DE102011106243A1 - Ammonia gas generator useful e.g. for producing ammonia from ammonia precursor solution, and in gas engines, comprises catalyst unit comprising catalyst and mixing chamber, injection device having nozzle, and outlet for formed ammonia gas - Google Patents

Abstract

Ammonia gas generator (100) comprises: a catalyst unit (70) comprising a catalyst (60) for decomposing and/or hydrolyzing ammonia precursors into ammonia, and a mixing chamber (51) arranged upstream of the catalyst; an injection device (40) for introducing the ammonia precursor solution into the mixing chamber, comprising a nozzle exhibiting a theoretical spray angle (alpha ) of 10-90[deg] ; and (iii) an outlet for formed ammonia gas. The catalyst has a catalyst volume. The mixing chamber has a mixing chamber volume. The distance of the nozzle opening to the end face of the catalyst is 15-2000 mm.

Description

The present invention relates to an ammonia gas generator for producing ammonia from an ammonia precursor substance 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 in particular rely on the 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 B1 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 the production of ammonia in the form of a tubular evaporator is described, which comprises a spray device, an evaporator with evaporator tubes 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.

Although ammonia gas generators have been known for some time, so far no realization of the technology in a vehicle or other use has been realized. 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 which overcomes these disadvantages of the prior art. A further object of the present invention is to provide an ammonia gas generator which has a simple structure and provides a high conversion rate of ammonia precursors into ammonia gas and allows 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.

These objects are achieved by an ammonia gas generator according to claim 1.

Thus, according to a first embodiment, an ammonia gas generator for producing ammonia from a solution of an ammonia precursor substance of the present invention, comprising a catalyst unit, wherein the catalyst unit in turn comprises a catalyst for decomposition and / or hydrolysis of ammonia precursors in ammonia and a mixing chamber upstream of the catalyst and the catalyst has a catalyst volume V _Kat and the mixing chamber has a mixing chamber volume V _mixed . Furthermore, the ammonia gas generator comprises an injection device for introducing the solution of ammonia precursor substance into the mixing chamber and an outlet for the ammonia gas formed, wherein the injection device in turn comprises a nozzle having a theoretical spray angle α of 10 ° to 90 ° and the distance of the nozzle opening to Face of the catalyst is 15 to 2,000 mm.

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 autonomously or can also be operated with the aid of exhaust gas side streams, although in each case a reduction of nitrogen oxides by means of ammonia takes place only in a subsequent process step. 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 produced with the ammonia gas generator according to the invention can thus be introduced into the exhaust gas flow as required. Any shortening of the service 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.

Essential to the invention here is the arrangement and geometry of the injection device and the hydrolysis within the ammonia gas generator, wherein the injection device in turn comprises a nozzle having a theoretical spray angle α of 10 ° to 90 °, and the distance of the nozzle opening to the end face of the catalyst of 15 to 2,000 mm.

Surprisingly, it has been found that only those ammonia gas generators with nozzles having a theoretical spray angle α of 10 ° to 90 ° allow an acceptable conversion of the ammonia precursor substances with a degree of ammonia AG greater than 95%, while the distance from the nozzle opening to the end face of the catalyst 15 to 2,000 mm. Exceeding or exceeding the distances to be set and / or the use of nozzles with other theoretical spray angles produce only an insufficient total amount of ammonia per unit time and / or show incomplete reaction of the ammonia precursor substance into ammonia and / or promote deposits on the inner wall of the generator or the catalyst endface formed from the precursor substance and / or degradation or reaction products thereof. The degree of ammonia AG is here and hereinafter defined as the molar amount NH ₃ produced in the process, based on the molar amount of ammonia theoretically to be produced upon complete hydrolysis of the ammonia precursor substance. An ammonia degree of> 95% is in accordance with the present invention as a complete reaction considered. These observations were surprisingly made independent of the solution used.

