JP2008019820A - Exhaust emission control system and exhaust emission control method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce a slip quantity of ammonia owing to improvement of ammonia oxidation capability, and at the same time to improve NOx emission control capability, in an exhaust emission control system with selective reduction NOx catalyst (selective catalytic reduction catalyst). <P>SOLUTION: The exhaust emission control system has an oxygen occlusion discharge function as it is provided with an oxidation catalyst 12 in a downstream of the selective reduction NOx catalyst 11 and Rh-CeO<SB>2</SB>is carried by the oxidation catalyst 12. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification system and an exhaust gas purification method capable of reducing the slip amount of ammonia and improving the NOx purification performance at low temperatures in an exhaust gas purification system provided with a selective reduction type NOx catalyst.

  In an exhaust gas purification system equipped with a selective reduction type NOx catalyst (SCR catalyst) for a diesel engine, as shown in FIG. 1, a selective reduction type NOx catalyst 11 and an oxidation catalyst (DOC) are sequentially entered into the exhaust gas passage 2 from the upstream side. An exhaust gas device 10 having a number 12 is disposed, and an ammonia-based solution W that generates ammonia, such as ammonia and aqueous urea, is selectively reduced from an ammonia-based solution injection device 20 disposed upstream of the exhaust gas device 10. The NOx is purified by supplying it to the NOx catalyst 11 and selectively reacting NOx and ammonia in the exhaust gas G (see, for example, Patent Document 1).

In such an oxidation catalyst 12 of the conventional exhaust gas purification device 10, as shown in FIG. 9, platinum (Pt) and aluminum oxide (aluminum oxide (Pt) are supported on a carrier 12a such as a porous ceramic honeycomb structure such as a cordierite honeycomb. Al ₂ O ₃ ) and zeolite are supported.

In this conventional apparatus, the NOx purification rate is improved by improving the ammonia (NH ₃ ) adsorption ability of the selective reduction type NOx catalyst 11 and adding a selective reduction function to the oxidation catalyst 1 2 disposed downstream. . Further, in order to prevent ammonia slip, which is the outflow of ammonia to the downstream side, unreacted ammonia and oxygen that have flowed out of the selective reduction type NOx catalyst 11 are reacted with nitrogen by the oxidation catalyst 12 provided on the downstream side to be nitrogenated. This reduces the amount of ammonia released.

However, in this conventional apparatus, if the ammonia adsorption capacity of the selective reduction type NOx catalyst is increased, the original NOx purification performance is lowered. Therefore, the ability to oxidize ammonia and reduce ammonia slip and the NOx purification rate are improved. It is difficult to achieve both of these, and also when the selective reduction function is imparted to the oxidation catalyst, there is a problem that it is difficult to achieve both.
JP 2006-051499 A

  The present invention has been made to solve the above-described problems, and an object of the present invention is to reduce the slip amount of ammonia and improve the NOx amount by improving the ammonia oxidation ability in an exhaust gas purification system equipped with a selective reduction NOx catalyst. An object of the present invention is to provide an exhaust gas purification system and an exhaust gas purification method capable of achieving both improvements in purification performance.

  An exhaust gas purification system for achieving the above object is an exhaust gas purification system comprising a selective reduction type NOx catalyst (SCR catalyst) in an exhaust gas passage, wherein an oxidation catalyst is provided downstream of the selective reduction type NOx catalyst. (DOC) and the oxidation catalyst is provided with an oxygen storage / release function (OSC function).

In the exhaust gas purification system, the oxidation catalyst can have an oxygen storage / release function by supporting the oxidation catalyst with Rh—CeO ₂ .

According to this exhaust gas purification system, even if the selective reduction type NOx catalyst cannot sufficiently purify NOx in a low temperature range where the ammonia cannot be supplied below the activation temperature of the selective reduction type NOx catalyst, Rh-CeO. An oxidation catalyst having an oxygen storage / release function, such as a ₂ / Pt—Al ₂ O ₃ catalyst, exhibits a NOx purification rate of about 30 to 60% at about 100 ° C. to 300 ° C., and thus can purify NOx. In this low temperature range, ammonia is not supplied, so NOx can be purified without worrying about reaction inhibition by ammonia.

