Method for the production of ammonia from an ammonia precursor substance in order to reduce nitrogen oxides in exhaust gases

Abstract

The present invention relates to a method for generating ammonia from an ammonia precursor substance and to the use thereof for reducing nitrogen oxides in exhaust from industrial facilities, from combustion engines, from gas engines, from diesel engines or from petrol engines.

Claims

1. A method for continuously generating ammonia for introducing into an exhaust gas line, wherein the ammonia is generated from a solution of an ammonia precursor substance using an ammonia gas generator, the generator comprising a catalyst unit and a mixing chamber, the catalyst unit comprising a heated catalyst for at least one of decomposing and hydrolysing ammonia precursor substances into ammonia, and the mixing chamber being upstream from the catalyst in a flow direction, the catalyst having a catalyst volume and the mixing chamber having a mixing chamber volume, the method comprising: introducing the solution of the ammonia precursor substance into the mixing chamber together with a carrier gas, the carrier gas and an optionally further additional energy source having a combined specific enthalpy flow of 8000-50000 kJ/kg with respect to a mass flow of the solution, such that an end face loading of the catalyst is 3.0 to 15 g/(h*cm.sup.2), wherein the ammonia gas generator is separate from the exhaust gas line.

2. The method of claim 1, further comprising: introducing the carrier gas into the mixing chamber separately from introducing the solution of the ammonia precursor substance.

3. The method of claim 1, further comprising: introducing the carrier gas into the mixing chamber separately from introducing the solution of the ammonia precursor substance and introducing the carrier gas tangentially to the solution of the ammonia precursor substance.

4. The method of claim 2, wherein the carrier gas comprises a partial stream of an exhaust, the partial stream containing less than 5 vol. % of a total exhaust.

5. The method of claim 1, wherein introducing the solution of the ammonia precursor substance further comprises spraying the solution into the mixing chamber from a reservoir container by a nozzle having a spray angle of 10° to 40°.

6. The method of claim 1, wherein introducing the solution of the ammonia precursor substance further comprises injecting the solution at a pressure of at least 0.5 bar; and further comprising: injecting atomisation air at a pressure of 0.5 to 2 bar.

7. The method of claim 1, further comprising: applying the solution of the ammonia precursor substance to the end face of the catalyst in the form of droplets having droplet diameters of less than 20 μm.

8. The method of claim 1, further comprising: introducing the carrier gas and atomisation air, wherein a volume ratio of the carrier gas to the atomisation air is 7:1 to 10:1.

9. The method of claim 1, wherein introducing the solution of the ammonia precursor substance further comprises: spraying the solution into the mixing chamber perpendicular to the catalyst end face.

10. A method for reducing nitrogen oxides in exhaust from at least one of industrial facilities, combustion engines, gas engines, diesel engines, and petrol engines, the method comprising: (i) providing an ammonia gas generator comprising a catalyst unit that comprises: (a) a heated catalyst for at least one of decomposing or hydrolysing ammonia precursor substances into ammonia, and (b) a mixing chamber upstream from the catalyst in a flow direction, the catalyst being of a catalyst volume and the mixing chamber being of a mixing chamber volume; and introducing a solution of an ammonia precursor substance into the mixing chamber together with a carrier gas, the carrier gas and an optionally further additional energy source having a combined specific enthalpy flow of 8000-50000 kJ/kg with respect to a mass flow of the solution, such that an end face loading of the catalyst is 3.0 to 15 g/(h*cm.sup.2), wherein the ammonia gas generator is separate from an exhaust gas line; and (ii) introducing the ammonia generated using the ammonia gas generator into the exhaust gas line.

11. The method of claim 1, wherein a ratio of the mixing chamber volume to the catalyst volume is 1.5:1 to 5:1.

12. The method of claim 1, wherein introducing the solution of the ammonia precursor substance further comprises: spraying the solution such that a spray cone diameter upon incidence on the catalyst end face is at least 80% and at most 98% of a diameter of the catalyst end face.

