DK167102B1 - PROCEDURE FOR TREATMENT OF EXHAUST GAS CONTAINING SO2, HCL AND EVENTS HF - Google Patents

Description

DK 167102 B1 i

The present invention relates to a process for treating waste gas containing SO 2, HCl and HF in a gas treatment tower, more particularly a method for wet treating such a waste gas, e.g.

5 a waste gas resulting from the combustion of coal.

In the case of desulfurization of smoke by the usual wet methods, the exhaust gas will sometimes contain not only SQx but also the harmful compounds HC1 and HF. In case of coal burning, the exhaust gas can typically contain approx. 1000 ppm SOx, 10 approx. 60 ppm HCl and approx. 40 ppm HF.

If such a waste gas is treated by the wet method in a gas treatment tower using CaCO 2 as the absorbent for SO 2, the following reactions will proceed:

CaCO3 + SO2 - *> CaSO3 + CO2 (1) CaCO3 + 2HC1 -> CaCl2 + CO2 + H2O (2)

CaCO3 + 2HF -> CaF2 + CO2 + H2O (3) In this case, however, reaction (2) will be the dominant one and the solution of CaCO

2 I

of the presence of CaCl2 derived Ca, which is formed by the reaction (2). As a result, reaction (1) will be hindered and SO 2 absorption capacity will decrease.

In addition, the CaSO 2 .2H 2 O formed by the desulfurization reaction will be deposited on the wall surfaces of the desulfurization reactor as a coating, with the result that the operation of the processing apparatus 25 will be disturbed. To overcome these drawbacks, it is considered effective to add an appropriate amount of sodium sulfate Na2SO4 or potassium sulfate K2SO4 to the reaction system in proportion to the amount of HCl, utilizing the following reactions to remove CaCl2 in the form of CaSO4.2H2O.

2 DK 167102 B1

Na2 SO4 + CaCl2 + aq. 2NaCl + CaSO4.2H2O (4) K2SO4 + CaCl2 + aq. 2KC1 + CaS04.2H20 (5)

The first reaction is well known, namely the reaction in which gypsum CaSO4.2H2O is prepared from a Na2SO4-5 solution by the addition of CaCl2.

The compounds Na 2 SO 4 and K 2 SO 4 necessary for the CaCl 2 removal reactions can be prepared by simply adding a basic sodium or potassium salt to the gas treatment tower used in the wet method. If 10 is used e.g. The following reactions take place:

Na2CO3 + SO2 ^ Na2SO3 + CO2 (6)

Na2 SO3 + 1/2 02— * Na2 SO4 (7)

The above-mentioned compound HF is converted to CaF2, as shown in the above formula (3), but CaF2 cannot be stabilized as a solid material of poor solubility.

CaF2 will dissolve on aluminum compounds contained in the exhaust gas dust, and reaction between the formed aluminum ions and fluorine ions will inhibit the dissolution of calcium carbonate. In order to eliminate this disadvantage 20, JA Provisional Publication No. 167023/1980 has proposed a process using a basic sodium salt.

Thus, based on the prior art mentioned above, it is to be expected that the addition of a basic sodium salt 25 in an amount which depends on the amounts of HC1 and HF present is effective in treating a waste gas containing SO2, HC1 and HF with a view to on alleviating the above-mentioned disadvantages of CaCl2, aluminum ions and fluorine ions.

3 DK 167102 B1

It has now been found that the addition of basic sodium or potassium salts to the reaction system in an amount equal to the amount of HCl, combined with air injection in the absorption solution used in the wet method, not only eliminates the above disadvantages but also causes a considerable increase in the reaction rate of the solution. CaCO ^ ·

The present invention, based on this disclosure, thus relates to a process for treating exhaust gas containing SC 2, HCl and optionally HF in a gas treatment tower which is characterized by adding one to the amount of HC1 and optionally the gas treatment tower. HF equivalent amount of at least one sodium and / or potassium compound which will react to NaCl 15 and KCl in the gas treatment tower, as well as a calcium compound as absorbent for SO2 and blow air into an absorption solution which is contacted with the exhaust gas.

Preferred embodiments, which are explained in more detail below, are described in claims 2 and 3.

Thus, the present invention can also be used in the treatment of gas which does not contain HF.

