Methods for reducing NOx during transient events in nitric acid plants.

Abstract

A method for reducing NOx emissions during start-up and shutdown of a nitric acid plant, the nitric acid plant comprising a tail gas treatment section (10) including a first catalyst layer (2) for removing NO followed by a second catalyst layer (3) for removing NOx, wherein during start-up or shutdown, operation of the treatment section (10) is temporarily modified by adding a NOx reducing agent (4) upstream of the first catalyst layer (2), so that the first catalyst layer (2) provides for the reduction of NOx.

Description

[Technical Field]

[0001] The present invention relates to the field of nitric acid production. In particular, the present invention relates to a method for reducing NOx emissions during transient events in a nitric acid plant. [Background technology]

[0002] The industrial production of nitric acid uses well-known processes described in the literature, for example in Ullmann's Encyclopedia of Industrial Chemistry, Edition 2012 vol. 24, "Nitric Acid, Nitrous Acid and Nitrogen Oxides".

[0003] Briefly, the process involves catalytic oxidation of ammonia with an oxidizing agent over a suitable catalyst, typically a platinum / rhodium catalytic gauze. The product of ammonia oxidation is a hot nitrogen oxide-containing gas, which is absorbed in water in a dedicated column to produce a nitric acid solution and a tail gas.

[0004] Tail gas contains nitrogen oxides (NO), a source of air pollution. x (NO and NO2) and dinitrogen oxide, N2O, which must be removed before the tail gas is released into the atmosphere.

[0005] Several abatement systems are available in the art for removing nitrogen oxides and dinitrogen oxides from tail gas during normal or steady-state plant operation. Typically, NOx and NO are removed catalytically. Some plants employ so-called tertiary abatement systems, which have multiple catalyst beds arranged in series downstream of an absorber. The term tertiary refers to the location downstream of the absorber.

[0006] More specifically, tertiary abatement systems typically comprise a first catalyst bed and a second catalyst bed arranged in series, the first catalyst bed being used at a temperature range of 300°C to 600°C to catalytically decompose NO into nitrogen and oxygen, and the second catalyst bed reducing nitrogen oxides (NOx) to nitrogen and water in the presence of a suitable reducing agent, typically ammonia and hydrocarbons.

[0007] Overall, a tertiary abatement system such as that described above can reach greater than 95% N2O and NOx abatement efficiency during normal plant operation. For example, the NOx content in the tail gas can be reduced to values ​​below 40 ppmv, e.g., 20 ppmv, which is considered satisfactory in most cases.

[0008] Unfortunately, during transient events, such as plant startup or shutdown transients, tail gas temperatures are insufficient to maintain adequate conversion of NO and adequate reduction of NO due to poor catalytic activity at low temperatures. For example, tail gas temperatures during startup transients can range from 180°C to 230°C.

[0009] The low conversion rate of NO during the transient event is not usually considered a problem because the amount of NO formed during the transient event is limited. However, the production of NO is problematic. In addition to such pollution, NO emissions can result in the formation of an undesirable yellow-to-reddish-brown plume stack. The formation of such a visible plume stack is undesirable because it causes anxiety to residents around the plant. Therefore, efforts are being made to provide plumeless start-up of nitric acid plants.

[0010] Prior art solutions have proposed adding items such as additional heaters to heat the process gas more quickly, especially during start-up, but these solutions have the drawback of requiring the installation of expensive items.

[0011] Therefore, it is highly desirable to find ways to reduce atmospheric emissions of nitrogen oxides, NOx, during the transient events of start-up and shutdown of nitric acid plants. Summary of the Invention

[0012] The present invention aims to overcome the above-mentioned drawbacks of the prior art. In particular, the present invention addresses the problem of high concentrations of NOx emitted during transient periods of start-up and shutdown of a nitric acid plant as a result of deviations from normal operating conditions. The present invention also aims to address the problem of how to reduce NOx emissions during transient periods without adding expensive items.

