JP3241209B2 - Flue gas desulfurization method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a flue gas desulfurization process for removing SO ₂ from the exhaust gas containing SO _2, such as coal fired exhaust gas or heavy oil combustion exhaust gas.

[0002]

2. Description of the Related Art A wet lime-gypsum method flue gas desulfurization apparatus which absorbs and removes SO ₂ from exhaust gas using limestone (CaCO ₃ ) as an absorbent and recovers gypsum as a by-product has been widely used. In Japan in the early 1970's, it was first put into practical use for heavy oil combustion exhaust gas treatment. Since the second energy crisis in 1982, boilers using coal have increased, and the coal-fired exhaust gas has been treated accordingly. It is well known that flue gas desulfurization equipment has increased. In the flue gas desulfurization process using these devices, CaCO ₃ as an absorbent is supplied as a powder, and neutralizes SO ₂ as an acidic gas while dissolving in an absorbent. However, in general, the dissolution rate of CaCO ₃ is relatively slow. Therefore, in order to perform desulfurization with high efficiency, the amount of Ca as an absorbent must be more than the reaction equivalent shown by the following formula.
It is necessary to add CO ₃ .

Embedded image SO_Two+ CaCO_Three+ 1 / 2O_Two→ CaSO_Four+ CO_Two (1) Generally, excessive CaCO_Three, The desulfurization rate increases
But excess CaCO _ThreeIs mixed into the by-product gypsum,
In addition to reducing the purity of gypsum, CaCO_ThreeIncrease consumption of
There were drawbacks to invite.

[0003] In order to solve this problem, various desulfurization auxiliaries have been proposed which increase the solubility of CaCO ₃ in the conventional wet desulfurization method and thereby improve the desulfurization rate. For example, Na ₂ SO ₄ , Na ₂ SO ₃ , N
NaOH, the method of using a sodium compound such as _{Na 2 CO 3, NaHCO 3,} NaHSO 3 JP 60-8
No. 4133, Japanese Patent No. 894725, Japanese Patent No. 903276, and Japanese Patent Application Laid-Open No. 53-12916.
7, JP-A-55-124530, JP-A-56-65
No. 615 and JP-A-51-97597, and a method of using a magnesium compound as a desulfurization aid is disclosed in JP-A-53-17565. Furthermore, the present inventors have newly found that ammonium salts, which have not been mentioned at all, show a significantly higher effect than conventionally used sodium salts and magnesium salts as desulfurization aids, A method for treating exhaust gas in which an ammonium salt is coexisted in an absorbing solution as an absorption aid is pending (Japanese Patent Application No. 05-25 / 1993).
2723, Japanese Patent Application No. 05-260561).

[0004]

However, in a process using an ammonium salt as an absorption aid and CaCO ₃ as an absorption agent, a technical problem has emerged that ammonia gas is released from the absorption liquid. Since the absorbing solution usually used in the lime-gypsum method is acidic, most of the ammonium salts are present as ammonium ions (NH ₄ ⁺ ) in the absorbing solution according to the following dissociation equilibrium equation and are released into the gas from the absorbing solution. Ammonia gas (NH ₃ )
Is very small.

Embedded image However, the effect of ammonium salts as absorption aids is
Since the higher the concentration, the higher the concentration, it is desirable to increase the ammonium salt concentration in the absorbing solution in order to obtain a high desulfurization efficiency. As can be seen from the above dissociation equilibrium equation, when the ammonium salt concentration increases, the amount of NH ₃ present in the absorbing solution increases in proportion thereto, and the amount of ammonia gas released from the absorbing solution into the gas increases. Become. If ammonia gas is entrained in the purified gas after SO ₂ removal, excessive consumption of ammonium salts may occur, and depending on the amount, secondary environmental problems such as odor may be caused.
The present invention solves the above-mentioned problems of the prior art, and the diffusion of ammonia gas into the processing gas even under conditions in which a high concentration of ammonium salt is dissolved in the absorbing solution in order to improve the effect of the ammonium salt on the absorption aid. It is an object of the present invention to provide a flue gas desulfurization method without any problem.

