lec. 01 - fire science

8/7/2019 Lec. 01 - Fire Science

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FIRE SCIENCE

Fire can destroy your house and all of your possessions in less than an hour, and itcan reduce an entire forest to a pile of ash and charred wood. It's also a terrifying

weapon, with nearly unlimited destructive power. Fire kills more people every yearthan any other force of nature.

Fire is one of the most important forces in human history.

But at the same time, fire is extraordinarily helpful. It gave humans the first form of

portable lightand heat. It also gave us the ability to cook food, forge metal tools,form pottery, harden bricks and drive power plants. There are few things that havedone as much harm to humanity as fire, and few things that have done as much

good. It is certainly one of the most important forces in human history.

What is Fire?

Typically, fire comes from a chemical reaction between oxygen in the atmosphereand some sort offuel (wood or gasoline, for example). Of course, wood and

gasoline don't spontaneously catch on fire just because they're surrounded byoxygen. For the combustion reaction to happen, you have to heat the fuel to its

ignition temperature.


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For many years, fire was described by the fire triangle, this simple of fire says thatthree things are required for fire to occur including Oxygen, Fuel and Heat of

Ignition.

Oxygen is normally available in the air in sufficient quantities for combustion tooccur, a fuel source is required to react with the oxygen. A fuel is usually, but notalways, a carbon-based materials that is consumed, or partially consumed, in the

combustion reaction. (hydrocarbon gas is an example of the fuel that is not carbonbased). Since the fuel and oxygen will not only react at a high temperature, a sourc

of heat is required to cause the reaction, or ignition, to occur. For the reaction to beself-sustaining or continue without the presence of an outside ignition source, theremust be sufficient fuel and oxygen, and it must be react at such a rate that is

produces its own heat of ignition to sustain the reaction.

It is necessary to have a complete understanding of these three components andtheir interaction in order to understand the combustion process.

HEAT OF

IGNITIONFUEL

OXYGEN (AIR)

The Fire Triangle


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FUEL

Generally, fuel can be defined as anything that will burn, when fuels react withoxygen they are oxidized, in the presence of heat, this oxidation reaction is the

burning process or combustion.

Fuels are classified chemically as follows:

Carbon-Based and Other Readily Oxidizable Nonmetals, The most commonfuels are carbon-based materials. Also included in this class are other readily

oxidizable nonmetals such as sulfur, phosphorus and arsenic.

Hydrocarbons, Compounds that are rich in carbon and hydrogen such as methane(CH4), ethane (C2H4) and propane (C2H10) are hydrocarbon fuel.

Compound Containing Carbon, Hydrogen and Oxygen, Fuel materials that

contain carbon, hydrogen and oxygen include alcohol, aldehydes, organic acids,cellulosic and lignin (wood and vegetable material).

Metals, Many metals and their alloys including sodium, potassium, magnesium,

aluminum, zinc, titanium, zirconium and uranium are classed as fuel.

Fuel may be solids, liquids or gases, however, combustion occurs only

with fuels in the vapor or gaseous state.

OXYGEN

Because it is readily available in the atmosphere, its become most difficult item orelement of the fire triangle to control in fire prevention. Ambient is made up ofabout 21 % oxygen, 79 % nitrogen and small amount of other rare gases such as

argon, xenon, krypton and neon. The oxygen amount (21%) is adequate for mfuel to react to burn. Different fuel required different amount of oxygen in order t

burn.

For example: wood may need 4 to 5 %, acetylene may need less than 4 % whereby

hydrocarbon fuels require more than 15 % of oxygen.

As we can see, the is ample oxygen in the air readily to react with any fuels to causburn.


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The air is not the only source of oxygen, oxygen is also available chemically in somecompounds called oxidizers. Oxidizers are very important consideration in the study

of fire because they can provide additional oxygen to the reaction. Some commonchemical oxidizers include: hypochlorates, oxides, peroxides and of course oxygen

gas. In areas where oxidizers may be present, extra caution is required, because anoxygen-enriched atmosphere will cause combustible materials to ignite more readily

spread flame faster and burn more vigorously. Oxygen-enriched atmosphere canoccur where chemical oxidizers are present or gaseous oxygen, such as might occurnear to leak oxyacetylene welding system, petro-chemical plant and etc. Oxygen an

chemical oxidizers react vigorously with hydrocarbon based materials and thereaction can generate heat for ignition to occur.

