JPH0722675B2 - Wet flue gas desulfurization equipment - Google Patents

Description

The present invention relates to a wet flue gas desulfurization apparatus that absorbs and removes sulfur oxides from exhaust gas and oxidizes and recovers calcium sulfite generated by the absorption into gypsum. Regarding

[Prior art and its problems]

In wet flue gas desulfurization, a solution or suspension of hydroxides, carbonates, sulfites or oxides of alkali metals, alkaline earth metals, ammonium etc. is used to absorb and remove sulfur oxides in the exhaust gas. A common method is to recover stable sulfate as a by-product.

With reference to FIG. 2, a conventional technique for recovering calcium sulfate (gypsum) using a calcium-based absorbent will be described. Exhaust gas 1 from a boiler or the like (not shown) is guided from a dust removal tower inlet duct 2 to a dust removal tower 3, where a dust removal circulation slurry containing calcium compounds (such as CaCO ₃ ) from a dust removal tower circulation tank 4 is sprayed. By doing so, the dust is removed and cooled. Then, the scattered mist is removed by the demister 5 and then sent to the absorption tower 6. In the absorption tower 6, the calcium-based absorption circulation slurry discharged from the absorption tower circulation tank 7 and supplied from the pipe 9 via the absorption tower circulation pump 8 is sprayed from the nozzle 10 to remove the sulfur oxides in the exhaust gas. Absorbed and removed. The entrained mist in the exhaust gas is removed by the demister 11, and the clean gas 12 is guided to the flue through the duct 13. On the other hand, the absorption circulation slurry containing the calcium-based absorbent that has absorbed the sulfur oxides becomes calcium sulfite in the absorption tower 6 and the absorption tower circulation tank 7, part of which is changed by the oxygen in the exhaust gas in the absorption tower 6. Partly oxidized to gypsum.
This absorption circulation slurry is supplied from the pipe 9 to the nozzle 10 in the upper part of the absorption tower via the absorption tower circulation pump 8 or to the dust removal tower circulation tank 4 via the bleed pump 14. The dust removal slurry in the dust removal tower circulation tank 4 comes into gas-liquid contact with the exhaust gas in the dust removal tower 3 to remove the sulfur oxides in the exhaust gas, thereby reducing the amount of unreacted limestone in the slurry and recovering the by-product gypsum. Is taken out. That is, first, the unreacted CaCO ₃ contained in the reaction tank 15 is converted into gypsum by adding sulfuric acid 16 therein, and the pH is adjusted to be suitable for the oxidation of calcium sulfite. This slurry is supplied to the oxidation tower 18 by the oxidation tower supply pump 17, where calcium sulfite is oxidized to gypsum by the air 19. The gypsum slurry thus obtained is guided to the thickener 20, is subjected to solid-liquid separation, and is dehydrated by the centrifugal separator 21 or the like to recover the gypsum 22. The filtered water 23 at the time of solid-liquid separation and dehydration is reused when preparing the limestone slurry. Limestone slurry, which is a sulfur oxide absorbent, is prepared in the limestone slurry tank 24 from limestone 25, filtered water 23 and makeup water 26, and is supplied into the absorption tower circulation tank 7 by a bleed pump 27.

As described above, in the conventional method, it is difficult to completely transform calcium sulfite into gypsum during the absorption process of the sulfur oxides in the absorption tower 6, so that the calcium sulfite generated in the absorption system is separately provided as described above. The method of making plaster in 18 has been adopted.

However, in recent years, many methods have been proposed in which the oxidation tower 18 is omitted and the oxidation of the sulfur compound to calcium sulfate is promoted in the sulfur oxide absorption section. As an example, a method of using an oxidation catalyst (Japanese Patent Publication No. 58-36619), a method of blowing air into an absorption tower circulation tank or a separately provided reaction tank (Japanese Patent Publication Nos. 55-116423 and 116424), JP-A-58-98126, JP-A-52452, JP-A-95543, JP-A-104619, and JP-A-58-95218) or a two-stage desulfurization method (JP-A-5
8-74126). However, the method using a catalyst is not economically viable unless it is recovered at a high rate, and in the air blowing method, unless a large amount of air is supplied as fine bubbles, the conventional oxidation tower 18 is used. It is not possible to oxidize potassium sulfite at such a rate that it can replace.

