JP2004000867A - Apparatus for denitrating exhaust gas, and urea vaporizer used therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a urea vaporizer which prevents deposition of cyanuric acid or isocyanic acid as pyrolysates of urea and can continuously be operated without inducing chalking, and to provide an apparatus for denitrating exhaust gas by using the vaporizer. <P>SOLUTION: The apparatus has a passage for exhaust gas containing nitrogen oxides, a supply means for a reducing agent for nitrogen oxides disposed in the passage, and a denitrating part for exhaust gas disposed in the slip stream side of the supply means of the reducing agent. The supply means for the reducing agent has a urea water injecting part 1 to inject urea water, a vaporizing part 6 to vaporize the urea water injected from the urea injecting part, a hydrolyzing part 6 to hydrolyze urea disposed in the slip stream side of the vaporizing part 6, and a reducing agent spraying part to spray and supply a gas containing ammonia produced in the above parts into the exhaust gas. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an exhaust gas denitration apparatus and a urea vaporizer used for the same, and particularly to an exhaust gas denitration apparatus using urea as a nitrogen oxide reducing agent in the treatment of an exhaust gas containing nitrogen oxides, for example, exhaust gas from a diesel engine, and an apparatus therefor. The present invention relates to a urea vaporizer to be used for urea.
[0002]
[Prior art]
In recent years, the problem of global warming has been highlighted, and there is an urgent need to reduce the emission of nitrogen oxides generated by consuming a large amount of fossil fuels. In particular, the gas discharged from a diesel engine is an exhaust gas containing PM (particles) and nitrogen oxides. Since the nitrogen oxide has a higher combustion temperature than that of a gasoline engine, it has a high Due to the high oxygen concentration in the gas, it is said that it is difficult to reduce the nitrogen oxide reduction effect using hydrocarbons in the exhaust gas, unlike a three-way catalyst for gasoline engine exhaust gas. As a measure for reducing such nitrogen oxides, a method of directly injecting and supplying urea water to an engine exhaust system has been proposed by Bosch Corporation or the like. Urea is suitable as a denitration agent because it is liquid and easy to handle, and is inexpensive as used in fertilizers, but its stable operation is difficult. Urea is a solid at room temperature, has a property of being easily dissolved in water, and melts at a low temperature when heated, but it is considered difficult to vaporize because a complex organic compound is generated. Spraying in a flue gas in a liquid state is often supposed to enable simple and stable operation, but it is actually said that nozzle clogging is likely to occur.
[0003]
FIG. 7 shows an example of a conventional exhaust gas denitration method by direct injection of urea water (JP-A-2001-317346). After the exhaust gas 14 of the diesel engine is oxidized in the oxidation chamber 8 filled with the oxidation catalyst, the exhaust gas 14 passes through the communication pipe 15 and enters the denitration chamber 9 filled with the denitration catalyst. Urea water 11 supplied from the tank 11 is sprayed. 10 is a pump. Since the atmospheric temperature exceeds 400 ° C., the urea blown into the exhaust gas is vaporized and reduces nitrogen oxides in the denitration chamber 9.
[0004]
However, in this device, the supply from the injection portion of the nozzle 12 to the catalyst portion is not sufficient, and the supply is performed in an environment in which urea decomposition is difficult, such as a change in ambient temperature. It was difficult to decompose urea and supply ammonia.
[0005]
[Problems to be solved by the invention]
In these urea blowing devices, cyanuric acid ((HNCO) ₃ ), isocyanic acid (3HNCO), which is a thermal decomposition product of urea, and burettes of solid matter are transported in the part of the spray nozzle 12 and the rear part thereof in the course of transport. There is a big problem that it adheres to the inside of the pipe and becomes blocked.
[0006]
An object of the present invention is to provide a urea vaporizer that prevents continuous adhesion of cyanuric acid and isocyanic acid, which are thermal decomposition products of urea, without causing blockage, and an exhaust gas denitration apparatus using the same. To provide.
[0007]
[Means for Solving the Problems]
The invention claimed in the present application to achieve the above object is as follows.
(1) A flow path for the nitrogen oxide-containing exhaust gas, a supply means for the nitrogen oxide reducing agent provided in the flow path, and an exhaust gas denitration unit provided on the downstream side of the supply means for the reducing agent. In the exhaust gas denitration apparatus having the above, the supply means for the reducing agent is provided in a urea water jetting section for jetting urea water, an evaporating section for evaporating urea water jetted from the urea water jetting section, and a downstream section thereof. An exhaust gas denitration apparatus comprising: a hydrolysis section for hydrolyzing urea; and a reducing agent injection section for injecting and supplying gas containing ammonia generated in these sections into the exhaust gas.
