JPS628203B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for removing NOx from combustion exhaust gas containing NOx and SOx.

Generally, in the dry ammonia catalytic reduction denitrification method, the following gas adsorption reactions are said to occur competitively on the catalyst.

NH ₃ NH ₂ +HNH+HN+H NO+ONO ₂ NO+NH ₂ →N ₂ +H ₂ O NO ₂ +NH ₂ →N ₂ +H ₂ O SO ₂ +O→SO ₃ SO ₂ +NO→SO ₃ +N SO ₃ +NH ₃ +H ₂ O→NH ₄ HSO ₄ (or NH ₄ H ₃
(SO ₄ ) ₂ , (NH ₄ ) ₄ H ₂ (SO ₄ ) ₃ , (NH ₄ ) ₃ H(SO ₄ ) ₂ ,
(NH ₄ ) ₂ SO ₄ )SO ₂ +NH ₃ +H ₂ O→NH ₄ HSO _3For these reactions, physical and chemical changes in the catalyst itself due to the reaction between SOx and catalyst components, e.g.
In Al ₂ O ₃ supported catalysts, Al ₂ (SO ₄ ) ₃ , NH ₄ Al
Deterioration in performance due to the production of (SO ₄ ) ₂ and performance deterioration due to reaction products of SOx and ammonia coating the catalyst surface have become major problems. In order to solve these problems, conventionally SOx
In order to prevent reactions between SOx and catalyst components, or between SOx and ammonia, there are two methods: (1) removing SOx in advance and (2) adjusting the temperature of the exhaust gas or catalyst surface to the thermal decomposition temperature of the ammonium sulfate compound. There is a way to maintain the temperature above 300℃. In addition, in order to remove the catalyst surface reaction products of SOx and ammonia, (3)
A method of washing and drying with either ammonia water, an inorganic alkaline aqueous solution, or an organic amine,
Methods (4) and (3) in which the catalyst is further impregnated with a catalyst component and fired, and (5) a method in which the catalyst is heated in air, reducing gas, or ammonia gas are known. However, both methods have the following drawbacks.
That is, the method (1) described above requires exhaust gas desulfurization equipment and exhaust gas heating equipment and increases running costs, and the method (2) requires exhaust gas heating equipment as described above and has high running costs. Method (3) requires washing equipment, drying equipment, and water treatment equipment, and also has high running costs. Furthermore, methods (4) and (5)
In addition to method (3), catalyst impregnation and calcination equipment and catalyst heating equipment are required, and both have the major drawback of requiring enormous equipment costs and running costs. The present invention aims to provide an extremely excellent method that eliminates these drawbacks.

In other words, the present invention focuses on the fact that the combination of SOx and ammonia is extremely slow in the gas phase, but almost instantaneously in the liquid phase. The purpose is to prevent the deterioration of the catalyst by absorbing and dissolving it and reacting with ammonia to form ammonium sulfate-based compounds, thereby preventing free SOx from reaching the catalyst surface.

The present invention will be explained below with reference to the drawings. FIG. 1 shows a process diagram according to the present invention. When the combustion exhaust gas containing NOx and SOx generated from the combustion device 1 is sent through the flue 2 to the denitrification catalyst tower 7, an ammonia water injection pipe 3 is installed in the flue 2 at a certain distance from the denitrification catalyst tower 7. The pipe 3 is connected to an ammonia water tank 4, and ammonia water is added in the form of a mist from a jet nozzle 6 via a pump 5 to an appropriate location in the flue 2 to achieve the object of the present invention. The exhaust gas introduced into the flue is reacted with SOx and aqueous ammonia to generate an ammonium sulfate compound, thereby preventing the reaction between SOx and ammonia on the surface of the catalyst layer 8, and then passing through the denitrification catalyst tower. NOx in the exhaust gas is removed in the catalyst tower 7 and then released from the chimney 9 via the flue 10. As in the above process, by reacting SOx in the exhaust gas with ammonia in advance, it forms on the catalyst surface.
In addition to preventing SOx from reaching the system, the reaction products are continuously denitrated without being taken out of the system at all times.To this end, it is necessary to prevent the temperature of the exhaust gas from decreasing as much as possible, so the minimum amount of ammonia water is required. It is introduced in the form of minute droplets so that it can be carried by airflow and react effectively. The reason why ammonia water was added in the form of minute droplets with a maximum particle size of 300μ or less is that when the reaction products of SOx and NH ₄ OH are produced in the same way, the larger the droplet size, the smaller the unit liquid. As the surface area per volume decreases, the concentration of reaction products near the mist surface increases, and the amount of NH ₃ evaporated increases due to long-term retention, resulting in SOx and NH ₄ OH
Because the reaction is rate-limiting on the liquid side, SOx and
This is because the reaction efficiency of NH ₄ OH decreases. In other words, the mist evaporates before SOx diffuses into the liquid, resulting in a decrease in reaction efficiency. Therefore, it is necessary to know the relationship between the evaporation rate of droplets and the instantaneous diffusion rate of SOx into aqueous ammonia, so the relationship between the evaporation rate or SOx diffusion rate and the mist diameter is shown in FIG. According to FIG. 2, when the exhaust gas temperature is 300°C and the droplet diameter is 300μ, the evaporation rate is greater than the SOx diffusion rate. In other words, at an exhaust gas temperature of 300℃
The curve shows that when the mist diameter is 300μ or less, the time required to complete evaporation is always longer than the time required for SO ₂ to diffuse throughout the mist. Of course, at an exhaust gas temperature of 150°C or 200°C, the time is always longer than the time required for SO ₂ to diffuse throughout the mist. Based on this relationship, when the exhaust gas temperature is limited to 150 to 300°C, the maximum mist diameter must be limited to 300μ as shown in Figure 2.

