In recent years, in response to the severe water shortage in Hebei Province and the serious constraints on power plant expansion, the Hebei Provincial Electric Power Survey and Design Institute has actively carried out research on water-saving technologies for thermal power plants. Progress has been made in the design and research of water-saving ash removal systems, using urban sewage as supplementary water for circulating water systems, and achieving "zero discharge" of wastewater from large thermal power plants. The water-saving effects are significant, with substantial environmental, economic, and social benefits. 1. Research and Application of Water-Saving Technology in Ash Removal Systems 1.1 Research Background The research on water-saving technology in ash removal systems was applied to the Matou Power Plant in 1995. Before its application, the plant had been forced to shut down its units due to water shortages. The ash removal system consumed a large amount of water; the total amount of ash slurry discharged from boilers #1 to #7 to the ash storage yard was 3339.2 m³/h, with an ash-to-water ratio of approximately 1.28–30, which increased the burden on the ash storage yard. Sometimes the water level in the ash storage yard reached as high as 4 m, seriously threatening the safety of the ash storage yard dam. In 1993, water leakage at the ash disposal site caused flooding of approximately 133 hm2, resulting in significant economic losses due to compensation claims. Farmers near the ash disposal site, in order to collect slag and cenospheres, arbitrarily altered the flow of ash water, causing multiple landslides at the ash disposal site and increasing maintenance costs. To conserve water and ensure safe and economical production, a water-saving renovation design study was conducted on the ash removal system during the expansion of boiler #8 at the plant in 1992 to ensure water resource security. 1.2 Scheme Formulation and Optimization Since vibrating screens can separate ash and slag, and thickening tanks can convert low-concentration ash slurry into higher-concentration ash slurry, and their overflow water can be directly recycled for use in the power plant's ash removal system, based on extensive investigation and research, several schemes were compared to address the main problems existing in the plant's ash removal system. a. All dust collectors in boilers #1 to #7 were replaced with electrostatic precipitators, and the ash removal system adopted a water-sealed agitator-elevated chute-centrifugal ash pump. This system is relatively simple, achieving an ash-to-water ratio of approximately 1:10, and also improving the dust removal efficiency of the dust collectors. However, replacing the dust collectors of all seven boilers with electrostatic precipitators is costly and space-constrained; furthermore, this modification affects many pieces of equipment operating normally, has a long modification cycle, and is difficult to implement. b. An ash and slag separation – thickening tank – volumetric slurry pump scheme. This system can achieve a very high ash-to-water ratio, generally around 1.2. This scheme is a typical high-concentration hydraulic ash removal system. The thickening tank and other equipment and facilities are constructed independently, with minimal interference from the existing equipment and system except for pipe interfaces. However, for the power plant, this scheme requires replacing the ash conveying pipeline. Because the ash-to-water concentration is too high (e.g., 1:2), the slurry flow rate will be drastically reduced. Even considering the ash and slag volume of boiler #8, the total slurry volume for the entire plant is transported by a single existing (DN500) pipeline, with a slurry flow velocity of only 0.5 m/s within the pipe, far below the specified 1 m/s. Low slurry flow rate will cause ash and slag to accumulate in the pipes, preventing normal operation. Replacing the existing pipes with two D-N350 pipes would increase investment by approximately 20 million yuan, which is difficult to implement. Furthermore, the volumetric slurry pump has strict requirements on the particle size of the inlet slurry, resulting in high maintenance costs. Therefore, this option is not recommended. c. Adopting an ash and slag separation – thickening tank – centrifugal ash and slag pump scheme. This scheme utilizes the existing three ash conveying pipes (DN500), requiring less modification work and reducing initial investment. The ash and slag flow rate of the #1 and #8 boilers after thickening is approximately 850 m³/h (calculated with an ash-to-water ratio of 1.6.6). Using one existing pipe, the flow velocity within the pipe is 1.2 m/s. For conveying such ash and slag slurry, a centrifugal ash and slag pump is perfectly adequate; therefore, this scheme is recommended. 