JP3369229B2 - Denitration method and apparatus using urea - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flue gas denitration system by a catalytic reduction method, and more particularly to a denitration method and apparatus using safe and easy-to-handle urea as a reducing agent.

[0002]

2. Description of the Related Art NOx in smoke emitted from power plants, various factories, automobiles, etc. is a causative substance of photochemical smog, and ammonia (NH
_The flue gas denitration method by selective catalytic reduction using ₃ ) as a reducing agent is widely used mainly in thermal power plants. Nowadays,
The number of cogeneration systems that use diesel engines, gas turbines, etc. is increasing mainly in central Tokyo. NOx emission regulations are being applied to these systems as well, and they will be strengthened in some regions. There is an urgent need for equipment for flue gas denitration equipment.
Since such denitration equipment for small-scale facilities is used in densely populated areas such as buildings, it is difficult to apply liquefied NH ₃ .
Therefore, a method of using urea, which is easy to handle and safe, instead of liquefied NH ₃ , is drawing attention.

In the denitration method using urea as a reducing agent, it is necessary to vaporize urea and mix it uniformly in the exhaust gas in order to obtain a high denitration rate, and various vaporization methods have been invented conventionally. As a typical method, (1) a method in which urea water is directly sprayed into exhaust gas to evaporate and vaporize (JP-A-63
-190623, JP-A-1-47427), (2) A method of injecting urea water or solid urea into a flue after vaporizing and decomposing it in a reaction tank in advance (JP-A-2-86).
No. 813, Japanese Patent Application Laid-Open No. 2-191528), or (3) a method in which urea powder is directly sprayed into exhaust gas and vaporized and decomposed (Japanese Patent Application Laid-Open No. 2-203923).

[0004]

In any of the above-mentioned prior arts, there are problems in complete vaporization of urea, uniform mixing with exhaust gas, operability, etc., and they have not been widely put into practical use. . For example, the method (1) of spraying urea water into the exhaust gas has a problem that the vaporization speed of the liquid droplets is slow so that it is not completely vaporized or that the spray becomes non-uniform and uniform mixing with the exhaust gas is difficult. . Further, the method of blowing solid urea into the exhaust gas as shown in (3) has a problem that it becomes a secondary pollutant because volatile substances such as melamine and biuret are easily generated in the vaporization process. It is not suitable for denitration of exhaust gas with a large fluctuation of NOx concentration or exhaust gas containing low concentration NOx because of the limit of quantitative supply of body.
Among them, (2) the method of vaporizing and decomposing urea in advance and injecting it into the exhaust gas in a gaseous state does not cause problems such as non-uniform mixing with the exhaust gas, generation of scale in the flue and generation of secondary pollutants. Not only is it advantageous, but it is also excellent in quantification, uniform mixing with exhaust gas, and followability to fluctuations in NOx concentration. However, in the conventional method for decomposing and vaporizing urea, it is necessary to provide scale prevention means because scale that is a heating product of urea is likely to be generated in the urea decomposing / vaporizing unit, and the solid urea in the vaporizing unit must be provided. However, there was a problem because it was difficult to supply a fixed amount of. An object of the present invention is to eliminate the above-mentioned drawbacks of the prior art, and to provide a denitrification method and apparatus in which urea, which is safer and easier to handle than liquefied ammonia, can be used as a reducing agent for denitrification similar to ammonia.

[0005]

The above objects of the present invention can be achieved by the following constitutions. That is, in the denitration method in which nitrogen oxides in exhaust gas are catalytically reduced by a denitration catalyst, it is composed of at least one carbonate, borate or hydroxide of alkali metal, alkaline earth metal or rare earth element. Among the compounds of either the basic compound or the acidic compound consisting of an inorganic strong acid or an ammonium salt of an organic strong acid, a catalyst solution consisting of an aqueous solution containing at least one compound is heated, and urea or urea is added to the heated catalyst solution. Aqueous solution is continuously supplied to generate ammonia and carbon dioxide and at the same time water is evaporated, and a mixed gas of generated ammonia, carbon dioxide and water vapor is injected into the exhaust gas, and ammonia in the mixed gas causes NOx removal method using urea that catalytically reduces nitrogen oxides on NOx catalyst, or nitrogen oxidation in exhaust gas In a denitrification device for catalytically reducing carbon dioxide by a denitrification catalyst layer provided in the exhaust gas flue, the denitrification device is composed of at least one carbonate, borate or hydroxide of alkali metal, alkaline earth metal or rare earth element. At least a reaction tank containing a catalyst solution consisting of an aqueous solution containing at least one compound of a basic compound or an acidic compound consisting of an inorganic strong acid or an ammonium salt of an organic strong acid, and a catalyst solution in the reaction tank. Heating means capable of heating to the boiling point, urea or urea aqueous solution supply means for continuously supplying urea or urea aqueous solution to the catalyst solution in the reaction tank heated by the heating means, and ammonia generated in the reaction tank, It is a denitration device using urea, which is provided with an injection means for injecting a mixed gas of carbon dioxide and water vapor into exhaust gas.

