Reducing the emission of nitrogen oxide when starting up systems for producing nitric acid

Abstract

A method of reducing NO.sub.x in tail gas obtained during startup of a plant for preparing nitric acid may involve heating the tail gas from a starting temperature T.sub.0, through a threshold temperature T.sub.G, to an operating temperature T.sub.B at which steady-state operation of the plant can occur (T.sub.0<T.sub.G<T.sub.B). NO.sub.x-containing tail gas may be passed through a storage medium and at least partially stored while the temperature of the tail gas is lower than the threshold temperature T.sub.G. The NO.sub.x may be released, preferably when the temperature of the tail gas has attained the threshold temperature T.sub.G. The NO.sub.x may be combined with a reducing agent in the presence of an SCR catalyst after the temperature of the tail gas has exceeded the threshold temperature T.sub.G, but not before, resulting in catalytic reduction of at least a portion of the NO.sub.x.

Claims

1. A method, comprising: generating nitrogen oxides (NO.sub.x) in a nitric acid production plant; directing the NOx into an absorption tower of the nitric acid production plant and removing tail gas comprising unabsorbed NO.sub.x from the absorption tower; and reducing the concentration of NO.sub.x, in the tail gas obtained during startup of the nitric acid production plant, wherein the tail gas is heated from a starting temperature, through a threshold temperature, and to an operating temperature at which steady-state operation of the nitric acid production plant for preparation of nitric acid can be effected, passing the tail gas through a storage medium for NO.sub.x and storing at least a portion of the NO.sub.x in the storage medium while the temperature of the tail gas is below the threshold temperature, wherein a selective catalytic reduction (SCR) catalyst serves as the storage medium; combining, when the temperature of the tail gas exceeds the threshold temperature, the NO.sub.x in the storage medium with a reducing agent for NO.sub.x in the presence of the SCR catalyst, which results in catalytic reduction of at least a portion of the NO.sub.x in the storage medium; measuring, when the temperature is below the operating temperature, a concentration of the NO.sub.x in the tail gas before the tail gas contacts the SCR catalyst; and metering, based on said measuring of the concentration of the NO.sub.x, an amount of the reducing agent sufficient to degrade the NO.sub.x already adsorbed on the SCR catalyst and the measured concentration of the NO.sub.x.

2. The method of claim 1 wherein at least one of the starting temperature is less than 120° C., the threshold temperature is greater than or equal to 120° C. and less than 300° C., or the operating temperature is greater than or equal to 300° C.

3. The method of claim 1 wherein the SCR catalyst comprises a catalytically active material selected from a group consisting of iron, compounds of iron, cobalt, compounds of cobalt, copper, and compounds of copper.

4. The method of claim 1 wherein the SCR catalyst comprises aluminum silicate.

5. The method of claim 4 wherein the aluminum silicate comprises a zeolite.

6. The method of claim 1 further comprising adjusting a level of oxidation of the NO.sub.x in the tail gas.

7. The method of claim 1 wherein the reducing agent for NOx comprises ammonia.

8. The method of claim 1 further comprising reducing a concentration of the NO.sub.x and N.sub.2O in tail gas obtained after the startup of the nitric acid production plant during the steady-state operation of the nitric acid production plant by way of a gas cleaning system of the nitric acid production plant for preparation of nitric acid.

9. The method of claim 8 wherein the SCR catalyst is disposed in the gas cleaning system.

10. The method of claim 1 comprising the steady-state operation of the nitric acid production plant, the steady-state operation comprising: breaking down N.sub.2O catalytically to O.sub.2 and N.sub.2 and reducing NO.sub.x catalytically in the presence of the reducing agent for NO.sub.x; or reducing N.sub.2O catalytically in the presence of a reducing agent for N.sub.2O and reducing NO.sub.x catalytically in the presence of the reducing agent for NO.sub.x; or breaking down N.sub.2O catalytically to O.sub.2 and N.sub.2, further reducing residual N.sub.2O catalytically in the presence of the reducing agent for N.sub.2O, and reducing NO.sub.x catalytically in the presence of the reducing agent for NO.sub.x.

11. The method of claim 10 wherein the nitric acid production plant includes a gas cleaning system for reducing the concentration of the NO.sub.x and N.sub.2O in the tail gas obtained during the steady-state operation, wherein the gas cleaning system includes a two-stage construction, wherein the N.sub.2O is broken down catalytically to O.sub.2 and N.sub.2 in a first stage in a flow direction of the tail gas during the steady-state operation of the nitric acid production plant, after the first stage the reducing agent for NOx is mixed with the tail gas and then, in a second stage, the NOx is reduced catalytically in the presence of the reducing agent; or during the steady-state operation of the nitric acid production plant the tail gas is mixed in the gas cleaning system with the reducing agent for the NOx and a hydrocarbon, carbon monoxide, hydrogen, or a mixture thereof as the reducing agent for the N.sub.2O, wherein the reducing agent is added in such an amount that is sufficient at least for complete reduction of the NOx, and that a gas mixture is passed through at least one reaction zone comprising the SCR catalyst for reduction of the NOx and for reduction of the N.sub.2O; or the gas cleaning system includes a two-stage construction, wherein during the steady-state operation of the nitric acid production plant in a flow direction of an offgas, in a first stage, the N.sub.2O is broken down catalytically to O.sub.2 and N.sub.2, and the offgas, after the first stage, is mixed with a reducing agent for the NOx and a hydrocarbon, carbon monoxide, hydrogen, or a mixture thereof as a reducing agent for the N.sub.2O, wherein the reducing agent is added in such an amount that is sufficient at least for complete reduction of the NOx, and that a gas mixture is passed through at least one reaction zone comprising a catalyst for reduction of the NOx and for reduction of the N.sub.2O.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a graph depicting a profile of NOx concentration against time during startup of a conventional HNO.sub.3 plant in a tail gas of a conventional nitric acid plant prior to entry thereof into any gas cleaning system.

(2) FIG. 2 is a graph depicting a profile of NOx offgas concentration beyond an installed conventional one-stage gas cleaning system based on a V.sub.2O.sub.5/TiO.sub.2 catalyst.

(3) FIG. 3 is a graph depicting a profile of NOx concentration against time for a nitric acid plant equipped with an EnviNOx® system as a two-stage gas cleaning system on startup.

(4) FIG. 4 is a graph depicting another profile of NOx concentration against time for a nitric acid plant equipped with an EnviNOx® system as a two-stage gas cleaning system on startup.

DETAILED DESCRIPTION

(5) Although certain example methods and apparatus have been described herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all methods, apparatus, and articles of manufacture fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents. Moreover, those having ordinary skill in the art will understand that reciting ‘a’ element or ‘an’ element in the appended claims does not restrict those claims to articles, apparatuses, systems, methods, or the like having only one of that element, even where other elements in the same claim or different claims are preceded by “at least one” or similar language. Similarly, it should be understood that the steps of any method claims need not necessarily be performed in the order in which they are recited, unless so required by the context of the claims. In addition, all references to one skilled in the art shall be understood to refer to one having ordinary skill in the art.