On the other hand, the present inventors have previously adopted an oxidation method in which air is supplied to the vicinity of the blades of the stirrer 28 attached to the circulation tank 7 of the absorption tower at the bottom of the absorption tower 6 to generate fine bubbles. It has been confirmed that the sulfur oxides in the exhaust gas can be recovered as gypsum without using a separate oxidation tower. In this method, by spraying the absorption circulation slurry containing CaCO ₃ as an absorbent into the absorption tower 6, not only the absorption of sulfur oxides but also the removal of dust can be performed at the same time (dust mixing method). By the way, it is known that a small amount of metal ions act as a catalyst in the oxygen oxidation reaction of calcium sulfite, but in this dust mixing method, elements such as iron are mixed with dust in the absorption system. And gives favorable results in carrying out the oxidation of calcium sulfite.

On the other hand, if a large amount of Cl, F, etc. is contained in the exhaust gas, a dust separation unit is provided separately to reduce the purity of gypsum, and a dust separation system that suppresses the mixing of the slurry between this dust removal unit and the absorption unit It is desirable to adopt. However, when the dust separation method is adopted as described above, the amount of the catalytic substance mixed with the dust into the absorption system is remarkably reduced, and as a result, the oxidation rate of calcium sulfite is reduced.

The object of the present invention is to recover sulfur oxides in exhaust gas as gypsum with high efficiency without separately installing an oxidation tower, and to prevent dust in the exhaust gas from mixing into the gypsum to obtain high-purity gypsum. It is to provide a wet flue gas desulfurization apparatus that can be obtained.

[Means and Actions for Solving Problems]

In order to achieve the above-mentioned object, the wet flue gas desulfurization apparatus according to the present invention has a dust removing section for removing dust in the exhaust gas by gas-liquid contacting a circulation slurry for dust removal containing a calcium compound and the exhaust gas, and a tower bottom part. An absorption circulation slurry containing a calcium compound stored in the absorption tower circulation tank provided is sprayed from the upper part of the tower to cause gas-liquid contact reaction with the exhaust gas that has passed through the dust removal section to absorb sulfur oxides into calcium sulfite. And an absorption / oxidation section for removing the calcium sulfite generated by this gas-liquid contact reaction and converting it into gypsum by supplying fine bubbles into the slurry in the absorption tower circulation tank to cause an oxidation reaction. A communication line for supplying the dust removal circulating slurry from the dust removing portion to the absorbing / oxidizing portion is provided.

According to the present invention, the circulation slurry for absorption containing the calcium compound is sprayed from the upper part of the tower, and the exhaust gas that has passed through the dust removal portion is subjected to gas-liquid contact reaction to convert sulfur oxides into calcium sulfite for absorption and removal. The calcium sulfite produced by this gas-liquid contact reaction drops together with the spray liquid into the absorption tower circulation tank provided at the bottom of the tower. In the absorption tower circulation tank, fine bubbles are supplied into the slurry in the tank to cause the calcium sulfite to undergo an oxidation reaction to form gypsum, and metal ions serving as an oxidation catalyst of the calcium sulfite in the oxidation reaction. Is effectively used by partially supplying the dust removal circulation slurry to the absorption / oxidation section through the communication line, and the inflow of the component that reduces the purity of the generated gypsum into the absorption / oxidation section is prevented.

〔Example〕

As shown in FIG. 1, the exhaust gas 1 from the boiler or the like is guided to the dust removing tower 3 from the dust removing tower inlet duct 2 of the dust removing section, where the dust removing circulation slurry from the dust removing tower circulation tank 4 is sprayed. After removing dust and cooling, and then removing the scattered mist by the demister 5, the absorption tower 6 of the absorption / oxidation section
Sent to. In the absorption tower 6, the calcium-based absorbent absorption circulation slurry, which is extracted from the absorption tower circulation tank 7 and is supplied from the conduit 9 via the absorption tower circulation pump 8, is sprayed from the nozzle 10 to Sulfur oxides are absorbed and removed. The entrained mist in the exhaust gas is removed by the demister 11, and the clean gas 12 is guided to the flue through the duct 13.

