JPH08192191A - Method for treating unsteady-state drainage of thermal power station - Google Patents

Abstract

PURPOSE: To easily treat unsteady-state drainage of a thermal power station without causing a lowering of performance of catalyst or without producing clogging of pipes of a treating device by a method wherein unsteady-state drainage of the thermal power station is subjected to an aggregation and solid-liquid separation process, and then ammoniacal nitrogen is oxidized and decomposed in the presence of catalyst. CONSTITUTION: Unsteady-state drain of a thermal power station stored in a storage tank 1 is led to a reaction tank 2, where alkali is added to the drain while it is being agitated, to make pH, for example, 10 or more. The treated water, in which metal hydroxide has been deposited and suspended matters have been precipitated, is sent to a coagulating sedimentation tank 3, where flocculant is added thereto and the water is slowly agitated and settled to precipitate the aggregates. Supernatant liquid is sent to a filter 5 by a pump 4 and the filtered liquid is sent to an adjusting tank 6 to adjust pH, for example, to 6-8 and oxidizing agent is added thereto. The resultant treated water is sent to a catalyst packed tower 10 via a heat exchanger 8 and a heater 9 to oxidize ammonium ion.

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 unsteady wastewater of thermal power plants. More specifically, the present invention makes it possible to remove nitrogen in unsteady wastewater of a thermal power plant by a catalytic decomposition method without deteriorating the treatment performance of the catalyst and clogging of pipes, etc. It relates to the treatment method for unsteady wastewater in a plant.

[0002]

2. Description of the Related Art In a thermal power plant, thermal energy obtained by burning coal, heavy oil, etc. is converted into mechanical energy by a boiler and a steam turbine, and further converted into electric energy by a generator. A water tube boiler is widely used as a power generation boiler, and a large number of water tubes are provided in the peripheral wall of a furnace for burning fuel and in the passage of combustion gas. The boiler uses a part of the residual heat of the combustion gas leaving the boiler to heat the feed water, an air heater that preheats the air supplied to the furnace with the residual heat of the combustion gas, and sends air to the furnace to burn the combustion gas. It is equipped with a ventilation device for carrying to the chimney, and an electric dust collector for collecting fine particles in the exhausted combustion gas. At a thermal power plant, after continuing operation for a certain period, all or part of the equipment is systematically shut down to perform inspections, inspections, repairs, maintenance, etc., but at that time, the air heater and the electrostatic precipitator were washed. Sometimes drainage is discharged. This wastewater is unsteady wastewater discharged at the time of periodic inspection, and once it is generated, it is temporarily stored in the storage tank, and the wastewater in the storage tank is often treated thereafter. Ammonia nitrogen, carbon,
It contains a relatively high concentration of metal salts. Removal of nitrogen in unsteady wastewater is subject to large fluctuations in wastewater load and also contains many coexisting substances such as metals and suspended substances.
It is difficult to apply commonly used biological treatments, and a method using hypochlorous acid as an oxidant is adopted. However, the nitrogen removal method using hypochlorous acid as an oxidizing agent has the following problems. (1) The amount of hypochlorous acid added is required to be 9 to 10 times the theoretical amount required to convert ammoniacal nitrogen into nitrogen gas. That is, the amount of chemicals used is large, and a large amount of residual chlorine remains in the treated water, which requires further post-treatment. (2) Part of ammonium ions becomes nitrate nitrogen,
Remains in treated water. Therefore, the development of a new nitrogen removal method has been demanded, and a catalytic decomposition method of ammonia nitrogen has been attempted.However, in the case where a large amount of coexisting substances such as metals and suspended substances are present, catalyst performance deterioration and This is not practical because it tends to cause clogging of the piping of the processing equipment.

[0003]

SUMMARY OF THE INVENTION According to the present invention, unsteady wastewater of a thermal power plant containing ammoniacal nitrogen, metals, suspended substances, etc. can be generated without deterioration of catalyst performance or clogging of pipes of treatment equipment. The purpose is to provide an easy processing method.

