In this chapter, we will discuss the following topics:
- What is Combustion?
- Ignition temperature
- Condition for Combustion
- Controlling the fire
- Types of combustion:
(i) Rapid combustion
(ii) Spontaneous combustion
- Zones of candle flame
- Fuel efficiency
What is Combustion?
The process in which a substance undergoes a chemical reaction in the presence of air (oxygen) to produce heat and light is called combustion.
Combustion is a chemical process.
- Burning of Wood or Coal to heat your home.
- Burning of Petrol or Diesel to run your Car.
- Combustion of Natural Gas or LPG to cook for on your stovetop
- For the production of energy in thermal power plants
- Substances that go under combustion are known as combustible substances. Combustible substances are also known as fuel.
- The fuel may be in a solid, liquid or gas state.
Example: Wood, Charcoal, LPG, Kerosene, Petrol, Diesel, etc.
- An ignition temperature is the lowest temperature at which a substance catches fire.
- Ignition temperatures are different for different substances.
- It is the minimum temperature to which a combustible substance must be heated, before it catches fire.
Therefore, we see that a combustible substance cannot catch fire as long as its temperature is lower than its ignition temperature e.g. Ignition temperature of phosphorus is 35 °C. So, unless phosphorus is heated to 35 °C, it will not catch fire. However, if temperature of air is 35°C or more, phosphorus will catch fire without heating.
Example: LPG, petrol, natural gas, etc. catch fire at very low temperatures and thus have a low ignition temperature, while wood, coal, etc. have a high ignition temperature.
Conditions for Combustion
There are three conditions necessary for combustion:
- Fuel: Fuel is the substance that undergoes combustion.
- Supply of Air: Oxygen helps in combustion. Air contains about 29% of oxygen, thus the supply of air makes the oxygen available which helps in combustion. Without oxygen, combustion will not take place.
- Ignition Temperature: For catching fire, a combustible substance must reach its ignition temperature. If a combustible substance does not reach or above its ignition temperature, it will not catch fire and combustion will not take place.
Thus, the above three conditions are necessary for combustion to take place. If any one of the three will not be available, combustion will not take place.
Controlling the Fire
Generally, water is used to control fire. Water brings down the temperature of the combustible substance below its ignition temperature. The water vapour surrounds the combustible material, thus helping in cutting off the supply of air. So, that the fire is extinguished.
Fire produced by the burning of oil or petrol cannot be controlled by throwing water on it because water being heavier than oil, settles down the oil and oil continues to burn
We know that there are 3 conditions necessary for producing and sustaining combustion.
- Presence of a combustible substance.
- Presence of a supporter of combustion.
- Attainment of ignition or kindling temperature.
Thus, fire can be controlled by removing one or more of these requirements of fire control.
- Many times homes, forest, shops, etc. catch fire. In such cases, it becomes necessary to put off the fire otherwise, it may cause huge monetary loss and loss of lives.
- Fire Brigade or Firemen are experts in controlling fire by using a fire extinguisher. To extinguish the fire, at least one out of three essential conditions for combustion must be removed. Supply of fuel, supply of air and ignition temperature are the three essential conditions for combustion.
- In case of fire in a building, the whole building becomes fuel. It is not possible to cut off the supply of fuel. Thus, firemen try to cut off the air supply and or bring down the temperature of combustible material below their ignition temperature.
- Water is one of the best, cheapest and oldest fire extinguishers. By pouring water over the combustible material, the temperature can be cooled down. Cooling down brings the combustible materials below their ignition temperature.
- In addition to this, water vapour surrounds the combustible material which stops the supply of air. Removal of these two conditions puts off the fire. This is the cause that firemen generally pour water over the materials which have caught fire.
Different Types of Fire Extinguisher
1. Controlling fire when electrical equipment is on fire
- Water is not suitable in the case when electrical equipment or oil catches fire.
- In the case of electrical equipment on fire, pouring water over them may prove disastrous because normal water is a good conductor of electricity.
- It may conduct electricity and can harm the persons who are trying to control the fire.
2. Controlling fire in the case of oil, petrol, etc.
- Water is heavier than oil. So when water is poured over oil. It comes on top and keeps on burning. In such cases, a fire extinguisher is used.
3. Carbon dioxide as a fire extinguisher
- Carbon dioxide does not support combustion and hence is considered the best fire extinguisher.
- Carbon dioxide is heavier than air and hence covers the material which is burning. By covering the material, the supply of oxygen is stopped. This puts off further combustion and fire is controlled.
- Under high pressure, carbon dioxide liquefies and takes less space because of compression. Liquid carbon dioxide is stored in cylinders.
