JP4895855B2 - Scrubber wastewater treatment method - Google Patents

Description

  The present invention relates to a method for treating scrubber wastewater discharged from a scrubber installed in a fuel recovery process for organic waste.

In the fuel recovery process for recovering fuel gas from organic waste containing nitrogen compounds, especially sewage sludge containing a large amount of protein, by pyrolysis or gasification, high concentration hydrogen cyanide (HCN) is obtained by gasification reaction of nitrogen compounds. ) And the like and ammonia (NH ₃ ) are by-produced. The fuel gas is purified through a scrubber and then recovered, but the cyan component and ammonia in the fuel gas are transferred into the scrubber wastewater, so that the cyan component in the scrubber wastewater needs to be treated. Further, it may be necessary to reduce the ammonia component.

  As a method for treating cyanide-containing wastewater, an alkali chlorine method using sodium hypochlorite is generally used. As for the ammonia component, a breakpoint method using sodium hypochlorite is known.

  However, these alkali chlorine methods and breakpoint methods use sodium hypochlorite as an oxidizing agent, so that sodium hypochlorite is used in excess due to the presence of oxidizable components in the wastewater, and pH adjustment There is a problem in that the alkaline agent used in the above method requires a large amount due to the buffering action, which increases the cost.

  Then, the cyan volatilization method to the gaseous-phase side by a diffusion tower or an aeration tower as shown by patent documents 1-3 is proposed. As for the ammonia component, there is an ammonia stripping method using a diffusion tower. However, the volatilization treatment of the cyan component and ammonia component to the gas phase side using these stripping towers and volatilization towers has a problem that a safe treatment method for the cyan component transferred to the gas phase side has not been established. It was.

Moreover, when the method of returning the volatilized cyan component shown in Patent Document 2 to the furnace is applied to the gasification furnace in the fuel recovery process, the decomposition of the cyan component proceeds because the gasification furnace has a low oxygen atmosphere. There was a risk that the cyan component in the fuel gas would be concentrated.
JP-A-11-302667 JP 7-148493 A Japanese Patent No. 54-156343

  An object of the present invention is to provide a scrubber wastewater treatment method capable of safely and inexpensively treating cyan components contained in scrubber wastewater discharged from a scrubber installed in a fuel recovery process for organic waste. It is to be.

The present invention was made to solve the above problems by adjusting the scrubber drainage discharged from the scrubber disposed downstream of the organic waste gasification furnace to a pH of 2 to 5 with a pH adjusting tank and an aeration tank. The cyan and ammonia components in the wastewater are aerated and transferred to the gas phase side, and the cyan and ammonia-containing gas generated thereby is provided in a boiler that burns a part of the pyrolysis gas obtained in the gasification furnace. The gas burner is supplied as combustion air and burned .

In addition, the transition from the scrubber drainage to the gas phase side of the cyan component can be performed by aeration, and in that case, air having a volume ratio of 20 to 100 times the scrubber drainage amount is blown into the scrubber drainage and aerated. Is preferred. Moreover, it is preferable to aerate after adjusting scrubber waste_water | drain to pH2-pH5. Further, it is preferable to use steam generated by a boiler that burns a part of the recovered fuel for drying organic waste supplied to the fuel recovery process.

  According to the present invention, the cyan component contained in the scrubber wastewater is transferred to the gas phase side, and the cyan-containing gas is combusted in the boiler, so that the cyan component can be treated safely and reliably. Further, if aeration is adopted as a means for transferring the cyan component to the gas phase side, it becomes possible to effectively transfer the cyan component to the gas phase side. In particular, if the scrubber drainage is adjusted to pH 2 to pH 5 in advance, the volatilization of cyan Is promoted. Moreover, waste of energy can be eliminated by using the steam generated from the boiler for drying organic waste.

Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is an explanatory view showing an embodiment of the present invention, and here shows a step of recovering fuel using dehydrated sludge generated from a sewage treatment plant as a raw material. However, organic waste is not limited to dehydrated sludge, and municipal waste and other biomass can also be used.

