Combustion and Flame

Combustion is a chemical process in which a substance reacts with oxygen (usually from the air) to produce heat and light. This process is also known as burning.

• Reactants: A substance (fuel) and an oxidizer (usually oxygen).
• Products: Heat, light (as flame or glow), and often other substances like carbon dioxide, water vapor, etc.
• Energy Release: Combustion is an exothermic reaction, meaning it releases energy in the form of heat and light.

Types of Substances based on Combustion

• Combustible Substances (Fuels):
• Substances that undergo combustion and produce heat and light.
• Also called fuels.
• Can be solid (wood, coal, charcoal), liquid (kerosene, petrol, alcohol), or gaseous (LPG, CNG, natural gas).
• Examples: Paper, cloth, wood, LPG, kerosene, petrol, charcoal.
• Non-combustible Substances:
• Substances that do not burn in the presence of air or oxygen.
• Examples: Glass, sand, iron nails, stone pieces, water.

Essential Conditions for Combustion

For combustion to occur, three conditions must be met, often referred to as the Fire Triangle:

1. Fuel: Presence of a combustible substance.
2. Oxygen (Air): Presence of a supporter of combustion (oxygen).
3. Heat: Attainment of ignition temperature.

• Removing any one of these conditions will extinguish the fire. This principle is crucial for fire control.

Ignition Temperature

• The lowest temperature at which a substance catches fire and starts burning is called its ignition temperature.
• Different substances have different ignition temperatures.
• Example: Paper has a lower ignition temperature than wood.
• A substance will not burn if its temperature is below its ignition temperature.
• This is why a matchstick doesn't burn on its own at room temperature; it needs friction to reach its ignition temperature.

Inflammable Substances

• Substances that have a very low ignition temperature and can easily catch fire with a flame are called inflammable substances.
• These substances require special care in storage and handling due to their high flammability.
• Examples: Petrol, alcohol, Liquefied Petroleum Gas (LPG), Compressed Natural Gas (CNG), ether.

Practical Application: Water and Paper Cup Experiment

• If you heat an empty paper cup, it burns quickly because the paper reaches its ignition temperature.
• If you heat a paper cup filled with water, the paper does not burn (or burns very slowly) even when heated.
• Reason: The heat supplied to the paper cup is transferred to the water by conduction.
• This keeps the temperature of the paper below its ignition temperature, preventing it from catching fire.
• If heating continues, the water can even boil without the paper burning.

Forest Fires

• Forest fires are a common example where dry leaves, grass, and wood have low ignition temperatures.
• During extreme heat, these materials can reach their ignition temperature naturally, leading to widespread fires.
• Factors like dry weather, high temperatures, and strong winds contribute to rapid spread.

Fire control involves removing one or more components of the fire triangle (fuel, oxygen, heat).

Methods of Fire Control

1. Removing the Fuel: This is often not practical, as the burning material itself is the fuel.
2. Cutting off the Air Supply (Oxygen):

• Smothering the fire with a blanket, sand, or foam.
• Using carbon dioxide (CO₂) extinguishers.

1. Cooling the Combustible Material (Heat):

• Bringing the temperature below the ignition temperature.
• Water is the most common agent for this.

Fire Extinguishers

• Devices used to extinguish small fires.
• Water Extinguishers:
• Most common for fires involving ordinary combustible materials like wood, paper, cloth (Class A fires).
• Works by cooling the fuel below its ignition temperature and creating water vapor that helps cut off air supply.
• DO NOT use on electrical fires (water conducts electricity) or oil/petrol fires (water is heavier than oil, so oil floats and continues to burn; also, water can spread the burning oil).
• Carbon Dioxide (CO₂) Extinguishers:
• Best for electrical equipment fires (Class C fires) and flammable liquid fires (Class B fires).
• CO₂ is heavier than oxygen, so it forms a blanket around the fire, cutting off the oxygen supply.
• It also cools down the fuel, though not as effectively as water.
• Leaves no residue, making it suitable for sensitive equipment.
• Foam Extinguishers:
• Used for flammable liquid fires (Class B fires).
• Creates a foam blanket that smothers the fire and cools the fuel.
• Dry Powder Extinguishers:
• Contain fine powder that smothers the fire and inhibits the chemical reaction.
• Effective for Class A, B, and C fires, and sometimes metal fires (Class D).

