JP2010194504A - System for treating exhaust gas by electron beam irradiation - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an apparatus for efficiently decomposing NO<SB>x</SB>contained in exhaust gas at a low cost. <P>SOLUTION: The apparatus for treating exhaust gas comprising NO<SB>x</SB>is characterized in that a feed pipe for feeding exhaust comprising NO<SB>x</SB>is connected to the inlet of a container including an inlet and an outlet and storing an adsorbent of NO<SB>x</SB>, and an electron beam device and a nitrogen gas feeder are connected to each other via a switching valve in the middle of the feed pipe. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a system for decomposing NOx by irradiating exhaust gas containing NOx with an electron beam.

  Conventionally, as a method for treating exhaust gas containing NOx, NOx-containing gas is caused to flow through an adsorbent to adsorb NOx, and after replacing the atmospheric gas with nitrogen gas, heating is performed to release NOx, and this gas is discharged. NOx is reduced by the above method (Patent Document 1), after NOx is adsorbed on the adsorbent, the atmosphere gas is replaced with nitrogen gas having an oxygen concentration of 10 vol% or less and a purity of 90 vol% or more, and then the adsorbent is not heated A method (Patent Document 2) in which plasma is applied to desorb NOx and regenerate the adsorbent is known.

  Further, in the method of removing nitrogen oxides and / or sulfur oxides in exhaust gas by adding ammonia to the exhaust gas and irradiating the electron beam, the position of ammonia addition is determined at the center of the electron beam irradiated into the reactor. A method (Patent Document 3) is also known in which the range of the electron beam is within 2.5 times the upstream side of the exhaust gas.

Further, as shown in FIG. 3, the engine exhaust is introduced into the adsorption tower A to adsorb NOx and the like, and then heated and introduced with nitrogen gas having oxygen of 10 vol% or less and purity of 90 vol% or more. A method is also known in which NOx is desorbed and nitrogen gas containing the desorbed NOx or the like is plasma treated to reduce NOx to N ₂ (Patent Document 4).

JP 2001-300409 A International Publication No. 2005 / 037412A1 Pamphlet JP-A-8-108037 JP 2007-321678 A

  The low-temperature plasma methods described in Patent Documents 1 and 2 have a problem in that the processing efficiency is poor, and in an ordinary exhaust gas containing about 10 vol% oxygen, an oxidation reaction proceeds rather than reduction, and reductive decomposition does not occur.

The electron beam processing method described in Patent Document 3 requires addition of ammonia because ammonia is added to produce and remove ammonium nitrate.
The method described in Patent Document 4 is a system for continuously processing NOx using the method described in Patent Document 2 as shown in FIG. 3, but the system is complicated and there is a problem in processing efficiency, and 10 vol. In ordinary exhaust gas containing about 1% of oxygen, there was a problem that the oxidation reaction progressed rather than reduction and the reductive decomposition did not occur.

  On the other hand, as shown in FIG. 4, the present inventor has applied a method of directly irradiating an exhaust gas with an electron beam by a low energy electron beam irradiation method or an N radical generated by irradiating an electron beam into nitrogen gas. Using the injection method, it was found that NOx reductive decomposition by N radicals was possible even in an oxygen environment with a concentration of 10 to 20 vol%. However, although the treatment efficiency in this method is very high as compared with the low temperature plasma method, NOx in the exhaust gas has a very low concentration of 100 to 1000 ppm (0.01 to 0.1%), and produced N There is a low probability of reacting with the radicals, and there is a problem that the processing efficiency is poor because N radicals in a necessary amount (NOx / N radical = 1/1) or more are necessary.

  As a result of intensive studies to develop a more efficient decomposition system for NOx, the present inventor concentrated NOx by bringing NOx-containing exhaust gas into contact with the NOx adsorbent and adsorbing it, and the above N radicals. It has been found that NOx can be efficiently decomposed by reducing and decomposing NOx by contacting nitrogen gas containing nitrogen.

