JPH04284826A - Method for purifying exhaust gas and apparatus therefor - Google Patents

Abstract

PURPOSE:To purify exhaust gas by catalytically decomposing nitrogen oxide in the exhaust gas into N2 and O2 in a relatively low temp. region under an oxygen rich atmosphere by a coexisting catalyst. CONSTITUTION:An exhaust gas purifying method consists of a reducing agent introducing process introducing hydrocarbon into exhaust gas as a reducing agent and a catalytic decomposition process bringing a catalyst composed of metal-containing zeolite containing at least one kind of an element selected from copper, manganese, cobalt, iron, nickel, chromium and vanadium capable of relatively easily changing in valence as oxide into contact with the exhaust gas into which hydrocarbon is introduced to decompose nitrogen oxide. The aforementioned hydrocarbon is characterized in that the number of carbon atoms is set to 8-16.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] This invention relates to an exhaust gas purification method and method for purifying nitrogen oxides in exhaust gas by catalytically decomposing them into N2 and O2 using a coexisting reducing agent in an oxygen-rich atmosphere. Regarding the device.

0002

[Prior Art] Conventionally, the technologies for removing nitrogen oxides, which are harmful components in exhaust gas from automobile engines, include purification methods using three-way catalysts such as Pt-Rh, and purification methods using ammonia, urea, etc. Purification methods that apply selective reduction methods, purification methods that use copper ion exchange zeolite catalysts, etc.
Various types are known.

However, in a purification method using a three-way catalyst such as a Pt-Rh system, nitrogen oxides can be removed on the rich side of the stoichiometric air-fuel ratio, but on the lean side (that is, in an oxygen-rich atmosphere), nitrogen oxides can be removed. , there is a fatal problem that cannot be removed. In addition, purification methods that apply selective reduction using ammonia, urea, etc. have a high purification rate of nitrogen oxides, but the equipment becomes large and new pollution occurs due to the secondary emission of ammonia. There is a problem with this. Furthermore, in purification methods using copper ion exchange zeolite catalysts, nitrogen oxides can be removed even on the lean side, unlike the purification methods using three-way catalysts such as the Pt-Rh system mentioned above. It has been pointed out that the purification efficiency is low and that the purification performance deteriorates at relatively low temperatures.

[0004] In recent years, in such conventional exhaust gas purification methods, even when oxygen coexists at a high concentration, such as in exhaust gas from a diesel engine or exhaust gas from a lean fuel gasoline engine, nitrogen oxidation As an exhaust gas purification method and catalyst that can stably decompose and remove substances,
A technique disclosed in Japanese Unexamined Patent Publication No. 100919/1983 is known. This conventional publication discloses that nitrogen oxides in the exhaust gas are removed by preparing a catalyst containing copper and bringing the exhaust gas containing nitrogen oxides into contact with the catalyst in the presence of hydrocarbons in an oxidizing atmosphere. and a catalyst for removing nitrogen oxides in exhaust gas in an oxidizing atmosphere, characterized in that copper is supported on a porous carrier such as alumina, silica, zeolite, etc. An exhaust gas purification catalyst is disclosed. As a result, by implementing the technical content disclosed in this prior art publication, a method and catalyst for efficiently removing nitrogen oxides in an oxidizing atmosphere will be provided.

[0005]

[Problems to be Solved by the Invention] However, even when such conventional technology is used, nitrogen oxides are decomposed by utilizing the high-temperature activity of the catalyst. In a low-temperature region where the exhaust gas temperature has not risen sufficiently, the catalyst cannot reach a sufficiently high temperature and cannot utilize its high-temperature activity, resulting in extremely poor purification performance. As a result, even when using the technology disclosed in the above-mentioned conventional publications, nitrogen oxides are released into the atmosphere without being decomposed until the catalyst temperature rises to a predetermined high temperature range. There remains a major issue.

Furthermore, as mentioned above, since the high-temperature activity of the catalyst is utilized, the catalyst itself is subject to severe high-temperature deterioration, and there remains a problem in terms of durability. This invention was made in view of the above-mentioned problems, and an object of the invention is to provide an exhaust gas purification method and an exhaust gas purification device that can obtain good nitrogen oxide purification performance from a relatively low temperature side. It is.

