Patent Application
20090120078
The present invention relates to an apparatus for treating or purifying exhaust gas of an internal combustion engine, with particular emphasis being given to reducing the nitrogen oxide content of the exhaust gas through the use of selective catalytic reduction (SCR). The present invention is concerned, in particular, with making available reducing agents such as, in particular, ammonia, which act selectively on nitrogen oxides.
The emission into the environment of substances contained in the exhaust gas of internal combustion engines, is not desirable. For example, in many countries the exhaust gas of internal combustion engines is only allowed to contain nitrogen oxides (NOx) up to a certain limit value. In addition to measures internal to the engine, by which the emission of nitrogen oxides can be reduced by selection of a most suitable possible operating point of the internal combustion engine, after-treatment methods by which further reduction of nitrogen oxide emission is possible, have become established.
One possible way of further reducing nitrogen oxide emission is known as selective catalytic reduction. With that method, a selective reduction of the nitrogen oxides to molecular nitrogen (N2) is carried out with the use of a reducing agent. A possible reducing agent is ammonia (NH3). In that case, the ammonia is often stored not in the form of ammonia, but rather an ammonia precursor is stored which is converted into ammonia as required. Possible ammonia precursors are, for example, urea ((NH2)2CO), ammonium carbamate, isocyanic acid (HCNO), cyanuric acid and the like. In particular, urea has proved simple to store. The urea is preferably stored in the form of an aqueous urea solution. Urea and, in particular, aqueous urea solution, is harmless to health and simple to distribute and store. Such a urea/hydrogen solution is marketed under the trademark “Ad Blue”.
A method in which an aqueous urea solution is metered into a partial stream of an exhaust gas of an internal combustion engine upstream of a hydrolysis catalytic converter, is known from German Published, Non-Prosecuted Patent Application DE 199 13 462 A1. Hydrolysis and thermolysis of the urea into ammonia take place during operation upon impingement on the hydrolysis catalytic converter. The ammonia is used as a reducing agent in an SCR catalytic converter located downstream. The method described in that document has the disadvantage that the hydrolysis catalytic converter is cooled by the evaporation of the aqueous urea solution. Cooling can take place, especially if large quantities of ammonia are required, at least in zones of the hydrolysis catalytic converter, to such an extent that in those zones the hydrolysis reaction no longer takes place completely or at all. In addition, according to that prior art, exhaust gas flows through the hydrolysis catalytic converter. As a result, relatively large mass flows are present, in view of which the hydrolysis catalytic converter must be of correspondingly large dimensions. That entails cost and requires installation space which, especially in modern cars, is frequently not available to that extent.
It is accordingly an object of the invention to provide an apparatus for treating exhaust gas of an internal combustion engine, which overcomes or at least mitigates the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, an apparatus for treating exhaust gas of an internal combustion engine. The apparatus comprises at least one feed conduit for feeding an aqueous solution, a hydrolysis catalytic converter connected to the feed conduit, an SCR catalytic converter through which exhaust gas can flow, and at least one rod-shaped heating element for heating at least one of the following components:
a) at least parts of the feed conduit, and/or
b) the hydrolysis catalytic converter.
A rod-shaped heating element is understood to be, in particular, an electrically heatable element in which a heat conductor is embedded in an appropriate medium. It may, in particular, be a tubular casing which is heated from inside by the corresponding heat conductor. A heating cartridge, such as a ceramic heating cartridge which includes a ceramic element with a heat conductor embedded therein, may preferably be provided in the tubular casing.
A hydrolysis catalytic converter is understood to be a component through which a flow can take place and which catalyses hydrolysis of a reducing agent precursor to yield a reducing agent. In particular, a hydrolysis catalytic converter is understood to mean a component which includes a catalytic coating through the use of which the hydrolysis of urea to ammonia can be catalyzed. The aqueous solution includes, in particular, at least one reducing agent precursor of a reducing agent for use in the SCR method. A reducing agent precursor is understood to mean, in particular, a substance which can separate the reducing agent or can be converted thereto. The aqueous solution preferably includes at least urea. Further substances are possible as contents of the aqueous solution and are, according to the invention, in particular substances through which the freezing point of the aqueous solution can be lowered. These are, in particular, formic acid and/or ammonium formate. The connection between the feed conduit and the hydrolysis catalytic converter is configured in such a way that completely evaporated solution enters the hydrolysis catalytic converter in operation.
