Combustion and Flame
Introduction
People generally use wood, cow dung cakes, crop residues,
kerosene oil, coke or LPG gas to cook food. For ruining vehicles, we use petrol
or diesel oil. In factories, we use coal or fuel oil or natural gas.
The materials, such as wood, coke, LPG (liquid petroleum gas),
petrol, diesel, natural gas have one property [n common, i.e., they produce
heat on burning which is then put into different uses. Furthermore, some
materials on burning produce flame and some do not. For example, a candle or
coal gas on burning produces a flame, but not the coal or charcoal.
Combustion
Some substances, like magnesium catches fire and burns 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 producing carbon dioxide, heat and light.
Air or oxygen which helps in burning is called supporter of
combustion and the chemical reaction which takes place with the release of heat
and light energy is called combustion.
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.
Fuel may be solid, liquid or gas. A combustion reaction maybe
written as-
(a) Charcoal burns in air to give carbon dioxide
and heat.
C
+ O2 → CO2 + Heat
(b) Methane bums in air forming carbon dioxide, water and heat.
CH4 +
2O2 → CO2 + 2H2O + Heat
Some interesting facts:
Q1. Why is the person caught in fire, is covered with a blanket?
Explanation
If the clothes of a person catch fire, he is immediately wrapped
in a thick blanket. The blanket cuts off the supply of air i.e. the supporter
of combustion and hence the fire is put off.
Let us we some more activities to team more about combustion.
Can a matchstick burn on its own? Can we bum a wood piece by a lighted
matchstick? The answer to these questions Is - No.
This shows that different substances bum at different
temperatures. Some minimum temperature is essential for burning a combustible
substance, before it catches fire. This minimum temperature is called 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 temperature of air is 35°C or more,
phosphorus will catch fire without heating. Similarly, Kerosene oil and wood do
not catch fire on their own at room temperature. But if both are heated a
little, than kerosene oil will catch fire and not wood. This shows that ignition
temperature of kerosene oil is lower than that of wood..
Q2.Why does the fire goes off when water is poured over burning
wood?
Explanation
It happens because water absorbs a large amount of heat energy,
which results in fall in the temperature of wood below its ignition temperature
and the fire goes off.
Types of combustion
Combustion is mainly of three types
(1) Rapid combustion (2) Spontaneous combustion (3) Explosion
1. Rapid combustion: The
combustion in which the gas burns rapidly and produces heat and light is called
rapid combustion.
For
example: When a burning matchstick is brought near a gas burner and
the gas tap is opened, the gas immediately starts burning with the production
of heat and light. Eg. Petrol, LPG, spirit, etc.,
2. Spontaneous combustion: The
combustion in which no external heat is given is known as spontaneous
combustion.
For
example: Burning of white phosphorus in air.
3. Explosion: The combustion in which large
amount of gases are evolved with the production of a tremendous amount of neat;
light and sound is called explosion,
For
example: When a cracker is ignited, a sudden reaction, oxidation
process takes place, and in which at high speed large amount of gas is evolved
with the production of tremendous amount of heat, light and sound.
Difference between rapid and spontaneous combustion
Forest fires are a result of which type of combustion and why ?
Explanation
Forest fires are the result of spontaneous combustion because of
the following factors
(i) presence of a
combustible substance
(ii) presence of right ignition temperature
(iii) presence of supporter of combustion in
air
Flame-
A flame is the
visible, gaseous part of a fire. It is caused by a highly exothermic reaction
taking place in a thin zone. Very hot flames are hot enough to have ionized
gaseous components of sufficient density to be considered plasma.
Why does a matchstick produce a flame on burning?
Explanation
When the matchstick catches fire or is burnt, the heat released
due to the burning of chemicals on the match head, partly decomposes the wood
to form wood gas. The wood gas then catches fire and produces a flame.
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 four zones.
(1) Blue zone: It is near
the base of the flame. Vaporised wax gets oxidised to carbon monoxide and carbon monoxide bums
completely with a blue flame in this zone.
(2) 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 unscrotched (unbumt) showing that
there is no ‘burning’ in this zone. However, some =burnt wax vapours are present in this zone.
(3) 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 unbumt carbon particles are left which give rise
to soot.
(4) 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.
Q. Why do goldsmiths use the outermost zone of the flame for
melting gold and silver?
Explanation
Goldsmiths, use the outermost zone or non-luminous part of the
flame for melting or shaping gold and silver ornaments. They direct the
non-luminous part of the flame of candle on the gold with the help of a blow
pipe. fps the temperature of this part of the flame is around 1300°C, it is
sufficient to melt gold at specific point and thus helps goldsmiths to give
proper shape to the gold ornaments.
