JPH07246319A - Method for removing nox in exhaust gas and device therefor - Google Patents

Abstract

PURPOSE:To optimize the decomposition condition of NOx for a long time by controlling the temp. of an oxidation catalyst bed to the temp. where the conversion efficiency of an org. compd. passing through the catalyst bed is controlled within a specific range based on the temp. difference between the inlet and outlet of the catalyst bed. CONSTITUTION:An exhaust gas is passed through an oxidation catalyst bed 2 together with a reducing agent consisting of org. compds. to decompose and remove the NOx in the exhaust gas. In this case, the difference between the inlet temp. of the catalyst bed 2 measured by a temp. sensor 22 and the outlet temp. of the bed 2 measured by a temp. sensor 21 is measured as a function of the conversion efficiency of the org. compd. obtained when the compd. is passed through the bed 2, and the temp. of the bed 2 is adjusted by the temp. difference so that the conversion efficiency is controlled to 10-90%. Consequently, NOx are efficiently removed even if the temp. and NOx concn. of the exhaust gas are fluctuated.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a nitrogen compound (hereinafter referred to as NOx) contained in various combustion exhaust gas such as diesel engine and boiler, and various exhaust gas such as exhaust gas generated during the production and use of nitric acid. The present invention relates to a method for decomposing and removing (hereinafter, also simply referred to as denitration method) and an apparatus therefor.

[0002]

2. Description of the Related Art At present, air pollution due to NOx emitted from internal combustion engines, boilers, etc.
There is a strong demand for the development of an exhaust gas purification system that includes exhaust gas.
As a method of removing NOx from these exhaust gases, a catalytic decomposition method of NOx has been proposed in which the exhaust gases pass through an oxidation catalyst layer together with a reducing agent composed of an organic compound (JP-A-2-149317). According to this method, the added organic compound reacts with NOx more preferentially than coexisting oxygen, and NOx can be decomposed into harmless nitrogen.

Since the optimum temperature range of this reaction is narrow, if the temperature is too low, the added organic compound will not be fully utilized and will not react with NOx and will go out of the system. On the other hand, if the temperature is too high, the organic compound reacts with the coexisting oxygen before burning with NOx and burns to be consumed unnecessarily. Therefore, in order to enhance the NOx removal effect, some means for keeping the catalyst layer at the optimum temperature is required. However, since the optimum temperature changes depending on the flow rate of exhaust gas, its composition, the amount of organic compounds added, the usage time of the catalyst, etc., an effective temperature control means cannot be found, which makes the denitration process difficult to put into practical use. there were.

[0004]

DISCLOSURE OF THE INVENTION The present invention provides a method for decomposing and removing nitrogen oxides contained in exhaust gas by passing the exhaust gas together with a reducing agent composed of an organic compound through an oxidation catalyst layer. In addition to providing a method capable of maintaining the decomposition condition of an object in an optimum range for a long time,
To provide a method for removing nitrogen oxides in exhaust gas by reaction from exhaust gas at a high decomposition reaction rate and not causing environmental pollution by treated exhaust gas, and further to provide an apparatus for carrying out the method. It is an issue.

[0005]

The present inventors have completed the present invention as a result of intensive studies to solve the above problems. That is, according to the present invention, in a method for decomposing and removing NOx contained in the exhaust gas by passing the exhaust gas together with a reducing agent composed of an organic compound through an oxidation catalyst layer,
The temperature difference ΔT value between the inlet temperature and the outlet temperature of the oxidation catalyst layer is measured as a function of the conversion rate of the organic compound obtained when the organic compound passes through the oxidation catalyst layer, and the temperature difference ΔT is measured. Based on the value, the temperature of the oxidation catalyst layer is adjusted so that the conversion rate of the organic compound passing through the catalyst layer is in the range of 10 to 90%, and NOx in the exhaust gas.
A removal method is provided.

Further, according to the present invention, a second oxidation catalyst layer is arranged on the downstream side of the oxidation catalyst layer (first oxidation catalyst layer), and the processing gas discharged from the first oxidation catalyst layer is treated as a second gas. There is provided a method for removing NOx in exhaust gas, which comprises passing through an oxidation catalyst layer and oxidizing and removing an organic compound contained in the first oxidation catalyst layer treatment gas. Further, according to the present invention, the exhaust gas is introduced and passed through the first oxidation catalyst layer together with the reducing agent composed of an organic compound, so that the organic compound is contained in an amount of 10 to 90%.
In the exhaust gas, which is characterized in that, after being oxidized at a conversion rate of 100%, the processing gas discharged from the first oxidation catalyst layer is passed through the second oxidation catalyst layer to oxidize and remove the organic compounds contained in the exhaust gas. Nox removal method is provided. Furthermore,
According to the present invention, the first oxidation catalyst layer and the second oxidation catalyst layer are provided on the downstream side of the exhaust gas supply port in the cylinder having the exhaust gas supply port at one end and the processing gas discharge port at the other end. An NOx removing device for exhaust gas is provided, which is sequentially arranged, and is provided with an exhaust gas temperature adjusting gas supply port and an organic compound supply port on the upstream side of the first oxidation catalyst layer.

