JPS6129776B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention deals with nitrogen oxides (hereinafter abbreviated as NOx) contained in exhaust gas from internal combustion engines such as heavy oil-fired boilers, coal-fired boilers, combustion furnaces attached to various chemical equipment, steel plants, diesel engines, and turbines. catalytic composition that effectively and economically reduces and detoxifies In particular, it is applied to exhaust gases that contain sulfur oxides and soot that are likely to poison the catalyst, in addition to NOx, and uses ammonia (hereinafter abbreviated as _NH3 ) as a reducing agent to catalytically reduce NOx at an appropriate temperature. In converting it into nitrogen (N ₂ ) and water (H ₂ O) and making it harmless,
The object of the present invention is to provide a catalyst that is inexpensive, free from secondary pollution, and efficiently exhibits its functions without being affected by the poisoning components. Methods for removing NOx from exhaust gas include solidification capture methods, adsorption methods, oxidation absorption methods, and reduction processes, and these processes can also be classified into wet methods and dry methods. Regarding solidification collection method and adsorption method, concentrated
Some kind of treatment of NOx is required, and the absorption method and wet reduction process require an oxidizing agent due to the stability of NO, which increases treatment costs, and the adsorption or absorption equipment becomes larger. Many problems remain before practical application, as it is necessary to regenerate the adsorbent or absorbent, treat by-products, and consider secondary pollution caused by waste liquid. Compared to these methods, the reduction method converts NOx into harmless N ₂ and H ₂ O using reducing gases such as ammonia, hydrogen, hydrogen sulfide, carbon monoxide, and hydrocarbons, and does not have the problems mentioned above. Development efforts are being made in various areas. One of these methods is to react NOx with reducing gas in a non-catalytic state at high temperatures.
It is not practical because the effective temperature range is limited under high temperatures, a large amount of residual reducing agent is emitted, and the amount of reducing agent required to make NOx harmless is many times that of NOx. It's not an effective method. by a catalytic reduction reaction applying a catalyst to the other one
There are NOx detoxification methods (hereinafter abbreviated as denitrification), which can be divided into two types depending on the choice of reducing agent.
The selective catalytic reduction method, which is not affected by the presence or absence of oxygen in the exhaust gas, is considered to be economically advantageous because the reaction proceeds at a relatively low temperature and with the same amount of reducing agent as NOx. The present invention relates to a catalyst composition applied to the reaction of NH ₃ as a reducing agent and NOx, and the features of the present invention will be described in detail below. Conventionally, the composition of the catalyst applied to the selective catalytic reduction process is alumina, silica, titania,
It is known that oxides or sulfates of base metals in addition to noble metals are effective as activators when using porous materials such as zirconia, mixtures thereof, diatomaceous earth, zeolites, etc. alone or in combination. However, since alumina-based catalysts are poisoned by sulfur oxides, they have a short catalyst life and may not be economical when treating a large volume of exhaust gas. In general, combustion exhaust gas containing sulfur oxides contains soot and dust, and when the process gas is passed through the catalyst bed in a reactor filled with normal granular catalysts, this dust causes blockage, making it difficult to put into practical use. do not have. To solve these problems, durable carriers such as titania and zirconia are used to treat poisoning caused by sulfur oxides, and moving bed reactors that move granular catalysts and specially shaped catalysts are used to deal with dust. For example, honeycomb shapes and hollow cylindrical shapes are used. However, with these measures, the cost of materials such as titania is high, the specific gravity of the catalyst is high and the reactor support frame needs to be strengthened, and the construction of a moving bed reactor is complicated and due to site constraints for installation. ,
There are many issues that need to be resolved, such as the difficulty of installing a denitrification device to existing equipment and the poor formability of titania and zirconia. Nevertheless, it cannot be ignored that titania exhibits extremely high performance by supporting oxides or sulfuric compounds of iron, cobalt, nickel, manganese, chromium, molybdenum, tungsten, vanadium, copper, etc. In view of these points, it is an object of the present invention to provide a catalyst that can be applied to any location, has free moldability, is durable against poisonous components, is easy to regenerate, and is lightweight and inexpensive. . In addition, the catalyst of the present invention can be formed into any shape and can be processed into a thin plate shape, and the specific gravity is also low.
