Thermal Power Plant Working, Layout and Advantage/Dis-advantage

The schematic arrangement of a modern steam power station is shown.

The layout of the Thermal Power Plant:

The whole arrangement can be divided into the following stages for the sake of simplicity :

1. Coal and ash handling arrangement
2. Boiler
3. Superheater
4. Economiser
5. Air pre-heater
6. Steam Turbine
7. condenser
8. Feed Water Pump
9. Alternator

1. Coal and ash handling arrangement:

• In a coal-based thermal power plant, coal is transported from coal mines to the generating station. Generally, bituminous coal or brown coal is used as fuel.
• The coal is stored in either 'dead storage' or in 'live storage'. Dead storage is generally 40 days of backup coal storage which is used when the coal supply is unavailable. Live storage is a raw coal bunker in the boiler house.
• The coal is cleaned in a magnetic cleaner to filter out any iron particles present which may cause wear and tear on the equipment.
• The coal from live storage is first crushed in small particles and then taken into a pulveriser to make it in powdered form. Fine powder coal undergoes complete combustion, and thus, pulverised coal improves the efficiency of the boiler.
• The ash produced after the combustion of coal is taken out of the boiler furnace and then properly disposed of. Periodic removal of ash from the boiler furnace is necessary for proper combustion.
• It is worthwhile to give a passing reference to the amount of coal burnt and ash produced in a modern thermal power station.
• A 100 MW station operating at a 50% load factor may burn about 20,000 tons of coal per month and ash produced may be to the tune of 10% to 15% of coal-fired i.e., 2,000 to 3,000 tons. In fact, in a thermal station, about 50% to 60% of the total operating cost consists of fuel purchasing and handling.

2. Boiler

• The mixture of pulverised coal and air (usually preheated air) is taken into the boiler and then burnt in the combustion zone.
• On ignition of the fuel, a large fireball is formed at the centre of the boiler, and a large amount of heat energy is radiated from it. The heat energy is utilised to convert the water into steam at high temperatures and pressure. Steel tubes run along the boiler walls in which water is converted into steam. The flue gases from the boiler make their way through the superheater, economiser, and air preheater and finally get exhausted to the atmosphere from the chimney.

3. Superheater:

The superheater tubes are hung at the hottest part of the boiler. The saturated steam produced in the boiler tubes is superheated to about 540 °C in the superheater. The superheated high-pressure steam is then fed to the steam turbine.

4. Economizer:

 An economizer is essentially a feed water heater which heats the water before supplying to the boiler.

5. Air preheater: 

The primary air fan takes air from the atmosphere and it is then warmed in the air pre-heater. Pre-heated air is injected with coal in the boiler. The advantage of pre-heating the air is that it improves the coal combustion.

6. Steam turbine:

High-pressure superheated steam is fed to the steam turbine which causes turbine blades to rotate. Energy in the steam is converted into mechanical energy in the steam turbine which acts as the prime mover. The pressure and temperature of the steam fall to a lower value and it expands in volume as it passes through the turbine. The expanded low-pressure steam is exhausted in the condenser.

7. Condenser:

The exhaust steam is condensed in the condenser by means of cold water circulation. Here, the steam loses its pressure as well as temperature and it is converted back into water. Condensing is essential because compressing a fluid which is in the gaseous state requires a huge amount of energy with respect to the energy required in compressing a liquid. Thus, condensing increases the efficiency of the cycle.

8. Feed Water Pump:

The condensed water is again fed to the boiler by a feed water pump. Some water may be lost during the cycle, which is suitably supplied from an external water source.

9. Alternator.

The steam turbine is coupled to an alternator. The alternator converts the mechanical energy of the turbine into electrical energy. The electrical output from the alternator is delivered to the bus bars through the transformer, circuit breakers, and isolators.

