The process control of water conservation is a prerequisite for zero discharge of wastewater in thermal power plants. Water balance testing and analysis of the potential for water conservation development provide technical guidance for process control of water conservation. The comprehensive utilization of tail end wastewater is important for zero discharge of wastewater in thermal power plants. At present, the key wastewater at the tail end of most thermal power plants includes flue gas desulfurization wastewater, cooling circulating water wastewater treatment water, and small amounts of recycled wastewater. The flue gas desulfurization wastewater has the characteristics of high salinity, strong corrosion, heavy metal pollution, and high suspended solid concentration. It is an extreme wastewater in water bodies in thermal power plants. Therefore, the comprehensive utilization of flue gas desulfurization wastewater is important for thermal power plants to achieve zero discharge of wastewater, and it is also a new challenge encountered in the design planning and environmental protection renovation of thermal power plants.

1. Water balance testing methods and contents for thermal power plants

1.1 Testing standards

Taking the water balance test of our factory in 2013 as an example, the water balance test is divided into two parts: static data system partition leak detection and dynamic water balance detection. After the detection is carried out, the discovered leaks are rectified, the rationalization of water use in the factory area is analyzed, and a water conservation improvement plan is clearly proposed. The dynamic testing of water purification equipment is mainly based on online measurement and calibration instrument panel testing. Temporary testing and basic theoretical calculations are necessary to fill in. Before the inspection, the original online steam flow meter was calibrated. The measurement points of pipelines without fixed fixed steam flow meters are accurately measured using portable ultrasonic flow meters during the horizontal stability period of the generator set load and system software. For pipelines with stable flow and continuous water use, choose different times or working conditions for repeated testing 3-5 times. Take the average value for statistical analysis. The flow rate of high-pressure cleaning water, the amount of water replenished from the bottom slag water buffer storage tank, and the amount of water replenished from the fire water storage tank are accurately measured using this method.

For water with small pipe diameter and small total flow rate, the capacity method is used. For some water consumption points that cannot be accurately measured, calculations are carried out based on design scheme information or water balance method, such as steam damage to generator units, moisture damage from flue gas desulfurization smoke and dust, etc.

The drainage volume of the rain pump room during the period of wearing a hat on a rainy mountain means a part of the wastewater discharge and leakage volume of the water system software. The precise measurement of the drainage volume of the rain pump room adopts a method that closely combines statistical analysis of pump operation time and precise measurement of instantaneous total flow rate.

1.2 Testing content

(1) Research on the total water usage and trend in the factory area. Including total water intake, water consumption, circulating system water consumption, reuse water consumption, cascade water consumption, consumption water, discharge water, etc., calculate the power generation, water consumption, reuse water rate, etc. in the factory area.

(2) Measurement of water consumption in each subsystem. Including low-voltage industrial production water system software, high-pressure cleaning water system software, desulfurization technology water, daily living water, fire safety water, etc.

(3) Measurement of wastewater discharge and reuse water volume. Detect the water volume of the wastewater treatment system software and the recovery, utilization, and discharge of cold water after treatment.

1.3 Analysis of the Development Potential of Water Conservation in Thermal Power Plants

(1) Comprehensive utilization of rainwater and drainage pipelines. Changing the drainage of sewage wells from discharge to landscaping water, and connecting the rainwater drainage pipeline to the industrial park landscaping water pipeline, can save 10 tons of water per day.

(2) Reduce water consumption while ensuring normal operation of machinery and equipment. Such as reducing damage to the exhaust pipe of the generator set, reducing the total sampling flow of the instrument panel, and moderately reducing the cooling water output in winter.

(3) Suggest identifying and eliminating leaks in the existing fire water pipe network, or carrying out updates and renovations to remove leaks and reduce fire water consumption. It is estimated that 200t/d of water can be saved.

(4) Reduce the hydration and moisturizing rate of the heating furnace. After calculation, the water replenishment and moisture retention rates of the condensers of each generator unit are: 0.31% for # 1 generator unit, 0.34% for # 2 generator unit, 0.68% for # 3 generator unit, and 0.61% for # 4 generator unit, all of which are lower than the water surface evaporation rate (1.5%) specified in the Water Conservation Guidelines for Thermal Power Plants. However, the water replenishment and moisture retention rates of # 3 and # 4 generator units are higher than those of # 1 and # 2 generator units, effectively adjusting the operation and technical improvement of the boiler, and reducing the amount of desalinated water replenishment. This not only saves water economic benefits, but also saves raw materials and reduces production and power generation costs.

(5) Daily water usage. The total water consumption of the domestic water system software is 53.6m3/h, of which the practical daily water consumption in the industrial zone is 38.6m3/h, and the average water consumption is 440L/person ˙ Day, far higher than the water budget quota of Zhejiang Province (120-180L/person for daily water use of urban residents) ˙ In Japan, there is consumption and certain leakage of domestic water. Technological updates, renovations, and the selection of energy-saving machinery and equipment can effectively reduce daily water leakage and consumption. At present, energy-saving environmental sanitation equipment and gate valves for daily water use are receiving attention and selection in power plants, including the use of energy-saving water faucets and power switches for foot pedals.

