Method and apparatus for removing nitrogen oxides from flue gases

Abstract

The invention relates to a method for removing nitrogen oxides from flue gases occurring in staged combustion processes, as well as to an apparatus for carrying out the method.

Description

[0001] METHOD AND DEVICE FOR DENOXIFICATION OF FLUE GASES

[0002] The present invention relates to the technical field of exhaust gas treatment, in particular the treatment of flue gases containing nitrogen oxides.

[0003] In particular, the present invention relates to a process for denitrification of flue gases.

[0004] Furthermore, the present invention relates to a device for denitrification of flue gases.

[0005] Furthermore, the present invention relates to the use of a device for denitrification of flue gases.

[0006] Combustion reactions in the presence of air produce metastable, generally toxic, and reactive oxides of nitrogen, known as nitrogen oxides. The formation of nitrogen oxides is enhanced by the combustion, thermolysis, and pyrolysis of organic and inorganic nitrogen-containing compounds, which occurs in large combustion plants such as combined heat and power plants or waste incineration plants.

[0007] Nitrogen oxides, in particular the compounds nitrogen monoxide and nitrogen dioxide known as nitrous gases, which are also abbreviated to NOx, are not only toxic and cause irritation and damage to the respiratory system, but also promote the formation of acid rain because they react with moisture to form acids.

[0008] However, the release of nitrogen oxides is also problematic for further environmental protection reasons, as they promote the formation of smog and harmful ground-level ozone, and as greenhouse gases they increase global warming.

[0009] Due to the adverse effects of nitrogen oxides on health and the environment, and not least the associated economic damage, early attempts were made to minimize or prevent the release of nitrogen oxides during combustion processes. In cars, for example, this is achieved through the use of catalytic converters, which allow for almost complete removal of nitrogen oxides from exhaust gases.

[0010] In order to reduce nitrogen oxide emissions from large-scale plants, particularly large-scale industrial combustion plants, various denitrification (DeNOx) processes have been developed, taking into account the respective legal situation and economic considerations. These processes, either alone or in combination, are intended to effectively reduce or prevent nitrogen oxides in exhaust gases, particularly flue gases.

[0011] The processes or measures for reducing the nitrogen oxide content of exhaust gases, especially flue gases, can be divided into primary and secondary measures:

[0012] As part of the primary measures, the combustion process is controlled to keep the nitrogen oxide content in the resulting exhaust gases as low as possible; the aim is to prevent nitrogen oxides from being formed in the first place. Examples of primary measures include flue gas recirculation, in which the flue gas is fed back into the combustion zone, as well as air or fuel stages, in which combustion is controlled so that various combustion zones with different oxygen concentrations are passed through. In addition, the formation of nitrogen oxides in flue gases can also be reduced by adding additives or by quenching, i.e., by injecting water to lower the temperature during the combustion process.

[0013] In contrast to primary measures, which are intended to prevent the formation of nitrogen oxides, secondary measures are intended to reduce the concentration of nitrogen oxides in the exhaust gases, especially flue gases. Secondary measures include, for example, separation processes in which the nitrogen oxides are chemically bound or washed out of the flue gas stream. A disadvantage of separation processes, however, is that large quantities of waste products, such as process water, which are often contaminated with other flue gas components, are retained and must be disposed of at great expense.

[0014] In modern large-scale plants, therefore, secondary measures are usually based on the reduction of nitrogen oxides to elemental nitrogen and result in only small amounts of waste materials, whereby a general distinction is made between catalytic and non-catalytic processes.

[0015] Selective catalytic reduction of nitrogen oxides (SCR) involves catalytic processes in which nitrogen oxides are converted into elemental nitrogen with the aid of metal catalysts. SCR processes generally achieve the best denitrification values, although the use of the catalyst makes the process significantly more expensive and less economically viable. Furthermore, the systems used to implement the SCR process are extremely expensive not only to purchase but also to maintain, as the sensitive catalysts must be serviced or replaced at short intervals. Especially in large combustion plants where the fuel composition can often only be inadequately determined, such as waste incineration plants, there is always a risk of catalyst poisoning due to flue gas contamination.This risk can only be reduced through additional cost-intensive measures.

[0016] Selective non-catalytic reduction (SNCR), on the other hand, is based on the thermolysis of nitrogen compounds, in particular ammonia or urea, which then react with the nitrogen oxides in a comproportionation reaction to form elemental nitrogen.

[0017] Selective non-catalytic reduction is significantly more cost-effective than selective catalytic reduction: the costs for purchasing and maintaining SNCR systems are just 10 to 20% of the costs of corresponding SCR systems.

[0018] The problem with SNCR processes, however, is that their effectiveness often does not compare to that of catalytic processes. Therefore, if, for example, the legally permitted limits for nitrogen oxides in exhaust gases, especially flue gases, were further reduced, many SNCR plants would no longer be able to operate. A further disadvantage of processes based on the selective non-catalytic reduction of nitrogen oxides is that the reducing agent must be used in excess and therefore does not react completely, resulting in a certain, potentially significant, ammonia load in the exhaust gas. Excess ammonia in the exhaust gas must either be separated or its content reduced through process engineering measures to such an extent that the exhaust gas stream can be released into the environment.

[0019] In addition, there are also processes that are based on both a catalytic mode of action and the use of reducing agents, although these processes cannot overcome the fundamental disadvantages of the respective processes (high costs for the use of catalytic processes or low effectiveness for the use of reducing agents).

[0020] Although there are now new SNCR systems based on the combined use of several reducing agents and which are almost equivalent to catalytic processes in terms of their effectiveness, they cannot deliver optimal results under all operating conditions.

[0021] SNCR plants that operate with a combination of multiple reducing agents are the exception. Typically, SNCR processes use equipment and systems designed and operated either with urea or ammonia or ammonia water. To adapt to fluctuating flue gas temperatures, systems equipped with suitable temperature measurement have proven effective. Individual injection lances or groups of injection lances are switched depending on the temperatures at the injection points, ensuring that the reactions always take place within the optimal temperature range.

[0022] If flue gas temperatures are too low, NH3 slip can be avoided or at least reduced by switching the corresponding injection lances to inject into a higher-temperature zone. This also results in a higher NOx removal efficiency and, generally, lower reducing agent consumption.

[0023] Particularly problematic, however, are combustion plants that operate with grate firing and inhomogeneous fuel compositions, which also cannot be evenly distributed on the grate, as is the case, for example, in waste incineration plants for domestic and industrial waste as well as for biomass. Due to the inhomogeneous fuels, the flue gas flows from the furnace are also very uneven in terms of

[0024] ■ the temperature distribution,

[0025] ■ the flow velocities and

[0026] ■ the distribution of NOx, O2, CO, SO2, SO3 and other components.

[0027] In addition, after service shutdowns, flue gas temperatures are too cold for a period of approximately two to four weeks due to the cleaned boilers, as basic contamination has not yet been able to build up on the heat exchangers. This has a potentially significant negative impact on NOx reduction and is accompanied by a significantly increased ammonia slip. For the operator, this can potentially mean the loss of their operating license if they cannot comply with the emission limits specified for the plant.

[0028] In addition, there have also been attempts to homogenize the flue gas stream and introduce the subsequent reducing agent together with the secondary air into the afterburning stage, as described in EP 1 077 077 A2. This process has never been applied industrially due to the complex process management.

[0029] The state of the art therefore still lacks a universally applicable and cost-effective denitrification process for flue gases, particularly in cases where the flue gas quantity and composition and / or the flue gas temperature and thus the nitrogen oxide emissions fluctuate significantly.

[0030] Particularly in the case of highly fluctuating nitrogen oxide emissions caused by an inhomogeneous fuel, neither SNCR nor SCR denitrification of the flue gases can guarantee compliance with the specified limit values. The systems cannot be controlled quickly enough to react to suddenly occurring nitrogen oxide emission peaks and valleys. In particular, highly fluctuating nitrogen oxide emissions with widely fluctuating combustion temperatures can result in both the upper limit for nitrogen oxide emissions and the upper limit for ammonia slip not being met. Therefore, the current state of the art still lacks a simple, cost-effective, and reliable process for reducing the nitrogen oxide load in flue gases, even in the case of highly fluctuating nitrogen emissions, without releasing ammonia-containing compounds into the environment.

[0031] An object of the present invention is therefore to avoid, or at least mitigate, the aforementioned disadvantages associated with the prior art.

[0032] In particular, it is an object of the present invention to provide a method for denitrification of flue gases which enables a reliable, simple and cost-effective reduction of the nitrogen oxide content even under unfavorable conditions, in particular strongly fluctuating boiler conditions and strongly fluctuating nitrogen oxide emissions and boiler imbalances, and which prevents the excessive release of ammonia.

