JP5299647B2 - Exhaust gas treatment equipment - Google Patents

Description

  The present invention relates to a processing apparatus for detoxifying NOx by irradiating an exhaust gas containing nitrogen oxides NOx with an electron beam.

Conventionally, many methods of denitration of exhaust gas containing nitrogen oxide NOx (NO, NO ₂ ) mainly containing NO such as coal-fired power generation and boilers have been developed. Different. For high-temperature exhaust gas at 800 to 1000 ° C., a non-catalytic denitration method in which ammonia is sprayed into the exhaust gas and decomposed into nitrogen and water is used. A selective catalytic reduction method (SCR) in which ammonia is sprayed at an exhaust gas temperature of 300 to 400 ° C. and then decomposed into nitrogen and water with a titanium vanadium catalyst is well known. Furthermore, in a low temperature exhaust gas of 50 to 100 ° C., a method of separating and recovering NOx as ammonium nitrate (NH ₄ NO ₃ ) by irradiating an electron beam after injecting ammonia is known (Patent Document 1).

As a method of using other than ammonia as a reducing agent at a low exhaust gas temperature of 100 ° C. or lower, low temperature (or non-equilibrium) plasma such as pulse corona discharge or dielectric barrier discharge is generated in exhaust gas to oxidize NO to NO ₂ . Thereafter, a method of absorbing and reducing in a scrubber using an aqueous solution containing a sulfite such as sodium sulfite or sodium sulfide is known (Patent Document 2). A method using sodium sulfite produced by a desulfurization method using sodium hydroxide has also been proposed (Patent Document 3). Alternatively, a method has been proposed for a long time, in which the atmosphere is taken in, ozone is generated by low-temperature plasma discharge, the ozone is blown into the exhaust gas, and NO is oxidized to NO ₂ (for example, Patent Document 4).

JP-A-8-108037 JP 2000-51653 A Japanese Patent Application Laid-Open No. 2000-117049 JP 51-81779 A

In recent years, NOx and SOx emission regulations have been strengthened for ship diesel engines, and the need for external denitration and desulfurization equipment is necessary for future significant emission reductions compared to the small reductions that can be handled by conventional engines. It is pressed for. Particularly in large marine diesel engines that navigate the open ocean, it is difficult to apply the conventional ammonia SCR method due to low exhaust gas temperature around 250 ° C, high NOx and high SOx concentration, ammonia replenishment, storage risk, etc. is there. In the electron beam irradiation method described in Patent Document 1, there is no problem of exhaust gas temperature, but there is a problem of ammonia in addition to the scale of equipment and cost, and ammonium nitrate and ammonium sulfate that can be reused as fertilizer are explosive raw materials, and there are problems with safety. There is. In the low-temperature plasma method described in Patent Documents 2 and 3, a large apparatus is difficult, and if NOx concentration is about 100 to 300 ppm, NO can be oxidized to NO ₂ by 80% or more. _Since the oxidation rate to ₂ was saturated at about 50% and reached an equilibrium state, there was a problem that the usual required denitration rate of 80% could not be obtained. Although ozone having strong oxidizing power described in Patent Document 4 can be processed even at high concentration NOx, there is a problem that the power efficiency of generating ozone itself is as low as 50 g / kWh and the power efficiency for processing is poor. .

As a result of intensive studies to develop an efficient treatment device for NOx, the present inventor made exhaust gas in a reducing atmosphere containing water mixed with a sulfite-based reducing agent such as sodium sulfite or magnesium sulfite. by irradiating an electron beam directly to the exhaust gas, the NO ₂ via oxidation to decomposition or once NO ₂ of NO by using various radical decomposition proceeds simultaneously to N _2, NOx and the very efficiently N ₂ It was found that it can be decomposed and the denitration rate can exceed 80%.

Accordingly, the present invention provides an apparatus for use in such a method, which is an apparatus for treating an exhaust gas containing nitrogen oxides, in which an electron emission portion comprising a field emission element is formed. An electron beam irradiation device comprising an anode, an electron beam extraction window, and a vacuum chamber for generating an electron beam, and a flow path for the exhaust gas to which the electron beam irradiation device is connected via the electron beam extraction window And a denitration apparatus having a device for supplying sulfite to the exhaust gas flow path. The present invention relates to an exhaust gas treatment apparatus containing nitrogen oxides.

