Method for operating an exhaust aftertreatment system of a motor vehicle

Abstract

Method for operating an exhaust aftertreatment device (6) for cleaning the exhaust gas of an internal combustion engine (4) of a motor vehicle (2), in which ammonia (NH3) is used for the selective catalytic reduction of nitrogen oxides (NOx) in the exhaust gas, wherein the ammonia (NH3) is produced by electrolysis from a mixture of potassium hydroxide (KOH) and sodium hydroxide (NaOH), wherein nanoparticles of iron(III) oxide (Fe2O3) dispersed in the mixture of potassium hydroxide (KOH) and sodium hydroxide (NaOH) are used as a catalyst, an electric direct voltage (U) of 1 V to 48 V is used for electrolysis, and wherein the electrolysis is carried out at an operating temperature of 150°C to 450°C and at an operating pressure of 5 bar to 50 bar, wherein waste heat from the internal combustion engine (4) is used to generate the operating pressure, wherein a coupling with a cooler of a cooling circuit for cooling the internal combustion engine is provided for this purpose.to generate steam using the waste heat from the internal combustion engine.

Description

[0001] The invention relates to a method for operating an exhaust aftertreatment device of a motor vehicle. The invention further relates to an exhaust aftertreatment device and a motor vehicle with such an exhaust aftertreatment device.

[0002] Exhaust aftertreatment devices are used to clean exhaust gases or combustion gases from internal combustion engines that power motor vehicles as traction engines in order to comply with pollutant limits.

[0003] Nitrogen oxides can be removed from exhaust gas by means of selective catalytic reduction (SCR). The chemical reaction at an SCR catalyst in such an exhaust aftertreatment system is selective, meaning that the nitrogen oxides are preferentially reduced, while undesirable side reactions, such as the oxidation of sulfur dioxide to sulfur trioxide, are largely suppressed. Ammonia, which is added to the exhaust gas, is required for the reaction. The products of the reaction are water and nitrogen. The reaction involves the comproportionation of nitrogen oxides with ammonia to form nitrogen.

[0004] In the exhaust aftertreatment systems of motor vehicles, an aqueous urea solution, such as AdBlue, is injected. During its further transport through the exhaust pipe, this solution hydrolyzes to produce ammonia. However, a fluid tank is required to store the urea solution, which takes up space in the vehicle. Furthermore, refilling is necessary.

[0005] Producing ammonia using the Bosch-Haber process requires an operating pressure of 300 bar to 500 bar and temperatures of 400°C to 500°C. However, such operating pressures and temperatures cannot be readily provided in a motor vehicle.

[0006] From DE 10 2014 204 307 A1, a method and an arrangement for treating exhaust gases from an internal combustion engine are known. For the treatment of exhaust gases from an internal combustion engine of a motor vehicle, the exhaust gases from the internal combustion engine are fed to an SCR catalyst for selective catalytic reduction, whereby nitrogen oxides contained in the exhaust gas are reduced to nitrogen at the SCR catalyst by reaction with ammonia. The ammonia is obtained at least partially by reacting urine provided in a storage tank with a bacterial catalyst.

[0007] A method for exhaust gas aftertreatment by means of selective catalytic reduction using urea electrolysis is also known from US 2011 / 0 243 823 A1.

[0008] From DE 10 2016 123 850 A1, an exhaust aftertreatment system is known which comprises H2 and NH3 produced by electrolysis. The exhaust aftertreatment system comprises an exhaust channel, a NOx storage catalyst provided in the exhaust channel, a tank containing the aqueous reactant, an electrochemical cell connected to the tank and configured to receive the aqueous reactant from it, wherein the electrochemical cell is configured to convert the aqueous reactant into a hydrogen exhaust aftertreatment fluid for purging the NOx storage catalyst, and a control unit connected to the electrochemical cell, wherein the control unit is configured to vary the amount of hydrogen exhaust aftertreatment fluid produced by the electrochemical cell.

