JP4507901B2 - Exhaust gas purification system and exhaust gas purification method thereof - Google Patents

Description

  The present invention relates to an exhaust gas purification system including a selective reduction type NOx catalyst for purifying nitrogen oxide in exhaust gas of an internal combustion engine, and an exhaust gas purification method thereof.

  Various studies and developments have been made on exhaust gas purification devices used in internal combustion engines such as diesel engines and lean burn gasoline engines, and selective reduction is performed to purify nitrogen oxides (NOx) in the exhaust gas. Type NOx catalyst (SCR (Selective Catalytic Reduction) catalyst) and the like have been proposed.

  This selective reduction type NOx catalyst is made of titania-vanadium, zeolite, chromium oxide, manganese oxide, molybdenum oxide, titanium oxide on a honeycomb structure carrier (catalyst structure) formed of cordierite, aluminum oxide, titanium oxide or the like. Further, it is formed by supporting tungsten oxide or the like.

In the exhaust gas purification system equipped with this selective reduction type NOx catalyst, in an oxygen-excess atmosphere, a reducing agent such as liquid ammonia, ammonia water or urea water is injected into the exhaust pipe to supply ammonia (NH ₃ ), NOx is reduced by selectively reacting with NOx in the exhaust gas with ammonia (see, for example, Patent Documents 1 to 3).

  Since liquid ammonia and aqueous ammonia are highly toxic and require careful handling, urea water, which is relatively easy to handle, is used. This urea is hydrolyzed and thermally decomposed to produce ammonia, which is then generated. NOx is reduced with the ammonia.

  However, when this urea water is used, there are the following problems. After urea is supplied into the exhaust pipe and before reaching the selective reduction type NOx catalyst, the amount of ammonia converted into ammonia is small, that is, the ammonia production rate is low, so the NOx purification rate is low. . In order to avoid this, if the distance from the urea supply position to the selective reduction type NOx catalyst is increased, it becomes difficult to dispose the exhaust gas purification device in the exhaust pipe, and the exhaust gas temperature also decreases.

  In particular, when the exhaust gas is below a predetermined temperature (about 150 ° C. to 170 ° C.), the ammonia production rate is low, so even if urea water is supplied, the NOx purification rate of the selective reduction type NOx catalyst only becomes low. And unreacted urea is released into the atmosphere. Therefore, in the prior art, NOx purification in the region where the exhaust gas temperature is low is given up and urea supply is stopped to prevent unreacted urea from being released into the atmosphere.

  In addition, in order to improve the ammonia production rate at low temperature, a hydrolysis catalyst such as alumina, titania, silica, zirconia, zeolite or the like is installed on the upstream side of the selective reduction type NOx catalyst, or between the hydrolysis catalyst and heater heating. Measures such as a combination are taken (for example, see Patent Document 4).

  Furthermore, although a method of adsorbing ammonia on the surface of the selective reduction type NOx catalyst has been tried, there is a problem that the effect cannot be sustained because the amount of adsorption of ammonia is limited.

On the other hand, the present inventor has found that silicon nitride used as a material for forming a carrier can greatly enhance the hydrolysis reaction of urea even at a low temperature of 100 ° C. to 150 ° C. Was obtained from our experiments. Moreover, the knowledge that hydrolysis could be further promoted by adding a moisture adsorbing substance such as titanium oxide to the silicon nitride was also obtained from the experiments of the present inventors.
JP 2003-232215 A JP 2003-293739 A JP 2004-162544 A JP 2004-60494 A

  The present invention has been made to solve the above-mentioned problems by obtaining the above knowledge, and the object thereof is an exhaust gas purification system equipped with a selective reduction NOx catalyst. Another object of the present invention is to provide an exhaust gas purification system and an exhaust gas purification method thereof that can improve the NOx purification rate even at low exhaust gas temperatures.

