EP3289197A1

EP3289197A1 - Exhaust gas treatment system

Info

Publication number: EP3289197A1
Application number: EP16786851.2A
Authority: EP
Inventors: David Raymand; Daniel Hjortborg
Original assignee: Scania CV AB
Current assignee: Scania CV AB
Priority date: 2015-04-29
Filing date: 2016-04-21
Publication date: 2018-03-07
Also published as: KR20170136610A; DE202016008865U1; SE1550513A1; WO2016175697A1; SE541476C2; EP3289197A4

Abstract

An exhaust gas treatment system (4) comprising a selective catalytic reduction catalyst (7) and a method for providing a desired ratio between NO₂ and NO_x in the exhaust gas intended to pass a selective catalytic reduction catalyst (7) are disclosed. The flow of exhaust gas is in the system is divided into two partial flows wherein NO is oxidised to a low degree in the first partial flow and to a high degree in the second partial flow. The exhaust gas treatment system (4) and the method enables a high conversion rate in the SCR for a large range of operating parameters in terms of temperature and mass flow.

Description

EXHAUST GAS TREATMENT SYSTEM

TECHNICAL FIELD The present disclosure relates in general to an exhaust gas treatment system comprising an oxidation catalyst and a selective catalytic reduction catalyst. The exhaust gas treatment system may for example be an exhaust gas treatment for a vehicle, especially a heavy vehicle such as a bus or a truck. The present disclosure also relates to a method for providing a desired ratio between N0₂ and NO in an exhaust gas flow intended to pass through a selective catalytic reduction catalyst for reduction of NO_x.

BACKGROUND

A combustion engine combusts a fuel and air mixture in order to generate a driving moment for powering for example a heavy vehicle, such as a bus or truck. The combustion process generates exhaust gases, which exit the engine and are transferred to an exhaust gas treatment system. The exhaust gases from the combustion engine comprise nitrogen containing gases (NO_x), carbon dioxide (C0₂), carbon monoxide (CO), hydrocarbon (HC), and particles. NO_x is a commonly used generic term to describe the nitrogen containing gases, which primarily comprises nitrogen monoxide (NO) and nitrogen dioxide (N0₂).

The exhaust gas treatment system often comprises a diesel oxygen catalyst (DOC) adapted to primarily oxidise hydrocarbons, but also carbon monoxide and nitrogen monoxide. Furthermore, the exhaust gas treatment system often comprises a selective catalytic reduction (SCR) catalyst in which a reduction agent and NO_x are converted into nitrogen and water, thereby reducing the amount of NO_x released to the surrounding atmosphere. The reduction agent used is usually a urea-containing aqueous solution, such as AdBlue, and is introduced into the system upstream of the SCR. The exhaust gas treatment system may typically further include one or more particulate filters, for example, a diesel particulate filter (DPF) and/or a catalysed soot filter (CSF), in order to trap and oxidise for example soot particles. Additional types of catalysts may also be provided in the exhaust gas treatment system, for example an ammonium slip catalyst (ASC).

The catalysts of the exhaust gas treatment system cooperates in order to clean the exhaust gases from emissions in terms of hydrocarbons (HC), particulate matter (PM), carbon monoxide (CO), NO_x and ammonium (NH₃). A major challenge in order to optimise an exhaust gas treatment system is to make the DOC and DPF cooperate in an optimal manner with the SCR catalyst. The performance of the SCR catalyst is highly dependent on the composition of the exhaust gas entering the SCR. It is well know that the highest conversion rate of NO_x in the SCR catalyst is achieved in case the exhaust gas comprises a ratio of N0₂ to NO of 50:50. The exhaust gases leaving the engine comprises mainly NO (usually about 90 %), but a part of the NO is oxidised to N0₂ when the exhaust gas passes the DOC and the DPF. Thus, many systems seek to optimise the oxidation of NO in the DOC and/or DPF to achieve the desired 50:50 ratio of N0₂ and NO in the exhaust gas when it enters the SCR. This is a difficult task since the catalytic substance normally used in DOC and DPF, i.e. platinum, has the activity for oxidising CO, HC and NO. However, for optimal performance the DOC and the DPF should oxidise all of the CO and the HC, but only about 50 % of the NO.

Furthermore, while the above-described ratio of N0₂ and NO is the ratio, which gives the highest conversion rate in the SCR, it may not always be suitable to control the exhaust gas treatment system towards such a composition of the exhaust gas. For example, in case the SCR catalyst is a V₂0₅-based catalyst, even a small excess of N0₂ (within normal fluctuation ranges during operation) may have a detrimental effect on the performance of the SCR and thereby result in considerably lower reduction degree of NO_x.