In the present invention, a low-energy uniform electron beam is generated by an electron beam irradiation apparatus, and nitrogen molecules and oxygen molecules are dissociated by this electron beam to generate N radicals and O radicals with high efficiency. NO is directly decomposed into N ₂ and O ₂ by the N radical, and NO is oxidized to NO ₂ by the O radical, and then reduced and decomposed to N ₂ by a sulfite-based reducing agent. On the other hand, the sulfite is considered to promote the direct decomposition reaction of NO by locally reducing the O radical and oxygen concentration. Any sulfite such as sodium thiosulfate may be used. Sodium bisulfite, sodium sulfide and the like can also be used.

  Unlike the conventional device, the device of the present invention does not require high-risk ammonia, and the product is not a conventional high-risk ammonium nitrate, but is a sulfate, so it is highly safe and has an impact on the environment. Because it is few, it can be disposed of in rivers and oceans. By continuously irradiating the exhaust gas with a uniform and high-density electron beam of low energy, generation of unnecessary radicals due to a thermal reaction or the like is suppressed, so that NOx in the exhaust gas can be efficiently decomposed. In addition, since the energy is lower than in the prior art, the radiation shielding of the apparatus is simple and the equipment can be made small.

It is a figure which shows schematic structure of the waste gas processing apparatus which is one Example of this invention. It is a figure which shows another example of the waste gas processing apparatus of this invention. It is a block diagram which shows an example of the exhaust gas processing system incorporating this exhaust gas processing apparatus. It is a figure which shows another example of the waste gas processing apparatus of this invention.

  The apparatus of the present invention comprises a cathode, an anode, a chamber, an electron beam extraction window attached to the chamber, and an apparatus for supplying a reducing agent to the exhaust gas flow path outside the electron beam extraction window.

  The cathode has an electron emission portion formed on a conductive base.

The base is conductive. This includes Ni alloys such as stainless steel and Fe-Ni alloys, metals such as Al, Cu, W, Ti, Co, Cr, Mo, Nb, Mn, Si, and alloys thereof, as well as glass and ceramics. There are those in which a metal or a conductive semiconductor is deposited on the surface by vapor deposition or the like. Examples of semiconductors include n-type oxide semiconductors such as ITO (tin-doped indium oxide), ZnO, SnO ₂ , and TiO ₂ with good conductivity. The shape and size of the base are determined according to the shape of the exhaust gas treatment device, etc., but the basic shape is usually a plate shape such as a circle, a quadrangle, or a rectangle.

  The surface on which the carbon nanotube aggregate is attached can be subjected to a pre-treatment such as mirror finishing or degreasing treatment, oxide film removal such as heat treatment ion bombardment.

  The electron emission part must have long life characteristics under high voltage and high current output conditions, and it can produce a higher dose at a lower voltage than conventional ones. Field emission cold cathodes such as nanotubes (CNT) and spint are effective. The electron emission portion made of carbon nanotubes is particularly preferably one obtained by forming a CNT film on a metal substrate and performing a raising treatment. As the CNT, a multilayer CNT manufactured by an arc discharge method having the highest allowable current density and excellent durability is most suitable. Known methods such as spray deposition, screen printing, and electrophoresis can be used as a method for forming a film on a metal substrate. However, chemical substances such as CNT dispersants and thickeners, and impurities such as resins. It is preferable for durability of the electron emission ability to have a high bonding property between the CNT and the substrate. There are a method of forming a film of a mixture of CNT and metal fine particles and imparting the bonding property by heat treatment, a method of increasing the bonding property by electron beam irradiation in vacuum, and the like.

  As a raising treatment method for causing the CNT film to exhibit electron emission, an ultrashort pulse high energy density laser irradiation method, a peeling raising method using an adhesive tape, or the like can be used. When tape-like CNTs are used, a method may be used in which the CNT tape is pressure-bonded to a substrate on which a soft metal is vapor-deposited and then peeled off, and fixing and raising are performed simultaneously.

  The planar shape of the electron emission portion is not particularly limited, but is, for example, a circle, a square, or a rectangle, and the size is about 1 to 100 mm in diameter (in the case of not being a circle, the area is converted to a circle).

  The anode is provided in a vacuum chamber that generates an electron beam. Usually, an electron beam extraction window attached to the vacuum chamber in order to send the electron beam to the exhaust gas flow path may be used as the anode.

  The electron beam extraction window needs to be light and strong to maintain the vacuum of the vacuum chamber and facilitate the transmission of the electron beam, and is low in the order of several microns to tens of microns of titanium, aluminum, silicon, beryllium, carbon, etc. A thin film of a high density material is used. A nickel alloy, stainless steel, or the like is used for the outer frame or vacuum chamber, and a ceramic or glass-based material is used for electrical insulation between the electron gun portion and the anode.

