JPH07116457A - Method and apparatus for controlling flue gas wet desulfurization treatment - Google Patents

Abstract

PURPOSE:To provide a flue gas desulfurization method which can solve problems concerning the lowering in the reactivity of limestone due to HC1 absorption in exhaust gas and the oxidation function of sulfite salt. CONSTITUTION:In wet desulfurization using absorbent slurry containing a highly concentrated Cl<-> ion, two units of an absorption tower are installed to attain high functions of desulfurization and sulfite salt oxidation, and the absorbent slurry in the first absorption tower 3 on the upstream side of an exhaust gas passage is operated at a low pH value. The Ca<2+> concentration is reduced by adding an alkaline non-calcium type compound 7 represented by dolomite into the first absorption tower 3, the reactivity of limestone and the oxidation function of sulfite salt are prevented from lowering, the absorbent slurry in the second absorption tower 9 is operated at a high pH value, and an alkaline calcium type compound 10 such as limestone is used as an absorbent.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flue gas desulfurization system, and more particularly to improvement in performance of a wet flue gas desulfurization system in which limestone is used as a desulfurizing agent and gypsum is recovered as a by-product.

[0002]

Exhaust gas from coal or heavy oil fired boilers usually contains sulfur oxides (SO ₂ ) with a concentration of several 100 ppm,
By treating this with a desulfurizer, it is possible to get a few
The most common desulfurization method is to release a gas containing about 0 ppm of SO ₂ . At present, a typical method of a flue gas desulfurization apparatus for boiler exhaust gas in a thermal power plant is a limestone-gypsum method wet desulfurization method in which limestone is used as an SO ₂ absorbent and gypsum is finally produced as a byproduct. is there.
Of the limestone-gypsum method wet desulfurization method, the one-tower type desulfurization method, which is currently becoming the mainstream, will be described with reference to FIG.

In FIG. 3, the exhaust gas 31 is guided from the absorption tower duct 32 to the absorption tower 33, and the contained SO ₂ is removed. The exhaust gas 31 that has passed through the absorption tower 33 has its entrained mist removed by a demister 34, is heat-exchanged with the exhaust gas guided to the absorption tower 33 by a gas-gas heater (not shown), and is then discharged from the chimney as purified gas. On the other hand, the limestone 35 that serves as an absorbent is supplied to the limestone slurry tank 36 and serves as an absorbent slurry. The supply of absorbent slurry is determined by the amount of SO ₂ that is treated. The absorbent slurry is transferred to the absorption tower circulation tank 38 by the absorption tower circulation pump 39.
Is discharged from the suction tower 33, sprayed into the absorption tower 33, dropped while coming into contact with the exhaust gas rising in the tower, and returned to the absorption tower circulation tank 38. The slurry at the time of returning to the circulation tank 38 contains calcium sulfite generated by absorption of SO ₂ , but in the system shown in FIG. 3, the agitation for oxidation that disperses the air 40 as fine bubbles in the slurry. Machine 4
1 and the slurry agitator 42, if necessary, the circulation tank 38
The calcium sulfite is oxidized to gypsum.

As described above, since the sulfite in the slurry dropped into the circulation tank 38 is rapidly oxidized, most of the solid substance in the slurry in the circulation tank 38 is gypsum, and the amount of limestone and sulfite is small. . A predetermined amount of the slurry in the circulation tank 38 is sent to the gypsum slurry tank 43. The slurry in the gypsum slurry tank 43 is sent to the gypsum separator 45 by the slurry pump 44, and the powdery gypsum 46 is collected. The gypsum separation mother liquor 47 generated in the gypsum separator 45 is
If necessary, the water is temporarily stored in the filtered water tank 48, and a predetermined amount thereof is introduced to the limestone slurry tank 36 by the pump 49 and used for preparing the limestone slurry. A part of the gypsum separation mother liquor 47 is sent to the wastewater treatment device 50 as wastewater as needed. In the above method, the dedicated oxidation tower that oxidizes sulfite to sulfate, which was installed by the conventional method, is omitted, and unreacted limestone and sulfuric acid added for the decomposition of sulfite are unnecessary. It has various advantages.

