DE3523820A1 - METHOD FOR TREATING EXHAUST GAS - Google Patents

Description

The present invention relates to a method for treating an exhaust gas, in particular a method for a wet treatment of an exhaust gas which contains SO ₂ , HCl and optionally HF, such as, for example, an exhaust gas from coal combustion.

When the desulfurization of smoke is carried out by a widely known wet coal process the resulting exhaust gas sometimes contains the compounds HCl as harmful components in addition to SOx and HF. An example of the coal combustion exhaust gas contains components of about 100 ppm SOx, about 60 ppm HCl and about 40 ppm HF.

When such exhaust gas is treated in a wet exhaust gas treatment tower using CaCO ₃ , which is an absorbent for SO ₂ , the follow-

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the reactions from:

CaCO 3 + SO ₃ -> CaSO ₃ 2 + co ₂ H ₃ O ( 1) CaCO 3 + 2HCl -> CaCl + + Co ₂ ( 2) CaCO 3 + 2HF ) CaF ₂ CO ₂ + ( 3)

In this case, however, the reaction of the equation prevails

(2) before and the dissolution of CaCO., Is due to the

2+
Prevents the presence of Ca from CaCl ₃ , which is formed in accordance with the reaction of equation (2). As a result, the reaction of equation (1) is hindered and the SO ₃ absorption capacity decreases. In addition, CaSO ₄ χ 2H ₃ O, which is formed simultaneously with the desulfurization reaction in the exhaust gas treatment tower, is deposited as flakes or pollution on the wall surfaces of an apparatus used, with the result that the exhaust gas treatment apparatus is impaired in its function.

To avoid these disadvantages, a method is considered to be effective in which, with regard to the amount of HCl, an appropriate amount of sodium sulfate Na ₇ SO ₄ or potassium sulfate K ₇ SO _{4 is added to} the above-mentioned reaction system and the following reactions are used: to remove CaCl ₃ in the form of CaSO ₄ χ 2H "0:

2NaCl + CaSO ₄ χ

Na ₃ SO ₄ + CaCl ₂ + aq

K ₃ SO ₄ + CaCl ₃ + aq.> 2KC1 + CaSO ₄ χ

2H ₂ O (5)

Incidentally, the reaction (4) is generally well known as the reaction according to which gypsum CaSO ₄ χ 2HpO is prepared from a Na ₂ SO ₄ solution by adding CaCl ₃ .

The compounds Na ₃ SO ₄ and K ₃ SO ₄ , which are necessary for the reactions to remove CaCl ₃ , can only be produced by supplying a basic sodium salt and a basic potassium salt to the tower for the wet treatment of exhaust gas. An example of reactions involving Na ₃ CO ₃ is given below:

Na ₃ CO ₃ + SO ₂ ^ Na ₃ SO ₃ + CO ₂ (6)

Na ₂ SO ₃ + 1/2 O ₂ > Na ₂ SO ₄ (7)

The above-mentioned compound HF is converted into CaF ₃ , which is mentioned in the above equation (3); however, the latter compound CaF ₃ cannot be stabilized as a solid with low solubility.

This compound CaF ₃ causes an aluminum component contained in the dust of the exhaust gas to dissolve, and a reaction of the obtained aluminum ions and fluorine ions prevents the dissolution of lime. In order to avoid such disadvantages, the method using a basic sodium salt has been proposed in Japanese Patent Publication No. 167023/1980.

From the above considerations it can be assumed that the addition of a basic sodium salt, the amount of which depends on the amounts of HCl and HF, is _{effective in the treatment of the exhaust gas containing SO 3} , HCl and HF in order to overcome the above-mentioned disadvantages due to CaCl ₃ To avoid aluminum ions and fluorine ions.

The inventors found in the course of experiments on this treatment method that when the basic Sodium or potassium salt to the reaction system in an amount depending on the amount of HCl and at Injection of air into an absorption solution into the exhaust gas treatment tower not only the above Disadvantages are avoided, but also a considerable

There was an improvement in the rate of the dissolution reaction of CaCO, can be achieved.