[0033] In particular, it is an object of the present invention to provide a method and a device which can be easily retrofitted and implemented on existing incineration plants, in particular waste incineration plants.

[0034] The subject matter of the present invention - according to a first aspect of the present invention - is thus a method according to claim 1; further advantageous embodiments of this aspect of the invention are the subject matter of the relevant subclaims.

[0035] A further subject matter of the present invention - according to a second aspect of the present invention - is a device according to claim 21; further advantageous embodiments of this aspect of the invention are the subject matter of the relevant subclaims.

[0036] Yet another subject matter of the present invention - according to a third aspect of the present invention - is the use of a device according to the invention according to claim 26. It goes without saying that special features, features, embodiments and embodiments as well as advantages or the like which are explained below only with regard to one aspect of the invention for the purpose of avoiding unnecessary repetition, naturally apply accordingly with regard to the other aspects of the invention without this requiring a corresponding mention.

[0037] In addition, all values ​​or parameters or the like mentioned below can generally be determined using standardized or explicitly specified determination procedures or using determination methods that are familiar to a person skilled in the art.

[0038] Furthermore, it goes without saying that all percentages based on weight or quantity are selected by the person skilled in the art in such a way that the total amounts to 100%.

[0039] With that in mind, the present invention will be described in more detail below.

[0040] The subject of the present invention - according to a first aspect of the present invention - is thus a method for denitrification of flue gases from staged combustion processes, wherein

[0041] (a) in a first process step, at least one nitrogen-containing reducing agent is introduced into the flue gases with the air for a subsequent combustion stage, in particular the secondary air, and

[0042] (b) in a second process step following the first process step, at least one nitrogen-containing reducing agent is introduced into the flue gases.

[0043] Within the scope of the present invention, it is particularly preferred if the second process step (b) is carried out in the form of an SCR process or an SNCR process, preferably an SNCR process, for denitrification of flue gases. The process according to the invention for denitrification of flue gases thus combines a novel process step of introducing a nitrogen-containing reducing agent with the air into the flue gases for a subsequent combustion stage with a known SCR or SNCR process, preferably an SNCR process, for denitrification of flue gases.

[0044] As the applicant has surprisingly discovered, it is possible to drastically reduce the nitrogen oxide content in flue gases and the ammonia slip by introducing the reducing agent, in particular ammonia water and / or urea solution, into the flue gases with the air for a subsequent combustion stage during staged combustion and then carrying out conventional flue gas denitrification using SCR or SNCR processes.

[0045] By introducing a nitrogen-containing reducing agent into the flue gases in the first process step (a) with the air for subsequent combustion stages in staged combustion, in particular the secondary air, such an efficient removal of nitrogen oxides from flue gases is achieved that often the limit value or a specified clean gas value of 90 mg / Nm 3h NOx is achieved. Especially in plants with highly fluctuating nitrogen oxide emissions, a base load of nitrogen oxides can be reliably removed by injecting the reducing agent into the air for further combustion stages, especially the secondary air. The subsequent SNCR or SCR process then only needs to be adjusted to the fluctuating residual nitrogen emissions.

[0046] By introducing the reducing agent into the air for subsequent combustion stages, especially the secondary air, not only is an outstanding reduction in the nitrogen oxide content of the flue gases achieved, but it is also possible to reduce the reducing agent consumption by 20% or more while simultaneously reducing ammonia slip. Surprisingly, almost no ammonia slip is generated in the first process step. Since significantly less reducing agent is required in the second process step compared to conventional processes, the ammonia slip of the overall process is also significantly reduced compared to conventional denitrification using SCR or SNCR processes.These positive effects of the process according to the invention are particularly surprising since the flue gases in the first process stage are too hot for denitrification to enable an effective reaction of nitrogen oxides and reducing agent in the region of the further combustion zone, in particular the burnout zone, so that the oxidation of reducing agents to nitrogen oxides would be expected. However, it has been shown that the large volume flow of air for the further combustion stage, in particular the secondary air, can reliably protect the reducing agents from oxidation until they reach a zone of optimal reduction conditions and react there with the nitrogen oxides to form nitrogen. On the one hand, this makes it possible to reduce the nitrogen oxide load of the clean gas, i.e. the flue gas after cleaning, to values ​​below the prescribed limit values, in particular below a frequently targeted clean gas value of 90 mg / hNm. 3NOx, and at the same time the ammonia slip - as mentioned above - can be completely or almost completely suppressed.

[0047] In the context of the present invention, flue gases are in particular the volatile, in particular gaseous, combustion products which are obtained during oxidation of the fuel, for example substitute fuels, coal, waste or biomass, preferably waste or biomass, in the presence of air, as well as particles contained in or entrained therein.

[0048] In staged combustion, the fuel is typically exposed to a deficient amount of oxygen, known as primary air, to prevent complete combustion of the fuel and the excessive formation of nitrogen oxides. This combustion stage is often referred to as the reduction zone, as only incomplete combustion occurs here.

[0049] This is followed by further combustion stages, often referred to as post-combustion stages, in which a further supply of liquid or gaseous fuels and / or additional air, so-called secondary, tertiary, quaternary air, etc., is added to complete the incomplete combustion in the first stage, reduce the carbon monoxide content, and fully utilize the fuel thermally. The formation of nitrogen oxides is minimized here because the temperatures in the subsequent combustion stages are usually not as high as in the primary combustion stage. In the last combustion stage(s) of the staged combustion, air is generally added to the incompletely burned flue gases, which have a high CO content, without any further fuel being added to achieve complete combustion. This zone is also called the burnout or burn-out zone.In many combustion processes, especially waste incineration plants, this burnout zone follows immediately after the primary combustion and is carried out with the supply of secondary air.

[0050] To minimize NOx formation in combustion plants, as previously explained, combustion air is usually introduced in stages as primary, secondary, or tertiary air. The lack of oxygen limits NOx formation in the primary combustion zone, as oxygen has a greater affinity for bonding with carbon. With the subsequently injected secondary or tertiary air, the unburned carbon components, particularly C and CO, are burned to CO2 in the flue gas flow. Further NOx formation is no longer possible or only possible to a very limited extent due to the now lower temperatures.

[0051] The present invention utilizes the sequence of combustion processes in staged combustions for the optimization of denitrification, in particular by means of the SNCR process, in that reducing agents are introduced into the flue gas to be denitrified with the aid of the air for subsequent combustion stages, in particular the secondary air, in particular through the secondary air openings, wherein the air for the subsequent combustion stages, in particular the secondary air, is used as a carrier medium for the reducing agents, in particular ammonia water and urea solution.

[0052] Within the scope of the process according to the invention, the flue gases, which are generally too hot for the SNCR process to function in this range, are mixed with air for subsequent combustion stages, in particular the secondary air, as is usual in staged combustion, to achieve complete combustion. On the one hand, the flue gas is first cooled, and on the other hand, the reducing agents are shielded from the excessively hot flue gases by the combustion air and carried further with the flue gas, thereby preventing the undesired combustion of the nitrogen components to NOx. The secondary air heats up through mixing with the flue gas, but also through post-combustion reactions, such as the oxidation of CO to CO2.

[0053] While the reducing agents, particularly ammonia water or urea solution, are carried downstream with the oxygen-rich flue gas fraction into the higher-lying area of ​​the combustion chamber, especially the afterburner chamber, the urea decomposes, especially at temperatures of approximately 130°C, into reactive NH2 radicals and CO. However, due to the initially too cold flue gas temperatures, the reducing agent cannot yet react with NOx. Only when the temperature window effective for the SNCR process is reached does the reduction of NOx to molecular nitrogen and water vapor begin.

[0054] In contrast to conventional SNCR and SCR systems, knowledge of the precise location of the effective temperature window for the injection of reducing agent and the function of the process is not important for the first process step of the process according to the invention. The reactants react, regardless of the boiler load and the temperature distribution across the cross-section, especially in the case of boiler tilt, only when sufficiently high temperatures are reached in the flue gas. Therefore, technically complex temperature measurement systems can be dispensed with for this process step.

[0055] Therefore, homogenization or mixing of the flue gas stream to achieve a uniform temperature distribution of the flue gases across the cross-section of the combustion chamber is neither necessary nor desirable within the scope of the present invention. Such homogenization and mixing of the flue gas stream unnecessarily complicate and increase the cost of the process and reduce efficiency.

[0056] With the method according to the invention, even with strongly fluctuating nitrogen oxide emissions, significantly more than 100 mg / mN 3 NOx is separated from the flue gases.

[0057] By combining the first process step with the second process step, in particular a conventional SNCR plant, NOx emissions of 50 to 80 mg / Nm 3 It is noteworthy that the NHs slip is always in the range of 5 mg / Nm 3This is due to the fact that in the first process step the reactions only take place when the reactive ammonia radicals have reached the optimal temperature window.

