What are the technologies for zero discharge of desulphurization wastewater in thermal power plants?

1 Overview

With the rapid development of coal-fired thermal power generation in my country in recent years, environmental problems have become increasingly prominent. Affected by the shortage of water resources, zero discharge of wastewater has attracted much attention.

At present, the comprehensive treatment process of high-salt wastewater in the thermal power generation industry has two main directions, one is evaporation and crystallization, and the other is the evaporation of flue gas waste heat. Because the cost of evaporative crystallizer is very high, and the evaporation of flue gas waste heat is limited by the amount of flue gas, the current comprehensive treatment process of high-salt wastewater is divided into three steps: the first step: pretreatment, removal of suspended solids, hardness, alkalinity, Heavy metals, etc.; the second step: concentration treatment to concentrate the amount of waste water to reduce the investment in terminal evaporation and crystallization or flue gas evaporation; the third step: evaporation desalination treatment, using evaporation crystallization or flue gas heating and evaporation, to achieve "zero discharge" of wastewater .

This article focuses on the pretreatment, concentration, and high-efficiency evaporation process for zero discharge of wastewater, and briefly introduces and analyzes the technology and economy.

2 Pretreatment process

The wastewater pretreatment system mainly removes suspended solids and Ca2+, Mg2+, and SO42-plasma in the desulfurization wastewater to meet the water inlet requirements of the subsequent system and avoid scaling and fouling of the subsequent treatment system.

It can be seen from Table 1 that although the dosage of the "sodium hydroxide + sodium carbonate" softening method is small, the cost of the dosage is based on the market price fluctuations of sodium hydroxide and lime. At this stage, the dosage of lime is large and the price is high. , "Sodium hydroxide + sodium carbonate" softening method has lower cost. The "sodium hydroxide+sodium carbonate" softening method is not effective in removing F ions, and the sedimentation effect of magnesium hydroxide is not good. It will happen that magnesium hydroxide precipitates in the water and cannot settle. The precipitation of magnesium hydroxide is small. Crystal particles will enter the lower-level system along with the wastewater, which will have a greater impact on the operation of the subsequent membrane system. Therefore, the "sodium hydroxide + sodium carbonate" softening method requires finer control and higher operating levels, while "lime + sodium carbonate" The softening method has better sedimentation effect, better drug safety, easier purchase, and higher drug addition cost.

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3 Concentration treatment process

At present, wastewater concentration is divided into two mainstream processes, thermal concentration and membrane concentration.

(1) The thermal treatment process is a relatively mature process route. It has been used for seawater desalination and wastewater treatment for many years, mainly including mechanical vapor recompression (MVC/MVR), low-temperature multi-effect distillation (MED), and multi-stage Flash evaporation (MSF) and other processes. After thermal concentration, the TDS of concentrated water can reach up to 200,000 mg/L. According to the technical and economic efficiency of thermal concentration, the current process for thermal power plants to be more suitable for thermal concentration of high-salt wastewater is the "low-temperature flue gas evaporative concentration" process.

The low-temperature flue gas evaporation and concentration processing unit draws out part of the low-temperature flue gas after the induced draft fan is used as a heat source to enter the evaporation concentrator. The supernatant of desulfurization wastewater treated by the triple box, acid-base regeneration wastewater and concentrated water from the circulating cooling system are led to the evaporative concentrator for atomization and directly contacted with the flue gas for heat exchange, and then evaporated and concentrated. The saturated wet flue gas is evaporated and concentrated. The mist eliminator in the device enters the main flue after processing. The high-salt wastewater is treated by low-temperature flue gas evaporation to form two parts: condensed water and concentrated liquid. The condensed water can be reused as desulfurization process water, and the concentrated liquid enters the terminal wastewater treatment process unit.

In the low-temperature flue gas evaporation concentration unit, desulfurization wastewater, acid-base regeneration wastewater and circulating cooling system drainage concentrated water adopt a large-flow circulation evaporation method to concentrate wastewater by 8-10 times. The schematic diagram of the processing system of the low-temperature flue gas evaporation unit is shown in Figure 1.

