JP2014084785A - Exhaust emission control system for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To keep the NOx eliminating rate of a whole system high during both regeneration and non-regeneration of an exhaust emission control filter.SOLUTION: An exhaust emission control system includes the exhaust emission control filter provided in an exhaust passage for trapping PMs from exhaust gas, a downstream catalyst converter provided downstream of the filter, a first SCR catalyst supported by the exhaust emission control filter, and storing NHfor eliminating NOx from the exhaust gas under the existence of the NH, a second SCR catalyst supported by a base material for the downstream catalyst converter, and storing NHfor eliminating NOx from the exhaust gas under the existence of the NH, and temperature increasing means for increasing the temperature of the exhaust emission control filter to burn and remove the PMs trapped by the filter. The first SCR catalyst has higher NOx eliminating performance when in a low temperature region than when in a high temperature region, and the second SCR catalyst has higher NOx eliminating performance when in a high temperature region than when in a low temperature region.

Description

  The present invention relates to an exhaust gas purification system for an internal combustion engine. More specifically, the present invention relates to an exhaust gas purification system for an internal combustion engine in which a plurality of catalysts that store a reducing agent and purify NOx in exhaust gas using the reducing agent are provided in an exhaust passage.

Conventionally, as an exhaust gas purification system for purifying NOx in exhaust gas, a system in which a selective reduction catalyst for selectively reducing NOx in exhaust gas with NH ₃ is provided in an exhaust passage has been proposed. For example, in the exhaust purification system of urea addition type, the NH ₃ by supplying urea water which is a precursor of NH ₃ from the upstream side of the selective reduction catalyst, thermal decomposition or hydrolysis in the exhaust heat from the urea water The NOx in the exhaust gas is selectively reduced by this NH ₃ . In addition to such a urea addition type system, for example, a system in which NH ₃ is generated by heating a NH ₃ compound such as ammonia carbide and this NH ₃ is directly added has also been proposed. Hereinafter, a urea addition type system will be described.

Such a selective reduction catalyst has the ability to store NH ₃ that has not been consumed in the reduction of NOx in the exhaust. That is, when the amount of urea water supplied is larger than the amount of NOx flowing into the selective reduction catalyst, the surplus NH _{3 that} is not consumed for the reduction of NOx is stored in the selective reduction catalyst. Conversely, when the supply amount of urea water is smaller than the NOx amount flowing into the selective reduction catalyst, NH ₃ stored in the selective reduction catalyst is consumed for the reduction of NOx.

  As for the selective reduction catalyst having both the reducing agent storage function and the NOx purification function as described above, there has been proposed a technique in which a plurality of types of selective reduction catalysts having different temperature characteristics are provided in the exhaust passage. For example, an exhaust gas purification system disclosed in Patent Document 1 includes a filter provided with a first selective reduction catalyst and a catalytic converter provided downstream of the filter and provided with a second selective reduction catalyst. In this exhaust purification system, a filter that exhibits high NOx purification performance in a high temperature range is used as a selective reduction catalyst for a filter that is exposed to a relatively high temperature, and a downstream second selective reduction catalyst that is exposed to a relatively low temperature. For this, a material that exhibits high NOx purification performance in a low temperature range is used.

Special table 2012-514157 gazette

However, in the system of Patent Document 1, the NOx purification performance of the entire system when the filter is regenerated is not fully studied. For example, when the regeneration of the filter is started, the temperature of the filter and the first selective reduction catalyst provided in the filter rapidly rises to about 600 ° C. At this time, from the first selective reduction catalyst, NH ₃ stored in the selective reduction catalyst is discharged downstream (NH ₃ slip) in a short time. This is because the NH ₃ storage capacity of a general selective reduction catalyst decreases as the temperature increases.

