WO2016079937A1

WO2016079937A1 - Exhaust gas purification system

Info

Publication number: WO2016079937A1
Application number: PCT/JP2015/005514
Authority: WO
Inventors: 卓也布施; 卓金子; 稲垣　孝治
Original assignee: 株式会社デンソー
Priority date: 2014-11-20
Filing date: 2015-11-03
Publication date: 2016-05-26
Also published as: JP2016098698A

Abstract

An exhaust gas purification system for purifying an exhaust gas discharged from an energy conversion part (100), the system comprising a reservoir part (4) which retains a reducing solution that reduces nitrogen oxides contained in the exhaust gas, a catalyst (22) which has been disposed in an exhaust gas passage (11) and accelerates the reduction reactions of the nitrogen oxides, and a supply part (3) which supplies, to the catalyst (22), the reducing solution supplied from the reservoir part (4). The reducing solution comprises urea, which is a reducing agent for reducing nitrogen oxides contained in the exhaust gas, a solute (70) which is not urea, and a solvent in which the reducing agent and the solute are soluble. The solute (70) is constituted of molecules each comprising: a first moiety (71), which, in cases when the temperature of the reducing solution has fallen to or below a predetermined reference temperature, comes selectively close to solid/liquid interfaces (80) within the solvent; and a second moiety (72), which has been connected to the first moiety (71) and has a poor affinity for the solvent.

Description

Exhaust purification system

Cross-reference of related applications

This application is based on Japanese Patent Application No. 2014-235512 filed on November 20, 2014, the contents of which are incorporated herein by reference.

The present disclosure relates to an exhaust purification system that purifies exhaust.

Conventionally, an exhaust purification system (urea SCR system) having a selective reduction catalyst (SCR catalyst) using urea is known as a technique for reducing nitrogen oxide (NOx) discharged from an internal combustion engine of a vehicle ( For example, see Patent Document 1).

In the exhaust gas purification system described in Patent Document 1, an SCR catalyst that selectively reduces NOx by the action of a reducing agent is disposed in an exhaust passage of an internal combustion engine, and the exhaust flow upstream of the SCR catalyst is disposed upstream. The urea aqueous solution serving as the reducing agent is jetted. As the urea aqueous solution used in this exhaust purification system, a 32.5% urea aqueous solution having the lowest freezing temperature (−11 ° C.) is used.

However, when the usage environment is extremely low, such as in a cold region or severe winter, the temperature around the urea water tank that stores the urea aqueous solution may drop to the freezing point (-11 ° C.) of the urea aqueous solution. . For this reason, the urea aqueous solution may be locally frozen or totally frozen in the urea water tank, and it is necessary to take measures against freezing at low temperatures.

On the other hand, in the conventional exhaust purification system, a heater is disposed in the urea water tank or in the urea water pipe connected to the urea water tank. Thereby, freezing of the urea aqueous solution is suppressed.

JP 2009-35644 A

However, in the above conventional technique, since the urea aqueous solution having the lowest freezing temperature is used, it is difficult to further increase the urea concentration in the urea aqueous solution. That is, when the urea concentration in the urea aqueous solution is increased, the freezing point is increased.

Therefore, in the above conventional technique, since the amount of water in the urea aqueous solution increases, it is necessary to increase the capacity of the urea water tank or increase the number of refills for injecting the urea aqueous solution into the urea water tank. Further, since the amount of water in the aqueous urea solution is large, the amount of heat required for evaporating the aqueous urea solution increases when injected to the upstream side of the SCR catalyst.

By the way, in the above conventional technique, the freezing of the urea aqueous solution is suppressed by operating the heater when the temperature of the urea aqueous solution is lowered to the freezing point. For this reason, when the freezing point of the urea aqueous solution is high, the frequency of operating the heater increases, and the power consumption by the heater increases. As a result, the fuel consumption of the vehicle deteriorates.

In view of the above points, an object of the present disclosure is to provide an exhaust purification system capable of improving the antifreeze performance of a reducing solution containing a reducing agent that reduces nitrogen oxides contained in exhaust gas.

In one aspect of the present disclosure, an exhaust purification system that purifies exhaust discharged from an energy conversion unit that generates exhaust including nitrogen oxides when energy conversion is performed based on fuel oxidation in the presence of nitrogen and oxygen includes: And a catalyst that promotes a reduction reaction of nitrogen oxides and a reduction that is supplied from the storage unit. The storage unit stores a reducing solution that reduces the nitrogen oxides contained in the exhaust gas. And a supply unit that supplies the working solution to the catalyst. The reducing solution has a reducing agent that reduces nitrogen oxides contained in the exhaust, a solute different from the reducing agent, and a solvent capable of dissolving the reducing agent and the solute. When the temperature is equal to or lower than a predetermined reference temperature, the first portion that is selectively close to the solid-liquid interface of the solvent and the first portion that is connected to the first portion and has a sparse relationship with the solvent It is composed of molecules having two sites.

