US20100236225A1

US20100236225A1 - Exhaust posttreatment arrangement with reducing agent reservoir, and method for posttreatment of exhaust gases

Info

Publication number: US20100236225A1
Application number: US12/677,059
Authority: US
Inventors: Thomas Wahl; Florian Wahl; Michael Koehler
Original assignee: Individual
Current assignee: Robert Bosch GmbH
Priority date: 2007-09-06
Filing date: 2008-08-14
Publication date: 2010-09-23
Also published as: EP2185797A1; DE102007042409A1; WO2009033911A1

Abstract

Proposed are an exhaust gas posttreatment arrangement and a method for the reduction of pollutants, in particular for the selective catalytic reduction of nitrogen oxides. The arrangement has an exhaust line for conducting the exhaust gas of an internal combustion engine to a treatment unit for the reduction of the pollutants, and at least one reservoir for storing a reducing agent for use in the treatment unit. The reservoir is arranged in the region of the exhaust line and extends parallel to the exhaust line, and is in heat-conducting contact with the exhaust line and/or with the exhaust gas in the exhaust line.

Description

PRIOR ART

The invention is based on an exhaust gas posttreatment arrangement for reducing pollutants and on a method for exhaust gas posttreatment as generically defined by the preambles to the independent claims.
To meet the demands for exhaust gas quality in vehicles, what is known as the SCR method (for “selective catalytic reduction”) is employed for reducing nitrogen oxides in diesel vehicles, and especially utility vehicles.
This method has already been used for some time in heating power plants for reducing nitrogen oxide emissions. There, ammonia is added to the untreated exhaust gas from the heating stage. The ammonia-laden exhaust gas is conducted via a catalytic converter, which selectively catalyzes the reaction of ammonia with nitrogen monoxide in the presence of oxygen. The temperatures of the catalytic converter are in a range from 250° C. to 500° C. As a rule, a catalytic converter which has a catalytically active coating of divanadium pentoxide on tungsten-oxide-stabilized titanium dioxide (the so-called anatase phase) is used. Ammonia and nitrogen monoxide react in the catalytic converter to harmless substances, specifically water and nitrogen.
In vehicles, instead of ammonia, urea is used as the reducing agent. The catalytic reduction of the nitrogen oxides is preceded by a catalytic hydrolysis of the urea into ammonia and carbon dioxide. After that, ammonia reacts with the nitrogen oxides further in accordance with the aforementioned reaction. Particularly in motor vehicles, still other suitable catalytic converter materials may be employed, such as transition metal compounds applied to gamma aluminum oxide, and iron-doped gamma aluminum oxide. Zeolites replaced or impregnated with transition metal ions, can also be employed.
In direct metering of gaseous ammonia into the exhaust system, there are advantages over denitrification of exhaust gas that is based on an aqueous urea solution with the tradename “AdBlue”, such as the avoidance of the use of a corrosive and freezable fluid. Moreover, famishing the reducing agent is independent of the exhaust gas temperature.
From International Patent Disclosure WO 99/01205, it is known to store ammonia by incorporation in salts and to initiate a thermal desorption as needed. It is furthermore known, for heating a solid reservoir medium, to use the waste heat of the engine coolant and/or of the exhaust gas and as needed to provide a buffer volume for ammonia that has already dissolved out of the ammonia reservoir.
From European Patent Disclosure EP 1 561 017, it is known to provide exhaust lines, discharging on opposed ends of an ammonia reservoir, in order to release an auxiliary exhaust gas posttreatment agent that originates in a reactor and is temporarily stored in the ammonia reservoir, and in a further step, to be able to transfer it into the exhaust system.