The present invention has the advantage that problems such as incompatibility with exhaust gas, scale generation in the flue and generation of secondary pollutants do not occur. Further, the present invention can reduce the utility such as hydrolysis at a low temperature around 100 ° C, and can reduce the utility, as compared with the conventional vaporization method in which powdered urea is vaporized on a high temperature catalyst of 300 ° C or higher. It is excellent in that it does not generate scale that is a heating product of urea in the decomposition vaporizer. However, there is still a problem in controlling the amount of generated ammonia. That is, according to the present invention, when the ammonia generation rate is controlled with respect to the load fluctuation of the NOx concentration, the response is poor only by controlling the supply amount of urea or the carrier gas amount as a reaction control method, and the ammonia generation rate is controlled. I can't. In the denitration method using liquid ammonia as a reducing agent, the accumulator method is used as a load response countermeasure against the load fluctuation of NOx concentration, but in the denitration method of the present invention, only the ammonia and carbon dioxide generated by the target gas are generated. Not only that, it also contains carrier gas and water vapor, so it cannot be used as a huge accumulator.

Therefore, in order to improve the load followability of the ammonia generation rate of the present invention, the denitration method using urea is improved to improve the heating capacity of the catalyst solution or the supply amount of urea or urea aqueous solution to the catalyst solution. A method for controlling the generation rate of ammonia by increasing or decreasing at least any one of them, or a urea decomposition capable of heating the reaction tank to the boiling point of the catalyst liquid in the reaction tank by improving the denitration apparatus using the urea The reaction area and the preliminary reaction area for keeping the temperature lower than the boiling point of the catalyst solution are divided into two tanks, and the catalyst solutions in the urea decomposition reaction area and the preliminary reaction area can move to each other's area. As described above, the connection means may be provided. Here, the volume and increase / decrease of the catalyst liquid are achieved by using one or more of the following. (1) Two tanks containing the catalyst liquid are connected to each other, and one of them is used as a reaction urea decomposition reaction tank and the other is used as a preliminary reaction tank, and the liquid is moved between these two tanks. Further, the urea decomposition reaction tank is heated to or near the boiling point, urea or an aqueous urea solution is continuously supplied to generate NH ₃ and CO ₂ , and at the same time water is evaporated, and the preliminary reaction tank is kept at a temperature lower than the boiling point. I'll do it. (2) The reaction tank containing the catalyst liquid is divided into a zone for heating at or near the boiling point and a zone for heating to a temperature lower than the boiling point, and the amount of liquid in each zone is increased or decreased and heated to or near the boiling point. Urea or urea aqueous solution is continuously supplied to the section to generate NH ₃ and CO ₂ and at the same time water is evaporated, and the section heated to a temperature lower than the boiling point is kept at a temperature lower than the boiling point.

In carrying out the present invention, the catalyst liquid used is a hydroxide or carbonate of an alkali metal, an alkaline earth metal, a rare earth element or the like, or a compound acting as a base catalyst such as an organic or inorganic weak acid salt, or On the contrary, either one of the compounds acting as an acid catalyst such as a salt consisting of a mineral acid such as sulfuric acid, hydrochloric acid, phosphoric acid or a strong organic acid or a weak base such as an ammonium salt thereof, or a mixture of an acid catalyst and a base catalyst is used. It can be used as an aqueous solution. The catalyst liquid in the reaction tank is preferably used in a state of containing a high concentration of the catalyst in order to increase its boiling point, and for this purpose, the catalyst used has a high solubility.
Further, when an acid or a base is used alone, it takes a long time because the decomposition of urea becomes steady because it forms a salt with NH ₃ or CO ₂ which is a decomposition product of urea. It is easy to obtain high results when used in combination with a salt or an ammonium salt such as ammonium sulfate or ammonium chloride. Further, by adding urea to the catalyst solution in advance, the steady state can be quickly achieved.

The concentration of the catalyst liquid in the reaction tank is not particularly limited, but in the case of alkali carbonate, 1 to 60 wt%,
It is preferable to select 10 to 50 wt% to obtain good results. When the concentration of the catalyst solution is low, the decomposition rate of urea decreases, and when it is too high, the salts in the catalyst solution tend to coagulate when the apparatus is stopped, which is not desirable. On the other hand, the urea concentration and supply amount are selected based on the required ammonia concentration and water evaporation amount. Further, the temperature of the heated catalyst liquid is preferably 100 ° C. or higher in order to generate water vapor in addition to ammonia and carbon dioxide. Further, a mixed gas of ammonia, carbon dioxide and water vapor generated in the reaction tank can be carried into the exhaust gas by a carrier gas such as air, nitrogen gas, or combustion exhaust gas.