In the absorption part of the wet flue gas desulfurization apparatus, HCl in the exhaust gas is also absorbed and removed, while the gypsum separation mother liquor 47 generated in the gypsum recovery system is circulated to the absorption system, so that a predetermined amount of waste water is discharged. Otherwise, the Cl ^- concentration in the absorption system will increase significantly. There is a problem that an increase in chloride concentration corrodes equipment materials and reduces the purity of gypsum, which is a by-product, but operating while keeping the chloride concentration at a low level replenishes the desulfurization equipment. It has the problems of increasing the amount of water and increasing the size of wastewater treatment facilities. Therefore,
If there are severe restrictions on securing water for use or discarding (discharging) the waste water, the device must be operated with the Cl ⁻ concentration in the absorption system being as high as possible. In an extreme case, there is a demand for a system that does not discharge wastewater at all or discharges wastewater in a solution state by drying and solidifying the wastewater. An increase in the Cl ^- concentration in the desulfurization agent slurry means an increase in Ca ²⁺ ions as counterions. When the concentration of CaCl ₂ increases, the reactivity of limestone decreases, and the oxidation rate of sulfite decreases.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a flue gas desulfurization system capable of solving the problems relating to the reactivity of limestone and the deterioration of the oxidation performance of sulfite due to the absorption of HCl in exhaust gas. is there. Further, an object of the present invention is to coexist with limestone a desulfurizing agent having a reactivity lower than that of limestone, such as dolomite, without reducing the desulfurization performance resulting from the absorption of HCl in exhaust gas and also reducing the oxidation performance of sulfite. It is therefore to provide a flue gas desulfurization system that can reliably use dolomite and the like.

[0007]

The above objects of the present invention can be achieved by the following constitutions. That is, in the wet flue gas desulfurization treatment method in which the absorbent supplied into the absorption tower is brought into contact with the combustion exhaust gas to absorb and remove chloride together with sulfur oxides in the exhaust gas, the exhaust gas is subjected to the first absorption on the upstream side of the exhaust gas duct. The column and the second absorption tower on the downstream side are sequentially introduced to desulfurize,
A wet flue gas desulfurization treatment method in which an absorbent containing one or more kinds of alkaline non-calcium compounds is supplied to the first absorption tower, and an absorbent containing alkaline calcium compounds is supplied to the second absorption tower, Alternatively, a wet flue gas desulfurization treatment apparatus provided with an absorption tower having a gas-liquid contact part for contacting the absorbent supplied into the absorption tower with combustion exhaust gas and a circulation tank for circulating the absorbent to the gas-liquid contact part In, the absorption tower comprises a first absorption tower provided on the upstream side of the exhaust gas duct and a second absorption tower provided on the downstream side, and the first absorption tower contains one or more kinds of alkaline non-calcium compounds. The wet flue gas desulfurization treatment apparatus is provided with a supply section for the absorbent to be contained and a supply section for the absorbent containing the alkaline calcium compound in the second absorption tower.

In the present invention, the absorbent-containing slurry in the liquid layer portion of the circulation tank of the first absorption tower is preferably maintained at a lower pH than the slurry containing absorbent in the liquid layer portion of the circulation tank of the second absorption tower. . Further, as the alkaline non-calcium compound of the present invention, oxides, hydroxides or carbonates of magnesium, sodium, potassium and ammonium are used, and as the alkaline calcium compound, carbonates, oxides or hydroxides of calcium are used. Etc. are used.

[0009]

[Function] Ca ²⁺ existing in the liquid phase portion in the absorption tower circulation tank
Higher concentrations reduce the reactivity of limestone and the oxidation performance of sulfite. The conventional general method is to adjust the amount of waste water discharged to the outside of the system so as to maintain the Cl ⁻ concentration in the liquid at a predetermined level. However, although the Cl ⁻ concentration is the dominant factor for the corrosion characteristics of the equipment materials, it was clarified that the above-mentioned deterioration of the desulfurization reaction characteristics is largely due to the increase of the Ca ²⁺ ion concentration. This has led to the present invention. The present invention is based on the basic principle of reducing the concentration of dissolved calcium salt in the absorbent in the circulation tank of the first absorption tower arranged on the upstream side of the exhaust gas passage, and specifically, magnesium, sodium, potassium,
An alkaline non-calcium compound such as an ammonium compound is added as an absorber of chloride contained in the exhaust gas.