13. A method for continuously generating ammonia for introduction into an exhaust gas line, wherein the ammonia is generated from a solution of an ammonia precursor substance using an ammonia gas generator, the ammonia gas generator comprising a catalyst unit, the catalyst unit comprising a heated catalyst for at least one of decomposing or hydrolysing ammonia precursor substances into ammonia, and a mixing chamber upstream from the catalyst in a flow direction, the catalyst having a catalyst volume and the mixing chamber having a mixing chamber volume, the method comprising: spraying the solution of the ammonia precursor substance into the mixing chamber together with a carrier gas, the carrier gas and an optionally further additional energy source having a combined specific enthalpy flow of 8000-50000 kJ/kg with respect to a mass flow of the solution, such that an end face loading is 1.0 to 15 g/(h*cm.sup.2), and such that a spray cone diameter of the solution upon incidence on an end face of the catalyst is at least 80% and at most 98% of a catalyst end face diameter, wherein the ammonia gas generator is separate from the exhaust gas line.

14. A method for continuously generating ammonia for introduction into an exhaust gas line, wherein the ammonia is generated from a solution of an ammonia precursor substance by an ammonia gas generator, the ammonia gas generator comprising a catalyst unit, the catalyst unit comprising a heated catalyst for at least one of decomposing or hydrolysing ammonia precursor substances into ammonia, and a mixing chamber upstream from the catalyst in a flow direction, the catalyst having a catalyst volume and the mixing chamber having a mixing chamber volume, the method comprising: spraying the solution of the ammonia precursor substance into the mixing chamber together with a carrier gas, the carrier gas and an optionally further additional energy source having a combined specific enthalpy flow of 8000-50000 kJ/kg with respect to a mass flow of the solution, such that a spray cone diameter of the solution upon incidence on an end face of the catalyst is at least 80% and at most 98% of a catalyst end face diameter, wherein the ammonia gas generator is separate from the exhaust gas line.

15. The method of claim 1, wherein the mixing chamber is bounded at one end by a catalyst end face of the catalyst.

16. The method of claim 1, wherein the catalyst comprises a catalyst end face that is an entry face for the catalyst; and further comprising: passing the solution of the ammonia precursor substance through the catalyst.

17. The method of claim 1, further comprising: passing ammonia gas generated by the ammonia gas generator from an outlet of the ammonia gas generator to the exhaust gas line.

Description

(1) In the following, the present invention is described in greater detail by way of drawings and associated examples, in which:

(2) FIG. 1 is a schematic axial cross-sectional view of a first ammonia gas generator;

(3) FIG. 2 shows a schematic construction of an exhaust system in a vehicle;

(4) FIG. 3 is a radial cross-section of the mixing chamber (plan view) in the region of the tangential carrier gas stream supply;

(5) FIG. 4 shows a diagram 1 of the conversion of the ammonia precursor solution into ammonia according to the end face loading; and

(6) FIG. 5 shows a diagram 2 of the conversion of the ammonia precursor solution into ammonia according to the specific enthalpy flow.

(7) FIG. 1 shows 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), a mixing chamber (51) and an outlet chamber (55). In the operating state, the ammonia precursor solution (B) is sprayed out of a reservoir container (20) via a metering pump (30) together with an atomisation air stream (A) via a two-substance nozzle (41) having a nozzle opening (42) into the mixing chamber (51) of the ammonia gas generator (100) at a defined spray angle, and distributed into fine droplets. Additionally, a hot transport gas stream (C) is introduced into the mixing chamber (51) tangentially via the inlet (56), causing an eddy mist flow comprising the droplets to be generated, which is passed axially in the direction of the hydrolysis catalyst (60) onto the hydrolysis catalyst end face (61). The catalyst (60) is configured in such a way that the first segment (62) is in the form of an electrically heatable metal carrier comprising a hydrolysis coating. This is followed by an unheated metal carrier catalyst (63), likewise comprising a hydrolysis coating and an unheated catalyst (64) comprising a hydrolysis coating configured as a mixer structure for better radial distribution. The generated ammonia gas (D) exits the generator (100) together with the hot carrier gas stream via the outlet chamber (55) comprising the outlet (80) and the valve (81). The generator may additionally be heated by a jacket heater (52) around the housing (54) of the catalyst unit. Apart from the head region in which the injection device (40) is located, the ammonia gas generator (100) is enclosed in a thermal insulation (53) of microporous cladding material.

