Use of chlorine for control of biological fouling in power plant cooling water system is the most effective way of controlling bio-fouling at reasonable concentration as chlorine is available at a low cost. Chlorine Gas feeders are simple and easy to control at low capital cost and requires minimum maintenance. The chlorine residual for shock dosing can be relatively short and in expensive. De-toxification (dechlorination) systems can be installed at a very nominal cost to meet the environmental requirement.
Total residual chlorine (TRC) as expressed in red book of United States of Environmental Protection Agency (EPA) specifies 0.2 mg/litres TRC. It should not exceed 160 minutes for 24 hours.
Point of chlorine dosing in Sea Water
Normally the sea water for power plant is taken deep in the sea at Sea water intake head. The sea water travels through intake pipe and reaches the CW pump sump from where the water is pumped to the condenser.
Continuous and Shock dosing chlorination
In Sea water application both continuous and shock dosing chlorination are applied for effective bio fouling.
At sea water intake head of inlet pipe, continuous dosing of approx. 1 - 2 ppm will be effective in control of Mollusks, algae, slime and weed. They constrict the flow of Sea water in the intake pipe. It also prevents sea shell deposition in the pipeline which is a most trouble-some to remove.
Continuous chlorine dosing is done near to the pump house ahead of bar racks and traveling water screen. 1-2 ppm is recommended so that the complete bar racks and traveling water screen can be kept clean and free from any bio fouling. This will also ensure the pump sump and pump suction side free from any slime growth. The free residual chlorine and the condenser outlet can be measured to determine the chlorine dosing as the chlorine demand of sea water varies according to the season and also the quality of the sea water.
Shock dosing chlorination - Dosing point No.3
Shock dosing of chlorination is done effectively at 1 - 5 ppm depending upon the quality of sea water. The recommended dosing time is one cycle time for every 8 hours shift which would be sufficient to keep the condenser cooling surface free from bio fouling and maintain the efficiency of the condenser and power generation.
Theory of Sea Water chlorination
In Sea water at pH 7.4 - 8.1 less than 50% of chlorine is available as fast acting Hypochlorous acid. However Sea water also contains 60 mg/litre (ppm) bromide ion which displaces the chlorine, being a stronger oxidizing agent to produce hypobromous acid.
Cl2 + H20 --------- HOCL + H+ + Cl-
HOCL + Br ---------- HOBr + Cl-
When chlorine is dissolved in water, it hydrolyzes rapidly according to the above equation. The rapidity of the reaction can be expressed that the chlorine molecule reacting with hydroxyl ion rather than with the water module.
Cl2 + OH- ------- HOCL + Cl