The vacuum chamber is preferably as high as possible in order to generate and accelerate an electron beam. The degree of vacuum is preferably 10 ⁻³ Pa or less, and more preferably 10 ⁻⁵ Pa level. This is because, when a gas containing oxygen is present, oxygen and CNT react with each other to be consumed and deteriorated, so that the lifetime of the electron emission performance is shortened. As a method for maintaining the inside in a vacuum state, there are a method of providing a vacuum pump or the like and a vacuum sealing tube using a vacuum tube technique.

  The shape of the flow path of the exhaust gas for mounting such an electron beam irradiation device is not limited, but is limited by the transmission distance (range) of the electron beam in the gas determined by the acceleration energy of electrons. The diameter of the electron beam from the circumference is limited to about 100 cm. In the case of a channel having a larger cross-sectional area, a method such as dividing the shape into a rectangular shape or a plurality of shapes is required.

  The reducing agent supply device attached to the exhaust gas passage is provided so as to supply the reducing agent to the electron beam irradiation unit. Although the kind of reducing agent is not ask | required, a sulfur type thing is preferable and can mention a sulfite, a thiosulfate, sulfide etc. as an example. Examples of the sulfite include sodium sulfite, sodium hydrogen sulfite, and magnesium sulfite. Examples of the thiosulfate include sodium thiosulfate. Examples of the sulfide include sodium sulfide. When the exhaust gas contains sulfur oxides or the like, it can be desulfurized to produce these compounds and used as a reducing agent.

  The apparatus for supplying such a reducing agent may be anything as long as it uniformly disperses the reducing agent in the exhaust gas. For example, a method of spraying it into the exhaust gas or spraying it into the exhaust gas may be used. . Alternatively, a method of disposing in an electron beam irradiation region in a slurry or solid state and evaporating or dispersing with an electron beam transmitted through exhaust gas may be used.

  In the treatment of exhaust gas using the apparatus of the present invention, the voltage for accelerating electrons is preferably 30 kV or more, preferably 50 kV or more, 300 kV or less, preferably 150 kV or less. This is defined as a practical minimum voltage at which electrons can be extracted into the atmosphere through a thin film extraction window, and an upper limit voltage from equipment costs such as X-ray shielding and power supply. The lower the acceleration voltage, the shorter the distance (electron range or stroke) of the electron beam that passes through the exhaust gas, and the higher the irradiation density, the better. However, since there are restrictions on the exhaust gas flow rate and pipe diameter in practical use, it is necessary to obtain optimum acceleration conditions from the necessary range, loss of extraction window, strength, and the like.

The current density of the electron emission portion, 0.1 mA / cm ² or more, preferably 0.3 mA / cm ² or more, 66 mA / cm ² or less, preferably to 33 mA / cm ² or less. This is because the current density is limited by the heat loss at the extraction window in the above voltage range, and the upper limit value of the current density of the electron emission source is set. In addition, the emission current per field emission device (one electron emission in the CNT) is 1 nA to 100 nA (if the multi-wall CNT having a diameter of 10 nm is 1.27 kA / cm ^{2 to} 1.27 MA / cm ² ), It is preferable that the number of effective releases is in the range of 1 × 10 ³ to 1 × 10 ⁸ per 1 cm ² .

  When the exhaust gas to be treated according to the present invention contains sulfur oxide, a desulfurization apparatus can be provided. The desulfurization equipment can be applied to any process such as lime / gypsum method or activated carbon absorption method, whether wet, semi-dry, or dry, but the denitration treatment according to the present invention is possible at a low exhaust gas temperature. Rather than desulfurization, denitration can be performed after desulfurization by a wet apparatus such as a scrubber. Further, since PM particles and SOx are removed at the time of desulfurization, there is little influence on the denitration process, which is preferable. In the case of the sodium hydroxide method, magnesium hydroxide method and ship engines where the reducing agent used in the present invention is produced as a by-product, there are restrictions on the storage and replenishment of water and chemicals at sea, and combined use with the seawater method is also possible Is suitable.

  Here, the sodium hydroxide method and the magnesium hydroxide method are methods in which an aqueous solution or slurry of hydroxide is sprayed into exhaust gas and SOx is removed as sulfite and sulfate, and storage and supply of the aqueous solution or water is required. It is.

  The seawater method is a simple method in which seawater is sprayed into exhaust gas, SOx is absorbed in seawater, and the absorbed solution is diluted with seawater to reduce the pH and COD (chemical oxygen demand) to the ocean. It is a system that only takes seawater and does not require storage of chemicals or water, and combined use with this method can save chemicals and water.

  Magnesium hydroxide can be generated by mixing slaked lime with seawater, and sodium hydroxide can also be generated by electrolysis of seawater.

