chemistry of fire
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fire
Definition
It is a chemical chain
reaction which takes place with
the evolution of heat and light.
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Fire triangle
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Essentials of Fire
Fuel
FIRE
Heat
Oxygen
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OXYGEN SOURCE
HEAT SOURCES
To Reach Ignition Temp.
Approx. 16% Required
Normal air contains 21% of oxygen
Some Fuels contains its own oxygen
supply
Oxygen
Open Flame, the Sun,Hot Surface,
Sparks & Arcs, Friction, Chemical
Action, Elec. Energy & Gas
Compression
Heat
Fuel
GASES
Natural Gas,
Propane, CO,
Butane, Hydrogen,
Acetylene,
LIQUIDS
Gasoline, Kerosene,
Turpentine, Alcohol,
Paint, Varnish, Olive
oil, Lacquer
FIRE TRIANGLE
SOLIDS
Coal, Wood, Paper,
Cloth, Wax, Grease,
Leather, Plastic,
Sugar, Grain, Hay,
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Fire Tetrahedron
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Principles of Fire Extinction
1. Starvation
Removal of un-burnt material from fire area
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Principles of Fire Extinction
2. Smothering
Cutting off the supply of oxygen from fire area
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Principles of Fire Extinction
3. Cooling
Removal of heat from the burning material/fire area
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Principles of Fire Extinction
4. Breaking of chain reaction
It is a chemical process
Mono-ammonium phosphate
NH3
&
H2O
Ortho-Phosphoric Acid
Meta-Phosphoric Acid
Flame
Steam
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Fig: Extinguishing mechanism of multipurpose mono-ammonium phosphate dry chemical
Phases of Burning
It depends on the following factors
i. The amount of time the fire has burnt.
ii. The ventilation characteristics of the
confining structure
iii. The amount and type of combustibles
present.
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Three progressives stages of burning
•Incipient or beginning phase
•Free burning phase
•Smoldering phase
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Oxygen Rich
Atmosphere
Oxygen Deficient
Atmosphere
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Incipient or beginning phase
•Oxygen plentiful
•Temperature has not built up
to high peak
•Thermal updraft rises,
accumulates at highest point
•Breathing not difficult
•Fire extinguishments :
–Direct application of water
at base of fire.
•Ventilation :not a problem
•Little steam production
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Free burning phase
•Fire has involved more fuel
•Oxygen supply is being depleted
• Heat accumulated at upper
areas
•Breathing difficult : Masks
recommended
•Fire extinguishments is reaching
the area of major involvement
•Ventilation : Not a definite need
•Good steam production
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Smoldering phase
•Oxygen supply not equal to
demands of fire
•Temperature throughout
building is very high
• Normal breathing is not
possible
•Oxygen deficiency may cause
back-draft
•Fire extinguishments indirect
method
•Ventilation : A must
•Maximum steam production
from water fog
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Smoldering phase
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Modes of spread of fire
Conduction
It occurs only in solids i.e.
metallic objects.
Convection
It occurs both in liquids &
gases
Radiation
It is neither conduction nor
convection. These are heated
rays emanating from the hot
object.
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Conduction
Highly Excited
Less Agitated
•Heat energy is passed on
from one molecule to the
next
•The molecule vibrates
above its mean position
•Pass on heat energy by
colliding with their
neighbors
•The ability to conduct
heat varies between
materials.
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Conduction
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conduction
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Convection
• When a liquid or gas heated,
it expands
• It becomes less dense
• The lighter fluid arises being
displaced by colder and
therefore denser fluid
• In turn becomes heated and
so a circulation is set up
• Heat energy is carried
throughout the fluid by
actual movements of
molecules.
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Fig: Fire Plume
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Radiation
• It does not involved
any contact between
bodies
• It is independent of
any material in the
intervening space
• These are the heated
rays, travels in
straight line in all
directions with same
intensity
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Other chemical reactions which
produce
1. Combustion (heat & light)
2. Combustion (without heat & light)
3. Combustion ( without heat)
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Rusting of iron
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Types of combustion
i.
Slow or incipient combustion- in which
the amount of heat & light emitted is
feeble.
ii. Rapid or active combustion- in which a
considerable amount of heat & light is
emitted within a short time
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Types of combustion…
iii. Deflagration - which
takes place with a
considerable rapidity,
evolving heat & light
iv. Explosion - is a very
rapid combustion with a
loud noise within an
extremely short time
with generation of very
high pressure &
temperature.
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Rate of Combustion
A rate of combustion or the spread of fire would
depends on :
•The area of solid/liquid in contact with air.
•The amount of heat generated to raise the
temperature of un-burnt portion.
•The ability of materials to conduct heat away.
•Atmospheric humidity.
•Wind velocity.
•Temperature.
•Atmospheric pressure.
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CHEMICAL REACTIONS
EXOTHERMIC REACTION
When heat is liberated in chemical reaction is known as
Exothermic Reaction.
Ex :
N2+3H2=2NH3+ H(-24Kcal)
C+O2=CO2+ H (-94.5 K cal)
Bond
Heat Liberated
Being Broken
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CHEMICAL REACTIONS…
ENDOTHERMIC REACTION
When heat is absorbed from the reacting
substances is known as Endothermic Reaction.
Ex:
2C + H2 = C2H2 + H (54 Kcal)
3O2
2O3 + H (69 Kcal)
Heat Absorbed
Bond
Being Formed
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SPECIFIC SURFACE
It is the surface area in square centimeter per
gram of that solid substance.
Liquids & gases have no specific surface, these
two take the shape of container.
On the basis of specific surface and fire
susceptibility, all combustible solids classified into
three :
»Tinder
»Kindling
»Bulk fuel
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SPECIFIC SURFACE
Ex: Paper
TINDER :
Solids with specific surface of more
than 20 Sq. cm/gm.
Tinder can be ignited by match stick.
Ex: Card Board
KINDLING :
Solids with specific surface of 2 to 20
Sq.cm/gm.
Kindling requires a burning tinder for
ignition.
Ex: Wood Block
BULK FUEL :
Solids with specific surface of 0.04 to
2 Sq. cm/gm.
Bulk Fuel requires burning Kindling for
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ignition.
IMPORTANT DEFINITIONS
•Flash Point : It is the lowest temperature at
which an inflammable substance gives off
sufficient vapours, so as to form a momentary
flash on application of a pilot flame.
•Fire Point : It is the lowest temperature at
which the heat from the combustion of burning
vapours is capable of producing sufficient
vapours to enable combustion to continue.
The Fire Point is generally above the Flash
Point.
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IMPORTANT DEFINITIONS
•Ignition Temperature : It is the lowest
temperature at which spontaneous combustion
can takes place without application of an
external heat.
•Auto Ignition Point : Refers to the temperature
to which a substance must reach, before it
ignite, in the absence of flame, but in presence
of air.
•Spontaneous Combustion : It occurs as a result
of heat generated by the reacting substances
without any external heat.
e.g. Hot glycerine + Potassium Permagnate
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IMPORTANT DEFINITIONS
Density :
The Density of substance is its mass per
unit volume. Unit of Density(D) is Kg/m3
or gms/cm3
Ex:
Water - 1000 Kg/m3 or 1 gm/cm3
Mercury - 13.6 gm/cm3
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IMPORTANT DEFINITIONS
Relative Density or Specific Gravity :
It is a ratio of the mass of any volume
of a substance to the mass of an equal
volume of water.
Specific gravity, or relative density
Material density to the density of water
S.G. < 1 floats on water
S.G. > 1 sinks in water
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IMPORTANT DEFINITIONS
Vapour Density : It is the ratio of the mass of a
given volume of the vapour to the mass of an equal
volume of air under the same temperature &
pressure.
Dense Vapor (V.D. >1) Hazards
• Choking, suffocation, death
• Distant ignition possible
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IMPORTANT DEFINITIONS
Vapour Pressure : It is the pressure exerted by
the vapour of the liquid at any given temperature.

