JPS6116490B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a wet lime method flue gas desulfurization device for desulfurizing exhaust gas containing SO ₂ such as coal-fired and coal-fired boiler exhaust gas using limestone or a slurry containing limestone and dolomite or lime. It concerns a method of wastewater treatment.

The drawback of the wet lime method flue gas desulfurization equipment that is currently in practical use is that the waste water is discharged, and in conjunction with the purification of the flue gas, it is necessary to take measures to prevent secondary pollution caused by the waste water.

In particular, when exhaust gas such as coal-fired boiler exhaust gas, which contains a large amount of halogen and dust as impurities in addition to SO ₂ , is treated by the wet lime method, measures have been taken to suppress the accumulation of impurities.

FIG. 1 shows a generally known conventional wet lime flue gas desulfurization process. Exhaust gas 1 enters an absorption tower 2 and is discharged outside the system as purified gas 3. Limestone 4 is supernatant liquid 6 from thickner 5
The slurry is made into a slurry in a limestone slurry tank 7, and is supplied to the absorption tower 2 via a line 8 to purify the exhaust gas. The slurry that has absorbed SO ₂ is sent to the thickener 5 via line 9, where it is mixed with supernatant liquid 6 and thick slurry 1.
0 and the concentrated slurry 10 is discharged to the outside of the system.

This thick slurry contains approximately 60wt% water, which becomes wastewater and is discharged outside the system, resulting in SO ₂
At the same time, the accumulation of impurities such as halogen compounds and dust collected from exhaust gas was also suppressed.

However, since water containing impurities in the discarded concentrated slurry becomes wastewater, various measures are required to treat it.

Figure 2 also shows the wet lime method flue gas desulfurization process, which has already been industrialized.

The exhaust gas 11 enters the first absorption tower 12, where impurities such as dust and halogen and a part of SO ₂ are removed, and then enters the second absorption tower 14 through the line 13 and becomes purified gas 15.
is discharged from the system as Limestone 16 is slurried with clarified liquid 18 from solid-liquid separator 17 and limestone slurry tank 19, and is supplied to second absorption tower 14 through line 20 to purify exhaust gas. The slurry that has absorbed SO ₂ is sent to the first absorption tower 12 via line 21, and the first absorption tower removes SO ₂ , dust, halogen, etc. from the exhaust gas. The slurry containing SO ₂ , dust, and halogen is sent to the solid-liquid separator 17 through a line 22 and separated into a clarified liquid 18, waste water 23, and a cake 24 mainly composed of gypsum.

As shown in Figures 1 and 2, the conventional method produces wastewater containing impurities.

The inventor conducted various studies and found that if wastewater is suppressed, molten impurities such as halogen compounds and compounds that increase COD will accumulate, causing problems such as a decrease in desulfurization rate and impeding the reactivity of limestone. I found out.

Coal-fired boiler exhaust gas mainly contains HC and HF as halogen compounds, and these react with limestone to produce CaC ₂ and CaF _2. The present inventors found that CaC ₂ and CaF ₂ significantly improve desulfurization performance. It was confirmed through various tests that it impairs the reactivity of SO ₂ with limestone, which is an absorbent, and reduces the reaction rate of limestone.

In particular, _CaC2 dissolves well in water, so if it is not discharged from the system, it will accumulate within the system, so for stable operation of the wet lime method flue gas desulfurization equipment,
It turns out that drainage cannot be avoided.

Therefore, the main reason why we were able to suppress the malfunction of the equipment by discharging waste water using the conventional method is that CaC ₂ , which has a particularly high melting degree, tends to accumulate in the water within the system, and it is important to understand this cause. After researching various processes that do not produce wastewater, we discovered the method of the present invention.

The method of the present invention adopts a method in which halogen compounds are accumulated to an extreme extent without discharging wastewater, contrary to the conventional method in which malfunctions of the equipment can be suppressed by discharging wastewater. In other words, in the process of collecting halogen compounds, the process of selectively collecting halogen compounds and dust and not absorbing much SO ₂ and the process of selectively absorbing only SO ₂ are incorporated into the same process. A flow has been established in which halogen compounds are accumulated to an extreme extent, and halogen compounds are not incorporated into the SO ₂ absorption process.

By the method of the present invention, even if the waste water from the wet lime flue gas desulfurization equipment is eliminated, the disadvantages of the conventional method can be overcome and stable operation can be achieved.

That is, the present invention relates to an exhaust gas treatment device for treating exhaust gas containing _SO2 , which includes a first absorption tower and a second absorption tower, and guides the exhaust gas from the first absorption tower to the second absorption tower and uses limestone or limestone as an absorbent. In a device that supplies a slurry containing dolomite or lime to a second absorption tower and sends the slurry from the second absorption tower to the first absorption tower to treat exhaust gas, the slurry extracted from the second absorption tower is classified using a thickener. The underflow concentrated slurry is sent to the first absorption tower, and the overflow supernatant liquid is added with absorbent limestone or limestone and dolomite or lime to form a slurry containing limestone or limestone and dolomite or lime, The slurry is supplied to the second absorption tower again, and the slurry extracted from the first absorption tower is separated into solid and liquid, the solid phase is discharged outside the system, and the separated liquid is sent to the first absorption tower together with the underflow thick slurry of the thickener. This is a method for eliminating wastewater in an exhaust gas treatment device that is characterized by being a liquid.