1.3 Effect of Ash Removal System Technical Modification The Matou Power Plant tested and recorded various technical indicators, and the results were satisfactory. The main features of this renovation are as follows: a. Significant water-saving effect, with substantial economic and social benefits. Nearly 20 million tons of water are saved annually, resulting in an annual increase in revenue and savings of over 10 million yuan. In addition, 14 million yuan was saved in infrastructure costs for Unit #8. The planned investment for this project was 32 million yuan, with an actual investment of 18 million yuan, meaning the entire investment can be recovered within two years. b. A reasonable water-cement ratio was selected, and the existing ash conveying pipelines were utilized, reducing engineering costs. c. Low-concentration ash slurry was separated by equipment and facilities in the ash separation station, increasing the ash slurry concentration and reducing the flow rate of ash slurry discharged to the ash storage yard, achieving the goals of water conservation and reducing slag wear on the pipelines. The slurry flow velocity inside the pipe decreased from the original 2.3 m/s (excluding the ash slurry flow rate of Unit #8) to 1.2 m/s (including the ash slurry flow rate of Unit #8). The ash conveying pipeline configuration changed from two operating and one standby to one operating and two standby. d. The design employs a cenosphere drainage pipe to automatically collect cenospheres distributed in two thickening tanks into a cenosphere retrieval pool at 0 m above ground. Compared to other power plants in China equipped with thickening tanks, this design reduces worker workload, improves working conditions, reduces environmental pollution, increases cenosphere collection rate, and, most importantly, provides sufficient guarantees for personal safety and civilized production. e. Simplified system for cost savings and efficiency. The design replaces the four-point water discharge ring pipe with a single-point direct discharge system. The modified system simplifies the system, saves approximately 230 m of 630 mm × 8 mm welded steel pipe, and saves approximately 273,000 yuan in material and installation costs. It also reduces construction difficulty, workload, and accelerates construction progress. f. Selecting suitable equipment ensures safety. Extensive research was conducted on vibrating screens. Based on the selection of a new multi-stage inertial vibrating screen, partial modifications were made to the diversion section before the screen inlet and the first-stage screen surface. Practice has shown that the modified vibrating screen, compared to similar models, improves operational stability, reliability, and service life. g. The system layout is compact and reasonable, requiring minimal land area. For the 1050 MW Matou Power Plant, the ash and slag removal site occupies only 1 hm2, which is more than 0.267 hm2 less than other similar power plants. h. The thickener adopts advanced anti-leakage measures, ensuring no leakage at the bottom and guaranteeing the safe use of facilities located at the bottom of the thickener (such as transformers, printing plants, and garages). 1.4 Other Applications In 2001, the research results on water-saving technology for the ash removal system were applied to Xingtai Power Generation Co., Ltd., reducing the water-ash ratio from 1.13 to 1:3, saving 13.84 million tons of groundwater and 16.608 million yuan in water costs annually. Simultaneously, 5000 tons of cenospheres and 250,000 tons of slag can be extracted, generating over 18.6 million yuan in annual benefits. This not only achieves the goals of water and electricity conservation but also protects the environment through the comprehensive utilization of ash and slag. 2. Research and Application of Urban Wastewater as Makeup Water for Circulating Water System 2.1 Research Background The research on using urban wastewater as makeup water for the circulating water system was applied to the Handan Thermal Power Plant. With the increase in industrial heat load and the decommissioning of some small units, the Handan Thermal Power Plant began a feasibility study for a large-scale replacement of small units technical upgrade project in 1993. In 1994, the project was approved by the higher authorities to construct two 200 MW extraction condensing units, equipped with two 670 t/h ultra-high pressure natural circulation coal-fired boilers. The Fuyang River was used as the primary source of makeup water for the circulating water system. However, since the flow rate of the Fuyang River could not meet 97% of the water demand after the large-scale replacement of small units, and groundwater extraction was prohibited, the secondary treated effluent from the Handan East Wastewater Treatment Plant was used as the secondary source of makeup water for the power plant's circulating water system. 