EP1444030A1

EP1444030A1 - Method and device for reducing nitrogen oxides present in exhaust gases

Info

Publication number: EP1444030A1
Application number: EP02802599A
Authority: EP
Inventors: Wolfgang Ripper; Gerd Scheying; Johannes Schaller
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2001-11-06
Filing date: 2002-10-18
Publication date: 2004-08-11
Also published as: KR20040060969A; US20040115110A1; WO2003039718A1; DE10154421A1; JP2005507985A

Abstract

The invention concerns a method and a device for reducing nitrogen oxides in exhaust gases, in particular in a motor vehicle exhaust gases. The method is characterized in that it consists in introducing an aqueous urea solution in a reactor (42) and converting it by thermal, catalytic or enzymatic treatment into a decomposition product containing ammonia and carbon dioxide, the ammonia being added to the exhaust gases. Th aqueous urea solution contains a substance for heat transmission and for lowering freezing point.

Description

Method and device for reducing nitrogen oxides in an exhaust gas

State of the art

The present invention is based on a method for reducing nitrogen oxides in an exhaust gas, in particular an exhaust gas of a motor vehicle, in accordance with the type defined in more detail in the preamble of patent claim 1 and on an apparatus for carrying out the method in accordance with the type defined in more detail in the preamble of patent claim 15.

In diesel combustion machines in particular, it has proven useful to add urea in aqueous solution to the exhaust gas generated by the internal combustion engine. In the case of known processes, a hydrolysis catalytic converter is used in which ammonia is obtained from the urea. The aqueous urea solution is upstream of the hydrolysis catalytic converter. tors injected into the relevant exhaust line. The ammonia then reacts on a so-called SCR catalyst with the nitrogen oxides contained in the exhaust gas to form molecular nitrogen and water.

When the aqueous urea solution is injected into the exhaust gas, compressed air can additionally be used, which aids in atomizing the solution, so that an aerosol is formed. Droplets of the aqueous urea solution are thus distributed in the exhaust gas.

The use of compressed air further reduces the risk of the urea crystallizing out of the aqueous urea solution and clogging the nozzle or a line leading to the nozzle.

In systems that work without additional compressed air, it has been shown that the urea cannot be reliably removed from the aqueous urea solution.

In a method known from practice for reducing nitrogen in an exhaust gas from a motor vehicle, a 32.5% urea solution is provided in a tank. In order to denitrify the exhaust gas, this solution is removed from the tank and sprayed into the exhaust gas. The droplets of the urea solution formed during the injection heat up in the exhaust gas, so that the water in the solution evaporates and the urea dries. This causes the urea to crystallize. The urea crystals formed in this way heat up further until the at 130 ° C Melting point of urea is reached. At this temperature, urea is thermally decomposed into ammonia (NH ₃ ) and isocyanic acid (HNCO). By using a hydrolysis catalyst, the isocyanic acid is converted to ammonia and carbon dioxide using water.

Particularly in the case of non-ideal operating conditions, crystals or undesired by-products can arise in the process described above. Such by-products are, for example, water-insoluble biuret, which is formed from isocyanic acid and urea, and cyanuric acid, which is the trimerization product of isocyanic acid.

A method of the type mentioned in the introduction is known from US 6,077,491. In this process, solid urea is added to a tank from a storage container and processed there together with water to form an aqueous urea solution. The aqueous urea solution is fed to a so-called hydrolysis reactor by means of a pump, where it is heated by means of a heating device and thus preconditioned. When heated, ammonium carbamate is produced as an intermediate product, which in turn produces ammonia and carbon dioxide, which are largely present in a gas phase. The gas phase is then fed via a check valve with the support of compressed air to a process gas stream, as a result of which nitrogen oxides contained therein are broken down.

Advantages of the invention The method for reducing nitrogen oxides of an exhaust gas, in particular an exhaust gas of a motor vehicle, with the features according to the preamble of claim 1, in which method the aqueous urea solution comprises a substance for heat transfer and freezing point depression, has the advantage in particular in use in motor vehicles that it can also be used at low ambient temperatures.

By using the method according to the invention, in which the aqueous urea solution comprising the substance for heat transfer and lowering the freezing point is preconditioned, it is possible to ensure a substantial reduction of nitrogen oxides in an exhaust gas down to temperatures of minus 40 ° C. without this would lead to the formation of urea crystals, which lines or nozzles could clog a device for carrying out the method.

