JP3335725B2 - Exhaust gas treatment method and apparatus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for wet-cleaning and removing exhaust gas containing acidic substances such as hydrogen chloride and harmful substances such as dust, and more particularly to a method and an apparatus for treating exhaust gas from an incinerator. is there.

[0002]

2. Description of the Related Art Exhaust gas discharged from an incinerator for incinerating industrial wastes and the like often contains harmful substances such as a large amount of hydrogen chloride, sulfur oxides, hydrogen fluoride and dust. Conventionally, as a method for removing such harmful substances from exhaust gas, a wet detoxification method using caustic soda (NaOH) as a neutralizing agent is known. In this method, the removed harmful substances are usually dissolved in aqueous solution as a reaction product such as sodium chloride, sodium sulfate, sodium fluoride, etc., and are discharged. The dust is separated from the discharged water. And processed separately. Such a wet method has an advantage that harmful substances can be removed with higher efficiency than a dry method in which a neutralizing agent such as slaked lime is blown into a furnace or a flue.

[0003] However, this method has a disadvantage that the processing cost is high because expensive caustic soda is used. For this reason,
In recent years, instead of caustic soda as a neutralizing agent,
As described in 242322, a method using inexpensive magnesium hydroxide has been proposed. In the method using magnesium hydroxide as a neutralizing agent, magnesium chloride, magnesium sulfate, and the like generated by the reaction between the components in the exhaust gas and the neutralizing agent are harmless like sodium salts and have high solubility. Because it can be done, there is no need for trouble and there is no environmental problem. Furthermore,
In the caustic soda method, harmful fluoride ions enter the wastewater because the generated sodium fluoride is water-soluble, but when magnesium hydroxide is used as a neutralizing agent, the generated magnesium fluoride is hardly soluble. Therefore, there is also an advantage that the concentration of harmful fluoride dissolved in the wastewater can be reduced.

[0004]

However, magnesium hydroxide has the disadvantage that its solubility in water is extremely low, and the rate of neutralization reaction with acidic substances is slower than that of the caustic soda method, so that the efficiency of gas absorption is low. For this reason, even in the above proposal, the entire amount of the neutralizing agent cannot be converted to magnesium hydroxide, and the absorption is divided into two stages. In the previous stage, hydrogen chloride is removed with magnesium hydroxide, and in the later stage, sulfuric acid is removed with caustic soda. The method of removing oxides is adopted.

[0005] Therefore, there has not yet been known a method of using only low-reactivity magnesium hydroxide as a neutralizing agent and efficiently removing harmful substances in exhaust gas with a compact device. The present inventors have studied various methods for efficiently using magnesium hydroxide having lower reactivity than caustic soda, and as a result, have found that the efficiency greatly differs depending on the method of contacting the exhaust gas with the absorbing solution. Invented the invention.

[0006]

That is, according to the present invention, exhaust gas containing harmful substances such as acidic substances and dust is brought into contact with an absorbing solution containing magnesium hydroxide to absorb the harmful substances in the exhaust gas. In the treatment method of the exhaust gas to be removed, the exhaust gas and the absorbent are brought into contact with each other in the absorption zone provided with a plurality of stages of non-weired perforated plates in counterflow, and the pH of the absorption fluid flowing down from the lowermost stage of the absorption zone becomes constant. Thus, there is provided a method for treating exhaust gas, characterized in that the supply amount of magnesium hydroxide to the absorption liquid supplied to the absorption zone is adjusted.

Further, according to the present invention, there is provided an absorption zone in which a plurality of non-weired perforated plates for bringing exhaust gas and an absorbing solution containing magnesium hydroxide into countercurrent contact are arranged, and an absorption zone supplied to the absorption zone. An exhaust gas treatment device is provided, comprising: pH control means for controlling the amount of magnesium hydroxide contained in the solution to control the pH of the absorbing solution flowing down from the lowermost stage of the absorption zone to a predetermined value.

