Combustion and flame

Combustion:

Some substances, like magnesium, catch fire and burn with a dazzling white flame when heated in

air. Similarly, when a piece of per is brought near a candle flame, it burns, producing heat and light.

The charcoal and coal bums in air produce carbon dioxide, heat and light.

A chemical process in which a substance reacts with oxygen to give off heat is called combustion.The

substances which undergo combustion are known as combustible substances. It may also be called a

fuel.

Sometimes, heat is accompanied by light, either as a flame or as a glow, during combustion.

Types of Combustion:

1. Rapid combustion – When combustion occurs rapidly, it is called rapid combustion.

2. Spontaneous combustion – The type of combustion in which material suddenly bursts into flames,

without the application of any apparent cause is called spontaneous combustion. Eg, Burning of

phosphorus.

3. Explosion – When a sudden reaction takes place with the release of heat and light and evolution of a

large amount of gas takes place it is called an explosion. Eg, firecrackers.

Conditions necessary for combustion:

● Presence of a combustible substance

● Presence of supporter of combustion

● Attainment of Ignition temperature

Ignition temperature: 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 the temperature of air is 35°C, fire can

be controlled by removing one or more of these requirements of fire control.

Principle of Fire extinguisher: The fire extinguisher also try to cut off the supply of air or bring

down the temperature of the fuel or both, to control the fire. The combustible substance as such

cannot be eliminated in most of the cases from the place of fire.

How can fire be stopped?

Fire can be controlled by removing one or more of these requirements of fire control. The fire

extinguisher also tries to cut off the supply of air or bring down the temperature of the fuel or

both, to control the fire. The combustible substance as such cannot be eliminated in most of the cases

from the place of fire.

There are various types of fire extinguishers used for controlling a fire. The job of a fire

extinguisher is to cut off the supply of air or bring down the temperature of the fuel.

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Soda - acid fire extinguisher: Carbon dioxide liberated by the reaction of sulphuric acid with

sodium bicarbonate solution comes out with a stream of liquid water at high pressure. Water puts off

the fire by lowering the temperature of the combustible material below its ignition temperature and

carbon dioxide cuts off the supply of air to the combustible substance.

Water : It cools the temperature of the burning substance below its ignition temperature. It also

cuts off the oxygen supply by forming water vapours.

Structure of a flame : In order to understand the structure of a flame, light a wax candle and

watch its flame. Carefully note the different coloured zones in the flame. Starting from the base of the

flame, a flame has three zones.

(1) Dark inner zone: Surrounding the wick is the dark zone. There is no burning in this zone. If

we pass a wooden splinter through the dark zone of the flame, it comes out unscratched (unburnt)

showing that there is no ‘burning’ in this zone. However, some burnt wax vapours are present in this

zone.

(2) Luminous zone: In this region of the flame hydrogen burns with a brilliant yellow luminous

flame. Burning hydrogen combines with oxygen to form water vapour. Carbon also burns in this zone

giving some luminosity to the flame and producing carbon dioxide. Some unburnt carbon particles are

left which give rise to soot.

(3) Outermost non-luminous zone: This zone is poorly visible and is slightly blue. It is the

hottest part of the flame where complete oxidation (burning) of the fuel is taking place.

Ideal Fuel

There is probably no fuel that could be considered as an ideal fuel. We should look for a fuel which

fulfills most of the requirements for a particular use.

Characteristics of a good fuel

(1) It should be cheap and readily available.

(2) It should be easy to store.

(3) It should bum at a slow rate and its combustion should be controllable

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(4) It should have a low ignition temperature.

(5) It should produce a very small amount of residues such as ash.

(6) It should have a large calorific value.

(7) It should not produce gases which pollute the air.

(8) It should not produce any hazards during transportation.

Classification of fuels

On the basis of physical state, fuels are classified into three parts.

1. Solid fuels: The fuels which occur In a solid state at room temperature are called solid

fuels.Example: Wood, agricultural residues, charcoal, coal, coke, etc.

2. Liquids fuels: The fuels which occur in a liquid state at room temperature are called liquid fuels.

Example: Liquefied hydrogen, petrol, oil, kerosene, diesel, etc.

3. Gaseous fuels: The fuels which occur in a gaseous state at room temperature are called

gaseous fuels.

Example: Water gas, producer gas, coal gas, compressed natural gas (CNG) and gobar gas, etc.

Fuel efficiency

The amount of heat energy produced on completely burning one Kilogram of fuel (in 02) is called the calorific

value of a fuel. The more the calorific value of a fuel, the more is the efficiency of the fuel. The calorific value of

the fuels is expressed in Kilojoules per kilogram (kJ/kg) or kilojoules per gram (kJ/g). The calorific value of

common solids, liquids and gaseous fuels is given below.

Pollution Caused by Combustion of Fuels

● Burning of coal and diesel releases sulphur dioxide gas. It is an extremely suffocating and

corrosive gas. Moreover, petrol engines give off gaseous oxides of nitrogen. Oxides of sulphur and

nitrogen dissolve in rain water and form acids.

● Carbon monoxide (CO) is a toxic air pollutant produced largely from vehicle emissions.

● Fuels like wood, coal, petroleum etc, which are carbon fuels produce unburnt carbon particles,

which cause respiratory diseases and skin diseases.

● Generally CO2 is released during burning of fuels. It causes the rise in the temperature of the

atmosphere of earth, which is called global warming.

● Incomplete combustion of fuel releases many poisonous gases such as carbon monoxide. It can

kill people sleeping in a room by causing suffocation.

● Combustion of coal and diesel generates sulphur dioxide. It can kill people sleeping in a room by

causing suffocation.

● Burning of fuel releases oxides of nitrogen and sulphur. They dissolve in rain water and form acid

rain.