JPH0416209B2 - - Google Patents

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to an improvement in a wet treatment method for exhaust gas containing harmful components such as sulfur compounds, halogen gas, and soot and dust, and particularly relates to improvement of a method for wet treatment of exhaust gas containing harmful components such as sulfur compounds, halogen gas, and soot. The aim is to reduce or eliminate wastewater and provide a method to eliminate wastewater from the equipment.

(Prior art) From the perspective of preventing air pollution, the wet lime-gypsum method has been widely put into practical use as a sulfur oxide removal device, and is used for exhaust gas treatment in heavy oil-fired boilers, coal-fired boilers, sintering plants, metal smelting plants, etc. It's being used. Further, in many cases, a dry type dust collector is also used as a device for removing dust from the exhaust gas.

Among all types of exhaust gas, coal-fired boiler exhaust gas contains a lot of soot, NOx, and halogen gas in addition to sulfur oxides, so the treatment of coal-fired boiler exhaust gas requires the most advanced technology.
In most cases, other exhaust gas treatments can be easily accomplished if the exhaust gas treatment technology is available.

Therefore, the case where the present invention is particularly applied to the treatment of coal-fired boiler exhaust gas will be explained below.

Conventionally, the purification treatment of coal-fired boiler exhaust gas was carried out in the fourth stage.
It is implemented in the manner shown. Exhaust gas from a coal-fired boiler 1 is guided through a duct 2 to a dry dust collector 3 to remove fly ash contained in the exhaust gas, and then discharged to the outside of the system through a line 4.

Next, the exhaust gas from which most of the fly ash has been removed is supplied to the gas heater 6 through the duct 5, and after being cooled by heat exchange with the exit gas of the wet exhaust gas treatment device flowing through the duct 10, it is supplied to the absorption tower 8 through the duct 7. . The processed gas heated by the gas heater 6 is discharged from the duct 11.

In the absorption tower 8, the absorbent sprayed from the line 12 comes into contact with the exhaust gas, the water in the absorbent evaporates, and the exhaust gas is lowered to the adiabatic cooling temperature.
Absorbs and removes halogens such as SO ₂ , fluorine, and chlorine.

The gas that exits the absorption tower 8 then enters the gas-liquid separator 9, where droplets in the gas are removed by inertial collision as it passes between the folded plates filled inside.
The removed droplets are returned via line 13 to the liquid circulation tank at the bottom of the absorption tower. Lime slurry and an oxidation promoter are supplied to the absorption tower 8 from the line 14 as necessary, SO ₂ is absorbed in the tower to produce calcium sulfite, and the calcium sulfite is further oxidized by oxygen in the exhaust gas to form gypsum. shall be. Since moisture evaporates in the absorption tower 8, make-up water corresponding to the evaporation is supplied from the line 22.

The gypsum slurry produced in the absorption tower 8 is supplied to a dehydrator 16 through a line 15, dehydrated by the dehydrator 16, and taken out as a by-product gypsum 17 for use outside the system. On the other hand, the filtrate from the dehydrator 16 is transferred to the line 18.
It is sent to a raw material tank 19 for preparing an absorbent through the line 20, where it is mixed with an absorbent such as limestone or slaked lime supplied through a line 20. Further, a part of the filtrate is drained from a line 21 to adjust the concentration of impurities in the system, and is sent to a wastewater treatment device 23.

In the wastewater treatment equipment 23, a wastewater neutralizer such as slaked lime is added through the line 24 to precipitate sulfate ions and dissolved metals contained in the wastewater as gypsum and hydroxide.
The solids are discharged as sludge 25. The waste liquid after solid precipitation is discharged through line 26.

The disadvantages of the conventional method shown in FIG. 4 are as follows.

(1) Drainage is performed to prevent impurities from accumulating in the exhaust gas treatment system.

The water discharged from the discharge line becomes a neutral liquid from which most of the sulfate ions, dissolved metals, and suspended solids have been removed, but chloride has a high solubility and remains as Cl ^- ions. Therefore, if this discharged water is used as a substitute for makeup water to the absorption tower,
This effluent water cannot be used as make-up water because Cl ^- ions are not removed and accumulate in the system at high concentrations, causing corrosion of equipment materials, deterioration of desulfurization performance, and problems that induce scaling. .

(2) Sludge treatment is required.

While by-product gypsum has utility as a raw material for cement and boards, sludge, which mainly contains a wide variety of metal hydroxides, gypsum, and fly ash, has no utility value and must be detoxified for disposal. is necessary.

(3) It is necessary to supply fresh water to match the evaporation and discharged water in the absorption tower, which wastes water resources and is uneconomical.