  An exhaust gas purification system for achieving the above object is the exhaust gas purification system in which a urea supply nozzle, a urea hydrolysis catalyst, and a selective reduction type NOx catalyst are arranged in the exhaust gas passage in order from the upstream side. A silicon nitride surface is formed on the contact surface of the cracking catalyst with the exhaust gas.

According to this configuration, by forming the silicon nitride surface on the contact surface with the exhaust gas of the urea hydrolysis catalyst having a large reaction promoting effect at a low temperature, the conventional hydrolysis catalyst has a low urea hydrolysis reaction rate. Even in the range of from 150 ° C. to 150 ° C., urea can be hydrolyzed to generate ammonia, and the NOx purification rate in the selective reduction type NOx catalyst can be improved. That is, the hydrolysis reaction of urea ((NH ₂ ) ₂ CO + H ₂ 0 → 2NH ₃ + CO ₂ ) can be promoted.

In the above exhaust gas system, the urea hydrolysis catalyst is provided with a moisture adsorbing substance. With this configuration, moisture that promotes the decomposition of urea can be adsorbed to the urea hydrolysis catalyst and the contact probability between urea and moisture can be increased. Therefore, in addition to the promotion of the urea hydrolysis reaction, , (6H ₂ O + Si ₃ N ₄ → 3SiO ₂ + 4NH ₃ ) and the like are promoted, so that the generation rate of ammonia (NH ₃ ) is increased. In particular, the latter reaction is likely to proceed at 100 ° C. to 200 ° C.

  In the exhaust gas purification system, urea is supplied without stopping the urea supply from the urea supply nozzle even when the temperature of the exhaust gas flowing into the urea hydrolysis catalyst is in the range of 100 ° C to 150 ° C. And NOx purification control means for performing the control.

  When the temperature of the exhaust gas is 150 ° C. or higher, the control for supplying urea from the urea supply nozzle is performed as in the exhaust gas purification system provided with the conventional selective reduction type NOx catalyst. In other words, in the present invention, control is performed by lowering the lower limit temperature of the exhaust gas temperature range for supplying urea from the urea supply nozzle to 100 ° C. Even if the urea supply range is lowered to 100 ° C., the urea generation reaction of silicon nitride improves the ammonia production rate and the NOx purification rate. It is possible to prevent the reaction from being discharged.

In the above exhaust gas purification system, the urea hydrolysis catalyst is provided with a moisture adsorbing substance. Titanium oxide (TiO ₂ ) is used as the moisture adsorbing substance.

  In the above exhaust gas purification system, the catalyst structure of the urea hydrolysis catalyst is formed of silicon nitride. Alternatively, the catalyst structure of the urea hydrolysis catalyst is formed by coating silicon nitride. With these structures, silicon nitride can be easily provided on the contact surface of the urea hydrolysis catalyst with the exhaust gas.

  Alternatively, an exhaust gas purification system for achieving the above object is the exhaust gas purification system in which a urea supply nozzle and a selective reduction type NOx catalyst are arranged in an exhaust gas passage in order from the upstream side. A silicon nitride surface is formed on the contact surface with the exhaust gas.

According to this configuration, the hydrolysis reaction of urea ((NH ₂ ) ₂ CO + H ₂ 0 → 2NH ₃ + CO ₂ ) can be promoted with silicon nitride, and the reaction from urea to ammonia can be promoted. Since ammonia can be supplied to the selective reduction type NOx catalyst, the NOx purification rate of the selective reduction type NOx catalyst, particularly the NOx purification rate at a low temperature, can be improved.

In the above exhaust gas purification system, the selective reduction type NOx catalyst is provided with a moisture adsorbing substance. Titanium oxide (TiO ₂ ) is used as the moisture adsorbing substance.

With this configuration, moisture that promotes the decomposition of urea can be adsorbed to the selective reduction type NOx catalyst, and the contact probability between urea and moisture can be increased. As a result, the reaction such as (6H ₂ O + Si ₃ N ₄ → 3SiO ₂ + 4NH ₃ ) is promoted, so that the generation rate of ammonia (NH ₃ ) is increased. In particular, the latter reaction is likely to proceed at 100 ° C. to 200 ° C.

