JPH084710B2 - Operation method of wet flue gas desulfurization equipment - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for operating a wet flue gas desulfurization apparatus, and in particular, oxidizes sulfite produced by absorbing sulfur oxides in exhaust gas with low utility. The present invention relates to a method for operating a wet flue gas desulfurization apparatus suitable for.

(Prior Art) In wet flue gas desulfurization, an aqueous solution or suspension of alkali metal, alkaline earth metal and other alkaline hydroxides, carbonates, sulfites and oxides is used to produce sulfur oxides in exhaust gas. A common method is to absorb and remove sulphate and recover stable sulfate as a by-product. With reference to FIG. 5, a conventional technique for producing and recovering calcium sulfate (gypsum) by using a calcium-based absorbent will be described.

Exhaust gas 101 from a boiler or the like is supplied to the absorption tower 2 after being dedusted and cooled in the dust removal tower 1, and the sulfur oxide in the exhaust gas is absorbed and removed in the absorption tower 2. Here, the absorbent slurry 102 used to absorb and remove the sulfur oxides in the exhaust gas is once held in the absorption tower circulation tank 3 and circulated to remove the sulfur oxides. Dust removal tower 1
If there is, an absorbent slurry 10 is placed in the dust removal tower circulation tank 4.
3 is retained and circulated to the dust removing tower 1 to remove dust and remove sulfur oxides. In each of the circulation tanks 3 and 4 of the absorption tower and the dust removal tower, the absorbent slurry is agitated by the agitators 5 and 6 to recover the SO ₂ absorption performance, and then the circulation pump 7 or 8 is used again to recycle the absorption tower or the dust removal tower. Is supplied to. By repeating this sequentially, the absorbent slurry absorbs the sulfur oxides in the exhaust gas to become calcium sulfite, and depending on a part or conditions thereof, the whole amount is oxidized by oxygen in the exhaust gas to become calcium sulfate (gypsum). The slurry is air-oxidized, concentrated and dehydrated in the gypsum manufacturing process and recovered as gypsum.

Thus, in the conventional process, it is difficult to always completely transform calcium sulfite into gypsum in the absorption process, and the absorbent slurry needs to be oxidized in an oxidation tower or the like.
The conversion of calcium sulfite to calcium sulfate in the absorption process is determined by the O ₂ and SO ₂ concentrations in the exhaust gas, the contact time between the exhaust gas and the absorbent slurry, the contact area, the residence time and the pH. In order to convert all the calcium into gypsum in the absorption tower, it is necessary to lower the absorption tower pH and increase the contact frequency and contact area of the absorbent slurry. In order to increase the number of contacts and the contact area, it is necessary to increase the liquid-gas ratio and the tower height, which leads to an increase in power costs and equipment costs.

In the above process, the most important control of desulfurization performance (desulfurization rate) in the desulfurization equipment is basically to detect the pH of the absorption tower circulation slurry and, depending on the deviation from the set value, the limestone slurry which is the absorbent. This is done by controlling the supply amount. That is, in order to improve the desulfurization performance, it is necessary to increase the pH of the absorption tower circulation slurry, that is, increase the limestone (CaCO ₃ ) concentration in the absorption tower circulation slurry. SO in the absorption tower circulation slurry in the above process
Calcium sulfite produced by absorbing ₂ (CaCO ₃ 1 / 2H
₂ O) is contained in a large amount (10 to several hundred mmol /), and the relationship between pH and desulfurization rate of the absorption tower circulation slurry having such a slurry composition is strongly influenced by pH as shown in FIG. Therefore, it is understood that the above process is an effective method for controlling the desulfurization rate.

Although the above process has stable desulfurization performance and has a large number of achievements, in recent years, there is a strong need for rationalization of desulfurization equipment, and the above process (hereinafter, referred to as “oxidation by-value process”) requires equipment cost. Therefore, it is not always possible to meet those needs. Therefore, the present inventors
Through intensive research and development, we proposed a wet flue gas desulfurization device having gas cooling, dust removal, absorption, and oxidation functions in one column (JP-A-62-193629, JP-A-62-193630). FIG. 6 is an explanatory diagram showing a flow of such a wet flue gas desulfurization apparatus. This apparatus includes a desulfurization tower 14 and the desulfurization tower 14
It comprises a circulation tank 15 provided in the lower part of the above, agitators 17 and 18 provided in the circulation tank, and a circulation pump 16 for circulating the absorbent slurry from the circulation tank 15 to the spray means 22. In addition, 17 is a stirrer for preventing slurry settling,
18 is a stirrer for gas dispersion, 19 is a demister.

