fire

THE CHEMISTRYOF FIRE

Fire Triangle

Fires Require

Air (oxygen)FuelHeat

Air

Air is required as a catalyst

Can be oxygen from the air

OR

From the Fuel itself

Fuel

For a fire to start there must be

something to Burn

Heat

For a Fire to Start, there must be a heat or

ignition source

TOGETHER, THEY PRODUCE A CHEMICALCHEMICAL CHAIN REACTION REACTION THAT

IS FIRE,

which produces heat and light.

Types of Fires

Fires Are Classified by the type FUEL they burn.

The 4 Types areABCD

Class A Fires (Carbonaceous)

Wood,

Paper,

Plastic,

Rags, etc

Combustible Material burn and ash is left behind

Class B Fires (Oil)

Gasoline,

Oil,

Grease,

Paint, etc

Water not to be used as oil is lighter and

will float

Class C Fires

Electrical Fires and Gases under fire Office Equipment,

Motors,

Switchgear,

Heaters,

Gases

ELECTRICITY ITSELF DOES NOT BURN. IT PROVIDES THE IGNITION TO START A FIRE

Class D Fires

Metals Potassium Sodium Aluminum Magnesium

PRODUCTS OF FIRE

SMOKE

HEAT

GASES

FLAME

BY- PRODUCTS OF FIRE

1. Smoke• Carbon Di Oxide• Carbon Mono Oxide• Hydrogen Cyanide• Hydrogen Chloride• Unburnt Carbon Particles

2. Heat - Burn and other physical effects3. Flames4. Sound5. Pressure • Effect on ears and other limbs}During Explosion

TERMS & DIFINITIONS

ATOMS AND MOLECULES

SMALLEST PART OF A SUBSTANCE WHICH CAN TAKE PART IN A CHEMICAL REACTION IS CALLED AN ATOM

SMALLEST PART OS A SUBSTANCE WHICH CAN EXIST FREELY IS CALLED A MOLECULE

CHEMICAL REACTION

WHEN TWO OR MORE SUBSTANCES COMBINE TOGETHER TO FORM A NEW SUBSTANCE OR SUBSTANCES, IT IS CALLED A CHEMICAL REACTION

A REACTION PRODUCES ENERGY, IE HEAT IS CALLED EXOTHERM REACTION

FIRE IS AN EXOTHERM REATION BETWEEN FUEL AND OXYGEN.

NEXT TO HEAT, A FIRE GENERALLY PRODUCES LIGHT, COMBUSTION GASES AND SOOT.

EXOTHERM REACTION

TO INITIATE A FIRE A CERTAIN AMOUNT OF ENERGY IS NEEDED.

TAKE A EXAMPLE OF A SIMPLE TEST WITH GASOLINE AND DIESEL FUEL. A MATCH HAS ENOUGH ENERGY TO LIGHT THE GASOLINE BUT IN THE DIESEL FUEL THE MATCH EXTINGUISHES.

IN CHEMISTRY THE ENERGY NEEDED TO START A REACTION IS CALLED THE ACTIVATION ENERGY.

ACTIVATION ENERGY

CHAIN REACTIONCHEMICAL REACTIONS NEEDS TO SURMOUNT ACTIVATION ENERGY BEFORE THE REACTION CAN TAKE PLACE .

IN A FIRE, THE INITIAL ENERGY SOURCES THAT CAUSE THE FIRE CAN BE MULTIPLE, E.G. A SPARK, AN OPEN FLAME, ELECTRICITY, SUNLIGHT, ETC

ONCE THE REACTION IS STARTED, HOWEVER, IT GENERATES MORE THAN ENOUGH ENERGY TO BE SELF-SUSTAINING, A CHAIN REACTION OCCURS. THE ENERGY GIVEN OFF IN EXCESS CAN BE SEEN AS LIGHT AND HEAT GENERATED BY THE FIRE.

PYROLYSIS

THE ENERGY LIBERATED IN THE COMBUSTION PROCESS CAUSES THE CHEMICAL COMPOSITION OF THE FUEL TO BREAK DOWN INTO SMALL MOLECULES

THIS PROCESS IS CALLED PYROLYSIS

THE PYROLYSIS PROCESS CAUSES THE EVAPORATION OF THE FUEL

MOLECULES EVAPORATE AND REACT WITH THE OXYGEN IN THE AIR.

COMPLETE COMBUSTION MEANS THAT JUST ENOUGH OXYGEN MOLECULES ARE PRESENT, TO OXIDISE THE FUEL MOLECULES.

COMPLETE REACTION

NEXT TO FUEL, OXYGEN AND ENERGY ONE SHOULD ALSO NOTE THE MIXING RATIO BETWEEN OXYGEN AND FUEL.