In order to accelerate the hydrolysis of urea, the present invention decomposes urea into ammonia (NH ₃ ) and carbon dioxide (CO ₂ ) by adding urea or an aqueous urea solution into a highly concentrated acid or base catalyst solution. The basic principle is to dilute the urea decomposition gas and purge it out of the system with water vapor generated by evaporating water. Therefore, the method of supplying urea is not limited to supplying urea to the catalyst solution as an aqueous solution in advance, but it is also possible to modify the method such as supplying solid urea and water separately to the catalyst solution to generate steam in the catalyst solution. It is within the scope of the invention. Further, the scope of the invention is not limited by the improvement such as providing a stirring blade in the reaction tank for decomposing urea containing the catalyst solution. Further, the denitration catalyst used to realize the denitration method of the present invention, vanadium in titanium oxide, molybdenum, a catalyst carrying oxides such as tungsten and zeolite transition metal or iron oxide molded article, Well-known honeycomb, plate, or granular catalysts can be used.

[0011]

[Function] In order to provide a denitrification method using urea, which has the same simplicity and controllability as the denitrification method using NH ₃ as a reducing agent, which is widely used at present, it is possible to mix solid urea with exhaust gas. It is desirable to use it in the form of gas that is easy and the injection amount can be controlled easily. As a result of detailed examination of the decomposition vaporization method of urea, the present inventor came to the conclusion that it is preferable to use a method in which urea water is hydrolyzed into NH ₃ and CO ₂ with an acid catalyst or a base catalyst as shown in formula (1). did. (NH ₂ ) ₂ CO + H ₂ O = 2NH ₃ + CO ₂ (1) However, the equilibrium of the reaction of the equation (1) is largely inclined to the left side (equilibrium constant is of the order of 1/10 ⁴ ), and diluted urea water or Not only under high temperature and high pressure conditions, a gas containing a high concentration of NH ₃ that can replace NH _{3 of the} conventional denitration method cannot be obtained, but also the apparatus becomes huge and a high pressure container is required. In order to avoid this, the generated NH ₃ and CO ₂
It is necessary to lower the partial pressure of and remove it to the outside of the catalytic reaction tank system promptly. The present invention achieves this by skillfully utilizing the evaporation of urea water.

Urea is hydrolyzed by the action of an acid catalyst or a base catalyst as shown in the formula (1), but normally only by heating, the partial pressure of NH ₃ and CO _{2 in} the hydrolysis reaction is not so high, Immediately, the liquid phase and the gas phase are in equilibrium, and the decomposition of urea hardly progresses any more, and the rate required for use as a reducing agent cannot be obtained. On the other hand, in the present invention, the concentration of the catalyst liquid is increased to maintain the catalyst liquid at or near the boiling point. By keeping the catalyst liquid at or near the boiling point, the water added as urea water can be quickly evaporated, and the generated steam lowers the partial pressure of NH ₃ and CO ₂ to accelerate the reaction (1). . Further, the generated gas component is immediately removed to the outside of the system.
The reaction of the formula is accelerated. In addition to this, by blowing N ₂ , air or combustion exhaust gas as a carrier gas into the catalyst liquid, not only the above-mentioned action is promoted but also the generated gas and steam mixture is constantly introduced into the exhaust gas flue due to the rise of the dew point of steam. You will be able to breathe in. The NH ₃ , CO ₂ , and steam mixture blown into the exhaust gas flue is mixed with the exhaust gas, but since NH ₃ is blown into the flue as a gas diluted with water vapor, the conventional solid urea or urea water is blown in. It is possible to easily achieve uniform mixing and to configure a high-performance denitration device without causing such a drift as described above or generating unvaporized urea droplets.

Furthermore, the present inventors have studied in detail the characteristics of the hydrolysis reaction of urea in an acid or alkali heating solution in order to improve the load followability of the ammonia generation rate from urea, and utilize the characteristics. Then, the method of easily controlling the ammonia generation rate was found. The hydrolysis reaction of urea of the formula (1) with an acid and an alkali catalyst proceeds as in the following formulas (2) and (3). (NH ₂ ) ₂ CO = NCO ⁻ + NH ₃ (2) NCO ⁻ + H ₂ O = NH ₃ + CO ₂ (3) At this time, the molar ratio of the ammonia generation amount and the urea supply amount is 2: 1 and the ammonia generation rate is urea. When the urea supply rate is doubled, ammonia is also generated twice, and when the urea supply rate is 1/2, the ammonia generation rate is also 1/2. However, as a result of studies by the present inventors, it was revealed that the generation rate of ammonia is proportional to the amount of unreacted urea and / or reaction intermediate (NCO ⁻ ) present in the catalyst. FIG. 9 shows the relationship between the ammonia generation rate and the nitrogen content in the catalyst solution (hereinafter referred to as N content). If the N rate in the catalyst solution does not change at the same time as the urea supply rate, the ammonia generation rate changes. I know I won't. As described above, in order to control the generation rate of ammonia, it is necessary to devise not only the supply rate of urea but also the amount of N in the catalyst liquid. Although it is difficult to change the amount of N in the catalyst solution rapidly, the present inventor has concluded that a method of changing the amount of the catalyst itself contained in the hydrolysis reaction system is preferable.

