JP2004313917A - Method and apparatus for waste gas denitrification using urea - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and an apparatus for waste gas denitrification capable of forming ammonia necessary for a denitrification reaction by efficiently hydrolyzing urea with reduced energy consumption at a relatively low temperature. <P>SOLUTION: The waste gas denitrification method reduces and decomposes the NOx contained in waste gas and comprises reducing and decomposing the NOx contained in the waste gas by; heating and pressurizing urea water; bringing the urea water into contact with a solid hydrolysis catalyst 7 to hydrolyze a part or the whole of the urea and to form ammonium carbonate; blowing an aqueous solution (tank 11) containing the ammonium carbonate into a waste gas flue to generate NH<SB>3</SB>; then bringing the NH<SB>3</SB>into contact with the waste gas in the presence of an NH<SB>3</SB>catalytic reduction denitrification catalyst. At this time, the catalyst prepared by adding titania or cerium to γ-alumina is used as the solid hydrolysis catalyst 7. Also, intermittent operation is made possible by heating or heat insulating a reactor and the apparatus on the post stream thereof. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method and an apparatus for exhaust gas denitration using urea, and more particularly to a method for safely using nitrogen oxides (NOx) contained in exhaust gas discharged from an internal combustion engine such as an automobile or an external combustion engine such as a power plant as a reducing agent. The present invention relates to an exhaust gas denitration method and apparatus for decomposing and removing using urea hydrolyzed water which is easy to handle.
[0002]
[Prior art]
As a method of removing nitrogen oxides (NOx) in flue gas discharged from power plants and factories as external combustion engines, a flue gas denitration method by selective catalytic reduction using ammonia (NH ₃ ) as a reducing agent is known. Widely used. In recent years, the number of cogeneration systems or vehicles using a diesel engine for a distributed power source, which is an internal combustion engine, has been increasing mainly in urban areas. Due to stricter regulations, installation of exhaust gas denitration equipment is urgently required as in large plants. Since such a small facility or automobile denitration apparatus is used in a densely populated area, the use of liquefied ammonia as used in power plants and factories is dangerous. For this reason, a method using urea, which has low toxicity, is easy to handle, and is inexpensive, has attracted attention.
[0003]
When urea is used as a reducing agent, there is a method in which urea or urea water is directly added to the exhaust gas flue to perform a denitration reaction. However, when denitration is performed at a low temperature of 400 ° C. or less, decomposition into ammonia is performed. There are problems such as a reduction in denitration reaction efficiency due to poor efficiency and adhesion of by-products derived from the reducing agent inside the flue. Therefore, in general, a method is used in which a urea is decomposed to generate ammonia by using a reducing agent decomposition apparatus filled with a reducing agent decomposition catalyst (hydrolysis catalyst) such as alumina, and then used for denitration. In this case, the problem is the decomposition rate of urea that produces ammonia. As a method for obtaining a high decomposition rate, for example, by setting the temperature of the reducing agent decomposition catalyst to a high temperature of 250 ° C. to 500 ° C. Urea is decomposed at a low temperature of 200 ° C. by using a method of improving the decomposition rate of urea (JP-A-5-15739) or using an alkali metal hydroxide or carbonate as a reducing agent decomposition catalyst. A method of decomposing to produce ammonia (JP-A-11-171535) is known.
[0004]
[Patent Document 1] Japanese Patent Application Laid-Open No. 05-015739 [Patent Document 2] Japanese Patent Application Laid-Open No. 11-171535 [0005]
[Problems to be solved by the invention]
However, in the former method, it is necessary to raise the temperature in order to obtain a high ammonia generation rate, which not only raises the running cost, but also makes it easy to generate a hardly decomposable substance and precipitates on a denitration catalyst or a pipe. There is a possibility that. On the other hand, in the conventional method, an alkali metal hydroxide or carbonate is used as a reducing agent decomposition catalyst, but since these catalysts are water-soluble, they are dissolved in urea water or ammonia water during heating or cooling. There is a problem that the catalyst performance is deteriorated in a short time, or the catalyst itself is disintegrated or clogged.
[0006]
On the other hand, a medium-sized diesel engine may be operated when power generation is necessary and stopped when power generation is unnecessary. When an intermittent operation test was performed using a pressurized hydrolysis device, it was sometimes impossible to start the pressurized hydrolysis device because the pipes, filters, pressure regulating valves, and the like were blocked by deposits at startup.