(6) One example object of the present disclosure concerns reducing emissions of nitrogen oxides obtained in the course of startup of plants for preparation of nitric acid.

(7) A first aspect of the invention relates to a method of reducing the concentration of NO.sub.x nitrogen oxides (NO, NO.sub.2) in tail gas obtained during the startup of a plant for preparation of nitric acid;

(8) wherein the tail gas contains NO.sub.x and wherein the tail gas is obtained during the startup of the plant and is heated preferably as a result of the measures for preparation of nitric acid from a starting temperature T.sub.0, passing through a threshold temperature T.sub.G and finally up to an operating temperature T.sub.B at which steady-state operation of the plant is subsequently effected (T.sub.0<T.sub.G<T.sub.B);

(9) wherein the method comprises the following steps: (a) passing the NO.sub.x-containing tail gas through a storage medium for NO.sub.x and storing at least a portion of the NO.sub.x in the storage medium for NO.sub.x while the temperature of the tail gas is lower than the threshold temperature T.sub.G; (b) optionally releasing the NO.sub.x stored in step (a), preferably when the temperature of the tail gas has attained the threshold temperature T.sub.G; (c) combining the NO.sub.x with a reducing agent for NO.sub.x in the presence of an SCR catalyst after the temperature of the tail gas has exceeded the threshold temperature T.sub.G, but not before, which results in catalytic reduction of at least a portion of the NO.sub.x, preferably to N.sub.2 and H.sub.2O.

(10) FIGS. 1 to 4 illustrate the invention in schematic form and by way of example on the basis of experimental data which are obtained in nitric acid plants of different configuration with different method regimes. FIGS. 1 and 2 illustrate conventional methods; FIGS. 3 and 4 illustrate the method of the invention.

(11) FIG. 1 shows a typical profile of the NO.sub.x concentration against time in the startup of the HNO.sub.3 plant in the tail gas of a nitric acid plant prior to entry thereof into any gas cleaning system, which thus shows the potential emission profile of the HNO.sub.3 plant without any gas cleaning system in the startup of the plant (solid line). Likewise shown is the temperature as a function of operating time (dotted line). The tail gas temperature at time zero corresponds to the temperature that has been established after a plant shutdown (T.sub.0). This depends on the ambient temperature and the shutdown time of the HNO.sub.3 plant and, in the comparative example according to FIG. 1, is about 125° C. In phase 1, the machine set of the air compressor is started and the plant is filled with air (pressurized). There is emission here of NO.sub.x which was still present in the plant, i.e. in corresponding pipelines and gas spaces of the apparatuses, according to peak 1. In parallel, there is a constant rise in temperature because of the compression work and the energy introduced from the steam system to an intermediate level depending on the particular plant. The temperature T.sub.G is exceeded here, which in the comparative example according to FIG. 1 is about 185° C. Over and above this temperature, it would be possible to put a downstream gas cleaning system in operation, in principle without formation and accumulation of NH.sub.4NO.sub.3. In phase 2, the absorption tower of the plant is filled with unbleached acid (containing dissolved NO.sub.x). Because of the lower pressure compared to standard operation, NO.sub.x is stripped (scrubbed) out of the acid and emitted to the environment. In phase 3, the actual production of acid is then commenced. For this purpose, NH.sub.3 is switched on and oxidized with the atmospheric oxygen over platinum mesh catalysts to NO. Because of the heat of reaction released, which is introduced via heat exchangers into the tail gas after it leaves the absorption tower, there is a constant rise in temperature here up to the ultimate operating temperature T.sub.B, which depends on the design of the plant. In the comparative example according to FIG. 1, T.sub.B is about 415° C. There is at first a drastic rise in the NO.sub.x emissions, since effectiveness of the absorption tower has not yet been attained. With increasing operating time, the effectiveness of the absorption tower increases and, as a result, there is a reduction in the NO.sub.x content in the offgas.

(12) FIG. 2 shows, in schematic form, a typical profile of the NO.sub.x offgas concentration beyond an installed conventional one-stage gas cleaning system based on a V.sub.2O.sub.5/TiO.sub.2 catalyst. The first peak (phase 1) has been shifted slightly to later times compared to FIG. 1, but there is no storage of NO.sub.x in the sense of the invention. From the threshold temperature T.sub.G, the gas cleaning system is then charged with reducing agent (NH.sub.3) and adjusted to an NO.sub.x concentration at the exit of the gas cleaning system of about 30 ppm by the variation of the amount of NH.sub.3 at the inlet. In this way, the emission from phase 2 can already be virtually completely eliminated. In phase 3, however, because of the abrupt rise in the NO.sub.x inlet concentration, there may be a time-limited rise in NO.sub.x emission. In the comparative example according to FIG. 2, T.sub.0 is about 135° C., T.sub.G is about 190° C. and T.sub.B is about 415° C.

(13) FIG. 3 shows a nitric acid plant equipped with an EnviNOx® system as two-stage gas cleaning system on startup. By contrast with FIGS. 1 and 2, the emission of NO.sub.x in phase 1 by the selected SCR catalyst (iron-laden zeolite) is reduced by physical adsorption (storage). With the crossing of the temperature threshold T.sub.G, in accordance with the invention, reducing agent (NH.sub.3) is applied to the system. As a result, the total emission of NO.sub.x is distinctly reduced compared to FIGS. 1 and 2. In this inventive example, the reducing agent for the reduction of the NO.sub.x, however, is applied to the reactor in a 1:1 NH.sub.3:NO.sub.x stoichiometry with respect to the continuous new additional NO.sub.x. Since, however, a greater amount of NO.sub.x has already collected at the catalyst beforehand, which is now desorbed in phase 3, the amount of reducing agent supplied is not entirely sufficient to reduce the total amount of NO.sub.x. Consequently, the result is another smaller NO.sub.x peak, i.e. an NO.sub.x emission because of thermally induced desorption in phase 3. In the inventive example according to FIG. 3, T.sub.0 is about 125° C., T.sub.G is about 185° C. and T.sub.B is about 415° C.

(14) FIG. 4 likewise shows a nitric acid plant equipped with an EnviNOx® system as two-stage gas cleaning system on startup, conducting the method of the invention in a preferred embodiment. The emission of NO.sub.x in phase 1 is again reduced because of physical adsorption (storage) at the SCR catalyst (iron-laden zeolite). Because of the improved NH.sub.3 dosage taking account of the total amount of NO.sub.x (already adsorbed at the catalyst+continuous new additional), a sufficient amount of reducing agent is now being metered in that, in parallel to the new additional NO.sub.x, a majority of the NO.sub.x adsorbed on the catalyst is additionally also degraded before the temperature is increased in phase 3. In this way, it is even possible to virtually completely prevent the desorption of NO.sub.x shown in FIG. 3.

