HK1192744A - Ammonia gas generator and method for producing ammonia in order to reduce nitrogen oxides in exhaust gases - Google Patents

Description

The present invention relates to an ammonia gas generator for producing ammonia from an ammonia precursor substance and a method for producing ammonia gas, as well as their use in exhaust gas aftertreatment systems for reducing nitrogen oxides in exhaust gases.

In the exhaust gases of internal combustion engines, substances are often present that are undesirable to emit into the environment. Therefore, in many countries, limit values are set for the emission of such pollutants, for example from industrial plants or vehicles, which must be adhered to. Among these pollutants are also nitrogen oxides (NOx), such as particularly nitrogen monoxide (NO) or nitrogen dioxide (NO2).

The reduction of these nitrogen oxides from exhaust gases of internal combustion engines can be achieved in various ways. Particularly noteworthy is the reduction through additional exhaust gas after-treatment measures, which mainly rely on selective catalytic reduction (SCR). These methods share the common feature that a reducing agent selectively acting on nitrogen oxides is added to the exhaust gas, leading to the conversion of nitrogen oxides in the presence of a suitable catalyst (SCR catalyst). In this process, the nitrogen oxides are converted into less environmentally harmful substances, such as nitrogen and water.

A reducing agent already in use today for nitrogen oxides is urea (H2N-CO-NH2), which is introduced into the exhaust gas as an aqueous urea solution. In the exhaust gas stream, the urea can decompose into ammonia (NH3), for example through the effect of heat (thermolysis) and/or through reaction with water (hydrolysis). The ammonia formed in this way is the actual reducing agent for nitrogen oxides.

The development of exhaust aftertreatment systems for automobiles has been ongoing for some time and is the subject of numerous publications. For example, European Patent EP 487 886 B1 describes a method for selective catalytic NOX reduction in oxygen-containing exhaust gases from diesel engines, in which urea and its thermal decomposition products are used as reducing agents. In addition, a device for generating ammonia in the form of a tube evaporator is described, comprising a spray device, an evaporator with evaporator tubes, and a hydrolysis catalyst.

Furthermore, the European patent document EP 1 052 009 B1 describes a method and a device for carrying out the method of thermal hydrolysis and dosing of urea or urea solutions in a reactor using a part of the exhaust gas stream. In the method, a portion of the exhaust gas is taken from the exhaust gas stream upstream of the SCR catalyst and passed through the reactor, where the portion of the exhaust gas loaded with ammonia after hydrolysis in the reactor is returned to the exhaust gas stream again upstream of the SCR catalyst.

In addition, the European patent document EP 1 338 562 B1 describes a device and a method that use the catalytic reduction of nitrogen oxides by ammonia. In this case, ammonia is obtained under conditions of flash thermolysis from solid urea and hydrolysis from cyanic acid, and is then directed into the exhaust gas stream of a vehicle.

Furthermore, the European patent application EP 1 348 840 A1 describes an exhaust gas purification system as a completely transportable unit in the form of a 20-foot container. The system is operated in such a way that a urea or ammonia solution is directly injected into the exhaust gas stream by means of an injection device. The reduction of nitrogen oxides contained in the exhaust gas takes place at an SCR catalyst.

Furthermore, the German patent application DE 10 2006 023 147 A1 describes a device for generating ammonia, which is part of an exhaust gas aftertreatment system.

In addition, the international applications WO 2008/077 587 A1 and WO 2008/077 588 A1 describe a method for the selective catalytic reduction of nitrogen oxides in exhaust gases of vehicles using aqueous guanidinium salt solutions. In these methods, a reactor is used that produces ammonia from the aqueous guanidinium salt solutions.

JP 2009-062860 and JP 2008-267269 describe exhaust gas aftertreatment systems including a device for producing ammonia by thermal decomposition of urea, which comprises a reactor chamber equipped with a spray nozzle for an aqueous urea solution and a hydrolysis catalyst.

EP 1 481 719 describes an exhaust gas aftertreatment system comprising a device for producing ammonia by thermal decomposition of urea, including a reactor chamber equipped with a spray nozzle for an aqueous urea solution and a catalyst for decomposing urea, wherein a portion of the exhaust gas is used as a carrier gas for the urea solution.

DE 42 03 807 describes a device for the catalytic reduction of NOx from oxygen-containing exhaust gases by using urea, and includes a hydrolysis catalyst consisting of fine flow channels that allow partial flows through bends, openings or slots, which are approximately perpendicular to the main flow, in order to achieve a uniform distribution of the urea solution and a very rapid heating of the solution.

Although ammonia gas generators have been known for some time, the technique has not yet been implemented in a vehicle or any other application. To date, the concept of directly injecting an ammonia precursor substance into the exhaust gas stream of an internal combustion engine has been pursued, where this ammonia precursor substance decomposes into the actual reducing agent through appropriate measures in the exhaust system. However, due to incomplete decomposition or side reactions of the decomposition products in the exhaust system, deposits are repeatedly observed, which impair the catalysts and filters still present in the exhaust system.

Therefore, the present invention is based on the object of providing an ammonia gas generator and a method for producing ammonia that overcomes the disadvantages of the prior art. Furthermore, the present invention is based on the object of providing an ammonia gas generator having a simple construction, capable of achieving a high conversion rate of ammonia precursor substances into ammonia gas, and allowing long-term operation without maintenance. Moreover, the ammonia gas generator should be universally applicable, enabling the use of various types of ammonia precursor substances. Additionally, the method for generating ammonia should be implementable using simple technical measures, provide a high conversion rate of ammonia precursor substances into ammonia gas, and allow long-term operation without maintenance.

These tasks are solved by an ammonia gas generator according to claim 1 and by a method for producing ammonia from a solution of an ammonia precursor substance using an ammonia gas generator according to claim 9. According to a first embodiment, an ammonia gas generator for producing ammonia from a solution of an ammonia precursor substance is the subject of the present invention, comprising a catalyst unit, wherein the catalyst unit itself includes a catalyst for decomposing and/or hydrolyzing the ammonia precursor substance into ammonia and a mixing chamber located upstream of the catalyst in the flow direction, with the catalyst having a catalyst volume VKat and the mixing chamber having a mixing chamber volume VMisch. Furthermore, the ammonia gas generator comprises an injection device for introducing a solution of the ammonia precursor substance into the mixing chamber and an outlet for the generated ammonia gas.wherein the ammonia gas generator includes an inlet for a carrier gas that generates a tangential carrier gas flow relative to the solution injected into the mixing chamber. The ammonia gas generator is characterized in that the catalyst unit comprises at least a two-part hydrolysis catalyst, wherein a first part of the hydrolysis catalyst, in the direction of flow, is formed as a heated catalyst, while the second part is designed as an unheated catalyst, or that two hydrolysis catalysts are arranged in series, wherein the first hydrolysis catalyst is a heated catalyst and the second hydrolysis catalyst is an unheated catalyst.

It should be emphasized at this point that an ammonia gas generator according to the present invention is a separate unit for producing ammonia from ammonia precursor substances. Such a unit can, for example, be used for reducing nitrogen oxides in industrial exhaust gases or for post-treatment of exhaust gases from internal combustion engines, such as diesel engines. This ammonia gas generator can operate autonomously or be operated by using exhaust side streams; however, in any case, the reduction of nitrogen oxides by means of ammonia occurs only in a subsequent process step. If an ammonia gas generator according to the invention is used as a separate component in an exhaust aftertreatment system of an internal combustion engine, for example a diesel engine, then it is possible to reduce the nitrogen oxides in the exhaust gas stream without introducing further catalysts for splitting ammonia precursor substances or other components into the exhaust gas stream itself. The ammonia generated by the ammonia gas generator according to the invention can thus be introduced into the exhaust gas stream as required. Any shortening of the service life of the SCR catalyst due to impurities in the form of deposits, for example, from ammonia precursor substances or products of the decomposition of ammonia precursor substances is also avoided.

