Heat exchanger system for treatment of a flow of exhaust gases in an exhaust gas aftertreatment system

Abstract

A heat exchanger system for treatment of a flow of exhaust gases in an exhaust gas aftertreatment system of a vehicle. The heat exchanger system includes a nitrogen monoxide (NO) oxidation site for oxidizing nitrogen monoxide to nitrogen dioxide (NO2). The NO oxidation site is positioned such that the flow of exhaust gases at a downstream end (40) of the NO oxidation site in use of the heat exchanger system is arranged to proceed at a temperature within a predetermined temperature interval corresponding to a desired NO to NO2 (NO:NO2) ratio interval in the flow of exhaust gases. An exhaust gas aftertreatment system and a vehicle including such a heat exchanger system, and a method for using such a heat exchanger system, are also provided.

Claims

1. A heat exchanger system for treatment of a flow of exhaust gases in an exhaust gas aftertreatment system of a vehicle, the heat exchanger system comprising a heat exchanger, wherein the heat exchanger comprises: a first guiding passage, a second guiding passage, and a flow reversing region arranged between the first guiding passage and the a second guiding passage, whereby the first guiding passage is arranged to guide the flow of exhaust gases along a first flow direction towards the flow reversing region, and the second guiding passage is arranged to guide the flow of exhaust gases from the flow reversing region in a second flow direction, such that heat may be exchanged between the flow of exhaust gases in the second guiding passage and the flow of exhaust gases in the first guiding passage, a nitrogen monoxide (NO) oxidation site for oxidising nitrogen monoxide (NO) to nitrogen dioxide (NO2) in the flow of exhaust gases, wherein the NO oxidation site comprises an NO oxidation catalyst, and a further oxidation site for oxidising a further constituent of the flow of exhaust gases, the further oxidation site comprises a catalyst for oxidising the further constituent, wherein the further oxidation site is arranged within the first guiding passage; the NO oxidation site is positioned within the second guiding passage, downstream of the further oxidation site; the NO oxidation site is positioned such that the flow of exhaust gases at a downstream end of the NO oxidation site in use of the heat exchanger system is arranged to proceed at a temperature within a predetermined temperature interval corresponding to a desired NO to NO2 (NO:NO2) ratio interval in the flow of exhaust gases, and the further oxidation site is a hydrocarbon oxidation site for oxidising methane (CH4) to mainly carbon dioxide (CO2).

2. The heat exchanger system according to claim 1, wherein the position of the downstream end of the NO oxidation site is adapted to the size and position of the further oxidation site.

3. The heat exchanger system according to claim 1, wherein the heat exchanger system is a counter-flow heat exchanger system, whereby the second guiding passage is arranged to guide the flow of exhaust gases in the second flow direction which is generally opposite to the first flow direction of the first guiding passage.

4. The heat exchanger system according to claim 1, wherein the NO oxidation site has an upstream end opposite to the downstream end, which upstream end is positioned immediately adjacent the flow reversing region.

5. The heat exchanger system according to claim 1, wherein the further oxidation site is positioned immediately adjacent the flow reversing region.

6. The heat exchanger system according to claim 1, in which the desired NO to NO2 (NO:NO2) ratio interval is 30:70-70:3.

7. The heat exchanger system according to claim 1, in which the predetermined temperature interval is 350-420 C.

8. The heat exchanger system according to claim 1, wherein an exhaust gas inlet is arranged to the first guiding passage, and an exhaust gas outlet is arranged to the second guiding passage.

9. The heat exchanger system according to claim 1, wherein the downstream end of the NO oxidation site is positioned within a middle third part between the flow reversing region and a downstream end of the second guiding passage.

10. The heat exchanger system according to claim 1, wherein a surface area of the second guiding passage is coated by the NO oxidizing catalyst to at least 33%.

11. The heat exchanger system according to claim 1, further comprising a third guiding passage for guiding at least part of the flow of exhaust gases from the exhaust gas inlet to a second flow reversing region, and a fourth guiding passage for guiding the flow of exhaust gases from the second flow reversing region to the exhaust gas outlet (4).

