CN108031287A - A kind of plasma enhancing Ag/Al2O3The method of catalyst removal nitrogen oxides - Google Patents

Abstract

This divisional application relates to a method for plasma-enhanced Ag/Al ₂ O ₃ catalyst removal of nitrogen oxides. The Ag/Al ₂ O ₃ catalyst is calcined by an equal-volume co-impregnation method, and filled in a plasma discharge device In the discharge area, the reaction gas containing C ₂ H ₂ and nitrogen oxides is passed into the plasma discharge device, the reaction temperature is 180-300°C, and a 5kHz-20kHz high-voltage power supply is used for discharge to realize the conversion of nitrogen oxides. The method provided by the invention can effectively solve the problem of low removal efficiency of nitrogen oxides by simply relying on Ag/Al ₂ O ₃ catalysis in the selective catalytic reduction of hydrocarbons at low temperature, and adopts plasma and 2% Ag/Al ₂ O ₃ catalyst The synergistic effect can increase the efficiency of reducing nitrogen oxides by 50%, and the conversion rate _{of C2H2} can reach 100%. Moreover, the method is simple and easy to operate, and the device of the invention is simple and has a wide application range.

Description

A method for removing nitrogen oxides by plasma enhanced Ag/Al2O3 catalyst

This application is a divisional application with the application number 2017107631037, the application date is August 30, 2017, and the title of the invention is "a method for plasma-enhanced Ag/Al ₂ O ₃ catalyst removal of nitrogen oxides".

technical field

The invention belongs to the field of air pollution _control , including the conversion _{of C2H2} and a method for selectively catalytically reducing nitrogen oxides, and relates to a method for converting nitrogen oxides in a low-temperature region under a _C2H2 system with a plasma combined with a catalyst .

Background technique

Nitrogen oxides (NO _x ) are the main pollutants that form acid rain and photochemical smog that damage the natural environment. At the same time, nitrogen oxides have a pungent odor that can cause hypoxia and nausea when inhaled by the human body. Nitrogen oxides are carcinogenic to a certain extent. Nitrogen oxides commonly referred to mainly include NO, NO ₂ , N ₂ O and N ₃ O ₄ . The sources of nitrogen oxides in nature are mainly volcanic eruptions, lightning and microbial oxidation and decomposition, while man-made nitrogen oxides are mainly emitted from coal-fired thermal power plants and heating high-temperature boilers in the form of stationary sources. In addition, it is also produced by means of mobile sources such as fuel vehicle exhaust emissions.

Nitrogen oxides are the main cause of smog in our country at present. As the main means of transportation in the daily life of residents, automobiles bring us convenience, and at the same time, the solution to the problem of exhaust pollution is imminent. my country has become the global automobile manufacturing center and the world's largest automobile consumer market. According to the latest statistics from the International Association of Automobile Manufacturers (OICA), since 2005 my country's automobile production and sales have increased from 5.71 million and 5.76 million to 24.50 and 24.6 million in 2015, with growth rates of 15.68% and 15.62% respectively. Because automobile exhaust contains a large amount of sulfide and nitrogen oxides, controlling the emission of pollutants in automobile exhaust is the fundamental problem to solve air pollution. At present, the relatively mature denitrification methods of motor vehicle exhaust in industry are mainly based on ammonia selective catalytic reduction and three-way catalyst. Since the 1980s, the world's first tail gas denitrification device using ammonia as a reducing agent has been operating in Kudamatsu Power Plant in Japan. Since then, the method of using ammonia selective catalytic reduction reaction for tail gas denitrification has become popular all over the world. NH ₃ -SCR is applied to the removal of heavy-duty vehicle exhaust. Due to the high operating cost of ammonia and its corrosiveness, urea is usually used to decompose ammonia to produce ammonia. However, regular replacement and maintenance are still additional operating costs for motor vehicles, and there is no The discharge of fully reacted ammonia to the atmosphere will cause secondary pollution, have a strong irritating effect on the ear, nose and throat of the human body, and even cause suffocation if inhaled for a long time. Therefore, there are still certain technical problems in its wide application. Three-way catalyst (Three Way Catalyst) is a noble metal catalyst supported mainly by platinum, rhodium and palladium. The operation of the three-way catalyst requires a strict air-fuel ratio (about 14.6) to work effectively, and its resistance to water and sulfur is relatively poor. The catalytic activity of the oxygen-enriched engine system currently used to meet the power requirements of motor vehicles is greatly challenged. At the same time, due to the high cost caused by precious metals, they cannot be widely used.

