Patent Application
20070274877
The present invention relates to gas treatment apparatus and, in particular, to gas treatment apparatus for treating the exhaust gases from a diesel engine of a vehicle. The invention extends to a vehicle equipped with such gas treatment apparatus.
Diesel engine exhaust gases contain a number of noxious gases, such as nitrogen oxides, sulphur oxides and carbon oxides, as well as un-burnt hydrocarbons, carbon and other particles. Some of these compounds can be treated so as to render them less obnoxious.
It is therefore common practice to pass the exhaust gases through one or more treatment elements such as catalytic converters and filters. The exhaust gases can be subjected to reduction of nitrogen oxides to nitrogen by injecting a reducing agent, typically urea dissolved in water, into the gas stream and then passing it through a catalytic treatment element to convert residual ammonia from the urea to nitrogen and water, which are acceptable exhaust emissions. This process is known as Selective Catalytic Reduction (SCR). In practice, the efficiency of the process is partly dependent upon the quality of the mixing of the reducing agent and gas before the mixture enters the catalyst.
Such a technology may also be combined with other technologies such as catalysed Diesel Particulate Filters, CRT™ (Continuously Regenerating Trap) technology or other treatment methods to further reduce undesirable emissions from diesel engines.
To ensure acceptable mixing of the reducing agent and gas stream a mixing region of the gas treatment apparatus can be large and therefore have a long residence time during which mixing and chemical breakdown of the urea-water solution can occur.
It is an object of the present invention to address some of the above issues.
According to the present invention there is provided a gas treatment apparatus for treating a gas stream, the gas treatment apparatus comprising addition means for adding a reducing agent to a gas stream passing through the apparatus, a guide chamber having an end wall, a mixing conduit within which the addition means are located and a catalytic treatment chamber containing a catalytic treatment element, wherein:
Positioning at least a portion of the mixing conduit within the guide chamber and including at least one aperture in the outer wall of the mixing conduit within the guide chamber causes gas to enter the mixing conduit substantially radially. In this case, the term ‘radially’ means that the gas must have a component of its flow velocity towards the mixing axis. It should be understood that the flow velocity could also include radial or axial components with respect to the mixing axis as the gas enters the gas treatment apparatus. The radial entry of the gas into the mixing conduit allows simple guide means to cause at least some of the gas to flow substantially circumferentially. The term ‘circumferentially’ means that the gas has a component of its flow velocity circumferentially around the mixing axis. It should be understood that the gas may still include an axial component to its motion so that the gas moves through the mixing conduit. Simple guide means are easy to construct and may have a low resistance to gas flow which enables a reduction in the pressure drop across the gas treatment apparatus caused as a gas stream flows through the apparatus. The reduction in pressure drop may increase the torque output of an engine to which the gas treatment apparatus is coupled. The circumferential flow, or swirl, of the gas within the mixing conduit increases turbulence within the gas which promotes mixing of the gas and reducing agent as the mixture passes along the mixing conduit.
By locating the addition means within the mixing conduit and having the rotating gas stream expand radially after leaving the mixing conduit the requirements for providing satisfactory mixing and satisfactory distribution of the mixture in downstream chambers are effectively de-coupled. This means that the mixing parameters or the flow distribution parameters can be altered substantially without affecting the other. This can be achieved because the rapidly rotating gas flow is turbulent and this promotes rapid mixing, but the turbulent flow through the mixing conduit has an overall flow pattern which is rotating about a mixing axis. This rotating overall flow pattern causes the gas exiting the mixing conduit to expand rapidly radially and therefore distribute the gas across the downstream chamber.
The guide chamber of the apparatus may be a conduit from an inlet into the apparatus, or from an upstream treatment chamber to the mixing conduit. In the guide region it is possible that no treatment is performed on a gas passing therethrough. The guide chamber may comprise means for removing particulates such as a filter, performing a catalytic treatment on a gas, or attenuating noise that may be generated as a gas flows through the apparatus, or transmitted by the gas flow. Gas may enter the guide chamber a any angle, for example axially or radially.
The catalytic treatment chamber contains a catalytic treatment element that preferably catalyses a reaction between a gas stream passing through the apparatus and the reducing agent added such that NOx in the gas stream is converted to N2 and water. The preferred reducing agent is a urea in water solution and this is preferably added through an injector into the gas stream. The reducing agent is preferably injected so that at least some of the reducing agent reaches the rapidly swirling gas flow near the wall of the mixing conduit as this results in more efficient mixing. Such an arrangement can be achieved in many ways, for example using a central injector injecting radially, or one or more injectors arranged adjacent the wall of the mixing conduit injecting reducing agent substantially axially. It should be noted that the catalytic treatment element may any other catalytic treatment element, for example it may act as a hydrolysis catalyst.