Next, the reason why the exhaust gas temperature is limited to 150°C to 300°C is that below 150°C, the denitration performance of the catalyst decreases and the desired denitration effect cannot be obtained at all.

In addition, in a temperature range exceeding 300℃, ammonium sulfate compounds, which are reaction products, decompose, and as shown in Figure 2, the evaporation rate of the mist becomes faster than the rate at which SO ₂ diffuses throughout the mist. The effective utilization of NH ₄ OH decreases, resulting in SO ₂ and NH ₄ OH in the mist.
This is because the rapid reaction with NOx is restricted, so SOx reaches the catalyst surface and the efficiency of NOx detoxification is significantly reduced. Furthermore, the reason why it takes at least 2 to 10 seconds for the mist to reach the catalyst is that when the mist reaches the catalyst, the temperature of the catalyst decreases due to its latent heat of vaporization, and the droplets adhere to the catalyst surface.
It impedes the adsorption of NOx and NH ₃ and extremely reduces catalytic performance. Therefore, as shown in Figure 2,
The time it takes for a 300μ mist to completely evaporate at 300℃ is 2
It takes about 10 seconds for a 300μ mist to completely evaporate at 150°C. From this relationship, when the temperature of exhaust gas is within the range of 150℃ to 300℃, it is 2 seconds to 10 seconds, so it is 2 seconds to 10 seconds before reaching the catalyst.
It is necessary to add it at least at the second position.

Examples according to the present invention will be described below.

Example 1 Ammonia is injected into the flue of the exhaust gas, NOx concentration 200ppm, SOx concentration 30ppm, exhaust gas temperature 200℃, exhaust gas flow rate 200Nm ³ /H.Ammonia concentration 250ppm, ammonia droplet diameter 200μ mist is placed 10 seconds before the denitrification catalyst tower. Addition was injected. As a result, the initial denitration rate was 95%, and the adhesion of ammonium sulfate compounds to the catalyst surface was reduced.
Even after 10 hours had passed, only a small amount was detected, and no decrease in the denitrification rate was observed.

Example 2 NOx concentration of exhaust gas is 200 ppm, SOx concentration is 30 ppm, exhaust gas temperature is 300°C, exhaust gas flow rate is 200 Ncm ³ /H, and ammonia is injected into the flue of the flue gas with a concentration of 200 ppm and a mist with an ammonia droplet diameter of 300 μm 2 seconds before the denitrification catalyst tower. The mixture was added and injected. As a result, the denitrification rate was 97
%, and the adhesion of ammonium sulfate compounds to the catalyst surface was only slightly detected even after 10 hours of use.

As described above, compared to the conventional addition method, the catalyst performance can maintain a high denitrification rate for a long time even at low temperatures, so it has become possible to significantly reduce the denitrification energy cost and the catalyst regeneration cost.

[Brief explanation of the drawing]

FIG. 1 shows a process diagram according to the present invention, and FIG. 2 shows a relationship curve between the evaporation completion time or the SO ₂ diffusion completion time and the mist diameter. 1... Combustion device, 2... Flue, 3... Ammonia water injection pipe, 4... Ammonia water tank, 5
... pump, 6 ... blowout nozzle, 7 ... denitrification catalyst tower, 8 ... catalyst layer, 9 ... chimney, 10 ... flue.

Claims (1)

[Claims] 1 NOx in combustion exhaust gas containing NOx and SOx
In the process of catalytically reducing and removing nitrification in the presence of a denitrification catalyst, ammonia water is used as a reducing agent in a mist state with a maximum particle size of 300μ or less at 150℃~
Combustion characterized by removing NOx while preventing the reaction between SOx and ammonia on the catalyst surface by adding it to exhaust gas at 300℃ at least 2 seconds to 10 seconds before it reaches the catalyst. How to treat exhaust gas.