The present invention has been carried out on the basis of the above knowledge and is directed to a method for treating an _{exhaust gas containing SO 9} , HCl and optionally HF, which is characterized in that a stoichiometric amount of a sodium compound and / or a potassium compound is applied to an exhaust gas treatment tower which becomes NaCl or KCl in the exhaust gas treatment _{tower and passes a calcium compound as an absorbent for SO 2} and blows air into an absorbent solution which is brought into contact with the exhaust gas. Furthermore, the present invention can be applied to an exhaust gas that does not contain HF.

The invention is described in more detail below:

If the sodium or potassium compound is not added to the system in an amount corresponding to the HCl, the dissolved CaCl compound will be present therein as mentioned above, and consequently the absorbing capacity of the CaCO 2 absorbent will be poor. In other words, the production of CaCl ₉ causes the

2+
Amount of dissolved Ca is increased. The latter compound reduces the solubility of CaSO ₄ χ 2H ₉ O (gypsum), which is formed simultaneously with the desulfurization and the oxidation reaction, which accelerates the growth of gypsum deposits. Further, since the partial pressure of SO ₉ becomes higher in the absorption solution _{containing CaCl 9} , its performance for absorbing SO _{9 is} deteriorated.

In other words, the concentration of calcium ions in the absorbent solution containing CaCl _{9 increases} and, as a result, the solubility of

SO, as from the formula for the solution equilibrium

(Ca ²⁺ ) (SO ₃ ² ") = Ksp.

2 -

emerges. A decrease in the concentration of SO, which is the solution component of the SO_ gas, means a deterioration in the solubility of the SO ₂ gas.

In other words, in such an absorption

2 _ solution, the concentration of SO, which is generated by absorption of the SO ^ gas, immediately reaches a saturation level and therefore the partial pressure of SO _{2 will become} slightly higher. Conversely, if dissolved chloride is present due to the addition of NaCl or KCl,

2-the saturation concentration of SO becomes higher; in other words, the dissolving power of the absorption solution becomes larger, so that the partial pressure of SO ₂ in the absorption solution is kept at a lower level.

Experimentally, air was blown into the absorption solution in the exhaust gas treatment tower, with interesting results were achieved. Namely, when the amount of air blown into the absorbent solution is increased, the rate of conversion of the absorbent CaCO, and the rate of absorption of SO- were improved.

In order to obtain confirmation that the effect of air blowing was produced by the addition of the sodium compound or potassium compound simultaneously with the blowing of air, experiments were carried out in which the amounts of air to be blown into the absorption solution were varied under such conditions that neither sodium nor potassium compound was added and that Cl could be present _{as CaCl 2 in the absorption solution.} According to the results of these experiments, increasing the amount of air allows an improvement in the rate of conversion of CaCO and

sorption rate of SO-; but the rate of calcium consumption was relatively lower compared to the case where Cl ~ was present as NaCl or KCl.

Therefore, Cl must finally be present in the absorption solution as NaCl or KCl.

The reason why the rate of conversion of CaCO ₃ and the rate of absorption of SO- can be increased by blowing air into the absorption solution has not yet been clearly seen; but the following can be assumed:

namely, when air is blown into the absorbing _{solution, HSO 3} , a weak acid, is converted to HSO ₄ , the ion of a strong acid, and so the solution reactivity of CaCO _{3 is} easily restored even though aluminum and fluorine ions are dissolved in the absorbing solution (can). As a result, both the rate of absorption of SO ₂ and the reactivity of CaCO _{3 are} improved.