The inventors of the present invention have conducted extensive studies on a method of decomposing and removing NOx in the exhaust gas by passing the exhaust gas together with an organic compound as a reducing agent through an oxidation catalyst layer, and as shown in FIG. The temperature range in which NOx removal is efficiently performed corresponds to the incomplete combustion temperature range that is lower than the temperature at which the organic compound added as a reducing agent becomes a completely stable final product such as carbon dioxide, water, and nitrogen gas. Focusing on this, the conversion rate (oxidation degree) of organic compounds in the oxidation catalyst layer
Is monitored using the temperature difference ΔT value between the inlet temperature and the outlet temperature of the oxidation catalyst layer, and the conversion of the organic compound is 10 to 95.
It has been found that the NOx decomposition and removal can be always maintained in the optimum condition range by adjusting the temperature of the oxidation catalyst layer so as to be in the range of 50%, preferably 50 to 90%.

Next, the principle of temperature control of the oxidation catalyst layer using the ΔT value in the present invention will be described with reference to the drawings. FIG. 1 shows a schematic diagram of a NOx decomposition and removal apparatus used in the present invention. In FIG. 1, 1 is a tubular body, 2 is an oxidation catalyst layer, 3 is an exhaust gas supply pipe, 4 is a treated exhaust gas exhaust pipe, 5 is a truncated cylinder-shaped short cylinder, 6 is a temperature control gas supply pipe, and 7 is a flow rate. A control valve, 8 is an organic compound supply pipe, 9 is a flow rate control valve, 10 is an exhaust gas generation source, and 13 is an exhaust gas flow meter.
Further, 20 is a controller, 21 is an oxidation catalyst layer outlet temperature sensor, 22 is an oxidation catalyst layer inlet temperature sensor, 23 is an exhaust gas / temperature control gas mixture temperature sensor, 24 is an exhaust gas temperature sensor, and 25 is an outside air temperature sensor. . In the apparatus shown in FIG. 1, the exhaust gas passes through the line 11 and the exhaust gas flow meter 13 and enters the tube body 1 from the exhaust gas supply pipe 3 to the tip portion of the short cylinder 5 and the tip nozzle portion of the temperature control gas supply pipe. After passing through the annular space between the two, through the oxidation catalyst layer 2,
It is discharged from the processing gas discharge pipe 4 and discharged into the atmosphere through the line 12. On the other hand, the organic compound as a reducing agent is added and mixed from the supply pipe 8 through the flow rate control valve 9 into the exhaust gas flowing in the cylindrical body 1.

Next, in the apparatus shown in FIG. 1, when the NOx is decomposed and removed with the supply amount of the temperature adjusting gas set to zero by the flow rate adjusting valve 7, the conversion rate of NOx to N ₂ (reduction and removal of NOx). 2) shows the relationship between the exhaust gas temperature and the conversion rate of the organic compound (oxidation degree of the organic compound) and the exhaust gas temperature. Further, in FIG. 2, the curve 1 is N
The conversion curve of Ox is shown, and the curve 2 shows the conversion curve of an organic compound. As can be seen from FIG. 2, in the NOx conversion rate curve 1, the NOx conversion rate started to increase rapidly when the organic compound conversion rate was 10% or more, and the peak was at a point where the organic compound conversion rate was about 50%. And the exhaust gas temperature in this case is about 315 ° C. On the other hand, the NOx conversion rate does not increase even if the conversion rate of the organic compound rises above about 50%,
On the contrary, it gradually decreases, but the conversion rate of the organic compound is 90 to 100.
Shows the highest value in%. However, the exhaust gas temperature is
Temperature to reach 100% conversion of organic compounds (about 360
(° C), the NOx conversion rate decreases sharply.

The present invention has an organic compound conversion rate of 10 to 95.
%, Preferably 40-95%, N
Focusing on the fact that the Ox conversion rate is maintained in a high range, the conversion rate of the organic compound in the oxidation catalyst layer is always 10%.
~ 95%, preferably 40-95%, more preferably 5
It is to be adjusted within the range of 0 to 90%. Hereinafter, a method for maintaining the conversion rate of the organic compound in the oxidation catalyst layer within the above range will be described in detail.