According to the invention, therefore, an ammonia precursor substance is not introduced into an exhaust gas stream, from which ammonia is then formed in situ and acts as a reducing agent in the exhaust gas stream. Instead, according to the invention, ammonia is directly introduced into the exhaust gas stream, which has been previously generated in a separate unit, namely the ammonia gas generator according to the invention. According to the invention, ammonia is thus first produced in an ammonia gas generator as a separate component unit from an ammonia precursor substance. This ammonia, and not the ammonia precursor substance, is then introduced into the exhaust gas stream, particularly to cause a reduction of nitrogen oxides there.

The introduction of ammonia is preferably carried out according to the invention before a SCR catalyst located in the exhaust gas stream. Furthermore, the introduction of ammonia is preferably carried out after the combustion engine. In another preferred embodiment, the introduction of ammonia takes place after an oxidation catalyst located in the exhaust gas stream.

The invention is characterized by the fact that the ammonia gas generator comprises an inlet for a carrier gas, which generates a tangential carrier gas flow relative to the solution injected into the mixing chamber.

The inlet for the carrier gas is preferably located in the mixing chamber.

Surprisingly, it has been found that by using a tangential carrier gas flow (hereinafter also referred to synonymously as transport gas flow), deposits on the walls of the catalyst unit in the area of the mixing chamber can be prevented, and a permanently good mixing of the carrier gas (hereinafter also referred to synonymously as transport gas) and the solution of the ammonia precursor substance can be ensured. If such a tangential carrier gas flow is not used, spraying of the solutions of the ammonia precursor substance into the mixing chamber may result in wetting of the walls of the catalyst unit in the area of the mixing chamber, and unwanted side reactions such as, for example, polymerization of the ammonia precursor substance may occur. These side reactions lead to unwanted deposits in the area of the mixing chamber, thereby making it permanently impossible to achieve the extremely important mixing of the carrier gas and the solution of the ammonia precursor substance for the operation of the generator. Furthermore, due to the poor mixing of the carrier gas with the solution, additional deposits can be observed in and on the catalyst itself. The tangential carrier gas flow generates a swirling mist flow with droplets, which is guided axially towards the front surface of the hydrolysis catalyst. This swirling mist flow enables a very good conversion into ammonia at the catalyst.

The tangential introduction of the carrier gas takes place in the head section of the generator, preferably at the level of the spray device of the ammonia precursor solution into the catalyst unit or mixing chamber. In this process, the gas flow is introduced as flat as possible along the wall of the mixing chamber, such that a downward-directed vortex flow is established in the catalyst unit toward the catalyst front surface.

The carrier gas, and in particular the tangential carrier gas flow, is preferably introduced into the mixing chamber at a temperature of up to 550 °C, preferably at a temperature of 250 to 550 °C, more preferably at a temperature of 250 to 400 °C, and particularly preferably at a temperature of 300 to 350 °C.

Ideally, that is, to achieve an conversion rate of the ammonia precursor substance into ammonia of more than 95% and to avoid contact between the precursor substance and the generator wall, some key conditions are preferably maintained during injection. It is preferred to inject the ammonia precursor substance into the mixing chamber in such a way that, given a certain catalyst frontal area, the spray cone diameter upon impact on the catalyst frontal area is at most 98%, preferably at most 95% of the catalyst diameter. On the other hand, the spray cone diameter is preferably at least 80%, preferably at least 83% of the catalyst frontal area diameter, in order to avoid an excessively high concentration for a given area and thus an excessive loading of the catalyst frontal area with precursor substance. An excessively high loading of the catalyst frontal area leads to insufficient contact with the catalyst and to excessive cooling caused by the evaporating liquid, thereby also resulting in incomplete conversion and unwanted side reactions associated with deposits. Therefore, ideally, preferred combinations to be maintained include a tangential carrier gas flow together with other parameters defined by the injection device. In this context, in particular, the type of injection device to be used, as well as the distance between the opening of the injection device and the given catalyst frontal area, should be mentioned.

In the context of the present invention, an "atomizing device" is intended to mean any device that sprays, mists or otherwise forms a solution, preferably an aqueous solution, of an ammonia precursor substance into droplets, wherein the solution of the ammonia precursor substance is formed in the form of droplets having a droplet diameter d32 of less than 25 µm. The droplet diameter d32 refers, in the context of the present invention, to the Sauter diameter according to the German Industrial Standard DIN 66 141.

According to a preferred embodiment of the present invention, it is provided that the injection device itself includes a nozzle which generates droplets with a d32 droplet diameter of less than 25 µm. According to the present invention, it is further preferably provided that the nozzle produces droplets with a d32 droplet diameter of less than 20 µm, and particularly preferably of less than 15 µm. At the same time or independently thereof, it is further preferably provided that the nozzle produces droplets with a d32 droplet diameter of greater than 0.1 µm and in particular greater than 1 µm. By using such nozzles, an ammonia formation degree AG of more than 95% (see above) can also be achieved. Furthermore, a particularly uniform distribution of the solution on the catalyst face can be achieved. The ammonia formation degree AG is defined here and in the following as the amount of moles of NH3 produced in the process, related to the amount of moles of ammonia that could theoretically be produced by complete hydrolysis of the ammonia precursor substance. According to the present invention, an ammonia formation degree of more than 95% is considered as complete conversion.

According to a particularly preferred embodiment, it may especially be provided that the injection device itself includes a nozzle which, according to the present invention, is a so-called two-component nozzle. A two-component nozzle here refers to a nozzle that uses a pressurized gas, generally air, for applying the liquid phase to a surface and thus for forming droplets. This pressurized gas is also referred to as atomizing air. This type of nozzle allows for a particularly fine distribution of the ammonia precursor substance with droplet diameters d32 smaller than 25 µm, preferably smaller than 20 µm.

The propellant, particularly the atomizing air, is preferably introduced together with the solution of the ammonia precursor substance through the same nozzle opening into the mixing chamber.

The dosing device can, independently or simultaneously, also have at least two nozzles for introducing the ammonia precursor substance into the mixing chamber, which can be particularly switched on simultaneously or separately from each other.

Alternatively, the dosing device may also be designed to include a so-called flash evaporator.

The spray cone according to the present invention refers to the cone of the solution to be sprayed, which can be generated by means of a nozzle or several nozzles with defined spray angles α. The spray cone diameter is the diameter obtained when the droplets hit the catalyst face. This is achieved by applying a liquid pressure of 0.1 to 10 bar to the solution to be sprayed at 25 °C, and optionally using atomizing air in the operating range of 0.5 to 10 bar (for two-component nozzles), employing a carrier gas.

In order to achieve a spray cone diameter of at most 98 % of the catalyst diameter, according to an improvement of the present invention, it can also be provided that the injection device itself comprises at least one nozzle, in particular a two-component nozzle, which has a theoretical spray angle α of 10 ° to 90 °. In particular, it can also be provided simultaneously or independently that the distance from the nozzle opening to the front face of the catalyst is between 15 and 2,000 mm.