If the sodium or potassium compound is not supplied to the system in an amount equal to the amount of HCl, the dissolved CaCl 2 compound as mentioned above will appear in the system and consequently the absorption capacity of the CaCO 2 absorbent will be poor. In other words, the formation of CaCl 2 will lead to an increase in the concentration of dissolved Ca 2+, which will decrease the solubility of CaSO 2 .2H 2 O (gypsum) formed by the desulfurization and oxidation reaction and accelerate the growth of gypsum deposits in the system. In addition, the partial pressure of SO 2 will be higher in an absorption solution containing CaCl 2, so that its SC 2 absorbency will be reduced.

Thus, when the absorption solution contains CaCl 2, the concentration of calcium ions will increase, and consequently, the solubility of SO-, as shown by the 2+ 2- 2 equation (Ca) (SO 2) = Ksp. A decrease in the concentration of SO 2 to which the gaseous SO 2 is converted by dissolving in aqueous medium results in a decreased solubility of the gaseous SO 2. In such an absorption solution, the concentration of the SO 2 formed by absorption of the gaseous SO 2 will immediately reach a saturation level so that the partial pressure of SO 2 can easily be higher. On the other hand, if dissolved chloride is obtained from the addition of 2-

NaCl or KCl, the saturation concentration of SO 2 will be greater, so that the solubility of the absorption solution will be greater, which means that the partial pressure of SO 2 in the absorption solution will be maintained at a lower level.

Experiments were conducted with blowing 20 air into the absorption solution in the gas treatment tower.

This produced interesting results. As the amount of air entrained in the absorption solution was increased, a greater rate of conversion of the CaCO 2 absorbent and a higher rate of absorption of SO2 · 25 were found to investigate whether the effect of the blown air is dependent on the addition of the sodium or potassium compounds added simultaneously. with air being blown in, experiments were conducted in which varying amounts of air were blown into an absorption solution to which no sodium or potassium compounds were added, so that Cl in the absorption solution was present as CaCl3. The results of these experiments showed that an increase in the amount of air blown in resulted in an increased rate of conversion of CaCO 2 and an increased rate of absorption of SO 2, but that the calcium utilization rate was relatively lower than it was in the case where Cl was in the form 5 of NaCl or KCl.

Therefore, Cl must be present in the absorption solution in the form of NaCl or KCl.

The reason for the increase in the conversion rate of CaCO 2 caused by air injection into the absorption solution is not completely clarified, but probably the following is done: When the air is blown into the absorption solution, HSO is converted to the HSO₂ residue of a strong acid so that the dissolution reactivity of CaCOCO can be readily restored even though dissolved aluminum and fluorine ions are present in the absorption solution. As a result, both the absorption rate of SO 2 and the reactivity of CaCO 2 will be improved.

The following chemical reactions are as follows: (Reactions in the gas treatment tower) SO2 (gas) + H2O H2SO3 (8) H2O3 H + + HS03 (9) (Reactions in the absorption solution tank) 25 HS03 "+ 1/2 02 (gas) HSO ^ - (10) H + + HSO4 - + CaCO3 (solid) + water

CaSO4.2H20 + CO2 ^ (11) 6 DK 167102 B1 (Reaction in the absorption solution tank without aeration) 2HS0 ^ _ +. Ca 2+ + CaCO 3 (solid) - 2CaSO3.l / 2 H 2 O + CO 2'T * (12)

Thus, as explained above, a number of 5 drawbacks can be overcome by determining the amount of HCl in the exhaust gas, adding a stoichiometric amount of a sodium and / or potassium compound to the treatment tower, where the compound will be converted to NaCl and KCl, and a calcium compound as an absorbent for SO2 and blow air 10 into the absorption solution used to treat the exhaust gas.

Remedied disadvantages include the decrease in HC1 and HF caused by HCl and HF, the material deposition problems caused by the deposition of CaSO2 .2H2O, and the decreased reactivity of the SO2 absorbent. In addition, a substantial increase in the absorption rate of SO2 and the SO2 conversion rate of the absorbent are provided.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is illustrated in more detail below with reference to the drawing, in which 1 shows schematically an embodiment of the method according to the invention, fig. 2 graphically shows experimental data, more specifically, the SO 2 concentration in the purified gas as a function of the amount of air supplied - partly without the addition of Na 2 CO 3 - partly according to an embodiment of the present invention, where Na 2 CO 3 is added in relation to HCl. stoichiometric amount, fig. 3 graphically shows experimental data, more specifically, the CaCO-j concentration in the absorption solution as a function of the amount of air supplied - partly without the addition of 2 2 2 - partly according to an embodiment of the present invention, where 5 Na 2 CO 2 in a stoichiometric amount relative to HCl is added, and FIG. Figure 4 graphically shows experimental data, more specifically, the SC₂ concentration in the purified gas and the CaCO₂ concentration in the absorption solution as a function of the Mn ion concentration in the absorption solution for values 10 up to ca. 400 mg / liter, at the same time adding ^ 2 ^ 2 in a stoichiometric amount relative to HCl, and injecting a predetermined amount of air into the absorption solution.