Referring again to FIG. 1, the oxidation catalyst layer outlet temperature T
Temperature difference between (21) and oxidation catalyst layer inlet temperature T (22) Δ
It is clear that the T value [ΔT = T (21) −T (22)] is generated by the amount of heat generated in the oxidation catalyst layer 2 if the oxidation catalyst layer 2 is held in an adiabatic state. The ΔT value is approximately represented by the following formula (1). ΔT = akQf / F (1) F in the above formula is the flow velocity of the exhaust gas supplied to the oxidation catalyst layer,
f is the supply rate of the organic compound, Q is the combustion heat per unit amount of the organic compound, and k is the constant heat specific heat coefficient of the exhaust gas. k fluctuates according to the exhaust gas temperature, but in practice k is 10
The average value of the constant pressure specific heat of air at 0 ° C to 500 ° C, preferably 200 ° C to 400 ° C may be used. When using a more accurate value, it may be measured. a represents the conversion of the organic compound, 0 when the added organic compound does not react at all, 1 when the organic compound is 100% oxidized,
In the middle of these, the number is a value between 0 and 1 depending on the conversion rate of the organic compound, which coincides with the conversion rate of the organic compound shown in FIG. When the organic compound is diluted and supplied, f is a value obtained by multiplying the supply rate by the dilution rate. Further, if the above equation (1) is expanded with respect to a, the following equation (2)
Can be expressed as a = F / (kQf) · ΔT (2) F in the above formula is uniquely determined by the operating conditions of the exhaust gas generation source, for example, the rotational speed and load of a diesel engine. Similarly, f is a preset numerical value.
Therefore, F / (kQf) in the equation (2) can be treated as a constant, and the ΔT value is a function of the conversion value a value of the organic compound. Therefore, the a value is proportional to the ΔT value, and the a value can be controlled by the ΔT value. On the other hand, as described above, in the case of the present invention, the value a representing the conversion rate of the organic compound is 0.1 to 0.1% in relation to the NOx conversion rate.
The value is set in the range of 0.95. For this purpose, the ΔT value is set to 0.ΔT (max) which is the ΔT value when a = 1.
1 to 0.95 times, that is, (0.1 to 0.95) ΔT (m
It may be controlled within the range of ax). In addition, ΔT
(Max) is an organic compound conversion rate of 100 in the case of an actual device
It can be easily calculated by operating under the condition of%. The conversion rate a value of the organic compound obtained when passing through the oxidation catalyst layer can be adjusted by the exhaust gas temperature entering the oxidation catalyst layer as can be seen from FIG. The rate goes up. This means that the conversion of the organic compound is promoted as the temperature of the oxidation catalyst layer rises. Therefore, when the ΔT value is set to a specific value or a value in a specific range in advance to perform denitration of exhaust gas at a specific temperature, the ΔT value and the oxidation catalyst layer temperature are monitored and By adjusting the temperature of the oxidation catalyst layer based on the T value, the ΔT value is constantly controlled within the specific value or within the specific range, whereby the denitration treatment is performed in a state where the NOx conversion rate is always maintained at a high value. be able to.

The temperature of the oxidation catalyst layer can be adjusted by an appropriate method. For example, a method of arranging a temperature controller inside the oxidation catalyst layer, a method of arranging a temperature control jacket around the outside of the oxidation catalyst layer, and a method of adjusting the temperature of the exhaust gas itself using the temperature controller. The most convenient method is to mix the temperature control gas into the exhaust gas flowing in the tubular body 1 through the introduction pipe 6 and the valve 7 as shown in FIG. . That is, in FIG. 1, the temperature control gas is ejected from the tip of the introduction pipe 6 through the introduction pipe 6 and the valve 7. On the other hand, the exhaust gas circulates in the annular space portion formed between the tip portion of the short tube 5 and the tip portion of the supply pipe 6 which are arranged in the tubular body 1. Since the exhaust gas flowing through the annular space becomes a turbulent flow, uniform mixing of the temperature adjusting gas and the exhaust gas is achieved and the exhaust gas is adjusted to a predetermined temperature. The exhaust gas mixed with the temperature adjusting gas and adjusted to a predetermined temperature passes through the oxidation catalyst layer 2 and is denitrated at that time, and the obtained denitration treatment gas is extracted from the exhaust pipe 4 thereof, and is discharged through the line 12. Through which it is discharged into the atmosphere.

When the temperature control gas is mixed with the exhaust gas, the temperature and the mixing ratio of the temperature control gas are appropriately determined according to the variation of the ΔT value. For example, the exhaust gas temperature becomes higher than the temperature in the steady operation of the device, and therefore the equation (2)
When the value of a increases and the ΔT value fluctuates higher than the value in the set range, a gas having a temperature lower than the exhaust gas temperature is used as a temperature control gas, and this gas is mixed with the exhaust gas to lower the exhaust gas temperature. Then, the value a is decreased, and the ΔT value is lowered to a value within the set range. On the other hand, the exhaust gas temperature becomes lower than the temperature during steady operation of the device, and therefore
When the value decreases and the ΔT value fluctuates below the value in the set range, a gas having a temperature higher than the exhaust gas temperature is used as a temperature control gas, and this gas is mixed into the exhaust gas to raise the exhaust gas temperature. , Increase the ΔT value to a value within the set range. The mixing ratio of the temperature control gas to the exhaust gas is appropriately selected according to the temperature of the temperature control gas and the variation value of the ΔT value.