(15) In the method of the invention, the tail gas contains NO.sub.x and is obtained during the startup of the plant for preparation of nitric acid. It is not absolutely necessary here that the NO.sub.x-containing tail gas is obtained continuously during the startup of the plant. For instance, it is likewise possible in accordance with the invention that, during the startup of the plant, the incidence of the NO.sub.x-containing residual gas is interrupted, possibly on more than one occasion, i.e., for example, occurs in intervals. In the inventive example according to FIG. 4, T.sub.0 is about 125° C., T.sub.G is about 185° C. and T.sub.B is about 415° C.

(16) A person skilled in the art will be able to distinguish the state of a plant for preparation of nitric acid during the startup thereof from the state of the plant during the steady-state operation thereof. Preferably, the tail gas during the startup of the plant has not yet attained its ultimate operating temperature T.sub.B. Correspondingly, the tail gas during the subsequent steady-state operation of the plant has attained its operating temperature T.sub.B, which then does not change any further thereafter beyond typical fluctuations. The startup of the plant precedes the steady-state operation of the plant, which directly follows the startup.

(17) At the commencement of the startup of the plant, the tail gas obtained has the starting temperature T.sub.0. If the plant has completely cooled down beforehand after the last phase of operation, the starting temperature T.sub.0 corresponds to the ambient temperature. However, it is also possible in accordance with the invention that the starting temperature T.sub.0 is above the ambient temperature, for instance when the duration of the temporary shutdown of the plant after the last phase of operation has not been sufficient for complete cooling of all devices and apparatuses. Preferably, the starting temperature T.sub.0 is in the range from ambient temperature to 170° C., more preferably from ambient temperature to 150° C., even more preferably from ambient temperature to 120° C. and especially preferably from ambient temperature to 100° C.

(18) In the course of the method of the invention, the NO.sub.x-containing tail gas is heated from the starting temperature T.sub.0, passing through a threshold temperature T.sub.G and finally up to the operating temperature T.sub.B. Accordingly, T.sub.0<T.sub.G<T.sub.B. The heating of the NO.sub.x-containing tail gas is preferably effected essentially constantly, but may also include minor, temporary phases of relative cooling. Preferably, the heating is effected exclusively by means of measures which are taken to recover the reaction energy obtained in ammonia oxidation in the standard process of nitric acid preparation, meaning that there is preferably no additional, active heating of the tail gas by suitable measures which as such would otherwise not be integrated into the process for preparing nitric acid.

(19) In step (a) of the method of the invention, the NO.sub.x-containing tail gas is passed through a storage medium for NO.sub.x and at least a portion of the NO.sub.x is stored in a storage medium for NO.sub.x while the temperature of the tail gas is less than the threshold temperature T.sub.G. It is not necessary here for NO.sub.x to be fed into the storage medium and/or stored therein continuously over the entire period during which the temperature of the tail gas is less than the threshold temperature T.sub.G. For instance, it is sufficient when the storage is effected during one or more intervals while the temperature of the tail gas is less than the threshold temperature T.sub.G, and when the storage is effected for a period over which the temperature of the tail gas is less than the threshold temperature T.sub.G.

(20) The duration of storage of the NO.sub.x need not be over the entire period over which the temperature of the tail gas rises from the starting temperature T.sub.0 until it is only slightly below the threshold temperature T.sub.G. Thus, it is sufficient in principle in accordance with the invention if there is storage of at least a portion of the NO.sub.x present in the tail gas in the storage medium over a particular period of time for which the temperature of the tail gas is less than the threshold temperature T.sub.G. Preferably, at least a portion of the NO.sub.x is stored in the storage medium at least until the temperature of the tail gas is only slightly below the threshold temperature T.sub.G, for example 5° C. below T.sub.G.

(21) It will be apparent to the person skilled in the art that a storage process can proceed dynamically, meaning that NO.sub.x molecules that have been stored at an early stage in the storage medium at which the temperature of the tail gas is less than the threshold temperature T.sub.G can be released again at a later stage at which the temperature of the tail gas is likewise smaller than the threshold temperature T.sub.G and possibly replaced by new NO.sub.x molecules.

(22) In addition, it is possible in principle that at least a portion of the NO.sub.x is additionally also stored when the temperature of the tail gas has already attained or exceeded the threshold temperature T.sub.G.

(23) Useful storage media for NO.sub.x include various apparatuses and materials.

(24) Preferably, the NO.sub.x is stored by physical adsorption (physisorption) at the surface of solids and/or by chemical adsorption or absorption (chemisorption), i.e. by chemical reaction of the NO.sub.x with the storage material. Preferably, the storage medium for NO.sub.x comprises solids composed of inorganic materials.

(25) Suitable materials for physical adsorption are especially adsorbents having high internal and/or external specific surface area that are known to those skilled in the art, such as various kinds of activated carbons, ashes, porous glasses, aluminas or silicatic materials, such as silica gel, clay minerals, e.g. montmorillonite, hydrotalcites or bentonites, or especially also natural or synthetic zeolites.

(26) Suitable materials for chemical adsorption or absorption are those which enter into a superficial chemical reaction, or one which penetrates the entire solid materials, with NO.sub.x and/or preferably with NO.sub.2. Such materials are known to those skilled in the art, for example, from the sector of gas treatment of automotive diesel exhaust gases. In this context, the term LNT (Lean NO.sub.x Trap) or NAC (NO.sub.x Absorber Catalyst) is used to refer to a wide variety of different materials for storage and/or reduction of ad- or absorbed NO.sub.x species. Examples include alkali metal and alkaline earth metal oxides, for example Na.sub.2O, K.sub.2O or MgO, CaO, SrO, BaO, CaO, which form corresponding nitrates with NO.sub.2 at various temperatures according to the metal, from which the NO.sub.x can then be released again by further thermal stress or else by addition of specific reducing agents in the optional step (b) of the method of the invention. Particular preference is given in the context of the invention to storage materials containing BaO, which can absorb NO.sub.2 and reversibly release it again according to the following reaction scheme: BaO+NO.sub.2BaNO.sub.3.

(27) The storage materials may comprise further components, for example noble metal dopants, for example platinum, which catalyze the oxidation of NO.sub.x to NO.sub.2, so that they can then react to exhaustion to give the corresponding nitrates. Also possible is doping with SCR-active transition metals or transition metal oxides, for example Rh or MnO.sub.2, as likewise known from the sector of gas treatment of automotive diesel exhaust gases. In this case, the chemisorbed NO.sub.x can also be released from the storage material in accordance with the invention by means of specific reducing agents such as ammonia or preferably hydrocarbons (HC) or HC mixtures. The nitrate species are preferably reduced here to nitrogen and water and optionally CO.sub.2, as shown in the following reaction scheme:
BaNO.sub.3+HC.fwdarw.BaO+N.sub.2+H.sub.2O+CO.sub.2.

