Mixed Systems
The novel mixing system with a manifold and injection rings ensures efficient mixing of reducing agents with NOx-containing gases, addressing low mixing efficiency issues in SCR systems, enhancing conversion and temperature uniformity in nitric acid plants.
Patent Information
- Application Number
- JP2024544382
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2022-01-26
- Filing Date
- 2023-01-20
- Publication Date
- 2025-02-10
AI Technical Summary
Conventional mixing devices for selective catalytic reduction (SCR) systems in nitric acid plants face low mixing efficiency when the flow rate of the reducing agent is significantly lower than the flow rate of the gas being treated, leading to ammonia slip, non-uniform temperature distribution, and catalyst wear, especially in space-constrained environments.
A novel mixing system with a manifold and static mixing unit, featuring a main injection ring and optional additional rings, oriented to inject the reducing agent in the same direction as the mainstream flow, and optionally using injection tubes and metering openings to ensure high mixing efficiency over a wide range of flow rates.
The system achieves uniform mixing of reducing agents like ammonia with NOx-containing gases, ensuring high conversion efficiency and uniform temperature distribution across the catalyst bed, suitable for renovation and modernization of nitric acid plants.
Smart Images
Figure 2025504268000001_ABST
Abstract
Description
[Technical field]
[0001] The present invention is in the field of mixing devices configured to inject a process gas into a mainstream to be treated. More particularly, the present invention relates to a mixing system configured for use in a system for the selective catalytic reduction of NOx contained in a fuel gas, such as the tail gas of a nitric acid plant. [Background technology]
[0002] Mixing devices are present in all devices requiring mixing between multiple streams, for example in combustion units, in catalytic synthesis converters, in catalytic purification units.
[0003] In a catalytic purification unit, a gas or fluid containing pollutants is mixed with a reducing agent before entering the catalyst bed to help convert the pollutants into harmless species.
[0004] Selective catalytic reduction (SCR) is widely used to treat exhaust gases (e.g. from combustion processes) and to convert nitrogen oxides into harmless nitrogen and water. For this purpose, a reducing agent, e.g. ammonia or urea, is continuously injected into the NOx-containing gas to be treated. The term nitrogen oxides usually refers to NO and NO2.
[0005] An application of great industrial interest is represented by a NOx removal reactor in a nitric acid plant. The industrial production of nitric acid is based on the Ostwald process, in which NOx-containing gas obtained by oxidation of ammonia is contacted with water in a suitable absorption column, where the NOx is absorbed by the water to produce nitric acid. The tail gas containing the residual NOx and possibly nitrous oxide (NO) is removed from the absorption tower and needs to be treated to reduce the NOx content to an acceptable level, such as below 100 ppm. For this purpose, a suitable NOx removal reactor needs to be arranged downstream of the absorption column.
[0006] A conventional SCR mixing system disposed within a NOx removal reactor includes a conductor, an injection unit, and a static mixer disposed downstream of the injection unit. The injection unit includes a cylindrical bore manifold configured to deliver a reductant into the mainstream of the tail gas, while the static mixer includes a honeycomb structure configured to facilitate mixing of the ammonia with the tail gas.
[0007] Unfortunately, the above-mentioned mixing devices are unable to achieve high mixing efficiency over a wide range of injected flow rates, especially when the flow rate of the injected reductant (e.g., ammonia) is orders of magnitude smaller than the flow rate of the gas being treated (e.g., tail gas).
[0008] Therefore, to achieve a satisfactory mixing efficiency and a homogeneous mixture, the mixing system must be housed in very long channels or conductors. Unfortunately, this solution is not always feasible in nitric acid plants, especially when carrying out work on the renovation or modernization of existing plants, since the physical space available to carry out this work is very often limited.
[0009] Poor mixing efficiency between the tail gas containing nitrogen oxides and the reductant may result in ammonia slip across the catalyst bed, especially if too much ammonia is added, or may result in reduced NOx conversion, especially if too little ammonia is added. In addition, poor mixing efficiency may also result in non-uniform temperature distribution across the catalyst bed due to the exothermic nature of the SCR reaction. The non-uniform temperature distribution described above may further affect the conversion of NOx to nitrogen and water, and may also cause non-uniform wear of the catalyst, which may lead to catalyst loss.