12. The heat exchanger system according to claim 11, wherein the third guiding passage comprises a further oxidation site for oxidising a further constituent of the flow of exhaust gases.

13. The heat exchanger system according to claim 12, wherein the further oxidation site of the third guiding passage is a hydrocarbon oxidation site for oxidising hydrocarbon (HC) to mainly carbon dioxide (CO2) and water (H20), more preferably for oxidising methane (CH4).

14. The heat exchanger system according to claim 12, wherein the further oxidation site of the third guiding passage comprises a catalyst for oxidising the further constituent.

15. The heat exchanger system according to claim 11, wherein the fourth guiding passage comprises a selective catalytic reduction site for selective reduction of NO and NO2 in the flow of exhaust gases to mainly nitrogen (N2).

16. The heat exchanger system according to claim 15, wherein the fourth guiding passage comprises a selective catalytic reduction catalyst.

17. The heat exchanger system according to claim 11, wherein the heat exchanger system further comprises a second heat exchanger device (1b) incorporating the third and fourth guiding passages and the second exhaust gas flow reversing region.

18. The heat exchanger system according to claim 11, wherein the flow of exhaust gases through the exhaust gas inlet is adapted to be distributed between the first and third guiding passages, wherein the flow of exhaust gases is adapted to be distributed generally evenly between the first and third guiding passages.

19. The heat exchanger system according to claim 11, further comprising an exhaust gas distribution device for controlling the amount of exhaust gas through the first and third guiding passages, respectively.

20. The heat exchanger system according to claim 19, wherein the exhaust gas distribution device is adapted to be controlled in accordance with a sensed NO amount and/or NO2 amount and/or the NO:NO2 ratio in the flow of exhaust gases.

21. The heat exchanger system according to claim 1, wherein the first and/or second flow reversing region is equipped with at least one urea injection device for injecting urea into the exhaust gas.

22. The heat exchanger system according to claim 1, wherein the first and/or second flow reversing region is equipped with at least one heater.

23. An exhaust gas aftertreatment system for controlling exhaust gas emissions of an internal combustion engine, comprising a heat exchanger system according to claim 1.

24. The exhaust gas aftertreatment system according to claim 23, wherein a NOx sensor is arranged downstream of the heat exchanger system for sensing an NO amount and/or NO2 amount and/or an NO:NO2 ratio in the flow of exhaust gases leaving the heat exchanger system.

25. The exhaust gas aftertreatment system according to claim 23, wherein the exhaust gas aftertreatment system further comprises a selective catalytic reduction unit, and wherein the heat exchanger system is arranged upstream of the selective catalytic reduction unit.

26. The exhaust gas aftertreatment system according to claim 23, wherein the exhaust gas aftertreatment system further comprises an oxidation catalyst, wherein the heat exchanger system is arranged downstream of the oxidation catalyst or in a bypass passage bypassing the oxidation catalyst.

27. A method for controlling exhaust gas emissions of an internal combustion engine comprising the step of using a heat exchanger system according to claim 1.

28. A vehicle, comprising a heat exchanger system according to claim 1.

29. The vehicle according to claim 28, wherein an engine of the vehicle is adapted to operate using compressed natural gas (CNG) or liquid natural gas (LNG).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following, the principle of the disclosure is explained by means of or by embodiments depicted in the figures. Thereby, the figures are exemplary only and shall not be intended to define the scope of protection. This scope of protection is solely defined by the appending claims.

(2) The figures show:

(3) FIG. 1 a schematic illustration of a heavy duty vehicle which is provided with an embodiment of a heat exchanger system according to the present disclosure,

(4) FIG. 2 a schematic illustration of a first embodiment of the heat exchanger system,

(5) FIG. 3 a schematic diagram of the temperature of a downstream end of a NO oxidation site and the resulting NO2/NOx ratio,

(6) FIG. 4 a schematic illustration of a second embodiment of the heat exchanger system,

(7) FIG. 5 a schematic diagram of a temperature along the axial length of an embodiment of the disclosed heat exchanger system,

(8) FIG. 6 a schematic illustration of a third embodiment of the heat exchanger system, and

(9) FIG. 7 a schematic illustration of a fourth embodiment of the heat exchanger system.