[0013] This problem is solved by the method according to claim 1.

[0014] The method reduces NOx emissions in a nitric acid plant during a transient event for plant startup or shutdown, The nitric acid plant comprises a synthesis section and an absorption section, wherein ammonia is catalytically oxidized in the synthesis section to obtain a nitrogen oxide-containing gas, and the nitrogen oxide-containing gas is absorbed in water in the absorption section to produce concentrated nitric acid, which is a tail gas containing NOx and NO; the nitric acid plant comprises a treatment section suitable for removing N2O and NOx from the tail gas before it is discharged into the atmosphere, the treatment section comprising a first catalyst bed and a second catalyst bed arranged so that the tail gas passes through them in succession, and during normal operation of the nitric acid plant, one of the two catalyst beds is used to remove N2O and the other of the two catalyst beds is used to remove NOx in the presence of a reducing agent; During the transient event, the tail gas passes through the catalyst bed to remove NO in the presence of an additional amount of reductant that is added only during the transient event, such that the catalyst bed temporarily operates as an additional bed for removing NO from the tail gas.

[0015] The present invention is applicable to nitric acid plants having a tail gas treater including a catalyst bed for removing NO and a catalyst bed for reducing NO in the presence of a reducing agent, such as ammonia. The method modifies the operation of the treater by introducing an additional amount of reducing agent so that the catalyst bed originally designed for removing NO during normal operation temporarily acts as an additional catalyst bed for the reduction of NO.

[0016] The first and second catalyst beds are passed sequentially by the tail gas. In a first general embodiment of the present invention, a first bed is for removing NO and a second bed for removing NO follows the first bed. Thus, in this first embodiment, the first bed is temporarily used as an additional bed for removing NO. In a second general embodiment, the arrangement is reversed from the first embodiment, i.e., a first bed is for removing NO and a second bed for removing NO follows the first bed. In this second embodiment, the second bed provides temporary additional de-NOx capacity during transient events.

[0017] The terms DeNOx and DeN2O refer to the removal of NOx and N2O, respectively.

[0018] In a broader sense, the present invention is based on the enlightened insight that in the above-mentioned setup, a catalyst bed originally designed for removing NO in the absence of a reducing agent can be used to complement the function of the other bed, i.e., the function of removing NO, during transient events, by supplying an appropriate additional amount of reducing agent to remove NO in the presence of a reducing agent. This can be done provided that the catalyst in said bed is also suitable for reducing NO in the presence of said reducing agent. This is the case for many catalysts for the decomposition of NO, including the widely used iron-zeolite catalysts.

[0019] A catalyst bed designed to remove NOx produced during normal operation may not be able to remove the much larger amounts of NOx that are transiently generated, particularly during start-up. The present invention provides additional de-NOx capacity temporarily provided by an NO de-NO catalyst bed. The invention is further based on the realization that during such transient periods, NO removal is not important and, in any event, the associated catalyst bed is catalytically less active at low temperatures, but the same bed can be advantageously used temporarily for NOx removal tasks. During transient periods, the present invention can improve de-NOx capacity by approximately 50% compared to normal operation.

[0020] In summary, during transient events, a catalyst bed originally designed for the decomposition of N2O is advantageously used for the reduction of NOx by temporarily adding a reducing agent upstream of said bed.

[0021] During transient events, a reducing agent can be added to the gas upstream of the catalyst bed for removal of NO, or can be added directly to the catalyst bed, which can be done in a suitable mixer upstream of the catalyst bed.

[0022] The reducing agent added between beds and the reducing agent added temporarily during the transient event can be the same or different reducing agents, with ammonia being a highly preferred reducing agent for both.

