JP2009541655A - Diesel engine exhaust pipe and desulfurization method - Google Patents

Abstract

  A method is provided for desulfurizing a NOx trap (6) in an exhaust line of a diesel engine, the method being aimed at removing at least a portion of the sulfur compounds stuck in the NOx trap during operation of the engine:- Generating reformed oil from engine fuel via a reforming device; and an injection point located downstream of the engine and upstream of the NOx trap until the temperature in the NOx trap reaches a predetermined desulfurization temperature. Preheating the NOx trap by injecting reformate into the exhaust gas stream (1) in (5), and then exhausting from a branch point (2) on the exhaust line upstream of the injection point. Performing the desulfurization by diverting the gas flow and continuing the injection of the reformed oil into the NOx trap. Never concentration of the mixture supplied to the diesel engine is affected by facilities.

Description

  The present invention relates to an exhaust pipe of a diesel engine and a desulfurization method of a NOx trap of the exhaust pipe, and the desulfurization method removes at least a part of a sulfur compound fixed in the NOx trap during operation of the engine. With the goal.

Many exhaust systems for automotive diesel engines are used in the presence of excess oxygen in the exhaust gas, i.e., under a concentration condition of less than 1, to reduce the level of nitric oxide in the exhaust gas emitted from the vehicle. A catalytic device generally called a NOx trap having a function of capturing nitric oxide NOx can be used. However, sulfur compounds, particularly SOx, generated from the fuel and lubricating oil are present in the exhaust gas, which is preferably absorbed on the catalyst region of the NOx trap and the region is blocked, causing the NOx trap to deposit NOx. In order to resume this activity, periodic regeneration of the NOx trap catalyst, ie, SOx emission, is required.
Such regeneration is achieved by raising the temperature of the NOx trap to a temperature above the minimum desulfurization temperature of 400-900, often above 550 degrees, and injecting reducing gas into the NOx trap to a concentration greater than 1. Consists of forming conditions there.

  WO 99/00588 discloses regeneration consisting of raising the temperature of the NOx trap by operating the engine with a mixture having a higher concentration than during normal operation. This method has the disadvantage of increasing fuel consumption and adversely affecting engine operation and vehicle driving comfort, which can result in dilution of engine oil. EP 1055806 and EP 1106798 also describe regeneration systems based on fuel post-injection and have the same drawbacks. US 2005/0000210 describes a system based on injecting diesel fuel or reformate after raising the temperature of the NOx trap by modifying the concentration of the mixture supplied to the engine. FR 2 838 770 and DE 19939807 describe a system based on injection of hydrogen upstream of a NOx trap and thus a system requiring a specific hydrogen storage vessel.

  It is an object of the present invention to provide a desulfurization process that does not inhibit the engine and minimizes excessive fuel consumption.

This object is achieved by a method of desulfurization of the NOx trap, which aims to remove at least a part of the sulfur compounds fixed in the NOx trap of the exhaust line of the engine during operation of the diesel engine. Is
-Generating reformate from engine fuel via a reforming device;
-Preheating the NOx trap by injecting the reformate into the exhaust gas stream at injection points located downstream of the engine and upstream of the NOx trap until the temperature in the NOx trap reaches a predetermined desulfurization temperature; And then-branching the exhaust gas flow from a branch point of the exhaust pipe located upstream of the injection point, and continuing the injection of the reformed oil into the NOx trap to achieve the desulfurization. In this method, the concentration of the mixture supplied to the diesel engine by the execution of the step is not affected.

Therefore, the implementation of the method according to the invention comprises an exhaust line comprising a detour from a branch point upstream of the NOx trap and means, such as a three-way valve, for branching the exhaust gas through this detour, and In other words, there is a need for a reformer that is a device capable of converting hydrocarbons, such as fuel for diesel engines, into a mixture of reducing gas called reformed oil mainly composed of CO, H2, and N2. To do.
The reformed oil can be produced by a technique selected from known techniques, partial oxidation reforming (POX, partial oxidation), steam reforming, and automatic thermal reforming (ATR).