Plasma is the fourth state of matter distinct from solids, liquids and gases. Atmospheric pressure low-temperature plasma has a relatively high electron temperature (10 ⁴ -10 ⁵ K), but the actual temperature can be as low as room temperature and can be generated under atmospheric pressure. Therefore, since the 1980s, atmospheric pressure non-equilibrium plasma has been used to treat the atmosphere Pollutants are prevalent around the world. Plasma treatment has the advantages of simple device, convenient and easy operation, high treatment efficiency, and wide range of action. Higher disadvantages. At present, methane, acetylene, ethylene, propylene and other hydrocarbons are used as the main reducing agent in combination with metal oxide catalysts, noble metal catalysts and molecular sieve catalysts to remove nitrogen oxides from automobile exhaust. However, when the reaction temperature is lower than 300 °C, the overall catalytic denitrification activity is low. Usually, the addition of atmospheric pressure non-equilibrium plasma is expected to move high-temperature reactions to low temperatures due to its higher electron temperature. Therefore, it is meaningful to apply plasma-combined catalysts to improve the denitrification of motor vehicle exhaust during cold start and belt speed.

Contents of the invention

In order to make up for the _deficiencies of the prior art, the purpose of the present invention is to provide a method for synergistically improving the low-temperature activity of the _{C2H2 selective} catalytic reduction nitrogen oxide system by using plasma and Ag/ _Al2O3 catalyst. In a system containing C ₂ H ₂ and nitrogen oxides, at low temperature (less than 300°C), the removal efficiency of nitrogen oxides is not high by simply relying on Ag/Al ₂ O ₃ catalysts, and plasma assistance is used to reduce nitrogen oxides The highest efficiency can be increased by about 50%, which can effectively treat nitrogen oxides in low temperature areas.

The present invention adopts following technical scheme:

The Ag/Al ₂ O ₃ catalyst was calcined by equal volume co-impregnation method, filled in the discharge area of the plasma discharge device, and the mixed gas containing C ₂ H ₂ and nitrogen oxides was passed into the plasma discharge device, The reaction temperature is 180-300°C, and a 5kHz-20kHz high-voltage power supply is used for discharge to realize the conversion of nitrogen oxides.

Preferably, the preparation method of the Ag/Al ₂ O ₃ catalyst is as follows: grind active Al ₂ O ₃ pellets, select 20-160um granular Al ₂ O ₃ , immerse in an aqueous solution of silver nitrate and let it stand at room temperature for 12 hours, and then It was placed in an infrared drying oven and treated at 80°C for 4 hours, then at 120°C for 4 hours, and finally placed in a muffle furnace for calcination at 550°C under air for 2 hours to obtain a 2wt% Ag/Al ₂ O ₃ catalyst.

Another object of the present invention claims the plasma discharge device applied to the above method, the discharge device includes a quartz reactor, a dielectric barrier discharge and catalyst combination treatment chamber, a fixed ring, a high voltage electrode, a ground electrode and an electrode protection tube.

The high-voltage electrode is made of stainless steel long straight tube, and the ground electrode is wrapped around the quartz reactor with dense-porous white steel mesh. Both the high-voltage electrode and the ground electrode adopt a coaxial wire barrel structure. The high-voltage electrode is located in the center of the quartz reactor, and the ground electrode is tightly wrapped around the outer wall of the quartz reactor and fixed with tungsten wire.

Preferably, the fixed ring is a tetrafluoro ring.

Preferably, the ground electrode is a stainless steel mesh.

Preferably, the quartz reactor is a quartz tube with a wall thickness of 4mm.

Both the air inlet and outlet are made of PTFE joints, which are closely connected with the quartz reactor. The working gas containing C ₂ H ₂ and nitrogen oxides flows into the quartz reactor through the air inlet, the reaction temperature is 180°C -300°C, and the discharge is performed after the system is stable. The plasma is generated in the discharge area of the coaxial wire barrel, and the Ag/Al ₂ O ₃ catalyst is placed in the plasma action area (in the discharge area of the high-voltage electrode and the ground electrode), so that the plasma enhances the activation of C ₂ H ₂ and is used for Conversion of nitrogen oxides over Ag/Al ₂ O ₃ catalysts.