For example, if a nozzle with a spray angle α greater than 90 ° is used, and the distance of 15 mm falls below a catalyst end face diameter of 30 mm, the inner wall of the catalyst unit is excessively wetted with the injected solution, so that deposits of unreacted ammonia precursor substance and / or degradation or reaction products thereof are formed and observed within the mixing chamber is a real Zuwachsen. If, on the other hand, a nozzle is used which has a spray angle α of less than 10 ° and the distance of 2000 mm has been exceeded at a catalyst end face diameter of 1000 mm, then a surface of the catalyst surface which is too small is wetted with the injected solution, so that larger drops are formed form the surface of the catalyst end wall, and an excessive concentration of ammonia precursor substance per unit area of the end face is observed and an increase in the catalyst end face is observed. In both extreme cases, only an ammonia degree of less than 95% is recorded.

Ideally, d. H. In order to achieve a conversion of the ammonia precursor substance in ammonia with more than 95% and to avoid contact of the precursor substance with the inner wall of the catalyst unit, some decisive conditions according to the invention must be observed during metering. If the precursor substance also wets the inner wall of the catalyst unit upstream of the hydrolysis catalyst, an insufficiently catalyzed decomposition can lead to undesirable side reactions and consequently to problematic deposits at these sites. It is therefore necessary to inject it in such a way that, for a given catalyst end face, the spray cone diameter when impinging on the catalyst end face is at most 98% of the catalyst diameter. In contrast, the spray cone diameter must be at least 80% of the catalyst face diameter in order to avoid too high a concentration for a given area and thus too high an end load on the precursor substance. Excessive loading of the catalyst end face leads to insufficient contact with the catalyst and to excessive cooling by the evaporating liquid and thus also to incomplete reaction and undesirable side reactions associated with deposits. In accordance with the invention, combinations of spray angle .alpha. Of the nozzle and spacing of the nozzle opening from the given catalyst end face are obtained.

Thus, in another aspect, there is also provided a process for producing ammonia from a solution of ammonia precursor using the ammonia gas generator of the present invention wherein the spray cone diameter is at most 98% and at least 80% of the diameter of the catalyst used. Particularly preferred is such a method in which the spray cone diameter is at most 95% and at least 80% and most preferably at most 95% and at least 85% of the diameter of the catalyst used.

The spray cone according to the present invention is the cone which can be produced by means of a nozzle with a defined spray angle α, the spray cone diameter being the diameter obtained on the occurrence of the droplets on the catalyst face.

In the context of the present invention, an injection device is understood here to mean any device which sprays, atomises 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 drops, which in particular form a Have droplet diameter d ₃₂ of less than 25 microns. The 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 ,

Thus, it is provided according to a preferred embodiment of the present invention that the injection device in turn comprises a nozzle which generates droplets having a droplet diameter d ₃₂ of less than 25 microns. According to the present invention, it is further preferred in this case for the nozzle to produce droplets having a droplet diameter d ₃₂ of less than 20 μm and very particularly preferably of less than 15 μm. At the same time or independently thereof, it is further preferred that the nozzle produces droplets having a droplet diameter d ₃₂ greater than 0.1 μm and in particular greater than 1 μm. Also by the use of such nozzles, a degree of ammonia formation of> 95% (see above) can be achieved. In addition, a particularly uniform distribution of the solution can take place on the catalyst end face.

Alternatively, however, it can also be provided that the injection device comprises a so-called flash evaporator.