The duration of the branching step can be predetermined, and in particular this duration can be 10 seconds to 20 minutes.
The duration of the preheating step can also be predetermined.

Further, the duration of at least one of the preheating step and the branching step can be determined by a temperature measurement value representing the temperature of the catalyst region of the NOx trap.
At the end of the branching step, exhaust gas can be reintroduced into the NOx trap and the production of reformate can be stopped.

According to another embodiment, while the generation of the reformed oil is continued, the preheating step and the branching step can be alternately performed periodically.
In the method according to the present invention, it is possible to regenerate a particulate filter generally called PF installed in the exhaust pipe downstream of the NOx trap. In this embodiment, the flow rate of the reformed oil The exhaust gas branched during the step of desulfurizing the NOx trap is adjusted so that the temperature downstream of the filter reaches a predetermined regeneration temperature, at a re-injection point located downstream of the NOx trap and upstream of the particulate filter. Re-introduce into the exhaust line.

This method is for regeneration of an exhaust line comprising two NOx traps mounted in parallel, a particulate filter mounted downstream of the NOx trap, and a valve system that distributes the exhaust gas flow to the two NOx traps. Can be applied by using a valve system that distributes the reformed oil from the reformer to each of two injection points arranged in parallel upstream of the first NOx trap and the second NOx trap. it can. Each of the two NOx traps can alternate between a preheating stage and a desulfurization stage during regeneration of the particulate filter.
By switching the valve system instantaneously in such an exhaust line, the other NOx trap can be placed in the desulfurization stage while exhausting NOx from one NOx trap.

  Other specific features and advantages of the present invention will become apparent to those skilled in the art from the following description of three embodiments and the accompanying drawings.

It is the schematic of the 1st Example of an exhaust pipe line. It is the schematic of the 2nd Example of an exhaust pipe line. It is the schematic of the 3rd Example of an exhaust pipe line.

FIG. 1 shows an exhaust line provided with a NOx trap 6 and a reformer 4, and the flow of reformed oil indicated by a dotted arrow is injected into the exhaust line at an injection point 5. The flow of the exhaust gas 1 discharged from the engine is indicated by a solid line arrow on the left side of FIG. These exhaust gases can be branched toward the bypass 2 by the valve system 3.
During the preheating step, reformate is injected into the exhaust gas stream that passes through the NOx trap 6. Since these exhaust gases contain oxygen, the H2 and CO components of the reformed oil that come into contact with the catalyst (noble metal) contained in the NOx trap are oxidized by oxygen. This reaction is highly exothermic and the heat released in this way can heat the NOx trap to the temperature required for desulfurization. To achieve the desired temperature range, the reformate flow rate is controlled by a control loop.

  When the temperature reaches a desulfurization temperature of 400 to 900 degrees, for example, about 650 degrees while the injection of the reformed oil into the NOx trap 6 is continued, the valve 3 is activated and the exhaust gas 1 is transferred to the bypass pipe 2. Branch toward. Under such conditions, the reformed oil stream having a concentration equal to the concentration of the air / fuel mixture supplied to the reformer 4 is passed through the NOx trap 6. For example, in the case of diesel POx, the optimum concentration can be 2.9. Under such conditions, sulfur present in the NOx trap is released in the form of compounds such as H2S, COS, and SO2. The duration of this desulfurization step can generally be from 10 seconds to several minutes, for example 20 minutes.