Next, in the apparatus shown in FIG. 1, a method of automatically adjusting the organic compound conversion rate a value to a value within a preset range will be described in detail. In the case of this NOx removal device, the fluctuation of the a value fluctuates so as to increase and the ΔT value also fluctuates accordingly. Therefore, a gas having a temperature lower than that of the exhaust gas is used as the temperature adjusting gas.
In the following description, an example will be shown in which outside air (atmosphere) is used as the temperature control gas.

In the apparatus of FIG. 1, when the ΔT value fluctuates higher than the predetermined range due to the rise of the value a, this fluctuation value is captured by the temperature sensors 21 and 22 and sent to the controller 20 as an electric signal. , Where the temperature control gas is converted into an electric signal for flow rate control of the flow rate control valve 7 and is sent to the valve 7 to open the valve 7 so that outside air at a predetermined flow rate passes through the pipe 6 inside the cylindrical body 1. Is mixed with the exhaust gas flowing through. The exhaust gas temperature drops due to the inflow of this outside air,
Correspondingly, the temperature of the oxidation catalyst layer decreases, the a value decreases,
Accordingly, the ΔT value also decreases. The ΔT value decreases due to the decrease of the a value, and when the ΔT value falls within a predetermined range, the denitration treatment of the exhaust gas is continued under this condition. When the outside air is mixed with the exhaust gas, the relationship between the a value and the ΔT value in the above formulas (1) and (2) is as follows. ΔT (n) = akQf / (F + Fn) = akQf / Ft (3) a = Ft / (kQf) · ΔT (n) (4) In the above formula, Fn represents the introduction rate of outside air, and ΔT ( n) indicates the temperature difference between the outlet temperature and the inlet temperature of the oxidation catalyst layer when the outside air is introduced, and Ft indicates the sum of the exhaust gas flow rate F and the outside air flow rate Fn. k, Q and f have the same meanings as given for formulas (1) and (2).

As can be seen from the equations (3) and (4), when the outside air is mixed with the exhaust gas, even if the value a is the same as in the equations (1) and (2), ΔT (n). The value becomes a value (ΔT (n) = F / Ft · ΔT) lower than in the case of the formulas (1) and (2) due to the introduction of outside air. Therefore,
When the outside air is mixed with the exhaust gas to reduce the value a, the ΔT (n) value shown in the equations (3) and (4) is used to determine whether this ΔT (n) is within a predetermined range. Need to be monitored. For example, when the apparatus is operated so that the a value is in the range of 0.5 to 0.9, the ΔT (n) value is set to ΔT
(N) (max) is kept within the range of 0.5 to 0.9. When the ΔT (p) value obtained by mixing the outside air with the exhaust gas of the flow rate F at a constant flow rate Fp is within the predetermined range, the apparatus is operated while continuing the supply of the outside air at the flow rate Fp. On the other hand, when the supply of outside air at the flow rate Fp is insufficient, the outside air is supplied at a higher flow rate Fm (Fm> Fp), and the ΔT (m) value at that time is within a predetermined range. Monitor whether or not. Hereinafter, the same operation is repeated to obtain the flow velocity Fn in which the ΔT (n) value is within the predetermined range.
The apparatus is operated while continuously introducing the outside air at this flow rate Fn. As described above, when the device is operated while continuously introducing the outside air at the flow velocity Fn, the ΔT (n) value at that time is within a predetermined range depending on the time such as the exhaust gas temperature falling. When it becomes lower, on the contrary, the flow velocity of the outside air to be introduced is reduced, and the flow velocity F at which the ΔT (q) value is within the predetermined range is obtained.
q (Fq <Fn) is obtained, and the apparatus is operated while continuing the supply of outside air at this flow rate Fq.

The rate of decrease of the a value can be adjusted by the flow rate of the outside air introduced into the apparatus. The higher the flow rate Fn, that is, the greater the temperature drop of the exhaust gas, the lower the rate of decrease of the a value. growing. Therefore, in order to finely adjust the decrease rate of the a value, it is necessary to finely adjust the flow velocity of the outside air. When adjusting the flow rate of the outside air stepwise, 5 to 50 ° C. per one step flow adjustment operation,
It is preferable that the temperature of exhaust gas be lowered by about 10 to 20 ° C. In the apparatus shown in FIG. 1, an example of adjusting the flow rate of the introduced gas so that the value a falls within the range of 0.5 to 0.9 is shown in Table 1 below.