2.2 Investigation and Testing of Water Quality Change Patterns Using urban sewage as makeup water for power plant circulating cooling water is still in the experimental research stage in my country's power system. Although many power plants have designed their circulating cooling water makeup water sources according to urban sewage in recent years, there is no precedent for a power plant operating normally. Currently, my country has not fully grasped the composition and water quality change patterns of urban sewage, especially the polluting items. In addition, the Fuyang River is also polluted by upstream industrial and domestic sewage, and the pollution level is quite serious. Its COD, free NH3, and unidentified heavy metals and phosphorus content are all high, and the salt content and organic matter content are similar to those of sewage treatment plant effluent. Suspended solids and color are even worse than those of sewage treatment plant effluent. However, sewage treatment plant effluent has a large volume and relatively stable water quality. Sewage reuse and multiple uses of water are the only way to save water and are also the development direction of industrial water sources. Therefore, the circulating water treatment system of Handan Thermal Power Plant is based on using polluted water and requires operation with both types of water. The technical solution should focus on the treatment of urban sewage to ensure the power plant is prepared for any eventuality and can operate safely and reliably in the long term. To ensure the safe operation of the unit, based on the water quality characteristics of the two water sources and the unit's requirements for circulating cooling water quality, data collection and investigation of water pollution sources and water quality change patterns of urban sewage and river water were conducted, along with one year of raw water quality testing. Through analysis and research of the water sources, combined with domestic and international sewage treatment experience, several advanced sewage treatment schemes were determined. In 1996, the Hebei Provincial Power Company invited renowned domestic water treatment experts to conduct three special demonstrations of the schemes. On this basis, a preliminary circulating water makeup water treatment scheme was drafted. Then, dynamic simulation tests were conducted to optimize the water treatment process, equipment, chemicals, and condenser pipes, especially for bactericides, algaecides, and clarifier filtration equipment, optimizing various equipment and operating conditions. Because organic pollutants, bacteria, algae, and colloidal substances in wastewater can grow, reproduce, and accumulate in the system to form organic sludge, directly corroding metals or producing corrosive substances after decomposition. These substances exist in the water as dissolved or insoluble colloids and suspended particles. At this point, the suspended solids in the water are no longer suspended solids in the general sense, making the removal of these organic suspended solids more difficult and the treatment effect less stable. Therefore, the optimization of dynamic simulation experiments is crucial for determining the circulating makeup water treatment scheme. The following dynamic simulation experiments were conducted: a. Water quality testing of two water sources (1a). b. Screening test for circulating water scale inhibitors. c. Selection test for bactericides. d. Selection and corrosion resistance test of condenser pipe materials. e. Sludge test in the circulating water system. f. Selection test of the circulating makeup water lime treatment process system. g. Comprehensive evaluation test of the circulating water system. 2.3 Determination of the Circulating Water Treatment System Through investigation and special demonstrations, referring to foreign experience in wastewater reuse treatment, and optimizing water treatment schemes, equipment, and dynamic simulation test data of the water treatment system, a lime treatment system was finally selected. Therefore, the circulating makeup water treatment scheme was determined to be a sterilization, lime, coagulation, clarification, acid addition, and filtration system. The specific process is as follows: Urban sewage → Sewage tank (or Fuyang River water) → Booster pump → Sodium hypochlorite sterilization → Mechanically accelerated sludge suspension clarification tank (high-purity lime slurry, polyferric iron, and coagulant are added to the tank) → Acid addition → Variable porosity filter → Clear water tank → Circulating water makeup pump → Circulating water makeup tank → Circulating water system. The circulating cooling water treatment adopts acid addition, water quality stabilizer addition, and bactericide addition, with a circulating cooling water concentration ratio of 2-2.5. 2.4 Operational Effect of the Circulating Water Treatment System Since its commissioning and operation began in September 1998, it has been running for approximately 3 years. The operational results show that the system design is reasonable, the equipment operates normally, and the effluent quality is stable, basically meeting the design requirements. The design of the clarifier, the setting of the sludge layer, the determination of the reagent addition location, and the optimization of the filter media particle size and specific gravity are correct and reasonable. The actual effluent water quality indicators achieved are: pH value 8-9, suspended solids 2-10 mg/L, Ca²⁺ 30-150 mg/L, Cl⁻ ≤150 mg/L, PO₄²⁻ <0.5 mg/L, ammonia nitrogen <0.1 mg/L, SO₄²⁻ ≤300 mg/L, COD <1 mg/L. From the above data, it can be seen that the actual effluent water quality indicators of the designed circulating water treatment system basically meet the water quality standards for circulating water treatment, with a turbidity removal rate of over 90%, a COD removal rate of 50%, and a total phosphorus