The oxidation capacity of NO in an oxidation catalyst is strongly dependent on both the mass flow of the exhaust gas and the temperature, and to a lower degree of the NO_x concentration. For a typical catalyst, the capability of oxidising NO increases with increasing temperature for temperatures below approximately 300°C. Above said temperature, the capability of oxidising NO decreases because the conversion is limited of the thermodynamic equilibrium between NO and N0₂. Exactly where the breaking point between increasing and decreasing NO/N0₂ ratio is depends on the DOC and DPF activities. The ability to oxidise NO also depends on the exhaust gas mass flow as disclosed above. The capability of oxidising NO decreases with increasing mass flow of the exhaust gas. The temperature of the exhaust gases and the mass flow varies greatly between different operation points, whereby it is difficult to find an activity which is suitable both in case of high mass flow and low temperature as well as in case of low mass flow and medium temperature.

The problem of achieving the optimal composition of the exhaust gas for the SCR is especially pronounced at low temperatures of the exhaust gas. An oxidation catalyst, which is capable of oxidising enough NO to N0₂ at low temperatures, such as about 200 to 250°C, would likely produce too much N0₂ at a medium exhaust gas temperature, such as about 350°C. Thus, when seeking to optimise the capability of an oxidation catalyst to oxidise NO it is therefore necessary to determine when the optimal composition of the exhaust gas is mostly needed: i) at high mass flow where the load on the SCR is the highest, thereby compromising the performance at low mass flow at which N0₂/NO will be too high, or ii) at low mass flow where the engine has most of its operation, which limits the performance for the SCR catalyst at high mass flow and transients.

Moreover, the NO_x-sensor used to determine the composition of the exhaust gas in order to determine the correct amount of reducing agent to be introduced for the reduction in the SCR catalyst is generally calibrated against "normal" N0₂/NO_x ratios. The deviation from a 50 % N0₂/ NO_x ratio is amplified by the SCR-catalysts' affinity for removing NO_x in a 1:1 NO:N0₂ ratio, e.g. if 90% of the NO_x is removed in the SCR from a 55% N0₂/NO_x mixture, the remaining fraction of N0₂ after the SCR is likely to have increased towards 100% N0₂/NO_x. For a 45% N0₂/NO_x ratio before the SCR the effect is opposite, where the ratio is likely to have decreased towards 0% N0₂/NO_x after the SCR. Therefore, a small change in the amount of N0₂ in the exhaust gas to the SCR, such as from <50 % to >50%, can result in a large impact on the reading of registered amount of NO_x in the NO_x-sensor, thereby affecting the amount of urea solution introduced into the system and consequently on the degree of reduction in the SCR. From the perspective of the sensor, it is therefore advantageous to limit the amount of N0₂/NO_x to < 50%. US 6,846,464 B2 discloses a method of reducing NO_x in exhaust gases of an internal combustion engine. The exhaust gases from the engine are converted to roughly 50:50 mixture of NO and N0₂ while simultaneously oxidising hydrocarbons which may interfere with the reduction of NO_x by urea. The method disclosed comprises selective oxidation of the exhaust gases of an engine in which the NO and hydrocarbons in a first portion of the exhaust gases are oxidised separately from the remaining second portion of the exhaust gases. In the second portion of the exhaust gases, only hydrocarbons are oxidised while NO is left essentially unreacted. This is achieved by the first and the second exhaust stream each separately allowed to pass through a different catalytic chamber. The first and second portions of the exhaust cases are then recombined in order to achieve the desired ratio of NO to N0₂ and passed through an SCR catalyst. The solution disclosed has however certain limitations. Firstly, it is optimised against a 50:50 ratio between NO and N0₂, which may not be optimal for a NOx-sensor as, discussed above. The NO_x -sensor is generally calibrated against intended ratios of NO and N0₂ and therefore even a very small change of the ratio between NO and N0₂ to the SCR catalyst will have a large impact on registered amount of NO_x in the sensor.

Moreover, the solution is based to the use of Pt-based catalyst in one of the two chambers, and Pd- based in the other. For this reason, the solution proposed is likely associated with a high cost. US 2010/0107610 Al discloses an exhaust system for an internal combustion engine, the system comprising two oxidation catalysts disposed upstream of a SCR catalyst. The two oxidation catalysts are configured for different activities and/or different temperature ranges. The exhaust gas flow through the two oxidation catalysts is adjusted by means of an actuator, more specifically a flap. The proposed system however has the disadvantage of the need for a movable part in the form of the flap inside the system in order to adjust the flows.

DE 10 2013 204 405 Al discloses an oxidation catalyst having different coating compositions in different segments in order to provide different catalytic activities between the segments. By means of a covering device rotatable around a central axis of the catalyst, the flow of exhaust gas is directed to the segment(s) intended for the specific operating condition such as to achieve the desired conversion degree. This solution also suffers from the disadvantage of the need of a movable part. Furthermore, it has the disadvantage of not being able to use the whole catalytic surface available during operation, thereby providing an unnecessarily bulky catalyst which also contributes to increased weight of the system.

SUMMARY The object of the present disclosure is to be able to achieve an optimised ratio of N0₂ and NO in an exhaust gas adapted to pass a selective catalytic reduction (SCR) catalyst for conversion of NO_x, which is optimised for a large range of operating conditions as regards to temperature and mass flow of the exhaust gas. The object is achieved by means of an exhaust gas treatment system and a method for providing a desired ratio between N0₂ and NO in an exhaust gas flow intended to pass through an SCR catalyst of an exhaust gas treatment system in accordance with the appended independent claims.

