AT526941B1 - Method for cleaning exhaust gases from an internal combustion engine and exhaust gas cleaning system therefor - Google Patents

Abstract

The invention relates to a method for cleaning exhaust gas from an internal combustion engine (1) with an exhaust gas cleaning system (2) connected thereto, which has a first SCR catalyst (4) and a downstream second SCR catalyst (8) as well as a particle filter (7) arranged between the first SCR catalyst (4) and the second SCR catalyst (8). In this process, a urea-containing reducing agent is metered into the exhaust gas using a first metering device (10) arranged on the inlet side of the first SCR catalyst (4) and/or using a second metering device (12) arranged on the inlet side of the second SCR catalyst (8), and nitrogen oxides contained in the exhaust gas are at least largely removed from the exhaust gas by selective reduction with ammonia released from the metered urea in the first and/or second SCR catalyst (4, 8). According to the invention, an exhaust gas temperature is determined in the area between the second metering device (12) and the second SCR catalyst (8) and a first metering rate metered by the first metering device (10) is increased in relation to a second metering rate metered by the second metering device (12) if the determined exhaust gas temperature is in a temperature range in which the formation of urea-based particles as a result of urea added to the exhaust gas by the second metering device (12) exceeds a predeterminable particle formation limit value (19). The invention further relates to an exhaust gas purification system with a control device (13) which is set up to control the implementation of a corresponding exhaust gas purification process.

Description

Description

METHOD FOR CLEANING EXHAUST GAS FROM AN INTERNAL COMBUSTION ENGINE AND EXHAUST GAS CLEANING SYSTEM THEREFOR

[0001] The invention relates to a method for cleaning exhaust gases from an internal combustion engine having the features of the preamble of claim 1 and to an exhaust gas purification system having the features of the preamble of claim 7.

[0002] To clean combustion engine exhaust gas, it is known to remove nitrogen oxides (NOx) contained in the exhaust gas by reduction with ammonia (NH) as a selectively acting reducing agent. As requirements become more stringent, exhaust gas purification systems have been proposed which have two SCR catalysts arranged one behind the other, each of which can selectively reduce NOx. In this case, a reducing agent containing urea, such as an aqueous urea solution with about 32.5% urea (Adblue ®), is usually used as the reducing agent. To simultaneously remove particles from the exhaust gas, a particle filter is usually also provided in the exhaust gas purification system. For example, WO 2019159151 A1 discloses a corresponding exhaust gas purification system.

[0003] The object of the invention is to provide an exhaust gas purification method and an exhaust gas purification system which enable further improved exhaust gas purification, in particular with regard to particle emissions.

[0004] This object is achieved by an exhaust gas purification method having the features of claim 1 and by an exhaust gas purification system having the features of claim 8. Advantageous embodiments and further developments of the invention are the subject of the respective subclaims.

[0005] In the method according to the invention for cleaning exhaust gas from an internal combustion engine with an exhaust gas purification system connected thereto, which has a first SCR catalyst and a downstream second SCR catalyst as well as a particle filter arranged between the first SCR catalyst and the second SCR catalyst, a urea-containing reducing agent is metered into the exhaust gas with a first metering device arranged on the inlet side of the first SCR catalyst and/or with a second metering device arranged on the inlet side of the second SCR catalyst, and nitrogen oxides contained in the exhaust gas are at least largely removed from the exhaust gas by selective reduction with ammonia released from the metered urea in the first and/or second SCR catalyst. According to the invention, an exhaust gas temperature is determined in the region between the second metering device and the second SCR catalyst, and a first metering rate metered by the first metering device is increased in relation to a second metering rate metered by the second metering device if the determined exhaust gas temperature is in a temperature range in which a formation of urea-based particles as a result of urea added to the exhaust gas by the second metering device exceeds a predeterminable particle formation limit value.

