JPS597493B2 - Exhaust gas desulfurization equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method for converting calcium compounds into sulfur oxides (hereinafter referred to as SO
In the wet flue gas desulfurization method, which is used as an absorbent for The present invention relates to an exhaust gas desulfurization device that can reduce

Conventionally, as a method for removing SOx from exhaust gas, lime-
Gypsum method, limestone-gypsum method is commonly used and practiced.

To explain this method based on FIG. 1, untreated exhaust gas that has flowed into the cooling/dust removal tower 1 is cooled by circulating liquid that is sprayed into the cooling/dust removal tower through the circulation pipe 2. At the same time, impurities such as dust contained in the exhaust gas are removed. The exhaust gas leaving the cooling/dust removal tower 1 passes through the mist eliminator 3 and then flows into the absorption tower 4.
In the absorption tower 4, SOx contained in the exhaust gas is removed by an absorption liquid containing calcium carbonate, calcium sulfite, etc., which is sprayed into the absorption tower through an absorption liquid circulation pipe 5. The exhaust gas leaving the absorption tower 4 passes through the mist eliminator 6 to capture the absorption liquid mist in the exhaust gas, and then is guided to the chimney T as treated exhaust gas and released into the atmosphere. The absorption liquid circulates between the absorption tower 4 and the circulation tanks 8 and 9 through the circulation pipe 5, absorbs SOx in the exhaust gas in the absorption tower 4, and is supplied to the circulation tank 8 through the calcium carbonate slurry pipe 10. A neutralization reaction takes place with calcium carbonate. The SOx absorbed in the absorption tower 4 is mainly contained in the absorption liquid in the form of calcium sulfite (CaS03) 5, and is supplied from the circulation tank 8 to the oxidation tower 12 through the liquid supply pipe 11, and in this oxidation tower 12, air is Blow 13
Calcium sulfite is oxidized to gypsum by the air blown into the oxidation tower.

Further, unreacted calcium carbonate contained in the absorption liquid is neutralized in the oxidation tower 12 by sulfuric acid supplied from the sulfuric acid supply pipe 14 to the oxidation tower, and becomes gypsum. The gypsum slurry that has exited the oxidation tower 12 flows through an overflow pipe 15 into a thickener (sedimentation concentration tank) 16, in which the slurry concentration is concentrated or adjusted to a predetermined value.
The gypsum cake is introduced into the centrifugal separator 18 through the extraction pipe 17 at the bottom of the gypsum cake, and is separated into a gypsum cake and a liquid. 20 is a gypsum cake extraction pipe, and 21 is a deep liquid extraction pipe.

The bottom liquid that has passed through the centrifugal separator 18 is returned to the circulation tank 8 through the transfer pipe 22 together with the overflow liquid from the liquid extractor 16, but a part of it is discharged from the system as waste water through the waste water extraction pipe 23. A part of the powdered calcium carbonate supplied from the calcium carbonate supply pipe 24 is supplied to a calcium carbonate slurry tank 25 for slurrying. In the cooling/dust removal tower 1, in order to prevent impurities contained in the flue gas such as captured dust from concentrating in the circulating fluid, a certain amount of the circulating fluid is discharged from the system as wastewater through the drainage extraction pipe 26. It needs to be drained.