[0010] In view of the above considerations, it is clear how important it is to establish conditions of uniform mixing between the tail gas and the reductant in the NOx removal reactor. It is therefore particularly desirable to provide a mixing device that can ensure high mixing efficiency over a wide range of injected flow rates and over short lengths of distribution channels.
[0011] Apparatuses suitable for injecting a reducing agent into the exhaust gas stream of a combustion engine for use in the automotive sector are disclosed in US 10,392,989 and DE 10 2011 078 181. An in-line mixing device for gases is described in US 3,702,619. Summary of the Invention [Problem to be solved by the invention]
[0012] The present invention aims to overcome the above-mentioned drawbacks of the prior art related to the low mixing efficiency of conventional mixing devices. The present invention aims to provide a new mixing system that is particularly suitable for mixing a process gas into a mainstream when the flow rate of the process gas is significantly lower than the flow rate of the mainstream. More specifically, the present invention aims to provide a mixing system that is particularly suitable for use in a reactor for the selective catalytic reduction of nitrogen oxides from a gas when said process gas is a reducing agent for the reduction of the nitrogen oxides. [Means for solving the problem]
[0013] The above object is achieved by a mixing system as claimed.
[0014] The mixing system comprises an injection unit, a static mixing unit, and a distribution channel having a main longitudinal axis that determines the main flow direction during operation.
[0015] The injection unit includes a manifold and a main injection ring, the manifold being disposed within the distribution channel and perpendicular to a main longitudinal axis of the distribution channel, the manifold being configured to receive the process gas through the main injection ring, the main injection ring having a circular or annular structure and provided with a plurality of injection orifices arranged to introduce the process gas in a mainstream flow direction.
[0016] An interesting application of the present invention is the use of the mixing system of the present invention in a reactor for removing NOx from a gas stream (NOx removal reactor). The NOx removal reactor comprises a catalyst suitable for catalytically reducing nitrogen oxides in the presence of a reducing agent. Such catalysts are known in the prior art and include iron-loaded zeolite catalysts, among others. The reducing agent used in the NOx removal reactor is preferably ammonia or comprises ammonia.
[0017] Thus, one aspect of the present invention is a conditioning system for removing nitrogen oxides from an input gas by selective catalytic reduction, the conditioning system comprising a catalytic reactor and a mixing system of the present invention, the reactor configured to catalytically reduce NOx contained in the input gas in the presence of a reducing agent, the mixing system configured such that a major portion is the input gas directed to said reactor and a process gas is the reducing agent.
[0018] In a particularly interesting application of the invention, the conditioning system is part of a nitric acid synthesis plant, more specifically part of a stage for conditioning NOx-containing tail gas before it is discharged to the stack, said tail gas usually being taken from an absorption column where nitric acid is produced.
[0019] The conditioning system can include multiple catalytic reactors. In some embodiments, the conditioning system can include a first reactor for removing N2O followed by a second reactor for removing NOx. In such a case, a mixing system for introducing a reducing agent is preferably disposed downstream of the first reactor and upstream of the second reactor to add the reducing agent to the effluent of the first reactor before the effluent of the first reactor enters the second reactor.
[0020] The present invention further relates to a nitric acid plant comprising an absorption column and a conditioning system as described above, in which in the absorption column, NOx-containing gas is absorbed in water to produce nitric acid and a tail gas containing residual NOx is taken from said column, in which the manifold of the mixing system is connected to a supply line of a reducing agent, preferably ammonia, and in which the distribution conduct of the mixing system is connected to the tail gas output of the absorption column.
[0021] The mixing system of the present invention makes it possible to achieve a high mixing efficiency when the flow rate of the reductant is significantly lower than the flow rate of the gas to be treated. Advantageously, the high mixing efficiency established in the mixing system allows a high conversion of the NOx contained in the tail gas and allows a uniform temperature distribution over the catalyst bed. Furthermore, a high mixing efficiency can be achieved over a very short distance of the mixing device making it suitable for retrofitting operations. In particular, the present invention can be useful for retrofitting and modernizing nitric acid plants to meet stricter limits on NOx emissions.
[0022] Detailed Description of the Invention The mixing system of the present invention is particularly suited for injecting small amounts of a process gas into the mainstream gas being treated. The process gas can be selected to effect desired conditioning of the mainstream gas, such as catalytic removal of pollutants or contaminants.