The substance for heat transfer and freezing point depression is basically characterized in that it does not react chemically with urea, has a boiling point that is higher than that of water, and has a freezing point that is lower than that of water. With an excess of ammonia and carbon dioxide contained in the aqueous solution, the substance preferably also serves to stabilize the solution. The substance for heat transfer and freezing point depression and H ₂ 0 form the carrier for the urea contained in the solution.

The heat transfer and freezing point lowering agent is selected, for example, from a group which includes ethylene glycol diethyl ether, diethylene glycol dibutyl ether, diethylene triamine, diethyl glycol and glycerol.

According to a preferred embodiment of the method according to the invention, the aqueous urea solution comprising the substance for heat transfer and freezing point reduction is a supersaturated solution, the urea and the water of the solution having a molar ratio of 1: 1. As a result, the urea can ideally be completely hydrolyzed during preconditioning in the reactor, with no additional water vapor being removed from the reactor.

The urea is thermohydrolysed according to the following formula:

(NH ₂ ) ₂ CO + H ₂ 0 → 2NH ₃ + C0 ₂

According to a preferred embodiment of the process according to the invention, the aqueous urea solution comprising the substance for heat transfer and freezing point depression is pumped into the reactor under pressure. All liquid components of the solution and the gases dissolved in the solution form the carrier for the urea. In this so-called high pressure process, the aqueous urea solution is preferably heated to a temperature above of about 180 ° C. Under these conditions, the urea is completely decomposed into ammonia and carbon dioxide in the aqueous solution. There are no undesirable by-products such as isocyanic acid. The decomposition product, which comprises the ammonia and the carbon dioxide, can then be atomized directly and thus added to the exhaust gas in question.

To separate a gas phase and a liquid phase, the gas phase comprising carbon dioxide, ammonia and, if appropriate, H ₂ O, the decomposition product is advantageously expanded downstream of the reactor. However, the pressure of the gas phase present after the expansion should be above the pressure of the exhaust gas in question, so that the gas phase can be added to the exhaust gas without further energy expenditure.

Alternatively, in a so-called low-pressure process, in which the aqueous urea solution is fed to the reactor, for example under atmospheric pressure, the decomposition product can be compressed downstream of the reactor, likewise to a pressure which is above the pressure of the waste gas in question.

In the low-pressure process, it may be sufficient to heat the aqueous urea solution comprising the material for heat transfer and freezing point lowering to a temperature between 80 ° C. and 150 ° C. Thermolysis, ie the thermal decomposition of urea into ammonia and isocyanic acid, takes place at these temperatures. The components remain in aqueous solution, which prevents the formation of unwanted by-products. The aqueous solution can then be sprayed into the exhaust gas and further heated by it. The unwanted isocyanic acid decomposes in aqueous solution in reaction with water to carbon dioxide and ammonia.

If the decomposition product comprises a liquid and a gaseous phase, the liquid phase can be returned to a tank for the aqueous urea solution comprising the substance for heat transfer and freezing point reduction. The liquid phase usually includes water, the substance and small amounts of dissolved ammonia and carbon dioxide.

The liquid phase is advantageously relaxed when it is returned to the tank, specifically to the pressure prevailing in the tank, for example atmospheric pressure.

If appropriate, it may be expedient to cool the aqueous urea solution preconditioned in the reactor downstream of the reactor. If a dosing valve is present, this should also be done upstream of the dosing valve in order to protect it from thermal damage.

Economic process control is ensured if the aqueous urea solution comprising the carrier is heated by means of the exhaust gas. In this case, the reactor is arranged, for example, in an exhaust line of a motor vehicle, so that the exhaust gas heats the reactor and thus the urea solution comprising the substance for heat transfer and freezing point reduction. The invention also relates to a device for carrying out such a method according to the invention. This device comprises a tank for an aqueous urea solution, a reactor in which the aqueous urea solution is preconditioned, and a valve by means of which a decomposition product comprising ammonia generated by the reactor is supplied to an exhaust gas line. The aqueous urea solution comprises a substance for heat transfer and freezing point depression.

The device according to the invention is particularly suitable for use in an exhaust line of a diesel internal combustion engine of a motor vehicle.

