JP2012087662A - Method and device for exhaust emission control - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method and device for exhaust emission control maintaining a high NOx reduction rate without deteriorating the performance even under the operation condition of a low exhaust gas temperature.SOLUTION: When an exhaust air-fuel ratio is lean, NOx in exhaust gas 3 is temporarily stored in a state of nitrate by a NOx storage reduction catalyst 5 in the middle of an exhaust pipe 4, and fuel 8 is added to the inlet side of the NOx storage reduction catalyst 5 to enrich the exhaust air-fuel ratio and allow NOx to be disassembled and discharged. The discharged NOx is reduced and controlled in emission by the reduction component generated from the added fuel to regenerate the NOx storage reduction catalyst 5. When doing this, by igniting the fuel 8 added to the inlet side of the NOx storage reduction catalyst 5, combustion is performed in a state that the exhaust air-fuel ratio is rich, atmospheric temperature at the combustion position is raised to a fuel reforming temperature or higher to generate Hand CO, and the NOx storage reduction catalyst 5 is regenerated by using the Hand CO as a reducing agent.

Description

  The present invention relates to an exhaust purification method and apparatus.

Conventionally, in a diesel engine, when the exhaust air-fuel ratio is lean, NOx in the exhaust gas is oxidized and temporarily stored in the form of nitrate, and when the O ₂ concentration in the exhaust gas decreases, unburned HC and CO For example, a NOx storage reduction catalyst having a property of decomposing and releasing NOx by the intervention of the above and the like to reduce and purify is installed in the middle of the exhaust pipe, and the NOx emission concentration is reduced by this NOx storage reduction catalyst.

However, in the NOx occlusion reduction catalyst, if the NOx occlusion amount increases and reaches the saturation amount, no more NOx can be occluded, and therefore the NOx occlusion reduction catalyst before the NOx occlusion amount reaches the saturation amount. It is necessary to decompose and release NOx by reducing the O ₂ concentration of the exhaust gas flowing into the exhaust gas with a reducing agent such as HC.

For example, when used in a gasoline engine, the operating air-fuel ratio of the engine is reduced (the engine is operated at a rich air-fuel ratio), thereby reducing the O ₂ concentration in the exhaust gas and unburned HC in the exhaust gas. It is possible to promote the decomposition and release of NOx by increasing reducing components such as CO and CO. However, when the NOx storage reduction catalyst is used as an exhaust gas purification device for a diesel engine, it is difficult to operate the engine at a rich air-fuel ratio. is there.

For this reason, when the NOx storage reduction catalyst is used as an exhaust gas purification device for a diesel engine, by adding fuel to the exhaust gas upstream of the NOx storage reduction catalyst, the HC generated from the added fuel is reduced. It is necessary to reduce the O ₂ concentration in the exhaust gas by reacting with O ₂ on the NOx storage reduction catalyst.

  As prior art document information related to this type of exhaust purification device, there is the following Patent Document 1 and the like.

Japanese Patent Laid-Open No. 2007-9718

However, in the system in which fuel is added upstream of the NOx storage reduction catalyst as described above, a part of the HC generated from the added fuel reacts with the O ₂ in the exhaust gas on the surface of the NOx storage reduction catalyst (combustion). However, since the NOx decomposition and release starts after the O ₂ concentration in the atmosphere around the NOx occlusion reduction catalyst becomes almost zero, the operation condition under low exhaust temperature (for example, in a city with a lot of traffic congestion) In slow running operation, etc.), the catalytic activity of the NOx storage reduction catalyst is low, so NOx cannot be efficiently decomposed and released from the NOx storage reduction catalyst, and the regeneration of the NOx storage reduction catalyst does not proceed efficiently. There was a problem that the recovery rate of the NOx storage site occupying in the volume of the storage space was reduced and the storage capacity was lowered.

  The present invention has been made in view of the above-described circumstances, and an object of the present invention is to provide an exhaust purification method and apparatus that can maintain a high NOx reduction rate without degrading performance even under operating conditions at low exhaust temperatures.

According to the present invention, when the exhaust air-fuel ratio is lean, NOx in the exhaust gas is temporarily occluded in the form of nitrate by the NOx occlusion reduction catalyst in the middle of the exhaust pipe, and fuel is added to the inlet side of the NOx occlusion reduction catalyst. An exhaust purification method for decomposing and releasing NOx by reducing the exhaust air-fuel ratio and reducing and purifying the released NOx with a reducing component generated from the added fuel to regenerate the NOx storage reduction catalyst, the NOx storage reduction catalyst the exhaust air-fuel ratio to ignite the additional fuel at the inlet side performs combustion in a rich state, the ambient temperature at the combustion positions to generate H ₂ and CO is raised above the fuel reforming temperature, these H ₂ and CO The NOx occlusion reduction catalyst is regenerated using NO as a reducing agent.

Thus, when the added fuel is ignited on the inlet side of the NOx occlusion reduction catalyst and combustion is performed with the exhaust air / fuel ratio being rich, the ambient temperature at the combustion position becomes the fuel reforming temperature (about 1000 ° C.). ) when it becomes equal to or greater than, H ₂ and CO are produced by decomposing the unburned fuel remaining in the atmosphere, these H ₂ and CO is to be supplied to the NOx storage reduction catalyst as a reducing agent.

At this time, the fuel consumed by combustion contributes to lowering the O ₂ concentration by consuming O ₂ in the exhaust gas, so that H ₂ and CO are supplied to the NOx occlusion reduction catalyst as reducing agents. Immediately after that, the NOx decomposition and release starts immediately after the O ₂ concentration in the atmosphere becomes zero, and the H ₂ and CO that are highly reactive on the surface of the NO x storage reduction catalyst as they are are used as compared with the case where HC is used as the reducing agent. NOx is efficiently reduced to N ₂ from a lower temperature.

  In addition, the present invention provides a fuel addition device that is equipped with a NOx storage reduction catalyst in the middle of an exhaust pipe, and that adds fuel to the inlet side of the NOx storage reduction catalyst as a reducing agent for regenerating the NOx storage reduction catalyst. An exhaust emission control device, an ignition device for igniting the added fuel from the fuel addition device, a temperature sensor for measuring an ambient temperature at a combustion position of the fuel ignited by the ignition device, and A detection signal is input to control the fuel addition amount of the fuel addition device so that the exhaust air-fuel ratio becomes rich in a temperature range above the fuel reforming temperature, and in the temperature range below the fuel reforming temperature, the fuel addition And a control device for controlling the fuel addition amount of the device to be relatively suppressed.

In this way, when the atmospheric temperature measured by the temperature sensor is equal to or higher than the fuel reforming temperature, the fuel addition amount of the fuel addition device is controlled by the control device so that the exhaust air-fuel ratio becomes rich. As a result, combustion with the exhaust air-fuel ratio in a rich state is surely performed under conditions above the fuel reforming temperature, and H ₂ and CO are efficiently generated.

On the other hand, when the atmospheric temperature measured by the temperature sensor is lower than the fuel reforming temperature, the amount of fuel added to the fuel addition device is relatively suppressed by the control device, and thus does not contribute to the generation of H ₂ and CO. In addition to reducing unnecessary fuel addition, it is possible to efficiently raise the ambient temperature to the fuel reforming temperature with a small amount of fuel addition, and combustion is performed by uniformly adding a large amount of fuel addition from the start of fuel addition. It becomes possible to increase the production amount of H ₂ and CO more than the case.

In other words, if a large amount of fuel is added from the beginning of fuel addition, the fuel becomes excessive in the initial stage of combustion, and the temperature drop due to the latent heat of vaporization affects the temperature of the fuel efficiently. Although it is difficult to increase the temperature, the amount of fuel added is suppressed until the fuel reforming temperature is reached, and if the fuel is added without excess or deficiency, the ambient temperature can be efficiently raised to the fuel reforming temperature even with a small amount of fuel added. As a result of the fact that the ambient temperature can be raised to the fuel reforming temperature earlier, the fuel reforming reaction takes place earlier, resulting in an increase in the amount of H ₂ and CO produced.

  In the present invention, a particulate filter may be interposed between the fuel addition device and the NOx occlusion reduction catalyst, so that the heat generated by the combustion of the fuel is absorbed by the particulate filter. Thus, it is possible to suppress the increase in the catalyst bed temperature in the NOx storage reduction catalyst, and the increase in the catalyst bed temperature of the NOx storage reduction catalyst prevents the NOx reduction rate from being reduced due to the accelerated release of the stored NOx. The

  That is, the saturated occlusion amount of the NOx occlusion reduction catalyst increases as the catalyst bed temperature rises until it reaches a predetermined peak temperature, but conversely decreases when the catalyst bed temperature rises beyond the peak temperature. NOx occlusion reduction because the NOx occlusion reduction may increase and the NOx reduction rate will decrease if the catalyst bed temperature of the NOx occlusion reduction catalyst increases excessively beyond the peak temperature. It is important to consider that the catalyst bed temperature of the catalyst does not become too high due to the heat generated by the combustion of the fuel.

  Further, if the particulate filter is arranged immediately after the fuel addition device in this way, it is also possible to regenerate the particulate filter by adding and burning the fuel from the fuel addition device, Moreover, it is possible to quickly increase the temperature during the regeneration.

  Furthermore, a heat storage material having a ventilation structure may be interposed between the particulate filter and the NOx occlusion reduction catalyst, and in this way, further heat absorption is performed by the heat storage material. An increase in the catalyst bed temperature in the storage reduction catalyst can be further suppressed.

Further, in the present invention, it is preferable to provide a selective reduction catalyst that can selectively react NOx with NH ₃ even in the presence of oxygen at the subsequent stage of the NOx occlusion reduction catalyst. even H ₂ and the NOx is that NH ₃ reacts generated Te, it is possible to reduce and purify NOx at a later stage of the selective catalytic reduction catalyst of this NH ₃ as a further reducing agent.

That is, when H ₂ is used as a reducing agent, a part of NH ₃ is produced by the reaction of H ₂ and NO x in the NO x storage reduction catalyst under the condition that the exhaust air-fuel ratio is in a deep rich state. If this NH ₃ is used as a reducing agent in the selective reduction catalyst in the subsequent stage, it becomes possible to further reduce NOx and to prevent leakage ammonia.

  According to the exhaust purification method and apparatus of the present invention described above, various excellent effects as described below can be obtained.

(I) According to the first and second aspects of the present invention, the added fuel is ignited on the inlet side of the NOx occlusion reduction catalyst, combustion is performed in a rich exhaust air-fuel ratio, and the atmosphere at the combustion position The temperature is raised above the fuel reforming temperature to produce H ₂ and CO, and these H ₂ and CO are used as a reducing agent and guided to the NOx storage reduction catalyst, so that NOx is reduced from a lower temperature than when HC is used as the reducing agent. Since the reduction treatment can be efficiently performed to N ₂ , a high NOx reduction rate can be maintained without causing performance degradation even under operating conditions with a low exhaust temperature.

  (II) According to the invention described in claim 3 of the present invention, the heat generated by the combustion of the fuel can be absorbed by the particulate filter and the increase in the catalyst bed temperature in the NOx storage reduction catalyst can be suppressed. The increase in the catalyst bed temperature of the NOx occlusion reduction catalyst can prevent the situation where the NOx occlusion is released and the NOx reduction rate decreases, and the particulate filter is regenerated by the heat generated by the combustion of the fuel. It is also possible to quickly increase the temperature during the regeneration.

  (III) According to the invention described in claim 4 of the present invention, it is possible to further suppress an increase in the catalyst bed temperature in the NOx storage reduction catalyst by causing the heat storage material to absorb further heat. A reduction in the NOx reduction rate due to an increase in the catalyst bed temperature of the NOx storage reduction catalyst can be prevented more reliably.

(IV) According to the invention described in claim 5 of the present invention, even if NH ₃ is produced by the reaction of H ₂ and NO x in the NO x storage reduction catalyst, this NH ₃ is used as a further reducing agent. Since NOx can be reduced and purified by the selective reduction catalyst in the latter stage, it is possible to further reduce NOx and prevent NH ₃ from being discharged outside the vehicle by consuming NH ₃ as a reducing agent. be able to.

It is the schematic which shows an example of the form which implements this invention. It is a graph which shows the conventional fuel addition pattern. It is a graph which shows the change of sensor temperature. It is a graph which shows the fuel addition pattern of this embodiment. Is a graph showing changes in concentration of H _2. It is the schematic which shows another form example of this invention. It is the schematic which shows another example of a form of this invention.

  Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an example of an embodiment for carrying out the present invention. In this embodiment, exhaust air is exhausted in the middle of an exhaust pipe 4 through which exhaust gas 3 discharged from a diesel engine 1 through an exhaust manifold 2 flows. When the fuel ratio is lean, the NOx in the exhaust gas 3 is oxidized and temporarily stored in the form of nitrate, and when the O ₂ concentration in the exhaust gas 3 decreases, the NOx is decomposed and released by the intervention of a reducing agent to reduce and purify. The NOx occlusion reduction catalyst 5 is mounted on the catalyst case 6.

  In addition, a fuel addition nozzle 10 that guides and injects fuel 8 (light oil) in the light oil tank 7 through the light oil pump 9 is provided at the inlet portion of the catalyst case 6, and these light oil tank 7, light oil pump 9 A fuel addition device 11 is configured by the fuel addition nozzle 10.

  Further, in the vicinity of the injection position of the fuel 8 by the fuel addition nozzle 10 of the fuel addition device 11, an igniter 12 (ignition device) for igniting the fuel 8 and the ambient temperature at the combustion position of the fuel 8 ignited by the igniter 12 And a detection signal 13a from the temperature sensor 13 is input to the control device 14.

In the control device 14, a temperature equal to or higher than the fuel reforming temperature (about 1000 ° C.) at which the fuel reforming reaction from the fuel 8 to H ₂ and CO can occur based on the detection signal 13 a from the temperature sensor 13. The fuel addition amount of the fuel addition device 11 is controlled by the control signal 11a so that the exhaust air-fuel ratio becomes rich in the region, and the fuel addition amount of the fuel addition device 11 is relative in the temperature region below the fuel reforming temperature. Therefore, the control signal 11a is controlled so as to suppress it.

  In the example shown in FIG. 1, a case where a catalyst-carrying particulate filter 15 carrying an oxidation catalyst is interposed between the fuel addition device 11 and the NOx occlusion reduction catalyst 5 is illustrated. The particulate filter 15 has a porous honeycomb structure made of ceramic such as cordierite so as to collect particulate matter (particulate matter) in the exhaust gas 3.

Thus, in this case, when the exhaust air-fuel ratio is lean, the NOx occlusion reduction catalyst 5 temporarily stores NOx in the exhaust gas 3 in the form of nitrate, so that the NOx occlusion reduction catalyst 5 enters. It is necessary to regenerate the NOx occlusion reduction catalyst 5 by adding the fuel 8 to the exhaust side so that the exhaust air-fuel ratio becomes rich and the NOx is decomposed and released, and the released NOx is reduced and purified by the reducing components generated from the fuel 8. However, at this time, if the fuel 8 is ignited by the igniter 12 on the inlet side of the NOx storage reduction catalyst 5 and combustion is performed with the exhaust air-fuel ratio being rich, the ambient temperature at the combustion position becomes the fuel reforming. when a temperature (about 1000 ° C.) or higher, the fuel 8 unburned remaining in the atmosphere is produced decomposition to H ₂ and CO, these H ₂ and CO is the NOx storage reduction catalyst 5 as a reducing agent Serving It is is will be.

At this time, the fuel consumed by combustion contributes to lowering the O ₂ concentration by consuming O ₂ in the exhaust gas 3, so that H ₂ and CO are used as a reducing agent in the NOx storage reduction catalyst 5. Immediately after being supplied, the O ₂ concentration in the atmosphere becomes zero and NOx decomposition and release is started immediately, and HC is used as a reducing agent by H ₂ and CO having high reactivity on the surface of the NO x storage reduction catalyst 5 as they are. Thus, NOx is efficiently reduced to N ₂ at a temperature lower than that of the case.

In the example shown in FIG. 1, a particulate filter 15 carrying an oxidation catalyst is interposed between the fuel addition device 11 and the NOx occlusion reduction catalyst 5, but H ₂ and CO are generated. At this stage, O ₂ in the exhaust gas 3 is consumed and is in an oxygen deficient state, so that H ₂ and CO reach the NOx occlusion reduction catalyst 5 without being oxidized by the particulate filter 15. become.

In particular, in the exhaust purification apparatus of the present embodiment, when the atmospheric temperature measured by the temperature sensor 13 is equal to or higher than the fuel reforming temperature, the fuel addition apparatus 11 is set so that the exhaust air-fuel ratio becomes rich by the control apparatus 14. Therefore, the combustion with the exhaust air-fuel ratio in a rich state is reliably performed under the condition above the fuel reforming temperature, and H ₂ and CO are efficiently generated.

On the other hand, when the atmospheric temperature measured by the temperature sensor 13 is lower than the fuel reforming temperature, the amount of fuel added to the fuel adding device 11 is relatively suppressed by the control device 14, and thus H ₂ and CO are generated. The amount of useless fuel 8 that does not contribute to fuel consumption is reduced, and the ambient temperature can be efficiently raised to the fuel reforming temperature with a small amount of fuel addition. It is possible to increase the amount of H ₂ and CO produced as compared with the case where combustion is performed with addition of.

  That is, as shown in FIG. 2, if the fuel addition amount is uniformly increased from the start stage of fuel addition, the fuel 8 becomes excessive at the initial stage of combustion, as shown by the chain line in the graph of FIG. However, although the temperature drop due to the latent heat of vaporization affects the atmospheric temperature, it becomes difficult to efficiently reach the fuel reforming temperature. However, as shown in FIG. If fuel is added without a shortage, it is possible to efficiently raise the ambient temperature to the fuel reforming temperature even with a small amount of added fuel. As indicated by the solid line in the graph of FIG. It is possible to raise the reforming temperature.

As a result, the fuel addition pattern in which the fuel addition is performed by adding the control as shown in FIG. 4 is faster than the fuel addition pattern in which the fuel addition is uniformly increased from the start stage of the fuel addition as shown in FIG. Since the atmospheric temperature rises to the fuel reforming temperature and the fuel reforming reaction takes place early, the amount of H ₂ and CO produced will increase, and focusing on the H ₂ concentration, as shown in the graph of FIG. those had been a concentration of H ₂ as a chain line in the fuel addition in FIG. 2, it is raised to concentration of H ₂ as a solid line in the fuel addition of 4 has been confirmed.

Therefore, according to the above embodiment, the fuel 8 added on the inlet side of the NOx storage reduction catalyst 5 is ignited to perform combustion in a rich exhaust air-fuel ratio, and the ambient temperature at the combustion position is equal to or higher than the fuel reforming temperature. H ₂ and CO are generated, and these H ₂ and CO are led to the NOx occlusion reduction catalyst 5 as reducing agents, so that NOx is efficiently reduced to N ₂ from a lower temperature than when HC is used as the reducing agent. Since it can be processed, a high NOx reduction rate can be maintained without causing performance degradation even under operating conditions with a low exhaust temperature.

  In fact, according to a verification experiment by the present inventor, when HC is used as a reducing agent without igniting the fuel 8, a NOx reduction rate of about 50% is applied to the fuel 8 in the same operation mode. When ignition was performed and combustion was performed in a rich exhaust air-fuel ratio, the NOx reduction rate could be improved to about 65%.

  In this embodiment, since the particulate filter 15 is interposed between the fuel addition device 11 and the NOx storage reduction catalyst 5, the heat generated by the combustion of the fuel 8 is absorbed by the particulate filter 15. The increase in the catalyst bed temperature in the NOx occlusion reduction catalyst 5 can be suppressed, and the increase in the catalyst bed temperature of the NOx occlusion reduction catalyst 5 prevents the storage NOx from being released and the NOx reduction rate decreases. can do.

  That is, the saturated occlusion amount of the NOx occlusion reduction catalyst 5 increases as the catalyst bed temperature rises until it reaches a predetermined peak temperature, but decreases when the catalyst bed temperature further rises beyond the peak temperature. If the catalyst bed temperature of the NOx occlusion reduction catalyst 5 exceeds the peak temperature and increases too much, the amount of NOx released may increase and the NOx reduction rate may decrease. It is important to consider that the catalyst bed temperature of the storage reduction catalyst 5 is not excessively increased by the heat generated by the combustion of the fuel 8.

  Further, when the particulate filter 15 is arranged immediately after the fuel addition device 11 as described above, the particulate filter 15 is regenerated by adding to the fuel 8 from the fuel addition device 11 and performing combustion. In addition, the temperature rise during the regeneration can be performed quickly.

  FIG. 6 shows another embodiment of the present invention, in which a heat storage material 16 having a ventilation structure is further interposed between the particulate filter 15 and the NOx storage reduction catalyst 5. In this way, since the heat storage material 16 can absorb more heat, the increase in the catalyst bed temperature in the NOx storage reduction catalyst 5 can be further suppressed, so the catalyst bed of the NOx storage reduction catalyst 5 can be reduced. A decrease in the NOx reduction rate due to the temperature rise can be prevented more reliably.

FIG. 7 shows still another embodiment of the present invention. The NOx occlusion reduction catalyst 5 is provided with a selective reduction catalyst 17 that can selectively react NOx with NH ₃ even in the presence of oxygen. ing.

In this way, even if the NH ₃ generated by the NOx storage reduction catalyst 5 by the reaction with H ₂ and NOx, the NOx in the downstream selective reduction catalyst 17 to the NH ₃ as a further reducing agent it is possible to reduce and purify, it is possible to achieve a further reduction of NOx, it is possible to prevent the discharge of the outside of the NH ₃ in the consumption of NH ₃ as the reducing agent.

That is, when H ₂ is used as a reducing agent, a part of NH ₃ is generated by the reaction between H ₂ and NO x in the NO x storage reduction catalyst 5 under the condition that the exhaust air-fuel ratio is in a deep rich state. However, if this NH ₃ is used as a reducing agent in the subsequent selective reduction catalyst 17, it is possible to further reduce NOx and to prevent leakage ammonia.

  It should be noted that the exhaust purification method and apparatus of the present invention are not limited to the above-described embodiments, and various modifications can be made without departing from the scope of the present invention.

3 Exhaust gas 4 Exhaust pipe 5 NOx storage reduction catalyst 8 Fuel 11 Fuel addition device 11a Control signal 12 Igniter (ignition device)
DESCRIPTION OF SYMBOLS 13 Temperature sensor 13a Detection signal 14 Control apparatus 15 Particulate filter 16 Thermal storage material 17 Selective reduction type catalyst

Claims (5)

When the exhaust air-fuel ratio is lean, NOx in the exhaust gas is temporarily stored in the form of nitrate by the NOx storage-reduction catalyst in the middle of the exhaust pipe, and the fuel is added to the inlet side of the NOx storage-reduction catalyst so that the exhaust air-fuel ratio Is an exhaust purification method for decomposing and releasing NOx and reducing and purifying the released NOx with a reducing component generated from the added fuel to regenerate the NOx storage reduction catalyst, on the inlet side of the NOx storage reduction catalyst exhaust air-fuel ratio to ignite the fuel addition is performed combustion in a rich state, the ambient temperature at the combustion position raised above the fuel reforming temperature to produce a H ₂ and CO, these H ₂ and CO as a reducing agent An exhaust purification method characterized by using to regenerate a NOx storage reduction catalyst.   An exhaust purification device equipped with a NOx occlusion reduction catalyst in the middle of the exhaust pipe and provided with a fuel addition device for adding fuel to the inlet side of the NOx occlusion reduction catalyst as a reducing agent for regenerating the NOx occlusion reduction catalyst An ignition device for igniting the added fuel from the fuel addition device, a temperature sensor for measuring the ambient temperature at the combustion position of the fuel ignited by the ignition device, and a detection signal from the temperature sensor are input. The fuel addition amount of the fuel addition device is controlled so that the exhaust air-fuel ratio becomes rich in the temperature range above the fuel reforming temperature, and the fuel addition amount of the fuel addition device is reduced in the temperature range below the fuel reforming temperature. An exhaust emission control device comprising a control device that performs control so as to be relatively suppressed.   The exhaust emission control device according to claim 2, wherein a particulate filter is interposed between the fuel addition device and the NOx occlusion reduction catalyst.   The exhaust emission control device according to claim 3, wherein a heat storage material having a ventilation structure is interposed between the particulate filter and the NOx occlusion reduction catalyst. An exhaust emission control device as claimed in claim 2, 3 or 4, characterized in that also provided with a selective reduction catalyst capable of selectively reacting NOx with NH ₃ in the presence of oxygen downstream of the NOx storage reduction catalyst.

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