EXHAUST SYSTEM AND METHOD FOR REDUCING PARTICULATE AND NO2 EMISSIONS
An exhaust system for reducing particulate and NO2 emissions from diesel engine exhaust gases. The exhaust system includes a diesel particulate filter, an inlet for receiving the exhaust gases, a first conduit for providing a first fluid connection between the inlet and the diesel particulate filter, and a second conduit for providing a second fluid connection between the inlet and the diesel particulate filter. The exhaust system also includes a diesel oxidation catalyst in the second conduit for catalysing hydrocarbon combustion and the formation of NO2 and having high I-IC activity and high NO2 activity, and a fuel injector for injecting fuel upstream of the diesel oxidation catalyst. A valve mechanism for selectively directing exhaust gases from the inlet to the diesel particulate filter through the first conduit or the second conduit is also included. A method for reducing particulate and NO2 emissions using the exhaust system is also provided.
Latest Eminox Limited Patents:
This application is a National Stage application of International Application No. PCT/EP2013/050236, filed on Jan. 8, 2013, which claims priority of Great Britain patent application number 1200230.9, filed on Jan. 9, 2012, both of which are incorporated herein by reference in their entireties.
BACKGROUNDa. Field of the Invention
The present invention relates to an exhaust system for reducing particulate matter and NO2 levels in exhaust gases from diesel engines, and to a method for using the system.
b. Related Art
Historically, NOx and particulate emissions have been of particular concern when developing diesel engines and aftertreatments. More recently, legislation and local air quality improvement schemes are now also targeting NO2 reduction.
SCRT is a technology that can control NOx, particulate, CO and HC (hydrocarbon) emissions. However it is difficult also to control NO2 emissions because significant quantities of NO2 are generated within the system in order to facilitate the passive regeneration of the CRT.
SCR has the potential to control NOx and NO2, but particulate reduction might be difficult to achieve downstream of this because there is limited energy available in the exhaust gas to enable particulate burn (regeneration) over the filter.
It is known to provide a system in which the DPF regenerates at low temperatures. The system is connected downstream of a turbocharger or engine and the exhaust gas is split into a main pipe and a bypass. The bypass has a heating element and means for injecting diesel fuel into the exhaust gas stream. A first DOC provides heating through the exothermic heat from catalytic fuel burning. Downstream of the first DOC, the two pipes join together. Gases from the combined pipes are fed to a second DOC and the DPF. The second DOC also serves to burn hydrocarbons, to increase the temperature to a level at which the filter can regenerate. Sensors are used to measure temperatures at various points, and relative gas flows through the pipes are adjusted, together with pre-heating using the electric heater, to ensure that gases reaching the second DOC are above a threshold temperature required for efficient hydrocarbon conversion. The system meters flow through the first DOC between 0% to 100% in order to achieve the ideal flow rate for maximum performance. The use of supplementary heating and the flow rate adjustment system has significant cost and development disadvantages. Moreover, because exhaust gases flow through the DOC, which generates NO2 it is necessary to use a special oxidation catalyst which is selected for low NO2 formation but which is less effective than others for catalysing HC conversion.
US 2010/0037607 describes a system with parallel exhaust gas flows. The system has a NO oxidation catalyst in one stream and a HC oxidation catalyst in the other. The HC catalyst acts as a heater when fuel is injected upstream. This allows selective heating of the DPF without heating of the NO catalyst. This is said to optimise the catalytic activity of the NO oxidation catalyst.
US 2003/0089104 describes an exhaust system which has an exhaust line with an oxidation catalyser and a catalytically-coated DPF. During a DPF regeneration phase, post-injection of fuel takes place to provide unburned HC to the oxidation catalyst which oxidises the HC. A bypass circuit and valve arrangement permits exhaust gas with unburned fuel to bypass the oxidation catalyst. When regeneration of the filter is triggered, a proportion of the exhaust gases with injected HC is diverted straight to the DPF without passing through the oxidiser. This arrangement is said to increase the combustion rate of particles trapped in the DPF.
Other prior art systems are described in the following documents: JP2010043577, US2006/117742, US2010/199634, JP2010209783, EP2014348, US2008/155968, EP2309103, JP2006233947, US2011/192143, US2009/277159, US2011/146233, US2010/242438, US2009/178393, WO07/136148, US2009/260346, US2005/031514.
An industry challenge is to develop a DPF system that can regenerate at low temperature, with low tailpipe NO2 and, if necessary, in combination with other aftertreatment systems, such as those for reducing NOx.
SUMMARY OF THE INVENTIONThe invention provides a system and method for reducing tailpipe NO2 emissions while providing for DPF regeneration. According to an aspect of the invention, an exhaust system for reducing particulate and NO2 emissions from diesel engine exhaust gases is provided. The exhaust system includes a diesel particulate filter, an inlet for receiving exhaust gases from a diesel engine, a first conduit capable of providing a first fluid connection between the inlet and the diesel particulate filter, and a second conduit capable of providing a second fluid connection between the inlet and the diesel particulate filter. The exhaust system also includes a diesel oxidation catalyst in the second conduit for catalysing hydrocarbon combustion and the formation of NO2 and having high HC activity and high NO2 activity. A valve mechanism for selectively directing exhaust gases from the inlet to the diesel particulate filter through the first conduit or the second conduit is also included, wherein exhaust gases that pass from the inlet to the diesel particulate filter through the first conduit but not the second conduit will not encounter a diesel oxidation catalyst. The exhaust system also includes a fuel injector for injecting fuel upstream of the diesel oxidation catalyst.