DISCLOSURE OF THE INVENTION

The exhaust gas posttreatment arrangement according to the invention and the method for exhaust gas posttreatment according to the invention, having the definitive characteristics of the bodies of the independent claims, have the advantage over the prior art of energy-efficient temperature control of a reducing agent reservoir, in particular an ammonia reservoir, and thus to furnish an energy-efficient arrangement for reducing nitrogen oxides contained in the exhaust gas, for instance, and to ensure energy-efficient exhaust gas posttreatment. In particular, the additional fuel consumption required for an electrical energy demand can be reduced. It can considered a further advantage that because of integration of the storage tank in the vicinity of the exhaust system, the waste heat of the exhaust gas can not only be optimally utilized for releasing the reducing agent, such as gaseous ammonia, from the reservoir or for releasing a reservoir substance contained in the reservoir, but also and at the same time, a space-saving arrangement can be furnished and furthermore, possible problems with the voltage stability of an electrical on-board system of the motor vehicle can be lessened or even avoided entirely.
By means of the provisions recited in the dependent claims, advantageous further refinements of and improvements to the arrangements and methods recited in the independent claims are possible. It is especially advantageous to provide a direct heat-conducting interaction, for instance by means of a special shaping of the reservoir, adapted to the longitudinal dimension of the exhaust system and disposed directly on or inside the exhaust gas-carrying line, as a result of which the heat transfer can be maximized on the one hand, and on the other, the space required for the entire arrangement can be minimized.
Further advantages will become apparent from further characteristics recited in the dependent claims and in the description.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are shown in the drawings and explained in further detail in the ensuing description.

FIG. 1 shows an exhaust gas posttreatment arrangement with a reservoir contacting the exhaust line;

FIG. 2 shows an arrangement with a reservoir disposed inside one path of the exhaust line;

FIG. 3 shows an arrangement with a reservoir inside a single-path exhaust line;

FIG. 4 a shows a reservoir, and FIG. 4 b shows a reservoir with a cartridge that can be introduced into a container.