The case where limestone is used as an absorbent will be described below, but the same applies to lime.

FIG. 3 shows the flow of an embodiment of the present invention.

Exhaust gas 2 containing SO ₂ , halogen compounds and dust
5 enters the first absorption tower 26, where impurities such as dust and halogen compounds, and a portion of SO ₂ are removed, and then enters the second absorption tower 28 through line 27, where most of the SO ₂ is removed. , and is discharged to the outside of the system as purified gas 29. A mixture 30 of limestone and dolomite is slurried with a supernatant liquid 32 from a thickener 31 in an absorbent adjustment tank 33, and is supplied to the second absorption tower 28 through a line 34. The slurry 35 extracted from the second absorption tower is supplied to the thickener 31 and separated into a supernatant liquid 32 and an underflow thick slurry 36. The concentrated slurry 36 is transferred to a solid-liquid separator 37
It is sent to the first absorption tower 26 along with the clarified liquid 38 from the line 39. Slurry containing dust, impurities such as halogen compounds, and a portion of SO ₂ from the exhaust gas in the first absorption tower 26 is extracted from the line 40, sent to the solid-liquid separator 37, and separated into a clear liquid 38 and a cake 41. be done.

By not discharging wastewater, soluble impurities such as halogen compounds and compounds that cause an increase in COD accumulate in the slurry in the first absorption tower 26, but almost none in the second absorption tower 28. Does not accumulate.

This is because halogen compounds and dust are more easily collected than SO ₂ and most of them are removed from the exhaust gas in the first absorption tower 26, while only SO ₂ is collected in the second absorption tower 28, and the solid-liquid separator This is to prevent the clarified liquid 38 containing many soluble impurities from returning to the second absorption tower 28 from the absorption tower 37 .

Therefore, in the second absorption tower 28, stable desulfurization performance can be obtained without being affected by impurities.

Further, even if halogen compounds accumulate in the first absorption tower 26 and the desulfurization rate decreases, there is no problem because the second absorption tower 28 has sufficient desulfurization performance.

In addition, even if the reactivity of limestone appears to be impaired due to the accumulation of halogen compounds, it is already detected in the second absorption tower 28.
Since more than 90% of the limestone has reacted, the amount of unreacted limestone in the cake 41 discharged to the outside of the system is not large enough to cause a large economic loss.

However, as shown in FIG. 3, if a mixture 30 of limestone and dolomite is supplied, the line 34→3
5 → 36, the Mg compound is transferred to the first absorption tower 2.
6, the halogen taken in from the exhaust gas reacts with the Mg compound in the first absorption tower 26 to form MgC ₂
The present inventors have found that the halogen-induced hindrance to the desulfurization rate and limestone reactivity is reduced or eliminated by the formation of halogen and MgF ₂ . It has also been found that the amount of dolomite supplied can be approximately equivalent to the amount of halogen flowing in from the exhaust gas.

Next, when high-temperature exhaust gas such as boiler exhaust gas is treated, water evaporates in the first absorption tower 26, so it is necessary to adjust the slurry concentration of the thick slurry 36, which is the underflow of the thickener 31, to maintain a water balance. The adjustment method is Thickner 31
A part of the supernatant liquid 32 may be mixed. (Not shown in Figure 3.) As described above, the wet lime method flue gas desulfurization method does not require waste water and does not impede the desulfurization rate or the reactivity of limestone even with the accumulation of impurities. The device became possible.

〔Example〕

A test was conducted using a 2000 Nm ³ /H scale pilot plant that processes heavy oil-fired boiler exhaust gas by the method of the present invention.

The flow sheet is shown in Figure 4.

SO ₂ concentration in exhaust gas 1800ppm, HCl concentration
70ppm, HF concentration 20ppm, exhaust gas 2000Nm ³ /H adjusted to 150℃ from No. 42.
Air was blown into the absorption tower 43. The exhaust gas comes into contact with slurries 45 and 46 sent from the pump 44, passes through the mist eliminator 47, and enters the second absorption tower 48. The flow rate of slurries 45 and 46 is 4 m ³ /H.

In the inlet gas of the second absorption tower 48,
SO ₂ was 1500 ppm, HCl and HF were 1 ppm or less, and most of the halogen compounds were collected in the first absorption tower 43.

The gas that has entered the second absorption tower 48 comes into contact with a slurry 50 of 24 m ³ /H sent from a pump 49, passes through a demister 51, and is discharged to the outside of the system as purified gas 52. SO ₂ contained in purified gas 52 is 80ppm
A desulfurization rate of over 95% was obtained.