(8) FIG. 2 shows a schematic material flow of an exhaust treatment on a combustion engine (10). In this context, the exhaust from the combustion engine (10) is passed through a charging unit (11) and in a counter flow incoming air (E) for the internal combustion engine is compressed. The exhaust (F) is guided over an oxidation catalyst (12), so as to achieve a higher NO.sub.2 concentration in relation to NO. The ammonia-containing gas stream (D) from the ammonia gas generator (100) can be supplied and mixed in both upstream and downstream from a particle filter (13). In this context, an additional gas mixer (14) in the form of a static mixer or for example a Venturi mixer may be used. The NOx is reduced at the SCR catalyst (15) by means of the reducing agent NH.sub.3 at an SCR catalyst (SCR=selective catalytic reduction). In this context, the ammonia gas generator may be operated using separate carrier gas or else using a partial exhaust stream.

(9) FIG. 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 enclosed in a thermal insulation (53) of microporous cladding material in the region of the mixing chamber (51). The tangential supply of the carrier gas (C) is provided in the head region of the ammonia gas generator or in the head region of the mixing chamber (51), at the level of the nozzle opening (42) of the nozzle (41). In this context, the inlet (56) for the carrier gas stream (C) is configured in such a way that the gas stream is introduced as shallowly as possible against the wall (54) of the mixing chamber, in such a way that a downwardly directed eddy current in the generator in the direction of the catalyst and thus a tangential carrier gas stream inside the catalyst unit sets in.

PRACTICAL EXAMPLE 1

(10) The construction basically corresponds to the ammonia gas generator shown in FIG. 1. The ammonia gas generator is configured for a metering amount of 10-100 g/h NH.sub.3 and is in the form of a cylindrical tubular reactor. A two-substance nozzle from Schlick, model 970 (0.3 mm), having a variable air cap and coated with amorphous Si, is arranged centrally in the head region. The ammonia precursor substance is metered in at room temperature through this nozzle and atomised in a full cone. The spray angle α is 30°. In this context, the liquid is entrained, by means of a pressurised air stream (0.5-2 bar) of approximately 0.8 kg/h which is passed through the nozzle, and atomised. The Sauter mean diameter of the resulting droplets below the nozzle is <25 μm. There is a uniform radial distribution of the solution of the ammonia precursor substance over the reactor cross-section in the hot transport gas stream upstream from the hydrolysis catalyst in a mixing chamber, without these touching the reactor wall in the process, which could lead to depositions. In the mixing chamber drops are already evaporating in such a way that upon incidence on the catalyst end face the drop diameter is reduced by up to 20%. As a result of the droplets which are still present, cooling of approximately 120-150° C. occurs at the catalyst end face. Therefore, the reactor is configured in such a way that the amount of heat supplied with the hot transport gas stream, the integrated heatable hydrolysis catalyst and further supplies of energy introduce sufficient energy that for the amount of solution metered in there is no cooling to below approximately 300° C. In this context, the metering amount of 50-280 g/h is controlled by means of a Bosch PWM valve. The pressure for conveying the liquid is generated from a pressurised air line in a reservoir container by overpressure, and therefore no additional conveyor pump is required.

