JP2009013932A - Exhaust emission control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain excellent NOx reducing effect from an exhaust temperature lower than a conventional one without causing a trouble in engine starting or the like after long time engine stop. <P>SOLUTION: An exhaust emission control device which includes at some midpoint of an exhaust pipe 4 a selective reduction type catalyst 5 capable of selectively reacting NOx with ammonia even in the presence of oxygen, and which reduces and controls NOx by adding urea water 6 as a reducing agent in the exhaust pipe 4 on the upstream side from the selective reduction type catalyst 5, is provided with a particulate filter 10 attaching an oxidation catalyst 9 to a front stage thereof in the exhaust pipe 4 on the upstream side from an adding position of the urea water 6, and NOx adsorbing catalyst 17 having a property in which NOx in the exhaust 3 is physically adsorbed at a temperature lower than an active lower limit temperature of the selective reduction type catalyst 5 and the absorbed NOx is discharged at a temperature near an active lower limit temperature of the selective reduction type catalyst 5, between the particulate filter 10 and an adding position of the urea water 6. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an exhaust emission control device.

  Conventionally, a diesel engine is equipped with a selective reduction catalyst having a property of selectively reacting NOx with a reducing agent even in the presence of oxygen in the middle of an exhaust pipe through which exhaust gas flows, and the selective reduction catalyst A required amount of a reducing agent is added to the upstream side of the catalyst so that the reducing agent undergoes a reduction reaction with NOx (nitrogen oxide) in the exhaust gas on the selective catalytic reduction catalyst, thereby reducing the NOx emission concentration. There is what I did.

On the other hand, in the field of industrial flue gas denitration treatment in plants and the like, the effectiveness of a method for reducing and purifying NOx using ammonia (NH ₃ ) as a reducing agent is already widely known. Since it is difficult to ensure safety with respect to traveling with ammonia itself, in recent years, the use of non-toxic urea water as a reducing agent has been studied.

  That is, if urea water is added to the exhaust gas upstream of the selective catalytic reduction catalyst, the urea water is thermally decomposed into ammonia and carbon dioxide gas in the exhaust gas, and NOx in the exhaust gas is converted into the selective catalytic reduction catalyst. It is reduced and purified well by ammonia (see, for example, Patent Document 1).

  On the other hand, when purifying exhaust gas from a diesel engine, it is not enough to remove NOx in the exhaust gas, and particulates contained in the exhaust gas are also collected through the particulate filter. However, when this type of particulate filter is employed, it is necessary to regenerate the particulate filter by appropriately burning and removing the particulate before the exhaust resistance increases due to clogging.

  For this reason, a flow-through type oxidation catalyst is attached to the preceding stage of the particulate filter, and fuel is added to the exhaust gas upstream from the oxidation catalyst when the amount of particulate accumulation increases. It is considered to force playback.

  In other words, if fuel is added to the exhaust gas upstream of the oxidation catalyst, the added fuel (HC) undergoes an oxidation reaction while passing through the preceding oxidation catalyst. The catalyst bed temperature of the particulate filter immediately after that is raised, the particulates are burned out, and the particulate filter is regenerated.

  In general, as a specific means for performing the fuel addition as described above, the post-injection is executed at the timing of non-ignition later than the compression top dead center following the main injection of fuel performed near the compression top dead center. It is considered that the fuel is added to the exhaust gas, but in order to efficiently utilize the added fuel for forced regeneration and oxidize the added fuel before the temperature of the exhaust gas drops as much as possible, It is considered preferable to dispose the particulate filter and the preceding oxidation catalyst upstream of the selective reduction catalyst.

  However, in the selective reduction type catalyst using urea water as a reducing agent as described above, an exhaust temperature of about 200 ° C. or higher is required to obtain sufficient catalytic activity during the reduction reaction. If the operation condition is low such that the temperature is below 200 ° C. (generally, the low exhaust temperature range extends to the low load operation range), there is a problem that the NOx reduction rate does not increase easily. For vehicles that run only on congested roads such as local buses, etc., the operation above the required predetermined temperature does not continue for a long time, so the NOx reduction rate remains low and good NOx reduction There was a possibility that the effect could not be obtained.

