WO2008072013A1

WO2008072013A1 - System and method for exhaust gas after-treatment

Info

Publication number: WO2008072013A1
Application number: PCT/GB2007/050750
Authority: WO
Inventors: Ronny Allansson
Original assignee: Johnson Matthey Public Limited Company
Priority date: 2006-12-12
Filing date: 2007-12-11
Publication date: 2008-06-19
Also published as: GB0624727D0

Abstract

An exhaust system (8) for treating exhaust gas from a lean-burn internal combustion engine comprising at least one exhaust treatment element (10), and at least one heat exchanger (12) arranged so that the exhaust gas can pass through the at least one heat exchanger (12) both before and after passing through the at least one exhaust treatment element (10), wherein valves (16, 18) are used to switch the flow of engine-out exhaust gas and/or exhaust gas exiting the at least one exhaust treatment element (10) so that such gas can by- pass the at least one heat exchanger (12). A method of treating exhaust gas using at least one heat exchanger (12) and at least one exhaust treatment element (10) is also disclosed.

Description

SYSTEM AND METHOD FOR EXHAUST GAS AFTER-TREATMENT

The present invention relates to an exhaust system for treating exhaust gas from a lean-burn internal combustion engine. In particular it concerns an exhaust system comprising at least one exhaust treatment element, such as a catalyst, a filter or a NO_x trap.

It is known to treat exhaust gas from a lean-burn internal combustion engine using a catalyst. Exhaust systems for treating such exhaust gas may be designed to reduce nitrogen oxides (NO_x) present in the exhaust gas of an internal combustion engine to nitrogen (N₂) by selective catalytic reduction (SCR) using a suitable reductant.

Two examples of SCR systems using reductant are ammonia SCR (NH₃-SCR) and hydrocarbon SCR (HC-SCR).

In NH₃-SCR systems several reactions occur, of which the overall desirable reactions (which reduce NO_x to N₂) are represented in equations (1) and (2). In practice reaction (2) is faster than reaction (1) and therefore is more desirable.

4 NO + 4 NH₃ + O₂ → 4 N₂ + 6 H₂O (1)

2 NO + 2 NO₂ + 4 NH₃ → 4 N₂ + 6 H₂O (2)

Catalysts for promoting reactions (1) and (2) include V₂Os/TiO₂/WO₃ and copper- or iron-substituted zeolites such as ZSM-5, beta or mordenite.

It will be appreciated that at lower temperatures, below about 100-200 ⁰C, NH₃ can also react with NO₂ to produce an explosive mixture of ammonium nitrate (NH₄NO₃) and ammonium nitrite (NH₄NO₂) and so, in use, the system is controlled to avoid conditions that promote the formation of this explosive mixture. Additionally, at least, reaction (1) occurs less readily at lower temperatures. In HC-SCR systems, hydrocarbons react with NO_x, rather than oxygen (O₂), to form nitrogen, carbon dioxide (CO₂) and water (H₂O) according to reaction (3):

{HC} + NO_x → N₂ + CO₂ + H₂O (3)

The competitive, undesirable non-selective reaction with oxygen is given by reaction (4):

{HC} + O₂ → CO₂ + H₂O (4)

A number of catalysts are known for selectively promoting reaction (3) including platinum on alumina, transition metal e.g. copper- or iron-substituted zeolites and silver supported on alumina. The peak activity of platinum on alumina may occur at 200-300 ⁰C, whilst the peak activity of base metal catalysts can occur at 400 ⁰C or above.

Another type of catalyst that can be used to treat exhaust gas from a lean-burn internal combustion engine is the diesel oxidation catalyst (DOC), which oxidises carbon monoxide (CO), gas phase hydrocarbons (HC) and the organic fraction of diesel particulates (SOF). Reaction (3), see above, describes the reaction that removes both gas phase and particulate hydrocarbons from the exhaust gas, whilst removal of CO occurs as follows:

DOCs tend to show little or no activity at low exhaust gas temperatures, but as the temperature increases so does the oxidation rate of CO, HC and SOFs.