[0007]

[Means for Solving the Problems] In order to achieve the above object, the exhaust gas purification method according to the present invention includes a reducing agent introduction step of introducing hydrocarbons as a reducing agent into the exhaust gas, and a step of introducing hydrocarbons as a reducing agent into the exhaust gas. The exhaust gas is brought into contact with a catalyst made of a metal-containing zeolite containing at least one of copper, manganese, cobalt, iron, nickel, chromium, and panadium whose valence as an oxide can be changed relatively easily, The method includes a catalytic cracking step for decomposing nitrogen oxides, and the hydrocarbon is characterized in that the number of carbon atoms is set to 8 to 16.

[0008] Furthermore, in the exhaust gas purification method according to the present invention, the hydrocarbon is a linear normal paraffin. Further, the exhaust gas purification device according to the present invention includes a reducing agent introducing means for introducing a hydrocarbon having a carbon number of 8 to 16 as a reducing agent into the exhaust gas, and a method in which the hydrocarbon coexists with the reducing agent introducing means. The exhaust gas is brought into contact with a catalyst made of metal-containing zeolite containing at least one of copper, manganese, cobalt, iron, nickel, chromium, and panadium whose valence as an oxide can be changed relatively easily, and It is characterized by comprising a decomposition means for decomposing the oxide.

Further, in the exhaust gas purification device according to the present invention, the reducing agent introduction means includes a fuel reforming catalyst that reformes the fuel supplied to the engine into hydrocarbons having a carbon number of 8 to 16. It is characterized by the fact that it is equipped with

0010

[Operation] The operation of the exhaust gas purification method according to the present invention will be explained in detail below. First, the oxidizing atmosphere defined in the present invention is a stable atmosphere that completely oxidizes reducing substances such as carbon monoxide, hydrogen, and hydrocarbons contained in exhaust gas and hydrocarbons introduced in the present invention. H2 O and C
This refers to a state in which oxygen is contained in excess of the amount of oxygen required to convert it into O2. That is, it refers to the atmosphere of exhaust gas from a diesel engine or the atmosphere of exhaust gas from a gasoline engine that burns a lean mixture with a large air-fuel ratio (that is, lean).

[0011]The exhaust gas purification method according to the present invention involves introducing a hydrocarbon as a reducing agent into the exhaust gas in the above-mentioned oxidizing atmosphere, and then bringing it into contact with a catalyst made of metal-containing zeolite. Based on the discovery that in decomposing nitrogen oxides, by limiting the number of carbon atoms in hydrocarbons to a range of 8 to 16, good nitrogen oxide purification performance can be achieved even at relatively low temperatures. ing.

That is, when oxygen generated in an oxidizing atmosphere and by the decomposition of nitrogen oxides is adsorbed to the active sites of metal-containing zeolite, it becomes difficult for oxygen to be desorbed and nitrogen oxides lose adsorption sites, resulting in nitrogen oxidation. Purification of substances is limited, and it becomes impossible to maintain high nitrogen oxide purification performance continuously. Here, if specific hydrocarbons in the range of carbon numbers mentioned above are made to coexist in the exhaust gas to be purified, the purification efficiency of nitrogen oxides will be dramatically improved, especially in the relatively low temperature range. It will be possible to demonstrate. especially,
The reason why the purification performance is extended to a relatively low temperature region by using hydrocarbons ranging from octane having 8 carbon atoms to cetane having 16 carbon atoms is thought to be due to the following reason.

That is, the conversion rate of each hydrocarbon to combustion is
In all cases, the conversion rate is higher than that of nitrogen oxides, but the temperature range of combustion roughly corresponds to the conversion range of nitrogen oxides, and the combustion of hydrocarbons depletes the oxygen on the surface of the metal contained in the zeolite. This is because it is believed that this prevents excessive oxidation and helps the progress of the decomposition reaction of nitrogen oxides. especially,
By using straight-chain normal paraffins as hydrocarbons, these hydrocarbons can diffuse into the pores of the zeolite, adsorb to the active species on the surface of the zeolite pores, and suppress the adsorption of nitrogen oxides. Oxygen and this hydrocarbon form a chemical bond through oxidative combustion, and the oxygen is explosively taken away from the active species. In this way, the number of adsorption sites for nitrogen oxides increases, thereby dramatically improving the purification efficiency of nitrogen oxides.