[0009] From DE 11 2014 004 331 T5, a method for operating an exhaust aftertreatment device for cleaning the exhaust gas of an internal combustion engine of a motor vehicle is disclosed, in which ammonia is used for the selective catalytic reduction of nitrogen oxides in the exhaust gas, wherein the ammonia is produced by electrolysis from a mixture of potassium hydroxide and sodium hydroxide. A direct current voltage of 1.2 V is used for the electrolysis, which is carried out at an operating temperature of 200°C to 250°C and at an operating pressure of 25 bar.

[0010] From the article Licht et al.: Ammonia Synthesis by N2 and steam electrolysis in molten hydroxide auspensions of nanoscale Fe2O3, a process is known in which nanoparticles of iron(III) oxide are used as a catalyst for ammonia production.

[0011] Therefore, there is a need to show ways in which ammonia can be generated in a motor vehicle for an exhaust aftertreatment device.

[0012] The object of the invention is achieved by a method for operating an exhaust aftertreatment device for cleaning the exhaust gas of an internal combustion engine of a motor vehicle, in which ammonia is used for the selective catalytic reduction of nitrogen oxides in the exhaust gas, wherein the ammonia is produced by electrolysis, wherein nanoparticles of iron(III) oxide dispersed in a mixture of potassium hydroxide and sodium hydroxide are used as the catalyst, an electrical direct voltage of 1 V to 48 V, in particular 1 V to 10 V and further in particular 1 V to 2 V, is used for electrolysis, and wherein the electrolysis is carried out at an operating temperature of 150°C to 450°C, in particular 200°C to 250°C, and at an operating pressure of 5 bar to 50 bar, in particular 10 bar to 30 bar and further in particular 20 bar to 25 bar, wherein waste heat from the internal combustion engine is used.to generate the operating pressure, whereby a coupling with a cooler of a cooling circuit for cooling the internal combustion engine is provided in order to generate steam using the waste heat of the internal combustion engine.

[0013] Water is used as the hydrogen source. An equimolar mixture of sodium and potassium hydroxide is heated to 150°C to 450°C, specifically 200°C to 250°C, to form a molten salt. Iron(III) oxide nanoparticles, approximately 40 millionths of a millimeter in size, are dispersed in the molten salt and act as a catalyst. This mixture is subjected to a direct current voltage of 1 V to 48 V, specifically 1 V to 10 V, and further specifically 1 V to 2 V, via two electrodes. As water vapor and air flow through this electrochemical cell, the water molecules split into oxygen and hydrogen. With catalytic assistance, the hydrogen combines with atmospheric nitrogen to form ammonia. In this way, ammonia can be supplied to an exhaust aftertreatment system at significantly lower temperatures and pressures. This eliminates the need to store a urea solution in a fluid tank, thus saving installation space.Refilling is also unnecessary.

[0014] Energy efficiency is increased by using waste heat from the internal combustion engine to generate the operating pressure.

[0015] According to one embodiment, waste heat from the internal combustion engine is used to heat it to operating temperature. For this purpose, for example, heat energy from the exhaust gases of the internal combustion engine is used, or a connection to a cooler in a cooling circuit for the internal combustion engine is provided. Thus, no heat source such as a heater is required, and no additional energy is consumed for heating, which increases energy efficiency.

[0016] According to another embodiment, a pump is used to generate the operating pressure. The pump can, for example, increase the pressure provided by using waste heat to supply the required operating pressure.

[0017] According to another embodiment, electrical energy from the vehicle's electrical system is used to provide the direct current (DC) voltage. This eliminates the need for a separate power supply. Furthermore, this allows, for example, the use of recuperated braking energy, which is temporarily stored in electrical form. It is also possible to utilize waste heat from a thermoelectric generator (TEG) to provide the DC voltage.

[0018] According to another embodiment, water is used that is obtained from water vapor in the exhaust gas of the internal combustion engine and / or that is collected rainwater or atmospheric humidity. This eliminates the need for manually refilling a storage tank for water.

[0019] According to another embodiment, the hydrogen produced by the decomposition of water is used to generate energy. This allows for the provision of additional energy, e.g., electrical energy, using a fuel cell, which can be used in normal operation and / or in boost mode with special power peaks or requirements.

[0020] Furthermore, the invention includes an exhaust aftertreatment device and a motor vehicle with such an exhaust aftertreatment device.