[0023] Once the transient event has ended, the catalyst bed for NO removal can be returned to its original function, i.e., decomposition of NO, by discontinuing the supply of the excess amount of reducing agent. The end of the transient event can be determined based on one or more operating parameters of the treatment unit, such as the temperature of the gas at a selected location, such as the outlet of the first catalyst bed, or the concentration of NO in the inlet gas. Preferably, the system is returned to normal operation when the temperature of the tail gas at the inlet of the bed for NO removal is above a set minimum temperature, preferably at least 250°C.

[0024] Another advantage is that catalyst downtime in the bed for N2O removal is minimized. In a typical tertiary abatement configuration, the catalyst is active for N2O decomposition at temperatures above 380°C, preferably above 400°C, and more preferably above 430°C. Especially during transient periods such as start-up or shutdown, the temperature of the gas entering the bed for N2O removal remains low. During this period, the catalyst is inactive for N2O decomposition. Applying the method of the present invention, the bed is used to reduce NOx, effectively utilizing the catalyst for NOx reduction at low temperatures.

[0025] A preferred reducing agent is ammonia. Another advantage is that during start-up, the exothermic reaction of reducing nitrogen oxides with ammonia heats the catalyst bed, helping to shorten start-up time. Yet another advantage is that if ammonia is added upstream of the first catalyst bed, the risk of ammonia slip is reduced because the ammonia passes through two catalyst beds in series. It should be noted that ammonia slip during start-up is particularly undesirable because low gas temperatures can lead to the formation and deposition of highly undesirable, explosive ammonium nitrate. This risk is particularly high downstream of the tail gas expander, where gas temperatures during start-up can fall below 100°C. The present invention reduces this risk. [Brief explanation of the drawings]

[0026] [Figure 1] 1 is a schematic diagram of an N2O and NOx abatement system for a nitric acid plant during normal operation, according to the prior art. [Figure 2] 2 illustrates the NOx abatement system of FIG. 1 when operated in accordance with the present invention during a transient period, such as a start-up or shutdown event. [Figure 3] FIG. 1 illustrates another embodiment of an N2O and NOx abatement system during normal operation. [Figure 4] FIG. 4 illustrates the abatement system of FIG. 3 when operated in accordance with the present invention during a transient period. DETAILED DESCRIPTION OF THE INVENTION

[0027] The present invention is applicable to newly designed nitric acid plants, but also to existing nitric acid plants that include a tertiary abatement system that includes a first catalyst bed and a second catalyst bed arranged in series, one of which is utilized for the decomposition of NO and the other for the reduction of NO.

[0028] In a first embodiment, the first catalyst bed is configured to remove NO and is followed by the second catalyst bed configured to remove NO. During normal operation, a reducing agent is added between the first and second catalyst beds to act as a reducing agent for NO in the second catalyst bed, and no reducing agent is added to the tail gas upstream of the first catalyst bed. During the transient event, the method includes temporarily adding a certain amount of reducing agent upstream of the first catalyst bed, such that the first catalyst bed during the transient event operates as an additional catalyst bed for the reduction of the NO in the presence of the reducing agent.

[0029] In a second embodiment, the first catalyst bed is configured to remove NOx and is followed by the second catalyst bed configured to remove NO. During normal operation, a reducing agent is added to the tail gas upstream of the first catalyst bed to act as a reducing agent for NOx in the first catalyst bed. During the transient event, the method includes temporarily adding an additional amount of reducing agent upstream of the first catalyst bed or between the first and second catalyst beds, such that the second catalyst bed during the transient event operates as an additional catalyst bed for the reduction of the NOx in the presence of the reducing agent.

[0030] Thus, the catalyst bed designed and used for the removal of N2O during normal operation may be the first bed or the second bed in the sequence.

[0031] The following description applies to both of the above general embodiments.

[0032] The catalyst bed designed for removing N2O contains a catalyst suitable for decomposing N2O and also suitable for reducing NOx in the presence of a suitable reducing agent.

