What Is Combustion?

Different substances are used as fuels for various purposes, such as cooking at home (e.g., LPG), in industries (e.g., coal, natural gas), and for running vehicles (e.g., petrol, diesel, CNG). When a substance burns, it undergoes a chemical process.

Combustion is defined as a chemical process in which a substance reacts with oxygen, releasing heat. In some cases, light is also produced during combustion, either as a flame or as a glow.

The substance that undergoes combustion is called a combustible substance, or simply a fuel. Fuels can exist in solid (e.g., wood, coal, charcoal), liquid (e.g., kerosene, petrol, diesel), or gaseous states (e.g., LPG, CNG, natural gas).

Examples of combustion include:

• Burning of magnesium: Magnesium reacts with oxygen to form magnesium oxide, producing heat and light.
• Burning of charcoal or coal: These substances burn in air (react with oxygen), producing carbon dioxide, heat, and light.

Food is also considered a fuel for our bodies, where it reacts with oxygen during respiration to produce energy (heat).

Combustion requires certain conditions to take place:

• Presence of a combustible substance: The material must be capable of burning.
• Presence of oxygen (air): Oxygen is typically necessary for combustion. Experiments with a burning candle covered by a chimney show that the flame flickers and eventually goes out when the supply of fresh air (oxygen) is restricted or cut off.
• Heat: The temperature of the combustible substance must reach its ignition temperature.

The ignition temperature is the lowest temperature at which a substance catches fire and starts burning. Different substances have different ignition temperatures. For instance, a matchstick does not catch fire on its own at room temperature but burns when rubbed against the side of the matchbox because the friction generates enough heat to raise its temperature to its ignition point. Similarly, dry leaves catch fire more easily than green leaves because dry leaves have a lower ignition temperature and less moisture.

Substances that have a very low ignition temperature and can easily catch fire with a flame are called inflammable substances. Examples include petrol, alcohol, and Liquified Petroleum Gas (LPG). Special care is needed when storing and handling inflammable substances.

Heating water in a paper cup over a candle flame demonstrates the concept of ignition temperature and heat transfer. The paper cup with water does not burn easily because the heat supplied by the candle is transferred to the water by conduction, preventing the paper's temperature from reaching its ignition point. The paper only heats up enough to burn if all the water evaporates or if the heat source is strong enough.

The history of the matchstick involves the use of chemicals with low ignition temperatures that could be easily ignited by friction.

How Do We Control Fire?

Fire can cause significant damage, and knowing how to control it is essential. Fire requires three main things to burn: fuel, air (oxygen), and heat (to reach ignition temperature). To extinguish a fire, one or more of these requirements must be removed.

A fire extinguisher is a device used to control fire by cutting off the air supply, lowering the fuel's temperature, or both. In many cases, the fuel cannot be removed (e.g., a burning building).

The most common fire extinguisher is water. Water cools the combustible material below its ignition temperature and the steam produced helps to block the air supply. However, water is not suitable for all types of fires:

• It cannot be used on fires involving electrical equipment because water conducts electricity and can cause electric shock.
• It is not effective on fires involving oil or petrol because water is heavier than oil and sinks below it, allowing the oil to continue burning on the surface.

For fires involving electrical equipment or inflammable liquids like petrol, carbon dioxide ($\text{CO}_2$) is the best fire extinguisher. $\text{CO}_2$ is heavier than oxygen, so it forms a blanket around the fire, cutting off the supply of air (oxygen) to the fuel. This stops the combustion. An additional benefit is that $\text{CO}_2$ usually does not damage electrical equipment.

$\text{CO}_2$ is stored under high pressure as a liquid in cylinders. When released, it expands greatly in volume and cools down, further helping to lower the temperature of the burning material. Chemicals like sodium bicarbonate (baking soda) or potassium bicarbonate can also be used; when heated by the fire, they release $\text{CO}_2$ gas.

Types Of Combustion

Combustion can occur in different ways, depending on the substance and conditions:

• Rapid Combustion: This is when a substance burns quickly and produces heat and light, often with a flame. Burning of LPG in a gas stove is an example of rapid combustion.
• Spontaneous Combustion: This occurs when a material suddenly bursts into flames without any external cause like applying heat or a spark. It happens when the substance reaches its ignition temperature on its own due to internal factors (e.g., oxidation, decomposition). Spontaneous combustion of coal dust has caused fires in mines. Forest fires can sometimes start spontaneously due to intense heat or lightning, but often they are caused by human carelessness.
• Explosion: A very rapid combustion reaction that produces a large amount of heat, light, and sound, along with the sudden release of a large volume of gas. Fireworks (crackers) exploding are an example of explosion. Explosions can also be caused by applying pressure on some materials.

Flame

Some substances burn with a flame, while others burn without one, only producing a glow. A flame is the visible, gaseous part of a fire.

Substances that vaporise during burning produce a flame. For instance, kerosene oil and wax in a candle melt, rise up the wick, turn into vapour, and burn with a flame. Charcoal, which does not vaporise easily upon heating, burns without a flame, producing only a glow.

