JP2006223937A - Decomposing catalyst for urea, and exhaust gas denitrification method and apparatus using the catalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst capable of efficiently decomposing ureal water into a gas at low temperature and at normal pressure, and to provide an exhaust gas denitrification method and apparatus using the catalyst by which the possibility of producing precipitates such as cyanuric acid even at low temperature. <P>SOLUTION: This decomposing catalyst for urea is the catalyst (1), which mainly consists of an oxide or oxo acid salt of vanadium (V), which reacts with NH<SB>4</SB>VO<SB>3</SB>or NH<SB>3</SB>to produce NH<SB>4</SB>VO<SB>3</SB>, and decomposes the urea in a urea aqueous solution into ammonia (NH<SB>3</SB>) or isocyanic acid (HCNO), or the catalyst (2), which is obtained by adsorbing/depositing NH<SB>4</SB>VO<SB>3</SB>or V<SB>2</SB>O<SB>5</SB>on a porous carrier containing TiO<SB>2</SB>or Al<SB>2</SB>O<SB>3</SB>. In the urea decomposing method, the urea is decomposed by suspending particles of the catalyst (1) or (2) in ureal water and heating catalyst them to the temperature of >100°C to the boiling point of urea. In the exhaust gas denitrification method, NOx in exhaust gas is reduced and removed by supplying the gas containing steam and the substances obtained by decomposing/gasifying urea by the method for decomposing urea to NOx-containing exhaust gas, and bringing the gas-supplied exhaust gas into contact with a catalytic reduction/denitrification catalyst. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  TECHNICAL FIELD The present invention relates to a catalyst for promoting a hydrolysis reaction of urea, and a method and apparatus using the hydrolysis product as a reducing agent, and in particular, a catalyst for highly efficiently hydrolyzing urea in urea water at a low temperature and the same. The present invention relates to an exhaust gas denitration method and apparatus.

NOx in flue gas discharged from power plants, various factories and automobiles is a causative substance of photochemical smog, and as an effective removal method, selective catalytic reduction using ammonia (NH ₃ ) as a reducing agent is used. The flue gas denitration method is widely used mainly in thermal power plants.

  Recently, cogeneration systems using diesel engines or gas turbines are increasing mainly in the city center of large cities, and NOx emission regulations are also applied to these systems. Due to stricter regulations, installation of exhaust gas denitration equipment is urgently required as in large plants. Since such denitration equipment for small-scale facilities is used in densely populated areas such as buildings, it is difficult to apply liquefied ammonia. Therefore, a method of injecting into the exhaust gas by spraying a safe and easy urea water instead of liquefied ammonia has been put into practical use (Japanese Patent Laid-Open Nos. 53-62772 and 53-). No. 64102, JP-A 53-112273, JP-A 53-115658, etc.). In recent years, many attempts have been made to use harmless urea water for removing nitrogen oxides from diesel engine vehicles.

In a method in which urea water is sprayed directly into exhaust gas and used for catalytic reduction of nitrogen oxides by a denitration catalyst, urea is not sufficiently vaporized and decomposed, and urea, melamine, cyanuric acid, etc. are precipitated as solids, There may be a problem that the injection nozzle is blocked, the gas passage of the denitration catalyst is blocked, or even the exhaust gas pipe is blocked. For this reason, a method of hydrolyzing urea water using an alkali carbonate or the like as a catalyst, converting it into ammonia and carbon dioxide gas and using it in a gaseous state (JP-A-6-165814, etc.), urea water as a catalyst There are inventions such as a method of spraying a solution obtained by hydrolyzing ammonia by pressure heating in the presence (JP 2004-202450 A, etc.).
JP-A-6-165814 JP 2004-202450 A

  All of these conventional techniques are excellent methods, but the method of directly spraying urea water into the exhaust gas has a problem that cyanuric acid and the like are precipitated because urea is insufficiently vaporized at an exhaust gas temperature of 200 ° C. or lower. It is likely to occur. On the other hand, there is no catalyst that can hydrolyze urea at a low temperature in the method of hydrolyzing urea using a catalyst. Therefore, a high-concentration alkali carbonate catalyst is used in the form of a high-concentration viscous solution at high temperatures or under pressure. -There were also points to be improved, such as the need to carry out at high temperatures.