  Further, in the above exhaust gas purification system, urea can be supplied without stopping the urea supply from the urea supply nozzle even when the temperature of the exhaust gas flowing into the selective reduction NOx catalyst is in the range of 100 ° C. to 150 ° C. A NOx purification control means for controlling the supply is provided.

  When the temperature of the exhaust gas is 150 ° C. or higher, the control for supplying urea from the urea supply nozzle is performed as in the exhaust gas purification system provided with the conventional selective reduction type NOx catalyst. In other words, in the present invention, control is performed by lowering the lower limit temperature of the exhaust gas temperature range for supplying urea from the urea supply nozzle to 100 ° C. Even if the urea supply range is lowered to 100 ° C., the urea generation reaction of silicon nitride improves the ammonia production rate and the NOx purification rate. It is possible to prevent the reaction from being discharged.

  In addition, an exhaust gas purification method for achieving the above-described object includes a urea supply nozzle in the exhaust gas passage in order from the upstream side, a urea hydrolysis catalyst in which silicon nitride is provided on the contact surface with the exhaust gas, and selective reduction. In the exhaust gas purification system provided with a NOx catalyst, even when the exhaust gas temperature flowing into the urea hydrolysis catalyst is in the range of 100 ° C. to 150 ° C., without stopping the supply of urea from the urea supply nozzle, Control for supplying urea is performed.

  Alternatively, in the exhaust gas purification method for achieving the above object, a selective reduction type NOx catalyst in which silicon nitride is provided on the contact surface between the urea supply nozzle and the exhaust gas in order from the upstream side is disposed in the exhaust gas passage. In the exhaust gas purification system, the control for supplying urea without stopping the supply of urea from the urea supply nozzle even when the temperature of the exhaust gas flowing into the selective reduction type NOx catalyst is in the range of 100 ° C. to 150 ° C. It is characterized by performing.

  In the exhaust gas purification method, the silicon nitride is regenerated by making the air-fuel ratio of the exhaust gas rich. Furthermore, in the exhaust gas purification system provided with the DPF device in the exhaust passage, the silicon nitride is regenerated when the DPF device is regenerated.

With this configuration, the carbon dioxide generated by the reaction such as (3O ₂ + Si ₃ N ₄ → 3SiO ₂ + 2N ₂ ) or (6CO ₂ + Si ₃ N ₄ → 3SiO ₂ + 6CO + 2N ₂ ) performed in parallel with the reaction of ammonia generation. Silicon (SiO ₂ ) can be returned to silicon nitride (Si ₃ N ₄ ) by a reaction of (3SiO ₂ + 2N ₂ → Si ₃ N ₄ + 3O ₂ ).

  Further, in the above exhaust gas purification method, the temperature of the exhaust gas flowing into the portion where the silicon nitride is provided is set to 300 ° C. or higher, and the air / fuel ratio of the exhaust gas is made rich so that silicon nitride is regenerated. . Here, the air-fuel ratio state of the exhaust gas does not necessarily mean the state of the air-fuel ratio in the cylinder, but the amount of air and the amount of fuel (cylinder (cylinder) supplied to the exhaust gas flowing into the exhaust gas purification device) This is the ratio to the amount of combustion in the cylinder).

  In the above exhaust gas purification method, in the exhaust gas purification system provided with a DPF device in an exhaust passage, the silicon nitride is regenerated at the time of regeneration of the DPF device. According to this configuration, when the exhaust gas temperature is increased in order to burn and remove PM trapped in the DPF during regeneration of the DPF, the PM trapped in the DPF burns and exhaust gas on the downstream side of the DPF. Since the air-fuel ratio becomes rich with less oxygen (reducing atmosphere), silicon dioxide is returned to silicon nitride using this. Therefore, in the exhaust gas purification system provided with the DPF, it is not necessary to increase the temperature and the rich air-fuel ratio of the exhaust gas for restoring the silicon nitride.