According to a first variant embodiment, after the desulfurization step, the reforming device is stopped and the valve system 3 returns to the initial position of the valve system such that the exhaust gas once again passes through the NOx trap.
According to the second modified embodiment, the reformed oil is continuously injected into the NOx trap while a plurality of steps are continuously performed. During the reformate injection, the valve system operates periodically between the two positions so that the exhaust gas flow through the NOx trap alternates between the low and high concentration stages. Is done. During the low-concentration phase, the exhaust gas enters the NOx trap, the reformed oil reacts with these exhaust gases on the catalyst surface (platinum or palladium) of the NOx trap, and the temperature of the NOx trap is maintained by an exothermic oxidation reaction. Is done. During the high-concentration phase, the exhaust gas is rerouted through the bypass tube 2 and only the reformate passes through the NOx trap so that sulfur deposited from the NOx trap is decontaminated. In general, the duration of the low concentration stage can be from 20 seconds to several minutes, and the duration of the high concentration stage can be from 10 seconds to several minutes. The duration of each stage is tailored to the NOx trap catalyst formulation and its thermal behavior. One of the main advantages of cyclically alternating high and low concentrations alternately when compared to the method of continuously bypassing exhaust gas is that temperature control in the NOx trap during desulfurization can be improved. It is. In this way, premature aging of the NOx trap can be avoided.

NOx trap desulfurization as described above can be advantageously combined with regeneration of the particulate filter. FIG. 2 shows an exhaust line with the same components 1 to 6 as in FIG. A particulate filter 8 with a catalyst is attached downstream of the NOx trap 6. The exhaust gas flowing into the bypass pipe 2 is supplied to the reinjection point 10 downstream of the NOx trap 6 and upstream of the particulate filter 8 via the branch pipe 7. A temperature sensor 9 provided slightly downstream of the particulate filter 8 is used to measure the temperature in the particulate filter 8. This temperature value is sent to the computer 11 of the reformer 4. The computer 11 also controls the valve system 3.
Regeneration of the particulate filter 8 requires a temperature of about 600 degrees to initiate burning of soot trapped by the filter. In the first step, the valve system 3 for rerouting the exhaust gas into the bypass pipe 2 is not activated, so that the exhaust gas flows into the NOx trap 6. The reformed oil is injected at the injection point 5 and this reformed oil comes into contact with the catalyst coating of the NOx trap 6 and is oxidized by oxygen present in the exhaust gas. This exothermic oxidation of the reformed oil enables both heating of the NOx trap 6 for desulfurizing the NOx trap 6 and heating of the particulate filter 8 by exhaust gas downstream of the NOx trap 6. In order to bring the temperature of the sensor 9 to the regeneration temperature, the flow rate of the reforming oil is adjusted to reach approximately 600 degrees, which is generally sufficient to allow the soot accumulated in the particulate filter 8 to be oxidized. The When the temperature reaches this regeneration temperature, the NOx trap has also reached a temperature sufficient for desulfurization.

In the second step, when the temperature reaches the regeneration temperature of the particulate filter 8, the valve system 3 is activated, the exhaust gas is rerouted from the NOx trap, and flows into the branch pipe 7. The reforming oil injection is maintained so that the NOx trap 6 is exposed to a flow having a high concentration of reducing components and the NOx trap is decontaminated mainly by releasing H2S and COS. The duration of this step can be longer than 10 minutes.
During desulfurization, most or most of the reformed oil does not react in the NOx trap 6, exits the NOx trap, mixes with the exhaust gas coming from the branch pipe 7, flows into the particulate filter 8, and enters the particulate filter. Reacts with oxygen in the exhaust gas when in contact with the 8 catalyst coating. This exothermic reaction makes it possible to maintain the particulate filter at its regeneration temperature during the desulfurization of the NOx trap 6.

In order to completely regenerate the particulate filter 8, the injection is generally performed for about 20 minutes. If the duration required for desulfurization is much shorter than the duration required for regeneration of the particulate filter, detouring can be stopped after the end of desulfurization. At this time, regeneration of the particulate filter 8 is continued by maintaining the injection of the modified oil. This modified oil
-On the catalyst surface of the NOx trap if detouring is stopped, or-on the direct catalyst surface of the particulate filter if detouring continues,
Oxidized by the exhaust gas, as a result, the particulate filter 8 is maintained at the regeneration temperature. The advantage gained by the second option (diversion is maintained) is that the over-consumption of fuel supplied to the reformer is minimized. Specifically, it is possible to immediately use the energy released by the exothermic oxidation reaction of the reformed oil by simultaneously oxidizing the reformed oil on the particulate filter and regenerating the soot of the particulate filter. Therefore, the heat loss that occurs between the NOx trap and the particulate filter can be minimized when processing is performed according to the first option. Such a process can optimize the fuel-related disadvantages caused by supplying fuel to the reformer.
During desulfurization, the odor of H2S may cause an unpleasant odor in the immediate environment of an automobile. However, since H2S and COS are oxidized by the exhaust gas in the particulate filter 8, this problem is solved.