On the other hand, when the filter is regenerated, the temperature of the second selective reduction catalyst provided on the downstream side of the first selective reduction catalyst also rises to the same level as that of the first selective reduction catalyst. When the regeneration of the filter is started, a large amount of NH ₃ slips from the first selective reduction catalyst and flows into the second selective reduction catalyst as described above. However, since the second selective reduction catalyst cannot exhibit sufficient NOx purification performance in a high temperature range, it cannot sufficiently purify NOx in the exhaust gas using the slipped NH ₃ . For this reason, from the second selective reduction catalyst, NH ₃ that has not been consumed in the reduction slips together with NOx that could not be purified. Therefore, in the system of Patent Document 1, during the regeneration of the filter, the NOx purification rate of the entire system including the first and second selective reduction catalysts is remarkably lowered, and excessive NH ₃ slip is also generated outside the system. It is thought that.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide an exhaust purification system capable of maintaining a high NOx purification rate in the entire system both during regeneration and non-regeneration of a filter.

  (1) An exhaust purification system (for example, an exhaust purification system 2 to be described later) of an internal combustion engine (for example, an engine 1 to be described later) of the present invention is provided in an exhaust passage (for example, a casing 35 to be described later) of the internal combustion engine. A filter (for example, an exhaust purification filter 33 described later) that collects particulate matter therein, a catalytic converter (for example, a downstream catalytic converter 34 described later) provided downstream of the filter, and supported by the filter, A first catalyst that stores the reducing agent and purifies NOx in the exhaust gas in the presence of the reducing agent (for example, a first SCR catalyst described later), and is supported on the base of the catalytic converter, stores the reducing agent, and reduces it. A second catalyst that purifies NOx in the exhaust in the presence of the agent (for example, a second SCR catalyst described later), and particulate matter collected by the filter by raising the temperature of the filter The comprises a heating device for removing combustion (e.g., the engine 1 and ECU4 below), the. When the first catalyst is in a predetermined first temperature range, the NOx purification performance is higher than when the first catalyst is in a second temperature range higher than the first temperature range, and the second catalyst is in the second temperature range. In some cases, the NOx purification performance is higher than in the first temperature range.

  (2) In this case, it is preferable that the first catalyst contains Cu zeolite and the second catalyst contains Fe zeolite.

(3) In this case, the exhaust passage further includes a reducing agent supply device (for example, a reducing agent supply device 32 described later) for supplying NH ₃ or a precursor thereof upstream of the filter. The two catalyst is preferably a selective reduction catalyst capable of reducing NOx using NH ₃ as a reducing agent and storing a predetermined amount of NH ₃ .

(1) In the present invention, a catalytic converter is provided downstream of the filter, the first catalyst is supported on the filter, and the second catalyst is supported on the catalytic converter. The first catalyst has a NOx purification performance that increases in the first temperature range, which is a relatively low temperature range, and the second catalyst has a NOx purification performance that has a relatively high temperature range. What became high in 2 temperature range was used. Thereby, when the filter and the catalytic converter are in a low temperature range, NOx in the exhaust gas can be purified mainly by the first catalyst of the filter.
Further, when the regeneration of the filter is started by the temperature raising means, the temperature of the filter and the downstream catalytic converter rises to a high temperature range. At this time, since the NOx purification performance of the first catalyst provided in the upstream filter is reduced, the first catalyst could not completely purify from the first catalyst to the second catalyst provided in the downstream catalytic converter. NOx slips with the reducing agent stored in the first catalyst. However, since the second catalyst has a high NOx purification performance at a relatively high temperature range with respect to the first catalyst, the NOx slipped from the first catalyst is converted into the reducing agent slipped from the first catalyst and the second catalyst until then. It can be purified using the reducing agent stored in Thereby, it is possible to maintain high NOx purification performance in the entire system including the first catalyst and the second catalyst both during regeneration and non-regeneration of the filter.

