JPS583730B2 - Exhaust gas treatment method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an exhaust gas treatment method capable of efficiently collecting dust such as fly ash in exhaust gas containing NOx, SOx, fly ash, etc. and removing nitrogen oxides. The present invention relates to a method for treating the above exhaust gas that can improve dust collection performance.

Currently, the mainstream method for removing NOx from exhaust gas containing SOx as well as NOx is a selective dry catalytic reduction method using NH3 as a reducing agent.

Therefore, NOx and SO such as coal-fired boiler exhaust gas, etc.
As a method for treating exhaust gas containing dust such as x and fly ash, there is a tendency to use a combined process of a NOx removal device according to the above method and an electrostatic precipitator for removing dust such as fly ash.

As a treatment process for coal-fired boiler exhaust gas that is currently being implemented, there is a process as shown in Figure 1.

That is, the exhaust gas emitted from the coal-fired boiler 1 enters the electrostatic precipitator 5, fly ash in the exhaust gas is removed by the electrostatic precipitator 5, and NH and injection means are added to the exhaust gas emitted from the electrostatic precipitator 5. After injecting NH3 from 2, it is introduced into a dry denitrification device 3 to remove NOx in the exhaust gas, and then the exhaust gas is introduced into a heat exchanger 4 for heat exchange to lower the temperature of the exhaust gas. It is sent to the wake deflow device or chimney.

According to this treatment process, since the electrostatic precipitator 5 is placed upstream of the heat exchanger 4, when fly ash with high electrical resistance is collected, reverse ionization occurs due to a decrease in exhaust gas temperature. However, the treatment process has the following drawbacks.

(1) The S component in the coal is burned in the coal-fired boiler 1, resulting in S
O2 is generated, and a part of it is further converted into SO3, which is generated in the heat exchanger 4, and NH3 leaked from the dry denitration equipment 3.
and acidic ammonium sulfate N, which reacts with moisture in exhaust gas.
Since H4HSO4 is generated, this acidic ammonium sulfate melts and adheres, causing clogging of the heat exchanger 4.

(2) Ammonium sulfate (NH4)2SO4 produced by the reaction of SO3, NH3, and moisture in exhaust gas
The reaction products of the system become fine dust at the outlet of the heat exchanger 4 because the exhaust gas temperature decreases, and for example, about 50 mg/Nm of dust is generated from 1 oppm of SO3, so the amount of dust in the exhaust gas increases.

Therefore, the present inventors first conducted the second to
We proposed the method shown in the flowchart in Figure 4 (Japanese Patent Application No. 1983).
-81144).

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 about 100 to 160°C. In some cases, the electrical resistance may exceed a so-called reverse ionization starting resistance value, and a reverse ionization phenomenon may occur within the electrostatic precipitator 5, significantly reducing the dust collection performance of the electrostatic precipitator 5.

For example, as shown in FIG. 5, in the conventional process shown in the flowchart of FIG. However, in the above-mentioned force method shown in the flowcharts of Figs. 2 and 3, the operating temperature range of the electrostatic precipitator 5 is within the range B, which is quite low, so the electrical resistance of the dust is equal to the reverse ionization resistance. It falls within range B, and the dust collection performance decreases significantly.

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 for this is that SO3 and SO, which are involved in surface conduction that mainly controls the electrical resistance of fly ash in this temperature range,
This is due to the fact that the amount of H2SO4 produced in No. 3 is not sufficient by combustion of the above-mentioned low sulfur coal.

Generally, in a coal-fired boiler, the amount of SO3 is considered to be approximately 1% of the total amount of SOx in the exhaust gas, and the amount of SOx is proportional to the combustible sulfur content in the coal.

On the other hand, the aforementioned selective option that is currently considered mainstream? The catalyst used in the catalytic reduction method includes NOx in the presence of NH3.
In addition to the reducing action of (6NO+4NH3g5N2+6H20), there are substances that promote the oxidizing action of SO (SO2→SO3).

However, conventionally, the use of such catalysts has been avoided due to the drawbacks mentioned in (1) and (2) above when generating a large amount of SO3 using such catalysts. It was there.

Therefore, the present invention aims to improve the dust collection performance of the electrostatic precipitator by imparting an appropriate SO2→SO3 conversion action to the dry denitrification device installed in front of the electrostatic precipitator, while also preventing secondary pollution. Excessive SO2 → SO
This was done with the aim of generating the amount of 803 necessary to improve the above-mentioned dust collection performance while preventing excessive SO3 generation during tertiary conversion and high sulfur coal combustion.