[0033] The method includes modifying operation of the treatment section during a transient event due to startup or shutdown by adding a reducing agent to a suitable location upstream of the catalyst bed for NO removal, so that the catalyst bed temporarily operates as a catalyst bed for NO reduction. Once the event ends, the addition of the reducing agent is discontinued and normal operation of the treatment section resumes. The temporarily added reducing agent is preferably ammonia.

[0034] The present invention is applicable to transient events in which the nitric acid synthesis process deviates significantly from normal operating conditions, and is particularly applicable to start-up events.

[0035] According to particularly preferred embodiments, ammonia is added during a transient event to achieve a target molar ratio of NH3 / NOx in the tail gas, which, according to various embodiments, may be about 1.0, or may be greater than 1.0, or may be less than 1.0.

[0036] Preferably, during a transient event, the NOx content in the tail gas passing through the first catalyst bed is reduced by at least 80% in the first catalyst bed. The NOx content in the tail gas passing through the second catalyst bed may be further reduced by at least 80%, the reduction rate in the second catalyst bed being based on the NOx contained in the gas at the inlet of the second catalyst bed.

[0037] During the transient event, the catalyst bed designed for N2O removal is primarily utilized for NOx removal, nevertheless, the catalyst bed may still decompose some N2O based on operating conditions.

[0038] In a preferred embodiment, during a transient event, the catalyst bed originally designed for the removal of N2O operates at a temperature below 250°C, preferably comprised between 180°C and 250°C.

[0039] In a further preferred embodiment, during a transient event, the catalyst bed for removing NOx operates at a temperature comprised between 180°C and 250°C, preferably between 180°C and 220°C.

[0040] The first and second catalyst beds may contain the same catalyst or different catalysts. Preferably, the catalyst bed for removing NO uses an iron-supported zeolite catalyst, more preferably an iron ferrierite catalyst (Fe-FER). The catalyst bed for removing NO may use a zeolite catalyst. The zeolite catalyst preferably contains metals and / or metal oxides, including copper, iron, vanadium, molybdenum, tungsten, or mixtures thereof.

[0041] This method is applicable during any kind of transient event, especially during start-up or shutdown. Those skilled in the art can distinguish normal operation of a plant from a start-up or shutdown or another transient event.

[0042] During normal operation, the tail gas temperature reaches an operating temperature of about 300 °C to about 600 °C and then does not change beyond normal fluctuations, but typically at start-up, the final operating temperature has not yet been reached. Normal operation of the plant occurs immediately after plant start-up.

[0043] A transient event may be identified by one or more of the following: the NOx content in the inlet tail gas at the inlet of the first catalyst bed, the detected temperature of the tail gas at the outlet of the catalyst bed for NO removal, the detected or estimated temperature of the catalyst bed for NO removal, the flow rate of the tail gas. During a transient event, all of these parameters will deviate from their normal values, and the start and end of the transient event may be identified based on such deviations.

[0044] The temperature of the catalyst bed may be estimated based on the inlet and outlet temperatures of the tail gas.

[0045] The temperature of the tail gas at the outlet of the catalyst bed for the removal of NO is a particularly preferred parameter for distinguishing between transient events and normal operation. A minimum value can be defined, for example, as 250°C, below which the decomposition of NO in the catalyst bed is significantly reduced or even negligible, making it attractive to use said catalyst bed according to the invention for the reduction of NOx. This temperature can also be easily and practically detected.

[0046] A system adapted to carry out the method of the present invention may identify the end of the start-up event by, for example, one or more of the following: the NOx content in the tail gas at the inlet of the first catalyst bed falling below a target value, the temperature of the tail gas leaving the catalyst bed for N2O removal being above a minimum target value, or the temperature of the catalyst bed for N2O removal being above a minimum target value.

[0047] In some embodiments, the temperature of the catalyst bed refers to the temperature of the catalyst within the bed, which may be the average temperature.