[0018]

[Table 1]

The flow rate control shown in Table 1 is performed by operating the valve to control the flow rate of the outside air in multiple stages from 0 to 6 stages.
It is set to drop 0 ° C. Therefore, in an apparatus including such a flow rate control, the ΔT value is displayed on the screen, and the flow rate control valve is adjusted to an appropriate step from 0 to 6 so that the ΔT value is within a predetermined range. Should be adapted to. As described above, when the operation of the outside air supply valve is performed in multiple stages in order to keep the value a in the range of 0.5 to 0.9, the controller is not always required, and An outside air flow rate controller and a ΔT value indicator may be attached, and the outside air flow rate controller may be manually adjusted so that the ΔT value falls within a predetermined range. The relationship between ΔT (n) (max) when the outside air is supplied and ΔT (max) when the outside air is not supplied can be expressed by the following equation for the same a value. ΔT (n) (max) = F / F (t) · ΔT (max) (5)

Although the outside air flow rate control valve can be manually operated as described above, it can be automatically operated by using the controller 20 as shown in FIG. When a controller is used, the valve operation can be performed in multiple stages or continuously. The principle of operating the apparatus by automatically operating the outside air flow rate control valve will be described in detail below.

As described above, the flow rate Fn of outside air is determined according to the desired rate of decrease in exhaust gas temperature. For example, the exhaust gas temperature is T (24) and the outside air temperature is T (25), and the outside air flow rate Fn required to lower the exhaust gas temperature by X by mixing this exhaust gas with the outside air is obtained as follows. To be The amount of heat held by the exhaust gas and the outside air before mixing is F × T (24) + Fn × T (25). On the other hand, the amount of heat possessed by the mixture obtained after mixing the exhaust gas and the outside air is (F + Fn) × T (23). Since the heat quantity of the mixture is equal to the heat quantity of each gas before mixing, the following formula is established. F × T (24) + Fn × T (25) = (F + Fn) × T (23) (6) By rearranging this equation, the following equation is established. Fn = [T (24) -T (23)] / [T (23] -T (25)] * F = X / [T (24) -T (25) + X] * F (7) [however, T (24) -T (23) = X] Further, the total gas amount Ft = F + Fn supplied to the oxidation catalyst layer.
Is expressed by the following equation. Ft = [T (24) -T (25)] / [T (23) -T (25)] F = [T (24) -T (25)] / [T (24)-(T (25 ) + X)] · F (8) Therefore, the expressions (3) and (4) are respectively expressed by the following expressions. Further, the equation (5) is represented by the following equation.

In the operation of the apparatus shown in FIG. 1, by automatically operating the outside air flow rate control valve 7, the a value is within a predetermined range, for example, a = 0.5 to 0.9 [0]. ．
5 to 0.9) ΔT (max)] while maintaining the value within the range, the controller 20 that stores the relational expression described above is used to operate the device. The operation method of the device will be described below. When operating the apparatus shown in FIG. 1, for example, a = 0.5 to 9.0 [0.5 to 9.0) ΔT (m
ax)], the a value becomes higher than a value within a predetermined range and the ΔT value fluctuates higher than a predetermined range due to a rise in exhaust gas temperature, etc. This is detected by the controller 20 which is in electrical communication with the temperature sensors 21,22. When the controller 20 detects that the ΔT value fluctuates higher than a predetermined value, this detection signal is converted into a flow rate control valve operation signal of the outside air flow rate control valve 7 and sent to the flow rate control valve 7. Operate the valve 7. In this case, the operation signal can be a signal for operating the valve in multiple stages or continuously. When operating the valve 7 in multiple stages,
The outside air is supplied to the device at a constant flow rate and at an increasing flow rate. Then, when the outside air flow velocity reaches a predetermined ΔT (n) value when Fn, the flow velocity F
The device is operated while supplying n outside air. On the other hand, when the valve 7 is continuously operated, the outside air is supplied to the device at a flow rate that increases at a constant rate. Then, when the outside air flow rate reaches a predetermined ΔT (n) value when Fn,
The device is operated while supplying the outside air having the flow rate Fn to the device.

In the above operation of the outside air flow rate control valve 7, the outside air flow rate Fn is expressed by the equation (7), and Δ at that time
The T (n) value is expressed by the equation (9), and Δ at that time
T (n) (max) is expressed by equation (11). Therefore, the controller 20 increases the outside air flow rate stepwise or continuously according to the equation (7) and monitors the fluctuation of the ΔT value at that time, and the ΔT value is expressed by the equation (11). When (0.5 to 0.9) ΔT (n) (max) is reached, the stepwise or continuous increase of the outside air flow rate is stopped, and the outside air is supplied to the apparatus at the outside air flow rate Fn at that time. In this way, the device will always be operated with a value in the predetermined range.

During the operation of the apparatus, the exhaust gas flow rate F itself may fluctuate. In this case, the above equation (1) is used.
Alternatively, as can be seen from (2), the ΔT value also fluctuates due to the fluctuation of the exhaust gas flow velocity F, even though the a value is a constant value. In order to prevent the fluctuation of the ΔT value due to the fluctuation of the exhaust gas flow rate F, the supply rate of the organic compound may be changed proportionally to the fluctuation of the exhaust gas flow rate F. That is, F / f in equation (1) or (2) may be held at a constant value. In this case, in FIG. 1, a signal from the exhaust gas flow meter 13 is sent to the controller 20, and a signal proportional to the fluctuation of this signal is sent to the flow control valve 9 for the organic compound.
To supply the organic compound in an amount proportional to the fluctuation of the flow rate of the exhaust gas.