EMBODIMENTS OF THE INVENTION

FIG. 1 shows an exhaust gas posttreatment arrangement 1 with an exhaust line 7 that conducts exhaust gas, arriving in the flow direction 5 from an internal combustion engine, such as a diesel engine, to a preparation unit 3 embodied as a catalytic converter for selective catalytic reduction of nitrogen oxides. Upstream of the SCR catalytic converter, in terms of the exhaust gas flow, the exhaust line 7 splits into a first path 9 and a second path 11; the two paths unite again, still upstream of the SCR catalytic converter, to form one gas-carrying line. On the side toward the engine, a first, in particular electrically triggerable exhaust gas valve 13, a so-called splitting exhaust gas valve, carries the exhaust gas stream selectively into one of the two paths, or on assuming an intermediate position, also simultaneously into the two paths 9 and 11. On the side toward the catalytic converter, via a second exhaust gas valve 15, one of the two paths can be selectively closed. In an intermediate position of the second exhaust gas valve, exhaust gas flows from both paths that unite upstream of the catalytic converter can also be carried to the catalytic converter 3. A reservoir 18 is provided for storing ammonia by means of a reservoir substance 19 comprising magnesium chloride. The storage tank 17 of the reservoir is disposed parallel to the exhaust line, along a wall 8 of the exhaust line 7, in the vicinity of the second path 11. The outside of the storage tank and the exhaust gas tube touch two-dimensionally and form a heat transfer region 27. The spatial dimension 29 of the reservoir or of the storage tank perpendicular to the flow direction 20 of the exhaust gas in the second path 11 is small, compared to its dimension along the second path 11 (for instance at a ratio in the range of 1:10 to 1:50, in particular in the range of 1:12 to 1:45). An electrically triggerable metering valve 21 is mounted on the reservoir and is connected on its open end to a supply line 23, protruding from the SCR catalytic converter 3 into the exhaust line, and between the metering valve and the open end of the supply line that protrudes into the exhaust line, there is a buffer container 24 for temporary storage of ammonia that has already emerged from the reservoir. This buffer container, on its end toward the open end of the supply line, is provided with a further closing means, not identified by reference numeral, such as a further electrically triggerable valve. The cleaned exhaust gas leaves the catalytic converter, on the side away from the delivery of reducing agent, in the flow direction 25 and then, optionally via further exhaust gas posttreatment arrangements or via the muffler, it reaches the open air. An electric heating element 31 is also disposed on the reservoir 18. An electric control unit 32 is connected to sensors, not identified by reference numeral, but furnish sensor signals 33, such as a nitrogen oxide sensor at the outlet of the SCR catalytic converter. The closing means, in particular the metering valve 21, on the reservoir or on the buffer container is supplied, along with the electric heater 31 and the exhaust gas valves 13 and 15, with control signals 34, via electric signal lines, not identified by reference numeral, from the control unit 32.
The exhaust system is embodied such that the exhaust gas flow can be carried away, controlled via valves, in two phases (so-called dual-flow system). The ammonia reservoir substance is accommodated in a storage tank, which is disposed on one of the two exhaust lines, in such a way that a good heat transfer from the exhaust gas to the reservoir substance takes place. The good heat transfer can be reinforced by a suitable choice from among materials with high thermal conductivity and by a suitable structural design, for instance by employing an intermeshing rib structure of the exhaust gas tube and the storage tank. By means of a heat input from exhaust gas flowing past into the ammonia reservoir substance, ammonia is released. By means of the released, gaseous ammonia (and possible byproducts if alternative reservoir materials are used), an overpressure is created in the storage tank embodied as a pressure vessel. Via the metering valve 21, or the closing means mounted on the buffer container, the gaseous ammonia is metered into the exhaust system. The exhaust gas flow for heating the ammonia reservoir substance can be adjusted via the splitting exhaust gas valve 13. The amount of heat that is imported into the ammonia reservoir substance is dependent on the temperature and the flow rate of the exhaust gas flowing through the path 11. Controlling the valve position and thus also the ammonia released can be done by means of a flow rate meter, a temperature sensor, or an exhaust gas pressure sensor. The measured value is detected in the control unit 32 and regulated, in accordance with the position of the splitting exhaust gas valve 13, via control signals 34. The exhaust gas flow in the path 11 is adjusted, at every operating point of the engine, such that whichever quantity of ammonia is required for reducing nitrogen oxides is available. If in certain operating points, the exhaust gas heat proves inadequate for furnishing the reducing agent for nitrogen oxide reduction, then the electric heating element 31 can additionally heat the reservoir, or the reservoir substance, for the release of ammonia. The electric heating can also be employed for an early-onset metering in cold starting of the engine, where otherwise, pollutant-laden exhaust gas leaving the motor vehicle would have to be expected, since the exhaust gas temperatures for releasing the ammonia from the reservoir substance are not yet high enough. In cooperation with the exhaust gas valve control, the heating element also increases the dynamic range of the possible metering of the reducing agent, both in terms of timing, or in other words the response time of the metering system to an electronically controlled demand for increased quantities of reducing agent, and quantitatively, in other words with regard to the maximum possible output quantity of reducing agent per unit of time. The buffer container 24 can also help to shorten the response time of the metering system by keeping a quantity of gaseous ammonia in reserve that can be called up within the briefest possible time, especially upon cold starting of the engine. A buffer volume also serves the general purpose of reserving gaseous ammonia for greater dynamic demands, or in other words in quantitative terms as well, as already explained in conjunction with the electric heating element.
In alternative versions, the arrangement may also be provided without electric heating or without a buffer container. In a further alternative version, the exhaust gas valves can be omitted. Furthermore, a buffer volume may be provided not in the form of a separate container but rather as a partial volume inside the storage tank. The reservoir substance can also be accommodated in a replaceable cartridge, instead of in a container, and the replaceable cartridge can in turn be introduced into the container. In a further variant, the storage tank itself may be embodied as a replaceable cartridge, which improves the heat transfer from the exhaust gas to the reservoir substance, because then, instead of three intermediate walls (exhaust gas tube, wall of the storage tank, wall of the cartridge), now only two intermediate walls divide the exhaust gas from the reservoir substance, namely the wall of the exhaust gas tube and the wall of the storage tank serving as a cartridge. The introduction of the ammonia reservoir substance, magnesium chloride, ensures simple refilling of the exhaust gas posttreatment arrangement with new ammonia by means of standardizably changing cartridges. The arrangement is furthermore suitable for many ammonia reservoir substances from which ammonia is released by means of thermal desorption or thermolysis, that is, the action of temperature. Suitable reservoir substances, in addition to magnesium chloride, may for instance be many other salts, in particular other chlorides and/or sulfates of one or more alkaline earth elements (such as CaCl₂) and/or one or more 3d subgroup elements, such as manganese, iron, cobalt, nickel, copper, and/or zinc. Moreover, organic adsorbers and ammonium salts, such as ammonium carbamate, are suitable ammonia reservoir substances that can be employed.
FIG. 2 shows a further exhaust gas posttreatment arrangement 40, in which components that are the same as or similar to the arrangement shown in FIG. 1 are provided with the same reference numerals and will not be described again. In a distinction from the arrangement in FIG. 1, in which a storage tank is indeed integrated in the vicinity of the exhaust system but is disposed outside the exhaust gas flow, the arrangement 40 has a storage tank 41 disposed inside the exhaust line 7, in the vicinity of the path 11. The storage tank is locked here by means of detachable struts 43, so that the exhaust gas, on at least one side, can flow between the reservoir wall and the wall of the exhaust line.
In the present introduction of the storage tank for the ammonia reservoir substance into the exhaust line, the container is exposed directly to the full flow of exhaust gas in the path 11, and as a result the heat transfer region, in comparison to the arrangement of FIG. 1, ensures improved heat-conducting contact between the exhaust gas and the reservoir substance. Refilling the storage tank can be done by connecting it to an ammonia gas reservoir, via a connection device not identified by reference numeral, in a factory or garage. Alternatively, a screw/flange connection may be provided, which makes it possible to replace the reservoir, together with the exhaust gas tube (path 11) with a filled reservoir.
FIG. 3 shows an exhaust gas posttreatment arrangement 46, in which, similarly to the arrangement of FIG. 2, the storage tank 41 is exposed to the full flow of exhaust gas, but not to the full flow of exhaust gas in a partial path but instead, to avoid splitting up the exhaust gas flow, to the entire exhaust gas flow originating in the engine. If the exhaust gas temperature is too high, which leads to pronounced desorption of ammonia, then cooling of the exhaust gas can be brought about by means of a cooling device 47 embodied as an air supply line for cooling air. Here as well, via an electric heater not identified by reference numeral, the reservoir substance can be heated in the event that the fundamental heat of the exhaust gas is inadequate.
FIG. 4 a again schematically and in enlarged form shows a detail of the preceding exhaust gas posttreatment arrangements, namely a reservoir 18 whose reservoir substance 19 is located in a storage tank 17. For withdrawing reducing agent, the metering valve 21 is disposed on one side of the reservoir. This storage tank 17 can be embodied as a replaceable and standardizable cartridge, as already discussed above. FIG. 4 b shows an alternative embodiment of a reducing agent reservoir that can be employed in one of the described arrangements or methods, in whose storage tank 17 it is not the ammonia reservoir substance that is introduced directly but rather a cartridge 51 that in turn contains the ammonia reservoir substance. In this case, the storage tank can be said to form a heat-transfer sheath, in the interior of which replaceable cartridges can be placed as needed.

