JPH0623347A - Treatment of desulfurization waste water - Google Patents

Abstract

PURPOSE:To convert desulfurization waste water to a directly disposable substance by a method wherein discharged desulfurization waste water is conc. and the residual conc. solution is dried using high temp. exhaust gas and the obtained dried powdery matter is blown into a flue to be mixed with and diluted with fly ash. CONSTITUTION:The pH of the desulfurization waste water 18 from wet desulfurization equipment is preliminarily adjusted to 5-6 by slaked lime and a gypsum seed is added to the waste water 18 and this waste water is introduced into a falling film type concentrator 5 and heated by steam to be circulated and conc. The obtained conc. slurry is sent to a rotary spray type dryer 10 by a pump 9. High temp. exhaust gas 15 is taken out of the flue between an economizer and an air heater 3 through a taking-out port 2 and introduced into the dryer 10 to perform the spray drying of the slurry. The dried powdery matter thus produced is continuously diffused into the flue through a rotary valve 12 and diluted with fly ash 17 to be taken out of the system along with the fly ash 17.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating waste water discharged from a wet flue gas desulfurization apparatus and related equipment. More specifically, the present invention not only desulfurizes wastewater, but also recovers water.

[0002]

2. Description of the Related Art Exhaust smoke produced when burning a fuel contains various harmful substances, especially sulfur compounds, and it is not allowed to release it as it is from a chimney to the atmosphere. Therefore, the flue gas is discharged into the atmosphere after being subjected to desulfurization treatment, etc., and the desulfurization wastewater generated at that time further reduces or removes harmful substances if necessary to meet the regulated values by national and local agreements and then public water bodies. Had been discharged to.

In addition, as the requirement for the regulation value becomes stricter in recent years, it is more strongly demanded to prevent the water pollution by avoiding the release to the public water area as much as possible.
Therefore, a method of concentrating the wet desulfurization effluent to some extent and then drying the concentrate to form a powdery material, and disposing of the powdery material by landfill or the like is also being sought.

[0004]

[Problems to be Solved by the Invention] With the recent increase in demand for electric power, the unit capacity of power generation has been gradually increased from the conventional 200 MW to 500 MW, 700 MW, and 1000 MW. The amount of water used per unit, the amount of wastewater, the amount of solid waste, etc. are expanding quantitatively, and the countermeasures are qualitatively different from those of small scale.

For example, in terms of water, the boiler wet flue gas desulfurization equipment is an equipment that requires a very large amount of make-up water, and the moisture for making up the evaporated water is 4/4 ECR 200MW power plant flue gas. What was about 21 tons per hour was 10
At 00 MW, it is about 105 tons per hour. Besides, the drainage is 200MW
It increases from about 5 tons per hour to about 25 tons per hour at 1000 MW. Therefore, the amount of make-up water for the desulfurizer reaches about 130 tons per hour, and the scale of one power plant is usually 2000 to 3000 MW. Even if they choose well, they can no longer rely solely on the supply from local governments. Inevitably, there is an urgent need to rationalize and reduce the amount of water used in power plants.

[0006] A further problem with the expansion of the scale of power generation is that the amount of desulfurization wastewater generated after desulfurization by introducing boiler flue gas into a wet desulfurization unit and contacting it with a calcium-based absorbent increases with the scale of power generation. By continuing,
There is an urgent need to reexamine the treatment method for desulfurization wastewater. In addition, there are many kinds of harmful substances contained in desulfurization wastewater,
For example, heavy metals such as cadmium and arsenic, fluorine compounds,
Dithionic acid, nitrogen-containing sulfur compounds, and soluble nitrogen (NO
₃ ^-, NH ₄ ^-) are also present. Soluble salts derived from the wet desulfurization process, such as MgSO ₄ , CaSO ₄ .2H ₂ O and CaCl _2, are 3.0
It is contained at a concentration of up to 8.0% by weight. Therefore, complicated processes and equipment are required to treat desulfurized effluent of these complicated compositions to efficiently remove regulated substances and satisfy the regulated values. Costs and even equipment maintenance costs continue to increase. In addition, if there are regulated values for nitrogen, phosphorus, etc. according to regional agreements, additional processes will be required. In addition, since each process facility for wastewater treatment occupies a fairly large area, it was uneconomical from the viewpoint of effective use of land.