[0008]

Hereinafter, the present invention will be described in detail. A structural feature of the present invention is that an absorbent solution containing magnesium hydroxide is brought into contact with exhaust gas containing at least acidic gases (substances) such as hydrogen chloride, sulfur oxides and hydrogen fluoride, and harmful substances such as dust. Another object of the present invention is to make the exhaust gas and the absorbing liquid come into contact with each other in counterflow in an absorption zone provided with a plurality of non-weired perforated plates. The difference between magnesium hydroxide and caustic soda is that
Since magnesium hydroxide is hardly soluble in water, it is necessary to cause an absorption reaction in a slurry state, and as a result, a difference in reactivity occurs depending on the type of acidic substance in the exhaust gas. Therefore, how this point relates to the absorption efficiency of the acidic substance will be described.

In the present invention, the main harmful substances to be eliminated are hydrogen chloride (HCl), sulfur oxides (SO ₂ , S
O ₃ ), hydrogen fluoride (HF), dust, and the like. Of these, HCl, SO ₂ and HF are gaseous, and HCl, which produces strong acids, is most easily absorbed by water,
₂ , HF becomes difficult to be absorbed in order. That is, it is sufficient that the pH of the absorbing solution is maintained at 2 or more in order to absorb HCl, but in order to absorb SO ₂ and HF,
It is necessary to maintain the pH above 4. On the other hand, since SO ₃ and dust are not in a gaseous state, the removal rate is determined only by physical collision between the exhaust gas and the absorbent, and has no relation to the pH of the absorbent.

When simultaneously absorbing each component having such a difference in absorption characteristics, if the neutralizing agent is caustic soda, the solubility is large, and it is easy to maintain the pH range necessary for absorbing all components. Thus, there is almost no difference in the removal efficiency depending on the type of absorption system. However, when the neutralizing agent is magnesium hydroxide, the solubility is low,
In order to simultaneously absorb each component, it is necessary to select an appropriate absorption method. That is, although magnesium hydroxide exists as a slurry in the absorbing solution, it must be dissolved in water to neutralize the acid gas, and it takes a certain amount of time to dissolve it. For this reason, it is preferable to dissolve the magnesium hydroxide as much as possible in the place of absorption of the acidic gas, and it is preferable that the absorption method be such that the residence time of the absorption liquid in the absorption zone can be long.

The exhaust gas absorption system is roughly classified into a system using a spray tower, a packed tower or a column tower. In this order, the residence time of the absorbing solution in the absorption zone becomes longer, and the magnesium hydroxide in the absorption zone becomes longer. The amount of dissolution can be increased. On the other hand, dust in the exhaust gas is also collected by the absorbing liquid, and the absorbing liquid becomes a dust slurry. However, it is necessary to prevent the dust from depositing and sticking in the apparatus. In this case, in a column tower having a weir and a downcomer to secure a stagnant liquid layer on the stage such as a bubble cap tray tower, or a packed tower using a filler such as terraret, the deposition of dust is likely to occur, There is a problem with long-term stable operation.

As a result of comparing and examining various absorption systems based on the above viewpoints, the present inventors have found that exhaust gas and absorption liquid are countercurrently flowing in an absorption zone in which non-weired perforated plates are provided in a plurality of stages. The method of contacting (feeding the absorbing liquid from the upper part to the lower part and the exhaust gas from the lower part to the upper part) has the following characteristics and has been found to be the most preferable. (1) Since the residence time of the absorbing solution in the absorption zone can be made relatively long, the amount of magnesium hydroxide dissolved in the absorption zone is large, and the pH of the absorbing solution due to the absorption of acidic gas (substance) does not decrease much. (2) Therefore, not only HCl but also SO ₂ and HF can be simultaneously absorbed and removed. That is, HCl is mainly in the lower part of the absorption band, and SO2 is mainly in the upper part of the absorption band.
_2. By controlling the pH of the absorbing solution at the outlet of the absorption zone to a specific value in the range of 2 to 6, preferably 4 to 5.5 so as to absorb and remove HF, water can be collected in a single tower with a compact device. Even when magnesium oxide is used as an absorbent, all harmful substances can be removed. If the pH of the absorbing solution is 2 or less at the outlet of the absorbing zone, the absorption of HCl at the lower portion of the absorbing zone becomes insufficient, and the p of the absorbing solution reaches the upper portion of the absorbing zone.
H decreases, and SO ₂ gas may not be completely absorbed. On the other hand, when the pH of the absorbing solution at the bottom of the absorbing zone is increased to 6 or more, the amount of undissolved magnesium hydroxide in the absorbing solution in the storage tank increases, and part of the absorbing solution is drained out of the system. When extracted, the magnesium hydroxide contained therein is unnecessarily extracted out of the system. (3) Further, as shown in FIG. 2, the slurry-like absorbing liquid 6 is always in a flowing state on each perforated plate 5 and does not stay even partially, so that the deposition and fixation of dust do not occur. Conceivable.