Particularly preferred is a nozzle, in particular a two-component nozzle, which has a theoretical spray angle α of at least 10°, particularly at least 20°, especially at least 25°, preferably at least 30°, particularly preferably at least 35°, particularly preferably at least 40°, and especially preferably at least 45°. At the same time or independently thereof, nozzles are also preferred which have a theoretical spray angle α of at most 90°, particularly at most 80°, especially at most 75°, particularly at most 70°, preferably at most 65°, particularly preferably at most 60°, particularly preferably at most 55°, and especially preferably at most 50°. As already explained, by using a nozzle with a defined spray angle α in a targeted manner, a uniform distribution of the solution to be sprayed can be achieved without causing deposits on the walls or on the catalyst face.

As a theoretical spray angle α (hereinafter also referred to as spray angle α) according to the present invention, such a spray angle is intended to be understood as the spray angle that is established at an operating pressure of 0.1 to 10 bar on the solution to be sprayed at 25 °C and, if applicable, the atomizing air in the operating range of 0.5 to 10 bar (for two-component nozzles) at the exit of the nozzle opening or nozzle openings, without the presence of a carrier gas or any other influence on the sprayed solution.

A similar effect is achieved when a nozzle is used that has a first number of nozzle openings for introducing the solution of the ammonia precursor substance into the catalytic unit, which is surrounded annularly by a second number of nozzle openings for introducing a carrier gas or atomization air into the catalytic unit.

Alternatively, it may also be provided that at least one inlet for the carrier gas is arranged around the nozzle, designed in such a way that the carrier gas forms a jacket around the solution introduced into the mixing chamber. Thus, the sprayed solution is enclosed by a layer of carrier gas, so that no wetting of the inner wall is observed.

In a further embodiment, the invention therefore relates to an ammonia gas generator comprising at least one inlet for a carrier gas. The inlet is preferably located in the mixing chamber and is particularly separate or distinct from the nozzle opening through which the solution of the ammonia precursor substance is introduced. Thus, the carrier gas can be introduced independently of the ammonia precursor substance solution. Preferably, the inlet generates a tangential or parallel carrier gas flow relative to the solution injected into the mixing chamber. For a parallel carrier gas flow, preferably one or more carrier gas inlet openings are arranged in the wall, which also contains the injection device for introducing the solution of the ammonia precursor substance.

In the present invention, it is further provided that the distance between the nozzle opening and the front face of the catalyst can be, in particular, from 15 to 1500 mm, particularly preferably from 15 to 1000 mm, and very particularly preferably from 15 to 800 mm. However, independently or simultaneously, it may also be provided that the distance between the nozzle opening and the front face of the catalyst is at least 30 mm, preferably at least 40 mm, particularly preferably at least 50 mm, especially preferably at least 60 mm, particularly preferably at least 100 mm, and very particularly preferably at least 300 mm, and furthermore independently or simultaneously at most 1500 mm, in particular at most 1000 mm, in particular at most 800 mm, in particular at most 500 mm, in particular at most 400 mm, particularly preferably at most 200 mm, and very particularly preferably at most 150 mm.

According to a further development of the present invention, it is also provided that the ratio of the volume of the mixing chamber VMisch to the volume of the catalyst VKat corresponds to a ratio ranging from 1.5:1 to 5:1. Surprisingly, it has been shown that the sprayed ammonia precursor substance can then be completely (conversion > 95%) decomposed into ammonia if the droplets of the solution are already partially evaporated before they hit the catalyst face. This can be ensured by making the volume of the mixing chamber larger than the volume of the catalyst. By partial evaporation of the droplets, sufficient energy is already supplied to the solution, thus avoiding excessive cooling at the catalyst face caused by too large droplets, and thereby preventing worse decomposition orCounteraction of by-product formation is achieved. Moreover, a suitable mixing chamber volume VMisch ensures that the sprayed ammonia precursor substance is evenly distributed as an aerosol across the cross-section of the transport gas flow over the catalyst, thus avoiding spots with excessive concentration, which would otherwise result in a poorer conversion. Particularly preferred is the provision that the ratio of the volume of the mixing chamber VMisch to the volume of the catalyst VKat ranges from 2.5:1 to 5:1, particularly preferably from 3:1 to 5:1, and most particularly preferably from 3.5:1 to 5:1.

The volume of the catalyst VKat is preferably 50 ml to 1,000 l. The volume of the mixing chamber VMisch is preferably at least 10 ml, preferably at least 50 ml, further preferably at least 100 ml, further preferably at least 200 ml, further preferably at least 1,000 ml, further preferably at least 2,000 ml, and further preferably at least 5,000 ml. At the same time or independently thereof, the volume of the mixing chamber VMisch is preferably at most 2.5 l, further preferably at most 10 l, further preferably at most 80 l, further preferably at most 500 l, further preferably at most 1,200 l, and further preferably at most 2,000 l.

Furthermore, according to the present invention, a catalytic unit is understood to be an assembly comprising a housing for accommodating a catalyst, a mixing chamber located upstream of the catalyst in the flow direction, and at least one catalyst for decomposing and/or hydrolyzing ammonia precursor substances into ammonia, wherein the catalyst has a catalyst volume VKat and the mixing chamber has a mixing chamber volume VMisch. Optionally, the catalytic unit may additionally include an outlet chamber located downstream of the catalyst in the flow direction for discharging the generated ammonia gas.

As a catalyst for the decomposition and/or hydrolysis of ammonia precursor substances, any catalyst can be used within the scope of the present invention that enables the release of ammonia from the precursor substance under catalytic conditions. A preferred catalyst hydrolyzes the ammonia precursor substance into ammonia and other harmless substances such as nitrogen, carbon dioxide, and water. Thus, the catalyst preferably is a hydrolysis catalyst.

For example, if a guanidinium salt solution, particularly a guanidinium formate solution, a urea solution or mixtures thereof are used, the catalytic decomposition to ammonia can be carried out in the presence of catalytically active, non-oxidizing coatings made of oxides selected from the group consisting of titanium dioxide, aluminum oxide and silicon dioxide as well as their mixtures, or/and hydrothermally stable zeolites which are completely or partially metal-exchanged, in particular iron zeolites of the ZSM-5 or BEA type. In this case, in particular, the transition elements and preferably iron or copper are suitable as metals. The metal oxides such as titanium dioxide, aluminum oxide and silicon dioxide are preferably applied on metallic support materials such as heating element alloys (in particular chromium-aluminum steels).

Particularly preferred catalysts are hydrolysis catalysts that particularly include catalytically active coatings made of titanium dioxide, aluminum oxide, and silicon dioxide as well as mixtures thereof.

Alternatively, the catalytic decomposition of the guanidinium formate solutions or other components can also result in ammonia and carbon dioxide, using catalytically active coatings made of oxides selected from the group consisting of titanium dioxide, aluminum oxide, and silicon dioxide as well as their mixtures, or/and hydrothermally stable zeolites which are completely or partially metal-exchanged, and which are impregnated with gold and/or palladium as oxidation-active components. The corresponding catalysts with palladium and/or gold as active components preferably have a noble metal content of 0.001 to 2 weight percent, particularly 0.01 to 1 weight percent. Using such oxidation catalysts, it is possible to avoid the unwanted formation of carbon monoxide as a byproduct during the decomposition of the guanidinium salt already at the ammonia production stage.

A catalytic coating containing palladium or/and gold as active components with a noble metal content of 0.001 to 2 wt.%, especially 0.01 to 1 wt.%, is preferably used for the catalytic decomposition of guanidinium formate and possibly further components.