Safety Measures

• Always know the location of fire extinguishers.
• Know the emergency contact number for fire services (e.g., 101 in India).
• In case of clothes catching fire, cover the person with a blanket to cut off air supply.

Combustion can be classified based on the rate and conditions under which it occurs:

1. Rapid Combustion:

• Occurs when a substance burns rapidly, producing a large amount of heat and light in a short period.
• Requires an external source of ignition (e.g., a matchstick).
• Examples: Burning of LPG in a gas stove, burning of a candle, burning of paper.

1. Spontaneous Combustion:

• The type of combustion in which a material suddenly bursts into flames without the application of any apparent cause (external heat).
• This happens when the substance reaches its ignition temperature on its own due to factors like ambient temperature, microbial action, or slow oxidation.
• Examples: Forest fires (due to heat from the sun or lightning striking dry leaves), spontaneous combustion of coal dust in mines, phosphorus burning in air at room temperature.

1. 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.
• Often occurs in a confined space, leading to a sudden increase in pressure.
• Examples: Bursting of firecrackers on ignition, explosion of methane gas in coal mines, detonation of explosives.

Comparison of Combustion Types

| Feature | Rapid Combustion | Spontaneous Combustion | Explosion | | :------------------ | :------------------------- | :------------------------- | :------------------------------- | | Speed | Fast | Slow build-up, then sudden | Very fast | | Ignition Source | External ignition needed | No apparent external source | External ignition often, but rapid gas expansion is key | | Products | Heat, light | Heat, light | Heat, light, sound, large amount of gas | | Pressure | No significant pressure build-up | No significant pressure build-up | Significant pressure build-up (if confined) | | Examples | LPG burning, candle flame | Forest fires, phosphorus | Firecrackers, dynamite |

What is a Flame?

• A flame is the visible, gaseous part of a fire.
• It is produced when gaseous substances burn.
• Substances that vaporize during burning produce a flame (e.g., wax, kerosene, LPG).
• Substances that do not vaporize during burning typically glow without producing a flame (e.g., charcoal, coal).

Structure of a Candle Flame

A candle flame typically has three main zones, each with different characteristics:

1. Outermost Zone (Non-luminous Zone):

• Hottest part of the flame.
• Appears blue due to complete combustion of fuel.
• Rich in oxygen, leading to full oxidation of carbon particles.
• Goldsmiths use this zone for melting gold and silver because it provides the maximum heat.

1. Middle Zone (Luminous Zone):

• Bright yellow and luminous.
• Moderate temperature.
• Partial combustion of fuel occurs here due to insufficient oxygen.
• Contains unburnt carbon particles that glow, giving it its yellow color and luminosity.
• Produces soot (carbon particles).

1. Innermost Zone (Dark Zone):

• Least hot part of the flame.
• Appears dark or black.
• Consists of unburnt wax vapors from the wick.
• No combustion occurs here due to the absence of oxygen.

Experiment: Glass Tube in Candle Flame

• When a thin glass tube is introduced into the dark zone of a non-flickering candle flame, and a lighted matchstick is brought near the other end of the tube, a flame appears at the other end.
• Observation: A flame is seen at the end of the glass tube.
• Explanation: The wax vapors from the dark zone of the candle flame are drawn up through the glass tube. These vapors are combustible and, upon reaching the ignition temperature at the other end, catch fire, producing a flame.
• This experiment confirms that the dark zone contains unburnt wax vapors.

What is a Fuel?

• A fuel is any substance that can be burned to produce energy (heat and light) for domestic and industrial purposes.
• Fuels are essential for cooking, heating, transportation, and electricity generation.
• Examples: Wood, charcoal, coal, petrol, diesel, kerosene, LPG, CNG, natural gas.