  According to the exhaust gas purification system and the exhaust gas purification method of the present invention, by using silicon nitride for hydrolysis of urea, even when the exhaust gas is at a low temperature of 100 ° C. to 150 ° C., ammonia is converted from urea to ammonia. Therefore, the ammonia production rate can be increased, and the NOx purification rate of the selective reduction type NOx catalyst can be improved. Moreover, the ammonia production rate can be further improved by combining a moisture adsorbing material such as titanium oxide with silicon nitride.

  Therefore, even when the exhaust gas is at a low temperature of 100 ° C. to 150 ° C., urea can be supplied to the exhaust gas passage to purify NOx, so that the NOx purification performance of the exhaust gas purification system at a low temperature of the exhaust gas. Can be improved.

  Hereinafter, an exhaust gas purification system and an exhaust gas purification method thereof according to an embodiment of the present invention will be described with reference to the drawings.

  Note that the air-fuel ratio state of the exhaust gas here does not necessarily mean the state of the air-fuel ratio in the cylinder (cylinder), but the amount of air supplied to the exhaust gas flowing into the exhaust gas purification device and the fuel This is the ratio to the amount (including the amount burned in the cylinder).

  FIG. 1 shows a configuration of an exhaust gas purification system 1 according to a first embodiment of the present invention. The exhaust gas purification system 1 is configured by disposing an exhaust gas purification device 10 in an exhaust passage 3 of an engine (internal combustion engine) 2. The exhaust gas purification device 10 includes a DPF (diesel particulate filter) 11, a urea water supply device 12, a urea hydrolysis catalyst 13, a selective reduction type NOx catalyst (SCR catalyst) 14, and an oxidation catalyst (DOC) 15. Is done.

  The DPF 11 is formed of a monolith honeycomb type wall-through filter in which the inlet and outlet of the porous ceramic honeycomb channel are alternately sealed, and collects and removes PM (particulate matter) in the exhaust gas. . The DPF 11 may apply an oxidation catalyst or a catalyst layer carrying the PM oxidation catalyst to the exhaust gas passage in order to promote the combustion removal of PM.

  The urea water supply device 12 includes a urea water injection nozzle 12a, a urea water storage tank 12b, a pipe 12c that connects the two, a pump 12d for supplying urea water, and the like. The urea water supply device 12 is controlled by a control device 30 that controls the exhaust gas purification device 10 together with the control of the engine 2, and urea water for NOx reduction is supplied into the exhaust passage 3 from the urea water injection nozzle 12a. .

In the present invention, the urea hydrolysis catalyst 13 is formed by supporting titanium oxide (TiO ₂ ) on the wall surface of a honeycomb made of silicon nitride (Si ₃ N ₄ ). Alternatively, a silicon nitride coating layer is applied to a carrier such as a honeycomb made of aluminum oxide or titanium oxide, and titanium oxide is supported on the coating layer. With these configurations, silicon nitride and titanium oxide are provided on the contact surface of the urea hydrolysis catalyst 13 with the exhaust gas. The urea hydrolysis catalyst 13 promotes hydrolysis of urea water supplied from the urea water supply device 12 and converts urea into ammonia. Further, since heat is generated by this hydrolysis, the temperature of the exhaust gas can be raised, and the downstream selective reduction type NOx catalyst 14 can be more activated.

The selective reduction type NOx catalyst 14 carries titania-vanadium, zeolite, chromium oxide, manganese oxide, molybdenum oxide, titanium oxide, tungsten oxide, etc. on a honeycomb structure carrier made of cordierite, aluminum oxide, titanium oxide or the like. Formed. This selective reduction type NOx catalyst 14 reacts this ammonia with NOx in the exhaust gas, and reduces and purifies NOx to nitrogen (N ₂ ).

