JP2001149744A - Scale controlling method in wet stack gas desulfurizing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for controlling the precipitation of the scale of calcium salts such as calcium sulfate or calcium sulfite formed in an absor bent solution of a wet stack gas desulfurizing device. SOLUTION: This scale controlling method in the wet stack gas desulfurizing device comprises adding polyphosphates to the absorbent solution of the device.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wet flue gas desulfurization apparatus for removing sulfur oxides (SOX) contained in flue gas, and relates to calcium sulfite and calcium sulfate produced in an absorbent of the wet flue gas desulfurization apparatus. It is intended to provide a method for suppressing the precipitation of calcium salts and the like as scale.

[0002]

2. Description of the Related Art Smoke exhaust from boilers such as oil factories, paper mills, and thermal power plants contains sulfur oxides, and the sulfur oxides are removed by a flue gas desulfurization unit before being discharged to the environment. ing. There are two types of flue gas desulfurization: wet and dry methods. Further, flue gas desulfurization by wet method involves absorption in lime or gypsum slurry (lime-gypsum method) and absorption in sodium hydroxide aqueous solution (soda method). , Magnesium hydroxide slurry (magnesium hydroxide method)
There is.

In the lime-gypsum method, the absorbing solution is a calcium compound itself, and in the magnesium hydroxide method, the magnesium hydroxide contains a calcium compound derived from impurities in the raw material magnesium oxide. Produces calcium sulfite and calcium sulfate in contact with sulfur oxides. These calcium sulphite and calcium sulphate have formed a hard scale on the inner walls of the absorption tower and the peripheral pipes, causing a problem that the operation is hindered.

[0004] In order to suppress the formation of such calcium scale, a method of adding a polymer electrolyte such as polyacrylic acid or phosphonic acid has been used for a long time.
Further, in the lime-gypsum method, fine crystals of calcium sulfate are added to a lime slurry in advance, and the crystals are used as seed crystals to deposit and deposit crystals thereon [Japanese Patent Laid-Open Publication No.
No. 108373] and a method of adding a water repellent to the slurry [Japanese Patent Publication No. Sho 62-54528] has been proposed. On the other hand, in the magnesium hydroxide method, the pH of the absorption solution of the magnesium hydroxide slurry is controlled to a specific range on the acidic side, the absorption wastewater is guided to an oxidation tower, and magnesium hydroxide is added to neutralize the pH while adding neutrality. A method of suppressing the formation of scale by discharging the aqueous solution out of the system as a magnesium sulfate aqueous solution (Japanese Patent Application Laid-Open No. Sho 60-261531) has been proposed.

[0005]

However, none of the conventional methods has a sufficient effect of suppressing the formation of scale, and it has been necessary to frequently stop the operation and peel off the scale manually.

Accordingly, an object of the present invention is to provide a method for efficiently suppressing the formation of scale due to calcium sulfite and calcium salts of calcium sulfate generated in an absorbent in such a wet flue gas desulfurization apparatus.

[0007]

Means for Solving the Problems The present inventors have conducted intensive studies on a method capable of efficiently suppressing the scale of calcium sulfite and calcium sulfate without adversely affecting the absorption liquid of the wet desulfurization apparatus. It has been found that this object can be achieved by adding specific polyphosphates to the present invention.

[0008] That is, claim 1 of the present invention is a method for suppressing scale in a wet flue gas desulfurization apparatus, which comprises adding polyphosphates to the absorbent of the wet flue gas desulfurization apparatus. The method according to claim 1, wherein the absorption liquid in the wet desulfurization apparatus is a magnesium hydroxide slurry.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention relates to a method for suppressing calcium scale deposited on a wet desulfurization apparatus, which comprises adding a polyphosphate to a desulfurization absorption liquid.

FIG. 1 is a conceptual diagram of a typical wet desulfurization apparatus. In the wet desulfurization device, the absorbing liquid is guided from the lower portion of the device to the upper portion of the device by a pump, and is showered and circulated at the upper portion of the device. Exhaust gas from the boiler is sent to the lower part of the device, comes into contact with the shower of the absorbing liquid in the device, removes sulfur oxides in the exhaust gas, and is discharged from the upper part of the device.

The wet flue gas desulfurization apparatus to which the present invention is applied is:
The lime-gypsum method and the magnesium hydroxide method have a problem of calcium scale. In these methods, lime or magnesium hydroxide is used as a slurry of 5 to 10% by weight as an absorbing liquid. In the soda method, since the absorption liquid does not substantially contain calcium, the scale problem does not occur and is not the object of the present invention.

In the lime-gypsum method, since the absorbing solution is a lime slurry, when reacted with sulfur oxide, calcium sulfite is further oxidized to produce calcium sulfate.

In the magnesium hydroxide method, the absorbing solution is a magnesium hydroxide slurry. Since magnesium hydroxide is usually made from naturally occurring magnesium oxide,
Calcium oxide contained as an impurity in natural magnesium oxide is introduced as calcium hydroxide. When such a magnesium hydroxide slurry reacts with sulfur oxides, calcium sulfite and calcium sulfate are generated from the coexisting calcium hydroxide.

When calcium sulfite and calcium sulfate are formed in the absorbing solution and the concentration thereof is increased, a hard scale precipitates on the inner wall surface of the absorbing tower or the peripheral piping in which the absorbing solution is circulated.