In the present invention, the perforated plate provided in the absorption zone needs to be a weir-free. As described above, when using a perforated plate or a bubble cap tray having a weir,
Dust deposition and sticking occur, making long-term stable operation difficult, which is not preferable. A plurality of perforated plates are provided in the absorption zone. The number of stages is not particularly limited, and although it depends on the tower diameter of the absorption zone and the distance between the upper and lower porous plates, usually three to ten stages are preferably installed. Although the aperture ratio of the perforated plate is not particularly limited, it is usually preferably 25 to 50%. Also, the hole diameter of the perforated plate is not limited, and is usually 4 to 15 m.
m is preferred. The exhaust gas treated by the method and apparatus of the present invention is often a high-temperature exhaust gas such as an incinerator exhaust gas. In the present invention, there is no particular limitation as long as the material and the like of the absorption zone are sufficiently high in temperature and can withstand the atmosphere. However, gas absorption is more efficient at lower temperatures, and materials that can withstand high-temperature acidic humid atmosphere are expensive.
Normally, the hot gas is cooled before being introduced into the absorption zone. Prior to the absorption and removal treatment with the harmful substance absorbing solution,
The method for cooling the high-temperature exhaust gas may be any of the conventional techniques. As such a cooling method, a high-temperature gas flows down the vertical flue, and cooling water is sprayed or sprinkled from the top in the gas flow direction, and cooling water is formed on the inner wall of the flue so as to form a wet wall. An adiabatic cooling method in which the fluid flows is preferably employed. In this case, the same or similar fluid as the harmful substance absorbing liquid can be used as the cooling water for forming the wetted wall. The degree of cooling is selected in relation to the material of the absorption tower and the absorption operation conditions. In the present invention, it is desirable that the solution is cooled to 100 ° C. or lower, preferably 80 ° C. or lower, and then guided to the absorbing section.

[0014]

Next, specific embodiments of the present invention will be described based on illustrated examples, but the present invention is not limited to the illustrated examples. In FIG. 1, 4 is an absorption tower, 2 is an absorption tower 4
1 shows a cooling tower installed on the upstream side of exhaust gas. The exhaust gas 1 to be absorbed and removed in the present invention is mainly generated from an incinerator, and has a relatively high temperature of 700 to 1000 ° C. Therefore, it is preferable to cool the exhaust gas before absorbing the gas. Various methods are known for this cooling. As shown in FIG. 1, a water spray 3 in the cooling tower 2 and a wet wall method in which water or the absorbing solution 6 flows down on the inner wall of the cooling tower 2 are used. A method of adiabatic cooling of exhaust gas in combination is preferable. As a result, the temperature of the exhaust gas is usually 80 ° C. or less.

The exhaust gas which has been humidified with cooling is supplied to the absorption tower 4
The harmful component is introduced into the lower absorption zone lower part, while coming up in the absorption zone where the multi-stage perforated plate 5 is arranged, and coming into countercurrent contact with the absorbing solution 6 containing magnesium hydroxide flowing down from the upper part. The exhaust gas 7 from which harmful substances have been removed is discharged from a chimney through a demister and an electric dust collector (not shown). The absorption liquid 6 is circulated from a circulation line 9 described later, is supplied substantially uniformly onto the uppermost porous plate 5 of the absorption tower 4 by spraying with a liquid spraying device 11, and flows down in the absorption zone of the absorption tower 4 while being acidic. Absorbs gas (substance).