[0008]
(2) The exhaust gas denitration apparatus according to (1), wherein a heater is provided inside and outside the apparatus at least in the evaporating section of the evaporating section and the hydrolysis section. (3) The exhaust gas denitration apparatus wherein the evaporating section and the hydrolyzing section are formed as separate vessels, and a temperature control device is provided for each of them.
(4) The exhaust gas denitration apparatus according to any one of (1) to (3), wherein the evaporating section is filled with metal or ceramic particles or honeycomb as a filler.
[0009]
(5) The hydrolysis section is filled with particles supporting γ-alumina or potassium carbonate (K ₂ CO ₃ ) as a filler for promoting the hydrolysis of urea. The exhaust gas denitration apparatus according to any one of (4).
(6) A urea vaporizer in which a urea water ejection section, a urea water evaporation section, a urea hydrolysis section, and a decomposition gas outlet section are sequentially formed in a cylindrical container, and a urea water spray nozzle is provided at the urea water ejection section. A urea water evaporator is formed by sequentially providing a space necessary for the spraying and a liquid receiving part for receiving the atomized urea particles at a lower part thereof, and promotes the evaporation of the urea water on the liquid receiver. A urea vaporizer characterized by being filled with a filling material.
[0010]
(7) The urea water jetting portion has a urea water jetting nozzle and a flange for mounting the urea water jetting nozzle on the urea vaporizer main body, and the flange surrounds the urea water jetting nozzle. The urea vaporizer according to (6), wherein a gas supply hole is provided so as to supply gas in a tangential direction along an inner periphery of the flange portion.
(8) The urea vaporizer according to (7), wherein the filler in the liquid receiving part is made of a substance having a higher thermal conductivity than the filler in the hydrolysis part.
[0011]
(9) The urea vaporizer according to (7) or (8), wherein the filling of the liquid receiving portion is metal or ceramic particles or honeycomb.
(10) The hydrolyzing section is filled with particles supporting γ-alumina or potassium carbonate (K ₂ CO ₃ ) as a filler for accelerating the hydrolysis of urea. The urea vaporizer according to any one of (9).
[0012]
In the present invention, when the container of the urea vaporizer is uniformly heated and urea water (urea concentration: 30% to 50%) is blown into the inside, the water and urea evaporate inside the container, and the catalyst particles filled in the inside For example, urea is hydrolyzed as shown in the following formula on the surface of γ-alumina particles, particles supporting potassium carbonate, etc., and ammonia gas is generated.
[0013]
(NH ₂ ) ₂ CO + H ₂ O → 2NH ₃ + CO ₂
The inside of the vaporizer should be maintained at 450 ° C or higher, and should be structured to promote rapid evaporation heating so that solids such as cyanuric acid and isocyanic acid generated in the process of thermal decomposition of urea are not by-produced. preferable. Normally, when urea water is injected into a container kept at a high temperature, the temperature at the surface receiving the liquid drops is reduced, solids are deposited, which leads to blockage, and it is extremely difficult to control the internal temperature. In the present invention, attention is paid to the evaporation portion in the liquid spray, and even if the urea spray flow rate changes significantly, the structure focused on promoting the heat transfer on the evaporation surface so as to suppress the temperature fluctuation inside the container, The upper part (or the upstream side) of the γ-alumina particles or the like filled in the decomposition part was filled with silicon carbide, iron, stainless steel spheres, metal honeycomb, or the like having a higher thermal conductivity than this. Since these fillers have good heat conduction, and have a spherical structure and a honeycomb structure, when urea water droplets collide, they are caught by the uneven portions, so that an increase in the amount of evaporation can be expected. . Further, when heaters are provided inside and outside the container, it contributes to improvement of the heat transfer coefficient from these heaters.
[0014]
Further, a gas supply hole is provided around the urea water spray nozzle, that is, in a flange portion for fixing the nozzle to the vaporizer main body, and a cooling gas (for example, air) is supplied downward along the inner periphery of the flange portion from here. This prevents clogging or the like due to deposits around the nozzle.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
【Example】
FIG. 1 shows one embodiment of the urea vaporizer of the present invention. This device forms a urea water inlet, a urea water evaporator (5), a urea hydrolysis unit (7), and a cracked gas outlet (31) in this order from the top of the cylindrical container 30 downward, A urea water injector 1 is installed at the urea water inlet, and a urea water spray section 6 and a liquid receiving section (screen 4) for receiving atomized urea particles are sequentially provided below the urea water injector 1 as a space required for spraying. The liquid receiving portion on the screen 4 is filled with a filler 5 that promotes the evaporation of urea water. The container 30 is uniformly heated by the inner heater 2 and the outer peripheral heater 3, and is controlled so that the temperature inside the container is as uniform as possible. The urea water is sprayed and supplied toward the inside of the container 30 by the injector 1 located at the uppermost part of the apparatus. The sprayed urea water collides with the evaporator filling 5 inside the apparatus and evaporates instantaneously. Here, the space from the injector 1 to the filling of the evaporating section (urea water spraying section 6) is a space necessary for spraying and atomizing urea water. The urea water once fills this space and collides and adheres to the walls of the container. However, since this space is also maintained at a high temperature by the outer peripheral heater 3, it contributes to the evaporation of urea. The evaporated water and urea reach the hydrolyzed packing 7 at the bottom of the container, where they are decomposed into ammonia and carbon dioxide (CO ₂ ), and the decomposed gas is taken out from the outlet 31 and jetted into the diesel exhaust system. In the figure, reference numeral 34 denotes a screen for supporting and fixing the filler.