[0007]

In order to solve various problems relating to wastewater treatment as described above, the present invention first treats the wastewater by about 25 to 40 which does not hinder the reheating of the treated flue gas in terms of heat balance. Concentrate to about wt% to recover water for reuse, dry the remaining concentrated liquid using high temperature flue gas from the boiler flue to form a powder, and mix this powder in the same boiler flue gas. We paid attention to the fact that if diluted, the concentration of harmful substances falls below the environmental standard and that they can be dumped together with fly ash by landfilling or other methods.

That is, the present invention is the first step of concentrating desulfurization wastewater discharged from a wet flue gas desulfurization apparatus and related equipment after adjusting the pH, if necessary, and drying the remaining concentrated liquid using high temperature flue gas from a boiler flue. The present invention provides a method for converting desulfurization wastewater into a substance that can be directly discarded, which comprises a second step and a third step of blowing the obtained powder into the flue and mixing and diluting it into fly ash.

In the method of the present invention, the water recovered in the concentration step is efficient because it can be reused in the desulfurization step or the like which requires a large amount of make-up water, the boiler flue gas can be effectively used, and the produced powdery form can be obtained. The product can be easily mixed with fly ash and diluted to reduce the concentration of harmful substances below the standard value, which is advantageous in that it can be discarded as it is. Another advantage of the method of the present invention is that it is sufficient to add simple additional equipment to the current boiler-related equipment including flue gas desulfurization equipment, so that it is much simpler and more economical than the conventional treatment method in terms of space and operation. Is.

At present, about 70% of fly ash is dumped at seaside, and about 30% is used for fly ash cement, cement admixture and other industrial applications. The former landfill is regulated by the Marine Pollution Control Law, but at present, the regulation value is satisfied by discarding the surface water with no treatment other than pH adjustment when the surface water flows out. The finally obtained fly ash containing the powdered material has passed the landfill standard values of the sea defense method and the industrial waste method, and therefore can be directly disposed of in landfills.

Further, regarding the items required by the recent regional agreements, the effluent control method can also be satisfied by subjecting the effluent from the ash separation type to F ^- removal treatment as described later. The present invention will be described in more detail below. The wastewater treated in the first step of the present invention is wastewater from the desulfurization apparatus and related equipment, and the desulfurization apparatus may be of ash mixing type or ash separation type. However, in the case of the ash separation method, the concentration of fluorine ions in the desulfurization effluent tends to be high.If the concentration exceeds 500 ppmw, the effluent is neutralized in advance with slaked lime, etc., and most of the fluorine ions are precipitated as CaF _2. The gypsum sludge produced is mixed with the gypsum slurry in the desulfurizer as a precipitate together with the resulting gypsum sludge, and then adjusted to pH 4 to 7 and subjected to a concentration step.

Wastewater can be concentrated in various ways. For example, a self-vapor compression heating type falling film double-effect can is advantageous because it has a small boiling point increase due to the liquid depth pressure and can be concentrated in a self-vapor compression energy saving mode. The desulfurization effluent to be treated contains solid fine particles from the beginning, and the soluble salts are supersaturated with the concentration of the liquid to cause precipitation and scaling, which tends to lower the overall heat transfer coefficient. It is preferable to add about 2 to 3% by weight of seed crystals to the slurry so that the crystals are deposited on the surface of the seed crystals and avoid scaling to heat transfer equipment.

Concentration is based on the total dissolved solid content in the wastewater (hereinafter TDS).
Abbreviated as "), preferably about 20 to 50% by weight, preferably about 25 to 40% by weight of an economical concentration without disturbing the heat balance for reheating the treated flue gas.
The concentrated liquid thus obtained has a slurry form and is sent to the subsequent drying step. The drying step, which is the second step, is usually performed by a spray drying method because of the advantage that a powdery substance can be obtained, and a rotary disk type device is usually used in order to cope with the change in the flow rate of the liquid to be dried. In the case of desulfurization drainage of a 700MW base power plant, a dryer with an inner diameter of about 5m and a height of about 5 to 6m can be used to perform drying sufficiently conveniently in terms of function.