  The oxidation catalyst 15 is formed of a monolith catalyst having a large number of polygonal cells formed of a structural material such as cordierite, silicon carbide, or stainless steel. The inner wall of the cell has a catalyst coat layer that has a large surface area, and a large surface of the cell coats a catalyst metal such as platinum, rhodium, or palladium.

  This oxidation catalyst 15 is supplied excessively with respect to NOx, oxidizes ammonia and urea that have passed through the selective reduction type NOx catalyst 14 without reacting with NOx, and these components are released into the atmosphere. To prevent.

As shown in FIG. 1, an upstream side exhaust temperature sensor 21 is provided upstream of the urea hydrolysis catalyst 13, and an upstream side NOx concentration sensor 22 is provided upstream of the selective reduction type NOx catalyst 14, A downstream NOx concentration sensor 23 is provided downstream of the selective reduction type NOx catalyst 14. Further, a differential pressure sensor 24 for monitoring the clogged state of the DPF 11 is provided for regeneration control of the DPF 11. Further, for the air-fuel ratio control of the exhaust gas, an upstream λ sensor 25 on the upstream side of the exhaust gas purification device 10, a downstream exhaust temperature sensor 26 on the downstream side, a downstream O ₂ sensor 27, etc., as necessary. Provided.

  As shown in FIG. 1, a control device (ECU: engine control unit) 30 that performs overall control of the operation of the engine 2 is provided. The control device 30 includes a DPF regeneration control unit 30 a that performs regeneration control of the DPF 11 and a NOx purification control unit 30 b that controls the NOx purification ability of the selective reduction type NOx catalyst 14. Detection values from the sensors 21 to 27 and the like are input to the control device 30, and the fuel injection valve and the intake throttle valve (intake air valve) of the EGR valve of the engine 2 and the common rail electronic control fuel injection device for fuel injection are input from the control device 30. A signal for controlling the urea injection nozzle 12a and the pump 12d of the urea supply device 12 is output.

  In the present invention, the NOx purification control means 30b does not stop the supply of urea from the urea supply nozzle 12a even when the temperature of the exhaust gas flowing into the urea hydrolysis catalyst 13 is in the range of 100 ° C to 150 ° C. The control is performed so as to supply urea.

By this control, the reaction shown in the upper diagram of FIG. 3 occurs in the urea hydrolysis catalyst 13 containing silicon nitride. That is, since silicon nitride (Si ₃ N ₄ ) adsorbs urea ((NH ₂ ) ₂ CO) and water (H ₂ O), the probability of contact between urea and water increases, and the hydrolysis reaction of urea (( NH ₂ ) ₂ CO + H ₂ 0 → 2NH ₃ + CO ₂ ) is promoted. Also, the moisture _{adsorption, (6H 2 O + Si 3} N 4 → 3SiO 2 + 4NH 3) reaction or the like occurs, to assist the generation of ammonia (NH _3).

In parallel with these reactions, reactions such as (3O ₂ + Si ₃ N ₄ → 3SiO ₂ + 2N ₂ ) and (6CO ₂ + Si ₃ N ₄ → 3SiO ₂ + 6CO + 2N ₂ ) occur. In the range of 200 ° C., the reaction ( 6H ₂ O + Si ₃ N ₄ → 3SiO ₂ + 4NH ₃ ) is likely to proceed. Then, by arranging the water adsorption material such as titanium oxide (TiO ₂₎ on the surface of the silicon nitride, it is possible to further accelerate the hydrolysis reaction of the urea.

On the other hand, the surface of silicon nitride is oxidized by these reactions to become silicon dioxide (SiO ₂ ) as shown in the lower diagram of FIG. Therefore, the NOx purification control means 30b sets the exhaust gas temperature to 300 ° C. or more and regenerates the silicon nitride, and the exhaust gas flowing into the urea hydrolysis catalyst 13 is in a rich state (for example, excess air). In a reduced atmosphere of .lambda.R = 1.1 to 0.8 in terms of rate, the reaction is returned to the original silicon nitride by the reaction of (3SiO ₂ + 2N ₂ → Si ₃ N ₄ + 3O ₂ ).

