DE102012025112A1 - Generator, used to produce ammonia gas from precursor solution, includes catalyst unit for decomposition and/or hydrolysis of precursors, and device that is adapted to inject solution into mixing chamber and has droplet forming nozzle - Google Patents

Abstract

The ammonia gas generator comprises a catalyst unit for decomposition and/or hydrolysis of ammonia precursors in a solution, a device (30) for injecting the solution of the ammonia precursor into a mixing chamber (51), and an outlet for the generate ammonia gas. The hydrolysis catalyst (60) is present in upstream of the mixing chamber, and has a volume. The mixing chamber has a volume. The injection device comprises a single-fluid nozzle or an injector for forming drops having a Sauter mean diameter of 26-49 mu m. The solution is supplied with compressed air. The ammonia gas generator comprises a catalyst unit for decomposition and/or hydrolysis of ammonia precursors in a solution, a device (30) for injecting the solution of the ammonia precursor into a mixing chamber (51), and an outlet for the generate ammonia gas. The hydrolysis catalyst (60) is present in upstream of the mixing chamber, and has a volume. The mixing chamber has a volume. The injection device comprises a single-fluid nozzle or an injector for forming drops having a Sauter mean diameter of 26-49 mu m. The solution is supplied with compressed air. The nozzle includes a first number of nozzle holes for introducing the solution into the mixing chamber, and a second number of annular nozzle holes for the introduction of compressed air into the mixing chamber. The first nozzle holes are surrounded by the surrounded by the second nozzle holes. A ratio of the volume of the mixing chamber to the volume of catalyst is 1:1-5:1. The generator further comprises an inlet for generating a flow of a carrier gas directed perpendicular to a catalytic face, a perforated disc arranged in a center of an opening of the nozzle, and a metering unit for dosage of the solution of the ammonia precursor. The metering unit is connected in upstream of the injection device. The perforated disc includes openings for in generating partial flows of carrier gas in parallel with the flow direction of the catalyst unit.

Description

The present invention relates to an ammonia gas generator for producing ammonia from an ammonia precursor substance, a process for the production of ammonia and its use in exhaust aftertreatment systems for the reduction of nitrogen oxides in exhaust gases.

Exhaust gases from internal combustion engines often contain substances whose emission into the environment is undesirable. Therefore, emission limits for such pollutants are defined in many countries, such as in the exhaust of industrial plants or automobiles, which must be adhered to. In addition to a number of other pollutants, these pollutants also include nitrogen oxides (NO _x ), in particular nitrogen monoxide (NO) or nitrogen dioxide (NO ₂ ).

The reduction of the emission of these nitrogen oxides from exhaust gases of internal combustion engines can be done in various ways. It should be emphasized at this point, the reduction by additional exhaust aftertreatment measures, which rely in particular on the selective catalytic reduction (selective catalytic reduction - SCR). These methods have in common that a selectively acting on the nitrogen oxides reducing agent is added to the exhaust gas, followed by a conversion of the nitrogen oxides in the presence of a corresponding catalyst (SCR catalyst). Here, the nitrogen oxides are converted into less polluting substances, such as nitrogen and water.

A reducing agent for nitrogen oxides already in use today is urea (H ₂ N-CO-NH ₂ ), which is added to the exhaust gas in the form of an aqueous urea solution. In this case, the urea in the exhaust stream in ammonia (NH ₃ ) decompose, for example by the action of heat (thermolysis) and / or by reaction with water (hydrolysis). The ammonia thus formed represents the actual reducing agent for nitrogen oxides.

The development of exhaust aftertreatment systems for automobiles has been going on for quite some time and has been the subject of numerous publications. For example, with the European patent specification EP 487 886 61 describes a method for selective catalytic NO _x reduction in oxygen-containing exhaust gases of diesel engines, are used in the urea and its Thermolyseprodukte as a reducing agent. In addition, a device for producing ammonia in the form of a tubular evaporator is described which comprises a spray device, an evaporator tube evaporator and a hydrolysis catalyst.

Furthermore, with the European patent specification EP 1 052 009 B1 a method and an apparatus for carrying out the method for the thermal hydrolysis and metering of urea or urea solutions in a reactor with the aid of a partial exhaust gas flow described. In the method, a partial stream of the exhaust gas is taken from an exhaust line upstream of the SCR catalyst and passed through the reactor, wherein the after hydrolysis in the reactor loaded with ammonia partial stream is also recycled back into the exhaust line upstream of the SCR catalyst.

In addition, with the European patent specification EP 1 338 562 B1 describes an apparatus and method which utilizes the catalytic reduction of nitrogen oxides by ammonia. The ammonia is here obtained under conditions of Blitzthermolyse from urea in solid form and the hydrolysis of isocyanic acid and fed to the exhaust stream of a vehicle.

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

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

In addition, will continue with the international application WO 2008/077 587 A1 and WO 2008/077588 A1 describe a process for the selective catalytic reduction of nitrogen oxides in exhaust gases of vehicles by means of aqueous guanidinium salt solutions. In these processes, a reactor is used which produces ammonia from the aqueous guanidinium salt solutions.

Recent developments in the field of exhaust aftertreatment are the subject of current work. For example, the concept of ammonia production outside a flue has also been pursued recently. Versions of generators and methods for operating these generators are the international applications PCT / EP2012 / 062757 . PCT / EP2012 / 062750 and PCT / EP2012 / 062752 (not published).

Although ammonia gas generators have been known for some time, it is still up Today, no realization of the technology has been realized in a vehicle or other use. To date, the concept of direct injection of an ammonia precursor substance has been pursued in the exhaust gas stream of an internal combustion engine, wherein this ammonia precursor substance is decomposed by suitable measures in the exhaust gas line into the actual reducing agent. However, due to incomplete decomposition or side reactions of decomposition products in the exhaust gas line, deposits will again and again occur which lead to impairment of the catalysts and filters which continue to be present in the exhaust gas system.