  In FIG. 1, 1 is a dewatered sludge dryer, 2 is a gasifier, 3 is a waste heat boiler, 4 is a dust collector, 5 is a gas purification scrubber, 6 is a blower, and 7 is a recovered fuel gas. A gas engine 8 is a generator.

  The dehydrated sludge is dried by the dryer 1 and then supplied to the gasification furnace 2. The dried sludge is thermally decomposed while being fluidly stirred in the gasification furnace 2, and is decomposed into a combustible pyrolysis gas.

  The obtained gas is exhausted and recovered by the exhaust heat boiler 3 to generate water vapor. This water vapor is used as a heat source for the dryer 1.

The obtained gas is ash removed by a dust collector 4 such as a ceramic filter and purified through a scrubber 5 to become a clean fuel gas, which is led to a gas engine 7 . By making gas-liquid contact with the cleaning water in the scrubber 5 , cyan components such as hydrogen cyanide (HCN) and ammonia in the reformed gas are transferred to the scrubber drain.

  The fuel gas purified by the scrubber 5 is supplied to the gas engine 7 by the blower 6 and used as an energy source for driving the generator 8. The exhaust gas from the gas engine 7 is supplied to the boiler 14 to generate water vapor, and then discharged from the chimney 16. When the dewatered sludge has a high water content or depending on the operating conditions, the amount of steam supplied to the dryer 1 from the exhaust heat boiler 3 or the boiler 14 is insufficient to dry the dewatered sludge in the dryer 1. The gasification efficiency of the gasification furnace 2 may be reduced. Therefore, in order to compensate for the shortage of heat supplied to the dryer 1, a burner 15 is provided in the boiler 14, and a part of the fuel gas recovered in the fuel recovery process is burned to serve as a heat source for the dryer 1. Yes.

  The scrubber wastewater discharged from the scrubber 5 is fed to the pH adjustment tank 10 by the pump 9. In addition, the water temperature of the scrubber drainage discharged | emitted from the scrubber 5 is 40 to 60 degreeC. The pH adjustment tank 10 is adjusted to pH 2 to 5 by introducing an acid such as sulfuric acid. By adjusting to such a pH range, cyan becomes easy to dissociate from water.

  The scrubber wastewater whose pH is adjusted in the pH adjusting tank 10 is guided to the aeration tank 11 which is a gas phase transfer means provided in the subsequent stage. This aeration tank 11 has a water depth of 1 to 4 m, and air is blown into the aeration means 11a such as a diffusion plate or a diffusion tube by blowing air through a blower 12 and aerated. By this aeration, harmful substances such as cyan and ammonia components contained in the scrubber drainage are volatilized and transferred to the gas phase side. In order to reliably transfer the harmful substances from the scrubber wastewater to the gas phase side, it is preferable to aerate by blowing air having a volume ratio of at least 20 to 100 times the scrubber wastewater amount into the scrubber wastewater. As described above, the scrubber drainage is adjusted to pH 2 to 5 in the pH adjusting tank 10 in order to quickly and surely transfer the harmful substances, particularly the cyan component, to the gas phase side in the aeration tank 17. . Further, by performing pH adjustment in the aeration tank, it is possible to omit the step of adjusting the pH in the pH adjustment tank 10, and in this case, the pH adjustment tank 10 is unnecessary.

  The aeration tank 11 is maintained at a negative pressure by a blower or the like so that the gas containing the cyan component transferred to the gas phase side does not leak to the outside due to aeration of the scrubber drainage in the aeration tank 11.

  Since the scrubber wastewater is warm water of 40 ° C. to 60 ° C., the migration of the harmful substances contained in the scrubber wastewater to the gas phase side is promoted. However, if necessary, in order to promote the transition of the harmful substances to the gas phase side, the aeration tank 11 may be provided with a heat retaining means for keeping the scrubber waste water or a warming means for warming the scrubber waste water. preferable. Moreover, it is preferable to use the exhaust heat of the organic waste gasification process, such as the heat obtained by the exhaust heat boiler 3, as the heat source of the heating means for heating the scrubber waste water. In this case, since the exhaust heat is used, the environmental load can be reduced.

