JP2940981B2 - Wet exhaust gas desulfurization apparatus and wet exhaust gas desulfurization method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a wet exhaust gas desulfurization apparatus and a wet exhaust gas desulfurization method, and more particularly to a method of using a calcium compound slurry as an absorbent and a slurry absorption liquid after use. The present invention relates to a wet-type exhaust gas desulfurization apparatus and a wet-type exhaust gas desulfurization method with reduced emission to the outside.

[Conventional technology]

Exhaust gas emitted from coal-fired thermal power plant boilers contains sulfur dioxide (SO ₂ ) as a harmful gas component.
In addition to nitrogen and nitric oxide (NO), halogen compounds such as chlorine and fluorine are contained.

In an exhaust gas desulfurization device, when a limestone slurry is brought into contact with an exhaust gas, SO ₂ gas and the like are absorbed, and at the same time, halogen compounds and the like are concentrated in the slurry. The absorbed SO ₂ gas is oxidized and collected as gypsum, but the halogen compound is in a dissolved state because of its high solubility. As the concentration of chlorine dissolved in the slurry increases, the rate of limestone dissolution decreases.

Therefore, in order to maintain the concentration of the halogen compound or the like in the slurry at a predetermined level or less, it is necessary to extract a part of the slurry and replace it with a new absorbent (makeup) instead. The wastewater contains COD substances such as dithionic acid in addition to the halogen compounds, and it is necessary to treat the wastewater and discharge it.

In recent years, studies have been made on a method of closing the desulfurization device so as not to discharge wastewater, and in Japanese Patent Application Laid-Open No. 50-72871, the entire amount of the effluent from the desulfurization device is included in the exhaust gas immediately before the dry dust collector. A method of a non-drainage system that sprays, evaporates to dryness, collects solids with a dust collector, and does not discharge wastewater to the system is being studied.

It should be noted that, in consideration of this type of non-drainage, related ones, besides Japanese Patent Application Laid-Open No. 50-72871,
No. 24669, JP-A-52-32896, JP-A-55-9722
No. 5 publication.

In addition, about the method of adding slaked lime and gypsum to Al ₂ O ₃ , SiO ₂ , Fe ₂ O _3, etc. which are the main components of limestone, kneading with water, performing a hydration reaction with steam to generate a cured product, It is already described in Material Chemistry of Cement (Dainihon Tosho Publishing) p120.

Hereinafter, the conventional technique of the wet desulfurization apparatus will be specifically described with reference to FIG.

The absorbent slurry solution supplied to the desulfurization device 1 for the exhaust gas 101 is recycled and concentrated, and then discharged as wastewater 100 such that the chlorine concentration in the system becomes a certain value or less. The reason for this is to prevent a decrease in desulfurization performance and to prevent corrosion of equipment materials. The desulfurized effluent discharged in this way contains SS (Suspended Solid) and F
(Fluorine), heavy metals and COD (Chemical Oxygen Dem)
), the flocculation agent (eg, Krihix (Kurita Corporation)) 110 is added for coagulation and precipitation in the SS removal step 10, and then F and heavy metal removal step 11 are performed.
Then, slaked lime 111, sodium carbonate 112, and caustic soda 113 are added to remove F and heavy metals. Further, the wastewater is added with sulfuric acid 114 in a COD removal step 12, and COD removal is mainly performed by an ion exchange resin.
Make H adjustment and release.

In the above prior art, the wastewater after being used as the absorbent slurry solution is detoxified and then discharged out of the system.However, in the case where the wastewater cannot be discharged as wastewater, the wastewater is discharged immediately before the dry dust collector. The limestone and wastewater are agglomerated and discarded.

[Problems to be solved by the invention]

In any of the non-drainage systems described above, the larger the amount of wastewater discharged from the desulfurization unit, the greater the equipment cost required for the wastewater treatment and the utility cost required for operation. On the other hand, if the amount of wastewater discharged from the desulfurization unit is to be reduced, the degree of recirculation of the water used in the unit, that is, the concentration of impurities, especially chlorine, will increase, and if the material of the unit is upgraded to take measures against edible food, No.

An object of the present invention is to efficiently reduce the above-mentioned contradictory problem, that is, the amount of wastewater that needs to be finally treated.