Because it is light at 0.2 to 0.5 g/cc, it is placed in the existing flue and the exhaust gas is brought into contact with a plate-shaped catalyst in parallel flow, and NOx in the exhaust gas can be rendered harmless using NH ₃ as a reducing agent, making it a special denitrification reaction. This eliminates the need for equipment, greatly reduces equipment costs, and facilitates the prevention of air pollution. Furthermore, because the exhaust gas is brought into contact with the plate-shaped catalyst in parallel flow, there is no reduction in activity due to adhesion or blockage of soot and dust, unlike in conventional fixed bed reactor catalysts that include a moving bed, and the catalyst has a long lifespan. It has become possible to make the device extremely inexpensive. Based on the above recognition, the present inventors have developed a catalyst for denitrification that not only has high activity, but also durability, especially resistance to poisoning by sulfur oxides, light weight, and the ability to be easily molded into any shape. In addition, we conducted extensive research to find new catalysts, keeping in mind the need to reduce catalyst costs, including material costs and manufacturing costs. Calcium acid, titania (TiO ₂ ), -B group (vanadine group elements), -B group (chromium group elements), -B group (manganese group elements) of the periodic table,
Supporting a catalyst component consisting of at least one of the oxides of base metals of the transition metal group and copper not only shows extremely good performance but also has the advantage of being able to be molded into various shapes due to good moldability. It was found that There are two methods for producing calcium silicate molded bodies, which are the base material of this catalyst.One is to suspend silicic acid raw materials and calcareous raw materials in water, and add clay and glass fiber to the slurry obtained. The other method is to add a reinforcing material such as asbestos and form it, then hydrothermally treat it in an autoclave to crystallize and harden the calcium silicate hydrate. A mixture obtained by suspending the mixture in water and adding reinforcing materials such as clay, glass fiber, and asbestos as necessary is subjected to a hydrothermal synthesis reaction while heating and stirring under pressure, or by intermittent stirring or discontinuing the stirring. This is a method of obtaining a molded body by molding and drying calcium silicate hydrate obtained by promoting crystallization. Although either method can be applied to the catalyst of the present invention, the latter is preferable since it can be molded into any shape. Possible methods for supporting the catalyst component include a method in which the catalyst component is coated on the surface of the molded article obtained as described above, and a method in which the catalyst component is mixed with calcium silicate slurry and molded. On the other hand, molybdenum, vanadium,
Oxides of manganese, iron, copper, and chromium alone or in combination (West German patent 1253685)
(1963.1.22), 1259298 (1964.8.29)) are reported to be valid. Furthermore, the effects of combining oxides of iron, cobalt, nickel, copper, vanadium, chromium, manganese, and tungsten with silica, titania, and zirconia have been reported in JP-A-49-97794. When applying these catalysts to SOx-containing exhaust gas, catalysts with TiO ₂ as a carrier are optimal because they have SOx resistance and high performance.
Particularly preferred is TiO ₂ having an anatase crystal structure. However, in order to form TiO ₂ into granular or hollow cylindrical shapes while keeping it in the anatase form, it is possible to granulate it using a rotary or tableting method, but when the temperature exceeds 700-800℃, the crystal structure changes to a rutile type. It is difficult to manufacture strong materials. Also, considering the blockage caused by soot and dust in the exhaust gas, reactors using granular catalysts are impractical, and extrusion molding into dust-free honeycomb or pipe shapes is not possible, but due to crystal structure limitations, high-strength carriers are not practical. Hard to get. Due to the difficulty of molding TiO ₂ to high strength, it is difficult to form TiO ₂ into plate-like products that are not affected by soot and dust, for example, 1 m x 1 m in size.
Molding 1m x 10mm thick is not easy either. moreover
TiO ₂ generally has a high specific gravity (approximately 1.0 to 1.2), making it difficult to handle, and has disadvantages such as the need to design a reactor with high strength and high cost. From the above point of view, the catalyst of the present invention has a composition in which TiO ₂ and active ingredients are added to the base material of calcium silicate synthesized from limestone and silica sand, which are abundant and inexpensive in Japan, and are extremely light. It has a specific gravity of about 0.3 to 0.6 g/cc, can be molded into any shape, provides an extremely inexpensive catalyst, and has excellent denitrification performance due to the addition of TiO ₂ and active ingredients. It has characteristics.
Therefore, there is an advantage that the support frame and other steel materials required for filling the reactor with the catalyst can be provided in small amounts. This will be explained in detail below using examples. Example 52 parts of silica sand powder, 44 parts of quicklime and 4 parts of asbestos are mixed with 1100 parts of water.
This was placed in a sealed autoclave with a stirrer and pressurized to an internal pressure of 12 kg/cm ² and a temperature of 191°C.
The reaction was carried out while stirring at a stirring rate of 100 times/min for 5 hours to obtain a calcium silicate slurry containing xonotlite (6CaO.6SiO _{2.H 2} O) as a main component. To 100 parts of this slurry, 10 parts of glass fiber and 1200 parts of anatase-type TiO ₂ powder (average particle size 1.2μ) were added, dehydrated and molded in a metal frame, and dried to a specific gravity of 0.4 and a thickness of 7.
A molded body with a weight ratio of TiO ₂ and 6CaO·6SiO ₂ ·H ₂ O of 9.2 was obtained using calcium silicate as a base material with a width of 1 m, a width of 1 m, and a length of 1 m. Here, adding 1.2 times more TiO ₂ powder to the slurry is done during dehydration molding.