Conventionally, there has been proposed a method aiming at improvement of desulfurization reaction characteristics by adding soluble sulfates such as magnesium, sodium, potassium and ammonium compounds, but like the present invention, the first absorption tower side There is no known method of preferentially adding Since HCl is much more soluble in water than SO ₂ , Mg (O 2
When H) ₂ or the like is used as an absorbent, HCl does not react with limestone in terms of average composition and forms MgCl ₂ to be collected. As a result, the increase in Ca ²⁺ ion concentration is suppressed. As described above, according to the method of the present invention, the increase of Ca ²⁺ ion concentration in the liquid is suppressed, so that SO ₂ absorption and the sulfite oxidation performance are not deteriorated. If the first absorption tower is operated under a lower pH condition than the second absorption tower, dolomite, which has lower reactivity than limestone, can also be used as an absorbent.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in more detail below with reference to the drawings. FIG. 1 is a diagram showing a configuration of a flue gas desulfurization apparatus according to an embodiment of the present invention. The exhaust gas 1 is heat-exchanged by the gas-gas heater 2 to have a low temperature, and is introduced into the first absorption tower 3. The absorbent slurry from the circulation tank 4 arranged in the lower part of the first absorption tower 3 is circulated and supplied into the first absorption tower 3 by the circulation pump 5, and comes into gas-liquid contact with the introduced exhaust gas. An absorbent slurry containing an alkaline non-calcium compound 7 is supplied from an absorbent slurry tank 6 to the circulation tank 4. Also,
A demister 8 is provided at the outlet of the first absorption tower 3 to remove the mist and then guide the exhaust gas to the second absorption tower 9. In the second absorption tower 9, since the absorbent slurry in the circulation tank 11 containing the alkaline calcium compound 10 is circulated and supplied to the gas-liquid contact portion by the circulation pump 12, the S in the exhaust gas 1
O ₂ is absorbed. Mist is removed from the purified exhaust gas by the demister 13, the temperature of the exhaust gas is raised via the gas-gas heater 2, and then the purified exhaust gas is discharged from the chimney 14. In addition, in the circulation tank 4 of the first absorption tower 3 and the circulation tank 11 of the second absorption tower 9, the agitators 16 and 17 for oxidation that disperse the introduced air 15 as fine bubbles in the slurry and the tanks 4 and 11 Slurry stirrers 18 and 19 for stirring the slurry are respectively provided. Then, the circulation tank 11 of the second absorption tower 9
The alkaline calcium compound 10 in the first absorption tower 3
Is supplied to the circulating tank 4 via line 20.

In this embodiment, a first absorption tower 3 and a second absorption tower 9 are provided, and magnesium, sodium as an absorbent,
It is characterized in that an alkaline non-calcium compound 7 such as potassium or ammonium is supplied to the first absorption tower 3 mainly for absorbing chloride contained in the exhaust gas 1.
This example will be described by taking the case of using dolomite as the non-calcium compound. The boiler flue gas 1 is introduced into the first absorption tower 3, and most of HCl in the flue gas 1 and SO
Part of ₂ is absorbed by the spray absorbent, but the alkaline absorbent supplied for these absorption reactions contains the alkaline calcium compound 10 supplied from the second absorption tower 9 via the line 20. Slurry and circulation tank 4
And the slurry supplied via the circulation pump 5 and the slurry containing the alkaline non-calcium compound 7 supplied from the slurry tank 6.

The exhaust gas leaving the first absorption tower 3 is demister 8
After the mist is removed by the method, it is guided to the second absorption tower 9 and the contained SO ₂ is removed. The exhaust gas that has passed through the second absorption tower 9 is removed of entrained mist by the demister 13. The alkaline non-calcium compound 7 serving as the absorbent can be directly supplied to the slurry circulation line of the first absorption tower 3. When the method of supplying the absorbent to the first absorption tower 3 is determined as described above, HCl and SO in the exhaust gas are
₂ is M (II) Cl ₂ , M (II) SOx, CaCl ₂ , C
It is absorbed as aSOx (M (II) represents a non-calcium element or compound represented by Mg). Calcium-based absorbent (limestone) 10 supplied from the second absorption tower 9
When the amount of alkali in the mixture is small, the production ratio of M (II) Cl ₂ and M (II) SOx is increased, but if the supply amount of the non-calcium compound 7 is also reduced, M (II) Cl ₂ is prioritized. To generate. This is because HCl is more easily absorbed than SO ₂ . That is, if the conditions are optimized, HCl can be fixed as the non-calcium chloride M (II) Cl _{2 and} the increase of Ca ²⁺ ion concentration in the absorbent slurry in the circulation tanks 4 and 11 can be suppressed. .

In the method shown in FIG. 1, the first absorption tower 3
The sulfite is oxidized in the circulation tank 4 by the oxidizing stirrer 16 and the air 15 supplied to the stirrer 16. Similarly, the sulfite is oxidized in the circulation tank 11 of the second absorption tower 9 by the oxidizing stirrer 17 and the air 15 supplied to the stirrer 17. Further, a predetermined amount of the absorbent slurry in the circulation tank 4 is extracted into the gypsum recovery system 21. This gypsum recovery operation is performed in the same manner as the conventional method.
Further, as shown in FIG. 2, the circulation tank 4 of the first absorption tower 3
It is also possible to use a system in which the slurry inside is flown into the circulation tank 11 of the second absorption tower 9. In this case, a predetermined amount of the absorbent slurry in the circulation tank 11 of the second absorption tower 9 is the gypsum recovery system 2
1, but the oxidizing stirrer 16 in the circulation tank 4 of the first absorption tower 3 can be omitted.

Next, the method of this embodiment will be described in more detail based on the results of specific experiments. Reference Experimental Example 1 90 ml of water was placed in a 100 ml beaker and immersed in a constant temperature water bath. Continue stirring with a magnetic stirrer and the liquid temperature becomes 5
When it was confirmed that the temperature reached 0 ° C., 2 mmol of limestone having an average particle size of 10 μm (based on alkali content) and gypsum of 0.2.
4 g was added. 1N sulfuric acid was added dropwise using an automatic titrator until the pH of the liquid reached 4.5, and thereafter 1N sulfuric acid was continuously added so that the liquid pH was maintained at 4.5 (pH stat method). Similarly, if the pH is set to 5.0, 5.5 and 6.0, p
An H-stat method titration experiment was performed. The residual amount C of limestone was plotted against the reaction time t, and the results were arranged assuming that the relationship of the following equation holds between C and t. C = C _o exp (−kt) As a result, k values at pH 4.5, 5.0, 5.5 and 6.0 were 4.19, 2.55 and 1.41, respectively.
And 0.75 h ^-1 were obtained.

Comparative Experimental Example 1 By the same method as in Reference Experimental Example 1 above, the influence of pH on the reaction between dolomite and acid was examined. The obtained k value is shown in Table 1 together with the k value in the case of limestone.

[Table 1] As is clear from this result, the reactivity of dolomite is lower than that of limestone. However, lowering the pH improves the reactivity with acid.

Comparative Experimental Example 2 The effect of chloride concentration on the reaction between limestone and acid at a predetermined pH was examined by the same method as in Reference Experimental Example 1 above. However, in this case, gypsum and limestone were added to a solution containing a predetermined amount of CaCl ₂ . The results are shown in Table 2.

[Table 2] As is clear from the results in Table 2, the reactivity of limestone decreases as the CaCl ₂ concentration increases.

Experimental Example 1 The effect of chloride concentration on the reaction between dolomite and acid at a predetermined pH was examined by the same method as in Reference Experimental Example 1 above. However, in this case, a predetermined amount of CaCl
_A sample solution (slurry) was prepared by adding gypsum and dolomite to a solution containing ₂ and MgCl ₂ . The obtained k value is shown in Table 3.

[Table 3] From the results of Table 3, total Cl ^- concentration be identical MgC
It can be seen that the reactivity increases as the proportion of l ₂ increases.

Comparative Example 3 800 ml of pure water was placed in a polyvinyl chloride reaction vessel having a volume of about 1500 ml, and a predetermined amount of calcium chloride (C
aCl ₂ · 2H ₂ O), calcium sulfite (CaSO ₃ ·
(1/2 H ₂ O) 0.1 mol and 10 g of gypsum were added with good stirring, and water was further added to prepare a 100 ml slurry. When the slurry was heated with stirring and it was confirmed that the temperature reached 30 ° C, 1 mol / liter H ₂ S
The pH was adjusted to 5 with O ₄ . Next, while adjusting the pH with 1 mol / liter Ca (OH) ₂ or 1 mol / liter H ₂ SO ₄ , a predetermined amount of air is supplied from the lower part of the stirring blade to generate fine air bubbles of the calcium sulfite. Was oxidized. When the oxidation rate was calculated by measuring the total sulfite concentration at predetermined time intervals, the oxidation reaction of sulfite proceeded in the zero order with respect to the sulfite concentration, and the results shown in Table 4 were obtained as the reaction rate.

