JPS61433A - Waste gas desulfurization - Google Patents

Abstract

PURPOSE:To keep desulfurization capacity while reducing the flow amount of air, by continuously detecting redox potential in the recirculation slurry of an absorbing tower and controlling the flow amount of oxygen-containing gas blown in the slurry to perfectly oxidize CaSO3 in the slurry. CONSTITUTION:SO2-containing exhaust gas is brought into contact with a Ca- compound-containing slurry in an absorbing tower 2. Air is blown in the recirculation slurry containing CaSO3 generated by the absorption of SO2 from dispersing nozzles 7 in a liquid sump 5 to form gypsum. The redox potential of the recirculation slurry is detected by an electrode 8 and the opening and closing signal of a control valve 12 is issued by a controller 10 corresponding to the deviation of the detection value and preset redox potential voltage to set the necessary min. amount of air supplied into the liquid sump 5. The redox potential voltage preliminarily inputted to the controller 10 is set from the result of the calibration curve formed by calculating the correlation of sulfite concn. and redox potential.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a flue gas desulfurization method, and more specifically, it involves removing SO2 from combustion flue gas using calcium compounds such as limestone, slaked lime, and dolomite as absorbent raw materials. This paper concerns improvements to the so-called wet lime/gypsum flue gas desulfurization method.

(Conventional technology) In the absorption process in wet lime/gypsum flue gas desulfurization equipment, 80
．． Exhaust gas containing 0a(OH)1, C1LOO2.

C! ILSO3113AH10, C1asO4”2H1
This method absorbs 803 from exhaust gas by contacting it with a slurry containing a calcium compound with low solubility such as So, but the absorption reaction can be expressed as S
O2+0a(OH%→C! !L 80B ”%T1
20+%H, OII@11@II (1) S 02 +
Oa 003 +3AH20→0aSO3”%H,o
+ a o2 - (2), and the following oxidation reaction also occurs partially due to oxygen in the exhaust gas.

C! a S OH” 3AH2O+ % OH+ 7
2 Hl O→Oa S 04 ” 2 Hl O”
(3) Although the overall reaction formula is simple, the actual reaction mechanism is not as simple as this, and involves various dissolved ions such as Oa'', MW'''', S9゜+
2- Na, Bog, H8O3, 003+ HCO
3-+ H2Some To 10- H2003+ CZ ”' + F-+ A Z
” +r Mn + 820s + H+
OH and other factors are involved in an extremely complex manner, and there are many aspects that remain unclear.

Conventionally, it is known that there are two types of methods for recovering gypsum by oxidizing sulfite compounds obtained by reacting calcium oxide, calcium hydroxide, calcium carbonate, etc. with So as an absorbent, as shown below. .

One of them is to carry out the reactions (1) and (2) above in an absorption device, and the resulting oxidation reaction (3
) is a method in which the oxidation device is provided separately from the absorption device. 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 1 to s.
kg/al to increase the oxygen utilization rate. 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 methods is that there is no way to continuously and easily measure the conversion rate of calcium sulfite to calcium sulfate. 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 concentration of calcium sulfite is measured by redox titration using iodine, and the air flow rate is intermittently adjusted to keep the concentration below a predetermined level. In addition to deteriorating the quality of gypsum, especially in the method in which oxidation is carried out in the absorber body, disadvantages such as a decrease in the absorption performance of So and a decrease in reactivity with the calcium compound that is the absorbent may occur. 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 Inventors) In the process of conducting detailed experimental studies on 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 slurry circulating in the sleeping tower It was found that there is a certain relationship between the concentration of calcium sulfite and the concentration of calcium sulfite in the slurry.

(Means for solving the problem) The present invention has been completed based on the above knowledge, and
In the method of desulfurization treatment by bringing exhaust gas containing O2 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 increase the redox potential of the slurry. This flue gas desulfurization method is characterized in that the flow rate of the oxygen-containing gas is controlled by the detection, and adjusted to completely oxidize calcium sulfite in the slurry.

FIG. 1 shows an example of the relationship between the ORP and the sulfite concentration of the circulating fluid when exhaust gas containing 1300 ppm of Boz 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 suddenly shows a high value as the sulfite concentration decreases.

In addition, we obtained experimental results showing that the sulfite concentration in the circulating fluid is correlated with the amount of air supplied to the liquid reservoir provided at the bottom of the absorption tower, as illustrated in Figure 2.

That is, it was discovered 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 focused on the facts shown in FIGS. 1 and 2 and came to propose the present invention. That is, conventionally, the relationship shown in FIG. 2 has been obtained by sampling the circulating fluid intermittently during operation and manually analyzing it by oxidation-reduction method using iodine, which naturally limits the frequency of analysis. The amount of exhaust gas, the concentration of S02, 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. Based on the analytical circumstances mentioned above, in order to maintain desulfurization performance in consideration of load fluctuations, etc.
The amount of air oxidation had to be set in excess of point B in FIG. 2, which was unfavorable in terms of running costs.

