Chemistry of combustion

transcript

• FIRE is an associated series of chemical reactions, which releases the energy stored in fuel, such as heat and light (flame).

Three Things Every Fire Needs

air (oxygen)

Three Ways That Heat Moves

ConductionInside a solid

material

ConvectionIn a gas or

liquid

Radiation

In anything clear

heat flows out to fill the volume

hot fluid

movesupwar

heat glows

outward in all

directions

For certain temperatures, we see this radiation

glow as light.

whiteyellow

Glow Color Tells the Fire’s Temperature

Visible Colors

orange or redleast hot

yellow or white hot

blue or violet very hot

Invisible colors

Infraredvery warm

Ultraviolethottest

A fire is hottest at the bottom and coolest at the top.

How a Fire Burns

air air

Hot spark starts the flame

1. Flame move down to find fresh fuel

2. Radiation heats the nearby air

3. Convection moves hot air upward

4. Upwelling air pulls fresh air in behind it

5. Upwelling air won’t let fresh air reach the flame from the side

6. Hot air loses heat by radiation as it rises: we see flame color!

BURNING ZONE

OLD CONCEPT OF FIRE

FIRE TRIANGLE THEORY

Solid, liquid and Gas

Vapour

Factors : Volatility of fuelGeometrical arrangementsConfigurationally arangementSurface area to Mass ratio

Oxygen : obtained from the atmosphere

Air regulated fire

Fuel regulated fire

Theoretical requirement of air:

3.38 Kg of air is required for combustion of 10,000 Btu of potential heat in solid fuel

MODERN CONCEPT OF FIRE

TETRAHEDRON THEORY

FORMULATED BY WALTER . M . HAESSLAR

1. OXIDIZING AGENT

2. REDUCING AGENT

3. TEMPERATURE

4. UN-INHIBITED

CHAIN REACTION

Fuel vapour

Solid fuel

Liquid fuel

Cracking zone

HC CHO

Combustion zone

Products of combustion I.e C,CO,CO2,O,N,HO

ANATOMY OF FIRE

PRINCIPLES OF FIRE EXTINCTION

FUEL AIR

COOLING

STARVATION

AIRFUELSMOOTHERING

METHODS OF EXTINCTION OF FIRE

1. STARVATION

2. COOLING

3. SMOOTHERING OR BLANKETING

4. CUTTING OFF THE CHAIN REACTION

CLASSIFICATION OF FIRE

• CLASS “A” :

FIRE IN SOLID CARBONACEOUS SUBSTANCES LIKE WOOD, PAPER, JUTE, COAL, CLOTH ETC.

• CLASS “B” :

LIQUID FIRE- FIRE IN FLAMMABLE LIQUID LIKE PETROL, KEROSENE OIL, ALCOHOL, BENZENE ETC.

• CLASS “C” :

GAS FIRE- FIRE IN GASES LIKE

L.P.G., HYDROGEN, ACETELENE ETC.

• CLASS“D” :

METAL FIRE- FIRE IN METALS Like Sodium, Magnesium, Aluminum etc.

• Fire of any class (i.e. Class A, B, C Or D) , when associated with Electrical power supply, shall be taken extra care.

Flash Point

• The minimum Temp. at which any substance gives off sufficient combustible vapours to produce a momentary flash on the application of any external source of flame.

Fire Point

• The minimum Temp. at which any substance gives off sufficient combustible vapours to continue the flame on the application of any external source of flame.

Ignition Temperature

• The minimum temp. at which any combustible material starts burning itself without any application of external source of flame.

SPONTANEOUS COMBUSTION

It occurs as a result of heat generated by the reacting substance themselves, the substance soon begin to fume & afterwards they burst in to flame.

SPECIFIC HEAT

It can be defined as the number of calories needed to raise the temperature of 01 Grm. Of any substance by 01 deg. Cel.

SPECIFIC SURFACE AREA

oThe area of substance in terms of square centimeter for each gram of it.

FLAMMABILITY LIMITS

• Limits within which the

vapours of fuel can produce a

flame after mixing with air.