  An object of the present invention is to provide a catalyst capable of efficiently decomposing urea water into gas even at a low temperature and normal pressure, and using the urea decomposition catalyst, an exhaust gas denitration method that does not cause a precipitate such as cyanuric acid even at a low temperature And to provide a device.

The above-mentioned subject is achieved by the following solution means.
The invention described in claim 1 relates to an oxide of vanadium that reacts with ammonium metavanadate or ammonia to produce ammonium metavanadate, such as vanadium pentoxide (V ₂ O ₅ ), or an oxo acid salt of vanadium, such as ammonium metavanadate. It is a urea decomposition catalyst that decomposes urea in an aqueous urea solution mainly composed of (NH ₄ VO ₃ ) into ammonia (NH ₃ ) or isocyanic acid (HCNO).

  The invention according to claim 2 is the urea decomposition catalyst according to claim 1, wherein ammonium metavanadate or vanadium pentoxide is adsorbed and supported on a porous carrier containing titanium oxide or aluminum oxide.

  The invention according to claim 3 decomposes urea by suspending the particles of the urea decomposition catalyst according to claim 1 or 2 in urea water and heating to a temperature between 100 ° C. and the boiling point of urea. This is a method for decomposing urea.

  According to a fourth aspect of the present invention, an ammonia catalytic reduction denitration catalyst is provided after supplying a gas containing urea decomposition vaporized by the urea decomposition method according to the third aspect and water vapor into the exhaust gas containing nitrogen oxides. It is an exhaust gas denitration method in which nitrogen oxides in exhaust gas are reduced and removed by contacting with NO.

  The invention according to claim 5 is a method in which the urea decomposition vaporized product obtained by the urea decomposition method according to claim 3 and water vapor in the gas containing water vapor are once condensed to form a liquid, This is an exhaust gas denitration method in which the liquid is supplied into an exhaust gas containing nitrogen oxides and vaporized, and the resulting vaporized product is brought into contact with an ammonia catalytic reduction denitration catalyst to reduce and remove nitrogen oxides in the exhaust gas.

  According to the sixth aspect of the present invention, urea water is passed through a reactor filled with the molded article of the urea decomposition catalyst according to the first or second aspect in an atmosphere of 100 to 250 ° C. and 1 to 20 atm. This is a method for decomposing urea to obtain a liquid containing decomposition products.

  According to a seventh aspect of the invention, a solution containing the hydrolysis product of urea obtained by the urea decomposition method according to the sixth aspect is vaporized by spraying it into an exhaust gas containing nitrogen oxides, and the vaporized product is ammonia. This is an exhaust gas denitration method for contacting with a catalytic reduction denitration catalyst.

  The invention according to claim 8 is a urea water slurry tank in which the particles of the urea decomposition catalyst according to claim 1 or 2 are suspended in urea water, a heating means for the urea water slurry in the urea water slurry tank, Urea supplying urea water decomposition gas generated by decomposition when urea in the urea water slurry tank is heated to a temperature between 100 ° C. and the boiling point of urea to an exhaust gas passage containing nitrogen oxides An exhaust gas denitration apparatus comprising a water decomposition gas supply means and a denitration catalyst layer in an exhaust gas flow path downstream of the urea water decomposition gas supply means.

  The invention according to claim 9 is a urea water slurry tank in which the particles of the urea decomposition catalyst according to claim 1 or 2 are suspended in urea water, a heating means for the urea water slurry in the urea water slurry tank, The urea water decomposition gas produced by decomposition when the urea in the urea water slurry tank is heated to a temperature between 100 ° C. and the boiling point of urea is cooled to decompose urea vaporized in the gas, Cooling / condensed liquid storage means for once condensing with water vapor and storing the liquid, condensed liquid supply means for supplying the liquid in the cooling / condensed liquid storage means to the exhaust gas flow path containing nitrogen oxide, and the condensed liquid An exhaust gas denitration apparatus including an ammonia catalytic reduction denitration catalyst layer in an exhaust gas flow channel on the downstream side of a supply unit.