[0007]
An object of the present invention is to provide a method and an apparatus for exhaust gas denitration that can efficiently hydrolyze urea and obtain high ammonia at a relatively low temperature with low energy consumption, and a pressurized hydrolysis apparatus used in the denitration apparatus. It is an object of the present invention to provide a method and an apparatus which can be used stably.
[0008]
[Means for Solving the Problems]
In order to achieve the above object, the present inventors disclosed in Japanese Patent Application No. 2002-377564, which was a prior application, by heating and pressurizing an aqueous solution of urea, bringing the aqueous solution into contact with a solid hydrolysis catalyst, and hydrolyzing the urea. An ammonium carbonate is generated, and an aqueous solution containing the ammonium carbonate is blown into the exhaust gas to come into contact with the exhaust gas in the presence of an NH ₃ catalytic reduction denitration catalyst to reduce and decompose nitrogen oxides contained in the exhaust gas. An exhaust gas denitration method was proposed.
[0009]
By the way, in order to spray 10 to 20% ammonia water required for exhaust gas denitration, a concentration of urea water of 20 to 40% by weight is necessary. However, when the concentration of urea water increases, a commercially available hydrolysis catalyst ( For example, in the presence of (γ-alumina), the amount of ammonia produced tended to slightly decrease. Therefore, the present inventors have conducted extensive research on a catalyst for the hydrolysis of urea under pressure, and found that a multi-component catalyst based on γ-alumina, particularly, titania based on γ-alumina and having a high specific surface area. A catalyst with a large increase in pore volume by addition, and a catalyst based on γ-alumina, to which cerium is added to greatly increase active sites such as hydrolysis, can obtain a high ammonia production rate. They have found that they can do this and have reached the present invention.
[0010]
That is, the invention claimed in the present application to solve the above-mentioned problem is as follows.
(1) An aqueous solution of urea is brought into contact with a solid hydrolysis catalyst in a liquid phase under heat and pressure to hydrolyze a part or all of the urea to form ammonium carbonate, and the aqueous solution containing the ammonium carbonate is blown into the exhaust gas containing nitrogen oxides, NH ₃ catalytic reduction denitration reducing nitrogen oxides contained in the exhaust gas in the presence of a catalyst, is the exhaust gas denitration method for decomposing, the solid hydrolysis catalyst, .gamma. An exhaust gas denitration method characterized by using a catalyst obtained by adding titania to alumina.
[0011]
(2) The exhaust gas denitration method according to (1), wherein the titania has a specific surface area of 100 m ² / g or more, and the added amount is 2 to 40% by weight.
(3) An aqueous solution of urea is brought into contact with a solid hydrolysis catalyst in a liquid phase under heat and pressure to hydrolyze a part or all of the urea to produce ammonium carbonate, blown into the exhaust gas containing nitrogen oxides, NH ₃ catalytic reduction denitration reducing nitrogen oxides contained in the exhaust gas in the presence of a catalyst, is the exhaust gas denitration method for decomposing, the solid hydrolysis catalyst, .gamma. An exhaust gas denitration method characterized by using a catalyst obtained by adding cerium to alumina.
[0012]
(4) The exhaust gas denitration method according to (3), wherein the amount of cerium added is 2 to 40% by weight.
(5) The method according to any one of (1) to (4), wherein the aqueous solution containing ammonium carbonate is heated to form a mixed gas containing ammonia gas, carbon dioxide gas, and water vapor, and then blown into the exhaust gas channel. The exhaust gas denitration method described in the above.
[0013]
(6) The method according to any one of (1) to (5), wherein the heating condition is 150 to 400 ° C. and the pressurizing condition is 0.5 to 10 MPa.
(7) The method according to any one of (1) to (5), wherein the heating condition is 200 to 400 ° C. and the pressurizing condition is 0.5 to 10 MPa.
[0014]
(8) An exhaust gas denitration device that reduces and decomposes nitrogen oxides in exhaust gas using urea, and heats an NH ₃ catalytic reduction denitration catalyst layer provided in an exhaust gas channel and an aqueous urea solution as a reducing agent. A means for contacting with a solid hydrolysis catalyst in a pressurized atmosphere to hydrolyze part or all of the urea to form ammonium carbonate, and directly or vaporizing the obtained aqueous solution containing ammonium carbonate, Means for injecting the gas flow passage into the upstream side of the NH ₃ catalytic reduction denitration catalyst layer, wherein the solid hydrolysis catalyst is a catalyst obtained by adding titania or cerium to γ-alumina. Exhaust gas denitration equipment.
