Cooling Towers/Precipitators

Cooling Towers

In thermal power stations, heat is used to produce steam which drives the turbine, which in turn is connected to and drives the generator. An important factor in power station design is the disposal of the heat which cannot be converted into electrical energy or otherwise used in the steam cycle. Some surplus heat is in the form of hot air which naturally rises to higher altitudes where it cools and recirculates back to ground level, but, most surplus heat is in the form of hot water. This is because water is used to cool equipment and also to cool exhaust steam from the turbine in the steam condenser where the steam must be converted back to water before it can again be used in the steam cycle.

The heat trapped in the cooling water must be removed before the cooling water can be reused in the power station or discharged to local rivers and streams. In the Latrobe Valley two methods are used to remove the waste heat from the cooling water. These are cooling ponds and cooling towers.

A cooling tower is a structure designed to induce a flow of cool air to remove surplus heat from the water being circulated through the tower. In the older power stations, cooling towers, built from wood, had fans installed to create a flow of air up past the falling water. With the advent of Yallourn W and Loy Yang Power Stations, natural draft cooling towers, built from concrete, were used and while the structure is different, the method of cooling the water is the same. Cool air entering the bottom of the tower absorbs heat as it flows past the falling hot water droplets, becomes less dense and rises up through the tower carrying with it some water vapour.

The water becomes heated to a temperature of 36 degrees Celsius as it passes through the turbine steam condensers and is piped to a point about 12 metres up inside the cooling tower. The hot water is then distributed to 2500 spray nozzles which breaks the water flow into millions of droplets to provide a large surface area for the air to come in contact with as the water falls into the pond below. In this cooling process, the water is cooled partly by the circulation of air through the water and partly by evaporation.

ELECTROSTATIC DUST PRECIPITATORS

When any fuel is burnt, there are by-products of combustion which must be either removed from the flue gases or neutralised in some way so they will not have any detrimental effect on the environment. The brown coal fired power stations is the Latrobe Valley use over 55 million tonnes of coal each year and after this is burnt, there are many by-products which must be considered. These by-products fall into two main areas – the solids or ash and gaseous by-products.

Most of the minerals and trace elements present in the coal are trapped in the ash and unburnt fuel solids. These amount to approximately 3% of the total amount of coal supplied to the power station. If this ash is not removed from the flue gas or boiler smoke, it would be discharged to atmosphere and become a dust problem in the local area.

Early power stations in the Latrobe Valley (Yallourn A & B) had only partially effective mechanical means of removing dust from the smoke so dust in the plumes was very noticeable. However, the newer stations have electrostatic precipitators to remove the dust from the boiler smoke.

But to keep the problem in perspective, we should remember that every 10 kg of coal from Hazelwood mine (for example) contains 6.05 litres of moisture, and only 158 grams of ash and can produce 7.4 kW of electricity. The moisture discharged into the air is not of major concern. Most effort is directed to containing or controlling the small amount of ash and dust produced in the combustion process.

HOW AN ELECTROSTATIC DUST PRECIPITATOR WORKS

If a comb is run through dry hair, it becomes sufficiently charged with static electricity to attract and hold small pieces of dust and/or paper. In an electrostatic precipitator, the dust particles are themselves charged with electricity and are the attracted to the collecting plates, in a similar way to which the dust us attracted to the comb or iron filings are attracted to a magnet.

Once on the collecting plate, the dust gives up its electrostatic charge and when the plate is hit or rapped with a mechanically driven hammer, the collecting plate vibrates and the dust slides down the plate to the collecting hopper underneath.