On the other hand, in order to replenish the water discharged outside the system as wastewater and the water that evaporates into the exhaust gas in the cooling/dust removal tower 1,
Fresh water is supplied from the fresh water supply pipe 27 to the cooling/dust removal tower 1 . A portion of this fresh water is used as cleaning water for the mist eliminator 3 and then supplied to the cooling/dust removal tower 1. On the other hand, clean water is supplied from the cleaning water supply pipe 28 to the mist eliminator 6 installed after the absorption tower 4, and after the element is cleaned, the cleaning waste water is sent to the mist drain together with the absorption liquid mist captured by the mist eliminator. Return pipe 30
It flows into the absorption tower 4 through. Instead of flowing into the absorption tower 4, the washing wastewater containing this mist may be sent to other equipment included in the desulfurization apparatus, such as the oxidation tower 12. Most of the water supplied to the process system after the absorption tower 4 is washed water from the mist eliminator 6, and the rest is pump seal water and, if necessary, gypsum to the centrifugal separator 1.
The gypsum cleaning water used for cleaning in step 8 can be considered. On the other hand, since the exhaust gas flowing into the absorption tower 4 does not have the ability to evaporate water, the water added to the process system contains a small amount of water that adheres to gypsum and comes out of the system, and water that evaporates inside the oxidation tower 12. There is no choice but to discharge everything except for this out of the system as waste water, and therefore it is necessary to discharge out of the system an amount of waste water approximately equivalent to the cleaning water of the mist eliminator 6. In the absorption tower 4, an absorption reaction as shown in the following formula (1) and a neutralization reaction as shown in the following formula G2) are carried out (the case where calcium carbonate is used as an absorbent in Ishio 1 is described). In this case, sulfite ions (SO') contained in the absorption liquid are
It is known that the higher the concentration of HSO, the better the absorption performance, and the higher the concentration of bisulfite (HSO), the higher the neutralization reaction rate.

It is also known that when magnesium is added to the absorption liquid (in the form of MgSO4), the concentration of sulfite ions and bisulfite ions in the absorption liquid increases dramatically, as shown in FIG.

This is considered to be because the presence of highly soluble cations (Mg2)+ also increases the concentration of the corresponding anions (SO?-HSOi). Therefore, when the magnesium concentration in the absorption liquid increases, the desulfurization performance significantly improves as shown in FIG. 3. In addition, in order to obtain the same sulfur rate, when the magnesium concentration in the absorption liquid is high, the absorber exhaust gas pressure drop and liquid-gas ratio (L/G) are smaller than when the magnesium concentration in the absorption liquid is low. That's enough. Note that the same effect as in the case of magnesium can be exerted even if sodium or ammonium is added to the absorption liquid instead of magnesium. Conveniently, calcium carbonate contains a certain amount of magnesium as an impurity, and this magnesium content is usually 0.5% as MgO.
It is about 2 to 0.5%.

A small amount of magnesium supplied together with this calcium carbonate is combined with SOx in the absorption tower 4 to become MgSO3 or Mg(HSO3)2, and after being oxidized to MgSO4 in the oxidation tower 12, it is added to the effluent. It is dissolved and circulated within the process system, and is mixed with the waste water from the waste water extraction pipe 23 and discharged outside the system. Therefore, the magnesium concentration in the process system is determined by the magnesium content in calcium carbonate and the amount of wastewater discharged from the wastewater extraction pipe 23, but since the amount of wastewater is usually large, the absorption liquid in the process system The magnesium concentration in MgSO4 is 2.00
It is difficult to maintain it above 0 ppm. In this way, in the conventional flue gas desulfurization method, the desulfurization performance of the absorption liquid is low.If the MgSO4 concentration in the absorption liquid is increased, the desulfurization performance can be greatly improved, but the amount of waste water is large, so there is a large loss of magnesium. In order to maintain the MgSO4 concentration in the absorption liquid at a constant level, a considerable amount of external magnesium source,
For example, it is necessary to replenish Mg(0H)2, which is extremely costly.

