Application of low-temperature and low-pressure evaporation and concentration technology in desulfurization wastewater treatment-Hezhong Environment (Beijing) Co., Ltd. was established in 2003

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Application of low-temperature and low-pressure evaporation and concentration technology in desulfurization wastewater treatment

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2020-12-09

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Currently, 70% of China's power generation relies on thermal power plants. The thermal power industry is a major water consumer. Statistics show that thermal power plants nationwide consume 60-70 billion cubic meters of water annually, with wastewater discharge reaching 300 million cubic meters. Desulfurization wastewater is the final collection point for various types of wastewater from thermal power plants. Its water quality composition is very complex, making treatment extremely difficult and a key challenge in industrial water treatment. With increasingly stringent environmental requirements, zero-discharge technology for desulfurization wastewater is gaining favor and widespread application. Concentration and reduction technology is a crucial step in zero-discharge of desulfurization wastewater, and it is divided into membrane concentration and thermal concentration. Membrane concentration includes reverse osmosis, forward osmosis, and nanofiltration. Thermal concentration includes multi-stage flash distillation (MSF), multi-effect forced circulation evaporation (MED), and mechanical vapor recompression (MVR).

This article introduces the application of a low-temperature, low-pressure evaporation and concentration technology in desulfurization wastewater treatment. Zero discharge adopts pretreatment + low-temperature, low-pressure evaporation concentration and reduction technology + solid-liquid separation system as the process route.

Desulfurization Wastewater Water Quality

Desulfurization wastewater is wastewater produced by wet flue gas desulfurization. Impurities in the wastewater mainly come from flue gas, limestone, and process water. Desulfurization wastewater not only has a complex water quality composition but also a large volume and significant fluctuations in water quality. In general, desulfurization wastewater has the following characteristics:

(1) Because desulfurization wastewater absorbs gases such as sulfur dioxide and carbon dioxide from the flue gas, it forms a weak acid in the water, resulting in acidic water with a pH between 4 and 6.

(2) The water contains a high concentration of suspended solids, mainly gypsum particles, silicon dioxide, iron salt suspensions, and aluminum salt suspensions. The solid content is generally between 1% and 3%, and sometimes even exceeds 5%.

(3) Desulfurization wastewater has high calcium and magnesium ion content, resulting in high water hardness.

(4) Desulfurization wastewater has a complex composition and contains many heavy metal ions, including mercury, cadmium, chromium, arsenic, lead, nickel, and zinc.

(5) The chloride ion content in desulfurization wastewater varies from 4000 to 20000 mg/L, making it highly corrosive to equipment.

(6) COD: mainly composed of unoxidized SO32-, S2O32-, S2O62-, and trace organic matter. Its content is related to the operating status of the desulfurization system.

Desulfurization Wastewater Treatment Process Flow

Before desulfurization wastewater enters the evaporation and concentration section, it undergoes pretreatment to remove suspended solids, heavy metal ions, calcium, magnesium, and sulfate as much as possible. The treated wastewater then enters the low-temperature, low-pressure evaporation and concentration stage. The condensate is reused, and the concentrated liquid undergoes solid-liquid separation. The process flow is shown in Figure 1:

Figure 1 Desulfurization Wastewater Treatment Process Flow

Pretreatment System

Desulfurization wastewater is weakly acidic and contains a large amount of SS particles, calcium, magnesium, sulfate, and heavy metal ions. To meet the requirements of subsequent treatment, heavy metal ions and scaling ions need to be removed from the desulfurization wastewater. The pretreatment uses the traditional "three-chamber" process, which is widely used, mature, and stable. It not only removes calcium and magnesium from the water but also effectively removes heavy metals. The process flow is shown in Figure 2:

Figure 2 Pretreatment Process Flow

Adjusting the pH of the wastewater to 9.0-9.5 allows most heavy metal ions to precipitate as hydroxides. The wastewater from the sedimentation tank is then treated in a flocculation tank, where flocculants are added to further flocculate and settle suspended matter and particles. After passing through the clarifier, it enters the low-temperature, low-pressure evaporation and concentration system.

Low-Temperature, Low-Pressure Evaporation and Concentration System

The low-temperature, low-pressure evaporation and concentration system uses a multi-effect evaporation system. Desulfurization wastewater that has passed through the pretreatment system enters the desulfurization wastewater buffer tank and is then pumped into the multi-effect evaporation system for evaporation and concentration. The concentrated liquid then enters the solid-liquid separation system. 。

The multi-effect evaporation system uses an external heat source and fully utilizes the secondary steam from each effect, thereby improving heat utilization efficiency. The entire system operates under negative pressure, reducing the boiling point and allowing the evaporation process to be completed at a lower temperature.

Advantages of Low-Temperature, Low-Pressure Evaporation and Concentration Technology

Compared with traditional multi-effect evaporation and concentration technology, the low-temperature, low-pressure evaporation and concentration technology operates under negative pressure, reducing the boiling point of water and improving evaporation efficiency. At the same time, the temperature requirement for the external heat source is lower than that of traditional technologies, saving energy. Moreover, the operating and maintenance costs of low-temperature, low-pressure evaporation and concentration technology are lower than those of traditional multi-effect evaporation technology.

Conclusion

Low-temperature, low-pressure evaporation and concentration technology is a new treatment technology for desulfurization wastewater. It has been applied in actual engineering projects and has achieved good treatment results, basically meeting the design objectives and requirements. The "zero discharge" of desulfurization wastewater ultimately requires finding a path that is both economically and ecologically beneficial, requiring continuous development, improvement, and innovation of new technologies.

Source: Hezhong Technology Department

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