It is therefore an object of the present invention to provide an ammonia gas generator and a process for producing ammonia which overcomes these disadvantages of the prior art. In particular, an ammonia gas generator is to be provided which can be arranged outside an exhaust line of an exhaust system of an internal combustion engine. It is a further object of the present invention to provide an ammonia gas generator which has a simple structure and a compact construction and provides a high conversion rate of ammonia precursors into ammonia gas and allows a long service life without maintenance. In addition, the ammonia gas generator should be universally applicable, and in particular also different types of ammonia precursor substances can be used.

This object is achieved by an ammonia gas generator according to claim 1 and a method according to claim 12.

Thus, an ammonia gas generator for generating ammonia from a solution of an ammonia precursor substance is the subject of the present invention comprising i) a catalyst unit comprising a catalyst for decomposing and / or hydrolysing ammonia precursors in ammonia and a mixing chamber upstream of the catalyst, wherein the catalyst is a catalyst volume V _Kat and the mixing chamber, a mixing chamber volume V _mixer having, ii) an injection device for introducing the solution of the ammonia precursor into the mixing chamber, and iii) an outlet for the formed ammonia gas, wherein the injection device comprises a nozzle, the droplets having a Sauter mean diameter D ₃₂ produced from 26 to 100 microns. In particular, the injection device may comprise a nozzle which generates drops with a Sauter diameter D ₃₂ of 26 to 49 microns.

It should be emphasized at this point that an ammonia gas generator according to the present invention is a separate unit for generating ammonia from ammonia precursor substances. Such a structural unit can be used for example for the reduction of nitrogen oxides in industrial exhaust gases or for the exhaust aftertreatment of exhaust gases from internal combustion engines, such as diesel engines. This ammonia gas generator can operate independently or operated with the aid of exhaust gas side streams, but in any case the ammonia formed is introduced into the exhaust gas flow and only in a subsequent process step, a reduction of nitrogen oxides by means of ammonia takes place. If an inventive ammonia gas generator is used as a separate component in an exhaust aftertreatment system of an internal combustion engine, such as a diesel engine, thus reducing the nitrogen oxides in the exhaust stream can be made without introducing additional catalysts for the separation of ammonia precursors or other components in the exhaust stream itself. The ammonia generated by the ammonia gas generator according to the invention can thus be introduced as needed into the exhaust gas stream. A possible shortening of the life of the SCR catalyst by impurities in the form of deposits of, for example, ammonia precursors or products of cleavage of ammonia precursors is also avoided.

Furthermore, in the context of the present invention, an injection device comprising a nozzle is to be understood as any device or nozzle which sprays, aerosolizes or otherwise forms drops of a solution, preferably aqueous solution, of an ammonia precursor substance, the solution of the ammonia precursor substance being in the form of is formed by drops having a droplet or droplet diameter D ₃₂ of 26 to 100 microns. The droplet or droplet diameter D ₃₂ refers in the context of the present invention to the Sauter diameter according to the German industrial standard DIN 66 141 ,

Furthermore, a catalyst unit according to the present invention should be understood to mean an assembly comprising a housing for accommodating a catalyst, a mixing chamber upstream of the catalyst and at least one catalyst for decomposition and / or hydrolysis of ammonia precursors in ammonia, wherein the catalyst Catalyst volume V _Kat and the mixing chamber has a mixing chamber volume V _mixed . The mixing chamber is limited in the flow direction limited by the catalyst end face on the one hand and on the other hand by the housing of the catalyst unit or, if present, by a perforated disc. Optionally, the catalyst unit may additionally comprise a catalyst in the flow direction downstream outlet chamber to the outlet of the ammonia gas formed.

It has been found that, despite optimal injection geometry (angle and distance), in particular in an application with carrier gas flow, a non-optimal uniform distribution of the droplets on the catalyst end wall is achieved. Completely surprisingly, this defect could be corrected by a nozzle is put to use, which produces droplets with a Sauter diameter D ₃₂ 26-100 microns. By the targeted selection of the nozzle or the targeted use of a nozzle that produces drops with a Sauter diameter of 26 to 100 microns, a distribution of the drops over almost the entire flow cross-section and thus a more uniform wetting of the catalyst end face. In turn, deposits are avoided and the turnover rate is improved. Without being bound by theory, it can be said that small size drops are too slow in inertia. As a result, the droplet-containing flow is applied only to a portion of the catalyst end face. Due to the greater inertia of the larger droplets, a droplet-containing flow is provided which wets almost 100% of the catalyst end face, especially in the presence of a carrier gas stream.

In addition, it was found that droplets with a Sauter diameter D ₃₂ of 26 to 100 microns can be deflected less by the carrier gas flow and it to a more homogeneous spray pattern, especially in the outer regions (range greater than 80% of the diameter) of the catalyst face and a more uniform overall Distribution on the catalyst comes. Keep droplets having a Sauter mean diameter D ₃₂ 26-100 microns by its inertia to the original predetermined by the nozzle spray angle until impact with the catalyst face at and it comes, therefore, to a more uniform wetting of the catalyst end surface corresponding to the given spray angle of the nozzle.

At the same time could be realized by the use of a nozzle, the drops with a Sauter diameter D ₃₂ of 26 to 100 microns, a simple construction and a compact design of the ammonia gas generator itself be realized.

Furthermore, completely unforeseeable, it could be stated that the necessary energy requirement or pressure for atomizing droplets with a larger Sauter diameter (corresponding to formation of less new liquid surface) is significantly lower. This results in a particularly simple construction of the generator.

According to a preferred embodiment of the present invention, the injection device may comprise a nozzle, the droplets having a Sauter mean diameter D ₃₂ of 26 microns, especially at least 30 microns and more preferably generates at least 32 microns, said droplets at the same time or independently thereof, a Sauter mean diameter of at most 100 microns , in particular at most 90 .mu.m, particularly preferably at most 80 .mu.m, particularly preferably at most 70 .mu.m, very particularly preferably at most 60 .mu.m and most preferably at most 49 .mu.m.