Although it is preferred that the mixing conduit discharges directly into the catalytic treatment chamber it should be understood that the apparatus may include other chambers between the mixing conduit and the catalytic treatment chamber. Gas must still flow through the mixing conduit to pass from the guide chamber to the catalytic treatment chamber, but may additionally have to pass through other chambers. It should be understood that no further mixing is required in these chambers as the gas and reducing agent leaving the mixing conduit are substantially well mixed.
It is preferred that the mixing conduit, catalytic treatment chamber, and guide chamber have substantially circular cross sections as this facilitates manufacture and reduces the likelihood of damage to the apparatus at weak spots that may occur at corners of the apparatus. It should be understood that the cross section of one or all of the sections mentioned above may not be circular, for example one or more may have elliptical cross sections, or other shapes depending upon the desired final use and location of the apparatus.
The outer wall of the mixing conduit preferably includes a plurality of apertures through which gas may flow, and each of which includes a guide means that causes at least some of the gas passing through the aperture to flow substantially circumferentially. All the guide means preferably cause the gas to flow circumferentially in the same direction about the mixing axis. It should be understood that there may be additional apertures through the outer wall of the mixing conduit which do not include guide means and, in this case, a circumferential flow of gas about the mixing axis would be created by the guide means provided.
If a plurality of apertures are provided through the outer wall of the mixing conduit, it is preferred that the apertures are substantially equally circumferentially spaced as this helps to maintain a substantially equal distribution of gas flow within the guide chamber and mixing conduit and this may help to avoid poor distribution of the reducing agent within the gas stream.
The guide means may be any size, shape or configuration but are preferably formed integrally with the outer wall rather than formed separately and then attached. A deflection wall which is angled such that at least some of a gas passing substantially radially through the aperture is deflected by a surface of the deflection wall and caused to flow substantially circumferentially is preferred due to the simplicity in construction and low pressure drop of such a design.
The apertures are preferably substantially rectangular and are preferably arranged such that a long axis of the rectangular aperture is substantially parallel with the mixing axis.
The apertures and guide means can be formed in a plurality of different ways. An embodiment of the aperture and guide means is a “louver” design which comprises a substantially rectangular aperture with a substantially rectangular deflection wall attached to a long edge of the aperture. A mixing conduit having such an embodiment can be readily fabricated using sheet metal, the sheet metal being stamped such that three of the sides that will define the rectangular aperture are cut (in a preferred example, two short sides and one long side) and the resulting flap of metal is bent so that the aperture is opened, the folded edge of the flap defines the aperture together with the three cut sides and the flap is arranged to form a deflection wall. A plurality of such apertures and deflection walls are formed in the sheet and the sheet is then rolled to form the outer wall of the mixing conduit. The seam where two ends of the sheet meet may then be welded to form the mixing conduit.
Other examples of aperture and guide means include a “twisted ribbon” design and an “out of plane” design. In the “twisted ribbon” design two slits are made through the wall of the mixing conduit. The slits are preferably made such that they are substantially parallel with the mixing axis and have a substantially equal length. The two slits define two sides of a ‘ribbon’ of material which is attached to the wall of the mixing conduit at each end. To create the guide means and aperture the ribbon is twisted by forcing a first side of the ‘ribbon’ into the mixing conduit and a second side of the ribbon out of the conduit, simultaneously opening an aperture through the wall and creating guide means.
The “out of plane” design is based on the “louver” design. Two adjacent sets of cuts through the wall of the mixing conduit are formed such that the resulting deflection walls are attached to adjacent long edges of the two apertures. One deflection wall is bent into the mixing conduit and one out of the mixing conduit.
The deflection walls can be curved or otherwise shaped to aid in the deflection of gas in the required direction. It should be understood that the deflection walls and apertures can be formed in a variety of other ways, which may include, for example, separate fabrication of deflection walls and subsequent attachment to the mixing conduit wall. It should be understood that the apertures may have any suitable shape.
The guide chamber and catalytic treatment chambers may be separate chambers, joined by an elongate mixing conduit, or the two chambers may have a common outer wall and a gas be prevented from passing from the guide chamber to the catalytic treatment chamber by a wall through which the mixing conduit passes. Such a wall is preferably substantially perpendicular to a chamber axis around which the common outer wall extends, but may be at any suitable angle.