The present invention has a structure in which the reaction tank and the preliminary reaction tank are connected and the amount of the catalyst liquid can be freely changed by circulating between the reaction tank and the preliminary catalyst tank. For example,
When the ammonia generation rate is halved from the current rate, the urea supply rate is halved and the catalyst solution in the reaction tank is halved.
Transfer the amount to the pre-catalyst tank. When the ammonia generation rate is doubled, the urea supply rate is doubled and the same amount of catalyst solution as the catalyst in the reaction tank is moved from the preliminary reaction tank to the reaction tank. Changing the amount of catalyst in this way is nothing but changing the amount of N in the catalyst liquid. By such an operation, the ammonia generation rate can be controlled easily and quickly.

At this time, unreacted urea and reaction intermediates in the catalyst solution that is temporarily not used and moved to the preliminary reaction tank may generate NH ₃ and CO ₂ by hydrolysis, which is caused by this hydrolysis. It can be easily suppressed by utilizing the property that the reaction hardly proceeds unless the catalyst liquid has the boiling point or near the boiling point. That is, the present invention is characterized by accelerating the hydrolysis of urea by an acid and basic catalyst by using a catalyst solution having a high concentration and a boiling point of 100 ° C. or higher. By maintaining the temperature lower than the boiling point, the progress of this reaction can be suppressed, and the use of preventing the generation of NH ₃ from the catalyst liquid not used in the reaction is utilized. As described above, by skillfully utilizing the characteristics of this reaction, the generation of NH ₃ from the catalyst not used in the reaction is suppressed, and the ammonia generation rate can be controlled by circulating the catalyst solution.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to the drawings. First, an example in the case of using a single reaction tank will be described. Overall Configuration FIG. 1 shows an example of the flow of a denitration apparatus for carrying out the invention using a single reaction tank. In FIG. 1, 1-40w
The urea water 1 is sent to the reaction tank 6 by the pump 3 from the tank 2 containing t% urea water 1 through the pipe 10. Urea in the urea water 1 is hydrolyzed by the catalyst liquid 9 having a concentration of several tens wt% which is heated to the boiling point or its vicinity by the heater 7 in the reaction tank 6. Urea is hydrolyzed by the action of an acid or base catalyst as shown in formula (1), but normally only by heating, the partial pressure of NH ₃ and CO _{2 in} the hydrolysis reaction is not so high, and urea is immediately converted into a liquid phase. In the equilibrium with the gas phase, the decomposition of urea hardly progresses any more, and the rate required for use as a reducing agent cannot be obtained. On the other hand, in this embodiment, in order to make the concentration of the catalyst liquid 9 high and maintain the boiling point at 100 ° C. or higher, the water added as urea water is rapidly evaporated,
The generated water vapor lowers the partial pressure of NH ₃ and CO ₂ to accelerate the reaction (1). Further, the generated steam is immediately removed from the system by the pipe 12, so that the reaction of the formula (1) is further promoted. In addition to this, as shown in FIG. 1, by blowing N ₂ or air as a carrier gas with the pump 4 from the pipe 11 into the catalyst liquid 9 in the reaction tank 6, the partial pressure of generated NH ₃ and CO ₂ is lowered and Not only the action of promptly discharging these products to the outside of the catalytic reaction system is promoted, but also the generated gas / steam mixture is constantly generated by the rise of the dew point of the steam through the pipe 12 from the inlet 5 to the exhaust gas flue 19 You will be able to blow into it. N blown into the flue 19 from the inlet 5
The H ₃ , CO ₂ , steam mixture is mixed with the exhaust gas,
Since H ₃ is blown into the flue in the form of a gas diluted with water vapor, it does not cause uneven flow like the conventional blowing of solid urea or urea water and does not generate unvaporized urea droplets, and it is easily uniform. Therefore, a high-performance denitration device can be constructed with which excellent mixing can be achieved.

The urea water / solid urea supply system and the water supply system may be separated as shown in FIG. 2 so that the supply amount of urea and the supply amount of water are independently changed to facilitate control. The catalyst liquid 9 is usually maintained at or near the boiling point, and the amount of heat corresponding to the amount of evaporation is given by the external heat or the internal heater 7. Further, a stirrer (not shown) may be provided in the reaction tank 6 to improve the gas-liquid contact. In addition, the water vapor concentration in the decomposed gas is lowered to lower the dew point of water, and the flue 1
Good results can be obtained by blowing air, nitrogen, exhaust gas or the like into the reaction tank 6 as a carrier gas in order to improve the gas supply to the inside. The supply amount may be any amount, but if it is too large, heat loss increases, which is not preferable. N which is a decomposition product of urea in the reaction tank 6 described above.
After H ₃ is injected into the exhaust gas and mixed, it is guided to the denitration catalyst layer 8 in the flue 19. NOx in the exhaust gas is reduced by NH ₃ , which is a decomposition product of urea. The denitration reaction temperature is 200 as in the case of using known ammonia as a reducing agent.
The temperature is up to 600 ° C, and the injection amount of urea is N in terms of ammonia.
It is used in the range of about 0 to 2 with respect to the amount of Ox.