It is thus provided according to the present invention that the injection device in turn comprises a nozzle which generates droplets with a droplet diameter D ₃₂ in a defined region. By using such nozzles, an ammonia degree AG of> 97% can be achieved. In addition, a particularly uniform distribution of the solution can take place on the catalyst end face. The degree of ammonia AG is here and hereinafter defined as the molar amount NH ₃ produced in the ammonia gas generator, based on the molar amount of ammonia theoretically to be produced upon complete hydrolysis of the ammonia precursor substance. An ammonia formation level of> 97% is considered as a complete reaction according to the present invention.

It is inventively avoided by a uniform droplet load exceeding the maximum end load in individual ring areas. This is the only way to ensure that an approximately complete conversion takes place over the entire catalyst end face and that there is no longer any prolonged use of annular deposits on the catalyst end face. Thus, additional maintenance can be avoided.

According to a particularly preferred variant, provision may in particular be made for the injection device in turn to comprise a nozzle, which according to the present invention is a so-called two-substance nozzle. In this case, a two-fluid nozzle is understood to mean a nozzle which uses a pressurized gas, generally air, as the propellant for the surface breakdown of the liquid phase and thus for droplet formation. This pressurized gas is also referred to as atomizing air. Thus, an ammonia gas generator is the subject of the present invention, which comprises an injection device, which is a two-fluid nozzle, in which the solution to be introduced into the mixing chamber is subjected to compressed air.

Alternatively or independently thereof, it may also be provided that the ammonia gas generator comprises a nozzle which has a first number Having nozzle openings for introducing the solution in the mixing chamber, which is surrounded by a second number of nozzle openings for introducing compressed air or carrier gas in the mixing chamber annular.

Alternatively it can also be provided that the ammonia gas generator comprises a nozzle which is operated without compressed air. This so-called single-fluid nozzle or injector works with a mechanical fluid break-up without additional air flow. Preference is given to a single-substance nozzle which generates drops with a Sauter diameter D ₃₂ of 26 to 100 μm. In this case, a single-substance nozzle with a plurality of openings or boreholes is particularly preferred. Very particularly preferred is the combination of more than one single-substance nozzle for the introduction of the solution into the ammonia gas generator.

According to a development of the present invention, it is also provided that the ratio of the volume of the mixing chamber V _mixing to the volume of the catalyst V _Kat corresponds to the ratio of 1: 1 to 5: 1. Surprisingly, it has been shown that the sprayed ammonia precursor substance can then be completely (conversion> 97%) decomposed in ammonia, if the droplets of the solution are already partially evaporated before impinging on the catalyst end face. This can be ensured by the fact that the volume of the mixing chamber is greater than the volume of the catalyst. By partially evaporating the droplets, sufficient energy is already supplied to the solution, so that overcooling at the catalyst end face due to excessively large drops is avoided and thus poorer decomposition or by-product formation is counteracted. In addition, it is ensured by a corresponding mixing chamber volume V _mixing that the sprayed ammonia precursor substance as an aerosol homogeneously distributed in the transport gas stream over the cross section impinges on the catalyst and spots with too high concentration, which in turn would result in a poorer implementation, can be avoided. Very particularly preferably, it is provided that the ratio of the volume of the mixing chamber V _mixing to the volume of the catalyst V _Kat of 2: 1 to 5: 1, more preferably from 1: 1 to 5: 1, more preferably 3: 1 to 5: 1 and most preferably 3.5: 1 to 5: 1.

According to a further preferred embodiment, the ammonia gas generator according to the present invention further comprises at least one inlet for a carrier gas, which in particular generates a carrier gas flow directed perpendicular to the catalyst end face. In this case, it may further be preferred for the ammonia gas generator to comprise a perforated disk, in the center of which the nozzle opening of the nozzle is arranged. In this case, the perforated disc further preferably has a multiplicity of openings through which the introduced carrier gas flow is split into a multiplicity of partial flows. The openings are particularly preferably designed such that a plurality of parallel carrier gas partial flows (parallel to the flow direction of the catalyst unit) can be generated.

Thus, regardless of an ammonia gas generator of the type described above, an ammonia gas generator for producing ammonia from a solution of an ammonia precursor substance of the present invention, the i) a catalyst unit comprising a catalyst for the decomposition and / or hydrolysis of ammonia precursors in ammonia and a catalyst upstream mixing chamber, said catalyst having a catalyst volume V _Kat and the mixing chamber, a mixing chamber volume V _compound, ii) an injection device for introducing the solution of the ammonia precursor into the mixing chamber, and iii) an outlet for the formed ammonia gas, the ammonia gas generator further iv) includes at least one inlet for a carrier gas, which generates a directed perpendicular to the catalyst end face carrier gas stream.

In particular, this ammonia gas generator also includes the injection device, which in turn comprises a nozzle which generates drops with a Sauter diameter D ₃₂ of 26 to 100 microns. In particular, the injection device may comprise a nozzle which generates drops with a Sauter diameter D ₃₂ of 26 to 49 microns.

Surprisingly, it has been found that in the operation of the ammonia gas generator with a guided perpendicular to the catalyst end face carrier gas flow (synonymously also transport gas flow) deposits on the walls of the catalyst unit in the mixing chamber can be prevented and a permanent good mixing of carrier gas (synonymous synonymous transport gas) and the solution of the ammonia precursor substance can be provided. If such a carrier gas stream conducted perpendicular to the catalyst end face is not used, the walls of the catalyst unit in the mixing chamber can be wetted by spraying the solutions of ammonia precursor substance into the mixing chamber and unwanted secondary reactions such as polymerization of the ammonia precursor substance take place. These side reactions lead to undesirable deposits in the mixing chamber, whereby a permanently important for the function of the generator mixing of carrier gas and solution of ammonia precursor substance is not possible. Due to the poor mixing of the carrier gas In addition, with the solution, further deposits can be observed in and on the catalyst itself. The carrier gas flow, which is conducted perpendicular to the catalyst end face, produces a fog flow with the droplets, which is guided axially in the direction of the hydrolysis catalytic converter onto the end face of the hydrolysis catalytic converter. This fog flow allows a very good conversion to the ammonia on the catalyst.