A method for reducing particulate and NO2 emissions from diesel engine exhaust gases using the exhaust system according to an aspect of the invention is also provided. According to an aspect of the invention, the method includes, in normal mode, operating the valve mechanism to direct most or all of the exhaust gases entering the inlet, to the diesel particulate filter via the first conduit; and in regeneration mode, operating the valve mechanism to direct most or all of the exhaust gases entering the inlet, to the diesel particulate filter via the second conduit.
DefinitionsWhen used herein, the following definitions define the stated term:
“CO” is carbon monoxide.
A “CRT” is a Continuously Regenerating Trap system for the removal of PM from the exhaust gas stream using a wall-flow filter. The system operates passively and is self cleaning. It achieves this by using a catalyst upstream of the filter to produce exhaust gas conditions that enable the carbon fraction of the PM to be burnt off at typical diesel exhaust gas temperatures.
A “DOC” is a Diesel Oxidation Catalyst, which is used to promote burning of diesel hydrocarbons in an exhaust gas flow.
A “DPF” is a Diesel Particulate Filter, which is used to remove PM from exhaust gases.
“HC” means hydrocarbon.
“High HC activity” means a catalyst which when fresh will cause or promote combustion of at least 70% (preferably at least 80%) of HC in an exhaust gas at 300° C.
“High NO2 activity” means a catalyst which when fresh will cause or promote conversion of at least 60% (preferably at least 70%) of NO to NO2 in an exhaust gas at 300° C.
“NO” is nitric oxide.
“NO2” is nitrogen dioxide, a major contributor to photochemical smog and acid rain. It can be removed from an exhaust gas stream by SCR.
“NOx” is a generic term for all oxides of nitrogen.
“PM” is Particulate Matter, the solid content of exhaust gases, primarily soot (carbon) and ash.
“SCR” is Selective Catalytic Reduction, a process for removing NOx by reducing with a reductant such as ammonia over a catalyst.
“SCRT”® is a combination of SCR and CRT in a single exhaust emissions reduction system. It is capable of removing NOx, PM, HC and CO. SCRT is a registered trade mark of Johnson Matthey PLC.
The invention will now be further illustrated, by way of example only, with reference to the following drawings, in which:
An exhaust system 2 comprises an inlet 6 for receiving exhaust gases from a diesel engine (not shown) and a first conduit 4 connected to a DPF module 10 via an outlet 8. The first conduit 4 is capable of providing a first fluid connection between the inlet 6 and the DPF 10. A second conduit 12 is capable of providing a second fluid connection between the inlet 6 and the DPF 10. The second conduit 12 houses a DOC 14. The DOC catalyst has high HC activity and high NO2 activity; when fresh, the DOC converts over 80% HC and over 70% NO2at 300° C. Suitable DOCs with high HC activity and high NO2 activity will be well known to those skilled in the art of exhaust emission control.
A fuel injector 16 is arranged to inject fuel into the exhaust gas stream upstream of the DOC. A valve mechanism 18 is adjustable for selectively directing exhaust gases from the inlet 6 to the DPF 10 through the first conduit 4 or the second conduit 12. The fuel injector 16 is between the valve mechanism 18 and the DOC 14. In this example, the fuel injector 16 is upstream of the valve mechanism 18 but could alternatively be in a separate stream within the second conduit 12. Gases which pass through the second conduit 12 encounter the DOC 14 before reaching the DPF 10. Gases which pass through the first conduit 4 reach the DPF 10 without encountering a diesel oxidation catalyst. Filtered exhaust gases exit the DPF 10 via a tailpipe 28.
In this embodiment, the exhaust system 2 is illustrated in combination with an upstream SCR unit 20. The SCR 20 has an injector 22 for introducing a reductant such as ammonia, and an optional inlet module oxidation catalyst 24 and an optional outlet module 26 with slip catalyst. It will be appreciated that the invention is not limited to use with an SCR. It may be used as a standalone active DPF system or may be combined with other aftertreatment or exhaust technologies.
Referring to
Referring now to
When the DOC achieves light-off, the system switches to regeneration mode, and fuel is injected via the fuel injector 16 to oxidise over the DOC (
As shown in
Tailpipe NO2 output is relatively high during the light-off mode, as illustrated in
In the embodiment illustrated in
The relatively low gas flow through the second conduit 12 can be eliminated completely by use of a second valve 19 in the second conduit, as illustrated in the embodiment shown in
Referring now to
The first conduit 4 and the second conduit 12 may be arranged in any convenient manner. In the embodiment illustrated in
The embodiment illustrated in
As illustrated, the DOC 14 is located in the inner conduit which functions as the second conduit 12, while the annular conduit functions as the first conduit 4. It is appreciated that the DOC could alternatively be provided in the annular conduit.