  In addition, since sulfites such as sodium sulfite absorb SOx, desulfurization is simultaneously possible when sulfite is used as a reducing agent in the denitration apparatus of the present invention. Further, it is possible to perform a process in which desulfurization and denitration are simultaneously performed by actively supplying a hydroxide together with a reducing agent according to the NOx and SOx concentrations.

  The exhaust gas treated by the apparatus of the present invention is an atmospheric exhaust gas containing nitrogen gas as a main component and containing NOx. The nitrogen content concentration of this exhaust gas is 40 vol% or more, usually 60 vol% or more, and the content of NOx is about 100 to 3000 ppm, usually about 300 to 1500 ppm. The other components of the exhaust gas vary depending on the type of exhaust gas, and include, for example, oxygen, carbon dioxide, carbon monoxide, water, and SOx. Examples of the exhaust gas include exhaust gas discharged from a diesel engine for ships, automobiles, power generation, etc., combustion exhaust gas generated from a thermal power plant using petroleum, coal, natural gas, etc., exhaust gas generated from a boiler, and the like.

  An example of the apparatus of the present invention is shown in FIG. In this apparatus, a box-type electron irradiation device is attached in the middle of a cylindrical exhaust gas flow path.

  In the back of the electron irradiation device, a cathode having carbon nanotubes fixed on a rectangular SUS304 base is provided.

  The side surface of the exhaust gas passage of the electron irradiation device is made of titanium, and an electron beam extraction window having a thickness of 2 to 15 μm is attached, which serves as an anode.

The cathode is a field emission type electron gun, and electrons are emitted from the electron emission portion at a current density of 0.3 to 33 mA / cm ² by a tunnel effect from the tip of the CNT concentrated in the electric field by a DC high voltage applied to about 50 to 150 kV. Is done. The electrons are further accelerated by the applied voltage in the vacuum vessel, and become high-energy electron beams and reach the electron extraction window that is the anode. The electron beam passes through a thin titanium metal film having a thickness of 2 to 15 microns in the extraction window and is irradiated into the exhaust gas. In addition, there are small holes in the exhaust gas duct wall other than the electron beam extraction window, from which sodium sulfite, a kind of sulfite, is extruded and supplied in small solid or semi-solid slurry. Yes.

In the exhaust gas pipe, exhaust gas containing nitrogen as a main component and 300 to 1500 ppm of NOx, each containing about 5 to 15% oxygen, carbon dioxide, water, and several hundred ppm of SOx flows in one direction. When an electron beam collides with nitrogen molecules in the exhaust gas, it is dissociated into atoms, and a large amount of nitrogen atoms having unpaired electrons called N radicals are generated. On the other hand, when an electron beam collides with an oxygen molecule, a large amount of oxygen atoms having unpaired electrons called O radicals are generated. This highly active N radical causes a reduction reaction with NO with a high probability, and decomposes NOx into N ₂ and O ₂ with high efficiency. Conversely, when O radicals come into contact with NO, they are oxidized to highly reactive NO ₂ . This NO ₂ is decomposed into N ₂ by a reduction reaction by contacting with sodium sulfite supplied to the inner wall of the exhaust gas duct opposite to the electron beam extraction window. The form of sodium sulfite is preferably in the form of powder or slurry. Sodium sulfite can be diffused into the gas by the flow of exhaust gas, moisture, or an electron beam that has passed through the exhaust gas, enabling an effective reaction. is there.

The decomposition efficiency by electron beam irradiation does not change greatly within the range of oxygen concentration of 0 to 20%. However, as the oxygen concentration increases, the generation of O radicals by the electron beam increases, and the amount of NO ₂ generated increases. It is necessary to increase the amount added. However, since the necessary amount of the reducing agent is equimolar to the NO concentration, the amount added is small.

  Another example of the device of the present invention is shown in FIG.

  The basic configuration is the same as that of the apparatus shown in FIG. 1, but is an apparatus example in a larger exhaust duct. An electron beam generator is disposed around the entire duct of the flue where exhaust gas flows vertically from top to bottom, and the electron beam is irradiated toward the center of the flue. As a result, the entire exhaust gas passing through this region can be uniformly irradiated with an electron beam, and an effective treatment is possible. Further, the reducing agent, sodium sulfite and magnesium sulfite, can be efficiently reduced by spraying downward in the form of powder, slurry, or solution from the upper center of the irradiated portion. The produced sulfate is deposited and collected downward, or is collected by a filter, scrubber or the like on the downstream side.