[0021] The invention will now be explained with the aid of a drawing. The drawing shows: Fig. 1 an internal combustion engine and an exhaust aftertreatment device of a motor vehicle for carrying out an embodiment of the method according to the invention.

[0022] The Fig. Figure 1 shows an internal combustion engine 4 and an exhaust aftertreatment device 6 of a motor vehicle 2.

[0023] In the present embodiment, the internal combustion engine 4 is a diesel engine. Alternatively, the internal combustion engine 4 can also be designed as a gasoline engine.

[0024] In the present embodiment, the exhaust aftertreatment device 6, which is located downstream of the internal combustion engine 4 in the exhaust gas flow direction, includes an SCR catalyst 10 for reducing nitrogen oxides (NOx) in the exhaust gas stream by selective catalytic reduction. The SCR catalyst 10 requires ammonia (NH3), which is supplied by a device 8 for generating ammonia. In addition to the SCR catalyst, the exhaust aftertreatment device 6 can also include other components, such as three-way catalysts and / or NOx storage catalysts. Furthermore, the exhaust aftertreatment device 6 can also include more than one SCR catalyst, in a deviation from the present embodiment.

[0025] The device 8 has an electrochemical cell in which an equimolar mixture of potassium hydroxide (KOH) and sodium hydroxide (NaOH), e.g. with a molar ratio of 0.5 NaOH / 0.5 KOH, is provided as a catalyst by dispersing nanoparticles of iron(III) oxide (Fe2O3) with a size of about 40 millionths of a millimeter.

[0026] Furthermore, nitrogen (N2), in this exemplary embodiment atmospheric nitrogen from the ambient air, and water (H2O), e.g. from a storage tank, can be supplied to the device. The storage tank can be filled with water (H2O), which is condensed water vapor from the exhaust gas, collected rainwater, or humidity.

[0027] During operation, the mixture is heated to an operating temperature of 150°C to 450°C, particularly 200°C to 250°C, and subjected to a DC voltage U of 1 V to 48 V, particularly 1 V to 10 V and further, particularly 1 V to 2 V, and a pressure of 5 bar to 50 bar, particularly 10 bar to 30 bar and further, particularly 20 bar to 25 bar. In the present embodiment, the operating temperature is 200°C and the DC voltage U is 1.2 volts. The pressure is in the range of 20 to 25 bar and the current density is 2 mA / cm². 2 At an operating temperature of 250°C and a pressure of 25 bar, the DC voltage U is at a current density of 2 mA / cm². 2 1 V.

[0028] To heat the mixture to the operating temperature, the device 8 in the present embodiment is coupled to the internal combustion engine 4 in such a way that, for example, thermal energy from the exhaust gases of the internal combustion engine 4 can be used. Alternatively, a coupling to a cooler of a cooling circuit for cooling the internal combustion engine 4 can be provided.

[0029] In the present embodiment, the DC electrical voltage U is provided by the vehicle's electrical system 2. Alternatively, a thermoelectric generator can also utilize waste heat from the internal combustion engine 4 to provide the DC electrical voltage U.

[0030] In this embodiment, waste heat from the internal combustion engine 4 is also used to generate the operating pressure. Specifically, the waste heat is used to generate steam, thus providing the operating pressure. A pump can be provided to assist this process, increasing the pressure generated by the waste heat to the required operating pressure.

[0031] During operation, water vapor and air flow through the electrochemical cell of device 10. There, water (H₂O) is split into oxygen (O₂) and hydrogen (H₂). With catalytic assistance, the hydrogen (H₂) combines with atmospheric nitrogen (N₂) to form ammonia (NH₃).

[0032] To compensate for peak demand for ammonia (NH3) and / or to prevent the unused release of ammonia (NH3) that is not currently needed, ammonia (NH3) is partially and temporarily stored. For this purpose, device 8 has a storage tank.

[0033] This allows ammonia to be supplied to an exhaust aftertreatment system at significantly lower temperatures and pressures. Therefore, there is no need to store a urea solution in a fluid tank, thus saving installation space. Refilling is also unnecessary. Reference symbol list 2 motor vehicles 4 Internal combustion engine 6 Exhaust aftertreatment device 8 Device 10 SCR catalyst H2O water N2 Nitrogen NH3 Ammonia U electric DC voltage

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