Surprisingly, it has further been shown that in the case of an ammonia gas generator with a NH ₃ power of 10-100 g / h NH _{3 produced} a catalyst having a catalyst diameter D _Kat of 30 to 80 mm and a nozzle with a theoretical spray angle α of 20 ° up to 60 ° turns out to be the best. Only these values allow spraying the solution of an ammonia precursor substance without wetting the inner wall of the catalyst unit and uniformly the catalyst end face for the reaction to ammonia available and it does not come to unwanted by-products and deposits. It is ensured with these parameters that in the thus determined catalyst volume of 50 ml to 1,000 ml a space velocity in the range of 5,000 1 / h to 30,000 1 / h is maintained. Measurements have shown that complete decomposition (conversion> 95%) of ammonia precursors, in particular guanidinium salts such as, for example, guanidinium formate, can be quantitatively carried out in ammonia only in the region of these space velocities. Thus, an ammonia gas generator with an injection device is the subject of the present invention, which comprises a nozzle having a spray angle α of 20 ° to 60 ° and the diameter of the catalyst D _{Kat is} 30 to 80 mm. It is further preferred that the distance of the nozzle opening to the end face of the catalyst is from 15 to 200 mm.

If, on the other hand, an NH ₃ power of 100-1000 g / h of NH ₃ generated is defined, it has been found that catalysts with a catalyst diameter D _Kat of 80 to 450 mm are necessary and in conjunction with a nozzle with a theoretical spray angle α of 20 ° to 60 °. Again, it is ensured that no droplets impinge on the surrounding inner wall of the catalyst unit and the droplets land sufficiently distributed on the catalyst end face and thus a complete conversion (> 95%) is possible without by-products and deposits. Likewise, with the resulting total catalyst volume of 1 to 100 liters, a space velocity of 5,000 l / h to 30,000 l / h is maintained. For only in the range of these space velocities can a complete decomposition (conversion> 95%) of ammonia precursor substances, in particular of guanidinium salts such as, for example, guanidinium formate, be carried out quantitatively in ammonia.

Thus, an ammonia gas generator with an injection device is the subject of the present invention, which comprises a nozzle having a spray angle α of 20 ° to 60 ° and the diameter of the catalyst D _{Kat is} 80 to 450 mm. It is further preferred that the distance of the nozzle opening to the end face of the catalyst is from 15 to 500 mm.

In the case of a NH ₃ output of 1,000-50,000 g / h NH _{3 produced} has shown that catalysts with a catalyst diameter D _Kat of 450 to 1,000 mm are necessary and in conjunction with a nozzle with a theoretical spray angle α of 20 ° to 60 °. From this catalyst size it follows that in turn can be sprayed in front of the catalyst so that no droplets impinge on the surrounding inner wall of the catalyst unit, these droplets land sufficiently distributed on the catalyst end face and consequently complete conversion (> 95%) without by-products and deposits is made possible , Likewise, with the resulting total catalyst volume of 100 to 1,000 liters, a space velocity of 5,000 l / h to 30,000 l / h is maintained. For only in the range of these space velocities can a complete decomposition (conversion> 95%) of ammonia precursor substances, in particular of guanidinium salts such as, for example, guanidinium formate, be carried out quantitatively in ammonia.

Thus, an ammonia gas generator with an injection device is the subject of the present invention, which comprises a nozzle having a spray angle α of 20 ° to 60 ° and the diameter of the catalyst D _{Kat is} 450 to 1000 mm. It is further preferred here that the distance between the nozzle opening and the end face of the catalyst is from 15 to 1500 mm.

In connection with the present invention and in particular with the special design for an ammonia gas generator is further provided that the distance of the nozzle opening to the end face of the catalyst of in particular 15 to 1500 mm and more preferably 15 to 1000 mm and most preferably 15 to 800 mm can. Independently or simultaneously, however, it may also be provided that the distance of the nozzle opening to the end face of the catalyst is at least 30 mm, more preferably at least 40 mm, especially. preferably at least 50 mm and very particularly preferably at least 60 mm and further 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 and very particularly preferably at most 200 mm.

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 1.5: 1 to 5: 1. Surprisingly, it has been shown that the sprayed ammonia precursor substance can only be completely decomposed (conversion> 95%) 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 carrier 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, avoids. Most 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 2.5: 1 to 5: 1, more preferably 3: 1 to 5: 1 and most preferably 3.5: 1 to 5: 1.