This drawback can be said to be the fate of the wet method, and as shown in FIG. 2, installing a gas heater also brings about the effect of saving water, which is one way to eliminate this drawback. However, due to the capacity of the gas heater, there is a limit to the cooling temperature of the gas introduced into the absorption tower.
Not enough.

(Problems to be Solved by the Invention) The present invention aims to eliminate the drawbacks of these conventional methods, reduce or eliminate makeup water supplied to a wet exhaust gas treatment device, and provide a method for eliminating waste water from the device. It is.

(Structure of the Invention) That is, the present invention provides an exhaust gas treatment method in which exhaust gas is introduced into a dry type dust collector to remove dust contained in the exhaust gas, and then introduced into a wet type exhaust gas treatment device for purification. The surface of the folded plate filled in the gas-liquid separator, which is one of the devices constituting the system, is forcibly cooled, and a portion of the absorption liquid is cooled by spraying onto the surface of the folded plate, and the liquid is cooled at the outlet of the wet processing equipment. The exhaust gas is cooled so that the amount of water vapor in the exhaust gas is less than that in the exhaust gas at the inlet of the device, and the entire amount of the waste liquid from the wet exhaust gas treatment device is injected from the upstream side of the dry dust collector, and the This is a method for treating exhaust gas, characterized in that solid matter obtained by evaporation to dryness by contacting with exhaust gas is collected by the dry dust collector.

Hereinafter, one embodiment of the method of the present invention will be explained with reference to FIG.

Reference numerals 1 to 21 are exactly the same as in FIG. 4. 1st
In the figure, 22 to 26 have been deleted, and a dryer 27, a duct 28, a heat exchanger 29 of the type that cools the bent plate surface filled in the gas-liquid separator 9, and seawater is sent to the heat exchanger 29 as a refrigerant. Enter line 30
A line 31 for discharging seawater after heat exchange and a line 32 for feeding a portion of the circulating absorption liquid to the gas-liquid separator 9 are added.

A portion of the absorption liquid 12 is supplied to the gas-liquid separator 9 via line 32, and enters via line 30 to line 3.
It comes into contact with the bent plate surface cooled by the refrigerant (seawater) emitted from 1, further absorbs and removes SO ₂ in the passing exhaust gas, cleans the dust adhering to the bent plate surface, and further cools the exhaust gas. Returns from line 13 to the tank at the bottom of absorption tower 8. Therefore, the exhaust gas that comes into contact with the absorption liquid in the absorption tower 8 and the gas-liquid separator 9 is
Duct 1 is cooled to a temperature lower than the adiabatic cooling temperature.
0 to the outside of the absorption tower 8.

Since the water vapor in the gas 10 at the outlet of the gas-liquid separator 9 comes into contact with the absorption liquid and has almost saturated humidity, the lower the temperature of the absorption liquid, the lower the temperature of the outlet gas 10 and the lower the water vapor concentration. If cooling is performed until the amount of water vapor in the outlet gas 10 becomes the same as that in the inlet gas 7, no evaporation of the absorption liquid will occur.
Furthermore, the amount of water vapor in the outlet gas 10 is
When the exhaust gas is cooled to a level smaller than that in the exhaust gas, the moisture in the exhaust gas is condensed in the absorption tower 8 and the gas-liquid separator 9, and water is produced. By operating in this way, the water supply to the absorption tower 8, which is the fate of the conventional wet method, can be significantly reduced or made unnecessary.

Normally, a condenser is installed downstream of the turbine attached to the boiler, and seawater is supplied as a refrigerant, but only a few percent of this amount is sufficient for the amount of seawater supplied to the heat exchanger 29, and the equipment is significantly reduced. It can be branched and used without any major changes, and the required power increases only slightly.

The amount of evaporated water saved by forced cooling of the absorption liquid is specifically shown below. That is,
In the conventional method where the gas temperature at the inlet of the absorption tower is 90°C and the water concentration is 8 vol%, and the gas temperature at the exit of the absorption tower is 47°C, it is necessary to supply water equivalent to 22ml of evaporation per 1 m ³ N of treated gas. On the other hand, if the gas temperature at the outlet of the absorption tower is forcedly cooled to 42° C. using the method of the present invention, evaporation will be eliminated and a considerable amount of water will not be necessary. Furthermore, for example, if the gas temperature at the outlet of the absorption tower is cooled to 39°C, it is possible to generate 12 ml of water per 1 m ³ N of treated gas, making it possible to generate water equivalent to the amount of wastewater, eliminating the need for any water supply from outside the system. Become.

