JP2018094529A - Mercury removal method in waste incineration exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a mercury removal method in waste incineration exhaust gas capable of removing mercury in waste incineration exhaust gas, efficiently in a smoke washing tower.SOLUTION: Ozone is supplied in a gaseous state to a humidifying cooling part 23 where an exhaust gas temperature is lowered to 100°C or lower, in a smoke washing tower 21 for washing waste incineration exhaust gas, and is brought into contact with exhaust gas, and thereby metal mercury in exhaust gas is converted into divalent mercury, dissolved into smoke washing water, and removed. Since contact between ozone and exhaust gas is carried out in a low temperature area, thermal decomposition of ozone is suppressed.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a method for effectively removing mercury in waste incineration exhaust gas using ozone.

Since a trace amount of mercury may be contained in the exhaust gas from the waste incinerator, it must be removed in the exhaust gas treatment process. Mercury in the incineration exhaust gas is divalent mercury such as HgO and HgCl ₂ that is soluble in water and metallic mercury that is insoluble in water. Among these, divalent mercury that is soluble in water can be removed by bringing the exhaust gas into contact with the smoke-washed water in the smoke-washing treatment tower. However, it is difficult to remove metallic mercury that is insoluble in water using a smoke treatment tower.

  Therefore, as disclosed in Patent Document 1, a method has been proposed in which metallic mercury is converted into divalent mercury using a catalyst and then removed by contact with an absorbing solution. However, since this method requires a new catalytic reaction facility to be added to the exhaust gas treatment facility, there is a problem that the exhaust gas treatment facility becomes complicated and the equipment cost increases.

  Therefore, as shown in Patent Document 2, a method of converting metal mercury into divalent mercury using ozone in a flue gas treatment tower and removing it with smoke washing water has also been proposed. As schematically shown in FIG. 4, the method of Patent Document 2 is configured in a circulation pipe 3 that draws smoke-washed water from the lower part of the smoke-washing treatment tower 1 with a circulation pump 2 and circulates it in the upper part of the smoke-washing treatment tower 1. Ozone is blown into this, and this ozone water is sprayed from the nozzle 4 at the top of the smoke-washing tower and brought into contact with the exhaust gas to form divalent mercury. This method has the advantage that it is not necessary to add a catalytic reaction facility as in Patent Document 1, and only an ozone generator may be attached to the smoke-washing treatment tower.

  However, in general, the exhaust gas temperature at the entrance of the smoke cleaning tower is 200 to 300 ° C, so even if ozone water is sprayed into the exhaust gas, the ozone is immediately decomposed by heat, and metal mercury in the exhaust gas is efficiently removed. It cannot be oxidized to divalent mercury. In addition, sewage treatment water is used as sewage water in the sewage treatment plant, but since this treatment water contains about 15 mg / L of BOD, ozone reacts with BOD in the shamwash water and is wasted. It will be consumed. Furthermore, the solubility of ozone in water is low. For this reason, in order to fully oxidize the metal mercury in the exhaust gas, a large amount of ozone and water must be added, and there is a problem that the running cost becomes very high. In addition, since ozone water promotes oxidative corrosion of the circulation pipe, there is a problem in terms of equipment life.

JP 2004-313833 A WO2005 / 005025 Publication

  Accordingly, an object of the present invention is to solve the above-mentioned conventional problems and to provide a mercury removal method in waste incineration exhaust gas that can efficiently remove mercury in waste incineration exhaust gas in a smoke-washing tower. It is.

  The mercury removal method for waste incineration exhaust gas according to the present invention, which has been made to solve the above-mentioned problems, is a method for removing smoke from a smoke cleaning tower for cleaning waste incineration exhaust gas by reducing the exhaust gas temperature to 100 ° C or lower. Ozone is supplied in a gaseous state to the humidified cooling section and brought into contact with the exhaust gas, and the metal mercury in the exhaust gas is converted to divalent mercury and dissolved in smoke-washed water to be removed.

