Process for nitric acid production

Abstract

A process for production of nitric acid including the steps of oxidation of ammonia in at least one ammonia burner producing a nitrous gas; cooling of said nitrous gas producing a stream of nitrogen oxides; feeding said stream of nitrogen oxides to an absorber obtaining a nitric acid product and an absorption tail gas which is withdrawn from the absorber; reintroducing a tail gas recirculation stream, which is a portion of the absorption tail gas, into the absorber; sending to the at least one ammonia burner a stream of conditioned tail gas, which is obtained by mixing a stream of absorption tail gas with an oxygen-containing stream.

Description

[0001] Process for nitric acid production

[0002] DESCRIPTION

[0003] Field of application

[0004] The present invention relates to the field of nitric acid production.

[0005] Prior art

[0006] The industrial process (Ostwald process) for production of nitric acid (HNO3) involves basically the following steps: ammonia is catalytically combusted with air in an ammonia oxidation reactor, also termed ammonia burner, to produce a nitrous gas mainly containing nitrogen oxides (NO and NO2); said nitrous gas is subject to cooling and condensation obtaining an acid condensate and a cooled gas stream; said acid condensate and cooled gas are sent to an absorber, usually an absorption column, where the nitrogen oxides are absorbed in water producing a liquid nitric acid and a tail gas. Various implementations of the process are described e.g. in the Ullmann’s Encyclopaedia, 2012 Edition, “Nitric Acid”.

[0007] The process is performed under pressure. In the single-pressure process, the ammonia oxidation and absorption are carried out at the same pressure, usually around 5 to 10 bar. In the dual-pressure process, the absorption step is carried out at higher pressure than the ammonia oxidation, for example ammonia oxidation at about 5 bar and absorption at about 10-12 bar. Consequently, a compressed air feed must be supplied to the ammonia oxidation reactor and to the absorption column. The compression of air represents a significant consumption of energy in the entire process.

[0008] The absorption column is designed to optimize the contact between gas and liquid to produce concentrated nitric acid. The absorption column typically includes a set of perforated sieve trays installed horizontally inside the column, creating multiple stages where the liquid and vapor can interact. The nitric-acid product collected from the absorption column is normally purified in at least onebleacher downstream the column.

[0009] The ammonia feed for the ammonia oxidation burner may be produced in an ammonia plant connected to the nitric acid plant. The production of ammonia typically starts from a make-up gas containing nitrogen and hydrogen in a suitable ratio. Traditionally, the make-up gas is produced by steam reforming of hydrocarbons, thus heavily relying on fossil fuels; nowadays there is a growing interest in the so-called “green” nitric acid production. With the term "green" nitric acid production we refer to a synthesis process of nitric acid wherein at least one feed, i.e. ammonia or oxygen, and / or the source of energy powering the plant comes at least in part from a renewable energy source. For example, a renewable source of energy can be used to produce hydrogen by electrolysis of water and nitrogen in an air separation unit.

[0010] The use of renewable energy sources however introduced the issue of the fluctuation of the available power. Accordingly, flexibility is crucial in green nitric acid plants powered by renewable energy as they must be able to run at reduced load to follow the generation profile of the energy source.

[0011] Typical loads of conventional nitric acid process are limited between 60% and 110% due to technical issues like limitations of the turbomachinery and minimum flow in the sieve trays of the absorption column, necessary to avoid weeping. Weeping is an unwanted condition occurring when the flow of gas across the sieve trays is low, causing the liquid to drop down across the holes. This causes a decrease in the liquid level on trays and consequently a decrease in the absorption efficiency. The column may suffer from weeping in a condition of low load caused by low renewable power.

[0012] In summary, a drawback of the prior art is that the absorption column has a poor efficiency at low load such as less than 50%. This affects the advantages of “green” production of nitric acid.

[0013] Another important challenge in the nitric acid process is the handling of the tail gas withdrawn from the absorber. Said tail gas contains significant amounts ofnitrogen oxides (NOx) and is treated in a tail gas conditioning section before venting. The tail gas treatment section includes normally one or more reactors with one or more catalytic beds for removing nitrogen oxides (NOx) and nitrous oxides N2O. The tail gas may be heated before treatment to reach the suitable temperature for the catalytic purification. Being under pressure, the tail gas may also be expanded after purification to recover some energy. The treatment of the tail gas is also a substantial source of cost in a modem nitric acid plant.