Next, a part of the filtrate from which the gypsum has been separated is sent to line 22.
is extracted and supplied to the dryer 27,
It is brought into contact with exhaust gas 2 from a coal-fired boiler 1, and the water in the filtrate is evaporated using the thermal energy of the exhaust gas 2. The components dissolved in the filtrate become solid particles in the form of CaCl ₂ , MgCl ₂ , etc. as water evaporates, and are discharged as solids from the dry dust collector 3 together with the fly ash 4 through the duct 28 to the outside of the system. be done.

As the dryer 27, a commonly used spray dryer can be used, but the amount of liquid to be evaporated is usually about 1/5 of the maximum amount that can be evaporated by the exhaust gas 2 from the boiler 1, so it can be used in the existing duct. In many cases, simply adding a spray nozzle is sufficient. In addition, the filtrate is brought into contact with the exhaust gas 2,
Other methods of evaporative concentration can also be used.

Next, a specific example of a gas-liquid separator applicable to the present invention will be shown.

FIG. 2 shows a conventional gas-liquid separator, which has a bent plate 36 for gas-liquid separation bent so as to have a zigzag cross section in a main body 35 having a gas inlet 33 and a gas outlet 34. A gas-liquid separator is used in which a cleaning water (absorbing liquid) pipe 37 is disposed upstream of the main body 36 and a cleaning water extraction pipe 38 is disposed below the main body 35. The exhaust gas containing liquid droplets collides with the inertia while passing between the bent plates 36, is collected and flows down as a liquid film, and is extracted from the pipe 38 together with the wash water and enters the bottom tank of the absorption tower. Washing water is supplied intermittently or continuously to prevent lime, gypsum, etc. contained in the droplets from adhering to the bent plate surface.

FIG. 3 shows an embodiment of a folded plate 36 having a heat exchange function applicable to the present invention. A cooling pipe 39 is provided on the bent plate 36, and the cooling pipe 3
9 is connected to a line 30 through which seawater as a refrigerant is supplied and a line 31 through which the refrigerant is discharged. The absorption liquid can be cooled by installing such a bent plate 36 in the gas-liquid separator 35 shown in FIG. 2 and spraying a portion of the absorption tower circulating liquid as washing water.

Since the 36 sides of the bent plate are kept at a temperature below the adiabatic cooling temperature of the exhaust gas, even if cleaning water does not reach the plate, the condensed water generated on the surface of the bent plate has a self-cleaning effect, and is also effective in preventing scale adhesion. It's convenient. Depending on the exhaust gas conditions, the bent plate 36
There are cases in which sufficient cooling cannot be expected by simply using the heat transfer surface as a heat transfer surface, but in this case, it is of course possible to additionally install only a cooling pipe. In any case,
By providing a heat exchange function to the gas-liquid separator, which has conventionally been installed downstream of the absorption tower, there is an advantage that the overall design can be made compact.

(Effects of the Invention) According to the method of the present invention, the embodiments of which are shown in FIGS. 1 to 3, no waste liquid is discharged from the exhaust gas treatment device, and concerns about secondary pollution caused by the waste liquid can be eliminated. Furthermore, it is possible to eliminate the discharge of sludge with a high moisture content, which is mainly composed of metal hydroxide, flyash, and gypsum, and moreover, by-product gypsum 17 with high utility value can be obtained.

Accumulation of Cl ^- ions in the system water, which is one of the main causes of unavoidable drainage in the past, is not a problem because it is discharged together with the fly ash 4 as solid chlorides in the method of the present invention.

Furthermore, since the boiler exhaust gas is used as the heat source for evaporating the waste liquid, there is no need to supply a large amount of energy required for evaporation from the outside, so it is economically advantageous.

As explained above, according to the embodiment to which the present invention is applied, the problems of water supply and drainage, which are the fate of wet exhaust gas treatment methods, can be solved at once, and the location requirements such as securing water supply and owning wastewater treatment equipment are significantly eased. can be expected.

Examples are shown below to clarify the effects of the present invention.

Example 1 The method of the present invention was carried out using a pilot plant of the embodiment shown in FIG. 1 for treating 4000 m ³ N/H of coal-fired exhaust gas. Table 1 shows the properties of the exhaust gas passing through the duct 7.

Table 1 Properties of exhaust gas at absorption tower inlet Gas volume 4000m ³ N/H Moisture concentration in gas 8.0vol% SO ₂ concentration 1200ppm Hcl concentration 20ppm HF concentration 6ppm Fly ash concentration 120mg/m ³ N Gas temperature 90°C From line 30 to heat exchanger 29 25℃ seawater to
When 6 t/h was supplied, the gas properties at the outlet of absorption tower 8 were as shown in Table 2.