  On the other hand, when the temperature of the exhaust gas rises and ammonia is supplied or adsorbed to the selective reduction type NOx catalyst sufficiently or more, the selective reduction type NOx catalyst exhibits a NOx purification rate of 60 to 90% or more. In some cases, ammonia flows out downstream of the selective reduction type NOx catalyst due to excessive ammonia. However, in this exhaust gas purification system, when NOx is purified by the selective reduction type NOx catalyst, the oxidation catalyst having the oxygen storage / release function on the downstream side functions as an oxidation catalyst, and excess ammonia is reduced to nitrogen. And can be purified.

Therefore, by disposing an oxidation catalyst having an oxygen storage / release function, such as a Rh—CeO ₂ / Pt—Al ₂ O ₃ catalyst, on the downstream side of the selective reduction type NOx catalyst, in a low temperature range where ammonia is not supplied, NOx The purification rate can be improved, and in the high temperature region where ammonia is supplied, ammonia flowing out downstream of the selective reduction type NOx catalyst can be efficiently decomposed and purified, so that the slip amount of ammonia can be reduced.

  Further, in the exhaust gas purification system, an ammonia supply device that supplies ammonia or an ammonia-based solution that generates ammonia is disposed upstream of the selective reduction type NOx catalyst, and a second oxidation device is disposed upstream of the ammonia supply device. Even in the case where the catalyst is provided, the hydrocarbon (HC), which is a reducing agent in the exhaust gas, is produced by the second oxidation catalyst at a low temperature at which the ammonia cannot be supplied below the activation temperature of the selective reduction NOx catalyst. , Carbon monoxide (CO) is purified, and NOx purification rate by catalytic reduction reaction using hydrocarbons is reduced, but even at low temperatures where this ammonia cannot be supplied by an oxidation catalyst having an oxygen storage / release function, The NOx purification rate can be improved.

  An exhaust gas purification method for achieving the above object is an exhaust gas purification method for purifying exhaust gas using a selective reduction type NOx catalyst, after passing the exhaust gas through the selective reduction type NOx catalyst. The exhaust gas is purified by passing through an oxidation catalyst having an oxygen storage / release function.

  By this method, as in the exhaust gas purification system described above, the NOx purification rate can be improved in the low temperature range where ammonia is not supplied, and the exhaust gas flows out downstream of the selective reduction type NOx catalyst in the high temperature region where ammonia is supplied. Ammonia slip amount can be reduced because the coming ammonia can be efficiently decomposed and purified.

  According to the exhaust gas purification system and the exhaust gas purification method of the present invention, by disposing an oxidation catalyst (DOC) having an oxygen storage / release function (OSC function) on the downstream side of the selective reduction type NOx catalyst (SCR catalyst). Even when the selective reduction type NOx catalyst is not activated, the oxidation catalyst can purify NOx passing through the selective reduction type NOx catalyst, and when the selective reduction type NOx catalyst is activated, the selective reduction type NOx catalyst is selected. Ammonia flowing downstream from the reduced NOx catalyst can be oxidized and detoxified. Therefore, it is possible to achieve both reduction of the slip amount of ammonia and improvement of NOx purification performance.

  Hereinafter, an exhaust gas purification system and an exhaust gas purification method according to embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows a configuration of an exhaust gas purification system 1 according to a first embodiment of the present invention. In this exhaust gas purification system 1, a selective reduction type NOx catalyst (SCR catalyst) 11 is provided upstream in an exhaust gas passage 2 of a diesel internal combustion engine or the like, and an oxidation catalyst having an oxygen storage / release function (OSC function) on the downstream side. An exhaust gas purifying device 10 having (DOC) 12 is arranged.

  This selective reduction type NOx catalyst 11 is formed on a carrier such as a honeycomb structure formed of cordierite, aluminum oxide, titanium oxide or the like, titania-vanadium, zeolite, chromium oxide, manganese oxide, molybdenum oxide, titanium oxide, tungsten oxide. Etc. are formed.

