Patent Application
20120233985
The present invention relates to an apparatus comprising a lean burn internal combustion engine, particularly diesel engines and particularly for vehicular applications, and an exhaust system for treating a flowing exhaust gas from the engine. In particular it relates to an apparatus comprising an exhaust system including a NOx adsorber catalyst (NAC) and a catalysed soot filter (CSF).
NAC are known e.g. from U.S. Pat. No. 5,473,887 (the entire contents of which is incorporated herein by reference) and are designed to adsorb nitrogen oxides (NOx) from lean exhaust gas (lambda>1) and to desorb the NOx when the oxygen concentration in the exhaust gas is decreased. Desorbed NOx may be reduced to N2 with a suitable reductant, e.g. diesel fuel, promoted by a catalyst component, such as rhodium, of the NAC itself or located downstream of the NAC. In practice, the oxygen concentration is adjusted to a desired redox composition intermittently in response to a calculated remaining NOx absorption capacity of the NAC, e.g. richer than normal engine running operation (but still lean of stoichiometric or lambda=1 composition), stoichiometric or rich of stoichiometric (lambda<1). The oxygen concentration can be adjusted by a number of means, e.g. throttling, injection of additional hydrocarbon fuel into an engine cylinder such as during the exhaust stroke or injecting hydrocarbon fuel directly into exhaust gas downstream of an engine manifold. More sophisticated common rail fuel injector systems in diesel engines can be used to meter very precise quantities of fuel to adjust exhaust gas composition.
A typical NAC formulation includes a catalytic oxidation component, such as platinum, a NOx-storage component, such as barium, and a reduction catalyst, e.g. rhodium. One mechanism commonly given for NOx-storage from a lean exhaust gas for this formulation is:
NO+½O2→NO2 (1);
and
wherein in reaction (1), the nitric oxide reacts with oxygen on active oxidation sites on the platinum to form NO2. Reaction (2) involves adsorption of the NO2 by the storage material in the form of an inorganic nitrate.
At lower oxygen concentrations and/or at elevated temperatures, the nitrate species become thermodynamically unstable and decompose, producing NO or NO2 according to reaction (3) below. In the presence of a suitable reductant, these nitrogen oxides are subsequently reduced by carbon monoxide, hydrogen and hydrocarbons to N2, which can take place over the reduction catalyst (see reaction (4)).
Ba(NO3)2→BaO+2NO+ 3/2O2 or Ba(NO3)2→BaO+2NO2+½O2 (3);
and
NO+CO→½N2+CO2 (and other reactions) (4).
In the reactions of (1)-(4) above, the reactive barium species is given as the oxide.
However, it is understood that in the presence of air most of the barium is in the form of the carbonate or possibly the hydroxide. The skilled person can adapt the above reaction schemes accordingly for species of barium other than the oxide.
A problem in the use of NAC, e.g. in diesel applications, is that diesel engine fuel also contains sulfur, and this is converted to sulfur dioxide (SO2) during fuel combustion. SO2 is oxidised to SO3 by the oxidation catalyst component of the NAC and the SO3 is adsorbed on the NOx adsorber by a similar mechanism to that of NO2. Of course, there are a finite number of active sites on the NOx storage component for adsorbing the NOx and so the presence of sulfate on the NOx storage component reduces the capacity of the NOx storage component as a whole to adsorb NOx. Therefore, in order to retain sufficient NOx storage capability, sulfur must be periodically removed from the NAC. However, sulfates of NOx storage components such as barium are more stable than nitrates in lean exhaust gas and generally higher temperatures and/or richer conditions for longer periods are required than for desorbing NOx.
A significant problem with desulfating a NAC using richer than normal exhaust gas compositions is that the sulfate is removed as hydrogen sulfide. This compound has a characteristic and unpleasant rotten egg odor and accordingly it is desirable to prevent its emission to atmosphere.
This problem has been reduced to a certain extent in recent years because Ultra Low Sulfur Diesel (fuel of maximum sulfur content of 15 ppm (wt.)) is now becoming available in the US. In Europe, diesel fuel with a maximum sulfur limit of 50 ppm has been available since the beginning of 2005 and also “sulfur-free” 10 ppm sulfur diesel fuel has been available in Sweden since 1991 and more recently in Denmark and UK. As a result, NAC desulfation procedures are required less often.
