JPH0359731B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a flue gas desulfurization method, and more specifically, it uses calcium compounds such as limestone, slaked lime, and dolomite as absorbent raw materials to remove SO ₂ from combustion flue gas.
The present invention relates to improvements in the so-called wet lime and gypsum flue gas desulfurization method that removes carbon dioxide. (Conventional technology) In the absorption process in wet lime/gypsum flue gas desulfurization equipment, flue gas containing SO ₂ and Ca(OH) ₂ , CaCO ₃ ,
This method absorbs SO ₂ from exhaust gas by contacting it with a slurry containing calcium compounds with low solubility such as CaSO ₃・1/2H ₂ O and CaSO ₃・_{2H 2} O. Expressed in the overall reaction formula, SO ₂ + Ca(OH) ₂ →CaSO ₃・1/2H ₂ O+1/2H ₂ O …(1) SO ₂ +CaCO ₃ +1/2H ₂ O→CaSO ₃・1/2H ₂ O+CO ₂ ..., and some of the following oxidation reactions also occur due to oxygen in the exhaust gas. CaSO ₃・1/2H ₂ O＋1/2O ₂ +5/2H ₂ O →CaSO ₄・2H ₂ O …(3) Although the overall reaction formula is simple as above, the actual reaction mechanism is not as simple as this. , various dissolved ions such as Ca ²⁺ , Mg ²⁺ , SO ₄ ^2- ₄ ,
Ma ⁺ , SO ^2- ₃ , HSO ^- ₃ , CO ^2- ₃ , HCO ^- ₃ , H ₂ SO ₃ ,
H ₂ CO ₃ , Cl ^- , F ^- , Al ³⁺ , Mn ²⁺ , S ₂ O ^2- ₆ , H ⁺ ,
OH ^- etc. are involved in an extremely complex manner,
There are many parts that are not explained. Conventionally, it is known that there are two types of methods for recovering gypsum by oxidizing sulfite compounds obtained by reacting with SO ₂ using absorbents such as calcium oxide, calcium hydroxide, and calcium carbonate, as shown below. . One method is to carry out the reactions (1) and (2) above in an absorption device, and the oxidation reaction (3) of the obtained calcium sulfite to be carried out in an oxidation device installed separately from the absorption device. be. Another method is to oxidize calcium sulfite in the absorption liquid by providing a mechanism for generating fine air bubbles in a liquid reservoir that circulates and supplies the absorption liquid to the absorption apparatus main body. In both cases, air is generally used as the oxidizing agent and an aeration tank is used as the oxidizing device, but various efforts have been made to improve the oxidation rate and reduce the amount of aeration. If the oxidizer is installed separately, the operating pressure should be increased to 1
A common method is to increase the oxygen utilization rate by increasing the oxygen utilization rate to 5 Kg/cm ² . When oxidation is performed in the absorber itself, the exhaust gas to be treated is at normal pressure, so it is economically disadvantageous to install a liquid reservoir that can operate under pressure, so the oxidation reaction is promoted. A method of adding a liquid phase catalyst such as manganese is known. A common drawback of both is that there is no way to continuously and easily measure the conversion rate of calcium sulfite to calcium sulfate, and therefore, if you want to maintain a high conversion rate of calcium sulfate, it is necessary to take into account load fluctuations, etc. The current situation is that an excessive amount of air flow must always be supplied. In other words, the circulating fluid is sampled from time to time, the calcium sulfite concentration is measured by the redox titration method using iodine, and the air flow rate is intermittently adjusted to keep the concentration below a predetermined concentration. Not only does the quality of the product gypsum deteriorate, but also disadvantages such as a decrease in SO ₂ absorption performance and a decrease in reactivity with the calcium compound that is the absorbent occur, especially in the method in which oxidation is performed in the absorption device itself. As mentioned above, it was necessary to supply an excessive amount of ventilation flow. Providing excessive aeration flow rate leads to increased running costs and has been a major drawback of conventional aerobic oxidation methods. (Problems to be Solved by the Invention) The present invention eliminates the drawbacks of the above-mentioned conventional methods, and particularly proposes an optimal method for carrying out an oxidation reaction within the absorption liquid circulation reservoir. (Findings of the Inventor) In the process of conducting a detailed experimental investigation into the effects of the various components mentioned above on desulfurization performance, the present inventors discovered that the oxidation-reduction potential (hereinafter referred to as ORP) of the absorption tower circulating slurry It was found that there is a certain relationship between the calcium sulfite concentration in the slurry. (Means for Solving the Problems) The present invention has been completed based on the above findings, and includes desulfurization treatment by bringing exhaust gas containing SO ₂ into contact with absorption tower circulation slurry containing calcium compounds in an absorption tower. In this method, a gas containing oxygen is blown into the slurry, and the flow rate of the gas containing oxygen is controlled by continuously detecting the redox potential of the slurry, and calcium sulfite in the slurry is controlled. This is a flue gas desulfurization method characterized by adjustment to achieve complete oxidation. Figure 1 shows an example of the relationship between the ORP and sulfite concentration of the circulating fluid when exhaust gas containing 1300 ppm SO ₂ is desulfurized by contacting it with a slurry containing a calcium compound. ORP is sensitively related to the sulfite concentration in the liquid, and while ORP shows a low value even in the presence of a very small amount of sulfite, it rapidly increases as the sulfite concentration decreases. In addition, experimental results were obtained in which the sulfite concentration in the circulating liquid was correlated with the amount of air supplied to the liquid reservoir provided at the bottom of the absorption tower, as illustrated in FIG. That is, it has been found that as the air flow rate increases, the oxidation rate increases, and therefore the sulfite concentration decreases, and then, when the point B in the figure is exceeded, the sulfite disappears.