Thus, an ammonia gas generator is also the subject of the present invention, which comprises a catalyst, particularly a hydrolysis catalyst, wherein the catalyst has a catalytically active coating impregnated with gold and/or palladium, preferably with a content of gold and/or palladium of 0.001 to 2 wt.% (based on the catalytic coating). Furthermore, preferably, this catalyst has a catalytically active coating made of oxides selected from the group consisting of titanium dioxide, aluminum oxide, and silicon dioxide as well as their mixtures, or/and hydrothermally stable zeolites, which are impregnated with gold and/or palladium, wherein preferably the content of gold and/or palladium is 0.001 to 2 wt.% (based on the catalytic coating).

According to the invention, the ammonia gas generator is characterized in that the catalyst unit comprises at least a two-part hydrolysis catalyst, wherein a first part of the hydrolysis catalyst, in the direction of flow, is formed as a heated catalyst, while the second part is formed as an unheated catalyst, or that two hydrolysis catalysts are arranged in series, wherein the first hydrolysis catalyst is a heated catalyst and the second hydrolysis catalyst is an unheated catalyst.

It is possible within the scope of the present invention to use a hydrolysis catalyst consisting of at least two sections in the flow direction, wherein the first section contains non-oxidation-active coatings and the second section contains oxidation-active coatings. Preferably, 5 to 90 vol.% of this catalyst consists of non-oxidation-active coatings and 10 to 95 vol.% consists of oxidation-active coatings. In particular, 15 to 80 vol.% of this catalyst consists of non-oxidation-active coatings and 20 to 85 vol.% consists of oxidation-active coatings. Alternatively, the hydrolysis can also be carried out in the presence of two catalysts arranged one after the other, wherein the first catalyst contains non-oxidation-active coatings and the second catalyst contains oxidation-active coatings. In the case of two catalysts arranged one after the other, the first hydrolysis catalyst is a heated catalyst and the second hydrolysis catalyst is a non-heated catalyst.

Alternatively, a hydrolysis catalyst is used, which consists of at least two sections, wherein the first section arranged in the flow direction is in the form of a heated catalyst and the second section arranged in the flow direction is in the form of an unheated catalyst. Preferably, the catalyst comprises 5 to 50 vol.-% from the first section and 50 to 95 vol.-% from the second section.

According to a particularly preferred embodiment of the present invention, it is provided that the ammonia gas generator comprises a catalytic unit with at least a two-part, preferably at least three-part, hydrolysis catalyst, wherein the first part in the flow direction is formed as a heated catalyst, preferably having direct electrical resistance heating and/or a jacket heating, while the second part is designed as an unheated catalyst, and, particularly preferably, a third part which is also an unheated catalyst with a mixer structure follows downstream.

Particularly preferred is an ammonia gas generator comprising a catalytic unit whose catalyst has a ratio of the catalyst diameter DKAT to the length L of the catalyst of 1:1 to 1:5, particularly 1:2 to 1:4 and especially 1:3. The catalyst diameter DKAT is preferably 20 to 2000 mm, particularly 30 to 1000 mm and even more preferably 30 to 100 mm. However, it may also be provided that the diameter DKat is 30 to 80 mm, 80 to 450 mm or 450 to 1,000 mm.

Furthermore, it is preferred that the catalyst has a length L of 30 mm to 2000 mm, particularly preferably 70 mm to 1000 mm, and very particularly preferably 70 mm to 700 mm.

It has been found that for a complete catalytic conversion of the ammonia precursor substances, catalysts with a catalyst cell density of at least 60 cpsi (cpsi: number of cells per square inch on the frontal surface of the catalyst) and the already described catalyst volumes are preferably used. The increasing back pressure (pressure loss across the catalyst) limits the catalyst cell density to a maximum of 800 cpsi for an application in an ammonia gas generator. Particularly preferred are such catalysts, especially hydrolysis catalysts, which have a catalyst cell density of 100 to 600 cpsi per square inch of frontal area, preferably 100 to 500 cpsi per square inch of frontal area, and particularly preferably 100 to 400 cpsi per square inch of frontal area of the catalyst.

Regarding the design of the catalyst unit, tests have shown that a cylindrical shape is particularly suitable. In this case, the tangential carrier gas flow can fully exert its effect. Other shapes, however, are less suitable because they result in excessive turbulence. Thus, an ammonia gas generator is also the subject of the present invention, which comprises a catalyst unit formed in the shape of a cylinder.

In addition, it has proven particularly advantageous if the ammonia gas generator includes a catalytic unit that in turn comprises at least one thermal insulation layer, particularly a thermal insulation layer made of microporous insulating material.

As ammonia precursor substances, according to the present invention, chemical substances are understood which can be converted into a solution and which can release ammonia by means of physical and/or chemical processes or in other forms. According to the present invention, ammonia precursor compounds can particularly be urea, urea derivatives, guanidines, biguanidines as well as salts of these compounds and salts of ammonia. In particular, according to the present invention, urea and guanidines or their salts can be used. In particular, such salts can be used which are formed from guanidines and organic or inorganic acids. Particularly preferred are guanidinium salts of the general formula (I), where R = H, NH2 or C1-C12-alkyl, XΘ = acetate, carbonate, cyanate, formiate, hydroxide, methylate or oxalate.

Guanidinium formate is particularly preferred.

In the context of the present invention, these guanidinium salts can be used as a single substance or as a mixture of two or more different guanidinium salts. According to a preferred embodiment, the guanidinium salts used according to the invention are combined with urea and/or ammonia and/or ammonium salts. Alternatively, aqueous urea solutions can also be used according to another embodiment of the present invention. The mixing ratios of guanidinium salts with urea as well as with ammonia or ammonium salts can be varied within wide limits. However, it has proven particularly advantageous if the mixture of guanidinium salt and urea contains a guanidinium salt content of 5 to 60 wt.%, and a urea content of 5 to 40 wt.%, preferably 5 to 35 wt.%. Furthermore, mixtures of guanidinium salts with ammonia or ammonium salts having a guanidinium salt content of 5 to 60 wt.% and an ammonia or ammonium salt content of 5 to 40 wt.% are considered preferable. Alternatively, however, an urea solution, in particular an aqueous urea solution, can also be used.

As ammonium salts, compounds of the general formula (II) have proven particularly effective: R-NH3⊕ XΘ (II), where R = H, NH2 or C1-C12-alkyl, and XΘ = acetate, carbonate, cyanate, formiate, hydroxide, methylate or oxalate.

The ammonia precursor substances used according to the invention, in particular guanidinium salts as well as, if applicable, further components consisting of urea or ammonium salts, are used in the form of a solution, wherein water and/or a C1-C4-alcohol are preferably used as solvents. The aqueous and/or alcoholic solutions preferably have a solid content of 5 to 85 weight-%, particularly 30 to 80 weight-%.

It has surprisingly turned out that, according to the present invention, both aqueous guanidinium formiate solutions at a concentration of 20 to 60 weight-%, as well as aqueous urea solutions at a concentration of 25 to 40 weight-%, and also aqueous mixtures of guanidinium formiate and urea solutions, wherein the mixture contains guanidinium formiate at a concentration of 5 to 60 weight-% and urea at a concentration of 5 to 40 weight-%, can be particularly effectively used.

The aqueous solutions of the ammonia precursor substances, particularly the guanidinium salts, mixtures of guanidinium salts, or guanidinium salts in combination with urea in water have a preferred ammonia formation potential of 0.2 to 0.5 kg of ammonia per liter of solution, particularly 0.25 to 0.35 kg of ammonia per liter of solution.