Must Read: SSC JE Previous Year Thermal Power Plant MCQ

Working of Thermal Power Plant:

• First, the pulverized coal is burnt in the furnace of the steam boiler.
•  High-pressure steam is produced in the boiler.
• This steam is then passed through the superheater, where it is further heated up.
• This superheated steam is then entered into a turbine at high speed. 
• In the turbine, this steam force rotates the turbine blades which means here in the turbine the stored potential energy of the high-pressure steam is converted into mechanical energy. 
• After rotating the turbine blades, the team has lost its high pressure, passes out of the turbine blades and enters into a condenser. 
• In the condenser, the cold water is circulated with the help of a pump which condenses the low-pressure wet steam. 
• This condensed water is then further supplied to a low-pressure water heater where the low-pressure steam increases the temperature of this feed water, it is then again heated in a high-pressure heater where the high pressure of steam is used for heating. 
• The turbine in a thermal power station acts as a prime mover of the alternator.

Advantages and Disadvantages of Thermal Power Plant

Advantages:

• Less initial cost as compared to other generating stations.
• It requires less land as compared to the hydropower plant.
• The fuel (i.e. coal) is cheaper.
• The cost of generation is less than that of diesel power plants.

Disadvantages:

• It pollutes the atmosphere due to the production of a large amount of smoke.
• This is one of the causes of global warming.
• The overall efficiency of a thermal power station is low (less than 30%).

Important Questions on Thermal Power Plant for SSC JE Electrical:

Select the correct statement regarding coal-fired boilers.
1)  Water-tube boilers are less expensive compared to fire-tube boilers.
2)  Water-tube boilers are more efficient than fire-tube boilers.
3)  Fire-tube boilers are typically used for high-pressure applications.
4)  Fire-tube boilers have higher thermal efficiency compared to water-tube boilers.

Answer: Water-tube boilers are more efficient than fire-tube boilers.

Water-tube boilers are generally preferred for high-pressure applications and can achieve higher efficiency due to their design. Fire-tube boilers, on the other hand, are usually used for lower pressure applications and tend to have lower efficiency compared to water-tube boilers.

In a thermal power plant, the overall efficiency can be determined using which of the following?

1)  Carnot Cycle Efficiency
2)  Rankine Cycle Efficiency
3)  Regenerative Cycle Efficiency
4)  Boiler Efficiency × Generator Efficiency × Turbine Efficiency

Answer: In a thermal power plant, the overall efficiency can be determined using:

Boiler Efficiency × Generator Efficiency × Turbine Efficiency.

This approach considers the efficiencies of the main components involved in the power generation process.

What type of boilers are suitable for low-maintenance cost, small size and low-pressure plants?
1)  High-pressure boilers
2)  Supercritical boilers
3)  Water tube boilers
4)  Fire tube boilers

Answer: The type of boilers suitable for low-maintenance cost, small size, and low-pressure plants are: Fire tube boilers.

Fire tube boilers are typically smaller, easier to maintain, and more cost-effective for low-pressure applications.

High-pressure boilers: 

These are designed to operate at high pressures, making them suitable for industrial applications that require steam generation at elevated pressures. They are typically larger and more complex, which can lead to higher maintenance costs.

Supercritical boilers:

These operate above the critical pressure and temperature of water, allowing for higher efficiency in converting thermal energy to electricity. They are often used in large power plants but are more complex and expensive to maintain.

Water tube boilers:

In these boilers, water circulates through tubes heated by combustion gases. They are suitable for high-pressure applications and can handle large steam capacities. However, they are typically larger and more expensive than fire tube boilers, which may not be ideal for low-pressure, small-scale plants.

In a steam power plant, the function of _______ is to utilise the waste heat of the flue gases for heating the feed water.
1)  a boiler
2)  a superheater
3)  an economiser
4)  a condenser

• Answer: In a steam power plant, the function of 3) an economiser is to utilize the waste heat of the flue gases for heating the feed water.
• The economiser improves the overall efficiency of the plant by preheating the feed water before it enters the boiler, reducing the energy needed to convert it into steam.