JPH07178314A - Wet desulfurizer and desulfurization method - Google Patents

Abstract

PURPOSE:To efficiently remove dust even under a low load where the the amt. of a mist absorbent is reduced by using this wet desulfurizer. CONSTITUTION:A pipe 19 for blowing steam into the main body 1 of a desulfurization tower or into the outlet duct 3 of the tower is provided. The main body 1 contains saturated steam due to the contact of an absorbent with waste gas. The gas in the main body 1 is kept at 40-60 deg.C, the steam at >=100 deg.C blown into the main body is cooled by the environmental gas to the saturation concn. at that temp., the surplus steam is condensed to form the droplet, most of the droplets are grown with the dust of several micron size suspending in the gas as the nucleus, and hence the fine particles are apparently enlarged. The fine particle in the gas detours the droplet and is collected by the absorbent only in a small amt., the enlarged particle is collided with the droplet by inertia and easily collected by the absorbent in a large amt., and the dust, etc., are collected more efficiently.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the treatment of exhaust gas from a combustion apparatus, and more particularly to a wet desulfurization apparatus and method suitable for effectively removing dust.

[0002]

2. Description of the Related Art Sulfur oxides, especially sulfur dioxide (SO ₂ ), in flue gas produced by burning fossil fuels in thermal power plants, etc., are a major cause of global environmental problems such as air pollution and acid rain. It is one of the main causes. For this reason, research on a flue gas desulfurization method for removing SO ₂ from flue gas and development of a desulfurization device have become extremely important subjects. As the desulfurization method, a wet method is mainly used, and there are a soda method using a soda compound as an absorbent, a calcium method using a calcium compound, and a magnesium method using a magnesium compound. In particular, a method using a relatively inexpensive calcium compound such as calcium carbonate is most often used for a flue gas desulfurization apparatus such as a large-scale boiler for power generation.

Taking the desulfurization system using this calcium compound as an absorbent, for example, it is roughly classified into three types, a spray system, a wet wall system and a bubbling system, depending on the difference in the gas-liquid contacting method. Each method has its own characteristics. Here, the spray method will be described as an example. As a spray-type desulfurization system, there have been conventional cooling towers for cooling and removing dust from exhaust gas, and SO in exhaust gas by spraying an absorbing liquid.
It was composed of three towers: a desulfurization tower for reacting with _2, and an oxidation tower for oxidizing calcium sulfite produced in the desulfurization tower. However, in recent years, the development of a single tower desulfurization tower in which the desulfurization tower has functions of cooling and oxidation has advanced, and recently, the single tower desulfurization system has become the mainstream of the spray system.

FIG. 4 shows an example of a desulfurization apparatus using a spray method. The desulfurization tower mainly comprises a tower body 1, an inlet duct 2, an outlet duct 3, a spray nozzle 4, an absorbent circulating pump 5, an oxidation tank 6, an oxidizing agitator 7, an air blowing pipe 8, a mist eliminator 9, and a limestone slurry tank 12. , Thickener 1
3, a centrifugal separator 17 and the like. A plurality of spray nozzles 4 are installed in the horizontal direction and a plurality of stages in the height direction. The number of stages of the spray nozzle 4 is 1 to 6
Although it is possible to install about four steps, in FIG. 4, it is decided to represent four steps. Further, the oxidizing stirrer 7 and the air blowing pipe 8 are installed in the oxidation tank 6 in the lower part of the desulfurization tower where the absorbing liquid stays, and the mist eliminator 9 is installed in the uppermost part of the desulfurization tower or in the outlet duct 3.