Further advantages and advantageous refinements of the subject matter according to the invention can be found in the description, the drawing and the patent claims.

drawing

Several embodiments of the method according to the invention and the device according to the invention are shown schematically simplified in the drawing and are explained in more detail in the following description. Show it

Figure 1 is a schematic diagram of a low pressure process for reducing nitrogen oxides in an exhaust gas;

Figure 2 is a schematic diagram of an apparatus for performing the method of Figure 1;

FIG. 3 shows a high-pressure process for reducing nitrogen oxides in an exhaust gas; Figure 4 is a schematic diagram of an apparatus for performing the method of Figure 3;

Figure 5 is a schematic diagram of an alternative device for performing the method of Figure 3;

FIG. 6 shows a schematic diagram of a special embodiment of a device for carrying out a high-pressure process for reducing nitrogen oxides in the exhaust gas of a motor vehicle; and

FIG. 7 shows a special embodiment of a high-pressure process for reducing nitrogen oxides in the exhaust gas of a motor vehicle.

Description of the embodiments

FIG. 1 shows a low-pressure process for preconditioning an aqueous urea solution which comprises a substance for heat transfer and freezing point depression. The method according to FIG. 1 can be carried out by means of a device shown in FIG. 2.

In the process according to FIG. 1, in a process step 1, an aqueous, for example 32.5% urea solution is stored in a tank 20, which comprises a substance for heat transfer and lowering of the freezing point. This substance is formed here, for example, from diethylene glycol diethyl ether.

The solution carried in the tank 20 is led in a process step 2 via a line 21 into a reactor 22 and heated there to a temperature between 80 ° C and 150 ° C.

The urea (NH ₂ ) ₂ CO contained in the solution decomposes by heating the solution in ammonia NH ₃ and isocyanic acid HNCO and is thus preconditioned. HNCO is unstable in aqueous solution and decomposes to NH ₃ and C0 ₂ . The decomposition product or preconditioning product therefore consists of ammonia, water, CO ₂ and the substance for heat transfer and lowering the freezing point, the ammonia, depending on the process pressure, being partially gaseous.

In a further process step 3, the decomposition product described above is injected by means of a pump 23 and a solenoid valve 24 into an exhaust line of a motor vehicle (not shown here) and further heated there in a process step 4, so that the water evaporates and therefore changes into a gaseous state. The gaseous ammonia NH ₃ reacts with nitrogen oxides, which are contained in exhaust gases flowing in the exhaust line, to form molecular nitrogen and water. The reaction takes place on a so-called SCR catalyst (SCR = Selective Catalytic Reduction).

FIG. 3 shows a high-pressure process for reducing nitrogen oxides in the exhaust gas of a motor vehicle. A device for carrying out the method is shown in FIG. 4 and FIG. 5. In the high-pressure process shown in FIG. 3, as in the low-pressure process, a supersaturated, aqueous urea solution with a molar ratio of urea: water of 1: 1 is produced in a tank 21, which comprises ethylene glycol diethyl ether as a substance for heat transfer and freezing point depression. However, in the high pressure process in a second process step 32 this solution is fed under pressure to a reactor 42 by means of a pump 41 and is heated there to a temperature of at least 180 ° C., for example 220 ° C., so that the urea in the aqueous solution increases Reacts ammonia NH ₃ and carbon dioxide C0 ₂ . The conversion or decomposition takes place completely.

In a third process step 33, the decomposed product or the gas phase of the decomposition product of ammonia NH ₃ and carbon dioxide C0 ₂ is eingedust via a solenoid valve 24 in an exhaust line of a motor vehicle with a diesel process brennungsmotor. There, the ammonia reacts on an SCR catalytic converter with nitrogen oxides contained in the exhaust gas to form molecular nitrogen and water. The SCR catalyst consists, for example, of coated zeolites or copper-exchanged zeolites.

FIG. 5 shows an alternative embodiment of a device for carrying out the method according to FIG. 3. This device differs from that according to FIG. 4 in that a solenoid valve 51 is arranged between the pump 41 and the reactor 42, by means of which the supply of the substances contained in the tank 21, the material for heat transfer and freezing point depression, the aqueous urea solution in the reactor 42 is controlled. Furthermore, a check valve 52 is arranged downstream of the reactor 42 instead of a solenoid valve, which opens as soon as the gas pressure generated in the reactor 42 exceeds the pressure of the exhaust gas prevailing in the exhaust line by a certain amount, for example 1 bar.