In general, the regeneration of the particulate filter can be performed every 500 km of the automobile traveled, and the NOx trap is desulfurized at the same time.
The method according to the invention can be applied to an exhaust line comprising two NOx traps mounted in parallel. Such an exhaust line is shown schematically in FIG. In this exhaust pipe, a valve system 12 having a function of distributing the reformed oil coming out of the reformer 4 to the two injection points 5 and 5b is used to improve the upstream of the two NOx traps 6 and 6b. Quality oil can be injected. The valve system 3 can be switched between the following positions.
-Position 1: All exhaust gas flows into NOx trap 6-Position 2: All exhaust gas flows into NOx trap 6b-Position 3: Exhaust gas is equally divided between NOx traps 6 and 6b The

Reproduction is performed as follows.
In the first step, the valve system 3 is in position 1 where all the exhaust gas flows into the NOx trap 6. The reformed oil is injected at the injection point 5 and is oxidized by oxygen contained in the exhaust gas on the catalyst surface of the NOx trap 6. By this exothermic reaction, both the NOx trap 6 and the particulate filter 8 are heated to the desulfurization temperature and the regeneration temperature, respectively. In the sensor 9 downstream of the particulate filter 8, the flow rate of the reforming oil is adjusted so that the temperature reaches a temperature sufficient to oxidize the soot in the particulate filter 8, generally about 600 degrees. When the temperature reaches this temperature, the valve system 3 switches to position 2, causing the exhaust gas to be rerouted into the NOx trap 6b, from which the exhaust gas flows into the particulate filter 8 via the branch pipe 7b.

Even after switching the valve system 3, the injection of the reformed oil into the NOx trap 6 is maintained, and at the desulfurization temperature, mainly H2S and COS are released, so that the sulfur accumulated from the NOx trap 6 is decontaminated. Is done. In order to ensure complete desulfurization of the NOx trap 6, the system 3 is maintained in this position for more than about 10 minutes.
During this stage of desulfurizing the NOx trap 6, NOx present in the exhaust gas accumulates in the NOx 6b. In order to prevent the NOx trap 6b from being saturated with NOx, NOx can be discharged from the NOx trap 6b. That is, the valve system 3 is instantaneously switched to position 1 and the reformed oil is injected into the NOx trap 6b.

During desulfurization, the majority of the reformate exits the NOx trap and is oxidized on the catalyst surface of the particulate filter 8 so that the particulate filter is maintained at a temperature sufficient for soot oxidation throughout the regeneration period. When regeneration of the particulate filter, which can take about 20 minutes, is completed, the computer 11 which is the computer of the reformer 4 stops the injection, the valve system is switched to position 3, and the exhaust gas flow is distributed to the two NOx traps. And NOx is processed.
It is possible to desulfurize each of the two NOx traps in sequence by changing the position of the valve systems 12 and 3 during the regeneration operation of the particulate filter, which typically occurs every 500 km of automobile travel. It is possible to alternately desulfurize only one of the traps and desulfurize the other particulate filter during the next desulfurization operation.