In the above description, the outside air is used as the cooling gas, and a method of supplying the outside air into the apparatus is as follows.
An example has been shown in which the outside air is sucked into the device by the orifice effect when the exhaust gas flows through the annular space formed between the tip part of the short cylinder 5 and the tip part 6 of the outside air supply pipe. The present invention is not limited to such an example, and can be performed using a metering pump, a compressor, or the like.

The apparatus shown in FIG. 1 is preferably operated in an adiabatic state as much as possible. For this purpose, the entire apparatus is covered with a heat insulating material, the cylinder 1 itself is made of a heat insulating material, or a cylinder is used. A method of heating the outer peripheral surface of the body to a temperature slightly lower than the reaction temperature can be adopted.

Since the apparatus shown in FIG. 1 is operated in an incomplete combustion state in which the value of a is lower than 1, unreacted organic compounds remain in the processing gas. Since the release of this organic compound into the atmosphere is undesirable, it is necessary to completely oxidize and remove it. In FIG. 3, N
An example of an apparatus in which a second oxidation catalyst layer for oxidizing and removing an organic compound is provided after an oxidation catalyst layer (first oxidation catalyst layer) for decomposing and removing Ox is shown. 3, the same reference numerals as those shown in FIG. 1 have the same meaning. Further, 14 in FIG. 3 indicates a second oxidation catalyst layer. First oxidation catalyst layer 2
As the catalyst in the second oxidation catalyst layer 14, various conventionally known oxidation catalysts can be used. As such an oxidation catalyst, there are various ones as exemplified in JP-A-2-149317. For example, a supported catalyst carrying one or more metals selected from copper, chromium, manganese, iron, nickel, cobalt, rhodium, palladium, platinum, vanadium and molybdenum can be mentioned. Further, it is preferable to use hydrogen type mordenite, clinobutilolite, activated alumina or the like as the carrier. Among the above, a rhodium-supported activated alumina catalyst is particularly preferable.

The present inventors have found that when a supported rhodium catalyst is used as an oxidation catalyst for NOx decomposition removal and a supported platinum catalyst is used as an oxidation catalyst for complete oxidation of organic compounds, the NOx removal rate is increased. It was This effect is observed when the mixed gas of the NOx-containing gas and the organic compound is contacted with the supported rhodium catalyst and then with the supported platinum catalyst, and the contact order is reversed, and it is not observed when both catalysts are used in combination. It is a phenomenon. The catalyst used in this two-step catalyst method is preferably both active rhodium and platinum based active alumina.

As the organic compound as the reducing agent, various conventionally known compounds can be used. Examples of such a compound include ethylene, ethane, propane, propylene, n-butane, isobutane, t-butane, and n-butane.
Examples include various hydrocarbons such as butene, dodecane, and light oil; alcohols such as methanol and ethanol; ethers such as methyl ethyl ether, dimethyl ether, diethyl ether; and ketones such as acetone and methyl ethyl ketone. The amount of the organic compound added to the exhaust gas is 0.05 to 10 mol, preferably 0.1 to 2 mol, per 1 mol of NOx contained in the exhaust gas.

The treatment temperature of the exhaust gas is appropriately determined according to the type of oxidation catalyst used. If the temperature of the exhaust gas of the exhaust gas source is higher than the operating temperature of the catalyst, the exhaust gas is brought into contact with the catalyst after lowering it to the operating temperature of the catalyst. The use temperature of the rhodium catalyst preferably used in the present invention is 250 to 350 ° C., preferably 280 to 330 ° C. When the exhaust gas temperature is higher than the temperature in this range, the exhaust gas is cooled to the catalyst use temperature. Perform denitration treatment.

Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

Reference Example 1 A simulated exhaust gas containing about 1000 ppm NO and 5% oxygen and the residual gas consisting of helium was prepared, and about 1000 ppm of propylene was added thereto to prepare a test gas. This gas was supplied to the NOx removal device shown in FIG. 1 and reacted. In this case, the apparatus is a stainless steel tubular body (inner diameter: 30 mm, length: 60 cm), and a cordierite honeycomb support (diameter: 3 cm, as a catalyst)
Alumina was deposited on the surface (height: 5 cm) and rhodium was supported on the catalyst (the amount of rhodium supported on the whole:
0.3% by weight) was used. Further, the reaction was carried out in a state where both the valve 7 and the valve 9 shown in FIG. 1 were closed. In the reaction experiment, the experimental gas was set at various temperatures, and this was fed into the apparatus at a rate of 5 liters / minute for denitration treatment. The result is shown in FIG. Figure 2
As can be seen from the results shown in (1), efficient decomposition and removal of NOx is achieved when the propylene addition rate to CO ₂ is in the range of 10 to 95%, preferably 50 to 90%.