Exhaust gas discharged from a combustion device such as a boiler is introduced into the desulfurization tower main body 1 through the inlet duct 2 and discharged through the outlet duct 3. The limestone is supplied to the limestone slurry tank 12 together with water and sent to the oxidation tank 6 in the form of a slurry. The absorption liquid containing calcium carbonate in the oxidation tank 6 is extracted by the absorption liquid circulation pump 5, sprayed by the spray nozzle 4 through the header 10, and the absorption liquid and the exhaust gas are brought into gas-liquid contact. At this time, the absorbing liquid is SO in the exhaust gas.
_It selectively absorbs ₂ to produce calcium sulfite. The absorption liquid that has generated calcium sulfite returns to the oxidation tank 6 again. The absorbing liquid in the oxidizing tank 6 is stirred by the oxidizing stirrer 7, and the calcium sulfite in the absorbing liquid is oxidized by the air supplied from the air blowing pipe 8 to produce gypsum. A part of the absorption liquid in the oxidation tank 6 in which calcium carbonate and gypsum coexist is sent to the spray nozzle 4 again by the absorption liquid circulation pump 5. A part of the absorbing liquid is sent from the oxidizing tank 6 to the thickener 13 by the absorbing liquid extracting pump 11, and is sedimented, dehydrated and concentrated. Clear water 1
Gypsum slurry concentrated except for 4 is gypsum slurry tank 1
5, sent to the centrifuge 17 by the pump 16,
It is dehydrated and collected as gypsum 18.

Of the absorbing liquid atomized and sprayed from the spray nozzle 4, those having a small droplet size are entrained in the exhaust gas, but are recovered by the mist eliminator 9 provided in the upper part of the desulfurization tower. The desulfurization tower not only absorbs and removes SO ₂ , but also removes dust. Most of the dust is removed by a dust collector (not shown) in the preceding stage of the desulfurization tower, but fine dust that is difficult to collect by a dry dust collector such as an electric dust collector will be collected by the desulfurization tower. However, in the operation of the desulfurization tower, the spray amount of the absorbing liquid is increased or decreased according to the load change of the combustion device of the boiler. Therefore, when the load is low, the desulfurization performance is maintained because the amount of the absorbing liquid sprayed decreases, but there is a problem that the dust removal rate decreases. Furthermore, there is a strong demand to reduce the dust concentration at the smoke outlet, and after desulfurizing the gas that has exited the electrostatic precipitator,
Furthermore, a method of installing a wet electrostatic precipitator is being implemented.

[0007]

The wet desulfurization device is generally attached after the dust collector, and at the same time as performing the desulfurization, it also has a function as a dust collector. Therefore, the total exhaust gas dust collection performance is determined by these two devices. In addition, improving the dust collecting performance of the desulfurizer has the effect of reducing the load on the dust collector. Further, the operation of the desulfurization tower according to the load change changes the amount of the absorbing liquid, but especially when the load is low, the amount of absorbing liquid is reduced and the desulfurization rate is kept constant, so that the dust collection performance is deteriorated. Even if such a situation occurs, the dust collector is planned to always maintain high performance so as not to reduce the dust collecting effect. Further, the removal of SO ₃ mist contained in a small amount in the exhaust gas has the same problem. Therefore, it is desirable that the wet desulfurization device has a function of removing dust independently of the desulfurization performance. However, the dust entering the desulfurization tower has a fine particle diameter of several micrometers or less, and it is difficult to expect the inertial dust collection effect due to the collision between the spray droplets of the absorbing liquid and the dust.

In order to solve these problems, a part of the water cooled in the desulfurization tower or the cooled absorption liquid is sprayed into the desulfurization tower, and dust is attached to the cooling water or the like in the desulfurization tower. A method for removing is proposed in Japanese Utility Model Publication No. 5-18630. The above-mentioned device is an excellent method for removing dust by a simple method, but in the above method, water vapor in the gas preferentially condenses and adheres to the surface of droplets such as cooling water having a low temperature.
It is not sufficient to enlarge the condensed water by using fine particles of relatively high temperature floating in the gas as nuclei. As a result, there is room for improvement in the dust removal performance of exhaust gas by the above method. Therefore, an object of the present invention is to provide a wet desulfurization apparatus and method capable of efficiently removing dust even when the load is small and the amount of the spray-absorbed liquid is small. Another object of the present invention is to provide a wet desulfurization apparatus and method in which dust is enlarged due to adhesion of condensed water in exhaust gas to improve dust removal efficiency.