According to another aspect, a method for producing ammonia from a solution of an ammonia precursor substance using an ammonia gas generator, particularly a method for continuous production of ammonia, is also preferred by means of the ammonia gas generator described herein. The subject of the present invention is this method. This ammonia gas generator comprises a catalyst unit, which itself includes a catalyst for decomposing and/or hydrolyzing the ammonia precursor substance into ammonia, and a mixing chamber located upstream of the catalyst in the flow direction, wherein the catalyst has a catalyst volume VKat and the mixing chamber has a mixing chamber volume VMisch. Furthermore, the ammonia gas generator includes an injection device for introducing the solution of the ammonia precursor substance into the mixing chamber and an outlet for the generated ammonia gas. The essential feature of the invention is that in the new method, the solution of the ammonia precursor substance is introduced separately from a carrier gas into the mixing chamber, and the carrier gas is introduced tangentially relative to the solution of the ammonia precursor substance. The ammonia gas generator is characterized in that the catalyst unit comprises at least a two-part hydrolysis catalyst, wherein the first part in the flow direction is designed as a heated catalyst, while the second part is designed as an unheated catalyst, or that two hydrolysis catalysts are arranged in series, wherein the first hydrolysis catalyst is a heated catalyst and the second hydrolysis catalyst is an unheated catalyst.

By separately introducing the solution of the ammonia precursor substance from the carrier gas, it is possible to achieve a targeted dosing of the required amount of energy or heat flow for a trouble-free, continuous operation of the generator. It has been found that the process can be carried out without forming unwanted by-products by providing a sufficient amount of energy at an appropriate temperature level. A complete decomposition of the applicable ammonia precursor solutions into ammonia requires, given a certain amount or volumetric flow of the solution, a corresponding amount or volumetric flow of energy in the form of heat at a temperature level necessary for complete decomposition. The temperature level is determined by the hydrolysis catalyst used. The majority of the energy introduced into the process preferably comes from the carrier gas stream.

According to the invention, an ammonia gas generator is technically and economically operated particularly when the energy introduced is used for decomposing the ammonia precursor solution from the waste heat of the carrier gas. The amount of carrier gas does not automatically correlate with the dosage of the liquid solution, since the usable energy quantity of the carrier gas varies with temperature. A carrier gas stream at a slightly lower temperature level, which is associated with a slightly smaller temperature difference between inlet and outlet in the ammonia gas generator, can, for example, be compensated by a higher carrier gas mass flow and thus a higher heat input into the generator.

It has been found that the carrier gas can also be, for example, a portion of an exhaust gas or a different carrier gas such as, for example, a portion of the engine's compressed air, pre-conditioned to a corresponding temperature level by means of a heat exchanger. If a portion of the exhaust gas is used, it has proven particularly advantageous if this portion contains less than 5% of the total exhaust gas. According to a further development, it may also be provided that a portion of the exhaust gas is used as the transport gas, which contains at least 0.1% of the total exhaust gas and preferably less than 4%, and particularly preferably less than 2% of the total exhaust gas.

As a secondary stream of the exhaust gas, the percentage is considered which is diverted as a mass percentage from the main exhaust stream and then passed through the generator as a transport or carrier gas stream.

In principle, any gas can be used as a carrier gas flow according to the invention. Since the carrier gas flow should preferably have a temperature of 250 °C to 550 °C, a gas that is already heated is preferably used for good energy efficiency, such as intake air or part of the exhaust gas stream. However, it is also possible to heat any desired carrier gas to the required temperature.

According to another advantageous embodiment of the method, it has been found that a particularly high efficiency of the process can be achieved when the solution of the ammonia precursor substance is injected at a pressure of at least 0.5 bar and the atomizing air is injected at a pressure of 0.5 to 2 bar.

According to a further preferred embodiment, it may particularly be provided that the solution is sprayed from the storage container into the mixing chamber by means of a pump and a nozzle with a theoretical spray angle α of 10 to 40 degrees.

It is particularly advantageous if the solution of the ammonia precursor substance is applied very finely distributed over the catalyst face. Therefore, a method for producing ammonia is also an object of the invention, in which the solution of the ammonia precursor substance is applied to the catalyst face in the form of droplets having a D32 droplet diameter of less than 25 µm. According to the present invention, it is further preferred that the nozzle produces droplets with a d32 droplet diameter of less than 20 µm, and particularly preferably of less than 15 µm. At the same time or independently thereof, it is further preferred that the nozzle produces droplets with a d32 droplet diameter of greater than 0.1 µm and in particular greater than 1 µm. By using such nozzles, an ammonia formation degree of more than 95% (see above) can be achieved. Furthermore, a particularly uniform distribution of the solution over the catalyst face can be achieved.

Furthermore, it has proven advantageous if the solution of the ammonia precursor substance is sprayed perpendicularly to the catalyst surface into the mixing chamber. Regardless of or simultaneously with this, the volume ratio of carrier gas to atomizing air can range from 7:1 to 10:1.

It has further been found to be crucial for the smooth and thus deposit-free operation of an ammonia gas generator that a specific amount of solution is preferably evenly distributed over a given catalyst front area within a defined period of time (= mass flow, dosing quantity). The impact and initial contact with the front part of the catalyst unit (=catalyst front area) significantly determine the complete decomposition of the ammonia precursor.

It has continued to be shown that the ratio of dosing amount to catalyst frontal area preferably lies within a range of 0.17 to 15 g/(h·cm²), especially between 0.2 and 15 g/(h·cm²), so that no excessive cooling occurs at the catalyst frontal surface and a too low conversion to ammonia takes place. The frontal surface load is defined as the quotient of the mass flow rate of the ammonia precursor solution arriving at the catalyst frontal surface within one hour, and the catalyst frontal area wetted by the spray cone.

Thus, according to another aspect, a method is also the subject of the present invention, wherein the solution of the ammonia precursor substance is introduced into the catalyst unit such that the face load of the catalyst ranges from 0.17 to 15 g/(h·cm²), particularly from 0.2 to 15 g/(h·cm²), preferably from 0.2 to 12 g/(h·cm²). Particularly preferred is a method in which the face load is at least 0.4 g/(h·cm²), at least 1.0 g/(h·cm²), particularly at least 2.0 g/(h·cm²), particularly at least 3.0 g/(h·cm²), and very particularly preferably at least 4.0 g/(h·cm²). At the same time or independently thereof, the face load can particularly be at most 12.0 g/(h·cm²), particularly at most 10.0 g/(h·cm²), particularly at most 9.0 g/(h·cm²), and very particularly preferably at most 8.0 g/(h·cm²).

It has been found that if an excessive mass flow of ammonia precursor solution were to strike the hot catalyst surface, it would cause excessive local cooling due to heating and evaporation of the liquid. This would prevent complete conversion. Measurements have shown that with excessively high dosing quantities on the catalyst surface, and thus an excessive surface load, a cooling of more than 100 K occurs at the wetted surface. As a result, the temperature level required for complete decomposition on the catalyst surface is exceeded, leading to spontaneous further reactions toward unwanted by-products.

If the catalyst frontal area is too large, resulting in a low frontal load, the ammonia gas generator becomes uneconomical because it would be operated with a catalyst that is too large for the application.

From further extensive investigations, it has been found that, in addition to a defined amount of ammonia precursor solution per catalyst frontal area, a corresponding amount of energy related to the quantity of the ammonia precursor solution is also necessary. Surprisingly, it has turned out that the total energy required for the complete, residue-free conversion of the ammonia precursor solution into ammonia is essentially independent of the ammonia precursor solution used. Only the mass flow rate of the solution of the ammonia precursor substance correlates with a specific energy flow in the form of an enthalpy flow (essentially a heat flow). It has been shown that for the endothermic process of complete conversion of the ammonia precursor solution into ammonia, a defined amount of energy must be available. Furthermore, it has been found that the temperature level at which this decomposition occurs does not need to be considered. It has been demonstrated that the required temperature level essentially depends on the hydrolysis catalysts used, which can lower the necessary decomposition temperatures without changing the total energy required for decomposition.