  Furthermore, if at least two containers containing NOx adsorbent are used to adsorb NOx on the one hand and NOx adsorbed on the other side is reduced and decomposed, continuous NOx treatment can be efficiently performed. I found it.

The present invention has been made based on these findings,
An inlet pipe for exhaust gas containing NOx is connected to the inlet side of a container containing a NOx adsorbent having an outlet and an inlet, and an electron beam device and nitrogen gas supply are provided in the middle of the inlet pipe via a switching valve. A NOx-containing exhaust gas treatment device, characterized in that the device is connected;
There are at least two containers containing NOx adsorbents, and the exhaust gas inlet pipe, the electron beam apparatus, and the nitrogen gas supply apparatus are all connected to each container via a switching valve,
NOx is adsorbed by bringing NOx-containing exhaust gas into contact with the NOx adsorbent, and then nitrogen gas is injected into the adsorbent and directly irradiated with an electron beam or irradiated with an electron beam to generate nitrogen radicals. A NOx-containing exhaust gas treatment method, wherein the NOx-containing exhaust gas treatment method,
NOx adsorbent is contained in at least two containers, one of which is fed with exhaust gas containing NOx to adsorb NOx, and the other that has already adsorbed NOx is injected with nitrogen gas. The present invention relates to the above method in which nitrogen gas generated by direct irradiation with an electron beam or irradiation with an electron beam to generate N radicals is fed to reduce and decompose NOx.

In the present invention, uniform electrons having higher energy than low-temperature plasma are generated by an electron beam device, and nitrogen molecules are dissociated by the electrons to generate N radicals with high efficiency. This N radical decomposes NOx directly into N ₂ and O ₂ .

  Unlike the conventional method of reacting with low-concentration NOx, the present invention injects nitrogen gas into a NOx adsorbent container that has been concentrated by adsorbing NOx and directly irradiates the adsorbent with an electron beam, By sending in nitrogen gas that has been irradiated to generate N radicals, NOx in the exhaust gas can be efficiently decomposed.

It is a figure which shows schematic structure of the apparatus which is one Example of this invention. It is a figure which shows schematic structure of the apparatus which is another Example of this invention. It is a figure which shows an example of the conventional waste gas processing apparatus. It is a figure which shows another example of the conventional waste gas processing apparatus.

The apparatus of the present invention comprises a container containing a NOx adsorbent, an electron beam apparatus, and a nitrogen gas supply apparatus.
Any known adsorbent can be used as the NOx adsorbent, and examples thereof include zeolite, activated carbon, and alumina. The shape is not limited as long as gas can be circulated, and may be granular, fibrous or the like. In addition, when zeolite is used, since zeolite also has a catalytic performance for decomposing NOx, a synergistic effect of adsorption and decomposition can be expected, and the processing efficiency may be further improved.

  The shape of the container containing the NOx adsorbent is not limited, but is usually a tower shape or a column shape. An inlet is provided near one end, an outlet is provided near the other end, and an exhaust gas inlet pipe is connected to the inlet.

An electron beam device and a nitrogen gas supply device are connected in the middle of the exhaust gas supply pipe or in the adsorbent storage container.
The electron beam apparatus includes a cathode, an anode, a chamber, and an electron beam extraction window attached thereto.

  The cathode has an electron emission portion formed on a conductive base. 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 electron beam extraction window, which is an anode, needs to be easy to transmit the electron beam and have high strength, and a thin film of low density and high strength material such as titanium, aluminum, silicon, beryllium, carbon, etc. of several to several tens of microns is used It is done. 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 oxygen and a gas containing oxygen are 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 keeping 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 nitrogen gas supply device is not particularly limited as long as it can supply a predetermined amount of nitrogen gas, and the simplest device is a device that supplies air as it is with a pump. However, the higher the nitrogen purity, the higher the efficiency of N radical generation, which is preferable. It is preferable to add a hygroscopic agent, a filter for separating N ₂ , a purification device, or the like. A nitrogen gas cylinder can be used as a supply source for a small amount of processing equipment.