An absorption liquid 6 containing magnesium hydroxide is supplied to the absorption tower 4. In the absorption zone, the number of stages of the perforated plate 5,
The residence time of the absorbing solution 6 in the absorption zone is adjusted by appropriately selecting the aperture ratio, the pore diameter, the tower diameter, and the like. The required amount of the absorbent 6 coming out of the absorption zone is supplied to the circulation line 9 of the absorbent 6 so that the pH thereof is detected by the pH detector 10 and the value thereof becomes a specific value in the range of 2 to 6. Magnesium hydroxide slurry 1
4 is regulated by the controlled addition. The magnesium hydroxide slurry 14 is usually available as a slurry of 30 to 40% by weight. The absorbent 6 having passed through the absorption zone in this manner enters the storage tank 12 (with a stirrer), and is pumped by the pump 13 through the circulation line 9 from the liquid dispersion device 11 above the absorption tower 4 onto the perforated plate 5. Circulated supply.

Most of the magnesium hydroxide contained in the absorption liquid 6 supplied to the uppermost stage of the absorption zone is dissolved while flowing down in the absorption zone, and is consumed by reacting with the acidic substance. Even after passing through the lowermost stage, a part remains without being dissolved, so that it is dissolved in the absorption liquid storage tank 12 with a stirrer. Further, as a characteristic of the incinerator exhaust gas, the flow rate of the acidic gas contained therein often fluctuates rapidly. In order to smoothly follow the supply amount of the magnesium hydroxide slurry 14 with respect to this fluctuation, it is preferable to detect the pH of the absorbing solution 6 at the outlet of the absorption zone that has passed the lowermost stage of the absorption zone. If the detection of the pH of the absorbing liquid 6 is performed in a line after the storage tank 12, a time delay occurs, and it becomes difficult to perform accurate pH control.

Further, a part of the absorbing liquid 6 which has come out of the storage tank 12 is drawn out as drainage 8 outside the system. The extraction flow rate of the drainage 8 is set so that the magnesium chloride or magnesium sulfate in the absorbing solution 6 generated by the reaction between the acidic gas and the magnesium hydroxide in the absorbing solution 6 does not exceed the saturation value.
Alternatively, the concentration is set so that the dust slurry concentration does not become too high.

Hereinafter, more specific results of the present invention will be described. (Example) The temperature generated from an industrial waste incinerator is 750 ° C.
Exhaust gas 1 (composition: O ₂ : 10%, CO ₂ : 8%, H
₂ O: 15%, HCl: 4000 ppm (dry basis), SO ₂ : 500 ppm (dry basis), HF: 4
5 ppm (dry basis), dust: 2.5 g / Nm ³ a (dry basis)) 10,000 nM ³ / H, and introduced into the cooling tower 2 of the exhaust gas processing device having the structure shown in FIG. The exhaust gas 1 in the cooling tower 2, after adiabatic cooling to 75 ° C. by the liquid amount 15 m ³ / H wetted wall by the absorbing liquid 6 from 5 m ³ / H of the water spray 3 and the circulation line 9, column diameter 1.3 m, The perforated plate 5 was introduced into the lower part of the absorption tower 4 having an absorption band composed of 7 stages with a hole diameter of 8 mm, an aperture ratio of the perforated plate of 30%, and seven stages of perforated plates.

On the other hand, the absorption liquid 6 at the absorption zone outlet 10
PH control set value of 5.0, 30% by weight in the circulation line 9 after the storage tank 12 with a stirrer having a capacity of 4 m ³
The absorption liquid 6 whose pH has been adjusted while adding the magnesium hydroxide slurry 14 is supplied from the liquid dispersion device 11 onto the uppermost porous plate 5 of the absorption tower 4 at a supply amount of 17 m ³ / H, and the absorption band is supplied. , The absorbent 6 and the exhaust gas 1 were brought into countercurrent contact. Pressure loss of the absorption band was about 130mmH ₂ O. A part of the absorbent 6 in the circulation line 9 is drained 8
It was extracted out of the system at g / H. The above exhaust gas treatment was performed for one month. As a result, the composition of the purified gas discharged from the absorption tower 4 and the properties of the wastewater 8 discharged outside the system were as follows.

