5.0 mol%, and in particular at least 6.0 mol%. Sentence 63: The method according to one of sentences 59 to 62, wherein the proportion of H2 in the mixture is at least 5.0 mol%; preferably at least 10 mol%, more preferably at least 20 mol%, more preferably at least 30 mol%, most preferably at least 40 mol%, and in particular at least 50 mol%. Sentence 64: The method according to one of sentences 59 to 63, wherein the molar ratio of H2: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, still more preferably 60:40 to 75:25, most preferably 65:35 to 70:30 or 70:30 to 75:25. Sentence 65: The method according to one of the preceding sentences, wherein the flammable gas preferably comprises a mixture of NH3 with hydrocarbon, preferably CH4, C3H8, or C4H8, after the addition of NH3 to the flammable gas. Sentence 66: The process according to sentence 65, wherein the proportion of hydrocarbon, preferably CH4, C3H8, or C4H8 in the mixture is at most 80 mol%; preferably at most 70 mol%, more preferably at most 60 mol%, even more preferably at most 50 mol%, most preferably at most 40 mol%, and in particular at most 30 mol%. Sentence 67: The process according to sentence 65 or 66, wherein the proportion of hydrocarbon, preferably CJK, C3H8, or C4H8 in the mixture is at most 30 mol%, preferably at most 25 mol%; more preferably at most 20 mol%, still more preferably at most 15 mol%, most preferably at most 10 mol%, and in particular at most 5.0 mol%. Sentence 68: The process according to one of sentences 65 to 67, wherein the proportion of hydrocarbon, preferably CH4, C3H8, or C4H8 in the mixture is at least 1.0 mol%; preferably at least 2.0 mol%, more preferably at least 3.0 mol%, even more preferably at least 4.0 mol%, most preferably at least 5.0 mol%, and in particular at least 6.0 mol%. Sentence 69: The process according to one of sentences 65 to 68, wherein the proportion of hydrocarbon, preferably CH4, C3H8, or C4H8, in the mixture is at least 5.0 mol%; preferably at least 10 mol%, preferably at least 20 mol%, more preferably at least 30 mol%, most preferably at least 40 mol%, and in particular at least 50 mol%. Sentence 70: The process according to one of sentences 65 to 69, wherein the molar ratio of hydrocarbon, preferably CH4, C3H8, or C4H8: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, still more preferably 60:40 to 75:25, most preferably 65:35 to 70:30 or 70:30 to 75:25. Sentence 71: The method according to one of the preceding sentences, wherein the exhaust gas after the addition of NH3 to the combustible gas has a different composition than before the addition of NH3 to the combustible gas. Sentence 72: The procedure according to one of the preceding sentences, wherein the exhaust gas NH3, NO X , N2O, N2 and H2O comprises, preferably after the addition of NH3 to the flammable gas. Sentence 73: The method according to one of the preceding sentences, wherein the exhaust gas contains NH3, preferably after the addition of NH3 to the combustible gas. Sentence 74: The method according to one of the preceding sentences, wherein the exhaust gas contains at least 0.005 mol.- % NH3; preferably at least 0.01 mol%, more preferably at least 0.02 mol%, even more preferably at least 0.04 mol%, most preferably at least 0.06 mol%, and in particular at least 0.08 mol%, preferably after the addition of NH3 to the combustible gas.

Claims

1. Patent claims:

1. A method for starting up an NH3 combustion plant comprising a combustion unit and an exhaust gas duct, wherein the method comprises the following steps: (a) Burning a flammable gas in the combustion device, producing an exhaust gas and transferring the exhaust gas into the exhaust duct; (e) Metering NH3 into the combustible gas as soon as the temperature of the exhaust gas in the combustion device and in the exhaust gas duct exceeds a limit temperature T G lies in the process where the formation of NH4NO3 deposits occurs from NH3 and NO contained in the exhaust gas. X is prevented.

2. The method according to claim 1, wherein the system, in addition to the combustion device and the exhaust duct, further comprises an exhaust gas treatment unit downstream of the exhaust gas duct and an outlet downstream of the exhaust gas treatment unit, and wherein the method further comprises the step of: (c) Passing the exhaust gas through the exhaust gas treatment unit and through the outlet; wherein the addition of NH3 to the combustible gas takes place in step (e) as soon as the temperature of the exhaust gas at or after leaving the exhaust gas treatment unit exceeds the limit temperature T G lies.

3. The method according to claim 1 or 2, comprising the additional step: (b) Addition of a reducing agent for NO X and / or for N2O into the exhaust gas downstream of the combustion device and preferably upstream of the exhaust gas treatment unit.

4. The method according to any of the foregoing claims, comprising the additional step: (d) Determining the temperature of the exhaust gas.

5. The method according to claim 4, wherein in step (d) the determination of the temperature T Ä of the exhaust gas at or after leaving the exhaust gas treatment unit; and wherein the addition of NH3 to the combustible gas takes place in step (e) as soon as the temperature T A the limiting temperature T G has exceeded (T Ä > T G ).

6. The method according to claim 4 or 5, wherein in step (d) the determination by measuring the temperature T Ä of the exhaust gas at or after leaving the exhaust gas treatment unit.

7. The method according to one of the preceding claims, wherein the combustion of the flammable gas is carried out to generate heat.

8. The method according to one of the preceding claims, wherein the combustion of the flammable gas is non-catalytic.

9. The method according to any of the preceding claims, wherein the flammable gas in step (a) preferably contains or consists substantially of the flammable gas H2 and / or hydrocarbon prior to the addition of NH3; preferably H2 and / or CH4, C3H8, or C4H8.

10. The method according to one of the preceding claims, wherein the flammable gas in step (a) preferably contains at most 10 mol% NH3 before the addition of NH3; preferably at most 8.0 mol%, more preferably at most 6.0 mol%, even more preferably at most 4.0 mol%, most preferably at most 2.0 mol%, and in particular at most 1.0 mol%; preferably the flammable gas in step (a), preferably before the addition of NH3, contains no NH3.

11. The method according to one of the preceding claims, wherein the exhaust gas - preferably before adding NH3 to the flammable gas, NO X, N2O, N2 and H2O and preferably does not contain NH3; and / or - preferably after the addition of NH3 to the flammable gas, NH3, NO X , N2O, N2 and H2O.

12. The method according to one of the preceding claims, wherein step (b) is only carried out after the temperature of the exhaust gas has reached the limit temperature T G has exceeded.

13. The method according to any one of the preceding claims, wherein the exhaust gas treatment unit comprises at least one catalyst; preferably an NO catalyst. x -Reduction catalyst.

14. The method according to one of the preceding claims, wherein the limiting temperature T G - is at least 150°C; preferably at least 160°C, more preferably at least 170°C, even more preferably at least 180°C, most preferably at least 190°C, and in particular at least 200°C; and / or is at most 450°C; preferably at most 400°C, more preferably at most 350°C, even more preferably at most 300°C, most preferably at most 250°C, and in particular at most 200°C; and / or - in the range of 185±40°C; preferably in the range of 185±35°C, more preferably in the range of 185±30°C, even more preferably in the range of 185±25°C, most preferably in the range of 185±20°C, and particularly in the range of 185±15°C.

15. The method according to one of the preceding claims, wherein the flammable gas preferably contains, after the addition of NH3, a mixture of NH3 with H2 and / or hydrocarbons; preferably a mixture of NH3 with H2 and / or CH4 (natural gas), C3H8, or C4H8.