CSFs are known from, e.g. U.S. Pat. No. 5,100,632 (the entire contents of which are incorporated herein by reference). Generally, CSFs are used with mixed passive-active regeneration, wherein the filter regenerates passively under some operating conditions, e.g. at high engine loads. Under other operating conditions, e.g. light load operation, the exhaust gas temperature is actively increased—typically to about 550-600° C.—to trigger periodic regeneration whenever a given soot load is reached in the filter (detected e.g. using a backpressure sensor) or after the vehicle has travelled a pre-determined distance. Increased temperatures are realized through such means as engine management or injection of fuel into the exhaust gas, followed by HC oxidation over a warm-up catalyst (which may be on the CSF itself).
A diesel exhaust system comprising a NAC and a downstream CSF is known e.g. from SAE 2001-01-2065 entitled “Cummins Light Truck Diesel Engine Progress Report” (the entire contents of which are incorporated herein by reference).
WO 01/12320 (the entire contents of which are incorporated herein by reference) discloses a wall-flow filter for an exhaust system of an internal combustion engine, which wall-flow filter comprising an oxidation catalyst on a substantially gas impermeable zone at an upstream filter end. Optionally, a NOR absorber can be located in a substantially gas impermeable zone at a downstream filter end.
There remains a need for an exhaust system for a lean burn internal combustion engine, particularly for a diesel engine, that emits carbonaceous soot particles and NOx, which exhaust system integrates NAC functionality with means for treating such carbonaceous soot particles and enables NAC desulfation optionally under rich conditions whilst reducing or preventing H2S emissions. We have investigated exhaust systems for lean burn internal combustion engines comprising both a NAC and a CSF and have now devised an arrangement that meets such needs.
According to one aspect, the invention provides an apparatus comprising:
By “enriched” herein, we mean “relative to normal running operation”. The enriched exhaust gas composition may therefore be net lean of stoichiometry (i.e. lambda>1), net stoichiometric (lambda=1) or net rich (lambda<1), wherein lambda=actual air-to-fuel ratio/stoichiometric air-to-fuel ratio. Preferably, however, the enriched state is net rich, i.e. lambda<1.
The filter substrate can be any suitable for the purpose of trapping and retaining diesel particulate matter, including wall-flow filters, sintered metal filters and partial filters such as those disclosed in EP 1276549 and EP 1057519 (the entire contents of both documents being incorporated herein by reference). The first substrate monolith can be a flow-through monolith substrate or a filter substrate, such as a partial filter substrate.
The CSF catalyst can be any suitable for the purpose including platinum and/or palladium supported on a suitable support material including alumina and ceria or a mixed oxide or composite oxide of ceria and zirconia. However, in a second embodiment according to the invention (see below), the first substrate monolith is the filter substrate, and the NAC composition performs the function of the CSF catalyst.
Typically, the NAC will comprise a catalytic oxidation component, such as platinum, palladium or both platinum and palladium, a NOx-storage component, such as an alkaline earth metal oxide, an alkali metal oxide or an oxide of a lanthanide metal, e.g. ceria, lanthana, yttria or mixtures of any two or more of such oxides, and a reduction catalyst, e.g. rhodium, although the rhodium may be located on outlet channels of the filter or downstream of the filter entirely e.g. located on a flow-through substrate downstream. Whilst the NAC is typically applied as a washcoat onto a honeycomb monolith substrate, it is also possible to provide the first substrate monolith as an extruded-type honeycomb. Where the NAC is present as an extrudate, it is possible to coat other catalyst coatings onto the extrudate to combine catalyst functionality in a single “brick”.
Desirably, the enriching means comprises a microprocessor (ECU), which can comprise part of an engine control unit.
In a first embodiment according to the invention, the filter substrate is located upstream of the first substrate monolith. In a particular embodiment, the first substrate monolith comprises the NAC and the compound effective to remove and/or convert at least some hydrogen sulfide. For example, the arrangement can be such that the first substrate monolith has a length extending between an inlet end and an outlet end thereof, wherein the
NAC is located in a first zone of substantially uniform length defined at an upstream end by the inlet end of the first substrate monolith and at an downstream end by a point more than one half way along the first substrate monolith measured from the inlet end and the compound effective to remove and/or convert at least some hydrogen sulfide is located in a second zone of substantially uniform length defined at an upstream end by a point more than one half way along the first substrate monolith length measured from the inlet end and at a downstream end by the outlet end of the first substrate monolith.
In another particular embodiment of the first embodiment according to the invention, a second substrate monolith is located downstream of the first substrate monolith, which second substrate monolith comprises the compound effective to remove and/or convert at least some hydrogen sulfide.