[0006] The inventors have recognized in extensive investigations that reactions of the metered urea produce particles as reaction products, which can result in increased particle emissions. Reactions are understood here to mean reactions of the urea to form higher molecular weight reaction products and reaction products that may be formed by further subsequent reactions. It is assumed that these particles are produced as reaction products by deposits of, for example, biuret or cyanuric acid. These particles are referred to below as urea-based particles. If, as provided according to the invention, this particle formation is taken into account when dividing the total reducing agent metering rate metered into the exhaust gas purification system between the first metering device and the second metering device, in particular for the urea metered by the second metering device, the emission of urea-based particles can be largely prevented.

prevented or at least reduced. A dosing rate is understood here as a time-related quantity of the dosed reducing agent. The dosing rates are based on a predeterminable NOx conversion rate of the respective SCR catalyst. The division of the total reducing agent dosing rate into the first and second dosing rates therefore corresponds to a division of a total NOx conversion rate of the exhaust gas purification system into a NOx conversion rate of the first SCR catalyst and a NOx conversion rate of the second SCR catalyst.

[0007] In the design of the method, the formation of particles with a size between 10 nm and 23 nm is specifically taken into account. The size of the particles is to be understood in particular as their hydraulic diameter. Particles of this particle fraction are also abbreviated to PN10 particles. As recent studies have shown, these fine particles can be said to have a particularly hazardous effect on health. If the formation of these fine particles is specifically taken into account during the conversion of the urea dosed by the second dosing device, their emission and thus the resulting danger can be significantly reduced.

[0008] In a further embodiment of the method, the first dosing rate is increased in relation to the second dosing rate if the above-mentioned formation of urea-based particles exceeds a predeterminable particle formation limit value. The NOx conversion rate of the first SCR catalyst is thus increased in comparison to that of the second SCR catalyst if the formation of urea-based particles due to the urea dosed by the second dosing device exceeds the predeterminable particle formation limit value. A particle formation limit value based on the number of particles or on the particle mass can be selected. It is preferably provided that if the particle formation limit value is exceeded, the entire reducing agent dosing rate is divided in such a way that a higher dosing rate is set for the first dosing device than for the second dosing device. For example, the first dosing rate can be set higher than the second dosing rate by a factor of 1.2 to 4, preferably by a factor of 1.5 to 3. In this way, the formation of urea-based particles as a result of the urea metered in by the second dosing device is reduced. On the other hand, urea-based particles which form as a result of the urea metered in by the first dosing device are removed from the exhaust gas by the particle filter arranged downstream of the first dosing device and downstream of the first SCR catalyst. The release of urea-based particles into the environment can thus be effectively limited.

[0009] It has proven to be advantageous if, in a further embodiment of the method, the ratio of the first dosing rate and the second dosing rate is set at least approximately proportional to the extent of the formation of the particles. As the predeterminable particle formation limit value for the formation of the urea-based particles as a result of the urea metered in by the second metering device is increasingly exceeded, the first dosing rate is thus increasingly increased in comparison to the second dosing rate, with a predeterminable proportionality relationship existing between the increases.

[0010] In a further embodiment of the method, characteristic curves or characteristic fields are used to quantify the formation of the urea-based particles, which at least depict a dependence of the formation of the particles on the temperature. The temperature is preferably the exhaust gas temperature. However, the catalyst temperature or the temperature of an exhaust line section upstream and/or downstream of the second metering device can also be taken into account. The temperature of a mixer arranged between the second metering device and the second SCR catalyst can also be used as the relevant temperature. By using corresponding temperature formation characteristic curves or fields, the knowledge that the formation of the urea-based particles is strongly dependent on temperature is taken into account.

[0011] In a further embodiment of the method, in the case of SCR catalysts at operating temperature, the total reducing agent dosing rate is divided into the first dosing rate and the second dosing rate in a ratio of approximately 1:1 for the majority of the operating time, if the

The formation of particles falls below the preset particle formation limit. This means that the dosing rates are adjusted in such a way that about half of the NOx raw emissions of the combustion engine are reduced at the first SCR catalyst and the other half at the second SCR catalyst. This ensures that there is sufficient NOx slip at the first SCR catalyst to enable passive soot regeneration at the downstream particle filter.

