JP3905359B2 - Exhaust gas purification device for gas engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a technique for reducing the emission amount of nitrogen oxides (NOx), particularly in an exhaust gas purification apparatus for a gas engine.
[0002]
[Prior art]
Gas engines that use CNG (compressed natural gas), LPG (liquefied natural gas), etc., containing hydrocarbons (C _m H _n ) as fuel, have an excellent feature of low NOx emissions. However, recently, the NOx emission regulation tends to be further strengthened, and it is necessary to further reduce the NOx emission.
[0003]
For this reason, for example, as disclosed in the 80th Catalysis Conference A Proceedings 3A07, P.67 (1997), a small amount of unburned hydrocarbons remaining in the exhaust gas are used as a reducing agent, and the exhaust passage An exhaust gas purification device that reduces and purifies NOx using a catalyst applied to a monolith intervening in the air has been developed. In view of the fact that unburned hydrocarbons in the exhaust gas are less than NOx to be purified, for example, as disclosed in Applied Catalysis B: Environmental 17 (1998) 333-345, unburned upstream of the monolith. A technique for adding a fuel containing hydrocarbons has also been devised. In this conventional technique, NOx in the exhaust gas is reduced and purified mainly by a chemical reaction of NO ₂ + HC (C _m H _n ) → N ₂ + CO ₂ + H ₂ O.
[0004]
[Problems to be solved by the invention]
However, in the prior art, the NOx reduction performance of a catalyst using hydrocarbons as a reducing agent is still insufficient, and therefore it has been required to further improve the exhaust properties through further reduction of NOx emissions. .
Therefore, in view of the conventional problems as described above, the present invention promotes the reduction and purification reaction of NOx in the catalyst by hydrogen (H ₂ ) generated by the steam reforming reaction, and further reduces the NOx emission amount. An object of the present invention is to provide an exhaust gas purification device for a realized gas engine.
[0005]
[Means for Solving the Problems]
For this reason, in the first aspect of the present invention, the first monolith interposed in the exhaust passage and the exhaust bypass passage that bypasses a part of the exhaust gas flowing through the exhaust passage and introduces it to the upstream side of the first monolith. A second monolith interposed in the exhaust bypass passage, the first and second catalysts applied to the first and second monoliths, respectively, and a hydrocarbon upstream of the second monolith Adding means for adding a kind, concentration detecting means for detecting the concentration of nitrogen oxide in the exhaust gas flowing through the exhaust passage located upstream of the first monolith, and the concentration of nitrogen oxide detected by the concentration detecting means And an addition control means for controlling the addition means to add hydrocarbons upstream of the second monolith only when the value is equal to or greater than a predetermined value. ,
While the second catalyst reacts the hydrocarbons added by the adding means with the steam in the exhaust gas to cause a steam reforming reaction such as the following formula,
C _m H _n + mH ₂ O → (m + n / 2) H ₂ + mCO
The first catalyst causes the nitrogen oxides in the exhaust to react with the hydrogen produced by the second catalyst and unconverted hydrocarbons to cause reduction reactions as shown in the following equations, respectively. NO _x + H ₂ → N ₂ + H ₂ O
NO _x + C _m H _n → N ₂ + CO ₂ + H ₂ O
It is characterized by that.
[0006]
According to such a configuration, the exhaust discharged from the gas engine is introduced into the first monolith via the exhaust passage, and a part thereof is introduced into the second monolith via the exhaust bypass passage. In the second monolith, the action of the second catalyst applied here causes hydrocarbons to react with the steam in the exhaust gas to cause a steam reforming reaction to generate hydrogen and carbon monoxide. . Hydrogen and carbon monoxide produced by the steam reforming reaction are introduced upstream of the first monolith together with hydrocarbons that have not been subjected to the steam reforming reaction. In the first monolith, by the action of the first catalyst applied here, hydrogen and hydrocarbons react with nitrogen oxides in the exhaust gas, respectively, and are converted into harmless nitrogen, carbon dioxide and water vapor.
[0007]
At this time, only when the concentration of nitrogen oxides in the exhaust gas is equal to or higher than a predetermined value, in other words, only when the amount of hydrocarbons added is insufficient and exhaust purification is insufficient, the hydrocarbon on the upstream side of the second monolith. Is added, so that its consumption is suppressed.
