SUBJECT**************

PHYSICS&

CHEMISTRY OF COMBUSTION

TOPIC

MODES AND TYPE OF COMBUSTION

WHAT IS COMBUSTION ? Combustion is an exothermic, self sustaining chemical reaction involving a solid, liquid or gas fuel.

The process is usually associated with the oxidation of a fuel by atmospheric oxygen with the emission of heat & light followed by smoke and fumes.

Combustion is a display of the conversion of chemical energy, locked up within the fuel molecules, to heat energy.

EXAMPLES :

1.Combustion of carbon :- i) C + O2 CO2 + Heat

Carbon oxygen carbon dioxide

ii) 2C + O2 2CO + Heat

Carbon oxygen Carbon monoxide

Combustion of paper (cellulose) :-

C6H10O5 + O2 5H2O + 6CO2

Paper Oxygen Water carbon oxide Combustion of Magnesium:-

2Mg + O2 2MgO Magnesium Oxygen Magnesium oxide

Combustion of Sodium:- 4Na + O2 2Na2O Sodium oxygen Sodium Oxide

Combustion of Potassium:- 4K + O2 2K2O Potassium Oxygen Potassium Oxide

MODES AND TYPES OF COMBUSTION

The combustion process is observed as occurring in either of two modes:

1. The Glowing Mode2. The Flaming Mode

THE GLOWING MODE***********************

The glowing mode is exhibited in the form of surface combustion in which the burning is localized to the interfacial surface between the fuel and the air .Pure examples of fuels that burn according to the glowing mode include numerous metallic and nonmetallic chemical elements, from which gases and vapours are not distilled and no flaming exists.

Examples of elements from which gases and vapour are not distilled are as follow:-Sodium PotassiumMagnesiumAluminiumLithiumTitanium Uranium PhosphorousBoron ZincCombustion process is subject to the stimulating effect of radiative feedback.

THE CONCEPT OF TRIANGLE OF FIRE

The glowing combustion / surface combustion mode can be symbolized correctly in the form of traditional Triangle, in which each of the three sides is contiguous with other two sides and each sides represents one of the three basic requirements:-

1. Fuel (Incandescent solid) 2. Oxygen 3. Heat

THE TRIANGLE OF FIRE************************

THE FLAMING MODE*********************

The flaming mode is a characteristics indicative of the free burning of either gases or vapours that are derived indirectly from liquid or solid fuel or directly from flammable gases.Fuel that burn according to the flaming mode include paper, wood, flammable liquids and flammable gases from which vapours are distilled / produced and flaming combustion exists.

THE FLAMING MODEIn combustion process when solid fuel is heated by input heat energy, it converted into finely divided particles or liquids and liquid converted into vapour / gas they mix with surrounding air and form mixture. If this mixture is in the flammable range it will ignite if sufficient ignition source received.

THE FLAMMING MODE

In the early stage of the fire , the chemical reaction are energy absorbing (Endothermic), and these are followed by energy emitting (Exothermic reaction ).After ignition, the pace of burning accelerates the combustion chain reaction.Combustion process , is subject to the stimulating effect of radiative feed back.

THE FLAMING MODE

With flaming solid fuels, vapours are derived from the pyrolytic decomposition through the effect of radiative feedback from the fuel itself.Radiative feedback also motivates other phases of combustion process such as “cracking” of complex fuel vapours, thus forming free radicals.Similarly , radiative feedback stimulates the interplay of chain reactions as well as the combustion of fire generated gases.

THE CONCEPT OF TETRAHEDRON OF FIRE

According to “Walter M. Haessler “ 1974 concept flaming combustion can be conceived as Tetrahedron. According to him there four rather than three factors are involved in flaming combustion process. Those are as follow:-

1. Reducing agent 2. Oxidizing agent 3. Temperature 4. Chemical chain reaction

THE LIFE CYCLE OF FIRE

According to Powel concept there are six factors are involved to complete the life cycle of fire . They are as follow:-

1. FUEL 2. OXYGEN 3. INPUT HEAT 4. PROPORTIONING 5. MIXING6. IGNITION CONTINUITY

1. FUEL Fuel are those substance that

will burn when heat is applied to them.

They have tendency:-• To take up oxygen or• electronegative element OR• radical.

FUELFuel may be in the form of :-• Solid (Wood, paper, textile) • Liquid ( Petrol ,Diesel,

Kerosene )• Gases (Methane, Ethane

Propane, Butane, Acetylene, Carbon monoxide etc.)

2. OXYGEN

OXYGEN IS A COLOURLESS AND TASTELESS GAS AT ORDINARY TEMPERATURES. OXYGEN SUPPORT TO FIRE WITHOUT OXYGEN ( BUT NOT NECESSARILY ) FIRE COULD NOT EXIST. IT IS CONSUMED DURING BURNING.

Sources of oxygen Air is the main source of oxygen. Air contain near about 20.9% oxygen .

Mostly substance burn in presence of air . Oxidizing agents like- potassium chlorate(kclo3), potassium nitrate (KNO3) may also main source of oxygen.

3. INPUT HEAT It is necessary to have a source of Input Heat to initiate a combustion process. Input heat to produce and distill the fuel vapour from solid fuel:- wood, paper , textile etc.Input heat is also necessary to rise the temperature of fuel to its flash point Example of such fuel are:- kerosene , diesel, and paraffin. Fuel vapour and air mixture being capable of ignited by a given source of Input Heat . Continue…

There must be continuing source of self generated heat if the fire is to continue to develop and spread to fuel vapour.

