JP2009243316A - Exhaust emission control device and its exhaust emission control method - Google Patents

Abstract

【Task】
Exhaust gas purification apparatus capable of improving the NOx purification rate without releasing the reducing agent into the atmosphere by controlling the ratio of the NO concentration and the NO ₂ concentration flowing into the SCR catalyst with high accuracy, and the exhaust gas thereof Provide a purification method.
[Solution]
An exhaust gas purification device provided in an exhaust passage of an internal combustion engine includes an oxygen supply means, an oxidation catalyst disposed downstream of the oxygen supply means, and a reducing agent addition means disposed downstream of the oxidation catalyst. And an SCR catalyst arranged on the downstream side of the reducing agent addition means.
[Selection] Figure 1

Description

  The present invention relates to an exhaust gas purification device for an internal combustion engine and an exhaust gas purification method thereof. In particular, the present invention relates to an exhaust gas purification device that purifies NOx in exhaust gas discharged from an internal combustion engine using an oxidation catalyst and an SCR catalyst, and an exhaust gas purification method thereof.

In recent years, from the viewpoint of measures against environmental pollution, in addition to low fuel consumption, internal combustion engines such as diesel engines (hereinafter also referred to as engines), nitrogen oxides (hereinafter referred to as NOx) contained in exhaust gas that is discharged. There is a strong demand for reducing the above. However, in order to meet both of these requirements, the response by the engine itself is already close to the limit, and an exhaust gas aftertreatment device (exhaust gas purification device) is required. In particular, since fuel consumption and NOx are in a trade-off relationship, fuel consumption can be improved if there is an exhaust gas purification device that can significantly reduce NOx.
Therefore, as one means for reducing this NOx, urea water is added as a reducing agent in the exhaust gas, and NOx is converted into nitrogen (N ₂ ) and water (NH ₃ ) by ammonia (NH ₃ ) generated by hydrolysis of urea. _2. Description of the Related Art An exhaust gas purification device that performs NOx reduction using a NOx catalyst that selectively reduces to H ₂ O), that is, a so-called urea SCR (Selective Catalytic Reduction) catalyst, is known.

  As an exhaust gas purification device as described above, for example, an exhaust gas purification device including an oxidation catalyst, an SCR catalyst, and a device that injects urea water as a reducing agent, which is disclosed in Patent Literature 1 and Non-Patent Literature 1, is known. It has been.

In this exhaust gas purification device, in order to efficiently purify NOx by the SCR catalyst, the NO component in the exhaust gas is oxidized to NO ₂ by the oxidation catalyst, and then urea water is injected into the exhaust gas, and then the SCR The catalyst is configured to reduce NOx to N ₂ and purify it.
Here, since the reaction with the SCR catalyst follows 2NH ₃ + NO + NO ₂ → 2N ₂ + 3H ₂ O, it is known that the purification efficiency is most improved when the molar ratio of NO and NO ₂ is 1: 1.
Further, since the NOx concentration in the exhaust gas normally discharged from the engine is higher than the concentration of NO ₂ , in the oxidation catalyst, a part of NO in the exhaust gas is obtained by the reaction of 2NO + O ₂ → 2NO _2. It is varied to NO _2. Thus, the NOx purification rate is improved by flowing into the SCR catalyst in a state where the concentration ratios of NO and NO ₂ are made uniform.
JP 2004-1000069 A Hirata Kiminobu et al., “Urea Selective Reduction System for Large Car Diesel”, Automotive Technology, VOL, 60, No. 9, 2006, PP28-33

However, it is difficult to finely adjust the ratio of the optimal NO concentration to the NO ₂ concentration described above by simply oxidizing NO with the oxidation catalyst disclosed in Patent Document 1 and Non-Patent Document 1. It was. That is, although the ratio of the NO concentration to the NO ₂ concentration in the exhaust gas varies depending on the temperature of the exhaust gas, the engine operating condition, etc., the SCR catalyst located downstream of the oxidation catalyst exhaust gas is not considered. It was supposed to flow into. For this reason, the SCR catalyst cannot be efficiently reduced, and the NOx purification rate cannot always be increased.