The Ag/Al ₂ O ₃ catalyst is filled in the dielectric barrier discharge and catalyst combination treatment chamber, and the two ends of the catalyst are sealed and fixed with quartz wool, and the nitrogen oxide gas containing C ₂ H ₂ is passed into the dielectric barrier discharge and catalyst combination treatment chamber. , the reaction temperature is 150-300°C.

The third object of the present invention is to protect the application of the above method in the treatment of motor vehicle exhaust gas. The Ag/Al ₂ O ₃ catalyst is loaded on the honeycomb ceramics, and the honeycomb ceramics are placed at the exhaust end of the motor vehicle exhaust gas, and the dielectric barrier discharge plasma The generation can be generated inside the honeycomb by using the array wire barrel discharge. A temperature sensor is also arranged at the exhaust end of the motor vehicle exhaust. Since the discharge power to achieve the best conversion of nitrogen oxides is different at different temperatures, low temperature requires a relatively high discharge power and high temperature requires a relatively low discharge power to effectively convert nitrogen oxides, so different temperatures can be set At the same time, when the temperature is higher than 300°C, the high-voltage power supply is turned off, and the effective conversion of nitrogen oxides can be achieved by relying on the action of a simple Ag/Al ₂ O ₃ catalyst. The conversion of incompletely burned hydrocarbons is realized, and the conversion of nitrogen oxides in the cold start stage is also improved, saving energy.

Beneficial effects: the method provided by the invention can effectively solve the problem of low removal efficiency of nitrogen oxides in the selective catalytic reduction of hydrocarbons by simply relying on Ag/Al ₂ O ₃ catalysis at low temperature, and adopts 2% Ag/Al ₂ O ₃ catalyst Synergistically working with plasma, the efficiency of nitrogen oxide reduction can be increased by 50%, and the conversion rate of C ₂ H ₂ can reach 100%. Moreover, the method is simple and easy to operate, and the device of the invention is simple and has a wide application range.

Description of drawings

Fig. 1 is a schematic diagram of the structure of the device of the present invention. 1. Quartz reactor, 2. Dielectric barrier discharge and catalyst combination treatment chamber, 3. Fixed ring, 4. High voltage electrode, 5. Ground electrode, 6. Electrode protection tube, 7. Air inlet, 8. Gas outlet.

Detailed ways

The present invention is described in detail below by means of drawings and specific examples, but the protection scope of the present invention is not limited. Unless otherwise specified, the experimental methods used in the present invention are conventional methods, and used experimental equipment, materials, reagents, etc. can be purchased from chemical companies.

Example 1

Put a long straight high-voltage electrode with a diameter of 1.5mm into a quartz electrode protection tube with an inner diameter of 2.5mm and fix it at the center of the quartz tube with a fixing ring. Use a quartz tube with a wall thickness of 4mm and an inner diameter of 8mm as the dielectric tube. A tungsten wire tightly wound stainless steel mesh is used as a grounding electrode. The high-voltage electrode and the grounding electrode both adopt the coaxial wire barrel structure.

Preparation of catalyst Ag/Al ₂ O ₃ : According to the mass ratio of Ag to Al ₂ O ₃ , weigh 5g of silver nitrate and 20-160um aluminum oxide particles required for 2wt%/Ag/Al ₂ O ₃ , take 5ml to remove Completely dissolve silver nitrate in ionized water to prepare silver nitrate aqueous solution. Immerse 20-160um aluminum oxide particles in the silver nitrate aqueous solution and let it stand for 12 hours at room temperature. Treat at 120° C. for 4 hours, and finally place it in a muffle furnace for calcination at 550° C. under air for 2 hours to prepare a 2wt%/Ag/Al ₂ O ₃ catalyst.

Removal of nitrogen oxides: Before the reaction starts, the 2% Ag/Al ₂ O ₃ catalyst is heated to 550°C in N ₂ atmosphere for activation treatment, and then the experiment is carried out in N ₂ up to 150°C. Put 1500ppm C ₂ H ₂ , 500 ppm NO, 10vt% O ₂ , N ₂ as the balance gas, feed the raw material gas with a space velocity of 15000h ^-1 into the reactor for reaction, turn on the high-voltage power supply, when the discharge voltage changes from peak to peak to 14kV, The conversion of nitrogen oxides was 54%, and the conversion of C ₂ H ₂ was 100% conversion. However, when Ag/Al ₂ O ₃ catalyst is only used, the nitrogen oxide conversion rate is only 5%.