(28) As well as a selection of material as such, the storage capacity of both the chemical and the physical storage materials can be varied and adjusted via the selection of the specific and geometric surface area of the solid materials.

(29) Especially in the case of physical adsorption, the NO.sub.x is typically better stored at a lower temperature of the tail gas below the threshold temperature T.sub.G than at a higher temperature of the tail gas below the threshold temperature T.sub.G, since there is an increase in desorption processes with rising temperature. In the case of chemical adsorption or absorption, a certain minimum temperature is generally required to overcome the corresponding chemical activation energies for the chemical reaction of the NO.sub.x with the storage material to proceed.

(30) It is considered to be a particular advantage of the invention that the storage medium for NO.sub.x can be disposed in spatial terms in the tail gas line of a nitric acid plant at a position where the temperature level resulting from the production process is particularly favorable for the reversible storage of the NO.sub.x. This is not necessarily the same site or the same temperature level at which the gas cleaning system is also operated. Preferably, the storage medium for NO.sub.x is arranged at a site such that its temperature, at any time during the startup, is below the particular temperature of the SCR catalyst at the same time. Preferably, the relative temperature differential is at least 5° C., more preferably at least 10° C.

(31) In a particularly preferred embodiment, the SCR catalyst which catalyzes the reduction of NO.sub.x with the reducing agent for NO.sub.x in step (c) of the method of the invention in turn serves as storage medium in step (a) of the method of the invention. This is preferably achieved in accordance with the invention by virtue of the SCR catalyst comprising or consisting of a material which is preferentially suitable for adsorption of NO.sub.x. Suitable SCR catalysts, especially suitable catalytic materials for reduction of NO.sub.x with a reducing agent for NO.sub.x, especially with NH.sub.3, are known to a person skilled in the art. These are preferably zeolites doped with transition metals, including the lanthanides, preferably zeolites doped with cobalt, especially with copper and most preferably with iron. Further possible transition metals which preferably occur in the zeolite together with cobalt, copper and/or iron are manganese, vanadium, chromium or nickel. The zeolites are preferably “high-silica” zeolites having high hydrothermal stability. Preferably, the zeolites are selected from the group of the MFI, BEA, FER, MOR and MEL types or mixtures thereof, and are preferably of the BEA or MFI type, more preferably a ZSM-5 zeolite.

(32) In this embodiment of the invention, it is optionally possible to dispense with the release or transfer of the NO.sub.x in step (b) of the method of the invention, since the NO.sub.x is already adsorbed in the direct spatial proximity of or directly on the catalytically active material, such that the catalytic reduction can be initiated by supply of the reducing agent for NO.sub.x in step (c) without requiring separate release and transfer of the stored NO.sub.x beforehand.

(33) Useful storage media for NO.sub.x, as an alternative to solid materials, are in principle also vessels in which the NO.sub.x-containing tail gas is accommodated temporarily, optionally under pressure.

(34) Preferably, the temperature of the stored NO.sub.x varies during its storage. Preferably, the temperature of the stored NO.sub.x varies in accordance with the change in the temperature of the tail gas obtained in the startup of the plant.

(35) Preferably, the storage medium for NO.sub.x is present in such an amount and with such a storage capacity that at least 20%, more preferably at least 30%, even more preferably at least 50%, most preferably at least 60%, and especially at least 70% of the total amount of the NO.sub.x obtained overall during the pressurization of the plant can be simultaneously stored temporarily, preferably by physical adsorption or by chemical adsorption or absorption. The total amount of the NO.sub.x obtained overall during the pressurization of the plant can be determined by a person skilled in the art by simple routine tests or else simple calculations.

(36) The terms “pressurization”, “filling” and “ignition” are known to a person skilled in the art in connection with plants for preparation of nitric acid and preferably have the meaning elucidated in the introductory part.

(37) Preferably, in step (a) of the method of the invention, the method goes through a state during which the temperature of the tail gas is less than the threshold temperature T.sub.G and at least a portion of the total molar amount, preferably at least 20%, more preferably at least 30%, even more preferably at least 50% and especially at least 70% of the total molar amount of the NO.sub.x obtained in total during the pressurization in the plant is stored. Preferably, the method goes through this state shortly before the temperature of the tail gas reaches the threshold temperature T.sub.G, for example 1 minute before.

(38) In the optional step (b) of the method of the invention the NO.sub.x stored in step (a) is released. In addition, step (b) of the method of the invention optionally includes the transfer of the NO.sub.x released to an SCR catalyst, at the surface of which the NO.sub.x can be adsorbed and then catalytically reduced in step (c) with supply of reducing agent for NO.sub.x.

(39) The stored NO.sub.x can be released by means of an active measure or else passively.

(40) In a preferred embodiment, the stored NO.sub.x is released passively, especially by means of the further heating of the tail gas to the threshold temperature T.sub.G and beyond. This embodiment is preferred especially when the storage of the NO.sub.x in the storage medium is based on physical adsorption or chemical adsorption or absorption. In this case, the NO.sub.x is preferably released by desorption, which is promoted with rising temperature of the NO.sub.x adsorbed or absorbed onto the storage medium. Preferably, in the method of the invention, the storage medium with the NO.sub.x adsorbed or absorbed thereon is exposed continuously to the NO.sub.x-containing tail gas, such that, with heating of the NO.sub.x-containing tail gas, it is heated thereby.

(41) Preferably, the NO.sub.x stored in the storage medium is released when the temperature of the tail gas has attained the threshold temperature T.sub.G. In the case of physical adsorption, it will be apparent to a person skilled in the art that, on the basis of the adsorption isotherm, the release in practice is not abrupt; instead, as a result of the preferably constant heating of the NO.sub.x-containing tail gas and hence also as a result of the constant heating of the NO.sub.x stored in the storage medium, significant desorption processes are initiated from a particular temperature, which lead to release of the NO.sub.x.

(42) In another preferred embodiment, the stored NO.sub.x is released actively, especially when the storage medium used is a vessel in which the NO.sub.x-containing tail gas is stored temporarily, optionally under pressure. In this case, the release can be effected, for example, by active opening of suitable valves and the NO.sub.x can be released from the storage medium.

(43) Active additional temporary heating of the storage medium or of the tail gas stream entering the storage medium, which is typically not envisaged in the process scheme of a nitric acid plant, is also usable in the context of the invention. This heating is preferably operated in addition to the promotion of the customary heating of the tail gas, in order to assure faster attainment of T.sub.G. The heating can be brought about, for example, by means of appropriate heat exchangers, burners or else electrical heating registers.