[0014] For example, in hydrocarbons with a large number of carbon atoms such as cetane, the amount of oxygen taken away during combustion increases, and some cetane is not completely combusted, resulting in combustion intermediates (low molecular hydrocarbons). By remaining near the zeolite surface in the form of , reactions with adsorbed oxygen occur continuously, making it possible to continuously purify nitrogen oxides. Note that if the number of carbon atoms in the hydrocarbon exceeds a specific value or if the molecular structure is a cyclic hydrocarbon with a three-dimensional structure, the efficiency of purifying nitrogen oxides will not improve. This is because it becomes difficult for these hydrocarbons to diffuse into the zeolite pores.

[0015] The catalyst used in the present invention is a metal-containing zeolite, not a metal ion-exchanged zeolite. In addition, the metals contained in zeolite are copper,
It is at least one type of transition metal such as manganese, cobalt, iron, nickel, chromium, and panadium, and the combination thereof is not limited at all. As described above, by configuring the means for solving the problem, nitrogen oxides in exhaust gas can exhibit good purification performance from a relatively low temperature range even in an oxidizing atmosphere, for example, Even if the engine has not been sufficiently warmed up immediately after starting, nitrogen oxides can be purified early, making it possible to suppress nitrogen oxides from being released into the atmosphere as much as possible. Also,
This exhaust gas purification method exhibits good purification performance from a relatively low temperature range, so for example, the catalyst is placed in the exhaust pipe far away from the engine to minimize the heat received from the engine and exhaust gas. Therefore, by maintaining the temperature of the catalyst itself in a relatively low temperature range, the life of the catalyst can be extended.

[0016]

Effects of the Invention As detailed above, the exhaust gas purification method according to the present invention includes a reducing agent introduction step of introducing hydrocarbons as a reducing agent into exhaust gas, and a reducing agent introduction step of introducing hydrocarbons as a reducing agent into exhaust gas, and Nitrogen oxides are decomposed by contacting with a catalyst made of metal-containing zeolite containing at least one of copper, manganese, cobalt, iron, nickel, chromium, and panadium, whose valence as an oxide can be changed relatively easily. The hydrocarbon is characterized in that the number of carbon atoms is set to 8 to 16.

Furthermore, in the exhaust gas purification method according to the present invention, the hydrocarbon is a linear normal paraffin. Further, the exhaust gas purification device according to the present invention includes a reducing agent introducing means for introducing a hydrocarbon having a carbon number of 8 to 16 as a reducing agent into the exhaust gas, and a method in which the hydrocarbon coexists with the reducing agent introducing means. The exhaust gas is brought into contact with a catalyst made of metal-containing zeolite containing at least one of copper, manganese, cobalt, iron, nickel, chromium, and panadium whose valence as an oxide can be changed relatively easily, and It is characterized by comprising a decomposition means for decomposing the oxide.

Further, in the exhaust gas purification device according to the present invention, the reducing agent introduction means includes a fuel reforming catalyst that reformes the fuel supplied to the engine into a hydrocarbon having a carbon number of 8 to 16. It is characterized by the fact that it is equipped with Therefore, according to the present invention, there is provided an exhaust gas purification method and an exhaust gas purification apparatus that can obtain good nitrogen oxide purification performance from a relatively low temperature side.

Furthermore, compared to the method of purifying exhaust gas using a three-way catalyst, it is possible to purify nitrogen oxides with high efficiency even in an oxidizing atmosphere, eliminating exhaust gas from diesel engines and gasoline engines. In addition, compared to selective reduction methods using ammonia, etc., the equipment is extremely small and inexpensive, and there is no problem of secondary pollution such as excessive ammonia discharge. Compared to replacement zeolite, it has superior thermal stability and can maintain stable purification performance for a long time even when used under conditions where the exhaust gas temperature is high.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The structure of an embodiment of the exhaust gas purification method and apparatus according to the present invention will be explained in detail below with reference to FIGS. 1 and 5. First, copper-containing type A zeolite constituting the catalyst used in this example was prepared in the following manner.