The supply of the carrier gas takes place in the head region of the generator, preferably in the amount of the injection device of the ammonia precursor solution into the catalyst unit or into the mixing chamber.

The carrier gas and in particular the carrier gas stream conducted perpendicular to the catalyst end face, is preferably 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 most preferably with introduced a temperature of 300 to 350 ° C in the mixing chamber. However, it can also be provided that the carrier gas and in particular the carrier gas stream guided perpendicular to the catalyst end face, with a temperature of less than 250 ° C, in particular less than 200 ° C, more preferably less than 150 ° C, particularly preferably less is introduced as 100 ° C and most preferably of less than 80 ° C in the mixing chamber, wherein the temperature should be at least 10 ° C, in particular at least 20 ° C and very particularly preferably 25 ° C.

Furthermore, it has surprisingly been found that the introduced carrier gas stream can be split into a multiplicity of partial streams through the use of a perforated disk, and thus a particularly uniform distribution of the droplets formed on the catalyst end face can be produced. The uniform distribution of the droplets formed on the catalyst end face can in particular be further improved if the nozzle opening of the nozzle is arranged in the center of the perforated disk. Thus, with the multitude of partial flows of the carrier gas, a jacket enveloping the sides for the sprayed drops can be provided. This jacket prevents the deposition of ammonia precursors or unwanted degradation products therefrom on the inner walls of the catalyst unit.

In particular, the present invention provides an ammonia gas generator which operates autonomously from the exhaust gas stream of an internal combustion engine or an industrial plant, that is to say without the aid of an exhaust gas stream or partial exhaust gas stream from a combustion gas as the carrier gas. In particular, in the ammonia gas generator of the present invention, ammonia is formed from an ammonia precursor substance in the absence of an exhaust gas flow.

Thus, according to a further aspect, a process for the production of ammonia, in particular for the continuous production of ammonia, from a solution of an ammonia precursor substance is the subject of the present invention by using the ammonia gas generator described herewith. In this process, the solution is introduced into the catalyst unit in such a way that the solution is injected in the form of droplets having a Sauter diameter D ₃₂ of 26 to 100 μm into the catalyst unit. Particularly preferred is a method in which a carrier gas is introduced into the catalyst unit in the form of a carrier gas stream guided perpendicular to the catalyst end face, in particular in the form of parallel beams. In this case, it can furthermore be particularly preferred that the solution of the ammonia precursor substance is introduced as aerosol into a plurality of parallel jets of the carrier gas.

Simultaneously or independently thereof, this process can be carried out with a carrier gas or with a carrier gas stream conducted perpendicular to the catalyst end face, which has a temperature of up to 550 ° C. Preferably, this carrier gas or this carrier gas stream has a temperature of from 250 to 550 ° C or 10 to 200 ° C. Surprisingly, it has been found that the ammonia gas generator can also be operated with a carrier gas or a carrier gas stream having a temperature below the decomposition temperature of the ammonia precursor substance. The energy required for decomposition is provided here essentially by the heatable catalyst. Surprisingly, this energy supply is sufficient to achieve a complete conversion and a high conversion rate of> 97%.

Thus, in particular, a method of the present invention, in which a carrier gas at a temperature T _(K) in the range of 10 to 200 ° C in the mixing chamber (51) is introduced, wherein the solution in the form of drops with a Sauter diameter D. _{32 is} injected from 26 to 100 microns in the catalyst unit.

Further preferably, the carrier gas and in particular the carrier gas stream guided perpendicular to the catalyst end face, with a temperature of less than 200 ° C, further preferably less than 150 ° C, more preferably less than 100 ° C and most preferably less than 80 ° C introduced into the mixing chamber or be injected, wherein the temperature should be at least 10 ° C, especially at least 20 ° C and very particularly preferably 25 ° C simultaneously.

As a catalyst for the decomposition and / or hydrolysis of ammonia precursor substances, any catalyst which enables the liberation of ammonia from the precursor substance under catalytic conditions can be used in the context of the present invention. A preferred catalyst hydrolyzes the ammonia precursor substance to ammonia and other innocuous substances such as nitrogen and carbon dioxide and water. The catalyst is thus preferably a hydrolysis catalyst.

If, for example, a guanidinium salt solution, in particular a guanidinium formate solution, a urea solution or mixtures thereof, is used, the catalytic decomposition to ammonia in the presence of catalytically active, non-oxidation active coatings of oxides selected from the group titanium dioxide, alumina and silica and 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. Suitable metals in this case are in particular the subgroup elements and preferably iron or copper. The metal oxides such as titanium oxide, alumina and silica are preferably supported on metallic support materials such. B. heating conductor alloys (especially chromium-aluminum steels) applied.

Particularly preferred catalysts are hydrolysis catalysts which comprise, in particular, catalytically active coatings of titanium dioxide, aluminum oxide and silicon dioxide and mixtures thereof.

Alternatively, the catalytic decomposition of the guanidinium formate solutions or the other components can also be carried out to ammonia and carbon dioxide, wherein catalytically active coatings of oxides selected from the group of titanium dioxide, alumina and silica and their mixtures, or / and hydrothermally stable zeolites, the complete or partially metal exchanged, are used, 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% by weight, in particular 0.01 to 1% by weight. With the aid of such oxidation catalysts, it is possible to avoid the undesired formation of carbon monoxide as a by-product in the decomposition of the guanidinium salt, in particular in the decomposition of formates, already in the production of ammonia.

For the catalytic decomposition of the guanidinium formate and optionally the further components, preference is given to using a catalytic coating with palladium or / and gold as active components having a noble metal content of 0.001 to 2% by weight, in particular 0.01 to 1% by weight.