Example 1 6 g of urea and 6.9 g of potassium carbonate dissolved in 50 g of water were placed in a beaker and heated in a sand bath for 2 hours to decompose urea while boiling the aqueous solution and to partially decompose water. Evaporated. The average temperature of the aqueous solution at this time was about 107 ° C. The weight of the remaining aqueous solution was measured, the concentration of urea was measured under the conditions shown in Table 1 by a high performance liquid chromatograph, and the decomposition rate of urea was determined from the obtained residual amount of urea by the formula (4). Urea decomposition rate (%) = (initial urea amount-urea residual amount) / (initial urea amount) × 100 (4) However, in the formula (4), the unit of the urea amount is gram.

[0019]

[Table 1]

Examples 2 to 6 In place of 6.9 g of potassium carbonate of Example 1, 5.3 g of sodium carbonate Na ₂ CO ₃ and barium hydroxide Ba (O) were used.
H) ₂ · 8H ₂ O and 16.3 g, cerium hydroxide Ce (O
H) ₂ 8.7 g, ammonium sulfate (NH ₄ ) ₂ SO ₄ 6.6 g, potassium carbonate 3.5 g, and ammonium chloride 3.3 g were used in the same manner as in Example 1 except that water was evaporated. The amount and the decomposition rate of urea were obtained.

Comparative Examples 1 to 5 The same aqueous solutions as in Examples 1 to 6 were used and placed in an oil bath heated to 99 to 101 ° C. for 30 minutes so as not to boil. Then, in the same manner as in Example 1, the evaporation amount of water and the decomposition rate of urea were obtained. The results obtained in Examples 1 to 6 and Comparative Examples 1 to 5 are summarized in Table 2.

[0022]

[Table 2]

In Table 2, in all of Examples 1 to 6, a high urea decomposition rate was obtained, whereas in Example 1, the sample was heated to the same temperature as in Examples 1 to 6 without boiling, and the evaporation amount of water was small. It can be seen that Comparative Examples 1 to 5 hardly decompose urea. This result shows that in order to accelerate the decomposition of urea by using an acid or base catalyst, it is essential to remove the product out of the system together with steam, and the method of the present invention utilizing this is It shows that it is excellent.

Example 7 In order to confirm steady decomposition of urea, an experimental apparatus having a system as shown in FIG. 3 was used, and 2.3 wt% urea aqueous solution was continuously fed under the conditions of Table 3. Ammonia and CO ₂
Was measured.

[0025]

[Table 3]

In FIG. 3, 2.3 wt% of urea water 1 is supplied from the tank 2 through the pipe 10 to the reaction tank 6 by the pump 3.
The urea in the urea water 1 is sent to the heater 7 in the reaction tank 6.
Heated up to or near the boiling point by tens of wt%
It is hydrolyzed by the catalyst liquid 9 composed of potassium carbonate and urea having a concentration of. The decomposition products are cooled by the cooler 16 provided in the pipe 13, the components condensed in the trap 17 are removed, and the generated CO ₂ is guided to the monitor 18 via the pipe 14 and measured. It should be noted that the urea is preliminarily added in order to make it steady quickly. Ammonia was trapped with a 5% sulfuric acid aqueous solution and quantified by ion chromatography, and CO
₂ was measured with an infrared absorption monitor. FIG. 4 shows the CO and NH ₃ generation rates and the decomposition rate of urea calculated from them with time. It can be seen that after about 2 hours, a urea decomposition rate of almost 100% was obtained, and ammonia corresponding to the injected urea was constantly obtained.

Example 8 A denitration test was conducted on the decomposition gas of urea generated in Example 7 under the conditions shown in Table 4 using a denitration apparatus having the system shown in FIG. The obtained denitration rate was 95% or more, which was the same as when the urea decomposition gas was replaced with ammonia gas.

[0028]

[Table 4]

Next, examples of the invention using a reaction tank and a preliminary reaction tank will be described. Overall Configuration In this example, the hydrolysis catalyst solution of urea is divided into a system used for the reaction and a system not used for the reaction, the catalyst solution of the system not used for the reaction is kept at a temperature lower than the boiling point, and the catalyst solution is The basic principle is to circulate between two systems. Therefore, in addition to the apparatus shown in the flow chart of FIG. 5, which is also provided with a reaction tank and a preliminary reaction tank, a method of providing three or more reaction tanks, a section in which one reaction tank heats at or near the boiling point and a temperature lower than the boiling point Modifications such as a device for dividing into a keeping section are also within the scope of the present invention. In FIG. 5, the urea water 21 is sent from a tank 22 containing 1 to 40 wt% of urea water 21 to a reaction tank 26 containing a catalyst solution 31 having a concentration of several tens wt% by a pump 23 via pipes 42 and 43. To be Reaction tank 26
The pre-reaction tank 27 is connected to the via pipe 48. The preliminary reaction tank 27 is provided with heating means 30 for keeping the catalyst liquid 31 in the tank at a constant temperature. The pump 33 is used to move the liquid between the reaction tank 26 and the preliminary reaction tank 27. Further, the trap 28 is provided to prevent backflow to the pump 33. Further, in the same manner as in the flow shown in FIG. 1 above, the reaction tank 26 generates NH ₃ and CO ₂ by blowing N ₂ or air as a carrier gas into the catalyst liquid 31 in the reaction tank 26 from the pipes 44 and 45 by the pump 24. Not only is the action of promptly discharging these products out of the catalytic reaction system promoted while the partial pressure is lowered, but also the generated gas / steam mixture is constantly introduced through the pipe 46 due to the rise of the dew point of the steam. It can be blown into the exhaust gas flue 39 in which the denitration catalyst 30 is arranged. In the flow of FIG. 5 having the above-described configuration, the urea in the urea water 21 introduced into the reaction tank 26 is heated to a boiling point or its vicinity by a heater 29 in the reaction tank 26 and is a catalyst having a concentration of several tens wt%. It is hydrolyzed by the liquid 31. This catalytic reaction is the same as the reaction in the reaction tank 6 in the flow of FIG.