The present invention is based on the fact that the exhaust gas flow is divided into at least two separate partial flows which are essentially parallel to each other (as opposed to arranged in sequence). The two partial flows are both subjected to oxidising conditions wherein hydrocarbons, as well as preferably CO, are oxidised to a degree as high as possible, preferably essentially fully oxidised. However, NO in the first partial flow is only oxidised to a low degree, i.e. 5-30 % of the total content of NO in the first partial flow, whereas the NO in the second partial flow is oxidised to a high degree of at least 70 %. The two partial flows are then recombined, before passing the SCR, such that the resulting exhaust gas flow constitutes a mixture of the two partial flows, thereby achieving the desired ratio of N0₂ to NO in the exhaust gas entering the SCR catalyst. This is achieved without the need for any mechanical movable parts inside the exhaust gas treatment system for diverting the exhaust gas flow into separate partial flows. Instead, the mass ratio between the two partial flows is adapted to be the same for any operating condition of the engine, i.e. any

composition/concentration, temperature and mass flow of the exhaust gas, and hence for any operating condition of the exhaust gas treatment system.

The fact that NO is oxidised in both the first partial flow and in the second partial flow, however to a different extent, ensures that a suitable composition of the recombined exhaust gas is achieved. Furthermore, lower demands can be made of the catalytic activity of an oxidation catalyst.

Therefore, it is also possible to use non-PGMs by means of the present invention if desired, thereby reducing the costs of the system compared to a system comprising PGM (Platinum Group Metal) Catalysts.

By controlling the mass ratio of exhaust gas diverted into the first partial flow and the second partial flow, respectively, it is possible to achieve the desired N0₂:NO ratio for the SCR catalyst used. The system and the method can be controlled by selecting the appropriate activities of the first and the second zones as well as the mass flow ratio between the two parallel partial flows such that a desired ratio of N0₂ to NO in the recombined exhaust gas is achieved before entry into the SCR catalyst. This can be achieved while avoiding the risk for arriving at a N0₂:NO ratio above a predetermined threshold value, such as 50:50. For example, it has been found possible to achieve 50 % N0₂ already at a temperature of about 240°C using two parallel oxidation catalysts, said catalysts being commercially available, and at the same time avoiding the risk of the amount of N0₂ increasing above said level for higher temperatures of the exhaust gas.

The exhaust gas treatment system according to the present invention comprises an inlet for introduction of a flow of exhaust gas into the system, such as a flow of exhaust gas from a combustion engine. The flow of exhaust gas, after entry into the system through the inlet, is divided into at least a first partial flow and at least a second partial flow, the second partial flow being essentially parallel to said first partial flow. The system is configured such that a mass ratio between the first partial flow and the second partial flow is essentially constant for all operating conditions of the system, i.e. for any mass flow, any composition or concentration, and any temperature of the exhaust gas entering the system through the inlet. The system is further configured such that the first partial flow passes a first zone having a catalytic activity adapted for oxidising 5 to 30 %, preferably 8-25%, of NO in the first partial flow. The system is further configured such that the second partial flow passes a second zone having a catalytic activity adapted for oxidising at least 70 %, preferably at least 75%, of NO in the second partial flow. The first and the second zones both have a catalytic activity adapted for oxidising hydrocarbons, preferably at least 70 % of the hydrocarbons in the respective partial flows. The system is further configured such that the first partial flow and the second partial flow, after oxidation of NO, are recombined into a single exhaust gas flow before the exhaust gas passes through the SCR.

The first partial flow may suitably constitute at least 30 % of the exhaust gas mass flow entering the exhaust gas flow treatment system, preferably 35-65 % of the exhaust gas mass flow entering the exhaust gas treatment system. Thereby, an appropriate mass flow ratio between the first partial flow and the second partial flow can easily be achieved to optimise the desired N0₂/NO_x ratio.

According to one aspect, the exhaust gas treatment system comprises a first oxidation catalyst comprising the first zone and a second oxidation catalyst comprising the second zone.

According to another aspect, the exhaust gas treatment system comprises an oxidation catalyst comprising said first zone and said second zone. Such an oxidation catalyst may comprise a plurality of said first zone and a plurality of said second zone. The zones may suitably be arranged parallel to each other and alternatively with respect to each other.

The SCR of the exhaust gas treatment system may suitably be a V₂0₅-based catalyst for reasons of economy, robustness and control strategy. The first zone may suitably comprise a metal oxide based catalytic substance for reasons of cost efficiency. Such a catalytic substance of the first zone may further comprise a platinum group metal (e.g. Pt or Pd). Furthermore, the second zone may comprise a metal oxide based catalytic substance. Such a catalytic substance of the second zone may preferably further comprise a platinum group metal (e.g. Pt or Pd). The metal of the metal oxide based catalytic substance (of any of the first and second zones) may for example be Cu, Mn, Fe, Co, or Ni. Alternatively, the metal of the metal oxide based catalytic substance can be rare earth metal(s).