[0020]

[Table 4] From Table 4, it can be seen that as the CaCl ₂ concentration increases, the oxidation rate of sulfite decreases. CaCl ₂ concentration 10.9%
In the case of, the oxidation rate is 35 mmol / liter · h, whereas at 0%, it is 77 mmol / liter · h, which is about twice the oxidation rate.

Experimental Example 2 Table 5 shows that CaC is adjusted so that the Cl ^- concentration becomes 3% (constant).
It was added l _2, whereas, shows the oxidation rate of sulfite in the case of changing the pH to 4.5,5.5,6.0.

[Table 5] The oxidation rate of sulfite at pH 6.0 is 24 mmol.
/ Liter · h, whereas at pH 4.5 73 m
It becomes mol / liter · h. As described above, when the Cl ⁻ concentration is high and the pH is low, the oxidation rate of sulfite shows a large value.

[0022]

According to the present invention, the absorbent slurry has a high Cl content.
^- even when the concentration, can be operated desulfurizer in a state in which the oxidation rate and / or limestone utilization sulfite was maintained at the same level as the conventional method.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a two-column wet flue gas desulfurization apparatus according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram of a two-column wet flue gas desulfurization apparatus according to an embodiment of the present invention.

FIG. 3 is an explanatory diagram of a conventional one-column wet flue gas desulfurization apparatus.

[Explanation of symbols]

1 ... Exhaust gas, 2 ... Gas-gas heater, 3 ... First absorption tower,
4 ... First absorption tower circulation tank, 6 ... Absorbent slurry tank,
7 ... Alkaline non-calcium type compound, 8, 13 ... Demister, 9 ... 2nd absorption tower, 10 ... Alkaline calcium type compound, 11 ... 2nd absorption tower circulation tank, 14 ... Chimney, 1
5 ... Air, 16, 17 ... Stirrer for oxidation, 18, 19 ... Slurry stirrer, 21 ... Gypsum recovery system

Claims (5)

[Claims] 1. A wet flue gas desulfurization treatment method in which an absorbent supplied to an absorption tower is brought into contact with combustion exhaust gas to absorb and remove chloride together with sulfur oxides in the exhaust gas. The first absorption tower and the second absorption tower on the downstream side are successively introduced and desulfurized, and in the first absorption tower, an absorbent containing one or more kinds of alkaline non-calcium compounds is supplied to the second absorption tower. A wet flue gas desulfurization treatment method, characterized in that an absorbent containing an alkaline calcium compound is supplied to. 2. The absorbent-containing slurry in the liquid layer of the circulation tank of the first absorption tower is maintained at a lower pH than the slurry containing the absorbent of the liquid layer of the circulation tank of the second absorption tower. The wet flue gas desulfurization treatment method according to claim 1. 3. The alkaline non-calcium compound is one of magnesium, sodium, potassium and ammonium oxides, hydroxides or carbonates, and the alkaline calcium compound is calcium carbonate, oxides or hydroxides. 3. The wet flue gas desulfurization treatment method according to claim 1, wherein the method is a wet flue gas desulfurization treatment method. 4. The wet flue gas desulfurization treatment method according to claim 3, wherein the alkaline non-calcium compound is dolomite. 5. Wet flue gas provided with an absorption tower having a gas-liquid contact portion for bringing the absorbent supplied into the absorption tower into contact with combustion exhaust gas, and a circulation tank for circulating the absorbent to the gas-liquid contact portion. In the desulfurization treatment apparatus, the absorption tower comprises a first absorption tower provided on the upstream side of the exhaust gas duct and a second absorption tower provided on the downstream side, and the first absorption tower contains one or more kinds of alkaline non-calcium. A wet flue gas desulfurization treatment apparatus characterized in that a supply section for an absorbent containing a system compound is provided, and a supply section for an absorbent containing an alkaline calcium compound is provided in the second absorption tower.