Article: Selection of Treatment Solutions for Circulating Cooling Water in Thermal Power Plants

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Selection of treatment schemes for circulating cooling water in thermal power plants Source: China Environmental Protection Network News In large-scale thermal power plants where the quality of circulating water is poor, the amount of wastewater generated from circulating cooling water is high； When the ash removal system is of the dry type, a large amount of circulating cooling water wastewater cannot be reused. To reduce the discharge volume from the circulating cooling water system and save water, it is necessary to appropriately increase the concentration ratio of the circulating water. However, if the quality of the make-up water is poor and a high concentration ratio for the circulating water is required, then the make-up water used for the circulating water must be treated. Currently, there are various treatment methods, each with its own advantages and disadvantages. Based on the detailed technical and economic comparisons, in large-capacity thermal power plants where the quality of circulating water is poor, meaning that the levels of HCO32-, Ca2+, and SO42- are high, a \"combined treatment system using partial weak acid resins and stabilizers\" is suitable for treating the water used to replenish the circulating water. 　　Taking the design water volume and water quality of a 2×2023 t/h boiler and 2×600 MW condensing steam turbine generators (with positive pressure pneumatic ash removal) as examples, this is illustrated through calculations and economic and technical comparisons. 1 Design flow rate of circulating water and quality of its feed water 1.1 Design flow rate of circulating water 　　 See Table 1. Table 1 Design water volume for circulating water and various loss amounts
Item | Water volume/(t·h-1) | Loss rate (%)
---|---|---
Total circulating water volume | 140,000 |
Water replenishment for circulating water | 2,559 | 1.75
Evaporation loss (average) | 2,069 | 1.478
Loss due to wind | 140 | 0.10
Wastewater discharge (average) | 350 | 0.25
Concentration ratio | 5 | 1.2
Design water quality for circulating water replenishment: See Table 2. Table 2 Design water quality for circulating water make-up water
Analysis parameter Quality concentration/(mg·L⁻¹) Concentration/(mmol·L⁻¹)
Cations Na+ 12.2 0.53 K+ 3.0 0.08 Ca²⁺ 90.2 4.51 Mg²⁺ 20.7 1.73
Anions OH⁻ 0 0 CO₃²⁻ 0 0 HCO₃⁻ 237.3 3.89 Cl⁻ 13.6 0.38 SO₄²⁻ 108.0 2.25 NO₃⁻ 20.0 0.32
Hardness Total hardness 5.20 Non-carbonate hardness (permanent hardness) 2.31 Carbonate hardness (temporary hardness) 3.89
Acidity and alkalinity Total alkalinity 3.89 pH 7.79
Others Total solids 456.0 Dissolved solids 413.4 Suspended solids 42.6 Solute loss on ignition 136.8 Total silica (SiO₂) 8.0 Active silica (SiO₂) 6.5 Colloidal silica (SiO₂) 1.5 Chemical oxygen demand (CODMn) 0.9 Conductivity (25°C), μS/cm Free CO₂ 6.6