The absorption circulation slurry provided for desulfurization is the absorption tower circulation tank 7
Although falling inside, in the method of the present invention, fine bubbles 29 made of air are supplied to the absorption tower circulation tank 7 so that calcium sulfite generated by absorption of sulfur oxides in the exhaust gas is oxidized. There is. As an air oxidation method of calcium sulfite in the absorption tower circulation tank 7, fine bubbles are generated in the tank 7, and the amount of calcium sulfite in the slurry is a desired value (in the case of recovering gypsum, calcium sulfite-containing Any method may be used as long as the amount is usually suppressed to 0.5% or less). Specifically, as shown in FIG. 1, the method of supplying the fine bubbles 29 to the vicinity of the blades of the stirrer 28 installed in the absorption tower circulation tank 7 provides an oxidation rate that matches this purpose. In order to achieve complete oxidation of calcium sulfite in the absorption tower circulation tank 7 by this oxidation method, it is necessary to optimize the shape of the stirring blade 28, stirring speed, air supply amount, slurry pH and the like. Of these, the slurry pH must be a condition that the absorption of sulfur oxides and the oxidation of the formed calcium sulfite proceed at a desired rate. The proper pH for absorption of sulfur oxides and the oxidation of calcium sulfite is 5.5 or higher and 4.0 to 5.5, respectively, and the proper pH common to both reactions is limited to around 5.5. On the other hand, the present inventors have found that the calcium sulfite in the slurry falling into the absorption tower circulation tank 7 is in a solution state, and if the oxidizing means in this tank 7 is appropriate, the calcium sulfite content in the vicinity of pH 6 will increase. It has been confirmed that the oxidation progresses sufficiently, and that the method of blowing air to the stirring blade 28 described above gives good results as the oxidation means. The present invention is a further improvement of this.

For the oxygen oxidation reaction of calcium sulfite, Fe ²⁺ , Mn ²⁺ ,
It is well known that metal ions such as Co ²⁺ and Cu ²⁺ act as a catalyst. Here, first, the function of the metal ions will be described. In normal wet desulfurization, these metal elements are mixed into the absorption system along with the used water, limestone, and dust in the exhaust gas, so it is not necessary to intentionally add catalyst ions. Oxidation of calcium sulfite proceeds faster than in pure water systems. Particularly in the dust mixing method, since the dust-removing circulating slurry extracted from the dust-removing section is conventionally guided to the oxidation tower 18, a catalytic effect of ions mainly composed of Fe ²⁺ appears. On the other hand, also in the case of the dust separation method, some dust passes through the dust removing section and mixes into the absorbing section, and metal ions derived from limestone also act as a catalyst. In addition, in either method, when calcium sulfite is oxidized in the oxidation tower 18, sulfuric acid is released according to the following formula and the pH is lowered to Ca (HSO ₃ ) ₂ + O ₂ → CaSO ₄ + H ₂ SO _4; The catalyst ions that were present as a solid will dissolve in the oxidation tower. Therefore, since the separation mother liquor of gypsum is circulated and used for the preparation of the absorbent, as long as it has a dedicated oxidation tower, the catalytic effect of metal ions is exhibited in the oxidation tower 18 in any of the methods of dust separation and dust mixing. become.

On the other hand, in the case of the dust mixing method among the methods of absorbing the sulfur oxides and oxidizing the calcium sulfite in the same tower, the metal elements due to the dusts are mixed into the absorption-oxidation system. In the case of the separation method, the amount of the metal element mixed due to the dust is extremely small. In addition, since absorption and oxidation are performed at the same pH (5.5 to 6.0), and the process of releasing acid as in the case where a dedicated oxidation tower is installed is not included, the absorption circulation in the absorption tower circulation tank 7 The concentration of catalyst ions in the slurry becomes significantly lower than that of other methods, and in some cases, the oxidation rate of calcium sulfite becomes lower than the desired value.

The present invention relates to a device for increasing the oxidation rate of calcium sulfite in the one-column desulfurization system of this dust separation system. The absorption circulation slurry in the absorption tower circulation tank 7 is transferred from the pipe 30 to the dust removal tower circulation tank 4. On the other hand, it is characterized in that a part of the dust removal circulation slurry is supplied from the dust removal section to the absorption tower circulation tank 7 of the absorption / oxidation section through a pipe line 31 serving as a communication line. 32 indicates a bleed pump. As a result, metal ions serving as an oxidation catalyst for calcium sulfite are supplied to the absorption / oxidation unit. The supply amount of the dust-removing circulation slurry is determined by the oxidation catalytic ability with respect to calcium sulfite, but if it is supplied in an unnecessarily large amount, the concentration of Cl or the like in the absorption system increases, which is not preferable. A part of the slurry extracted from the absorption tower circulation tank 7 is guided to the thickener 20 through the pipe 33, separated into solid and liquid, and then dehydrated by the centrifuge 21 or the like to recover the gypsum 22. The filtrate 23 at the time of solid-liquid separation and dehydration is used for preparation of limestone slurry and the like. The limestone 25, which is a sulfur oxide absorbent, was added to the limestone slurry preparation tank 24, and filtered water was added.
The limestone slurry is mixed with 23 and makeup water 26 and introduced into the absorption tower circulation tank 7 via a bleed pump 27.