Claims

1-25. (canceled)

26. An exhaust gas posttreatment arrangement for reducing pollutants contained in an exhaust gas of an internal combustion engine, in particular for selective catalytic reduction of nitrogen oxides, having

an exhaust line for conducting the exhaust gas to a preparation unit for reducing the pollutants, and having

at least one reservoir for storing a reducing agent for use in the preparation unit, the reservoir being disposed in the vicinity of the exhaust line and extending parallel to the exhaust line and being in heat-conducting contact with the exhaust line and/or with the exhaust gas contained in the exhaust line, wherein

the exhaust line splits upstream of the preparation unit, in terms of the exhaust gas flow, into a first path and a second path, and the heat-conducting contact is effected via at least one of the first path and the second path.

27. The exhaust gas posttreatment arrangement as defined by claim 26, wherein the reservoir and the exhaust line are disposed such that the reservoir interacts in directly heat-conducting fashion with the exhaust line and/or the exhaust gas.

28. The exhaust gas posttreatment arrangement as defined by claim 26, wherein the heat-conducting contact is arranged for thermal release of the reducing agent from a reservoir substance of the reservoir, in particular for inducing a thermal desorption of the reducing agent out of the reservoir substance and/or for inducing an at least partially thermolytic decomposition of the reservoir substance, forming the reducing agent.

29. The exhaust gas posttreatment arrangement as defined by claim 27, wherein the heat-conducting contact is arranged for thermal release of the reducing agent from a reservoir substance of the reservoir, in particular for inducing a thermal desorption of the reducing agent out of the reservoir substance and/or for inducing an at least partially thermolytic decomposition of the reservoir substance, forming the reducing agent.

30. The exhaust gas posttreatment arrangement as defined by claim 28, wherein the reservoir substance is in the solid phase at room temperature.

31. The exhaust gas posttreatment arrangement as defined by claim 29, wherein the reservoir substance is in the solid phase at room temperature.

32. The exhaust gas posttreatment arrangement as defined by claim 30, wherein the reservoir substance is formed from at least one salt and/or at least sulfate of one or more alkaline earth elements and/or of one or more 3d subgroup elements and/or from at least one organic adsorber.