[0023] The present invention can be used in particular to inject a reducing agent for treating mainstream gas in a selective catalytic reduction process for removing pollutants. A particularly preferred application is the removal of nitrogen oxides NOx from exhaust gas, in which case the process gas is preferably ammonia. The mixing system of the present invention is therefore used to mix a reducing agent such as ammonia into the NOx-containing gas to be treated in order to reduce NOx to N2 and H2O by a suitable catalyst.
[0024] The mixing system of the present invention can be placed inside a purification reactor and above a catalyst bed. For example, the mixing system of the present invention can be placed inside a NOx removal reactor of a nitric acid plant to remove nitrogen oxides from the tail gas leaving the absorption column.
[0025] Particularly interesting results have been found in embodiments where ammonia is used as a reducing agent to treat a NOx-containing gas, particularly tail gas obtained from the production of nitric acid, where the mass ratio of ammonia to tail gas sent to the mixing system does not exceed 1:1000, preferably does not exceed 1:5000, more preferably is within the range of 1:5000 to 1:7000, for example 1:6500.
[0026] In the mixing system, the injection unit is preferably located above the static mixing unit, and the injection unit and the static mixing unit are preferably located inside a conduct, which is preferably circular and has a main longitudinal axis that determines the main flow direction during operation.
[0027] The orientation of the injection orifices of the main injection ring determines the direction of injection of fluid or gas into the main flow. According to preferred but non-limiting embodiments, the orifices are oriented such that the fluid or gas injected by them has the same or substantially the same direction as the main flow. In some embodiments, the orifices are oriented such that the flow emitted by them is parallel to the main longitudinal axis of the distribution conduct or forms a small angle with said longitudinal axis.
[0028] According to an interesting application of the invention, the mixing system comprises, in addition to the main injection ring, several additional rings, preferably one or two in number, arranged concentrically with respect to the main injection ring.
[0029] Preferably, the additional rings are in fluid communication with the manifold by way of one or more additional manifolds arranged to allow the passage of fluids or gases from one ring to another.
[0030] The one or more additional rings are provided with a plurality of injection orifices arranged to introduce the process gas into the main flow. Thus, the flow of the fluid or gas is injected into the main flow in the same direction as the latter. Thus, in some embodiments, a first portion of the process gas is introduced by a first set of orifices arranged on the main injection ring, and a second portion of the process gas is introduced through the orifices of the one or more additional injection rings.
[0031] The injection orifices of the main injection ring and, if provided, of any additional rings, can be oriented in the flow direction of the main flow. According to an embodiment, said orifices can be oriented parallel to said longitudinal axis or form a small angle with said axis, said angle being preferably less than or equal to 30°, more preferably less than or equal to 10°. Injecting the process gas according to the flow direction of the main flow facilitates the dispersion of the process gas into the main flow.
[0032] According to one embodiment of the invention, the main ring and the one or more additional rings lie in a plane perpendicular to a main longitudinal axis of the distribution conduct, and preferably the main injection ring and the one or more additional rings are equidistant from each other.
[0033] Preferably, the main injection ring and / or the one or more additional rings are integrated with the distribution channel by means of one or more supports, which can be fastened or welded onto the distribution channel.
[0034] According to an embodiment of the invention, the injection unit further comprises an injection tube extending longitudinally from the manifold. The injection tube may be provided with a plurality of metering openings. The metering openings may be arranged longitudinally along the length of the injection tube and may be oriented in the main flow direction. Also, the metering openings of the injection tube may be oriented parallel to said longitudinal axis of the conductor or to form a small angle with said longitudinal axis of the conductor.
[0035] According to another embodiment of the invention, the injection unit further comprises an injection conduct extending longitudinally from said manifold, the injection conduct being provided with a plurality of injection tubes arranged longitudinally in at least one row along the length of the injection conduct.
[0036] The injection tubes may be parallel to one another and may be arranged such that for each row of tubes, each pair of consecutive tubes is made up of two tubes of different lengths.
[0037] Preferably, each injection tube is provided with an aperture configured to eject fluid or gas in a flow direction perpendicular to the major longitudinal axis of the distribution channel.
[0038] According to an interesting application of the invention, the number of rows is two, said two rows being placed on opposite sides of the injection conduct.
