JPS6211617B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an exhaust gas treatment catalyst for removing NOx in boiler exhaust gas containing NOx, SOx, fly ash, etc. and at the same time efficiently removing dust from fly ash, etc., and particularly improves the dust collection performance of fly ash, etc. dust. The present invention relates to a catalyst for treating exhaust gas. in exhaust gas
Selective catalytic reduction using NH ₃ as a reducing agent is currently the mainstream method for removing NOx. Therefore, such as coal-fired boiler exhaust gas, etc.
The above method is used to treat exhaust gas containing dust such as NOx, SOx and fly ash.
There is a trend toward using a process that combines a NOx removal device and an electrostatic precipitator for removing dust such as fly ash. This is the first treatment process for coal-fired boiler exhaust gas that is currently being implemented.
There is something like the one shown in the figure. In other words, the exhaust gas emitted from the coal-fired boiler 1 is
After entering the electrostatic precipitator 5, the fly ash in the exhaust gas is removed by the electrostatic precipitator 5, and NH ₃ is injected from the NH ₃ injection means 2 into the exhaust gas exiting from the electrostatic precipitator 5. into the exhaust gas.
After removing NOx, the exhaust gas is introduced into the heat exchanger 4 for heat exchange, the temperature of the exhaust gas is lowered, and the exhaust gas is sent to a downstream desulfurization device or a chimney. According to this treatment process, since the electrostatic precipitator 5 is placed upstream of the heat exchanger 4, when collecting flyash with high electrical resistance, reverse ionization occurs due to a decrease in exhaust gas temperature. However, the treatment process has the following drawbacks. (1) The S content in the coal was oxidized in the coal-fired boiler 1 and SO ₂ was generated, and some of the SO ₃ was further converted and leaked from the denitrification device 3 in the heat exchanger 4. Since it reacts with NH ₃ and moisture in the exhaust gas to generate acidic ammonium sulfate (NH ₄ HSO ₄ ), this acidic ammonium sulfate melts and adheres, causing clogging of the heat exchanger 4. (2) The reaction product of ammonium sulfate (NH ₄ ) ₂ SO 4 produced by the reaction of SO ₃ with NH ₃ and moisture in the exhaust gas becomes fine dust at the outlet of the heat exchanger ₄ as the exhaust gas temperature decreases. ,for example
Approximately 50 mg/Nm ³ of dust is generated from 10 ppm of SO ₃ , which increases the amount of dust in the exhaust gas. Therefore, with the aim of providing an exhaust gas treatment method for coal-fired boilers that does not have the above-mentioned drawbacks,
We proposed a method as shown in the flowchart in Figure 4 (see Japanese Patent Application No. 52-81144 and Japanese Unexamined Patent Publication No. 16737-1983).
In FIGS. 2 to 4, the same symbols as in FIG. 1 are the same devices as in FIG. 1, and 6 is a mechanical dust collector. However, the above method has been found to have the following drawbacks through subsequent studies by the present inventors. That is, since the electrostatic precipitator 5 is installed downstream of the heat exchanger 4, the temperature of the exhaust gas entering the electrostatic precipitator 5 is usually lowered to 100 to 160°C, which changes the properties of the dust in the exhaust gas. As a result, the electrical resistance exceeds a so-called reverse ionization starting resistance value, and a reverse ionization phenomenon occurs within the electrostatic precipitator 5, which may significantly reduce the dust collection performance of the electrostatic precipitator 5. For example, as shown in curve 1 in FIG. 5, when low sulfur coal is burned in the conventional process shown in the flowchart in FIG. 1, the operating temperature range of the electrostatic precipitator 5 is a quite high range, Is the electrical resistance of the dust within the normal electrostatic collection resistance range [A]?
In the above method shown in the flowcharts of Figs. 2 and 3, the operating temperature range of the electrostatic precipitator 5 is quite low (b), so the electrical resistance of the dust falls within the reverse ionization resistance range [B] and the dust is collected. Dust performance is significantly reduced. This performance degradation phenomenon occurs noticeably when low-sulfur coal is burned in a coal-fired boiler, as shown in curve 1 in Figure 5 (curve 2 in Figure 5 shows the case when high-sulfur coal is burned). ). The reason is,
This is due to the fact that the amount of SO ₃ and H ₂ SO ₄ generated by SO ₃ , which are involved in surface conduction which mainly controls the electrical resistance of the fly ash in this temperature range, is not sufficient in the combustion of the above-mentioned low sulfur coal. Generally, in coal-fired boiler 1, the amount of _SO3 is considered to be approximately 1% of the total amount of SOx in the exhaust gas, and SOx
The amount is proportional to the combustible sulfur content in the coal. On the other hand, the catalyst used in the above-mentioned selective catalytic reduction method, which is currently considered to be mainstream, has a NOx reduction effect in the presence of NH ₃ (4NO + 4NH ₃ + O ₂ → 4N ₂ +
6H ₂ O), as well as the oxidizing effect of SO ₂ (2SO ₂ + O ₂ →
2SO ₃ ). However, conventionally, such catalysts have been used to reduce SO ₃
There has been a tendency to avoid the use of such catalysts because the generation of large amounts of these catalysts causes the drawbacks mentioned in (1) and (2) above. However, it is possible to improve the dust collection performance of the electrostatic precipitator by providing an appropriate SO ₂ → SO ₃ conversion effect in the denitrification device installed in front of the electrostatic precipitator, as well as to avoid excesses that may cause secondary pollution. of
Excess during SO ₂ → SO ₃ conversion and high sulfur coal combustion
An application has already been filed for an invention aimed at generating an amount of SO ₃ necessary for improving the above-mentioned dust collection performance while preventing SO ₃ generation. (Special application 1973-3308
(Refer to Special Publication No. 58-3730). That is, (the invention of Patent Application No. 53-3308 is an apparatus for treating exhaust gas using a denitrification device for catalytic reduction method and an electrostatic precipitator installed downstream of the denitrification device via a heat exchanger, This exhaust gas treatment device is characterized in that the denitrification device generates SO ₃ by being filled with a catalyst for the reduction reaction of NOx and a catalyst having an action of converting SO ₂ into SO ₃ . In order to carry out such a device, it is necessary to use a conventional catalyst for the reduction reaction of nitrogen oxides using NH ₃ as a reducing agent and simultaneously convert SO ₂ coexisting in the exhaust gas into SO ₃ (2SO ₂
There is a need for a catalyst that promotes +O ₂ →2SO ₃ ) with high efficiency. In addition to the high denitrification reaction, the catalyst of the present invention also reduces SO ₂ to SO ₃
This makes it possible to adjust SO ₃ as necessary even in coal combustion, which produces dust with extremely poor collection properties. This contributes to effective exhaust gas treatment by making it more compact. In addition, in the case of a process that targets high-dust exhaust gas as shown in Figures 2 to 4, the denitration equipment 3
Even if SO ₃ is increased, acidic sulfur ammonium will be absorbed into the heat exchanger 4 due to the brushing effect of the fly ash.
This invention is based on the fact that it can be used without adhering to the surface. The catalyst performance of the present invention will be explained in detail below using Examples. Examples Gas compositions for measuring the catalyst performance of the present invention are shown in Table 1, and test conditions are shown in Table 2.