EXAMPLE, A LOG OF WOOD WILL NOT SUSTAIN A FIRE IF IT IS LIT WITH A MATCH. AN AMOUNT OF WOOD SHAVINGS WILL BECAUSE THIS CAUSES A BETTER MIXTURE BETWEEN FUEL AND AIR, WHICH FAVORS COMBUSTION. LARGER SURFACE OF THE FUEL IN CONTACT WITH THE AIR CAUSES A GREATER REACTION SURFACE , WHICH FACOURS COMBUSTION

REACTION SURFACE

IN A FIRE A CHEMICAL CHAIN REACTION OCCURS, RADICALS OF FUEL REACT WITH RADICALS OF OXYGEN HEAT AND COMBUSTION PRODUCTS ARE FORMED.

ADDING A CHEMICAL MOLECULE, WHICH REACTS WITH THOSE RADICALS WITHOUT SUSTAINING THE COMBUSTION PROCESS CAN STOP THE FIRE. THIS CHEMICAL IS CALLED INHIBITOR

THIS PRINCIPLE IS USED IN DRY CHEMICAL EXTINGUISHERS WICH CONTAIN E.G. POTASSIUM OR SODIUM BICARBONATE OR IN THE NOW BANNED HALON EXTINGUISHERS

INHIBITOR

CATALYST

CATALYST HAS THE OPPOSITE EFFECT OF AN INHIBITOR CATALYST IS A SUBSTANCE, WHICH PROMOTES THE REACTION (WITHOUT BEING ALTERED OR USED IN THE REACTION)

E.G. ADDING METAL SHAVINGS TO OIL RAGS AIDS THEIR COMBUSTION

THE IGNITION TEMPERATURE OF A SUBSTANCE (SOLID, LIQUID OR GASEOUS) IS THE MINIMUM TEMPERATURE TO WHICH THE SUBSTANCE EXPOSED TO AIR MUST BE HEATED IN ORDER TO CAUSE COMBUSTION OR FIRE

IGNITION TEMPERATURE

THE LOWEST TEMPERATURE AT WHICH A SUBSTANCE CONTINUES TO BURN IS (USUALLY A FEW DEGREES ABOVE ITS FLASHPOINT) AND IS CALLED FIRE POINT OR FLASH POINT

A SPECIFIC IGNITION TEMPERATURE FOR SOLIDS IS DIFFICULT TO DETERMINE BECAUSE THIS EPENDS UPON MULTIPLE ASPECTS SUCH AS HUMIDITY (WET WOOD VERSUS DRY WOOD), COMPOSITION (TREATED OR NON-TREATED WOOD) AND PHYSICAL FORM

FIRE OR FLASH POINT

AUTO-IGNITION TEMPERATURE IS THE LOWEST TEMPERATURE AT WHICH POINT A SOLID, LIQUID OR GAS WILL SELF-IGNITE WITHOUT AN IGNITION SOURCE.

AUTO IGNITION TEMPERATURE OF SUBSTANCES EXCEEDS ITS FLASHPOINT. SUCH CONDITIONS CAN OCCUR DUE TO EXTERNAL HEATING - A FRYING PAN THAT OVERHEATS CAUSING THE OIL TO AUTOIGNITE, AN EXHAUST-PIPE FROM A CAR DRIVING OVER DRY GRASS OR STRAW CAN CAUSE IT TO AUTO-IGNITE- OR THEY CAN OCCUR DUE TO CHEMICAL OR BIOLOGICAL PROCESSES - A SILO FIRE CAN OCCUR BECAUSE OF THE BIOLOGICAL PROCESSES IN HUMID ORGANIC MATERIAL.

AUTO IGNITION TEMPERATURE

VOLATILITY IS HOW READILY A LIQUID WILL EVAPORATE.

THE VOLATILITY OF A SUBSTANCE IS CLOSELY LINKED TO ITS BOILING POINT. THE HIGHER THE BOILING POINT OF A LIQUID THE HARDER IT WILL BE FOR THE LIQUID TO EVAPORATE.

THE LOWER THE BOILING POINT, THE GREATER THE FIRE RISK.

VOLATILITY

THE FEROCITY OF AN EXPLOSION DEPENDS ON THE SPEED OF THE FLAME IF THE FLAME SPREAD REMAINS LOWER THAN 340 M/S THE EXPLOSION IS CALLED A DEFLAGRATION. IF THIS SPEED EXCEEDS 340 M/S -AND THEY CAN REACH UP TO 1800 TO 2000 M/S- IT IS CALLED DETONATION.

THE DIFFERENCE IS BEING FASTER OR SLOWER THAN THE SPEED OF SOUND, SUPERSONIC AND SUBSONIC.

DEFLAGRATIONN & DETONATION

IN A COMPARTMENT FIRE THERE CAN COME A STAGE WHERE THE TOTAL THERMAL RADIATION FROM THE FIRE PLUME, HOT GASES AND HOT COMPARTMENT BOUNDARIES CAUSES THE GENERATION OF FLAMMABLE PRODUCTS OF PYROLYSIS FROM ALL EXPOSED COMBUSTIBLE SURFACES WITHIN THE COMPARTMENT.