The mixing conduit has a smaller cross sectional area for gas flow than the catalytic treatment chamber. This allows the swirling gas flow from within the mixing conduit to expand radially upon entry into the catalytic treatment chamber. This slows the rotation of the gas flow and since the cross sectional area for flow is increased, the axial speed of the gas flow is also reduced which allows a longer residence time within the apparatus. The catalytic treatment chamber may include an entry portion located between the mixing conduit and the catalytic treatment element. This allows gas leaving the mixing conduit to disperse across the entire cross section of the catalytic treatment chamber before entering the catalytic treatment element. This radial dispersal reduces the rotational velocity of the gas stream which reduces the pressure loses associated with a rapidly rotating gas flow entering a catalytic treatment element.
The mixing conduit has a smaller cross sectional area for gas flow than the guide chamber. This means that the average gas speed is increased as a gas stream flowing through the apparatus passes from the guide chamber to the mixing conduit. The increased gas speed is both axial and circumferential and this results in greater turbulence within the mixing region and therefore more efficient mixing.
In a particularly preferred embodiment the guide chamber, mixing conduit and catalytic treatment chamber have substantially circular cross sections and are therefore substantially cylindrical in shape, with the mixing conduit have a smaller diameter than the guide chamber and catalytic treatment chamber. It is preferred that the portion of the mixing conduit within the guide chamber is substantially co-axial with guide chamber and that the exit from the mixing conduit is substantially co-axial with the downstream chamber into which the rotating gas flow passes. This facilitates control of the distribution of the mixed gas flow across the cross section of downstream chamber and catalytic treatment element.
The invention will now be further described, by way of example, with reference to the accompanying drawings, in which:
Although the inlet 2 and outlet 3 are shown in
The mixing region 10 includes a mixing conduit 18 which includes an outer wall 22 that surrounds a mixing axis 20 and an endwall 24. The outer wall 22 extends into the guide chamber 8. The addition means 14 extends through the surrounding wall 4 and through the endwall 24 of the mixing conduit 18 so that outlets 26 from the addition means 14 are arranged such that reducing agent 12 is injected into the gas stream substantially radially outwardly from the mixing axis 20. In this case there are ten outlets 26 substantially equally circumferentially distributed around an outlet end 28 of the addition means 14. It should be understood that any suitable number of injector outlets may be used and injection may occur in directions other than radially.
The mixing conduit 18 also includes apertures 30 through the outer wall 22 through which gas from the guide chamber 8 can enter the mixing conduit 18. In this example the endwall 24 includes no apertures, and prevents gas from entering the mixing conduit without passing through the apertures 30 in the outer wall 22. It should be understood that the endwall 24 may also include apertures.
Guide means 32 are associated with each of the apertures 30. The guide means 32 (in this case guide walls 34) cause at least some of the gas passing through the apertures 30 to flow circumferentially around the mixing axis 20. The guide means 32 and apertures are better shown in
The guide chamber 8 and catalytic treatment chamber 16 are separated by a wall 13 through which the mixing conduit 18 passes. The wall 13 is, in this case, both the end wall of the guide chamber 8 and the catalytic treatment chamber 16.
Gas passing from the guide chamber 8 into the mixing conduit 18 through the aperture 30 may contact the deflection wall 34 and be deflected so that it moves substantially circumferentially around the mixing axis 20. The deflected gas from the apertures 30 cause the gas flow within the mixing conduit 18 to swirl or rotate about the mixing axis 20 which increases turbulence and therefore promotes mixing.
The deflection wall 34 of
The deflection wall 34 helps to re-entrain droplets of reducing agent 12 that may form within the mixing conduit.
FIGS. 6 to 9 show different arrangement of gas treatment apparatus. In each case, the different parts of the apparatus that have the same function as in the apparatus 1 shown in
The swirling gas flows through the mixing conduit 318 and then into the transfer conduit 52. The transfer conduit 52 directs the gas into the catalytic treatment chamber 320 where the gas decelerates before entering the catalytic treatment element 320. In this arrangement, the gas flow through the catalytic treatment chamber 316 in a direction opposite to that in which the gas flows through the guide chamber 308. It should be understood that this need not be the case, the transfer conduit 52 could include bends to redirect the gas and/or the catalytic treatment chamber 316 could be orientated in a different direction.
It should be understood that the invention has been described above by way of example only and that modifications in detail may be made without departing from the scope of the invention as described in the claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 04166917.3 | Jul 2004 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB05/02934 | 7/26/2005 | WO | 11/16/2006 |