(44) If the NO.sub.x is already stored in the direct spatial proximity or directly in the catalytically active material, especially adsorbed thereon, it is optionally possible to dispense with the release of the NO.sub.x in step (b) of the method of the invention, since the catalytic reduction can be initiated by supply of the reducing agent for NO.sub.x in step (c) of the method of the invention without requiring separate release and transfer of the stored NO.sub.x beforehand.

(45) In step (c) of the method of the invention, the NO.sub.x is combined with a reducing agent for NO.sub.x in the presence of an SCR catalyst is once the temperature of the tail gas has exceeded the threshold temperature T.sub.G, but not before, which results in catalytic reduction of at least a portion of the NO.sub.x, preferably to N.sub.2 and H.sub.2O. For this purpose, the SCR catalyst is supplied with a reducing agent for NO.sub.x after, preferably as soon as the temperature of the tail gas has exceeded the threshold temperature T.sub.G, but not before, which results in catalytic reduction of at least a portion of the NO.sub.x, preferably to N.sub.2 and H.sub.2O.

(46) Suitable reducing agents for NO.sub.x are known to a person skilled in the art. These may be any nitrogen-containing reducing agent having a high activity for reduction of NO.sub.x. Examples are azanes, hydroxyl derivatives of azanes, and amines, oximes, carbamates, urea or urea derivatives. Examples of azanes are hydrazine and very particularly ammonia. One example of a hydroxyl derivative of azanes is hydroxylamine.

(47) Examples of amines are primary aliphatic amines, such as methylamine. One example of carbamates is ammonium carbamate. Examples of urea derivatives are N,N′-substituted ureas, such as N,N′-dimethylurea. Ureas and urea derivatives are preferably used in the form of aqueous solutions. Particular preference is given to using ammonia as reducing agent for NO.sub.x.

(48) Suitable SCR catalysts, especially suitable catalytic materials for reduction of NO.sub.x with a reducing agent for NO.sub.x, especially with NH.sub.3, are known to a person skilled in the art. These are preferably zeolites doped with transition metals, including the lanthanides, preferably zeolites doped with cobalt, especially with copper and most preferably with iron. Further possible transition metals which preferably occur in a zeolite together with cobalt, copper and/or iron are manganese, vanadium, chromium or nickel.

(49) The zeolites are preferably high-silica zeolites having high hydrothermal stability. The zeolites are preferably selected from the group of the MFI, BEA, FER, MOR and MEL types or mixtures thereof, and are preferably of the BEA or MFI type, more preferably a ZSM-5 zeolite.

(50) Exact details of the makeup or structure of the zeolite types used in accordance with the present disclosure are given in the Atlas of Zeolite Structure Types, Elsevier, 4th revised edition 1996, which is hereby incorporated by reference in its entirety.

(51) In addition, preference is given to using “steamed” zeolites, i.e. zeolites where, following a hydrothermal treatment, some of the aluminum lattice atoms have moved to interstitial lattice sites. The person skilled in the art is aware of such zeolites and the mode of preparation thereof.

(52) The content of transition metals in the zeolites may, based on the mass of zeolite, vary within wide ranges, preferably up to 25%, but preferably 0.1% to 10%, and especially 2% to 7%.

(53) The zeolites can be doped with the transition metals, for example, proceeding from the H or preferably NH.sub.4 form of the zeolites by ion exchange (in aqueous phase or by solid-state reaction) with appropriate salts of the transition metals. The SCR catalyst powders obtained are typically calcined in a chamber furnace under air at temperatures in the range from 400 to 650° C. After the calcination, the transition metal-containing zeolites are washed vigorously in distilled water, and the zeolite is filtered off and then dried. These and other relevant methods of loading or doping of zeolites with transition metals are known to the person skilled in the art. Finally, the transition metal-containing zeolites thus obtained can be admixed and mixed with suitable auxiliaries for plasticization and binders, for example aluminosilicates or boehmite, and, for example, extruded to give cylindrical SCR catalyst bodies.

(54) The SCR catalyst may take the form of shaped bodies of any size and geometry, preferably geometries which have a high ratio of surface to volume and generate a minimum pressure drop on flow-through. Typical geometries are all those known in catalysis, for example cylinders, hollow cylinders, multihole cylinders, rings, crushed pellets, trilobes or honeycomb structures. The size of the SCR catalyst particles or shaped catalyst bodies used may vary within wide ranges. Typically, these have an equivalent diameter in the range from 1 to 10 mm. Preference is given to equivalent diameters of 2 to 5 mm. The equivalent diameter is the diameter of a sphere of equal volume.

(55) According to the invention, NO.sub.x and reducing agent for NO.sub.x are only combined in the presence of the SCR catalyst once the temperature of the NO.sub.x-containing tail gas has attained the threshold temperature T.sub.G, but not before. If the combination were already to take place beforehand, the unwanted formation and accumulation of NH.sub.4NO.sub.3 could not be effectively prevented, since the temperature of the tail gas is not yet high enough. According to the invention, step (c) of the method of the invention can be commenced as soon as the temperature of the tail gas has attained the threshold temperature. However, it is also possible that step (c) of the method of the invention is commenced only a while after the temperature of the tail gas has exceeded the threshold temperature, for example only once the temperature of the tail gas is 5° C. or 10° C. above the threshold temperature T.sub.G.

(56) According to the invention, step (c) of the method of the invention, however, is preferably commenced before the temperature of the tail gas has attained the operating temperature T.sub.B. Preferably, the performance of step (c) of the method of the invention has already commenced before the NH.sub.3 burner of the plant for preparation of nitric acid is started (ignited).

(57) According to the invention, the threshold temperature T.sub.G is preferably therefore that temperature of the NO.sub.x-containing tail gas at which, under the given conditions in the particular plant for preparation of nitric acid, there is just no formation and accumulation of NH.sub.4NO.sub.3 when the NO.sub.x is combined with the reducing agent for NO.sub.x in the presence of the SCR catalyst. This threshold temperature T.sub.G is known to the person skilled in the art from the literature (for example Iwaki et al, Appl. Catal. A, 390 (2010) 71-77 or Koebel et al, Ind. Eng. Chem. Res. 40 (2001) 52-59) or can be determined by corresponding simple routine tests. Preferably, the threshold temperature T.sub.G is in the range from 170° C. to 200° C. and is therefore preferably 170° C., 171° C., 172° C., 173° C., 174° C., 175° C., 176° C., 177° C., 178° C., 179° C., 180° C., 181° C., 182° C., 183° C., 184° C., 185° C., 186° C., 187° C., 188° C., 189° C., 190° C., 191° C., 192° C., 193° C., 194° C., 195° C., 196° C., 197° C., 198° C., 199° C. or 200° C. According to the SCR catalyst used, the threshold temperature T.sub.G may, however, also be lower or higher.