According to the aforementioned second embodiment according to the invention, the first substrate monolith is the filter substrate. This combination of exhaust system features promotes space saving within an exhaust system, which is useful in vehicular applications, and heat retention within the filter substrate, which can promote catalysed chemical reactions therewithin. In a particular embodiment, the filter substrate comprises both the NAC and the compound effective to remove and/or convert at least some hydrogen sulfide. So, in one such particular embodiment, the arrangement is such that the filter substrate has a length extending from an inlet end to an outlet end thereof, wherein the NAC is located in a zone of substantially uniform length defined at an upstream end by the inlet end of the filter substrate and at an downstream end by a point more than one half way along the filter substrate measured from the inlet end and the compound effective to remove and/or convert at least some hydrogen sulfide is located in a fourth zone of substantially uniform length defined at an upstream end by a point more than one half way along the filter substrate length measured from the inlet end and defined at a downstream end by the outlet end of the filter substrate itself.
In another particular embodiment of the second embodiment according to the invention, a second substrate monolith may be located downstream of the filter substrate, which second substrate monolith comprises the compound effective to remove and/or convert at least some hydrogen sulfide.
By a third embodiment according to the invention, the filter substrate has an inlet end and an outlet end and the inlet end of the filter substrate is located downstream of the first substrate monolith in a flow direction of the exhaust gas. The compound effective to remove and/or convert at least some hydrogen sulfide can be located on a third substrate monolith disposed between the first substrate monolith and the filter substrate and/or the first substrate monolith can comprise both the NAC and the compound effective to remove and/or convert at least some hydrogen sulfide. In an embodiment according to the latter arrangement, the first substrate monolith has a length extending between an inlet end and an outlet end thereof, wherein the NAC is located in a first zone of substantially uniform length defined at an upstream end by the inlet end of the first substrate monolith and at an downstream end by a point more than one half way along the first substrate monolith measured length from the inlet end and the compound effective to remove and/or convert at least some hydrogen sulfide is located in a second zone of substantially uniform length defined at an upstream end by the NAC and at a downstream end by the outlet end of the first substrate monolith.
In a further embodiment according to the third embodiment of the invention, the filter substrate comprises the compound effective to remove and/or convert at least some hydrogen sulfide. In embodiments, the compound can be coated throughout the entire length of the filter substrate, in a zone defined at an upstream end by an inlet end of the filter substrate, in a zone defined at a downstream end by an outlet end of the filter substrate or in a zone at any position between the inlet end and the outlet end of the filter substrate defined neither by the inlet end itself nor the outlet end itself. In a particular embodiment, the filter substrate has a length extending from an inlet end to an outlet end thereof, and the compound effective to remove and/or convert at least some hydrogen sulfide is located in a fifth zone of substantially uniform length defined at an upstream end by the inlet end itself and at a downstream end by a point up to one half way along the filter substrate length measured from the inlet end.
According to another particular embodiment, the filter substrate has a length extending from an inlet end to an outlet end thereof, wherein the compound effective to remove and/or convert at least some hydrogen sulfide is located in a zone of substantially uniform length defined at an upstream end by a point more than one half way along the filter substrate length measured from the inlet end and defined at a downstream end by the outlet end of the filter substrate itself.
In addition to the or each location for the compound effective to remove and/or convert at least some hydrogen sulfide disclosed hereinabove in connection with the second or third embodiments of the invention, a fourth substrate monolith comprising the compound effective to remove and/or convert at least some hydrogen sulfide may be disposed downstream of the filter substrate.
In summary, the compound effective to remove and/or convert at least some hydrogen sulfide can be located at one of the following positions in the exhaust system:
(1) between the NAC and the filter substrate;
The exhaust system can comprise an oxidation catalyst disposed between the engine and the NAC. The oxidation catalyst can oxidize unburned hydrocarbons and soluble organic fraction hydrocarbons adsorbed on diesel soot particles and carbon monoxide in the exhaust gas. The composition of such oxidation catalysts is well known in the art and includes, for example, U.S. Pat. No. 5,491,120 (the entire contents of which are incorporated herein by reference). The oxidation catalyst can also be formulated to oxidize nitrogen oxide in addition to oxidizing HCs and CO. Generally, DOCs are coated on a flow-through monolith substrate, but they may also be coated on a diesel particulate filter to form a catalysed soot filter (CSF). In the second embodiment according to the invention, wherein filter substrate comprises the NAC, the oxidation catalyst can oxidise NO to NO2 and particulate matter trapped on the filter substrate can be oxidised in NO2, as described in WO 01/12320.
In one set of embodiments (which may be used in combination with any of the first, second and third embodiments of the invention), the oxidation catalyst can be located on a separate substrate monolith disposed between the engine and the first substrate monolith.