It is also known to treat exhaust gas from a lean-burn internal combustion engine using a filter. Exhaust gases from lean burn internal combustion engines comprise particulate matter (PM); even modern lean burn gasoline engines emit large numbers of very small (<0.1 μm generally 10-100 nm) PM. Since such gas-borne particulates can cause health problems if inhaled various National and European regulations have been introduced to control the quantity of particulates emitted from diesel engines.

Johnson Matthey has patented and commercially introduced a device marketed as CRT^® for diesel engines, especially suited for use with heavy duty diesel engines (as defined by the relevant European, US Federal or Californian legislation) see EP 0 341 832 and US 4,902,487 (the entire contents of which are incorporated herein by reference). This device is a self-regenerating diesel particulate filter (DPF) that makes use of the process whereby nitrogen monoxide (NO) in the exhaust gas is oxidised to nitrogen dioxide (NO₂) and PM on the filter is combusted in the NO₂ at temperatures of up to 400 ⁰C. Alternatively, the PM may be combusted on a catalysed soot filter (CSF) in the presence of oxygen by intermittently increasing the temperature of the filter by injecting hydrocarbon fuel into exhaust gas upstream of the CSF for combustion on the CSF.

A characteristic of light duty diesel engines, however, is that their exhaust gas temperature is appreciably lower than that of heavy duty diesel engines. A number of engine design modifications introduced for both fuel efficiency and pollution control reasons, and the use of small turbocharged diesel engines, have reduced the exhaust gas temperatures and NO or NO₂ levels to the point where even the CRT^® does not produce sufficient NO₂ passively and/or the reaction proceeds efficiently only under certain operating conditions when the exhaust gas temperature rises. Additionally, a known problem with CSFs is that PM can build up on the CSF during periods when the exhaust gas temperature is relatively cool, e.g. 150-200 ⁰C, such as during extensive periods of idling and/or in slow driving conditions. In such circumstances, backpressure in the system can rise undesirably as PM collects on the CSF.

NO_x traps are used to adsorb and store NO_x present in exhaust gases at low temperatures, and release the NO_x at higher temperatures when it may be converted to N₂. The adsorption process involves two steps represented by equations (6) and (7) in which barium oxide, a typical NO_x storage material, is used by way of illustration only:

BaO + NO₂ + Vi O₂ → Ba(NO₃)₂ (7)

In the first step, the nitric oxide (NO_x emissions from a diesel engine are typically composed of 90-95% nitric oxide) reacts with oxygen in the presence of an oxidation catalyst, e.g. platinum, to form NO₂. The NO₂ thus produced is then adsorbed by the storage material to form an inorganic nitrate in a second reaction step. When the engine runs under conditions wherein the exhaust gas is enriched or at elevated temperatures the nitrate species become thermodynamically unstable and decompose, producing NO or NO₂, according to reactions (8a) and (8b):

Ba(NO₃)₂ → BaO + 2 NO + 1 Vi O₂ (8a)

Ba(NO₃)₂ → BaO + 2 NO₂ + Vi O₂ (8b)

Under rich conditions, these nitrogen oxides are subsequently reduced by carbon monoxide, hydrogen, and hydrocarbons to N₂. One reaction pathway is illustrated in equation (3); another is shown in equation (9):

This is a simplified view of NO_x trap chemistry (a more detailed analysis would include other reaction paths and species, for example carbonates may be formed in reactions between barium nitrate and carbon dioxide) but is sufficient to demonstrate the importance of temperature for NO_x trap operation. Most NO_x traps are at their most efficient between 200 ⁰C and 400 ⁰C and therefore, in contrast to DOCs or filters, the temperature of exhaust gas is at times too high to enable efficient operation of this particular type of exhaust gas component. WO 2000/028196 discloses a heat exchanger enclosing a series of exhaust treatment elements, the purpose of which is to recover heat from the reactions taking place within the exhaust treatment elements to preheat the incoming exhaust stream thereby to maintain a higher temperature within the exhaust treatment elements. In Figure 1 of WO ' 196 the exhaust treatment elements are depicted as a HC-SCR catalyst, a DPF and a DOC, arranged in series, all surrounded by a single heat exchanger in a spiral configuration. This arrangement does not enable the flow of exhaust gas to bypass the heat exchanger whilst still flowing through the or each exhaust treatment element, nor does it allow for multiple exhaust treatment elements to be maintained at different temperatures.