[0008]
In the invention according to claim 2 , the addition control means increases or decreases the addition amount of hydrocarbons added upstream of the second monolith based on the nitrogen oxide concentration detected by the concentration detection means. Features.
According to such a configuration, the amount of hydrocarbons added to the upstream side of the second monolith is increased or decreased based on the nitrogen oxide concentration in the exhaust gas, so that unreacted hydrocarbons are discharged as they are. Is suppressed, it is possible to perform stable exhaust purification over a wide range, and the exhaust properties are improved.
[0009]
The invention according to claim 3 is characterized in that the first catalyst is platinum-supported zeolite or silver-supported alumina.
According to this configuration, by using platinum-supported zeolite or silver-supported alumina as the first catalyst, hydrogen and hydrocarbons can be effectively reacted with nitrogen oxides in the exhaust.
[0010]
The invention according to claim 4 is characterized in that the second catalyst is rhodium-supported zirconia or rhodium-supported alumina.
According to this configuration, the steam reforming reaction is effectively performed by using rhodium-supported zirconia or rhodium-supported alumina as the second catalyst.
The invention according to claim 5 is characterized in that the exhaust bypass passage is provided with a forced bypass means for forcibly bypassing a part of the exhaust gas flowing through the exhaust passage.
[0011]
According to such a configuration, a part of the exhaust gas flowing through the exhaust passage is forcibly introduced into the second monolith that performs the steam reforming reaction, so that the hydrogen used for the reduction and purification of nitrogen oxides is surely obtained. Generated.
The invention according to claim 6 is characterized in that the addition control means operates the forced bypass means only when adding hydrocarbons upstream of the second monolith.
According to this configuration, the forced detour means is activated only when hydrocarbons are added upstream of the second monolith.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 shows the overall configuration of a first embodiment of a gas engine exhaust gas purification apparatus (hereinafter referred to as “exhaust gas purification apparatus”) according to the present invention.
A first monolith 14 having a honeycomb-shaped cross section is interposed in the exhaust passage 12 of the gas engine 10. Further, the exhaust passage 12 is provided with an exhaust bypass passage 16 that bypasses a part of the exhaust gas flowing through the exhaust passage 12 and introduces the exhaust to the upstream side of the first monolith 14. A second monolith 18 having a honeycomb-shaped cross section is interposed in the exhaust bypass passage 16.
[0013]
The gaseous fuel stored in the fuel tank 20 is supplied to the gas engine 10 and is added to the exhaust bypass passage 16 upstream of the second monolith 18 through the introduction passage 22. Here, CNG, LPG, etc. containing hydrocarbons are used as the gaseous fuel. The adding means includes the fuel tank 20 and the introduction passage 22.
[0014]
The second monolith 18 reacts with the gaseous fuel added from the fuel tank 20 to the water vapor in the exhaust gas introduced through the exhaust gas bypass passage 16 to perform a water vapor reforming reaction as shown in the following equation. A second catalyst to be awakened is applied. As the second catalyst, for example, rhodium-supported zirconia (Rh / ZrO ₂ ) or rhodium-supported alumina (Rh / Al ₂ O ₃ ) is desirably used in order to cause an effective steam reforming reaction.
[0015]
C _m H _n + mH ₂ O → (m + n / 2) H ₂ + mCO
On the other hand, the first monolith 14 is reacted with hydrogen produced in the second monolith 18 and unconverted (unreacted) gaseous fuel with respect to nitrogen oxides in the exhaust gas, as shown in the following formula: A first catalyst for reducing and purifying nitrogen oxides is applied. As the first catalyst, for example, platinum-supported zeolite (Pt / ZSM-5) or silver-supported alumina (Ag / Al ₂ O ₃ ) is preferably used in order to effectively reduce and purify nitrogen oxides.
[0016]
NO _x + H ₂ → N ₂ + H ₂ O
NO _x + C _m H _n → N ₂ + CO ₂ + H ₂ O
Reference numeral 24 not described above is a holding material for holding the first monolith 14 and the second monolith 18 in the exhaust passage 12 and the exhaust bypass passage 16, respectively.
[0017]
Next, the operation of the exhaust emission control device having such a configuration will be described.
