Short Answer:

Combustion is a chemical process in which a fuel reacts with oxygen to produce heat and light energy. It is also called burning. During combustion, the fuel is oxidized, and the products formed are usually carbon dioxide, water vapor, and heat. This process is widely used in engines, power plants, and boilers to generate energy.

In simple words, combustion is the process of burning a fuel like coal, oil, or gas in the presence of air (oxygen) to release heat energy. This heat energy is then used for various purposes, such as producing steam in boilers, driving engines, and generating electricity in power plants.

Detailed Explanation :

Combustion

Combustion is a rapid chemical reaction between a fuel and oxygen that results in the production of heat and light energy. It is one of the most important processes in thermal engineering because it provides the necessary heat energy for mechanical and electrical power generation. Combustion is used in boilers, internal combustion engines, gas turbines, and furnaces.

In a typical combustion process, the fuel contains carbon, hydrogen, and sometimes sulfur. When these elements combine with oxygen from the air, they form carbon dioxide (CO₂), water vapor (H₂O), and heat. If combustion is incomplete due to a lack of oxygen, it may also produce carbon monoxide (CO) and soot, which are undesirable.

The basic chemical equation for complete combustion of a hydrocarbon fuel (like methane) is:

1. Principle of Combustion

The principle of combustion is based on the chemical combination of fuel and oxygen. For combustion to occur, three essential elements are required, commonly known as the “fire triangle”:

1. Fuel: The combustible material (solid, liquid, or gas) such as coal, diesel, or natural gas.
2. Oxygen (Air): The oxidizing agent that reacts with fuel to produce heat.
3. Heat (Ignition Source): The minimum temperature required to start and sustain combustion.

When these three conditions exist in the proper proportion, combustion takes place and releases energy. If any one of them is removed, the combustion process stops.

2. Stages of Combustion

Combustion takes place in the following stages:

1. Ignition:
   • This is the beginning of combustion.
   • The fuel is heated to its ignition temperature, the minimum temperature at which it starts to burn.
2. Flaming Combustion:
   • After ignition, the fuel burns rapidly, producing a flame.
   • In this stage, chemical reactions occur between oxygen and fuel vapors, releasing heat and light.
3. Smoldering Combustion:
   • In some cases, especially for solid fuels, the reaction continues slowly without a flame.
   • This happens when oxygen supply becomes limited.
4. Complete Combustion:
   • Occurs when sufficient oxygen is available.
   • Produces maximum heat and only CO₂ and H₂O as by-products.
5. Incomplete Combustion:
   • Occurs when oxygen supply is insufficient.
   • Produces CO, soot, and unburnt hydrocarbons, leading to energy loss and pollution.

3. Types of Combustion

Combustion can be classified into several types based on speed, nature, and the presence of flame.

1. Rapid Combustion:

• Occurs quickly and releases a large amount of heat and light in a short time.
• Example: Burning of petrol in an engine or LPG in a stove.

1. Slow Combustion:

• Takes place at a slow rate, releasing little heat over a long period.
• Example: Rusting of iron or fermentation process.

1. Spontaneous Combustion:

• Occurs without any external ignition source when a material self-heats to its ignition temperature.
• Example: Combustion of oily rags or coal dust piles.

1. Explosion:

• A very rapid combustion accompanied by a sudden release of gas and heat, causing pressure rise and noise.
• Example: Explosion of gunpowder or internal combustion engine cylinder firing.

4. Conditions Necessary for Combustion

For efficient and continuous combustion, the following conditions must be satisfied:

1. Proper Fuel-Air Ratio:
   • The right mixture of fuel and air is essential. Too much air leads to cooling losses, and too little air causes incomplete combustion.
2. Adequate Mixing:
   • Air and fuel must mix thoroughly to ensure complete burning.
3. Sufficient Temperature:
   • The temperature of the fuel-air mixture should be above the ignition point.
4. Continuous Supply of Fuel and Air:
   • To sustain combustion, both fuel and oxygen must be continuously supplied.
5. Removal of Products:
   • The combustion gases must be properly vented to prevent interference with the process.

5. Products of Combustion

The products of combustion depend on the type of fuel and the amount of oxygen available.

• Complete Combustion Products:
  • Carbon dioxide (CO₂)
  • Water vapor (H₂O)
  • Heat energy
• Incomplete Combustion Products:
  • Carbon monoxide (CO)
  • Soot (unburnt carbon particles)
  • Hydrocarbons (unburnt fuel)

Complete combustion is preferred because it gives maximum heat and minimum pollution.

6. Importance of Combustion in Engineering

Combustion plays a vital role in mechanical and power engineering. Some of its key applications include:

1. Power Generation:
   • In thermal power plants, combustion of coal or oil produces heat to generate steam, which drives turbines.
2. Internal Combustion Engines:
   • Petrol and diesel engines depend on controlled combustion of fuel to produce mechanical power.
3. Industrial Heating:
   • Used in furnaces, kilns, and boilers for various manufacturing processes.
4. Domestic Use:
   • Combustion of LPG and natural gas is used for cooking and heating.
5. Transportation:
   • Airplanes, ships, and vehicles rely on combustion engines for propulsion.

7. Efficiency and Control of Combustion

To achieve efficient combustion, engineers aim to maximize heat release and minimize fuel wastage. This is done by:

• Using preheated air (from air preheaters).
• Maintaining the correct fuel-air ratio.
• Ensuring complete mixing of air and fuel.
• Reducing excess air to prevent energy losses.
• Using modern combustion control systems for continuous monitoring.

Efficient combustion not only saves fuel but also reduces harmful emissions like CO and NOx, contributing to environmental protection.

Conclusion

Combustion is the chemical process of burning a fuel in the presence of oxygen to produce heat and light energy. It is essential for energy production in power plants, engines, and industrial furnaces. Efficient combustion depends on proper fuel-air mixing, sufficient oxygen, and maintaining the correct temperature. By ensuring complete combustion, engineers can maximize energy output, minimize pollution, and improve the overall efficiency of thermal systems. Combustion remains the heart of all heat-based energy conversion systems in mechanical engineering.