[0039] According to a particularly interesting application of the present invention, the mixing system is used to remove NOx from the tail gas of a nitric acid production process, the manifold is connected to an ammonia feed and the distribution conduct is connected to the tail gas output line of an absorption column, so that the fluid or gas is the ammonia feed and the mainstream is the tail gas containing NOx.
[0040] The dimensions of the injection rings and the diameter of the injection orifices can be optimized by the person skilled in the art depending on the flow rate of the gas or liquid circulating in the manifold, depending on the flow rate of the main flow entering the mixing device, and depending on the injection speed required to establish high mixing efficiency in the mixing system. Preferably, all additional rings have the same diameter as the main ring mentioned above, and preferably the diameter of each ring is comprised between 20 and 30 mm.
[0041] Preferably, the number of injection orifices arranged on the main injection ring is comprised between 8 and 20, more preferably between 10 and 15.
[0042] Preferably, the number of injection orifices arranged in each of the one or more additional rings is comprised between 3 and 15, more preferably between 4 and 8.
[0043] Preferably, the number of injection tubes arranged on the injection conduct is comprised between 5 and 20, more preferably between 8 and 16.
[0044] According to a particularly preferred embodiment, the injection unit is symmetrical along an axis perpendicular to the main longitudinal axis of the distribution channel.
[0045] The cross-section of the main injection ring and of any additional rings, if provided, may be circular or non-circular depending on the different embodiments. In one embodiment, said main ring or any additional ring may have a non-circular cross-section with a convex side facing in the main flow direction. Particularly preferably, such a cross-section is a semicircular cross-section. Said semicircular cross-section may include a semicircular side facing in the main flow direction and a flat side, in which case the injection orifice is located on the flat side.
[0046] Applicants have discovered that a non-circular cross-section of the injection ring provides particularly effective mixing. The improved mixing is believed to result from the formation of a vortex pattern, possibly a Von Karman vortex street, around the injection ring. This vortex pattern enhances mixing of the main flow with the injected medium, such as gas and ammonia.
[0047] In embodiments with multiple injection rings, eg, coaxial injection rings, the above features of the non-circular cross-section may apply to one ring, some of the rings, or all of the rings.
[0048] In embodiments where ammonia gas is routed to more than one item, such as a main ring and an additional ring, a suitable ammonia gas distributor may be provided. [Brief description of the drawings]
[0049] [Figure 1] 1 illustrates a conventional mixing system according to a representative prior art configuration. [Diagram 2] 1 shows a cross-sectional view of an injection unit according to one embodiment of the present invention. [Diagram 3] 1 shows a cross-sectional view of an injection unit according to a preferred embodiment of the present invention. [Figure 4] 1 shows a cross-sectional view of an injection unit according to an alternative embodiment of the invention. [Diagram 5] 1 shows a cross section of an injection unit according to an alternative embodiment of the present invention. [Figure 6] 1 illustrates a cross section of an injection ring according to one embodiment. [Figure 7] Details of FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0050] 1 shows a conventional mixing system according to a typical prior art configuration. The mixing system 1 comprises a distribution channel / conduct 4, an injection unit 5 and a static mixing unit 30 located downstream of the injection unit 5. The distribution conduct 4 is circular and has a main longitudinal axis 16.
[0051] The injection unit further comprises a cylindrical bore manifold 18 configured to deliver ammonia 2 as a reductant into the main stream of tail gas 3 via a plurality of apertures 19. The ammonia 2 is preferably gaseous ammonia. The static mixing unit 30 comprises a honeycomb structure configured to promote mixing of the reductant 2 with the tail gas 3.
[0052] FIG. 2 shows a cross-sectional view of an injection unit 5 according to an embodiment of the present invention.
[0053] The injection unit 5 comprises a manifold 6 and a main injection ring 7. The manifold 6 is disposed inside the distribution channel 4 and is arranged perpendicular to the main longitudinal axis 16 of the distribution channel 4 shown in FIG.
[0054] Ammonia 2 is fed to the manifold, which is injected into the main tail gas stream 3 via a main injection ring 7 .
[0055] The injection ring 7 has a circular structure and is provided with a number of injection orifices 10 oriented in the flow direction of the main flow 3 such that the ammonia gas 2 is injected into the tail gas 3 having the same or substantially the same flow direction as the latter.
[0056] FIG. 3 shows a cross-sectional view of an injection unit 5 according to a preferred embodiment of the present invention.