Studies have shown that the supplied heat flow can be taken both from a hot gas stream, for example, hot exhaust gas from an internal combustion engine used as a transport gas, and can also be introduced into the ammonia gas generator through additional active heating (electrically, heat exchanger, heat pipe or other heat transfer devices using conduction or radiation).

According to the invention, a preferred specific enthalpy flow is in the range of 8,000 to 50,000 kJ/kg. Here, the specific enthalpy flow is defined as the quotient of the enthalpy flow introduced into the ammonia gas generator and the dosing mass flow of the ammonia precursor solution fed to the catalyst unit per unit time. The necessary energy is mainly introduced into the generator in the form of heat.

For a too high mass flow rate of the dosing stream with a given enthalpy flow, the specific enthalpy flow is undershot according to this invention, since not enough energy is supplied for the endothermic reaction. This leads to an insufficient conversion of the ammonia vapor and thus to deposits or the formation of unwanted by-products, which make continuous generator operation impossible. Similarly, it has been shown that a too high specific enthalpy flow leads to an unnecessary load on the ammonia gas generator, thereby causing an uneconomical operation or excessive stress on the components used.

Thus, a method is also the subject of the present invention, in which the solution of the ammonia precursor substance and a carrier gas are introduced into the mixing chamber, wherein the carrier gas and, if necessary, an additional energy source together provide a specific enthalpy flow of HTG/mPrecursor of 8,000 to 50,000 kJ/kg (enthalpy flow based on the mass flow rate of the introduced solution). Particularly preferred is a method in which the specific enthalpy flow is at least 10,000 kJ/kg, particularly preferably at least 12,000 kJ/kg, and very particularly preferably at least 15,000 kJ/kg. At the same time or independently thereof, it may be provided that the specific enthalpy flow is at most 45,000 kJ/kg, particularly preferably at most 40,000 kJ/kg, and very particularly preferably at most 35,000 kJ/kg.

Other parameters that are preferably maintained during the operation of the inventive ammonia gas generator are as follows: The dosing mass flow of the solution of the ammonia precursor substance is preferably between 50 g/h and 280 g/h, particularly between 100 g/h and 200 g/h. The mass flow of the carrier gas is preferably 1 to 10 kg/h, particularly 3 to 7 kg/h. The mass flow of the atomizing air is preferably 0.14 to 1.43 kg/h, particularly 0.5 to 1 kg/h. The additional heating energy amount is preferably between 0 and 150 W, particularly between 50 and 100 W. The catalyst front surface temperature is preferably set to 280 to 500 °C, particularly to 300 to 400 °C. The catalyst outlet temperature is preferably set to 250 to 450 °C, particularly to 280 to 380 °C. The catalyst space velocity is preferably 5,000 to 30,000 1/h, particularly 10,000 to 20,000 1/h. The dosing pressure of the liquid of the ammonia precursor substance is preferably 1 to 8 bar, particularly 1.5 to 3 bar. The catalyst front surface load per hour is preferably 0.53 to 3.45 g/(h x cm²), particularly 1 to 2 g/(h x cm²). The specific enthalpy flow is preferably 8,000 to 25,000 kJ/kg, particularly 10,000 to 20,000 kJ/kg.

The ammonia gas generators described herein are particularly suitable for use in industrial plants, in combustion engines such as diesel and gasoline engines, as well as in gas engines, due to their compact design. Therefore, the present invention also includes the use of an ammonia gas generator according to the described type, as well as the use of the described method for reducing nitrogen oxides in exhaust gases from industrial plants, from combustion engines such as diesel and gasoline engines, as well as from gas engines. In the following, the present invention will be explained in more detail with reference to drawings and corresponding examples. Herein: Figure 1: a schematic view of a first ammonia gas generator in axial cross-section Figure 2: a schematic representation of an exhaust system in a vehicle Figure 3: a radial cross-section of the mixing chamber (top view) in the area of the tangential carrier gas flow inlet Figure 4: a diagram 1: conversion of the ammonia precursor solution into ammonia depending on the frontal load Figure 5: a diagram 2: conversion of the ammonia precursor solution into ammonia depending on the specific enthalpy flow.

In Figure 1, a first ammonia gas generator (100) according to the present invention is shown. The generator (100) is in the form of a cylinder and comprises an injection device (40), a catalyst unit (70), and an outlet (80) for the generated ammonia gas. The catalyst unit (70) consists of a multi-part hydrolysis catalyst (60), a mixing chamber (51), and an exit chamber (55). In the operating state, the ammonia precursor solution (B) from a storage tank (20) is injected via a metering pump (30) together with an atomizing air stream (A) through a two-component nozzle (41) with nozzle opening (42) into the mixing chamber (51) of the ammonia gas generator (100) at a defined spray angle and distributed into fine droplets. Additionally, a hot transport gas stream (C) is introduced tangentially through the inlet (56) into the mixing chamber (51).resulting in a vortex flow of droplets that is guided axially toward the hydrolysis catalyst (60) and onto the hydrolysis catalyst face (61). The catalyst (60) is designed such that the first segment (62) represents an electrically heatable metal carrier with a hydrolysis coating. Next follows an unheated metal carrier catalyst (63), also with a hydrolysis coating, and an unheated catalyst (64) with a hydrolysis coating designed as a mixing structure for better radial distribution. The generated ammonia gas (D) leaves the generator (100) together with the hot carrier gas stream through the outlet chamber (55) with the outlet (80) and valve (81). The generator can be additionally heated by a jacket heating (52) around the housing (54) of the catalyst unit.Except for the head area where the injection device (40) is located, the ammonia gas generator (100) is surrounded by a heat insulation (53) made of microporous insulating material.

In Figure 2, a schematic material flow of exhaust gas aftertreatment for an internal combustion engine (10) is shown. The exhaust gas coming from the internal combustion engine (10) is guided through a charge air unit (11) and compressed in counter-current with intake air (E) for the internal combustion engine. The exhaust gas (F) is guided through an oxidation catalyst (12) to achieve a higher NO2 concentration relative to NO. The ammonia-containing gas stream (D) coming from the ammonia gas generator (100) can be introduced and mixed both before and after a particulate filter (13). In this case, an additional gas mixer (14) in the form of a static mixer or, for example, a Venturi mixer can be used. At the SCR catalyst (15), the NOx are reduced using the reducing agent NH3 at an SCR catalyst (SCR = selective catalytic reduction). In this context, the ammonia gas generator can be operated with separate carrier gas or also with a portion of the exhaust gas.

Figure 3 shows a detailed view of the mixing chamber (51) in the area of the tangential carrier gas inlet. The housing (54) of the catalytic unit is surrounded by a heat insulation (53) made of microporous insulating material in the region of the mixing chamber (51). The tangential introduction of the carrier gas (C) occurs in the head section of the ammonia gas generator or in the head section of the mixing chamber (51), at the height of the nozzle opening (42) of the nozzle (41). The inlet (56) for the carrier gas stream (C) is designed such that the carrier gas stream is introduced as flat as possible along the wall (54) of the mixing chamber, thereby creating a downward-directed vortex flow in the generator towards the catalyst, thus establishing a tangential carrier gas stream within the catalytic unit.