We now provide a flexible exhaust system design believed to be especially suitable for engine exhausts where the temperature may, at times, be too low for effective performance of the various oxidation/combustion reactions and/or too high for effective NO_x adsorption. According to one aspect, the invention provides an exhaust system for treating exhaust gas from a lean-burn internal combustion engine, which system comprising at least one exhaust treatment element, and at least one heat exchanger arranged so that the exhaust gas can pass through the at least one heat exchanger both before and after passing through the at least one exhaust treatment element, whereby heat generated from an exotherm over the at least one exhaust treatment element can be transferred by the at least one heat exchanger from gas downstream of the at least one exhaust treatment element to gas upstream of the at least one exhaust treatment element and/or whereby heat in the exhaust gas upstream of the at least one exhaust treatment element can be transferred by the at least one heat exchanger to gas downstream of the at least one exhaust treatment element, wherein the exhaust system comprises at least one of: a) a first valve for switching the flow of engine-out exhaust gas between a first position, wherein the exhaust gas flow passes through the at least one heat exchanger before passing through the at least one exhaust treatment element, and a second position, wherein the exhaust gas flow by-passes the at least one heat exchanger; and b) a second valve for switching the flow of exhaust gas exiting the at least one exhaust treatment element between a first position, wherein said exhaust gas passes through the at least one heat exchanger, and a second position, wherein the exhaust gas by-passes the at least one heat exchanger.

Any exhaust treatment element may be used within an exhaust treatment system according to this invention. Nonetheless, in certain embodiments the at least one exhaust treatment element is a catalyst, a filter or a NO_x trap. Furthermore, in exhaust systems in which multiple exhaust treatment elements are used each exhaust treatment element may be associated with a heat exchanger, or two or more exhaust treatment elements may be grouped and associated with a single heat exchanger. That is, the inherent flexibility within an exhaust system according to this invention allows for different exhaust treatment elements to be maintained at different temperatures depending on the position of the first valve relative to the second valve, and depending upon how the exhaust treatment elements are grouped and whether or not they are associated with multiple heat exchangers.

In addition to being able to increase or decrease the temperature of an exhaust treatment element to enable effective performance of the various reactions performed within each element, using an exhaust system according to the invention may also ensure that less heat leaves the exhaust system, thereby reducing the amount of energy wasted. This recycling of waste heat energy is especially useful when a vehicle passes from a period of high engine load to a period of low engine load thereby resulting in a dip in engine exhaust temperature. An exhaust system according to the invention may be used to ensure that some of the heat generated during the period of high engine load is used to preheat the gas upstream of an exhaust treatment element, resulting in a more energy efficient system.

The first and/or second valve may be controlled using means that when in use enables the switching of the first and/or second valve in order to optimise the activity of an exhaust treatment element, such as a catalyst, filter or NO_x trap. It follows that the control means may switch the first valve into the second position and/or the second valve into the second position at relatively high exhaust gas flow rates and/or during periods of relatively high engine-out exhaust gas temperature in order to raise the temperature of the catalyst and/or filter (when present in the exhaust system). Additionally, the control means may switch the first valve into the first position and/or the second valve into the first position at relatively high exhaust gas flow rates and/or during periods of relatively high engine-out exhaust gas temperature in order to lower the temperature of the NO_x trap (when present in the exhaust system) and more generally to avoid backpressure problems whatever the exhaust treatment element. The control means may comprise an electronic control unit (ECU).