2 Selection of design scheme
This system uses dry ash removal, and the reuse rate of wastewater from circulating water is very low. To save water and energy, it is necessary for the circulating water to operate at a high concentration ratio. 　　At a concentration ratio of 5, the calculated Langley saturation index for the circulating water was 3.74, and the Reznor stability index was 1.58. It can be determined that the circulating water is in a severe scaling condition. Therefore, the make-up water for the circulating water must be properly treated to keep the condenser in good operating condition. 2.1 Common scale prevention methods ① Simple sulfuric acid treatment: Adding H2SO4 to the make-up water for circulating water, using H2SO4 to neutralize the alkalinity in the water in order to ensure the stable operation of the circulating water. 　　② Treatment with sulfuric acid and stabilizers: First, a certain amount of H2SO4 is added to the make-up water for circulating water to reduce its alkalinity to a specified level; then, water quality stabilizers are used to ensure the stable operation of the circulating water. ? ③ Lime treatment: Lime slurry is added to the clarifier to reduce the alkalinity and carbonate hardness in the water, thereby preventing the formation of CaCO3 scale. 　　④ Full weak acid resin treatment: Utilizes weak acid cation exchange resins to remove carbonate hardness and part of the alkalinity from water. It reduces the hardness and alkalinity in circulating water. Corrosion inhibitors are then used to prevent corrosion in the circulating water system. 　　⑤Treatment with weak acid resin for a portion of the water: A portion of the make-up water is treated with weak acid resin, and stabilizers are added to prevent scaling caused by carbonates in the circulating water. This method is also known as the “combined treatment system using partial weak acid resins and stabilizers”. ? 2.2 Comparison of Common Scale Prevention Methods For methods ① and ②, since the environmental category of cooling towers in most projects is Category I, an SO42- content of up to 500 mg/L is allowed in the circulating water; exceeding this level causes moderate corrosion to the concrete. It is generally advisable for this system to keep the SO42- content in the circulating water below 1500 mg/L. The SO42- content in the make-up water is 108.0 mg/L. Calculations show that if methods ① and ② are used, the SO42- content in the circulating water will exceed 1500 mg/L; systems with high SO42- content in their make-up water are not suitable for the \"acid addition treatment\" approach. 　　For Option 3, due to the complexity of the system, poor operating conditions, and the tendency for pipes to get clogged, it is difficult to achieve automation of the metering system. Therefore, it is clearly unsuitable for large-capacity units with few operators and a high level of automation. 　　Therefore, the economically viable treatment options are only ④ the full weak acid resin treatment system and ⑤ the partial weak acid resin treatment system. 3 Determination of the weak acid resin treatment scheme 3.1 Full weak acid resin treatment system 3.1.1 Suitability for water quality Detailed calculations based on relevant data show that when the average calcium ion concentration in the effluent from the weak acid resin exchanger is controlled at 0.670 mmol/L, the average alkalinity is 0.17 mmol/L. If water with such quality is used as make-up water for circulating water, when the concentration ratio of the circulating water is 5, its Langley index is 0 and its Reznik index is 7.7. This indicates that the carbonates in the circulating water are in a stable state. 　　When the average alkalinity of the effluent from a weak acid ion exchanger is 0.17 mmol/L, calculations based on relevant data show that for a dual-flow weak acid bed with a resin packing height of 1000 for the upper layer and 1300 for the lower layer, the working exchange capacity of the upper layer resin is 2000 mol/m3 (for D113 resin). 　　The key issue is that, in actual operation, it is quite difficult to keep the circulating water in a state free from both scaling and corrosion. This can be seen from the control of the Langley index and the Rezner saturation index. Theoretically, when the Langley index is 0 and the Reznor index is 6.0, the circulating water is in a state free from both scaling and corrosion; however, it is difficult to achieve both of these conditions simultaneously. Furthermore, the ratio of all weak acid-treated water / {＋} (in mg/L) becomes far less than 1, which is not conducive to controlling pitting. Therefore, if water treated with all weak acids is used as make-up water for circulating water, it should be supplemented with corrosion inhibitors to control corrosion. Especially at the beginning of unit startup, when the circulating water concentration ratio is low, it is crucial to add corrosion inhibitors. 　　In summary, it is technically feasible to use water treated with weak acid resins as the make-up water for circulating water. As long as the control system maintains the water quality within appropriate limits, and corrosion is addressed through the use of commonly employed corrosion inhibitors, the circulating water can be kept in a stable operating condition. ?? 3.1.2 Comparison of Advantages and Disadvantages The advantage of using full softening treatment is that it removes not only the alkalinity from water but also a portion of the hardness, thereby fundamentally solving the problem of scaling. The system is simple, operates under favorable conditions, and is easy to automate and control. The disadvantages are high capital investment, high operating costs, and large land occupancy. Corrosion is likely to occur if not properly controlled. The large volume of wastewater discharge, at around 172 t/h, increases the burden on the wastewater treatment system and raises its costs. 3.2 Partially weak acid resin treatment systems 3.2.1 Suitability for water quality Based on water quality data and other relevant information. After detailed calculations, it was found that when 60% of the treated water containing weak acid resin is mixed with 40% of raw water, the quality of the resulting mixture is shown in Table 3. When this water is used as make-up water for circulating water, the Langley index of the circulating water is 2.37, the Reznik index is 3.81, the pH value is 8.5, and the carbonate hardness is 8.3 mmol/L. Based on the index, scaling in this water is relatively severe; however, stabilizers are currently being used in the power plant’s circulating water. It is capable of maintaining the carbonate hardness in the circulating water at above 9 mmol/L. Therefore, by adding the appropriate stabilizers, the circulating water can be kept in good operating condition, free from both scaling and corrosion. 　　Furthermore, this treatment method has less stringent requirements regarding the balance between scaling and corrosion in circulating water compared to full weak-acid treatment – it is not necessary to keep the Langley index of the circulating water at 0. The design control value for the Langley index is generally set at 2.5; in practice, the condition to prevent scaling requires a value higher than 2.5, thus allowing the water quality to fluctuate within a certain range. Furthermore, the pitting index /{＋｝ is much higher than that under all weak acid treatments, which is very beneficial for controlling pitting. 3.2.2 Comparison of Advantages and Disadvantages The advantage of using partial softening treatment is that it results in favorable operating conditions for the system and facilitates automation. The investment and floor space required are reduced by nearly 20% compared to full weak acid treatment systems, and operating costs are also lowered by about 34%. Water quality is easy to control during operation; the system is safe and reliable, with low wastewater discharge. The downside is that its operation is slightly more complex compared to full weak acid treatment. 3.3 Economic comparison of the two options The main economic indicators for the two options are shown in Table 4. Table 3 Comparison of Raw Water and Average Effluent Quality from Various Systems
Parameter | Raw Water Quality | Full Weak Acid Resin Treatment | 60% Weak Acid Resin Treatment | System Effluent Quality | Expected Circulating Water Quality | System Effluent Quality | Expected Circulating Water Quality
Na+/(mg.L-1) | 12.2 | 12.0 | 61 | 12.2 | 61
K+/(mg.L-1) | 3.0 | 3.0 | 15 | 3.0 | 15
Ca2+/(mg.L-1) | 90.2 | 13.4 | 67 | 44.2 | 221
Mg2+/(mg.L-1) | 20.7 | 20.7 | 103.5 | 20.7 | 103.05
Cl-/(mg.L-1) | 13.6 | 13.6 | 68 | 13.6 | 68
SO42-/(mg.L-1) | 108.0 | 108.0 | 540 | 108.0 | 540
NO3-/(mg.L-1) | 20.0 | 20.0 | 100 | 20.0 | 100
HCO3-/(mg.L-1) | 237.3 | 10.37 | 51.85 | 101.26 | 506.3
CO32-/(mg.L-1) | 0 | 0 | 0 | 0 | 0
OH-/(mg.L-1) | 0 | 0 | 0 | 0 | 0
CO2/(mg.L-1) | 6.6 | 163.68 | 1.5 | 98.12 | 1.5
SiO2/(mg.L-1) | 8.0 | 8.0 | 40 | 8.0 | 40
Total Solids/(mg.L-1) | 456.0 | 372.95 | 1047.85 | 429.08 | 1656.3
pH | 7.79 | — | 7.7 | — | 8.5