According to the present invention, the formation of calcium scale is suppressed by adding polyphosphates to an absorbing solution.

The polyphosphates are sodium or potassium salts of polyphosphoric acids (usually having a degree of condensation of 10 to 50) such as pyrophosphoric acid, acidic pyrophosphoric acid, tripolyphosphoric acid, tetrapolyphosphoric acid, hexametaphosphoric acid, and polymetaphosphoric acid. May be used in combination of two or more.

The polyphosphates may be added to the desulfurizer absorption liquid as they are, but it is more practically preferable to add them in an aqueous solution.

The concentration of polyphosphates required to suppress the formation of calcium scale varies depending on the calcium concentration in the desulfurizer absorption liquid, the sulfur oxide concentration in the flue gas, and cannot be uniformly determined. However, in order to sufficiently exhibit the effect of the present invention, usually 0.5 to 500 p
pm, preferably 10 to 100 ppm, more preferably 30 to 70 ppm. 500p of polyphosphates
Even if added at more than pm, the effect of suppressing the formation of calcium scale is sufficient, but an effect commensurate with the added amount cannot be expected, and it is often economical to lose. Also, 0.5pp
If the amount is less than m, the effects of the present invention may not be sufficiently exhibited.

The method of adding the polyphosphates is not particularly limited, but is optional such as continuous addition or intermittent addition. What is important is that the polyphosphates be maintained at the above concentration in the absorbing solution.

[0020]

The present invention will be described in detail with reference to the following examples.
The present invention is not limited by the following examples.

Example 1 A magnesium hydroxide slurry used in a flue gas desulfurization step of a paper mill [slurry concentration: 5% by weight, calcium concentration in the slurry; 4,4
00 ppm (as CaCO3)], and a calcium salt precipitation evaluation test was performed. 125 mL of magnesium hydroxide slurry in a mayonnaise bottle
Was collected, a predetermined amount of a scale inhibitor was added, and the mixture was capped and allowed to stand at 50 ° C. for 18 hours. Next, No. The resulting filtrate was filtered through a filter of No. 6, and the calcium concentration of the filtrate was determined by EDTA titration [JIS K0101]. When calcium crystals grow, they remain on the filter, but when they have a scale-inhibiting effect, they do not grow as crystals but come out in the filtrate. Therefore, the scale deposition suppression rate was calculated by the following equation.

[0022]

(Equation 1) Here, I (Ca); the calcium hardness (as ppm. CaCO3) of the test solution before the start of the test, B (Ca): the residual calcium hardness (as ppm. CaCO3) after the test without the addition of the scale inhibitor, S (Ca): Residual calcium hardness (pp) after testing with scale inhibitor added
m. (As CaCO3).

The results obtained are shown in Table 1.

[Table 1]

These results show that the polyphosphates of the present invention have a higher calcium precipitation inhibitory rate than the acrylic acid polymers, phosphonic acids, and phosphinic acids used for comparison, and have an excellent calcium scale inhibiting effect. Was.

Example 2 Exhaust gas amount 55 t / h, magnesium hydroxide slurry [slurry concentration: 3% by weight, calcium concentration in slurry: 2900 ppm (CaCO3
In a paper mill boiler flue gas exhaust system using)) as an absorbing liquid, when a scale inhibitor is not used (no treatment), there is a large amount of scale adhesion, a thickness of 3 mm in 3 months, and a pressure loss rise during this period of 2 kPa. It was intense. On the other hand, when 7 ppm of sodium hexametaphosphate was added to the magnesium hydroxide slurry for the purpose of controlling the scale, the formation of scale was greatly reduced, the scale thickness was operated for 3 months, and the operation was performed with no pressure loss change of 0.1 mm or less. did it.

[0026]

[Effects of the Invention] By adding polyphosphates to the absorbent of the wet flue gas desulfurization apparatus of the present invention, even if the concentration of calcium sulfite or calcium sulfate in the absorbent becomes high, the inner wall of the absorption tower or the peripheral piping can be used. This makes it possible to suppress the precipitation as a hard scale, and to operate the flue gas desulfurization device safely and efficiently for a long period of time.

[Brief description of the drawings]

Fig. 1 Conceptual diagram of a typical wet desulfurization unit

[Explanation of symbols]

 1. Boiler exhaust gas 2. Absorption liquid tank 3. 3. Treatment agent addition position 4. Magnesium hydroxide slurry circulation line Exhaust gas 6. Filling tank

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Junichi Nakajima 6-6-9, Yokkaichi City, Mie Prefecture F-Term in Yokkaichi Research Laboratories, Hakuto Co., Ltd. 4D002 AA02 AC01 BA02 CA01 CA07 DA05 DA06 DA12 DA16 DA18 EA07 EA13 GA01 GB01 GB06 GB08 GB20 HA05

Claims (2)

[Claims] 1. A method for suppressing scale in a wet flue gas desulfurization apparatus, comprising adding a polyphosphate to the absorbent of the wet flue gas desulfurization apparatus. 2. The method for controlling scale in a wet flue gas desulfurization device according to claim 1, wherein the absorption liquid in the wet desulfurization device is a magnesium hydroxide slurry.