Furthermore, as shown in FIG. 2, it was observed that the desulfurization rate significantly improved up to point B. The present inventors have focused on the facts shown in FIGS. 1 and 2, and have proposed the present invention. In other words, the relationship shown in FIG. 2 has conventionally been obtained by sampling the circulating fluid at faults during operation and manually analyzing it using oxidation-reduction methods using iodine, which naturally limits the frequency of analysis. The amount of exhaust gas, the concentration of SO ₂ , etc. vary greatly depending on the load conditions of the boiler, etc. that is the emission source, and the rate of change is generally rapid. From the above analytical circumstances, in order to maintain desulfurization performance while taking into account load fluctuations, it is necessary to set the air oxidation amount to an excess level compared to point B in Figure 2, which is unfavorable in terms of running costs. Ta. Once the correlation between ORP and sulfite concentration shown in Figure 1 is determined and a verification line is obtained, it is possible to ensure that the sulfite concentration in the circulating fluid disappears.
It becomes possible to continuously set the air flow rate so that it becomes Am▽ in the figure. In other words, if ORP is less than Am▽, the air flow rate will be increased according to the deviation.
When ORP exceeds Am▽, so-called proportional control can be applied to reduce the air flow rate according to the deviation. As shown in Figures 1 and 2, desulfurization performance is significantly affected by the sulfite concentration in areas where the sulfite concentration is low, but at the same time, ORP changes are also significant in this area. Therefore, ORP can detect slight changes in sulfite concentration, and the aforementioned proportional control can adjust the air flow rate to the minimum required for sulfite to disappear, thereby maintaining the necessary desulfurization performance. becomes possible. ORP can be measured extremely easily by simply immersing the electrode in the circulating fluid, and there is no measurement time delay, so there is no need to supply excess air to account for measurement delays, and it follows load fluctuations well. . Constantly supplying the minimum amount of air required is very advantageous in terms of reducing running costs. Next, in order to clarify the embodiments of the present invention, the third
This will be explained based on the diagram. In FIG. 3, exhaust gas containing SO ₂ is introduced into an absorption tower 2 through a duct 1, and the purified gas is discharged into the atmosphere through a duct 3. In the absorption tower 2, the exhaust gas and the absorption liquid sprayed into the absorption tower 2 through the line 4 come into contact, and SO ₂ is absorbed into the absorption liquid and becomes calcium sulfite (CaSO ₃ ). The absorption liquid containing CaSO ₃ comes into contact with the air that is blown in the form of fine bubbles from the dispersion nozzle 7 through the line 6 in the circulating liquid reservoir 5 at the bottom of the absorption tower 2, producing gypsum (CaSO ₄ ). . Of course, due to the oxygen present in the exhaust gas,
Although some of the CaSO ₃ is oxidized, the oxygen concentration is usually low and air blowing into the reservoir 5 is often necessary to complete the oxidation. of circulating fluid
ORP is detected by an electrode 8 installed in the liquid reservoir 5. The installation position of the ORP is not limited to the liquid reservoir 5, but it can also be installed in the middle of the line 4.
Of course it is possible. As the electrode 8, a commonly used platinum electrode can be used. The ORP detected by electrode 8 is transferred to controller 10 by line 9.
It sends an opening/closing signal for the control valve 12 through the line 11 according to the deviation from the preset ORP voltage. The minimum necessary air flow rate to be supplied into the liquid reservoir 5 is set by the control valve 12. The absorption liquid in which CaSO ₃ has been oxidized and disappeared is sent to the circulation pump 14.
The water is then sprayed into the absorption tower 2 again through the line 4. The pH of the absorption liquid is adjusted by the calcium carbonate slurry supplied through line 13, and a portion of the circulating liquid is withdrawn through line 15 to separate the gypsum. It is necessary to input the ORP voltage into the controller 10 in advance, but this needs to be set based on the results of a calibration curve created by determining the correlation between the sulfite concentration and ORP. At this time, since ORP is affected to some extent by dissolved solution components other than sulfite, it is necessary to create a calibration curve specific to the target evacuation device. Next, examples will be shown to clarify the effects of the present invention. Example In the embodiment shown in Fig. 3, a portion of the boiler exhaust gas was fractionated at approximately 8000 m ³ N/h and the exhaust gas was continuously treated for 24 hours. The average value of the air flow rate supplied from line 6 was as follows. The fluctuation range was ±20m ³ N/h, including during load following.

