Study of the settling charactristics of fly ash-water slurry and designing of a settling pond

Abstract

Fly ash is a very fine material which is produced by burning of pulverized coal in boilers of thermal power plants. Worldwide, more than 65% of fly ash produced from coal power stations is disposed off in landfills and ash ponds. The fly ash is sent to ash ponds in the form of slurry with water since it is economical. This fly ash being finer and lighter than river sand has lower settlement rate, which can be increased by adding a suitable polymer to the ash slurry in the pond. It is desired that the rate of of settling is fast, so that the water can be easily drained out form the ash pond. If the water height is built up for a long period of time, then it will result in the building up off the hydrostatic pressure which may damage the pond and lead to leakage of fly ash-water slurry from the pond causing various industrial hazards.
The objective of this report is to provide a detailed study of the settling rates of fly ash in ash pond for polymer (carboxy methyl cellulose) added, and at different concentration levels. This report also aims at suggesting the aspects to be considered while constructing an ash pond for the fly ash disposal. The turbidity of the fly ash-water slurry along with the polymer mixed to it is studied at some specific time intervals with the help of a Nephelo turbidity meter to determine the rate of decrease of turbidity of the clear liquid at the top of the fly ash-water slurry. This helps in determining the optimum concentration of polymer dosage for faster settlement of fly ash. From the experiments conducted and results obtained, it is concluded that the optimum concentration of the polymer solution to be added is 2ppm (2ml of 10-2 range polymer solution). The gravitational settling rate of the fly ash is calculated after the addition of the above concentration of the polymer. It is concluded that the settling rate of fly ash in ash pond of thermal power plants can be increased by the addition of the polymer of optimum concentration.