[0016]
FIG. 2 is an explanatory view of the urea water spray injector 1 in FIG. 1 and a mounting portion thereof, and FIG. 3 is a plan view cut along the line III-III in FIG.
[0017]
The injector 1 includes an injector body 1A provided with a urea water introduction pipe 31 and a nozzle 20. A pair of flanges are provided around the injector nozzle 20 from which the urea water is jetted at an interval such that air flows. 21 and 22 are provided. The injector body 1 has a step 32, which is supported on the upper flange 21. The upper flange 21 is provided with a gas supply hole 24, and the lower flange 22 is provided with a gas ejection hole 23 communicating with the gas supply hole 24. As shown in FIG. 3, the gas ejection hole 23 opens in a gap between the nozzle 20 and the flange 21 or 22, supplies tangential air around the nozzle 20, and cools the nozzle by an air flow. At the same time, the sprayed gas-liquid is cooled. That is, the cooling air is supplied to the upper flange 21 from the gas supply hole 24 of the upper flange, passes through the flow path in the upper flange, is supplied to the flow path between the upper flange and the lower flange, and then is supplied to the central portion. It is supplied from the gap with the injector 1 toward the evaporator filling 5 below.
[0018]
As the nozzle 20 for spraying urea water, either a one-fluid nozzle for spraying urea water alone or a two-fluid nozzle for mixing and spraying urea water and gas may be used. The flow rate of the injector 1 may be adjusted by an electromagnetic (solenoid) type in addition to a mechanical type. In either method, when the temperature rises (no spraying is performed), the temperature of the nozzle itself becomes high, and the urea water remaining inside the nozzle is thermally decomposed to form a solid and block the nozzle. In the case of the electromagnetic type, since the heat-resistant temperature is as low as 130 ° C. or less, for example, there is a problem of failure. By-products of cyanuric acid and isocyanic acid are prevented, and stable operation of the injector becomes possible. Accordingly, the present invention has an effect that a stable operation can be continued for a long period of time because the solid matter does not adhere and grow inside the vaporizer in addition to the fact that the pipe of the diesel combustion exhaust gas can be prevented from being clogged.
[0019]
FIG. 4 is an explanatory diagram showing an example of an exhaust gas purification system using the exhaust gas denitration apparatus of the present invention. This apparatus includes a pipe 15 provided at the outlet of the oxidation catalyst reactor 8 as a flow path of the nitrogen oxide-containing exhaust gas, and a means for supplying a nitrogen oxide reducing agent provided in the flow path (nozzle 12). And an exhaust gas denitration reactor 9 provided downstream of the reducing agent supply means in the exhaust gas flow path, wherein the reducing agent supply means is a urea vaporizer as shown in FIG. 1 is used. The urea water is pressurized from a tank 11 to about 0.3 MPa by a pump 10, vaporized by an injector 1 and a vaporizer 13, supplied to a diesel exhaust gas from a nozzle 12, and then enters a denitration reactor 9. And reacts on the NO reduction catalyst surface, and NO in the exhaust gas is reduced to N ₂ .
[0020]
As a comparison, when the filler is not used in the vaporizer, the sprayed droplets of urea directly collide with the heater surface or wall and partially evaporate, but the temperature drops due to the low heat capacity of the evaporating part. Drops, forming and accumulating cyanuric acid, isocyanic acid and other solids, and coarse droplets reach the bottom of the container without evaporation. Since this position does not have a heater, urea crystals are easily generated at a low temperature.
[0021]
FIG. 5 shows another embodiment of the present invention. The difference from FIG. 1 is that a metal honeycomb is used as the filler for promoting the evaporation, and the deterioration of the filler 7 (γ-alumina) in the hydrolysis section is reduced. The structure is such that the hydrolysis section is separated from the vaporizer body 13 in order to suppress it. According to this embodiment, deterioration of the filler in the hydrolysis section can be suppressed, and the life can be prolonged.
[0022]
FIG. 6 shows an embodiment in which the vaporizer main body 30 in FIG. 1 is installed at an angle. In this embodiment, since the descent of the spray liquid is reduced by the inclination of the apparatus main body, the screen 4 of the liquid receiving section provided in FIG. 1 is omitted.
[0023]
【The invention's effect】
According to the first to fifth aspects of the present invention, denitration of a nitrogen oxide-containing gas such as diesel engine combustion exhaust gas is stably performed while preventing solid matter from adhering to a pipe or the like using urea water. be able to.
[0024]
According to the invention of claims 6 to 10, in addition to the fact that urea water can prevent cyanuric acid, isocyanic acid and other by-products generated in the thermal decomposition process, it is possible to avoid clogging the pipe of diesel combustion exhaust gas. Since there is no growth of the solid matter inside the vaporizer, stable operation can be continued for a long period of time.
[Brief description of the drawings]
FIG. 1 is an explanatory view showing one embodiment of a urea vaporizer of the present invention.
FIG. 2 is a detailed cross-sectional view around the urea water injector nozzle of FIG. 1;
FIG. 3 is a plan view in the direction of arrows along the line III-III in FIG. 2;
FIG. 4 is a system diagram showing one embodiment of an exhaust gas denitration apparatus using the urea vaporizer of the present invention.
FIG. 5 is an explanatory view showing another embodiment of the urea vaporizer of the present invention.
FIG. 6 is an explanatory view showing another embodiment of the present invention in which the urea vaporizer is inclined.
FIG. 7 is an explanatory view showing an example of a conventional urea direct injection type exhaust gas denitration apparatus.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Injector, 2 ... Internal heater, 3 ... Outer periphery heater, 4 ... Screen, 5 ... Evaporation part filling, 6 ... Urea water spraying part, 7 ... Hydrolysis part filling, 9 ... Denitrification reactor, 12 ... Blowing Nozzle, 13: vaporizer main body, 30: cylindrical container, 31: cracked gas outlet.

Claims (10)

Exhaust gas denitration having a nitrogen oxide-containing exhaust gas flow path, a nitrogen oxide reducing agent supply means provided in the flow path, and an exhaust gas denitration unit provided downstream of the reducing agent supply means An apparatus, wherein the reducing agent supply means includes: a urea water ejection unit for ejecting urea water, an evaporation unit for evaporating the urea water ejected from the urea water ejection unit, and a urea An exhaust gas denitration apparatus, comprising: a hydrolysis section for hydrolyzing the water; and a reducing agent injection section for injecting and supplying gas containing ammonia generated in these sections into the exhaust gas. The exhaust gas denitration apparatus according to claim 1, wherein a heater is provided inside and outside of the apparatus at least in the evaporation section of the evaporation section and the hydrolysis section. An exhaust gas denitration apparatus, wherein the evaporating section and the hydrolysis section are formed as separate vessels, and a temperature control device is provided for each of them. The exhaust gas denitration apparatus according to any one of claims 1 to 3, wherein the evaporating section is filled with metal or ceramic particles or honeycomb as a filler. 5. The hydrolyzing section is filled with particles supporting γ-alumina or potassium carbonate (K ₂ CO ₃ ) as a filler for accelerating the hydrolysis of urea. An exhaust gas denitration apparatus according to any one of the above. A urea vaporizer in which a urea water ejection unit, a urea water evaporation unit, a urea hydrolysis unit, and a decomposition gas outlet unit are sequentially formed in a cylindrical container, and a urea water spray nozzle is installed in the urea water ejection unit. A space portion necessary for the spraying and a liquid receiving portion for receiving finely divided urea particles are sequentially provided at a lower portion thereof to form a urea water evaporating portion, and a filler for promoting evaporation of urea water is provided on the liquid receiving portion. A urea vaporizer characterized by being filled with: The urea water ejection section has a urea water ejection nozzle, and a flange portion for mounting the urea water ejection nozzle to the urea vaporizer main body, the flange portion surrounds the urea water ejection nozzle, and 7. The urea vaporizer according to claim 6, wherein a gas supply hole is provided so as to supply gas tangentially along an inner periphery of the flange portion. 8. The urea vaporizer according to claim 7, wherein the filler in the liquid receiving part is made of a substance having a higher thermal conductivity than the filler in the hydrolysis part. 9. The urea vaporizer according to claim 7, wherein the filler in the liquid receiving portion is metal or ceramic particles or a honeycomb. 10. The method according to claim 6, wherein the hydrolyzing section is filled with particles supporting γ-alumina or potassium carbonate (K ₂ CO ₃ ) as a filler for promoting the hydrolysis of urea. A urea vaporizer as described in Crab.

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