Application of "zero discharge" technology for flue gas desulfurization wastewater

With the rapid development of social economy, thermal power plants play an increasingly important role in China's national economy. However, the coal-fired power plant has seriously damaged the ecological environment in its operation, especially the industrial wastewater. In order to improve this situation, thermal power plants have to take effective measures to achieve the goal of "zero discharge". In the new situation, the utilization technology of water resources in China's thermal power plants is relatively backward, which is not optimistic. I hope the analysis of this paper has reference value for the development and research of the industry.

Application of "zero discharge" technology for flue gas desulfurization wastewater

With the rapid development of China's modern industry, environmental problems continue to highlight, air, water and soil pollution incidents repeatedly. Due to the shortage of water resources, zero discharge of wastewater has attracted much attention. In the process of industrial development, it is different to obtain economic benefits at the expense of the environment. The national environmental protection department advocates that industrial enterprises strictly implement desulfurization wastewater technology to achieve the goal of zero emission.

1、 Overview

Due to the high hardness and poor stability of desulfurization wastewater, which contains a large number of heavy metals and high chloride, it has strong scaling and corrosiveness, which is also the technical difficulty of wastewater treatment. Based on the application of domestic industrial wastewater and desulfurization zero discharge technology, the process systems are compared. On the basis of conventional operation parameters of 300MW Thermal Power Plant, the economic benefit of wastewater treatment is increased. At present, there are mainly 3 kinds of treatment technologies: (1) the technology of evaporating treatment of desulphurization wastewater; (2) the technology of spray evaporation treatment; (3) the evaporation membrane treatment technology of high salinity wastewater.

2、 Discharge characteristics and treatment difficulties of desulfurization wastewater

1. Emission characteristics

    The amount of process make-up water and water quality, the quality of limestone, the content of fly ash in boiler flue gas and the concentration of C1 slurry in desulfurization absorption tower directly determine the amount of limestone gypsum desulfurization wastewater discharge. In the actual operation of thermal power plant, the discharge of wastewater is generally determined by controlling the concentration of C1 slurry in the desulfurization tower. For example, if the C1 concentration of slurry is 20kg / m3 in a 600MW generator unit, the discharge of desulfurization wastewater is 17.3m3/h. If the quality of process water is not high, or the concentration of C1 slurry is lower, the discharge will increase.

    To a certain extent, the types and contents of pollutants in desulfurization wastewater are restricted by coal type, desulfurization process and method, dust content in flue gas, limestone quality and dehydration effect, ammonia volatilization rate and other factors. Even for the same generator set, the quality of desulfurization wastewater is quite different.

    Application of "zero discharge" technology for flue gas desulfurization wastewater

Effluent quality of desulfurization wastewater from triple header of 600 MW unit in a power plant

2. Handling difficulties

    In general, factors such as coal type, desulfurization process, make-up water quality and discharge cycle directly affect the composition of wastewater pollution. There are great differences in different regions of thermal power plants, and the discharge time is different, so is the discharge of the same power plant. Desulfurization wastewater is intermittent discharge with unstable water quantity fluctuation; the hardness of wastewater is high, which is easy to form evaporation structure; the concentration of slurry C1 - is relatively high, which causes corrosion to the system.

    Although the desulfurization wastewater treatment system has treated the water quality in the advanced treatment system, which reduces the hardness of suspended solids and calcium, the hardness of calcium, especially magnesium, is too high. At the same time, the solid solubility of C1 - and SO42 - in the slurry is very high, and the ion concentration is high, which leads to the scaling and corrosion of the advanced treatment system, which is not conducive to the stability and safety of the system operation.

3、 The current situation of reuse of desulfurization wastewater

In the process of desulfurization wastewater treatment, the commonly used treatment process is desulfurization wastewater, which generally has high chloride ion concentration and salt content. In the process of reclaimed water reuse, it is easy to appear corrosion and some scaling phenomenon, which seriously hinders the reuse effect of desulfurization wastewater. In domestic power plants, the utilization efficiency of desulfurization wastewater is still a problem A short board.

(1) Generally used for hydraulic ash flushing or ash field spraying, coal-fired power plants with hydraulic ash flushing system can use water for ash flushing after some treatment. (2) For the hydraulic slag removal system or wet slag removal system in coal-fired power plant, desulfurization wastewater is used to make up water. This way is limited by the closed circulating water volume of the slag removal system, which is easy to cause system blockage and equipment pipeline corrosion, thus affecting the operation reliability of the system. Therefore, the utilization of desulfurization wastewater is also limited.