Fire Extinguisher
1. Foam
type fire extinguisher
2. Soda
acid type extinguisher
Foam type fire extinguisher-
It consists of two cylinders:
Outer cylinder: It is made of metal (contains baking soda).
Inner cylinder: It is made up of glass which contains aluminium sulphate and aluminium sulphate is mixed with saporin.
Both cylinders are bound by common knob. When the knob is
pressed, the glass cylinder breaks. Then both chemicals, including baking soda
and aluminium sulphates get
mixed, react and as a result, produce carbon dioxide that comes out with great
pressure, cuts off oxygen supply and fire gets extinguished.
NaHCO3 + Al2(SO4)3 (Aluminium sulphate) →
Na2SO4 + Al2(CO3) + CO2 + H2O
Hence, we can say that carbon dioxide is a non
supporter of combustion. Saporin which is
mixed is useful in formation of Carbon Dioxide and it form Carbon Dioxide in
the form of foam.
Soda acid type extinguisher-
It consists of two cylinders:
Outer Cylinder: It is made up of metal, which contains
baking soda.
Inner Cylinder: It is made up of glass which contains
sulphuric acid.
Both cylinders are bound by a
common knob. When the knob is pressed, the glass cylinder breaks and both
chemicals including baking soda and sulphuric acid are mixed, react and as a
result, produce carbon dioxide that comes out with great pressure, cuts off
oxygen supply and fire gets extinguished. In this process, the carbon dioxide
produced is in the form of gas.
What is Fuel?
Any substance which is easily available and burns in air at a
moderate rate, producing a large amount of heat energy, without leaving behind
any undesirable residue is called fuel.
Example: wood, charcoal, petrol, kerosene, etc.
Why is sulphur not used as a fuel even though it can bum in air
to produce heat?
Explanation
Sulphur is easily available in nature and can bum in air to
produce heat. However, it is not a fuel because on burning, it forms a
poisonous gas, SO2 , which can cause
serious respiratory problems and can even be fatal.
Ideal Fuel
There is probably no fuel that could be considered as an ideal
fuel. We should look for a fuel which fulfils 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
(4) It should have low ignition temperature.
(5) It should produce very small amount of
residues such as ash.
(6) It should have 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 fueils: 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 is
the calorific value of a fuel, 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.
Calorific value of solid fuels
Calorific value of liquid fuels
Calorific value of gaseous fuels
Harmful effects of burning fuels
The increasing fuel consumption has harmful effects on the
environment. The main products formed during the fuel combustion which produce
harmful effect are:
1. Carbon fuels like wood, coal, petroleum
release unburnt carbon particles. These fine particles are dangerous pollutants
causing respiratory disease, such as asthma.
2. Incomplete combustion forms carbon monoxide
gas. It is very poisonous gas. It is dangerous to burn coal in a closed room.
The carbon monoxide gas produced can kill persons sleeping in that room.
3. Combustion of most fuels releases carbon
dioxide in the environment. Increased percentage of carbon dioxide in the air
causes global warming. Global warning is the rise in
temperature of the earth. This result in rrrre1ting of polar glaciers. This
leads to rise in sea level and floods in the sea coast.
4. Burning of coal and diesel release sulphur
dioxide gas. It is an extremely suffocating and corrosive gas. Sulphur dioxide
and nitrogen oxide dissolve in rain water to form acid. Such rain is called
acid rain. It is very harmful for crops, buildings and soil.
5. Wood is also used as a fuel. Burning of wood
gives a lot of smoke which causes air pollution and is also very harmful for
humans. It may lead to many respiratory problems. Cutting of trees for
obtaining wood Leads to deforestation which is quite harmful to environment.
6. Carbon particles of smoke or the ash get
suspended in the air. Excessive amount of them in the air causes breathing
problems.
Acid Rain
Burning of coal and diesel produces sulphur dioxide gas which
can cause suffocation and also it can lead to acid rain.
Or
Due to burning of coal and diesel, Chemicals like sulphur dioxide and nitrogen dioxide are released into the
air. The pollutants reacts with the water vapour present in the air and form
sulphuric and nitric acid. When it rains, these acids are also present. Such
kind of rain is called Acid Rain. It is very harmful for crops, buildings
and soil. It also affects aquatic organisms and can cause monument cancer.
Prevention from Acid Rain-
The use of diesel and petrol as fuels in automobiles is
being replaced by CNG (Compressed Natural Gas), because CNG produces harmful products
in very small amounts. CNG is a cleaner fuel.