Example 1. 250% 10 gypsum slurry was placed in a polyvinyl chloride tank having a diameter of 80 cm, and Ca (HSO ₃ ) ₂ was added at 0.35 mol /, Fe:
A solution containing 1.7 ppm, V: 100 ppb, Cu: 20 ppb, Ni: 20 ppb, Mn: 40 ppb was supplied at a flow rate of 16.7 / h. Change the pH of this slurry to CaCO
By adding ₃ powders or (1 + 3) H ₂ SO ₄
Maintained at 5.5 and 1.2Nm ³ / h ・ unit (total 4.8Nm ³ ) of air to 4 agitators (blade diameter 12cm) mounted on the side of the tank.
/ h) and stirred at a speed of 1,000 rpm, and the slurry was extracted at a flow rate of 16.7 / h (corresponding to a retention time of 15 h). The oxidation rate of calcium sulfite was 99% or more after 5, 10 and 20 hours from the start of the reaction.

Example 2. When the pH of the slurry in the tank was adjusted to 6.0 after the experiment of Example 1, the oxidation rates of calcium sulfite after 5 and 10 hours were 99.1 and 98.8%, respectively.

Comparative Example 1. When the supply of Fe, V, Cu, Ni, and Mn was omitted in Example 1, the oxidation rates of calcium sulfite after 5 and 10 hours were 97.0 and 95.1%, respectively.

Comparative Example 2. In Example 1, calcium sulfite was used instead of CaSO ₃
When supplied using 1 / 2H ₂ O powder, the oxidation rate of calcium sulfite after 5 hours was 60%.

Comparative Example 3. The pH of the slurry in Example 1 was 6.0, and the addition of calcium sulfite was CaCO ₃ 1 / 2H ₂ O for 5 hours.
The subsequent oxidation rate of calcium sulfite was 10%.

〔The invention's effect〕

According to the present invention, the dust removal circulating slurry is absorbed from the dust removal unit.
By providing a communication line to supply to the oxidation part, the metal ions, which are contained in the exhaust gas and serve as the oxidation catalyst for calcium sulfite, can be effectively used by partially absorbing and supplying the circulation slurry for dust removal to the oxidation part. It is possible to prevent the components that reduce the purity of the generated gypsum from flowing into the absorption / oxidation part. Therefore, it is possible to highly efficiently recover the sulfur oxides in the exhaust gas as gypsum without separately installing an oxidation tower, and it is possible to obtain high-purity gypsum by preventing dust in the exhaust gas from being mixed into the gypsum. .

In other words, absorption of sulfur oxide
The oxidation reaction can be carried out in the same tower, and high-purity gypsum can be obtained.

[Brief description of drawings]

FIG. 1 shows a block diagram of an apparatus according to the present invention, and FIG. 2 shows a block diagram of a conventional example. 1 ... Exhaust gas, 4 ... Dust removal tower circulation tank, 6 ... Absorption tower, 7 ... Absorption tower circulation tank, 22 ... Gypsum, 29 ... Fine bubbles, 31 ... Communication line.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location C01F 11/46 102 9040-4G

Claims (1)

[Claims] 1. A dust removing section for removing dust in exhaust gas by bringing a circulation slurry for dust removal containing a calcium compound and exhaust gas into gas-liquid contact, and a calcium system stored in an absorption tower circulation tank provided at the bottom of the tower. A circulation slurry for absorption containing a compound is sprayed from the upper part of the tower to cause a gas-liquid contact reaction with the exhaust gas that has passed through the dust removal section to absorb and remove sulfur oxides into calcium sulfite, and at the same time, a sulfurous acid produced by this gas-liquid contact reaction The absorption tower circulation tank is provided with an absorption / oxidation unit for supplying gypsum by supplying fine bubbles into the slurry in the tank to cause an oxidation reaction, and absorbs the dust removal circulation slurry from the dust removal unit. A wet flue gas desulfurization device characterized in that a communication line for supplying to the oxidizer is provided.