Application of Water Treatment Technology in Circulating Water Systems of Thermal Power Plants

Abstract: Dushanzi Thermal Power Plant has three generating units: 25MW, 25MW, and 50MW, with a total generating capacity of 100MW. It has three hyperbolic natural draft cooling towers with a total circulating water volume of 10,300 m³/h and a holding capacity of 11,000 m³. Since its commissioning, these towers have remained untreated and are connected to a fishpond, resulting in severe corrosion and biofouling problems. Corrosion causes up to 200 condenser copper tube leaks annually. Due to biofouling, cleaning with rubber balls is required quarterly, sometimes necessitating high-pressure water jetting, significantly increasing maintenance costs. Because of insufficient cooling, water users add raw water for cooling during hot weather, increasing water consumption. To address these issues, we conducted a comprehensive survey and adopted a water treatment solution involving disconnecting the fishpond and chemical dosing. This improved the vacuum level of the turbine condenser and the utilization rate of water resources, achieving economical power generation.

Keywords: Thermal power plant, Circulating water, Water treatment technology

1. Introduction

Dushanzi Thermal Power Plant has three generating units: 25MW, 25MW, and 50MW, with a total generating capacity of 100MW. It has three hyperbolic natural draft cooling towers with a total circulating water volume of 10,300 m³/h and a water holding capacity of 11,000 m³. Since its commissioning, no water treatment has been carried out. Furthermore, it is connected to a fishpond, resulting in serious corrosion and biological sludge problems. Corrosion causes up to 200 copper tube leaks in the condenser annually. Due to biological sludge, cleaning with rubber balls is required every quarter, sometimes necessitating high-pressure water jetting, significantly increasing maintenance costs. Because of insufficient cooling, water users add raw water for cooling during hot weather, increasing water consumption. To address these problems, we conducted a comprehensive survey and adopted a water treatment solution involving disconnecting the fishpond and chemical dosing. This improved the vacuum level of the turbine condenser and the utilization rate of water resources, achieving economical power generation.

2. Overview of the circulating water system in the thermal power plant

The operating parameters of the circulating water system in the thermal power plant are shown in Table 1.

Table 1 Operating parameters of the circulating water system in the thermal power plant

3. Water Treatment Technology Solution

3.1 Sterilization and Stripping Cleaning

The purpose of sterilization and stripping is to remove slime and deposits adhering to the system, cutting off their barrier effect on the chemicals and maximizing their corrosion and scale inhibition effects.

A. Lower the water level in the collection tank to the minimum safe level to save on chemical usage.

B. Add 400mg/L of slime stripping agent for sterilization and stripping.

C. Observe the removal of slime, bacteria, and algae from the water distribution device at the top of the cooling tower and the inner wall of the tower, the easing of blockage in the outlet holes, and the disappearance of green algae on the inner wall of the tower. Test the change in turbidity of the circulating water. If the turbidity remains unchanged for 2-4 hours, the sterilization and stripping can be completed. The makeup water and drain valves can be opened to replace and discharge the wastewater.

Test items: Turbidity, once every 2 hours; pH value, once per hour.

3.2 Normal Operation Dosing Scheme

(1) Scale and Corrosion Inhibitor: DL-6, dosage concentration 20 mg/L. After the initial dosing, the scale and corrosion inhibitor should be continuously and evenly added to the system using a dosing device to maintain a stable concentration. Large fluctuations in concentration are detrimental to the operation of the circulating water system; low concentrations affect the effectiveness of the agent, while high concentrations waste the agent.

(2) Bactericide: Non-oxidizing and oxidizing bactericides are used alternately.

Non-oxidizing bactericide: Dosing once a month at a concentration of 50 mg/L.

Oxidizing bactericide: Dosing once a day at a concentration of 50 mg/L.

3.3 Water Quality Control Indicators and Analysis Frequency

Table 2 Water Quality Control Indicators and Analysis Frequency

4. Application Effect of Water Treatment Technology

After sterilization, stripping, and cleaning, the circulating water system of the thermal power plant was normally treated with water treatment agents. The system is currently operating well, and the various analytical monitoring data of the circulating water quality are basically controlled within the target range. The concentration ratio of the circulating water system has increased ideally to approximately 5-6. The Corrosion Prevention Center of the Research Institute conducted system corrosion rate monitoring, and the monitoring results are shown in Table 3. The monitoring results show that the circulating water system of the thermal power plant is operating well, and the brass corrosion rate is within acceptable limits.