[0004]

Means for Solving the Problems As a result of intensive studies to solve the above-mentioned problems, the present inventors have conducted coagulation-sedimentation treatment of unsteady wastewater of a thermal power plant and then used ammonia nitrogen in the presence of a catalyst. It is easy to process by oxidizing
Moreover, they have found that they can be carried out efficiently, and have completed the present invention based on this finding. That is, the present invention provides (1) a first step of coagulating and solid-liquid separating unsteady wastewater of a thermal power plant, and a second step of oxidatively decomposing ammoniacal nitrogen in the treated water of the first step in the presence of a catalyst. The present invention provides a method for treating unsteady wastewater of a thermal power plant, which comprises steps. Furthermore, as a preferred embodiment of the present invention, (2) in the first step, the pH of the unsteady wastewater of the thermal power plant is set to 10 or more, and then a coagulant is added to the unsteady water of the thermal power plant according to (1). Wastewater treatment method, and
(3) In the second step, there may be mentioned a method for treating unsteady wastewater of a thermal power plant according to the above (1) to (2), which uses nitrous acid or its derivative or hydrogen peroxide as an oxidizing agent. .

In the first step of the method of the present invention, solid-liquid separation is carried out after the unsteady wastewater of a thermal power plant is subjected to coagulation treatment. In coagulation treatment, the pH of unsteady wastewater from thermal power plants
Is adjusted to 10 or more, the mixture is vigorously stirred for 30 minutes or more, then the stirring is loosened, then the flocculant is added, the mixture is gently stirred for 2 to 3 minutes, and the mixture is allowed to stand to precipitate the flocculate. The pH can be adjusted, for example, by adding an alkali such as sodium hydroxide or potassium hydroxide in the form of an aqueous solution or a solid. By setting the pH of unsteady wastewater of a thermal power plant to 10 or more, metals such as iron, zinc, copper, nickel, manganese, and magnesium contained in the wastewater are precipitated as hydroxides. At this time, suspended substances that coexist in the wastewater also precipitate due to the coprecipitation phenomenon. Then, a coagulant is added to the wastewater in which the metal hydroxide is deposited as alkaline, and the metal hydroxide is coagulated to promote sedimentation. The aggregating agent used is not particularly limited, and inorganic aggregating agents such as polyaluminum chloride and ferric sulfate, sodium polyacrylate, anionic polymer aggregating agents such as polyacrylamide partial hydrolysates, polyaminoalkyl methacrylate, polyethylene Examples thereof include cationic polymer flocculants such as imines, and nonionic polymer flocculants such as polyacrylamide and polyethylene oxide. After the precipitate is aggregated and settled, the supernatant is left as it is, or if necessary, fine suspended substances which have not been settled and separated by filtration are removed. The filter to be used is not particularly limited, and for example, a filter filled with sand or anthracite, a cartridge filter, a membrane separation device or the like can be used. The supernatant or filtrate from which the precipitate has been separated and removed is then adjusted to pH 4 to 4 in a pH adjusting tank.
It is adjusted to 9, preferably 6 to 8. The acid used for pH adjustment is not particularly limited and, for example, sulfuric acid, hydrochloric acid, etc. can be used. In the second step of the method of the present invention, an oxidizing agent is added to the treated water to oxidize ammoniacal nitrogen. Ammoniacal nitrogen exists primarily in the form of ammonium ions in the treated water. The oxidation is preferably carried out by heating in the presence of a catalyst. The oxidizing agent used is not particularly limited, and nitrite, hydrogen peroxide and the like can be preferably used. If the oxidant used is nitrite,
The amount of nitrite used is preferably 0.8 to 1.0 times the amount of ammonium ions present in the treated water. When the amount of nitrite used is less than 0.8 times the molar amount of ammonium ions, the amount of ammonium ions remaining without being decomposed by nitrite increases. When the amount of nitrite used exceeds 1.0 mol times that of ammonium ions, excess nitrite remains in the waste water. When the oxidizing agent used is hydrogen peroxide, the amount of hydrogen peroxide used is preferably 1.0 to 3.0 times the molar amount of ammonium ions present in the treated water. When the amount of hydrogen peroxide used is less than 1.0 mol times that of ammonium ions, the ammonium ions may not be completely decomposed by hydrogen peroxide and may remain in the waste water. Even when the amount of hydrogen peroxide used exceeds 3.0 mol times that of ammonium ions, the removal efficiency of the remaining ammonium ions does not improve in proportion to the increase in the amount of hydrogen peroxide added. In the method of the present invention, it is also possible to use nitrite and hydrogen peroxide together as an oxidizing agent, first to decompose ammonium ions using nitrite, and then to add hydrogen peroxide to decompose the remaining ammonium ions. is there.