[0048] In a preferred embodiment, a transient event, particularly a start-up event, can be identified by one or more of the following conditions: the tail gas contains more than 600 ppm NOx; the temperature of the tail gas at the outlet of the catalyst bed for NO removal is below 250°C, particularly in the range of 180°C to 230°C; the tail gas flow rate is 80% or less, or 70% or less of the nominal flow rate.

[0049] Thus, in the above example, if the incoming tail gas (at the inlet to the first bed) is detected to contain less than 600 ppm NOx, and / or the tail gas leaving the bed reaches at least 250°C to remove NO, and / or the tail gas flow rate reaches at least 70% of the design flow rate.

[0050] In a preferred embodiment of the present invention, start-up is identified by the NOx content in the tail gas taken from the absorber being 600 to 1500 ppm, more preferably 600 to 800 ppm.

[0051] ppm indicates parts per million by volume.

[0052] According to various embodiments, during a transient event, most or all of the reductant added to the catalyst bed can be added upstream of the first catalyst bed. In some embodiments, if the bed for removing NOx is a second bed, the addition of reductant after the first bed and upstream of the second bed can be reduced or discontinued during the transient event and returned to operational values ​​when the transient event, such as start-up, is complete.

[0053] When ammonia is used as the reducing agent, adding most or all of the ammonia before the first catalyst bed is advantageous to reduce the risk of ammonia slip during unfavorable start-up conditions when low temperatures increase the risk of ammonium nitrate formation. In one embodiment, the amount normally introduced between the catalyst beds is also temporarily introduced before the first catalyst bed.

[0054] The present invention may be applied to the retrofit of existing plants. In such cases, the retrofit method may include providing a mixer configured to mix the tail gas with a reductant during a transient event. The method may further include providing a control system adapted to implement the above-described method of the present invention.

[0055] The treatment section described above is part of a tertiary abatement system of a nitric acid plant. The nitric acid plant may be a single pressure or dual pressure plant. Dual pressure plants are known in the art as plants in which absorption occurs at a higher pressure than ammonia oxidation.

[0056] Another aspect of the present invention is a method for starting up a nitric acid synthesis process, comprising the steps of: The nitric acid synthesis process includes catalytically oxidizing ammonia to obtain a nitrogen oxide-containing gas and absorbing the nitrogen oxide-containing gas in water to produce nitric acid, a tail gas containing NOx and NO; nitric acid treatment includes treating a tail gas to remove N2O and NOx before it is discharged into the atmosphere, said treating the tail gas including passing the tail gas over a first catalyst bed and a second catalyst bed, one of the two beds adapted to remove N2O and the other adapted to remove NOx; and adding a reducing agent to the tail gas before it passes through the catalyst bed for removing NOx; The start-up method includes temporarily adding, at an appropriate location, an additional amount of reducing agent to the tail gas, so that a catalyst bed configured for removing NO during normal operation temporarily provides additional reduction of NO in the presence of reducing agent, said bed including a catalyst suitable for decomposing NO and also suitable for reducing NO in the presence of said reducing agent.

[0057] The abatement system of Figure 1 comprises a first catalyst bed 2 and a second catalyst bed 3 arranged in series, the first catalyst bed 2 containing a catalyst suitable for the decomposition of NO and the second catalyst bed 3 containing a catalyst suitable for the reduction of NO, which is supplied with ammonia 6, which acts as a reducing agent for NO.

[0058] This abatement system basically functions as follows: Tail gas 1, which contains N2O and NOx and is the exhaust gas from the absorption tower (not shown) of a nitric acid plant, is supplied to a first catalyst bed 2, where N2O is decomposed at a temperature range of 300°C to 600°C. Typically, during normal plant operation, a 98% N2O reduction efficiency can be achieved in the first catalyst bed 2. Note that no reducing agent is added to the first catalyst bed.