[0012] In a further embodiment of the method, a nitrogen oxide content of the exhaust gas fed to a respective SCR catalyst is determined and, when setting the first and/or the second dosing rate, a mole fraction a of the reducing agent calculated from ammonia and fed to a respective SCR catalyst and the nitrogen oxide content is specified. An a value of 1 corresponds to an equimolar ratio of NH3 and NOx. When specifying an a value for the reducing agent dosing, this is also referred to as a-based dosing. When setting the dosing rate, in particular of the second dosing device, load-controlled dosing can also be carried out. In this case, the dosing rate is set so that a predeterminable amount of NH3 stored in the SCR catalyst results. a values or loading values are selected so that a predeterminable NOx conversion rate of the respective SCR catalyst results.

[0013] The exhaust gas purification system according to the invention for a motor vehicle internal combustion engine has a first SCR catalyst and a downstream second SCR catalyst, a particle filter arranged between the first SCR catalyst and the second SCR catalyst, and a first metering device arranged on the inlet side of the first SCR catalyst and a second metering device arranged on the inlet side of the second SCR catalyst for metering a reducing agent containing urea into the exhaust gas. According to the invention, a control device is provided which is designed to control the implementation of an exhaust gas purification method according to one of claims 1 to 7.

[0014] Advantageous embodiments of the invention are illustrated in the drawings and are described below. The features mentioned above and those to be explained below can be used not only in the combination of features specified in each case, but also in other combinations or on their own, without departing from the scope of the present invention.

[0015] Show:

[0016] Fig. 1 is a schematic block diagram of a motor vehicle internal combustion engine and connected exhaust gas purification system and

[0017] Fig. 2 is a characteristic curve illustrating the temperature dependence of the formation of urea-based particles.

[0018] Fig. 1 shows a block diagram of a system with an internal combustion engine 1 of a motor vehicle (not shown) and associated exhaust gas purification system 2, merely schematically and by way of example. The internal combustion engine 1 is preferably designed as an air-compressing internal combustion engine, referred to simply as a diesel engine below. However, it can also be a gasoline or gas-powered Otto engine. The exhaust gas emitted by the diesel engine 1 flows over a turbine of an exhaust gas turbocharger (not shown) and is taken up by an exhaust line 3. In the exhaust line 3, a first SCR catalyst 4, a first oxidation catalyst 5, a second oxidation catalyst 6, a particle filter 7, a second SCR catalyst 8 and a third oxidation catalyst 9 are arranged one behind the other in the exhaust gas flow direction. Preferably, both the first SCR catalyst 4 and the first oxidation catalyst 5 as well as the second SCR catalyst 8 and the third oxidation catalyst 9 are arranged closely adjacent in a respective common housing. This is also provided for the second oxidation catalyst 6 and the particle filter 7.

[0019] The first SCR catalyst 4 and the first oxidation catalyst 5 are arranged close to the engine, i.e. in an engine compartment close to the exhaust gas outlet from the diesel engine 1 or the exhaust gas turbocharger turbine. In contrast, the second SCR catalyst 8 and the third oxidation catalyst 5 are

on catalyst 9 is arranged remote from the engine, outside the engine compartment and preferably in an underbody area of the motor vehicle.

[0020] The exhaust gas purification system 2 also comprises a first metering device 10 and a second metering device 12, which are connected upstream of the first SCR catalyst 4 and the second SCR catalyst 8, respectively. A reducing agent containing urea can be sprayed into the exhaust line 3 as required via the metering devices 10, 12. The metering devices 10, 12 are connected to a reducing agent container for this purpose, which is not shown separately. An aqueous urea solution is preferably used as the reducing agent. NH3 can be released from the urea added to the exhaust gas, in particular by thermolysis and/or hydrolysis, in the exhaust gas and can reduce nitrogen oxides contained in the exhaust gas at a respective SCR catalyst 4, 8. For a particularly uniform distribution of the urea solution supplied to the exhaust gas via the metering devices 10, 12, mixers can be provided between the first metering device 10 and the first SCR catalyst 4 or the second metering device 12 and the second SCR catalyst 8 in the exhaust line 3, which is not shown separately here. A quantitatively controlled metering of the reducing agent can be carried out by appropriate control of the metering devices 10, 12 and/or a respective delivery unit (not shown).