2. Application of RO reverse osmosis technology in the wastewater industry

RO technology is a purposeful filtration of particulate matter and organic compounds in a solution, achieving practical effects such as purification and separation. It has the advantages of small total area occupied by machinery and equipment, high output of enterprises, and low energy consumption. Therefore, this technology has been widely used in water treatment methods. Because RO has strict regulations on the water body of seepage, this technology is generally applied in conjunction with ultrafiltration membranes, microfiltration, and other technologies to achieve better actual stripping effects.

At present, in the application of wastewater treatment in thermal power plants, RO reverse osmosis technology is suitable for the treatment of wastewater from cooling circulating water, acid pickling and passivation wastewater from heating furnaces, and comprehensive wastewater from power plants.

A certain power plant uses the Continuous Microfiltration (CMF)+RO Reverse Osmosis (RO) process to treat the cooling circulating water in the circulation system. The solved water volume is 8000m3/d, the reused water volume is 7000m3/d, and the water output basically meets the water body requirements for filling and replenishing the cooling system of the circulation system. A certain air-cooled power plant in the northern region selected the "steam flotation oil lime powder soft acting biological filter ion exchange method soft RO reverse osmosis" treatment process to treat wastewater from the boiler auxiliary refrigeration system, chemical water production workshop, cleaning water, and other wastewater. After the system software was put into use, the overall water output of the system software met the design requirements.

However, in the specific operation process, it was found that due to the poor water quality of the wastewater from thermal power plants, especially for high salinity and high corrosion flue gas desulfurization wastewater, the ultrafiltration membrane is very easy to be polluted by the environment, and the desalination characteristics are suddenly reduced, resulting in frequent cleaning and reconstruction. This not only reduces the service life of the ultrafiltration membrane, reduces the utilization rate of wastewater, but also greatly increases the operating cost. Therefore, RO reverse osmosis technology is rarely used independently for solving flue gas desulfurization wastewater.

3. The application of evaporative crystallization technology in the field of zero discharge of wastewater

Multi effect evaporation technology is based on a series of methods to extract wastewater, which is then subjected to multi effect evaporation. The steam is then collected through cooling, and the salt crystals are dried and transformed into industrial salt, achieving the goal of zero discharge of wastewater.

3.1 Multi effect evaporation technology

The basic multi effect evaporation technology is the multi effect evaporation (MED) crystallization technology, which is generally divided into four modules: hot keying module, waste heat recovery module, crystallization module, and attached system software enterprise. The basically solved wastewater is transformed into salt slurry through the heating and extraction of multi-level volatilization chambers, and the salt slurry is centrifuged and dried to become industrial salt and transported to the factory for sale or burial.

In 2009, Guangdong Heyuan Power Plant used this technology to complete the zero discharge project of flue gas desulfurization wastewater. The design output was 20m3/h, and the TDS of the volatilization system software was less than 30mg/L. The liquid crystalline salt produced by reusing the cooling circulating water of the power plant's circulation system met the secondary industrial salt standards and was sold at a price of about 80 yuan per ton. Although this technology is relatively complete, its high energy consumption may limit its progress and marketing promotion.

3.2 Steam compression technology for mechanical equipment

To reduce energy consumption, researchers have developed condensers using mechanical equipment vapor compression (MVR or MVC) technology MVR (MVC) technology is the process of reducing secondary steam through heat transfer and sending it out of the heating chamber. The reduced steam temperature rises and can be used as a heat source again, significantly reducing steam usage and energy consumption.

The Sanshui Hengyi Power Plant has introduced China's first set of MVR (MVC) technology machinery and equipment from the United States. This technology adopts a two-stage vertical MVC+two-stage vertical MED processing process, with a design output of 20m3/h, to solve the problems of epoxy resin regeneration wastewater and flue gas desulfurization wastewater. The selected condenser is a vertical spray level tube thin film evaporator, with a horizontal setting. The wastewater flows through the tube, and the heated steam flows through the tube. The liquid is sprayed onto the outside of the heat exchanger tube through the nozzle to produce a plastic film, which is heated to produce steam. The extracted liquid is then sent to the crystallization system software to solve the problem.

This technology has relatively low energy consumption, but in the specific operation process, it is found that on the one hand, due to the lack of deep preparation and treatment system software, the product is a complex mixed salt, which can only be solved as a hazardous solid waste, with extremely high cost; On the other hand, water seepage has not been sufficiently softened, resulting in severe scaling and frequent cleaning of scale, which has also increased costs. Through improvements by overseas enterprises, the use of column type falling film evaporators can effectively address the serious scaling and energy consumption issues of vertical condensers. This technology has already been used in several specific wastewater zero discharge engineering projects around the world.

4. Conclusion

With the continuous improvement of environmental protection regulations in China, the rapid development and use of zero discharge technology for wastewater from thermal power plants are urgent. It is particularly important to select suitable zero discharge technology for wastewater treatment in thermal power plants. If the deep preparatory treatment+biological fiber facial mask high normal saline extraction technology+heating steam compression column type forced circulation system continuous casting mold technology is selected, zero discharge of wastewater can be achieved. Moreover, the system software has the advantages of high automation technology, energy-saving and low cost operation, which provides a technical reference for the basic construction of a more clean and environmentally protected thermal power plant. It can be expected that the system software has a broad application prospect in zero discharge of wastewater in thermal power plants.

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