In addition, the amount of water discharged is determined by the overall water balance of the absorption section, oxidation section, gypsum separation section, and raw material adjustment section, but since a cooling and dust removal tower is installed before the absorption tower, the water evaporation capacity of the absorption tower is Therefore, it is necessary to drain out of the system an amount of waste water equivalent to most of the water added to the system. in this case,
A large amount of water is used in the system for washing the mist eliminator installed at the rear of the absorption tower, and reducing this washing water can lead to a reduction in the amount of wastewater, but this also has its limitations. have. The present invention was made in view of the above points, and the mist eliminator is cleaned intermittently, and when cleaning the mist eliminator, all or part of the cleaning wastewater containing the captured mist is transferred to the outside of the device or to a cooling or cooling device. On the other hand, when the mist eliminator is not cleaned, the captured mist drain is returned to equipment in the desulfurization equipment system other than the cooling/dust eliminator, such as an absorption tower, to remove magnesium ( For example, MgOH) 2), sodium, and ammonium can be maintained at high concentrations (1% or more) in the absorption solution without supplementing or with a small amount of make-up, resulting in low cost. It is an object of the present invention to provide a method and apparatus for desulfurizing exhaust gas that exhibits high desulfurization performance. Hereinafter, the configuration of the present invention will be explained based on the embodiment shown in FIG. 1
4 is a cooling/dust removal tower that cools and removes dust from the exhaust gas containing SOx; 4 is an absorption tower that absorbs and removes SOx in the cooled and dust-removed exhaust gas using an absorption liquid containing a calcium compound;
2 is a mist eliminator provided between the cooling dust removal tower and the absorption tower, 6 is a mist eliminator that captures absorption liquid mist in the exhaust gas exiting the absorption tower, and 28 is connected to the top or side part of the mist eliminator 6. Wash water supply pipe, 3
0 is a mist drain return pipe connected between the bottom of the mist eliminator 6 and the absorption tower.

The above structure is the same as that of a conventional wet flue gas desulfurization device. Although not shown, the circulation line, circulation tank, oxidation tower, shaker, centrifuge, etc. around the cooling/dust removal tower 1 and the absorption tower 4 are also the same as in the case of FIG. 1. The absorption liquid mist mixed in the exhaust gas exiting the absorption tower 4 is
The mist eliminator 6 installed in the downstream section captures the mist and returns it into the absorption tower 4 through the mist drain return pipe 30.

Fresh water for cleaning the elements of the mist eliminator 6 is sprayed from a spray nozzle toward the elements through the cleaning water supply pipe 28. In order to reduce the amount of washing water, a washing water supply on/off valve 31 is provided in the washing water supply pipe 28 so that the on/off valve 31 is opened intermittently, that is, only during washing. On the other hand, Mist Eliminator 6
In order to directly drain the cleaning wastewater that collects at the bottom of the tank without returning it to the absorption tower 4, a cleaning wastewater extraction pipe 32 is branched from the mist drain return pipe 30 going to the absorption tower, and a new cleaning drainage pipe is installed. The cleaning wastewater is introduced into a tank 33, and is discharged from the cleaning drainage tank 33 to the outside of the system by a cleaning drainage pump 34.

In order to minimize the amount of absorption liquid mist mixed into the wastewater discharged outside the system, a mist drain return pipe 30,
A mist drain switching valve for the equipment line in the desulfurization apparatus system, for example, a mist drain switching valve for the absorption tower line and a washing wastewater extraction switching valve 36, is provided for each of the cleaning wastewater extraction pipe 32, and while the elements of the mist eliminator 6 are being cleaned. In other words, while the wash water supply on/off valve 31 is open, the absorbing mist drain switching valve 35 is closed and the washing water discharge switching valve 36 is open, so that a small amount of absorption liquid mist captured by the element is contained. The cleaning waste water is led to the cleaning waste water tank 33. On the other hand, while the element is not being cleaned, the on/off valve 3 for supplying cleaning water
1 is closed, in order to return the absorption liquid mist captured by the element to the absorption tower 4, the absorption column mist drain switching valve 35 is opened and the washing wastewater extraction switching valve 36 is closed. The net time for cleaning the elements of the mist eliminator 6 may be as short as 1/20 to 1/100 of the total operating time, so most of the absorbent mist captured by the elements is returned to the mist drain. tube 30
The water returns to the absorption tower 4 through the water, and a small portion thereof is led to the washing waste water tank 33 together with the washing waste water and is discharged to the outside of the system. Note that all or part of this washing wastewater may be sent to the cooling/dust removal tower 1 in some cases. Washing water supply on/off valve 31, absorption column mist drain switching valve 35, washing wastewater extraction switching valve 36
Typically, a diaphragm valve operated by pneumatic pressure is used, and the supply of pressurized air is controlled by opening and closing a solenoid valve. As mentioned above, the element of the mist eliminator 6 provided at the downstream part of the absorption tower 4 is intermittently cleaned, and only the cleaning waste water containing a small amount of absorption liquid mist captured by the element during the cleaning time is transferred to the mist eliminator. 6 to the cleaning drainage tank 33 and then discharged to the outside of the system. Therefore, the amount of absorbent that mixes in this drainage and goes out of the system is very small. It is also possible to suppress the amount of magnesium present to be discharged to a very small amount.