That is, the present invention provides a device for treating exhaust gas using a denitrification device using a dry catalytic reduction method and an electrostatic precipitator installed downstream thereof via a heat exchanger.
What is a catalyst that has a reducing effect on x? The reverse ionization phenomenon in the electrostatic precipitator can be prevented by incorporating a catalyst having the action of converting O2 into SO3 and adjusting the contact area of the catalyst part having the action of converting SO into SO3. The gist of the present invention is to provide a method for treating exhaust gas, characterized in that the amount of S03 required for this is generated in the denitrification device.

Hereinafter, the method of the present invention will be explained in more detail with reference to the accompanying drawings.

FIG. 6 is a flow sheet showing an embodiment of the method of the present invention.

In Figure 6, N from combustion equipment 1 such as a coal-fired boiler
After NH3 is added from the NH3 injection means 2, the exhaust gas containing dust such as Ox, SOx and fly ash enters the denitrification device 3' according to the present invention at a temperature range of usually 300 to 400°C, and is denitrified in this temperature range. NOx in exhaust gas is N
H3 is decomposed into N2 and H20 by the action of the catalyst, and at the same time, SO2 in the exhaust gas is converted to SO3 by the action of the catalyst.

This catalyst can be a base metal based catalyst that is active with oxides or a noble metal based catalyst that is active with metal elements, which has a NOx reduction effect, or a denitrification catalyst based on a dry catalytic reduction method, or something similar thereto, depending on the usage conditions. Vanadium pentoxide is used alone or in combination depending on the purpose of use, and has the effect of converting SO2 to SO3. A substance having an effect of converting SO into SO3 is used.

These substances that have a reducing effect on NOx, SO2 and SO
As described later, the substances having the conversion effect to 3 may be supported on separate carriers and arranged separately in parallel or in series, or they may be mixed in the required ratio. May be used.

The exhaust gas treated in the denitrification device 3' is heat exchanged in a heat exchanger 4 and cooled to 100 to 170°C, and then led to an electrostatic precipitator 5 to remove dust.

FIG. 1 is a diagram showing a specific example of a denitrification device 3' according to the present invention, in which 11 is an inlet nozzle, 12 is an outlet nozzle, and 15 is an N
A catalyst 16 is a mixture of a catalyst having an Ox reduction effect (hereinafter referred to as a NOx reduction catalyst) and a catalyst having an effect of converting SO2 into SO3 (hereinafter referred to as an SO2 conversion catalyst); reference numeral 16 indicates the extraction and supply of the catalyst 15; It is a means.

When the performance of the extraction and supply means 16 deteriorates due to poisoning of the catalyst 15, However, in the present invention, an electrostatic precipitator is used. In order to convert SO to SO3 in a quantitative range that is necessary to maintain the dust collection performance of the system and in which leakage of SO3 to the outside of the system is not a problem (i.e., contact with the catalyst part that has the function of converting SO2 to SO3). The catalyst 15 is equipped with a function to change the extraction and supply amount of the catalyst 15 using the S content in the fuel used or the SO3 amount at the entrance of the electrostatic precipitator as a selection condition so as to adjust the gas area.

For example, the particle size of the NOx reduction catalyst and the SO2 conversion catalyst may be changed (e.g., the NOx reduction catalyst may have a large particle size, and the
O2 conversion catalyst of small particle size), hybrid catalyst 15
After being extracted, it is sieved and the supply amounts of both catalysts are adjusted.

According to the results of an experiment conducted by the present inventors regarding the amount of SO3 at the inlet of the electrostatic precipitator among the above selection conditions, as shown in FIG. 8, when the amount of SO3 is 20 to 30 ppm, Operating temperature 12 of the electrostatic precipitator 5 in the flow shown
It can be seen that normal dust collection performance can be maintained at 0 to 170°C.

Figure 8 is a chart showing the relationship between the amount of SO3 at the electrostatic precipitator inlet and dust collection performance when low sulfur coal-fired boiler exhaust gas fly ash is used as an example. This is synonymous with A and B in the figure.

FIG. 9 is a diagram showing another specific example of the denitrification device 3' according to the present invention, in which 13 is a NOx reduction catalyst arranged in a layered, plate-like, zigzag, etc. shape depending on the situation, and 14 is the catalyst 13. The SO2 conversion catalyst is arranged in the same manner as above, and a bypass flow rate adjustment damper 17 is provided in the catalyst 14.

The bypass flow rate adjusting damper 17 is a means for adjusting the air contact area of the catalyst 14. When it is fully closed, the entire amount of exhaust gas passes through the catalyst 14, increasing the amount of SO3, and when it is fully open, the amount of SO3 increases. Due to the ventilation resistance of the catalyst 14, almost all of the exhaust gas is bypassed and the amount of 803 is reduced.