Evaluation of Circulating Water Stability
Saturation pH (pHs): 5.32 | 7.7 | 6.18
Langley Index: 3.74 | 0 | 2.37
Reznor Index: 1.58 | 7.0 | 3.81
Stability: Severe scaling; Corrosion even with slight fluctuations (anticorrosive agents required); Scaling (stabilizers required).

Table 4 Key Performance Indicators of the Two Treatment Methods
Parameter | Full Weak Acid Resin Treatment System | 60% Weak Acid Resin Treatment System
Floor Area/m³ | 1875 | 1500
Total Power Consumption of Softening Equipment/(t.h-1) | 2738 | 1643
Total System Investment/10,000 yuan | 1500 | 1200
Annual Depreciation Cost of Equipment/10,000 yuan | 30 | 24
Annual Chemical Consumption
98% Sulfuric Acid/t, 10,000 yuan | 3220 t, 193.2 million yuan | 1932 t, 115.92 million yuan
Water Stabilizer/t, 10,000 yuan | 45 t, 62.1 million yuan
Anticorrosive Agent/t, 10,000 yuan | 30 t, 70.8 million yuan
Annual Chemical Cost/10,000 yuan | 264 | 178.02
Annual Electricity Consumption/kWh | 728,000 | 436,800
Annual Electricity Cost/10,000 yuan | 15.29 | 9.17
Annual Resin Replacement Amount/t | 4.3 | 2.58
Annual Resin Cost/10,000 yuan | 12.9 | 7.74
Annual Increased Water Consumption/t | 879,000 | 527,400
Annual Increased Water Consumption and Treatment Cost/(10,000 yuan.a-1) | 70.32 | 42.19
Annual Total Operating Cost/10,000 yuan | 392.51 | 261.12

Waste Disposal
Wastewater Discharge Volume/(m³.d-1) | 4116 | 2470
Sludge Discharge Volume/t.d-1 | 14.87 (containing CaSO4 in wastewater) | 8.92 (containing CaSO4 in wastewater)
Concentration Ratio | 5 | 5 | 4

Conclusion
Based on the previous analyses as well as technical and economic comparisons, it can be seen that for the treatment of circulating cooling water in large-capacity thermal power plants where the levels of HCO32-, Ca2+, and SO42- in the make-up water are relatively high, a “combination treatment system using partial weak acid resins and stabilizers” is the appropriate approach for make-up water treatment. This treatment method features a high concentration ratio during operation, low waste discharge, easy control, and is safe and reliable ； It features low wastewater discharge, as well as low investment and operating costs. 　 This post was last edited by hesonchang214 on 2009-4-9 20:26.]