The minimum gas & air composition below which flame propagation is not possible, is called the lower flammability limit (LFL)

Lower limit

The maximum gas & air composition above which flame propagation is not possible, is called the upper flammability limit (UFL)

Upper limit

Flammability limits

FUEL LFL UFL

• H2 4.0 75.0

• CO 12.5 74.2

• CH4 5.0 15.0

• C3H8 2.2 9.5

• C4H10 1.5 9.0

(% by Volume)

Fire Suppression Procedures

PRIMARY

• Removal of Fuel

• Reduction of Heat

• Reduction of Air

SECONDARY

• Shock Wave

• Critical Vibration

• Ion Separation

(strong electric field)

PRIMARY FIRE SUPRESSION

COOLING (limitation of temperature)

SMOTHERING (limitation of oxygen)

STARVATION (limitation of fuel)

COOLING

• The cooling principle is the one most commonly

employed for fire extinction

• The extinguishing medium operates by

absorbing heat from the fire.

• Its temperature is raised.

• It is converted to the vapour state.

• For Fire fighting, Water is the best coolant

WHY WATER?• Easily available.

• Comparatively cost is less than other extinguishing media.

• High latent heat of vapourisation i.e. 536 cal/ gm

• After evapourisation it expands enormously (1:1700 times).

• Thus removing the oxygen from the scene of Fire.

Starvation

• By removing combustible material from the neighborhood of the fire.

• By removing the fire from the neighborhood of combustible material .

Smothering

• If the oxygen content of the atmosphere in the immediate neighborhood of burning material can be sufficiently reduced, combustion will cease.

• Agents- Foam, CO2, DCP, Sand etc

SECONDARY FIRE SUPRESSION

• Shock Wave

• Critical Vibration

• Ion Separation (strong electric field)

SHOCK WAVE

• Shock Waves are normally generated by explosion,

Resulting in

– Rise in temp. & Pressure in a coffined space.

– Time lag between Ignition and destructive pressure.

– Suppression system can react between this time lapse.

• Explosion may be single or Multiple

CRITICAL VIBRATIONS

• It is done to disrupt the mixing of fuel vapour and oxygen in a proper ratio. This disruption will create a time lag and that is the time for activating primary fire suppression system.

• Precision is essential.

ION SEPARATION Fire is a chemical process between

Reducing and Oxidising agents. Chemically, positive and negative charged ions participate in the exothermic reaction i.e,. Fire. Creation of strong electric field in the vicinity of Fire, will force the ions to move towards the electrodes, thus not allowing them to mix up and react with each other.

Hence fire will not sustain.

• Bad house-keeping

• Electrical

• Careless smoking

• Poor maintenance of machinery e.g: Mechanical heat, Spillage of hot oils/ lubricants.

Contd…..

• Unsafe handling of Hazardous chemicals

and gases

• Lack of periodical maintenance of

pipelines carrying steam, gas, fuels

• Poor awareness about fire safety systems

• Poor manpower training in Fire fighting

• Scorch marks on furniture or fittings.

• Discoloured or charred electrical plugs,

sockets.

• Cigarette burns.

DO NOT LOSE SIGHT

• Hazards–Arcing

–Overheating

–Shock

• Causes–Improper installation

–Lack of maintenance

–Improper use

–Carelessness or over-sight

• Prevention

– Use of proper size conductor appropriate to

the load

– Suitable insulation of environment

– Correct rated fuse/MCB

– Provision of ventilation/air circulation

– Prevention /Dissipation of static charges

• Good house keeping

• Separation of process from storage.

• Preventive maintenance of machinery and periodical lubrication.

• Regular fire safety checks and fire drills.

• Standard work practices.

• Maintenance of fire safety systems.

• Training of personnel.

Diffusion Flame

• Flame in which reactants are

mixed by diffusion, hence called

Diffusion flame.

Pre- Mixed Flame

• Occurs when a Fuel is well mixed

with an oxidising agent before the

ignition source is applied

BE ALERT AND AVERT FIRE LOSSES

THANK YOU

Chemistry of combustion

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