  In this embodiment, the aeration tank 11 is used as the vapor phase transfer means, but is not limited to this, and a multistage diffusion tower having a tower height of 4 to 8 m, a packed tower filled with Raschig rings, or the like is used as the gas phase transfer means. It can also be used.

The cyan-containing gas discharged from the aeration tank 11 is supplied as the combustion air of the burner 15 provided in the boiler 14 described above, and is subjected to combustion processing. As described above, since the fuel gas is oxidized and burned at a high temperature, harmful substances such as cyan and ammonia components can be completely burned. When the cyan-containing gas is returned into the gasification furnace, there is a concern about the concentration of cyan and ammonia and the generation of warming gases such as N ₂ O, but such a problem does not occur in the present invention.

  Moreover, since the combustion heat is recovered by the boiler 14 and used as a heat source for drying the organic waste by the dryer 1, the efficiency of gasification of the organic waste in the gasification furnace 2 using the exhaust heat. It is possible to improve the environmental load.

  The scrubber waste water from which the cyan component and the like are removed in the aeration tank 11 is guided to the neutralization tank 13. The neutralization tank 13 is charged with an alkali such as sodium hydroxide to neutralize the scrubber waste water. This scrubber effluent is returned to the first sedimentation site of a sewage treatment plant and processed.

The scrubber effluent discharged from the scrubber installed in the sewage sludge gas conversion facility shown in FIG. 1 contained hydrogen cyanide (HCN) with a concentration of 300 mg / L and ammonia with a concentration of 1000 mg / L. The water temperature is 60 ° C. Sulfuric acid was added to the scrubber effluent to adjust the pH to 2, and the water temperature was kept at 60 ° C. and aerated with 30 times the volume of air. When hydrogen cyanide (HCN) and ammonia concentrations in water 3 hours after the start of aeration were measured, the hydrogen cyanide (HCN) concentration was reduced to 0.1 mg / L and the ammonia concentration was reduced to 300 mg / L. The cyanide-containing gas discharged from the aeration tank was used as combustion air for the boiler burner and subjected to combustion treatment. It was confirmed that both hydrogen cyanide (HCN) and ammonia (NH ₃ ) in the exhaust gas discharged from the boiler were below the detection limit.

  Although the present invention has been described above in connection with the most practical and preferred embodiments at the present time, the present invention is not limited to the embodiments disclosed herein. The scrubber wastewater treatment method and scrubber wastewater treatment apparatus with such changes are also within the technical scope, as long as they do not contradict the gist or concept of the invention that can be read from the claims and the entire specification. It must be understood as included.

It is explanatory drawing which shows embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Dryer 2 Gasifier 3 Waste heat boiler 4 Dust collector 5 Scrubber 6 Blower 7 Gas engine 8 Generator 9 Pump 10 pH adjustment tank 11 Aeration tank 11a Aeration means 12 Blower 13 Neutralization tank 14 Boiler 15 Burner 16 Chimney

Claims (3)

Scrubber effluent discharged from the scrubber placed downstream of the organic waste gasification furnace is adjusted to pH 2 to pH 5 in the pH adjustment tank and then aerated in the aeration tank to remove cyan and ammonia components in the effluent. The gas containing cyan and ammonia generated thereby is supplied as combustion air to a gas burner provided in a boiler for burning a part of the pyrolysis gas obtained in the gasification furnace, and is subjected to combustion treatment. A method of treating scrubber wastewater.   The scrubber wastewater treatment method according to claim 1, wherein air having a volume ratio of 20 to 100 times the scrubber wastewater is blown into the scrubber wastewater and aerated.   The scrubber wastewater treatment method according to claim 1, wherein steam generated in a boiler is used for drying organic waste supplied to a gasification furnace.

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