[0058] Within the scope of the present invention, it is usually provided that before, in particular upstream of, the introduction of the reducing agent into the flue gases in process step (a), no process step for flue gas homogenization is carried out, in particular no oxygen-free or low-oxygen mixing medium, in particular no water vapor or recirculated flue gas, is introduced into the flue gases.

[0059] A special feature of the process according to the invention is that the reducing agent(s) can be introduced into the flue gas stream with the secondary air in the first process step (a) immediately following primary combustion. It has been shown that it is not necessary to homogenize the flue gas stream, since the reducing agent and the nitrogen oxides only react at optimal temperatures. Premature oxidation of the reducing agent is prevented by cooling or shielding by the air, especially secondary air.

[0060] Within the scope of the present invention, it is usually further provided that the reducing agent is introduced into the flue gases in the first process step (a) with the air for the combustion stage, in which no further fuel is introduced into the flue gases, in particular into the burnout zone.

[0061] In the context of the present invention, the reducing agent is therefore preferably introduced in the first process step (a) into the combustion zone in which the burnout takes place.

[0062] Within the scope of the present invention, it is preferably provided that the reducing agent is introduced into the flue gases in the first process step (a) with the secondary air and / or tertiary air, preferably the secondary air.

[0063] According to a further preferred embodiment of the present invention, it is provided that the method according to the invention is carried out as a method for denitrification of flue gases, wherein fuels, in particular solid fuels, preferably substitute fuels, coal, biomass and / or waste, are subjected to a thermal treatment, in particular combustion, with the supply of primary air, whereby flue gases are formed, wherein in a first method step (a) of denitrification, the flue gases are mixed downstream with secondary air, wherein at least one nitrogen-containing reducing agent is added to the secondary air and / or wherein at least one reducing agent is introduced into the flue gases with the secondary air, in particular wherein no method step for flue gas homogenization is carried out before, in particular upstream of, the introduction of the reducing agent into the flue gases,in particular, no oxygen-free or low-oxygen mixing medium, in particular no water vapor or recirculated flue gas, is introduced into the flue gases and / or in particular, wherein the mixing of the flue gases with the secondary air containing at least one reducing agent takes place immediately after the thermal treatment, and wherein in a second process step (b) of denitrification following the first process step (a) of denitrification, at least one nitrogen-containing reducing agent is introduced into the flue gases.

[0064] In the context of the present invention, denitrification is understood to mean a reduction in the content of nitrogen oxides, in particular NO and NO2.

[0065] In the context of the present invention, it has also proven useful if the air for the further combustion stage, in particular the secondary air, is introduced into the flue gases, in particular into the flue gas stream, in an amount of at least 5%, in particular at least 10%, preferably at least 20%, based on the total amount of air introduced for combustion.

[0066] In particular, it can be provided that the air for the further combustion stage, in particular the secondary air, is introduced into the flue gases, in particular the flue gas stream, in an amount of 5 to 60%, in particular 10 to 50%, preferably 20 to 40%, based on the total amount of air introduced for combustion. The total amount of air introduced for combustion corresponds to the amount of air supplied across all combustion stages.

[0067] Typically, within the scope of the present invention, the air for the further combustion stage, in particular the secondary air and / or the tertiary air, is injected into the flue gases. Within the scope of the present invention, it is preferred if the air for the further combustion stage, in particular the secondary and / or tertiary air, is introduced into the flue gases at a high volume flow rate, as this achieves effective cooling of the flue gases and sufficient shielding of the reducing agent.

[0068] Furthermore, it can be provided within the scope of the present invention that the air for the further combustion stage, in particular the secondary air and / or the tertiary air, before introduction, in particular injection, into the flue gases, has a temperature of at most 300 °C, in particular at most 250 °C, preferably at most 200 °C, preferably at most 150 °C, particularly preferably at most 100 °C, very particularly preferably at most 50 °C.

[0069] Furthermore, it can be provided that the air of the further combustion stage, in particular the secondary air and / or the tertiary air, has a temperature in the range from 10 to 300 °C, in particular 10 to 250 °C, preferably 10 to 200 °C, more preferably 10 to 150 °C, particularly preferably 10 to 100 °C, most preferably 10 to 50 °C, before being introduced, in particular injected, into the flue gases.

[0070] Within the scope of the present invention, it is therefore preferred if the air for the further combustion stage, in particular the secondary air and / or tertiary air, is not heated before being introduced into the flue gases. In this way, the air for the further combustion stage, in particular the secondary air and / or tertiary air, can cool the reducing agent introduced into the flue gas for as long as possible, so that it can reach the temperature ranges optimal for reduction.

[0071] As previously stated, the first process step of the process according to the invention is carried out as an SNCR process.

[0072] In the selective non-catalytic reduction (SNCR) of nitrogen oxides, reducing agents are usually injected into the hot flue gases in aqueous solutions, such as ammonia water or aqueous urea solutions, or in gaseous form, such as ammonia. The reducing agents then react with the nitrogen oxides to form molecular nitrogen, water, and carbon dioxide, as illustrated by the following reaction equations (1) and (2) for the reducing agents ammonia and urea.

[0073] (NH2)2CO + 2 NO + 1 / 2O2-> 2 N2+ CO2+ 2 H2O (1)

[0074] The optimal temperature range for achieving a significant reduction in nitrogen oxides is typically between 850 and 1,100 °C, depending on the flue gas composition. Above this temperature range, ammonia is increasingly oxidized, meaning additional nitrogen oxides are formed.

[0075] At temperatures below this, the reaction rate decreases, resulting in a so-called ammonia slip, which can lead to the formation of ammonia salts or ammonium salts further down the flue gas path and thus to secondary problems. The ammonia slip should therefore be kept as low as possible.

[0076] The goal of all NOx capture processes is to achieve a high NOx capture efficiency with the lowest possible reducing agent consumption and low NH3 slip. In principle, all ammonia-releasing substances, such as urea, ammonia, ammonia water, etc., can be used for NOx capture in flue gases from combustion plants. For optimal NOx capture with minimal NH3 slip, the reducing agents must be thoroughly mixed with the flue gases within the optimal temperature range. To achieve this, the reducing agents must be evenly distributed throughout the flue gas.

[0077] Within the scope of the present invention, an ammonia- and / or urea-based SNCR process is used for the first process step. For ammonia-based SNCR processes, ammonia can be used either in gaseous form or in the form of an aqueous solution. For urea-based SNCR processes, urea is typically used in the form of aqueous solutions.

[0078] The urea-based SNCR process consists of the following four steps:

[0079] 1. Distribution of urea-containing water droplets in the flue gas

[0080] 2. Evaporation of the water droplets in which the urea is dissolved

[0081] 3. Decomposition of urea into reactive radicals 4. Gas phase reaction between NH2 and NOx

[0082] The ammonia-based SNCR process operates similarly, but with one significant difference. Since ammonia does not need to be decomposed to react, the chemical reactions for NOx capture take place immediately after the ammonia water is injected into the flue gas.

[0083] In addition to the distribution and mixing of the reducing agents or the water droplets containing them in the flue gas, the size of the droplets is also usually of great importance in the SNCR process:

[0084] Droplets that are too small would evaporate too quickly and potentially react at too high a temperature range or too close to the colder boiler walls. Both of these can negatively impact NOx capture and / or lead to increased NH3 slip. Droplets that are too large would evaporate too slowly and lead to reactions at low temperatures or even outside the temperature window, increasing NH3 slip and decreasing NOx capture.

[0085] Ammonia slip refers specifically to the portion of ammonia that does not react with nitrogen oxides to form elemental nitrogen. The ammonia originates either from an overdose of ammonia or is a degradation product of the thermolysis of nitrogen-containing reducing agents, such as urea.

[0086] For most SNCR applications, either urea or ammonia water is used as the reducing agent, although gaseous ammonia can also be used. For optimal nitrogen oxide removal with minimal ammonia slip, the reducing agent must be evenly mixed with the flue gases within the optimal temperature range. To achieve the momentum required for complete mixing, ammonia water requires considerably more energy than urea, since ammonia has a significantly higher vapor pressure.

[0087] In addition, aqueous solutions of urea and ammonia exhibit different reaction kinetics, which is mainly due to the fact that the urea dissolved in water can only be split into reactive radicals when the water surrounding the urea particles has completely evaporated, which is why a high penetration depth into the exhaust gases is ensured with relatively low energy expenditure.

[0088] When using aqueous ammonia solutions, however, the ammonia evaporates from the individual water droplets immediately after entering the flue gases, so that the reaction takes place preferentially near the boiler walls. The partial pressure of ammonia reaches 1 bar at 38 °C. Because the ammonia is then in gaseous form, the momentum required for the optimal penetration depth of the reducing agent can only be achieved with greater energy expenditure due to its lower mass compared to a water droplet. This requires a significant increase in the corresponding steam or air volume. In addition to the higher operating costs resulting from the higher energy consumption, the investment costs for a plant operated with ammonia water are significantly higher due to safety requirements, as ammonia is a toxic gas that dissolves easily in water at ambient temperature.