An exhaust system according to the invention may comprise temperature sensing means for sensing the temperature of the engine-out exhaust gas and/or the exhaust gas exiting the at least one exhaust treatment element in order to control the position of the first and/or the second valve thereby to optimise heat transfer and control the exhaust treatment element inlet temperature as appropriate. Furthermore, an exhaust system according to the invention may comprise flow rate sensing means for sensing the flow rate of the engine-out exhaust gas and/or the exhaust gas exiting the at least one exhaust treatment element to avoid backpressure problems by switching out the heat exchanger during periods of relatively high engine load, e.g. acceleration.

In one embodiment of the invention the exhaust system comprises a catalyst, wherein the catalyst is an SCR catalyst. The SCR catalyst can be an ammonia-SCR catalyst or a hydrocarbon-SCR catalyst. In another embodiment of the invention the exhaust system comprises a catalyst, wherein the catalyst is a DOC. Alternatively, the exhaust system can comprise a filter, such as a DPF or a CSF.

In another embodiment of the invention a catalyst and a filter are grouped such that a catalyst for oxidising NO to NO₂ is followed by a DPF, i.e. the CRT-type configuration.

According to a further aspect, the invention provides a lean-burn internal combustion engine comprising an exhaust system as described above.

According to another aspect, the invention provides a method of treating an exhaust gas of a lean-burn internal combustion engine flowing in an exhaust gas aftertreatment system, which method comprising passing said exhaust gas through at least one heat exchanger both before and after passing said exhaust gas through at least one exhaust treatment element, whereby heat generated from an exotherm over the at least one exhaust treatment element is transferred from exhaust gas downstream of the at least one exhaust treatment element to exhaust gas upstream of the at least one exhaust treatment element and/or whereby heat in the exhaust gas upstream of the at least one exhaust treatment element is transferred by the at least one heat exchanger to gas downstream of the at least one exhaust treatment element, which method comprising one of the following steps: a) bypassing the heat exchanger with the engine-out exhaust gas; b) bypassing the at least one heat exchanger with the exhaust gas exiting the at least one exhaust treatment element; or c) both of steps a) and b).

In addition sensors may be used to sense the temperature of the engine-out exhaust gas, the temperature of the exhaust gas exiting the at least one exhaust treatment element, and/or the temperature of the at least one heat exchanger and the flow rate of the engine-out exhaust gas and/or the flow rate of the exhaust gas exiting the at least one exhaust treatment element in order to optimise exhaust gas temperature at the exhaust treatment element inlet and to avoid excessive backpressure problems respectively.

In order that the present invention may be more fully understood, embodiments thereof will be described with reference to the accompanying drawings, in which:

Figure 1 is a schematic of a first embodiment according to the invention with first and second valves both closed;

Figure 2 is a further schematic of the system of Figure 1 with first and second valves both open; and Figures 3a, 3b, 3c and 3d are schematics of a second embodiment wherein the valve positions of the first and second valves are independent of one another depending on the temperature of the exhaust gas at various points within the exhaust system.

Figures 1 and 2 depict a system 8 comprising an exhaust treatment element 10 and a heat exchanger 12 linked by conduits 14 through which exhaust gas of a lean- burn internal combustion engine may pass. The system is arranged such that, when in use, the exhaust gas passes through the heat exchanger 12 both before and after passing through the exhaust treatment element 10. Where exhaust treatment element 10 is an oxidation catalyst for oxidising NO to NO₂, a second exhaust treatment element 11 is a DPF, i.e. the exhaust system element is a CRT^®, as disclosed in EP 0 341 832.

A first valve 16 determines whether or not the engine-out exhaust gas passes through the heat exchanger 12 before passing through the exhaust treatment element. A second valve 18 determines whether or not the exhaust gas exiting the exhaust treatment element 10 passes through the heat exchanger before exiting to atmosphere. In Figure 1 both the first valve 16 and the second valve 18 are open, thereby ensuring that the exhaust gas passes through the heat exchanger 12 both before and after passing through the exhaust treatment element 10. In Figure 2 both the first valve 16 and the second valve 18 are closed, thereby ensuring that the exhaust gas bypasses the heat exchanger 12 both before and after passing through the exhaust treatment element 10.