(11) A hot transport gas stream of approximately 1-5 kg/h is likewise introduced tangentially in the head region of the ammonia gas generator in such a way that it is laid in a mist stream around the reactor wall and is passed through the mixing chamber in a spiral shape. By means of this gas stream, on one hand axial transport through the reactor is achieved at a defined retention time (reciprocal value of the space velocity) and on the other hand, sprayed droplets are prevented from coming into contact with the reactor wall. The diameter of the mixing chamber in the head region of the reactor is 70 mm. The length of the mixing chamber is 110 mm. The mixing chamber is additionally heated from the outside via an electric resistance heating casing (heating time max. 1 min.)—model Hewit 0.8-1 kW, 150-200 mm. The temperature is regulated in connection with temperature sensors (type K) which are arranged in and downstream from the catalyst and on the catalyst end face. All of the outer surfaces of the reactor are enclosed by Microtherm superG insulation. In this context, the Microtherm superG filling is embedded between glass fibre meshing which is wound around the reactor. Only the head region in which the solution is injected is uninsulated, for better heat dissipation. The surfaces in the mixing chamber are coated with catalytically active TiO.sub.2 washcoats (anatase structure).

(12) A heatable metal carrier catalyst of 55 mm diameter and 400 cpsi (Emitec Emicat, maximum power 1.5 kW, volume approximately 170 ml) is flange-mounted downstream from the mixing chamber. Said catalyst is in the form of a hydrolysis catalyst, likewise coated with catalytically active TiO.sub.2 (anatase, washcoat approximately 100 g/l, from Interkat/Südchemie), and is regulated in such a way that the temperature at the catalyst end face is between 300 and 400° C. In this context, only enough energy is supplied to compensate the cooling resulting from the evaporation of the droplets. To achieve a space velocity of up to at least 7000 1/h, a further hydrolysis catalyst of 400 cpsi is connected downstream, resulting in a total catalyst volume of approximately 330 ml.

(13) The ammonia generated at the hot hydrolysis catalyst flows freely in the foot region via the outlet chamber, centrally from an outlet opening from the reactor end piece. In this context, the outlet region is preferably shaped conically, so as to prevent eddy formation at edges and thus depositions of possible residues. The gas mixture from the ammonia gas generator is preferably supplied to the motor exhaust stream upstream from the SCR catalyst at a temperature >80° C. to prevent ammonium carbonate depositions, and distributed homogeneously in this exhaust stream by way of a static mixer.

(14) 1.4301 (V2A, Din X 5 CrNi 18-10) or alternatively 1.4401 (V4A, DIN X 2 CrNiMo 17-12-2), 1.4767, or other Fe Cr Al alloys typical of exhaust catalysts are used as the material for all of the metal components.

(15) In the following, the influence of the end face loading and the specific enthalpy flow on the continuous generation of the ammonia is set out, the ammonia gas generator from example 1 having been used. These generators were operated with a 60% guanidinium formate solution and with a 32.5% aqueous urea solution as well as with mixtures of the two. In this context, the results for these ammonia precursor solutions are approximately identical (±1%).

(16) TABLE-US-00001 TABLE 1 Processes according to the end face loading V1 V2 V3 V4 V5 Distance from nozzle opening to 100 100 100 100 100 catalyst end face [mm] Spray cone diameter [mm] 54 54 54 54 54 Metering mass flow of the solution of the 50 160 280 4 400 ammonia precursor substance per hour [g/h] Catalyst end face loading per hour 2.1 7.0 12.0 0.17 17.5 [g/(h * cm.sup.2)] Specific enthalpy flow 8000 12000 16000 16000 16000 Ammonia formation level AG [%] ≧95% ≧95% ≧95% ≧95% <90% Depositions on catalyst end face none none none none yes Depositions on the mixing wall chamber none none none none none

(17) By setting the catalyst end face loading to at least 0.17 g/(h*cm.sup.2) (cf. V4), a process can be provided in which depositions are also not formed over a time period of >100 h. Even if the end face loading is 2.1 g/(h*cm.sup.2) or 7.0 g/(h*cm.sup.2) or 12.0 g/(h*cm.sup.2) over a time period of >100 h, no depositions are observed, a continuous process being ensured thereby. If the end face loading is set to a value of 17.5 g/(h*cm.sup.2) (cf. V5), depositions on the catalyst end face are observed. A continuous process is thus no longer possible.