That is, predetermined amounts of copper nitrate, sodium aluminate, water glass, and sodium hydroxide are mixed in an ice bath,
Al/Si=1.0, Cu/Si=0.1 obtained by stirring
A gel with a composition of ~2.2 (preferably 0.5) was heated at 85°C.
Copper-containing type A zeolite was prepared by hydrothermal synthesis for 6 hours at a constant temperature of . This copper-containing type A zeolite was washed with water, dried, and then compressed into tablets, crushed into 15 to 24 mesh, and
．． 5 g was used for the reaction after standardization treatment in a He stream. When performing hydrogen reduction treatment, the temperature was raised from room temperature at 400° C./1.5 h in a 100% H 2 gas stream, and maintained for 0.5 h. Due to this long bronze quality, copper weighs 15wt as CuO.
% (12 wt% as Cu) of copper-containing type A zeolite was obtained.

[Example 1] For the reaction, a normal pressure flow reactor was used, and the temperature was raised and lowered at a constant rate of 2.5°C/min.
= 2500h-1, nitrogen monoxide as nitrogen oxide (
NO), octane (C8 H18), which is a methane series hydrocarbon of normal paraffins having 8 carbon atoms, and He were passed through the mixture to contact the copper-containing type A zeolite prepared as described above.

The gas that came into contact with this copper-containing type A zeolite was analyzed using gas chromatography and an infrared gas analyzer, and nitrogen (
The amount of N2) produced was measured, and the conversion rate of nitrogen monoxide was calculated from the measured amount of nitrogen produced. This result is shown in Figure 1●
Indicated with a mark. Here, the octane concentration is 6500 ppm
The oxygen concentration was set to 11.0%, and the nitrogen monoxide concentration was set to 9600 ppm.

[0024] As a result, as shown by the ● mark in FIG.
In the coexistence of nitrogen monoxide decomposition, we obtained results suggesting that by introducing octane, the activation temperature of the catalyst can be extended to around 200°C, which is a relatively low temperature range. That is, octane was found to be suitable as the hydrocarbon of the present invention.

[Example 2] Under the same conditions as in Example 1, the hydrocarbon introduced into the mixed gas was changed to decane (C10H22) having 10 carbon atoms, and the mixed gas was circulated. Then, the conversion rate of nitrogen monoxide was calculated in the same manner.

[0026] The results are shown in Fig. 1 with a □ mark. here,
The decane concentration is 4100 ppm and the oxygen concentration is 11
．． 0%, and the nitric oxide concentration is set at 9600 ppm. As a result, as shown by the □ mark in FIG. 1, it was found that the conversion rate of nitrogen monoxide when decane was introduced was temperature dependent, and the purification performance was exhibited from about 200°C, similar to octane.

[Example 3] Under the same conditions as in Example 1, the hydrocarbon introduced into the mixed gas was changed to cetane (C16H34) having 16 carbon atoms, and the mixture was circulated. Then, the conversion rate of nitrogen monoxide was calculated in the same manner.

[0028] The results are shown in Fig. 2 with a black mark. here,
The cetane concentration is 2600 ppm and the oxygen concentration is 11
．． 0%, and the nitric oxide concentration is set at 9600 ppm. As a result, as shown by the symbol ■ in Figure 1, the temperature dependence of the conversion rate of nitrogen monoxide when cetane was introduced appeared, and as in the case of Examples 1 and 2 described above, the temperature dependence was approximately 20%.
It was found that purification performance was exhibited from 0°C.

In this Example 3, the dependence of the concentration of coexisting cetane on the conversion of nitrogen monoxide under oxygen-excess conditions was investigated. In this investigation, the oxygen concentration was 11.0% and the nitric oxide concentration was 9600 ppm.
, and GHSV=2500h-1. The results are shown in FIG. In FIG. 2, the ● mark indicates the case where the cetane concentration was set to 2600 ppm, the ○ mark indicates the case where the cetane concentration was set to 700 ppm, and the ▲ mark indicates the case where the cetane concentration was set to 190 ppm.

From this result, under oxygen-excess conditions,
It is understood that as the concentration of coexisting cetane increases, the purification efficiency of nitrogen monoxide improves. This is because the more cetane diffuses into the pores of the zeolite, the more it adsorbs to the active species on the surface of the zeolite pores, and the oxygen that was suppressing the adsorption of nitrogen monoxide and this cetane are due to oxidative combustion. By chemically bonding, more oxygen is taken away from the active species, and as a result, the number of adsorption sites for nitric oxide increases, making it possible to purify nitric oxide more efficiently. Because it will be.