(58) Step (c) of the method of the invention is preferably additionally also conducted at much higher temperatures above the threshold temperature T.sub.G, preferably over the entire temperature range from the threshold temperature T.sub.G up to the operating temperature T.sub.B of the tail gas, but not at temperatures of the tail gas below the threshold temperature T.sub.G. Since the operating temperature T.sub.B is typically at temperatures of the tail gas of at least 300° C., more preferably at least 350° C. and especially preferably at least 400° C., step (c) of the method of the invention is preferably also effected at temperatures of the tail gas distinctly exceeding the threshold temperature T.sub.G.

(59) The supply of a reducing agent for NO.sub.x to the SCR catalyst in step (c) can be effected by customary measures known to a person skilled in the art. In a preferred embodiment, the NO.sub.x is already in the presence of the SCR catalyst beforehand and is more preferably already adsorbed thereon. This embodiment is preferred especially when the SCR catalyst in step (c) of the method of the invention has already acted beforehand in step (a) of the method of the invention as storage medium for NO.sub.x. It has been found that, surprisingly, the stored (adsorbed) NO.sub.x or NO.sub.2 is rapidly removed again from the SCR catalyst by the supply of a particular amount of reducing agent. This is also true when this SCR storage catalyst is supplied with further NO.sub.x in parallel.

(60) Preferably, the metered addition of the reducing agent for NO.sub.x, preferably the ammonia, is regulated or adjusted/controlled such that maximum reduction of the sorbed NO.sub.x and of any which is still yet to arrive at the storage medium or the SCR catalyst is brought about, without occurrence of an unwanted breakthrough (slip) of ammonia. The amounts of reducing agent required for the purpose are dependent on the nature of the reducing agent and the type and nature of the SCR catalyst and other operating parameters such as pressure and temperature. Especially when the storage medium used and/or the SCR catalyst used is capable of storing not only NO.sub.x but additionally also NH.sub.3, which is the case for the transition metal-laden zeolite catalysts that are particularly preferred in accordance with the invention, it should be ensured in accordance with the invention that, in step (c), the amount of reducing agent (NH.sub.3) metered in is not more than required for the reduction of the NO.sub.x. Otherwise, there is the risk of unintended slip of NH.sub.3 during the heating phase.

(61) In the case of ammonia as reducing agent for NO.sub.x, it is customary to add such an amount of NH.sub.3 as to result in, based on the NH.sub.3 and NO.sub.x components sorbed on the storage medium or still to arrive there, a molar NH.sub.3/NO.sub.x ratio of 0.8 to 2.5, preferably of 0.9 to 2.0, more preferably of 1.0 to 1.8.

(62) Contrary to the regulation of the amount of NH.sub.3 applied with respect to the desired NO.sub.x exit concentration which is typically employed in gas cleaning systems, in the simplest case, it is preferred in accordance with the invention to employ ratio control with regard to the NO.sub.x inlet concentration. In the case of this control method, the NO.sub.x concentration is determined upstream of the gas cleaning system (i.e. at the inlet) and NH.sub.3 is metered in according to the aforementioned molar NH.sub.3:NO.sub.x ratio. It is thus possible to ensure that a coreactant for the NH.sub.3 applied is constantly available and that unwanted accumulation of NH.sub.3 on the SCR catalyst is effectively prevented.

(63) Accordingly, step (c) of the method of the invention preferably comprises the measurement of the concentration of NO.sub.x in the tail gas before the tail gas is contacted with the SCR catalyst, with metered addition (control) of the amount of reducing agent for NO.sub.x supplied as a function of the measured concentration of NO.sub.x, such that accumulation of the reducing agent on the SCR catalyst is prevented. Preference is accordingly given to supplying no more reducing agent for NO.sub.x than is consumed by the catalytic reduction of NO.sub.x.

(64) Preferably, the reducing agent for NO.sub.x, preferably NH.sub.3, is fed to the tail gas line directly upstream of the SCR catalyst and guided onto the surface of the SCR catalyst where the NO.sub.x is adsorbed, such that the NO.sub.x can be catalytically reduced there, preferably to N.sub.2 with simultaneous formation of H.sub.2O.

(65) Preferably, the reduction is effected at least partly by the “fast” SCR process, i.e. according to the reaction scheme:
2NH.sub.3+NO+NO.sub.2.fwdarw.2N.sub.2+3H.sub.2O
or the “normal” SCR process, i.e. according to the reaction scheme:
4NH.sub.3+4NO+O.sub.2.fwdarw.4N.sub.2+6H.sub.2O.

(66) The oxidation level of the NO.sub.x entering the SCR catalyst is preferably 30% to 70%, more preferably 40% to 60%, especially preferably about 50%.

(67) If the oxidation level of the NO.sub.x is too high, i.e. when the proportion of NO.sub.2 in the mixture with NO is too great, in a preferred embodiment of the method of the invention, the oxidation level of the NO.sub.x is reduced and in this way adjusted to a desired lower value. Preferably, the oxidation level is adjusted such that it does not exceed 80%, more preferably 75%, even more preferably 70% and especially 65%. Suitable measures for reducing the content of NO.sub.2 are known to a person skilled in the art. The the equilibrium between NO.sub.2 and NO is temperature-dependent, it is possible, for example by varying the temperature, in the presence of suitable catalysts (e.g. platinum), to shift the thermodynamic equilibrium in the desired direction.

(68) Since NO.sub.2 can generally be better stored than NO, a comparatively high oxidation level for the storage in step (a) of the method of the invention is advantageous. In step (c) of the method of the invention, by contrast, a high oxidation level can have an adverse effect, for which reason it is preferable in accordance with the invention to reduce the oxidation level over the course of the method from high values (e.g. >70%) for step (a) to moderate values (e.g. 70%) for step (c).

(69) In a preferred embodiment of the method of the invention, the relationship of T.sub.0, T.sub.G and T.sub.B with respect to one another is as follows: T.sub.0<170° C. and/or 170° C.≤T.sub.G<300° C. and/or 300° C.≤T.sub.B.

(70) Preferred embodiments A.sup.1 to A.sup.B for ranges of the starting temperature T.sub.0, the threshold temperature T.sub.G and the operating temperature T.sub.B with T.sub.0<T.sub.G<T.sub.B are summarized in the following table:

(71) TABLE-US-00001 [° C.] A.sup.1 A.sup.2 A.sup.3 A.sup.4 A.sup.5 A.sup.6 A.sup.7 A.sup.8 T.sub.0 <170 ≤150 ≤120 ≤120 ≤100 ≤100 ≤100 ≤100 T.sub.G 170-220 170-220 170-210 170-210 170-200 180-200 180-200 180-190 T.sub.B ≥300 ≥310 ≥320 ≥330 ≥340 ≥350 ≥375 ≥400

(72) Preferably, steps (a) and (c) of the method of the invention proceed one after the other, but it is possible that the optional step (b) is effected partly or completely simultaneously with step (c).