[0089] Ammonia water is therefore assigned to water hazard class 2 and is also subject to the technical guidelines for steam boilers due to its high risk potential for the environment.

[0090] Due to the chemical bonding of ammonia in the urea molecule, urea solutions can be heated to temperatures up to 106 °C without evaporating ammonia gas. The decomposition of urea into ammonia and carbon dioxide gas only begins at 130 °C and reaches a maximum at approximately 380 °C. Since these high temperatures cannot be reached during storage, the safety precautions required for ammonia water are unnecessary. According to the German Water Resources Act (WHG), urea solution is only classified as Water Hazard Class 1, meaning that it is only necessary to ensure that urea cannot enter the groundwater. A collecting tray for the storage tank is sufficient for this purpose.

[0091] However, urea solutions have the disadvantage that if overdosed, it deposits in solid form on parts of the system and leads to unwanted corrosion. The urea dissolved in water can only split into reactive radicals once the water surrounding the urea particles has completely evaporated. The size of the water droplets and the resulting penetration depth can be used to predetermine the location in the flue gas where the reactions are to take place. If the water droplet is large enough and carried far enough, this allows, for example, injection into a spot that is too hot for NOx capture, enabling the reaction to take place at a cooler spot in the flue gas. The mass of the dilution water, which is used as a carrier medium in both urea solutions and ammonia water, ensures a high penetration depth with relatively low energy expenditure.

[0092] By introducing the reducing agent into the flue gases with the air for a subsequent combustion stage, in particular the secondary air, in the first process step of the process according to the invention, the precise adjustment of a droplet profile or droplet size is not necessary. Instead, the reducing agent is conveyed with the air for a subsequent combustion stage, in particular the secondary air, into temperature ranges optimal for reduction. Therefore, in the first process step, the reducing agent is preferably introduced into the flue gases in the form of the finest possible droplets or as a gas. The first process step of the process according to the invention thus represents a significant simplification compared to conventional SNCR processes.

[0093] Within the scope of the present invention, it can be provided that identical or different reducing agents are used for the first process step (a) and the second process step (b). Preferably, different reducing agents are used. Identical reducing agents mean that the reducing agents used, in particular ammonia and urea, have the same chemical structure and are used in the same concentrations. Different reducing agents mean that the reducing agents differ in their chemical composition or concentrations.

[0094] Within the scope of the present invention, it is usually provided that the reducing agent is selected from the group of ammonia, urea and mixtures thereof. Within the scope of the present invention, it has proven useful if urea and / or ammonia is used as the reducing agent in the first process step (a). Furthermore, it is preferred within the scope of the present invention if the reducing agent in the second process step (b) is urea and / or ammonia. Preferably, chemically identical compounds, in particular urea or ammonia, are used as the reducing agent in different concentrations for the first and second process steps. Within the scope of the present invention, the reducing agents used in the first and second process steps therefore preferably differ in their concentrations.

[0095] With regard to the introduction of the reducing agent into the flue gases in both process steps, it has proven effective to introduce the reducing agent into the flue gases in the form of an aqueous solution or dispersion, in particular a solution. The reducing agent is therefore particularly preferably introduced into the flue gases in the form of an aqueous ammonia and / or urea solution. The introduction of the reducing agent into the flue gas stream in the form of aqueous dispersions or solutions, in particular solutions, has the further advantage that the water evaporates upon heating, thus cooling the reducing agent and thus protecting it from premature reaction.

[0096] This effect is particularly pronounced when using urea solutions, since the water initially evaporates completely and powdered urea remains, which subsequently decomposes into ammonia radicals, which in turn react with the nitrogen oxides.

[0097] The introduction of the reducing agent into the flue gases containing nitrogen oxides, in particular the air from further combustion stages in the first process step (a), can be achieved by a variety of technical measures. However, it has proven useful within the scope of the present invention if the reducing agent is introduced, in particular sprayed or injected, preferably injected, in the first process step (a) in a finely distributed manner into the flue gas stream, in particular the air for a further combustion stage. In particular, injection can achieve a fine distribution of the reducing agent while simultaneously providing an excellent penetration depth of the reducing agent into the flue gas stream, thereby enabling a particularly effective and efficient reduction of the nitrogen oxides.It has proven advantageous if, in the first process step (a), the reducing agent is introduced into the flue gas stream, in particular the air for a further combustion stage, by means of injection devices, in particular injection lances.

[0098] The nozzles of the injection device can be designed as single-fluid or dual-fluid nozzles. The pressure required for injection in dual-fluid nozzles is typically generated by compressed air or steam.

[0099] In the context of the present invention, it has further proven useful if, in the first process step (a), the reducing agent, in particular the aqueous solution or dispersion of the reducing agent, is introduced into the flue gases, in particular the secondary and / or tertiary air, via one or more injection devices, in particular injection lances.

[0100] It is possible for each injection direction to have one or more, in particular 1 to 20, preferably 1 to 15, preferably 1 to 10, particularly preferably 1 to 5, nozzles for introducing the reducing agent into the secondary air.

[0101] Furthermore, it can equally be provided that the injection devices can be controlled individually or in groups. It is particularly preferred within the scope of the present invention if the injection devices in process step (a) can be controlled in groups.

[0102] The injection of the reducing agents into the air of further combustion stages, in particular the secondary air in the first process step (a), is preferably carried out, as far as possible, into the main channels of the air supply line of further combustion stages, in particular the secondary air supply line, or into the injection openings for injecting the air of further combustion stages, in particular the secondary air nozzles.

[0103] Within the scope of the present invention, it is therefore preferably provided that the injection devices for introducing the reducing agent are assigned to individual injection openings for introducing the air for a further combustion stage, in particular for introducing the secondary air and / or tertiary air, preferably the secondary air, into the flue gases. The injection devices for introducing the reducing agent into the flue gases in the first process step (a) are thus preferably arranged in the region of injection openings for introducing the air for further combustion stages, in particular the secondary air and / or tertiary air, into the flue gases, in particular the flue gas stream. In this way, it can be achieved that the air for the further combustion stage, in particular the secondary air and / or tertiary air, cools and surrounds the reducing agent and protects it from premature oxidation.This embodiment is particularly suitable for introducing aqueous urea solutions into the flue gas stream.

[0104] Equally, however, it is also possible for the injection devices for introducing the reducing agent in the first process step (a) to be assigned to collecting containers and / or main supply lines for introducing the air of a further combustion stage, in particular the secondary air and / or the tertiary air, into the flue gases. Within the scope of the present invention, it is thus also possible for the reducing agent, in particular the aqueous solution of the reducing agent, to be injected into collecting containers or main supply lines, so-called collectors, from which the injection openings for introducing, in particular injecting, the air for the further combustion stage, in particular the secondary air and / or the tertiary air, into the flue gases branch off. This embodiment is particularly suitable for introducing ammonia, either in the form of aqueous solutions or in gaseous form, into the flue gas stream.

[0105] Within the scope of the present invention, it can further be provided that the temperatures of the flue gases in the first process step (a) in the region of introduction of the air for the further combustion stage, in particular the secondary air and / or the tertiary air, preferably secondary air, in particular the burnout zone, in particular at least in some regions is at least 850 °C, in particular at least 1,000 °C, preferably at least 1,100 °C, preferably at least 1,200 °C. Furthermore, it can equally be provided that the temperature of the flue gases in the region of introduction of the air for the further combustion stage, in particular the secondary air and / or tertiary air, preferably secondary air, in particular at least in some regions is 850 °C to 1,600 °C, in particular 1,000 °C to 1,500 °C, preferably 1,100 °C to 1,450 °C, preferably 1,200 °C to 1,400 °C.Within the scope of the present invention, it is therefore preferably provided that in the first process step (a), the reducing agent is introduced with the air from the further combustion stage into a region of the flue gases that is fundamentally too hot, in which no reduction of nitrogen oxides is to be expected, but rather an oxidation of the introduced reducing agent. However, by cooling or protecting the reducing agent by the cooler air of the further combustion stage, the oxidation or any other reaction of the reducing agent is delayed until the reducing agent is in a temperature range optimal for the reduction of nitrogen oxides.

[0106] As already stated above, the second process stage of the process according to the invention is preferably carried out in the form of a conventional SCR or SNCR denitrification, with an SNCR procedure being preferred.