Therefore, by adjusting the opening degree of the bypass flow rate adjustment damper 17, it is possible to arbitrarily generate SO3 that is necessary to maintain the dust collection performance of the electrostatic precipitator and that does not cause problems with leakage to the outside of the system. can.

Furthermore, the present invention is not limited to this bypass flow rate adjusting damper 17, and any means that can adjust the bypass flow rate may be used.

Furthermore, the catalyst 13 and the catalyst 14 are not limited to being arranged with a space between them as shown in the figure, but both catalysts 13 and 14 may be arranged in contact with each other.

FIG. 10 shows a configuration in which two denitrification devices 3' according to the present invention are installed together. In this case, each denitrification device 3' has a catalyst 1 41 having a layer thickness that is changed to convert SO2 into SO3,
1 42 and adjust the exhaust gas flow rate to each denitrification device 3' with the output damper 18, 19.
This is to adjust the air contact area of the catalyst portion that has the function of converting SO2 to SO3.

Furthermore, there is no need to install two denitrification devices 3l, but only one denitrification device 3l may be installed as long as a mechanism is provided that can change the exhaust gas flow rate distribution ratio to the catalysts 141 and 142 having different SO2→SO3 conversion rates.

FIG. 11 shows a modification of FIG. 9, in which FIG. 11A is an overall view and FIG. 11B is a detailed view of the catalyst 14 portion.

That is, the catalyst 14 has a plurality of plate-like configurations, and each catalyst 14
A shielding means 20 is attached so as to be movable in the direction of the arrow in FIG. 11B, and the contact area of the catalyst 14 is adjusted by moving the shielding means 2o.

Further, FIG. 12 shows a method for adjusting the air contact area when the catalysts 14 are arranged in a zigzag pattern.

That is, the damper 2 is connected to the catalyst 14 arranged in a zigzag pattern.
1 and closing the damper 21 can increase the flow velocity in other parts and reduce the contact area.

EXAMPLE Using the denitrification apparatus 3' shown in FIG. 9, the following experiment was conducted according to the flow shown in FIG.

Inlet SO2 concentration 200ppm, 500ppm, 100ppm
0 ppm, and varied the opening degree of the damper 17.
The contact area of the O2 conversion catalyst 14 (i.e., the catalyst 14?
change the airflow rate), and change the contact area and electrostatic precipitator (EP)5.
We investigated the relationship with the amount of SO3 in the population.

The results were as shown in FIG.

In Figure 13, 101 is SO-200ppm, 102 is S
O2=500ppm, 103 is SO2=1000ppm
This is the result for the case.

As is clear from Fig. 13, the electrostatic precipitator 5 inlet SO3
When the target value for the amount is 30 ppm (dotted line 104 in Figure 3), high S coal (S content 1.2%, SO2 is about 1000
When using low S coal (with a S content of 0.25% and SO2 generation of approximately 200 ppm) as fuel, the above target SO3 amount can be obtained by setting the air contact area to approximately 25%. It can be seen that when using SO3 as fuel, the above target amount of SO3 can be obtained by setting the contact area to about 100%.

When using coal with a lower S content as fuel than the above-mentioned 0.25% S content coal, the amount of catalyst 14 used is increased and the contact area is increased to 10%.
This can be dealt with by increasing the conversion rate at 0%.

[Brief explanation of drawings]

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 Figure 6 is a diagram showing a diagram of the apparatus of the present invention. Flowcharts showing usage examples, Figures 7, 8, 10 and 11 are explanatory diagrams showing specific examples of the denitrification equipment according to the present invention, and Figure 12 shows the conversion action of SO2 to SO3 in the denitrification equipment according to the present invention. FIG. 8 is a diagram showing the relationship between the operating temperature of the electrostatic precipitator, the amount of SO3 at the inlet, and the electrical resistance of dust, and FIG. This is a chart showing the results.

Claims (1)

[Claims] 1. A method in which exhaust gas is treated in a denitrification device using a dry catalytic reduction method and an electrostatic precipitator installed downstream thereof via a heat exchanger, in which the denitrification device is equipped with a catalyst having an NOx reduction effect and an SO3 of SO2. By incorporating a catalyst having the action of converting SO2 into SO3; and adjusting the contact area of the catalyst portion having the action of converting SO2 into SO3, the amount necessary to prevent the reverse ionization phenomenon in the electrostatic precipitator. A method for treating exhaust gas, characterized in that SO2 is generated in the denitrification device.