  Further, when NOx purification is not sufficiently performed, a part of urea added as a reducing agent is not used and may be released into the atmosphere as ammonia (ammonia slip phenomenon), which is avoided. Therefore, there is a problem that an oxidation catalyst for removing ammonia is further required on the exhaust gas downstream side of the SCR catalyst.

The present invention relates to an exhaust gas purification apparatus capable of improving the NOx purification rate without releasing a reducing agent into the atmosphere by controlling the ratio of NO concentration and NO ₂ concentration flowing into the SCR catalyst with high accuracy. And an exhaust gas purification method thereof.

  In order to achieve the above object, an exhaust gas purifying apparatus according to a first aspect of the present invention is provided in an exhaust passage of an internal combustion engine, and includes an oxygen supply means, an oxidation catalyst disposed downstream of the oxygen supply means, A reducing agent adding means disposed downstream of the oxidation catalyst, and an SCR catalyst disposed downstream of the reducing agent adding means.

According to this invention, by supplying oxygen from the oxygen supply means to the oxidation catalyst, the oxidation reaction from NO to NO ₂ in the oxidation catalyst can be promoted. In other words, even if the NO concentration is high due to the operating conditions of the engine and the like, the oxidation reaction to NO ₂ could not be sufficiently performed due to the rate of oxygen supply, so that it can be efficiently oxidized and is optimal for the reduction reaction in the SCR catalyst. such closer to actively molar ratio of NO and NO _2, it is possible to improve the purification rate of NOx.

Note that NO and NO ₂ are often converted to each other by chemical reaction or the like and exhibit relatively similar characteristics, and it is more convenient to handle them together, so NOx here is referred to as NO and NO ₂ Refers to two types of gas.

  The exhaust gas purifying apparatus according to a second aspect of the present invention is the exhaust gas purifying apparatus according to the first aspect, wherein the oxygen supply means includes a solid electrolyte and a porous electrode provided so as to sandwich the solid electrolyte. It is configured.

  According to the present invention, one electrode is installed outside (external air side) of the exhaust system and the other electrode is exposed inside the exhaust system (exhaust gas side), and the voltage is applied to take in from the outside air side. Oxygen ions are conducted in the solid electrolyte and released into the exhaust gas as oxygen, so that oxygen can be actively supplied and the NOx purification rate can be improved. Further, by making the electrode porous, it is possible to increase the contact area between the solid electrolyte outside air and the exhaust gas, to supply oxygen more efficiently, and to further improve the NOx purification rate.

  In the exhaust gas purifying apparatus according to the third invention, in the second invention, the solid electrolyte is stabilized zirconium, and the electrode is made of Pt. Thus, the high-temperature exhaust gas discharged from the engine has excellent heat resistance and corrosion resistance, can stably supply oxygen, and can maintain a high NOx purification rate.

The exhaust gas purification apparatus according to a fourth aspect of the present invention further comprises a measuring means for measuring the NO concentration and the NO ₂ concentration in the exhaust gas flowing into the SCR catalyst in the first to third aspects of the invention. The measurement means includes at least one NOx sensor installed upstream of the SCR catalyst, and NOx concentration in the exhaust gas flowing into the SCR catalyst from the value of the NOx concentration measured by the NOx sensor. Computing means for calculating the concentration and the NO ₂ concentration.

  As a result, the NOx concentration in the exhaust gas can be measured directly and in real time, and the NOx purification rate can be checked, and the NOx purification rate can be increased by feeding back the result. it can.

  The exhaust gas purifying apparatus according to a fifth aspect of the present invention further comprises a temperature sensor on the upstream side of the SCR catalyst in the fourth aspect of the invention.

Thus, the NOx concentration can be measured with higher accuracy by directly measuring the temperature in the exhaust gas, and the NOx purification rate can be further increased. Further, by preparing data of NOx concentration corresponding to temperature in advance and calculating it, it becomes possible to simultaneously measure NO concentration and NO ₂ concentration with one NOx sensor, and the number of NOx sensors Can also be reduced.