[Means for solving the problem]

The above object is achieved by contacting a flue gas containing sulfur oxides with a calcium compound slurry absorbent as an absorbent to absorb and remove sulfur oxides in the flue gas. A drainage adjustment tank for adding hydrochloric acid to the calcium compound slurry absorption liquid after absorbing the oxide, and a wastewater concentration tower for concentrating the slurry absorption liquid after addition of hydrochloric acid discharged from the drainage adjustment tank with a heating device are provided. A wet exhaust gas desulfurization method, comprising: contacting a combustion exhaust gas containing sulfur oxides with a calcium compound slurry absorbing solution as an absorbent to absorb and remove sulfur oxides in the exhaust gas; A drainage adjustment process in which a part of the slurry absorption liquid after absorption is extracted and mixed with hydrochloric acid, and a wastewater in which the slurry absorption solution after the drainage adjustment process is heated and concentrated Concentration step, a kneading step of adding and kneading an alkaline calcium compound and coal ash to the slurry absorbing solution after the wastewater concentration step, and a step of drying the kneaded material, which is achieved by a wet exhaust gas desulfurization method characterized by having a step of drying. You.

[Action]

The desulfurization effluent is usually saturated with gypsum (CaSO ₄ ), so if it is concentrated, gypsum crystals will precipitate, leading to scaling, but add hydrochloric acid to the desulfurization effluent as in the following reaction formula As a result, the gypsum becomes calcium chloride and has a high solubility. Therefore, even if the gypsum is concentrated to a considerable concentration, crystals such as gypsum do not precipitate. Therefore, by adding hydrochloric acid to the waste water and then bringing the waste water into contact with an exhaust gas whose water content is not saturated, it can be easily concentrated.

CaSO ₄ + 2HCl → CaCl ₂ + H ₂ SO ₄ Example An example of the present invention will be described below with reference to FIG. SO
The boiler exhaust gas 101 containing x is brought into contact with a slurry containing limestone as an absorbent to absorb and remove SOx in the exhaust gas. The desulfurization effluent 100 discharged from the desulfurization device 1
The hydrochloric acid 102 necessary for solubilizing Ca in the wastewater is added and mixed. Wastewater mixed with hydrochloric acid 21
Is led to the wastewater concentration tower 3 where the moisture-unsaturated exhaust gas
In contact with 101a, the wastewater is concentrated. The wastewater 31 concentrated to a predetermined concentration is extracted from the concentration tower, guided to the kneader 4, where slaked lime 103 and coal ash 104 are added and kneaded, and the kneaded material 41 is dried in the molding machine 5. After sizing to an appropriate shape and dimensions, hot air drying using exhaust gas is performed in the dryer 6, and finally the drainage hardened material 105
To be collected.

A feature of the present invention is that concentration is performed after adding hydrochloric acid to desulfurization wastewater. This addition of hydrochloric acid converts the gypsum component in the waste water to CaCl ₂ and, when the vinegar and the desulfurized untreated exhaust gas are brought into contact in the concentration step, the SO in the exhaust gas.
_It also has the effect of not absorbing ₂ and not increasing the sulfite / sulfate groups in the wastewater. FIG. 2 shows the results of a bench-scale exhaust gas treatment experiment performed by the present inventors, and shows that SO ₂ in exhaust gas is not absorbed by setting the pH of waste water to 2 or less. . Since the increase in sulfite / sulfate in the wastewater in the wastewater concentration step causes scaling in the concentration tower 3, the present invention is superior in preventing desulfurization reaction from occurring in the concentration tower.

The concentrated wastewater obtained in this way can increase the hardness of the cured product due to the increased chlorine concentration during agglomeration with coal ash and slaked lime. This is due to the hydration reaction of chlorine in the wastewater with slaked lime and coal ash, resulting in β-3CaO
_This is because it is stably solidified as a hydrate such as ₂ O ₃ .CaCl ₂ .XH ₂ O.

Hereinafter, the results of the tests performed by the present inventors will be described in detail.

 The following test of the beaker scale was performed.

Test A (method of the present invention) After adding hydrochloric acid to the wastewater from the wet desulfurization device, the wastewater was concentrated using simulated exhaust gas. Hydrochloric acid was added so that the pH of the waste water was 2.

[Simulated exhaust gas properties]

SO _2: 1000ppm (dry) Temperature: 130 ° C. [desulfurization effluent properties] ^{Cl -: 7270ppm Ca 2+: 4050ppm} F -: 20ppm pH: 5.5 Cl in the concentrated waste water ^- concentration was concentrated operations until 70,000ppm However, no precipitation of solids was observed, and no scale was deposited in the concentration bottle.