- A nozzle is attached to the cylinder to release carbon dioxide. When the nozzle is opened, carbon dioxide starts coming out from the cylinder because of high pressure. It expands and covers the combustible materials as a blanket.
- This cuts off the supply of oxygen to combustible materials. Because of expansion, the temperature of carbon dioxide decreases which decrease the temperature and brings down the combustible material below their ignition temperature. Thus, the stoppage of the supply of oxygen and bringing down the temperature below the ignition temperature of combustible materials put off the fire.
4. Sodium bicarbonate or potassium bicarbonate as a fire extinguisher
- Sodium bicarbonate (baking soda) or potassium bicarbonate releases carbon dioxide on heating.
- Thus, when powder of sodium bicarbonate or potassium bicarbonate is spread over or near the fire, they release carbon dioxide which covers the burning materials and cuts off the supply of oxygen to them. This puts off the fire.
Types of Combustion
- Rapid energy needs external heat energy for the reaction to occur.
- Combustion produces a large amount of heat and light energy and does so rapidly.
- The combustion will carry on as long as the fuel is available.
- An example is when you light a candle. The heat energy is provided when we light the candle with a matchstick. And it will carry on till the wax burns out. Hence it is a rapid combustion.
- As the name suggests, the combustion occurs spontaneously. This means that it requires no external energy for the combustion to start.
- It happens due to self-heating.
- A substance with low-ignition temperatures gets heated, and this heat is unable to escape.
- In the presence of sufficient oxygen, the temperature rises above the ignition point and combustion will happen.
- The reaction of alkali metals with water is an example.
- Explosive Combustion happens when the reaction occurs very rapidly.
- A large amount of gas formed in the reaction is liberated. Such a reaction is called an explosion.
- The reaction occurs when something ignites to produce heat, light and sound energy.
- Some classic examples are firecrackers or blowing up of dynamite.
Zones of Candle Flame
When you light a candle a combustion reaction takes place with the wax of the candle which is the fuel and the air which contains oxygen. The flames are the are in which this combustion reaction is taking place. The release of heat and light energy from this exothermic reaction happens through the flame.
Starting from the base of the flame, a flame has four zones.
1. Inner Part
- This is the innermost part of the flame.
- It is the part closest to the wick.
- You might assume that this is the hottest part of the flame. However, it is the least hot.
- This is the black part of the flames that contains unburnt particles of the carbon from the wick i.e. unburnt fuel.
2. Middle Part
- This is the biggest part of the flame.
- The colors in this are varying shades of yellow and orange.
- This is the luminous flame because it emits light.
- This part is also not extremely hot. This is because this part gets a limited supply of oxygen.
- Incomplete combustion takes place here which is why it burns orange and is luminous.
3. Outer Part
- This is the hottest part of the flame.
- This part has an unlimited supply of oxygen. So complete combustion takes place here. Hence it is the hottest part of the flame.
- Also, this part of the flames burns with a blue color.
- It is the non-luminous, i.e. does not emit light.
Zones of a Candle Flame
Fuel is any substance that can provide heat and produce energy when it is burned. This energy that releases is generally in the form of chemical energy or heat energy.
The recent invention of nuclear technology means that now even nuclear energy may be released due to nuclear fission or fusion.
Examples of Fuel
- Natural gas.
Types of Fuel
1. Solid Fuel
- These are solid materials that combust to produce energy.
- Some examples of Solid fuel are coal, charcoal, soot, wood etc.
- These were most likely the first fuels utilized by mankind.
2. Liquid Fuel
- These are the fuels we burn to produce mechanical energy and kinetic energy.
- Most liquid fuels such as crude oil form due to exposure to intense heat and pressure to fossilized remains of plants and animals
3. Fuel Gas
- Fuel Gas as the name suggests are fuels that are in a gaseous state under normal conditions.
- Some examples are methane, carbon monoxide, propane etc.
- They have an advantage that they can be easily transported to the place of consumption.
- Biofuel can be solid, liquid or a gas.
- The only condition is that it must be derived from Biomass.
- So essentially it is fuel derived from living matter that can be replenished.
- One such example is ethanol made from sugarcane wastes
5. Fossil Fuel
- These are conventional fuels. They are also non-replenishable.
- They have formed over thousands of years in the earth’s core where fossilized remains of animals and plants have been exposed to high pressure and temperatures.
- These are fast depleting and our dependence on them is a major concern for all of humanity.
Fuel efficiency is measured as the amount of heat that 1 kg of fuel (any fuel) produces on combustion. This is known as the calorific value of the fuel. The unit of measurement of fuel efficiency is kilojoules per kg, i.e. kJ/kg.