FIG. 6 shows a special embodiment for carrying out the method according to FIG. 3. This device comprises a tank 21 for an aqueous urea solution comprising a substance for heat transfer and freezing point depression, which can also serve to stabilize the solution. A pump 41 is connected downstream of the tank 21, by means of which the aqueous urea solution comprising the substance is conveyed into a coiled, thus spiral reactor 61.

The reactor 61 is arranged in an exhaust line 62 of a motor vehicle with a diesel internal combustion engine. The direction of flow of an exhaust gas in the exhaust line 62 is indicated by an arrow x. By means of the exhaust gas, the urea solution contained in the reactor 61 and comprising the substance for heat transfer and lowering the freezing point can be heated to a temperature above 180 ° C., ideally to 220 ° C. As a result, the urea contained in the solution decomposes into ammonia and CO ₂ .

Downstream of the reactor 61, a line 63 leads out of the exhaust line 62 to a heat exchanger 64. In the heat exchanger 64, the decomposition product heated in the reactor 61, which is designed as a wound tube, is removed. cools so that a downstream metering valve 65, again leading into the exhaust line 62, is protected against thermal damage.

Downstream of the metering valve 65 there is an SCR catalytic converter (not shown) in the exhaust line, on which the ammonia injected by means of the metering valve 65 reacts with nitrogen oxides contained in the exhaust gas to form molecular nitrogen and water.

FIG. 7 shows a further embodiment of a method according to the invention using a schematically indicated device for metering ammonia into an exhaust gas system (not shown here) of a motor vehicle with a diesel internal combustion engine.

The device shown in FIG. 7 comprises a tank 21 for an aqueous solution which comprises urea and diethylene glycol dibutyl ether as a substance for heat transfer and freezing point depression. The solution is in the tank 21 under a pressure p ₀ and is a saturated solution, which has the advantage that the concentration of urea in the solution is always constant.

For filling, the tank 21 is equipped with a first supply line 71 for supplying solid urea and a second supply line 72 for supplying water and the material for heat transfer and lowering of the freezing point. The supply line 71 is provided with a check valve 73 and the supply line 72 with a check valve 74. The tank 21 is connected via a line 75, in which a pump 41 is arranged, to a reactor 42, in which a heating element 76 is arranged. Downstream, the reactor 42 is connected to a line 77 in which a throttle 78 is arranged and which leads to an intermediate store

79 leads. The intermediate storage 79 is via a metering valve

80 connected to a metering tube 81, which leads into the exhaust line of the motor vehicle.

Furthermore, the intermediate store 79 is connected to the tank 21 for the urea solution comprising diethylene glycol dibutyl ether via a return line 82, in which a throttle 83 is arranged.

The method carried out by means of the device according to FIG. 7 described above is carried out in such a way that the solution stored in the tank 21, which comprises urea, diethylene glycol dibutyl ether and water, is fed under pressure into the reactor 42 by means of the pump 41 via the line 75 and there by means of the Heater 76 is heated to a temperature greater than 200 ° C. The heating converts the urea (NH ₂ ) ₂ C0 and the water H ₂ 0 of the solution to ammonia NH ₃ and carbon dioxide C0 ₂ . The solution or the decomposition product resulting from the solution is at a pressure pi in the reactor 42 which is greater than the pressure p ₀ prevailing in the tank 21.

Downstream of the reactor 42, the decomposition product, which comprises ammonia, carbon dioxide, water and the substance for heat transfer and freezing point depression, is expanded by means of the throttle 78 and supplied to the intermediate store 79. leads. A binary phase system consisting of a liquid and a gaseous phase is thus present in the intermediate store 79. The gaseous phase includes ammonia, carbon dioxide and water. With appropriate process control, the gas phase contains ammonia and carbon dioxide in a ratio of 2: 1. The liquid phase contains water with the substance for heat transfer and freezing point depression and a dissolved portion of ammonia and carbon dioxide. The gas pressure p? is lower than the pressure pi prevailing in the reactor 42, but higher than the pressure p ₀ prevailing in the tank 21. The pressure p? the gas phase is sufficient to overcome the exhaust gas back pressure acting via the metering tube 81 and to be able to supply the gases directly into the exhaust gas line of the motor vehicle. The gas phase comprising ammonia, carbon dioxide and water is cooled when it is removed from the intermediate store 79, so that the water contained in the gas phase condenses, so that only ammonia and carbon dioxide in the gaseous phase are led into the exhaust gas line. The ammonia reacts there with nitrogen oxides contained in the combustion gas of the internal combustion engine.