[0015]
(9) A device for producing an ammonia and carbon dioxide gas by contacting an aqueous solution of urea with a solid hydrolysis catalyst in a liquid phase under heat and pressure in a reactor to hydrolyze a part or all of the urea. The reactor according to (8), wherein the reactor of the device, the flow pipe located in the downstream portion thereof, and the auxiliary equipment which comes into contact with the reaction product have a heating means or a heat retaining means. Disassembly device.
[0016]
(10) An operation method characterized in that, upon starting up the apparatus of (9), the heating or heat retaining means according to (9) is operated before the reaction product of the reactor comes into contact with piping or auxiliary equipment.
[0017]
(11) In the operation of the apparatus of (9), when the pressure gauge becomes equal to or higher than a predetermined value, the temperature of the heating of the reactor, the flow path piping located in the downstream portion thereof, and the auxiliary equipment in contact with the reaction product is increased. A driving method characterized in that:
[0018]
In the present invention, as a catalyst containing γ-alumina as a base and another component added, titania having a high specific surface area of 100 m ² / g or more is 2 to 40% by weight (particularly 5 to 30% by weight) based on the total weight. %) Added gamma-alumina is preferred. When titania having a high specific surface area is added to γ-alumina to form a granular catalyst, the pore volume increases 1.5 to 2 times as compared with a case where γ-alumina alone is granulated. It is considered that the contact area of γ-alumina as a solid acid point increases due to the passage of urea water under pressure and heating into the increased pores, thereby promoting the urea hydrolysis reaction. . Here, when titania having a specific surface area of less than 100 m ² is added, the addition amount of titania must be increased to, for example, 70% by weight, and the solid acid point of γ-alumina becomes insufficient even if the pore volume increases. Therefore, the ammonia generation rate relatively decreases. Also, if the addition amount of titania having a high specific surface area of 100 m ² or more is too small as less than 2% by weight, high ammonia production cannot be achieved because the pore volume increases only by about 10%, for example. If titania having a high specific surface area of 100 m ² / g or more is added in excess of 40% by weight, the pore volume increases, but the solid acid sites of γ-alumina are insufficient, so that high ammonia production cannot be achieved.
[0019]
As a catalyst which greatly increases the active site of the hydrolysis reaction by adding another component based on γ-alumina, cerium is added to γ-alumina in an amount of 2 to 40% by weight (particularly 5 to (30% by weight) are preferred. It is considered that the hydrolysis of urea is accelerated by increasing the number of active components (solid acid sites) while maintaining the high specific surface area of γ-alumina. Active substances include alkali or alkaline earth metals, but many substances dissolve in hot alkaline water when a hydrolysis reaction occurs. A catalyst obtained by adding less than 2% by weight of cerium to γ-alumina cannot achieve high ammonia production because the increase in the active component is small. The catalyst in which cerium is added to γ-alumina in an amount exceeding 40% by weight increases the active component, but cannot achieve high ammonia production because cerium decreases the specific surface area and pore volume.
[0020]
In order to spray ammonia water of 10 to 20%, the concentration of urea water is appropriately 20 to 40% by weight as described above. However, when an alumina / titania catalyst is used, for example, the temperature is 175 ° C. and the pressure is 175 ° C. Under the conditions of 2 MPa and a urea water flow rate of 10 mL / min and a catalyst loading amount of 280 mL, a high ammonia production rate of about 90% was obtained.
[0021]
In actual diesel power generation, when power generation is unnecessary, the diesel engine and the pressurized hydrolysis device are stopped. Therefore, when a repeated test of start / stop of the pressurized hydrolysis apparatus was performed, there was no major problem when the apparatus was stopped. However, when the apparatus was temporarily stopped and the temperature in the apparatus reached room temperature, and then restarted, the apparatus was restarted. Clogging of the piping from the reactor to the downstream side, the filter, the pressure regulating valve, and the recovery vessel in some cases. If the piping to the piping, the filter, the pressure regulating valve, and the collection container is closed, it is necessary to disassemble the piping once and then remove it by heating, which is a major obstacle. After the clogged portion was decomposed, it was collected and subjected to thermal analysis. As a result, it was found that the precipitate (clogged material) was ammonium carbonate. After urea water is hydrolyzed under pressure, it becomes ammonia, carbon dioxide, and water, but some of it is present as ammonium carbonate. At the same hydrolysis reaction rate, the higher the concentration of urea water, the more ammonium carbonate is generated.