The process of the present invention was carried out in the 15 of FIG. 1.

As shown in FIG. 1, exhaust gas 1 is passed from a coal-fired boiler to a gas treatment tower 2. The exhaust gas has previously passed a NOx remover, an electrostatic dust filter and a heat exchanger. These devices are not shown in the drawing.

A gas detector 3 is located at the gas inlet of the gas treatment tower 2 and this detector measured that the uncleaned gas contained approx. 1000 ppm SO2, approx. 60 ppm HCl and approx.

40 ppm HF. The untreated gas was introduced at a flow rate of approx. 4000 Nm 2 / h.

The gas treatment tower 2 was packed with shakes. By means of a circulation pump 4 for the absorption solution, this was injected at the top of the tower at a speed of 60 m 2 / h. Thereby, SO2, HCl and HF in the untreated gas were absorbed by the absorption solution, and the gas was extracted from the tower 2 as purified gas 6 via a mist remover 5.

8 DK 167102 B1 Measurements showed that the purified gas 6 contained approx. 100 ppm SO2J and the content of HC1 and HF was less than 1 ppm, which was the lowest detectable value.

The CaCO 2 used as an absorbent for SO2 was fed to the tower through a tube 7 at a rate of approx.

17 kg / h, and at the same time Na 2 CO 2 was fed through a tube 8 with a flow rate of at least 0.52 kg / h, which corresponded to the stoichiometric amount of HCl.

In addition, air is blown in an amount of approx. 20 Nm 2 / h a 10 tank 9 at the bottom of the treatment tower 2 to oxidize the sulfite to sulfate formed by absorption of SO2.

The absorption solution in tank 9 was available as a suspension containing crystalline CaSO 2 .2H 2 O and some powdery CaCO 3. The water balance was adjusted by adding water to the absorption solution so that the concentration of the slurry was approx. 18 weight- o.

Part of the absorption solution was fed via a pump 11 to a separator 12, where crystalline CaSO 2 .2H 2 O (gypsum) was removed from the system to balance the absorbed SOg. · The gypsum formed was recovered as a by-product in the separator 12 of the resulting filtrate was discharged via a tube 14, while the remaining portion of the filtrate was returned to the gas treatment tower 2.

During stable operation, a concentration of chloride ions of approx. 280 mmol / liter in the absorption solution as well as a concentration of sodium ions which was never less than 280 mmol / liter, which corresponds to the equivalent in chloride ions. Furthermore, it was found that the concentration of fluoride ions in the absorption solution did not exceed 5 mmol / liter, and since the fluoride ion concentration would be approx. 190 mmol / liter (estimated value), if all fluoride ions were present in dissolved form, it is seen that fluoride ions were removed from the system in the form of solid CaF 2.

When Na 2 O 2 was added in an amount less than the amount corresponding to the HCl amount, an inferior SO 2 absorption was found and the pH of the absorption solution decreased. The poorer SO 2 absorption could not be alleviated by increased addition of CaCO 2 absorbent. When the 10 Na 2 CO 2 supply was adjusted, a remarkable decrease in the absorption rate of SO 2 and the reaction rate of CaCO 2 were observed, with the result that unusually large plaster deposits occurred in the gas treatment tower.

Speaking as sodium or potassium compounds used in the process of this invention, in addition to Na 2 O 2, any compound which is acceptable provided it can produce NaCl or KCl by reaction with HCl and all such readily available chemicals 20 have been used successfully.

