Power plants

UNIT – IV

Chapter - 11

POWER PLANTS

 

UTILIZATION OF ENERGY RESOURCES

Power plants are used for the generation of electric power from the various sources of energy. Fuel represents the largest operating expense in electric power generation. This Chapter deals with the utilization of energy resources to generate electricity in power plants.

 

THERMAL POWER PLANT

Working Principle

A Thermal Power Plant is also known as Steam Power Plant. It is using steam as the working fluid. Superheated high pressure steam is generated in a Boiler using coal as fuel. That is, Chemical Energy of coal is converted into Heat Energy of steam by burning coal.

Heat energy of steam is converted into Mechanical Energy by expanding the steam in a Steam Turbine (Prime Mover). This mechanical energy is converted into Electrical Energy in the Electrical Generator.

Fig. 1 shows the stages of energy transformation in a thermal power plant.

 

1. LAYOUT OF A MODERN THERMAL POWER PLANT

Fig. 2 shows the layout of a thermal power plant. It consists of Four Circuits. These are:

1. Coal and Ash Circuit

2. Air and Flue Gas Circuit

3. Feed Water and Steam Circuit

4. Cooling Water Circuit

1. Coal and Ash Circuit

Coal from the mines is stored in the Coal Storage Yard. It is transferred to the Boiler Furnace by means of coal handling equipment like belt conveyor, bucket elevator, etc. Coal is burnt in the boiler furnace. Hot ash formed due to the combustion of coal in the furnace is removed to the Ash Storage Yard by means of ash handling equipment.

Ash Disposal: Indian coal contains about 40% ash. A power plant of 100 MW capacity produces about 25 tonnes of hot ash per hour. Hence, sufficient space near the power plant is essential to dispose such large quantities of hot ash.

2. Air and Flue Gas Circuit

Air is taken in from the atmosphere to Air Pre-heater. Air is heated in the air pre-heater by the flue gases from the economizer. Then the hot air from the air pre-heater is supplied to the furnace of the Boiler for combustion of coal. Coal contains elements which get oxidized during reaction with Oxygen supplied by air.

The flue gases after combustion in the furnace, pass around the boiler tubes to generate superheated steam. The flue gases then flow through an Economizer and the Air Pre-heater.

Economizer: The function of an economizer is to absorb the heat of the outgoing flue gases, to raise the temperature of the feed water before it enters the boiler.

Finally, the flue gases from air pre-heater are exhausted to the atmosphere through the Chimney. By this method, the heat of the flue gases which would have been wasted otherwise is used effectively in both the economizer and the air pre-heater. Thus, the overall efficiency of the power plant is improved.

Air Pollution

The pollution of the surrounding atmosphere is caused by the emission of the objectionable gases and dust through the chimney. The air pollution causes nuisance to people surrounding the plant. In fact, air pollution is a health hazard.

3. Feed Water and Steam Circuit

Steam Turbine

The high pressure steam generated in the boiler is supplied to the Steam Turbine. Work is done by the expansion of steam in the turbine. Hence, the pressure of steam is reduced.

The function of the steam turbine is to convert the heat energy in the steam into rotational power of the shaft on which the turbine is supported. The rotational speed of the turbine shaft is set by the frequency of the electricity supply and is 3000 revolutions per minute (50 r.p.s.) corresponding to an alternating electric supply at 50 hertz (c.p.s.).

Electric Generator

The electric generator is directly coupled to the turbine shaft. It converts the mechanical energy of turbine shaft into electrical energy. It consists of two electrical windings. One is mounted on the turbine shaft rotating with it, and is called the Rotor. The other is arranged as a shroud around the rotor, fixed to the floor and is called the Stator. The relative motion of rotor and stator generates the electricity at 11,000 volts.

Condenser

The expanded low pressure steam from the turbine passes to a Condenser, where it is condensed to water by cooling. The condenser is a large vessel containing a number of brass tubes. Cold water is circulated through these tubes continuously for condensing the steam flowing outside the surface of the tubes.

The condensate (i.e., condensed water) leaving the condenser is first heated in a L.P. Water Heater by using the steam taken from the low pressure extraction point of the turbine. Some of the steam and water is lost while passing through different components of the system. Therefore, feed water is supplied by a Feed Pump from an external source to compensate this loss. The external source of water is used as a make-up to the feed water system.

Again, steam taken from the high pressure extraction point of the turbine is used for heating the feed water in the H.P. Water Heater. The hot feed water is passing through the economizer, where it is further heated by the flue gases. The feed water which is sufficiently heated by the L.P. and H.P. feed water heaters as well as economizer is fed into the boiler.

4. Cooling Water Circuit

The condenser condenses the exhaust steam from the turbine to water by cooling. The volume occupied by the condensate is very much less than that of the low pressure steam. Thus, the pressure of the condensate reduces to vacuum. When the exhaust steam is passed to the condenser, its pressure automatically drops to vacuum that is existing in the condenser.

Hence, the steam in the turbine expands to vacuum condenser pressure, instead of to atmospheric pressure. This increase in the amount of pressure drop in the turbine increases the amount of work done. Thus, the efficiency of the plant is improved by the condenser by lowering the exhaust pressure of steam and also by providing hot feed water to the boiler.

Abundant quantity of cooling water (called coolant) is required for condensing the steam in the condenser. The condensed water is reused in the cycle.

Water circulating through the condenser may be taken from sources such as nearby river or lake, provided adequate water supply is available from the river or lake throughout the year. If adequate quantity of water is not available at the plant site, the hot coolant from the condenser is cooled in the Cooling Tower and re-circulated again.

Cooling Tower

The hot cooling water from the condenser passes on to the top of the Cooling Tower from where it is sprayed downwards through nozzles. It is cooled in contact with the atmospheric air entering along the periphery of the tower from the bottom and traveling in the upward direction. The hot coolant gives up its heat to the air. It becomes cool and is collected at the bottom of the tower. This cold water is again circulated by coolant pump to the condenser.

 

2. ADVANTAGES

1. Renewable Energy: Water is a natural recourse. It is a renewable energy source. It is the cheapest source of energy. It is a free gift by Nature.

2. Low Initial Cost: Initial Cost is low compared with hydel power plant.

3. Location: The thermal power plant can be located near the load center. Therefore, the transmission cost and transmission losses are considerably reduced.

4. The generation of power is not dependent on the nature's mercy like hydel plant.

5. Lesser Period of Time: The construction, erection and commissioning of thermal plant requires lesser period of time than a hydel plant.

 

3. DISADVANTAGES

1. Non-renewable Source of Energy: The fuel (coal) used in thermal plant will one day get ... exhausted by gradual use, since it is a non-renewable source of energy.

2. Part Load Efficiency: Its part load efficiency decreases very rapidly with decreasing load.

3. Transportation: Transportation of coal is difficult, if the plant is located away from coal mines.

4. Power Generation Cost: Power generation cost is high compared with hydel plant.

5. Air Pollution: Burning coal in the plant produces smoke. The smoke is exhausted through the chimney into the atmosphere. This causes air pollution.

6. Life of the Plant: Life of the thermal power plant is hardly about 30 - 40 years compared with the life of the hydel power plant (i.e., about 100 – 125 years).

7. Decrease in Efficiency: Efficiency of the plant decreases to less than 10% after its life period.

8. Running Speed: The turbines in thermal plant run at a speed of 3000 to 4000 rpm. Therefore, they require special alloy steel materials and rigid construction, compared to hydel plant which has a low running speed of 300 to 400 rpm.