CH707551A1 - Exhaust aftertreatment system for treating exhaust gas of diesel engine of mobile working machine e.g. crane, has first feed system that is provided for supplying precursor product of reducing agent to exhaust gas - Google Patents

Abstract

The exhaust aftertreatment system has a catalyst (2) that is provided for converting nitrogen oxide by reacting nitrogen oxide contained in the exhaust gas with a reducing agent such as ammonia to the exhaust gas added. A first feed system (4) is provided for supplying a precursor product of a reducing agent to the exhaust gas. A second feeding system (5) is provided for the direct supply of the reducing agent to the exhaust gas. The first feed system is provided with a first metering system (6) for metering a quantity of the exhaust gas added previous product. Independent claims are included for the following: (1) a mobile working machine; and (2) a method for operating exhaust gas aftertreatment system.

Description

The present invention relates to an exhaust gas aftertreatment system with a catalyst for converting NOx by reacting NOx contained in the exhaust gas with a reducing agent added to the exhaust gas. Such catalysts are used in particular in diesel engines for exhaust gas purification.

In known systems, the reducing agent, usually ammonia, is added as a precursor product in the form of an aqueous urea solution. With standard SCR systems, 32.5% urea is usually dissolved in water and metered into the exhaust gas. The water is then evaporated in the exhaust gas flow and the urea is converted to ammonia by thermolysis and hydrolysis. This is illustrated by the following three formulas:

[0003] Evaporation of water

Urea thermolysis

Isocyanic acid hydrolysis

The reaction to convert the previous product into the reducing agent and in particular the evaporation of the water requires a lot of energy. This energy is withdrawn from the exhaust gas flow, which reduces the temperature of the exhaust gas at the inlet of the catalytic converter.

[0007] On the one hand, this can lead to insufficient conversion of the NOx taking place in the catalytic converter. Furthermore, if there is insufficient energy in the exhaust gas flow, the aqueous predecessor product cannot be completely converted into the reducing agent, which can lead to the formation of deposits. The dosing quantity of the dosing system is therefore severely limited by the exhaust gas temperature.

Known SCR systems can therefore only insufficiently or not at all reduce the NOx emissions of the engine at low exhaust gas temperatures. This makes it necessary to artificially increase the exhaust gas temperatures, for example by increasing the back pressure in the exhaust system using an exhaust flap. But there are also limits to such measures.

Alternatives to adding the ammonia in the form of an aqueous urea solution are also known from the prior art. However, with the available installation space and the given weight restriction, especially with large diesel engines, these can often not be used sensibly.

It is therefore the object of the present invention to provide an exhaust gas aftertreatment system which nevertheless achieves a reduction in NOx emissions at low exhaust gas temperatures with the smallest possible installation space.

According to the invention, this object is achieved by an exhaust gas aftertreatment system according to claim 1.

The exhaust gas aftertreatment system according to the invention has a catalyst for converting NOx by reacting NOx contained in the exhaust gas with a reducing agent added to the exhaust gas. A first supply system is provided for supplying a predecessor product of the reducing agent to the exhaust gas, as well as a second supply system for direct supply of the reducing agent to the exhaust gas. The two supply systems thus allow a separate, separately meterable and controllable supply of the preceding product or the reducing agent to the exhaust gas.

This embodiment has the advantage that the metered quantity of the second feed system is not limited by the exhaust gas temperature, but only by the size of the second feed system. The second feed system can therefore be used in particular when the first feed system cannot provide sufficient reducing agent due to the low temperatures of the exhaust gas. At the same time, however, the second feed system does not have to be dimensioned so large that it can take over the entire feed of the reducing agent in all operating situations. Rather, especially at high exhaust gas temperatures, the reducing agent can be made available by adding the previous product via the first supply system. This saves space and weight. A further advantage is that, at high exhaust gas temperatures, no energy has to be expended in order to make the reducing agent available in the second supply system.

The present invention can be used in particular with one of the following types of catalyst: - SCR cat - DOC + SCR cat - DOC + DPF + SCR-Kat, DOC + SCR-Kat + DPF - SCRF - DOC + SCRF SCR Selective Catalytic Reaction DOC Diesel Oxidation Catalyst DPF Diesel Particle Filter SCRF SCR on Filter (DPF with SCR coating)

The present invention is also preferably used in a system in which ammonia is used as the reducing agent and an aqueous urea solution as the predecessor product. In particular, the urea solution can be 32.5% urea in aqueous solution.

The first feed system can have a first metering system for metering an amount of the previous product added to the exhaust gas. In particular, a pump and / or valves can be provided.

The first feed system is advantageously designed in such a way that it sprays the liquid precursor product into the exhaust gas flow.