[0022]
Therefore, the conditions under which the crystals of ammonium carbonate were not precipitated (clogged) in the pressure hydrolysis apparatus were examined, and a part of the apparatus was kept at a predetermined temperature (58 to 130 ° C.) or the pressure hydrolysis apparatus was started. After heating a part to a predetermined temperature (58 to 130 ° C.) beforehand, it has been found that the intermittent operation can be performed without blocking by starting the pump.
[0023]
Ammonium carbonate has a solubility of 40% by weight in water at 65 ° C, but a solubility of 20% by weight at 15 ° C. When the pressurized hydrolysis apparatus is operated under the conditions of, for example, a temperature of 150 ° C. or more and a flow rate of urea water of 5 mL / min or more, when stopped, each part on the downstream side from the reactor is at about 80 ° C. or more. Therefore, the solubility of ammonium carbonate is high and it is difficult to precipitate. However, once the pressure hydrolysis device has reached room temperature, ammonium carbonate precipitates. If the pump is started as it is, the precipitated ammonium carbonate accumulates / blocks in the narrow pipe, the filter, the pressure regulating valve, and the pipe to the recovery vessel.
[0024]
Ammonium carbonate, when present as a solid, decomposes at 58 ° C into ammonia, carbon dioxide and water, but the pressurized hydrolysis reaction proceeds mainly in the reactor filled with the catalyst, so that it flows downstream from the reactor. Keep all sides warm. Alternatively, it was found that the intermittent operation can be performed by heating before starting without closing the inside of the apparatus by crystallization.
[0025]
If the heat retention temperature exceeds 130 ° C., urea water becomes vapor in the pressure hydrolysis apparatus, and urea crystallizes or becomes hardly decomposable precipitates such as cyanuric acid and precipitates.
[0026]
Hereinafter, the present invention will be specifically described with reference to examples.
[0027]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is an explanatory diagram showing an example of a urea pressure hydrolysis apparatus used in the present invention. This apparatus comprises a tank 1 for storing urea water, a reactor 5 connected to the tank 1 via a high-pressure pump 2 and a preheater 4, a hydrolysis catalyst layer 7 provided in the reactor 5, It is mainly composed of an electric heater 6 as a heating means, a filter 9 and a recovery container 11 sequentially connected to the downstream of the reactor 5. 3 is an electric heater, 8 is a pressure gauge, and 10 is a pressure control valve. The urea water in the tank 1 is preheated by the electric heater 3 by the preheater 4 by the pump 2 and then introduced into the reactor 5. The reactor 5 in the urea water heated to a predetermined temperature by an electric heater 6 of the flows into the hydrolysis catalyst layer 7, where some or all of the urea is hydrolyzed NH ₃ occurs. The generated liquid containing NH ₃ is collected in the collection container 11 via the filter 9 and the pressure control valve 10.
[0028]
The composition of the recovered aqueous solution is ammonium carbonate water as shown in the following formula.
(NH ₂ ) ₂ CO + 3H ₂ O → (NH ₄ ) ₂ CO ₃ · H ₂ O
The obtained aqueous ammonium carbonate is decomposed at 70 ° C. and easily NH ₃ Generate
[0029]
FIG. 2 is an explanatory diagram in which an exhaust gas denitration apparatus according to one embodiment of the present invention is applied to a mobile internal combustion engine. In FIG. 2, the denitration apparatus includes an NH ₃ catalytic reduction denitration catalyst layer 16 provided in an exhaust pipe 14 of a diesel engine 15 which is an internal combustion engine, and urea water 2 as a reducing agent at 200 to 400 ° C. and 0.5 ° C. A tank 1, a pump 2 and a reactor as a means for bringing a solid hydrolysis catalyst into contact with a liquid phase in a liquid phase in an atmosphere of 10 to 10 MPa (subcritical conditions) to hydrolyze a part or all of the urea to produce ammonium carbonate. 5. The hydrolysis catalyst layer 7, the electric heater 6, and the back pressure valve 12 provided in the reactor 5, and the obtained aqueous solution of ammonium carbonate is fed upstream of the NH ₃ catalytic reduction denitration catalyst layer 16 in the exhaust pipe 14. And a reactor outlet pipe 20 as a means for blowing into the side. 17 is an exhaust.