Thus, an ammonia gas generator is the subject of the present invention, which comprises a hydrolysis catalyst having a catalytically active coating which is impregnated with gold and / or palladium, in particular with a content of gold and / or palladium of 0.001 to 2 wt .-% (relative on the catalytic coating). Further preferably, this catalyst has a catalytically active coating of oxides selected from the group of titanium dioxide, alumina and silica and mixtures thereof, and / or hydrothermally stable zeolites which is impregnated with gold and / or palladium, wherein further preferably the content of gold and or palladium 0.001 to 2 wt .-% (based on the catalytic coating).

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

In addition, it can be provided that one uses a hydrolysis catalyst, which consists of at least two sections, wherein the arranged in the flow direction of the first section in the form of a heated catalyst and arranged in the flow direction second section is in the form of a non-heated catalyst. Preferably, the catalyst is 5 to 50 vol% of the first portion and 50 to 95 vol% of the second portion.

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

It has been found that, for complete catalytic conversion of the ammonia precursor substances, preference is given to using catalysts having a catalyst cell count of at least 60 cpsi (cpsi: cells per square inch) and the catalyst volumes already described above. The increasing back pressure (pressure drop across the catalyst) limits the number of catalyst cells to at most 800 cpsi for use in an ammonia gas generator. Especially preferred are those catalysts, in particular hydrolysis catalysts having a catalyst cell number of 100 to 600 cpsi per inch ² end face, from 100 to 500 cpsi per inch ² face and very particularly preferably from 100 to 400 cpsi per inch ² end face of the catalyst.

With regard to the configuration of the catalyst unit, it has been found in the tests that a cylindrical design is particularly well suited. In this case, the carrier gas flow can develop its full effect. Other designs, however, are less suitable, since in this case an excessive turbulence is observed. Thus, an ammonia gas generator is also an object of the present invention comprising a catalyst unit formed in the shape of a cylinder.

Moreover, it has proved to be particularly advantageous if the ammonia gas generator comprises a catalyst unit, which in turn has at least one thermal insulation layer, in particular a thermal insulation layer of microporous insulating material.

Furthermore, it can be provided that the ammonia gas generator further comprises a metering unit for metering the solution of the ammonia precursor substance, which is connected upstream of the injection device. Thus, a precise control of the ammonia to be generated via this metering unit. If, for example, an increased emission of nitrogen oxides in the exhaust gas of an engine is recorded, a defined amount of ammonia can be liberated via the targeted control of the amount of precursor substance injected with the injection device.

wobei
R = H, NH₂ oder C₁-C₁₂-Alkyl,
X^⊝ = Acetat, Carbonat, Cyanat, Formiat, Hydroxid, Methylat oder Oxalat, bedeuten.According to the present invention, ammonia precursor substances are understood as meaning chemical substances which can be converted into a solution and which can split off the ammonia by means of physical and / or chemical processes or release it in any other form. As ammonia precursor compounds according to the present invention, in particular urea, urea derivatives, guanidines, biguanidines and salts of these compounds and salts of ammonia can be used. In particular, urea and guanidines or salts thereof can be used according to the present invention. In particular, it is possible to use those salts which are formed from guanidines and organic or inorganic acids. Particularly preferred are guanidinium salts of the general formula (I),

in which
R = H, NH ₂ or C ₁ -C ₁₂ -alkyl,
X ^⊝ = acetate, carbonate, cyanate, formate, hydroxide, methylate or oxalate.

Particularly preferred is guanidinium formate.

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, according to a further embodiment of the present invention, however, aqueous urea solutions can also be used. The mixing ratios of guanidinium salt with urea and ammonia or ammonium salts can be varied within wide limits. However, it has proved to be particularly advantageous if the mixture of guanidinium salt and urea a guanidinium salt content of 5 to 60 wt .-% and a urea content of 5 to 40 wt .-%, in particular from 5 to 35 wt. % owns. Furthermore, mixtures of guanidinium salts and ammonia or ammonium salts with a content of guanidinium salt of 5 to 60 wt. % and of ammonia or ammonium salt of 5 to 40 wt .-% to be regarded as preferred. Alternatively, however, it is also possible to use a urea solution, in particular an aqueous urea solution.

wobei
R = H, NH₂ oder C₁-C₁₂-Alkyl,
X^⊝ = Acetat, Carbonat, Cyanat, Formiat, Hydroxid, Methylat oder Oxalat bedeuten.In particular, compounds of the general formula (II) have proven to be suitable as ammonium salts

in which
R = H, NH ₂ or C ₁ -C ₁₂ -alkyl,
X ^⊝ = acetate, carbonate, cyanate, formate, hydroxide, methylate or oxalate.

The ammonia precursor substances used according to the invention, in particular guanidinium salts and possibly the other components consisting of urea or ammonium salts, are used in the form of a solution, with water and / or a C ₁ -C ₄ -alcohol being the preferred solvents. The aqueous and / or alcoholic solutions in this case have a preferred solids content of 5 to 85 wt .-%, in particular 30 to 80 wt .-%, on.

It has surprisingly been found that according to the present invention, both aqueous Guanidinformiatlösung in the concentration of 20 to 60 wt .-%, and aqueous urea solution in the concentration of 25 to 40 wt .-% and aqueous mixtures of guanidinium formate and urea Solutions, wherein guanidinium formate and urea in a concentration of 5 to 60 wt .-% guanidinium and 5 to 40 wt .-% urea in the mixture contained, can be used particularly well.

The aqueous solution of the ammonia precursor substances, in particular the guanidinium salts, the mixtures of guanidinium salts or guanidinium salts in combination with urea in water in this case have a preferred ammonia formation potential of 0.2 to 0.5 kg of ammonia per liter of solution, in particular 0.25 to 0 , 35 kg of ammonia per liter of solution.

The ammonia gas generators described herein are particularly suitable for use in industrial plants, in internal combustion engines such as diesel engines and gasoline engines, and 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 and the use of the described method for the reduction of nitrogen oxides in exhaust gases from industrial plants, internal combustion engines such as diesel engines and gasoline engines, as well as gas engines. Furthermore, the present invention thus also includes an exhaust aftertreatment system comprising an ammonia gas generator of the type described herewith.