GIVEN A SOURCE OF IGNITION, THIS WILL RESULT IN THE SUDDEN AND SUSTAINED TRANSITION OF A GROWING FIRE TO A FULLY DEVELOPED FIRE....... THIS IS CALLED 'FLASHOVER'......' MAIN FEATURE OF A 'FLASHOVER' IS THAT THERE IS TRANSITION TO A STATE OF TOTAL FIRE

FLASHOVER

A TIME COMES WHEN THE FLAMES CEASE TO BE LOCALISED AND SPREADS TO THE WHOLE COMPARTMENT VOLUME.

FIRE CHANGES FROM A SURFACE PHENOMENON TO A VOLUME PROCESS

LIMITED VENTILATION CAN LEAD TO A FIRE IN A COMPARTMENT PRODUCING FIRE GASES, CONTAINING SIGNIFICANT PROPORTIONS OF PARTIAL COMBUSTION PRODUCTS AND UNBURNT PYRO LYSIS PRODUCTS (UNDER-VENTILATED FIRE).

IF THESE ACCUMULATE, THEN THE ADMISSION OF AIR WHEN AN OPENING IS MADE TO THE COMPARTMENT CAN LEAD TO A SUDDEN INCREASE OF THE FIRE.

THIS INCREASE IN FIRE, MOVING THROUGH THE COMPARTMENT AND OUT OF THE OPENING, IS CALLED BACKDRAFT

BACKDRAFT

FIRE SPREAD

• FIRE SPREADS QUICKLY & GEOMETRICALLY

• RELEASED HEAT INCREASES TEMPERATURE IN THE VICINITY

• HIGH TEMPERATURE CAUSES SELF IGNITION IN OTHER FUELS

FIRE SPREAD

FIRE SPREAD

CONDUCTIONCONVECTIONRADIATION

THE HEAT LIBERATED BY THE FIRE ALSO CAUSES THE SURROUNDING MATERIALS TO WARM UP. THE HEAT TRANSFER IS ACCOMPLISHED BY THREE MEANS, USUALLY SIMULTANEOUSLY:

CONDUCTION• DIRECT THERMAL ENERGY TRANSFER DUE

TO CONTACT.• MATERIALS CONDUCT HEAT AT VARYING

RATES. METALS ARE VERY GOOD CONDUCTORS WHILE CONCRETE AND PLASTICS ARE VERY POOR CONDUCTORS, HENCE GOOD INSULATORS

• NEVERTHELESS A FIRE IN ONE SIDEWALL OF A COMPARTMENT WILL RESULT IN THE TRANSFER OF HEAT TO THE OTHER SIDE OF THE WALL.

CONVECTION• HEAT TRANSFER THROUGH A LIQUID OR

GAS • CAUSED BY DENSITY DIFFERENCE OF THE

HOT MOLECULES COMPARED TO THE COLD ONES (eg. BOILING WATER)

• HOT AIR, GASES EXPAND AND RISE. • CONVECTION NORMALLY DETERMINES THE

GENERAL DIRECTION OF THE FIRESPREAD.• CONVECTION CAUSES FIRES TO RISE AS

HEAT RISES

.

RADIATION

• ELECTROMAGNETIC WAVE TRANSFER OF HEAT TO AN OBJECT.

• WAVES TRAVEL IN ALL DIRECTIONS FROM THE FIRE AND MAY BE REFLECTED OR ABSORBED BY A SURFACE.

• ABSORBED HEAT RAISES THE TEMPERATURE OF THE MATERIAL BEYOND ITS IGNITION POINT, CAUSING IT TO IGNITE.

FIRE SPREAD- CUBE MODEL

HIGHEST DANGER

LESSDANGER

LESSDANGER

LEAST DANGER

STAGES OF COMBUSTION

FACTORS AFFECTING FIRE DEVELOPMENT

PROVIDING THAT THERE IS SUFFICIENT FUEL AND OXYGEN AVAILABLE THE FIRE MAY TOTALLY INVOLVE THE COMPARTMENT. THE ITEMS FIRST IGNITED IS SUFFICIENTLY FLAMMABLE TO ALLOW FLAME SPREAD OVER ITS SURFACES. THE HEAT FLUX FROM THE FIRST FUEL PACKAGE.IS SUFFICIENT TO IRRADIATE ADJACENT FUEL PACKAGES WHICH IN TURN WILL BEGIN TO BURN. SUFFICIENT FUEL EXISTS. OPEN OR WELL-SEALED COMPARTMENT.

FLASH OVER

A TIME COMES WHEN THE FLAMES CEASE TO BE LOCALISED AND SPREADS TO THE WHOLE COMPARTMENT VOLUME.

FIRE CHANGES FROM A SURFACE PHEMENON TO A VOLUME PROCESS

FLASHOVER

FLASHOVER