Limestone and dolomite powders, which are absorbents, are mixed at a weight ratio of 20:1 and charged into an absorbent adjustment tank 54 through a line 53, and tap water 55 and overflow liquid 57 from a thickener 56 are used to create a 7wt% absorbent slurry. was adjusted, and further gypsum cake was added through line 58 so that the gypsum concentration was 1wt%.

A slurry containing limestone, dolomite, and gypsum was supplied to the second absorption tower 48 through a feed line 60 using a pump 59 so that the slurry pH was 5.7. After the slurry in the second absorption tower 48 that had absorbed SO ₂ was extracted by a pump 61, the slurry was sent to a liquid cyclone 64 via a line 62 and the remaining liquid was sent to a thickener 56 via a line 63.

The underflow of the hydrocyclone 64 contained a large amount of gypsum particles, and was returned to the second absorption tower 48 through a line 65 as a material for preventing scale in the second absorption tower 48, and the overflow 66 was supplied to the thickener 56.

The underflow from Thickner 56 had a slurry concentration of approximately 20 wt% and contained calcium sulfite, gypsum, and limestone in a molar ratio of 65:25:10.

The thick slurry was sent from the pump 67 to the tank 69 through the line 68.

In the tank 69, the clarified liquid 71 from the filter 70 and a portion 73 of the gypsum slurry extracted from the oxidation tower 72 are used to form a slurry of approximately 8 wt% and are supplied to the first absorption tower 43 from the line 75 by a pump 74. did. The slurry pH in the first absorption tower 43 was approximately 4.2.

The slurry in the first absorption tower 43 was extracted from the line 76 and sent to the oxidation tower 72. In the oxidation tower 72, air 77 was introduced from the bottom at a rate of about 20 Nm ³ /H, and the calcium sulfite in the slurry was oxidized to gypsum by forming fine bubbles using a rotary atomizer. The gas discharged from the top of the oxidation tower 72 was introduced into the first absorption tower 43 through a line 78.

The slurry extracted from the oxidation tower 72 is transferred to line 7.
3, the liquid was supplied to the tank 69, and the liquid was sent to the filter 70 through the line 79.

The slurry liquid volume ratio between line 73 and line 79 was approximately 1:2.

The gypsum cake taken out from the filter 70 is approximately
It contained 20wt% of attached water. plaster cake 80
and fly ash 81 collected from coal-fired boiler exhaust gas are put into a mixer 82 at a weight ratio of 3:1, and when mixed, water is adsorbed to the fly ash and becomes a solid 82 that is easy to handle, preventing attached water from flowing out. Truck transportation became possible.

On the other hand, in the slurry of the first absorption tower 43, C ^- accumulated as a soluble impurity to about 8%, but in the slurry of the second absorption tower 48, it was only about 1000 ppm, which caused a decrease in the desulfurization rate and the reactivity of limestone. We were able to demonstrate a wet flue gas desulfurization method that does not cause any problems and does not require drainage.

Next, other preferred embodiments of the present invention are summarized.

1 In the method described in the claims, after oxidizing the slurry extracted from the first absorption tower to form gypsum slurry, a part of the gypsum slurry is supplied to the first absorption tower, and the remainder is converted into solid-liquid. How to separate.

2 In the method described in the claims, a part of the slurry extracted from the second absorption tower is sent to a hydrocyclone, the underflow of the hydrocyclone is supplied to the second absorption tower as a seed crystal, and the overflow is supplied to the second absorption tower. A method in which the slurry is fed to the thickener along with the remainder of the slurry extracted from the tower.

3. A method of mixing fly ash into a solid-liquid separated cake according to the method described in the claims.

[Brief explanation of the drawing]

Figures 1 and 2 show the conventional mixed lime desulfurization method;
FIG. 4 is a flowchart of an embodiment of the present invention. 2, 12, 14, 26, 28, 43, and 48 are absorption towers.

Claims (1)

[Claims] 1 Regarding an exhaust gas treatment device that processes exhaust gas containing SO ₂ , a first absorption tower and a second absorption tower are provided, and the exhaust gas is guided from the first absorption tower to the second absorption tower using limestone or limestone and dolomite or lime as an absorbent. In a device that processes exhaust gas by supplying slurry containing the liquid to a second absorption tower and sending the slurry from the second absorption tower to the first absorption tower, the slurry extracted from the second absorption tower is classified by a thickener to remove the thick underflow. The slurry is sent to the first absorption tower, and the overflow supernatant liquid is added with absorbent limestone or limestone and dolomite or lime to form a slurry containing limestone or limestone and dolomite or lime, and then sent to the second absorption tower again. At the same time, the slurry extracted from the first absorption tower is separated into solid and liquid, the solid phase is discharged outside the system, and the separated liquid is sent to the first absorption tower together with the underflow thick slurry of the thickener. A method for eliminating wastewater in exhaust gas treatment equipment.