The following reactions take place one after the other: (reactions in the exhaust gas treatment tower)

SO ₂ (gas) + H ₂ O> ^H 2 ^SO 3 ⁽⁸⁾

H ₃ SO ₃ > H ⁺ + HSO ₃ "(9)

(Reactions in the absorption solution tank) HSO ₃ "+ 1/2 O ₂ (gas)> HSO ₄ ~ (10)

H ⁺ + HSO ₄ "+ CaCO ₃ (solid) + aq. -> CaSO ₄ χ 2H-0 + CO- (11)

(Reaction in the absorption solution tank without aeration)

2HSO ₃ "+ Ca ²⁺ + CaCO ₃ (solid)

> 2CaSO ₃ χ 1/2 H ₃ O + CO ₃ (12)

From the above it can be seen that by determining the amount of HCl in the exhaust gas; Supplying a stoichiometric amount of a sodium and / or potassium compound, which becomes NaCl or KCl in the exhaust gas treatment tower, to the exhaust gas treatment tower together with a calcium compound as an absorbent for SO 2; and blowing air into the absorption solution which is brought into contact with the exhaust gas, problems such as deterioration in S0 ₂ absorption performance due to HCl and HF, difficulty with scales or buildup caused by the deposition of CaSO ₄ χ can be solved 2H ₂ O, and the reduction in the reactivity of the absorbent for SO ₃ ; in addition, the rate of conversion of the absorbent for S0_ and the rate of absorption of S0 ~ can also be effectively increased.

Fig. 1 is the schematic plan of an embodiment of the present invention;

Fig. 2 is a graph showing experimental values obtained in an example of the present invention and showing changes in the concentration of SO ₂ contained in a purified gas 6 depending on the flow rates of air entering the absorbing solution is blown, in the event that Na ₃ CO _{3 is} only added in a stoichiometric amount, based on the amount of HCl, and in the event that no Na-CO-. is added to;

Fig. 3 shows the graph of experimental values obtained in the example of the present invention

and shows changes in the concentration of CaCO ₃ in the absorption solution depending on the flow rates of air blown into the absorption _{solution in the event that Na 7} CO. is added only in a stoichiometric amount based on the amount of HCl is, and in the event that no Na-CO, is added; and

Fig. 4 shows the graphical representation of trial. ten obtained in the example of the present invention and gives the dependencies between SO "concentrations in the purified gas 6 and CaCO" concentrations in the absorption solution in terms of concentrations of Mn ions in the absorption solution up to a level of about 400 mg / 1, in the event that Na-CO., is only added in a stoichiometric amount, based on the amount of HCl, and in the event that that a certain amount of air in the absorption solution is blown.

Using an apparatus shown in Fig. 1, an embodiment of the method according to the invention practically carried out.

In Fig. 1, an exhaust gas from a coal-fired boiler is passed to an exhaust gas treatment tower 2 through a denitration device, an electric dust collector and a heat exchanger (not shown in the QQ drawing).

A sensor was provided at the inlet of the exhaust gas treatment tower 2; this element 3 stated that the Exhaust gas 1, the about 1000 ppm SO ", about 60 ppm HCl and contained about 40 ppm HF, was introduced with a flow rate of about 4000 Nm / h.

The exhaust gas treatment tower 2 was packed with grids, and an absorption solution was sprayed from the top of the tower 2 at m / h via an absorption solution circulation pump 4. _{At that time, SO 0} , HCl and HF present in the exhaust gas were absorbed by the absorption solution, and the exhaust gas was discharged from the tower 2 as purified gas 6 through a vapor separator 5.

According to the results of the detector, the purified gas 6 contained about 100 ppm SO ₃ ; however, the HCl and HF levels were below 1 ppm, the lower limit of detection.

With regard to the absorption amount of SO ", CaCO, was fed to the tower 2 through line 7 at about 17 kg / h, and at the same time Na ₃ CO., Was fed through line 8 at a flow rate of not less than 0.52 kg / h. h, which corresponds to the stoichiometric amount, based on the amount of HCl, given up.

In the tank 9 in a lower region of the exhaust gas treatment tower 2, air is blown in at about 20 m N / h To oxidize sulphite, which was produced by absorption of SO_, to sulphate.