Therefore, the inventors of the present invention have given the oxidation catalyst provided in the preceding stage of the particulate filter the property of physically adsorbing NOx in the exhaust gas, and the temperature of the exhaust gas is the lower limit temperature of the selective catalytic reduction catalyst. We are studying NOx reduction by adsorbing NOx to the oxidation catalyst in an operating region lower than (about 200 ° C.).
JP 2004-218475 A

  However, when the oxidation catalyst having NOx adsorption ability as described above is mainly composed of zeolite or the like and verified, this kind of oxidation catalyst is lower than about 200 ° C. which is the lower limit active temperature of the selective catalytic reduction catalyst. While NOx in exhaust gas is physically adsorbed at a low catalyst bed temperature, it has been confirmed that adsorbed NOx is released from around 200 ° C, but at the start after a long engine stop (cold start) In this case, even if the upstream catalyst bed temperature of the upstream oxidation catalyst rises to about 200 ° C., the particulate filter is interposed by the particulate filter between the oxidation catalyst and the selective reduction catalyst. The increase in the catalyst bed temperature of the selective reduction catalyst is delayed by the heat capacity of the filter, and the catalyst bed temperature of the selective reduction catalyst slowly rises to about 200 ° C. with a required time delay. Therefore, due to the time delay of the rise in the catalyst bed temperature, there occurs a time zone in which NOx cannot be reduced and purified by the selective catalytic reduction catalyst even if the release of adsorbed NOx from the oxidation catalyst is started, and NOx emission in this time zone It has been found that the problem of increasing amounts can occur.

  The present invention has been made in view of the above-described circumstances, and is intended to obtain a good NOx reduction effect from an exhaust temperature lower than that of the conventional one without causing any trouble at the time of starting after a long engine stop. It is aimed.

  The present invention provides a selective reduction catalyst capable of selectively reacting NOx with ammonia even in the presence of oxygen in the middle of an exhaust pipe, and urea water is added as a reducing agent in the exhaust pipe upstream of the selective reduction catalyst. An exhaust gas purification device that reduces and purifies NOx, and in the exhaust pipe upstream from the urea water addition position, a particulate filter with an oxidation catalyst attached to the preceding stage is disposed, and the particulate filter and Between the addition position of the urea water, NOx in the exhaust gas is physically adsorbed at a temperature lower than the activity lower limit temperature of the selective reduction catalyst, and the adsorbed NOx is adsorbed to the activity lower limit temperature of the selective reduction catalyst. The NOx adsorption catalyst having the property of releasing at a temperature close to is disposed.

  Thus, in this way, even when the catalyst bed temperature of the selective catalytic reduction catalyst does not reach the lower activity limit temperature, NOx in the exhaust gas is physically adsorbed by the NOx adsorption catalyst. NOx emissions will be suppressed even under low exhaust temperature conditions where NOx cannot be reduced and purified by the type catalyst, and after the catalyst bed temperature of the selective reduction type catalyst reaches the activation lower limit temperature, NOx is added by adding urea water. Reduction and purification can be performed.

  That is, when urea water is added to the upstream side of the selective catalytic reduction catalyst, the urea water is thermally decomposed into ammonia and carbon dioxide gas in the exhaust gas, and the selective reduction is in an active state under a temperature condition that is equal to or higher than the activation lower limit temperature. The NOx in the exhaust gas effectively reacts with ammonia on the type catalyst and is favorably reduced and purified.

  On the other hand, when starting after a long engine stop (cold start), etc., the NOx adsorption catalyst placed at the latter stage of the particulate filter includes the heat capacity of the preceding particulate filter (including the heat capacity of the oxidation catalyst). As a result, the catalyst bed temperature becomes difficult to rise, and the release temperature is reached later than when the oxidation catalyst in the previous stage of the particulate filter has NOx adsorption ability.

  For this reason, the timing at which the NOx adsorption catalyst reaches the release temperature and the release of the adsorbed NOx starts, and the selective catalytic reduction catalyst reaches the activation lower limit temperature close to the release temperature of the NOx adsorption catalyst, and NOx is reduced by adding urea water. Even when the timing for purifying conditions approaches, the time period during which NOx cannot be reduced and purified by the selective reduction catalyst even if the release of adsorbed NOx from the NOx adsorption catalyst is started is greatly shortened, and the amount of NOx emissions increases during this time period. The situation of doing so will be avoided as much as possible.

  According to the above-described exhaust purification apparatus of the present invention, even when the catalyst bed temperature of the selective catalytic reduction catalyst has not reached the activation lower limit temperature, NOx in the exhaust gas is physically adsorbed by the NOx adsorption catalyst, and the NOx emission amount As a result, even if the vehicle is operated in such a manner that the operation state with a low exhaust temperature continues for a long time, it is possible to obtain a good NOx reduction effect from the exhaust temperature lower than before, and Because NOx released from the NOx adsorption catalyst, such as when starting after a long engine stop (cold start), cannot be reduced and purified by the selective catalytic reduction catalyst, the time period during which NOx is discharged is extremely short. The outstanding effect that the situation which quantity increases can be avoided as much as possible can be produced.