Further, as shown in FIG. 3, the oxidation catalyst 12 is formed on a carrier 12a such as a porous ceramic honeycomb structure such as a cordierite honeycomb, and rhodium (Rh), cerium oxide (CeO ₂ ), platinum (Pt). And Rh—CeO ₂ / Pt—Al ₂ O ₃ catalyst supporting aluminum oxide (Al ₂ O ₃ ). With this Rh—CeO ₂ , the oxidation catalyst 12 can exhibit an oxygen storage / release function.

Further, an ammonia supply device 20 that supplies ammonia (NH ₃ ) or an ammonia-based solution W that generates ammonia such as urea water or ammonia water is disposed upstream of the exhaust gas purification device 10. Alternatively, the ammonia-based solution W is injected into the exhaust gas passage 2 and ammonia is supplied to the selective reduction type NOx catalyst 11.

  When the exhaust gas G passes through the exhaust gas purification device 10, the exhaust gas G receives the purification action of the selective reduction type NOx catalyst 11 and the oxidation catalyst 12 and is discharged as purified exhaust gas Gc.

  Next, FIG. 2 shows an exhaust gas purification system 1A of the second embodiment. In this exhaust gas purification system 1A, in addition to the configuration of the exhaust gas purification system 1 of the first embodiment, a second oxidation catalyst (pre-stage oxidation catalyst) 13 is disposed upstream of the ammonia supply device 20. That is, the upstream oxidation catalyst 13, the ammonia-based solution injection device 20, the selective reduction NOx catalyst 11, and the downstream oxidation catalyst 12 are arranged in order from the upstream side of the exhaust passage 2. The upstream oxidation catalyst 13 may be provided as part of a continuous regeneration type diesel particulate filter device (continuous regeneration type DPF device).

In this exhaust gas purification system 1A, the rear-stage oxidation catalyst 12 has an oxygen storage / release function, but the front-stage oxidation catalyst 13 does not have an oxygen storage / release function. This pre-stage oxidation catalyst 13 is a Pt—Al ₂ O ₃ catalyst or the like in which platinum (Pt) or aluminum oxide (Al ₂ O ₃ ) is supported on a support such as a porous ceramic honeycomb structure such as a cordierite honeycomb. It is formed.

  Next, the NOx purification action in the exhaust gas purification systems 1 and 1A of the first and second embodiments will be described focusing on the role and function of the oxidation catalyst 12 having an oxygen storage / release function.

In the selective reduction type NOx catalyst 11, ammonia or an ammonia-based solution W is supplied from the ammonia supply device 20 when the catalyst activation temperature is higher than, for example, 170 ° C. in an oxygen-excess atmosphere, and the exhaust gas G is exhausted from the exhaust passage. 2 is mixed with ammonia, and NOx and ammonia in the exhaust gas G are selectively reduced (4NH ₃ + 4NO + O ₂ → 4N ₂ + 6H ₂ O, 4NH ₃ + 2NO ₂ + O ₂ → 3N ₂ + 6H ₂ O, etc.) NOx is reduced to nitrogen (N ₂ ) and purified.

  The selective reduction type NOx catalyst 11 does not require a control such as regeneration control for restoring the storage capacity unlike the storage reduction type NOx catalyst, so that NOx can be purified continuously. When the temperature of the exhaust gas G rises and ammonia is supplied or adsorbed to the selective reduction NOx catalyst 11 more than enough, the selective reduction NOx catalyst 11 exhibits a NOx purification rate of 60 to 90% or more. In the high temperature region where ammonia is supplied, the ammonia flowing out downstream of the selective reduction type NOx catalyst 11 is efficiently decomposed and purified by the oxidation catalyst 12, so that the amount of ammonia slip can be reduced.