A 1 to 40 wt% urea aqueous solution is injected by a pump 23 into a catalyst liquid 31 having a concentration of several tens wt% heated to or near the boiling point, and urea in the reaction tank 26 acts as an acid or base catalyst. Is hydrolyzed by. The NH ₃ and CO ₂ generated in the reaction tank 26, and the steam generated by the evaporation of the water added as urea water together with the carrier gas blown into the reaction tank 26 pass through the pipe 46 and the flue 3
It is blown into 9. At this time, if it is necessary to increase or decrease the NH ₃ generation amount with respect to the fluctuation of the NOx concentration in the exhaust gas, the following operation is performed. To reduce the amount of NH ₃ generated, the pump 33 is used to send a part of the catalyst liquid 31 in the reaction tank 26 to the preliminary reaction tank 27 connected to the reaction tank 26 by the pipe 48 to reduce the urea supply amount. Let On the contrary, when it is necessary to increase the NH ₃ generation amount, the catalyst liquid 31 is sent from the preliminary reaction tank 27 to the reaction tank 26 to increase the urea supply amount. When increasing or decreasing the NH ₃ generation amount, a method of increasing or decreasing the carrier gas amount in addition to the movement of the catalyst liquid 31 between the reaction tank 26 and the preliminary reaction tank 27 or the heater 29 is used.
The method of increasing or decreasing the amount of steam generated by adjusting the heating degree may be used in combination.

As a means for moving the catalyst liquid 31 between the reaction tank 26 and the preliminary reaction tank 27, a method of pressurizing / depressurizing with a gas is used in addition to the method of sucking / pushing the catalyst liquid 31 with the pump 33. May be. Further, the position of the pump 33 may be between the reaction tank 26 and the preliminary reaction tank 27, or two or more pumps may be provided and divided into an increasing line and a decreasing line.
It is necessary to keep the temperature of the preliminary reaction tank 27 at a temperature lower than the boiling point, but if kept too low, the solubility of the catalyst will be lowered and the catalyst will be deposited, or the reaction temperature will decrease when it is returned to the reaction system. Be careful because it causes problems. If the catalyst in the preliminary reaction tank 27 is kept at a temperature lower than the boiling point, hydrolysis of unreacted urea and reaction intermediates contained therein can be suppressed.
Since some decomposition will occur by stirring 1 etc.,
It is preferable not to perform vigorous stirring or gas bubbling. The urea decomposed by the above-described decomposition method is injected into the exhaust gas, mixed, and then introduced into the denitration catalyst tank 30 in the flue 39. NOx in the exhaust gas is NH, which is a decomposition product of urea.
Reduced by ₃ . The denitration reaction temperature is 200 to 200, which is the same as in the case of using known NH ₃ as a reducing agent, as in the flow of FIG.
The temperature is 600 ° C., and the amount of urea to be injected is in the range of about 2 in excess of 0 with respect to the amount of NOx in terms of NH ₃ .

Example 9 Using a device having a flow as shown in FIG. 5, the amounts of ammonia and CO ₂ generated were measured when the aqueous urea solution was continuously fed into the reaction tank 26 under the condition A shown in Table 5.

[0033]

[Table 5]

Ammonia was trapped with a 5% sulfuric acid aqueous solution and quantified by ion chromatography, and CO ₂ was measured by an infrared absorption monitor. After confirming that the decomposition rate of urea constantly becomes 100%, 1/2 of the catalyst liquid 31
The amount was sucked into the preliminary reaction tank 27 by the pump 33. At this time, the catalyst holding temperature in the preliminary reaction tank 27 was 100 ° C. Simultaneously with the movement of the catalyst liquid 31, all of the conditions A in Table 5, such as the urea supply amount, were switched to the condition B of ½ amount thereof. After switching, the state is maintained for 2 hours, then the whole catalyst liquid 31 in the preliminary reaction tank 27 is moved to the reaction tank 26 by the pump 33, and at the same time, the condition A is switched to measure the amount of ammonia and CO ₂ generated during that time. did. FIG. 6 shows the changes over time in the generation rates of ammonia and CO ₂ . It can be seen that the generation rates of ammonia and CO ₂ immediately after moving the catalyst liquid 31 have reached a predetermined amount immediately. For comparison with this example, the apparatus having the flow of FIG. 1 was used to measure the amounts of ammonia and CO ₂ generated when the aqueous urea solution was continuously fed into the reaction tank 26 under the conditions shown in Table 6.