[Example 4] Next, in order to investigate the difference in the effect of addition of linear and cyclic hydrocarbons in the purification reaction of nitrogen monoxide, they were introduced into the mixed gas under the same conditions as in Example 1. The hydrocarbons were changed to normal hexane (C6 H14), which is a linear hydrocarbon, and cyclohexane, which is a cyclic hydrocarbon, and then distributed. Then, the conversion rate of nitrogen monoxide was calculated in the same manner. Here, G
HSV=2500 h-1, oxygen concentration is 11.0%, and nitrogen monoxide concentration is set to 0.9%. Further, the concentration of normal hexane added is set to 0.64%, and the concentration of cyclohexane added is set to 0.67%.

The results are shown in FIG. In FIG. 3, the symbol ● indicates the case of linear normal hexane, and the symbol ▲ indicates the case of cyclic cyclohexane. As is clear from Figure 3, in the case of linear normal hexane, it was confirmed that nitrogen monoxide purification performance was exhibited in a relatively low temperature range, but in the case of cyclic cyclohexane, It was not possible to confirm the remarkable purification performance of nitrogen oxide. This is because cyclohexane is cyclic, so its molecular structure makes it difficult for it to enter the pores formed in the zeolite, and it cannot adsorb to the active species on the surface of the zeolite pores, suppressing the adsorption of nitrogen monoxide. This is thought to be because the oxygen that was present in the atmosphere cannot be chemically combined with this cetane due to oxidative combustion.

[Example 5] Next, in order to investigate the difference in the effect of adding n- and i-hydrocarbons in the purification reaction of nitrogen monoxide, they were added to the mixed gas under the same conditions as in Example 1. The introduced hydrocarbons were changed to n-octane (C6 H14), which is an n-hydrocarbon, and i-octane, which is an i-hydrocarbon, and then circulated. Then, the conversion rate of nitrogen monoxide was calculated in the same manner. Here, GHSV
=2500h-1, the oxygen concentration is 12.0%, and the nitric oxide concentration is set to 0.9%. Also, n
The concentration of -octane added is set to 0.65%, and the concentration of i-octane added is set to 0.60%.

The results are shown in FIG. In FIG. 4, ● indicates the case of n-octane, and ▲ indicates the case of i-octane. As is clear from FIG. 4, n-
In the case of octane, it was confirmed that nitrogen monoxide purification performance was exhibited in a relatively low temperature range, but in the case of i-octane, no significant nitrogen monoxide purification performance could be confirmed. This is because i-octane has relatively long branched arms, so its overall molecular structure makes it difficult for it to enter the pores formed in the zeolite, making it unable to adsorb to the active species on the surface of the zeolite pores. This is thought to be because the oxygen that suppressed the adsorption of nitrogen monoxide and this cetane cannot be chemically combined through oxidative combustion.

As is clear from Examples 1 to 5 detailed above, a linear hydrocarbon having a carbon number of 8 to 16 was introduced into the exhaust gas as a reducing agent. At least one of copper, manganese, cobalt, iron, nickel, chromium, and panadium, whose valence as an oxide can change relatively easily, is used as an exhaust gas into which hydrocarbons have been introduced.
By bringing the nitrogen oxides into contact with a catalyst made of metal-containing zeolite, nitrogen oxides can be efficiently decomposed in a relatively low temperature range.

[One Embodiment] Next, referring to FIG. 5, the structure of one embodiment of the exhaust gas purification apparatus according to the present invention will be described in detail.

In the exhaust gas purification device 10 of this embodiment, the copper-containing type A zeolite applied in the above-mentioned embodiments 1 to 3 is used as the catalyst 16 in the middle of the exhaust pipe 14 from the gasoline engine 12. A first catalytic converter 18 is provided. On the downstream side of the first catalyst device 18 in the exhaust gas flow direction, there is a small-capacity oxidation catalyst 2 for purifying excess hydrocarbons discharged during transient periods.
A second catalytic device 22 with 0 is provided.