The liquid phase contained in the intermediate store 79 is returned to the tank 21 via the return line 82. The consumption of urea is compensated for by adding urea via the feed line 71 into the tank 21.

Claims

Expectations

1. A method for reducing nitrogen oxides in an exhaust gas, in particular in an exhaust gas from a motor vehicle, in which method an aqueous urea solution is fed to a reactor (22, 42, 61) and converted by thermal, catalytic or enzymatic treatment to a decomposition product comprising ammonia and carbon dioxide is and at least the ammonia is added to the exhaust gas, characterized in that the aqueous urea solution comprises a substance for heat transfer and lowering of the freezing point.

2. The method according to claim 1, characterized in that the substance for heat transfer and freezing point depression is selected from a group comprising diethylene glycol diethyl ether, diethylene glycol dibutyl ether and diethylene triamine.

3. The method according to claim 1 or 2, characterized in that the aqueous urea solution is a supersaturated Solution is and the urea and the water of the solution have a molar ratio of 1: 1.

4. The method according to any one of claims 1 to 3, characterized in that the aqueous heat urea solution comprising the substance for heat transfer and lowering the freezing point is added ammonia.

5. The method according to any one of claims 1 to 4, characterized in that the aqueous heat urea solution comprising the substance for heat transfer and lowering the freezing point is pumped under pressure into the reactor (22, 42, 61).

6. The method according to any one of claims 1 to 5, characterized in that the material for heat transfer and freezing point lowering, aqueous urea solution in the reactor (42, 61) to a temperature above a value of at least approximately 180 ° C, preferably to a Temperature of about 220 ° C, is heated.

7. The method according to claim 5 or 6, characterized in that the decomposition product is expanded downstream of the reactor (42).

8. The method according to any one of claims 1 to 4, characterized in that the decomposition product is compressed downstream of the reactor (22).

9. The method according to any one of claims 1 to 5 or 7 or 8, characterized in that the aqueous heat urea solution comprising the substance for heat transfer and freezing point lowering in the reactor (22) to a temperature in a range between 80 ° C and 150 ° C is heated.

10. The method according to claim 9, characterized in that in the reactor (22) isocyanic acid is decomposed on a hydrolysis catalyst in carbon dioxide and ammonia.

11. The method according to any one of claims 1 to 10, characterized in that a liquid phase of the decomposition product occurring downstream of the reactor (42) is returned to a tank (21) for the aqueous urea solution comprising the substance for heat transfer and lowering the freezing point.

12. The method according to claim 11, characterized in that the liquid phase is relaxed when it is recycled.

13. The method according to any one of claims 1 to 12, characterized in that the decomposition product is cooled downstream of the reactor (61).

14. The method according to any one of claims 1 to 13, characterized in that the aqueous heat urea solution comprising the substance for heat transfer and lowering the freezing point is heated by means of the exhaust gas.

15. Device for carrying out the method according to claim 1, comprising a tank (21) for an aqueous urea solution, a reactor (22, 42, 61) for preconditioning the aqueous urea solution and a valve (24, 52, 65, 80) for injecting a decomposition product comprising ammonia and produced by means of the reactor (22, 42, 61) into an exhaust line (62), characterized in that the aqueous urea solution comprises a substance for heat transfer and freezing point depression.

16. The apparatus according to claim 15, characterized in that the reactor is designed as a spiral tube (61).

17. The apparatus of claim 15 or 16, characterized in that the reactor (61) is arranged in the exhaust line (62).

18. Device according to one of claims 15 to 17, characterized in that a heat exchanger (64) for cooling the preconditioned, aqueous urea solution is arranged downstream of the reactor (61).

19. Device according to one of claims 15 to 18, characterized in that a pump (41) is arranged upstream of the reactor (42, 61).

20. Device according to one of claims 15 to 19, characterized in that a pump (23) is arranged downstream of the reactor (22).

21. The apparatus according to claim 20, characterized in that a valve (51) is arranged between the pump (41) and the reactor (42).

22. Device according to one of claims 15 to 21, characterized in that the valve for injecting the decomposition product comprising ammonia is designed as a check valve (52).