An essential component of the present invention is the injection device, which comprises at least one nozzle for introducing the solution of the ammonia precursor substance into the mixing chamber. This nozzle according to the present invention may be a so-called single-fluid nozzle or a two-fluid nozzle. Alternatively, however, it can also be provided that the injection device comprises a nozzle called a flash evaporator. In a flash evaporator additional energy is supplied in the form of heat in the liquid, so that z. T. adjusts a supercritical state and results after the throttle point in the expansion in the nozzle a sudden or faster phase transition. However, particularly preferred is a two-fluid nozzle.

According to a particularly preferred variant, provision may in particular be made for the injection device in turn to comprise a nozzle which 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 called atomizing air. This type of nozzle allows a particularly fine distribution of the ammonia precursor substance and a droplet diameter d ₃₂ of less than 25 microns.

Independently or simultaneously, the injection device can also have at least two nozzles for introducing the ammonia precursor substance into the mixing chamber, which can be switched simultaneously or separately from one another.

According to a development of the ammonia gas generator, it is provided that the nozzle has a spray angle α of at least 10 °, in particular at least 20 °, in particular at least 25 ° and very particularly preferably at least 30 °. At the same time or independently thereof, preference is furthermore given to those nozzles which have a theoretical spray angle α of at most 90 °, in particular not more than 80 °, in particular not more than 70 °, in particular not more than 75 ° and very particularly preferably not more than 60 °. As already stated, by the targeted use of a nozzle with a defined spray angle α, a uniform distribution of the solution to be sprayed can be achieved without causing deposits on the walls or the catalyst end face.

The spray cone according to the present invention is the cone which can be produced by means of a nozzle with a defined spray angle α, the spray cone diameter being the diameter obtained on the occurrence of the droplets on the catalyst face. This is based on the liquid pressure of 0.1 to 10 bar on the solution to be sprayed at 25 ° C and optionally the atomizing air in the operating range of 0.5 to 10 bar (two-fluid nozzles).

As a further measure, that the inner wall of the catalyst unit is not wetted with the solution of the ammonia precursor substance, it can be provided according to the invention that the ammonia gas generator comprises a further inlet for a carrier gas, which injects a tangential carrier gas flow with respect to the solution into the mixing chamber Solution generated. Alternatively it can also be provided that at least one inlet for the carrier gas is provided around the nozzle, which is designed such that the carrier gas forms a jacket around the solution introduced into the mixing chamber. Thus, the sprayed solution is wrapped with a sheath of carrier gas, so that no wetting of the inner wall is observed.

Surprisingly, it has been found that deposits on the walls of the catalyst unit in the area of the mixing chamber can be further prevented by the tangential carrier gas flow and a permanently good mixing of carrier gas and solution of the ammonia precursor substance can be provided. Thus, wetting of the wall of the catalyst unit in the region of the mixing chamber is almost completely prevented. Due to the tangential carrier gas flow, a vortex veil flow with the droplets is generated, which is guided axially in the direction of the hydrolysis catalytic converter to the end face of the hydrolysis catalytic converter. This vortex flow allows a very good conversion to the ammonia on the catalyst. The tangential supply of the carrier gas takes place in the head region of the generator, in the amount of the spray device of the ammonia precursor solution into the catalyst unit or into the mixing chamber. In this case, the gas stream is introduced as flat as possible on the wall of the mixing chamber in such a way that sets a downward vortex flow in the catalyst unit in the direction of the catalyst end face.

A similar effect is produced when using a nozzle having a first number of nozzle orifices for introducing the ammonia precursor solution into the catalyst unit annularly surrounded by a second number of nozzle orifices for introducing a carrier gas or atomizing air into the catalyst unit.

In particular, the present invention provides an ammonia gas generator which operates autonomously from the exhaust gas flow, that is to say without the aid of an exhaust gas flow, from a combustion gas as the carrier gas. However, it can also be provided that the carrier gas as a partial flow of the exhaust gas to be released from nitrogen oxides is used. It has been shown that an ammonia gas generator according to the present invention should be operated with at most 20%, in particular at most 15%, in particular 10% and very particularly preferably at most 5% proportion of a partial flow. In addition, eleven ammonia gas generators according to the invention can have at least one thermal insulation layer.