Example 1 As the NOx removing device, the one shown in FIG. 3 was used. This device has a structure in which, in the device shown in FIG. 1, a second oxidation catalyst layer 14 is formed after the rhodium catalyst layer 2 is filled with a 0.3 wt% platinum-supported catalyst having the same structure as the rhodium catalyst layer. belongs to. For this device, simulated exhaust gas heated from the exhaust gas supply pipe 3 (NO: 1000 ppm, O
₍₂ : 5%, balance: He gas) was supplied into the apparatus at a flow rate F, and the apparatus was operated while supplying propylene from the organic compound supply pipe 8 through the flow rate control valve 9 at a flow rate f. . In the operation of the apparatus, the value a in the first oxidation catalyst layer 2 is about 0.5 [ΔT (n) = about 0.
The operation was performed so that 5ΔT (n) (max)].
In this case, the exhaust gas supply rate F was 5 liters / minute, and the propylene supply rate f was 5 milliliters / minute. The simulated exhaust gas temperature is raised stepwise by 50 ° C. every hour, and the propylene concentration and NOx concentration in the treated gas obtained from the exhaust pipe 4 are measured at that time to measure the propylene conversion rate and NOx conversion rate (removal rate). I checked. The result is shown in FIG. Further, in the operation of the device, the flow velocity of the outside air flow rate control valve 7 is adjusted to ΔT while detecting the ΔT value.
The valve 7 was adjusted so that the (n) value was about 0.5 of ΔT (n) (max). The above experimental results are shown in FIG.
Shown in. In FIG. 4, a curve 1 shows the conversion rate of NOx, a curve 2 shows the conversion rate of propylene, and a curve 3 shows the external air mixing ratio (vol%) with respect to the simulated exhaust gas. As can be seen from the results shown in FIG. 4, in the case of the present invention, even if the simulated exhaust gas temperature rises, the NOx conversion rate can be kept high by introducing the outside air accordingly.
Further, in FIG. 4, when the same simulated exhaust gas temperature is compared, the NOx conversion rate is higher than the data in FIG. 2, and propylene is completely oxidized in the entire range where efficient NOx removal is performed. This is because the platinum catalyst layer 14 is provided downstream of the decomposition removal oxidation catalyst layer 2.

Example 2 An experiment was conducted in the same manner as in Example 1 except that the simulated exhaust gas temperature was kept constant and the supply of outside air was stopped. As a result, the propylene conversion rate is 10 to 90.
At temperatures of 250 ° C. and 350 ° C. showing temperatures in the range of 53%, NOx conversions of 53% and 40% were obtained, respectively.

[0035]

According to the method for removing NOx of claim 1, the temperature of the oxidation catalyst layer for NOx decomposition and removal is adjusted so that the conversion rate of the organic compound passing through the layer is in the range of 10 to 90%. Therefore, NOx is efficiently removed even if there is a change in the exhaust gas temperature or a change in the NOx content rate in the exhaust gas. Therefore, the present invention is a method suitable for various kinds of exhaust gas treatment, and is a preferable method for removing NOx in exhaust gas in which the fluctuation range of the NOx content ratio in the exhaust gas is large, particularly in diesel engines and lean burn engines. . N of claim 2
The Ox removal method is a practical NOx removal method that is simple and lightweight because the temperature of the oxidation catalyst layer for NOx decomposition removal is controlled by adjusting the outside air supply rate, and is compatible with mobile exhaust gas sources such as diesel vehicles. Is the preferred method.

In the method for removing NOx according to claim 3, since the exhaust gas from the oxidation catalyst layer for decomposing and removing NOx is further oxidized, the organic compounds in the gas are sufficiently oxidized and removed, and the remaining organic compounds are used. It is a NOx removal method that does not have a risk of secondarily polluting the environment. According to the method for removing NOx of claim 4, since the rhodium catalyst is used for decomposition and removal of NOx and the platinum catalyst is used for complete oxidation of the organic compound, the combined effect of both catalysts results in a higher NOx removal rate than the conventional catalyst. Has become. In the NOx removing method according to claim 5, the NOx decomposition and removal oxidation catalyst layer is a first oxidation catalyst layer at a front stage and a second oxidation catalyst layer at a rear stage.
Divided into the oxidation catalyst layer, 10 to 90% of the NOx reducing organic compound contained in the exhaust gas is oxidized in the preceding catalyst layer, and the remaining organic compound is oxidized and removed in the latter catalyst layer. Therefore, it is a NOx removal method that does not have a risk of secondary pollution of the environment due to residual organic compounds.