Embodiment 1:

The design basically corresponds to the ammonia gas generator shown in Figure 1. The ammonia gas generator is designed for a dosing rate of 10–100 g/h NH3 and is implemented as a cylindrical tubular reactor. In the head section, a two-component nozzle from Schlick, model 970 (0.3 mm), with a variable air cap, coated with amorphous silicon, is centrally arranged. The ammonia precursor substance is metered through this nozzle at room temperature and atomized into a full cone. The spray angle α is 30°. The distance between the nozzle opening and the catalyst face is 100 mm, and the spray head cone diameter is 54 mm.

The liquid is carried along and atomized by a compressed air stream (0.5–2 bar) at a rate of about 0.8 kg/h. The Sauter diameter of the droplets formed below the nozzle is less than 25 µm. There is a uniform radial distribution of the solution of the ammonia precursor substance across the reactor cross-section in the hot transport gas stream before the hydrolysis catalyst in a mixing chamber, without the solution coming into contact with the reactor wall, which could lead to deposits. In the mixing chamber, there is already some evaporation of the droplets such that the droplet size is reduced by up to 20% upon impact on the catalyst front surface. Due to the remaining droplets, the catalyst front surface is cooled by approximately 120–150 °C. For this reason, the reactor is designed in such a way that the heat supplied via the hot transport gas stream, the integrated heated hydrolysis catalyst, and additional energy inputs provide enough energy to ensure that the temperature of the dosed amount of solution does not drop below approximately 300 °C. The dosing rate of 50–280 g/h is controlled using a Bosch PWM valve. The pressure for pumping the liquid is generated by overpressure from a compressed air line in a pre-storage container, so no additional pumping device is required.

A hot carrier gas stream (transport gas stream) of approximately 1–5 kg/h is introduced tangentially into the head section of the ammonia gas generator in such a way that it forms a curtain flow around the reactor wall and is guided spirally through the mixing chamber. This prevents sprayed droplets from coming into contact with the reactor wall. The diameter of the mixing chamber in the head section of the reactor is 70 mm. The length of the mixing chamber is 110 mm. The mixing chamber is additionally heated from the outside by an electric resistance heating jacket (maximum heating time 1 minute) - Model Hewit 0.8 - 1 kW, 150 - 200 mm. Temperature control is carried out in conjunction with temperature sensors (Type K) arranged on the catalyst front surface, as well as inside and after the catalyst. All outer surfaces of the reactor are surrounded by an insulation layer of Microtherm superG. The Microtherm superG granulate is embedded between glass fiber fabric wrapped around the reactor. Only the head section, where the solution injection takes place, is not insulated for better heat dissipation. The surfaces in the mixing chamber are coated with a catalytically active TiO2 washcoat (anatase structure).

Following the mixing chamber, a heated metal carrier catalyst with a diameter of 55 mm and 400 cpsi is mounted (Emitec Emicat, maximum power 1.5 kW, volume approx. 170 ml). This catalyst is designed as a hydrolysis catalyst, also coated with catalytically active TiO2 (anatase, washcoat approx. 100 g/l, company Interkat/Südchemie), and is controlled so that the temperature on the catalyst front surface remains between 300 and 400 °C. Only as much energy is supplied as is required to compensate for the cooling caused by evaporation of the droplets. To achieve a space velocity of up to a minimum of 7,000 1/h, another hydrolysis catalyst with 400 cpsi is connected downstream, resulting in a total catalyst volume of approximately 330 ml.

The ammonia produced by the hot hydrolysis catalyst flows freely through the outlet chamber at the foot area, centrally exiting from an outlet opening of the reactor end section. Preferably, the outlet area is conically shaped to avoid vortices at edges and thus prevent deposits of possible residues. The gas mixture from the ammonia gas generator is preferably introduced into the engine exhaust stream before the SCR catalyst at a temperature above 80 °C to prevent ammonium carbonate deposits, and is evenly distributed within this exhaust stream via a static mixer.

As material for all metallic components, 1.4301 (V2A, DIN X 5 CrNi18-10) or alternatively 1.4401 (V4A, DIN X 2 CrNiMo 17-12-2), 1.4767 or other typical exhaust catalyst Fe-Cr-Al alloys are used.

This generator was operated both with a 60% guanidinium nitrate solution and with a 32.5% aqueous urea solution, as well as with mixtures of both. The results of these ammonia precursor solutions are approximately identical (+/- 1%).

The following are the operational parameters that should be maintained when operating the ammonia gas generator. Tabelle 1: Übersicht über die weiteren Betriebsparameter

Dosiermassenstrom der Lösung der Ammoniakvorläufersubstanz pro Stunde		[g/ h]	50	150	280
Massenstrom Trägergas		[kg/ h]	1	5	10
Massenstrom Zerstäubungsluft		[kg/ h]	0,14	0,71	1,43
Zuheizenergiemenge		[J/ S] = [W]	0	70	150
Katalysatorstirnflächentemperatur		[°C]	280	350	500
Katalysatorauslasstemperatur		[°C]	250	320	450
Katalysatorraumgeschwindigkeit	RG	[1/ h]	5.000	15.000	30.000
Dosierdruck der Flüssigkeit		[bar]	1	2	8
Katalysatorstirnflächenbelastung pro Stunde			0,53	1,59	3,45
spezifischer Enthalpiestrom		[kJ/ kg]	8.000	16.000	25.000

By introducing the gas stream at a tangent relative to the solution injected into the mixing chamber, and by separately introducing the solution and the carrier gas, it was prevented that any deposits formed on the catalyst front surface or on the mixing chamber wall even over a period of more than 100 hours. Thus, the generator and the method can be classified as low-maintenance.

The following describes the influence of frontal surface load and specific enthalpy flow on the continuous production of ammonia, using the ammonia gas generator employed in Example 1. This generator was operated both with a 60% guanidinium formate solution, a 32.5% aqueous urea solution, and also with mixtures of both. The results of these ammonia precursor solutions are approximately identical (+/-1%). The formation of ammonia as a function of frontal surface load is shown in Figure 4. Tabelle 2: Verfahren in Abhängigkeit von der Stirnflächenbelastung

Abstand Düsenöffnung bis Katalysatorstirnfläche [mm]	100	100	100	100	100
Spraykegeldurchmesser [mm]	54	54	54	54	54
Dosiermassenstrom der Lösung der Ammoniakvorläufersubstanz pro Stunde [g/ h]	50	160	280	4	400
	2,1	7,0	12,0	0,17	17,5
spezifischer Enthalpiestrom	8000	12000	16000	16000	16000
Ammoniakbildungsgrad AG [%]	≥95%	≥95%	≥95%	≥95%	< 90%
Ablagerungen an Katalysatorstirnfläche	keine	keine	keine	keine	ja
Ablagerungen an der Mischwandkammer	keine	keine	keine	keine	keine

By setting the catalyst frontal load to at least 0.17 g/(h·cm²) (see V4), a process can be provided in which no deposits are formed over a period of more than 100 hours. Even if the frontal load is 2.1 g/(h·cm²) or 7.0 g/(h·cm²) or 12.0 g/(h·cm²) over a period of more than 100 hours, no deposits are observed, thereby ensuring a continuous process. If the frontal load is set to a value of 17.5 g/(h·cm²) (see V5), deposits can be observed on the catalyst frontal surface. Therefore, a continuous process is no longer possible. Tabelle 3: Verfahren in Abhängigkeit vom spezifischen Enthalpiestrom

Abstand Düsenöffnung bis Katalvsatorstirnfläche [mm]	100	100	100	100	100
Spraykegeldurchmesser [mm]	54	54	54	54	54
Dosiermassenstrom der Lösung der Ammoniakvorläufersubstanz pro Stunde [g/ hl	160	160	160	160	160
	7,0	7,0	7,0	7,0	7,0
spezifischer Enthalpiestrom [kJ/ kg]	8.000	12.000	16.000	2.000	20.000
Ammoniakbildungsgrad AG [%]	≥95%	≥95%	≥95%	< 90%	≥0:95%
Ablagerungen an Katalysatorstirnfläche	keine	keine	keine	ja	keine
Ablagerungen an der Mischwandkammer	keine	keine	keine	ja	keine

By setting the specific enthalpy to at least 8,000 kJ/kg (see V1, V2, V3, and V5), a process can be provided in which no deposits are formed over a period of more than 100 hours, thereby enabling a continuous process. When the specific enthalpy is set to 2,000 kJ/kg (see V4), deposits can be observed on the mixing chamber wall and the catalyst front surface. The formation of ammonia as a function of the specific enthalpy flow is shown in Figure 5.