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, C₁-C₁₂-Alkyl,
X^⊖ = Acetat, Carbonat, Cyanat, Formiat, Hydroxid, Methylat und 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 be split off or released in other forms by means of physical and / or chemical processes. In particular, urea, urea derivatives, guanidines, biguanidines and salts of these compounds and salts of ammonia can be used as the amine-manganese precursor compounds according to the present invention. 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 , C ₁ -C ₁₂ -alkyl,
X ^⊖ = acetate, carbonate, cyanate, formate, hydroxide, methylate and oxalate.

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 mixed with urea and / or or ammonia and / or ammonium salts combined. 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 that the mixture of guanidinium salt and urea has a guanidinium salt content of 5 to 60 wt .-% and a urea content of 5 to 35 wt .-%. Furthermore, mixtures of guanidinium salts and ammonia or ammonium salts having a guanidinium salt content of 5 to 60% by weight and of ammonia or ammonium salt of 5 to 40% by weight are to be regarded as preferred. Alternatively, however, it is also possible to use a urea solution, in particular an aqueous urea solution.

In particular, compounds of the general formula (II) have proven to be suitable as ammonium salts R-NH ₃ ^⊕ X ^⊖ (II) in which
R = H, NH ₂ , C ₁ -C ₁₂ -alkyl,
X ^⊖ = acetate, carbonate, cyanate, formate, hydroxide, methylate and oxalate
mean.

The guanidinium salts used according to the invention and optionally 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 Guanidiniumformiatlösung 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.

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 . Optionally, the catalyst unit may additionally comprise an outlet chamber downstream of the catalyst in the flow direction to the outlet of the ammonia gas formed.

As the catalyst for the decomposition and / or hydrolysis of ammonia precursors, any catalyst which allows the ammonia precursor substance to release ammonia from the precursor under catalytic conditions can be used in 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. For example, if a guanidinium salt solutions, in particular guanidinium formate solution, or urea solutions or mixtures thereof used, the catalytic decomposition to ammonia in the presence of catalytically active, non-oxidation coatings of oxides selected from the group of 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 help of such oxidation catalysts, it is possible to avoid the undesirable formation of carbon monoxide as a by-product in the decomposition of the guanidinium already in the ammonia production.

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.

It has been found that for a complete catalytic conversion of the ammonia precursor substances, catalysts having a catalyst cell count of at least 60 cpsi (cells per square inch, number of cells at the end face of the catalyst) and the catalyst volumes already described above are required. 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. Particular preference is given to those catalysts which have a catalyst cell number of 60 to 600 cpsi, of 60 to 500 cpsi and most preferably of 60 to 400 cpsi.

In the context of the present invention, it is possible to use a hydrolysis catalyst which consists of two sections in the flow direction, the first section containing 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. 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 employs a hydrolysis catalyst, which consists of two sections, wherein the first section arranged in the flow direction is part in the form of a heated catalyst and the second section of a non-heated catalyst. Preferably, the catalyst is from 5 to 50% by volume from the first section and from 50 to 95% by volume from the second section.

With regard to the configuration of the catalyst unit, it has been found in the tests that a cylindrical design is particularly suitable. Thus, the tangential carrier gas flow can exert 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.

Furthermore, it can be provided according to the present invention that the ammonia gas generator has at least one thermal insulation layer.

The ammonia gas generators described here are due to their compact design particularly suitable for use in industrial plants, in internal combustion engines such as diesel engines and gasoline engines, and gas engines. Therefore, the scope of the present invention also includes the use of an ammonia gas generator according to the described type 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.