The exhaust system shown in Figures 1 and 2 is particularly useful for treating exhaust gas from a bus. Typically, in use, a drive cycle is characterised by hard acceleration away from a bus stop to maximum speed. In order to make available maximum power for the hard acceleration phase of the drive cycle, the exhaust system valves switch the heat exchanger out of the system (valve configuration shown in Figure 1) and then switch the heat exchanger back into the system (valve configuration shown in Figure 2) when the engine has achieved the vehicle's cruising speed. Figures 3a-3d depict a system identical to that in Figures 1 and 2, except that Figures 3a-3d also include temperature sensors T1-T3 and that the position of the first and second valves is controlled independently in response to the temperature differentials between the three sensors.

In use the system 8 depicted in Figure 1 transfers heat generated from an exotherm over an exhaust treatment element 10, such as a catalyst or filter or NO_x trap, from the gas downstream of the exhaust treatment element 10 to the gas upstream of the exhaust treatment element 10 via the heat exchanger 12, thereby enabling the exhaust treatment element 10 to more readily reach or maintain the appropriate temperature needed for it to function efficiently. Figure 2 shows the system in a configuration that prevents the transfer of heat, since valves 16/18 located within the conduits 14 are closed, which enables the gas upstream or downstream of the exhaust treatment element 10 to bypass the heat exchanger 12 when transfer of heat is not required.

The system 8 depicted in Figures 3a-3d uses temperature sensors T1-T3 to determine the temperature of the gas upstream of the catalyst or filter, the temperature of the gas downstream of the exhaust treatment element and the temperature of the heat exchanger. The temperature information acquired is used by control means (not shown) to determine whether to open or close the valves 16/18. Additionally flow rate sensors (not shown) may be used to determine the flow rate of the gas at different points within the conduits 14 and the information acquired is used by control means (not shown) to determine whether to open or close the valves 16/18.

Multiple catalysts and/or filters and/or NO_x traps may be used. Additionally, the heat exchanger may be duplicated with exhaust treatment elements having similar optimal operating temperatures grouped together in association with each heat exchanger.

Claims

1. An exhaust system (8) for treating exhaust gas from a lean-burn internal combustion engine, which system (8) comprising at least one exhaust treatment element (10) and at least one heat exchanger (12) arranged so that the exhaust gas can pass through the at least one heat exchanger (12) both before and after passing through the at least one exhaust treatment element (10), whereby heat generated from an exotherm over the at least one exhaust treatment element (10) can be transferred by the at least one heat exchanger (12) from gas downstream of the at least one exhaust treatment element (10) to gas upstream of the at least one exhaust treatment element (10) and/or whereby heat in the exhaust gas upstream of the at least one exhaust treatment element (10) can be transferred by the at least one heat exchanger (12) to gas downstream of the at least one exhaust treatment element (10), wherein the exhaust system (8) comprises at least one of: a) a first valve (16) for switching the flow of engine-out exhaust gas between a first position, wherein the exhaust gas flow passes through the at least one heat exchanger (12) before passing through the at least one exhaust treatment element (10), and a second position, wherein the exhaust gas flow by-passes the at least one heat exchanger (12); and b) a second valve (18) for switching the flow of exhaust gas exiting the at least one exhaust treatment element (10) between a first position, wherein said exhaust gas passes through the at least one heat exchanger (12), and a second position, wherein the exhaust gas by-passes the at least one heat exchanger (12).

2. An exhaust system (8) according to claim 1, wherein the or each at least one exhaust treatment element (10) is a catalyst, a filter or a NO_x trap.