The absorption solution in the tank 9 was in the form of a crystalline CaSO. χ 2H ₂ O and some CaCO ^ powder containing suspension, and the equilibrium of water was then adjusted by making up the absorption solution with water so that the concentration of the slurry was about 18% by weight.

Then a part of the absorption solution was conveyed by a pump 11 to a separator 12 in order to convert crystalline CaSO. ^{Remove X} 2H ₂ O (gypsum) from the system by balancing the absorption of SO _2. In the separator 12, the plaster of paris produced was from the

Product recovered and a portion of the resulting filtrate removed through line 14; what remains Filtrate was returned to the exhaust gas treatment tower 2.

While the steady state process was continued, chlorine ions were at a concentration of about 280 mmol / 1 in the absorption solution; against it the concentration of sodium ions was never less than 280 mmol / l, which is the equivalent for the chloride ions was. Further, according to the analysis results, the concentration of fluoride ions in the absorbing solution exceeded not the level of 5 mmol / 1. Taking into account the fact that the concentration of fluoride ions to the The point in time when all the fluoride ions are dissolved in it, was about 190 mmol / 1 (estimated value), it is understandable, that some fluoride ions in the form of the solid CaF "have been removed.

If Na ₃ CO _{3 was added} in a smaller amount than the absorption amount for HCl, the absorption capacity of SO_ and the pH value of the absorption solution decreased. The reduced SO ~ absorption capacity did not regain its old value, although the rate of absorption of the absorbent CaCO .. for SO- was increased. Further, when the supply of Na 1 CO-, was stopped, the rate of absorption of SO ₉ and the rate of conversion of CaCO, were markedly decreased, with the result that gypsum deposits occurred excessively in the waste gas treatment tower.

As the sodium or potassium compound which can be used in addition to Na ₃ CO ₃ in the present invention, any compound is acceptable so long as it can generate NaCl or KCl by reaction with HCl; now readily available chemicals are generally used.

In the above-mentioned example, air was tentatively blown into the absorbent solution through an air nozzle 10 while the amount of air was gradually increased; the results obtained were very interesting. Namely, as the amount of air to be blown into the absorption _{solution became larger and larger, the rate of conversion of CaCO 3} and the rate of absorption of SO_ were increasingly increased. Fig. 2 shows the change in the S0 ₂ concentration that was present in the purified gas 6 during the step-wise aeration, via a change in the flow rate of air blown into the absorption solution from 0 to about 12 0 m N / h. In other words, FIG. 2 shows the values obtained in the experiment in which the sodium compound was only added in a stoichiometric amount, based on the amount of HCl. Further, in Fig. 2, for comparison, the results obtained in the case where no sodium compound was added are shown.

In addition, in a similar experiment, the changes in the concentration of the remaining CaCO., In the absorption solution with respect to the amount of air blown in were examined; the results obtained are shown in FIG. 3, where the presence of the sodium compound is used as a parameter as in FIG. From the results of Figs. 2 and 3, it can be seen with certainty that if the amount of air to be blown into the absorbing solution is increased, the SO ₂ absorption performance and the CaCO 2 conversion rate are correspondingly improved.

This improvement in performance through the injection of air can also be obtained in the experiment in which none Sodium compound was added; however, if it is necessary that the SO «absorption rate and the CaCO -, - conversion rate are at the same level as im

In the event that the sodium compound is added, the amount of air blown must be increased, resulting in a large consumption of fan power and thus an increase in processing costs.

If the sodium compound was still not added, considerable gypsum deposition occurred on the inner walls of the The exhaust treatment tower and the absorption solution tank.

In an experiment in which the amount of Mn ions in the absorption solution is gradually increased by adding MnSO. in the above example was increased,. it was found from the results that the SO "absorption rate and the CaCO, conversion rate by increasing the concentration of Mn ions in the absorbing solution on condition that the flow rate into the absorption solution air to be injected was constant, were increasingly improved.

In Fig. 4, the change in the concentration of SO- in the purified gas 6 during the gradual addition of MnSO ₄ to the absorbing solution to increase the concentrations of Mn ions from 0 to a level of about 400 mg / l, and the change in concentration _{of CaCO 3} remaining in the absorption solution.

As can be seen from the above, it could be confirmed that the S0 _? -Absorption performance and the conversion rate of the absorbent is increased when treating exhaust gas containing HCl, SO and optionally HF, if a sodium or potassium compound is added to the absorbent solution in an amount corresponding to that of HCl, if a calcium compound is used as an absorbent in an amount corresponding to that of S0_ is added, and when air is blown into the absorbent solution, to remove the contained therein

Oxidize sulfite to sulfate.

In the example, the flow rate of air to be injected was in the range from 5 to 110 m N / h, ie about 0.125 to 2.75%, based on the flow rate of the exhaust gas under the condition that the flow rate of the exhaust gas was 4000 m N / h and the The concentration of SO 2 in the inlet was 1000 ppm. However, if the total concentration of SO ₂ in the inlet is higher, the amount of sulfite to be oxidized increases and thus the flow rate of air must be controlled depending on the amount of sulfite.

The easiest way to regulate the flow rate of blowing air in the present example is to use suitable means to measure the concentration of sulfite in the absorption solution and to blow air in so that the sulfite concentration is 10 mmol / l or less. Accordingly, in the experiments in which the SO2 concentration in the purified gas was within the range of 50 ppm or less in FIGS. 2 and 4, the sulfite concentration in the absorbing solution was 10 mmol / l or less. Therefore, it is desirable that the air blowing flow rate be determined by measuring the concentration of sulfite in the absorbing solution and increasing the air flow rate so that the sulfite concentration may be 10 mmol / l or less. Examples of effective manganese compounds that are added include MnSO ₄ , MnOOH, MnO ", and MnCl-. It was found that manganese in the present invention was effective at a concentration at which the oxidation-reduction reaction was effected in the absorption solution; therefore, no particular limitation is necessary on the anion of the manganese compound.

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Claims (1)

Qg) FEDERAL REPUBLIC GERMANY GERMAN PATENT OFFICE Disclosure document DE 3523820 al § File number: Filing date: Disclosure date: P 35 23 820.8
3. 7.85
30. 1.86 © Int. Cl. ⁴ : BOI D 53/34 B 01 D 53/14
C 01 F 11/46
B 01 D 47/02 '' i '; in CM OO CJ CM U> Union priority: © @ (3Ϊ)
07/03/84 JP 136510/84 Applicant: Mitsubishi Jukogyo K.K., Tokyo / Tokyo, JP Representative: Kühnen, R., Dipl.-Ing .; Wacker, P., Dipl.-Ing. Dipl.-Wirtsch.-Ing., 8050 Freising; Luderschmidt, W., Dipl.-Chem. Dr.phil.nat., Pat.-Anw., 6370 Oberursel Inventor: Shinoda, Naoharu; Tatani, Atsushi; Onizuka,
Masakazu; Okino, Susumu, Hiroshima, JP; Shimizu,
Taku, Tokyo / Tokyo, JP A request for examination in accordance with Section 44 PatG has been submitted @ Process for treating exhaust gas A method for treating exhaust gas, which contains SO ₂ , HCl and optionally HF, is characterized in that a stoichiometric amount of a sodium and / or potassium compound, which is NaCl or KCI in the exhaust gas treatment tower, and a calcium compound as Introduces absorbent for SO ₂ , and blows air into the absorbent solution, which is brought into contact with the exhaust gas. CM CO CO CM IA UJ BUNDESDRUCKEREI 12. 85 508 065/558 Determines absorption solution and the flow rate adjusted by air so that the concentration of sulfide becomes 10 mmol / l or less.