  Invention of Claim 10 is filled with the urea water tank, the molded object of the urea decomposition catalyst of Claim 1 or 2, and is 100-250 degreeC and the reactor in the atmosphere of 1-20 atmospheres, The said urea A liquid storage tank for storing a liquid containing urea hydrolysis products obtained by passing urea water from the water tank to the reactor, and an exhaust gas flow path containing nitrogen oxide for the liquid in the liquid storage tank An exhaust gas denitration apparatus comprising: a condensed liquid supply means for supplying to the catalyst; and an ammonia catalytic reduction denitration catalyst layer in an exhaust gas flow path downstream of the condensed liquid supply means.

  The heating temperature of the urea or urea water does not rise above the boiling point of urea (normally around 130 ° C., depending on the urea concentration), but is not preferable because the water evaporation rate becomes too fast if heated too much. The pressure may be normal, but the reaction rate can be increased by increasing the reaction temperature while increasing the boiling point to prevent boiling by increasing the pressure to 20 atmospheres or less. If the pressurizing pressure is too high, the apparatus becomes undesirably large.

(Function)
The inventor of the present invention has continued to search for a catalyst capable of efficiently decomposing urea (CO (NH ₂ ) ₂ ) in urea water at a low temperature, and as a result, has reached the inventions of claims 1 and 2.

The feature of the catalyst of the present invention is that the main component is a vanadium compound that reacts with ammonium metavanadate or NH ₃ to produce ammonium metavanadate, and the ammonium metavanadate acts as follows in the aqueous solution. It is presumed that the reaction similar to the decomposition reaction is promoted.

It is known that the reaction of the following formula (2) in which ammonium metavanadate (NH ₄ VO ₃ ) decomposes to generate NH ₃ proceeds at 100 ° C. or lower. The product vanadic acid (HVO ₃ ) efficiently extracts NH ₃ from urea according to the reaction of the following formula (3) to produce highly volatile isocyanate (HCNO). In general, the thermal decomposition reaction represented by the following formula (4) is promoted. Further, it is considered that a part of the isocyanic acid is hydrolyzed as in the reaction of the following formula (5) to generate NH ₃ and CO ₂ .

NH ₄ VO ₃ → NH ₃ + HVO ₃ (2)
CO (NH ₂ ) ₂ + HVO ₃ → NH ₄ VO ₃ + HCNO (3)
CO (NH ₂ ) ₂ → NH ₃ + HCNO (4)
HCNO + H ₂ O → NH ₃ + CO ₂ (5)
The catalyst used for this reaction does not need to be in the form of ammonia metavanadate, and proceeds even in the state of vanadic acid or vanadium pentoxide adsorbed on a porous carrier such as titania. The catalyst based on this discovery is the invention described in claim 2.

  When these catalysts are suspended in urea water as in the invention described in claim 3 and the urea water is heated, urea decomposes at a low temperature.

The urea decomposition product obtained by heating urea water in the presence of the decomposition catalyst as in the invention according to claim 4 (the urea decomposition product obtained by the above equation (4) which is a general reaction formula) ) Is introduced into the exhaust gas containing nitrogen oxide and brought into contact with the catalytic reduction denitration catalyst, so that both NH ₃ and isocyanic acid act as a reducing agent to reduce nitrogen oxide in the exhaust gas (denitration). ) Is possible.

  When the salt using the urea decomposition catalyst in a dissolved state is used as a catalyst, the viscosity of the solution is increased or the concentration cannot be increased beyond the solubility of the catalyst salt. When used in a suspended state, the reaction rate can be improved by increasing the catalyst / urea water ratio. Furthermore, problems such as solidification of the reaction solution during cooling do not occur.

  Further, according to the method of the invention described in claims 5 to 10, according to the method of once condensing the gas, which is a decomposition product obtained by decomposing urea by the catalyst of the present invention, together with water vapor, and then blowing it into the exhaust gas with a nozzle or the like, The reducing agent can be injected following a sudden change in the concentration of nitrogen oxide and the amount of gas, and the response characteristics of the denitration rate can be improved.

  When the catalyst according to the first and second aspects of the invention is used, it is possible to obtain a denitration reducing gas that is as easy to handle as ammonia gas at a temperature as low as about 100 ° C. For this reason, there is no fear of generation | occurrence | production of deposits, such as a melamine and cyanuric acid which arise when urea is directly sprayed in waste gas and urea is decomposed | disassembled.