[0107] The introduction of the reducing agent into the nitrogen oxide-containing flue gases in the second process step (b) can be achieved by a variety of technical measures. However, it has proven to be effective within the scope of the present invention if the reducing agent is introduced into the flue gas stream in a finely distributed manner, in particular by spraying or injecting it. In particular, by injecting it, a fine distribution of the reducing agent can be achieved while simultaneously achieving an excellent penetration depth of the reducing agent into the flue gas stream, thus enabling a particularly effective and efficient reduction of the nitrogen oxides.

[0108] It has proven advantageous if the reducing agent is introduced into the exhaust stream in the second process step (b) using injection devices, in particular injection lances. The pressure required for injection is usually generated by compressed air or steam.

[0109] It is possible for each injection device to have one or more, in particular 1 to 20, preferably 1 to 15, more preferably 1 to 10, and particularly preferably 1 to 5, nozzles for introducing the reducing agent into the flue gas stream. A plurality of nozzles per injection device achieves a particularly fine and uniform distribution of the reducing agent in the flue gas stream.

[0110] Likewise, it has proven advantageous in this regard if the injection devices in the second process step (b) are arranged in 1 to 10, in particular 1 to 7, preferably 1 to 5, injection planes. By arranging the injection devices in injection planes, it can be ensured that the entire reactor cross-section or the entire width of the flue gas stream can be covered by the reducing agent(s).

[0111] It is possible for each injection level to have 1 to 20, in particular 1 to 15, preferably 1 to 12, injection devices.

[0112] In general, in the second process step (b), the reducing agent is introduced into the flue gas stream via 1 to 200, in particular 2 to 100, preferably 5 to 60, injection devices.

[0113] In the context of the present invention, it has proven particularly advantageous if, in the second process step (b), the injection devices for introducing the reducing agent are controlled individually and / or in groups, preferably individually.

[0114] Particularly good results are achieved if, in the second process step (b), the discharge of the reducing agent from the injection devices is controlled individually for each injection device and / or for groups of injection devices. The individual injection devices are preferably controllable individually or at least in groups, since the flue gas stream is not a homogeneous entity but is subject to local and temporal fluctuations in its temperature and composition.

[0115] To ensure particularly efficient use of the reducing agent, the injection devices in the second process step (b) are advantageously individually controllable, i.e., the injection devices can advantageously be individually activated or deactivated, whereby the pressure and thus the penetration depth of the reducing agent into the exhaust gas stream can also be individually controlled for each injection device. It is particularly advantageous if the individual injection devices are not only individually controlled with regard to their use and injection pressure, but if the composition of the injected reducing agent, i.e., either individual reducing agents or mixtures thereof, can be individually controlled and tailored to the respective conditions.The aforementioned individual controllability of all injection devices with regard to their operating parameters and the composition of the reducing agent leads to the best results, but also increases the process-technical effort and thus the costs of denitrification.

[0116] Equally very good results - but with a significantly lower percentage of technical effort - are obtained if, in the second process step (b), the composition or the mixing ratio of the reducing agents is set jointly for all injection devices or at least is set jointly for all injection devices of an injection level, but the individual injection devices can be individually controlled with regard to the injection pressure and the operating conditions or operating ratios.

[0117] Likewise, very good results can be achieved if the injection devices are controlled in groups. A group of injection devices is understood to be a defined and / or jointly controllable and / or combined unit of a plurality of injection devices, in particular injection lances.

[0118] Within the scope of the present invention, it is particularly provided that the injection device in the first method step (a) and in the second method step (b) can be controlled independently and / or separately from one another.

[0119] As far as the temperatures during the introduction of the reducing agent in the second process step (b) are concerned, these can vary within a wide range.

[0120] Typically, within the scope of the present invention, the temperatures during the introduction of the reducing agent in the second process step (b) are in the range of 50 to 1,200 °C, in particular 50 to 1,150 °C, preferably 50 to 110 °C. In these temperature ranges, denitrification is possible using both SCR and SNCR processes.

[0121] If the second process step (b) is carried out in the form of an SCR process, the temperatures of the flue gas when introducing the reducing agent in the second process step (b) are usually in the range of 50 to 500 °C, in particular 50 to 450 °C, preferably 50 to 400 °C. In the event that the second process step is carried out in the form of an SNCR process, it has proven useful if the temperatures of the flue gas when introducing the reducing agent in the second process step (b) are in the range of 750 to 1,200 °C, in particular 800 to 1,150 °C, preferably 850 to 1,100 °C.

[0122] In these temperature ranges, particularly good results are obtained with aqueous ammonia or urea solutions.

[0123] As far as the regulation of the method according to the invention is concerned, this can be done in a variety of ways.

[0124] Within the scope of the present invention, however, it is usually provided that in the first process step (a) and / or in the second process step (b), in particular in the first process step (a) and / or in the second process step (b), the introduction of the reducing agent into the flue gases, in particular the air of the further combustion stage, is controlled as a function of the load signal, the flue gas temperature, the nitrogen oxide emission, the setting of the combustion output control of online calculation models and combinations thereof.

[0125] Within the scope of the present invention, it is at least usually provided that in the first method step (a) the introduction of the reducing agent into the flue gases is controlled as a function of the load signal, the flue gas temperature, the nitrogen oxide emission, the setting of the combustion power control of online calculation models and combinations thereof.

[0126] Depending on the availability and the need for control quality, the following data, in particular online data, are preferably used for control purposes:

[0127] ■ Emission data such as NOx, NH3, O2, CO

[0128] ■ Boiler data, such as load, steam quantity, steam parameters or flue gas quantity

[0129] ■ Flue gas temperatures, measured with thermocouples, acoustic or optical temperature measuring systems

[0130] ■ Settings of the combustion output control, and

[0131] ■ Online calculation model of the plant or furnace. According to a preferred embodiment of the present invention, it can be provided that the introduction of the reducing agent into the flue gas stream is controlled by evaluating the load signal and / or by determining the flue gas temperature and / or by comparing a measured value for the residual nitrogen oxide content of the clean gas resulting from treatment with a predetermined target value. Furthermore, it is also possible to additionally determine the ammonia slip and also take this into account in the process control.

[0132] In the context of the present invention, a load signal is understood to be the indication of the respective load at which a combustion device, such as a large combustion plant, in particular a boiler, is operated. The load corresponds to the power released by the combustion device and is typically specified as a percentage, with full load (100%) corresponding to the power for which the combustion device is designed with optimal combustion and filling.

[0133] Within the scope of the present invention, it is preferably provided that in the second method step (b) the introduction of the reducing agent into the flue gases is controlled as a function of the load signal, the flue gas temperature, the nitrogen oxide emission, the setting of the combustion power control of online calculation models and combinations thereof.

[0134] Within the scope of the present invention, it has further proven particularly advantageous for carrying out the second process step (b) if the temperature of the flue gases is determined during the process duration at least at defined and / or predetermined measuring points and / or if at least one temperature profile of the exhaust gases is created, in particular by means of acoustic and / or optical temperature measurement or by means of thermocouples.

[0135] Particularly good results are obtained within the scope of the present invention if, in the second process step (b), the exhaust gas stream is divided into sections based on the determined temperature values ​​of the flue gases and / or the determined temperature profile of the flue gases. Defined individual injection devices and / or defined groups of injection devices for introducing the reducing agent are assigned to the sections. This ensures that the reducing agent reaches the most effective locations for a reaction, even with flue gas temperatures fluctuating, and that the system is always operated within the optimal range with regard to the nitrogen oxide separation efficiency, ammonia slip, and reducing agent consumption.

[0136] According to a particularly preferred embodiment of the invention, in the second method step (b), the injection devices for introducing the reducing agent are controlled, in particular individually or in groups, on the basis of the determined temperature values ​​of the flue gases and / or the determined temperature profile of the flue gases and / or the load signal and / or a comparison between a measured value for the residual nitrogen oxide content of the clean gas resulting after the treatment on the one hand and a predetermined target value on the other hand and / or on the basis of the settings of the combustion output control and / or on the basis of online models.

[0137] A further subject matter of the present invention - according to a second aspect of the present invention - is a device for denitrification of flue gases from staged combustion processes, wherein the device has at least one first injection device for introducing at least one reducing agent into a flue gas stream with the air for a subsequent combustion stage, in particular the secondary air, and wherein the device has at least one second injection device for introducing at least one reducing agent into a flue gas stream downstream of the first injection device.

[0138] Combustion processes primarily involve combustion processes in technical, especially large-scale, facilities such as power plants and waste incineration plants. These processes primarily involve the combustion of solid materials.

[0139] Within the scope of the present invention, it is preferably provided that the device has one or more, preferably several, first injection devices for introducing at least one reducing agent into a flue gas stream. Within the scope of the present invention, it is particularly preferred if the device has 2 to 20, in particular 4 to 40, preferably 4 to 6, first injection devices for introducing at least one reducing agent into the flue gas stream.

[0140] Furthermore, it is preferred within the scope of the present invention if the outlet of the reducing agent from the first injection devices can be regulated individually for each injection device and / or for groups of injection devices.