3. An exhaust system (8) according to claim 1 or 2, comprising temperature sensing means for sensing the temperature of the engine-out exhaust gas and/or for sensing the temperature of the exhaust gas exiting the at least one exhaust treatment element (10), thereby to control the position of one or both of the first and second valves (16, 18) in response to the detected temperature.

4. An exhaust system (8) according to any of claims 1 to 3, comprising flow rate sensing means for sensing the flow rate of the engine-out exhaust gas and/or for sensing the flow rate of the exhaust gas exiting the at least one exhaust treatment element (10), thereby to control the position of one or both of the first and second valves (16, 18) in response to the detected flow rate.

5. An exhaust system (8) according to any preceding claim, comprising means, when in use, for controlling the switching of the first and/or second valve (16, 18) in order to optimise the activity of the at least one exhaust treatment element (10).

6. An exhaust system (8) according to claim 5 when dependent on claim 2, said exhaust system (8) comprising a catalyst and/or filter, wherein the control means switches the first valve (16) into the second position and/or the second valve (18) into the second position at relatively high exhaust gas flow rates and/or during periods of relatively high engine-out exhaust gas temperature.

7. An exhaust system (8) according to claim 5 when dependent on claim 2, said exhaust system (8) comprising a NO_x trap, wherein the control means switches the first valve (16) into the first position and/or the second valve (18) into the first position at relatively high exhaust gas flow rates and/or during periods of relatively high engine-out exhaust gas temperature.

8. An exhaust system (8) according to claim 5, 6 or 7, wherein the control means comprises an electronic control unit.

9. An exhaust system (8) according to any preceding claim, wherein the at least one exhaust treatment element (10) comprises a selective catalytic reduction (SCR) catalyst.

10. An exhaust system (8) according to claim 9, wherein the SCR catalyst is an ammonia-SCR catalyst.

11. An exhaust system (8) according to claim 9, wherein the SCR catalyst is a hydrocarbon-SCR catalyst.

12. An exhaust system (8) according to any preceding claim, wherein the at least one exhaust treatment element (10) comprises a diesel oxidation catalyst.

13. An exhaust system (8) according to any preceding claim, wherein the at least one exhaust treatment element (10) comprises a diesel particulate filter.

14. An exhaust system (8) according to claim 13, comprising a catalyst for oxidising NO to NO₂ followed by a diesel particulate filter.

15. An exhaust system (8) according to any preceding claim, wherein the at least one exhaust treatment element (10) comprises a catalysed soot filter.

16. A lean-burn internal combustion engine comprising an exhaust system (8) according to any preceding claim.

17. A method of treating an exhaust gas of a lean-burn internal combustion engine flowing in an exhaust gas aftertreatment system (8), which method comprising passing said exhaust gas through at least one heat exchanger (12) both before and after passing said exhaust gas through at least one exhaust treatment element (10), whereby heat generated from an exotherm over the at least one exhaust treatment element (10) is transferred from exhaust gas downstream of the at least one exhaust treatment element (10) to exhaust gas upstream of the at least one exhaust treatment element (10) and/or whereby heat in the exhaust gas upstream of the at least one exhaust treatment element (10) is transferred by the at least one heat exchanger (12) to gas downstream of the at least one exhaust treatment element (10), which method comprising one of the following steps: a) bypassing the at least one heat exchanger (12) with the engine-out exhaust gas; b) bypassing the at least one heat exchanger (12) with the exhaust gas exiting the at least one exhaust treatment element (10); or c) both of steps a) and b).

18. A method according to claim 17, comprising the step of sensing the temperature of the engine-out exhaust gas, the temperature of the exhaust gas exiting the at least one exhaust treatment element (10) and/or the temperature of the at least one heat exchanger (12) and controlling the position of one or both of the first and second valves (16, 18) in response to the detected temperature.

19. A method according to claim 17 or 18, comprising the step of sensing the flow rate of the engine-out exhaust gas and/or the flow rate of the exhaust gas exiting the at least one exhaust treatment element (10) and controlling the position of one or both of the first and second valves (16, 18) in response to the detected flow rate.