4、 Application technology of "zero discharge" of wastewater

Evaporation process

Through evaporation technology, the solution is concentrated to obtain certain solid solute and pure solvent, which is widely used in chemical industry, seawater desalination and food industry. In the actual evaporation, the amount of vaporization heat is relatively large, so this process is also a process of large heat consumption. At present, the chemical industry mainly through the multi effect evaporation technology to improve the efficiency of heating steam, improve the heat transfer conditions, reduce the unit energy consumption.

The newly developed mechanical steam recompression technology can effectively reduce steam consumption. In this technology, the secondary steam is compressed and adiabatically sent to the heating evaporator by the mechanical driven compressor. After compression, the temperature of the secondary steam keeps rising, and there is a thermal temperature difference between the secondary steam and the boiling liquid in the evaporator. Therefore, it can be used as a heating agent. In this case, sufficient compression power can be added to make full use of the potential heat energy in the secondary steam Quantity.

2. Flue treatment technology

The technology mainly refers to the treatment of flue water by spray evaporation technology, which is widely used in food and chemical industries, but has not been widely used in wastewater treatment. In the desulfurization wastewater, through the flue evaporation technology, the spray technology is firstly selected to atomize the desulfurization wastewater and introduce it into the flue before dust removal. After the high-temperature flue gas is heated, the wastewater in the form of small droplets evaporates rapidly. The suspended solids and soluble solids contained in the wastewater will turn into fine solid particles, which will flow into the dust collector and be removed under the entrainment effect, so as to realize zero discharge of desulfurization wastewater Set the process target.

3. organic combination of desulfurization wastewater and fly ash technology

In the operation of thermal power plant, landfill treatment of fly ash, and desulfurization wastewater has a certain humidification effect on fly ash, so the volume of dust can be reduced in transportation. If fly ash is used in brick making or cement additives, the Cl content is relatively low. Meanwhile, the technology can transfer heavy metals contained in wastewater to fly ash, which will affect its utilization effect.

4. Build constructed wetland

    In order to reduce the concentration of metals, nutrients and suspended particulates in wastewater, constructed wetland was constructed under the action of plants, soil and microorganisms. Constructed wetlands contain many plant and bacterial components, and thermal power plants can choose the components reasonably according to their own pollutant treatment. In order to effectively reduce the concentration of metals, nutrients and suspended particulates in wastewater, the constructed wetland must ensure low chlorine content.

5. Steam concentration technology

The technology is to evaporate and concentrate the waste water to form a certain distillation and concentrated water, and to continuously evaporate the concentration by means of crystallizers or spray dryers, thus forming distilled water and solid wastes, and recyclable or landfill materials to form this part. In order to prevent the evaporator from scaling, the wastewater should be pretreated to remove the calcium and magnesium ions.

5、 Process design scheme

In the operation of thermal power plant, according to the actual requirements of water quality, reasonable reuse technology is used to meet the requirements of wastewater discharge. Recycle wastewater, improve its reuse efficiency, reduce waste discharge, and protect new water resources as much as possible. According to the overall planning of water supply and drainage of thermal power plant in the relevant national regulations, reasonable reuse of wastewater, improve the efficiency of reclaimed water reuse, guide the system itself to achieve self-sufficiency of water supply as far as possible, save existing water resources and reduce water demand.

According to the demand of water quality, the cooling water cycle is used to achieve different uses of water and improve the efficiency of recycling. From a long-term point of view, the establishment of water-saving awareness is not only to consider the operation safety of thermal power plants, but also to strengthen the propaganda of water-saving by enterprise leaders.

From the perspective of the development of thermal power plants, the main existing sewage treatment systems of thermal power plants include industrial wastewater and domestic sewage treatment stations. At present, the industrial and agricultural wastewater treatment station mainly collects and processes the wastewater produced in the industrial and agricultural production, and then discharges it again after meeting the discharge standard. The separation or flocculation sedimentation process is used to treat the wastewater in the form of oil and boiler acid formed in the production of other enterprises, so as to effectively control the content of various substances in the wastewater and keep it below the national standard The waste water can be used in the recycling system of thermal power plant to improve its utilization efficiency.

Therefore, for power plant enterprises, according to their own water quality needs, they can respectively use sewage treatment systems such as domestic sewage, oily wastewater and desulfurization wastewater to treat and purify all wastewater formed in the production of enterprises, and strive for recycling, so as to achieve the goal of zero discharge of wastewater.

6、 Conclusion

To sum up, with the increasing environmental pollution, environmental protection has been paid attention to by all walks of life. In the operation process of thermal power plants, desulfurization wastewater reuse technology must be implemented. In practical operation, not only to ensure that the evaporation energy consumption is reduced, but also to reduce the sludge discharge, as far as possible without the use of chemicals. A large number of practices have proved that in the prevention of membrane fouling technology, the effect of vibrating membrane is remarkable, and the concentration technology has certain feasibility and economic value, which promotes the country to achieve the goal of "zero emission".