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Analysis of Conventional Treatment and Zero Discharge Technology of Desulfurization Wastewater
2019-2-2
Source:Publication
Clicks:  6801        作者:未知
  • 1. Analysis of the Drainage Water Quality of Desulfurization Wastewater
    As coal mines and other minerals will produce a large number of heavy metals and other harmful substances after combustion, these harmful substances are discharged into the atmosphere after flue gas desulfurization, but some of them are discharged with the desulfurization wastewater. Therefore, the relevant desulfurization devices should discharge the relevant desulfurization wastewater, and then effectively ensure the normal operation of other systems, and achieve better safety and reliability. The state has also issued relevant regulations and policies, which require some enterprises, such as power generation, to have a special and separate desulfurization wastewater treatment system.

    At the same time, the desulfurization wastewater has low density and weak acidity. The chemical oxygen consumption is different from that of common wastewater. The inorganic ions that produce COD are reductive ions. The CODCr value is generally in the range of 150-300 mg/l. The suspended matter content in desulfurization wastewater is relatively high. After separation by waste water cyclone, the concentration is generally about 1.5%. According to the concentration of incoming water and the separation effect of cyclone, the maximum concentration can reach 32000mg/l. The soluble salt content in desulfurization wastewater is about 40 000 mg/l, mainly Cl-, followed by Ca2+ and SO2-4.

    2. Conventional desulfurization wastewater treatment methods
    In view of the above characteristics of desulfurization wastewater, chemical or mechanical methods are generally used to separate heavy metals and other sedimentable substances in wastewater treatment. Sedimented sludge is dewatered and the remaining cake is transported to the slag yard for comprehensive treatment. The desulfurization wastewater is pumped into the neutralization, sedimentation and flocculation triple tank from the wastewater buffer box. The pH value of the wastewater is adjusted to about 9 by adding lime milk in the neutralization box, which makes most of the heavy metals in the wastewater form hydroxides and precipitate. The calcium ion in the lime milk reacts with the fluoride ion in the wastewater to form calcium fluoride precipitation with less solubility, and complexes with As3+ to form Ca3(AsO 3)2 and other insoluble substances.

    Organic sulfur (TMT-15) was added to the settling tank to react with the remaining PB2+, Hg2+ in water to form metal sulfides with lower solubility and deposit them. FeClSO4 was added into the flocculation box to coagulate suspended solids or colloidal impurities into micro-flocs. Under the slow and smooth mixing action, micro-flocs formed slightly larger flocs in the flocculation box. Cationic polymer electrolyte was added at the outlet of the flocculation box to reduce the surface tension of particles.

    The wastewater flows automatically into the clarification and concentration tank, and the floc separates from the water in the clarification and concentration tank. The flocs deposit at the bottom because of their high density, and then concentrate into sludge by gravity. Most of the sludge is transported to the sludge dewatering system by sludge pump. The upper part of the clarification and concentration tank is clean water, which flows automatically to the outlet through the overflow around the clarification and concentration tank. According to the measured water pH value, the pH value is adjusted to 6.0-9.0 by adding hydrochloric acid.

    Finally, the treated wastewater is fed into the hydraulic slag removal system by the waste water discharge pump, and discharged with the slag flushing water. In addition to the above neutralization reaction system and sludge dewatering system, the desulfurization wastewater treatment process also includes chemical dosing system, including lime milk dosing system, organic sulfur (TMT-15) dosing system, polyferric chloride sulfate (FeClSO4) dosing system, coagulant dosing system and hydrochloric acid dosing system.

    3. Analysis of Zero Emission Technology
    3.1 Evaporation and Crystallization Technology
    The evaporation system is divided into four units: heat input unit, heat recovery unit, crystallization unit and subsidiary system unit. The heat input unit is connected with steam from the main plant area, and after temperature and pressure reduction, it becomes a low-pressure steam storage tank. When the desulfurization wastewater needs to be treated in depth, the steam is sent to the heating chamber to heat the wastewater.

    After heat exchange, the condensate enters the condensate tank, which is divided into two branches, one of which provides cooling water to the desuperheater and decompressor on the steam pipeline through the desuperheater pump. The desulfurization wastewater after conventional treatment is heated and concentrated by four-stage evaporation chamber and then sent to the salt slurry box. Two salt slurry pumps are sent to the swirl station. The swirler enters the centrifuge after the swirler crystallizes large particles of salt. The salt crystals are separated by centrifuge and then sent to drying bed by screw conveyor for heating and drying.

    The slurry separated from the swirl station and centrifuge is returned to the heating system for evaporation and concentration. The dried salt crystals are transported out of the factory. The treatment process needs to establish a separate set of wastewater evaporation and drying system, which consumes a certain amount of steam and auxiliary power. Because it belongs to the terminal process, it will not affect other equipment and systems of power plant, but the construction and operation costs are high.

    3.2 Salt Concentration Process
    Salt concentration process can treat desulfurization wastewater from conventional treatment system and produce pure distilled water through steam compression falling film evaporator, which can be used to recover FGD system as process supplementary water. The by-product of this system is calcium chloride solution, which is suitable for application in dust control, soil stabilization, ice control and other areas related to highway construction.

    Hydrochloric acid and scale inhibitors are needed for conventional treatment of desulfurization wastewater. It is then preheated to remove air, heated to near boiling, and fed to the evaporator, where a recirculating concentrated salt solution is mixed. The slurry is pumped into a salt solution condenser, where the slurry is distributed to a thin film on the inner wall of the titanium alloy tube. Water evaporates when the slurry film flows down the pipe.


    The condensate is collected by distillation tank and cooled by heat conduction with saturated steam and returned to FGD system. With the evaporation of falling film, calcium sulfate began to crystallize. Calcium sulfate crystals provide nuclei to prevent pipe scaling. Controlling the concentration of suspended solids and dissolved solids in evaporator canals is critical to prevent the formation of secondary salts and the scaling of the products in evaporator pipes.

    The recovered salt solution flowing on one side is treated by a cyclone. The underflow returns to the salt concentration tank. The overflow can be recovered to a brine concentrator or transferred to a finished tank based on the dissolved solid concentration. The recovered salt solution flowing on the other side is transferred to the finished product tank to control the concentration of solid suspended solids. Thirty-three percent of the salt solution products are then cooled and transported to the market by truck.

    4. Conclusion
    In summary, through detailed analysis of the application of conventional treatment methods and zero-discharge technologies for desulfurization wastewater, it is concluded that these conventional treatment methods have reached a certain level after years of practical application and improvement, and the process flow and operation technology are relatively mature, which are widely used in modern desulfurization industry.

    However, the advanced treatment technology of desulfurization wastewater can not be carried out well because of the limitation of production conditions of enterprises. Relevant enterprises should strengthen the development and application of zero-discharge technology, learn from foreign advanced desulfurization water treatment technology, combine with their own regulations, gradually optimize and improve China's desulfurization water treatment technology, and select zero-discharge of desulfurization water more in line with the actual situation. Releasing process, so as to achieve the best desulfurization with the least cost input, has a certain positive effect on the development of industrial wastewater treatment technology and environmental protection work in China.

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