The exhaust discharged from the gas engine 10 is introduced into the first monolith 14 via the exhaust passage 12 and a part thereof is diverted to the exhaust bypass passage 16. The exhaust gas divided into the exhaust bypass passage 16 is mixed with the gaseous fuel added via the introduction passage 22 to become an air-fuel mixture, and is introduced into the second monolith 18. When the air-fuel mixture passes through the second monolith 18, it comes into contact with the second catalyst applied here, and a steam reforming reaction such as the following equation occurs. That is, in the steam reforming reaction, hydrocarbons (C _m H _n ) contained in the gaseous fuel react with steam (H ₂ O) in the exhaust gas, and hydrogen (H ₂ ), carbon monoxide (CO), and Is generated.
[0018]
C _m H _n + mH ₂ O → (m + n / 2) H ₂ + mCO
Hydrogen and carbon monoxide generated in the steam reforming reaction are introduced into the exhaust passage 12 upstream of the first monolith 14 together with hydrocarbons that have not been subjected to the steam reforming reaction. Hydrogen, carbon monoxide, and hydrocarbons introduced into the exhaust passage 12 are mixed with the exhaust flowing through the exhaust passage 12 to increase the hydrogen content in the exhaust.
[0019]
Then, when the exhaust gas having an increased hydrogen content passes through the first monolith 14, it comes into contact with the first catalyst applied here, and a reduction and purification reaction such as the following equation occurs.
NO _x + H ₂ → N ₂ + H ₂ O
NO _x + C _m H _n → N ₂ + CO ₂ + H ₂ O
Therefore, nitrogen oxides in the exhaust gas react with hydrogen generated by the steam reforming reaction and hydrocarbons not subjected to the steam reforming reaction, and harmless nitrogen (N ₂ ), carbon dioxide (CO ₂ ) and converted to water vapor (H ₂ O). For this reason, the reduction and purification reaction of nitrogen oxides is promoted by hydrogen, and the emission amount of nitrogen oxides can be further reduced.
[0020]
Note that carbon monoxide generated by the steam reforming reaction in the second monolith 18 is discharged as it is from the exhaust purification device, but the discharge amount is extremely small and can be treated with an oxidation catalyst or the like. Exhaust properties are not affected.
FIG. 2 shows an overall configuration of a second embodiment of the exhaust emission control device according to the present invention. In the second embodiment, in order to suppress the consumption of gaseous fuel as a reducing agent as much as possible, a mechanism for controlling the addition of gaseous fuel according to the nitrogen oxide concentration in the exhaust is added to the first embodiment. Is. In addition, about the same structure as previous 1st Embodiment, the description is abbreviate | omitted by attaching | subjecting the same code | symbol.
[0021]
The exhaust bypass passage 16 upstream of the second monolith 18 is provided with, for example, a pump 30 (forced bypass means) driven by an electric motor in order to ensure the introduction of exhaust into the second monolith 18. . The introduction passage 22 is provided with an electromagnetic on-off valve 32 that opens and closes the introduction passage 22 in order to control the amount of gaseous fuel added to the exhaust bypass passage 16. On the other hand, the exhaust passage 12 is provided with a NOx sensor 34 (concentration detection means) for detecting the nitrogen oxide concentration in the exhaust. The output from the NOx sensor 34 is input to a control unit 36 incorporating a microcomputer, and drive control of the pump 30 and the on-off valve 32 is performed by processing to be described later. Note that the addition control means is realized by software by the control unit 36.
[0022]
FIG. 3 shows the processing contents that are repeatedly executed at predetermined time intervals in the control unit 36.
In Step 1 (abbreviated as “S1” in the figure, the same applies hereinafter), the NOx sensor 34 detects the nitrogen oxide concentration in the exhaust gas.
In step 2, it is determined whether or not the detected nitrogen oxide concentration is equal to or higher than a predetermined value, that is, whether or not exhaust gas purification is insufficient due to insufficient gaseous fuel as a reducing agent. If the nitrogen oxide concentration is equal to or higher than the predetermined value, the routine proceeds to step 3 (Yes), and the on-off valve 32 is opened and the pump 30 is operated to add gaseous fuel to the exhaust bypass passage 16. . On the other hand, if the nitrogen oxide concentration is less than the predetermined value, the routine proceeds to step 4 (No), and the on-off valve 32 is closed and the pump 30 is stopped to suppress the consumption of gaseous fuel.
[0023]
According to the processing of Step 1 to Step 4 described above, the gaseous fuel is added to the exhaust bypass passage 16 only when the nitrogen oxide in the exhaust gas is greater than or equal to a predetermined value, so that the consumption amount of the gaseous fuel is suppressed as much as possible. be able to.