The hot flue gas used for drying is flue gas having a temperature of about 400 to 320 ° C. from a boiler flue, preferably the flue between the economizer and the air heater. This flue gas can be used as it is, or can be simply dust-removed using a cyclone. It is preferable to install a simplified cyclone on the outlet side of the dryer to roughly remove dust in the smoke discharged from the dryer to prevent corrosion of the induction fan downstream. The flue gas temperature at the outlet of the dryer is 180 to 120 ° C, preferably 15
The powder is most efficiently formed when adjusted to 0 to 130 ° C and dried. The particle size of the resulting powder is about 30-80 μm.

The dry powder thus produced, the powder collected by the cyclone and the smoke from the dryer are discharged in the third step at some stage in the flue, for example, between the air heater and the low temperature electrostatic precipitator. And continuously introduce it into the flue and dissipate it. Since the powdery matter has a high fluidity, it is uniformly mixed with a large amount of flue gas in the flue and collected together with fly ash by the low temperature electrostatic precipitator in the next step. In the case of a 700MW base coal-fired boiler, the desulfurization effluent is about 17m ³ (about 20 tons) per hour, and when this is concentrated and dried, about 1.3 to 1.5 tons of powder is produced per hour.

The amount of fly ash varies depending on the amount of ash in the coal used, and the relation between the amount of ash in the coal and the amount of fly ash generated is approximately as follows. Ash content in coal (% by weight) Amount of fly ash Dry powder in fly ash (%) 10 About 24.0 tons / hour About 6.0 12.5 About 30.0 〃 About 4.8 15 About 37.0 〃 About 3.9 18 About 44.0 〃 About 3.3 20 About 50.0 〃 2.9 About 10%, which is an example of low ash content in fuel coal, the ratio of dry powder to fly ash is about 6% by weight. In practice, the ash content in the fuel coal is often 10% or more, so that the fly ash amount is also large, and therefore, the amount of the particulate matter mixed in the fly ash is often lower than 6% by weight.

The above is an example in which the drying device is installed separately from the flue gas flue to separate the moisture content, thereby preventing corrosion in the flue, generation of adhered matter, and formation of dumplings. If the size of the flue is larger than 4 m in height, it may be possible to spray the concentrated liquid directly into the flue gas to dry it. The present invention will be described in more detail with reference to the following examples and the accompanying drawings.

[0018]

1 is a flow diagram showing the outline of the method of the present invention, and FIG. 2 is a view showing an example of a falling film type concentrator equipped with a special distributor having slits. The concentrator shown in Fig. 2 is currently patent pending (Japanese Patent Application No. 4-17
No. 3227), which is a self-vapor compression heating type falling film type concentrator and is a double effect can. This concentrator introduces steam from the outside to the jacket side only at the start, and heats it, but after normalizing the operation, it is sufficient to supply heat only by the steam compressed by the vapor compressor, so there is no need to supply steam from the outside.

Wastewater from wet desulfurization equipment 18 (solids concentration 500 p
pmw or less) was adjusted to pH 5 to 6 with slaked lime in advance, and about 2% by weight of gypsum seed crystal (average particle size 40 μm) was added.
Then, it was introduced into a falling film type concentrator 5 (inner tube of heat exchange pipe 50 mm, length 1.5 m, inner tube material titanium) in FIG. 2 at a flow rate of 1000 L to 2000 L / hour per 1 m of the inner tube peripheral length. The jacket side 34 was heated with steam, and the pressure difference between the inner tube 33 and the jacket was controlled to 0.15 to 0.2 kg / cm ² for circulation and concentration.

During the concentration, the feed liquid and the vaporized water vapor flowed down in parallel, and were separated into gas and liquid in the lower sump. After removing the mist, the water vapor was compressed by the vapor compressor 6 (the temperature difference between the inside and outside of the heat transfer pipe was about 5 ° C.) and sent to the condenser jacket. The liquid that flowed down was sent to the liquid distribution part at the top of the concentrator by the circulation pump 7 together with the new liquid supply, and was circulated and concentrated. Surface area of seed crystals in concentrate
Concentration operation was performed by adjusting the liquid depth of the feed liquid so that no scale is generated on the inner wall of the heat transfer tube in response to the heated concentration of the liquid while the (precipitated salt deposition surface) is passing through the heat transfer tube. Concentration is carried out in a continuous operation, and a part of the concentrated liquid is sent from the discharge side of the circulation pump 7 to the concentrated liquid storage tank 8, but a part of the liquid is separated into seed crystals by a hydroclone to make the seed crystal concentration in the circulating liquid constant. I kept it. The boiling point of the concentrate is 3-5 ° C, and the overall heat transfer coefficient is about 2400 kcal / m ² hr.
It was ℃.