[0009]

The above objects of the present invention can be achieved by the following constitutions. That is, in a wet desulfurization apparatus including a desulfurization tower that makes gas-liquid contact with an absorbent such as an alkali metal or an alkaline earth metal in order to remove sulfur oxides contained in exhaust gas from a combustion apparatus such as a boiler,
Wet desulfurization equipment provided with a steam pipe for blowing water vapor into the desulfurization tower or the desulfurization tower outlet duct, or spraying an absorbent such as alkali metal or alkaline earth metal into the desulfurization tower, and then from a combustion device such as a boiler. In the wet desulfurization method of removing sulfur oxides contained in the exhaust gas, the wet desulfurization method of removing dust in the exhaust gas by blowing steam into the desulfurization tower or the desulfurization tower outlet duct. In the present invention, a demister may be installed in the desulfurization tower on the downstream side of the steam pipe or in the desulfurization tower outlet duct. Further, the steam blown into the desulfurization tower or the desulfurization tower outlet duct is preferably steam having a temperature of 100 ° C. or higher.

[0010]

[Function] The particle size of the dust in the exhaust gas supplied to the desulfurization tower is very small, less than a few micrometers, and even if the absorption liquid is sprayed in the desulfurization tower, the dust rides on the gas flow and the spray droplets of the absorption liquid The dust and the droplets are less likely to be collected by colliding with each other due to inertial collision. Therefore, if the particle diameter of the dust can be increased apparently by causing water to condense with the dust as the nucleus, the dust collection efficiency can be increased.
The exhaust gas is sent to the desulfurization tower at a temperature of about 100 to 200 ° C. and is cooled by coming into contact with the absorbing liquid. At this time, the water content in the absorbing liquid was evaporated and the exhaust gas contained saturated water vapor.
It becomes a gas of 60 to 60 ° C. When steam of 100 ° C or higher is blown into a gas of 40 ° C to 60 ° C containing saturated steam in the desulfurization tower, it is cooled by the surrounding exhaust gas, and the steam in the gas becomes a saturated concentration at that temperature, and excess steam is condensed. And become droplets. That is, fog is generated in the desulfurization tower,
Most of this mist grows with dust or the like suspended in a gas having a particle diameter of several micrometers or less as a core, so that these fine particles are apparently enlarged and become particles of several tens to several hundreds of micrometers. The fine particles in the gas flow around the spray droplets of the absorbing liquid, so the ratio of being trapped in the absorbing liquid is small, but when the particles become large, the droplets collide due to inertia and are easily collected, such as dust. The collection efficiency of is increased.

However, as in the prior art, an object having a temperature lower than the exhaust gas temperature, such as cooling water, is placed around the desulfurization tower in which there is a gas of 40 ° C. to 60 ° C. containing saturated steam due to the contact between the absorbing liquid and the exhaust gas. When sprayed, the water vapor selectively condenses on the surface of the object such as cooling water, so that the condensation of dust on the surface disappears. That is, when cooling water is supplied to the desulfurization tower, the gas in the vicinity of the cooling water is cooled and the water contained in the gas is condensed and adheres to the cooling water. When the water concentration in the cooled gas decreases, the water in the surrounding gas (having a saturated water concentration higher than the water concentration in the cooled gas) diffuses and enters the low temperature gas, Furthermore, since the condensation is repeated, the condensation of water in the gas does not occur.