  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 the exhaust purification apparatus of this embodiment, the exhaust pipe 4 through which the exhaust gas 3 discharged from the diesel engine 1 through the exhaust manifold 2 flows is shown. In addition, a selective catalytic reduction catalyst 5 having the property of selectively reacting NOx with ammonia even in the presence of oxygen is provided.

A urea water addition injector 7 (urea water addition means) for injecting urea water 6 as a reducing agent is installed in the exhaust pipe 4 upstream of the selective reduction catalyst 5 and the selective reduction catalyst. Immediately after 5, an NH ₃ slip catalyst 8 that oxidizes excess ammonia as a countermeasure against leaked ammonia is provided.

  Further, an oxidation catalyst 9 having an enhanced function of oxidizing unburned fuel in the exhaust gas 3 is provided in the exhaust pipe 4 upstream of the urea water 6 addition position by the urea water addition injector 7. In addition, immediately after the oxidation catalyst 9, a particulate filter 10 that integrally carries the oxidation catalyst is also provided.

  Immediately after the particulate filter 10, NOx in the exhaust gas 3 is physically adsorbed at a temperature of about 150 ° C. or lower which is lower than about 200 ° C. which is the lower limit activation temperature of the selective catalytic reduction catalyst 5, and A NOx adsorption catalyst 17 having a property of releasing the adsorbed NOx at a temperature close to the lower limit temperature of the selective catalytic reduction catalyst 5 (about 200 ° C.) is provided.

The NOx adsorption catalyst 17 referred to here is a NOx occlusion reduction catalyst that has already been put into practical use (NOx in the exhaust gas 3 is oxidized and temporarily occluded in the form of nitrate when the exhaust air-fuel ratio is lean). In addition, when the O ₂ concentration in the exhaust gas 3 is lowered, it has a property that is completely different from that having the property of decomposing and releasing NOx through the inclusion of unburned hydrocarbons, CO, etc. This means a flow-through type catalyst made of zeolite having excellent ability to physically adsorb NOx and hydrocarbons in gas 3.

  Zeolite is an alumina silicate porous crystal material, characterized by having uniform molecular level pores oriented regularly in the crystal, through which various molecules are inserted into cavities or channels. In addition to these properties, it has a molecular sieving action based on the uniform pores (only adsorbs molecules smaller than the pore diameter), It also has the property of strongly adsorbing polar substances by the action of cations and anions, and the property of having a catalytic action.

  In addition, this type of zeolite is classified into many types based on the type of its skeletal structure, but it has the ability to adsorb NOx and hydrocarbons into the pores under low exhaust temperature conditions, and the adsorbed hydrocarbons are low exhaust. What is necessary is just to select the thing which was excellent in the ability to make it react with oxygen in exhaust gas slowly under temperature conditions, and to make it a sub-oxide hydrocarbon, and high heat resistance and high durability. It is also possible to select from zeolite-related compounds.

  On the other hand, the accelerator of the driver's seat (not shown) is provided with an accelerator sensor 11 (load sensor) that detects the accelerator opening as a load of the diesel engine 1, and the rotational speed is set at an appropriate position of the diesel engine 1. The rotation sensor 12 to detect is equipped, The accelerator opening signal 11a and the rotation speed signal 12a from these accelerator sensor 11 and the rotation sensor 12 are input into the control apparatus 13 which comprises an engine control computer (ECU: Electronic Control Unit). It has come to be.

  Further, in the control device 13, the fuel injection timing and injection toward the fuel injection device 14 that injects the fuel into each cylinder according to the current operation state determined from the accelerator opening signal 11 a and the rotational speed signal 12 a. A fuel injection signal 14a for commanding the amount is output.

  Here, the fuel injection device 14 is constituted by a plurality of injectors (not shown) provided for each cylinder, and the electromagnetic valves of these injectors are appropriately opened by a fuel injection signal 14a from the control device 13. Thus, the fuel injection timing and the injection amount (valve opening time) are appropriately controlled.

  However, in the present embodiment, the control device 13 determines the fuel injection signal 14a in the normal mode based on the accelerator opening signal 11a and the rotation speed signal 12a, while the forced regeneration of the particulate filter 10 is performed. When it is necessary to perform this, the normal mode is switched to the regeneration mode, and the non-ignition timing (start timing) later than the compression top dead center following the main injection of fuel performed near the compression top dead center (crank angle 0 °). The fuel injection signal 14a is determined so as to perform post-injection at a crank angle of 90 ° to 130 °.