[Table] The 24-hour average air flow rate was 379 m ³ N/h.
The inlet SO ₂ was kept constant at approximately 2000 ppm, and the desulfurization rate was maintained at over 96%. The total amount of calcium carbonate as an absorbent supplied during the operation period was equivalent to 1.04 molar ratio of the total amount of SO ₂ absorbed from the exhaust gas. In addition, for confirmation, we sampled the circulating fluid once per hour and measured the sulfite concentration.
It was less than 0.5 mmol/. Comparative Example In the embodiment shown in FIG.
2 was adjusted manually, and the exhaust gas was treated under the same conditions as in the example. The 24-hour average air flow rate was 415 m ³ N/h, and the fluctuation range was ±60 m ³ N/h. Note that the inlet SO ₂ is about 2000 ppm, the same as in the example.
The desulfurization rate was 94% or more. The total amount of calcium carbonate supplied during the operation period was equivalent to a 1.08 molar ratio of the total amount of SO ₂ absorbed from the exhaust gas. The air flow rate was adjusted by sampling the circulating fluid once/hr, but the maximum value of the sulfite concentration was
It was 3.5 mmol/. From the above Examples and Comparative Examples, it is clear that the method of the present invention can reduce the air flow rate and maintain desulfurization performance.

[Brief explanation of drawings]

Figures 1 and 2 are correlation diagrams showing the relationship between the sulfite concentration in the circulating fluid and ORP, and the sulfite concentration in the circulating fluid and the amount of air supplied to the reservoir, which are the basis for proposing the present invention. FIG. 1 is a diagram showing one embodiment of the present invention.

Claims (1)

[Claims] 1 In a method of desulfurization treatment by bringing exhaust gas containing SO ₂ into contact with absorption tower circulation slurry containing calcium compounds in an absorption tower, a gas containing oxygen is blown into the slurry to continuously maintain the redox potential of the slurry. A flue gas desulfurization method characterized in that the flow rate of the oxygen-containing gas is controlled by detecting the oxygen content and adjusted to completely oxidize calcium sulfite in the slurry.