  In the control for making the air-fuel ratio of the exhaust gas rich, the EGR valve is controlled to increase the EGR amount, the intake throttle valve is controlled to decrease the new intake amount, and the post-cylinder injection is performed. The temperature of the exhaust gas is raised or the air-fuel ratio in the exhaust gas is lowered by injection or the like. By these controls, the exhaust gas is brought into a predetermined target air-fuel ratio state, and in a predetermined temperature range (300 ° C. to 500 ° C.), the NOx purification ability is recovered, and the NOx catalyst is regenerated.

On the other hand, the DPF regeneration control means 30a controls the EGR valve to detect the EGR amount when the differential pressure sensor 24 detects that the accumulated amount of PM in the DPF 11 has increased and the clogged state has deteriorated in the regeneration control of the DPF 11. Add fuel to the exhaust gas by reducing the air-fuel ratio of the exhaust gas or by post-injection in cylinder injection, etc. . Then, the fuel is oxidized by an oxidation catalyst supported on the DPF, and the temperature of the exhaust gas and the DPF 11 are raised using the heat of the oxidation reaction, and the NO generated by the oxidation reaction of NO in the exhaust gas. ₂ is used to oxidize and remove the PM collected in the DPF 11.

  When the DPF 11 is regenerated, if the exhaust gas temperature is raised in order to burn and remove the PM collected in the DPF 11, the PM collected in the DPF 11 is combusted, and the air-fuel ratio of the exhaust gas downstream of the DPF 11 becomes oxygen. Therefore, a reaction to return silicon dioxide to silicon nitride also occurs at this time. Therefore, at the time of regeneration of the DPF 11, the silicon nitride for hydrolysis can be restored. Therefore, in the exhaust gas purification system 1 provided with the DPF 11, it is not necessary to increase the temperature and the rich air-fuel ratio of the exhaust gas for restoring the silicon nitride. When the DPF 11 is regenerated, the air-fuel ratio of the exhaust gas flowing into the urea hydrolysis catalyst 13 does not reach a sufficiently rich state (for example, λR = 1.1 to 0.8 in terms of excess air ratio). Performs post-injection or the like to make a reducing atmosphere with less oxygen to return silicon dioxide to silicon nitride.

  Next, an exhaust gas purification system 1A according to a second embodiment of the present invention will be described.

  As shown in FIG. 2, the exhaust gas purification system 1A is configured by disposing an exhaust gas purification device 10A in an exhaust passage 3 of an engine (internal combustion engine) 2. In the exhaust gas purification device 10A, urea hydrolysis is performed. Instead of providing the cracking catalyst 13, silicon nitride is provided on the contact surface of the selective reduction NOx catalyst 14 with the exhaust gas. That is, the DPF 11, the urea supply nozzle 12 a, the selective reduction type NOx catalyst 14 </ b> A, and the oxidation catalyst 15 are arranged in the exhaust gas passage 2 in order from the upstream side. Others are configured similarly to the first embodiment.

  In this configuration, when the catalyst structure of the selective reduction type NOx catalyst 14A is formed of silicon nitride, urea selective reaction and NOx reduction reaction as shown in FIG. 4 occur in the selective reduction type NOx catalyst 14A. And, by using silicon nitride as a base material as a catalyst carrier, the amount of ammonia and water adsorbed is increased, and although it is very slight, a NOx decomposition reaction occurs on the surface of silicon nitride. The effect for improving the NOx purification rate is increased.