The apparatus according to the invention includes, in particular, an SCR catalytic converter which is provided in the exhaust gas line and through which exhaust gas can therefore flow, while the feed conduit and the hydrolysis catalytic converter are provided outside the exhaust gas line and exhaust gas does not flow through them. It is thereby possible to construct hydrolysis catalytic converters having a volume which can be substantially reduced in comparison to conventional hydrolysis catalytic converters through which exhaust gas can flow. For example, conventional hydrolysis catalytic converters have a volume of at least 500 ml and more, whereas according to the present invention the hydrolysis catalytic converter can have a volume of less than 100 ml.
In particular, the apparatus according to the invention is operated in such a way that the feed conduit can be heated such that evaporation of the aqueous solution takes place therein. In this case, a configuration is preferred in which complete evaporation, that is evaporation of at least 90 wt. %, preferably at least 95 wt. %, especially preferably at least 98 wt. % of the aqueous solution takes place. In this case, the heating is effected substantially through the rod-shaped heating element. The apparatus according to the invention can advantageously be used for treating or purifying the exhaust gases of stationary and/or mobile internal combustion engines, in particular for treating or purifying the exhaust gases of motor vehicles such as, in particular cars, passenger vehicles, trucks or heavy goods vehicles, motorized two-wheelers and/or so-called all-terrain vehicles, water craft and/or aircraft.
In accordance with another feature of the invention, the feed conduit has a mean surface roughness of 8 to 12 microns at the inside.
The mean surface roughness is usually denominated as Rz. In this case, a distance of measurement points of the surface in relation to a reference line or level is measured. This measurement is performed on five measurement tracks having the same length. For each of these measurement tracks, the difference of maximal and minimal value is gathered. The mean surface roughness is e.g. the average value of these five differences.
The mean surface roughness of 8 to 12 microns was found to be particularly advantageous because it causes a good contact of the solution with the wall of the feed conduit and thus a good evaporation efficiency.
In accordance with a further feature of the invention, the feed conduit is formed of a material having a thermal conductivity of more than 200 W/(m K) (Watts per meter and Kelvin).
This thermal conductivity is advantageous since it allows a compact construction of the apparatus according to the invention and a dynamic fast evaporation.
In accordance with an added feature of the invention, the feed conduit is formed of a material including aluminum.
Aluminum was found to be particularly advantageous because the aluminum oxides created on the surface promote the hydrolysis of the reducing agent precursor, in particular urea, to reducing agent, in particular ammonia, without the need for an additional catalytically active coating. This increases the efficiency of the conversion of reducing agent precursor to reducing agent and allows downsizing of the respective hydrolysis catalytic converter.
In accordance with an additional feature of the invention, at least one of the following components is disposed or formed around the rod-shaped heating element:
a) at least parts of the feed conduit, and/or
b) the hydrolysis catalytic converter.
In this case, the feed conduit may be provided, in particular, around the rod-shaped heating element. An embodiment is preferred in which the feed conduit is substantially spiral-shaped, and an axis of symmetry of the spiral is disposed coaxially with the longitudinal axis of the rod-shaped heating element. The hydrolysis catalytic converter may be in the form of a honeycomb body which is formed around the rod-shaped heating element, preferably in the form of an annular honeycomb body delimited by an inner and an outer tubular casing, but it may also include channels which are worked directly into the heating element and/or into a tubular casing surrounding the heating element.
This configuration of the apparatus according to the invention permits a very compact structure thereof, so that even in situations with only little installation space as, for example, especially in motor vehicles, effective evaporation of the aqueous solution can be achieved and at the same time a very compact structure of the apparatus according to the invention.
In accordance with yet another feature of the invention, the feed conduit includes at least one channel which is formed at least partially by a tubular casing of the heating element.
A tubular casing is understood in this case to mean, in particular, a sleeve which is provided outside the electrical heat conductor of the rod-shaped heating element. The tubular casing may be made of metallic, ceramic, vitreous or similar material which can withstand temperatures of preferably 400° C. and above, especially preferably 600° C. and above.
In accordance with yet a further feature of the invention, the at least one channel is provided or formed in the tubular casing.
This means, in particular, that channels are worked into the tubular casing by abrasive manufacturing methods, for example, and/or directly during production of the respective tubular casing, by suitable extrusion, for example.
In accordance with yet an added feature of the invention, the at least one channel is delimited on the inside by the tubular casing of the heating element and on the outside by a sleeve which is provided coaxially with the heating element.
In particular, in this case, this may be one or more channels which is/are formed substantially helically around the rod-shaped heating element and/or has/have a cross section in the form of an annular gap which is delimited on the inside by the tubular casing and on the outside by the sleeve. A configuration in which the cross section of the feed conduit, in particular of the channel or channels, changes, is also preferred. For example, there may be provided a channel in the form of an annular gap, the diameter of which increases monotonically or strictly monotonically along the rod-shaped heating element.