Example 2

Using the device in Example 1, the 2wt% Ag/Al ₂ O ₃ catalyst was heated to 550°C under N ₂ atmosphere for activation before the reaction started, and then the experiment was carried out in N ₂ up to 250°C. Put 1500ppm C ₂ H ₂ , 500ppm NO, 10% O ₂ , N ₂ as the balance gas, feed the raw material gas with a space velocity of 15000h ^-1 into the reactor for reaction, turn on the high voltage power supply, due to the exothermic phenomenon during the plasma action , when the discharge voltage is from peak to peak value of 12kV, the whole system is stabilized at 250°C by using the programmable temperature controller, at this time the conversion rate of nitrogen oxides is 78%, and the conversion rate of nitrogen monoxide is 81%. Similarly, the reaction temperature is controlled at 250°C. As the discharge voltage gradually increases from 12kV to 16kV, the conversion of nitrogen oxides decreases from 78% to 28%. At the same time the conversion _{of C2H2} is always 100% conversion.

Example 3

Using the device of Example 1, the 2wt% Ag/Al ₂ O ₃ catalyst was heated to 550°C in N ₂ atmosphere for activation before the reaction started, and then the experiment was carried out in N ₂ up to 300°C. 1500ppm C ₂ H ₂ , 500ppm NO, 10% O ₂ , N ₂ are used as balance gas, and the raw material gas with a space velocity of 15000h ^-1 is connected to the reaction gas for reaction. When the discharge voltage is from the peak value to 12kV, the conversion rate of nitrogen oxides The conversion rate of nitric oxide was 68%, and the conversion rate of nitric oxide was 73%. As the discharge voltage changes from 12kV to 16kV, the conversion of nitrogen oxides drops to 26%, while the conversion of nitrogen oxides can reach 76% under the action of pure Ag/Al ₂ O ₃ . Therefore, at this temperature and above, nitrogen oxides can be converted and removed simply by catalyst action.

In summary, when the temperature exceeds 300°C, the 2%Ag/Al ₂ O ₃ catalyst obtained by the impregnation method has a good effect of removing nitrogen oxides, but when the temperature is lower than 300°C, the pure catalyst does not work well. Therefore, at this time, the high-voltage power supply of the device of the present invention is turned on, so that the plasma and the catalyst work together to reduce nitrogen oxides.

In the temperature range of cold start and belt speed (180-300°C) of motor vehicles, the efficiency of removing nitrogen oxides by using dielectric barrier discharge plasma and catalyst to act together is greatly improved compared with pure catalyst action. In the temperature range after 300°C, nitrogen oxides can be better removed by simple catalysis under the C ₂ H ₂ system.

The above is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto, and any person familiar with the technical field within the technical scope disclosed in the present invention, according to the technical solution of the present invention Any equivalent replacement or change of the inventive concepts thereof shall fall within the protection scope of the present invention.

Claims (2)

1. A plasma-enhanced Ag/Al ₂ O ₃ catalyst method for removing nitrogen oxides, characterized in that the Ag/Al ₂ O ₃ catalyst is obtained by calcining through equal-volume co-impregnation, and it is filled in the plasma discharge The high-voltage electrode and ground electrode discharge area of the device; the high-voltage electrode and the ground electrode of the plasma discharge device adopt a coaxial wire barrel structure, and the high-voltage electrode is directly inserted into the electrode protection tube and fixed with a fixing ring so that the high-voltage electrode is in the center of the quartz reactor position, the grounding electrode is fixed on the outer wall of the quartz reactor; _the mixed gas containing _C2H2 and nitrogen oxides is passed into the plasma discharge device, the reaction temperature is 180-300°C, and the discharge is carried out by a 5kHz-20kHz high-voltage power supply to realize the conversion of nitrogen oxides. 2. A method for plasma-enhanced Ag/Al ₂ O ₃ catalyst removal of nitrogen oxides according to claim 1, characterized in that, the preparation method of the Ag/Al ₂ O ₃ catalyst is as follows: grinding active Al ₂ O ₃ balls, choose 20-160um granular Al ₂ O ₃ , impregnate them in silver nitrate aqueous solution and let them stand at room temperature for 12 hours, then put them into an infrared drying oven and treat them at 80°C for 4 hours, then at 120°C for 4 hours , and finally placed in a muffle furnace for calcination at 550° C. under air for 2 h to obtain a 2 wt % Ag/Al ₂ O ₃ catalyst.