According to the invention of claim 3, denitration reducing gas can be easily obtained from urea water.
NH ₃ is a gas at room temperature, and isocyanic acid has a boiling point as low as 23 ° C. and is easily hydrolyzed to NH ₃ and carbon dioxide. For this reason, according to invention of Claims 4-10, a precipitate is not produced even at 200 degrees C or less, the reducing agent for denitration is obtained, and high exhaust gas denitration performance is realizable.

  Further, the catalyst according to the first or second aspect of the invention is used as the catalyst for hydrolytic hydrolysis described in Japanese Patent Application Laid-Open No. 2004-202450 as in the case of the sixth aspect of the invention. It is also possible to perform exhaust gas denitration by using a reducing agent as in the present invention.

Embodiments of the present invention will be described with reference to the drawings.
In the present invention, the catalyst mainly composed of vanadium compound that produces ammonium metavanadate or NH ₃ in the present invention is an ammonium metavanadate powder, a molded product thereof, or a porous carrier such as silica or alumina. Can be used. The method of use includes a method in which fine powder is dispersed in a urea aqueous solution and used in a slurry state, a method in which a container filled with a molded body is filled with urea water, a method in which urea water is heated and sprayed onto the catalyst, etc. Used. The catalyst of the present invention decomposes urea in urea water, but it is not necessary to immerse all of the catalyst in an aqueous solution, and includes a case where part of the catalyst decomposes in contact with urea water. The decomposition temperature is usually the boiling point of urea water, and it is convenient to decompose while boiling because the decomposition product is transported together with water vapor, but it is not necessary to react in the boiling state. Further, a gas such as air may be blown by bubbling or the like in order to carry the decomposition product out of the system. In addition, since the larger the amount of catalyst used relative to urea water, the better results are obtained, the effective catalyst / urea solution ratio is obtained by contacting the catalyst molded body with urea water and sucking the urea water into the catalyst by capillary action. Can be improved.

  As a method of adsorbing and supporting ammonium metavanadate on a titania carrier, a method of kneading titanium oxide and ammonium metavanadate in the presence of water, a method of supporting ammonium metavanadate after dissolving it with a complexing agent, etc. can be adopted. is there.

In a flat bottom flask, 40 g of urea (CO (NH ₂ ) ₂ ) and 60 g of water were dissolved, and 25 g of ammonium metavanadate (NH ₄ VO ₃ ) powder was suspended and heated on a sand bath and boiled. . 10 minutes after boiling, 50 cc of the mixed gas of steam and decomposition products discharged from the mouth of the flask is sampled with a gas sampler, and 10 ml of 0.1 N sulfuric acid is added and shaken well. NH ₃ and isocyanate gas Was absorbed. NH ₃ in the obtained liquid was measured by an ion chromatography method.
When isocyanic acid is dissolved in sulfuric acid, it is hydrolyzed and measured as NH ₃ .

In an evaporating dish, 50 g of titanium oxide (specific surface area 330 m ² / g) and 4 g of ammonium metavanadate, 200 ml were placed and heated on a sand bath with stirring. With the heating, a part of ammonium metavanadate is adsorbed on titanium oxide and turns yellow, and a part loses NH ₃ and turns brown. When stirring is continued in this state, the vanadium compound turned brown is gradually dissolved and adsorbed on titanium oxide to obtain a uniform yellow product. After evaporating and drying as it was, it was calcined at 500 ° C. for 2 hours, and the cooled product was pulverized and used as a catalyst.
In a flat bottom flask, 40 g of urea (CO (NH ₂ ) ₂ ) and 60 g of water were dissolved and 25 g of the previously prepared catalyst powder was suspended, and the NH ₃ contained in the boiling steam was suspended as in Example 1. And the total amount of isocyanic acid was measured.

The addition amount of the ammonium metavanadate powder of Example 1 was changed from 25 g to 75 g, and the total amount of NH ₃ and isocyanic acid was measured in the same manner except that.

Comparative Example 1

The total amount of NH ₃ and isocyanic acid was measured in the same manner except that ammonium metavanadate in Example 1 was not added.