[0141] Likewise, it can be provided that each first injection device for introducing the reducing agent into the flue gas stream has one or more, in particular 1 to 20, preferably 1 to 15, preferably 1 to 10, particularly preferably 1 to 5, nozzles.

[0142] In this context, it has proven useful if the device has 1 to 20, in particular 2 to 10, preferably 4 to 8, first injection devices for introducing the reducing agent into the flue gas stream.

[0143] According to a preferred embodiment of the present invention, the first injection devices for introducing reducing agent into the flue gas stream are arranged in 1 or 2, preferably 1, injection levels.

[0144] In this case, it can be provided in particular that each injection level has 1 to 10, in particular 1 to 5, preferably 2 to 4, first injection devices for introducing the reducing agent into the flue gas stream.

[0145] Furthermore, it is preferred within the scope of the present invention if the first injection devices are designed for introducing, preferably injecting, aqueous solutions of the reducing agent, in particular aqueous ammonia and / or urea solutions.

[0146] In general, it is provided within the scope of the present invention that the first injection device of a combustion device, in particular a device for carrying out a first combustion stage of a staged combustion, preferably a combustion chamber, is connected downstream of, in particular immediately downstream of, the primary combustion process, ie the combustion process taking place with the supply of primary air.

[0147] In general, it is provided within the scope of the present invention that the device has one or more second devices for introducing, in particular injecting, at least one reducing agent into the flue gas stream.

[0148] Likewise, it can be provided that every second injection device for introducing the reducing agent into the flue gas stream has one or more, in particular 1 to 20, preferably 1 to 15, more preferably 1 to 10, particularly preferably 1 to 5, nozzles.

[0149] In this context, it has proven useful if the device has 1 to 200, in particular 2 to 100, preferably 5 to 60, second injection devices for introducing the reducing agent into the flue gas stream.

[0150] According to a preferred embodiment of the present invention, the second injection devices for introducing reducing agent into the flue gas stream are arranged in 1 to 10, in particular 1 to 7, preferably 1 to 5, injection levels.

[0151] In this case, it can be provided in particular that each injection level has 1 to 20, in particular 1 to 15, preferably 1 to 12, second injection devices for introducing the reducing agent into the flue gas stream.

[0152] Furthermore, it is preferred within the scope of the present invention if the second injection devices are designed for introducing, in particular spraying, preferably injecting, aqueous solutions of the reducing agent, in particular aqueous ammonia and / or urea solutions.

[0153] In this context, it has proven particularly advantageous if the second injection device is arranged between a combustion device, especially adjacent to all devices for staged combustion, a combustion chamber, and an afterburner chamber, and a heat exchanger device. This allows for the best separation rates for the removal or separation of nitrogen oxides while simultaneously minimizing the consumption of reducing agent and ammonia slip.

[0154] Alternatively, however, it can also be provided that the second injection device is connected downstream of a combustion device, in particular all devices for staged combustion, preferably a combustion chamber and a post-combustion chamber, and extends into the area of ​​a heat exchange device. In particular, if existing combustion boilers or heating boilers are equipped with the device according to the invention, the temperature distribution and the structural conditions may result in the device according to the invention, in particular the injection lances, being located in the area of ​​the heat exchange devices or the heating surfaces of the system or the large-scale combustion plant.

[0155] Within the scope of the present invention, it is usually provided that the device has at least one storage device, in particular a storage vessel, for storing and / or dispensing at least one reducing agent, preferably connected via at least one supply line to the first injection device and / or the second injection device, preferably the first and the second injection device.

[0156] If several, in particular two, reducing agents are introduced into the flue gas stream using the injection devices for introducing the reducing agent, it has proven advantageous if the device comprises: at least one first storage device, in particular a first storage vessel, preferably connected via at least one supply line to the first injection device and / or the second injection device, preferably the first and second injection devices, for storing and / or dispensing at least one first reducing agent and at least one second storage device, in particular a second storage vessel, preferably connected via at least one supply line to the first injection device and / or the second injection device, preferably the first and second injection devices, for storing and / or dispensing at least one second reducing agent different from the first reducing agent.

[0157] According to a preferred embodiment of the present invention, the device according to the invention is part of a plant for the thermal treatment, in particular for the combustion, of fuels, in particular solid fuels, such as refuse-derived fuels, coal, biomass, or waste. The plant for the thermal treatment, in particular combustion, of fuels, preferably has a combustion chamber in which at least one fuel, in particular a solid fuel, preferably a solid waste, is thermally treated, in particular combusted, with the supply of primary air, producing flue gases.

[0158] The system preferably further comprises a post-combustion chamber or post-combustion chamber arranged downstream of the combustion chamber, in which the flue gases produced in the combustion chamber are post-combusted by supplying additional air and / or additional fuels. In particular, the system comprises a post-combustion stage in which no additional fuel, but only air, is added to the flue gases, the so-called burnout zone. Preferably, the air for further combustion stages is introduced into the post-combustion chamber, in particular into the burnout zone, via injection openings. The air from the further stages is guided via main supply lines and / or collecting containers, in particular collectors, to air injection openings for further combustion stages.

[0159] Within the scope of the present invention, it is now preferably provided that the first injection device for introducing the reducing agent into the flue gas flow, in particular into the air of further combustion stages, is assigned to individual injection openings for introducing the air of further combustion stages into the post-combustion chamber and / or that the first injection device for introducing the reducing agent into the flue gases, in particular into the air for further combustion stages, is assigned to main supply lines and / or a collecting container, in particular a collector, for supplying the air for further combustion stages to the injection openings for introducing the air for further combustion stages.

[0160] Likewise, it has proven advantageous within the scope of the present invention if the device has at least one water storage device for storing and / or dispensing water.

[0161] In this case, it can be provided, in particular, that the water storage device is connected, preferably via at least one supply line, to the first injection devices and / or the second injection devices, preferably the first and second injection devices, for introducing the reducing agent. The water is required in particular within the scope of the process according to the invention to precisely adjust the specific concentration ratios of the reducing agent prior to injection or introduction into the flue gases.

[0162] Furthermore, within the scope of the present invention, it is generally provided that the device has at least one gas storage device for storing and / or dispensing optionally compressed gases, in particular compressed air and / or steam. In this case, it can be provided that the gas storage device is connected via at least one supply line to the first injection devices and / or the second injection devices, preferably the first and second injection devices, for introducing the reducing agent into the flue gas stream.

[0163] Likewise, it is generally provided according to the present invention that the pressurization of the first injection devices for discharging the reducing agent into the flue gas stream can be carried out by means of the gases stored in the gas storage device and / or that the pressurization of the first injection devices and / or second injection devices, preferably the first and second injection devices, for discharging the reducing agent into the flue gas stream can be carried out by means of the gases stored in the gas storage device.

[0164] It has proven particularly advantageous within the scope of the present invention if the device has at least one dosing and / or mixing device.

[0165] In this context, it can be provided that the dosing and / or mixing device is connected to the storage devices for providing the reducing agent and to the first injection devices and / or second injection devices, preferably the first and second injection devices, for providing the reducing agent and to the first injection devices and / or second injection devices, preferably the first and second injection devices, for introducing the reducing agent into the flue gas stream and to the water storage device, if present.

[0166] Furthermore, it can be provided that the dosing and mixing device is designed such that the concentrations of the aqueous solutions of the reducing agent can be regulated identically and / or individually for individual injection devices, in particular for each injection device for introducing the reducing agent into the flue gas stream and / or for groups of injection devices for introducing the reducing agent into the flue gas stream, preferably for each injection device for introducing the reducing agent into the flue gas stream. Within the scope of the present invention, it is usually provided at least that the first injection device and the second injection device can be regulated independently of one another.

[0167] In general, it is provided within the scope of the invention that the device has at least one control unit for controlling the introduction of the reducing agent through the first injection device and the second injection device.

[0168] According to a preferred embodiment of the present invention, it is provided that the introduction of the reducing agent into the flue gas stream is controllable by determining temperature values ​​of the exhaust gases and / or by determining a temperature profile of the exhaust gases and / or by a load signal and / or by a comparison between a measured value for the residual nitrogen oxide content of the clean gas resulting after the treatment on the one hand and a predetermined target value on the other hand and / or by setting the combustion output control and / or by online calculation.

[0169] According to a particularly preferred embodiment of the present invention, the device comprises at least one measuring device for determining temperature values ​​of the flue gases and / or for determining a load signal and / or for determining a value for the residual nitrogen oxide content of the clean gas resulting after treatment, in particular for the purpose of regulating the introduction of the reducing agent into the flue gas stream. The device for determining the temperature can, in particular, be one or more thermocouples, an acoustic temperature measuring system, or an optical temperature measuring system.