[0059] The effluent from the first catalyst bed 2 is subsequently mixed with ammonia 6 and then fed to the second catalyst bed 3, where the nitrogen oxides NOx are chemically reduced by ammonia at a temperature range of 400°C to 600°C. During normal operation of the plant, a NOx reduction efficiency of at least 95% can be achieved in the second catalyst bed. The effluent from the second catalyst bed 3 is purified gas 7, which can be discharged to the atmosphere. Typically, the purified gas 7 is expanded in a tail gas expander to recover energy before being discharged.

[0060] 1 is known to function adequately during normal plant operation, however, during transient events, such as start-up, the temperature of the tail gas 1, and therefore the temperatures of the two catalyst beds 2 and 3, are too low to sustain sufficient decomposition of NO and reduction of NO.

[0061] FIG. 2 shows a transient event when, for example, the temperature of the tail gas removed from the absorber is below 300°C or below 250°C and NO production in the nitric acid process is negligible compared to NO production.

[0062] During a transient event, the tail gas 1 containing NOx and N2O is temporarily mixed with ammonia 4 to achieve a suitable molar ratio in the gas, e.g., NH3 / NO greater than 1.0. x The gas to which ammonia 4 has been added is fed to a first catalyst bed 2, which temporarily operates as an additional bed for reducing NOx. The first catalyst bed 2 is labeled "DeNOx" in FIG. 2 to emphasize that the upstream addition of ammonia 4 essentially operates as a bed for reducing nitrogen oxides (NOx).

[0063] The tail gas effluent of the first catalyst bed 2 may again be mixed with the ammonia stream 6 before entering the second catalyst bed 3. In some embodiments, the ammonia flow 6 may be temporarily reduced or stopped during a transient event.

[0064] The effluent of the second catalyst bed is a purified gas that can be released into the atmosphere without the risk of creating a plume stack.

[0065] At the end of the transient event, for example once start-up is complete, the transient ammonia flow 4 is discontinued so that the first catalyst bed 2 returns to normal operation.

[0066] In the embodiment of Figure 3, the locations of catalyst beds 2 and 3 are reversed from Figure 1. That is, under normal conditions, the first catalyst bed 2 reduces NOx in the presence of ammonia, and the second catalyst bed 3 removes NO. In such cases, ammonia is typically added before the first catalyst bed, and during transient events, additional ammonia is added upstream of the first catalyst bed 2 and / or at the same location between the two catalyst beds, as shown in Figure 4.

Claims

1. 1. A method for reducing NOx emissions in a nitric acid plant during a transient event for start-up or shutdown of the nitric acid plant, comprising: The nitric acid plant comprises a synthesis section and an absorption section, wherein ammonia is catalytically oxidized in the synthesis section to obtain a nitrogen oxide-containing gas, and the nitrogen oxide-containing gas is absorbed in water in the absorption section to produce concentrated nitric acid, which is a tail gas (1) containing NOx and NO; the nitric acid plant comprises a treatment section (10) suitable for removing NO and NO from the tail gas (1) before it is discharged into the atmosphere, the treatment section (10) comprising a first catalyst bed (2) and a second catalyst bed (3) arranged so that the tail gas passes through them in succession, and during normal operation of the nitric acid plant, one of the two catalyst beds is used to remove NO and the other of the two catalyst beds is used to remove NO in the presence of a reducing agent (6); During the transient event, the tail gas passes through the catalyst bed to remove NO in the presence of an additional amount of reductant added only during the transient event, such that the catalyst bed temporarily operates as an additional bed for removing NO from the tail gas. method.

2. the first catalyst bed is a bed used to remove NO during normal operation, and the second catalyst bed follows the first catalyst bed, which is a bed used to remove NO; During normal operation, a reducing agent (6) is added between the first catalyst bed (2) and the second catalyst bed (3) to act as a reducing agent for NOx in the second catalyst bed (3), and no reducing agent is added to the tail gas upstream of the first catalyst bed (2); temporarily adding a quantity of reducing agent (4) upstream of the first catalyst bed (2) during the transient event, so that the first catalyst bed (2) during the transient event operates as an additional catalyst bed for the reduction of the NOx in the presence of the reducing agent. The method of claim 1.