[0030]
In such a configuration, the urea water stored in the tank 1 is withdrawn by the pump 3 and press-fitted into the reactor 5, and is adjusted to, for example, a temperature of 200 ° C. and a pressure of 3 MPa by the electric heater 6 and the back pressure valve 12. It flows into the decomposition catalyst layer 7 and urea is hydrolyzed to produce ammonium carbonate. The aqueous solution containing ammonium carbonate is sprayed in a liquid state to the exhaust pipe 14 via the reactor outlet pipe 20 and the back pressure valve 12, mixed with the exhaust gas, and then flows into the denitration catalyst layer 16, where NOx in the exhaust gas is converted to ammonium carbonate. Is reduced and decomposed by decomposed NH ₃ .
[0031]
The present invention can be applied to an exhaust gas treatment device for a stationary internal combustion engine used in a factory or the like.However, compared to a case where the present invention is applied to a mobile internal combustion engine, the amount of exhaust gas increases, It is necessary to increase the supply.
[0032]
Hereinafter, specific examples of the present invention, although described hydrolysis method and apparatus of the urea, the exhaust gas denitration apparatus, as shown in FIG. 2, NH ₃ Hydrolysis apparatus urea It can be easily implemented by using it as an ammonia generation source of a catalytic reduction denitration apparatus.
[0033]
【Example】
Example 1
After mixing 120 g of boehmite (aluminum hydroxide), 30 g of titanium oxide having a specific surface area of 250 m ² and 180 g of water to form a slurry, the water was evaporated on a sand bath to solidify. After firing this lump at 500 ° C., it was pulverized to a size of 1 to 2 mm to obtain a pressure hydrolysis catalyst. After filling 150 mL of the catalyst into a stainless steel reactor having an outer diameter of 44 mmΦ, an inner diameter of 30 mmΦ, and a length of 300 mm, 30% by weight urea water was introduced into the reactor at a flow rate of 20 mL / min using a pump, and the reactor was downstream of the reactor. The pressure was maintained at 3 MPa by a pressure regulating valve installed in the section, and the periphery of the reactor was heated to 220 ° C. in an electric furnace. The recovered ammonia carbonated water was analyzed by ion chromatography, and as a result, the ammonia generation rate was 100%.
[0034]
Example 2
After mixing 160 g of boehmite (aluminum hydroxide), 100 g of cerium nitrate and 240 g of water to form a slurry, the water was evaporated on a sand bath and solidified. After firing this lump at 500 ° C., it was pulverized to a size of 1 to 2 mm to obtain a pressure hydrolysis catalyst. After charging 150 mL of the catalyst using the same reactor and apparatus as in Example 1, 30% by weight urea water was introduced into the reactor at a flow rate of 20 mL / min using a pump, and placed in the downstream part of the reactor. The pressure was maintained at 3 MPa with a pressure regulating valve, and the periphery of the reactor was heated to 220 ° C. in an electric furnace. The recovered ammonia carbonated water was analyzed by ion chromatography, and as a result, the ammonia generation rate was 100%.
[0035]
Example 3
160 g of boehmite (aluminum hydroxide), 50 g of cerium nitrate, 15 g of titanium oxide having a specific surface area of 250 m ² and 220 g of water were mixed to form a slurry, and the water was evaporated on a sand bath to solidify. After firing this lump at 500 ° C., it was pulverized to a size of 1 to 2 mm to obtain a pressure hydrolysis catalyst. After charging 150 mL of the catalyst using the same reactor and apparatus as in Example 1, 30% by weight urea water was introduced into the reactor at a flow rate of 20 mL / min using a pump, and placed in the downstream part of the reactor. The pressure was maintained at 3 MPa with a pressure regulating valve, and the periphery of the reactor was heated to 220 ° C. in an electric furnace. The recovered ammonia carbonated water was analyzed by ion chromatography, and as a result, the ammonia generation rate was 100%.
[0036]
Example 4
A pressure hydrolysis test of urea water in the present invention was performed. Using an alumina / titania catalyst, a high ammonia production rate of about 90% was obtained at a temperature of 175 ° C., a pressure of 2 MPa, a concentration of urea water of 35% by weight, a flow rate of urea water of 10 mL / min, and a catalyst loading of 280 mL. The entire downstream side of the reactor 5 of the apparatus in FIG. 1 was kept at 80 ° C. using a ribbon heater (always at 80 ° C.), and after stopping, the intermittent operation until the apparatus reached room temperature was repeated 20 times. As a result, the pressurized hydrolysis device could be operated intermittently without any blockage.
