The functions of using economizer in thermal power station is to

A. Heat air.

B. Preheat steam.

C. Remove feed water.

D. Heat feed water.

Answer: D. Heat feed water.

Explanation: 

Economizer: 

• An economizer is essentially a feedwater heater and derives heat from the flue gases for this purpose.
• The feed water is fed to the economizer before supplying to the boiler.
• The economizer extracts a part of the heat of flue gases to increase the feedwater temperature.

Additional Information

Thermal (Steam) Power Plant :

A generating station that converts heat energy of coal combustion into electrical energy is known as a Thermal/Steam power station.

1. Coal and ash handling plant:

• The coal is transported to the power station by road or rail and is stored in the coal storage plant. 
• From the coal storage plant, coal is delivered to the coal handling plant where it is pulverized in order to increase its surface exposure, thus promoting rapid combustion without using a large quantity of excess air. 
• The coal is burnt in the boiler and the ash produced after the complete combustion of coal is removed to the ash handling plant and then delivered to the ash storage plant for disposal.

2. Steam generating plant:

• The steam generating plant consists of a boiler, super heater, and economizer for the production of steam and other auxiliary equipment for the utilization of flue gases.

(i) Boiler: 

• The heat of the combustion of coal in the boiler is utilized to convert water into steam at high temperatures and pressure. 
• The flue gases from the boiler make their journey through super heater, economizer, air pre-heater and are finally exhausted to atmosphere through the chimney.

(ii) Super heater: 

• The steam produced in the boiler is wet and is passed through a super heater where it is dried and superheated by the flue gases on their way to the chimney. 
• Superheating provides two principal benefits. Firstly, the overall efficiency is increased. 
• Secondly, too much condensation in the last stages of the turbine is avoided. 
• The superheated steam from the super heater is fed to the steam turbine through the main valve.

(iii) Economizer: 

• An economizer is essentially a feedwater heater and derives heat from the flue gases for this purpose. 
• The feed water is fed to the economizer before supplying to the boiler. 
• The economizer extracts a part of the heat of flue gases to increase the feedwater temperature.

(iv) Air preheater: 

• An air preheater increases the temperature of the air supplied for coal burning by deriving heat from flue gases. Air is drawn from the atmosphere by a forced draught fan and is passed through the air preheater before supplying to the boiler furnace. 
• The air preheater extracts heat from flue gases and increases the temperature of the air used for coal combustion.

3. Steam turbine:

• The dry and superheated steam from the super heater is fed to the steam turbine through the main valve. 
• The heat energy of steam when passing over the blades of a turbine is converted into mechanical energy.

4. Alternator:

• The steam turbine is coupled to an alternator. 
• The alternator converts the mechanical energy of the turbine into electrical energy.