Solid Fuels: Fuels that are in a solid state at room temperature and require some form of combustion or heating to release their energy.

Examples: coal, wood, and biomass.

Liquid Fuels: These are fuels that are in a liquid state at room temperature and can be easily transported and stored.

Examples : gasoline, diesel, and kerosene.

Gaseous Fuels:These are fuels that are in a gaseous state at room temperature and can be easily transported through pipelines or compressed into cylinders.

Examples : natural gas, propane, and hydrogen.

Reliability: Solid fuels (e.g coal and wood) are readily available in many parts of the world, and their supply is less susceptible to disruptions compared to liquid or gaseous fuels.

Affordability: They are often cheaper than other types of fuels, making them accessible to low-income households and industries.

Energy density: They have a high energy density, they contain a large amount of energy per unit weight or volume, making them efficient for heating and power generation.

Versatility: They can be used in a variety of applications, from household heating to large-scale power generation.

Environmental impact: Burning solid fuels can release pollutants such as particulate matter, sulfur dioxide, and carbon monoxide, which can harm human health and contribute to air pollution and climate change.

Storage and transportation: Solid fuels are often bulky and require large storage facilities, which can be expensive and difficult to maintain. Transporting solid fuels over long distances can also be challenging and costly.

Safety hazards: Handling and storing solid fuels can pose safety risks such as fire and explosions, especially if they are not properly ventilated or contained.

Ash and waste disposal: Burning solid fuels generates ash and waste that require proper disposal, which can be expensive and contribute to environmental degradation if not managed properly.

Merits :

1. Higher calorific value. 

2. Lower storage capacity required.  

 3. Better economy in handling. 

4. Better control of consumption by using valves.  

5. Better cleanliness and freedom from dust. 

6. Practically no ashes. 

7. Non-deterioration in storage.  

8. Non-corrosion of boiler plates. 

9. Higher efficiency. 

Demerits:

1. Higher cost. 

2. Greater risk of fire. 

3. Costly containers are required for storage and transport.

Merits:

1. The supply of fuel gas, and hence the temperature of furnace is easily and accurately controlled. 

2. The high temperature is obtained at a moderate cost by pre-heating gas and air with heat of waste gases of combustion. 

3. They are directly used in internal combustion engines. 

4. They are free from solid and liquid impurities. 

5. They do not produce ash or smoke. 

6. They undergo complete combustion with minimum air supply.

Demerits:

1. They are radially inflammable. 

2. They require large storage capacity. #

1. A good fuel should have a low ignition point. 

2. It should have a high calorific value. 

3. It should freely burn with a high efficiency, once it is ignited. 

4. It should not produce harmful gases. 

5. It should produce least quantity of smoke and gases.  

6. It should be economical, easy to store and convenient for transportation.

Calorific Value of Fuel refers to the amount of heat energy that can be obtained per unit mass or volume of a fuel when it is completely burned.

Units: joules per kilogram (J/kg) or British thermal units per pound (BTU/lb).

Gross or Higher Calorific Value is the total amount of heat energy that is obtained from a unit mass of fuel when it is completely burned, assuming that all the water vapor produced during combustion is condensed and the heat released during this process is recovered.

Net or Lower Calorific Value is the amount of heat energy that is obtained from a unit mass of fuel when it is completely burned, assuming that the water vapor produced during combustion is not condensed and the heat released during this process is not recovered.