Claims (14)

A method of desulfurizing a NOx trap (6) in an exhaust pipe of a diesel engine, the objective is to remove at least a portion of sulfur compounds fixed in the NOx trap during operation of the engine,
-Producing reformed oil from the engine fuel via a reforming device (4);
-By injecting the reformate into the exhaust gas stream at an injection point (5) located downstream of the engine and upstream of the NOx trap until the temperature in the NOx trap reaches a predetermined desulfurization temperature; Preheating the NOx trap, and then-branching the exhaust gas flow from the branch point of the exhaust line located upstream of the injection point, and continuing the injection of the reformed oil into the NOx trap to prevent the desulfurization. And the concentration of the mixture supplied to the diesel engine is not affected by performing the steps of the method.   The method according to claim 1, characterized in that the duration of the branching step is predetermined, in particular the duration of the branching step is between 10 seconds and 20 minutes.   3. A method according to claim 1 or 2, characterized in that the duration of the preheating step is predetermined.   The method of claim 1, wherein the duration of at least one of the preheating step and the branching step is determined by a temperature measurement.   5. The method according to claim 1, wherein exhaust gas is reintroduced into the NOx trap (6) at the end of the branching step and generation of reformed oil is stopped. 6. the method of.   5. The method according to claim 1, wherein the plurality of preheating steps and the branching steps are alternately performed periodically, and the generation of the reformed oil is continued during the cycle. The method according to item.   The method according to any one of claims 1 to 6, wherein the reformed oil is produced by a technique selected from partial oxidation reforming, steam reforming and automatic thermal reforming.   A method for simultaneously regenerating a particulate filter installed in the exhaust line downstream of the NOx trap (6), wherein the flow rate of the reformed oil during the step of preheating the NOx trap (6) Between the step of bringing the temperature downstream of the particulate filter to a predetermined regeneration temperature and the step of desulfurizing the NOx trap, the repositioning located downstream of the NOx trap and upstream of the particulate filter. 8. A method according to any one of the preceding claims, characterized in that exhaust gas is reintroduced into the exhaust line at the injection point (10).   Two NOx traps (6, 6b) mounted in parallel, a particulate filter (8) mounted downstream of the two NOx traps, and a valve system for distributing an exhaust gas stream to the two NOx traps ( A method of using the method of claim 8 to regenerate the exhaust line comprising 3), wherein the reformed oil exiting from the reforming device (4) is fed into the first NOx trap and the second, respectively. Method of use, characterized by the use of a valve system (12) for distributing to two injection points (5, 5b) arranged in parallel upstream of the NOx trap.   Use according to claim 9, characterized in that during the regeneration of the particulate filter (8), each of the two NOx traps (6, 6b) is alternately subjected to a preheating stage and a desulfurization stage.   11. The desulfurization stage of the other NOx trap is performed while exhausting NOx from one NOx trap by momentarily switching the valve system (3, 12). how to use.   An exhaust pipe of a diesel engine including a NOx trap (6) and a particulate filter (8) disposed downstream of the NOx trap, the valve system (3) and the branch pipe (7) upstream of the NOx trap ), The exhaust gas can bypass the NOx trap and be reinjected upstream (10) of the particulate filter, the presence of the reforming device (4), An exhaust line for a diesel engine, characterized in that there is a computer (11) capable of controlling the method according to claim 8.   An exhaust line of a diesel engine for implementing the method according to any one of claims 9 to 11, comprising two NOx traps (6, 6b) mounted in parallel, of the two NOx traps. A particulate filter (8) mounted downstream, and a valve system (3) for distributing the exhaust gas stream to two NOx traps, the presence of the reforming device (4), and the reforming device ( 4) a valve system (12) for distributing the reformed oil from 4) to two injection points (5, 5b) arranged in parallel upstream of the first NOx trap and the second NOx trap, respectively. Exhaust line characterized in that it exists and that there is a computer (11) for carrying out said method. A method for regenerating a particulate filter (8) in an exhaust line of a diesel engine according to claim 12,
-Generating reformate from the engine fuel via the reforming device (4) and injecting the reformate into the exhaust line downstream of the valve system (3);
-Branching the exhaust gas flow into the branch pipe (7) and reinjecting the gas into the particulate filter (8);
Oxidising the reformate with exhaust gas on the active surface of the particulate filter.

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