Test B The same operation was performed as in Test A except that hydrochloric acid was not added. As a result, a small amount of solid matter was observed in the obtained concentrated wastewater, and a thin scale was also observed in the concentrated bottle.

As is clear from the results of Tests A and B, concentration can be achieved without causing a problem such as scaling by adding a small amount of hydrochloric acid before the concentration step.

According to the method of the present invention, since the wastewater is concentrated after leaving the apparatus without excessively increasing the degree of water circulation (use of recycling) in the desulfurization apparatus, the material of the desulfurization apparatus body is upgraded. No need.

On the other hand, when the wastewater concentrating method according to the present invention is used, even if a normal wastewater treatment apparatus that does not solidify wastewater is used, the amount of wastewater required for treatment is reduced, and the apparatus scale can be reduced.

As a wastewater concentration operation, a concentration method using exhaust gas as a heat source has been described as an example, but this concentration operation is not limited to this, and even if an evaporator using steam as a heat source is used,
Similar effects can be obtained.

〔The invention's effect〕

According to the present invention, regardless of whether the wastewater is solidified or the wastewater is treated by the wastewater treatment device, the amount of wastewater that needs to be finally treated can be reduced. Chemicals can be reduced.

In particular, in the wastewater solidification method, kneading by wastewater concentration,
Not only can the scale of the drying equipment be reduced, but also the chlorine concentration can be increased, and the hardness and stability of the finally recovered solid can be increased.

[Brief description of the drawings]

FIG. 1 is a system diagram of an embodiment of the present invention, and FIG.
FIG. 3 is a diagram showing a relationship between a pH value and a desulfurization rate, and FIG. 3 is a diagram of a wastewater treatment system of a conventional wet exhaust gas desulfurization apparatus. DESCRIPTION OF SYMBOLS 1 ... Desulfurization apparatus, 2 ... Drainage adjustment tank, 3 ... Drainage concentration tower, 4 ... Kneader, 5 ... Molding machine, 6 ... Dryer, 21 ...
... hydrochloric acid added wastewater, 31 ... concentrated wastewater, 41 ... kneaded material, 100
... desulfurization wastewater, 101, 101a ... boiler exhaust gas, 102 ... hydrochloric acid, 103 ... slaked lime, 104 ... coal ash, 105 ... hardened material.

Continued on the front page (72) Inventor Atsushi Katagawa 6-9 Takara-cho, Kure-shi, Hiroshima Babcock Incorporated Kure Plant (58) Field surveyed (Int.Cl. ⁶ , DB name) B01D 53/50

Claims (5)

(57) [Claims] 1. A combustion exhaust gas containing a sulfur oxide is brought into contact with a calcium compound slurry absorbing liquid as an absorbent,
In a wet exhaust gas desulfurization unit that absorbs and removes sulfur oxides in the exhaust gas, a drainage adjustment tank that adds hydrochloric acid to the calcium compound slurry absorbent after absorbing the sulfur oxides discharged from the desulfurization unit, and is discharged from the drainage adjustment tank A wet exhaust gas desulfurization apparatus comprising: a wastewater concentration tower for concentrating a slurry absorption liquid after addition of hydrochloric acid by a heating device. 2. A wet exhaust gas desulfurization apparatus according to claim 1, wherein the heating apparatus for the wastewater concentration tower is a combustion exhaust gas heating apparatus. 3. A wet exhaust gas desulfurization method for bringing a combustion exhaust gas containing a sulfur oxide into contact with a calcium compound slurry absorbing solution as an absorbent to absorb and remove the sulfur oxide in the exhaust gas. A drainage adjustment step of extracting a part of the slurry absorption liquid and mixing with hydrochloric acid, and a wastewater concentration step of heating and concentrating the slurry absorption liquid after the drainage adjustment step,
A wet exhaust gas desulfurization method comprising: a kneading step of adding and kneading an alkaline calcium compound and coal ash to a slurry absorption solution after a wastewater concentration step; and a step of drying the kneaded material. 4. A kneading step according to claim 3, wherein slaked lime and coal ash are added to the slurry absorbing liquid after the concentration step and kneaded to adjust the slurry pH to 7 or more, and the kneaded slurry is contacted with combustion exhaust gas. And a drying step of drying and curing. 5. The wet exhaust gas desulfurization according to claim 3, further comprising a step of forming the kneaded material into a predetermined shape after the kneading step, and a drying step of drying and curing the formed kneaded material. Method.