In order to improve the mixing of the partial flows when they are recombined into a single exhaust gas flow, the exhaust gas treatment system may further comprise stationary means for creating and/or increasing turbulence in the recombined exhaust gas flow upstream of the SCR. The method for providing a desired ratio between N0₂ and NO_x in an exhaust gas flow intended to pass through an SCR catalyst of an exhaust gas treatment system according to the present invention comprises dividing the exhaust gas flow, upstream of the SCR, into a first partial flow and a second partial flow, the second partial flow being essentially parallel to the first partial flow. The mass ratio between the first partial flow and the second partial flow is essentially constant for all operating conditions of the exhaust gas treatment system. The method further comprises oxidising the first partial flow such that 5 to 30 %, preferably 8-25%, of the NO in the first partial flow is oxidised to N0₂, and oxidising the second partial flow such that at least 70 %, preferably at least 75%, of the NO in the second partial flow is oxidised to N0₂. After oxidation of NO in the respective partial flows, the first partial flow is combined with the second partial flow in order to achieve a recombined exhaust gas flow having the desired ratio between N0₂ and NO_x.

The desired ratio between N0₂ and NO_x may suitably be 20 % < N0₂/NO_x < 50%, preferably the desired ratio between N0₂ and NO_x is 30 % < N0₂/NO_x < 45%. Thereby, a high conversion rate in the SCR may be achieved while avoiding the problems associated with the reading by the NO_x-sensor.

The method may suitably also comprise oxidising hydrocarbons both in the first partial flow and in the second partial flow, thereby reducing the problems associated with hydrocarbons in the SCR.

The present disclosure further relates to a diesel oxidation catalyst comprising a first zone extending from an inlet of the oxidation catalyst to an outlet of the oxidation catalyst, the first zone having a catalytic activity capable of oxidising 5 to 30 % of NO in a gas flow passing trough said first zone, and a second zone arranged parallel to said first zone, the second zone having a catalytic activity capable of oxidising at least 70 % of NO in a gas flow passing through said second zone, the catalytic activity of the first zone and the catalytic activity of the second zone both capable of oxidising hydrocarbons. The diesel oxidation catalyst is suitable for use in the method as disclosed above as well as in the exhaust gas treatment system disclosed above. The present invention also relates to a vehicle comprising the exhaust gas treatment system as disclosed above.

BRIEF DESCRIPTION OF DRAWINGS Fig. 1 schematically illustrates a side view of a vehicle comprising an internal combustion engine and an exhaust gas treatment system.

Fig. 2 schematically illustrates an exhaust gas treatment system.

Fig. 3 schematically illustrate N0₂/NO_x ratio vs. temperature of exhaust gas after oxidation of NO using different oxidation catalysts.

Fig. 4 schematically illustrates an exhaust gas treatment system according to one exemplifying embodiment comprising two parallel oxidation catalysts.

Fig. 5a schematically illustrates a cross sectional view of an oxidation catalyst comprising zones having different catalytic activities according to an exemplifying embodiment. Fig. 5b schematically illustrates a cross sectional view of an oxidation catalyst comprising zones having different catalytic activities according to another exemplifying embodiment.

DETAILED DESCRIPTION In the following, the invention will be described in more detail with reference to certain exemplifying embodiments and the drawings. However, the invention is not limited to the exemplifying embodiments discussed and shown in the drawings but may be varied within the scope of the appended claims. Furthermore, the drawings shall not be considered drawn to scale as some features may be exaggerated in order to more clearly illustrate certain features.

In accordance with the present invention, an exhaust gas flow is divided into a first partial flow and a second partial flow, wherein the second partial flow is essentially parallel to the first partial flow. In the present context, parallel partial flows are intended to mean partial flows, which are simultaneous and continuously present. Thus, the partial flows are divided out of a single exhaust gas flow, at a single common point in the system, and recombined at a common point in the system. Hence, the fact that the partial flows are parallel distinguishes from partial flows arranged in sequence one after another. However, parallel partial flows does not necessarily mean that the geometrical flow directions of the partial flows are necessarily parallel (unless otherwise explicitly given) as the parallel partial flows can have different geometrical flow directions inside the exhaust gas treatment system. The exhaust gas treatment system according to the present invention comprises an inlet for introduction of a flow of exhaust gas into the system, such as a flow of exhaust gas from a combustion engine. The flow of exhaust gas, after entry into the system through the inlet, is divided (at a common diving point) into at least a first partial flow and at least a second partial flow, the second partial flow being essentially parallel to said first partial flow. The system is configured such that a mass ratio between the first partial flow and the second partial flow is essentially constant for all operating conditions of the system, i.e. for any mass flow, any composition and any temperature of the exhaust gas entering the system through the inlet. The system is further configured such that the first partial flow passes a first zone having a catalytic activity adapted for oxidising 5 to 30 % of NO in the first partial flow. The system is further configured such that the second partial flow passes a second zone having a catalytic activity adapted for oxidising at least 70 % of NO in the second partial flow. The first and the second zones both have a catalytic activity adapted for oxidising hydrocarbons, preferably at least 70 % of the hydrocarbons in the respective partial flows. The system is further configured such that the first partial flow and the second partial flow, after oxidation of NO in the respective partial flows, are recombined into a single exhaust gas flow before the exhaust gas passes through the SCR.