  An example of the exhaust gas treatment system incorporating this exhaust gas treatment device is shown in FIG. In this system, a cylindrical desulfurization tower and a denitration tower are both connected in series. An exhaust gas inlet is provided at the lower part of the desulfurization tower, and an outlet is provided at the top. A seawater storage tank containing hydroxide is also provided. In this storage tank, magnesium hydroxide produced by adding extinguishing coal to seawater or the one that sodium hydroxide is generated by electrolysis of seawater is stored. From this storage tank, hydroxide is transferred to the lower part of the desulfurization tower. Supplied with seawater or mixed with water. Then, it is pumped from this lower part to the upper part of the desulfurization tower, sprayed into the tower from there, absorbs sulfur oxide as sulfite, and then accumulates at the bottom of the tower, and is sent to the upper part of the tower by a pump. Is circulating.

  The exhaust gas treatment apparatus of FIG. 2 is attached to the center of the denitration tower, and an exhaust gas inlet is provided at the lower part, and is connected to the inlet from the outlet of the desulfurization tower. An exhaust gas outlet is provided at the top of the denitration tower. The denitration tower also has a liquid reservoir at the bottom, and a circulation line is formed through which the liquid is sent to the top by a pump and sprayed into the tower. And a part of it is withdrawn from the circulation line of the desulfurization tower and sent to the circulation line of this denitration tower. The sulfite contained in the liquid is used as a reducing agent in the denitration reaction in the tower, is converted to sulfate, and a part is extracted from the tower.

  This system shows an example of a simultaneous denitration and desulfurization system that is optimal for marine moving objects such as large marine diesel engines.

  SOx and NOx-containing exhaust gas with a high concentration of about 150-250 ° C coming out of a large diesel engine or boiler is first absorbed by 80 to 90% or more of SOx in an absorption tower using sodium hydroxide or magnesium hydroxide as an absorbent. To do. In the case of exhaust gas cooling or high concentration SOx, it is preferable to install a scrubber with seawater before that because the amount of chemicals can be suppressed. In this step, PM and other fine particles contained in the exhaust gas are also absorbed at the same time, and the NOx-containing exhaust gas containing dust and sulfite is removed at a low temperature of about 60 to 70 ° C. This exhaust gas is an optimum gas component for the denitration process according to the present invention described above, and it is not necessary to add a sulfite reducing agent if the concentration of NOx is low. Further, even when the sulfite is added, it can be extracted from the SOx absorption tower and used, and storage and replenishment of NOx treatment chemicals are unnecessary. Further, when the seawater method is used for desulfurization, it can be used by extracting or synthesizing sulfite.

  Further, when the concentration of either NOx or SOx is low, the reaction tower can be made one by mixing and spraying the chemicals in an appropriate amount ratio.

  Moreover, magnesium hydroxide which is a chemical | medical agent for desulfurization can be produced | generated by mixing slaked lime with seawater. Sodium hydroxide (caustic soda) can be produced by electrolyzing seawater. Power required for electrolysis can be supplied by a solar cell to save energy.

  FIG. 4 shows an example of an apparatus when the amount of electron beam reaching the center of the flue decreases with a larger flue. By disposing a sulfite supply part in the central part, the gas flow in the central part can be restricted to increase the reaction efficiency. In addition, it is possible to assist in the diffusion of sulfite by the collision of electron beams that have passed through the gas and thermal energy.

  According to the present invention, since NOx and SOx contained in the exhaust gas can be efficiently processed at low cost, it can be widely applied to the treatment of exhaust gas containing NOx and SOx.

Claims (5)

An apparatus for treating exhaust gas containing nitrogen oxides, comprising a cathode having an electron emission portion formed of a field emission element, an anode, an electron beam extraction window, and a vacuum chamber for generating an electron beam. A denitration apparatus comprising: an electron beam irradiation apparatus; a flow path for the exhaust gas to which the electron beam irradiation apparatus is connected via the electron beam extraction window; and a device for supplying sulfite to the flow path for the exhaust gas. An exhaust gas treatment apparatus containing nitrogen oxides . The apparatus for treating exhaust gas containing nitrogen oxide according to claim 1, wherein the field emission element is an element using carbon nanotubes . Processing apparatus of an exhaust gas containing nitrogen oxides according to claim 1 or 2, wherein the exhaust gas is further characterized by a-law containing sulfur oxides. Before or after the previous SL denitration device, arranged desulfurizer by-produced a sulfite, contains nitrogen oxides according to claim 3, wherein the use of nitrous sulfate as a reducing agent in the NOx removal device Exhaust gas treatment equipment . The apparatus for treating exhaust gas containing nitrogen oxide according to claim 4, wherein the desulfurization apparatus uses seawater and / or a hydroxide generated from seawater as an absorbent for desulfurization .

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