The desulfurization process is one in which the main functions of the limestone-gypsum wet flue gas desulfurization process (dust removal / cooling) such as absorption and oxidation are added to one tower. Not only optimization of gas ratio, spray structure and arrangement, slurry pH, etc., but the oxidation state of sulfite is the most important factor. As a method for controlling desulfurization performance in this process, a method of detecting pH and controlling the supply amount of limestone as an absorbent can be applied as in the case of the separate oxidation process described above, but when this method is used in this process CaCO in the recovered gypsum
_{3 The} amount increases and decreases, and it is not necessarily a good method for recovering good quality gypsum. In order to use the gypsum produced as a by-product for structural and construction purposes, it is generally desirable that the CaCO ₃ content in the gypsum be 1% or less, but in the oxidation separate process, sulfuric acid is used before feeding to the oxidation tower and unreacted. Since the decomposition of CaCO ₃ and the optimization of the oxidation conditions are performed, there is no such concern.

As described above, in the integrated process of (dust removal) -absorption-oxidation, it is not always a good idea to use only the limestone supply amount control method as a method for keeping the desulfurization rate constant,
A new control system that solves the above problems has been desired.

Furthermore, the amount of exhaust gas and the concentration of SO _{2 at} the inlet largely depend on operating conditions such as the boiler, and in many cases maximum oxidation performance is not always required. On the other hand, in the above process, the number of revolutions of the agitator for oxidation, the amount of air, and the number of operating units are constant,
Depending on the exhaust gas conditions, it is often not necessary,
May spend extra utilities.

(Problems to be Solved by the Invention) An object of the present invention is to eliminate the above-mentioned drawbacks of the prior art and to maintain a desired desulfurization rate at a low utility in a single-column wet flue gas desulfurization apparatus. Another object of the present invention is to provide a method for operating a wet flue gas desulfurization apparatus.

Means for Solving the Problems The present invention not only supplies the absorbent slurry in proportion to the amount of SO ₂ removed, but also changes the number of agitators for oxidation in accordance with the amount of SO ₂ removed. Is achieved. That is, according to the present invention, the means for removing the sulfur oxides in the exhaust gas and the sulfite produced thereby are oxidized by blowing air into the vicinity of the blades of two or more agitators installed on the side surface of the liquid reservoir in the lower part of the absorption tower. In a method for operating a wet flue gas desulfurization device having a means in one tower, the amount of exhaust gas to the tower and the SO ₂ concentration removed are measured, and the operating number of the agitator for oxidation is controlled by the difference between the product of both and the set value. It is characterized by doing.

(Operation) A plurality of air-mixing agitators for oxidation, which are installed in multiple units, detect the amount of exhaust gas and the concentration of SO ₂ and calculate based on the signals to obtain the required amount of oxidation. By determining, the total amount of sulfite generated by absorbing SO ₂ in the exhaust gas can be oxidized without using a wasteful utility. In addition, it is preferable to stop the air supply to the stopped stirrer. The reason is that merely blowing air hardly contributes to oxidation and becomes a waste utility. Further, it is preferable that the amount of air supplied to the operated stirrer is constant. This is because when the air amount is changed under a constant agitator rotation speed, the shaft power of the agitating blade changes, and especially when the air amount is reduced below the set air amount, the shaft power becomes high and the load on the motor is increased. This is because it may cause a load and eventually cause the stirrer to malfunction. As a result, the sulfite ion in the absorption liquid becomes almost zero, and the absorption performance can be improved, and at the same time, the driving power of the agitator and the air supply power, which are utilities required for oxidation, can be saved. Also,
The limestone slurry as an absorbent only needs to be supplied in an approximately equal amount to the amount of SO ₂ removed, the CaCO ₃ concentration in the recovered gypsum is kept low, and the quality of the gypsum is not deteriorated.

 Hereinafter, the present invention will be specifically described with reference to FIG.

This device is the exhaust gas of the single tower desulfurization device shown in FIG.
A gas flow meter 24 and a sulfur oxide concentration detector 23 are provided on the supply line 101, and the operating number of the agitators 17 and 18 for oxidation is controlled by the deviation of the product of these and the set value. Different from the conventional device.