[0021] Furthermore, a fuel addition device 11 is provided downstream of the first oxidation catalyst 5 and upstream of the second oxidation catalyst 6. This can be used to meter a fuel, preferably the fuel used by the internal combustion engine 1, into the exhaust gas to support a thermally induced oxidation of soot deposited in the particle filter. The reaction heat generated by the oxidation of the metered-in fuel can bring the particle filter 7 to a temperature required for soot combustion. If necessary, the fuel addition device 11 can also be dispensed with and the particle filter 7 can be heated accordingly by engine measures.

[0022] In the exhaust gas purification system 2, sensors for temperature, NOx and NHs concentration (not shown separately here) are also provided, the signals of which are evaluated by a control device 13 and are used to adjust operating parameters and in particular the reducing agent dosing quantities.

[0023] A control device 13 of the exhaust gas purification system 2 is used to control its operation. For this purpose, the control device 13 receives input signals 14, processes them and generates output signals 15 from them. The input signals 14 include in particular signals from the sensors installed in the exhaust gas purification system 2 for temperature and NOx or NHs content in the exhaust gas. However, the control device 13 can also process input signals representing operating variables of the diesel engine 1. The output signals 15 include in particular control signals for controlling the metering devices 10, 12. The control device 13 can, for example, be designed as a microcontroller that can access characteristic maps or characteristic curves stored in an associated memory for dependencies on operating variables of the exhaust gas purification system 2. A sequence of the control and regulating processes effected by the control device 13 is preferably effected by a stored program.

[0024] The catalysts 4, 5, 6, 8 and 9 are preferably designed as ceramic honeycomb bodies with continuous, parallel channels. The channel walls that come into contact with the exhaust gas are coated with a carrier substance with a high specific surface area, which contains a specific catalytically active substance. The catalytically active substance of the oxidation catalysts 5, 6, 9 can be a finely dispersed metal from the platinum group such as platinum, palladium and/or rhodium.

[0025] In the second oxidation catalyst 6, the oxidation-catalytically effective coating causes, on the one hand, an oxidation of a portion of nitrogen monoxide (NO) contained in the exhaust gas to nitrogen dioxide (NO2), which enables passive oxidation of soot particles deposited in the particle filter 7. On the other hand, by oxidizing the fuel supplied to the exhaust gas via the fuel addition device 11, an increase in the exhaust gas temperature for

a thermal soot regeneration of the particle filter 7 can be achieved. The oxidation-catalytically effective coatings of the oxidation catalysts 5, 9 bring about in particular an oxidation of any NHs residues remaining in the exhaust gas or, for example, in the case of a rapid increase in temperature of the NH3 desorbing from the SCR catalysts 4, 8. The oxidation catalysts 5, 9 can therefore also be referred to as ammonia barrier catalysts.

[0026] The particle filter 7 is preferably designed as a wall-flow filter in a honeycomb structure with channels that are alternately closed on the inlet side and outlet side. The particle filter 7 can have a coating that is particularly effective in oxidation catalysis, which is applied to the raw gas sides and/or clean gas sides of the filter-effective channel walls or infiltrated into them.

[0027] As far as the SCR catalysts 4, 8 are concerned, an SCR catalytically effective coating is provided for them, which can catalyze a reduction of NOx even in the presence of an excess of oxygen in the exhaust gas with stored and/or supplied NH3 as a NOx-selective reducing agent. The SCR catalytically effective coating is preferably a zeolite which contains an SCR catalytic active component. For the first SCR catalyst 4, an SCR catalytically effective coating is preferably provided which contains vanadium as an SCR catalytic active component. The second SCR catalyst 8 preferably has a zeolitic coating containing copper or iron. However, copper as an SCR catalytic active component is preferred due to its comparatively high conversion capacity at low temperatures and lower nitrous oxide formation.

[0028] A preferred operating method for the exhaust gas purification system 2 according to the invention is explained in more detail below. In the following, it is assumed that the SCR catalysts 4, 8 are at operating temperature and that aqueous urea solution is metered into the exhaust gas emitted by the diesel engine 1 via the metering devices 10, 11 as a reducing agent for reducing NOx. As has been determined, chemical reactions of the urea added to the exhaust gas produce reaction products of higher molecular weight in particular, such as biuret or cynauric acid. In further subsequent reactions or through deposits, particles are again formed, whereby this particle formation is quantified in the present case by characteristic curves or characteristic maps. The corresponding characteristic curves or characteristic maps can be stored in the control device 13 so that they can be used when operating the exhaust gas purification system 2. Such a characteristic curve is shown in a diagram shown in Fig. 2.