It will be appreciated that various sensors and control systems may be employed to optimise performance of the exhaust system. For example, the temperature of exhaust gases may be monitored at or downstream of the DOC to determine when light-off occurs, and to trigger fuel injection at or after light-off. Accurate control of the fuel injection rate is preferred in order to limit unburned fuel passing through the DOC. The pressure difference over the DPF may be monitored, and regeneration mode triggered when a threshold Δp value is reached, corresponding to a threshold level of channel blockage within the DPF.
The system may be open loop, closed loop, feedforward, feedback or use other suitable monitoring and control methods.
The system is simple in terms of hardware and control, providing cost and operating advantages. For example, whereas the prior art system requires control of flow through the first catalyst anywhere between 0% and 100% in order to achieve desired flow rates through the DOC, this is not necessary in the present system. The valve mechanism can simply be switched between zero and maximum flow depending on the operating mode required.
The system uses fewer components than prior art systems, and operates differently to achieve both DPF regeneration and low NO2 levels at the tailpipe. The described prior art system cannot achieve low tailpipe NO2 because there will always be exhaust gas flowing through a catalyst that will generate NO2.
The prior art system requires a special oxidation catalyst selected for good HC conversion and low NO2 formation. An advantage of the present system is that it can use a very active catalyst that typically has both high HC and NO2 conversion, lighting off at lower temperatures and having higher efficiency. The layout and operation of the system is such that low tailpipe NO2 is achieved relative to NO2 going into the system.
It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately, or in any suitable combination.
What has been described above are preferred aspects of the present invention. It is of course not possible to describe every conceivable combination of components or methodologies for purposes of describing the present invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to embrace all such alterations, combinations, modifications, and variations that fall within the spirit and scope of the appended claims.
Claims
1. An exhaust system for reducing particulate and NO2 emissions from diesel engine exhaust gases, the system comprising:
- a diesel particulate filter;
- an inlet for receiving exhaust gases from a diesel engine;
- a first conduit capable of providing a first fluid connection between the inlet and the diesel particulate filter;
- a second conduit capable of providing a second fluid connection between the inlet and the diesel particulate filter;
- a diesel oxidation catalyst in the second conduit, the diesel oxidation catalyst catalysing hydrocarbon combustion and the formation of NO2 and having high HC activity and high NO2activity;
- a valve mechanism for selectively directing exhaust gases from the inlet to the diesel particulate filter through the first conduit or the second conduit; wherein exhaust gases that pass from the inlet to the diesel particulate filter through the first conduit but not the second conduit will not encounter a diesel oxidation catalyst; and
- a fuel injector for injecting fuel upstream of the diesel oxidation catalyst.
2. The exhaust system according to claim 1, wherein the fuel injector is upstream of the valve mechanism.
3. The exhaust system according to claim 1, wherein the fuel injector is arranged and adapted to direct most or substantially all of the injected fuel directly into the second conduit.
4. The exhaust system according to claim 1, further comprising an exhaust gas aftertreatment unit upstream of the inlet.
5. The exhaust system according to claim 4, wherein the aftertreatment unit is an SCR unit.
6. The exhaust system according to claim 1, wherein the first conduit and the second conduit are substantially concentric.
7. The exhaust system according to claim 6, wherein the second conduit is disposed centrally within the first conduit.
8. The exhaust system according to claim 6, wherein the first conduit is disposed centrally within the second conduit.
9. A method for reducing particulate and NO2 emissions from diesel engine exhaust gases using the system of claim 1, the method comprising:
- in normal mode, operating the valve mechanism to direct most or all of the exhaust gases entering the inlet, to the diesel particulate filter via the first conduit;
- in regeneration mode, operating the valve mechanism to direct most or all of the exhaust gases entering the inlet, to the diesel particulate filter via the second conduit.
10. The method according to claim 9, further comprising, in regeneration mode, injecting fuel via the fuel injector to oxidise on the diesel oxidation catalyst and increase the temperature of exhaust gases reaching the diesel particulate filter to burn at least some particulate matter accumulated therein.
11. The method according to claim 10, further comprising monitoring the temperature of exhaust gases at or downstream of the diesel oxidation catalyst to determine when light-off occurs, and commencing the fuel injection at or after light off.
12. The method according to claim 9, further comprising providing a catalyst to the diesel engine fuel, to catalyse the combustion of particulate matter on the diesel particulate filter.
13. The method according to claim 9, further comprising monitoring a pressure difference over the diesel particulate filter and triggering regeneration mode when a preset pressure difference is reached.
14. (canceled)
15. (canceled)
Type: Application
Filed: Jan 8, 2013
Publication Date: Jan 29, 2015
Applicant: Eminox Limited (Gainsborough)
Inventor: Phillip Bush (Gainsborough)
Application Number: 14/371,074
International Classification: F01N 3/20 (20060101);