On the other hand, in the present invention, although the temperature of the steam supply pipe and the temperature of the steam immediately after the supply are high, the temperature of the steam supply pipe and the temperature of the steam immediately after the supply are higher than the temperature of the steam in the desulfurization tower containing the gas of 40 to 60 ° C. Because of the low uniform temperature, when the steam supplied to the desulfurization tower is cooled by the surrounding gas,
Condensation of water vapor occurs, and droplets (fog) composed of fine water droplets are generated in the gas. At this time, most of the droplets grow with dust or the like suspended in a gas of several microns or less as a nucleus, and thus these fine particles are apparently enlarged, and the particle size of several tens to several hundreds of microns. Become particles.
The fine particles in the gas in the desulfurization tower flow around the spray droplets of the absorption liquid, so the ratio of collection in the absorption liquid is small, but
When the particles are large, inertially causes the droplets to collide with each other and is easily collected, and the collection efficiency of dust and the like increases. In particular,
When fine particles such as dust are present in the gas, droplets grow using these particles as nuclei, so that the dust and the like in the gas are apparently enlarged. If the particle size becomes large, it will be easily collected in the droplets of the absorbing liquid due to inertial collision.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to the drawings. In the embodiment shown in FIG. 1, the exhaust gas containing SO _{2 and the} like from the boiler combustion device is supplied to the desulfurization tower from the inlet duct 2 and enters the desulfurization tower main body 1. In the desulfurization tower, it comes into contact with the absorbing liquid sprayed from the spray nozzle 4, and SO ₂ in the exhaust gas is removed. The mist of the absorbing liquid is further removed by the mist eliminator 9 from the exhaust gas from which SO ₂ has been removed, and the exhaust gas is discharged to the outside of the system through the outlet duct 3. The sprayed absorbent absorbs SO ₂ and then falls into the oxidation tank 6. Oxidation tank 6
The absorption liquid inside is extracted by the circulation pump 5, and the header 1
It is sent again to the spray nozzle 4 through 0. The absorbing liquid sprayed from the lower spray nozzle 4 absorbs SO ₂ by coming into contact with the exhaust gas, and at the same time from 100 ° C. to 20 ° C.
Reduce the temperature of the exhaust gas, which is 0 ° C. At this time, the water in the absorbing liquid evaporates and the exhaust gas contains saturated steam. When steam at a temperature of 100 ° C. or higher is blown into the gas in the desulfurization tower in a saturated state of water from the steam pipe 19, a part of the steam is cooled and condensed by the surrounding gas to have a particle diameter of several tens to several hundreds of micrometers. Grow into droplets consisting of. These droplets grow with a small amount of fine dust contained in the exhaust gas as the nucleus, so that the apparent particle size becomes large. Although the droplets containing the increased dust are sent along with the exhaust gas, they are collected by colliding with the droplets of the absorbing liquid sprayed from the upper spray nozzle 4. The collecting performance at this time becomes higher as the diameter of the droplet becomes larger. Further, the collection performance can be freely changed by controlling the amount of water vapor supplied from the water vapor pipe 19. Further, since the condensed droplets of the absorbing liquid have a small particle size and a wide absorbing area, the ability to absorb SO ₂ is large and the desulfurization performance is improved.

2 and 3 show another embodiment. Figure 2
In each of the examples shown in FIG. 3 and FIG. 3, a steam pipe 19 for spraying steam is arranged in the outlet duct 3 of the desulfurization tower body 1. In the example shown in FIG. 2, a mist eliminator 9 is installed in the outlet duct 3, and a steam pipe 19 is installed upstream of this. Further, a washing water spray nozzle 21 for washing the mist eliminator 9 is attached. Water is supplied to the cleaning water spray nozzle 21 from a water tank 22 via a water supply pipe 23. Then, the water in the water tank 22 is partially supplied to the oxidation tank 6 through the water tank drain pipe 24. When steam is blown into the outlet duct 3 from the steam pipe 19, the exhaust gas is in a saturated state as in the desulfurization tower main body 1. Therefore, a part of the supplied steam is condensed to grow into large droplets with dust as a nucleus. The mist eliminator 9 can easily collect these, and the mist collecting efficiency is improved.

The example shown in FIG. 3 is also a method of supplying steam into the outlet duct 3, but in this case, the cleaning water is sprayed from the cleaning water spray nozzle 21 to the downstream of the steam pipe 19 to generate steam containing dust. The collection efficiency of the mist is enhanced by collecting the enlarged droplets due to the condensation. At the same time, the mist scattered from the desulfurization tower body 1 is also collected at the same time. As in the case shown in FIG. 2, the wash water is circulated and used after being collected in the water tank 22, and a part is sent from the water tank withdrawal pipe 24 to the desulfurization tower body 1. Water is supplied to the water tank 22 from a water supply pipe 23. The comparison of the dust removal performance when steam is supplied into the wet desulfurization apparatus of the above-mentioned example is shown. S in the exhaust gas at the entrance
O ₂ is 1000 ppm, dust concentration is 100 mg / m ³ N
The amount of absorption liquid sprayed in
m is ^3. The temperature of the absorption liquid has reached the saturation temperature and 55
℃. Under this condition, the amount of steam is 0.2 of the amount of exhaust gas,
As a result of conducting tests with 0.4 and 0.8%, respectively, the dust concentrations at the desulfurization tower outlet are 47%, 61%, and 75%, respectively, when steam is not supplied. It can be seen that the concentration can be reduced.