The second feed system, however, is advantageously designed so that it introduces the reducing agent in gaseous form directly into the exhaust gas flow.

The second supply system can have a reducing agent generator, which extracts the reducing agent from a previous product. In particular, the gaseous reducing agent can be obtained from a liquid predecessor product. In particular, the previous product can also be an aqueous urea solution and the reducing agent can be ammonia. A conversion catalyst into which the intermediate product is sprayed and in which the intermediate product is converted into the reducing agent can in particular be used as the reducing agent generator. The reducing agent generator advantageously has a heating device which provides the temperature required for the conversion.

Furthermore, the second feed system can have a second metering system for metering the precursor product added to the conversion catalyst, by means of which the amount of the reducing agent fed directly to the exhaust gas stream is adjusted. This metering system can also have a pump and / or valves.

The exhaust gas aftertreatment system according to the invention advantageously has a controller which controls the first and the second supply system. The first and the second supply system are advantageously controlled as a function of the temperature of the exhaust gas.

Advantageously, the control is designed so that it increases the amount of reducing agent that is added directly via the second feed system at lower temperatures of the exhaust gas. Conversely, the amount of reducing agent which is added directly via the second feed system can be reduced with increasing temperatures.

The control of the two supply systems can also depend on other operating parameters, in particular on operating parameters of the engine, which have an influence on the amount of NOx in the exhaust gas. Furthermore, a NOx sensor can also be provided upstream of the catalytic converter, the values of which are included in the control of the two supply systems.

The first and the second supply system can add reducing agent or the previous product to the exhaust gas both simultaneously and alternately.

The present invention further comprises an engine with an exhaust gas aftertreatment system as described above. In particular, the engine is a diesel engine. This is advantageously a diesel engine with high power.

[0026] The present invention further comprises a mobile work device with an engine with an exhaust gas aftertreatment system according to the invention. In particular, the motor can serve as a propulsion drive and / or to drive the work equipment of the mobile work device. For example, the motor can be used to drive a hydraulic system of the implement.

The working device can be, for example, a handling device such as a crane and / or earthmoving and / or construction machinery such as hydraulic excavators, dump trucks or the like. The present invention can also be used in diesel-electric systems, such as are also used, for example, in a dump truck.

The present invention further comprises a method for operating an exhaust gas aftertreatment system in which NOx is converted in a catalyst by reacting NOx contained in the exhaust gas with a reducing agent added to the exhaust gas. At the same time or alternately, a predecessor product of the reducing agent is added to the exhaust gas, which is converted there into the reducing agent, and the reducing agent is added directly to the exhaust gas.

The predecessor product can be added in liquid form by spraying it into the exhaust gas. In contrast, the reducing agent can be passed into the exhaust gas in gaseous form.

Advantageously, the predecessor product and the reducing agent are added as a function of the temperature of the exhaust gas.

In particular, at lower temperatures of the exhaust gas, the amount of reducing agent which is added directly can then be higher than at higher temperatures of the exhaust gas.

The method according to the invention is preferably carried out in the way that has already been described above with regard to the exhaust gas aftertreatment system according to the invention. This is advantageously a method for operating such an exhaust gas aftertreatment system.

The present invention will now be illustrated in more detail using an exemplary embodiment and a figure.

The single figure shows an embodiment of an exhaust gas aftertreatment system according to the invention.

In the single figure, an embodiment of an exhaust gas aftertreatment system according to the invention is shown. The exhaust gas aftertreatment system serves to treat the exhaust gas of a diesel engine 1 and, in the exemplary embodiment, comprises an SCR catalytic converter 2, in which NOx is converted by reaction with ammonia. However, the invention could also be used with other types of catalyst.

A first feed system 4 is provided, through which an aqueous urea solution is metered into the exhaust gas. A 32.5% urea dissolved in water can be used, which is known under the name AdBlue. The water is then evaporated in the exhaust gas flow and the urea is converted to ammonia by thermolysis and hydrolysis. This is illustrated by the following three formulas:

Evaporation of water

Urea thermolysis

Isocyanic acid hydrolysis

Since the metered amount of this metering system is severely limited by the exhaust gas temperature, a second feed system 8 is also provided for direct feeding of ammonia into the exhaust gas. The second supply system has an ammonia generator which generates gaseous ammonia, which can be dosed directly into the exhaust gas.

As a result, no evaporation or conversion is necessary in the exhaust gas flow, so that there is also no temperature loss. The entire exhaust gas temperature can thus be used for the DeNOx reaction in the SCR.

In the exemplary embodiment, the ammonia generator generates gaseous ammonia from a precursor product. This predecessor product is converted into gaseous ammonia in the ammonia generator. Since this process is independent of the exhaust gas temperature and independent of the engine load, the metered quantity of the second feed system is not limited by the exhaust gas temperature, but only by the size of the ammonia generator.