【table】

[Table] It is known from US Pat. No. 3,279,884 (1964) that V ₂ O ₅ is effective as a denitrification catalyst.
Alumina (Al ₁ O ₃ ) is known as a carrier, but when SOx is treated as in this example, it is converted to alumina sulfate (Al ₂ SO ₄ ) and becomes deactivated. The inventors aimed at a titania (TiO ₂ ) carrier having SOx resistance, but found it difficult to granulate it. We searched for a molded titania carrier that has excellent granulation properties, maintains high activity, and found that the carrier manufactured by Japanese Patent Publication No. 1983-44431 is superior. V ₂ O ₅ was added to this carrier (particle size 4 to 6 mm) at 1, 5,
Prepare catalysts with 7, 9, and 13% (by weight) added,
The performance evaluated under the conditions shown in Tables 1 and 2 is shown in FIG.
It is sufficient that the amount of V ₂ O ₅ on TiO ₂ that affects the denitrification performance is about 7% by weight. If the amount of V ₂ O ₅ increases, the reaction from SO ₂ to SO ₃ will be promoted, but there is no merit in increasing the amount to 10% or more because the denitrification performance will decrease. Furthermore, if the amount of V ₂ O ₅ is extremely small, the reaction from SO ₂ to SO ₃ will be slight and the denitrification performance will be low, so the amount of V ₂ O ₅ can be said to be 3 to 10% by weight, preferably 5 to 8% by weight. . Next, of these catalysts, V ₂ O ₅ is 7% (by weight)
Added CuSO ₄ to 1, 3, 5,
Figure 7 shows the results of evaluating the denitrification performance and the reaction rate of SO ₂ → SO ₃ when 7. (weight)% was added under the conditions shown in Tables 1 and 2. This test shows that although V ₂ O ₅ alone, which is used in conventional sulfuric acid catalysts, promotes the reaction from SO ₂ to SO ₃ , the addition of CuSO ₄ has a significant effect. However, even if the amount of CuSO ₄ added is increased to 5% by weight or more, the effect becomes small and the denitrification performance decreases. The cause of this decline in denitrification performance is currently being determined, but
It is thought that NH ₃ , which is a NOx reducing agent, is being burned at the reaction temperature of 380°C, which is not a desirable phenomenon. Also, if the amount of CuSO ₄ added decreases, the reaction rate of SO ₂ → SO ₃ will decrease, so from the purpose of the present invention,
The amount of _CuSO5 added is 1 to 5% by weight, preferably 3 to 5%.
% by weight. However, the amount of these catalyst components added depends on the required denitrification performance and SO ₂ →
It is determined by the SO ₃ reaction rate, and the optimum amount also varies depending on the selection of processing conditions. Reference example Next, a commercially available K-V catalyst (5S manufactured by Nippon Shokubai Kagaku Kogyo Co., Ltd.) was crushed into 4 to 6 meshes.
Evaluation was made under the conditions shown in 1 and 2, and the conversion rate of SO ₂ to SO ₃ was 25%, but the denitrification rate was 10%. From this, it can be seen that the catalyst of the present invention is a novel catalyst having an action completely different from that of conventional denitrification catalysts and sulfuric acid catalysts, even though they are of the same V type.

[Brief explanation of the drawing]

Figures 1 to 4 are flowcharts showing a conventional coal-fired boiler exhaust gas treatment method, Figure 5 is a chart showing the relationship between the operating temperature of an electrostatic precipitator and the electrical resistance of dust, and Figures 6 to 7 are diagrams of the present invention. 1 is a graph showing the catalyst characteristics of . 1...Boiler, 2...Ammonia injection means, 3
... Denitrification device, 4 ... Heat exchanger, 5 ... Electrostatic precipitator, 6 ... Mechanical dust collector.

Claims (1)

[Claims] 1 In exhaust gas treatment, denitrification reaction using ammonia as a reducing agent is carried out, and at the same time, the coexisting SO ₂ is removed.
TiO ₂ − characterized by promoting its conversion to SO ₃
Catalyst consisting of V ₂ O ₅ −CuSO ₄ .