[0014] Currently, there is a demand for enhanced green nitric acid production facilities able to adapt to fluctuation of energy. The challenge is to make a nitric acid process running efficiently also at low load conditions. The main problem related to low load is the possibility of maldistribution in the sieve trays of the absorber causing weeping and consequently a decrease in the plant efficiency. Furthermore, there is a need to explore ways to retrofit current nitric acid plants to take advantage of green ammonia production, reducing carbon emissions and improving energy consumption.

[0015] US 2024 / 0336481 discloses a dual pressure system for producing nitric acid and a method of operating thereof. CN 117500752 discloses a single pressure system for producing nitric acid and method of operating the same. Additional prior art can be found in EP3365276A1, EP4209454A1, WO2023287294A1 , W02019038045A1.

[0016] Summary of the invention

[0017] The invention addresses the problem of how to increase the flexibility of the nitric acid process, to improve the useability in connection with a fluctuating source of energy. Particularly, the invention addressed the problem of how to improve the efficiency of the absorber at low load. The invention further addresses the problem of how to improve the processing of the tail gas.

[0018] These problems are solved with a process according to the claims.

[0019] The process includes that a tail gas recirculation stream is reintroduced into theabsorber increasing the flow of gas in conditions of low load. Additionally, a portion of the tail gas withdrawn from the absorber is sent to the ammonia oxidation reactor after addition of oxygen by mixing with an oxygen-containing stream. The invention requires therefore an oxygen-containing stream to condition the tail gas recycled to the ammonia oxidation reactor. In a very interesting application, this oxygen-containing stream is obtained from electrolysis of water and / or from air separation.

[0020] The invention makes an ingenious use of the oxygen stream which is obtained as a by-product of production of hydrogen from water or nitrogen from air. This is typically the case of green ammonia-nitric acid plants where the production of the ammonia make-up gas relies on water electrolysis and air separation powered by renewable energy. The invention uses this oxygen to condition a tail gas stream, by increasing its oxygen content. This stream is then sent to the ammonia oxidation reactor. Preferably, the conditioned tail gas may have a content of oxygen close to the content of air. Accordingly, as part of the oxygen required for ammonia oxidation is introduced with the conditioned tail gas, the air feed and related power compression required by the ammonia burner are reduced. In some embodiments, said oxygen stream provides all the oxygen required by the oxidation of ammonia and further oxidation of nitrogen monoxide NO to nitrogen dioxide NO2, and a compressed air feed is not necessary.

[0021] The tail gas recirculation stream sent to the absorber, in addition, increases the gas flow in the absorber in a condition of low load, thus reducing the problem of weeping in the absorption column.

[0022] Preferably, the tail gas recirculation stream is sent to a bleacher downstream the absorber. In the bleacher, said tail gas act as a stripping agent and the tail gas emerging from the bleacher is then sent to the absorber.

[0023] Description of the invention

[0024] The invention discloses a process for production of nitric acid including the steps of: oxidation of ammonia in at least one ammonia burner producing a nitrous gas;cooling of said nitrous gas in a nitrous gas cooling / condensation section producing a stream of nitrogen oxides; feeding said stream of nitrogen oxides to an absorber, where nitrogen oxides are absorbed in water obtaining a nitric acid product and an absorption tail gas which is withdrawn from the absorber; feeding the nitric acid product effluent from the absorber to a bleacher for purification. The process further includes the following steps: a tail gas recirculation stream, which is a portion of the absorption tail gas withdrawn from the absorber, is reintroduced into the absorber; a stream of conditioned tail gas, which is obtained by mixing a stream of absorption tail gas with an oxygen-containing stream, is sent to said at least one ammonia burner; said oxygen-containing stream having a content of oxygen of at least 30% by volume.

[0025] The tail gas can be sent to the absorber and to the ammonia oxidation reactor by means of a tail gas compressor. Said tail gas compressor may be included in a tail gas conditioning section.