DETAILED DESCRIPTION

(10) In the following same or functionally similar elements are indicated by the same reference numerals.

(11) FIG. 1 shows a schematic illustration of a heavy duty truck 100 which is provided with an embodiment of a heat exchanger system 110 according to the present disclosure. Also other types of vehicles may be provided with the heat exchanger system 110 of the present disclosure. The truck 100 is provided with an internal combustion engine 101 which preferably is run on either compressed natural gas (CNG) or liquefied natural gas (LNG) comprising methane (CH4). Other fuels or fuel combinations may be possible. Also a hybrid vehicle comprising both an internal combustion engine 101 of the above kind and an electric machine with a battery pack may be contemplated. The internal combustion engine 101 operates according to otherwise known principles and produces exhaust gases which generally are in need of aftertreatment due to emission regulations before being released through a not shown exhaust system and exhaust pipe 102 to the ambient.

(12) FIG. 2 shows schematically a principle illustration of a first embodiment of the heat exchanger system 1 10. The figure shows a heat exchanger system 1 10 having a heat exchanger through which a flow of exhaust gasses 6 passes. The heat exchanger 1 can in this embodiment be of any otherwise known kind and the details thereof are hence left out in this figure. A NO oxidation site 1 is located within the heat exchanger 1. The NO oxidation site 18 has a downstream end 40 which is located downstream of the NO oxidation site 18 with regard to the flow direction of the flow of exhaust gases 6. Merely as an illustrative example, the NO oxidation site 18 may have a surface area which in the figure be seen as the length along the flow of exhaust gasses 6 (i.e. horizontally in the figure) multiplied by the length in the cross-wise direction, which is the direction going, into the figure (i.e. not visible in the figure).

(13) The flow of exhaust gases 6 comprises several different constituents, of which nitrogen monoxide (NO) and nitrogen dioxide (NO2) are two ones for which this heat exchanger system 1 10 is designed to handle. Nitrogen monoxide (NO) and nitrogen dioxide (NO2) are often referred to as nitrogen oxides (NOx) with a single, common expression. It may for several reasons be desirous to control the relationship between nitrogen monoxide (NO) and nitrogen dioxide (NO2), and especially the ratio there between: NO:NO2. In some contexts the ratio is instead expressed as one of the constituents in relation to nitrogen oxides: e.g. NO2:NOx, but this is merely a simple recalculation of the earlier mentioned ratio, since NOx is generally held to be the sum of NO and NO2 and the relationship between the two expressions is thus singular. A ratio of 50:50, or 1:1, in terms of the NO:NO2 ratio may consequently be expressed as e.g. 50% as the NO2:NOx ratio. Other ways to express these ratios exist. Within this disclosure only the expressions NO:NO2 ratio and NO2:NOx ratio will be used.

(14) FIG. 3 discloses a diagram showing on the horizontal axis a temperature which in the diagram is the temperature of the downstream end 40 of the NO oxidation site 18 (NO oxcat end). On the vertical axis the NO2:NOx ratio is disclosed for the temperatures of the horizontal axis. Within the diagram there are three curves represented, A, B and C. The uppermost curve A represents the NO2:NOx ratio at chemical equilibrium under optimum conditions for each temperature if enough time is given for the chemical reaction to take place. The higher the temperature at the downstream end of the NO oxidation site 18, the lower the amount of NO2 in relation to NOx becomes, or in other words, the lower the amount of NO2 in relation to NO becomes. When the temperature thus is increased, more NO is produced at the expense of NO2. The opposite is also true, with a lowered temperature the higher the NO2 amount in the NO2:NOx ratio becomes. This means in general terms that when knowing the temperature distribution within the heat exchanger system 110 it is possible to design the NO oxidation site 18 such that its downstream end 40 is located at a temperature where the NO2:NOx (or the NO:NO2) ratio is within the desired interval. It should also be noted with regard to any one of the embodiments of the disclosure herein, that in order not to produce undesirably much NO in relation to NO2, it is advantageous to release some heat from the catalyst not to overheat it. In the case of providing the NO oxidation site 18 in a heat exchanger system, or even more advantageously in a counter flow heat exchanger system, the risk of overheating the NO oxidation site 18 is hence lowered.