Further, in the exhaust gas purifying apparatus according to the sixth invention, in the fourth invention or the fifth invention, the oxygen supply means sets the oxygen supply amount in accordance with the values of the NO concentration and the NO ₂ concentration. Setting means is further provided.

As a result, the amount of oxygen to be supplied can be accurately adjusted according to the actual NOx concentration value in the exhaust gas, and the NO concentration and NO ₂ concentration flowing into the SCR catalyst can be brought close to the ideal ratio described above. Furthermore, the NOx purification rate can be increased.

Further, in the exhaust gas purifying apparatus according to the seventh invention, in the fourth invention to the sixth invention, the reducing agent adding means adds the reducing agent in an amount corresponding to the values of the NO concentration and the NO ₂ concentration. There is further provided setting means for setting.

  Thereby, the reducing agent (for example, ammonia) is not reacted while maintaining a high NOx purification rate by accurately adjusting the injection amount of the reducing agent according to the actual NOx concentration value in the exhaust gas. It can be prevented from being released into the atmosphere. In addition, a catalyst for purifying the unreacted reducing agent is not required, and the apparatus can be further reduced in size and cost.

An exhaust gas purification method according to an eighth aspect of the present invention is an exhaust gas purification method for an internal combustion engine that purifies exhaust gas discharged from the internal combustion engine using the exhaust gas purification device of the first aspect of the invention. An exhaust gas purification method that measures NO concentration and NO ₂ concentration in an inflowing exhaust gas and sets an oxygen supply amount from the oxygen supply means in accordance with the NO concentration and the NO ₂ concentration value. .

Accordingly, the amount of oxygen to be supplied is accurately adjusted according to the actual NOx concentration value in the exhaust gas, thereby bringing the NO concentration and NO ₂ concentration flowing into the SCR catalyst closer to the ideal molar ratio described above. And the NOx purification rate can be increased.

According to the present invention, the exhaust gas capable of improving the NOx purification rate without releasing the reducing agent into the atmosphere by controlling the ratio of the NO concentration and the NO ₂ concentration flowing into the SCR catalyst with high accuracy. A purification device and an exhaust gas purification method thereof can be provided.

(Embodiment 1)
FIG. 1 is a diagram for schematically explaining an exhaust gas purifying apparatus according to Embodiment 1 of the present invention. The exhaust gas purification device 30 is a device using a urea selective reduction catalyst that reduces and purifies NOx in exhaust gas to nitrogen gas or the like using urea. Moreover, in FIG. 1, the engine 11 is a diesel engine, for example.

As shown in the figure, the exhaust gas purification device 30 includes an oxidation catalyst 16 that oxidizes NO in the exhaust gas discharged from the engine 11 into NO ₂ , and downstream of the NOx in the exhaust gas with N ₂ and H _The SCR catalyst 21 is reduced to ₂ O.

Here, an oxygen supply device 15 is provided in the vicinity of the inlet of the oxidation catalyst 16. Thereby, the oxidation reaction from NO to NO ₂ in the oxidation catalyst 16 can be promoted by supplying predetermined oxygen. The supply amount of oxygen is controlled by adjusting a voltage by a control unit (ECU) 13.

  In addition, a urea addition device 19 for adding urea as a reducing agent is provided at the inlet of the SCR catalyst 21, and the amount of urea spray required for purification by NOx reduction at the SCR catalyst is controlled by a control device ( ECU) 14. Urea is stored in the urea tank 20.

Further, NOx sensors 17a and 17b are arranged between the oxidation catalyst 16 and the SCR catalyst 21. The current change rates from the two NOx sensors are read by a control device (ECU) 13 that controls the supply amount of oxygen and a control device (ECU) 14 that controls the spray amount of urea, and are input in advance. Based on this, the NO concentration and the NO ₂ concentration are calculated. Based on this NO concentration and NO ₂ concentration, the oxygen supply device 15 operates to supply an optimal amount of oxygen, so that the molar ratio of NO and NO ₂ in the exhaust gas after passing through the oxidation catalyst 16 is 1: It is controlled to be 1. Thereby, the NOx reduction reaction in the SCR catalyst 21 is efficiently performed, and the NOx purification rate can be improved.