This is because losses due to outflow of TiO ₂ powder are expected. In addition, a method of converting into anatase-type TiO ₂ by heating a base material carrier mixed with titanium hydroxide obtained by treating a slurry of calcium silicate crystals with titanium sulfate or titanium chloride, or a slurry of calcium silicate crystals. The desired molded product can also be obtained by adding glass fiber to the product, dehydrating it in a metal frame, drying it, and then applying titanium sulfate, titanium chloride, or titanium alcoholate to it and heating it. Furthermore, it is also possible to obtain the desired molded body by applying anatase-type TiO ₂ powder in a slurry state to the surface of the molded body. Although there are many types of crystal forms of calcium silicate obtained here, it has been found that the value of x/y of xCaO·ySiO ₂ is preferably 5/6 to 2 from the relationship between formability and strength as a catalyst. In addition, from the viewpoint of fire resistance, the smaller the value of [H ₂ O]b, the more stable it is, and the preferable and practical b/a is 1 or less. Next, regarding the required amount of TiO ₂ , if c/a is less than 1, the activity is poor, and if c/a is more than 10, the weight of the catalyst carrier becomes large and the cost of raw materials increases, which is not preferable from an economic point of view. Regarding the activation of the carrier obtained in the above manner, Fe ₂ O ₃ , Co ₂ O ₃ ,
NiO, _MnO2 , _Cr2O3 , _MoO3 , _WO3 , _{V2O5 , CuO}
Ammonium salts that can be used as raw materials for active substances such as
The purpose is achieved by dissolving sulfate, nitrate, etc. in a solvent such as water and impregnating it, or by mixing these raw material powders or the powder of the oxide itself with TiO ₂ and adding it during the carrier manufacturing process and firing. Regarding the amount of active substance, d/a in [MO] d
If it is less than 0.005, sufficient activity cannot be obtained,
If it is more than 1.0, it is not desirable because the economical efficiency deteriorates compared to the improvement in activity ability. When selecting a catalytically active component, it is necessary to use an activator that has excellent catalytic performance and suppresses side reactions other than the intended reaction, particularly reactions that adversely affect rear equipment. Catalytic properties due to differences in catalytic active components are largely influenced by the properties of the exhaust gas used, such as the presence or absence of SOx and H ₂ O, but are also greatly influenced by gas temperature. In other words, an active component that exhibits excellent catalytic properties over a wide temperature range is required for exhaust gas that changes in load, such as in a commercial boiler, while an active component that exhibits excellent catalytic properties over a wide temperature range is required when the load is kept constant, such as in a private boiler. Active ingredients that exhibit good performance at temperature are sufficient. SO ₂ is often present in the exhaust gas that normally requires denitrification, and reacts with the coexisting O ₂ to convert SO ₂ to SO ₃ on the catalyst. At this time, the reaction with NH ₃ used as a reducing agent in the denitrification reaction results in (NH ₄ ) ₂ SO ₄ or NH ₄ HSO ₄ adhering to the gas, resulting in gas blockage, corrosion of metal materials, and increased dust to strengthen the dust collector. Alternatively, problems such as SO ₃ being released into the atmosphere may arise, so the conversion of SO ₂ to SO ₃ must be suppressed as much as possible. In addition to the above-mentioned Co ₂ O ₃ , MoO ₃ , and WO ₃ , oxides that have this suppressing effect include SnO ₂ and ZnO, and these can also be added as SO ₂ oxidation inhibitors. In order to confirm the catalytic properties of the catalyst of the present invention due to differences in the active components, anatase-type TiO ₂ and the catalytic active components shown in Table 2 were added to the above-mentioned calcium silicate-based molded body with a size of 500 mm x 500 mm. Catalysts 1 to 12 supporting the above were obtained. Note that the ratio of TiO ₂ to the catalytically active component at this time was 9:1. These catalysts were placed in a reactor with a catalyst layer length of 5 m in which the catalyst spacing was 12 mm, and the amount of gas to be treated (Nm ³ /H)/catalyst area (m ² ) = 10
NH ₃ /NOx = 1.0, linear velocity 4
Heavy oil-fired boiler exhaust gas (gas amount) shown in Table 1 in m/sec
500Nm ³ /H) was brought into contact with the catalyst in parallel, and the catalyst performance shown in Table 2 was obtained. The gas temperature can be changed by operating the boiler and using a heater.
It shows the catalyst characteristics with respect to temperature.

【table】

[Table] Next, in order to confirm the oxidation suppressing effect of SO ₂ , the results of measurements for catalysts 1, 6, 7, 8, 10, 11, and 12 are shown in Table 3. The test conditions at this time were the same as those used when measuring the denitrification performance, and the conversion rate of SO ₂ to SO ₃ was calculated as: outlet SO ₃ - inlet SO ₃ /inlet SO ₂ ×100. It can be seen that the addition of WO ₃ and MoO ₃ greatly suppresses the generation of SO ₃ .

【table】

【table】

Claims (1)

[Claims] 1. [xCaO・ySiO ₂ ]a [H ₂ O]b [TiO ₂ ]c as a catalyst applied to a process for catalytic reduction removal of nitrogen oxides using ammonia as a reducing agent.
[MO] has the structure of d, b/a=O~1, c/
a=1 to 10, d/a=0.005 to 1.0, x/y is preferably 5/6 to 2, and MO is a base metal of -B group, -B group, -B group, group of the periodic table. A catalyst composition comprising at least one oxide of a transition metal and copper.