On the other hand, in the oxidation catalyst 12 having an oxygen storage / release function, nitrogen oxides (NOx), hydrocarbons (HC), and carbon monoxide (CO) are adsorbed on the catalyst surface, and (CeO ₂ → (x / 2) O The reaction of ₂ + 2CeO _2-x ) reduces cerium oxide (ceria: CeO ₂ ). Next, NOx is reduced on the noble metal surface of rhodium (Rh) by a reaction of (2NO → 2N + O ₂ ). Next, CeO _2-x is oxidized by oxygen (O ₂ ) generated by this reduction by ((x / 2) O ₂ + CeO _2-x → CeO ₂ ). This reaction is repeated to purify NOx. In the case of the interaction between rhodium and cerium oxide, the reaction proceeds more easily when the amount of HC and CO is equal to or less than the amount of NOx.

  The activation temperature (light-off temperature) for hydrocarbons is lowered to about 150 ° C. due to the interaction between rhodium and cerium oxide in the oxidation catalyst 12, that is, the direct reduction action of NOx on the rhodium surface. The reduction reaction occurs from 150 ° C. Further, since the direct reduction reaction of NOx also occurs due to the interaction between rhodium and cerium oxide, the NOx purification window is widened. Moreover, the NOx purification rate is about 45% at the maximum.

  In the exhaust gas purification systems 1 and 1A, even when the selective reduction type NOx catalyst cannot sufficiently purify NOx in a low temperature range where the ammonia cannot be supplied below the activation temperature of the selective reduction type NOx catalyst 11, oxygen The oxidation catalyst 12 having the occlusion / release function can exhibit a NOx purification rate of about 30 to 60% at about 100 ° C. to 300 ° C. In this low temperature range, ammonia is not supplied, so NOx can be purified without worrying about reaction inhibition by ammonia.

  Therefore, according to the exhaust gas purification systems 1 and 1A, the selective reduction NOx catalyst 11 is selectively reduced by the oxidation catalyst 12 having the oxygen storage / release function downstream of the selective reduction NOx catalyst 11 at a low temperature at which the selective reduction NOx catalyst 11 is not activated. NOx that passes through the NOx catalyst 11 can be purified, ammonia slip can be prevented, and the NOx purification rate is improved. Further, when the selective reduction type NOx catalyst 11 is activated at a high temperature, the ammonia slipping the selective reduction type NOx catalyst 11 can be oxidized and detoxified. Therefore, it is possible to achieve both reduction of the ammonia slip amount and improvement of the NOx purification performance.

As an example of the present invention, a Rh—CeO ₂ / Pt—Al ₂ O ₃ catalyst made of rhodium (Rh), cerium oxide (CeO ₂ ), platinum (Pt), and aluminum oxide (Al ₂ O ₃ ) is used as a cordierite honeycomb. The support coated in was installed in the converter as an oxidation catalyst and attached to the exhaust pipe of a diesel engine. As a comparative example, instead of the above oxidation catalyst, a support in which a zeolite / Pt—Al ₂ O ₃ catalyst was coated on a cordierite honeycomb was incorporated as an oxidation catalyst in a converter and attached to an exhaust pipe of a diesel engine.

Then, NOx purification experiments were conducted using simulated gases of hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxide (NOx), respectively 500 ppm, oxygen (O ₂ ) 10%, and remaining nitrogen (N ₂ ). went. FIG. 4 shows the NOx purification rate of the experiment using this simulated gas with only the above oxidation catalyst.

In the example (solid line A), the hydrocarbon light-off (reaction activation) temperature is lowered by the interaction between rhodium and cerium oxide (CeO ₂ ), which directly reduces NOx on the rhodium (Rh) surface ( In FIG. 4, the solid line A is 150 ° C.), and the selective reduction reaction of hydrocarbon and NOx ( ₂ × NO + 4HxCy + (3x + y) O ₂ → xN ₂ + 4xCO ₂ + 2yH ₂ O) occurs from 150 ° C. NOx direct reduction reaction (CeO ₂ → (x / 2) O ₂ + 2CeO _2−x , 2NO → 2N + O ₂ , (x / 2) O ₂ + CeO _2−x → CeO ₂ ) occurs, so the NOx purification window is wide Become. In addition, the maximum purification rate was about 45%.