(73) Preferably, the method of the invention is operated and the amount of reducing agent required is metered in such that at least 50%, more preferably at least 60%, even more preferably at least 70%, most preferably at least 80%, and especially at least 90% of the total amount of NO.sub.x obtained during the startup of the plant for preparation of nitric acid, i.e. during the pressurization and filling, is reduced.

(74) In a preferred embodiment, the nitric acid plant whose startup causes the NO.sub.x to be obtained comprises a gas cleaning system for reducing the concentrations of NO.sub.x and N.sub.2O in the tail gas which is formed after the startup of the plant during the steady-state operation of the nitric acid plant. The gas cleaning system accordingly especially (also) pursues the purpose of reducing the concentration of nitrogen oxides (NO, NO.sub.2 and N.sub.2O) in the tail gas in the steady-state operation of the plant.

(75) In a particularly preferred embodiment, steps (a), optionally (b) and (c) of the method of the invention are effected within this gas cleaning system. For this purpose, preferably, the SCR catalyst in whose presence the reduction of the NO.sub.x is effected in step (c) of the method of the invention is disposed in the gas cleaning system. In this case, the gas cleaning system accordingly pursues two purposes, namely of reducing the concentration of the nitrogen oxides obtained in the tail gas both in the startup and in the steady-state operation of the plant.

(76) A further aspect of the invention relates to a method of reducing the nitrogen oxide concentrations (preferably NO.sub.x and N.sub.2O) in the tail gas which are obtained both during the startup of a plant for preparation of nitric acid and thereafter during the steady-state operation of the plant. This method of the invention comprises the above-described method of reducing the concentration of nitrogen oxides in the tail gas which are obtained during the startup of a plant for preparation of nitric acid.

(77) All preferred embodiments which have been described above in connection with the method of the invention for reducing the concentration of nitrogen oxide in tail gas obtained during the startup of a plant for preparation of nitric acid also apply analogously to the method of the invention for reducing the concentration of nitrogen oxides (preferably NO.sub.x and N.sub.2O) in tail gas obtained both during the startup of a plant for preparation of nitric acid and thereafter during the steady-state operation of the plant, and are therefore not repeated.

(78) In a preferred embodiment, during the steady-state operation of the plant, (i) N.sub.2O is decomposed catalytically to O.sub.2 and N.sub.2 and NO.sub.x is reduced catalytically in the presence of a reducing agent for NO.sub.x; or (ii) N.sub.2O is reduced catalytically in the presence of a reducing agent for N.sub.2O and NO.sub.x is reduced catalytically in the presence of a reducing agent for NO.sub.x; or (iii) N.sub.2O is broken down catalytically to O.sub.2 and N.sub.2 and then residual N.sub.2O is further reduced catalytically in the presence of a reducing agent for N.sub.2O and NO.sub.x is reduced catalytically in the presence of a reducing agent for NO.sub.x.

(79) Preferably, the two above-described embodiments (i), (ii) and (iii) are variants of what is called EnviNOx® technology, which is also referred to hereinafter and in the context of the invention as (i) EnviNOx® technology “variant 1”, and (ii) EnviNOx® technology “variant 2” and (iii) EnviNOx® technology “variant ½”, and the mode of operation of which or the plant design of which is described in EP 01 905 656.3-2113. EP 1 370 342 B1, EP 1 515 791 B1, EP 2 286 897 B1 (all variant 1), EP 1 497 014 (variant 2) and in DE 10 2005 022 650 A1 (variant ½).

(80) More preferably, for this purpose, the plant has a gas cleaning system for reducing the concentration of NO.sub.x and N.sub.2O in tail gas obtained during the steady-state operation of the plant for preparation of nitric acid, wherein (i) the gas cleaning system has a two-stage construction, wherein N.sub.2O is broken down catalytically to O.sub.2 and N.sub.2 in the first stage in flow direction of the tail gas during the steady-state operation of the plant, after the first stage a reducing agent for NO.sub.x is mixed with the tail gas and then, in a second stage, NO.sub.x is reduced catalytically in the presence of the reducing agent; or (ii) during the steady-state operation of the plant, the tail gas is mixed in the gas cleaning system with a reducing agent for NO.sub.x and a hydrocarbon, carbon monoxide, hydrogen or a mixture of these gases as reducing agent for N.sub.2O, wherein the reducing agent is added in such an amount which is sufficient at least for complete reduction of the NO.sub.x, and that the gas mixture is passed through at least one reaction zone comprising an SCR catalyst for the reduction of NO.sub.x and for the reduction of N.sub.2O; or (iii) the gas cleaning system has a two-stage construction, wherein, during the steady-state operation of the plant, in flow direction of the offgas, in the first stage, N.sub.2O is broken down catalytically to O.sub.2 and N.sub.2, and the offgas, after the first stage, is mixed with a reducing agent for NO.sub.x and a hydrocarbon, carbon monoxide, hydrogen or a mixture of these gases as reducing agent for N.sub.2O, wherein the reducing agent is added in such an amount which is sufficient at least for complete reduction of the NO.sub.x, and that the gas mixture is passed through at least one reaction zone comprising a catalyst for the reduction of NO.sub.x and for the reduction of N.sub.2O.

(81) In the gas cleaning system, it is possible to use various already known cleaning methods arranged downstream of the absorption tower in flow direction of the tail gas discharged from the plant for preparation of nitric acid. These may be conventional SCR methods in which residues of NO.sub.x are removed catalytically and with use of reducing agents for NO.sub.x, preferably of NH.sub.3, from the tail gas of the plant for preparation of nitric acid. Typical SCR catalysts contain transition metal oxides, especially V.sub.2O.sub.5 supported on TiO.sub.2, noble metals, especially platinum, or zeolites laden with transition metals, especially zeolites laden with iron. Such methods that are suitable for the steady-state operation of the plant for preparation of nitric acids are known per se (cf., for example, WO 01/51181 A1, WO 03/105998 A1, and WO 03/084646 A1 described). A general overview of SCR catalysts for NO.sub.x reduction can be found, for example, in the description in G. Ertl, H. Knözinger J. Weitkamp: Handbook of Heterogeneous Catalysis, vol. 4, pages 1633-1668, VCH Weinheim (1997)).

(82) Preferably, the gas cleaning system is based on EnviNOx.sup.® technology, variant 1, variant 2, or variant ½. This is a method in which NO.sub.x and N.sub.2O are removed catalytically from the tail gas and wherein at least the NO.sub.x is reduced by supply of NH.sub.3. In the EnviNOx.sup.® method, preference is given to using iron zeolite catalysts; these have particularly good suitability for the catalytic reduction, especially the complete catalytic reduction, of NO.sub.x, surprisingly simultaneously have the required property as a storage medium, by contrast with conventional SCR catalysts based on V.sub.2O.sub.5/TiO.sub.2, and offer the additional advantage that they can also be used at higher temperatures compared to the aforementioned conventional SCR catalysts, which opens up the option of simultaneous N.sub.2O reduction.