[0026] Said oxygen-containing stream may be introduced in the tail gas conditioning section by means of one or more inlet(s). Preferably, said oxygen-containing stream contains more than 50% oxygen by volume, more preferably at least 70%. According to a highly preferred embodiment, said oxygen-containing stream is a product of an air separation process and / or of hydrolysis of water.

[0027] Said oxygen-containing stream, mixed with tail gas, may provide all the oxygen required for the oxidation of ammonia or only a part thereof. The ammonia oxidation reactor may also receive a separate input of oxygen, for example with a stream of air.

[0028] The absorber typically includes at least one absorption column which comprises a set of perforated sieve trays installed horizontally inside the column. Preferably, the absorber converts nitrogen oxides to nitric acid by adding process water on the top trays of said perforated sieve trays.

[0029] According to an embodiment, the absorption tail gas withdrawn from the absorber is compressed in a tail gas compressor and the compressed tail gas delivered bysaid compressor is split to form said tail gas recirculation stream and a remainder tail gas stream, wherein at least a portion of said remainder tail gas stream is mixed with the oxygen-containing stream to form the conditioned tail gas sent to said at least one ammonia burner.

[0030] Preferably, said tail gas recirculation stream is reintroduced into the absorber after mixing with said stream of nitrogen oxides effluent from the cooling / condensation section. The flow rate of the tail gas recirculation stream may be regulated as a function of the flow rate of the stream of nitrogen oxides after mixing with the tail gas recirculation stream. The control of this stream is necessary to keep the flow rate of the stream directed to the absorber higher than a target threshold, depending on the process pressure and / or the process load.

[0031] According to a preferred embodiment, said conditioned tail gas stream is mixed with fresh ammonia and the resulting mixture is introduced as ammonia feed stream in the at least one ammonia burner. Preferably, the amount of the oxygencontaining stream added to said stream of absorption tail gas is controlled such that the conditioned tail gas sent to the ammonia burner has an oxygen content of 15% to 25% by volume. The conditioned tail gas may be suitably heated before being introduced in the ammonia burner.

[0032] According to an embodiment, said conditioned tail gas is sent partially to the at least one ammonia burner and partially to said bleacher to act as stripping agent. The tail gas withdrawn from the bleacher may be sent to the absorber. The bleacher in a nitric acid production process serves the critical role of purifying the final product by removing dissolved nitrogen oxides, thereby improving the quality and concentration of the nitric acid.

[0033] Preferably, the stream of absorption tail gas, added with oxygen, is sent to said bleacher. The advantage of this embodiment is to use the same oxygen injection both for the stream directed to the ammonia burner and for the stream directed to the bleacher.

[0034] According to an embodiment, a separate addition of an oxygen-containing streamis introduced to the tail gas stream directed to the bleacher, said addition of oxygen-containing stream is controlled to obtain a target content of oxygen in the absorption tail gas at the outlet of the absorber, preferably said target content being less than 10% by volume, more preferably less than 5% by volume. With this embodiment two different oxygen injections are used for the stream directed to the ammonia burner and for the stream directed to the bleacher. The advantage of this embodiment is the possibility to separately control the level of oxygen directed to the burner in order to have a composition close to air, and to control the level of oxygen added to the tail gas stream directed to the bleacher to have an oxygen level in the tail gas effluent from the absorber below the target content. Therefore, the tail gas directed to the bleacher may be taken before or after the addition of the oxygen-containing stream.

[0035] According to a preferred embodiment, a portion of absorption tail gas withdrawn from the absorber, before addition of oxygen, is sent to a tail gas purification section for removal of nitrogen oxides (NOx) and nitrous oxides (N2O) obtaining a purified tail gas. Said purified tail gas is optionally expanded and discharged. This embodiment allows to purify small amount of gas, reducing the dimensions of the catalyst used for purification and associated costs.

[0036] The addition of inert gas to a reactor that hosts an exothermic reaction is necessary to control the system's temperature. According to an embodiment, the process includes the step of introducing an inert gas stream to the absorption tail gas effluent from the absorber, before and / or after compression of the tail gas, preferably said inert gas stream includes nitrogen. Inert gases do not participate in the chemical reaction but they are used to absorb part of the heat generated, reducing the risk of uncontrolled temperature increase and preventing the dangerous phenomena of "thermal runaway". The stream of inert gas may decrease the temperature of the reactor slowing down the reaction itself. Therefore, during start-up, inert gases may be sent to the portion of absorption tail gas purged from the process improving heating of the catalyst. Preferably, the process includes that at least part of the purged tail gas is heated before being treated. Treatment of the purged tail gas is necessary in order to avoid emittingnitrogen oxides and other pollutants in the atmosphere.