In accordance with yet an additional feature of the invention, there is provided, outside the heating element, a sleeve which is at least partially in thermal contact with at least one of the following components:
a) at least parts of the feed conduit, and/or
b) the hydrolysis catalytic converter.
In this way the sleeve, which is provided radially outside the heating element and preferably also radially outside at least parts of the feed conduit and/or of the hydrolysis catalytic converter, the feed conduit of the hydrolysis catalytic converter and/or the at least one channel, makes it possible to introduce heat from outside. In particular, the sleeve includes a further heat conductor and is configured, in particular, as an annular heating element or includes an annular heating element. In this way, very uniform distribution of the heat around the rod-shaped heat conductor can be ensured, which leads to very uniform evaporation and/or hydrolysis results.
In accordance with again another feature of the invention, the sleeve is heatable.
For this purpose the sleeve has, in particular, like the rod-shaped heating element, electrical connections through the use of which the sleeve and/or the rod-shaped heating element can be supplied with current. This enables heating of the respective channels from both inside and outside.
In accordance with again a further feature of the invention, the hydrolysis catalytic converter includes an annular honeycomb body which has channels through which a fluid can flow between an inner tubular casing and an outer tubular casing.
In this case, the honeycomb body preferably includes at least one at least partially structured, in particular corrugated, metallic layer by which the hydrolysis channels are formed. The channels have a coating which catalyses the hydrolysis of a reducing agent precursor to a reducing agent. A configuration is preferred in which at least the inner tubular casing of the hydrolysis catalytic converter is in thermal contact with the rod-shaped heating element, preferably is similarly connected thereto and/or is configured to rest against the same. A further heating element is preferably provided radially outside the hydrolysis catalytic converter, through the use of which heating element heat can be introduced into the hydrolysis catalytic converter through the outer tubular casing.
In accordance with again an added feature of the invention, the hydrolysis catalytic converter is formed at least partially by at least one hydrolysis channel which is formed at least partially by a tubular casing of the heating element.
In this case, the at least one hydrolysis channel, similarly to the channel described above, may be configured as implementing the feed conduit in the tubular casing of the heating element or may, for example, be delimited on the inside by the tubular casing of the heating element. In this case, a configuration is preferred in which, in particular, the channels or the feed conduit and the hydrolysis channel or channels may merge with one another, in particular may include the same channel, having a through-flow cross section which optionally changes and which has a hydrolysis coating in at least a partial region.
An embodiment is preferred in which the hydrolysis channel is formed in the tubular casing.
In particular, in this case a channel is formed, for example, by abrasive techniques in the structure, referred to herein as the tubular casing, which surrounds the heat conductors of the rod-shaped heating element.
In accordance with a concomitant feature of the invention, the hydrolysis channel is delimited on the inside by the tubular casing of the heating element and on the outside by a sleeve which is formed coaxially with the heating element.
In particular, the sleeve located on the outside is also heatable, so that heat transfer into the hydrolysis channel takes place from both the inside and the outside, so that hydrolysis which is as complete as possible of the reducing agent precursor to yield the reducing agent can take place with temperature regulation that is as uniform as possible. An embodiment is preferred in which the sleeve is in thermal contact with the hydrolysis channel.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an apparatus for treating exhaust gas of an internal combustion engine, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.
Referring now to the figures of the drawings in detail and first, particularly, to
The apparatus 1 according to the invention further includes an SCR catalytic converter 7 through which the exhaust gas 2 can flow. For this purpose, the SCR catalytic converter 7 is provided in an exhaust gas line 8, while the hydrolysis catalytic converter 5 and the feed conduit 4 are provided outside the exhaust gas line 8. In addition, the apparatus 1 according to the invention includes at least one rod-shaped heating element 9 through the use of which at least parts of the feed conduit 4 and/or of the hydrolysis catalytic converter 5 can be heated.
The feed conduit 4 and the reservoir 6 are connected to one another by a feed line 10. In addition, non-illustrated conveying devices may be provided, for example pumps, in particular metering pumps, with which the aqueous solution of at least one reducing agent precursor can be conveyed from the reservoir 6 into the feed conduit 4.