In particular, the present invention also includes an exhaust aftertreatment system which further comprises a Venturi mixer, wherein the outlet for the ammonia gas formed and the Venturi mixer are connected by a line, in particular directly connected. Most preferably, the invention also includes an exhaust aftertreatment system, further comprising a Venturi mixer, wherein the outlet for the ammonia gas formed and the Venturi mixer are connected by a line, in particular directly connected, and the venturi is part of the exhaust pipe of a vehicle, wherein the Venturi mixer is arranged upstream of an SCR catalyst in the flow direction of the exhaust gas.

It has been found particularly surprising that a pressure gradient can be established by the direct connection between the outlet of the ammonia gas generator for the ammonia gas and a venturi mixer in an exhaust system of a motor vehicle, with which the generated ammonia can be introduced into the exhaust system without further aids. It is essential here that the ammonia gas is introduced directly into the Venturi mixer. Thus, at the same time, a sufficiently high turbulence of the exhaust gas to be reduced and the ammonia gas can take place.

In the following, the present invention will be explained in more detail with reference to drawings and associated examples. This shows

1 FIG. 2: a schematic view of an ammonia gas generator according to the invention in axial cross section, FIG.

2 FIG. 2: a schematic construction of an exhaust system in a vehicle, FIG.

3 : a radial cross section of the mixing chamber (top view) in the region of the carrier gas stream supply.

With 1 is a first ammonia gas generator ( 100 ) according to the present invention. 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 ammonia gas formed. The catalyst unit ( 70 ) consists of a multi-part hydrolysis catalyst ( 60 ), the mixing chamber ( 51 ) and an exit chamber ( 55 ). The mixing chamber is in Longitudinal direction (flow direction) bounded by a perforated disc ( 48 ) with a multiplicity of openings (in the operating state, the ammonia precursor solution (B) is removed from a storage tank ( 20 ) via a metering pump ( 30 ) together with an atomizing air flow (A) via a two-fluid nozzle ( 41 ) with nozzle opening ( 42 ) into the mixing chamber ( 51 ) of the ammonia gas generator ( 100 ) sprayed with a defined spray angle and distributed in fine droplets. In addition, a carrier gas stream (C) is introduced via the inlet (FIG. 56 ) into the mixing chamber ( 51 ). This carrier gas stream (C) is placed on a perforated disk ( 48 ), in the center of which the nozzle opening ( 42 ) of the nozzle ( 41 ) is arranged. The nozzle opening ( 42 ) is at the height of the perforated disc. Through the perforated disc ( 48 ), which have a multiplicity of openings ( 49 ), the carrier gas stream is generated in a plurality of partial streams of carrier gas parallel to the flow direction of the catalyst unit, whereby a fog flow with droplets of the injected ammonia precursor solution is generated, which axially in the direction of the hydrolysis ( 60 ) to the hydrolysis catalyst face ( 61 ) to be led. The catalyst ( 60 ) is designed such that the first segment ( 62 ) represents an electrically heatable metal support with hydrolysis coating. This is followed by a non-heated supported metal catalyst ( 63 ) also with hydrolysis coating and a non-heated catalyst ( 64 ) with hydrolysis coating as a mixer structure for better radial distribution. The generated ammonia gas (D) leaves the generator together with the hot carrier gas stream ( 100 ) via the outlet chamber ( 55 ) with the outlet ( 80 ) and valve ( 81 ). The generator can be powered by a jacket heating ( 52 ) around the housing ( 54 ) of the catalyst unit are additionally heated. Except for the head area in which the injection device ( 40 ), the ammonia gas generator ( 100 ) with a thermal insulation ( 53 ) surrounded by microporous insulation material.

With 2 is a schematic flow of an exhaust aftertreatment on an internal combustion engine ( 10 ). In doing so, that of the combustion engine ( 10 ) coming exhaust gas via a charger ( 11 ) and countercurrent supply air (E) compressed for the internal combustion engine. The exhaust gas (F) is passed through an oxidation catalyst ( 12 ) to achieve a higher NO ₂ concentration relative to NO. The from the ammonia gas generator ( 100 ) coming ammoniacal gas stream (D) can both before and after a particulate filter ( 13 ) are added and mixed. An additional gas mixer ( 14 ) in the form of a static mixer or z. B. a Venturi mixer can be used. It can also be provided that the ammonia-containing gas stream (D) in the amount of the additional gas mixer ( 14 ) or in the additional gas mixer ( 14 ) or after the additional gas mixer ( 14 ) is introduced into the exhaust system. In any case, however, the ammonia-containing gas stream (D) before the SCR catalyst ( 15 ) introduced into the exhaust system. On the SCR catalyst ( 15 ), the reduction of the NOx is carried out with the aid of the reducing agent NH ₃ (SCR = selective catalytic reduction). In this case, the ammonia gas generator can be operated with a separate carrier gas or else with an exhaust gas partial stream.

With 3 is a detailed view of the mixing chamber ( 51 ) in the region of the carrier gas stream supply shown. The housing ( 54 ) of the catalyst unit is in the region of the mixing chamber ( 51 ) with a thermal insulation ( 53 ) surrounded by microporous insulation material. The supply of the carrier gas (C) takes place in the head region of the ammonia gas generator or in the head region of the mixing chamber (FIG. 51 ). The inlet ( 56 ) for the carrier gas flow (C) such that the inlet ( 56 ) in the flow direction of the catalyst behind the perforated disc ( 48 ) is arranged. At the height of the nozzle opening ( 42 ) of the nozzle ( 41 ) is a perforated disc ( 48 ) with a large number of holes ( 49 ) arranged. This perforated disc generates a multiplicity of carrier gas substreams, which in turn adjusts a downward veiling flow in the generator towards the catalyst.