  That is, when post-injection is performed at a non-ignition timing later than the compression top dead center following main injection, unburned fuel (mainly HC: hydrocarbon) is added to the exhaust gas 3 by this post-injection. The unburned fuel undergoes an oxidation reaction while passing through the oxidation catalyst 9, and the catalyst bed temperature of the particulate filter 10 immediately after that rises due to the inflow of the exhaust gas 3 heated by the reaction heat. Thus, the particulates are burned off.

  Further, in the control device 13, the number of revolutions of the diesel engine 1 and the amount of fuel injection determined from the output value of the fuel injection signal 14a are extracted, and diesel is generated from the particulate generation map based on the number of revolutions and the amount of injection. The basic generation amount of particulates based on the current operation state of the engine 1 is estimated, and the basic generation amount is multiplied by a correction coefficient considering various conditions relating to the generation of particulates, and the current operation state The final generation amount is obtained by subtracting the particulate processing amount at, and the final generation amount is accumulated momentarily to estimate the particulate deposition amount, and the deposition amount reaches a predetermined set value. Sometimes, it is determined that the particulate filter 10 needs to be forcibly regenerated, and the exhaust temperature is determined based on the detection signal 15 a from the temperature sensor 15. There has been a switch from the normal mode to the heating mode waiting to become active lower limit temperature of the oxidation catalyst 9 (about 150 about ° C.) or higher.

  There are various ways of estimating the amount of particulate deposition, and it is of course possible to estimate the amount of particulate deposition using a method other than the estimation method exemplified here. It is also possible to estimate the accumulated amount of particulates based on the differential pressure before and after the curate filter 10, or to estimate the accumulated amount of particulates based on the operation time and travel distance.

  The control device 13 also estimates the amount of NOx generated based on the rotational speed of the diesel engine 1 and the amount of fuel injected, and the addition of a required amount of urea water 6 commensurate with the amount of NOx generated is the addition of urea water. The urea water injection signal 7 a is directed to the injector 7, but the urea water 6 is added according to the exhaust gas temperature immediately before the selective catalytic reduction catalyst 5 determined from the detection signal 16 a from the temperature sensor 16. The amount can be modified accordingly.

  Thus, in this way, even when the catalyst bed temperature of the selective catalytic reduction catalyst 5 does not reach the lower limit of activation temperature (about 200 ° C.), NOx in the exhaust gas 3 is physically transferred to the NOx adsorption catalyst 17. Therefore, the NOx emission amount is suppressed even under low exhaust temperature conditions where NOx cannot be reduced and purified by the selective catalytic reduction catalyst 5.

  When it is confirmed by the control device 13 based on the detection signal 16a from the temperature sensor 16 that the temperature of the exhaust gas 3 is equal to or higher than the lower limit temperature of the selective catalytic reduction catalyst 5 (about 200 ° C.), the control device. 13 outputs a urea water injection signal 7a to the urea water addition injector 7 and the urea water 6 is injected from the urea water addition injector 7 and is thermally decomposed into ammonia and carbon dioxide gas in the exhaust gas 3, thereby lowering the lower limit of activity. The NOx in the exhaust gas 3 reacts effectively with ammonia on the selective catalytic reduction catalyst 5 that is in an active state under temperature conditions (about 200 ° C.) or higher, and is reduced and purified well. .

  Moreover, at the time of starting after a long engine stop (during cold start) or the like, the NOx adsorption catalyst 17 arranged at the subsequent stage of the particulate filter 10 is equivalent to the heat capacity of the preceding particulate filter 10 (of the oxidation catalyst 9). The catalyst bed temperature is difficult to rise as much as the heat capacity can be included), and the release temperature (about 200 ° C.) is reached later than when the oxidation catalyst 9 in the previous stage of the particulate filter 10 has NOx adsorption ability. Become.

  For this reason, the timing at which the NOx adsorption catalyst 17 reaches the release temperature (about 200 ° C.) and the release of the adsorbed NOx starts, and the selective lower limit catalyst 5 has a lower activation limit temperature (close to the release temperature of the NOx adsorption catalyst 17). NOx is reduced by the selective catalytic reduction catalyst 5 even when the release of adsorbed NOx from the NOx adsorbing catalyst 17 starts when the conditions for reducing and purifying NOx by the addition of urea water 6 are approached. The time zone that cannot be purified is greatly shortened, and the situation where the amount of NOx emission increases during this time zone is avoided as much as possible.