The constant mass flow ratio of the first partial flow and second partial flow for all operating conditions of the exhaust gas treatment system may be achieved by selecting the appropriate dimensions and shapes of the tubes or conduits of the respective partial flows in case of a plurality of oxidation catalysts, or the cross sectional sizes of the respective zones when present in a single oxidation catalyst. Thereby, the exhaust gas treatment according to the present invention does not need any movable parts in order to divert or control the mass flow ratio between the first and the second partial flows. Thereby, a more robust system is achieved and the need for maintenance measures minimised.

Preferably, the catalytic activity of the first zone is adapted for oxidising 5 to 30 % of NO in the first partial flow at least in a temperature range of 200 to 500 °C. Moreover, the catalytic activity of the second zone is preferably adapted for oxidising at least 70 % of NO in the second partial flow at least in the temperature interval of 250 to 300 °C, preferably in the temperature interval of 225 to 325 °C.

Figure 1 depicts a vehicle 1, here in the form of a truck, in a schematic side view. The vehicle may however be any other motor driven vehicle, for example a bus or a passenger car. The vehicle comprises a combustion engine 2, which powers the vehicle's tractive wheels 3 via a gearbox (not shown), and a propeller shaft (not shown). The engine is provided with an exhaust gas treatment system 4. The engine is powered by fuel supplied to it via a fuel system, which comprises a fuel tank 5. It should be pointed out that the invention is suitable for application in any appropriate exhaust gas treatment system comprising an SCR catalyst and is therefore not limited to systems of motor vehicles. The system and the method according to the invention are well suited to exhaust gas treatment systems other than for land borne motor vehicles, such as watercraft or stationary systems. The watercraft may be of any suitable type, such as motor boats, ships, ferries or vessels. Examples of stationary systems may be industrial engines or engine-powered industrial robots, power plants or the like.

Figure 2 schematically illustrates one exemplifying exhaust gas treatment system 4 comprising a diesel oxidation catalyst (DOC) 6 and a selective catalytic reduction (SCR) catalyst 7 arranged downstream of the DOC in the flow direction of the exhaust gas through the exhaust gas treatment system. While not shown in the figure, the exhaust gas treatment system 4 may further include one or more additional catalysts as well as one or more particulate filters as previously known. By way of example only, a particulate filter may be arranged between the DOC and the SCR, a particulate filter may be arranged downstream of the SCR, and/or an ammonium slip catalyst (ASC) may be arranged downstream of the SCR.

As explained above, the highest conversion rate in the SCR is achieved in the case of a 50:50 ratio of N0₂ and NO in the exhaust gas entering the SCR. The desired ratio of N0₂ in the exhaust gas is achieved by the oxidation of NO to N0₂ in the catalyst(s) provided upstream of the SCR. The catalytic activity of the oxidation catalyst(s) may be selected in order to achieve the desired oxidation of NO. However, the capability of oxidation is dependent on the temperature of the exhaust gas as well as the mass flow as previously explained. Figure 3 schematically illustrates one example of the temperature dependence of the capability of two different commercially available oxidation catalysts to oxidize NO to N0₂. Line 10 illustrates the thermodynamic equilibrium of N0₂/NO_x, and thus sets the upper limit for the oxidation of NO to N0₂. Line 11 represents a catalyst having the ability to oxidize NO to a high degree even at low temperatures, such as about 250°C, whereas line 12 represents a catalyst, which is also capable of oxidizing NO but to a much lower degree compared to the first catalyst illustrated by line 11. It is clear from Figure 3 that neither of the oxidation catalysts provides a desirable degree of oxidation for all temperature ranges to which a conventional gas exhaust treatment system may be subjected during operation thereof. While Figure 3 only illustrates the temperature dependence on the oxidation capacity, the dependence of mass flow has a similar dependence on the oxidation capacity as previously discussed.

In accordance with the present invention, the exhaust gas flow is divided into a first partial flow and a second partial flow. This may according to one aspect of the invention be achieved as shown in Figure 4 illustrating a first oxidation catalyst 6a and a second catalyst 6b arranged essentially parallel to the first oxidation catalyst. After the exhaust gas flow is divided into a first partial flow and a second partial flow at a dividing point 8 in the system, the first partial flow is allowed to flow through the first oxidation catalyst 6a whereas the second partial flow is allowed to flow through the second oxidation catalyst 6b. After the partial flows have passed the respective oxidation catalysts, they are recombined at a recombination point 9 such that the two partial flows are mixed into one single exhaust gas flow which thereafter is allowed to pass through the SCR 7. In the exemplifying embodiment shown, the first oxidation catalyst 6a comprises the first zone and the second oxidation catalyst 6b comprises the second zone.