[0037] The oxidation of ammonia in the ammonia burner is preferably performed in a first catalyst supported by one or more gauzes and in a second catalyst installed under said gauzes. The first catalyst is used for oxidation of ammonia producing NOx and byproducts as nitrous oxides (N2O). The second catalyst is used to convert N2O in nitrogen and oxygen so that N2O does not accumulate in the plant. This allows a reduction of nitrous oxides concentration in the purge stream and consequently lower emissions in the atmosphere.

[0038] According to a preferred embodiment, the flow rate of conditioned tail gas which is recycled to the at least one ammonia burner is controlled as a function of a detected temperature of the burner to maintain the temperature of the burner in a target range. The advantage of this embodiment is to keep constant process conditions in the burner avoiding temperature oscillation on the gauzes. Moreover, the control of the burner’s temperature is crucial to control the inlet temperature of the absorber in order to avoid weeping.

[0039] According to a preferred embodiment, the amount of purge and / or the amount of inert gas added to the purge stream is used as a means to control the pressure in the process. The advantage is to have a process operating at variable loads but at the same pressure.

[0040] According to a preferred embodiment, a flow rate control regulates the flow rate of the portion of conditioned tail gas directed to the bleacher.

[0041] Preferably, the ammonia feed sent to the at least one ammonia burner is produced from conversion of a make-up gas containing nitrogen and hydrogen; wherein at least part of the hydrogen for the make-up gas is produced by electrolysis of water and / or nitrogen for the make-up gas is produced in an air separation unit, wherein said oxygen-containing stream added to the tail gas is retrieved from said air separation unit and / or from said electrolysis of water.

[0042] In an embodiment, part of the N2O contained in the nitrous gas is removed in asecondary N2O removal stage between the ammonia burner and the absorber. According to established nomenclature in the field of nitric acid production, a secondary removal of N2O denotes that N2O is removed from the process gas (obtained from ammonia oxidation) before the absorption step, wherein tertiary abatement denotes that N2O is removed from the tail gas after absorption and before expansion. A secondary abatement of N2O is preferred to reduce the risk of accumulation of N2O in the loop.

[0043] An interesting feature of the invention is that the oxygen-containing stream added to the tail gas may provide a major part of the oxygen required for oxidation of ammonia, for example at least 50% or at least 70% or even all the required oxygen. Another interesting feature of the invention is that the oxygen-containing stream added may provide also part of the oxygen required for oxidation of NO to NO2 between the ammonia burner and the absorber gas exit, for example at least 50% or at least 70% or even all the required oxygen. In the latter case when all the oxygen is provided by the oxygen stream, the process does not require an inlet of compressed air for normal operation. Accordingly, the plant does not need an air compressor which is an expensive item. During startup, the plant may be pressurized by a suitable compressed gas, for example compressed air provided by a start-up air compressor or compressed nitrogen provided by an available air separation unit (ASU). A start-up compressor, if provided, is smaller and less expensive than an air compressor required for normal operation. Said start-up compressor or ASU may be external to the plant.

[0044] The invention may include compressing the tail gas in a tail gas compressor. Preferably, the tail gas withdrawn from the absorber is heated to a temperature at least 5 °C above the dew point, before compression, to avoid damage of the compressor. Preferably said heating is to at least 10 °C or at least 15 °C above the dew point of the gas.

[0045] In some embodiments, a purged tail gas is heated before being treated. This step is performed in a dedicated purge gas heater. The heat exchange duty of said purge gas heater may be determined by the startup and / or shutdown;accordingly, said gas heater may be over-designed with respect to the duty required by normal operation.

[0046] The flow rate of the tail gas compressor may be controlled by inlet guide vanes (IGV) and / or by controlling the flow through an antisurge bypass line.