There are various source of input heat .

Example - small open flame , arc, spark etc.

4. PROPORTIONING

The fuel vapour to be proportioned with air or within Limit of Flammability.

The limits of Flammability is usually measured as % by volume of vapour in air

5. MIXINGThe fuel vapour and air have to be properly mixed before the diffusion flame will be initiated .The fuel vapour in the centre of combustion zone are not thoroughly mixed or proportioned with oxygen (from air).Due to inadequate mixing of fuel vapour and air incomplete combustion of fuel take place.Proper mixing and proportioning is necessary to initiate combustion.

6. IGNITION CONTINUITY

Input heat from the fire and the rising temperature of fuel and oxygen mixture are prime determinants of ignition continuity.

Heat of both radiative and convective nature is transferred from the flame to the surface layer of fuel.

continue…..

Thus chemical energy is converted to heat energy.

The rate of conversion of heat is higher than the rate of heat loss.

Thus the rate of reaction goes – up and the fire is sustained when the rate of heat conversion fall below than the rate of dissipation ,the fire goes out.

STAGES OF DEVELOPMENT OF FIRE

Fire as a phenomenon has got four chronologically sequenced stages of development as study early.

They are as follow:- Continue…

1. Incipient Stage :- After ignition has occurred, and invisible products of combustion are being released i.e. non-visible smoke.

2. Smoldering stage : visible smoke formation but no flame .

3. Flaming Stage :- Radiant energy and light.Infrared radiationUltra violet radiationVisible light

4. Heating stage :- Distribution of heat energy to surroundings.

SMOKEThe process of combustion releases very large number of solid carbon and liquid particle into the atmosphere due to incomplete combustion of fuel.These particle when suspended in air called aerosols and produce by fire usually called smoke.

SMOKE

Statement by Walter M. Haessler , who said “smoke usually refers to a gaseous disperse system consisting of particle of low vapour pressure produced as a result of incomplete combustion process. it takes the form of solid carbon particle and carbonaceous matter.

EFFECTS OF SMOKEDIFFICULTY IN BREATHING COUGHING BURNING SENSATION IN CHEST AND THROATIRRITATION IN EYESABSORPTION AND SCATTERING OF LIGHT FROM VISIBLE SMOKE PARTICLE VISIBLE SMOKE CAN REDUCE VISIBILITY TO NEAR ZERO IN SOME CASES.

EFFECTS OF SMOKE

Loss of orientation in fire situation often result in panic behavior.Person becoming lost and trapped in dead end corridors.In extreme cases person become “paralysed” and taken no life saving action at all.Smoke is more dangerous than flame Number of death occurs due inhalation of smoke and toxic gases.

TOXIC GASES WHICH MAY BE PRODUCED BY COMBUSTION OF MATERIALS

NAME OF FUEL1. All combustible

material containing carbon.

2. Wool , silk, leather, plastics, cellulose materials, rayon.

3. Rubber , Thiokol, carbon disulphide, sulphur.

NAME OF TOXIC GAS1. Carbon dioxide,

carbon monoxide.

2. Hydrogen cyanide

3. Sulphur dioxide

TOXIC GASES WHICH MAY BE PRODUCED BY COMBUSTION OF MATERIALS

NAME OF FUEL1. P.V.C. , Plastics,

fluorinated plastics

2. Nylon , urea, resins

3. Phenol, polyesters4. Poly urethane

foam

NAME OF TOXIC GAS1. Halogenated acids-

Hcl, HBr, HF vapour

2. Ammonia3. Aldehyde vapours4. Isocynates

EFFECTS OF TOXIC GASES**************

EFFECT OF CARBON MONOXIDE (CO)

When a person breaths in a polluted atmosphere containing carbon monoxide , the carbon monoxide gas goes inside along with other gases in the lungs .Carbon monoxide has 210 times more affinity than oxygen to react with the Hemoglobin (Hb) of blood and form stable compound after reacting with the Hb of the blood.CO + Hb COHb (Carboxy hemoglobin)

ROLE OF OXYGEN INHUMAN BODYOn breathing in a normal and unpolluted atmosphere ,air is filled in the lungs of a person.The oxygen from air joins with the hemoglobin (hb) of the blood and forms Hbo. O2 + 2Hb 2HbO

This oxidize hemoglobin (HbO) flow with the blood to all parts of and cell of the body.

ROLE OF OXYGEN IN HUMAN BODY

The blood act as a carrier of oxygen to all cells of the body , reacts with them and oxidize them.Hbo + Cell Oxidized cell + HbThus , Hb becomes free and flowing with the blood through heart again reaches in the lungs and repeats the process. All the tissues and muscles of each part of the body get oxidized and generate energy in this process.

EFFECT OF OXYGEN DEPLETION

Concentration Exposure time Effects

21-17 % Indefinite Difficulty in

thinking .

17-14 % 2 Hrs. Rapid pulse

and dizziness.

14-11% 30 minutes Nausea, vomiting

and paralysis

6 % 1-2 minutes death within few min.

EFFECT OF HEAT

BURN INJURRIES EFFECTMENTAL CONFUSION PERFORMANCE LEVEL DROPDUE TO INHALATION OF HOT SMOKE AND TOXIC GASES DIFFICULTY IN BREATHING .DIFFICULTY IN RESCUE AND FIRE FIGHTING WORK.