[0029] The diagram shown in Fig. 2 shows only schematically and as an example a characteristic curve 18 which shows the formation of the urea-based particles mentioned as a function of an operating parameter. A number of urea-based particles with a diameter between 10 nm and 23 nm (PN10 particles) is plotted on the ordinate 16, based on a mass unit of urea introduced into the exhaust gas. An exhaust gas temperature is plotted on the abscissa 17 as the operating parameter that is relevant here. As can be seen, the characteristic curve 18 passes through a pronounced maximum.

[0030] If the particles are formed by reactions of urea added to the exhaust gas by means of the first metering device 10, they can be at least largely removed from the exhaust gas by the downstream particle filter 7. On the other hand, particles formed by reactions of urea added to the exhaust gas by means of the second metering device 12 can enter the environment in an undesirable manner. Thus, the amount of urea-based PN10 particles released into the environment is ultimately caused by the urea added to the exhaust gas by means of the second metering device 12.

[0031] As long as this particle formation falls below a particle formation limit 19 shown in Fig. 2 by a dashed line, the corresponding particle emission can be considered tolerable or harmless. In this case, i.e. if the exhaust gas temperature determined in particular in the area between the second metering device 12 and the second SCR catalyst 8 is outside a temperature range of

For example, if the temperature is between about 275 °C and about 375 °C, the emission of PN10 particles that would be considered critical is not reached and the exhaust gas purification system 2 is intended to operate as normal.

[0032] In this normal operation, it is typically provided that nitrogen oxides emitted by the diesel engine 1 are reduced approximately equally by the first SCR catalyst 4 and the second SCR catalyst 8, so that overall they are at least almost completely removed from the exhaust gas. For this purpose, the metering devices 10, 12 are controlled by the control device 13 to deliver an approximately equal metering rate. Preferably, a so-called a-based metering is carried out in each case. In this case, the urea metering rate for the first metering device 10 and the second metering device 12 is set in such a way that a predeterminable mole fraction a of NH3 and a respective NOx content in the exhaust gas results. When the SCR catalysts 4, 8 are at operating temperature, this is approximately 0.5 for the metering rate of the first metering device 10. This typically results in a division of the entire reducing agent metering rate between the first metering rate set for the first metering device 10 and the second metering rate set for the second metering device 12 in a ratio of approximately 1:1. Depending on the current conversion capacity of the SCR catalysts 4, 8, however, a metering rate can also be set for the first metering device 10 which corresponds to a more or less strong deviation from this, i.e. from an a value of 0.5. In particular, it can be provided to set the a value depending on the temperature of the second SCR catalyst 8. For the second metering device 12, a mole fraction of a = 1.0 is preferably set. As a result, nitrogen oxides still present in the exhaust gas on the inlet side of the second SCR catalyst 8 can be practically completely removed. However, the metering rate of the second metering device 12 can also be set in such a way that a predetermined loading of NH3 stored in the second SCR catalyst 8 results.

[0033] If the formation of PN10 particles as a result of urea added to the exhaust gas by the second metering device 12 exceeds the particle formation limit value 19, then a particle-limiting operation is provided. In this particle-limiting operation, the metering rate of the first metering device 10 is increased in relation to that of the second metering device 12. This increases the amount of nitrogen oxides reduced at the first SCR catalyst 4 in relation to the amount of nitrogen oxides reduced at the second SCR catalyst 8. Typically, a higher metering rate is set for the first metering device 10 than for the second metering device 12. The larger proportion of the total nitrogen oxides emitted by the diesel engine 1 is thus reduced by the first SCR catalyst 4.