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

1 : A schematic view of a first ammonia gas generator in axial cross-section

2 : a schematic structure of an exhaust system in a vehicle

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

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 ). In operation, the ammonia precursor solution (B) from a reservoir ( 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 hot carrier gas flow (C) is tangential across the inlet (FIG. 56 ) into the mixing chamber ( 51 ), whereby a vortex veil flow is produced with the droplets moving axially in the direction of the hydrolysis catalyst ( 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 Ammaniakgasgenerator ( 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. On the SCR catalyst ( 15 ), the reduction of the NOx with the aid of the reducing agent NH _{3 is carried out} on an SCR catalyst (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 tangential carrier gas stream supply. 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 tangential 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 ), at the level of the nozzle opening ( 42 ) of the nozzle ( 41 ). The inlet ( 56 ) designed for the carrier gas stream (C) such that the carrier gas stream as flat as possible on the wall ( 54 ) is introduced into the mixing chamber in such a way that sets a downward eddy current in the generator in the direction of the catalyst and thus a tangential carrier gas flow within the catalyst unit.

Embodiment 1

The structure corresponds to the principle with 1 reproduced ammonia gas generator. The ammonia generator is designed for a dosage of 10-100 g / h of NH ₃ and designed as a cylindrical tubular reactor. In the middle of the head is a two-fluid nozzle of the company Schlick Model 970, (0.3 mm) with variable air cap, coated with amorphous Si. The ammonia precursor substance is metered at room temperature through this nozzle and atomized in a full cone. It has been shown that aqueous guanidinium formate solution in the concentration of 20% to 60% and aqueous urea solution in the concentration of 25% to 40% as well as aqueous mixtures of guanidinium formate can be used for this setup as ammonia precursor solution and urea.

The liquid is entrained by means of a guided through the nozzle compressed air flow (0.5-2 bar) of about 0.8 kg / h and atomized. The Sauter diameter of the resulting droplets below the nozzle is <25 microns. There is a uniform radial distribution of the solution of the ammonia precursor substance over the reactor cross-section in the hot carrier gas stream upstream of the hydrolysis catalyst in a mixing chamber, without them touching the inner wall and leading to deposits. In the mixing chamber is already a drop evaporation in the way that the drop diameter is reduced by up to 20% when hitting the catalyst face. 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 catalyst and further energy supplies bring in so much energy that no cooling takes place below about 300 ° C. for the metered amount of solution. The dosing rate of 50-280 g / h is controlled by a Bosch PWM valve. The pressure for the promotion of the liquid is generated by means of positive pressure from a compressed air line in a storage tank, which is why no additional pump is required.

A hot carrier gas flow of about 1-5 kg / h is also introduced tangentially in the head region of the ammonia gas generator so that it lies in a veiling flow around the inner wall of the catalyst unit and is passed spirally through the mixing chamber. This prevents sprayed droplets from coming into contact with the inner wall. The diameter of the mixing chamber in the head region of the reactor is 70 mm. The length of the mixer chamber is 110 mm. The mixing chamber is additionally heated externally via an electrical resistance heating jacket (heating time max. 1 min.) - Model Hewit 0.8-1 kW, 150-200 mm. 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 a Microtherm superG insulation. The Microtherm superG filling is embedded between glass fiber fabric, which is wound around the reactor. Only the head area in which the injection of the solution is located is not isolated for better heat dissipation. The surfaces in the mixing chamber are coated with catalytically active TiO ₂ washcoat (anatase structure).

Following the mixing chamber is a heatable metal catalyst with 55 mm diameter and 400 cpsi flanged (Emitec Emicat, maximum power 1.5 kW, volume about 170 ml). This is carried out as a hydrolysis, likewise coated with catalytically active TiO ₂ (anatase, washcoat about 100 g / l, Fa. Interkat / Südchemie) and is controlled so that the temperature at the catalyst end face between 300 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, so that a total catalyst volume of about 330 ml results.