The water content concentrated to about 35% by weight and recovered was about 5 L / h. The obtained concentrated slurry was sent to the dryer 10 by the pump 9. Next, from the flue between the economizer and the air heater 3, high temperature flue gas is exhausted through the outlet 2.
Take out 15 and use rotary disc spray dryer 10
The slurry obtained in the previous step was spray-dried by swirling flow. The drying conditions were as follows.

Concentrate temperature 100 ° C. Concentrate concentration 35 wt% Disk atomizer diameter 5.08 cm (2 inches) High temperature flue gas inlet temperature 350 ° C. Flue gas outlet temperature 150 ° C. Dry residence time 2 seconds Dry product is a completely dry powder It is a substance, particle size 30 to 80
It was μm.

The bottom plate of the dryer and the air slide plate 14 are set as an inclined bottom plate, and a part of the high-temperature smoke is blown from the lower part of the air slide plate. Is moved to the rotary valve 12, through which the flue between the air heater 3 and the low-temperature electrostatic precipitator 4 is continuously diffused and diluted, and the fly ash 17 and the outside of the system are used. I took it out. The dryer exhaust was exhausted into the flue by the attracting fan 11 via the cyclone 13, and the powdery matter collected by the cyclone 13 was also released into the flue from the rotary valve 12.

The powder obtained in this example was added to fly ash obtained from a power plant at 6, 8 and 10% by weight, respectively.
A sample added at the concentration of was prepared and an elution test at the time of disposal was performed. The results are shown in Table 1 below. At the same time, Table 1 shows the emission standards (Water Control Law), landfill (Industrial Waste Law) and marine dumping (Marine Protection Law) regulation values related to the elution test of coal ash.

[0025] As can be seen from Table 1, it was found that in all cases, the national landfill standard values of the Industrial Waste Law and the Marine Defense Law were satisfied, and after adjusting the pH, the living environment items under the flood control law required by the regional agreement values were also satisfied. In practice, the ash content in fuel coal is often 10% or more, so the fly ash amount is also larger, and therefore the amount of powdered substances mixed in fly ash is often below 6% by weight. The fly ash containing particulate matter can be sufficiently discarded by landfilling.

[0026]

EFFECTS OF THE INVENTION According to the present invention, desulfurization effluent can be efficiently treated and dumped together with fly ash, and moisture can be recovered. Therefore, it is efficient and economical in terms of treatment equipment, operational aspect, occupied area, etc. An environmentally and environmentally advantageous method is provided.

[Brief description of drawings]

FIG. 1 is a flow diagram showing an example of the method of the present invention in which waste water is concentrated, dried, and diluted in fly ash.

FIG. 2 is a diagram showing an example of a falling film type concentrator used in the concentration step of the present invention.

[Explanation of symbols]

1 Denitration device 2 High temperature flue gas outlet 3 Air heater 4 Electrostatic precipitator 5 Falling film type concentrator 6 Vapor compressor 7 Concentrated liquid circulation pump 8 Concentrated liquid tank 9 Concentrated liquid supply pump 10 Rotary disk spray dryer 11 Induction fan 12 Rotary valve 13 Cyclone 14 Air slide plate 15 High temperature boiler flue gas 16 Untreated flue gas 17 Fly ash 18 Desulfurization drainage 19 Concentrated slurry 20 Evaporative vapor 21 Condensed water 22 Condensed water tank 23 Heat exchanger 24 Water storage tank 25 Degassing tank 31 Distributor 32 Top liquid tank 33 Inner tube 34 Jacket 35 Circulating liquid tank 36 Circulating liquid 37 Magnetic flow rate indicating controller 38 Steam pressure indicating controller 39 Slit 40 Steam 41 Peephole 42 Steam trap

Claims (1)

[Claims] 1. A first step of concentrating desulfurization effluent discharged from a wet flue gas desulfurization apparatus and related equipment, a second step of drying the remaining concentrated liquid using hot flue gas from a boiler flue, and A third step of blowing the dry powdery substance into the flue and mixing and diluting it into fly ash.