Embodiment Example 2:

In Embodiment 2, the reactor is designed such that the reactor is partially heated by counter-current heat exchange with the supplied hot transport gas stream. In this process, the transport gas stream is first guided through a double jacket below the reactor head, against the direction of flow inside the double jacket, along the reactor wall and flows around it on its way to the reactor head. At the reactor head, the main flow enters the reactor interior through several bores or alternatively through an annular gap in the area of the nozzle at the reactor head. Additionally, an electrical resistance heater may be located in the double jacket.

Embodiment 3:

In Embodiment 3, the reactor is designed such that the heating of the reactor does not take place from the outside via an electrical resistance heater, but rather through heat exchange with hot components of an internal combustion engine, separate burners for exhaust gas heating, or hot gas streams. In this case, heat can also be transported over a certain distance to the reactor via heat pipes.

Embodiment 4:

In Embodiment 4, the reactor is designed such that no external heating of the reactor takes place, but rather heat is supplied directly inside the reactor via an electrically heated catalyst Emicat from company Emitec. Alternatively, heat can be generated in the reactor using glow plugs of model Champion (60W, 11V).

Embodiment Example 5:

With preheating of the liquid solution of the ammonia precursor substance - when using an injector with critical superheating (flash evaporator).

Claims (18)

1. Ammonia gas generator (100) for producing ammonia from a solution of an ammonia precursor substance, comprising:

   - a catalyst unit (70) which comprises a catalyst (60) for decomposing and/or hydrolysing ammonia precursor substances into ammonia, and a mixing chamber (51) upstream of the catalyst (60) in the flow direction, the catalyst (60) having a catalyst volume V_Kat and the mixing chamber (51) having a mixing chamber volume V_Misch,

   - an injection device (40) for introducing the solution of the ammonia precursor substance into the mixing chamber (51), and

   - an outlet (80) for the ammonia gas formed,

   the ammonia gas generator comprising an inlet (56) for a carrier gas which produces a tangential carrier gas stream with respect to the solution injected into the mixing chamber (51), characterised in that, the catalyst unit (70) comprises a hydrolysis catalyst which is divided into at least two parts, the first part of which in the flow direction is in the form of a heated catalyst, while the second part is in the form of an unheated catalyst, or in that two hydrolysis catalysts are arranged one behind the other, the first hydrolysis catalyst being a heated catalyst and the second hydrolysis catalyst being an unheated catalyst.
2. Ammonia gas generator (100) according to at least one of the preceding claims, characterised in that the catalyst unit (70) is cylindrical.
3. Ammonia gas generator according to at least one of the preceding claims, characterised in that the ratio of the diameter D of the catalyst (60) to the length L of the catalyst is 1:3.
4. Ammonia gas generator according to claim 1, characterised in that the injection device (40) comprises a nozzle (41) which has a first number of nozzle openings for introducing the solution into the catalyst unit (70), which first number of nozzle openings is annularly surrounded by a second number of nozzle openings for introducing atomising air into the catalyst unit (70).
5. Ammonia gas generators according to at least one of the preceding claims, characterised in that the catalyst (60) is a hydrolysis catalyst (62, 63) having a catalyst cell count of at least 100 cpsi to at most 400 cpsi catalyst cells for each end face of the hydrolysis catalyst.
6. Ammonia gas generator (100) according to at least one of the preceding claims, characterised in that the catalyst (60) has a catalytically active coating which is impregnated with gold and/or palladium.
7. Ammonia gas generator according to at least one of the preceding claims, characterised in that the catalyst unit (70) comprises a hydrolysis catalyst that is divided into at least two parts, the first part of which in the flow direction is in the form of a heated catalyst that has direct electrical resistance heating and jacket heating, while the second part is in the form of an unheated catalyst which is followed downstream by an unheated catalyst that has a mixer structure.
8. Ammonia gas generators (100) according to at least one of the preceding claims, characterised in that the catalyst unit (70) comprises at least one thermal insulation layer (53) made of microporous insulating material.
9. Method for producing ammonia from a solution of an ammonia precursor substance by means of an ammonia gas generator (100), comprising

   - a catalyst unit (70) which comprises a catalyst (60) for decomposing and/or hydrolysing ammonia precursor substances into ammonia, and a mixing chamber (51) upstream of the catalyst (60) in the flow direction, the catalyst having a catalyst volume V_Kat and the mixing chamber having a mixing chamber volume V_Misch,

   - an injection device (40) for introducing the solution of the ammonia precursor substance into the mixing chamber (51),

   - an outlet (80) for the ammonia gas formed,

   the solution of the ammonia precursor substance being introduced into the mixing chamber (51) separately from a carrier gas and the carrier gas being introduced tangentially to the solution of the ammonia precursor substance, characterised in that the catalyst unit (70) comprises a hydrolysis catalyst that is divided into at least two parts, the first part of which in the flow direction is in the form of a heated catalyst, while the second part is in the form of an unheated catalyst, or in that two hydrolysis catalysts are arranged one behind the other, the first hydrolysis catalyst being a heated catalyst and the second hydrolysis catalyst being an unheated catalyst.
10. Method according to at least one of the preceding claims, characterised in that a partial flow of an exhaust gas is used as the carrier gas, which partial flow contains less than 5 % of the entire exhaust gas.
11. Method according to at least one of the preceding claims, characterised in that the solution is sprayed from the storage container (20) into the mixing chamber (51) by means of a pump (30) at a spray angle of from 10 to 40°.
12. Method according to at least one of the preceding claims, characterised in that the solution is applied to the end face (61) of the catalyst (60) in the form of droplets having a droplet diameter D₃₂ of less than 20 µm.
13. Method according to at least one of the preceding claims, characterised in that the ratio of carrier gas to atomising air is from 7:1 to 10:1.
14. Method according to at least one of the preceding claims, characterised in that the solution is injected at a pressure of at least 0.5 bar and the atomising air is injected at a pressure of from 0.5 to 2 bar.
15. Method according to at least one of the preceding claims, characterised in that the solution is introduced into the catalyst unit (70) such that the end face load of the catalyst is from 0.2 to 12 g/(h*cm²).
16. Method according to at least one of the preceding claims, characterised in that the solution is injected into the mixing chamber (51) perpendicularly to the catalyst surface.
17. Method according to at least one of the preceding claims, characterised in that the solution is introduced into the mixing chamber (51) together with a carrier gas, the carrier gas and optionally an additional energy source having in total a specific enthalpy flow of H_TG/M_Precursor of 8,000 - 50,000 kJ/kg (enthalpy flow based on the mass flow of solution introduced).
18. Use of an ammonia gas generator (100) according to at least one of the preceding claims or of a method according to at least one of the preceding claims for reducing nitrogen oxides in exhaust gases from industrial plants, internal combustion engines, gas engines, diesel engines or petrol engines.