It has been found in other experiments with this structure that a spray angle of 20 ° at the above-mentioned mixing chamber length and catalyst end face leads to a non-uniform wetting of the catalyst end face and thus undesirable deposits on this. It has been shown that for a spray angle of 20 °, the length of the mixing chamber would have to be increased to about 150-160 mm. In contrast, for a spray angle of 60 °, a mixing distance of about 40-60 mm would be sufficient.

The ammonia generated on 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 area is conically shaped to prevent vortex formation on edges and thus deposits of possible residues. The gas mixture from the ammonia gas generator is at a temperature> 80 ° C to avoid Ammoniumcarbonatablagerungen the engine exhaust gas upstream of SCR catalyst and distributed in this exhaust stream homogeneously via a static mixer.

The material used for all metallic components is 1.4301 (V2A, DIN X 5 CrNi18-10), alternatively 1.4401 (V4A, DIN X 2 CrNiMo17-12-2), 1.4767 or other catalytic converter type Fe-Cr-Al alloys. Table 1: Geometries and performance of the ammonia gas generators used (Example 1 corresponds to the embodiment 1, generator 2 and 3 are constructed analogously to the generator 1 and differ in the dimensions indicated herewith) Generator 1 Generator 2 Generator 3 Distance nozzle opening to catalyst end face [mm] 100 150 50 Spray angle α [°] 30 20 60 Catalyst diameter [mm] 55 55 55 Catalyst volume V _KAT [ml] 330 330 330 Spray cone diameter [mm] 54 54 54 Mixing chamber length [mm] 110 160 60 Ammonia degree AG [%] > 95% > 95% > 95% Deposits on the catalyst face none none none Deposits on the mixing wall chamber none none none

These generators 1-3 were operated both with a 60% guanidinium formate solution and with a 32.5% aqueous urea solution and also with mixtures of both. The results of these ammonia precursor solutions are approximately identical (+/- 1%). All generators were operated essentially continuously and maintenance-free, since a degree of ammonia formation of> 95% was achieved, with no deposits being observed on the catalyst end face or on the wall of the mixing chamber.

The following are the operating parameters that should be followed when operating the ammonia gas generator. Table 2: Overview of further operating parameters description formula units Area from medium to Dosing mass flow of the solution of the ammonia precursor substance per hour m _red [G / h] 50 150 280 Mass flow of carrier gas m _exhaust [Kg / h] 1 5 10 Mass flow atomizing air m _nozzle [Kg / h] 0.14 0.71 1.43 Zuheizenergiemenge E _heating [Y / s] = [W] 0 70 150 Catalyst face temperature T _a [° C] 280 350 500 catalyst outlet T _off [° C] 250 320 450 Catalyst space velocity RG [1 / h] 5000 15,000 30,000 Dosing pressure of the liquid p _red [bar] 1 2 8th Catalyst face load per hour m _Red / A _cat [g / (hcm ² )] 0.53 1.59 3.45 specific enthalpy current H _TG / m _Red [KJ / kg] 8000 16,000 25,000

If these parameters are in the appropriate range, operation of the ammonia gas generator without deposits and with almost complete conversion of the ammonia precursor solution is provided.

Is z. As the introduced specific enthalpy below the range listed in Table 2, so no complete decomposition of the Ammoniakvarläufers is given more. Deposits in the catalyst occur and sales drop below 90%.

If the catalyst end face load is exceeded (eg metered amount at 500 g / h), too much cooling occurs at the catalyst end face. If the temperature at the catalyst end face is too low, the impinging droplets are no longer sufficiently vaporized and unwanted side reactions (triazine formation) occur at the catalyst end face.

Embodiment 2:

In embodiment 2, the reactor is designed in such a way that the reactor is partially co-heated by countercurrent heat exchange from the supplied hot carrier gas stream. It is the Carrier gas stream initially passed below the reactor head via a double jacket against the flow direction in the interior of the double jacket to the reactor wall and flows around this on the way to the reactor head. At the reactor head, the main flow from the reactor double jacket enters the interior of the reactor via a plurality of bores or alternatively via an annular gap in the region of the nozzle at the reactor head. In addition, an electrical resistance heater can be located in the double jacket.