  Here, according to the results of experiments conducted by the present inventors, the catalyst bed of the oxidation catalyst 9 is shown in FIG. 2 as a graph showing the transition of the exhaust gas temperature at the start after a long engine stop (cold start). The temperature changes as shown by curve A, the catalyst bed temperature of the NOx adsorption catalyst 17 changes as shown by curve B, and the catalyst bed temperature of the selective catalytic reduction catalyst 5 changes as shown by curve C. As can be seen from the graph, the catalyst bed temperature of the NOx adsorption catalyst 17 follows the rise of the catalyst bed temperature of the oxidation catalyst 9 with a significant time delay, while the selective reduction type against the rise of the catalyst bed temperature of the NOx adsorption catalyst 17. The catalyst bed temperature of the catalyst 5 follows with a slight time delay.

For this reason, if the oxidation catalyst 9 in the previous stage of the particulate filter 10 has the NOx adsorption ability, the catalyst bed temperature of the oxidation catalyst 9 reaches about 150 ° C. at the stage of t ₁ and the adsorption of NOx is completed. and, when the release of adsorbed NOx starts reaches here about much time is about 200 ° C. at the stage of t ₂ has not yet passed, the catalyst bed temperature of the selective reduction catalyst 5 at the stage of t ₅ a lapse of considerable time from here Therefore, a time zone x ′ in which the released NOx cannot be reduced and purified until the lower limit of activation temperature reaches about 200 ° C. is formed.

In contrast, as in this embodiment, if you place the NOx adsorption catalyst 17 downstream of the particulate filter 10, that the catalyst bed temperature continues to adsorb NOx until the stage of t ₃ after considerably from t ₁ In addition, the NOx adsorption catalyst 17 reaches about 200 ° C. at which NOx starts to be released. The t5 slightly before t _{5 when the} selective catalytic reduction catalyst 5 reaches about 200 ° C., which is the lowest activity temperature. since the _fourth stage, the time period x which can not reduce and purify the NOx emission in the selective reduction catalyst 5 be initiated adsorption NOx from the NOx adsorption catalyst 17, NOx adsorbed in the oxidation catalyst 9 in front of the particulate filter 10 It will be greatly shortened compared with the case of having the ability.

  Therefore, according to the above embodiment, NOx in the exhaust gas 3 is physically adsorbed to the NOx adsorption catalyst 17 even when the catalyst bed temperature of the selective catalytic reduction catalyst 5 has not reached about the activation lower limit temperature of about 200 ° C. Therefore, even if the vehicle is operated in such a manner that the operation state with a low exhaust temperature continues for a long time, a favorable NOx reduction effect can be obtained from a lower exhaust temperature than before. In addition, the time zone x during which NOx released from the NOx adsorption catalyst 17 cannot be reduced and purified by the selective reduction catalyst 5 at the start after a long engine stop (during cold start) or the like becomes extremely short. Therefore, it is possible to avoid as much as possible the situation in which the NOx emission amount increases in this time zone x.

  Note that the exhaust emission control device of the present invention is not limited to the above-described embodiment, and it is needless to say that various modifications can be made without departing from the gist of the present invention.

It is the schematic which shows an example of the form which implements this invention. It is a graph which shows the temperature transition of an oxidation catalyst, a NOx adsorption catalyst, and a selective reduction catalyst.

Explanation of symbols

1 Diesel engine (engine)
DESCRIPTION OF SYMBOLS 3 Exhaust gas 4 Exhaust pipe 5 Selective reduction type catalyst 6 Urea water 7 Injector for urea water addition 9 Oxidation catalyst 10 Particulate filter 17 NOx adsorption catalyst

Claims (1)

  A selective reduction catalyst that can selectively react NOx with ammonia even in the presence of oxygen is provided in the middle of the exhaust pipe, and urea water is added as a reducing agent in the exhaust pipe upstream of the selective reduction catalyst to reduce NOx. An exhaust purification device for purification, wherein a particulate filter having an oxidation catalyst attached to the upstream stage is disposed in an exhaust pipe upstream from a urea water addition position, and the particulate filter and the urea water Between the addition position, NOx in the exhaust gas is physically adsorbed at a temperature lower than the lower limit active temperature of the selective catalytic reduction catalyst, and the adsorbed NOx is at a temperature close to the lower active limit temperature of the selective catalytic reduction catalyst. An exhaust gas purification apparatus comprising a NOx adsorption catalyst having a releasing property.

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