Short Answer:

Solid fuels are fuels that remain in solid form under normal conditions and are commonly used in thermal power plants to generate heat energy. These fuels include coal, lignite, peat, wood, and charcoal, which are burned in boilers to produce steam. The steam then drives turbines to generate electricity.

In simple words, solid fuels are natural or processed substances like coal or wood that release heat when burned. In thermal power plants, they are preferred because they are cheap, easily available, and produce high heat output, making them suitable for large-scale power generation.

Detailed Explanation :

Solid Fuels Used in Thermal Power Plants

Solid fuels are those fuels that are in a solid physical state and can burn to produce heat energy when combined with oxygen. These fuels have been widely used in thermal power plants since the beginning of industrialization because of their availability, high calorific value, and lower cost compared to liquid or gaseous fuels.

In a thermal power plant, solid fuels are burned in a boiler furnace to generate heat. The heat converts water into high-pressure steam, which drives a steam turbine connected to a generator that produces electric power. Among all solid fuels, coal is the most commonly used due to its high heat-producing capacity and abundance.

The major solid fuels used in thermal power plants are:

1. Coal
2. Lignite
3. Peat
4. Wood
5. Charcoal

1. Coal

Coal is the most important and widely used solid fuel in thermal power plants. It is a black or brownish-black sedimentary rock formed from the remains of ancient plants through heat and pressure over millions of years.

Types of Coal:
Coal is classified based on its carbon content and heating value into:

• Anthracite: Highest quality coal with high carbon content (about 90%) and high calorific value (~33,000 kJ/kg).
• Bituminous: Medium quality, widely used in power plants, with a calorific value of about 30,000 kJ/kg.
• Sub-bituminous: Lower carbon and calorific value than bituminous coal.
• Lignite: Brown coal with high moisture and low calorific value (~20,000 kJ/kg).

Uses in Power Plants:
Coal is pulverized into a fine powder and burned in the boiler to produce heat for steam generation. It provides steady and reliable power output.

Advantages:

• High energy output per kilogram.
• Easily available in large quantities.
• Suitable for continuous large-scale power generation.

Disadvantages:

• Produces ash and harmful gases.
• Requires large storage and handling facilities.

2. Lignite

Lignite, also known as brown coal, is a low-grade solid fuel formed from partially decomposed plant material. It has high moisture content (30–40%) and low carbon content (around 60–70%), which results in a lower calorific value compared to bituminous coal.

Calorific Value: Around 15,000–20,000 kJ/kg.

Characteristics:

• Burns easily but produces more smoke.
• Abundant and cheaper than other types of coal.
• Used mainly in power plants located near mining areas to reduce transportation cost.

Example:
Lignite is used in power stations like Neyveli Thermal Power Station in Tamil Nadu, India.

3. Peat

Peat is the lowest grade of solid fuel formed in the early stage of coal formation. It consists of partially decayed plant material found in swampy regions. Peat contains a large amount of moisture (up to 50%) and very low carbon content (less than 60%), making its calorific value quite low.

Calorific Value: Around 10,000–14,000 kJ/kg.

Characteristics:

• Produces more smoke and ash during combustion.
• Difficult to handle due to high moisture content.
• Not widely used for large power plants but may serve as a fuel for small local power stations or domestic heating.

Use:
Used only in regions where other fuels are scarce or expensive.

4. Wood

Wood is one of the oldest solid fuels used by humans. It consists mainly of cellulose, hemicellulose, and lignin. Though largely replaced by coal in modern power plants, it is still used in small-scale thermal systems and biomass-based plants.

Calorific Value: Around 15,000–18,000 kJ/kg (for dry wood).

Characteristics:

• Readily available and renewable.
• Contains volatile matter, making it easy to ignite.
• Produces moderate heat and more smoke if not properly dried.

Use in Power Plants:
Used in biomass thermal power plants where wood waste or pellets are burned to generate electricity.

Advantages:

• Renewable and carbon-neutral when sourced sustainably.
• Produces less sulfur emissions compared to coal.

Disadvantages:

• Requires large quantities due to lower calorific value.
• Moisture content affects burning efficiency.

5. Charcoal

Charcoal is a solid residue obtained by heating wood or other organic matter in the absence of air (a process known as pyrolysis). It contains mostly carbon and small amounts of ash and volatile matter.

Calorific Value: Around 28,000–33,000 kJ/kg.

Characteristics:

• Burns with a clean, smokeless flame.
• Produces more heat than wood for the same weight.
• Lighter in weight and easier to transport.

Use in Power Plants:
Used in small-scale thermal applications and as a supplementary fuel where high-temperature heating is required.

Advantages:

• High calorific value and low smoke.
• Easy to store and handle.

Disadvantages:

• Expensive to produce compared to raw wood.
• Limited large-scale use due to cost.

6. Comparison of Solid Fuels

While all the above solid fuels can be used in thermal power plants, coal and lignite are the most common because of their high calorific values, wide availability, and suitability for large boilers. Fuels like wood and charcoal are used in smaller or biomass-based plants, while peat is rarely used due to its poor efficiency.

7. Advantages of Using Solid Fuels in Thermal Power Plants

1. Low Cost: Solid fuels like coal are cheaper compared to liquid and gaseous fuels.
2. Abundant Supply: Available in large quantities across many regions.
3. Storage and Transport: Can be stored for long durations without significant loss of energy.
4. Reliable Performance: Provides continuous and steady energy output for power generation.

8. Disadvantages of Solid Fuels

1. Pollution: Produces smoke, ash, and harmful gases like CO₂ and SO₂.
2. Handling Problems: Requires large storage areas and heavy handling equipment.
3. Low Efficiency: Some solid fuels have high moisture content, reducing efficiency.
4. Environmental Impact: Mining and combustion cause land degradation and air pollution.

Conclusion

Solid fuels used in thermal power plants mainly include coal, lignite, peat, wood, and charcoal. Among these, coal is the most widely used because of its high calorific value, easy availability, and cost-effectiveness. These fuels are burned in boilers to generate steam, which drives turbines for electricity production. Although solid fuels provide reliable energy, their usage must be managed carefully to minimize pollution and environmental harm. In modern systems, efficient combustion and emission control technologies are used to make power generation cleaner and more sustainable.