  (2) The first catalyst containing Cu zeolite exhibits particularly excellent NOx purification performance in a low temperature range of about 250 to 350 ° C., but the NOx purification performance decreases at higher temperatures. On the other hand, the second catalyst containing Fe zeolite exhibits particularly excellent NOx purification performance in a high temperature range of about 550 to 600 ° C., but the NOx purification performance decreases at lower temperatures. Therefore, while the filter is not being regenerated and the filter and the catalytic converter are in the low temperature range, NOx in the exhaust gas is purified mainly by the first catalyst provided in the filter. Further, while the filter is being regenerated and the filter and the catalytic converter are in the high temperature range, the NOx in the exhaust gas is purified mainly by the second catalyst provided on the downstream side of the filter.

(3) Although the selective reduction catalyst is particularly excellent in the ability to store NH ₃ as a reducing agent, the storage ability has a characteristic of decreasing as the temperature increases. For this reason, when the temperature of the selective reduction catalyst rapidly increases, a large amount of NH ₃ stored so far slips downstream. In the present invention, such a selective reduction catalyst is used as the first and second catalysts, and as described above, the second catalyst is used whose NOx purification performance is relatively high in a high temperature range. Thus, even when a large amount of NH ₃ slips from the first catalyst whose temperature has suddenly increased during regeneration of the filter, NOx can be reduced by the second catalyst using the slipped NH ₃ as a reducing agent.

It is a mimetic diagram showing composition of an engine and its exhaust gas purification system concerning one embodiment of the present invention. It is a figure which shows the temperature characteristic of the NOx purification rate of a 1st and 2nd SCR catalyst. Is a diagram showing the temperature specified maximum NH ₃ storage amount of the first and second 2SCR catalyst.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic diagram showing a configuration of an internal combustion engine (hereinafter referred to as “engine”) 1 and an exhaust purification system 2 thereof according to an embodiment of the present invention. The engine 1 is a lean burn operation type gasoline engine or diesel engine, and is mounted on a vehicle (not shown).

  The exhaust purification system 2 includes a catalyst purification unit 3 containing a plurality of catalysts for purifying exhaust from the engine 1, and an electronic control unit (controlling fuel injection amount and fuel injection timing from a fuel injector (not shown) of the engine 1). (Hereinafter referred to as “ECU”) 4.

  The catalytic purification unit 3 includes an upstream catalytic converter 31, a reducing agent supply device 32, an exhaust purification filter 33, and a downstream catalytic converter 34 arranged in series in this order in a cylindrical casing 35. Is done. The upstream catalytic converter 31 side of the casing 35 is connected to an exhaust manifold 11 extending from an exhaust port (not shown) of the engine 1. As a result, the inside of the casing 35 becomes a part of the exhaust passage of the engine 1. The catalyst purification unit 3 is provided immediately below the engine 1, more specifically, adjacent to the engine 1.

The upstream catalytic converter 31 is configured by using a flow-through honeycomb structure as a base material and supporting an oxidation catalyst on the base material. HC and CO contained in the exhaust discharged from the engine 1 are oxidized by the action of the oxidation catalyst in the process of passing through the upstream catalytic converter 31. Further, NO contained in the exhaust is also oxidized to NO ₂ in the process of passing through the upstream catalytic converter 31. Generate almost all places not included and NO ₂ is almost a NO _(NO 2 / NOx ratio are substantially 0), and oxidizes NO in the upstream catalytic converter 31 NO ₂ among NOx contained in the exhaust immediately after the engine 1 As a result, the NO ₂ / NOx ratio of the exhaust gas flowing into the exhaust gas purification filter 33 can be increased to about 0.5 where the NOx purification performance of the SCR catalyst described later is optimized.

  The exhaust purification filter 33 includes a honeycomb structure having a plurality of cells partitioned by a porous wall, and plugs provided alternately on the upstream side and the downstream side with respect to each cell. Particulate matter containing carbon as a main component (hereinafter referred to as “PM”) contained in the exhaust discharged from the engine 1 is collected in the process of passing through the pores of the porous wall of the exhaust purification filter 33.