As the catalyst used in the second step, platinum, palladium, ruthenium, rhodium, indium, iridium, silver, gold, cobalt, nickel and tungsten, and water-insoluble or sparingly water-soluble metals of these metals are used as catalytically active components. Compounds such as cobalt monoxide, nickel monoxide, ruthenium dioxide, rhodium trioxide, palladium monoxide, iridium dioxide, tungsten dioxide and other oxides, as well as chlorides such as ruthenium dichloride and platinum dichloride, ruthenium sulfide. , One or more selected from the group consisting of sulfides such as rhodium sulfide, α-alumina, γ-alumina, activated carbon, titania, zirconia, zeolite, glass, silica, silica-alumina, ion exchange resin, etc. Supported on the carrier . The supported amount of the metal or its compound in the supported catalyst is usually 0.05 to 5% by weight of the carrier.
It is desirable that the amount is 25% by weight, preferably 0.5 to 3% by weight. Such supported catalysts are spherical, pelletized,
It can be used in various forms such as a columnar shape, a crushed piece shape, a honeycomb shape, and a powder shape. In particular, a catalyst in which platinum is supported on a granular carrier such as titania or γ-alumina is preferable. It is preferable that these catalysts are packed in a column and the reaction is carried out by passing ammonium ion-containing wastewater to which an oxidizing agent is added while heating, and in this case, an upward flowing liquid is desirable. In the second step, the decomposition treatment temperature is 70 to 300 ° C., preferably 80 to 250 ° C., more preferably 14
It is 0 to 180 ° C. Also, SV is 0.5 to 20 hr ^-1 ,
It is preferably 2 to 5 hr ^-1 . By this oxidation treatment, ammonium ions become nitrogen gas according to the following formula, for example. NH ₄ ⁺ + NO ₂ ⁻ → N ₂ + 2H ₂ O 2 NH ₄ ⁺ + 3H ₂ O ₂ → N ₂ + 2H ⁺ + 6H ₂ O The method of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic view of an example of an apparatus for carrying out the method of the present invention. First, the unsteady wastewater of the thermal power plant stored in the storage tank 1 is introduced into the reaction tank 2, and the pH is adjusted to 10 or more by adding alkali with stirring in this tank. The treated water in which the metal hydroxide is precipitated and the suspended substance is co-precipitated is then subjected to the coagulation sedimentation tank 3
Then, the flocculant is added to the mixture, the mixture is gently stirred, and then the mixture is allowed to stand to precipitate the flocculate. The supernatant liquid is sent to the filter 5 by the pump 4 and the fine suspended matter is removed by filtration. The filtrate that has passed through the filter is guided to the adjusting tank 6, where the pH is adjusted to 6-8.
And add an oxidant. After the pH adjustment, the treated water added with the oxidizing agent is then sent by the pump 7 to the catalyst packed column 10 via the heat exchanger 8 and the heater 9 to perform the ammonium ion oxidizing treatment. The treated water exiting the catalyst packed tower uses the residual heat in the heat exchanger and then the pressure regulating valve 1
It is sent to the gas-liquid separation column 12 via 1 and separated into nitrogen gas and treated water.

[0007]

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. Example 1 (First Step, Coagulation Treatment and Solid-Liquid Separation) As the unsteady wastewater of the thermal power plant, the coagulation sedimentation treatment of the air heater cleaning wastewater and the electrostatic precipitator cleaning wastewater was performed. Table 1 shows the results of water quality analysis of the cleaning wastewater before treatment. Sodium hydroxide was added to each wastewater to adjust the pH to 11.5. 3
Stir vigorously for 0 minutes. Thereafter, the stirring was loosened, 2 mg / liter of an anionic flocculant (polyacrylamide partial hydrolyzate) was added, and the mixture was further stirred for 3 minutes. After allowing to stand for 1 hour to allow the aggregate to settle, the supernatant was filtered with No. 5A filter paper. Water quality analysis was performed on the filtrate, and the results are shown in Table 1. In both the air heater cleaning wastewater and the electrostatic precipitator cleaning wastewater, all the metals in the treated water except calcium were removed to a concentration of 1 mg / liter or less.