  Therefore, according to the exhaust gas purification system 1 having the above-described configuration and the exhaust gas purification method thereof, even when the exhaust gas is at a low temperature of 100 ° C. to 150 ° C. by using silicon nitride for hydrolysis of urea, The hydrolysis reaction from urea to ammonia can be promoted. Therefore, the ammonia production rate can be increased, and the NOx purification rate of the selective reduction type NOx catalyst can be improved. Moreover, the ammonia production rate can be further improved by combining a moisture adsorbing material such as titanium oxide with silicon nitride.

  As a result, even when the exhaust gas is at a low temperature of 100 ° C. to 150 ° C., urea can be supplied to the exhaust gas passage to purify NOx. The NOx purification performance can be improved.

  Example 1a was formed with a hydrolysis catalyst using a silicon nitride honeycomb, and Example 1b was formed with a hydrolysis catalyst having titanium oxide supported on the walls of a silicon nitride honeycomb. Further, Comparative Example 1 was formed with a hydrolysis catalyst in which aluminum oxide was supported on a cordierite honeycomb.

  A urea solution having a concentration of 32.5% is injected in front of each of the hydrolysis catalysts of Examples 1a and 1b and Comparative Example 1, and a gas containing the urea solution is heated to 100 ° C. to 250 ° C. The ammonia production rate was calculated by measuring the ammonia concentration behind. Further, as Comparative Example 2, the ammonia production rate when no hydrolysis catalyst was used was measured. These results are shown in FIG. 5 for Examples 1a and 1b and Comparative Examples 1 and 2, respectively, as A1, A2 and B1, B2. As shown in FIG. 5, Example 1a (A1) and Example 1b (A2) showed high ammonia production rates from low temperatures.

Further, an exhaust gas purification system as shown in FIG. 1 was configured using each of these hydrolysis catalysts, and the NOx purification rate of the selective reduction type NOx catalyst was measured. The result is shown in FIG. Example 1a (A1) and Example 1b (A2) show higher NOx purification performance from low temperature than Comparative Example 1 (B1) and Comparative Example 2 (B2), and Example 1b (A2) is the most. High performance was shown.

The selective reduction type NOx catalyst of Example 2 (C1) was formed of a silicon nitride honeycomb, and on the wall surface of the silicon nitride honeycomb, aluminum oxide was supported on the lower layer, and platinum and zeolite were supported on the upper layer. In addition, the selective reduction type NOx catalyst of Example 3 (D1) is formed of a silicon nitride honeycomb, and on the wall surface of the silicon nitride honeycomb, the lower layer is aluminum oxide, and the upper layer is titanium oxide and vanadium oxide (TiO ₂ —V ₂ O). ₅ ) was supported. Further, the selective reduction type NOx catalyst of Comparative Example 3 (D2) was formed of a cordierite honeycomb, and on this wall surface, aluminum oxide was supported on the lower layer, and platinum and zeolite were supported on the upper layer.

Further, an exhaust gas purification system as shown in FIG. 2 was configured using each of these selective reduction type NOx catalysts, and the NOx purification rate of the selective reduction type NOx catalyst was measured. The result is shown in FIG. Example 2 (C1) showed the highest purification performance compared to Example 3 (D1) and Comparative Example 3 (D2). In particular, it has been found that the effect is large at low temperatures.

It is a figure which shows the structure of the exhaust-gas purification system of 1st Embodiment which concerns on this invention. It is a figure which shows the structure of the exhaust-gas purification system of 2nd Embodiment which concerns on this invention. It is a figure which shows typically the mode of reaction of the silicon nitride, titanium oxide, and urea in the hydrolysis catalyst of 1st Embodiment. It is a figure which shows typically the mode of reaction of the silicon nitride and urea in the selective reduction type NOx catalyst of 2nd Embodiment. It is a figure which shows the relationship between the exhaust gas temperature of the catalyst inlet_port | entrance of Example 1a, 1b and Comparative Examples 1 and 2, and an ammonia production rate . It is a figure which shows the relationship between the exhaust gas temperature of the catalyst inlet of Example 1a, 1b and Comparative Examples 1 and 2, and a NOx purification rate. FIG. 6 is a graph showing the relationship between the exhaust gas temperature at the catalyst inlet and the NOx purification rate in Example 2, Example 3, and Comparative Example 3.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Exhaust gas purification system 2 Engine 3 Exhaust passage 10 Exhaust gas purification apparatus 11 DPF
DESCRIPTION OF SYMBOLS 12 Urea supply apparatus 13 Urea hydrolysis catalyst 14 Selective reduction type NOx catalyst 15 Oxidation catalyst 30 Control apparatus 30a DPF regeneration control means 30b NOx purification control means