Examples

Embodiment 1

The structure corresponds to the principle with 1 reproduced ammonia gas generator. The ammonia generator is designed for a dosage of 50-2000 g / h NH ₃ and designed as a cylindrical tubular reactor. In the middle of the head area there is a two-component nozzle from Albonair (dosing system Albonair) with a spraying angle of alpha = 20 °. The Sauter diameter in the selected operating range is D ₃₂ = 37 μm. The length of the mixing chamber is about 250 mm.

Around the nozzle opening there is a perforated disk with 90 mm diameter over the entire cross section with a cutout of 30 mm around the centrally arranged nozzle. The nozzle opening is located at the height of the perforated disc. The holes of the perforated disc are every 5 mm and distributed evenly over the cross section.

In another embodiment, the holes from the inside (3 mm) to the outside (8 mm) towards larger. Through the perforated disc in front of the nozzle outlet, a low back pressure of 0.2 to 2 mbar is generated, thereby forcing a uniform axial flow of the transport gas flow and thus, especially in the edge region, a flow which forms a protective covering around the wall area (fog flow). This will prevent that Droplets are thrown against the wall due to excessive turbulence, lead to deposits and the overall ammonia formation level drops from the usual 97% to below 90%.

The entire area of the mixing chamber, including the metallic perforated disk, is coated with a hydrolysis catalyst (catalytically active TiO ₂ , anatase, washcoat about 100 g / l, from Interkat) in order to avoid undesirable side reactions of any sprayed solution that lands on the surfaces.

It has been shown that the uniform distribution of the droplets on the catalyst end surface does not lead to an undesired, pointwise overloading of the catalyst surface. In this case, the entire area is used for the reaction of the solution in ammonia without by-products. This can not lead to deposits, as has been shown in tests with maturities of several hundred hours. If no deposits show complete conversion of the ammonia precursor solution. Maintenance regarding deposits is no longer necessary in this case.

It has been found that both aqueous Guanidiniumformiatlösung in the concentration of 20% to 60% can be used for this construction as ammonia precursor solution and aqueous urea solution in the concentration of 25% to 40% and aqueous mixtures of guanidinium and urea.

Due to the remaining droplets, a cooling of about 120-150 ° C occurs at the catalyst end face. For this reason, the reactor is designed in such a way that the amount of heat supplied with the hot carrier gas stream, the integrated heatable hydrolysis catalytic converter and other energy supplies bring in so much energy that no cooling takes place below about 280 ° C. for the metered amount of solution.

In addition to the sprayed solution, a hot carrier gas flow of about 1-5 kg / h is also introduced in the head region of the ammonia gas generator such that it lays in a veiling flow around the inner wall of the catalyst unit and is passed through the mixing chamber in a laminar manner. This further prevents sprayed droplets from coming into contact with the inner wall. The carrier gas stream is branched off before turbo and fed via a throttle device to the ammonia generator. Due to the higher pressure before turbo, the necessary pressure difference of 10 mbar is overcome via the reactor to the mixing point in the exhaust gas flow upstream of the SCR catalyst and thus the carrier gas flow is moved through the reactor. Depending on the operating condition, the temperature of the partial exhaust gas stream is between 250 ° C and 550 ° C. The temperature control takes place in conjunction with temperature sensors (type K) arranged at the catalyst end face, in and after the catalyst. All outer surfaces of the reactor are surrounded by insulation. Only the head area in which the injection of the solution is located is not isolated for better heat dissipation.

Following the mixing chamber is a heated metal support catalyst with 90 mm diameter and 300 cpsi flanged (Emitec Emicat, maximum power 900 W). This is designed as a hydrolysis, coated with catalytically active TiO ₂ (anatase, washcoat about 100 g / l, Fa. Interkat) and is controlled so that the temperature at the catalyst end face between 280 and 400 ° C. In this case, only enough energy is supplied that the cooling is compensated by the evaporation of the droplets. To achieve a space velocity of up to a minimum of 7,000 1 / h, a further hydrolysis catalyst with 400 cpsi is connected downstream, resulting in a total catalyst volume of about 900 ml.

The ammonia generated at the hot hydrolysis catalyst flows freely in the foot region, centrally from an outlet opening out of the reactor end piece. In this case, the outlet region is preferably conically shaped in order to avoid vortex formation on edges and thus deposits of possible residues. The gas mixture from the ammonia gas generator is preferably at a temperature> 80 ° C to avoid ammonium carbonate deposits the engine exhaust gas stream before SCR catalyst admit and distributed homogeneously in this exhaust stream via a static mixer.

The material used for all metallic components is 1.4301 (V2A, DIN X 5 CrNi18-10) or alternatively 1.4401 (V4A, DIN X 2 CrNiMo 17-12-2), 1.4767 or other catalytic converter type Fe-Cr-Al alloys.

Embodiment 2:

In embodiment 2, the ammonia generator is designed in such a way that, instead of a two-substance nozzle, a single-substance nozzle (injector, Hilite Gen 2 Liquid Only) with a comparable spray pattern is used. The atomization is achieved completely without additional compressed air. The Sauter diameter in this design is D ₃₂ = 29 μm.

Accordingly, no compressed air or additional air pump for the operation of the atomization is necessary in this embodiment.