  The smoke cleaning tower includes a lower gas humidification cooling section and an upper exhaust gas cleaning section, or a gas dehumidification cooling section, and ozone is in a gas state above the gas humidification cooling section. It is preferable to supply by. Further, it is preferable that the smoke-washing water at the bottom of the smoke-washing tower is pumped up by a circulation pump and circulated in the lower gas humidification cooling unit. Furthermore, it is preferable to measure the mercury concentration in the exhaust gas treated in the smoke washing tower and to automatically control the supply amount of ozone.

  According to the method for removing mercury from the waste incineration exhaust gas of the present invention, metallic mercury can be oxidized into divalent mercury inside the smoke washing tower and dissolved in smoke washing water for removal. For this reason, it is only necessary to incorporate ozone supply means in the smoke-washing tower, and there is no need to add a catalyst reaction device or the like.

  Further, according to the method for removing mercury from the waste incineration exhaust gas according to the present invention, ozone is supplied to the humidification cooling section of the gas where the exhaust gas temperature of the smoke treatment tower is lowered to 100 ° C. or less to contact the exhaust gas. As described above, since the exhaust gas temperature in contact with the ozone gas is lowered, the thermal decomposition of ozone is suppressed, and the ozone supply amount can be reduced as compared with the method of Patent Document 2.

  Further, according to the method for removing mercury from the waste incineration exhaust gas of the present invention, ozone is supplied in a gas state. Therefore, compared with the case of supplying ozone into water, the contact time between ozone and gas in the smoke treatment tower is reduced. Can be long. Therefore, in the method of Patent Document 2, the amount of ozone that decomposes in water is large, and unless a large amount of ozone is supplied, the metal mercury in the exhaust gas cannot be oxidized. Metallic mercury can be oxidized. For this reason, it becomes possible to reduce running cost.

It is a whole block diagram of an incinerator and its waste gas treatment equipment. It is typical sectional drawing of the smoke-washing processing tower of embodiment. It is typical sectional drawing of the smoke-washing processing tower of other embodiment. It is explanatory drawing of the mercury removal technique in the conventional smoke-washing processing tower.

(overall structure)
A preferred embodiment of the present invention will be described below. First, an overall configuration of an incinerator and its exhaust gas treatment facility will be described with reference to FIG.

  FIG. 1 is an overall configuration diagram of an embodiment, and 10 is an incinerator for incinerating waste. The incinerator 10 includes a fluidized air supply means 11 in the lower part, and is a fluidized bed furnace that incinerates sludge while flowing a fluid medium. The dewatered sludge introduced into the furnace by the sludge pump 12 is incinerated at a high temperature of 850 ° C. or higher, and the generated waste incineration exhaust gas (exhaust gas) enters the fluidized air preheater 13. In the present invention, the type of the incinerator 10 is not particularly limited, and an incinerator such as a stalker furnace may be used. As described above, this exhaust gas contains a trace amount of mercury derived from waste as metallic mercury and divalent mercury.

  The fluidized air preheater 13 preheats the air blown by the fluidized air blower 14 by heat exchange with high-temperature exhaust gas, and supplies the fluidized air supply means 11 with it. The exhaust gas that has passed through the fluidized air preheater 13 then enters the white smoke preventer 15 and heats the air supplied from the white smoke preventive fan 16. The heated air is sent to the chimney 17 as white smoke prevention air, and the exhaust gas discharged from the chimney 17 into the atmosphere is heated to prevent white smoke. Both the fluid air preheater 13 and the white smoke preventer 15 are heat exchangers that exchange heat between exhaust gas and air.

  The exhaust gas that has passed through the white smoke preventer 15 is cooled in the cooling tower 18. The cooling tower 18 includes a watering nozzle 19 for spraying cooling water, and cools the exhaust gas to 200 to 300 ° C. Since all cooling water evaporates, there is no drainage. The cooling tower 18 can be omitted. The cooled exhaust gas is sent to the dust collector 20, where dust is removed.