(15) The lowermost curve C generally represents the same as curve A, but the conversion for a catalytical reactor. For curve C the NO2:NOx ratio initially increases with increased temperature, up to a maximum point, after which the NO2:NOx ratio again decreases with further increased temperature. The reason for the inverted V-shape of the curve is that the chemical reaction is not given enough time to reach equilibrium due to that the kinetics limits the oxidation speed.

(16) The middle curve, curve B, gently represents the same as curves A and B, in this case when NO is converted in a catalytically coated heat exchanger. Curve B generally follows curve C at the higher temperature region, since the kinetics are fast enough to reach equilibrium. At the lower temperature reason curve B departs from curve C, thanks to the catalytical process taking place at higher temperatures (except for at the outlet of the heat exchange) and that the kinetics thus are faster such that the oxidation process reaches further.

(17) Going back to FIG. 2, it is, based on the knowledge of FIG. 3, consequently possible to design the heat exchanger system 110 such that the NO oxidation site 18 is positioned such that the flow of exhaust gases 6 at a downstream end 40 of the NO oxidation site 18 in use of the heat exchanger system 110 is arranged to proceed at a temperature within a predetermined temperature interval corresponding to a desired NO:NO2 ratio interval in the flow of exhaust gases 6. The desired NO:NO2 ratio interval may be any ratio interval which is reachable within the boundaries of FIG. 3 for the particular heat exchanger system 110 at hand. It should be noted that depending on operating conditions of the heat exchanger system 110, the desired NO:NO2 ratio interval may be relatively seen wider or narrower, even narrow enough to approximately represent a singular NO:NO2 ratio.

(18) FIG. 4 shows schematically a principle illustration of a second embodiment of the heat exchanger system 110. As can be seen, the heat exchanger 1 included in the heat exchanger system 110 comprises an inlet 2 and an outlet 4 for the exhaust gas and its flow 6 produced in an internal combustion engine (not shown) and guided through an exhaust gas pipe (not shown). Thereby, the heat exchanger system 110 may be part of an exhaust gas aftertreatment system (not shown) for controlling environmentally harmful exhaust gas emissions. It should be noted that all or only part of the exhaust gas streaming through the exhaust gas aftertreatment system may be guided to the heat exchanger system 110.

(19) The heat exchanger 1 itself comprises first guiding passage 8, e.g. at least one channel or a channel system as e.g. be present in a common honeycomb heat exchanger, for guiding exhaust gas 6 from the inlet 2 along a first flow direction (see arrow 10) to a first exhaust gas flow reversing region 12. In the flow reversing region 12, the exhaust gas from the first guiding passage 8 is reversed and guided through a second guiding passage 14 to the outlet 4. Also the second guiding passage 14 may be at least one channel or channel system as mentioned above. For exchanging heat, the first guiding passage 8 and the second guiding passage 14 are substantially arranged in parallel, thereby providing a counter-flow heat exchanger system 110.

(20) Moreover, the first and second guiding passages 8, 14 are at least, partially coated by a catalytic material, wherein the first guiding passage 8 is coated with a hydrocarbon oxidation catalyst, preferably a methane oxidation catalyst 16. Further, the second guiding, passage 14 is coated with a NO oxidation catalyst 18. However, as can be seen in FIG. 4, not all of the active surface of the second guiding passage 14 is coated by the NO oxidation material, but only as much as is necessary for oxidizing enough NO to NO2 to gain a resulting desired NO:NO2 ratio (or desired NO:NO2 ratio interval) of for example approx. 50:50 (or for example approx. 40:60-60:40) downstream of the heat exchanger system.