Embodiment 3

In Embodiment 3, the ammonia generator is operated such that ambient air is sucked as a carrier gas stream through the ammonia generator. The generator corresponds to the embodiment in Example 1. As the carrier gas flow, cold ambient air is used instead of a hot exhaust gas secondary flow. The temperature of the ambient air is between 10 ° C and 25 ° C. The pressure difference across the reactor to the mixing point in the exhaust stream is overcome by means of a Venturi nozzle in the exhaust stream. In the exhaust gas stream, a venturi gas mixer is installed upstream of the SCR catalytic converter in such a way that a corresponding negative pressure draws in ambient air as carrier gas flow into and through the reactor and mixes ammonia-enriched gas into the exhaust gas flow. In this case, the mixing point of the ammonia-containing gas from the reactor is located centrally in the venturi integrated in the exhaust gas flow in the narrowest cross-section. Through several holes in the Venturidüsenengstelle the gas is sucked out of the reactor and radially distributed evenly distributed to the exhaust gas stream.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 48788661 [0005]
• EP 1052009 B1 [0006]
• EP 1338562 B1 [0007]
• EP 1348840 A1 [0008]
• DE 102006023147 A1 [0009]
• WO 2008/077587 A1 [0010]
• WO 2008/077588 A1 [0010]
• EP 2012/062757 [0011]
• EP 2012/062750 [0011]
• EP 2012/062752 [0011]

Cited non-patent literature

• DIN 66 141 [0017]

Claims (19)

Ammonia gas generator ( 100 ) for producing ammonia from a solution of an ammonia precursor substance comprising i) a catalyst unit ( 70 ), which is a catalyst ( 60 ) for the decomposition and / or hydrolysis of ammonia precursors in ammonia and a catalyst ( 60 ) upstream mixing chamber ( 51 ), wherein the catalyst ( 60 ) a catalyst volume V _Kat and the mixing chamber ( 51 ) has a mixing chamber volume V _mixing , ii) an injection device ( 40 ) for introducing the solution of the ammonia precursor substance into the mixing chamber ( 51 ), and iii) an outlet ( 80 ) for the ammonia gas formed, characterized in that the injection device ( 40 ) a nozzle ( 41 ), which produces drops with a Sauter diameter D ₃₂ of 26 to 100 microns. Ammonia gas generator ( 100 ) according to claim 1, characterized in that the injection device ( 40 ) a nozzle ( 41 ) which produces drops with a Sauter diameter D ₃₂ of 26 to 49 microns. Ammonia gas generator ( 100 ) according to at least one of claims 1 or 2, characterized in that the injection device ( 40 ) a two-fluid nozzle ( 41 ) is, in which the solution to be introduced into the mixing chamber is supplied with compressed air. Ammonia gas generator ( 100 ) according to at least one of the preceding claims, characterized in that the nozzle ( 41 ) has a first number of nozzle openings for introducing the solution into the mixing chamber, which is surrounded by a second number of nozzle openings for introducing compressed air into the mixing chamber annular. Ammonia gas generator ( 100 ) according to at least one of the preceding claims, characterized in that the nozzle is a single-fluid nozzle or an injector. Ammonia gas generator ( 100 ) according to at least one of the preceding claims, characterized in that the ratio of the volume of the mixing chamber V _mixing to the volume of the catalyst corresponds to the ratio of 1: 1 to 5: 1. Ammonia gas generator ( 100 ) according to at least one of the preceding claims, characterized in that the ammonia gas generator has at least one inlet ( 56 ) for a carrier gas, which in particular generates a directed perpendicular to the catalyst end face carrier gas stream. Ammonia gas generator ( 100 ) according to at least one of the preceding claims, characterized in that the ammonia gas generator a perforated disc ( 48 ), in whose center the nozzle orifice of the nozzle ( 41 ) is arranged. Ammonia gas generator ( 100 ) according to at least one of the preceding claims, characterized in that the ammonia gas generator a perforated disc ( 48 ) having a plurality of openings, which in particular generate a plurality of partial flows of carrier gas parallel to the flow direction of the catalyst unit. Ammonia gas generator ( 100 ) according to at least one of the preceding claims, characterized in that the catalyst ( 60 ) is a hydrolysis catalyst. Ammonia gas generator ( 100 ) according to at least one of the preceding claims, characterized in that the ammonia gas generator further comprises a dosing unit ( 30 ) for metering the solution of the ammonia precursor substance, the injection device ( 40 ) is connected upstream. Process for the production of ammonia from a solution of an ammonia precursor substance using an ammonia gas generator ( 100 ) comprising i) a catalyst unit ( 70 ), which is a catalyst ( 60 ) for the decomposition and / or hydrolysis of ammonia precursors in ammonia and a catalyst ( 60 ) upstream mixing chamber ( 51 ), wherein the catalyst ( 60 ) a catalyst volume V _Kat and the mixing chamber ( 51 ) has a mixing chamber volume V _mixing , ii) an injection device ( 40 ) for introducing the solution of the ammonia precursor substance into the mixing chamber ( 51 ), and iii) an outlet ( 80 ) for the ammonia gas formed, characterized in that in the method, the solution of the ammonia precursor substance in the form of drops having a Sauter diameter D ₃₂ of 26 to 100 microns in the mixing chamber ( 51 ) is introduced, in particular injected. A method according to claim 11, characterized in that in the method, a carrier gas or a carrier gas stream, in particular a guided perpendicular to the catalyst end face Carrier gas flow, with a temperature T _(K) in the range of 10 to 200 ° C in the mixing chamber ( 51 ) is introduced, in particular injected. A method according to claim 11, characterized in that in the method, a carrier gas or a carrier gas stream, in particular a perpendicular to the catalyst end surface guided carrier gas stream having a temperature T _(K) in the range of 250 to 550 ° C in the mixing chamber ( 51 ) is introduced, in particular injected. A method according to any one of claims 11 to 13, characterized in that the solution of the ammonia precursor is introduced as an aerosol into a plurality of parallel beams of the carrier gas. Aftertreatment system for exhaust gas lines of vehicles comprising an ammonia gas generator ( 100 ) according to at least one of the preceding claims. Exhaust after-treatment system according to claim 11, characterized in that the system further comprises a Venturi mixer, wherein the outlet ( 80 ) are connected for the ammonia gas formed and the Venturi mixer by a line, in particular directly connected. Exhaust after-treatment system according to claim 11 or 12, characterized in that the Venturi mixer is part of the exhaust pipe of a vehicle. Use of an ammonia gas generator ( 100 ) according to at least one of the preceding claims for the reduction of nitrogen oxides in exhaust gases, in particular exhaust gases from internal combustion engines, gas engines, diesel engines or gasoline engines.

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