Table 2 Characteristics of the exhaust gas at the outlet of the absorption tower Moisture concentration in gas 8.2vol% SO ₂ concentration 30ppm Hcl concentration 0.1ppm or less HF concentration 0.1ppm or less Fly ash concentration 10mg/m ³ N Gas temperature 42℃ At this time, the amount supplied to the absorption tower in line 14 and the amount of absorption tower drawn out from line 15, and the absorption tower 8
When the amount of evaporated water was measured, it was found to be almost zero. There was no moisture condensation.

Also, the amount of water extracted from line 22 is 41l/h.
The properties were as shown in Table 3.

Table 3 Properties of gypsum separation filtrate PH 4.5 Solids concentration 0.5% by weight Dissolved Cl ^- concentration 2700ppm Dissolved Mg ²⁺ concentration 1390ppm Dissolved SO ₄ ^2- concentration 5420ppm Dissolved Ca ²⁺ concentration 705ppm Installed approximately in the center of a circular duct with an inner diameter of 400 mmφ The filtrate having the above properties was sprayed into the exhaust gas 2 while blowing air from a two-fluid nozzle using air. The filtrate sprayed at 41 l/h was immediately evaporated to dryness and collected as a solid substance in a dry dust collector 3 (using an electrostatic precipitator) together with fly ash.

In this example, the amount of water that needed to be replenished from outside the system in order to balance the water in the entire system was 46 liters on average.
It was H.

Example 2 Under the same conditions as in Example 1, the properties of the exhaust gas at the inlet of the absorption tower 8 were supplied, and when 16 t/h of 25°C seawater was supplied from the line 30 to the heat exchanger 29, the properties of the exhaust gas at the exit of the absorption tower were as shown in Table 4. It became.

Table 4 Properties of exhaust gas at the outlet of the absorption tower Moisture concentration in gas 6.7vol% SO ₂ concentration 28ppm Hcl concentration 0.1ppm HF concentration 0.1ppm Fly ash concentration 8mg/m ³ N Gas temperature 58.5°C At this time, the absorption tower was prepared in the same manner as in Example 1. When we measured the water balance inside the tank, we found that 45 l/h of water was coagulating.

The extraction of water from line 22 was exactly the same as in Example 1.

In this example, the water balance of the entire system was balanced and stable operation was possible without replenishing water from outside the system.

(Comparative Example) The properties of the exhaust gas at the inlet of the absorption tower 8 were the same as in Example 1, but when the supply of seawater from the line 30 was stopped, the properties of the exhaust gas at the outlet of the absorption tower were as shown in Table 5.

Table 5 Characteristics of the exhaust gas at the outlet of the absorption tower Moisture concentration in gas 10.5vol% SO ₂ concentration 32ppm Hcl concentration 0.1ppm or less HF concentration 0.1ppm or less Fly ash concentration 10mg/m ³ N Gas temperature 47℃ At this time, absorption was performed in the same manner as in Example 1. When we measured the water balance inside the tower, we found that 90 l/h of water was evaporating.

Extraction of moisture from line 22 is carried out in Example 1.
It was exactly the same.

In this comparative example, in order to balance the water in the entire system,
The average amount of water that needed to be replenished from outside the system was
It was 135l/H.

From the above Examples and Comparative Examples, it has been confirmed that the method of the present invention can significantly reduce make-up water or simultaneously achieve no water supply and no drainage.

[Brief explanation of drawings]

Figure 1 is a flowchart of an embodiment of the present invention, Figure 2 is a conventional gas-liquid separator cited to explain the gas-liquid separator used in the present invention, and Figure 3 is a flowchart of a gas-liquid separator used in the present invention. FIG. 4, a detailed view of the folded plate surface filled in the liquid separator, shows the flow of a conventional wet exhaust gas treatment method.

Claims (1)

[Claims] 1 In an exhaust gas treatment method in which exhaust gas is introduced into a dry type dust collector, the dust contained in the exhaust gas is removed, and then introduced into a wet type exhaust gas treatment device for purification, one of the devices constituting the wet type exhaust gas treatment device. The amount of water vapor that occupies the exhaust gas at the outlet of the wet treatment equipment by forcibly cooling the bent plate surface filled in a certain gas-liquid separator and spraying a portion of the absorption liquid onto the bent plate surface. In addition to cooling the exhaust gas so that it is lower than that in the exhaust gas at the entrance of the equipment,
By injecting the entire amount of the waste liquid from the wet exhaust gas treatment device from the upstream side of the dry dust collector and bringing it into contact with the exhaust gas, the solid matter obtained by evaporation to dryness is collected by the dry dust collector. An exhaust gas treatment method characterized by: