Layout of Steam Power Plant1

The working of a steam power plant can be explained under four main circuits. They are:I. Fuel (coal) and ash circuit,2. Air and flue gas circuit,3. Feed water and steam flow circuit, and4. Cooling water flow circuit.1. Fuel (coal) and Ash CircuitThis circuit includes coal delivery, preparation of coal handling to boiler furnace, ash handling and ash storage. The coal from coal mines is delivered by ships, trains or by trucks to power station. The coal is received in coal storage yard of power station. This coal is sized by crushers, breakers, etc. The sized coal is then stored in coal storage, called stock yard. From the stock yard, the coal is transferred to the boiler furnace by means of conveyors, elevators, etc.

The coal is burnt in the boiler furnace and ash is formed by burning of coal. Ash coming out of the furnace will be too hot, dusty and accompanied by some poisonous gases. The ash is collected and transferred to ash storage. Usually the ash is mixed with water to form slurry and disposed off by pumping.2. Air and Flue Gas CircuitThis circuit includes forced draught fan, air preheater, boiler, furnace, superheater, economiser, dust collector, induced draught fan, chimney, etc. The forced draught fan supplies the atmosphere air to the air preheater by the action of a forced draught fan. This air is preheated by the flue gases in the air preheater. This pre­heated air is supplied to the furnace to perform the combustion of the fuel. Due to the combustion of fuel, hot gases (flue gases) are formed.The flue gases from the furnace pass over the boiler tubes and superheater tubes. In boiler, wet steam is generated and in superheater the wet steam is superheated by the flue gases. Then the flue gases pass through economiser to heat the feed water. It is then passed through the air preheater to prevent the incoming air. Finally, it is sent to the atmosphere through chimney, after the dusts in the flue gases are arrested in the dust collector.Feed Water and Steam Flow CircuitThis circuit consists of feed pump, economiser, boiler drum, superheater, turbine, condenser, etc. Feed water is pumped to the economiser from the hot well. This water is preheated by the flue gases in the economiser. This preheated water is then supplied to the boiler drum. Heat is transferred to the water by the burning of coal. Hence this water is converted into steam.The steam produced in the boiler is sent to superheater where it is superheated by the flue gases. This superheated steam is then expanded in a steam turbine to do mechanical work to produce electric power. The expanded (exhaust) steam is then passed through the condenser. In the condenser the steam is condensed into water and recirculated.4. Cooling Water Flow CircuitThis circuit includes a pump, condenser, cooling tower, etc. The exhaust steam from the turbine is condensed in a condenser. In the condenser, cold water is circulated to condense the steam into water. The steam is condensed by losing its latent heat to the circulating cold water. Thus the circulating water is heated.This heated circulating water is then taken to the cooling tower. In the cooling tower, the water is sprayed in the form of droplets through nozzles. The atmospheric air enters the cooling tower and

1 www.vidyarthiplus.com

removes the heat from the water.This cooled water is collected in the cooling pond. This cold water is again circulated through the pump; condenser and cooling tower. Thus the cycle is repeated again and again. Some amount of water may be lost during the circulation due to vaporisation and other losses. Hence, make up water is added to the pond by means of a pump.Advantages and Disadvantages of Steam Power Plants

Advantages Disadvantages1. Capital (initial) cost is low when compared

to Hydro­electric power plant.The fossil fuel (coal) used in this plant may be exhausted after some time since it is non­renewable source of energy.

2. The power generation is independent of the Nature.

Power generation cost is considerably high compared to hydro­electric plant.

3. The plant may be located near the load centre so that transmission losses and transmission cost are law.

Efficiency of the plant decreases with time

4. The erection and commissioning of a steam power plant consumes lesser time when compared with hydro­electric power plant

It cannot be used as peak load plant as its part load efficiency decreases very rapidly with decreasing l.oad.

5. In case the plant is located away from the coal fields, then transportation of fuel is major problem.

6. The smoke produced by the burning of fuel causes air pollution.

7. The life of the steam power plant (25 to 35 years) is lesser than that of hydro electric power plant (100 to 150 years).