  If PM accumulates on the exhaust purification filter 33, the pressure loss increases and the fuel consumption may deteriorate. Therefore, when the amount of PM deposited on the exhaust purification filter 33 exceeds a predetermined amount, the exhaust gas purification filter 33 is heated to perform a filter regeneration process for burning and removing the PM collected by the filter 33. More specifically, when the PM accumulation amount exceeds a predetermined amount, the ECU 4 performs post injection (fuel injection in the exhaust process) by the fuel injector of the engine 1 to burn unburned fuel in the upstream catalytic converter 31. Then, the temperature of the filter 33 is raised to a predetermined forced regeneration temperature (about 600 ° C.), and the PM collected by the filter 33 is burned and removed.

The honeycomb structure of the exhaust purification filter 33 carries a first selective reduction catalyst (hereinafter referred to as “first SCR catalyst”). This first SCR catalyst uses NH ₃ as a reducing agent and selectively reduces NOx in the exhaust in an atmosphere in which NH ₃ exists. Specifically, when NH _{3 is} supplied from a reducing agent supply device 32 described later, this NH ₃ selectively reduces NOx in the exhaust according to the following three reaction formulas.
NO + NO ₂ + 2NH ₃ → 2N ₂ + 3H ₂ O
4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O
6NO ₂ + 8NH ₃ → 7N ₂ + 12H ₂ O

Further, the first 1SCR catalyst has a function of reducing NOx in the exhaust in NH _3, has a function of storing NH ₃ by a predetermined amount. Hereinafter, the amount of NH ₃ stored in the SCR catalyst called NH ₃ storage amount, the limitations of this NH ₃ storage amount of up to NH ₃ storage amount. When the NH ₃ storage amount of the SCR catalyst exceeds the maximum NH ₃ storage amount, NH ₃ slips downstream. The NH ₃ stored in the first SCR catalyst in this manner is appropriately consumed for the reduction of NOx in the exhaust gas together with the NH ₃ supplied from the reducing agent supply device 32. Note that when a large amount of NH ₃ is present in the first SCR catalyst, the reactivity with the inflowing NOx is improved. Therefore, the NOx purification rate of the first SCR catalyst becomes higher as the amount of NH ₃ storage increases.

  Further, regarding the NOx purification performance of the entire exhaust purification system 2, the first SCR catalyst preferably contains Cu zeolite because the exhaust purification filter 33 mainly plays a role of NOx purification on the low temperature region side.

The downstream catalytic converter 34 has a flow-through type honeycomb structure as a base material and a second selective reduction catalyst (hereinafter referred to as “second SCR catalyst”) supported on the base material. The second SCR catalyst has the same NH ₃ storage function and NOx reduction function as the first SCR catalyst. Therefore, the detailed description about the effect | action is abbreviate | omitted.

  Further, regarding the NOx purification performance of the exhaust gas purification system 2 as a whole, unlike the exhaust gas purification filter 33, the downstream catalytic converter 34 mainly plays a role of NOx purification on the high temperature region side. Therefore, the second SCR catalyst is Fe zeolite. It is preferable to contain.

The reducing agent supply device 32 is provided between the exhaust purification filter 33 and the upstream catalytic converter 31 in the casing 35, and NH ₃ serving as a reducing agent for the first and second SCR catalysts is disposed upstream of the exhaust purification filter 33. Or a urea aqueous solution that is a precursor of NH ₃ is supplied. The urea water is thermally decomposed or hydrolyzed by the heat of the exhaust and the heat of the exhaust purification filter 33 to become NH ₃ .

Next, the difference in temperature characteristics between the first SCR catalyst carried on the exhaust purification filter 33 and the second SCR catalyst provided downstream thereof will be compared.
FIG. 2 is a graph showing the temperature characteristics of the NOx purification rates of the first and second SCR catalysts.

  As shown in FIG. 2, the NOx purification rate of the first SCR catalyst containing Cu zeolite exhibits an upwardly convex characteristic so as to be maximum in a low temperature range of about 250 to 300 ° C. with respect to the temperature. On the other hand, the NOx purification rate of the second SCR catalyst containing Fe zeolite increases as the temperature increases, and becomes maximum in a high temperature range of about 550 to 600 ° C. That is, when the first SCR catalyst is in the low temperature range, the NOx purification rate is higher than when it is in the high temperature range where the NOx purification rate of the second SCR catalyst is maximized. Further, when the second SCR catalyst is in the high temperature range, the NOx purification rate is higher than when it is in the low temperature range where the NOx purification rate of the first SCR catalyst is maximized.

