JPS6113060Y2 - - Google Patents

Description

[Detailed Description of the Invention] This invention relates to an exhaust gas treatment device, and more particularly to an exhaust gas treatment device that removes nitrogen oxides by ammonia reduction in the presence of a catalyst.

Methods to remove and reduce harmful components from exhaust gas containing nitrogen oxides (NOx) and sulfur oxides (SOx), such as boiler exhaust gas, are called denitrification and desulfurization, and the practical use of equipment for each is progressing. ing. For denitrification, ammonia reduction in the presence of a catalyst at 250 to 400°C is widely used, and for desulfurization, a wet absorption method using limestone slurry is the main method.

Since denitrification is a catalytic reaction, improving the activity of the catalyst is key to improving the performance of the equipment. Some of the catalysts that have been put into practical use so far are poisoned by SOx in exhaust gas, resulting in a decrease in catalytic activity. In response, sulfur oxide-resistant catalysts have been developed, for example, based on titanium oxide (TiO ₂ ),
Catalysts to which transition elements such as vanadium (V), molybdenum (Mo), and tungsten (W) are added have been put into practical use.

A phenomenon common to these catalysts is that when sulfur trioxide (SO ₃ ) coexists in the gas, the catalytic activity actually increases, but conversely, in a low-concentration SOx atmosphere, the activity decreases over time. This phenomenon occurs particularly with titanium-molybdenum catalysts (Ti-Mo) and titanium-tungsten catalysts (Ti-W).

On the other hand, dust in the exhaust gas generated from large combustion equipment is usually removed using an electrostatic precipitator, but if we take lime combustion exhaust gas as an example, the dust is alkaline and has an apparent specific electrical resistance of 10 ¹² Ωcm. Since it is a high value of 30 to 40 ppm
of SO ₃ was added from outside the system, and the specific electrical resistance was reduced to 10 ¹¹ Ω.
Humidity control is being carried out to improve dust collection efficiency by lowering the humidity to less than cm. For this, SO ₃ in the system
If it were possible to generate and control humidity, the process would be extremely streamlined.

The purpose of this invention is to eliminate the drawbacks of the conventional technology mentioned above, and to eliminate NOx without supplying SO ₃ from outside the system.
It is an object of the present invention to provide an exhaust gas treatment device that improves the activity of a reduction catalyst and also makes it possible to control the humidity of alkaline dust such as lime combustion exhaust gas.

Even in titanium oxide-based catalysts that are resistant to sulfur oxides, catalysts that do not contain sulfate groups have low activity. Therefore, as a way to improve activity, we developed a catalyst using a substance containing sulfates as a starting material. As a result, it was found that under gas conditions with low SOx concentration, especially low SO ₃ partial pressure, the number of sulfate groups in the catalyst decreases over time, resulting in a decrease in catalytic activity.
This is because the sulfate groups in the catalyst interact with the humidity during the denitrification reaction.
It increases and decreases depending on the SO ₃ partial pressure. Therefore, from these results, it was found that the catalyst activity can be maintained at a high level by keeping the SO ₃ concentration in the gas at an appropriate level.

In a titanium oxide-based denitrification catalyst device, the sulfur dioxide oxidation catalyst layer is installed upstream of the nitrogen oxide reduction catalyst layer in the denitrification reactor in order to increase the concentration of SO ₃ in the reaction gas to a certain level or higher. This prevents the above-mentioned decrease in sulfate groups over time and the decrease in catalytic activity caused by this.

Incidentally, it is generally considered undesirable to increase the SO ₃ concentration because it causes corrosion of steel materials and accumulation of dust in low-temperature parts. However, when containing strongly alkaline dust such as coal combustion exhaust gas, the influence of SO ₃ is small at low temperatures, and furthermore, in an electrostatic precipitator that removes dust, it reduces the specific electrical resistance of the dust, To improve the dust collection effect, SO ₃ is injected into the gas from outside the system to control humidity. The present invention allows SO ₂ to be removed within the system without injecting SO ₃ from the outside as described above.
oxidizes and produces NOx as SO ₃ concentration above a certain concentration.
This not only improves the activity of the reduction catalyst, but also makes it possible to control the humidity of alkaline dust such as coal combustion exhaust gas.

This invention will be explained in more detail below with reference to drawings and examples.

FIG. 1 is a system diagram of an apparatus showing an embodiment of this invention. The gas discharged from the combustion device 1 is added with ammonia 12 while passing through the flue 11 and then introduced into the denitrification reactor 2. Denitration reactor 2
consists of a nitrogen oxide reduction catalyst layer 22 and a sulfur dioxide oxidation catalyst layer 21 provided on the gas upstream side thereof. In this reactor 2, a part of the SO ₂ in the exhaust gas is converted into SO 3 by the SO ₂ oxidation catalyst layer 21 installed on the upstream side of the gas, and the SO 2 is converted into SO ₃ in the denitrification catalyst layer 22 with the SO ₃ concentration increased. The NOx reduction reaction progresses. The denitrified exhaust gas is led to the air preheater 3 through the flue 23, where heat is exchanged, and then led from the flue 31 to the electrostatic precipitator 4 to remove dust. The dust-removed exhaust gas is discharged from the flue 41 to the chimney. Note that ammonia, which is a reducing agent for the denitrification reaction, is mixed into the flue gas through the line 12, but may be injected into the low temperature region of the combustion device in order to improve gas mixing.