(21) This kind of heat exchanger system 110 is particularly preferred in connection with an internal combustion engine which is operated by natural gas as fuel, e.g. compressed natural gas (CNG) or liquid natural gas (LNG). The main disadvantages of using such fuels are the methane emissions in the exhaust gas, which should be avoided. Disadvantageously, the methane CH4 contained in such fuels generally cannot be oxidized by the ordinary oxidation catalysts due to the required high temperatures for the methane oxidation. Therefore, it has been suggested to coat the first guiding passage 8 of the heat exchanger 1 by a methane oxidation catalyst material 16 and use the heat of the highly exothermic oxidation reaction to heat the exhaust gas in the first guiding passage 8 by exchanging heat from the second guiding passage 14 back to the first guiding passage 8, which in turn increases the efficiency of the methane oxidation. Additionally, it has been realized that not only the methane oxidation is more efficient, but the produced heat may also be used for oxidizing NO means of the NO oxidation catalyst 18 coating in the second guiding passage 14.

(22) From the state of the art, e.g. DE 02 21 174, it has been known to arrange a NO.sub. storage catalyst or a selective catalytic reduction unit in a heat exchanger, respectively. Disadvantageously, this has the drawback that the storage has to be cleared from time to time and the NO:NO2 ratio is not adapted by the methane oxidation catalyst for an efficient working of the selective catalytic reduction unit. By arranging an NO oxidation site 18 in the heat exchanger system 1, a desired NO:NO2 ratio of approx. 50:50 may be produced which can be used in a selective catalytic reduction unit (not shown) arranged downstream of the heat exchanger system 1.

(23) Thereby, it is preferred to coat only half of the length of the second guiding passage 14 with the NO oxidation site 18 from the flow reversion region 12 to avoid excessive oxidation of NO, which in turn reduces the efficiency of the selective catalytic reduction unit. By doing so the downstream end of the NO oxidation site 18 is located where a temperature within the heat exchanger system is predicted to be of a range creating the desired NO:NO2 ratio or ratio interval.

(24) By providing such a heat exchanger 1 in an exhaust gas aftertreatment system emissions may be controlled over a wide range of applications by securing an optimal NO:NO2 ratio.

(25) This is further elucidated in FIG. 5, in which on the horizontal axis represents the location along the axial extension of the counter-flow heat exchanger system 110 of FIG. 4 as viewed in this figure. Similar figures may be developed for any heat exchanger 1. The inlet 2 and outlet 4 are consequently represented on the left hand side, and the flow reversion region 12 is represented on the right hand side. The expected temperature at each location within the counter-flow heat exchanger system 110 of FIG. 4 is generally represented on the vertical axis. The location within the counter-flow heat exchanger system 110 at which to locate the downstream end of the NO oxidation site 18 is consequently found by taking the appropriate temperature for the desired NO:NO2 ratio e.g. from FIG. 3, and from the vertical axis going towards the right hand side, and to use the location where this temperature meets the curve as represented on the horizontal axis. Hereby not only a location of the downstream end of the NO oxidation site 18 for a desired NO:NO2 ratio may be found, but any location, for any ratio or ratio interval as desired may be found.

(26) Instead of coating only the half of the length of the second guiding passage 14 by a NO oxidation catalyst 18 it has also been suggested to split the heat exchanger 1 into two separate devices 1a and 1b as can be seen in FIG. 6. Thereby, the first heat exchanger device 1a incorporates the first and second guiding passage 8, 14 and the first exhaust gas flow direction reversion region 12, wherein the second heat exchanger device 1b incorporates a third guiding passage 20, a fourth guiding passage 22, and an optional second exhaust gas flow direction reversing region 24. While the first heat exchanger device 1a comprises a NO oxidation catalyst 18 in the second guiding passage 14, such an oxidation, catalyst is omitted in the second heat exchanger device 1b.

(27) Consequently, the NO amount of the exhaust gas streaming through the second heat exchanger device 1b comprises an almost unchanged NO amount compared to the NO amount present in the exhaust gas entering the heat exchanger system 110. On the other hand, the first heat exchanger device 1a provides the NO oxidation into NO2. Dependent on the size of the NO oxidation cata-lyst 18 in the second guiding passage 14, an almost complete conversion of NO to NO2 may be gained. This may be reached e.g. by coating almost the complete second guiding passage 14 with an NO oxidation catalyst 18, as depicted in FIG. 6. When both exhaust gas streams are mixed in a mixing region 26 downstream of the second and fourth guiding passage 14, 20, respectively, the exhaust gas exiting the heat exchanger system 110 has the desired NO:NO2 ratio of roughly 50:50, again.