  Here, the vicinity of about 250 to 300 ° C. at which the NOx purification rate of the first SCR catalyst is maximized is a temperature range that can be realized when the filter is not regenerated and during low load operation. On the other hand, the vicinity of about 550 to 600 ° C. at which the NOx purification rate of the second SCR catalyst is maximized is a temperature range that can be realized during regeneration of the filter or during high load operation.

FIG. 3 is a graph showing temperature characteristics of the maximum NH ₃ storage amount of the first and second SCR catalysts.
As shown in FIG. 3, the maximum NH ₃ storage amount of both the first and second SCR catalysts decreases as the temperature increases. Therefore, both the first and second SCR catalysts have a lower NH ₃ storage capacity as the temperature is higher, and NH ₃ slip is likely to occur.

  Next, functions of the exhaust purification filter 33 and the downstream catalytic converter 34 at the low temperature and the high temperature of the exhaust purification system 2 configured as described above will be described.

First, the case of low temperature will be described.
When the exhaust purification filter 33 is not being regenerated and running at a low load, both the exhaust purification filter 33 and the downstream catalytic converter 34 are in a low temperature range of about 250 to 300 ° C. When these are in the low temperature range, as described with reference to FIG. 2, the NOx purification rate of the second SCR catalyst of the downstream catalytic converter 34 is reduced, but the NOx purification rate of the first SCR catalyst of the exhaust purification filter 33 is Maximized. Therefore, NOx in the exhaust at a low temperature is reduced mainly by the first SCR catalyst of the exhaust purification filter 33.

Further, at the time of low temperature, as described with reference to FIG. 3, the maximum NH ₃ storage amount of the first and second SCR catalysts is high. Therefore, although the second SCR catalyst at low temperature cannot exhibit a sufficient NOx purification function as compared with the first SCR catalyst, NH ₃ slipped from the first SCR catalyst is stored and consumed for reduction of NOx, so that NH ₃ is reduced. Exhaust from the exhaust purification system 2 can be prevented. That is, at a low temperature, the second SCR catalyst can function exclusively as a slip suppression catalyst that suppresses the slip of NH ₃ to the outside of the exhaust purification system 2. In addition, by making the second SCR catalyst function as a slip suppression catalyst in this way, the first SCR catalyst can store a large amount of NH ₃ to the extent that NH ₃ slips downstream, and its NOx purification rate can be increased. It can be even higher.

Next, the case of high temperature will be described.
When the exhaust purification filter 33 is being regenerated or running at a high load, both the exhaust purification filter 33 and the downstream catalytic converter 34 are in a high temperature range of about 550 to 600 ° C. When these are in the high temperature range, as described with reference to FIG. 2, the NOx purification rate of the first SCR catalyst of the exhaust purification filter 33 decreases, but the NOx purification rate of the second SCR catalyst of the downstream catalytic converter 34 is Maximized. Therefore, NOx in the exhaust at high temperature is reduced mainly by the second SCR catalyst of the downstream catalytic converter 34.

Further, immediately after the start of regeneration of the exhaust purification filter 33 or immediately after shifting to the high load operation state, the temperature of both the filter 33 and the downstream catalytic converter 34 rises rapidly, and as described with reference to FIG. The maximum NH ₃ storage amount of each decreases rapidly.
Therefore, NH ₃ stored in the first SCR catalyst of the exhaust purification filter 33 will slip to the second SCR catalyst of the downstream catalytic converter 34, but the NOx purification rate becomes high when the second SCR catalyst becomes high temperature. consisting Therefore, since the NOx is reduced by using the NH ₃ which has slipped from the upstream side and the NH ₃ which has been stored in the 2SCR catalyst so far, a large amount of NH ₃ to the outside of the exhaust purification system 2 There is no slip. In addition, when the temperature of the second SCR catalyst also rises rapidly, the maximum amount of NH ₃ storage decreases sharply. As described above, the second SCR catalyst functions exclusively as an NH ₃ slip suppression catalyst while in the low temperature range. Therefore, unlike the first SCR catalyst, it is not necessary to store a large amount of NH ₃ . For this reason, a large amount of NH ₃ does not slip out of the exhaust purification system 2 from the second SCR catalyst during the transition to the high temperature range.