Comparative Example 2

Only the titanium oxide powder used in Example 2 was calcined at 500 ° C. for 2 hours was used as a catalyst, and the total amount of NH ₃ and isocyanic acid was measured in the same manner except that.
The obtained results are summarized in Table 1. As is clear from this table, the mixed gas of high concentration NH ₃ and water vapor containing isocyanic acid gas at about 110 ° C., which is the boiling condition of urea water, by the catalyst based on ammonium metavanadate according to the present invention. Is obtained. Further, comparing Example 1 and Example 3, the concentration of cracked gas increases in proportion to the increase in the amount of catalyst to be suspended, and it is an excellent method that can easily obtain a higher concentration of cracked gas. I understand.

FIG. 1 shows an example in which a gaseous substance obtained by decomposing urea water using the catalyst of the present invention is used as a reducing agent for exhaust gas denitration. The urea water 1 in the tank 2 is sent by a pump 3 to a reactor 4 containing a catalyst solution 5 in which the catalyst of the present invention is suspended in water, and is heated and decomposed by a heater 6 installed in the reactor 4. . The cracked gas and water vapor generated in the reactor 4 are sent to a nozzle 8 provided in the exhaust gas duct 13 together with a carrier gas such as air or nitrogen sent from the gas supply line 7 as necessary. Into the exhaust gas containing nitrogen oxides. The cracked gas and water vapor injected into the exhaust gas are guided to the denitration catalyst layer 9 provided in the exhaust gas duct 13, and the nitrogen oxides are reduced to harmless nitrogen by the denitration catalytic action.
As the denitration catalyst 9 used here, a known catalyst such as a Ti—W—V catalyst or a Ti—Mo—V catalyst mainly composed of titanium oxide is used.

  In the apparatus of Example 4, it is difficult to maintain high denitration performance by following fluctuations in the amount of gas and the concentration of nitrogen oxides because of the time delay until the generation of urea decomposition gas in the reactor 4 changes. . FIG. 2 shows the gas and water vapor generated from the reactor 4 cooled, converted into a solution containing ammonia and isocyanic acid and stored in the buffer tank 11, and then pumped in accordance with changes in the gas amount and nitrogen oxide concentration. 12, the responsiveness to fluctuations in conditions is enhanced by blowing into the exhaust gas from the nozzle 8 provided in the exhaust gas duct 13. As a result, it is possible to deal with exhaust gas purification in which the amount of gas or the concentration of nitrogen oxides in an automobile or the like changes greatly.

NH ₃ is a gas at room temperature, and isocyanic acid has a boiling point as low as 23 ° C. and is easily hydrolyzed to NH ₃ and carbon dioxide. For this reason, even if the condensate is sprayed into the exhaust gas as in the present embodiment, no precipitate is formed, and high denitration performance is obtained even at 200 ° C. or lower.

  As shown in FIG. 3, the urea water 1 in the tank 2 includes a catalyst layer 16 filled with the urea decomposition catalyst of Example 1 or 2 by a high-pressure pump 15 and is sent to a reaction vessel 18 that can be heated by a heater 6 to react. The container 18 is decomposed under the conditions of 100 to 250 ° C. and 0.1 to 2 MPa. The cracked gas and water vapor generated in the reaction vessel 18 are sent from the flow path 17 having the valve 20 to the nozzle 8 provided in the exhaust gas duct 13 by the pump 12, and from the nozzle 8 into the exhaust gas containing nitrogen oxides. Injected. The cracked gas and water vapor injected into the exhaust gas are guided to the denitration catalyst layer 9 provided in the exhaust gas duct 13, and the nitrogen oxides are reduced to harmless nitrogen by the denitration catalytic action.

  INDUSTRIAL APPLICABILITY The present invention has applicability to a urea vaporizer that does not cause precipitation of hardly decomposed products and exhaust gas denitration using the device in fields where maintenance-free operation such as denitration of diesel engine exhaust gas is desired.