On the other hand, in order to introduce specific hydrocarbons into the exhaust gas introduced into the first catalyst device 18, the exhaust pipe 14 between the engine 12 and the first catalyst device 18 is provided with:
One end of a fuel bypass passage 28 is connected to a fuel supply pipe 26 that connects the engine 12 and the fuel tank 24 to each other, and the other end of the fuel bypass passage 28 is connected to the fuel supply pipe 26 . In the middle of this fuel bypass passage 28, there is a third fuel reforming catalyst 30 equipped with a fuel reforming catalyst 30 for polymerizing low molecular hydrocarbons in, for example, gasoline and converting the fuel flowing therethrough into high molecular hydrocarbons. Catalyst device 3
2 is interposed. In this embodiment, the fuel reforming catalyst 30 is, for example, a well-known iron-silicate catalyst (
Specifically, it is configured to reform a lower olefin having 2 to 4 carbon atoms into a higher olefin having 4 to 11 carbon atoms (high octane gasoline).

By providing the third catalyst device 32 in this way, hydrocarbons having 4 to 11 carbon atoms coexist in the exhaust gas introduced into the first catalyst device 18, and as a result, In the first catalyst device 18, the above-mentioned Example 1
As shown in Example 3, the improvement in the purification efficiency of nitrogen oxides present in exhaust gas, which is based on the coexistence of hydrocarbons with a carbon number of 8 or more, was extended to a relatively low temperature range. This will be achieved in the state.

As described above, in the exhaust gas purification device 10 of this embodiment, the first to third catalyst devices 18, 2
2 and 32, it becomes possible to provide a more effective system for suppressing all emissions of hydrocarbons, carbon monoxide, and nitrogen oxides.

[0041] In the above-mentioned embodiment, the exhaust gas purification device was described as being applied to a gasoline engine, but the present invention is not limited to such a configuration, but can also be applied to a diesel engine. It is also possible.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the influence of coexisting hydrocarbons on nitrogen monoxide decomposition in the coexistence of O2 when the exhaust gas purification method according to the present invention is applied.

FIG. 2 is a diagram showing the concentration dependence of coexisting cetane on nitric oxide under oxygen-excess conditions.

FIG. 3 is a diagram showing the difference in the effects of addition of linear and cyclic hydrocarbons in the nitric oxide decomposition reaction.

FIG. 4 is a diagram showing the difference in the effects of adding n- and i-hydrocarbons in a nitrogen monoxide decomposition reaction.

FIG. 5 is a diagram schematically showing the configuration of an embodiment of an exhaust gas purification device according to the present invention.

[Explanation of symbols]

10 Exhaust gas purification device, 12 Gasoline engine, 14 Exhaust pipe, 16 Copper-containing type A zeolite catalyst, 18
These are a first catalyst device, 20 an oxidation catalyst, 22 a second catalyst device, 24 a fuel tank, 26 a fuel supply pipe, 28 a fuel bypass passage, 30 a fuel reforming catalyst, and 32 a third catalyst device.

Claims (4)

[Claims] 1. A reducing agent introduction step of introducing a hydrocarbon as a reducing agent into the exhaust gas, and the exhaust gas into which the hydrocarbon has been introduced is made of copper, whose valence as an oxide can be changed relatively easily. manganese, cobalt, iron, nickel, chromium,
a catalytic cracking step in which nitrogen oxides are decomposed by bringing them into contact with a catalyst made of metal-containing zeolite containing at least one type of panadium; Characteristic exhaust gas purification method. 2. The exhaust gas purification method according to claim 1, wherein the hydrocarbon is a linear normal paraffin. [Claim 3] Adding 8 carbon atoms to the exhaust gas as a reducing agent.
A reducing agent introducing means for introducing hydrocarbons set to 1 to 16 is used to convert exhaust gas containing hydrocarbons to copper, manganese, whose valence as an oxide can be changed relatively easily. cobalt, iron, nickel, chromium,
1. An exhaust gas purification device comprising decomposition means for decomposing nitrogen oxides by bringing them into contact with a catalyst made of metal-containing zeolite containing at least one type of panadium. 4. The reducing agent introducing means includes a fuel reforming catalyst for reforming the fuel supplied to the engine into a hydrocarbon having a carbon number of 8 to 16. The exhaust gas purification device according to item 3.