  In this embodiment, the dust collector 20 has a structure using a ceramic filter having excellent heat resistance, and since the exhaust gas temperature hardly decreases in the dust collector 20, the exhaust gas temperature passing through the dust collector 20 is 200 to 300 ° C. The exhaust gas is then sent to the smoke cleaning tower 21 for processing, sucked by the suction fan 22 and released from the chimney 17 into the atmosphere. In the present invention, mercury is removed in the smoke cleaning tower 21. Hereinafter, the structure of the smoke cleaning tower 21 and the mercury removal method will be described in detail.

(Smoke washing tower)
FIG. 2 is a schematic cross-sectional view of the smoke cleaning tower 21 used in the present invention. The tower has a two-stage structure including a lower gas humidification cooling unit 23 and an upper gas dehumidification cooling unit 24. The lower gas humidification cooling unit 23 is an area for pumping water from the bottom smoke washing water tank 25 with a circulation pump 26, sprinkling water from a water spray nozzle 27, and cooling exhaust gas sent from an exhaust gas supply port 28 at the lower part of the tower body. The exhaust gas temperature fed from the exhaust gas supply port 28 is 200 to 300 ° C., but is cooled while rising inside the tower body, and is cooled to 60 to 100 ° C. above the gas humidification cooling unit 23. A temperature sensor 29 is installed at this position, and the amount of circulating water in the gas humidification cooling unit 23 is controlled so that the exhaust gas temperature is kept at 100 ° C. or lower. Note that the temperature sensor 29 may be omitted, the gas temperature and the gas amount on the inlet side may be obtained by calculation, and the circulating water amount may be controlled based on these values.

The upper gas dehumidifying / cooling section 24 includes a plurality of trays 30 and supplies makeup water from nozzles 31 and 32. Among these, the replenishing water is always supplied from the replenishing nozzle 31. On the other hand, when the tray 30 and the demister 34 are washed from the supply nozzle 32, supply water is supplied as necessary. Exhaust gas is washed in contact with the makeup water while rising between these trays 30, containing dust, such as NO _X and SO _X is removed. An alkali supply means 33 is provided above the gas dehumidifying / cooling unit 24 and sprays NaOH. As a result, the pH of the water in the tower is kept alkaline and the gas cleaning efficiency is increased. The exhaust gas that has passed through the gas dehumidifying cooling section 24 passes through the demister 34 at the upper end of the tower body, and is sucked into the chimney 17 at 40 to 60 ° C.

(Mercury removal method)
As shown in FIG. 2, the ozone supply means 35 is installed in the upper position of this gas humidification cooling part 23, and ozone is supplied in a gas state. As described above, the temperature of the exhaust gas fed from the exhaust gas supply port 28 is 200 to 300 ° C., but is cooled to 100 ° C. or less while ascending the gas humidification cooling unit 23. In the present invention, ozone is supplied in a gaseous state at this position. For this reason, the thermal decomposition of ozone is suppressed, and ozone stays in the tower for a long time, so the metallic mercury in the exhaust gas is oxidized to divalent mercury and dissolved in the circulating water together with the divalent mercury in the exhaust gas. Can be removed. Mercury in the circulating water is treated at the subsequent water treatment facility.

  Moreover, since ozone is supplied in a gas state, it can be mixed and contacted with exhaust gas for a long time. Moreover, since the tray 30 is provided at the ceiling of the ozone supply means 35, the generated divalent mercury can be quickly dissolved in water. Furthermore, gaseous ozone does not react with BOD in water, so it is not wasted.

  For this reason, according to this invention, compared with the method of supplying ozone in circulating water to water conventionally, the amount of ozone used can be reduced significantly. According to the model experiment conducted by the present inventor, the ozone supply amount necessary for obtaining the same mercury removal effect could be reduced to about 30% of the conventional amount. Moreover, the piping of the circulating water is not corroded by ozone.

  As shown in FIG. 1, the mercury concentration in the exhaust gas is continuously analyzed at the outlet of the chimney 17 by the mercury continuous analyzer 36, and the ozone supply amount from the ozone generator 37 is controlled according to the mercury concentration. Therefore, it becomes possible to efficiently remove metallic mercury in the exhaust gas with a small amount of ozone. Since the ozone generator 37 consumes a large amount of electric power, the reduction of the amount of ozone used greatly contributes to the reduction of the running cost.

