Industrial enterprises are big users of water, and most of them use water for process cooling. Taking thermal power plants as an example, the water consumption of circulating cooling water system accounts for almost more than 90% of the total water consumption of thermal power plants. At present, more and more circulating water systems of wet cooling thermal power units use reclaimed water and other water sources as supplementary water, so the risk of circulating water pollution also increases.
After circulating water is polluted, improper treatment will lead to corrosion and scaling of condensers and various heat exchangers. Corrosion of condenser will cause perforation and cracking of cooling water pipe, increase maintenance times of equipment, shorten service life of equipment and increase power generation cost. However, the scaling of condenser will reduce the heat exchange efficiency of heat exchanger and increase the energy consumption of power generation. If it is necessary to stop and clean the heat exchanger, the economic loss will be greater. Therefore, it is very important to adopt quick and effective coping strategies to deal with the pollution of circulating water system.
1 General situation of unit and circulating water system
In a thermal power plant, two 300 MW subcritical coal-fired self-provided wet cooling generator sets provide power and process steam for aluminum smelter. The circulating water system uses treated pharmaceutical wastewater reuse water, surface water and steam condensate (hereinafter referred to as condensate) of aluminum smelter as supplementary water sources. The make-up water of each unit is about 500 t/h, of which the condensed water is about 80 t/h, and the rest is treated wastewater reuse water and surface water. See table 1 for the water quality of each make-up water of circulating water.
From the table 1, it can be seen that the water quality of condensate is better than the other two water sources, but the condensate is easily polluted by some technological processes in aluminum electrolysis plant. According to the material of each heat exchanger (see Table 2), the circulating cooling water is treated by adding sulfuric acid, water stabilizer and bactericide.
2 Circulating water pollution situation
On March 6th, 20 18, it was found that the circulating water of No.2 water tower was abnormal, and the turbidity was grayish white, while the water quality of No.2 water tower 1 was normal. According to the analysis of the appearance of circulating water and the source of make-up water, it is considered that the most likely reason is that the polluted condensate from aluminum smelter is supplied to the No.2 water tower, which leads to the pollution of the circulating water in the whole No.2 water tower. The feedback from the aluminum factory also verified the above analysis.
The water samples of circulating water and condensed water of No.2 water tower were analyzed, and it was found that the phenolphthalein alkalinity of condensed water exceeded 30 mmol/L, and the main control indexes of circulating water quality were abnormal (see Table 3), indicating that the circulating water of No.2 water tower was polluted by strong alkaline substances. COD and ammonia nitrogen in circulating water did not exceed the requirements of the Integrated Wastewater Discharge Standard (GB 8978— 1996), which were 36.8 mg/L and 0. 1 mg/L respectively.
3 Response and treatment of circulating water pollution
While cutting off the pollution sources, corresponding measures were taken according to the alkaline pollution sources and the quality of circulating water. (1) Increase the addition of sulfuric acid and slow-release scale inhibitor to reduce the scaling risk of circulating water; (2) Increase the sewage discharge and water replenishment of No.2 water tower, and replace and discharge the polluted water as soon as possible; (3) In order to avoid the deposition of suspended solids in circulating water in the condenser tube, causing condenser scaling and corrosion under scale, the rubber ball cleaning system of the condenser of No.2 water tower runs continuously.
On March 7th, the main water quality indexes of circulating water in No.2 water tower were analyzed (see Table 4). When the total alkalinity drops to 6.7 mmol/L, the water quality is basically normal, and the appearance of circulating water in No.2 water tower gradually returns to normal.