According to this, when the temperature of the reducing solution falls below the reference temperature, the first part of the solute is adsorbed in close proximity to the solid-liquid interface of the solvent. Thereby, since the growth of the solidification nucleus of the solvent is inhibited by the first site adsorbed on the solid-liquid interface of the solvent, the progress of freezing can be suppressed. Furthermore, since the second portion having a sparse relationship with the solvent prevents the solvent from approaching the solid-liquid interface, the progress of freezing can be further suppressed. Therefore, the antifreeze performance of the reducing solution can be improved.

The above and other objects, features, and advantages of the present disclosure will become more apparent from the following detailed description with reference to the accompanying drawings. The drawing
FIG. 1 is a schematic diagram illustrating an overall configuration of a urea SCR system according to an embodiment of the present disclosure. FIG. 2 is an explanatory diagram for explaining the configuration of the urea aqueous solution in the embodiment of the present disclosure. FIG. 3 is a characteristic diagram showing the relationship between the temperature of the urea aqueous solution and the number of frozen parts.

Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. In the present embodiment, an example in which the exhaust purification system of the present disclosure is applied to a urea SCR system of an internal combustion engine of a vehicle will be described.

As shown in FIG. 1, an exhaust passage 11 through which exhaust gas discharged from the internal combustion engine 100 flows is connected to an internal combustion engine (ENG) 100 of the vehicle. The internal combustion engine 100 is an energy conversion unit that converts fuel energy into motive power, which is another form of energy, based on fuel oxidation in air, that is, in the presence of nitrogen and oxygen. The internal combustion engine 100 generates exhaust gas containing nitrogen oxides when performing energy conversion. In the present embodiment, a diesel engine is employed as the internal combustion engine 100.

The exhaust discharged from the internal combustion engine 100 passes through the exhaust aftertreatment device 1 installed in the exhaust passage 11 and is then released to the outside of the vehicle. The exhaust aftertreatment device 1 is a first oxidation catalyst 21, a selective reduction catalyst (SCR catalyst) 22, a second oxidation catalyst in order from the upstream side of the exhaust flow as a catalyst for purifying NOx (nitrogen oxide) in the exhaust. 23.

The first oxidation catalyst 21 converts nitrogen monoxide (NO) in the exhaust gas to nitrogen dioxide (NO ₂ ) to increase the NO ₂ ratio in NOx and facilitate the NOx reduction reaction. At the same time, the first oxidation catalyst 21 also functions to oxidize hydrocarbon (HC) and carbon monoxide (CO) in the exhaust gas.

SCR catalyst 22 selectively reduces and purifies NOx by the action of a reducing agent. That is, the SCR catalyst 22 is a catalyst that promotes the reduction reaction of NOx. Between the first oxidation catalyst 21 and the SCR catalyst 22, an addition valve 3 is provided as a supply unit that supplies a reducing agent to the SCR catalyst 22. In this embodiment, urea, which is a precursor of ammonia, is used as the reducing agent, and is injected and supplied from the addition valve 3 into the exhaust passage 11 in a urea aqueous solution that is easy to handle.

The urea aqueous solution of the present embodiment is a mixed solution having urea as a reducing agent and water as a solvent capable of dissolving urea, and corresponds to the reducing solution of the present disclosure.

The second oxidation catalyst 23 is for suppressing the ammonia generated from urea without being released without reacting with NOx, and oxidizes, decomposes and detoxifies the ammonia that has passed through the SCR catalyst 22. In the present embodiment, the SCR catalyst 22 and the second oxidation catalyst 23 are provided integrally.

The urea aqueous solution supplied to the addition valve 3 is stored in a tank 4 as a storage part. The tank 4 is a sealed container having a predetermined capacity.

Inside the tank 4, a pump 41 that sucks out the urea aqueous solution stored in the tank 4 is provided. Connected to the tank 4 is a urea aqueous solution supply path 31 that connects the pump 41 and the addition valve 3. The urea aqueous solution supply path 31 is provided with a filter 42 for removing foreign matters mixed in the urea aqueous solution.

When the pump 41 is driven, the urea aqueous solution stored in the tank 4 is sucked out and is pumped to the addition valve 3 via the filter 42. The pump 41 is an electric pump whose rotation speed (urea aqueous solution supply amount) is controlled by a control signal output from the control device 5.