(Other embodiments)
The embodiment described above is assumed to be used in a sewage treatment plant where water is abundant, and the water supplied to the upper gas dehumidification cooling unit 24 is supplied to the lower gas dehumidification cooling unit 23, Scrubber drainage. However, unlike a sewage treatment plant, water is not abundant at a garbage disposal plant, so it is necessary to reduce the amount of water used as much as possible.

  FIG. 3 is a schematic cross-sectional view of the smoke cleaning tower 21 intended for use in a treatment plant that is not rich in water. In this embodiment, the inside of the smoke cleaning tower 21 is divided into an upper gas dehumidifying cooling unit 24 and a lower gas humidifying cooling unit 23 by a partition wall 40 through which gas can pass. Ozone is supplied in a gas state from ozone supply means 35 at an upper position of the gas humidification cooling unit 23. In the lower gas humidification cooling unit 23, water in the bottom smoke washing water tank 25 is pumped by the circulation pump 26, sprayed from the water spray nozzle 27, and exhaust gas sent from the exhaust gas supply port 28 in the lower part of the tower body is cooled. A part of the circulating water is taken out to the smoke washing drain 41 and drained.

  On the other hand, the upper gas dehumidifying cooling unit 24 mixes makeup water and alkali in the dehumidifying cooling tank 42, sprays water from the nozzle 44 by the pump 43, and cools the gas. The makeup water is received by the partition wall 40 and collected in the dehumidifying cooling tank 42. A dehumidifying cooler 45 that cools the makeup water is installed between the pump 43 and the nozzle 44. In this manner, the upper gas dehumidifying / cooling section 24 also collects makeup water without waste, cools it, and circulates it for gas cooling. This embodiment is different only in the use of makeup water, and the mercury recovery by ozone is the same as the embodiment described above.

  As described above, according to the present invention, mercury in waste incineration exhaust gas can be efficiently removed in the smoke cleaning tower.

DESCRIPTION OF SYMBOLS 1 Smoke-washing tower 2 Circulation pump 3 Circulation piping 4 Nozzle 10 Incinerator 11 Fluid air supply means 12 Sludge pump 13 Fluid air preheater 14 Fluid air blower 15 White smoke preventer 16 White smoke prevention fan 17 Chimney 18 Cooling tower 19 Watering Nozzle 20 Dust collector 21 Smoke washing tower 22 Suction fan 23 Gas humidification cooling unit 24 Gas dehumidification cooling unit 25 Smoke washing water tank 26 Circulation pump 27 Sprinkling nozzle 28 Exhaust gas supply port 29 Temperature sensor 30 Tray 31 Nozzle 32 Nozzle 33 Alkali supply means 34 Demister 35 Ozone supply means 36 Mercury continuous analyzer 37 Ozone generator 40 Bulkhead 41 Smoke-washing drainage tank 42 Dehumidifying cooling tank 43 Pump 44 Nozzle 45 Dehumidifying cooler

Claims (4)

  Supplying ozone in a gas state to the gas humidification cooling section of the smoke-washing tower that cleans the waste incineration exhaust gas whose exhaust gas temperature has been lowered to 100 ° C or less to bring it into contact with the exhaust gas. A method for removing mercury from waste incineration exhaust gas, which is converted to valence mercury and dissolved in smoke-washed water for removal.   The smoke cleaning treatment tower includes a lower gas humidification cooling unit and an upper gas dehumidification cooling unit, and supplies ozone to an upper part of the gas humidification cooling unit. The method for removing mercury in waste incineration exhaust gas according to 1.   3. The method for removing mercury in waste incineration exhaust gas according to claim 2, wherein smoke cleaning water at the bottom of the smoke cleaning tower is pumped up by a circulation pump and circulated in a lower gas humidification cooling unit.   The mercury concentration in the waste incineration exhaust gas according to any one of claims 1 to 3, wherein the mercury concentration in the exhaust gas treated in the smoke treatment tower is measured and the supply amount of ozone is automatically controlled. Method.

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