33. The exhaust gas posttreatment arrangement as defined by claim 31, wherein the reservoir substance is formed from at least one salt and/or at least sulfate of one or more alkaline earth elements and/or of one or more 3d subgroup elements and/or from at least one organic adsorber.

34. The exhaust gas posttreatment arrangement as defined by claim 32, wherein the salt is an ammonium salt.

35. The exhaust gas posttreatment arrangement as defined by claim 33, wherein the salt is an ammonium salt.

36. The exhaust gas posttreatment arrangement as defined by claim 26, wherein a spatial dimension of the reservoir in a direction perpendicular to a flow direction of the exhaust gas is small in comparison to its spatial dimension in the flow direction of the exhaust gas.

37. The exhaust gas posttreatment arrangement as defined by claim 26, wherein the reservoir has a storage tank which extends along a wall of the exhaust line.

38. The exhaust gas posttreatment arrangement as defined by claim 37, wherein the storage tank contacts and/or is secured to the wall of the exhaust line.

39. The exhaust gas posttreatment arrangement as defined by claim 26, wherein the reservoir has a replaceable cartridge.

40. The exhaust gas posttreatment arrangement as defined by claim 37, wherein the storage tank is arranged for receiving a replaceable cartridge.

41. The exhaust gas posttreatment arrangement as defined by claim 38, wherein the storage tank is arranged for receiving a replaceable cartridge.

42. The exhaust gas posttreatment arrangement as defined by claim 38, wherein a replaceable cartridge is formed by the storage tank.

43. The exhaust gas posttreatment arrangement as defined by claim 38, wherein a replaceable cartridge is formed by the storage tank.

44. The exhaust gas posttreatment arrangement as defined by claim 39, wherein the replaceable cartridge is formed by a storage tank.

45. The exhaust gas posttreatment arrangement as defined by claim 26, wherein the reservoir is disposed inside the exhaust line.

46. The exhaust gas posttreatment arrangement as defined by claim 26, wherein both the first path and the second path discharge into the preparation unit.

47. The exhaust gas posttreatment arrangement as defined by claim 46, wherein a switchable exhaust gas valve is provided, by way of which the first path and the second path are joined together upstream of the preparation unit.

48. The exhaust gas posttreatment arrangement as defined by claim 26, wherein switchable splitting exhaust gas valve is arranged for selectively introducing the exhaust gas into the first path or into the second path.

49. The exhaust gas posttreatment arrangement as defined by claim 48, wherein the splitting exhaust gas valve is arranged for being able to assume an intermediate position as well, so that exhaust gas can be introduced into both the first path and the second path.

50. The exhaust gas posttreatment arrangement as defined by claim 26, wherein a heating element is provided for heating the reservoir independently of a heat derived from the exhaust gas.

51. The exhaust gas posttreatment arrangement as defined by claim 26, wherein the reservoir is disposed upstream of the preparation unit, in terms of the exhaust gas flow.

52. The exhaust gas posttreatment arrangement as defined by claim 26, wherein a cooling device is provided for cooling the reservoir.

53. The exhaust gas posttreatment arrangement as defined by claim 26, wherein an electrically triggerable metering valve is disposed on the reservoir.

54. The exhaust gas posttreatment arrangement as defined by claim 53, wherein an electric control unit is provided for triggering the metering valve.

55. The exhaust gas posttreatment arrangement as defined by claim 26, wherein for intermediate storage of reducing agent that has already emerged from the reservoir before it is metered into the exhaust line, a buffer container is provided.

56. The exhaust gas posttreatment arrangement as defined by claim 26, wherein the preparation unit has a catalytic converter, in particular a catalytic converter arranged for selective catalytic reduction.

57. A method for exhaust gas posttreatment for reducing pollutants contained in an exhaust gas of an internal combustion engine, in particular for selective catalytic reduction of nitrogen oxides, comprising the steps of

conducting the exhaust gas via an exhaust line from the engine to a preparation unit for reducing the pollutants;

storing a reducing agent for use in the preparation unit in a reservoir which is disposed in the vicinity of the exhaust line; and

extending the reservoir parallel to the exhaust line and having the reservoir contacting the exhaust line and/or the exhaust gas in heat-conducting contact, wherein the exhaust line splits upstream of the preparation unit, in terms of the exhaust gas flow, into a first path and a second path, and the heat-conducting contact is effected via at least one of the first path and the second path.