On the other hand, if the cleaning water of the mist eliminator 6 is discharged to the outside of the system as it is without returning it to the process system such as the absorption tower 4, the amount of water consumed at locations other than the mist eliminator 6 will be reduced. Since the amount of water is small, it is almost balanced with water discharged from the system such as water adhered to gypsum, and the first
It is no longer necessary to discharge the absorption system waste water from the waste water extraction pipe 23 in the conventional device shown in the figure separately from the washing waste water of the mist eliminator 6 (the waste water from the washing waste water pump 34 in FIG. 4). Therefore, although the amount of drainage is the same as in the conventional method, the method and apparatus of the present invention can significantly reduce the amount of absorbed liquid discharged outside the system compared to the conventional method. At the same time, it becomes possible to significantly increase the magnesium concentration in the absorption liquid compared to conventional methods. Next, →l is 250MW
In the case of a limestone-gypsum method flue gas desulfurization device for coal-fired power plants, the magnesium concentration in the absorption liquid was calculated and compared between the conventional method and the method of the present invention.

However, in the conventional method, the mist eliminator for the absorber is cleaned intermittently, similar to the method of the present invention. The preconditions for calculation are as follows. Calcium carbonate usage
2.0 tons/H M in raw material calcium carbonate
Magnesium content as gO 0.3% by weight
Drainage volume (excluding cooling tower drainage)] 0 tons/H Mist Elimine for absorption tower - 9.5 tons/H Evening cleaning water volume (average) ゜Cleaning time 3 minutes Cleaning interval 2 hours Cleaning liquid mist amount in exhaust gas 20t/Nm3 Above As a result of calculation under these conditions, the magnesium concentration in the absorption liquid was 360 ppm for the conventional method and 7.0 ppm for the method of the present invention.
It became 600ppm.

As explained above, according to the present invention, the magnesium concentration in the absorption liquid can be dramatically increased without adding magnesium separately from the outside, and therefore, as mentioned above, equipment costs and operating costs can be reduced. It is possible to easily achieve a reduction in the desulfurization rate and an improvement in the desulfurization rate as shown in FIG. The present invention is applicable not only to the above-mentioned limestone-gypsum method flue gas desulfurization apparatus, but also to all desulfurization processes that use absorbed calcium raw materials. Therefore, the process may use lime instead of limestone as the raw material for the absorbent, and the by-products include not only gypsum but also calcium sulfite (CaSO3.l).
/2H20) can also be applied to a process in which it is discharged to the outside of the system. Furthermore, when sodium or ammonium is included in the absorption liquid instead of magnesium, the same effect as with magnesium can be obtained, and in addition to using magnesium contained as an impurity in calcium carbonate, it is possible to add magnesium or ammonium to calcium carbonate in advance. Sodium, ammonium, etc. may also be added. Fifth
The figure shows another embodiment of the exhaust gas desulfurization apparatus of the present invention.

In this embodiment, an absorption tower mist eliminator 6a is provided inside the absorption tower 4.
The bottom part a and the inside of the absorption tower 4 are connected by a mist drain return pipe 30 having a mist drain switching valve 35 for the absorption tower, and the bottom part of the mist eliminator 6a and the cleaning drainage tank 33 are connected.
and a washing waste water extraction pipe 3 having a washing waste water extraction switching valve 36.
2 is connected. Other structures and operations are the same as in the embodiment shown in FIG. Since the present invention is configured as described above, by using only magnesium contained as an impurity in a calcium-based alkaline raw material (desulfurization agent) such as calcium carbonate, magnesium in the absorption liquid can be absorbed without adding magnesium from the outside. The concentration can be maintained high, and the desulfurization performance (SO2 absorption performance) and alkali neutralization performance of the absorption liquid can be increased.