[0008]

[Table 1]

Example 2 (Second Step, Oxidative Decomposition) Oxidative decomposition was carried out on the treated water of the air heater cleaning wastewater which had been subjected to the coagulating sedimentation treatment in Example 1. The treated water contained 700 mg / liter of ammoniacal nitrogen. Sulfuric acid was added to the treated water to adjust the pH to 6.4, sodium nitrite was added to adjust the nitrite nitrogen to 700 mg / liter, and titania spheres having a diameter of 1.5 mm and carrying 0.5% by weight of platinum were filled. SV = 3 hr ^-1 was passed through the catalyst packed tower, and 16
Oxidation treatment was performed at 0 ° C. The concentration of ammoniacal nitrogen in the treated water flowing out from the catalyst packed tower was 5 mg / liter or less, and the concentration of nitrite nitrogen was also 5 mg / liter or less. In addition, the quality of the treated water was stable even after the double-passage of 1,800 BV. Example 3 (Second Step, Oxidative Decomposition) The treated water of the electrostatic precipitator cleaning wastewater that had been subjected to the coagulating sedimentation treatment in Example 1 was subjected to oxidative decomposition. This treated water contained 1,800 mg / liter of ammoniacal nitrogen. Sulfuric acid was added to this treated water to adjust the pH to 6.4, sodium nitrite was added to adjust the nitrite nitrogen to 1,800 mg / liter, and a diameter of 1.5 mm carrying 0.5% by weight of platinum.
It was passed at SV = 3 hr ⁻¹ to a catalyst packed column packed with the titania spheres of No. ¹ and subjected to an oxidation treatment at 160 ° C. The concentration of ammoniacal nitrogen in the treated water flowing out from the catalyst packed tower was 5 mg / liter or less, and the concentration of nitrite nitrogen was also 5 mg / liter or less. Also, the quality of treated water is 1.8
It was stable after 00BV.

[0010]

According to the method of the present invention, by incorporating a coagulation-sedimentation treatment step into the nitrogen removal treatment of unsteady wastewater of a thermal power plant, wastewater containing many coexisting substances such as unsteady wastewater of a thermal power plant is incorporated. However, the catalyst performance is not deteriorated, and it is possible to remove nitrogen by the catalytic decomposition method, which has been considered difficult in the past.

[Brief description of drawings]

FIG. 1 is a schematic view of an example of an apparatus for carrying out the method of the present invention.

[Explanation of symbols]

 1 Storage Tank 2 Reaction Tank 3 Coagulation Sedimentation Tank 4 Pump 5 Filter 6 Adjustment Tank 7 Pump 8 Heat Exchanger 9 Heater 10 Catalyst Packing Tower 11 Pressure Control Valve 12 Gas-Liquid Separation Tower

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C02F 9/00 503 G 504 B E B01J 23/42 M C02F 1/52 ZAB K 1/72 ZAB B (72) Inventor Tomoyuki Asada, 3-3-22 Nakanoshima, Kita-ku, Osaka City, Kansai Denryoku Co., Ltd. (72) Inventor, Hiroshi Kimoto 3-3-22, Nakanoshima, Kita-ku, Osaka City, Kansai Denryoku Co., Ltd. ( 72) Inventor Toshiji Nakahara 3-4-7 Nishishinjuku, Shinjuku-ku, Tokyo Kurita Industry Co., Ltd. (72) Yuko Kitami 3-4-7 Nishishinjuku, Shinjuku-ku, Tokyo Kurita Industry Co., Ltd. (72 ) Inventor Yasuhiko Takabayashi 3-4-7 Nishishinjuku, Shinjuku-ku, Tokyo Kurita Industry Co., Ltd.

Claims (1)

[Claims] 1. A thermal power plant comprising a first step of coagulating and solid-liquid separating unsteady wastewater of a thermal power plant, and a second step of oxidatively decomposing ammonia nitrogen in the treated water of the first step in the presence of a catalyst. How to treat unsteady wastewater from power plants.