Breakup can be very explosive

Pressure + weakened container  container breaks

Vapor Pressure Hazards
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IMPORTANT DEFINITIONS
Latent Heat : It is the amount of heat energy required
to change the state of a matter without raising the
temperature.
Latent Heat of Vapourization : It is the heat energy
which is absorbed by the liquid at its boiling point, to
convert from its liquid state to gaseous state, without
raising the temperature.
Ex: Water - 2260000 Joules/Kg
Latent Heat of Fusion : It is the heat energy which is
required to change the state from solid to liquid at
melting point of substance without raising the
temperature.
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Ex: Water - 336000 Joules/Kg
IMPORTANT DEFINITIONS
Thermal Capacity : The thermal capacity or heat capacity
of a body is the heat required to raise its temperature by
1 degree Centigrade.
Ex: Water - 4.2 KJ/Kg/degree Centrigrade
Heat Energy can be transferred from a place of higher
temperature to one at lower temperature. When heat is
added to a body the temperature rises.
Specific Heat : It is the ratio of thermal capacity of a
substance to that of water.
Material with low specific heat will heat up more
rapidly in fire conditions.
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IMPORTANT DEFINITIONS
Thermal Expansion of Solids :
When a solid is heated, it expands in length, breadth &
thickness.
Solids which are homogeneous expand uniformly.
Co-efficient of Linear Expansion :
The amount with which unit length of substance
expands when its temperature is raised by 1 degree
Centigrade is called the Co-efficient of Linear
Expansion of the substance.
Ex: Steel - 0.000012 per degree Centigrade.
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IMPORTANT DEFINITIONS
Co-efficient of Cubical Expansion :
Solid - it is 3 times the co-efficient of linear expansion.
Liquid - only the co-efficient of volume expansion is
applicable.
Gases - can be measured either as an increase in volume
at constant pressure or as the increase in pressure at
Gas or
constant volume.
Vapor
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Any Questions
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Any Questions
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THE END
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