Embodiment 3

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

Embodiment 4

In embodiment 4, the reactor is designed such that no heating of the reactor is carried out from the outside but is supplied via heat via an electrically heatable catalyst Emikat from Fa. Emitec directly inside the reactor. Alternatively, heat in the reactor can be generated by model glow plugs (60 W, 11 V).

Embodiment 5:

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

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 487886 B1 [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]

Cited non-patent literature

• DIN 66 141 [0022]

Claims (14)

Ammonia gas generator ( 100 ) for producing ammonia from a solution of an ammonia precursor substance comprising - 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 , - 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, characterized in that the injection device ( 40 ) a nozzle ( 41 ), which has a theoretical spray angle α of 10 ° to 90 ° and the distance of the nozzle opening ( 42 ) to the face ( 61 ) of the catalyst ( 60 ) of 15 to 2,000 mm. Ammonia gas generator ( 100 ) according to at least one of the preceding claims, characterized in that the injection device ( 40 ) a nozzle ( 41 ) having a spray angle α of 20 ° to 60 ° and the diameter of the catalyst Da is 30 to 80 mm. Ammonia gas generator ( 100 ) according to at least one of the preceding claims, characterized in that the injection device ( 40 ) a nozzle ( 41 ) having a spray angle α of 20 ° to 60 ° and the diameter of the catalyst D _{Kat is} 80 to 450 mm. Ammonia gas generator ( 100 ) according to at least one of the preceding claims, characterized in that the injection device ( 40 ) a nozzle ( 41 ) having a spray angle α of 20 ° to 60 ° and the diameter of the catalyst D _{Kat is} 450 to 1,000 mm. 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 V _Kat corresponds to the ratio of 1.5: 1 to 5: 1. Ammonia gas generator ( 100 ) according to at least one of the preceding claims, characterized in that the injection device ( 40 ) comprises at least two of these for introducing the solution of the ammonia precursor substance in the mixing chamber, which are simultaneously or separately switchable. Ammonia gas generator ( 100 ) according to at least one of the preceding claims, characterized in that the ammonia gas generator ( 100 ) another inlet ( 56 ) for a carrier gas containing a tangential carrier gas flow with respect to the in the mixing chamber ( 51 ) injected solution produced. Ammonia gas generator ( 100 ) according to at least one of the preceding claims, characterized in that the nozzle ( 41 ) a two-fluid nozzle and the catalyst ( 60 ) is a hydrolysis catalyst. Ammonia gas generator ( 100 ) according to at least one of the preceding claims, characterized in that the nozzle ( 41 ) a first number of nozzle openings for introducing the solution into the catalyst unit ( 70 having a second number of nozzle openings for introducing a carrier gas into the catalyst unit (US Pat. 70 ) is surrounded annularly. Ammonia gas generator ( 100 ) according to at least one of the preceding claims, characterized in that the ammonia gas generator ( 100 ) a dosing unit ( 30 ) for metering the solution of the ammonia precursor substance, the injection device ( 40 ) is connected upstream. Ammonia gas generator ( 100 ) according to at least one of the preceding claims, characterized in that the catalyst ( 60 ) is a hydrolysis catalyst having a catalyst cell number of at least 60 cpsi to at most 800 cpsi. Ammonia gas generator ( 100 ) according to at least one of the preceding claims, characterized in that the catalyst unit ( 70 ) a two-part hydrolysis catalyst ( 62 . 63 ) in the flow direction of the first part in the form of a heated catalyst ( 62 ) and its second part in the form of a non-heated catalyst ( 63 . 64 ) is present. Ammonia gas generator ( 100 ) according to at least one of the preceding claims, characterized in that the ammonia gas generator at least one thermal insulation layer ( 53 ) having. 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 from industrial plants, internal combustion engines, gas engines, diesel engines or gasoline engines.

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