The inventive combination of a known metering system for metering a previous product with the ammonia generator makes it possible to provide enough ammonia for the full functionality of the SCR system in the entire temperature range of the diesel engine, and at the same time to meet the space restriction for such drives .

In the exemplary embodiment, the first feed system comprises a container for the preceding product, a feed line 5 for the preceding product from this container, a pump 6 for metering the preceding product, from which the preceding product is metered into the exhaust gas flow at a metering point 7.

The second feed system has an ammonia generator 11, which generates gaseous ammonia and from which it reaches a metering point 12 in the exhaust gas flow. The ammonia generator 11 can in particular be a conversion catalyst. This can be heated to provide the temperatures required to convert a previous product into ammonia. The previous product is advantageously also an aqueous urea solution, and in particular likewise a 32.5% solution of urea in water.

This previous product can reach the pump 10 via a feed line 9, which pump 10 takes over the dosing of the previous product into the ammonia generator 11.

The feed line 9 can branch off from a separate container for the preceding product, or from the same container which also provides the preceding product for the first feed system.

Thus, according to the invention, two metering systems are provided, each of which can be used to separately meter the predecessor product or the reducing agent to the exhaust gas flow.

Advantageously, the control of the two supply systems or the metering systems contained in them takes place as a function of the temperature of the exhaust gas, wherein the second supply system can be used in particular at low exhaust gas temperatures. If the exhaust gas temperatures are high, however, the first feed system can mainly be used.

The SCR catalytic converter 2 used in the exemplary embodiment has several catalytic converter sections with an SCR function 3. An ammonia oxidation catalytic converter 4 can optionally be provided, which is arranged at the exit of the catalytic converter and prevents the escape of ammonia.

The exhaust gas aftertreatment system according to the invention can also be combined with an exhaust gas flap, which increases the counter pressure in a heat mode and thereby achieves higher exhaust gas temperatures.

Claims (15)

An exhaust aftertreatment system comprising a catalyst (2) for converting NOx by reacting NOx contained in the exhaust gas with a reducing agent added to the exhaust gas, with a first supply system (4) for supplying a predecessor product of the reducing agent to the exhaust gas, and with a second supply system (5) for the direct supply of the reducing agent to the exhaust gas. The exhaust aftertreatment system of claim 1, wherein the reducing agent is NH 3 and the precursor product is an aqueous urea solution. 3. Aftertreatment system according to claim 1 or 2, wherein the first feed system (4) comprises a first metering system (6) for metering an amount of the precursor product added to the exhaust gas. 4. exhaust aftertreatment system according to any one of the preceding claims, wherein the first supply system (4) is designed so that it injects the liquid precursor product in the exhaust stream. 5. exhaust aftertreatment system according to one of the preceding claims, wherein the second supply system (8) is designed so that it introduces the reducing agent in gaseous form in the exhaust gas stream. 6. Exhaust after-treatment system according to one of the preceding claims, wherein the second supply system (8) comprises a reducing agent generator (11) which extracts the reducing agent from a predecessor product, in particular a conversion catalyst, wherein the second supply system (8) advantageously a second metering system (10) for dosing the precursor product added to the conversion catalyst. 7. exhaust aftertreatment system according to one of the preceding claims with a controller which controls the first and the second supply system in dependence on the temperature of the exhaust gas. The exhaust aftertreatment system of claim 7, wherein the controller increases the amount of reductant added directly via the second delivery system at lower temperatures of the exhaust gas. 9. Engine with an exhaust aftertreatment system according to one of the preceding claims, in particular diesel engine (1). 10. Mobile work unit with an engine according to claim 9, wherein the motor (1) serves as a travel drive and / or to drive the working equipment of the mobile implement, and in particular for driving a hydraulic system. 11. A method for operating an exhaust aftertreatment system in which NOx in a catalyst (2) is converted by reaction of NOx contained in the exhaust gas with a reducing agent added to the exhaust gas, where simultaneously or alternately both a precursor product of the reducing agent is added to the exhaust gas, which is converted there into the reducing agent, as well as the reducing agent is added directly to the exhaust gas. 12. The method of claim 11, wherein the addition of the precursor product in liquid form by spraying into the exhaust gas takes place and the reducing agent is passed in gaseous form into the exhaust gas. 13. The method of claim 11 or 12, wherein the addition of the precursor product and the reducing agent takes place in dependence on the temperature of the exhaust gas. 14. The method of claim 13, wherein at lower temperatures of the exhaust gas, the amount of reducing agent, which is added directly, is higher than at higher temperatures of the exhaust gas. 15. The method according to any one of the preceding claims for operating an exhaust aftertreatment system according to one of claims 1 to 9.