Examples of materials that form a flame on burning include candle wax, magnesium, camphor, and kerosene (in a stove). Charcoal is an example of a material that burns without a flame.

Structure Of A Flame

A typical flame, like that of a candle, has distinct zones, each with different characteristics:

• Innermost Zone (Dark Zone): This is the region closest to the wick. It consists of unburnt wax vapours. It is the least hot part of the flame and appears black or dark.
• Middle Zone (Luminous Zone): This is the large, bright yellow part of the flame. It is formed by the partial combustion of fuel vapours. Due to incomplete burning, unburnt carbon particles are present in this zone, which glow and make the flame luminous. Placing a glass plate in this zone results in a black deposit (soot) of these unburnt carbon particles. This zone is moderately hot.
• Outermost Zone (Non-luminous Zone): This is the thin, often faint blue part of the flame at the outer edge. Here, complete combustion of the fuel vapours takes place due to sufficient oxygen supply. This zone is the hottest part of the flame and is non-luminous because there are no unburnt glowing particles.

Goldsmiths use the outermost zone of the flame for melting gold and silver. This is because the outermost zone is the hottest part of the flame, providing the high temperature needed to melt these metals.

Inserting a thin glass tube into the dark zone of a candle flame allows unburnt wax vapours to travel up the tube. Bringing a burning matchstick to the other end of the tube ignites these vapours, showing that the flame is produced by the combustion of fuel vapours.

What Is A Fuel?

Substances that provide heat energy when they undergo combustion are called fuels. Common examples include wood, charcoal, petrol, kerosene, LPG, and natural gas. Fuels can be solid, liquid, or gas.

An ideal fuel would possess several desirable characteristics:

• Readily available.
• Cheap.
• Burns easily in air at a moderate rate.
• Produces a large amount of heat (high fuel efficiency).
• Does not leave behind any undesirable or harmful substances.

No single fuel is perfectly ideal; fuels are chosen based on the specific requirements of their use. Fuels also vary in cost.

Here is a list of fuels classified by their state:

S. No.	Solid Fuels	Liquid Fuels	Gaseous Fuels
1.	Coal	Kerosene oil	Natural gas
2.	Wood	Petrol	LPG
3.	Charcoal	Diesel	CNG
4.	Cow dung cake		Biogas

Fuel Efficiency

Different fuels produce different amounts of heat when burned completely. The amount of heat energy released upon the complete combustion of 1 kg of a fuel is called its calorific value. Calorific value is a measure of fuel efficiency and is expressed in a unit called kilojoule per kg (kJ/kg).

Comparing the calorific values helps in choosing a more efficient fuel. For example, LPG has a higher calorific value than wood or coal, meaning it produces more heat per unit mass, making it a more efficient domestic fuel.

Here are the approximate calorific values of some common fuels:

Fuel	Calorific Value (kJ/kg)
Cow dung cake	6000-8000
Wood	17000-22000
Coal	25000-33000
Petrol	45000
Kerosene	45000
Diesel	45000
Methane	50000
CNG	50000
LPG	55000
Biogas	35000-40000
Hydrogen	150000

Note: Hydrogen has a very high calorific value but is not commonly used as a fuel due to storage and handling challenges.

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Burning Of Fuels Leads To Harmful Products

While burning fuels provides essential energy, it also has negative environmental impacts due to the products of combustion:

1. Unburnt Carbon Particles: Fuels containing carbon (wood, coal, petroleum) release fine, unburnt carbon particles (soot) into the air, especially during incomplete combustion. These particles are hazardous pollutants that can cause respiratory problems like asthma.
2. Carbon Monoxide: Incomplete combustion of carbon fuels produces carbon monoxide ($\text{CO}$), a highly poisonous gas. Burning coal or wood in a closed room is dangerous because the accumulation of $\text{CO}$ can lead to suffocation and death.
3. Carbon Dioxide and Global Warming: The combustion of most fuels releases carbon dioxide ($\text{CO}_2$) into the atmosphere. An increase in atmospheric $\text{CO}_2$ concentration is linked to global warming, which is the rise in the Earth's average temperature. Global warming can cause polar ice caps to melt, leading to rising sea levels and flooding in coastal areas.
4. Oxides of Sulphur and Nitrogen and Acid Rain: Burning fuels like coal and diesel releases sulphur dioxide ($\text{SO}_2$), a suffocating and corrosive gas. Petrol engines also produce gaseous oxides of nitrogen ($\text{NO}_x$). These oxides dissolve in rainwater to form acids (sulphuric acid and nitric acid), resulting in acid rain. Acid rain is harmful to crops, soil, buildings (especially those made of marble), and aquatic life.

Replacing fuels like diesel and petrol with Compressed Natural Gas (CNG) in vehicles helps reduce pollution because CNG produces significantly smaller amounts of these harmful substances, making it a cleaner fuel.

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