[0037]
Example 5
Using an alumina / titania catalyst, a high ammonia production rate of about 90% was obtained at a temperature of 175 ° C., a pressure of 2 MPa, a concentration of urea water of 35% by weight, a flow rate of urea water of 10 mL / min, and a catalyst loading of 280 mL. A ribbon heater was installed on all components downstream from the reactor 5 of the apparatus of FIG. 1, and after restarting after stopping, after heating to 80 ° C. for 10 minutes, the intermittent operation of starting the pump was repeated 20 times. As a result, the pressurized hydrolysis device could be operated intermittently without any blockage.
[0038]
Example 6
Using an alumina / titania catalyst, a high ammonia production rate of about 90% was obtained at a temperature of 175 ° C., a pressure of 2 MPa, a concentration of urea water of 45% by weight, a flow rate of urea water of 10 mL / min, and a catalyst loading of 280 mL. A ribbon heater was installed on all components downstream from the reactor 5 of the apparatus shown in FIG. 1, and after restarting after stopping, heating was performed at 95 ° C. for 10 minutes, and then an intermittent operation of starting the pump was repeated 20 times. As a result, the pressurized hydrolysis device could be operated intermittently without any blockage.
[0039]
Example 7
Using an alumina / titania catalyst, a high ammonia production rate of about 90% was obtained at a temperature of 175 ° C., a pressure of 2 MPa, a concentration of urea water of 35% by weight, a flow rate of urea water of 10 mL / min, and a catalyst loading of 280 mL. In the apparatus shown in FIG. 1, the intermittent operation was repeated 20 times using an apparatus in which all parts except the urea water tank 1, the recovery liquid tank 11, and the pressure gauge 8 were arranged in a thermostat kept at 80 ° C. As a result, the pressurized hydrolysis device could be operated intermittently without any blockage.
[0040]
Example 8
Using an alumina / titania catalyst, a high ammonia production rate of about 90% was obtained at a temperature of 175 ° C., a pressure of 2 MPa, a concentration of urea water of 35% by weight, a flow rate of urea water of 10 mL / min, and a catalyst loading of 280 mL. A pressure sensor was attached to the apparatus shown in FIG. 1, and when the pressure became 2.5 MPa or more, the heater installed on the downstream part of the reactor 5 was set to be heated to a temperature of 120 ° C. Intermittent operation was repeated 20 times using this device. As a result, the pressurized hydrolysis device could be operated intermittently without any blockage.
[0041]
Comparative Example 1
Commercially available γ-alumina was pulverized to a size of 1 to 2 mm and used as a pressure hydrolysis catalyst. After filling 150 mL of the catalyst using the same equipment as in Example 1, 30% by weight urea water was introduced into the reactor at a flow rate of 20 mL / min using a pump, and a pressure regulating valve installed in the downstream of the reactor was used. , And the periphery of the reactor was heated to 220 ° C in an electric furnace. The collected ammonia carbonate water was analyzed by ion chromatography, and as a result, the ammonia generation rate was 80%.
[0042]
Comparative Example 2
Using an alumina / titania catalyst, a high ammonia production rate of about 90% was obtained at a temperature of 175 ° C., a pressure of 2 MPa, a concentration of urea water of 35% by weight, a flow rate of urea water of 10 mL / min, and a catalyst loading of 280 mL. In the apparatus shown in FIG. 1, the ribbon heater for heat retention / heating used in Example 5 was removed, and the intermittent operation was repeated 20 times. As a result, the pressure hydrolysis apparatus was closed 6 times out of 20 times.
[0043]
Comparative Example 3
Using an alumina / titania catalyst, a high ammonia production rate of about 90% was obtained at a temperature of 175 ° C., a pressure of 2 MPa, a concentration of urea water of 45% by weight, a flow rate of urea water of 10 mL / min, and a catalyst loading of 280 mL. In the apparatus shown in FIG. 1, the ribbon heater for heat retention / heating used in Example 6 was removed, and the intermittent operation was repeated 20 times. As a result, the pressurized hydrolysis device was closed 14 times out of 20 times.