The NOx removing method of claim 6 uses a rhodium catalyst in the oxidation catalyst layer for NOx decomposition and removal in the preceding stage described in claim 5, and uses a platinum catalyst in that of the latter step. NO that has the same effect as the removal method
x removal method. An apparatus suitable for carrying out the NOx removal method according to any one of claims 1 to 6. N of claim 8
Since the first oxidation catalyst layer of the NOx removal device according to claim 7 comprises a rhodium catalyst and the second oxidation catalyst layer comprises a platinum catalyst, the Ox removal device is improved in NOx removal efficiency and oxidation removal efficiency of residual organic compounds. It is a NOx removal device. The NOx removing device is capable of automatically adjusting the temperature of the first oxidation catalyst layer because it is provided with the controller.

[Brief description of drawings]

FIG. 1 is a schematic view showing one embodiment of a device for removing nitrogen oxides from exhaust gas of the present invention.

FIG. 2 is a diagram showing a relationship between an exhaust gas temperature and a conversion rate of nitric oxide and a relationship between an exhaust gas temperature and a conversion rate of an organic compound.

FIG. 3 is a schematic diagram showing another embodiment of the device for removing nitrogen oxides from exhaust gas of the present invention.

FIG. 4 is a diagram showing a reaction result obtained by the method of the present invention.

[Explanation of symbols]

 1 Cylinder 2 Oxidation catalyst layer 3 Exhaust gas supply pipe 4 Process gas discharge pipe 5 Short cylinder 6 Temperature control gas (or outside air) supply pipe 7, 9 Flow rate control valve 8 Organic compound supply pipe 10 Exhaust gas source 13 Exhaust gas flow meter 20 Controller 21-25 Temperature sensor

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location B01D 53/86 ZAB F01N 3/08 ZAB B 3/24 ZAB A 3/28 ZAB 301 G B01D 53 / 34 129 B 53/36 ZAB 102 A 102 H (72) Inventor Atsushi Ogata 16-3 Onogawa, Tsukuba City, Ibaraki Prefecture Institute of Industrial Science and Technology

Claims (9)

[Claims] 1. A method for decomposing and removing nitrogen oxides contained in the exhaust gas by passing the exhaust gas together with a reducing agent composed of an organic compound through the oxidation catalyst layer, the temperature of an inlet temperature and an outlet temperature of the oxidation catalyst layer. The difference ΔT value is measured as a function of the conversion rate of the organic compound obtained when the organic compound passes through the oxidation catalyst layer, and the temperature of the oxidation catalyst layer is determined based on the temperature difference ΔT value. A method for removing nitrogen oxides in exhaust gas, which comprises adjusting the conversion rate of the organic compound passing through the catalyst layer to be in the range of 10 to 90%. 2. The method according to claim 1, wherein the temperature of the oxidation catalyst layer is adjusted by introducing the outside air into the exhaust gas and adjusting the amount of the outside air introduced. 3. A second oxidation catalyst layer is disposed on the downstream side of the oxidation catalyst layer (first oxidation catalyst layer), and the processing gas discharged from the first oxidation catalyst layer is passed through the second oxidation catalyst layer, 3. The method according to claim 1, wherein the organic compound contained in the first oxidation catalyst layer processing gas is removed by oxidation. 4. The method according to claim 3, wherein the first oxidation catalyst layer comprises a rhodium catalyst and the second oxidation catalyst layer comprises a platinum catalyst. 5. Exhaust gas is introduced and passed through a first oxidation catalyst layer together with a reducing agent made of an organic compound to oxidize the organic compound at a conversion rate of 10 to 90%, and then discharged from the first oxidation catalyst layer. A method for removing nitrogen oxides in exhaust gas, which comprises passing the treated gas through the second oxidation catalyst layer to oxidize and remove organic compounds contained in the exhaust gas. 6. The method according to claim 5, wherein the first oxidation catalyst layer comprises a rhodium catalyst and the second oxidation catalyst layer comprises a platinum catalyst. 7. A first oxidation catalyst layer and a second oxidation catalyst layer are provided in this order in a cylinder having an exhaust gas supply port at one end and a processing gas exhaust port at the other end, downstream of the exhaust gas supply port. An apparatus for removing nitrogen oxides in exhaust gas, which is provided with an exhaust gas temperature adjusting gas supply port and an organic compound supply port on the upstream side of the first oxidation catalyst layer. 8. The apparatus according to claim 7, wherein the first oxidation catalyst layer comprises a rhodium catalyst and the second oxidation catalyst layer comprises a platinum catalyst. 9. A first oxidation catalyst layer inlet temperature sensor disposed at an inlet portion of the first oxidation catalyst layer, a first oxidation catalyst layer outlet temperature sensor disposed at a first oxidation catalyst layer outlet portion, and exhaust gas. 9. The apparatus according to claim 7, further comprising a temperature control gas supply valve and a controller electrically connected to the temperature sensors and the exhaust gas temperature control valve.