The Chemistry of Fire

• The fire tetrahedron

• Fires can be prevented by suppression of any one of these

Application of the Fire Tetrahedron

• Fuel-– Shut off the natural gas supply

• Oxidizer– Close the windows, smother the fire with a blanket

• Heat– Pour water on the flame, use CO2 extinguisher

• Free radical chain reaction-– Adsorb radicals with chemical suppressants, salts

Fuels• Organic – C, O, H and sometimes N

– Wood is 40-50% cellulose and hemicellulose• (5 and 6 carbon sugars)

Wood tends to produce oxygenated combustion products

Turpenes in turpentine

Nowadays, apart from specialised grades mainly for artists’ use (balsam turpentine) turpentine is a by-product of the production of pulp. Sulphate turpentine is chilled from the gas emitted when wood chips are pre-heated with steam before digestion. Pine and spruce have the best turpentine yields.Turpentine consists of 45-75% a-pinene (1), 5-30% b-pinene (2), 2-40% 3-carene (3), other turpentines such as limonene and camphene and their oxidation products, such as alcohols and aldehydes.

http://apps.kemi.se/flodessok/floden/kemamne_eng/terpentin_eng.htm

Used in moderation, turpentine is an ideal accelerant for arson

Other fuels

• Cotton – mostly cellulose –

• Hydrocarbons (CH2)n

Other fuels II

• Inorganic fuels– Mg, Al, S, Zn, etc

– Note surface area is important • Wood dust ignites easily• Diesel ignites in a spray, but is difficult to light in a pool• Metals used in pyrotechnic devices are finely powdered

Oxidizing Agents

• Usually oxygen from air– In medical facilities can be accelerated

• Temperature greatly affects a fires need for oxygen– @25C need 14-16% O2

– @900-1100C flashover conditions- (spontaneous ignition of entire room) nearly 0% oxygen is needed

• This is the effect of the flammable limit, the upper and lower concentrations of a flammable gas and air expressed in % fuel that can be ignited at a specific temperature and pressure

Types of Fire

• Flaming combustion– Common open flame fires like

gas burners – Gas to gas reaction, fuel must

be in gaseous state– Liquids and solids don’t burn in

an open flame– these must undergo chemical

or phase change first– Oxygen must be above 10%

Smoldering Combustion• Glowing combustion

occurs without the generation of flames– It is a solid to gas reaction – Surface of solid reacts

directly with oxidizer– Often due to a deficiency

of oxidizer– Less than 10% oxygen is

needed

Partial or incomplete combustion

• Stoichiometric ratios are rarely involved in combustion– Oxidation reactions often don’t go to

completion– This is called incomplete combustion

• C4H10 + 13/2 O2

Types of Fire

• Flaming combustion– Common open flame fires like

gas burners – Gas to gas reaction, fuel must

be in gaseous state– Liquids and solids don’t burn in

an open flame– these must undergo chemical

or phase change first– Oxygen must be above 10%

Smoldering Combustion• Glowing combustion

occurs without the generation of flames– It is a solid to gas reaction – Surface of solid reacts

directly with oxidizer– Often due to a deficiency

of oxidizer– Less than 10% oxygen is

needed

Flammable limits

• Fuel/air ratio must be correct for combustion to occur• There is a minimum and maximum level

– Measured at 0 C and 1 ATM

– Gasoline -1.4 - 7.6% in air– Acetylene- 4-100%– H2 4-75%

– – for this reason it is practically impossible for a full or partly full gas tank to explode or even burn

– The danger comes as temperature increases causing the range of flammable limits to expand

– http://www.youtube.com/watch?v=NOTWg3Krww0

Examples

• Flash point– http://www.youtube.com/watch?

v=yE5LdCyN0aE&feature=related

• Flammable limit– http://www.youtube.com/watch?v=ICsvddmYMr4

• Auto ignition– http://www.youtube.com/watch?v=lFIiTxqolZk

• Backdraft– http://www.youtube.com/watch?v=91R6MLcf-WQ

Partial or incomplete combustion

• Stoichiometric ratios are rarely involved in combustion– Oxidation reactions often don’t go to

completion– This is called incomplete combustion and

results in formation of carbon monoxide.

Complete vs incomplet combustion in a gas pilot light

Effect of Fuel on a Fire

• Fires have either excess air or excess fuel

Effect of venting

• Venting a fire has important effects– Gases inside a room may be oxygen starved– Gases venting to outside may ignite– Gases venting into an enclosed room will not

spread a fire

– Opening doors and windows may cause a smoldering fire to reignite

Heat

• Sufficient heat is required to produce a transition from solid to liquid to vapor phase – only vapors burn

• Additional energy is also required to initiate the chemical reaction

• Once initiated such reactions are exothermic with a large increase in entropy

Initiation of Fire

• The heat required to initiate a fire is a critical step– Matchespaperstickswood– Each step is critical and the underlying

process is to get the wood hot enough that it produces volatile gases that burn.

Types of ignition

• Spontaneous ignition– Chemical or biological processes that create

sufficient heat to ignite the reacting material– Basically heat is produced faster than it can

be dissipated.– Common with vegetable oils, hay

Spontaneous combustion of hay in a barn

Auto-ignition

• Ignition of a material in the absence of flame or spark (non-piloted ignition)

• All combustible materials must reach their autoignition temperature to burn– Thus one could light paper two ways:

• Use a match to heat a small section to ignition• Heat the entire piece of paper in an oven.

Flash Point

• There is a temperature above which a fuel will flash when presented with a flame – this is the flash point

• @ 10-20 degrees above the flash point there is sufficient vapor pressure to sustain a flame

• The auto-ignition temperature is the temperature of spontaneous ignition– For kerosene, the flash point is 100F and the ignition

temperature is 410F.

Chemical Chain Reactions

• Reactions become self sustaining when sufficient heat from exothermic reactions radiates back to cause ignition away from source

• The burning process involves pyrolysis, the breakdown of solids to produce gases and free radicals

Removal of Free Radicals

• Halon fire extinguishers work by shutting down the propagation of radicals

Bromine and chlorine quickly shut down free radicals

Types of fire extinguishers• Type A - Water/firehoses

– Cools fire as water converts to steam– Causes damage and is dangerous in electrical and metal fires

• Type BC - Powder extinguishers – 80% NaHCO3

– Starve oxygen and cool by release of CO2

– 6-8 meter range• Type BC - CO2 gas extinguishers

– Leave no residue for expensive cleanup– