JP4345484B2 - Exhaust gas purification method and exhaust gas purification system - Google Patents

Description

  The present invention provides a NOx occlusion reduction type catalyst, and in an exhaust gas purification system for purifying NOx in engine exhaust gas, it is possible to determine the progress of deterioration of the noble metal supported on this NOx occlusion reduction type catalyst. The present invention relates to an exhaust gas purification method and an exhaust gas purification system that can optimize the lean control time in accordance with the progress of deterioration of the exhaust gas.

  Exhaust gas purifier for purifying exhaust gas by removing PM (particulate matter) and NOx (nitrogen oxide) from exhaust gas of automobile internal combustion engine and stationary internal combustion engine, etc. Various studies and proposals have been made, and in particular, NOx storage catalysts such as NOx storage reduction catalysts and three-way catalysts are used for NOx in order to purify exhaust gas from automobiles and the like.

This NOx occlusion reduction type catalyst is formed by a monolith honeycomb 30M or the like having a structure as shown in FIG. 5, and this monolith honeycomb 30M is a structural material made of cordierite or stainless steel as shown in FIG. A plurality of polygonal cells 30S are formed on the carrier 31. As shown in FIGS. 6 and 7, a porous catalyst coat layer 34 serving as a catalyst support layer formed of alumina (Al ₂ O ₃ ) or zeolite is provided on the wall surface of the cell 30S. A supported noble metal (catalytically active metal) 32 and a NOx occlusion material (NOx occlusion material: NOx occlusion agent: NOx absorbent) 33 are carried on the surface of the catalyst coat layer 34 which has a surface area to be contacted, thereby generating a catalytic function. I am letting.

  8 and 9 show the arrangement of the catalyst substances 32 and 33 on the surface of the support layer of the NOx occlusion reduction catalyst and the NOx occlusion reduction mechanism. This NOx occlusion reduction type catalyst has a catalyst coat layer 34 with a supported noble metal 32 such as platinum (Pt) having an oxidation function and potassium (K), sodium (Na), lithium (Li), cesium (NOx occlusion function). NOx occlusion material 33 formed from some of alkali metals such as Cs), alkaline earth metals such as barium (Ba) and calcium (Ca), and rare earths such as lanthanum (La) and yttrium (Y). It is supported and has two functions, NOx occlusion and NOx release / purification, depending on the oxygen concentration in the exhaust gas.

As shown in FIG. 8, when the air-fuel ratio of the exhaust gas containing oxygen (O ₂ ) in the exhaust gas of a normal diesel engine, lean-burn gasoline engine or the like is a lean air-fuel ratio state, Oxygen contained in the gas causes nitrogen monoxide (NO) discharged from the engine to be oxidized into nitrogen dioxide (NO ₂ ) by the oxidation catalyst function of the supported noble metal 32. Then, the nitrogen dioxide is occluded in the form of nitrate in the NOx occlusion material 33 such as barium having NOx occlusion function to purify NOx.

  However, in this state, the NOx occlusion material 33 having the NOx occlusion function is all changed to nitrate and loses the NOx occlusion function. Therefore, by changing the operating conditions of the engine or injecting fuel into the exhaust passage, there is no oxygen in the exhaust gas, the exhaust gas having a high carbon monoxide (CO) concentration and a high exhaust temperature, that is, , Create rich combustion exhaust gas and send it to the catalyst.

Then, as shown in FIG. 9, when the exhaust gas has no oxygen, the carbon monoxide concentration is high, and the exhaust gas temperature is raised to a rich air-fuel ratio state, the nitrate that occludes NOx releases nitrogen dioxide and releases the original nitrogen dioxide. Return to barium etc. Since the released nitrogen dioxide has no oxygen in the exhaust gas, carbon monoxide, hydrocarbons (HC), hydrogen (H ₂ ) in the exhaust gas are used as a reducing agent by the oxidation function of the supported noble metal 32. Reduce to water (H ₂ O), carbon dioxide (CO ₂ ), nitrogen (N ₂ ) and purify.

  However, this NOx occlusion reduction type catalyst has a problem of sulfur poisoning and deterioration relating to the NOx occlusion material and a problem of deterioration relating to the supported noble metal.

This sulfur poisoning of the NOx occlusion material occurs because the NOx occlusion material selectively occludes sulfur dioxide (SO ₂ ) contained in the exhaust gas caused by sulfur in the fuel rather than NOx. For poison, measures to reduce the amount of sulfur in the fuel and when NOx purification is hindered by sulfur poisoning, the exhaust gas is heated and made oxygen-free and sulfur desorbed ( Measures are taken such as recovering the NOx storage capacity of the NOx storage material by sulfur purge).

  Further, as one of the methods for estimating the decrease in the storage capacity of the NOx storage agent, an oxygen concentration sensor is disposed downstream of the NOx storage reduction catalyst, and based on the magnitude of the output value of the oxygen concentration sensor during rich combustion. An exhaust emission control device for an internal combustion engine having an estimation means for estimating NOx storage capacity has been proposed (see, for example, Patent Document 1).

  Further, as a means for determining the deterioration of NOx storage materials other than sulfur poisoning, NOx that cannot be refreshed due to thermal deterioration such as cracking in the NOx absorbent other than sulfur poisoning of NOx storage materials that can be refreshed. Deterioration determination of the exhaust gas purification catalyst for determining deterioration of the storage material itself when the NOx concentration on the outlet side in the lean control operation after performing sulfur desorption is larger than the reference value. An apparatus has been proposed (see, for example, Patent Document 2).

  However, in these apparatuses, since the progress of the deterioration of the NOx storage reduction catalyst is determined by the change in the NOx storage amount, the deterioration of the NOx storage material is determined. There is a problem that this determination is not made.

  The deterioration of the supported noble metal occurs because the supported noble metal is sintered (sintered) due to SOx poisoning or thermal deterioration. When the supported noble metal is deteriorated, the oxidation / reduction function of the supported noble metal is lowered. Therefore, nitrogen dioxide is removed from the NOx occlusion material by the rich control operation at the time of regeneration or desulfurization (sulfur purge) of the NOx occlusion reduction type catalyst. In the case of release, the released nitrogen dioxide cannot be sufficiently reduced, causing NOx to flow out downstream of the catalyst.

  Further, since the NOx purification performance also decreases due to the deterioration of the supported noble metal, the deterioration as the NOx occlusion reduction type catalyst cannot be accurately determined only by the determination of the deterioration of the NOx occlusion material in the prior art. There is a problem that the NOx purification performance as a whole deteriorates because it is impossible to appropriately deal with the deterioration.

  On the other hand, the present inventors obtained the following knowledge regarding the deterioration of the supported noble metal through experiments and the like.

As schematically shown in FIG. 4, in the NOx occlusion reduction type catalyst, when the lean control operation is switched to the rich control operation, the exhaust gas air-fuel ratio is switched to the rich state, and at the same time, the downstream NOx concentration rapidly increases. To increase. This is because NOx released from the NOx occlusion material under a rich condition is reduced and purified by the supported noble metal, but a part of the released NOx is not reduced to N ₂ but flows out downstream of the catalyst (NOx slip). It is to do.

Since the amount of NOx flowing out downstream of the catalyst increases due to a reduction in reduction ability due to deterioration of the supported noble metal, the deterioration state of the supported noble metal is measured by measuring the NOx concentration at a predetermined time tm during the rich control operation. Can be determined. However, the amount of NOx flowing downstream of the catalyst does not depend solely on the deterioration of the supported noble metal, but is also affected by the oxygen concentration in the exhaust gas, the catalyst temperature, and the previous (previous cycle) lean control time.
JP 2000-34946 A JP 2000-230417 A (page 3, page 8)

  The object of the present invention is to determine the progress of deterioration of the supported noble metal by monitoring the maximum NOx slip amount at the initial stage when the lean control operation is switched to the rich control operation, based on the above knowledge. In addition, by setting the lean control time in the next lean control operation to a time commensurate with the progress of the deterioration of the supported noble metal, the exhaust gas purification method and the exhaust gas purification that can prevent the NOx purification rate from being lowered due to the deterioration of the supported noble metal To provide a system.

  More specifically, according to the present invention, in the NOx storage reduction catalyst, the NOx storage material for storing NOx and the supported noble metal for reducing and purifying NOx released from the NOx storage material cause different deterioration. In view of this, it is possible to determine the deterioration of the supported noble metal even if the NOx storage material is not deteriorated by determining the deterioration degree of the supported noble metal, not the sulfur poisoning deterioration, thermal deterioration, change with time, etc. Accordingly, the lean control time, which is the time for storing NOx, is corrected, and the NOx occlusion amount stored within the lean control time, that is, the NOx release amount in the rich state is adjusted to the reducing ability of the supported noble metal, so that It is an object of the present invention to provide an exhaust gas purification method and an exhaust gas purification system that can prevent the release of NOx to the side.

An exhaust gas purification method for achieving the above object is an exhaust gas purification system that purifies NOx in engine exhaust gas, and stores NOx when the air-fuel ratio of the inflowing exhaust gas is lean. And a NOx occlusion reduction type catalyst that releases and reduces NOx in a rich state, and further includes NOx concentration detection means that detects the NOx concentration downstream of the NOx occlusion reduction type catalyst, and performs lean control operation and rich control operation. In the exhaust gas purification system that repeats the above, by comparing the value determined by the NOx concentration detected by the NOx concentration detecting means during the rich control operation with the NOx release allowable value determined by the immediately preceding lean control time , This is a method characterized in that the progress of deterioration of the noble metal supported on the NOx storage reduction catalyst is determined.

  According to this exhaust gas purification method, it is possible not only to determine whether or not the supported noble metal has deteriorated by comparing the measured NOx concentration and the NOx release allowable value, but also the measured NOx concentration and the NOx release allowable value. Based on the difference, it is possible to accurately determine the progress of deterioration of the noble metal supported on the NOx storage reduction catalyst. Further, when the difference between the downstream NOx concentration and the upstream NOx concentration is used, the accuracy is higher than when the downstream NOx concentration is used.

Further, in the above exhaust gas purification method, the NOx concentration detection means includes means for detecting the upstream NOx concentration of the NOx storage reduction catalyst, and the difference between the downstream NOx concentration and the upstream NOx concentration, and the NOx release. The progress of deterioration of the supported noble metal is determined by comparison with an allowable value.
In the exhaust gas purification method described above, the greater the deterioration of the supported noble metal of the NOx occlusion reduction catalyst, the shorter the next lean control time, so that the lean control operation time corresponding to the reduction capability of the supported noble metal is reduced. Since the NOx occlusion amount, that is, the NOx release amount during the rich control operation can be set, it is possible to prevent the NOx purification rate from being lowered particularly during the rich control operation.

Further, Oite the exhaust gas purification method described above, the NOx emission tolerance, at least, of the NOx storage reduction catalyst during the rich control operation the catalyst temperature, and, of the NOx storage reduction catalyst during the rich control operation By determining the upstream oxygen concentration, it is possible to determine the deterioration of the noble metal supported on the NOx storage reduction catalyst with higher accuracy. The catalyst temperature may be measured directly, but is estimated from the upstream (inlet side) exhaust gas temperature, the upstream exhaust gas temperature and the downstream (outlet side) exhaust gas temperature of the NOx storage reduction catalyst. Also good.

  In the exhaust gas purification method, the allowable NOx release value is determined such that it increases as the preceding lean control time increases, decreases as the catalyst temperature increases, and increases as the upstream oxygen concentration increases. Thus, it is possible to determine the deterioration of the supported noble metal of the NOx storage reduction catalyst with higher accuracy.

An exhaust gas purification system for achieving the above object is an exhaust gas purification system that purifies NOx in the exhaust gas of the engine. When the air-fuel ratio of the inflowing exhaust gas is lean, the NOx A NOx storage reduction catalyst that stores NOx in a rich state and releases and reduces NOx, and has a NOx concentration detection means and a catalyst deterioration determination means for detecting the NOx concentration downstream of the NOx storage reduction catalyst. in the exhaust gas purification system provided with a control means to repeat the lean control operation and the rich control operation, the catalyst deterioration determining means, definitive during the rich control operation, the value determined by NOx concentration detected by the NOx concentration detecting means , by comparison with the NOx emission allowable value determined by the lean control time immediately before, of the NOx storage reduction catalyst responsible Configured to determine the progress of the precious metal of deterioration. According to this configuration, it is possible to accurately determine the deterioration of the noble metal supported on the NOx storage reduction catalyst.

In the above exhaust gas purification system, the NOx concentration detecting means includes means for detecting the upstream NOx concentration of the NOx storage reduction catalyst, and the difference between the downstream NOx concentration and the upstream NOx concentration, and the NOx release. The progress of deterioration of the supported noble metal is determined by comparison with an allowable value.
Further, in the exhaust gas purification system, a decrease in the NOx purification rate can be prevented by shortening the next lean control time as the deterioration of the noble metal supported on the NOx storage reduction catalyst increases.

  According to the exhaust gas purification method and exhaust gas purification system of the present invention, it is possible to accurately determine the progress of deterioration of the supported noble metal of the NOx storage reduction catalyst, and based on the progress of deterioration of the supported noble metal. In consideration of the reduced NOx reduction capability, the next lean control time is set so that the NOx occlusion amount corresponds to the decrease in the NOx reduction capability, so that the NOx release amount during the next rich control operation is deteriorated. It is possible to match the NOx reduction ability of the supported noble metal, and to prevent NOx outflow to the downstream side of the NOx occlusion reduction type catalyst. Accordingly, it is possible to prevent a decrease in the NOx purification rate.

  Hereinafter, an exhaust gas purification method and an exhaust gas purification system according to embodiments of the present invention will be described with reference to the drawings. Since the present invention has a problem of deterioration of the supported noble metal, in the following embodiments, the deterioration of the NOx storage material is omitted from the description, but the actual exhaust gas purification method and exhaust gas purification system are omitted. Then, determination of deterioration of NOx occlusion material and measures against deterioration are used in combination.

  FIG. 1 shows a configuration of an exhaust gas purification system 1 according to an embodiment of the present invention. The exhaust gas purification system 1 includes a NOx occlusion reduction type catalyst 10. An upstream side (inlet side) oxygen concentration sensor 11, an upstream side ( An inlet side) NOx concentration sensor 12 and an upstream side (inlet side) exhaust gas temperature sensor 13 are provided, and an upstream side (outlet side) exhaust gas temperature sensor 14 and an upstream side (outlet side) NOx are provided on the downstream side, that is, the outlet side. A density sensor 15 is provided. And this engine (internal combustion engine) E is comprised with an engine with a common rail electronically controlled injection apparatus.

The NOx occlusion reduction type catalyst 10 is formed of a monolithic catalyst 30M as shown in FIGS. 4 to 7, and has an oxidation function with respect to NOx on a porous catalyst coat layer 34 such as alumina (Al ₂ O ₃ ). Supported noble metal (catalyst metal) 32 such as platinum (Pt), and alkali metals such as sodium (Na), potassium (K) and cesium (Cs), and alkaline earths such as calcium (Ca) and barium (Ba) A NOx occlusion material 33 having a NOx occlusion function made of one or some combination of metals, rare earths such as yttrium (Y) and lanthanum (La) is supported.

In the NOx occlusion reduction type catalyst 10, in the lean control operation under the exhaust gas condition (lean air-fuel ratio state) in which the oxygen concentration in the exhaust gas is high, as in the normal operation state of a diesel engine, the lean combustion gasoline engine, etc., as shown in 8, by oxidation function of nitrogen monoxide (NO) is a catalyst metal to be discharged, the oxygen (O ₂₎ in the oxidized nitrogen dioxide (NO ₂₎ becomes contained in the exhaust gas, the NO ₂ is Since the NOx occlusion material 33 is occluded in the form of chloride, the exhaust gas is purified.

However, if this NOx occlusion continues, the NOx occlusion material 33 such as barium changes to nitrate, gradually saturates and loses the function to occlude NO ₂ . Therefore, it is necessary to release the NOx stored when the air-fuel ratio of the exhaust gas is in the lean state before the NOx storage capacity reaches saturation. Then, an exhaust gas (rich spike gas) having a low oxygen concentration and a high carbon monoxide concentration and a high exhaust temperature is generated and supplied to the NOx storage reduction catalyst 10.

In this exhaust gas rich air-fuel ratio state, as shown in FIG. 9, the NOx occlusion material 33 that has occluded NO ₂ and changed to nitrate releases the occluded NO ₂ and returns to the original barium or the like. Since this released NO ₂ does not contain O _{2 in} the exhaust gas, carbon monoxide (CO), hydrocarbon (HC), hydrogen (H ₂ ) in the exhaust gas is used as a reducing agent on the supported noble metal 32. It is reduced, converted into nitrogen (N ₂ ), water (H ₂ O), carbon dioxide (CO ₂ ) and purified.

  Further, the output values of the sensors 11, 12, 13, 14, and 15 are used to perform general control of the operation of the engine E, and at the same time, regeneration control and desulfurization (sulfur purge) to recover the NOx purification ability of the NOx storage reduction catalyst 10. ) A control device (ECU: engine control unit) 50 that performs control and the like, and a control signal output from the control device 50 causes a common rail electronic control fuel injection device, a throttle valve, and an EGR valve for fuel injection of the engine E Etc. are controlled.

  A control device of the exhaust gas purification system 1 is incorporated in the control device 50 of the engine E, and controls the exhaust gas purification system 1 together with the operation control of the engine E. The control device of the exhaust gas purification system 1 includes an exhaust gas purification system control means C1 having an exhaust gas component detection means C10, a NOx storage reduction catalyst control means C20, and the like as shown in FIG. The

  The exhaust gas component detection means C10 is a means including oxygen concentration (or excess air ratio λ) detection means in the exhaust gas and NOx concentration detection means for detecting the NOx concentration. The upstream oxygen concentration sensor 11 and the upstream side The NOx concentration sensor 12, the downstream NOx concentration sensor 15 and the like are configured.

  The NOx occlusion reduction type catalyst control means C20 is a means for controlling regeneration, desulfurization, etc. of the NOx occlusion reduction type catalyst 10, and a regeneration start judgment means C21, a regeneration control means C22, a desulfurization start judgment means C23, a desulfurization control means. C24, desulfurization completion determination means C25, and the like are provided, but the present invention further includes supported noble metal deterioration determination means C26 and lean control time setting means C27.

  In the control means C20 of the NOx occlusion reduction type catalyst 10, the regeneration start judging means C21 calculates the NOx purification rate from the NOx concentration detected by the exhaust gas component detecting means C10, and this NOx purification rate is lower than a predetermined judgment value. In this case, it is determined that regeneration of the NOx storage reduction catalyst 10 is started. When it is determined that the regeneration is started, the regeneration control means C22 changes the exhaust gas state to a predetermined rich air-fuel ratio state and a predetermined state by post injection, EGR control, intake throttle control, etc. in the fuel injection control of the engine E. In the temperature range (approximately 200 ° C. to 600 ° C. depending on the catalyst), and the rich control operation for recovering the NOx purification capacity, that is, the NOx storage capacity, is restored for a predetermined time or the NOx catalyst is recovered. Until it is judged.

  Further, desulfurization is performed by the desulfurization start determination means C23, the desulfurization control means C24, and the desulfurization end determination means C25. In this desulfurization, the desulfurization start determining means C23 calculates the engine exhaust sulfur amount based on the fuel consumption amount and the sulfur amount contained in the fuel (market actual value sulfur concentration, etc.), and integrated sulfur adsorption amount The desulfurization start determination value is compared, and when the integrated sulfur adsorption amount exceeds the desulfurization start determination value, it is determined that desulfurization is started.

  When it is determined that the desulfurization is started, the rich control operation is performed by the desulfurization control means C24. The rich control operation in this desulfurization is the temperature detected by the upstream temperature sensor 13 or the downstream temperature sensor 14 by performing multi-stage injection including pilot injection or post injection by engine fuel injection, EGR control or intake air control. The amount of fuel in multi-stage injection is adjusted while monitoring the above, and feedback control is performed so that the temperature of the exhaust gas flowing into the NOx occlusion reduction type catalyst 10 is equal to or higher than the desulfurization temperature (about 600 ° C. to 650 ° C.). . With this exhaust gas temperature increase, the temperature of the NOx storage reduction catalyst 10 is increased. Note that HC (hydrocarbon) or the like in the exhaust gas supplied to the exhaust passage by multi-stage injection burns due to the oxidizing action of the catalyst, so that the temperature of the catalyst is further promoted by this oxidation reaction reaction heat.

  Further, the oxygen concentration Coup (or excess air ratio λ) detected by the upstream oxygen concentration sensor 11 is monitored, and a predetermined oxygen concentration (or a stoichiometric air-fuel ratio (stoichiometric air-fuel ratio)), or a predetermined oxygen concentration (or stoichiometric air-fuel ratio) is obtained. Multi-stage injection control, EGR control, intake air control, etc. are feedback-controlled so that the excess air ratio is obtained. Thus, sulfur is efficiently desorbed in a low oxygen and high temperature state.

  Then, the desulfurization end judging means C25 calculates the desulfurization amount in the rich control operation for desulfurization, and when the integrated desulfurization amount obtained by integrating the desulfurization exceeds the integrated sulfur adsorption amount, the desulfurization control is ended as desulfurization is completed. To do.

  In the present invention, as described below, the supported noble metal deterioration determining means C26 determines the deterioration degree of the supported noble metal, not the sulfur poisoning deterioration, thermal deterioration, change with time, etc. of the NOx storage material, The lean control time setting means C27 corrects the lean control time, which is the time for storing NOx, in accordance with the deterioration of the supported noble metal, and matches the NOx release amount in the rich control operation with the reducing ability of the supported noble metal.

  In these exhaust gas purification systems 1, the exhaust gas purification method according to the present invention is performed with a lean control time setting flow as illustrated in FIG. 3. In the flow for setting the lean control time in FIG. 3, the degree of deterioration of the supported noble metal, that is, the progress of deterioration is determined by the supported noble metal deterioration determining means C26, and the lean control time tlean of the next lean control operation is determined by the lean control time setting means C27. It is a flow for setting.

  The control flow includes a lean control flow for performing an operation in a normal lean state, a regeneration control flow for regenerating the NOx storage capacity, a desulfurization control flow for desulfurizing the NOx storage reduction catalyst 10, and the entire exhaust gas purification system 1. The lean control time tlean is set by judging the progress of deterioration of the supported noble metal. As a result, in the next lean control operation, during the set lean control time tlean, NOx is occluded by the NOx occlusion material and the exhaust gas is purified.

  When the control flow of FIG. 3 starts, the previous lean control time tlean is input in step S11, and in the next step S12, the oxygen concentration, NOx concentration, and catalyst temperature in the rich control operation such as the regeneration control operation and the desulfurization operation. Measure and calculate.

  In this measurement and calculation, the catalyst temperature Tc is calculated from the upstream exhaust gas temperature Tgup measured by the upstream exhaust gas temperature sensor 13 and the upstream exhaust gas temperature Tgdown measured by the downstream exhaust gas temperature sensor 14. The upstream oxygen concentration Coup is measured by the oxygen concentration sensor 11, the upstream NOx concentration Cnup is measured by the upstream NOx concentration sensor 12, and the downstream NOx concentration Cndown is measured by the downstream NOx concentration sensor 15. Further, the NOx concentration difference Cn is calculated by subtracting the upstream NOx concentration Cnup from the downstream NOx concentration Cndown, and the maximum NOx concentration difference Cnmax, which is the maximum value, is obtained.

  The maximum NOx concentration difference Cnmax occurs during a predetermined time tm after the start of the rich control operation. Therefore, even if measurement and calculation are performed only during the predetermined time tm, the amount of calculation is reduced. Control may be simplified. Further, if the catalyst temperature Tc can be measured, the catalyst temperature may be directly measured, or may be estimated only by the upstream side exhaust gas temperature Tgup although the accuracy is somewhat lowered.

  This NOx concentration difference Cn (= Cndown−Cnup) is an index indicating how much NOx released by the rich control is not reduced by the supported noble metal but is released, and the larger the reduction amount, the smaller the reduction amount. The amount of reduction increases. Therefore, the maximum NOx concentration difference Cnmax indicates the amount of release when the amount of NOx released without being reduced by the NOx storage reduction catalyst 10 is maximum.

  That is, as shown in FIG. 4, due to the maximum NOx concentration difference Cnmax within a predetermined time tm when the lean control operation is switched to the rich control operation, it is released from the NOx occlusion material and is not reduced by the supported noble metal. The amount of NOx can be estimated. Then, the deterioration state of the supported noble metal is determined based on the maximum NOx concentration difference Cnmax related to the maximum value of the outflow NOx amount.