[0047] The invention is particularly useful for nitric acid plants powered with renewable energy. In condition of low energy source, the nitrous gas flow rate sent to the absorber may be lower than a target threshold, causing weeping and decreasing the efficiency of the absorption. The absorption tail gas withdrawn from the absorber may be used as an additional input to the nitrous gas absorber. The inventive recycle of tail gas disclosed by this invention allows to operate the nitric acid plant in conditions of low load, e.g. < 50% of nominal load. The increase of the flow rate in the absorber reduces the risk of weeping in the absorption column and improves the absorption efficiency. Furthermore, the recycle of tail gas allows a reduction of nitrogen oxides emissions in the atmosphere which are high polluting.

[0048] The recycle of the tail gas effluent from the absorber is particularly convenient when an oxygen-rich gas stream is available, as in ammonia plants where hydrogen and nitrogen are produced from an electrolysis unit and / or an air separation unit, and oxygen is a byproduct not used in the production of ammonia. The addition of oxygen to the tail gas generates a gas stream that can be used instead of fresh air in ammonia burners of nitric acid process. Therefore, the recycle of tail gas and oxygen is advantageous as it allows the use of oxygen that otherwise would be a byproduct of ammonia plants and it reduces the need of fresh air in nitric acid process.

[0049] The process disclosed by this invention can be used also for revamping nitric acid plants to improve their efficiency in conditions of variable load.

[0050] Description of the figures

[0051] Fig. 1 is a scheme of a nitric acid process according to a first embodiment of theinvention wherein a portion of tail gas is recirculated to the absorber and another portion of tail gas is added with oxygen and sent to the ammonia burner.

[0052] Fig. 2 is a scheme of an embodiment featuring a stream of tail gas added with oxygen and sent to a bleacher of the nitric acid product.

[0053] Fig. 3 illustrates a variant of the scheme of Fig. 2 wherein a stream of tail gas is sent partially to the absorber and partially to the bleacher.

[0054] Fig. 4 illustrates a variant of the embodiment of Fig. 3.

[0055] Fig. 5 illustrates a preferred variant of the embodiment of Fig. 2.

[0056] Fig. 1 to Fig. 5 illustrate a schematic flow of a nitric acid production process according to various embodiments of the invention. All embodiments comprise an ammonia burner 1, a cooling / condensation section 2, a nitrous gas absorber 3, a bleacher 8 and a tail gas compressor 4. The tail gas compressor 4 is included in a tail gas conditioning section 20.

[0057] In the ammonia burner 1 , an ammonia feed 21 is subject to oxidation producing a nitrous gas 13. Oxygen for the oxidation of the ammonia feed 21 is provided by an oxygen-containing stream 5 and optionally by a separate stream of air. The ammonia feed 21 and the oxygen-containing stream 5 are mixed with a tail gas stream 6 as described below.

[0058] The stream of nitrous gas 13 is directed to the cooling / condensation section 2 forming a stream of nitrogen oxides 14 and an acid stream 32, both directed to the absorber 3.

[0059] The absorber 3 converts the nitrogen oxides to nitric acid by adding additional process water 25 on the top trays. The absorber produces a liquid stream 10 containing nitric acid and a stream of tail gas 15. The liquid stream 10 is sent to a bleacher 8 for purification obtaining a purified nitric acid product 23. A gaseous stream 24 withdrawn from the bleacher 8 is sent back to the absorber 3.Part of the tail gas 15 effluent from the absorber 3 is purged through line 11 , the remainder tail gas is compressed in a tail gas compressor 4 and recycled within the plant.

[0060] In Fig. 1 , the compressed tail gas stream 16 effluent from the tail gas compressor 4 is split in a tail gas recirculation stream 7 directed to the absorber 3 and a tail gas stream 6 directed to the ammonia burner 1.

[0061] The tail gas recirculation stream 7 is reintroduced into the absorber 3 mixing up with the stream of nitrogen oxides 14 effluent from the cooling / condensation section 2. The flow of the recirculation stream 7 mixed with the stream of nitrogen oxides 14 is measured with a flow controller 19 and regulated through a valve 18.

[0062] The tail gas stream 6 is mixed with the oxygen-containing stream 5 to form a conditioned tail gas 22. Said conditioned tail gas 22 is mixed with ammonia 21 and the mixture is sent to said ammonia burner 1.