Alternatively, wherein the first substrate monolith has a length extending between an inlet end and an outlet end thereof, the oxidation catalyst can be located in a sixth zone of substantially uniform length defined at an upstream end by the inlet end of the first substrate monolith and at a downstream end by the NAC, i.e. a zone comprising the NAC may be defined at an upstream end by the downstream end of the zone comprising the oxidation catalyst. This arrangement may also be used with any of the first, second and third embodiments of the invention.
So, in one particular embodiment, the compound effective to remove and/or convert at least some hydrogen sulfide is located in a seventh zone of substantially uniform length defined at an upstream end by a point more than one half way along the first substrate monolith length or the filter substrate length measured from the inlet end and defined at a downstream end by the outlet end of the first substrate monolith or the filter substrate itself.
The means for enriching the exhaust gas can comprise an engine management unit configured intermittently to cause the engine to emit an enriched exhaust gas. Alternatively, or additionally, it can comprise an injector for injecting a reductant into a flowing exhaust gas carried by the exhaust system downstream of the engine. Alternatively or additionally, the exhaust gas can be enriched with hydrogen and carbon monoxide formed e.g. by feeding engine fuel and a portion of exhaust gas into a reformer catalyst and then feeding the resulting mixture back into the exhaust gas upstream of the NAC. Catalysts suitable for this reaction are known as reforming catalysts, illustrative examples of which include catalysts based on platinum group metals (PGMs) and nickel (Ni). For further details, reference can be made to D. L. Trimm and Z. I. Onsan in Catalysis Reviews—Science and Engineering, vol. 43 (2001) pp 31-84 (the entire contents of which is incorporated herein by reference).
Further suitable reforming catalysts with utility according to this embodiment of the invention include up to 2 wt% , e.g. 1 wt %, rhodium dispersed on a refractory oxide support material which comprises cations of cerium and zirconium—see for example our WO 99/48805, the entire contents of which is included herein by reference. In addition to the supported up to 2 wt % Rh, such as up to 1 wt % Rh, other catalysts include low loadings of Pt (up to 0.5 wt%, e.g. 0.1 wt %) and Rh-Pt comprising up to 2 wt % Rh (e.g. up to 1 wt % Rh) and up to 0.5 wt % Pt (e.g. up to 0.1 wt %). Supports for the Rh, Pt, Rh—Pt and Ni include alumina, titania, ceria, zirconia, silica, silica-alumina and mixtures and mixed oxides containing any two or more thereof.
The hydrogen sulfide removing and/or adsorbing compound can be any material capable of storing and/or converting hydrogen sulfide under rich conditions. In one embodiment, the hydrogen sulfide removing and/or converting compound is selected from the group consisting of NiO, CaO, Fe2O3 and BaO.
The adsorption of H2S in a hydrogen sulfide removing and/or converting compound is illustrated in reaction (5) and the desorption in lean conditions is illustrated in reaction (6):
2H2S+NiO+½O2→NiS2+2H2O (5)
NiS2+2½O2→NiO+2SO2 (6)
Whilst the hydrogen sulfide removing and/or converting compound may coexist with platinum group metal oxidation catalyst components, e.g. platinum and/or palladium, of the NAC and/or the CSF, the preferred compound, NiO, can poison the hydrocarbon and carbon monoxide activity of the PGM catalyst. Hence, it is desirable to segregate such hydrogen sulfide removing and/or converting compound by locating the materials in separate discreet zones or substrates.
The enriching means can comprise means for controlling a temperature of the NAC during enrichment to remove sulfate from the NAC.
In one embodiment, wherein the enriching means provides a net rich exhaust gas composition (lambda<1), the temperature controlling means comprises means for intermittently adjusting the exhaust gas composition to the lean side (lambda>1) during exhaust gas enrichment to remove sulfate adsorbed on the NAC. Such means for intermittently adjusting the exhaust gas composition to the lean side (lambda>1) can control: an engine air-to-fuel ratio; an injector for injecting air into exhaust gas downstream of the engine; and/or supply of diesel fuel and exhaust gas to a reforming catalyst. This has the additional benefit that sulfur stored on the hydrogen sulfide removing and/or converting compound is released as sulfur dioxide, which can be emitted to atmosphere as such, particularly where the hydrogen sulfide removing and/or converting compound is located on the downstream end of the CSF or on a separate substrate monolith disposed downstream of the CSF. Sulfur dioxide released upstream of the CSF can be oxidised to sulfur trioxide on the CSF catalyst, which sulfur trioxide when combined with water (steam) in the exhaust gas can form fine particles of sulfuric acid that can contribute to total particulates detected in a test cycle for meeting a relevant emission standard.