[0044]
【The invention's effect】
According to the invention as set forth in claims 1 to 8, since the amount of the catalyst for the decomposition of the reducing agent is small, the energy is small at a relatively low temperature, and the hydrolysis reaction tank is compact and the temperature control is easy, Urea can be efficiently hydrolyzed, and exhaust gas denitration can be performed at a high ammonia production rate.
[0045]
In addition, according to the inventions described in claims 9 to 11, the pressure hydrolysis apparatus can be intermittently operated in a stable state without being blocked by crystallization of the reaction product.
[Brief description of the drawings]
FIG. 1 is an explanatory view of a pressurized hydrolysis apparatus for carrying out the method of the present invention.
FIG. 2 is an explanatory view showing an embodiment in which the exhaust gas denitration method of the present invention is applied to a mobile internal combustion engine.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Urea water tank, 2 ... Pump, 3 ... Electric heater, 4 ... Preheater, 5 ... Reactor, 6 ... Electric heater, 7 ... Catalyst, 8 ... Pressure gauge, 9 ... Filter, 10 ... Adjustment Pressure valve, 11: Collection liquid tank.

Claims (11)

An aqueous solution of urea is brought into contact with a solid hydrolysis catalyst in a liquid phase under heat and pressure to hydrolyze a part or all of the urea to form ammonium carbonate. Exhaust gas denitration method for reducing and decomposing nitrogen oxides contained in the exhaust gas in the presence of an NH ₃ catalytic reduction denitration catalyst by blowing the solid hydrolysis catalyst into γ-alumina with titania An exhaust gas denitration method characterized in that the catalyst is a catalyst to which is added. Said titania has a higher specific surface area 100 m 2 ^/ g, the exhaust gas denitration process according to claim 1, wherein the amount added is 2 to 40 wt%. An aqueous solution of urea is brought into contact with a solid hydrolysis catalyst in a liquid phase under heat and pressure to hydrolyze a part or all of the urea to produce ammonium carbonate. Exhaust gas is blown into an exhaust gas containing, and the nitrogen oxides contained in the exhaust gas are reduced and decomposed in the presence of an NH ₃ catalytic reduction denitration catalyst, wherein the solid hydrolysis catalyst comprises cerium An exhaust gas denitration method characterized in that the catalyst is a catalyst to which is added. The exhaust gas denitration method according to claim 3, wherein the addition amount of the cerium is 2 to 40% by weight. The exhaust gas denitration method according to any one of claims 1 to 4, wherein the aqueous solution containing ammonium carbonate is heated to be a mixed gas containing ammonia gas, carbon dioxide gas and water vapor, and then blown into the exhaust gas passage. . The method according to any one of claims 1 to 5, wherein the heating condition is 150 to 400 ° C and the pressurizing condition is 0.5 to 10 MPa. The method according to claim 1, wherein the heating condition is 200 to 400 ° C. and the pressurizing condition is 0.5 to 10 MPa. An exhaust gas denitration apparatus for reducing and decomposing nitrogen oxides in exhaust gas using urea, comprising heating an NH ₃ catalytic reduction denitration catalyst layer provided in an exhaust gas channel and an aqueous urea solution as a reducing agent under a pressurized atmosphere. A means for contacting with a solid hydrolysis catalyst at a temperature to produce ammonium carbonate by hydrolyzing a part or all of the urea, and directly or vaporizing the obtained aqueous solution containing ammonium carbonate, Means for blowing into the upstream side of the NH ₃ catalytic reduction denitration catalyst layer of the channel, wherein the solid hydrolysis catalyst is a catalyst obtained by adding titania or cerium to γ-alumina. Exhaust gas denitration equipment. A device for heating an aqueous solution of urea in a reactor and bringing it into contact with a solid hydrolysis catalyst in a liquid phase under pressure to hydrolyze a part or all of the urea to produce ammonia and carbon dioxide gas, 9. The pressurized hydrolysis apparatus used in the apparatus according to claim 8, wherein the reactor of the apparatus, the flow pipe located in the downstream portion thereof, and the auxiliary equipment that comes into contact with the reaction product have heating means or heat retaining means. 10. An operating method, wherein the heating or heat retaining means according to claim 9 is activated before the reaction product of the reactor comes into contact with a pipe or ancillary equipment when the apparatus according to claim 9 is started. In the operation of the apparatus according to claim 9, when the pressure gauge becomes equal to or higher than a predetermined value, the heating of the reactor and the flow path piping located in the downstream thereof, and the auxiliary equipment in contact with the reaction product, is made higher. And driving method.

Images