Similarly, the urea addition device 19 operates based on the NO concentration and the NO ₂ concentration to supply an optimal amount of urea, so that the urea amount required for the NOx reduction reaction in the SCR catalyst 21 is increased. Can be controlled appropriately. As a result, it is possible to prevent a so-called ammonia slip phenomenon in which urea that has not been used in the reaction is released into the atmosphere as ammonia, and an oxidation catalyst downstream of the SCR catalyst that is installed to purify the slipped ammonia. And the apparatus can be further reduced in size and cost.

Here, the calculation of the NO concentration and the NO ₂ concentration has been described using two NOx sensors, but the current change detected from the NOx sensor using one NOx sensor and one temperature sensor (not shown). Based on the rate and temperature data from the temperature sensor, it is also possible to calculate the NO concentration and the NO ₂ concentration based on the sensitivity inputted in advance.
Note that any type of NOx sensor may be used, and a known NOx sensor can be used.

  Further, this diesel engine is provided with an exhaust gas recirculation device (EGR) (not shown), and a control device (ECU) 12 is arranged for controlling fuel injection. Therefore, the control device (ECU) 13 that controls the oxygen supply amount and the control device (ECU) 14 that controls the spray amount of urea can be shared with the control device (ECU) 12.

  As described above, according to the exhaust gas purification method using the exhaust gas purification device 30, the oxygen supply with high accuracy is performed in real time following the state of the NOx concentration in the exhaust gas which changes every moment according to the operating conditions of the engine 11. By controlling the amount and the amount of urea sprayed, it is possible to improve the NOx purification rate and prevent the occurrence of the ammonia slip phenomenon.

(Embodiment 2)
FIG. 2 is a cross-sectional view of a main part configuration of the oxygen supply device 15 according to Embodiment 1 of the present invention. As shown in the figure, the oxygen supply device 15 includes a solid electrolyte 31, two porous electrodes 32a and 32b that sandwich the solid electrolyte 31, and a wiring 33 that applies a voltage to the electrodes 32a and 32b.

  Here, the solid electrolyte 31 is formed of stabilized zirconium, and as shown in the drawing, one surface of the solid electrolyte 31 is disposed in the exhaust pipe and the other surface is disposed on the atmosphere side. The electrodes 32a and 32b are made of Pt, the electrode 32a is installed in the exhaust pipe, and the electrode 32b is installed on the atmosphere side.

  The electrodes 32 a and 32 b are connected to a control device (ECU) 13 that controls the supply amount of oxygen by a wiring 33, and a predetermined voltage is applied by the control device (ECU) 13. When a voltage is applied to the electrodes 32a and 32b, oxygen ions are conducted through the solid electrolyte from the atmosphere side into the exhaust pipe, and as a result, oxygen is taken into the exhaust pipe from the atmosphere side and oxygen is supplied into the exhaust gas. Will be.

  In addition, since the electrodes 32a and 32b are porous, the surface area of the solid electrolyte 31 in contact with the exhaust gas and the atmosphere can be increased, which is advantageous in that oxygen is supplied efficiently.

Further, as described above, the control device (ECU) 13 determines the NO concentration and NO ₂ based on the sensitivity inputted in advance based on the current change rate detected from the NOx sensors 17a and 17b and the temperature data from the temperature sensor. The concentration is calculated, and a voltage is applied to the electrodes 32a and 32b so that the molar ratio of NO to NO ₂ in the exhaust gas after passing through the oxidation catalyst 16 is 1: 1, and control is performed to supply appropriate oxygen. ing.

  By using the oxygen supply device 15 described above, NOx purification can be achieved by supplying an appropriate amount of oxygen in real time following the state of the NOx concentration in the exhaust gas, which constantly changes according to the operating conditions of the engine 11. It can be carried out. Moreover, since it is excellent in heat resistance and corrosion resistance, the NOx purification rate can be stably maintained over a long period of time.

(Embodiment 3)
FIG. 3 is a cross-sectional view showing the principal part of a modification of the oxygen supply device according to Embodiment 2 of the present invention. 2 denote the same or corresponding parts, and a description thereof will be omitted. FIG. 2 is different in that the oxygen supply device 15 is integrated with the oxidation catalyst 16.