[0170] For further details on the device according to the invention, reference can be made to the above statements on the methods according to the invention, which apply accordingly with regard to the device according to the invention.

[0171] A further object of the present invention - according to a third aspect of the present invention - is the use of a previously described device for denitrification of flue gases containing nitrogen oxides.

[0172] For further details on this aspect of the invention, reference can be made to the above statements on the method according to the invention or on the device according to the invention, which apply accordingly with regard to the use according to the invention.

[0173] The method according to the invention and the device according to the invention for treating flue gases containing nitrogen oxides are illustrated in the attached figures by way of example and in a non-limiting manner.

[0174] Further advantages, properties, aspects and features of the present invention will become apparent from the following descriptions of the preferred embodiment of the invention illustrated in the drawing.

[0175] Fig. 1 shows a schematic representation of a plant 1, in particular an incineration plant, which has the device according to the invention.

[0176] The plant 1 has a combustion chamber 2, which is depicted in the form of a grate furnace. A fuel 4 is fed into the combustion chamber 2, moved in stages over the grate 3, and removed from the plant 1 after combustion as waste or combustion residue 7.

[0177] The supplied fuel 4 is incompletely combusted with the aid of primary air 5. The flue gases 6 produced during the incomplete combustion in the combustion chamber 2, which typically have temperatures in the range of 1,200 to 1,600 °C, rise in the combustion chamber 2 to reach subsequent combustion stages, in particular the post-combustion chamber 8.

[0178] In the post-combustion chamber 8, air for a further combustion stage, in particular secondary air 9, is blown into the flue gas stream 6 together with the reducing agent 11, in particular urea and / or ammonia, preferably an aqueous urea solution and / or aqueous ammonia solution. The secondary air 9 is introduced into the post-combustion chamber 8 via a supply system 10. The high volume flow of the secondary air 9 protects the reducing agent 11 from the effects of excessively high temperatures and from premature reaction. In this way, it is possible for the reducing agent 11 to come into contact with the nitrogen oxides in a temperature range optimal for the reduction of nitrogen oxides, so that, in particular, unwanted oxidation of the reducing agent 11 is avoided.

[0179] The introduction of the reducing agent 11 into the flue gases, in particular the secondary air 9, can be carried out in a variety of ways, in particular by means of first injection lances 13.

[0180] In addition, the device 1 has second injection lances 14 for introducing a reducing agent into the flue gas stream 6. The second injection lances 14 are used in particular for carrying out a known or conventional SCR process or SNCR process for denitrification of flue gases, preferably for carrying out an SNCR process for suffocation of flue gases.

[0181] The second injection lances 14 are located downstream of the first injection lances 13 for introducing the reducing agent 11, in particular downstream of the post-combustion chamber 8. The injection lances 14 can be arranged in several levels, in particular 1 to 5, preferably 2 to 4, injection levels in the system 1. The device preferably further comprises a temperature measuring device 15 for determining the flue gas temperature.

[0182] Fig. 2 shows a section along plane A through the afterburning chamber 8 of the system 1 according to Fig. 1 in a plan view. The secondary air 9 is guided into the afterburning chamber 8 via a piping and collection system, in particular a supply system 10, via injection openings 12. In the region of the injection openings 12 for injecting the secondary air 9 into the afterburning chamber 8 there are first injection lances 13 for introducing a reducing agent 11. The first injection lances 13 for introducing reducing agents 11 into the flue gas stream 6 or the secondary air 9 usually have a plurality of nozzles, in particular 1 to 10, preferably 1 to 5, nozzles. The first injection lances 13 can be formed in the region of the injection opening 12 for injecting the secondary air 9 into the afterburning chamber 8. However, it is also possible that the first injection lances 13 are inserted through the injection opening 12 to inject secondary air 9 into the afterburning chamber

[0183] 8 or are set back from them, in particular in the supply system 10 which transports the secondary air 9.

[0184] Fig. 3 shows an alternative, likewise preferred embodiment of the device according to the invention for introducing reducing agents 11 into a flue gas stream 6. Fig. 3 again shows a section along the plane A in plan view through the system 1 according to Fig. 1, wherein the first injection lances 13 for injecting the reducing agent 11 into the secondary air 9 are arranged at a central location in the supply system 10 for transporting the secondary air 9. The secondary air

[0185] 9 is then transported via the feed system 10 into the afterburning chamber 8 and carries the reducing agent 11 with it.

[0186] The device comprises, in particular, storage vessels (not shown) for storing and / or dispensing one or more reducing agents 11. The reducing agent 11 is preferably an aqueous urea solution and / or an aqueous ammonia solution and / or gaseous ammonia, in particular an aqueous urea solution and / or an aqueous ammonia solution.

[0187] The storage vessels for storing and / or dispensing one or more reducing agents are preferably connected to the first injection lances 13 and / or the second injection lances 14, preferably the first injection lances 13 and the second injection lances 14.

[0188] The first injection lances 13 for injecting the reducing agent 11 are preferably connected via a supply line to a storage vessel (not shown) for delivering compressed air and / or steam. Compressed air is then supplied through the supply line to the respective injection lances 13 and / or 14, thereby specifically adjusting the respective outlet pressure and thus the targeted penetration depth and droplet size of the reducing agent 11. Alternatively, however, it is also possible in this case to have the compressed air control determined via a mixing and dosing device together with the respective reducing agent mixture.

[0189] The device further comprises storage vessels (not shown) for storing and / or dispensing water. The storage vessels for storing and / or dispensing water are preferably connected to the first injection lances 13 and / or the second injection lances 14, preferably the first injection lances 13 and the second injection lances 14.

[0190] Furthermore, the first injection lances 13 and / or the second injection lances 14, in particular the first injection lances 13 and the second injection lances 14, are preferably connected to a mixing device (also not shown), which is connected via supply lines to the storage vessels containing the reducing agent 11, in particular ammonia and / or urea, preferably urea in the form of their aqueous solutions. Furthermore, the mixing device is preferably connected to a water storage vessel via a supply line.

[0191] The first injection lances 13 and / or the second injection lances 14, preferably the first injection lances 13 and the second injection lances 14, can be connected to the mixing device individually or in groups, in particular via supply lines.

[0192] The device further comprises a control unit for controlling the introduction of the reducing agent 11 into the flue gases 6, in particular the secondary air 9.

[0193] Within the scope of the present invention, it can be provided, in particular, that the control of the introduction of the reducing agent 11 into the flue gases 6, in particular the secondary air 9, is carried out by determining the load signal, the nitrogen oxide content of the resulting clean gas in comparison to a target value, the setting of the firing output, the temperature of the flue gases, online calculations of the combustion and the system or combination. For this purpose, it can be provided that the system has devices for determining the temperature of the flue gases, in particular by means of acoustic or optical methods or by means of thermocouples, and / or that the system has devices for determining the nitrogen oxide content of the clean gas and / or devices for determining the load signal.

[0194] In addition, the device preferably has a temperature measuring device 15, which is arranged immediately upstream of the second injection lances 14 for injecting the reducing agent into the flue gas stream 6.

[0195] In the process according to the invention, the procedure is now preferably such that the flue gases 6 generated in the combustion chamber 2 rise into the post-combustion chamber 8, where, based on the nitrogen oxide emissions, the load signal, the temperatures of the flue gases, the firing output setting, or on the basis of online calculations, reducing agent 11 is injected into the secondary air 9 by means of the first injection lances 13. This achieves a significant and uniform reduction in the nitrogen oxide load of the flue gases 6, while at the same time ammonia slip can be completely or at least almost completely avoided.

[0196] Downstream of the post-combustion chamber 8, the flue gas temperatures are preferably determined, in particular spatially resolved, so that further reducing agent 11 is introduced into the flue gas stream 6 in a location-selective manner by means of individual injection lances 14 or by means of groups of injection lances 14 in order to also specifically reduce the strongly varying proportion of nitrogen oxide emissions remaining after the first process step.

[0197] Further embodiments, modifications, variations and special features of the present invention will be readily apparent and achievable to a person skilled in the art upon reading the description, without departing from the scope of the present invention.

[0198] The present invention is illustrated by the following exemplary embodiments, without, however, limiting the present invention thereto. Exemplary embodiments:

[0199] In a waste incineration plant with particularly extreme and fluctuating flue gas conditions, the guaranteed clean gas values ​​for NOx < 90 mg / Nm 3 and NH3 < 10 mg / Nm 3 cannot be achieved with conventional SNCR technology, especially since irregular amounts of leachate containing ammonia are evaporated in addition to the household waste.

[0200] Injecting a reducing agent, especially urea solution or ammonia water, just above the grate to at least limit ammonia slip is not feasible, as the nitrogen contained in the reducing agents would oxidize to further NOx at temperatures above approximately 1100 °C. This method would therefore be counterproductive. Furthermore, for structural reasons, the area above the grate is not suitable for the installation of additional lances, or only with considerable effort.