[0005]

According to the present invention, there is provided a flue gas desulfurization method for wet-removing SO ₂ by bringing exhaust gas containing SO ₂ into contact with an absorbent slurry containing limestone and ammonium salt.
Acid liquid is continuously sprayed into the flue on the gas inlet side of the mist eliminator installed in the flue behind the absorption tower, and the pH of the collected mist discharged from the mist eliminator is adjusted.
A flue gas desulfurization method and an exhaust gas containing SO ₂ , wherein a value obtained by adding an acid to a part of the collection mist discharge liquid is used as the acidic liquid while maintaining the value at 5 or less. in a flue gas desulfurization method absorption liquid is contacted with the slurry to wet removal of SO ₂ and containing limestone and ammonium salts, continuous acid liquid to the gas inlet side of the flue of the mist eliminator installed downstream of the flue of the absorption tower A liquid in which acid is added to a part of the collected mist effluent as the acidic liquid while maintaining the pH value of the collected mist effluent discharged from the mist eliminator at 5 or less. Wherein the remaining portion of the collected mist effluent is added to the absorbing solution.

[0006]

The method of the present invention will be described below in detail with reference to the drawings. FIG. 1 is an explanatory diagram showing an example of an embodiment of the present invention. In the process of FIG. 1, the flue gas is guided from the flue 1 to the absorption tower 2, where SO ₂ is removed, and then discharged through the mist eliminator 3 as a purified gas from the flue 4 to the outside of the system. In the absorption tower 2, the exhaust gas comes into contact with the absorption liquid sent through the line a by the circulation pump 5, and the SO ₂
Is absorbed and removed. The limestone powder is supplied from the line b into the absorption liquid in the tower bottom tank 6 and the ammonium salt as an absorption aid is supplied from the line c to desulfurize and remove SO ₂ with high efficiency. The supply amount of limestone powder depends on the pH of the absorbing solution (usually,
(pH 4.5 to 6.5). Air is supplied to the bottom tank 6 from the line e at the bottom of the tank, and the sulfite ions generated by SO ₂ absorption are oxidized into gypsum. Replenishing water is added from the line d, and the slurry concentration in the absorbing solution is kept constant.

The exhaust gas from which SO ₂ has been removed in the absorption tower 2 is discharged from a flue 4 provided with a mist eliminator 3.
A plurality of spray nozzles 7 are arranged in the gas inlet side flue of the mist eliminator 3 toward the mist eliminator,
As a result, the acidic liquid supplied by the liquid supply pump 8 via the line f is continuously sprayed into the gas. The sprayed acidic liquid reaches the mist eliminator 3 together with the gas, where it is collected together with a part of the absorbing liquid splashed from the absorption tower. Normally, the mist eliminator 3 has two stages, so that its collection efficiency is extremely high, and almost all of the sprayed acidic liquid and the absorbed liquid absorbed are collected, and the collected mist is discharged from the line g as a collected mist discharge liquid. Is discharged.
The ammonia gas partially diffused in the exhaust gas after the removal of SO ₂ comes into contact with the sprayed acidic liquid and is absorbed and removed. Therefore, the collected mist discharge liquid contains an ammonium salt.

Here, if the pH of the trapping mist effluent containing the ammonium salt is high, the partial pressure of ammonia in the effluent increases due to the dissociation equilibrium shown in the above equation (2).
It becomes impossible to absorb ammonia gas by spraying the acidic liquid. The inventors of the present invention have shown an example in FIG. 2 and have conducted a thorough study by measuring the equilibrium partial pressure of the ammonia gas and the pH of the collected mist effluent. It has been found that by maintaining the pH of the mist discharged liquid at 5 or less, the ammonia gas concentration can be reduced to an extremely low concentration, for example, 1 ppm or less, and based on this fact, the present invention has been completed. That is, the pH value of the collected mist effluent is detected by the pH meter 9 and the pH value is set to 5
The amount of an acid such as sulfuric acid or hydrochloric acid to be added to the acidic liquid tank 10 from the line h is adjusted so that the pH of the acidic liquid sprayed from the spray nozzle 7 into the exhaust gas is adjusted as follows. Here, by circulating and using a part of the collected mist discharged from the line g, the amount of the acidic liquid sprayed from the spray nozzle 7 can be increased and the efficiency of removing ammonia gas from exhaust gas can be improved. The remaining part of the collected mist discharge liquid is supplied to the tower bottom tank 6 via the line i and used as an absorbing liquid, so that ammonium salts can be effectively used. In the solid-liquid separation device 11, the by-product gypsum 12
Most of the filtrate containing the ammonium salt after is separated is circulated to the bottom tank 6 via the line j, and a part is discharged out of the system via the line k.

[0009]

EXAMPLES The method of the present invention will be described more specifically with reference to the following examples. (Example 1) Coal-fired exhaust gas was treated by an absorber of the type shown in FIG. The absorber is a pilot scale and the test conditions are shown in Table 1. The model of the mist eliminator was a folded plate type and was installed in two stages. The amount of ammonium sulfate supplied from the line c was adjusted so that the ammonium salt concentration in the absorbing solution was 0.18 [mol / l asNH ₄ ⁺ ]. The pH value of the collected mist discharged from line g is 4.
Sulfuric acid was added to the acidic liquid tank 10 to adjust the value to 0. As a result of measuring the ammonia gas concentration and the desulfurization rate in the purified gas in the steady state, the ammonia concentration in the purified gas was 0.2 ppm and the desulfurization rate was 98.8%. The ammonia concentration was measured by the JIS K0099 method in which the ammonia gas was absorbed by 0.5% boric acid water and then quantified by the indophenol method. Compared to a comparative example described later,
The ammonia concentration in the purified gas was significantly reduced.

[0010]

[Table 1]

(Embodiment 2) Of the pilot plant test operation conditions of Embodiment 1, the pH value of the trapped mist discharge after sulfuric acid addition was set to 5.0, and the other operation was carried out under exactly the same conditions as in Embodiment 1. . The ammonia gas concentration in the purified gas in a steady state was 0.3 ppm, and the desulfurization rate was 98.8%. Compared with Example 1, it was found that the ammonia concentration in the purified gas was reduced to the same order.

(Comparative Example 1) Among the pilot plant test operating conditions of Example 1, the operation was performed under exactly the same conditions as in Example 1 except that the pH value of the trapped mist discharge after sulfuric acid addition was set to 6.0. . The ammonia gas concentration in the purified gas in a steady state was 3 ppm, and the desulfurization rate was 98.9%. As compared with Examples 1 and 2, it was found that the ammonia concentration in the purified gas was significantly increased. This is presumed to be due to the fact that the pH of the trapped mist effluent was lower than in Examples 1 and 2 and the efficiency of ammonia absorption and removal by the acidic liquid was reduced. It became clear that it was necessary to spray an acidic solution so that

Comparative Example 2 The same apparatus as in Example 1 was used, and the operation was carried out under the same conditions as in Example 1 except that the spraying of the acidic liquid was stopped. The ammonia concentration in the purified gas in a steady state was 5 ppm, and the desulfurization rate was 98.8%.

[0014]

According to the present invention, the ammonium salt added as an absorption aid is prevented from being diffused as ammonia gas into the purified gas without impairing the remarkable effect of the ammonium salt as an absorption aid. be able to.
Further, the collected ammonium salt can be reused as an absorption aid, so that the problem of excessive consumption of ammonium salt can be solved and the problem of odor due to the release of ammonia gas can be solved.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing one embodiment of the present invention.

FIG. 2 is a graph showing a relationship between a pH value of a collected mist discharge and an ammonia concentration in an equilibrium gas.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Koichiro Iwashita 2-5-1 Marunouchi, Chiyoda-ku, Tokyo Mitsubishi Heavy Industries, Ltd. Headquarters (56) References JP-A-7-108131 (JP, A) JP-A Heisei 7-112117 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) B01D 53/34 B01D 53/50 B01D 53/77

Claims (2)

(57) [Claims] 1. A flue gas desulfurization method for wet-removing SO ₂ by bringing exhaust gas containing SO ₂ into contact with an absorbent slurry containing limestone and an ammonium salt, the gas of a mist eliminator installed in a flue downstream of the absorption tower. Acid liquid is continuously sprayed into the flue on the inlet side so that the pH value of the collected mist discharged from the mist eliminator is maintained at 5 or less, and the collected mist is discharged as the acidic liquid. A flue gas desulfurization method comprising circulating a liquid obtained by adding an acid to a part of the liquid. 2. A flue gas desulfurization method in which exhaust gas containing SO ₂ in contact with the absorption liquid slurry containing limestone and ammonium salts wet removal of SO _2, the gas mist eliminator installed downstream of the flue of the absorption tower Acid liquid is continuously sprayed into the flue on the inlet side so that the pH value of the collected mist discharged from the mist eliminator is maintained at 5 or less, and the collected mist is discharged as the acidic liquid. A flue gas desulfurization method comprising circulating a liquid obtained by adding an acid to a part of the liquid, and adding the remaining part of the collected mist discharged liquid to the absorbing liquid.