[0034] In an advantageous mode of operation, when the particle formation limit value 19 is exceeded, the dosing rate of the first dosing device 10 is increased in relation to that of the second dosing device 12 in such a way that the ratio is set proportionally to the extent of particle formation, i.e. proportionally to the number of PN10 particles formed by a unit mass of urea. The ratio of the first dosing rate and the second dosing rate can also be set proportionally to the particle formation limit value 19 being exceeded. This means that the more pronounced the particle formation is, or the more the particle formation limit value 19 is exceeded, the greater the ratio of the first and second dosing rates is set.

[0035] Irrespective of the specific procedure for increasing the first dosing rate in relation to the second dosing rate, this is in any case carried out in such a way that a predeterminable or predetermined limit value for the tailpipe emission of PN10 particles released into the environment is undercut.

[0036] Overall, the method according to the invention can thus comply with or even exceed the lowest emission limits for NOx and particles.

LIST OF REFERENCE SYMBOLS

1 Motor vehicle internal combustion engine 2 Exhaust gas purification system

3 Exhaust pipe

4 First SCR catalyst

5 First oxidation catalyst 6 Second oxidation catalyst 7 Particle filter

8 Second SCR catalyst

9 Third oxidation catalyst 10 First dosing device

11 Fuel addition device 12 Second dosing device

13 Control device

14 input signals

15 output signals

16 Ordinates

17 Abscissa

18 Characteristic curve

19 Particle formation limit

Claims (7)

Patent claims 1. Method for cleaning exhaust gas of an internal combustion engine (1) with an exhaust gas purification system (2) connected thereto, which has a first SCR catalyst (4) and a downstream second SCR catalyst (8) as well as a particle filter (7) arranged between the first SCR catalyst (4) and the second SCR catalyst (8), in which a metering of a urea-containing reducing agent into the exhaust gas is carried out with a first metering device (10) arranged on the inlet side of the first SCR catalyst (4) and/or with a second metering device (12) arranged on the inlet side of the second SCR catalyst (8), and nitrogen oxides contained in the exhaust gas are at least largely removed from the exhaust gas by selective reduction with ammonia released from the metered urea in the first and/or in the second SCR catalyst (4, 8), characterized in that an exhaust gas temperature in the range between the second Dosing device (12) and the second SCR catalyst (8) is determined and a first dosing rate dosed by the first dosing device (10) is increased in relation to a second dosing rate dosed by the second dosing device (12) if the determined exhaust gas temperature is in a temperature range in which a formation of urea-based particles as a result of urea added to the exhaust gas by the second dosing device (12) exceeds a predeterminable particle formation limit value (19). 2, Method according to claim 1, characterized in that the formation of particles with a size between 10 nm and 23 nm is taken into account. 3. Method according to claim 1 or 2, characterized in that the ratio of the first dosing rate and the second dosing rate is set at least approximately proportional to the extent of the formation of the particles. 4. Method according to one of claims 1 to 3, characterized in that for quantifying the formation of the particles, use is made of characteristic curves or characteristic maps which depict at least a dependence of the formation of the particles on the temperature. 5. Method according to one of claims 1 to 5, characterized in that in the case of SCR catalysts (4, 8) at operating temperature, the entire reducing agent metering rate is divided into a first metering rate and a second metering rate in a ratio of approximately 1:1 for the majority of an operating time if the formation of particles falls below the predeterminable particle formation limit value (19). 6. Method according to one of claims 1 to 5, characterized in that a nitrogen oxide content of the exhaust gas supplied to a respective SCR catalyst (4, 8) is determined and, when setting the first and/or the second dosing rate, a mole fraction a of reducing agent calculated from ammonia and supplied to a respective SCR catalyst (4, 8) and nitrogen oxide content is specified. 7. Exhaust gas purification system (2) for a motor vehicle internal combustion engine (1) which has a first SCR catalyst (4) and a downstream second SCR catalyst (8), a particle filter (7) arranged between the first SCR catalyst (4) and the second SCR catalyst (8), and a first metering device (10) arranged on the inlet side of the first SCR catalyst (4) and a second metering device (12) arranged on the inlet side of the second SCR catalyst (8) for metering a reducing agent containing urea into the exhaust gas, characterized in that a control device (13) is provided which is designed to control implementation of an exhaust gas purification method according to one of claims 1 to 6. 2 sheets of drawings