The addition valve 3 may have a known air assist type injection valve structure, for example. In the air assist type, the urea aqueous solution supply path 31 is connected to the addition valve 3, and an air supply path (not shown) provided with an air compressor is connected, and the nozzle portion 3 a is opened and closed by an actuator to exhaust the exhaust flow path 11. A urea aqueous solution is injected into the air together with assisting air.

The addition valve 3 is attached to the wall of the exhaust passage 11 so as to be inclined as shown. At this time, the injection direction of the nozzle portion 3 a protruding into the exhaust flow path 11 is parallel to the exhaust flow, and the urea aqueous solution is evenly supplied to the entire inlet side end surface of the SCR catalyst 22.

The urea aqueous solution supply path 31 is provided with a pressure regulator 6 having an on-off valve for adjusting the supply pressure. The pressure regulator 6 is configured such that when the set pressure is exceeded, the on-off valve is opened so that excess urea aqueous solution returns to the tank 4 from a return path 61 connected to the upper portion of the tank 4.

In the tank 4, a tank heater 43 for heating the urea aqueous solution in the tank 4 is provided. The urea aqueous solution supply path 31 is provided with a supply path heater 33 that heats the urea aqueous solution flowing through the urea aqueous solution supply path 31. Hereinafter, the tank heater 43 and the supply path heater 33 are also collectively referred to as

heaters

33 and 43.

In the present embodiment, as the

heaters

33 and 43, electric heaters that heat the urea aqueous solution by supplying electricity are employed. The

heaters

33 and 43 are controlled in operation (heat generation amount) by a control signal output from the control device 5.

The control device 5 is composed of a well-known microcomputer including a CPU, ROM, RAM, etc. and its peripheral circuits. Then, various operations and processes are performed based on the control program stored in the ROM to control the operation of various devices connected to the output side.

The above-described addition valve 3, pump 41,

heaters

33, 43, and the like are connected to the output side of the control device 5.

On the input side of the control device 5, a pressure sensor 51 that detects the pressure of the urea aqueous solution flowing through the urea aqueous solution supply path 31, a temperature sensor 52 that detects the temperature of the urea aqueous solution flowing through the urea aqueous solution supply path 31, and the internal combustion engine 100. A sensor group such as a water temperature sensor 53 for detecting the cooling water temperature of the cooling water flowing out from the outside, and an outside air temperature sensor 54 for detecting the outside air temperature is connected.

In FIG. 1, when the urea aqueous solution of the present embodiment is injected from the addition valve 3 into the exhaust passage 11, the urea in the injected urea aqueous solution is caused by exhaust heat as shown in the following equation (i). It is pyrolyzed to generate ammonia (NH ₃ ).

(NH ₂ ) ₂ CO → NH ₃ + CHNO (i)
At this time, the simultaneously generated cyanic acid (CHNO) is further hydrolyzed as shown in the following formula (ii) to generate ammonia and carbon dioxide.

CHNO + H ₂ O → NH ₃ + CO ₂ (ii)
The generated ammonia acts as a NOx reducing agent on the SCR catalyst 22 and promotes the NOx reduction reaction, as shown in the following formula (iii).

NO + NO ₂ + 2NH ₃ → 2N ₂ + 3H ₂ O (iii)
On the other hand, the ammonia that has passed through the SCR catalyst 22 without contributing to the reduction of NOx is purified by the second oxidation catalyst 23 as shown in the following equation (iv).

4NH ₃ + 3O ₂ → 2N ₂ + 6H ₂ O (iv)
Next, the urea aqueous solution used in the urea SCR system according to the present embodiment will be described. The urea aqueous solution of this embodiment includes urea, which is a reducing agent that reduces nitrogen oxides contained in exhaust gas, a solute 70 different from urea (reducing agent), and water that is a solvent capable of dissolving urea (reducing agent). It is comprised by the solution which has.

As shown in FIG. 2, the solute 70 contained in the urea aqueous solution is composed of molecules including a head 71 as a first part and a tail 72 as a second part. The head 71 is a part that is selectively close to the solid-liquid interface 80 of water as a solvent when the temperature of the urea aqueous solution becomes equal to or lower than a predetermined reference temperature. The tail 72 is a part connected to the head 71 and having a sparse relationship with water as a solvent.

Further, one of the head 71 and the tail 72 constituting the molecule of the solute 70 has hydrophilicity, and the other has lipophilicity (hydrophobic). In the present embodiment, among the molecules of the solute 70, the head 71 has hydrophilicity and the tail 72 has lipophilicity.