[0035]

[Table 6]

After confirming that the decomposition rate of urea was constantly 100%, only the urea supply amount was switched to 1/2. After switching, the state was maintained for 2 hours, after which the urea supply amount was switched to the initial amount again, and the amounts of ammonia and CO ₂ generated during that period were measured. As shown in FIG. 7, it can be understood that it takes more than two hours until the amount of NH ₃ generated changes to a predetermined amount by changing only the urea supply amount.

Test Example 1 Using the apparatus having the flow shown in FIG. 1, the amounts of ammonia and CO ₂ generated when urea aqueous solution was continuously fed under the conditions shown in Table 6 were measured, and ammonia and CO ₂ were constantly steadily added. It was confirmed that At this time, the generation rates of ammonia and CO ₂ were 1.6 and 0.8 mmol / cm 2 respectively in the steady state.
It was min.

Example 10 The catalyst liquid of which the steady state was confirmed in Test Example 1 is shown in FIG.
The temperature of the catalyst solution is kept at 80 ° C. in the reaction tank 26 of the apparatus having the flow shown in Fig. 1 and the time-dependent change in the amount of ammonia and CO ₂ generated is measured 1 minute, 10 minutes and 60 minutes after the start of the reaction. did.

Examples 11 and 12 The same operation as in Example 9 was carried out, and the temperature of the catalyst liquid 31 was adjusted to 90
1 minute after the start of the reaction when maintained at ℃ and 100 ℃, 1
The changes with time of the amounts of NH ₃ and CO ₂ generated after 0 minutes and 60 minutes were measured.

Comparative Examples 6 and 7 When the same operation as in Examples 10 to 12 was carried out and the temperature of the catalyst liquid 31 was maintained at 70 ° C. and 110 ° C., NH after 1 minute, 10 minutes and 60 minutes from the start of the reaction. ₃ and CO ₂ generation rates were measured over time. The results obtained in Examples 10 to 12 and Comparative Examples 6 and 7 are summarized in Table 7.

[0041]

[Table 7]

In Table 7, Examples 10 to 1 according to the present invention
No 2 produced almost no NH ₃ and CO ₂ ,
Further, in all cases, no bubbles were generated in the catalyst liquid 31 and no precipitate was generated in the catalyst liquid 31. On the other hand, in Comparative Examples 6 and 7 in which the holding temperature is 70 ° C., N
Almost no generation of H ₃ and CO ₂ was observed,
Precipitates were generated in the catalyst liquid 31. Further, in Comparative Example 7 in which the holding temperature is 110 ° C., NH ₃ and CO ₂ are remarkably generated even after the supply of urea to the reaction tank 26 is stopped, and the catalyst liquid 31 shows innumerable amounts due to boiling. Bubbles were generated. This result shows that by keeping the temperature of the catalyst liquid 31 lower than the boiling point and higher than the solubility of the catalyst, the catalyst liquid 31
It can be maintained in a steady state.

Embodiment 13 Another embodiment relating to the invention using a reaction tank and a preliminary reaction tank is shown in FIG. The supply system of urea, carrier gas, generated NH ₃ and CO ₂ to the exhaust gas introduced into the reaction tank 26 is the same as the flow shown in FIG. Heaters 55 are provided in multiple stages in the reaction tank 26 so that the temperature can be individually set. The heater 55 is set so that the upper portion of the reaction tank 26 is at or near the boiling point and the lower portion is at a temperature lower than the boiling point. Figure 8
When increasing or decreasing the amount of NH ₃ generated in the reaction tank 26 in the flow shown in FIG. When the amount of generated NH ₃ is reduced, the set temperature of each heater 55 is adjusted to reduce the amount of the catalyst liquid 31 that is heated at or near the boiling point, and the amount of urea supplied from the tank 22 is reduced. On the contrary, when the NH ₃ generation amount is increased, the set temperature of each heater 55 is adjusted to increase the amount of the catalyst liquid 31 heated to or near the boiling point, and the tank 2
Increase the urea supply from 2. Further, the method of keeping the temperature of the heater 55 constant and moving the catalyst liquid 31 itself by increasing or decreasing the pressure is within the range of this embodiment.

Example 14 A denitration test was conducted on the decomposition gas of urea generated in Example 9 using the denitration apparatus having the flow shown in FIG. 5 under the conditions shown in Table 5.
When the NOx concentration changes, the movement of the catalyst liquid 31 causes NH
_{3 No} response delay of the denitrification rate was observed when the generation rate was controlled.

[0045]

EFFECTS OF THE INVENTION According to the present invention, it is possible to inject urea into the flue in a gaseous state in the form of gas, which is safer and easier to handle than liquefied ammonia, for denitration. This makes it possible to completely perform the vaporization and mixing of urea, which has been a problem in the denitration method using urea, and also to improve the load followability of the NH ₃ generation rate. A highly reliable denitration method and apparatus similar to the denitration method used for a reducing agent can be provided. In particular, for a fixed source of NOx in a city and its suburbs where liquefied ammonia cannot be used, providing a safe and high-performance denitration device makes a great social contribution.