(28) For dividing the exhaust gas to the first and second heat exchanger devices 1a, 1b an exhaust gas distribution device 28 may be present which may be designed as a 3-way valve. As can be further seen in FIG. 6 both heat exchangers devices 1a, 1b comprise a methane oxidation catalyst 16.

(29) Even if FIG. 6 shows the heat exchanger system 110 as separate devices 1a, 1b, the heat exchangers may be arranged in a single common device. It should be further mentioned that the heat exchanger may be designed as the known multi-channel heat exchanger, where a first part of the channels of the heat exchangers belongs to the first heat exchanger device 1a, and a second part of the channels belongs to the second heat exchanger device 1b, wherein the corresponding channels are coated with the corresponding oxidation catalysts. It should be further noted that in this case it may also be possible to provide only a single flow reversing region.

(30) According to a further preferred embodiment, which is depicted in FIG. 7, the second heat exchanger device 1b may comprise a selective catalytic reduction coating 30 in the fourth guiding passage 20 for exploiting the heat produced by the methane oxidation reaction. Since for the selective catalytic reduction reaction a reducing agent such as urea or ammonia is needed, the exhaust gas should be equipped with the reducing agent upstream of the fourth guiding means. Even if in principle also a urea injection upstream of the complete heat exchanger system 110 is possible, it is preferred to inject urea into the second reversing region 24 (see arrow 31). This has at least two advantages. The first advantage is that urea is not wasted in the first heat exchanger device 1a. Secondly, it should be taken into account that the temperature of the exhaust gas upstream of the heat exchanger 1 is significantly less than the temperature of the exhaust gas in the second flow reversing region 24. Since urea tends to form de-posits of solid urea in case the temperature is too low, the urea injection in the second reversing region 24 avoids such deposits. Additionally, urea may be converted to ammonia due the high temperatures which in turn increased the efficiency of the selective catalytic reduction reaction in the fourth guiding passage 20 of the second heat exchanger device 1b.

(31) For ensuring that even at low temperatures and low load applications, the temperature of the exhaust gas upstream of the urea injection is high enough, a heater may be arranged in the second exhaust gas flow direction reversing region 24, which is operated in case the exhaust gas temperature is not in the required temperature range. Thereby, the heater unit may be arranged upstream or downstream of the urea injection device. It goes without saving that such a heater may also be arranged in the first flow reversing region 12.

(32) Even if a selective catalytic reduction coating 30 may be arranged in the second heat exchanger device 1b, it may also be preferred to arrange a particulate filter 32 and/or a further selective catalytic reduction unit 34 downstream of the heat exchanger system 110. Thereby, it is also possible to add a further urea injection device (not shown) for injecting urea to the SCR unit arranged downstream of the heat exchanger 1 for ensuring that sufficient urea is provided in the exhaust gas for the NOx reduction in the second selective catalytic reduction unit 34.

(33) The heat exchanger system 110 may be arranged directly downstream of the internal combustion engine, but it is also possible to arrange the heat exchanger system 110 downstream of a further oxidation catalyst (not shown) or in a bypass (not shown) bypassing the oxidation catalyst. Thereby it can be ensured that sufficient NO is converted to NO2 for optimal emission controls.

(34) In general, the heat exchanger system 110 and the exhaust gas after-treatment system save space and are rather costless. Moreover, since the desired NO:NO2 ratio or ratio interval can be provided, the legal requirements concerning the exhaust gas emissions may be met.

(35) Many alterations may be made to the disclosed heat exchanger system 110 without departing from the scope and idea behind it. Such alterations may be to apply not a single guiding passage, but a set of smaller guiding passages, each having the features of the disclosed embodiments. The guiding passages may also have any cross-sectional shape, such as rectangular, triangular or circular. Further, if nothing else is said the whole width of each guiding passage, from its upstream end to its downstream end, is provided with a coating such as the NO oxidation site 18. It may however be that a lesser width is coated if found advantageous. Also, an outer housing may be present to cover and protect the disclosed heat exchanger system. Such a housing may take any form, such as cuboid or cylinder. The flow reversing region may also be angular or rounded.