Considering the possibility of the first oxidation catalyst 6a in Figure 4 constituting the oxidation catalyst as illustrated with line 12 of Figure 3 and the second oxidation catalyst 6b of Figure 4 constituting the oxidation catalyst as illustrated by the line 11 in Figure 3, the recombined exhaust gas flow will (for a preselected mass ratio between the first partial flow and the second partial flow) comprise an amount of N0₂ as illustrated by line 13 in Figure 13. Thus, it is clear that by selecting the appropriate catalysts, or more correctly the catalytic activities of the respective catalysts, as well as the mass ratio between the first and the second partial flows enables tailoring the N0₂/NO_x ratio to a much broader extent than previously possible. The fact that the two partial flows are oxidized using different catalytic activates with regard to NO levels out the temperature dependence of the recombined exhaust gas composition as compared to a case where only one of the oxidation catalysts is used. Moreover, the fact that NO is oxidized in both the first and the second oxidation catalysts ensures that a predetermined desired breaking point (for example 50% N0₂/NO_x as indicated in Figure 3) is not exceeded for any temperature or mass flow of the exhaust gas. Figure 4 schematically illustrates an example wherein the two oxidation catalysts are arranged essentially parallel to each other. However, the oxidation catalysts as such need not necessarily be arranged in parallel inside the exhaust gas treatment system as long as the first and the second partial flows are essentially parallel. For example, the oxidation catalysts need not have their respective central axis (also coinciding with the flow direction through the catalysts) being parallel. The catalyst may thus be arranged in any convenient manner depending on factors such as space, for example inside a common housing of the exhaust gas treatment system, and/or the other catalysts of the exhaust gas treatment system. However, it is essential that the exhaust gas flowing through the exhaust gas treatment system is simultaneously and continuously, i.e. at all times, divided into the first partial flow and the second partial flow, and that the partial flows are fully recombined to a single flow of exhaust gas before the exhaust gas flow enters into the SCR.

While Figure 4 illustrates an example comprising two parallel oxidation catalysts, the first partial flow and the second partial flow need not necessarily pass separate oxidation catalysts but may alternatively both pass through a single oxidation catalyst. This may be achieved by the oxidation catalyst comprising a first zone and a second zone arranged parallel to the first zone in the flow direction through the catalyst. In order to be able to oxidize different amounts of NO in the first partial flow and the second partial flow, the first zone has a different catalytic activity than that of the second zone. This may for example be accomplished by providing the respective zones with different coatings having the intended catalytic activities for the respective zones. For example, a first wash coat may be applied/deposited to a first zone whereas the other zone is masked during such application/deposition step, thereafter the first zone is masked and another wash coat is applied/deposited to the second zone. Masking may for example be made by a suitable wax or the like which is melted and removed in a subsequent processing step of the oxidation catalyst. As apparent to the skilled person, it is not necessary to mask the entire surface of a zone when a desired wash coat is applied to the other zone. It is sufficient that just the openings of the respective zone is masked such that the wash coat intended for the other zone cannot flow into the cells or channels of the first zone.

Fig. 5a illustrates a cross sectional view of an exemplifying single oxidation catalyst 6. As can be seen from the figure, the oxidation catalyst comprises a plurality of cells or channels 8 through which the exhaust gas flows. The cells or channels 8 are arranged in a honeycomb pattern and extend through the oxidation catalyst from the inlet to the outlet. The cells or channels may also be arranged in other cross sectional patterns as known in the art. The oxidation catalyst 6 comprises two first zones 61 having a first catalytic activity and two second zones 62 having a second catalytic activity. The first and second zones are arranged as circle sectors as seen in the cross sectional view of the oxidation catalyst 6. While the figure illustrates two first zones 61 and two second zones 62, the catalyst may comprise only one first zone and one second zone. It is also plausible that the oxidation catalyst comprises more than two of the respective zones without departing from the scope of the present invention. In the case of a plurality of each of the zones, the first zones 61 and the second zones 62 are arranged in an alternating manner in the cross section of the oxidation catalyst. Each of the zones preferably extends from the inlet end of the oxidation catalyst to the outlet end of the catalyst, the inlet and outlet ends defined by the flow direction of exhaust gas through the oxidation catalyst.

In the exemplifying embodiment shown in Figure 5a, the cross sectional surface of the first zones 61 and the second zones 62 have essentially the same size. Thereby, the mass ration between the first partial flow and the second partial flow will necessarily be 50:50 (supposing that the channels have the same cross sectional opening size). In case another mass ratio between the first partial flow and the second partial flow is desired, the respective cross sectional size of the zones may be selected accordingly.