In this embodiment, as the head 71 of the solute 70, any one of a quaternary ammonium group, a sulfo group, an ester group, a carboxyl group, and a hydroxyl group is employed. In addition, as the tail 72 of the solute 70, one having a plurality of carbons as a main chain and 4 or less hydrophilic groups bonded to each carbon is employed.

Specifically, a compound in which the head 71 is a trimethylammonium group and the tail 72 is a linear hydrocarbon group having 16 or less carbon atoms is employed as the solute 70 of the present embodiment. More specifically, hexadecyltrimethylammonium bromide (hereinafter referred to as C ₁₆ TAB) is employed as the solute 70.

In addition to the C ₁₆ TAB, the solute 70 of this embodiment includes polyoxyethylene (10) octylphenyl ether (Triton (registered trademark) X-100), polyoxyethylene (25) octyldodecyl ether (emulgen ( (Registered trademark) 2025G), polyoxyethylene sorbitan oleate (Tween (registered trademark) 80), stearic acid PEG-150, myristyl sulfobetaine, sodium cholate may be employed.

Here, a solution obtained by adding 0.10% of C ₁₆ TAB to the current 32.5% urea aqueous solution (AdBlue (registered trademark)) is referred to as a urea aqueous solution according to the present embodiment. The inventor conducted a freezing experiment using the urea aqueous solution according to the present embodiment. Specifically, a metallic plate-like member having 36 dimples is prepared, and after the urea aqueous solution according to the present embodiment is injected into each dimple, the temperature of the plate-like member is lowered to obtain a plate-like member. The relationship between the temperature (i.e., the temperature of the urea aqueous solution) and the number of dimples in which the urea aqueous solution was frozen was verified. The results are shown in the white circle plot of FIG.

In addition, the present inventor conducted a freezing experiment similar to the urea aqueous solution according to the above-described embodiment using the current 32.5% urea aqueous solution (AdBlue (registered trademark)) as the urea aqueous solution according to the comparative example. It was. This result is shown in the white square plot of FIG.

Note that the temperature shown on the horizontal axis in FIG. 3 is the temperature of the plate member, that is, the temperature of the urea aqueous solution. The frozen number frequency shown on the vertical axis in FIG. 3 is an index indicating how many dimples out of a total of 36 dimples the urea aqueous solution is frozen. The case where the aqueous solution is frozen is defined as 100%.

As is clear from FIG. 3, the urea aqueous solution according to this embodiment can lower the freezing temperature by 7 ° C. relative to the urea aqueous solution according to the comparative example.

Therefore, when the urea concentration (weight percent concentration) of the urea aqueous solution according to the present embodiment is 32.5%, which is the same as the urea concentration of the urea aqueous solution according to the comparative example, the freezing point (freezing temperature) can be lowered. For this reason, since the operating frequency of the

heaters

33 and 43 decreases, the power consumption of the

heaters

33 and 43 can be reduced by 30%.

Further, when the freezing point of the urea aqueous solution according to the present embodiment is set to -11 ° C. which is the same as the freezing point of the urea aqueous solution according to the comparative example, the urea concentration can be increased by 10%. Therefore, the capacity of the tank 4, the number of refills injecting the urea aqueous solution into the tank 4, and the heat of evaporation required to evaporate the urea aqueous solution when the urea aqueous solution is injected from the addition valve 3 are each reduced by 10%. can do.

Incidentally, the concentration of the solute 70 contained in the urea aqueous solution is smaller than the saturated dissolution concentration of the solute 70 with respect to water. According to this, it can suppress that the solute 70 recrystallizes and ice grows with the crystal | crystallization as a nucleus. Furthermore, by setting the concentration of the solute 70 contained in the urea aqueous solution to be equal to or lower than the critical micelle concentration of the solute 70 with respect to water, it is possible to suppress the solute 70 from becoming micelle and growing ice using the micelle as a nucleus.

As described above, in this embodiment, the solute 70 different from urea is mixed in the urea aqueous solution. The solute 70 includes a head 71 that is selectively close to the solid-liquid interface 80 of water and a tail 72 that is connected to the head 71 and has a hydrophobic property when the temperature of the urea aqueous solution becomes a reference temperature or lower. It is made up of molecules.

According to this, when the temperature of the urea aqueous solution decreases and becomes lower than the reference temperature, the head 71 of the solute 70 is adsorbed in close proximity to the solid-liquid interface 80 of the water. Since the growth of ice nuclei (solidified nuclei) of water is inhibited by the head 71 adsorbed on the solid-liquid interface 80 of water, the progress of freezing can be suppressed. Furthermore, since the tail 72 having hydrophobicity suppresses water from approaching the solid-liquid interface 80, the progress of freezing can be further suppressed. Therefore, it becomes possible to improve the antifreeze performance of the urea aqueous solution.