At present, low-temperature flue gas evaporation and concentration treatment technology has been put into operation in Guodian Taizhou Power Plant and Liaocheng Xinyuan Group's desulfurization wastewater low-temperature flue gas evaporation and concentration projects for nearly a year. For the low-temperature flue gas evaporation and concentration process, the desulfurization wastewater, acid-base regeneration wastewater and circulating cooling system drainage concentrated water after the triple box treatment can be mixed and sent directly to the low-temperature flue gas evaporation concentration system for evaporation and concentration without softening treatment.

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(2) Membrane concentration process. At present, the more mainstream deep concentration membrane treatment processes mainly include: high pressure reverse osmosis (DTRO), forward osmosis (FO), and electrodialysis (ED). Among them, there is only one application case of the forward osmosis process in the power system, and it is not successful. This time the membrane concentration is mainly used for comparison between DTRO and ED, as shown in Table 2.

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4 Key technologies for zero emission of desulfurization wastewater flue evaporation

At present, there are three mainstream concentrated salt evaporation and desalination treatment processes, namely, the spray evaporation drying technology of the flue gas waste heat of the main flue, the bypass evaporator flue gas spray evaporation drying technology, and the evaporation crystallization process.

The spray evaporation drying technology of main flue gas waste heat has many application cases in the treatment of concentrated brine at the end. It has the advantages of simple system, low investment and operation cost, and no crystal salt disposal problems. However, the stability of the system operation is affected by the load of the unit. Greater impact.

The zero discharge technology of desulfurization wastewater evaporated by the bypass flue has the advantages of high automation and convenient operation, which improves the operation and maintenance level of the system; the design of the inlet and outlet isolation doors of the bypass flue can be isolated from the main body of the power plant. Does not affect the daily operation of the power plant. The zero discharge technology of desulfurization wastewater based on bypass flue evaporation is feasible. In this technology, pretreatment is the foundation, membrane reduction is the guarantee, and bypass flue evaporation is the core. When applying this technology, the membrane concentration factor should be reversed according to the allowable evaporation of water, and reasonable pretreatment process parameters should be designed. This technology uses high-temperature flue gas to achieve high-efficiency evaporation of desulfurization wastewater, no additional heat source is required, and low operating energy consumption; and the bypass flue can make full use of the gap between the flues, and has a small footprint and low engineering investment. It can reduce the problems of fouling and fouling in the operation of the system to a certain extent, but it has a slight impact on the efficiency of the boiler. Bypass evaporator flue gas spray evaporation drying technology has low investment and operating costs, no problems such as crystallization salt treatment and disposal, and less impact on air preheaters and fly ash quality. In recent years, the number of thermal power plants put into operation has gradually increased.

The steam mechanical recompression evaporation crystallization process has the problems of large area, high operating cost, and treatment and disposal of crystalline salt, so there are few considerations for the application of power plants.

Ash yard spray evaporation is used as a means to absorb the concentrated salt water at the end. There is no new investment and the operating cost is lower. However, it faces greater environmental protection policy risks, as shown in Table 3.

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5 concluding remarks

In summary, the technical route for zero wastewater discharge needs to be selected in accordance with the production characteristics of the power plant. The pretreatment process and operating parameters are the basis for the zero-discharge treatment of desulfurization wastewater. Concentration reduction can effectively reduce the processing load of the evaporative solidification section and ensure the efficient evaporation of the subsequent system. It is the key to achieving zero discharge of desulfurization wastewater. Compared with thermal concentration, membrane concentration equipment is simple, covers a small area, and consumes less energy. . High-temperature flue gas evaporation is the core of zero-emission treatment. Rotary atomization evaporation technology does not require additional heat sources, has high efficiency, small footprint, simple and easy automatic control, and does not require pretreatment, and has little impact on other equipment in the power plant. It has great promotion prospects.

At present, the domestic zero-discharge desulfurization wastewater technology is in the stage of extensive research and preliminary application. The investment cost of the existing zero-emission technology is generally higher, and the operating cost is higher. How to combine existing processes to optimize the realization of zero discharge of low-cost desulfurization wastewater will be the focus of zero discharge of desulfurization wastewater in the future.