Fig. 5b illustrates a cross sectional view of an alternative exemplifying single oxidation catalyst 6 comprising a plurality of first zones 61 having a first catalytic activity and a plurality of second zones 62 having a second catalytic activity. The first and second zones are arranged in a chess-like pattern with regard to the cross sectional view of the oxidation catalyst 6. Each zone preferably extends from the inlet end of the oxidation catalyst to the outlet end of the oxidation catalyst, the inlet and outlet ends defined by the flow direction trough the catalyst.

It will be readily apparent to the skilled person that other cross sectional patterns of the

arrangement of the first and second zones 61, 62 than the ones illustrated in Figures 5a and 5b are plausible. However, the first and second zones 61, 62, when arranged in a single oxidation catalyst, should be essentially parallel to each other with respect to the flow direction of exhaust gas through the oxidation catalyst.

Irrespective of whether the partial flows pass through a single oxidation catalyst or pass through two or more oxidation catalysts, the first and the second partial flows, after having passed the respective first and second zones, are mixed into a single flow of exhaust gas before the exhaust gas flows into the SCR catalyst. In view of the fact that the first partial flow will have a different N0₂ content than the second partial flow as a result of the different catalytic activities of the first and the second zones, the recombined exhaust gas flow will constitute a mixture of the first partial flow and the second partial flow. It is desired to achieve as homogenous mixture of the recombined exhaust gas as possible to ensure an appropriate reading in a NO_x-sensor as well as ensure the appropriate conversion in the subsequent SCR. Thus, it may in certain instances be desirable to provide stationary means adapted to create and/or increase turbulence of the gas flow. Such stationary turbulence increasing means may suitably be arranged in a conduit at, or close to, the point in the system where the two partial flows are recombined. Examples of such turbulence increasing means may for example be one or more radially extending rods or partitions, or a circumferential radially extending flange or the like. It is also plausible to create or increase the turbulence in the respective partial flows before they are recombined such as to facilitate mixing when they are recombined. This may suitably also be accomplished by stationary means in a similar manner as described above. By creating turbulence in the gas flow upstream of the SCR, the partial flows are better mixed.

Alternatively or additionally, increasing the number of first zones and second zones in a single catalyst may also increase the mixing of the first partial flow and the second partial flow.

The fact that NO is oxidised in both partial flows in accordance with the present invention, however to a different extent, ensures that a suitable composition of the recombined exhaust gas is achieved before the exhaust gas enters the SCR. As a comparison, the method disclosed in US 6,846,464 B2 discloses the use of a Pt-only and a Pd-only catalyst respectively, and thereby oxidises NO only in the Pt catalyst. Therefore, in contrast to the method as disclosed in the above identified document, the present invention is based on the fact that both partial flows are subjected to oxidation of NO, but to a different degree. By the fact that both partial flows are oxidised with respect to NO, lower demands can be made of the catalytic activity. Thereby, it is also possible to use non-PGMs by means of the present invention if desired. This could reduce the cost of the system.

From the perspective of practical industrialisation, all that is required from the catalytic activities of the first and the second zones is that one of the zones (the second zone) has a catalytic activity above a certain level (with no other limitation on its activity for NO-oxidation), while the other zone (the first zone) has a small catalytic activity with regard to NO-oxidation. The determination of the upper and lower threshold limits for the respective catalytic activities can be done with respect to cost and performance demands.

The system and method according to the present invention retain the possibility to maximize the activity of one of the zones to increase the durability of the system, without compromising SCR- performance. The ability to specify the activity of the one of the zones (the zone which is passed by the second partial flow) without any upper limit may be critical when using non-PGM catalysts, since these may, or may not, require this to achieve adequate durability. To maximize activity is not possible for a conventional single-catalyst system (having one single catalytic activity) for the reasons as previously discussed.

As mentioned above, the first and the second zone should both have as high catalytic activity as possible with regard to oxidation of hydrocarbons as well as carbon monoxide. The development of oxidation catalyst during recent years have resulted in catalysts fulfilling both the requirements with regard to oxidation of HC and CO, as well as the respective capabilities with regard to oxidation of NO. Such oxidation catalysts are now commercially available and will therefore not be further discussed in the present disclosure.

In view of the fact that the capability of a catalyst to oxidise compounds depend on the catalytic activity which is achieved by the coating of the substrate of the catalyst, it is thus also possible to obtain a single oxidation catalyst comprising a plurality of zones which have different catalytic activities. This is achieved by selecting wash coats providing different catalytic activities and applying said wash coats to the different zones of an oxidation catalyst by masking such as described above.

The exhaust gas treatment system as disclosed above can be used for a method for providing a desired pre-determined ratio between N0₂ and NO_x in an exhaust gas flow intended to pass through a selective catalytic reduction catalyst. The method comprises, upstream of the SCR, dividing an exhaust gas flow (intended to pass the SCR) flow into a first partial flow and a second partial flow, the second partial flow being essentially parallel to the first partial flow. The method is further configured to provide a mass ratio between the first partial flow and the second partial flow, which is essentially constant for all operating conditions, such as temperature, mass flow and composition or concentration of the exhaust gas. The method further comprises oxidising the first partial flow such that 5 to 30 % of the NO in the first partial flow is oxidised to N0₂, and oxidising the second partial flow such that at least 70 % of the NO in the second partial flow is oxidised to N0₂. After oxidation of NO in the respective partial flows, the method comprises combining and mixing the first partial flow with the second partial flow in order to achieve a recombined exhaust gas flow having the desired ratio between N0₂ and NO_x.