Table 3 System Corrosion Rate Monitoring (1)

Overall, in the nearly four months since the application of circulating water treatment technology in the power plant, all operational indicators and monitoring data have met or exceeded the national standard GBJ50-83 and the industrial water management system for oil refining and chemical enterprises of China National Petroleum Corporation.

5. Economic Benefit Assessment

5.1 Reduced Coal Consumption

Based on the equivalent heat drop theory, it can be calculated that for every 1% increase in vacuum, turbine efficiency increases by 1%, and overall plant efficiency increases by 1%. As shown in the aforementioned data, the vacuum in September and October this year was -7 kPa higher than the average of -82 kPa in the same period last year, representing an 8.5% increase in vacuum degree. The average annual increase is expected to be over 10.25%. Therefore, both turbine efficiency and overall plant efficiency will increase by 10.25%. Given that the power plant's annual coal consumption is over 50 million yuan, the resulting reduction in coal consumption = 50 million × 10.25% = 5.125 million yuan.

5.2 Savings in New Water Volume and Reduction in Wastewater Discharge

After the application of water treatment technology in the power plant's circulating water system, the concentration ratio was significantly improved, increasing from 2 to 4. As shown in Table 4, the makeup water volume decreased from 276 m³/h to 166 m³/h, saving 110 m³/h. Based on 8,000 hours per year, this translates to a water saving of 88 × 10⁴ m³, equivalent to 792,000 yuan. The wastewater discharge decreased from 138 m³/h to 28 m³/h, reducing wastewater discharge by 110 m³/h. Based on 8,000 hours per year, this reduces wastewater discharge by 880,000 m³, equivalent to 2.64 million yuan in wastewater discharge fees. However, not all discharged water is sent to sewers; some is used for watering trees, ash removal, etc., so this portion of water costs should be deducted from the overall benefits.

Table 4: Relationship between Concentration Ratio and Makeup Water/Wastewater Discharge (Circulating Water Volume: 10,000 m³/h)

5.3 Savings in Maintenance Costs

After chemical treatment, the corrosion rate can be reduced to 0.005 mm/a (copper pipe), thus extending the life of the copper pipe by more than 10 years. This can save 100,000 yuan in cleaning costs annually. Due to reduced copper pipe leakage, condensate pollution is also avoided, saving 50,000 yuan in condensate costs.

6. Existing Problems and Suggestions

Although the application of water treatment technology in the circulating water system of thermal power plants has achieved good results, there are still many problems. In terms of hardware, there is a lack of side-stream filtration devices and monitoring heat exchangers; in terms of software, there is a lack of a comprehensive management and supervision system, such as the absence of equipment overhaul ledgers, records of equipment corrosion conditions, and effective supervision and inspection of water treatment agents. In response to these issues, it is recommended that thermal power plants:

(1) Add side-stream filtration devices to create conditions for further improving water treatment effects. Adding side-stream filtration devices requires a large initial investment, but the operating costs are low and the treatment effect is good. (1) By allowing 2-5% of the circulating water to flow through a side-filter on the reuse pipe, suspended algae, microbial remains, and scale in the circulating water can be effectively removed, reducing turbidity.

(2) Improve monitoring methods. During major overhauls, appropriate locations should be selected to install monitoring heat exchangers and monitoring finned devices to accurately and promptly understand the treatment effect of the circulating cooling water and ensure the normal and safe operation of the condenser.

(3) Establish an equipment overhaul log, collect scale samples, perform scale composition analysis, and record the equipment corrosion status to provide a basis for adjusting the water treatment formula.

(4) Strengthen the supervision and inspection of water treatment agents. In cooling water systems with copper equipment, BTA in water treatment agents protects not only copper equipment but also carbon steel equipment. This is because once copper equipment corrodes, copper ions in the water will displace iron, causing copper deposition and resulting in crevice corrosion and pitting corrosion of carbon steel. Therefore, the BTA content in water treatment agents is crucial. Strict quality control is required, and each batch of agents entering the plant should be subject to quality monitoring and performance evaluation to ensure high-quality water treatment agents.

References

[1] Cooling Water Analysis and Test Methods. Production and Development Department, China Petroleum & Chemical Corporation, 1990, 440.

Author Biography: Lü Hui, female, born in May 1972, engineer at the Corrosion Prevention Research Center, Research Institute of Dushanzi Petrochemical Company, China Petroleum & Chemical Corporation, mainly engaged in water treatment technology research. Address: Corrosion Prevention Research Center, Research Institute, Dushanzi Petrochemical Company, Xinjiang, Postcode: 833600, Tel: 0992-3869975, Email: yjy_lh@petrochina.com.cn