In addition, even when adding magnesium, sodium, ammonium, etc. externally for the purpose of improving the desulfurization performance of the absorption liquid, it is possible to maintain the concentration of magnesium, sodium, ammonium, etc. in the absorption liquid at a high level with a small amount of replenishment. I can do it. Therefore, the desulfurization performance and alkali neutralization/bending ability of the absorption liquid can be greatly improved without increasing the cost of raw materials and chemicals.In addition to improving the desulfurization rate,
Since the absorption tower liquid-gas ratio is smaller, the circulation pump capacity can be reduced, and the absorption tower gas residence time can be shortened, so the absorption tower can be made smaller and unreacted alkali is reduced. Therefore, the amount of sulfuric acid consumed for neutralization can be reduced, and other parts of the equipment such as the circulation tank can be downsized, and overall equipment costs and operating costs can be significantly reduced. There is.

[Brief explanation of drawings]

Figure 1 is an explanatory diagram of a conventional wet flue gas desulfurization equipment, Figure 2 is a graph showing the relationship between MgSO4 concentration in the absorption liquid and sulfite solubility, and Figure 3 is a graph showing the relationship between the Mg2+ concentration in the absorption liquid and the desulfurization rate. FIG. 4 is an explanatory diagram showing one embodiment of the exhaust gas desulfurization apparatus of the present invention, and FIG. 5 is an explanatory diagram showing another embodiment of the exhaust gas desulfurization apparatus of the present invention. 1... Cooling/dust removal tower, 3... Mist eliminator, 4... Absorption tower, 5...
Absorbent circulation pipe, 6.6a...Mist eliminator, 8...Circulation tank, 10...
Calcium carbonate slurry pipe, 12...oxidation tower,
13... Air blower, 14... Sulfuric acid supply pipe, 15... Overflow pipe, 16...
...Shitsukuna, 18...Centrifugal separator, 20.
... Gypsum cake removal pipe, 21 ... Finished liquid removal pipe, 23 ... Drainage removal pipe, 24 ...
・Calcium carbonate supply pipe, 25...Calcium carbonate slurry tank, 26...Drainage discharge pipe, 2
7... Fresh water supply pipe, 28... Washing water supply pipe, 30... Mist drain return pipe, 31...
...On-off valve for supplying washing water, 32 ... Washing water discharge pipe, 33 ... Washing water tank, 34
. . . Washing drainage pump, 35 . . . Absorption column mist drain switching valve, 36 . . . Washing drainage extraction switching valve.

Claims (1)

[Claims] 1 Cooling and dust removal for exhaust gas containing sulfur oxides
A dust removal tower, an absorption tower that absorbs and removes sulfur oxides in the cooled and dust-removed exhaust gas using an absorption liquid containing a calcium compound, and a mist eliminator that captures absorption liquid mist in the exhaust gas that has exited the absorption tower. In wet flue gas desulfurization equipment, a cleaning water supply pipe is connected to the top or side of the mist eliminator, and a mist drain return pipe is connected between the bottom of the mist eliminator and the equipment in the desulfurization equipment system. Connect the cleaning drainage extraction pipe to the bottom of the device to take out the cleaning drainage outside the equipment, install the cleaning water supply on/off valve on the cleaning water supply pipe, and install the cleaning water supply on/off valve on the mist drain return pipe while it is open. A mist drain switching valve for equipment in the desulfurization equipment system is installed, which is open while the on-off valve for supplying wash water is closed, and it is open while the on-off valve for supplying wash water is open, An exhaust gas desulfurization device characterized by being provided with a wash water discharge switching valve that is closed while a wash water supply on-off valve is closed.