Once you have determined the correlation between ORP and sulfite concentration shown in Figure 1 and obtained a verification line, you can air air so that the sulfite concentration in the circulating fluid disappears, that is, ORP is submerged with Arn'7 in Figure 1. It becomes possible to set the flow rate continuously. In other words, if ORP is less than AInV, the air flow rate is increased by one according to the deviation1, and ORP is AI'
If the difference is 7 or more, so-called proportional control can be applied to reduce the air flow rate according to the deviation.

As shown in FIGS. 1 and 2, desulfurization performance is significantly affected by the sulfite concentration in the region where the sulfite concentration is low, but at the same time, the change in ORP is also significant in this region. Therefore O
The RP detects slight changes in the sulfite concentration, and the proportional control described above allows the setting to be adjusted to the minimum air flow rate required for the sulfite to disappear, thereby maintaining the necessary desulfurization performance. It 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, an embodiment of the present invention will be explained based on FIG. 3 in order to clarify the embodiment.

In FIG. 5, exhaust gas containing SO2 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 80° is absorbed into the absorption liquid and becomes calcium sulfite (aaso). OaS
The absorption liquid containing 03 comes into contact with the air that is blown into fine bubbles from the dispersion nozzle 7 through the line 6 in the circulating liquid retainer 5 at the bottom of the absorption tower 2, and forms gypsum (C!A8).
04).

Of course, due to the oxygen present in the exhaust gas, C! Although a portion of the aE 103 is oxidized, the oxygen concentration is usually low and it is often necessary to blow air into the reservoir 5 to complete the oxidation. The ORP of the circulating fluid is detected by an electrode 8 installed in the reservoir 5. The installation position of the ORP is not limited to the inside of the liquid reservoir 5, and it is of course possible to install it in the middle of the line 4. As the electrode 8, a commonly used platinum electrode can be used.

The ORP detected by the electrode 8 is sent to a controller 10 via a line 9, which sends an opening/closing signal for a control valve 12 via a line 11 depending on the deviation from the previously set ORP voltage. The minimum required air flow rate to be supplied into the liquid retainer 5 by the control pulp 12 is set.

The absorption liquid in which Oa803 has been oxidized and disappeared is sprayed into the absorption tower 2 again through the cylinder line 4 by the circulation pump 14. The pH of the absorption liquid is adjusted by calcium carbonate slurry supplied through line 13, and a portion of the circulating liquid is taken out through line 15 to separate the gypsum.

It is necessary to input the ORP voltage into the controller 10 in advance, but it is necessary to determine the correlation between the sulfite concentration and ORP, create a calibration curve, and set it based on the results. At this time, since ORP is influenced to some extent by components of the solution 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.
After continuous processing over time, the average value of the air flow rate supplied from line 6 was as follows, and the fluctuation range was ±20 m''N/h, including during load following.

The 24-hour average air flow rate was 579 m''H/h.

Note that the inlet EIO is constant at about 2000 ppm,
The desulfurization rate was maintained at 96% or higher. 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 absorbed from the exhaust gas.

In addition, for confirmation, the circulating fluid was sampled once per hour and the sulfite concentration was measured, and in all cases it was α5 m
It was less than moL/t.

Comparative Example (In the embodiment shown in Fig. 3, the ORP air control system, that is, electrode 8, controllers 1 and 0, and pipe 12, were manually adjusted, and the exhaust gas was treated under the same conditions as in the example. However, the 24-hour average air flow rate is 415 m"l.
J/h, the fluctuation range was ±60 m”N/h. The entrance So, was approximately 2 o o o, the same as in the example.
The desulfurization rate was constant at ppm, and the desulfurization rate was 94% or more.

The total amount of calcium carbonate supplied during the operation period was equivalent to 1.08 molar ratio of the total amount of EIO absorbed from the exhaust gas.

The air flow rate was adjusted by sampling the circulating fluid once/hr, but the maximum sulfite concentration was 55.
mmot/L.

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, the ORP fi, 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. Among the sub-agents: 1) Akihabara agent Ryo Hagiwara - Concentration of sulfuric acid in circulating fluid 0"/4! ;) Supply to 1F
(Target value) Figure 3

Claims (1)

[Claims] In a method of desulfurizing exhaust gas containing SO_2 by contacting it with an absorption tower circulation slurry containing calcium compounds in an absorption tower, blowing a gas containing oxygen into the slurry,
A flue gas desulfurization method characterized in that the flow rate of the oxygen-containing gas is controlled by continuously detecting the redox potential of the slurry so as to completely oxidize calcium sulfite in the slurry.