In practice, contacting the NAC with a rich exhaust gas composition followed by a lean exhaust gas can be accomplished in a cyclical fashion or a non-cyclical manner, e.g. controlled by negative feedback so that the NAC is maintained within a desired temperature window to effect optimum desulfation whilst limiting any hydrothermal deactivation of the NAC itself.
The lean burn internal combustion engine can be a diesel engine such as a light-duty diesel engine or a heavy duty diesel engine, as defined by relevant legislation. However, it may also be a gasoline lean burn engine, as desired.
According to a second aspect, the invention provides a vehicle comprising an apparatus according to the invention.
According to a third aspect, the invention provides a method of desulfating a NOx adsorber catalyst (NAC) in an exhaust system of a lean burn internal combustion engine comprising a first substrate monolith comprising a NOx adsorber catalyst (NAC); and a catalysed soot filter (CSF) comprising a filter substrate, which method comprising:
In one embodiment, the method comprises the step of releasing sulfur dioxide from the compound effective to remove and/or convert at least some hydrogen sulfide from the rich exhaust gas by intermittently contacting said compound with a lean exhaust gas during step (i).
In order that the invention may be more fully understood, embodiments whereof will now be described with reference to the accompanying drawings, in which:
In ordinary operation, engine control unit 24 comprising a microprocessor controls the engine's fuel injection regime intermittently (e.g. 2-3 minutes) to enrich the exhaust gas in the exhaust system momentarily (e.g. up to a few seconds) during normal lean running operation when it is determined that the capacity of NAC 18 to adsorb NOx is reduced and requires regeneration.
Over the course of e.g. a few thousand miles of vehicle service, sulfur in the fuel and lubrication oil can become adsorbed on the NAC, reducing the finite capacity of the NAC to adsorb NOx. When it is determined that the NAC should be desulfated, the engine control unit instigates a desulfation regime, e.g. of up to 10 minutes duration, wherein the engine fuel injection is controlled to make the exhaust gas rich (lambda<1). Combustion of unburned hydrocarbons and CO on DOC 16 and the oxidation catalyst component of the NAC 18 itself raises the temperature of the NAC to temperatures at which sulfur is removed from the NAC as hydrogen sulfide in rich exhaust gas. At least some hydrogen sulfide is removed from the exhaust gas contacting the nickel-containing washcoat component in zone 22 on the downstream end of filter 20.
In order to regenerate the hydrogen sulfide adsorber at zone 22 by releasing stored sulfur as sulfur dioxide and to prevent excessive hydrothermal damage to the NAC, the exhaust gas composition is controlled during the desulfation regime to return briefly to the lean side (lambda>1). In the present embodiment, lean exhaust gas composition is obtained by controlling the engine air-to-fuel ratio by means of engine control unit 24. However, lean exhaust gas can also be obtained by providing an injector for injecting air into the exhaust gas downstream of the engine controlled by engine control unit 24.
Reference numeral 44 refers to a fuel injector arranged to inject a supply of hydrocarbon reductant from reservoir 46 into exhaust gas flowing in the exhaust system 30 between engine 12 and substrate monolith 34. Flow of reductant to an injector nozzle is controlled by actuator 48, which is in turn controlled by engine control unit 24. Air injector 28 is provided to inject air into exhaust gas flowing in the exhaust system 30 between substrate monolith 34 and the hydrogen sulfide adsorber-coated monolith substrate 40. Air injection is actuated by air pump 52, which is controlled by the engine control unit 24.
The use of the embodiment shown in
The following Example is provided by way of illustration only.
0.01 g powdered 33% NiO/Al2O3 catalyst prepared by calcining particulate Al2O3 impregnated with an appropriate concentration of aqueous nickel nitrate in air at 650° C. for 2 hours was combined with the same weight of powdered cordierite. The resulting powder mixture was exposed in a Synthetic Catalyst Activity Test (SCAT) apparatus repeatedly to a rich gas mixture of 140 ppm H2S, 1% H2, balance He/N2 for 41.7 minutes followed by a lean gas mixture of 0.8% O2, balance He for 8.3 minutes.
| Number | Date | Country | Kind |
|---|---|---|---|
| 60876284 | Dec 2006 | US | national |
| 60876970 | Dec 2006 | US | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB07/50781 | 12/21/2007 | WO | 00 | 5/29/2012 |