  Although the accuracy is somewhat reduced, the upstream NOx concentration Cnup during the rich control operation can be estimated from the conditions of the rich control operation. Therefore, the estimated upstream NOx concentration Cnup ′ may be used instead of the measured value. . Further, although the accuracy is lowered, the downstream NOx concentration Cndown may be used instead of the maximum NOx concentration difference Cnmax when the following NOx release allowable value Cnp is calculated in consideration of the conditions during the rich control operation. it can. In this case, since the measured value of the upstream NOx concentration sensor 12 is not used, the upstream NOx concentration sensor 12 is not necessary in this deterioration determination, and the control is simplified.

  In step S13, the NOx release allowable value Cnp is calculated from the previous lean control time tlean, the catalyst temperature Tc, and the upstream oxygen concentration Coup based on the map data Cnmap set in advance and input in advance. As a result, the previous lean control time tlean, catalyst temperature Tc, and oxygen concentration Coup in the exhaust gas that affect the maximum NOx concentration difference Cnmax can be taken into consideration.

  Next, in step S14, the supported noble metal is checked for deterioration. This check of deterioration of the supported noble metal is performed when the maximum NOx concentration difference Cnmax calculated from the measured value is equal to or less than the NOx release allowable value Cnp (YES), and it is determined that the deterioration of the supported noble metal has not progressed. The process returns with the control time tlean set to the last lean control time tlean.

  When the maximum NOx concentration difference Cnmax is larger than the NOx release allowable value Cnp (NO), it is determined that the deterioration is progressing, and the display of the catalyst deterioration in step S15 indicates that the catalyst is deteriorated. In steps S16 and S17, the next lean control time tlean is changed and set again.

  The next lean control time tlean is set by estimating the progress of deterioration (degree of deterioration) from the NOx concentration deviation ΔCn calculated by subtracting the NOx release allowable value Cnp from the maximum NOx concentration difference Cnmax in step S16. Based on the NOx concentration deviation ΔCn, which is an indicator of the progress of the deterioration of the supported noble metal, a correction coefficient R is calculated in step S17 based on the map data Rmap of the correction coefficient R inputted in advance. Multiply the reference lean control time tlean0 by R to set the next lean control time tlean, and then return.

  According to the control flow for setting the lean control time, the progress of deterioration of the supported noble metal is estimated from the actually measured NOx concentrations Cnup and Cndown and the map data Cnmap inputted in advance, and the degree of deterioration of the supported noble metal. The next lean control time tlean can be set based on the map data Rmap input in advance according to ΔCn.

  The control flow for setting the lean control time is preferably performed in parallel with the rich control operation in the regeneration control flow or the desulfurization control flow, but immediately after the regeneration control flow or the desulfurization control flow, that is, the lean control flow. You may implement before a control driving | operation or immediately after a start. However, in this case, the measurement value in the rich control operation in step S12 of FIG. 3 is performed and stored in these rich control flows, and when the control flow for setting the lean control time is executed, in step S12 Configure to read.

  According to the exhaust gas purification method and the exhaust gas purification system 1 having the above-described configuration, it is possible to determine the progress of the deterioration of the supported noble metal, not the sulfur poisoning deterioration, heat deterioration, change with time, etc. of the NOx storage material.

  And even if the NOx occlusion material has not deteriorated, the lean control time, which is the time to occlude NOx, is corrected according to the deterioration of the supported noble metal, and the reduced amount of NOx released in the rich state is reduced. Since the capacity can be matched, the outflow of NOx to the downstream side (outlet side) of the NOx storage reduction catalyst can be reduced.

  In the above configuration, the exhaust gas purification system 1 using only the NOx storage reduction catalyst 10 has been described. However, the exhaust gas is a combination of a NOx storage reduction catalyst and a DPF that collects particulate matter (PM) of a diesel engine. The present invention can be applied to both a gas purification system and an exhaust gas purification system that combines a NOx storage reduction catalyst and a three-way catalyst. In short, the present invention can be applied to any exhaust gas purification system including a NOx storage reduction catalyst.