[0057] The injection unit 5 comprises a main injection ring 7 and an additional injection ring 9. The additional injection ring 9 is connected to the main injection ring 7 by a manifold 11, which is fastened to the main injection ring 7.
[0058] The main injection ring 7 and the additional injection ring 9 are supplied with ammonia gas 2. Each of said rings 7, 9 is provided with a number of injection orifices 10 preferably oriented in the same direction as the main longitudinal axis 16 of the distribution conduct. The ammonia gas 2 is thus injected into a main flow 3 having the same or substantially the same flow direction as the latter.
[0059] The ammonia gas 2 is distributed between the main ring 7 and the additional ring 9 by a distributor 33 which is further described below with reference to FIG.
[0060] FIG. 4 shows a cross-sectional view of an injection unit 5 according to an alternative embodiment of the present invention.
[0061] The injection unit 5 comprises a main injection ring 7 and an injection tube 13. The injection tube 13 is fastened to the main injection ring 7 and extends longitudinally from the manifold 6.
[0062] Fill tube 13 is provided with a plurality of metering openings 14 disposed longitudinally along the length of fill tube 13 .
[0063] Similar to the orifice 10 described above, the metering opening 14 is preferably oriented in the flow direction of a major longitudinal axis 16 of the distribution conduct 4 so as to inject the ammonia reducing agent 2 in the same flow direction as the tail gas 3.
[0064] FIG. 5 shows a cross-sectional view of an injection unit 5 according to an alternative embodiment of the present invention.
[0065] The injection unit 5 comprises a main injection ring 7 and an injection conduct 15. Ammonia gas 2 is distributed between said ring 7 and the conduct 15 by a distributor 33, similar to FIG.
[0066] The injection conduct 15 extends longitudinally from the manifold 6 and comprises a plurality of injection tubes 20 arranged longitudinally in two rows along the length of the injection conduct 15. The rows are located on opposite sides of the injection conduct 15.
[0067] The injection tubes 20 are parallel to one another and are arranged such that for each row of tubes, each pair of consecutive tubes consists of two tubes of different lengths.
[0068] Furthermore, each injection tube 20 is provided with an aperture 17 configured to eject ammonia gas 2 in a flow direction perpendicular to the main longitudinal axis 16 of the distribution channel 4 .
[0069] Figure 6 shows an embodiment of an injection ring 7 with a semicircular cross section, said cross section having a convex side 31 facing towards the mainstream gas 3 and a flat side 32 on which the orifice 10 is located. The arrows V in Figure 6 represent the formation of a vortex around the edge of the semicircular ring, improving the mixing of the gas 3 with the ammonia 2. The embodiment of Figure 6 can be applied to additional rings as well, such as ring 9 of Figure 3.
[0070] FIG. 7 shows the distributor 33 of the embodiment of FIG. 3. A similar distributor can be used in the embodiment of FIG. 7. The distributor 33 has one or more first passages 34 for feeding ammonia gas to the main ring 7 and one or more second passages 35 for feeding ammonia gas to the additional ring 9. These passages are preferably calibrated holes whose diameter is determined to achieve a targeted distribution of the ammonia gas flowing between the rings. In particular, the size of the passages 34, 35 can be determined taking into account the expected pressure losses in the rings, so that the desired flow rate of ammonia gas in each ring is ensured. For example, in the embodiment of FIG. 7, the distributor 33 has two lateral holes 34 feeding the main ring 7 on both sides and an axial hole 35 feeding the manifold 11 of the additional ring 9.
Claims
1. A mixing system (1) configured to mix a process gas (2) into a main flow (3), comprising: The mixing system (1) comprises: A distribution channel (4), An injection unit (5), A static mixing unit (3), said distribution channel (4) having a main longitudinal axis (16) which determines the flow direction of said main flow (3) during operation; The injection unit (5) comprises a manifold (6) and a main injection ring (7); The manifold (6) is disposed inside the distribution channel (4) and perpendicular to the main longitudinal axis (16) of the distribution channel (4); the manifold (6) is configured to receive the process gas (2) via the main injection ring (7); The mixing system (1), wherein the main injection ring (7) has a circular or annular structure, and the main injection ring (7) is provided with a plurality of injection orifices (10) arranged to introduce the process gas (2) into the main flow (3).