As described above, the present invention has the following advantages when applied to a wet desulfurization apparatus installed in a boiler system. That is, exhaust gas emitted from a boiler (not shown) is cooled to about 100 to 200 ° C. by an air preheater (not shown) for combustion air and sent to a dust collector (not shown) to remove most of dust. It After that, the exhaust gas is introduced into the desulfurization tower, and the desulfurization tower has a dust removal function by blowing in steam as described above, and the overall dust collection performance of the boiler system is determined by the dust collection performance of the dust collector and the desulfurization tower. To be done. If the dust collection performance in the desulfurization tower is improved, the load on the dust collector can be reduced. Moreover, even if the amount of sprayed absorption liquid is reduced when the SO ₂ concentration in the exhaust gas is low or the load of the boiler is reduced and the amount of treated gas is reduced, the amount of steam supplied to the desulfurization tower can be changed. It is possible to obtain a predetermined dust removal performance independent of the desulfurization performance. That is, it is not necessary to circulate and supply an excessive amount of absorbing solution for removing dust. At present, stricter dedusting performance than the current standard is required, and a method of installing a wet electrostatic precipitator after the desulfurization tower has been embodied.However, if this method is used, these facilities are also unnecessary. .

[0017]

According to the present invention, since the desulfurization tower itself has a dust removing function, even if the amount of sprayed absorption liquid is reduced when the SO ₂ concentration in the exhaust gas is low or the amount of exhaust gas decreases. The dust collection performance in the desulfurization tower does not deteriorate. as a result,
Even if severe dust removal performance is required, there is no need to install a wet electrostatic precipitator after the desulfurization tower.

[Brief description of drawings]

FIG. 1 is a system diagram of a wet desulfurization apparatus according to an embodiment of the present invention.

FIG. 2 is a system diagram of a wet desulfurization device according to another embodiment of the present invention.

FIG. 3 is a system diagram of a wet desulfurization device according to another embodiment of the present invention.

FIG. 4 is a system diagram of a conventional wet desulfurization device.

[Explanation of symbols]

1 ... Desulfurization tower main body, 3 ... Exit duct, 4 ... Spray nozzle,
6 ... Oxidation tank, 9 ... Mist eliminator, 19 ... Steam pipe, 21 ... Wash water spray nozzle, 22 ... Water tank, 2
3 ... Water supply pipe, 24 ... Water tank extraction pipe

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical indication location B01D 53/18 ZAB E 53/34 ZAB B01D 53/34 125 D 125 Q (72) Inventor Hirofumi Yoshikawa 3-6 Takaracho, Kure City, Hiroshima Prefecture Kure Research Institute, Babcock Hitachi Ltd. (72) Inventor Shigeru Nozawa 6-9 Takaracho, Kure City, Hiroshima Prefecture Babcock Hitachi Ltd., Kure Factory

Claims (4)

[Claims] 1. A wet desulfurization apparatus provided with a desulfurization tower for making gas-liquid contact with an absorbent such as an alkali metal or an alkaline earth metal in order to remove sulfur oxides contained in exhaust gas from a combustion apparatus such as a boiler. A wet desulfurization apparatus characterized in that a steam pipe for injecting steam is provided inside the desulfurization tower or inside the desulfurization tower outlet duct. 2. The wet desulfurization apparatus according to claim 1, wherein a demister is installed in the desulfurization tower on the downstream side of the steam pipe or in the desulfurization tower outlet duct. 3. A wet desulfurization method for removing sulfur oxides contained in exhaust gas from a combustion device such as a boiler by spraying an absorbent such as an alkali metal or an alkaline earth metal into the desulfurization tower. A wet desulfurization method, characterized in that dust in exhaust gas is removed by blowing water vapor into the interior of the desulfurization tower outlet duct. 4. The wet desulfurization method according to claim 3, wherein the steam blown into the desulfurization tower or the desulfurization tower outlet duct is steam having a temperature of 100 ° C. or higher.