It is a figure which shows the structure of the exhaust gas purification system of embodiment which concerns on this invention. It is a figure which shows the structure of the control means of the exhaust gas purification system of embodiment which concerns on this invention. It is a figure which illustrates the control flow for the lean control time setting of embodiment which concerns on this invention. It is a figure which shows the time series of the NOx density | concentration at the time of rich control, and an air excess rate. It is a figure which shows a monolith honeycomb. It is the elements on larger scale of a monolith honeycomb. It is an enlarged view of the cell wall part of a monolith honeycomb. It is a figure which shows typically the structure of a NOx occlusion reduction type | mold catalyst, and the purification mechanism of the state at the time of lean control. It is a figure which shows typically the structure of a NOx storage-reduction type | mold catalyst, and the purification mechanism of the state at the time of rich control.

Explanation of symbols

E engine 1 exhaust gas purification system 10 NOx occlusion reduction type catalyst 11 upstream oxygen concentration sensor 12 upstream NOx concentration sensor 13 upstream exhaust gas temperature sensor 14 downstream NOx concentration sensor 15 downstream exhaust gas temperature sensor 50 control unit (ECU) )

Claims (8)

An exhaust gas purification system for purifying NOx in engine exhaust gas, wherein NOx is stored when the air-fuel ratio of the inflowing exhaust gas is lean, and NOx is released and reduced when the air-fuel ratio is rich. In an exhaust gas purification system that includes a NOx storage reduction catalyst and NOx concentration detection means that detects a NOx concentration downstream of the NOx storage reduction catalyst and repeats lean control operation and rich control operation,
In the rich control operation, the noble metal supported by the NOx occlusion reduction catalyst is determined by comparing the value determined by the NOx concentration detected by the NOx concentration detecting means with the NOx release allowable value determined by the immediately preceding lean control time. The exhaust gas purification method characterized by determining the progress of deterioration of the exhaust gas. The NOx concentration detection means includes means for detecting the upstream NOx concentration of the NOx storage reduction catalyst, and the supported noble metal is determined by comparing the difference between the downstream NOx concentration and the upstream NOx concentration and the allowable NOx release value. The exhaust gas purification method according to claim 1 , wherein the progress of deterioration is determined . The exhaust gas purification method according to claim 1 or 2 , wherein the next lean control time is shortened as the deterioration of the noble metal supported on the NOx storage reduction catalyst increases . The NOx emission tolerance, at least, the catalyst temperature of the NOx storage reduction catalyst during the rich control operation, and, claims, characterized in that determined by the upstream oxygen concentration of the NOx occlusion reduction type catalyst in the rich control operation Item 4. The exhaust gas purification method according to Item 1, 2 or 3 . 5. The exhaust gas according to claim 4, wherein the NOx release allowable value is determined so as to increase as the immediately preceding lean control time increases, to decrease as the catalyst temperature increases, and to increase as the upstream oxygen concentration increases. Gas purification method. An exhaust gas purification system for purifying NOx in engine exhaust gas, wherein NOx is stored when the air-fuel ratio of the inflowing exhaust gas is lean, and NOx is released and reduced when the air-fuel ratio is rich. Exhaust gas provided with an NOx storage reduction catalyst, a NOx concentration detection means for detecting the NOx concentration downstream of the NOx storage reduction catalyst, and a catalyst deterioration determination means, and a control means for repeating a lean control operation and a rich control operation In the gas purification system,
The catalyst deterioration determining means, definitive during the rich control operation, the value determined by NOx concentration detected by the NOx concentration detecting means, by comparing the NOx emission allowable value determined by the lean control time immediately before the NOx occlusion An exhaust gas purification system for determining a progress of deterioration of a noble metal supported on a reduction catalyst. The NOx concentration detection means includes means for detecting the upstream NOx concentration of the NOx storage reduction catalyst, and the supported noble metal is determined by comparing the difference between the downstream NOx concentration and the upstream NOx concentration and the allowable NOx release value. The exhaust gas purification system according to claim 6 , wherein the progress of deterioration of the exhaust gas is determined . The exhaust gas purification system according to claim 6 or 7, wherein the next lean control time is shortened as the deterioration of the supported noble metal of the NOx storage reduction catalyst increases.

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