  The electron beam device and the nitrogen gas supply device are configured so that the electron beam is irradiated to the nitrogen gas supplied from the nitrogen gas supply device, that is, the nitrogen gas supply device is located at the same position as the electron beam device or upstream thereof. Provide.

  Then, in the container containing the NOx adsorbent, first, exhaust gas containing NOx is introduced for adsorption, and then the exhaust gas supply is stopped and supplied from the nitrogen gas supply device. A switching valve is provided in the middle of the feeding pipe so that the nitrogen gas irradiated with the line can be fed into the container.

  It is preferable to provide at least two containers containing NOx adsorbent, and NOx is adsorbed by feeding exhaust gas containing NOx to one of them. When adsorption is completed, nitrogen gas generated by irradiating an electron beam to generate N radicals is sent to reduce and decompose NOx. Meanwhile, the exhaust gas feed pipe is switched to the other container to send the exhaust gas. The NOx is adsorbed, and this is sequentially repeated.

Next, the adsorption-completed container is heated to desorb NOx, and nitrogen gas generated by generating an radical by irradiation with an electron beam is fed to reduce and decompose NOx. Alternatively, nitrogen gas is fed into the adsorbent storage container and directly irradiated with an electron beam to release NOx by heating the adsorbent or by desorption reaction with the electron beam, and at the same time generate N radicals to reduce and decompose NOx. .
Nitrogen gas uses pure nitrogen gas, but may contain oxygen, carbon dioxide, water, etc. up to about 10 vol%.

  The voltage for accelerating electrons in the electron beam apparatus is 30 kV or more, preferably 50 kV or more and 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 .

  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 the exhaust gas is 40 vol% or more, usually 60 vol% or more, and the NOx content is about 10 to 3000 ppm, usually about 100 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. When oxygen is contained, the decomposition efficiency of NOx is slightly reduced, but NOx can be decomposed even when the oxygen-containing concentration is about 15 vol%. A preferable upper limit of the oxygen-containing concentration is 30 vol%. Examples of the exhaust gas include exhaust gas discharged from automobiles, ships, diesel engines for power generation, combustion exhaust gas generated from a thermal power plant using petroleum, coal, natural gas, etc., exhaust gas generated from a boiler, and the like.

  The pressure of the exhaust gas is about atmospheric pressure, usually about 1 atmosphere ± 10%. The method of the present invention can be applied even at a lower atmospheric pressure than this, but is characterized in that it can be applied without reducing the pressure of exhaust gas.

FIG. 1 shows a schematic configuration of the apparatus according to the present invention.
In this embodiment, the exhaust gas path of the diesel engine is connected to two exhaust pipes into which a container containing NOx adsorbent is inserted. As the adsorbent, zeolite that selectively adsorbs NOx is used, and only NOx is selectively adsorbed while exhaust gas passes through the adsorbent. A switching valve is provided at the connection portion, and exhaust gas containing NOx is fed into one side to adsorb NOx. Then, depending on conditions such as NOx concentration and flow rate, after a few minutes to several hours have passed, if the allowable adsorption amount is reached, the nitrogen gas generated by irradiating an electron beam to generate N radicals is sent in to reduce and decompose NOx. In the meantime, the exhaust gas feed pipe is switched to the other container, the exhaust gas is fed, NOx is adsorbed, and this is repeated sequentially.

The electron beam irradiation apparatus using the carbon nanotubes shown in FIG. 4 was used for the CNT electron source of FIG. The cathode is a field emission type electron gun, and electrons are emitted from the electron emission portion at a current density of 1 to 5 mA / cm ² by the tunnel effect from the tip of the CNT concentrated in the electric field by a DC high voltage applied to about 50 to 100 kV. The 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 metal film having a thickness of 2 to 15 microns in the extraction window and is irradiated into a high-concentration nitrogen gas atmosphere fed by a nitrogen gas supply device. When an electron beam collides with nitrogen molecules in this atmosphere, they are dissociated into atoms, and a large amount of nitrogen atoms having unpaired electrons called N radicals are generated. Nitrogen gas containing N radicals at a concentration corresponding to the irradiation amount is introduced into the adsorbent storage container. The amount of NOx adsorbed may be very small, as long as the amount of adsorbed NOx is processed during the time until the next adsorption step. This highly active N radical causes a reduction reaction with NOx with high probability, and decomposes NOx into N ₂ and O ₂ with high efficiency. In order to make the reduction reaction proceed efficiently, it is better to heat the adsorbent and desorb NOx, but it is also possible to proceed the reduction reaction directly on the surface of the adsorbent without heating.