Although one embodiment of the present invention has been described above, the present invention is not limited to this.
In the above embodiment, an example in which the SCR catalyst is applied as the first catalyst and the second catalyst has been described. However, the present invention has both a function of storing the reducing agent and a function of purifying NOx by the reducing agent. Any catalyst other than the SCR catalyst can be used as long as the catalyst is provided. As such a catalyst, for example, a NOx occlusion reduction type catalyst described in Japanese Patent Application Laid-Open No. 2008-030003 by the applicant of the present application can be cited.

This NOx occlusion reduction type catalyst temporarily stores NOx in exhaust under a lean air-fuel ratio exhaust, and generates NOx temporarily stored when the exhaust air-fuel ratio is switched to rich by a water gas shift reaction in hydrogen causes converted to NH ₃ re store the NH _3. When the air-fuel ratio of the exhaust gas is switched to lean again, NOx in the exhaust gas is reduced using the re-stored NH ₃ as a reducing agent. When such NOx storage reduction catalyst also includes Cu zeolite, its NOx purification rate is maximized in a relatively low temperature range, and when it contains Fe zeolite, its NOx purification rate is relatively high. Maximized in the temperature range. For this reason, even if the NOx occlusion reduction type catalyst containing Cu zeolite is used as the first catalyst and the NOx occlusion reduction type catalyst containing Fe zeolite is used as the second catalyst, the same effects as those of the above embodiment can be obtained. Alternatively, an SCR catalyst may be used as the first catalyst and a NOx occlusion reduction type catalyst may be used as the second catalyst, or an NOx occlusion reduction type catalyst may be used as the first catalyst and an SCR catalyst may be used as the second catalyst.

1 ... Engine (internal combustion engine, temperature raising means)
2 ... Exhaust purification system 3 ... Catalyst purification unit 32 ... Reducing agent supply device 33 ... Exhaust purification filter (filter, first catalyst)
34 ... Downstream catalytic converter (catalytic converter, second catalyst)
4 ... ECU (temperature raising means)

Claims (3)

A filter provided in an exhaust passage of the internal combustion engine for collecting particulate matter in the exhaust;
A catalytic converter provided downstream of the filter;
A first catalyst carried on the filter for storing the reducing agent and purifying NOx in the exhaust in the presence of the reducing agent;
A second catalyst carried on the base material of the catalytic converter, storing the reducing agent and purifying NOx in the exhaust in the presence of the reducing agent;
An exhaust gas purification system for an internal combustion engine, comprising: a temperature raising means for burning and removing particulate matter collected by the filter by raising the temperature of the filter;
When the first catalyst is in a predetermined first temperature range, the NOx purification performance is higher than when it is in a second temperature range higher than the first temperature range,
The exhaust gas purification system for an internal combustion engine, wherein the second catalyst has higher NOx purification performance when in the second temperature range than when in the first temperature range. The first catalyst comprises Cu zeolite;
The exhaust purification system for an internal combustion engine according to claim 1, wherein the second catalyst contains Fe zeolite. A reducing agent supply device for supplying NH ₃ or a precursor thereof upstream of the filter in the exhaust passage;
Said first and second catalyst, an internal combustion engine exhaust gas purification according to claim 1 or 2, characterized in that the reducing life-and-death NH ₃ to NOx and NH ₃ as the reducing agent is a selective reduction catalyst capable predetermined storage amount system.

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