[0063] The oxygen-containing stream 5 is preferably the product of an electrolysis unit or of an air separation unit. Said electrolysis unit and air separation unit may be used to produce hydrogen and nitrogen necessary for the synthesis of the ammonia feed 21 in a tied-in ammonia plant.

[0064] Fig. 2 shows an embodiment wherein a portion 9 of the conditioned tail gas 22 is sent to the bleacher 8 to act as a stripping agent. Before entering the bleacher 8, additional oxygen can be introduced to stream 9 through line 5B. The tail gas sent to the bleacher passes into the absorber 3 via the line 24. This embodiment allows to separately control the addition of oxygen through line 5 and through line 5B and, therefore, the amount of oxygen introduced in the ammonia burner 1 and introduced in the bleacher 8 and absorber 3, respectively.

[0065] Fig. 3 shows another embodiment, wherein a portion of the tail gas recirculation stream 7 is sent to the bleacher through line 7B. Accordingly a part of said tail gas recirculation stream 7 goes directly to the absorber 3 via the valve 18 and the remainder goes to the bleacher 8.Fig. 4 shows an embodiment wherein the flow of the purge stream withdrawn from the process through line 11 is regulated by means of a control valve 26. The control valve 26 controls the pressure in the absorber 3. The pressure in the line of the tail gas 15 is measured by a pressure controller 27 which governs the valve 26.

[0066] Addition of inert gas to the process is preferred to compensate inert gas exiting in the purge line 11. According to an embodiment, a stream of inert gases may be introduced through line 12 to the tail gas effluent from the absorber 3 after compression of the tail gas. During start-up, the stream of inert gas could decrease the temperature of the reactor slowing down the reaction. In such situation, the inert gases may be rerouted in the purge stream through line 12B improving heating of the catalyst.

[0067] Fig. 5 shows other controls that may be performed to the process. A control valve 30 is added to regulate the temperature of the conditioned tail gas 22, which is measured by means of the temperature controller 31. This control is used to stabilize the inlet temperature of the burner 1 avoiding overheating. A flow controller 29 measures the flow of the stream 9 of the conditioned tail gas directed to the bleacher 8. The flow rate of this stream is regulated by means of a control valve 28.

[0068] Example 1

[0069] The table below shows a comparison of data of a nitric acid plant with a nominal load of 500 MTPD, operating according to different plant loads.

[0070] The Case 1 refers to the plant operating at 100% load which, as shown in the table below, does not lead to any maldistribution of the flow in the sieve trays.

[0071] In Case 2, the plant is operated at 30% of its nominal load and a severe maldistribution of the flow is observed on the sieve trays from the 15thto 34th.

[0072] Case 3 illustrates a prior art solution where the drawback of weeping or flow maldistribution is avoided by decreasing the pressure of the process.Case 4 illustrates a solution according to an embodiment of the invention, wherein 10% of the tail gas is recycled to the absorber. The recycle of the tail gas avoids weeping and flow maldistribution in the sieve trays and has a further advantage, compared to Case 3, in that it reduces the NOx emissions released from the top of the absorption column.

[0073]