Exhaust gas 101 from a boiler or the like is guided to a desulfurization tower 14, comes into contact with the sprayed calcium-based absorbent slurry, is subjected to dust removal, cooling, and desulfurization, and thereafter, a demister 19 removes entrained mist and is discharged from the desulfurization tower. On the other hand, the limestone slurry 104 as the absorbent is supplied to the circulation tank 15 in proportion to the amount of SO _{2 to be} removed, and contacts the exhaust gas to reduce the hydrogen ions (H ⁺ ) in the absorbent slurry having a lowered pH. The circulation tank 15 has
A plurality of stirrers 17, 18 for air refining and dispersion are installed to oxidize the sulfite ion (SO ₃ ^-2 ) of the slurry that has absorbed SO ₂ and has a high SO ₂ equilibrium partial pressure. Reduce the pressure, S
Calcium sulfate (gypsum) is obtained as well as recovery of O ₂ absorption performance. The absorbent slurry thus regenerated is supplied to the spray section by the circulation pump 16 and drops into the circulation tank 15 while making countercurrent contact with the exhaust gas. Thus, [SO ₂ absorption] …… (pH decrease, SO ₃ ²⁻ increase) − [SO ₃ ²⁻ oxidation] ……
(Ca (HSO ₃ ) ₂ + O ₂ + 2H ₂ O → CaSO ₄・ 2H ₂ O + H ₂ SO ₄ , CaSO
_{3 · 1 / 2H 2 O +} 1 / 2O 2 + 3 / 2H 2 O → CaSO 4 · 2H 2 O, CaCO 3 + H 2 SO
₄ + H ₂ O → CaSO ₄・ 2H ₂ O, etc.)-[SO ₂ absorption]. At this time, as shown in the control block diagram of FIG. 1A, the SO ₂ concentration detector 23 at the inlet of the apparatus detects the SO ₂ concentration, and the gas flow meter 24 detects the exhaust gas amount (G), and it is necessary for the product. The amount of oxidation (removed SO ₂ amount) is calculated, and the number of operating agitators for air refining (M) (agitator M and air Air) is changed in accordance with the amount, and the optimum oxidation state is always maintained to eliminate waste. Without spending utility
Gypsum of stable quality can be obtained by oxidizing sulfite and stabilizing the desulfurization performance.

A part of the absorption slurry thus circulated and used is withdrawn from the circulation tank 15, concentrated by the Shokuna 20, finally dehydrated by the centrifuge 21, and recovered as gypsum with adhered water of 10% or less. . In some cases, the thickener 20 may be omitted and the centrifugal separator 21 may be directly supplied.

The present invention makes it possible to reduce the concentration of sulfite ion (SO ₃ ² -concentration), which is the most important determinant of desulfurization performance in a wet flue gas desulfurization process, in which the functions of (dust removal), absorption and oxidation are integrated in one tower, with low utility and In order to set the value to 0, by controlling the number of operating agitators for air atomization dispersion, which is a means for oxidizing SO ₃ ^2- , the oxidizable amount is controlled so that an optimum oxidation state can always be obtained. Is. The relationship between the number of agitators operating and the oxidizable amount, that is, the oxidation rate is shown in FIG. 3. This is the result of experiments and research conducted by the present inventors and is a feature of the present invention. It is a thing.

As a result of intensive studies on the (dust removal) / absorption / oxidation one-column type process, the present inventors have found that the oxidation separate type process and this process are constituted by a desulfurization mechanism which is clearly different from each other, and arrived at the present invention. It was. That is, in the oxidation separate type process in which the absorbing solution contains a large amount of calcium sulfite (CaSO ₃ 1 / 2H ₂ O), the desulfurization rate and the absorbing solution pH are
As shown in FIG. 2, the desulfurization rate has a high dependency on pH. However, as a result of the inventors performing a test on the absorption mono-oxide one-column process, as shown in FIG. The desulfurization rate has little dependence on pH and shows a nearly constant desulfurization rate in the pH range of 4 to 6.5. Therefore, the desulfurization rate can only be controlled by detecting the effective pH in the oxidation separate process and controlling the limestone supply rate. I found it difficult to control.
In addition, in the absorption / oxidation one-column process, gypsum is collected from the lower circulation tank, so if more CaCO ₃ is added to the circulation tank to increase the desulfurization rate, unreacted gypsum will be recovered. CaCO ₃ is mixed in with a large amount and deteriorates the quality of by-product gypsum.

Based on the above experimental facts, the present inventors have conducted diligent research on the control method of the absorption / oxidation one-column desulfurization process. In this process, the desulfurization performance greatly changes depending on the sulfite ion concentration in the absorption liquid. I found that. FIG. 4 shows an example of the results obtained by the inventor's experiment, and it can be seen that in this process, the desulfurization rate is strongly affected by the sulfite ion concentration in the absorbent.
In this way, the absorption / oxidation one-column desulfurization process is strongly influenced by the sulfite ion concentration. When the sulfite ion concentration is high, the SO ₂ partial pressure in the absorbing solution becomes high and the SO ₂ absorbing capacity of the absorbing solution decreases. This is because.

As described above, in the absorption / oxidation one-column process, the unreacted CaCO ₃ concentration in the absorption liquid was lowered to operate at a pH of 4.0 to 4.0.
Operate at 6.0, preferably 5.0 to 5.5, but in this case, in order to maintain high absorption capacity, contact with exhaust gas
It is important to reduce the SO ₂ equilibrium partial pressure of the absorbing solution by absorbing almost all the sulfite ions of the absorbing solution that have absorbed SO ₂ and dropped into the circulation tank to sulphate.

In this process, multiple oxidizing stirrers were installed on the side of the circulation tank as this sulfite ion oxidizing means,
A method is used in which air is supplied to the vicinity of the blade and the air is atomized by the shearing force generated by the rotation of the stirrer blade to efficiently dissolve oxygen in the air serving as an oxidizer to oxidize.
In the present system, the object of the present invention can be achieved by controlling the number of stirrers in order to obtain an oxidation state suitable for the amount of SO ₂ removed. In the practice of the present invention, it is preferable that the agitators for oxidation are arranged in equal parts and that the agitators that are operated as much as possible have no deviation when viewed from the horizontal cross section of the tank when changing the number of agitators. Further, when the stirrer is stopped, it is preferable to carry out an operation such that the air is also stopped accordingly, or the air is kept to such an extent as to prevent backflow of the slurry.

When the present invention is applied to an actual device, a plurality of units will be used so that the sulfite will be almost 0 even under the most severe conditions to be assumed (depending on the plant, depending on the amount of treated gas, SO ₂ concentration at the inlet of the device, tank liquid depth, etc.). Install 2 to 20 units.

An example of the method of implementing the present invention is shown in FIG. in this case,
Six agitators A to F are installed as shown in FIG. The table below shows examples of usage.

At the maximum set value, all the agitators are operated, but in reality, the removed SO ₂ amount (Q ′ / Q) is calculated based on the exhaust gas amount, inlet SO ₂ concentration, and outlet SO ₂ concentration given as signals, and The optimum number of operating machines is selected depending on which area the calculated value is in. here (G: amount of treated gas, Sin: concentration of SO _{2 at} inlet, Sout: concentration of SO _{2 at} outlet, Q: maximum amount of SO ₂ removed (installed value), Q ′: amount of SO ₂ removed at any time), Q ′ / Q × When 100 = 40%, the number of operating vehicles is set to 0, which means that the amount of oxygen that is naturally oxidized by O _{2 in} exhaust gas is balanced (that is, forced oxidation is unnecessary).

As a method of more effectively using the present invention, there is a method of continuously analyzing the SO ₃ ²⁻ concentration in the absorbing liquid and determining the number of operating units based on the deviation between the signal and the set value.

As described above, by using the present invention, it is possible to cope with the most severe exhaust gas conditions and even when forced oxidation is not required, with a low utility.

Further, in order to clarify the content of the present invention, a detailed description will be given using examples.

(Example) Example 1 An experiment was conducted using an absorption / oxidation single-column flue gas desulfurization apparatus having a treatment gas amount of 580 Nm ³ / h (rated). The desulfurization tower has a diameter of 0.3 m,
The height is about 9.5 m, the circulation tank at the bottom of the tower is φ1.0 m, and the height is 1.
At 5 m, four side stirrers for sulfite chlorination and slurry settling prevention agitation were installed in equal parts at a height of 150 mm above the tank bottom. The stirrer used was a 3-blade propeller type blade with a diameter of 120 mm. The rotation speed of this agitator is 1500 rpm
Is constant at 0.75 Nm ³ / h
And About 20 wt% limestone slurry as an absorbent was supplied to the circulation tank. CaCO the circulation tank ₃ and CaSO ₄ · 2H
An absorbent slurry containing ₂ O as a main component was held by 410, and was taken out from the tank by a circulation pump and circulated at a flow rate of 8.68 t / h. Limestone slurry was supplied so that the pH of the circulation tank was 5.5.

When all four stirrers were operated under exhaust gas conditions of exhaust gas amount: 580 Nm ³ / h, inlet SO ₂ concentration: 740 ppm, the desulfurization rate was 90%, and the SO ₃ ²⁻ concentration in the absorbing liquid was always 0.5 mmol / h.
Kept below.

Example 2 Under the exhaust gas conditions of exhaust gas amount: 305 Nm ³ / h, inlet SO ₂ concentration: 630 ppm (corresponding to 1/4 load as boiler load), two agitators were operated, and the desulfurization rate was 92. %, And the SO ₃ ²⁻ concentration in the absorbing solution was always kept at 0.5 mmol / or less.

Reference Example 1 Under the same exhaust gas conditions as in Example 2, the desulfurization rate was 92.0% even when the number of agitators operated was 4, and the SO in the absorbent was
_{The 3} ^2- concentration was 0.5 mmol / or less as in Example 2. The CaCO ₃ concentration in the gypsum recovered in Examples 1 and 2 was always 1% or less.

(Effects of the Invention) According to the present invention, in the (dust removal) / absorption / oxidation one-column type process, important sulfite is efficiently oxidized,
And a stable desulfurization rate can be obtained. In particular, it is possible to control the amount of oxidation according to the required amount while keeping the concentration of CaCO ₃ in the recovered gypsum constant, that is, to minimize the utility spent for oxidation. In the case of the conventional oxidation separate process, even if the exhaust gas conditions change, there is a large time delay until the absorbent slurry is actually supplied to the oxidation tower, and it is difficult to apply this control method.

According to the present invention, when the boiler has a low load, or when coal having a low S content is burned, it is possible to reduce the number of operating agitators for oxidation, and to operate the agitator with such operating power and Air supply power can be saved.

[Brief description of drawings]

FIG. 1 is a diagram showing a flow sheet of a wet flue gas desulfurization apparatus according to the present invention, FIG. 1A is a block diagram of operation control of its agitator, and FIG. 2 is an absorption / oxidation one-column process and an oxidation separate type. Diagram showing the relationship between process slurry pH and desulfurization rate, No. 3
FIG. 4 is a diagram showing the relationship between the number of operating agitators for oxidation and the oxidizable amount, FIG. 4 is a diagram showing the relationship between the sulfite ion concentration in the absorption liquid and the desulfurization rate, and FIG. 5 is the oxidation separate process. FIG. 6 is a diagram showing a flow sheet of the absorption / oxidation one-column type process, and FIG. 7 is an example of a setting standard of the operating number of the oxidation agitator in the embodiment of the present invention. FIG. 8 is a diagram illustrating the arrangement of the stirrer as an example. 14 …… desulfurization tower, 15 …… circulation tank, 16 …… circulation pump,
17, 18 ... Stirrer, 19 ... Demister, 20 ... Thickener,
21 ... Centrifuge, 23 ... Sulfur oxide concentration detector, 24 ...
… Gas flow meter, 25 …… Valve, 101 …… Exhaust gas, 102 ……
Circulating slurry, 104 …… Limestone slurry, 106 …… Air, 107
... filtered water, 108 ... plaster.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location B01D 53/34 ZAB (72) Inventor Satoshi Nishimura 6-9 Takara-cho, Kure-shi, Hiroshima Babkotsu Hitachi Co., Ltd. Kure Factory (56) Reference JP-A-62-204828 (JP, A)

Claims (3)

[Claims] 1. A means for removing sulfur oxides in exhaust gas and a means for oxidizing the sulfite produced thereby by blowing air into the vicinity of the blades of two or more agitators installed on the side surface of the liquid storage under the absorption tower. In a method for operating a wet flue gas desulfurization apparatus in one tower, measuring the amount of exhaust gas to the tower and the concentration of SO ₂ removed, and controlling the operating number of the agitator for oxidation by the difference between the product of both and the set value. And a method for operating a wet flue gas desulfurization apparatus. 2. The method for operating a wet flue gas desulfurization apparatus according to claim 1, wherein the air to be blown is supplied only to the oxidizing agitator in operation. 3. The method for operating a wet flue gas desulfurization apparatus according to claim 1 or 2, wherein the amount of air supplied to one operating agitator for oxidation is constant. .