(Composition of Purified Gas 7)

(Composition of drainage 8)

As is clear from the above results, the purification rate of HCl, SO ₂ , HF and dust from the exhaust gas is high, and during the one-month test, an increase in the pressure drop of the absorption tower and a decrease in the removal performance are observed. And stable operation could be continued. Further, after the test, the inside of the absorption tower was inspected, but no scaling, accumulation of dust and clogging were observed.

[0024]

As described above, according to the present invention,
In the treatment of exhaust gas containing harmful substances such as hydrogen chloride, sulfur oxide, hydrogen fluoride and dust, only inexpensive magnesium hydroxide is used, and the exhaust gas is efficiently removed without depositing and sticking of slurry and dust. Can be processed.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing an embodiment of an exhaust gas treatment apparatus according to the present invention.

FIG. 2 is an explanatory diagram showing a contact state between exhaust gas and an absorbent on a non-weird perforated plate of the present invention.

[Explanation of symbols]

Reference Signs List 1 exhaust gas, 2 cooling tower, 3 water spray, 4 absorption tower, 5 perforated plate, 6 absorption liquid, 7 purification gas, 8 drainage, 9 circulation line, 10 pH detection device, 11 liquid dispersion device, 12 storage tank, 13 pump , 14 magnesium hydroxide slurry

──────────────────────────────────────────────────続 き Continuation of the front page (51) Int.Cl. ⁷ Identification symbol FI B01D 53/77 B01D 53/34 134D (56) References JP-A-3-26316 (JP, A) JP-A-61-157329 ( JP, A) JP-A-3-4919 (JP, A) JP-A-5-111615 (JP, A) JP-A-51-39463 (JP, A) JP-A-50-104436 (JP, A) (58) ) Surveyed field (Int.Cl. ⁷ , DB name) B01D 53/34

Claims (4)

(57) [Claims] 1. Hydrogen chloride, sulfur oxide, hydrogen fluoride and
The incinerator exhaust gas containing dust is contacted with the absorption liquid containing magnesium hydroxide, in the processing method of an exhaust gas to remove absorbed harmful substances in the exhaust gas, the exhaust gas in the absorption band having a perforated plate Museki in a plurality of stages And the absorbent in contact with each other in countercurrent ,
The absorbent after contact is circulated to the absorption zone by the circulation line
It is allowed Rutotomoni, the pH of the absorption liquid flowing down from the absorption zone bottom to detect the absorption zone outlet, of the pH is constant
Subjected magnesium hydroxide into the circulation line to be in the range
A method for treating incinerator exhaust gas , wherein the amount of magnesium hydroxide supplied is adjusted based on the detected pH. 2. The method for treating exhaust gas according to claim 1, wherein the exhaust gas is cooled in advance by a water spray and a wetting wall in a cooling zone and then sent to an absorption zone to be brought into contact with an absorbing liquid. 3. The pH of the absorbent flowing down from the lowermost stage of the absorption zone.
The exhaust gas treatment method according to claim 1 or 2, wherein is a specific value within a range of 2 to 6. 4. Hydrogen chloride, sulfur oxide, hydrogen fluoride and
Incinerator exhaust gas treatment equipment for treating incinerator exhaust gas containing dust
An absorption zone in which a plurality of non-weired perforated plates for countercurrently contacting the incinerator exhaust gas and the absorption solution containing magnesium hydroxide are arranged, and the absorption solution after contact is used as the absorption zone.
A circulation line for circulating a detecting means for detecting the pH of the absorbing liquid which has passed through the absorption zone bottom at the absorption zone outlet, the
Circulation line based on the pH detected by the detection means.
The amount of magnesium hydroxide contained in the absorption liquid supplied to the absorption zone by supplying magnesium hydroxide to
Control exhaust gas treatment apparatus characterized by having a pH control means for controlling the absorption liquid pH to control to flowing down the absorption zone bottom.