In the above example, air was blown into the absorption solution through an air nozzle 10. The amount of air was gradually increased, and the results obtained were very interesting. As the amount of air entrained in the absorption solution became larger and larger, the rate of conversion of CaCO 2 and the rate of absorption of SO 2 increased more and more. In FIG. 2, the SO 2 concentration in the purified gas 6 is shown as a function of the air flow rate for various values in the range of 0 to approx. 120 30 Nm / h. The two curves of FIG. 2 shows the measured function dependence partly without the addition of Na 2 CO 2 and partly with Na 2 CO 2 added in stoichiometric amount to HCl.

10 DK 167102 B1

In another experiment, concentration variations of CaCO 2 in the absorbent fluid as a function of the air flow were investigated. The results obtained are shown in the two curves of FIG. 3, with or without addition respectively. 2 and 3, that an increase in the amount of blown air leads to a corresponding improvement in the SO 2 absorption and the conversion rate of CaCO 2.

This improvement is also provided when no sodium compound is added, but if one wants the same SO 2 absorption rate and CaCO 3 conversion without using sodium addition as that obtained during the addition of the sodium compound, the amount of air injected must be increased. leads to high energy consumption and corresponding cost increases.

In addition, significant gypsum deposition occurred on the interior walls of the gas treatment tower and absorption solution tank when no sodium compound was added.

In addition, an experiment was carried out in which 20 was added ^ 2 ^ 2 in an amount corresponding to the amount of HCl and air was added in an amount of 20 Nm3 / n, while at the same time, gradually adding MnSO4 so that the amount of Mn ions in the absorption solution were gradually increased. The results showed that the SO 2 absorption rate and the CaCO 2 conversion rate were increased more and more by increasing Mn-ion concentration in the absorption solution at a maintained value for the blown air volume.

In FIG. 4, the concentration of SO2 in the purified gas 6 is shown as a function of the added amount of MnSO4 which was added stepwise, so that the Mn ion concentration increased from 0 to approx. 400 mg / liter (lower curve). FIG. 4 also shows the concentration of the remaining CaCO 2 in the absorption solution as a function of the Mn ion concentration (upper curve).

Thus, it has been found that the absorbance and the rate of conversion of the absorbent of SC 5 by treatment of a waste gas containing HF, HCl and increased by adding a sodium or potassium compound in an amount equal to the amount of HCl, if a calcium compound is added in an amount corresponding to the amount of SC 2, and if air is blown into the absorption solution to oxidize sulfite to sulfate.

In the examples, air was blown at a rate in the range of 5 to 110 Nm 2 / h, ie. an amount from approx. 0.125 to 2.75% based on the amount of the treated gas, the feed rate thereof being 4000 Nm / h with a SO₂ concentration in the untreated gas of 1000 ppm.

When the SO 2 concentration in the untreated gas is greater, the amount of sulfite to be oxidized is increased and the amount of air blown must then be adjusted taking into account the amount of sulfite.

The adjustment of the amount of the blown air can most conveniently be carried out by successive sulfite concentration measurements in the absorption solution, with air being injected so that the sulfite concentration is kept at a value less than equal to 10 mmol / liter. In the experiments whose results are shown in FIG. 2 and 4, the sulfite concentration in the absorption solution was less than or equal to 10 mmol / liter under the test conditions where the SO 2 concentration in the purified gas was less than or equal to 50 ppm. Therefore, it is desirable to determine the flow rate of the blown air by determining the sulfite concentration in the absorption liquid, and then increase the blown air flow, until the sulfite concentration becomes less than or equal to 10 mmol / liter. Examples of manganese compounds which may be advantageously added include MnSO4, MnOOH, MnO2 and MnCl2 · It has been found that manganese is an effective adjuvant in the process of this invention when used at concentrations promoting the oxidation / reduction reaction in the absorption solution, since the anion in the manganese compound does not play any significant role.

Claims (3)

13 DK 167102 B1 Patent claims: A process for treating exhaust gas containing SO 2, HCl and optionally HF in a gas treatment tower, characterized in that a gas treatment tower is supplied to the amount of HC1 and optionally HF corresponding to 5 of at least one sodium and / or potassium compound which will be reacted. to NaCl and KCl in the gas treatment tower, as well as a calcium compound as absorbent for SD2 and blowing air into an absorption solution which is contacted with the exhaust gas. Process according to claim 1, characterized in that the absorption solution contains manganese ions at a concentration of 400 mg / liter or less. Process according to claims 1-2, characterized in that the sulphite concentration is measured in the absorption liquid and the amount of the blown air is adjusted so that the sulphite concentration is 10 mmol / liter or less.