(18) The formation of ammonia according to the end face loading is reproduced in FIG. 4.

(19) TABLE-US-00002 TABLE 2 Processes according to the specific enthalpy flow V1 V2 V3 V4 V5 Distance from nozzle opening to 100 100 100 100 100 catalyst end face [mm] Spray cone diameter [mm] 54 54 54 54 54 Metering mass flow of the solution of the 160 160 160 160 160 ammonia precursor substance per hour [g/h] Catalyst end face loading per hour 7.0 7.0 7.0 7.0 7.0 [g/(h * cm.sup.2)] Specific enthalpy flow [kJ/kg] 8000 12000 16000 2000 20000 Ammonia formation level AG [%] ≧95% ≧95% ≧95% <90% ≧95% Depositions on catalyst end face none none none yes none Depositions on the mixing wall chamber none none none yes none

(20) By setting the specific enthalpy to at least 8000 kJ/kg (cf. V1, V2, V3 and V5), a process can be provided in which depositions are also not formed over a time period of >100 h, it being possible to provide a continuous process thereby. If the specific enthalpy is set to 2000 kJ/kg (cf. V4), depositions on the mixing chamber wall and the catalyst end face are observed. The formation of ammonia according to the specific enthalpy flow is reproduced in FIG. 5.

(21) The operating parameters which should be adhered to during operation of the ammonia gas generator are specified in the following.

(22) TABLE-US-00003 TABLE 3 Overview of further operating parameters For- Range Name mula Unit from average to Carrier gas m.sub.Abα [kg/h] 1 5 10 mass flow Atomisation air m.sub.Dü.sub.se [kg/h] 0.14 0.71 1.43 mass flow Heating energy E.sub.Heiz [J/s] = [W] 0 70 150 Catalyst end T.sub.ein [° C.] 280 350 500 face temperature Catalyst outlet T.sub.aus [° C.] 250 320 450 temperature Catalyst space RG [1/h] 5000 15000 30000 velocity Metering P.sub.Red [bar] 1 2 8 pressure of the liquid

PRACTICAL EXAMPLE 2

(23) In practical example 2, the reactor is configured in such a way that the reactor is additionally heated in part as a result of counter flow heat exchange by the supplied hot carrier gas stream. In this context, the carrier gas stream is initially passed below the reactor head, via a double casing, counter to the flow direction in the inside of the double casing, to the reactor wall, and flows around said wall on the way to the reactor head. At the reactor head, the primary flow from the reactor double casing enters the reactor interior from the reactor double casing via a plurality of holes or alternatively via an annular gap in the region of the nozzle at the reactor head. In addition, an electrical resistance heater may be located in the double casing.

PRACTICAL EXAMPLE 3

(24) In practical example 3, the reactor is configured in such a way that the reactor is heated from the outside by heat exchange with hot components of a combustion engine or of a separate burner for exhaust heating or by hot gas flows, rather than by means of an electrical resistance heater. In this context, the heat can also be transported to the reactor via a heating tube over some distance.

PRACTICAL EXAMPLE 4

(25) In practical example 4, the reactor is configured in such a way that heat is supplied directly in the interior of the reactor by means of an electrically heatable Emikat catalyst from Emitec, instead of the reactor being heated from the outside. Alternatively heat can be generated in the reactor by glow plugs, model Champion (60 W, 11 V).

PRACTICAL EXAMPLE 5

(26) With preheating of the liquid solution of the ammonia precursor substance—when an injector having critical superheating (flash evaporator) is used.