The gaseous fuel added to the exhaust bypass passage 16 may be set in multiple stages by referring to a map, for example, based on the nitrogen oxide concentration in the exhaust. In this case, for example, the on-off valve 32 may be duty-controlled according to the set addition amount.
[0024]
In this way, the amount of gaseous fuel added is controlled in multiple stages in accordance with the concentration of nitrogen oxides in the exhaust, so that unreacted hydrocarbons are prevented from being discharged as they are, and over a wide range. Stable exhaust purification can be performed, and exhaust properties can be improved.
[0025]
【The invention's effect】
As described above, according to the first aspect of the invention, the reduction and purification reaction of nitrogen oxides is promoted by the hydrogen generated by the steam reforming reaction, so that the emission amount of nitrogen oxides is further reduced. be able to. At this time, since the hydrocarbons are added to the second monolith only when the addition amount of the hydrocarbons is insufficient and the exhaust purification is insufficient, the consumption amount can be suppressed.
[0026]
According to the second aspect of the present invention, it is possible to suppress unreacted hydrocarbons from being discharged as they are, and it is possible to perform stable exhaust purification over a wide range and to improve exhaust properties.
According to invention of Claim 3 , hydrogen and hydrocarbons can be made to react effectively with the nitrogen oxide in exhaust_gas | exhaustion.
[0027]
According to invention of Claim 4 , a steam reforming reaction can be performed effectively.
According to the fifth aspect of the present invention, hydrogen used for reduction and purification of nitrogen oxides can be reliably generated.
According to the sixth aspect of the present invention, the forced bypass means can be operated only when hydrocarbons are added upstream of the second monolith.
[Brief description of the drawings]
FIG. 1 is an overall configuration diagram of a first embodiment of an exhaust emission control device according to the present invention. FIG. 2 is an overall configuration diagram of an exhaust emission control device according to a second embodiment of the present invention. Flow chart [Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 Gas engine 12 Exhaust passage 14 1st monolith 16 Exhaust bypass passage 18 2nd monolith 20 Fuel tank 22 Introduction passage 30 Pump 32 On-off valve 34 NOx sensor 36 Control unit

Claims (6)

A first monolith interposed in the exhaust passage;
An exhaust bypass passage that bypasses a part of the exhaust gas flowing through the exhaust passage and is introduced upstream of the first monolith;
A second monolith interposed in the exhaust bypass passage;
First and second catalysts respectively applied to the first and second monoliths;
Adding means for adding hydrocarbons upstream of the second monolith;
Concentration detecting means for detecting the concentration of nitrogen oxide in the exhaust flowing through the exhaust passage located upstream of the first monolith;
An addition control means for controlling the addition means only when the nitrogen oxide concentration detected by the concentration detection means is a predetermined value or more, and adding hydrocarbons upstream of the second monolith;
An exhaust purification device for a gas engine configured to include:
While the second catalyst reacts the hydrocarbons added by the adding means with the steam in the exhaust gas to cause a steam reforming reaction such as the following formula,
C _m H _n + mH ₂ O → (m + n / 2) H ₂ + mCO
The first catalyst causes the nitrogen oxides in the exhaust to react with the hydrogen produced by the second catalyst and unconverted hydrocarbons to cause reduction reactions as shown in the following equations, respectively. NO _x + H ₂ → N ₂ + H ₂ O
NO _x + C _m H _n → N ₂ + CO ₂ + H ₂ O
An exhaust emission control device for a gas engine. The said addition control means increases / decreases the addition amount of the hydrocarbons added to a said 2nd monolith upstream based on the nitrogen oxide density | concentration detected by the said density | concentration detection means . Exhaust gas purification device for gas engines. The exhaust purification device for a gas engine according to claim 1 or 2, wherein the first catalyst is platinum-supported zeolite or silver-supported alumina . The exhaust purification device for a gas engine according to any one of claims 1 to 3, wherein the second catalyst is rhodium-supported zirconia or rhodium-supported alumina . 5. The forced bypass means for forcibly bypassing a part of the exhaust gas flowing through the exhaust passage is interposed in the exhaust bypass path. Gas engine exhaust purification device. 6. The exhaust gas purification apparatus for a gas engine according to claim 5, wherein the addition control means operates the forced bypass means only when adding hydrocarbons upstream of the second monolith .

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