HEAT OF IGNITION

To complete the fire triangle, heat energy is required to initiate and sustain thechemical reaction between the fuel and oxygen. The amount of heat energy requirevaries with the type of fuel. Solids and some liquids have to be heated to the point

where they decompose or vaporize sufficiently to give off enough flammable vaporsand gases to form an ignitable mixture in the air. This thermal decomposition ofsolid is referred to as pyrolysis and in liquid fuels is a vaporization. Flammable gases

require no decomposition energy because they are already in a form that is capableof combining with the oxygen in the air.

In initiating combustion, the process of pyrolysis is endothermic, or, heat isabsorbed by the fuel from the source of ignition. Initially, as heating of solid fuels

takes place, moisture is given off, as further heating takes place, combustible andflammable gases and vapors are given off, when there are enough gases to form an

ignitable moisture in air, ignition is possible, then the reaction turn exothermic, orheat is given off, this is known as ignition temperature.

The minimum temperature to which the substance in air must be heated inorder to initiate self-sustain combustion.

At the ignition temperature, the combustion reaction is self-sustaining in that itrequires no further heat input from an outside energy source.


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HEATHEAT

Type of ignition:

Spontaneous Ignition, Spontaneous heating is a phenomenon that can occur in

gas, liquid and as well as solid fuels. It involves the combination of materials withoxygen which evolves heat. In its most common usage, spontaneous ignition refers

to the ignition of combustible solids by the action of internal heating caused by anexothermic reaction. Under confined conditions, when the heat that is generated bya slow oxidation reaction cannot be dissipated, ignition can occur. It is a complex

situation.

Oxidation Reaction, An oxidation reaction in which the fuel and oxygen react in

the present of heat to produce heat and other products of combustion, the reactionoccurs when the three sides of the fire triangle are present in the proper amounts,

the reaction can take place very slowly as in spontaneous ignition, or it can occurexplosively under some combinations and conditions.

Example: CH4 + 2O2 CO2 + 2H2O +HEAT

The methane + oxygen (react in the presence of heat) to producecarbon dioxide + water vapor + heat (assuming - idle reaction)

OXYGEN

DIFFUSION

THERMAL

DECOMPOSITIONFUEL

Ignitable

Mixture

Solid

Liquid

Pyrolysis

Vaporization

Gases

Combus


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Reduction Reaction, is a reaction involving the removal of oxygen from thesubstance containing oxygen. In fire science , this reaction is important because

when combustion and consumption of available oxygen occur in a closedenvironment, reduction reaction take place, when there is insufficient oxygen

available, incomplete combustion take place, and carbon monoxide, a reductionagent is produced. In addition to carbon monoxide being a toxic gas, it is flamma

and highly reactive,


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THE CHAIN REACTION

The fire triangle described three components that are necessary for fire to occur, thcombustion reaction using methane as the fuel and yielding carbon dioxide and

water appears to be very simple, however, it is really only an abbreviation of a verycomplex series of chain reactions, it was found that these chain reaction must takeplace for fire to occur and the interruption of the chain reaction would result in the

extinguishment of the fire. Therefore, the simple fire triangle had to be expanded toa tetrahedron is the chain reaction ( Figure below). As noted, combustion is a

chemical reaction, the chemical bonds that hold the fuel molecules together arebroken, or broken bond, are highly reactive. They proceed to react with othermolecules, causing additional broken bonds and free radicals, some of which are

with oxygen, forming carbon dioxide and water. Because the bonds that hold thefuel molecules together are stronger than the bonds that hold the carbon dioxide

and water molecules, these is excess energy left over from this reaction. This excesenergy is given off as heat resulting in what is known as an exothermic reaction.

HEAT OF

FUEL

OXYGEN (AIR)

CHAIN

REACTION

The Fire Tetrahydron


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In simple example of combustion, methane reaction is used:

CH4 + 2O2 CO2 + 2H2O +HEAT

The carbon atom is bonded to four hydrogen atoms

In the presence of heat, the chemical bonds are broken, releasing the four

hydrogens as reactive H radicals.

These radicals react with oxygen molecules causing the bonds of their two atoms to

break and they are either released as O radicals, or an oxygen atom may bond to aH radical to form an OH radical.

These radical are short lived, they play an important role in the consumption of thefuel.

They are constantly being regenerated and consumed in a series of chain reactions.


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PRUDUCTS OF COMBUSTION

HEAT

LIGHT

SMOKE

FIRE GASES

PHYSIOLOGICAL EFFECTS

TOXIC EFFECTS OF FIRE GASES

lec. 01 - fire science

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