[0201] By injecting an aqueous urea solution into the secondary air, significantly more than 100 mg / mN 3 NOx can be separated. In combination with the additionally installed conventional SNCR system, NOx emissions of 50 to 80 mg / Nm 3 reached.

[0202] It is noteworthy that the NHs slip is always in the range of 5 mg / mN 3 which is due to the fact that the reactions only take place when the reactive ammonia radicals have reached the optimal temperature window.

[0203] Example

[0204] The boiler is operated at a load of approximately 108%, and the flue gases are treated for 3.5 hours each using conventional SNCR technology (standard SNCR) and by injecting urea into the secondary air stream. With the same reducing agent flow rate of 120 l / h, the secondary air injection achieves a NOx removal rate of 43% and an NH3 slip of less than 8 mg / Nm3. 3achieved, compared to a NOx separation of 27% and approx. 20 mg / Nm 3 Slip with the standard SNCR system. In addition, a combination of both systems achieves a higher NOx removal rate of 57% with significantly lower NH3 slip. The values ​​mentioned are summarized in Table 1. Table 1: Comparison of standard SNCR with the injection of reducing agent into the secondary air and a combination of both processes.

[0205] List of reference symbols: System 9 Secondary air combustion chamber 10 Feed system combustion grate 11 Reducing agent fuel 12 Injection opening primary air 13 First injection lance flue gases 14 Second injection lance combustion residue 15 Temperature measuring device afterburner chamber

Claims

Patent claims:

1. A process for denitrification of flue gases from staged combustion processes, characterized in that (a) in a first process step, at least one nitrogen-containing reducing agent is introduced into the flue gases with the air for a subsequent combustion stage, in particular the secondary air, and (b) in a second process step following the first process step, at least one nitrogen-containing reducing agent is introduced into the flue gases.

2. Process according to claim 1, characterized in that the second process step (b) is carried out in the form of an SCR process or an SNCR process, preferably an SNCR process, for denitrification of flue gases.

3. Process according to claim 1 or 2, characterized in that before, in particular upstream of the first process step (a), no process step for flue gas homogenization is carried out, in particular no oxygen-free or low-oxygen mixing medium, in particular no water vapor or recirculated flue gas is introduced into the flue gases.

4. Method according to one of claims 1 to 3, characterized in that in the first method step (a) the reducing agent is introduced with the air for the combustion stage into the flue gases, in which no further fuel is introduced into the flue gases, in particular into a burnout zone.

5. Process according to one of claims 1 to 4, characterized in that in the first process step (a) the reducing agent is introduced into the flue gases with the secondary air and / or tertiary air, preferably the secondary air.

6. A process for denitrification of flue gases, in particular a process according to one of claims 1 to 5, characterized in that fuels, in particular solid fuels, preferably substitute fuels, biomass and / or waste, are subjected to a thermal treatment, in particular combustion, with the supply of primary air, whereby flue gases are formed, wherein in a first process step (a) of denitrification the flue gases are mixed downstream with secondary air, wherein at least one nitrogen-containing reducing agent is added to the secondary air and / or wherein at least one reducing agent is introduced into the flue gases with the secondary air, in particular wherein no process step for flue gas homogenisation is carried out before, in particular upstream of, the introduction of the reducing agent into the flue gases, in particular no oxygen-free or low-oxygen mixing medium, in particular no water vapour or recirculated flue gas,is introduced into the flue gases and / or in particular wherein the mixing of the flue gases with the secondary air containing at least one reducing agent takes place immediately after the thermal treatment, and wherein in a second process step (b) of denitrification following the first process step (a) of denitrification, at least one nitrogen-containing reducing agent is introduced into the flue gases.

7. Method according to one of the preceding claims, characterized in that in the first method step (a) the air for a further combustion stage, in particular the secondary air and / or the tertiary air, is injected into the flue gases.

8. Method according to one of the preceding claims, characterized in that in the first method step (a) air for the further combustion stage, in particular the secondary air and / or the tertiary air, before introduction, in particular injection, into the flue gases has a temperature of at most 300 °C, in particular at most 250 °C, preferably at most 200 °C, preferably at most 150 °C, particularly preferably at most 100 °C, very particularly preferably at most 50 °C.

9. Method according to one of the preceding claims, characterized in that in the first method step (a) air for the further combustion stage, in particular the secondary air and / or the tertiary air, before introduction, in particular injection, into the flue gases, has a temperature in the range of 10 to 300 °C, in particular 15 to 250 °C, preferably 20 to 200 °C, preferably 20 to 150 °C, particularly preferably 20 to 100 °C, most particularly preferably 20 to 50 °C.

10. Process according to one of the preceding claims, characterized in that the reducing agent is selected from the group of ammonia, urea and mixtures thereof. 11 . Process according to one of the preceding claims, characterized in that the reducing agent is introduced into the flue gases in the form of an aqueous solution or dispersion, in particular a solution.

12. Process according to one of the preceding claims, characterized in that in the first process step (a) the reducing agent, in particular the aqueous solution or dispersion of the reducing agent, is introduced into the flue gases, in particular the secondary and / or tertiary air, via one or more injection devices, in particular injection lances.

13. The method according to claim 12, characterized in that in the first method step (a) the reducing agent is introduced into the exhaust gases via one to 50, in particular 2 to 40, preferably 4 to 30, preferably 5 to 20, injection devices.

14. Method according to claim 12 or 13, characterized in that in the first method step (a) the injection devices can be controlled individually or in groups, in particular in groups.

15. Method according to one of the preceding claims, characterized in that in the first method step (a) the injection devices for introducing the reducing agent are assigned to individual injection openings for introducing the air for the further combustion stage, in particular the secondary air and / or tertiary air, preferably secondary air, into the flue gases and / or that the injection devices for introducing the reducing agent are assigned to collecting containers and / or main supply lines for introducing the air for the further combustion stage, in particular the secondary air and / or tertiary air, preferably secondary air, into the flue gases.

16. Method according to one of the preceding claims, characterized in that in the first method step (a) the temperature of the flue gases in the region of the introduction of the air for a further combustion stage, in particular the secondary air and / or tertiary air, preferably secondary air, in particular at least in some regions, is at least 850 °C, in particular 1,000 °C, preferably 1,100 °C, preferably 1,200 °C.

17. Method according to one of the preceding claims, characterized in that in the first method step (a) the temperature of the flue gases in the region of the introduction of the air for a further combustion stage, in particular the secondary air and / or tertiary air, preferably secondary air, in particular at least in regions is 850 °C to 1,600 °C, in particular 1,000 °C to 1,500 °C, preferably 1,100 °C to 1,450 °C, preferably 1,200 °C to 1,400 °C.

18. Method according to one of the preceding claims, characterized in that in the second method step (b) the temperature of the flue gases in the region of the introduction of the reducing agent is, in particular at least in some regions, at most 1,200 °C, in particular 1,150 °C, preferably 1,100 °C.

19. Process according to one of the preceding claims, characterized in that in the second process step (b) the temperature of the flue gases in the region of the introduction of the reducing agent is, in particular at least in regions, 50 °C to 1,200 °C, in particular 50 °C to 1,150 °C, preferably 50 °C to 1,100 °C.

20. Method according to one of the preceding claims, characterized in that the introduction of the reducing agent into the flue gases, in particular the air for a further combustion stage, is controlled as a function of the load signal, flue gas temperature, nitrogen oxide emission, online calculation models and combinations thereof.

21. Device for denitrification of flue gases from staged combustion processes, characterized in that the device comprises at least one first injection device (13) for introducing at least one reducing agent (11) into a flue gas stream (6) with the air for a subsequent combustion stage, in particular of the secondary air (9), and that downstream of the first injection device (13) for introducing at least one reducing agent (11) into a flue gas stream, it has at least one second injection device (14) for introducing at least one reducing agent (11) into a flue gas stream (6).

22. Device according to claim 21, characterized in that the device has one or more, preferably several first injection devices (13) for introducing at least one reducing agent (11) into a flue gas stream (6).

23. Device according to claim 22, characterized in that the device has 2 to 20, in particular 4 to 40, preferably 4 to 6, first injection devices (13) for introducing at least one reducing agent (11) into the flue gas stream (6).

24. Device according to one of claims 21 to 23, characterized in that the device has one or more, preferably several second injection devices (14) for introducing at least one reducing agent (11) into a flue gas stream (6).

25. Device according to claim 24, characterized in that the device has 2 to 200, in particular 4 to 100, preferably 6 to 50, second injection devices (14) for introducing at least one reducing agent (11) into the flue gas stream.

26. Use of a device according to one of claims 21 to 25 for denitrification of flue gases from staged combustion processes.

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