  Nitrogen gas supplied to the nitrogen gas supply device may be air, but if other gas concentrations such as oxygen and water are high, the electron beam is consumed to generate O radicals and OH radicals, and the N radical decomposition reaction is also inhibited. Therefore, the nitrogen purity is 80 vol% or higher, preferably 95 vol% or higher. In this apparatus, the electron beam is consumed to generate only N radicals, and since it directly contacts and reacts with highly concentrated NOx, a very efficient decomposition process was possible.

FIG. 2 shows a schematic configuration of another apparatus according to the present invention.
In this embodiment, the exhaust gas path of the boiler is connected to two exhaust pipes into which a container containing NOx adsorbent is inserted. As the adsorbent, activated carbon that selectively adsorbs NOx is used, and only NOx is selectively adsorbed while exhaust gas passes through the adsorbent. A switching valve is provided at the connection portion, and exhaust gas containing NOx is fed into one side to adsorb NOx. Depending on conditions such as NOx concentration, flow rate, etc., after an elapse of several minutes to several hours, when the allowable adsorption amount is reached, an electron beam is directly irradiated into the adsorbent storage container with nitrogen gas being fed into the adsorbent storage container. Then, NOx that generates N radicals is reduced and decomposed, and during that time, the exhaust gas feed pipe is switched to the other container, the exhaust gas is fed, NOx is adsorbed, and this is sequentially repeated.

The electron beam irradiation apparatus using the carbon nanotube shown in FIG. 4 was used for the CNT electron source of FIG. The cathode is a field emission type electron gun, and electrons are emitted from the electron emission portion at a current density of 1 to 5 mA / cm ² by the tunnel effect from the tip of the CNT concentrated in the electric field by a DC high voltage applied to about 50 to 100 kV. The 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 metal film having a thickness of 2 to 15 microns in the take-out window, and is directly irradiated into the adsorbent storage container into which nitrogen gas is introduced by a nitrogen supply device. The NOx adsorbed by this electron beam irradiation is decomposed into N ₂ and O ₂ very efficiently.

According to the present invention, NOx contained in the exhaust gas can be efficiently and inexpensively decomposed into N ₂ and O ₂ , so that it can be widely applied to the treatment of exhaust gas containing NOx.

Claims (4)

  An inlet pipe for exhaust gas containing NOx is connected to the inlet side of a container containing a NOx adsorbent having an outlet and an inlet, and an electron beam device and nitrogen gas supply are provided in the middle of the inlet pipe via a switching valve. NOx-containing exhaust gas treatment device, characterized in that the device is connected   The apparatus according to claim 1, wherein there are at least two containers containing NOx adsorbent, and the exhaust gas inlet pipe, the electron beam apparatus, and the nitrogen gas supply apparatus are all connected to each container via a switching valve.   NOx is adsorbed by bringing NOx-containing exhaust gas into contact with the NOx adsorbent, and then nitrogen gas is injected into the adsorbent and directly irradiated with an electron beam or irradiated with an electron beam to generate nitrogen radicals. NOx-containing exhaust gas treatment method, wherein NOx is reduced and decomposed by feeding   NOx adsorbent is contained in at least two containers, one of which is fed with exhaust gas containing NOx to adsorb NOx, and the other already adsorbing NOx is injected with nitrogen gas. The method according to claim 3, wherein the NOx is reduced and decomposed by direct irradiation with an electron beam or nitrogen gas generated by irradiation with an electron beam to generate N radicals.

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