[Brief description of drawings]

FIG. 1 Examples 1 to 7 using a single reaction tank of the present invention
FIG.

FIG. 2 Examples 1 to 7 using a single reaction vessel of the present invention
FIG.

FIG. 3 is a system diagram of Example 7 of the present invention.

FIG. 4 is a diagram showing the results of Example 7 of the present invention.

FIG. 5 is a system diagram of Examples 8 to 12 using the reaction tank of the present invention and the preliminary reaction tank.

FIG. 6 is a diagram showing changes with time of generated NH ₃ and CO ₂ in Example 8 of the present invention.

FIG. 7: N generated when a single reaction tank of the present invention is used
Is a diagram showing the time course of H ₃ and CO _2.

FIG. 8 is a system diagram of Example 13 using the reaction tank and the preliminary reaction tank of the present invention.

FIG. 9 is a diagram showing the relationship between the NH ₃ generation rate and the amount of N in the catalyst liquid of the present invention.

[Explanation of symbols]

1, 21 ... Urea water, 5, 25 ... Injecting port, 7, 29, 55
... heater, 8, 30 ... denitration catalyst, 9 ... catalyst liquid, 16 ... cooler, 17 ... trap, 18 ... CO ₂ monitor, 19, 3
9 ... Flue

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-2-191528 (JP, A) Chemistry Dictionary 6 Miniature edition, Japan, Kyoritsu Publishing Co., Ltd., December 15, 1963, first edition edition , P. 823 (58) Fields surveyed (Int.Cl. ⁷ , DB name) B01D 53/90 B01D 53/56

Claims (8)

(57) [Claims] 1. A denitration method in which nitrogen oxides in exhaust gas are catalytically reduced by a denitration catalyst, wherein a carbonate, borate or hydroxide of at least one selected from alkali metals, alkaline earth metals and rare earth elements is used. A basic compound consisting of a compound or an acidic compound consisting of an ammonium salt of an inorganic strong acid or an organic strong acid, a catalyst liquid consisting of an aqueous solution containing at least one compound is heated, Urea or an aqueous urea solution is continuously supplied to generate ammonia and carbon dioxide, and at the same time water is evaporated, and a mixed gas of the generated ammonia, carbon dioxide and water vapor is injected into the exhaust gas, and ammonia is mixed in the mixed gas. A NOx removal method using urea, which comprises catalytically reducing nitrogen oxides in exhaust gas on a NOx removal catalyst. 2. The urea according to claim 1, wherein the generation rate of ammonia is controlled by increasing or decreasing at least one of the heating capacity of the catalyst solution and the supply amount of urea or an aqueous urea solution to the catalyst solution. The denitration method used. 3. The heating capacity of the catalyst liquid is increased or decreased by increasing or decreasing the amount of liquid in each section of the reaction region containing the catalyst solution, which is divided into a section for heating at or near the boiling point and a section for heating to a temperature lower than the boiling point. The urea according to claim 2, wherein urea or an aqueous urea solution is continuously supplied to a section heated to or near the boiling point to generate ammonia and carbon dioxide and at the same time evaporate water. The denitration method used. 4. The denitration method using urea according to claim 1, wherein the temperature of the heated catalyst liquid is 100 ° C. or higher. 5. The method for denitration using urea according to claim 1, wherein the temperature of the heated catalyst liquid is at or near the boiling point of the aqueous solution. 6. The mixed gas of generated ammonia, carbon dioxide and water vapor is conveyed into the exhaust gas by a carrier gas composed of air, nitrogen gas, combustion exhaust gas or the like. A denitration method using urea according to 1. 7. A denitration device for catalytically reducing nitrogen oxides in exhaust gas by a denitration catalyst layer provided in an exhaust gas flue, wherein at least one carbonic acid selected from alkali metal, alkaline earth metal and rare earth element is used. salt, and a basic compound consists of borate or hydroxide or a strong inorganic acid or reaction vessel where the acidic compound Neu either the compounds comprising an ammonium salt of an organic strong acid put catalyst solution comprising an aqueous solution containing at least one or more Heating means capable of heating the catalyst liquid in the reaction tank to at least its boiling point, and urea or urea for continuously supplying urea or an aqueous urea solution to the catalyst liquid in the reaction tank heated by the heating means Equipped with an aqueous solution supply means and an injection means for injecting a mixed gas of ammonia, carbon dioxide and water vapor generated in the reaction tank into the exhaust gas A denitration device using urea characterized in that 8. The urea decomposition reaction region capable of heating the reaction tank to the boiling point of the catalyst liquid in the reaction tank and the preliminary reaction region for keeping the temperature lower than the boiling point of the catalyst liquid, 8. The denitration device using urea according to claim 7, further comprising a connecting means so that the catalyst liquids in the urea decomposition reaction region and the preliminary reaction region can move to each other.