For this reason, it is possible to increase the urea concentration of the aqueous urea solution while maintaining the freezing point of the aqueous urea solution at the same temperature as in the past, that is, while ensuring the same antifreezing performance as in the past. Thereby, it is possible to reduce the capacity of the tank 4, the number of refills for injecting the urea aqueous solution into the tank 4, and the evaporation heat necessary for evaporating the urea aqueous solution when the urea aqueous solution is injected from the addition valve 3. Become.

On the other hand, when the urea concentration of the urea aqueous solution is equal to the conventional one, the freezing point of the urea aqueous solution can be lowered. Thereby, the power consumption of the

heaters

33 and 43 can be reduced.

Therefore, according to the configuration of the present embodiment, at least one of an increase in the urea concentration of the urea aqueous solution and a reduction in the power consumption of the

heaters

33 and 43 can be achieved.

By the way, in the conventional technique described above, antifreeze performance is ensured by mixing about 10% of an alcohol-based organic solvent with the urea aqueous solution. However, in this conventional technique, since the concentration of the alcohol-based organic solvent contained in the urea aqueous solution is high, there is a possibility that the composition of the exhaust gas changes due to the alcohol or the alcohol adheres to the SCR catalyst 22. .

On the other hand, in this embodiment, since the concentration of C ₁₆ TAB mixed with the urea aqueous solution is 0.1%, the amount of the solute 70 mixed with the urea aqueous solution is about 1/100 of the conventional amount. However, antifreeze performance can be ensured. For this reason, it can suppress that the composition of exhaust_gas | exhaustion changes under the influence of the solute 70, and can suppress that the solute 70 adheres to the SCR catalyst 22. FIG.

(Other embodiments)
The present disclosure is not limited to the above-described embodiment, and can be variously modified as follows, for example, within a range not departing from the gist of the present disclosure.

(1) In the above embodiment, an example in which a diesel engine is employed as the energy conversion unit has been described, but the energy conversion unit is not limited to this. For example, a gasoline engine may be employed as the energy conversion unit.

(2) In the above embodiment, an example in which the exhaust purification system of the present disclosure is applied to a urea SCR system has been described. However, the present invention is not limited thereto, and may be applied to other exhaust purification systems.

(3) In the above-described embodiment, the example in which the SCR catalyst 22 and the second oxidation catalyst 23 located on the downstream side of the exhaust flow of the addition valve 3 are integrally described has been described. The second oxidation catalyst 23 may be provided separately.

Although the present disclosure has been described with reference to the embodiments, it is understood that the present disclosure is not limited to the embodiments and structures. The present disclosure includes various modifications and modifications within the equivalent range. In addition, various combinations and forms, as well as other combinations and forms including only one element, more or less, are within the scope and spirit of the present disclosure.

Claims

An exhaust purification system that purifies the exhaust discharged from an energy conversion unit (100) that generates exhaust containing nitrogen oxide when energy conversion is performed based on fuel oxidation in the presence of nitrogen and oxygen,
A reservoir (4) for storing a reducing solution for reducing the nitrogen oxides contained in the exhaust;
A catalyst (22) that is provided in an exhaust passage (11) through which the exhaust flows, and that promotes a reduction reaction of the nitrogen oxides;
A supply unit (3) for supplying the reducing solution supplied from the storage unit (4) to the catalyst (22);
The reducing solution includes a reducing agent that reduces the nitrogen oxides contained in the exhaust, a solute (70) that is different from the reducing agent, and a solvent that can dissolve the reducing agent and the solute (70). Have
The solute (70) is
A first portion (71) that is selectively close to the solid-liquid interface (80) of the solvent when the temperature of the reducing solution is equal to or lower than a predetermined reference temperature;
An exhaust purification system configured by molecules having a second part (72) connected to the first part (71) and having a sparse relationship with the solvent.
The exhaust purification system according to claim 1, wherein one of the first part (71) and the second part (72) has hydrophilicity and the other has lipophilicity.
The exhaust purification system according to claim 1 or 2, wherein a concentration of the solute (70) is smaller than a saturated dissolution concentration of the solute (70) with respect to the reducing solution.
The exhaust purification system according to any one of claims 1 to 3, wherein a concentration of the solute (70) is equal to or lower than a critical micelle concentration of the solute (70) with respect to the reducing solution.
The reducing agent is urea;
The exhaust purification system according to any one of claims 1 to 4, wherein the solvent is water.