Claims (12)

  In an exhaust gas purification system in which a urea supply nozzle, a urea hydrolysis catalyst, and a selective reduction type NOx catalyst are arranged in this order from the upstream side in the exhaust gas passage, a silicon nitride surface is provided on the contact surface of the urea hydrolysis catalyst with the exhaust gas. An exhaust gas purification system characterized by being formed.   The exhaust gas purification system according to claim 1, wherein the urea hydrolysis catalyst is provided with a moisture adsorbing substance.   In the exhaust gas purification system, the urea supply control is performed without stopping the urea supply from the urea supply nozzle even when the exhaust gas temperature flowing into the urea hydrolysis catalyst is in the range of 100 ° C to 150 ° C. The exhaust gas purification system according to claim 1 or 2, further comprising NOx purification control means for performing the operation.   The exhaust gas purification system according to any one of claims 1 to 3, wherein the catalyst structure of the urea hydrolysis catalyst is formed of silicon nitride.   The exhaust gas purification system according to any one of claims 1 to 3, wherein the catalyst structure of the urea hydrolysis catalyst is formed by coating silicon nitride.   In the exhaust gas purification system in which the urea supply nozzle and the selective reduction NOx catalyst are arranged in the exhaust gas passage in order from the upstream side, a silicon nitride surface is formed on the contact surface with the exhaust gas of the selective reduction NOx catalyst. A featured exhaust gas purification system.   The exhaust gas purification system according to claim 6, wherein the selective reduction type NOx catalyst is provided with a moisture adsorbing substance.   In the exhaust gas purification system, control is performed to supply urea without stopping the supply of urea from the urea supply nozzle even when the temperature of the exhaust gas flowing into the selective reduction NOx catalyst is in the range of 100 ° C. to 150 ° C. The exhaust gas purification system according to claim 6 or 7, further comprising NOx purification control means for performing the operation.   In the exhaust gas purification system, in which the urea supply nozzle, the urea hydrolysis catalyst provided with silicon nitride on the contact surface with the exhaust gas, and the selective reduction type NOx catalyst are disposed in the exhaust gas passage sequentially from the upstream side. Even when the temperature of the exhaust gas flowing into the cracking catalyst is in the range of 100 ° C. to 150 ° C., the urea supply control is performed without stopping the urea supply from the urea supply nozzle. Exhaust gas purification method.   In an exhaust gas purification system in which a selective reduction type NOx catalyst in which silicon nitride is provided on the contact surface between the urea supply nozzle and the exhaust gas is arranged in the exhaust gas passage sequentially from the upstream side, the exhaust gas flows into the selective reduction type NOx catalyst. An exhaust gas purifying method characterized in that control for supplying urea is performed without stopping urea supply from the urea supply nozzle even when the exhaust gas temperature is in the range of 100 ° C to 150 ° C.   The temperature of the exhaust gas flowing into the portion where the silicon nitride is provided is set to 300 ° C or higher, and the silicon nitride is regenerated by making the air-fuel ratio of the exhaust gas rich. The exhaust gas purification method according to 9 or 10. The exhaust gas purification system according to claim 11, further comprising: regenerating the silicon nitride at the time of regeneration of the DPF device in an exhaust gas purification system having a DPF device in an exhaust passage.

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