On the other hand, in the comparative example (dotted line B), when ammonia (NH ₃ ) is not included in the exhaust gas, it is higher than the light-off temperature of the oxidation catalyst hydrocarbon (200 ° C. in dotted line B in FIG. 4). Then, a selective reduction reaction of hydrocarbon (HxCy) and NOx contained in the exhaust gas (2xNO + 4HxCy + (3x + y) O ₂ → xN ₂ + 4xCO ₂ + 2yH ₂ O) occurs. At that time, a maximum NOx purification rate of about 30% can be obtained, but the NOx purification window is very narrow.

Moreover, the NOx purification rate and ammonia (NH ₃ ) of Example (A) and Comparative Example (B) of the experimental results of the actual exhaust gas in a state where the selective reduction type NOx catalyst and the above oxidation catalyst are combined. The outflow amounts are shown in FIG. 5 and FIG. 6, respectively, and the average NOx purification rate in the JE05 mode (transient running mode based on the running situation in the city) is shown in FIG.

Further, in FIG. 8, the upstream oxidation catalyst, the urea injection device, the selective reduction type NOx catalyst (SCR catalyst), and the downstream oxidation catalyst are arranged in order from the upstream side in the exhaust passage, and the NOx purification rate is increased with the exhaust gas of the actual machine. The measurement results are shown. The lower limit temperature for injecting urea in front of the selective reduction type NOx catalyst was 175 ° C. According to FIG. 8, in the example (solid line A), in the low temperature range of 175 ° C. or lower where urea cannot be injected by disposing the oxidation catalyst supported by Rh—CeO ₂ with the oxygen storage / release function in the post-stage oxidation catalyst. Even if it exists, it turns out that a NOx purification rate can be improved from about 130 degreeC.

It is a figure showing typically composition of an exhaust-gas purification system of a 1st embodiment concerning the present invention. It is a figure which shows typically the structure of the exhaust-gas purification system of 2nd Embodiment which concerns on this invention. It is a figure which shows typically the structure of the oxidation catalyst which has an oxygen storage / release function. It is a figure which shows the NOx purification rate in the simulation gas of the Example and comparative example only by an oxidation catalyst. It is a figure which shows the NOx purification rate of the Example by the combination of a selective reduction type NOx catalyst and an oxidation catalyst, and a comparative example. It is a figure which shows the ammonia slip amount of the Example by the combination of a selective reduction type NOx catalyst and an oxidation catalyst, and a comparative example. It is a figure which shows the NOx purification rate in the JE05 mode of the Example by the combination of a selective reduction type NOx catalyst and an oxidation catalyst, and a comparative example. It is a figure which shows the NOx purification rate of the Example by the combination of a front | former stage oxidation catalyst, a selective reduction type NOx catalyst, and an oxidation catalyst, and a comparative example. It is a figure which shows typically the structure of the oxidation catalyst of a prior art.

Explanation of symbols

E engine 1, 1A exhaust gas purification system 2 intake passage 10 exhaust gas purification device 11 selective reduction type NO catalyst 12 oxidation catalyst (rear-stage oxidation catalyst)
13 Oxidation catalyst (previous oxidation catalyst: second oxidation catalyst)
20 Ammonia supply device G Exhaust gas Gc Purified exhaust gas W Ammonia-based solution

Claims (4)

  In the exhaust gas purification system having a selective reduction type NOx catalyst in an exhaust gas passage, an oxidation catalyst is provided downstream of the selective reduction type NOx catalyst, and the oxidation catalyst has an oxygen storage / release function. Exhaust gas purification system. The exhaust gas purification system according to claim 1, wherein the oxidation catalyst is provided with an oxygen storage / release function by supporting Rh-CeO ₂ on the oxidation catalyst.   An ammonia supply device that supplies ammonia or an ammonia-based solution that generates ammonia is disposed upstream of the selective reduction type NOx catalyst, and a second oxidation catalyst is provided upstream of the ammonia supply device. Item 3. The exhaust gas purification system according to Item 1 or 2. In an exhaust gas purification method for purifying exhaust gas using a selective reduction type NOx catalyst, after passing the exhaust gas through the selective reduction type NOx catalyst, the exhaust gas is purified by passing through an oxidation catalyst having an oxygen storage / release function. An exhaust gas purification method characterized by the above.

Images