The oxygen supply device 15 is disposed on the upstream side of the catalyst carrier 22 of the oxidation catalyst 16, and is installed so that one surface of the solid electrolyte 31 is in the oxidation catalyst (in exhaust gas) and the other surface is on the atmosphere side. The porous electrode 32a is installed in the oxidation catalyst (in the exhaust gas), and the electrode 32b is installed on the atmosphere side.
By integrating with the oxidation catalyst 16 in this way, the exhaust gas purification device can be further downsized. Further, the oxidation catalyst 16 and the oxygen supply device 16 approach each other, whereby the oxidation reaction can be promoted more efficiently, and NOx purification can be performed more efficiently as a whole.

  The structure of the embodiment of the present invention disclosed above is merely an example, and the scope of the present invention is not limited to the scope of these descriptions. The scope of the present invention is indicated by the description of the scope of claims, and further includes meanings equivalent to the description of the scope of claims and all modifications within the scope.

  The exhaust gas purification apparatus of the present invention can purify NOx in exhaust gas generated from an internal combustion engine such as a vehicle or a ship.

It is a figure for demonstrating typically the exhaust-gas purification apparatus in Embodiment 1 of this invention. It is principal part structure sectional drawing of the oxygen supply apparatus which concerns on Embodiment 2 of this invention. It is principal part structure sectional drawing which shows the modification of the oxygen supply apparatus which concerns on Embodiment 2 of this invention.

Explanation of symbols

11 Engine 12 Controller (ECU)
13 Control unit (ECU)
14 Control unit (ECU)
DESCRIPTION OF SYMBOLS 15 Oxygen supply apparatus 16 Oxidation catalyst 17a NOx sensor 17b NOx sensor 19 Urea spray apparatus 20 Urea tank 21 SCR catalyst 22 Oxidation catalyst support body 30 Exhaust gas purification apparatus 31 Solid electrolyte 32a Electrode 32b Electrode 33 Wiring

Claims (8)

An exhaust gas purifying device provided in an exhaust passage of an internal combustion engine,
An oxygen supply means, an oxidation catalyst arranged downstream of the oxygen supply means, a reducing agent addition means arranged downstream of the oxidation catalyst, and an SCR catalyst arranged downstream of the reducing agent addition means And an exhaust gas purifying device.   The exhaust gas purification device according to claim 1, wherein the oxygen supply means is an oxygen supply device having a solid electrolyte and a porous electrode provided so as to sandwich the solid electrolyte.   The exhaust gas purification apparatus according to claim 2, wherein the solid electrolyte is stabilized zirconium, and the electrode is formed of Pt. A measuring means for measuring NO concentration and NO ₂ concentration in the exhaust gas flowing into the SCR catalyst;
The measuring means includes
At least one NOx sensor installed upstream of the SCR catalyst;
Calculating means for calculating the NO concentration and the NO ₂ concentration in the exhaust gas flowing into the SCR catalyst from the value of the NOx concentration measured by the NOx sensor;
The exhaust gas purification device according to any one of claims 1 to 3.   The exhaust gas purification device according to claim 4, further comprising a temperature sensor on the upstream side of the SCR catalyst. The exhaust gas purifying apparatus according to claim 4 or 5, wherein the oxygen supply means further includes a setting means for setting the oxygen supply amount in accordance with the values of the NO concentration and the NO ₂ concentration. Said reducing agent addition means, the NO concentration in correspondence with the value of the NO ₂ concentration, any one of claims 4-6, further comprising a setting means for setting the amount of the reducing agent The exhaust gas purifying apparatus according to 1. An exhaust gas purification method for an internal combustion engine for purifying exhaust gas discharged from the internal combustion engine using the exhaust gas purification device according to claim 1,
The NO concentration and the NO ₂ concentration in the exhaust gas flowing into the SCR catalyst are measured, and the oxygen supply amount from the oxygen supply means is set in accordance with the values of the NO concentration and the NO ₂ concentration. Exhaust gas purification method.

Images