2. The injection unit (5) further comprises one or more additional rings (9) arranged concentrically with respect to the main injection ring (7), The one or more rings (9) are in fluid communication with the manifold (6) and the main injection ring (7) by one or more additional manifolds (11); the one or more additional manifolds (11) are configured to receive the process gas (2) via the one or more additional rings (9); The mixing system (1) according to claim 1, wherein the one or more additional rings (9) are provided with a number of injection orifices (10) for introducing the process gas (2) into the main flow (3).
3. 3. The mixing system (1) according to claim 1 or 2, wherein the injection orifices of at least one element of the main injection ring (7) and any additional rings (9) are oriented in the flow direction of the main flow.
4. said main ring (7) and said one or more additional rings (9) lie in a plane perpendicular to said main longitudinal axis (16) of said distribution conductor (4); The mixing system (1) according to any one of claims 1 to 3, wherein the main injection ring (7) and the one or more additional rings (9) are equally spaced from each other.
5. The mixing system (1) according to any one of claims 1 to 4, wherein the main ring (7) and / or the one or more additional rings (9) are integrated with the distribution channel (4) by one or more supports (12).
6. The injection unit (5) further comprises an injection tube (13); the fill tube (13) extends longitudinally from the manifold (6), the fill tube (13) being provided with a plurality of metering openings (14) longitudinally disposed along the length of the fill tube (13); The mixing system (1) according to any one of the preceding claims, wherein the metering openings (14) are preferably oriented in the flow direction of the forward main flow (3).
7. 7. The mixing system according to any one of claims 1 to 6, wherein at least one of the main injection ring (7) or any additional ring (9) has a non-circular cross section with a convex surface (31) facing in the flow direction of the main flow.
8. At least one of the main injection ring (7) or any additional ring (9) has a semicircular cross section including a semicircular convex surface (31) facing in the flow direction of the main flow and a flat surface (32); The mixing system of claim 7 , wherein the injection orifice is disposed on the flat surface.
9. The injection unit (5) further comprises an injection conductor (15) extending longitudinally from the manifold (6), the injection conductor (15) being provided with a plurality of injection tubes (20) arranged longitudinally forming at least one row along the length of the injection conductor (15); the injection tubes (20) are parallel to one another and arranged such that for each row of tubes, each pair of consecutive tubes is made up of two tubes of different lengths; 9. The mixing system (1) according to any one of claims 1 to 8, wherein each injection tube (20) is provided with an aperture (17) configured to eject the process gas (2) in a flow direction perpendicular to the main longitudinal axis (16) of the distribution channel (4).
10. 10. The mixing system according to claim 9, wherein the number of rows is two, said two rows being arranged on opposite sides of said injection conductor (15).
11. The mixing system according to any one of the preceding claims, wherein the number of injection orifices (10) arranged on the main injection ring (7) is comprised between 8 and 20, preferably between 10 and 15.
12. 12. The mixing system according to any one of claims 2 to 11, wherein the number of injection orifices (10) arranged in each of said one or more additional rings (9) is comprised between 3 and 15, preferably between 4 and 8.
13. The mixing system according to any one of claims 9 to 12, wherein the number of injection tubes (20) arranged on said injection conductor (15) is comprised between 5 and 20, preferably between 8 and 16.
14. The mixing system (1) according to any one of the preceding claims, wherein the injection unit (5) is symmetrical along an axis (8) perpendicular to the main longitudinal axis (16) of the distribution channel (4).
15. 1. A conditioning system for removing nitrogen oxides from an input gas by selective catalytic reduction, comprising: The system comprises: A catalytic reactor; A mixing system according to any one of claims 1 to 11, the reactor is configured to catalytically reduce NOx contained in the input gas in the presence of a reducing agent; The mixing system is configured such that the main flow (2) is the input gas directed to the reactor and the process gas is the reducing agent.
16. The system of claim 15, wherein the manifold (6) is connected to an ammonia supply line such that ammonia is used as the reducing agent.
17. 1. A nitric acid plant comprising: The nitric acid plant comprises an absorption column and a conditioning system according to claim 15 or 16, In the absorption column, the NOx-containing gas is absorbed in water to produce nitric acid, and a tail gas containing residual NOx is produced; the manifold (6) of the mixing system is connected to a supply line of the reducing agent; The distribution conductor (4) is connected to a tail gas output of the absorption column such that the conditioning system is arranged to remove NOx from the tail gas of the absorption column.