Characteristics of a Good Fuel

An ideal fuel should possess the following qualities:

• Readily Available: Easily accessible and in sufficient quantity.
• Cheap: Economical to purchase and use.
• Moderate Rate of Combustion: Burns at a controlled rate, not too fast or too slow.
• High Calorific Value: Produces a large amount of heat per unit mass.
• Easy to Store and Transport: Convenient to handle and move.
• Safe to Handle: Should not be highly explosive or toxic.
• Leaves No Undesirable Residue: Should burn completely without leaving harmful ash or pollutants.
• Does Not Cause Environmental Pollution: Minimal emission of harmful gases.

Calorific Value

• The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value.
• It is a measure of the efficiency of a fuel.
• The unit of calorific value is kilojoule per kilogram (kJ/kg).
• Higher calorific value means more heat is produced from the same mass of fuel.

| Fuel | Calorific Value (approx. kJ/kg) | | :------------ | :------------------------------ | | Cow Dung Cake | 6,000 - 8,000 | | Wood | 17,000 - 22,000 | | Coal | 25,000 - 33,000 | | Petrol | 45,000 | | Kerosene | 45,000 | | Diesel | 45,000 | | Methane | 50,000 | | LPG | 55,000 | | CNG | 50,000 | | Hydrogen | 150,000 |

• Example: LPG has a higher calorific value than wood, meaning 1 kg of LPG produces more heat than 1 kg of wood. This makes LPG a more efficient fuel for cooking.

While fuels are essential, their combustion, especially of fossil fuels, has several detrimental environmental impacts.

1. Carbon-containing Fuels and Air Pollution

• Unburnt Carbon Particles: When carbon-containing fuels (wood, coal, petrol) burn incompletely, they release fine unburnt carbon particles (soot).
• These fine particles are dangerous pollutants, causing respiratory diseases like asthma.
• Carbon Monoxide (CO): Incomplete combustion of carbon fuels produces carbon monoxide gas.
• CO is a highly poisonous gas. Breathing air containing CO can be fatal because it reduces the oxygen-carrying capacity of blood.
• This is why it's dangerous to burn coal in a closed room.
• Carbon Dioxide (CO₂): Complete combustion of most fuels releases carbon dioxide.
• CO₂ is a major greenhouse gas.

2. Global Warming

• The increased concentration of carbon dioxide in the atmosphere due to the burning of fossil fuels is believed to cause global warming.
• Greenhouse Effect: CO₂ traps heat radiation reflected from the Earth's surface, preventing it from escaping into space. This leads to a gradual increase in the Earth's average temperature.
• Consequences of Global Warming:
• Melting of polar glaciers, leading to a rise in sea level.
• Coastal flooding.
• Extreme weather events (droughts, floods, heatwaves).
• Disruption of ecosystems and agriculture.

3. Acid Rain

• Burning of coal and diesel releases sulphur dioxide (SO₂) gas.
• Petrol engines release oxides of nitrogen (NOx).
• These oxides (SO₂ and NOx) are acidic in nature.
• They dissolve in rainwater to form sulphuric acid and nitric acid, respectively.
• This acidic rainwater is called acid rain.
• Consequences of Acid Rain:
• Damage to crops, soil, and aquatic life.
• Corrosion of buildings (especially marble structures like the Taj Mahal) and historical monuments.
• Damage to forests.

4. Deforestation

• Extensive use of wood as fuel leads to deforestation.
• Deforestation has severe environmental impacts:
• Reduced oxygen production and increased CO₂ levels.
• Soil erosion and loss of fertility.
• Loss of biodiversity.

Cleaner Fuels

• CNG (Compressed Natural Gas): Considered a cleaner fuel because it produces harmful products in very small amounts compared to petrol and diesel.
• It burns more completely and produces less soot, carbon monoxide, and oxides of sulphur and nitrogen.
• LPG (Liquefied Petroleum Gas): Also a relatively cleaner fuel for domestic use.

Steps to Reduce Pollution from Fuels

• Using cleaner fuels like CNG and LPG.
• Improving engine efficiency.
• Developing renewable energy sources (solar, wind, hydro).
• Planting more trees (afforestation).