The pre-determined desired ratio is adapted for achieving a desired conversion rate while avoiding the problems associated with the NO_x-sensor as previously disclosed. More specifically, desired ratio between N0₂ and NO_x may suitably be 20 % < N0₂/NO_x < 50%. Preferably, the desired ratio between N0₂ and NO_x is 30 % < N0₂/NO_x < 45%. As disclosed above, the exhaust gas treatment system and the method ensures that a high ratio of N0₂/NO_x is achieved irrespective of the temperature and mass flow while at the same time avoiding the risk of temporarily exceeding an upper threshold value which could risk incorrect reading of the NO_x by the NO_x-sensor.

Claims

1. Exhaust gas treatment system comprising an inlet for introduction of a flow of exhaust gas into the system and an SCR catalyst, wherein the flow of exhaust gas after entry into the system through the inlet is divided into at least a first partial flow and at least a second partial flow, the second partial flow being essentially parallel to said first partial flow, characterised in that a mass ratio between the first partial flow and the second partial flow is essentially constant for all operating conditions of the system, the first partial flow passing a first zone having a catalytic activity adapted for oxidising 5 to 30 % of NO in the first partial flow, the second partial flow passing a second zone having a catalytic activity adapted for oxidising at least 70 % of NO in the second partial flow, wherein the first zone and the second zone both have a catalytic activity adapted for oxidising hydrocarbons, and wherein the first partial flow and the second partial flow are recombined into a single exhaust gas flow before passing through the SCR catalyst.

2. Exhaust gas treatment system according to claim 1, wherein the first partial flow constitutes at least 30 % of the exhaust gas mass flow entering the exhaust gas flow treatment system.

3. Exhaust gas treatment system according to any of claim 1 and 2, comprising a first oxidation catalyst comprising the first zone, and a second oxidation catalyst comprising the second zone.

4. Exhaust gas treatment system according to any of claim 1 or 2, comprising an oxidation

catalyst comprising said first zone and said second zone.

5. Exhaust gas treatment system according to claim 4, wherein the oxidation catalyst comprises a plurality of said first zone and a plurality of said second zone.

6. Exhaust gas system according to any of the preceding claims, wherein the first zone has a catalytic activity adapted for oxidising 8-25 % of the NO in the first partial flow.

7. Exhaust gas treatment system according to any of the preceding claims, wherein the second zone has a catalytic activity adapted for oxidising at least 75 % of the NO in the second partial flow.

8. Exhaust gas treatment system according to any of the preceding claims, wherein the SCR catalyst is a V₂0₅-based catalyst.

9. Exhaust gas treatment system according to any of the preceding claims, wherein the first zone and/or second zone comprises a metal oxide based catalytic substance.

10. Exhaust gas treatment system according to claim 9, wherein said catalytic substance in the second zone further comprises a platinum group metal.

11. Method for providing a desired ratio between N0₂ and NO_x in an exhaust gas flow intended to pass through an SCR catalyst of an exhaust gas treatment system, the method comprising: upstream of the SCR, dividing the exhaust gas flow into a first partial flow and a second partial flow, the second partial flow being essentially parallel to the first partial flow, and wherein the mass ratio between the first partial flow and the second partial flow is essentially constant for all operating conditions of the exhaust gas treatment system, oxidising the first partial flow such that 5 to 30 % of the NO in the first partial flow is oxidised to N0₂, oxidising the second partial flow such that at least 70 % of the NO in the second partial flow is oxidised to N0₂, after oxidation of NO in the respective partial flows combining the first partial flow with the second partial flow in order to achieve a recombined exhaust gas flow having the desired ratio between N0₂ and NO_x.

12. Method according to claim 14, wherein the desired ratio between N0₂ and NO_x is 20 % <

N0₂/NO_x < 50%, preferably wherein the desired ratio between N0₂ and NO_x is 30 % < N0₂/NO_x < 45%.

13. Method according to any one of claims 14 and 15, further comprising oxidising hydrocarbons in the first partial flow and in the second partial flow.

14. Diesel oxidation catalyst, characterised in comprising a first zone extending from an inlet of the oxidation catalyst to an outlet of the oxidation catalyst, the first zone having a catalytic activity capable of oxidising 5 to 30 % of NO in a gas flow passing trough said first zone, and a second zone arranged parallel to said first zone, the second zone having a catalytic activity capable of oxidising at least 70 % of NO in a gas flow passing through said second zone, the catalytic activity of the first zone and the catalytic activity of the second zone both capable of oxidising hydrocarbons.

15. Vehicle comprising the exhaust gas treatment system according to any one of claims 1 to 10.