[0074] Table 1

Claims

CLAIMS1 ) A process for production of nitric acid including the steps of:oxidation of ammonia in at least one ammonia burner (1) producing a nitrous gas (13);cooling of said nitrous gas in a nitrous gas cooling / condensation section (2) producing a stream of nitrogen oxides (14);feeding said stream of nitrogen oxides (14) to an absorber (3), where nitrogen oxides are absorbed in water obtaining a nitric acid product (10) and an absorption tail gas (15) which is withdrawn from the absorber;feeding the nitric acid product (10) effluent from the absorber (3) to a bleacher (8) for purification;the process further includes the following steps:a tail gas recirculation stream (7), which is a portion of the absorption tail gas withdrawn from the absorber, is reintroduced into the absorber (3);a stream of conditioned tail gas (22), which is obtained by mixing a stream of absorption tail gas with an oxygen-containing stream (5), is sent to said at least one ammonia burner (1 );said oxygen-containing stream having a content of oxygen of at least 30% by volume.2) A process according to claim 1 , wherein said oxygen-containing stream (5) contains more than 50% oxygen by volume, preferably at least 70%.3) A process according to claim 1 or 2, wherein said oxygen-containing stream (5) is a product of an air separation process and / or of hydrolysis of water.4) A process according to any of claims 1 to 3, wherein the absorption tailgas (15) withdrawn from the absorber (3) is compressed in a tail gas compressor (4) and the compressed tail gas (16) delivered by said compressor is split to form said tail gas recirculation stream (7) and a remainder tail gas stream, wherein at least a portion of said remainder tail gas stream is mixed with the oxygen-containing stream (5) to form the conditioned tail gas (22) sent to the at least one burner.5) A process according to any of the previous claims, wherein said tail gas recirculation stream (7) is reintroduced into the absorber (3) after mixing with said stream of nitrogen oxides (14) effluent from the cooling / condensation section (2).6) A process according to any of the previous claims, wherein the flow rate of the tail gas recirculation stream (7) is regulated as a function of the flow rate of the stream of nitrogen oxides (14) after mixing with the tail gas recirculation stream (7).7) A process according to any of the previous claims, wherein said conditioned tail gas stream (22) is mixed with fresh ammonia (21) and the resulting mixture is introduced as ammonia feed stream in the at least one ammonia burner (1).8) A process according to any of the previous claims, wherein the amount of the oxygen-containing stream (5) added to said stream of absorption tail gas is controlled such that the conditioned tail gas (22) sent to the at least one ammonia burner has an oxygen content of 15% to 25% by volume.9) A process according to any of the previous claims, wherein said conditioned tail gas (22) is sent partially to the at least one ammonia burner (1) and partially (9) to said bleacher (8) to act as stripping agent.10)A process according to claim 9, wherein the tail gas (24) withdrawn from the bleacher (8) is sent to the absorber (3).11)A process according to claim 10 further comprising that a stream ofabsorption tail gas (15), added with oxygen, is sent to said bleacher (8).12)A process according to claim 11 including a separate addition of an oxygen-containing stream (5B) to the tail gas stream (9) directed to the bleacher (8), said addition of oxygen-containing stream being controlled to obtain a target content of oxygen in the absorption tail gas (15) at the outlet of the absorber (3), preferably said target content being less than 10% by volume, more preferably less than 5% by volume.13)A process according to any of the previous claims wherein the tail gas directed to the bleacher is taken before (7B) or after (9) the addition of the oxygen-containing stream (5).14)A process according to any of the previous claims, wherein a portion of the absorption tail gas (15) withdrawn from the absorber, before addition of oxygen, is sent to a tail gas purification section for removal of nitrogen oxides (NOx) and nitrous oxides (N2O) obtaining a purified tail gas, and said purified tail gas is optionally expanded and discharged.15)A process according to any of the previous claims, wherein the process includes the step of introducing an inert gas stream to the absorption tail gas (15) effluent from the absorber (3), before and / or after compression of the tail gas, preferably said inert gas stream includes nitrogen.16)A process according to any of the previous claims, wherein at least part of the purged tail gas is heated before being treated.17)A process according to any of the previous claims, wherein the flow rate of conditioned tail gas (22) which is recycled to the at least one ammonia burner is controlled as a function of a detected temperature of the burner to maintain the temperature of the burner in a target range.18)A process according to any of the previous claims, wherein the amount of purge and / or the amount of inert gas added to the purge stream is used as a means to control the pressure in the process.)A process according to any of the previous claims, wherein a flow rate control regulates the flow rate of the portion (9) of conditioned tail gas directed to the bleacher (8).)A process according to any of the previous claims, wherein the ammonia feed sent to the at least one ammonia burner is produced from conversion of a make-up gas containing nitrogen and hydrogen; wherein at least part of the hydrogen for the make-up gas is produced by electrolysis of water and / or nitrogen for the make-up gas is produced in an air separation unit, wherein said oxygen-containing stream added to the tail gas is retrieved from said air separation unit and / or from said electrolysis of water.)A process according to any of the previous claims, wherein a part of the N2O contained in the nitrous gas (13) is removed in a secondary N2O removal stage between the ammonia burner (1) and the absorber (3). )A process according to any of the previous claims wherein said oxygen- containing stream (5) provides at least 50%, preferably at least 70%, more preferably all the oxygen necessary for oxidation of ammonia and for oxidation of NO to NO2.

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