Example 1 Using the above equipment system (hereinafter referred to as A system) with a capacity of 50Nm ³ /h, crude oil combustion exhaust gas containing 100ppm SOx, 150ppm nitric oxide (NO) and 2% oxygen (SO ₃ concentration is 1ppm or less) A denitrification experiment was conducted. In reactor 2, a vanadium pentoxide system was used as an SO ₂ oxidation catalyst, and a Ti-Mo system was used as an NO reduction catalyst, each of which was formed into a 1 mm thick plate shape using a substrate, and gas was supplied at equal intervals of 5 mm. The reactor was loaded in an array parallel to the flow. The SO ₂ oxidation catalyst was installed on the upstream side of the gas flow, and the NO reduction catalyst was placed on the downstream side. The density of the catalyst layer was set to be 20,000 and 2,000 h ^-1 in terms of gas space velocity, respectively. SO ₂ for comparison
The B system was a device that was the same as the A system except that no oxidation catalyst was loaded. The gas temperature at the combustion device outlet was 350°C, and the reactor and flue were kept warm.

The SO ₂ oxidation rate in system A was set to approximately 5%, the initial SO ₃ concentration in the gas was maintained at around 5 ppm, and ammonia was injected at 150 ppm (NO/HN ₃ ratio = 1) upstream of the reactor to reduce NO. I went. As a result of investigating the change in NO reduction catalyst over time, the ratio of the reaction rate constant to the initial value, which indicates the change in activity over time, was 0.97 for the A system and 0.93 for the B system after 500 hours, and after 1000 hours. It was 0.95 for A strain and 0.86 for B strain.

These results revealed that when the SO ₃ concentration was increased using the SO ₂ oxidation catalyst (A system), the activity of the NO reduction catalyst could be maintained at a high level.

Example 2 A coal combustion exhaust gas test was conducted using the same equipment and catalyst as in Example 1. This exhaust gas SOx and
NO is 1500ppm and 250ppm respectively,
Oxygen was 2%. Reaction temperature is the same as Example 1
The temperature was 350℃.

The SO ₂ in the exhaust gas is 25 ppm, the SO ₂ oxidation rate in the A system is about 5%, and the total SO ₃ in the gas is
It was 100ppm.

After carrying out the NO reduction reaction under these conditions, the dust collection efficiency in the electrostatic precipitator was measured for the heat-exchanged exhaust gas that was cooled to around 150°C. As a result, the dust of system B, which does not have an SO ₂ oxidation catalyst, has poor dust collection performance, and the dust collection efficiency of system A is lower.
It was 0.8. In order to obtain the same efficiency as system A, approximately 40 ppm from outside the system must be removed at the dust collector inlet.
It was necessary to inject SO ₃ .

As is clear from these results, it was found that by increasing the SO ₃ concentration in the gas using an SO ₂ oxidizing catalyst, it was possible to efficiently collect dust in coal combustion exhaust gas without injecting SO ₃ from outside the system. .

As described above, according to this invention, the activity of the nitrogen oxide reduction catalyst, typified by the titanium oxide-based catalyst, can be maintained at a high level over time. This is particularly effective in treating exhaust gases with low SOx concentrations, such as crude oil and low-sulfur heavy oil exhaust gas. Furthermore, for dust that is alkaline and has high specific electrical resistance, such as coal combustion exhaust gas, it is possible to control the humidity within the system, increasing dust collection efficiency without injecting SO ₃ from outside the system.

[Brief explanation of the drawing]

FIG. 1 is a system diagram of an exhaust gas treatment apparatus showing an embodiment of this invention. 1... Combustion device, 2... Denitration reactor, 3... Air preheater, 4... Electrostatic precipitator, 21... SO ₂ oxidation catalyst layer, 22... Nitrogen oxide reduction catalyst layer.

Claims (1)

[Scope of Claim for Utility Model Registration] (1) In an exhaust gas treatment device that removes nitrogen oxides in exhaust gas containing nitrogen oxides and sulfur oxides by ammonia reduction in the presence of a catalyst in a denitrification reactor, An exhaust gas treatment device characterized in that a sulfur dioxide oxidation catalyst is installed upstream of a nitrogen oxide reduction catalyst installed inside the exhaust gas treatment device. (2) The exhaust gas treatment device according to claim 1 of the utility model registration claim, characterized in that an electrostatic precipitator is installed downstream of the denitrification reactor.