(83) It has been found that, surprisingly, compared to conventional plants, there is a change in the NO.sub.x emission characteristics when the plant for preparation of nitric acid is equipped with a gas cleaning system based on EnviNOx.sup.® technology (variant 1, variant 2 or variant ½). There is a reduction here in NO.sub.x emission (first NO.sub.x peak during the pressurization of the nitric acid plant) to a maximum concentration of below 100 ppm and it seems to be slightly delayed. The second peak (filling of the absorption tower) is likewise delayed, but is approximately the same in terms of intensity. By contrast, the third peak (production of HNO.sub.3) appears to be distinctly enhanced at the start. It has been found that, surprisingly, the iron-zeolite catalysts used with preference in the EnviNOx® method are capable of physically absorbing (storing) NO.sub.2 primarily at temperatures below 250° C. If the NO.sub.2 absorption capacity of the SCR catalyst has been attained after a while, the gas cleaning system merely brings about a slight delay in the NO.sub.x emission characteristics.

(84) Through the preferred combination of properties of both at first acting as storage medium (absorption medium) for NO.sub.x and then as SCR catalyst for the reduction of NO.sub.x, the zeolite catalysts that are particularly preferred in accordance with the invention enable a distinct reduction in NO.sub.x emissions during the startup of the plant for preparation of nitric acid. The minimum temperature (˜200° C.) for performance of the reduction of NO.sub.x is often already attained after phase 1 and before phase 2 (cf. FIG. 1). It is thus possible, after the exceedance of this temperature (T.sub.G), to feed reducing agents, especially NH.sub.3, by metering into the reaction space and in this way to reduce the nitrogen oxides obtained in phase 2 and phase 3 of the startup. Overall, the use of such a gas cleaning system thus offers distinct advantages over systems based on conventional SCR catalysts which do not have storage properties. The ability of the SCR catalysts used (e.g. iron-zeolite catalysts) to store (adsorb) NO.sub.x at first reduces the emission directly after the starting of the plant.

(85) This in principle also gives rise to the advantage that the gas cleaning system can be integrated into the plant for preparation of nitric acid at points where, in steady-state operation, there are temperatures (T.sub.B) of 400 to 500° C. It is thus already possible in air operation of the plant for preparation of nitric acid to attain a temperature of more than 200° C., for example of 210-230° C., at the inlet of the gas cleaning system, which enables supply of reducing agent in step (c) even in air operation.

(86) Because the gas cleaning system based on EnviNOx® technology (variant 1, variant 2 or variant ½) is designed not just for reduction of NO.sub.x but additionally also brings about the lowering of N.sub.2O, the amount of SCR catalyst provided is increased compared to conventional systems, which has a favorable effect on the storage capacity of the SCR catalyst for NO.sub.x. One advantage of this procedure is that additional NO.sub.2 molecules and NH.sub.3 molecules which have not reacted to exhaustion by the SCR reaction over the SCR catalyst are adsorbed in an unexpectedly large amount on the SCR catalyst and hence are not emitted into the environment. Because the concentration of NO.sub.x during the pressurization operation occurs only as a peak and hence is limited in terms of volume, the storage capacity of conventional gas cleaning systems based on EnviNOx® technology (variant 1, variant 2 or variant ½) is frequently already sufficient to effectively prevent emission. A further advantage can be achieved when the adsorbed NO.sub.2 is removed again from the SCR catalyst through the addition of a predetermined amount of reducing agent which has preferably been determined beforehand.

(87) A further aspect of the invention relates to an apparatus for reducing the concentration of nitrogen oxides in tail gas obtained during the startup of a plant for preparation of nitric acid, comprising the elements: A) a storage medium for NO.sub.x having a capacity sufficient such that at least 5.0% by volume of the NO.sub.x obtained in total during the startup of the plant for preparation of nitric acid can be stored in the storage medium; B) optionally means of releasing the stored NO.sub.x; C) an SCR catalyst for reduction of NO.sub.x; D) a control device comprising a temperature measurement device for determining the temperature of the tail gas and a metering device for metering in a reducing agent for NO.sub.x as a function of the measured temperature of the tail gas; and E) means of combining the reducing agent for NO.sub.x with the NO.sub.x in the presence of the SCR catalyst for catalytic reduction of at least a portion of the NO.sub.x.

(88) Preferably, the apparatus of the invention serves to reduce the concentration of nitrogen oxides (preferably NO.sub.x and N.sub.2O) in tail gas, which are obtained both during the startup of a plant for preparation of nitric acid and thereafter during the steady-state operation of the plant.

(89) All preferred embodiments that have been described above in connection with the two methods of the invention also apply analogously to the apparatus of the invention, and are therefore not repeated.

(90) In a preferred embodiment, the apparatus of the invention comprises, as additional elements, F) means of adjusting the oxidation level of the NO.sub.x, which are preferably suitable for adjusting the oxidation level of the NO.sub.x such that it does not exceed 80%, more preferably 75%, even more preferably 70% and especially 65%.

(91) In a preferred embodiment of the apparatus of the invention, the SCR catalyst for reduction of NO.sub.x C) also acts as storage medium A). Suitable SCR catalysts, especially suitable catalytic materials for reduction of NO.sub.x with a reducing agent for NO.sub.x, especially NH.sub.3, are known to a person skilled in the art. Preferably, the catalytically active material is selected from the group consisting of iron, compounds of iron, cobalt, compounds of cobalt, copper and compounds of copper. Iron-zeolite catalysts are particularly preferred. Preferably, the SCR catalyst for NO.sub.x is present in such an amount and with such a storage capacity that, in its effect as a storage medium for NO.sub.x, it can temporarily and simultaneously store at least 20%, more preferably at least 30%, even more preferably at least 50%, and especially at least 70%, of the total molar amount of NO.sub.x obtained overall during the pressurization of the plant, preferably by physisorption and/or chemisorption.

(92) Preferably, storage medium for NO.sub.x and SCR catalyst for reduction of NO.sub.x are one and the same element and/or are arranged in a gas cleaning system into which the tail gas coming from the absorption tower of the plant for preparation of nitric acid is introduced.

(93) Most preferably, the gas cleaning system comprises an iron-laden zeolite as SCR catalyst and the gas cleaning system is positioned downstream of the absorption tower in the tail gas line of the plant for preparation of nitric acid at such a point where the tail gas in steady-state operation has a temperature (T.sub.B) of at least 300° C., preferably at least 350° C., especially at least 400° C.

(94) A further aspect of the invention relates to the use of the above-described apparatus of the invention in one of the two above-described methods of the invention.

(95) All preferred embodiments which have been described above in connection with the two methods of the invention and the apparatus of the invention also apply analogously to the use of the invention and are therefore not repeated.