During operation, a gaseous mixture of substances is produced in the apparatus according to the invention 1 by evaporation of the aqueous solution in the feed conduit 4 through the use of the rod-shaped heating element 9 and subsequent hydrolysis in the hydrolysis catalytic converter 5. The gaseous mixture of substances includes at least one reducing agent for the selective catalytic reduction of nitrogen oxides. This is preferably ammonia. This gaseous substance mixture, which is produced especially as a result of the complete evaporation of the aqueous solution in the feed conduit 4 and subsequent hydrolysis in the hydrolysis catalytic converter 5, is fed at a mouth, opening-out or orifice region 11 into the exhaust gas line 8. In this case, a device 12 is provided in the mouth region 11 for generating a low-pressure-region in the exhaust gas line 8. The device 12 prevents exhaust gas 2 from entering the hydrolysis catalytic converter 5 and/or the feed conduit 4 from the exhaust gas line 8 in normal operation. The device 12 generates a calming or dead zone in the flow, which causes the corresponding low-pressure in the mouth region 11.
After the hydrolysis, a vapor stream 22 which contains reducing agent, in particular ammonia, leaves the channel 3. A sleeve 15 may be pushed over the tubular casing 14, as is indicated by an arrow 23. This sleeve 15 may, for example, itself have suitable heat conductors 18, so that the sleeve 15 is also heatable, and the channel 3 is thus heated from both outside and inside. Alternatively or cumulatively, the sleeve 15, after being brought into contact with the tubular casing 14, preferably by a material connection, in particular by soldering, brazing, welding, pressing or the like, is in thermal contact with the tubular casing 14. This may be achieved, for example, through the use of rib portions 24 between individual turns of the channel 3. In this way, with sufficient thermal conductivity through the rib portions 24, heating can also be effected by the sleeve 15 without an active heating device being provided thereon. The sleeve 15 has a narrowed portion 25 which serves to reduce the conduction of heat in the sleeve 15 between the first zone 20 and the second zone 21. The sleeve 15 can thereby form part of a separate regulating loop in each of the zones 20, 21, so that the hydrolysis catalytic converter 5 and the feed conduit 3 can be heated separately from one another. Independently thereof, it is advantageously possible for the rod-shaped heating element 9 and/or the sleeve 15 to have different heating zones, so that the heating output can be configured variably. This means, in particular, that preferably in the longitudinal direction of the rod-shaped heating element 9 and/or of the sleeve 15, zones can be formed which are subjected to different heating output. In this way, in particular, the different processes, especially during complete evaporation of the aqueous solution 16, can be taken into account. In this case, heating of the aqueous solution 16 takes place first, then evaporation and then, preferably, superheating of the resulting vapor, with different quantities of heat needing to be introduced in each case. This can be provided for by different heating zones of the rod-shaped heating element 9 and/or of the sleeve 15. In particular, a configuration of the rod-shaped heating element 9 and/or of the sleeve 15 is possible and advantageous in which, depending on the operating parameters, in particular depending on the quantity of aqueous solution 16 to be evaporated, variation of the lengths of the different heating zones can be achieved during operation.
In the present exemplary embodiment, the Peltier element 29 is used, in particular, to cool the feed line 10 and is therefore appropriately connected through the electrical connections 17. The second thermal insulator 28 and the Peltier element 29 are preferably provided only in the region of the feed line 10 and in an entry or connection zone thereof with the channel 3. The channel 3 preferably opens into a hydrolysis channel 36. In this case, the cross section of flow of the channel 3 may correspond to that of the hydrolysis channel 36. Furthermore, the cross section of the hydrolysis channel 36 may be larger than that of the channel 3. The at least one hydrolysis channel 36 is preferably also provided, in a similar way to the channel 3, in the tubular casing 14 of the rod-shaped heating element 9.
The apparatus 1 according to the invention permits the construction, as a compact structure, of a feed conduit 4 heated by a rod-shaped element 9, and of a corresponding hydrolysis catalytic converter 5, with which an aqueous solution containing urea can be evaporated and then hydrolyzed to a gas stream 22 containing ammonia. The gas stream 22 serves as a reducing agent in the SCR process. The compact configuration allows installation even in very confined space conditions. Due to the use of the hydrolysis catalytic converter 5, through which exhaust gas does not flow, the volume of the hydrolysis catalytic converter 5 can be decisively reduced, since in this case significantly smaller mass flows of gas have to be hydrolyzed.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2006 023 148.1 | May 2006 | DE | national |
This is a continuing application, under 35 U.S.C. § 120, of copending International Application No. PCT/EP2007/004360, filed May 16, 2007, which designated the United States; this application also claims the priority, under 35 U.S.C. § 119, of German Patent Application DE 10 2006 023 148.1, filed May 16, 2006; the prior applications are herewith incorporated by reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2007/004360 | May 2007 | US |
| Child | 12272343 | US |