It is a block diagram at the time of using the urea decomposition catalyst of this invention for a denitration apparatus. It is a block diagram at the time of using the urea decomposition catalyst of this invention for a denitration apparatus. It is a block diagram at the time of using the urea decomposition catalyst of this invention for a denitration apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Urea water 2 Tank 3 Pump 4 Reactor 5 Catalyst slurry 6 Heater 7 Carrier gas introduction line 8 Decomposition gas blowing nozzle 9 Denitration catalyst 10 Cooler 11 Buffer tank 12 Pump 13 Exhaust gas duct 15 High pressure pump 16 Catalyst layer 17 Flow path 18 Reaction Container 20 Valve

Claims (10)

Urea in urea aqueous solution mainly composed of vanadium oxide or vanadium oxoacid salt which reacts with ammonium metavanadate or ammonia to produce ammonium metavanadate is decomposed into ammonia (NH ₃ ) or isocyanic acid (HCNO) Urea decomposition catalyst.   2. The urea decomposition catalyst according to claim 1, wherein ammonium metavanadate or vanadium pentoxide is adsorbed and supported on a porous carrier containing titanium oxide or aluminum oxide.   3. Urea decomposition by suspending the urea decomposition catalyst particles according to claim 1 or 2 in urea water and heating to a temperature between 100 ° C. and the boiling point of urea. Method.   A gas containing urea decomposition vaporized by the urea decomposition method according to claim 3 and water vapor is supplied to the exhaust gas containing nitrogen oxides, and then brought into contact with an ammonia catalytic reduction denitration catalyst. An exhaust gas denitration method comprising reducing and removing nitrogen oxides.   The urea decomposition vaporized product obtained by the urea decomposition method according to claim 3 and water vapor in the gas containing water and the water vapor are once condensed to form a liquid, and the liquid contains nitrogen oxides. An exhaust gas denitration method characterized by supplying and vaporizing the exhaust gas to an exhaust gas to be contacted, and contacting the obtained vaporized product with an ammonia catalytic reduction denitration catalyst to reduce and remove nitrogen oxides in the exhaust gas.   A liquid containing a hydrolysis product of urea is obtained by passing urea water through a reactor filled with a molded article of the urea decomposition catalyst according to claim 1 or 2 in an atmosphere of 100 to 250 ° C. and 1 to 20 atm. The urea decomposition method characterized by the above-mentioned.   A solution containing the hydrolysis product of urea obtained by the urea decomposition method according to claim 6 is sprayed and vaporized in an exhaust gas containing nitrogen oxides, and the vaporized product is brought into contact with an ammonia catalytic reduction denitration catalyst. An exhaust gas denitration method characterized by A urea water slurry tank in which particles of the urea decomposition catalyst according to claim 1 or 2 are suspended in urea water;
Means for heating the urea water slurry in the urea water slurry tank;
The urea water decomposition gas generated by decomposition when urea in the urea water slurry tank is heated to a temperature between 100 ° C. and the boiling point of urea is supplied to the exhaust gas passage containing nitrogen oxides. Urea water cracking gas supply means;
An exhaust gas denitration apparatus comprising a denitration catalyst layer in an exhaust gas flow path downstream of the urea water decomposition gas supply means. A urea water slurry tank in which particles of the urea decomposition catalyst according to claim 1 or 2 are suspended in urea water;
Means for heating the urea water slurry in the urea water slurry tank;
The urea water decomposition gas generated by decomposition when the urea in the urea water slurry tank is heated to a temperature between 100 ° C. and the boiling point of urea is cooled to decompose and vaporize urea in the gas Cooling and condensing liquid storage means for once condensing with water vapor and storing as liquid,
Condensed liquid supply means for supplying the liquid in the cooling and condensed liquid storage means to the exhaust gas flow path containing nitrogen oxides;
An exhaust gas denitration apparatus comprising an ammonia catalytic reduction denitration catalyst layer in an exhaust gas flow path downstream of the condensed liquid supply means. A urea water tank;
A reactor filled with a molded article of the urea decomposition catalyst according to claim 1 or 2 and in an atmosphere of 100 to 250 ° C. and 1 to 20 atm;
A liquid storage tank for storing a liquid containing a hydrolysis product of urea obtained by passing urea water from the urea water tank to the reactor;
Condensed liquid supply means for supplying the liquid in the liquid storage tank to the exhaust gas flow path containing nitrogen oxides;
An exhaust gas denitration apparatus comprising an ammonia catalytic reduction denitration catalyst layer in an exhaust gas flow path downstream of the condensed liquid supply means.

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