removal rate ≥95%. The conclusions are as follows. a. The operational results show that it is feasible to use urban sewage as makeup water for the power plant's circulating water after secondary sewage treatment, sterilization, coagulation, lime clarification, and deep filtration. The system operates stably and reliably, and the effluent quality meets the standards for reuse. b. The selection of mechanical accelerated clarification tanks and variable porosity filters, as well as the selection and addition methods of coagulants and scale inhibitors, should be determined by dynamic simulation tests. There should be more than two locations for adding bactericides. c. The reuse of urban sewage opens up new avenues for multiple uses of water and reduction of environmental pollution. d. The use of lime treatment in the deep treatment system for urban sewage is technically feasible, with relatively low operating costs and no new environmental pollution. Therefore, using urban sewage as makeup water for the Handan Thermal Power Plant's circulating water has significant economic, environmental, and social benefits. 3 Research and Application of Zero Discharge Technology for Wastewater from Large Thermal Power Plants 3.1 Research Project Background The research on zero discharge technology for wastewater, applied to Xibaipo Power Generation Co., Ltd. (4×300 MW units), is one of the major scientific and technological research projects of the State Power Corporation. According to the original design of the first phase of the project, industrial wastewater was to be discharged into Huangbizhuang Reservoir after reaching the Class III surface water standard. However, with the increasing demand for water from industry, agriculture, and population development, Huangbizhuang Reservoir was changed from an agricultural irrigation source to a source of drinking water for the city. The Hebei Provincial and Shijiazhuang Municipal Environmental Protection Bureaus issued documents prohibiting the power plant from continuing to discharge industrial wastewater into Huangbizhuang Reservoir. Therefore, in the design of the second phase of the project, it is necessary to study technologies for wastewater treatment, multiple uses of water, and recycling to achieve zero wastewater discharge. 3.2 Formulation and Optimization of Technical Solutions 3.2.1 Formulation of Water Balance Accurate water balance is key to achieving zero discharge from the power plant. Therefore, before determining the wastewater treatment plan, it is essential to understand the water volume and quality of the plant's supply and drainage to facilitate the reuse of treated wastewater. Through investigation and actual testing of the power plant, a relatively realistic water balance was established, laying the foundation for determining the wastewater treatment plan. 3.2.2 Optimization of Wastewater Treatment Scheme Based on water balance and investigation, four schemes were selected: lime + reverse osmosis, high concentration ratio 4; weak acid + reverse osmosis, high concentration ratio 4; lime + reverse osmosis, 2 low and 2 high; weak acid + reverse osmosis, 2 low and 2 high. The design concept of the first two schemes is to ensure that the circulating cooling water of units #1 to #4 operates at a high concentration ratio. The circulating wastewater is treated by reverse osmosis and then supplied to the boiler feedwater. The wastewater within the system is used for ash removal. Its advantages are: high circulating water concentration ratio, convenient water saving; good effluent quality; reverse osmosis effluent replaces deep well water; similar lime and weak acid systems have operational experience in China. Its disadvantages are: large site occupation; large circulating water treatment capacity, high annual operating cost; large infrastructure investment; reverse osmosis treatment requires testing. The design concept of the latter two schemes is as follows: To reduce the number of treatment equipment and infrastructure investment costs, the treatment scheme adopts two concentration ratios for the circulating cooling water. Specifically, units #1 and #2 operate at a low concentration ratio, and their circulating wastewater is treated and supplied to the circulating makeup water of units #3 and #4. Units #3 and #4 operate at a high concentration ratio for the circulating cooling water, and their circulating wastewater is used for ash removal. The advantages are: low infrastructure investment and small footprint; small circulating water treatment capacity and low operating costs; high circulating water concentration ratio, resulting in significant water savings; and the low-concentration circulating wastewater can be discharged for agricultural irrigation, with the design already considering necessary discharge measures for agricultural irrigation. The disadvantages are: there are no similar systems operating in China; and the treatment effect requires dynamic simulation testing. After review by the Hebei Provincial Power Company, the two-low-two-high scheme was approved. Subsequently, a thorough investigation and further technical and economic comparison were conducted on the utilization of weak acid, lime, and lime sludge.