3. the first catalyst bed is a bed used to remove NOx during normal operation, followed by the second catalyst bed, which is a bed used to remove NO; During normal operation, a reducing agent (6) is added to the tail gas upstream of the first catalyst bed (2) to act as a reducing agent for NOx in the first catalyst bed; temporary addition of an additional amount of reducing agent (4) upstream of the first catalyst bed or between the first catalyst bed (2) and the second catalyst bed (3) during the transient event, such that the second catalyst bed (3) during the transient event operates as an additional catalyst bed for the reduction of the NOx in the presence of the reducing agent; The method of claim 1.

4. 4. The method of claim 1, further comprising: upon termination of the transient event, interrupting the introduction of the additional amount of reducing agent and resuming normal operation of the treatment unit.

5. A method according to any one of claims 1 to 4, wherein during the transient event the tail gas at the outlet of the absorption section contains at least 600 ppm NOx, such as from 600 ppm to 1500 ppm or from 600 ppm to 800 ppm.

6. 6. The method according to any one of claims 1 to 5, wherein the first catalyst bed (2), used for removing NO during normal operation, is operated at a temperature below 250°C, preferably comprised between 180°C and 250°C, during the transient event.

7. The method of any one of claims 1 to 6, wherein during the transient event, the tail gas flow rate is no more than 80% of the normal operating flow rate.

8. 8. The method according to any one of claims 1 to 7, wherein the second catalyst bed (3), used for removing NOx during normal operation, is operated during the transient event at a temperature comprised between 180°C and 250°C, preferably between 180°C and 220°C.

9. 9. The method of claim 1, wherein the end of the transient event is identified by one or more of the following: the NOx content in the tail gas at the inlet to the first catalyst bed falling below a target value; the temperature of the tail gas leaving a catalyst bed normally used for NO removal being at or above a minimum target value; or the temperature of a catalyst bed normally used for NO removal being at or above a minimum target value.

10. 10. The method of any one of claims 1 to 9, wherein the end of a start-up event is identified by one or more of the following: the inlet tail gas at the inlet of the first catalyst bed containing less than 600 ppm NOx; the tail gas exiting a catalyst bed normally used for NO removal reaching a temperature of at least 250°C; and the tail gas flow rate reaching at least 70% of design flow rate.

11. 11. The method according to any one of claims 1 to 10, wherein during normal operation the first catalyst bed is a bed for removing NO and the second catalyst bed is a bed for removing NO, and during the transient event the amount of reducing agent (6) added between the first catalyst bed (2) and the second catalyst bed (3) is reduced or zero compared to the amount added during normal operation.

12. 12. The method of any one of claims 1 to 11, wherein the reducing agent is ammonia and the reducing agent temporarily added during the transient event is also ammonia.

13. 13. The process according to any one of claims 1 to 12, wherein the first catalyst bed (2) normally used for the removal of NO contains an iron-supported zeolite catalyst, preferably an iron ferrierite catalyst Fe-FER.

14. 14. The method according to any one of claims 1 to 13, wherein the second catalyst bed (3) normally used for the reduction of NOx contains a zeolite catalyst containing metals and / or metal oxides, preferably comprising copper, iron, vanadium, molybdenum, tungsten or mixtures thereof.

15. During normal operation, the first catalyst bed is a bed for removing NO and the second catalyst bed is a bed for reducing NO, and during the transient event, ammonia (4) is added upstream of the first catalyst bed (2), the amount of ammonia added being determined by the target molar ratio NH ３ The method according to any one of claims 1 to 14, wherein the method is determined to obtain / NOx.

16. The method according to any one of the preceding claims, wherein the treatment section (10) is part of a tertiary abatement system of a nitric acid plant, the nitric acid plant being a single pressure or dual pressure plant.

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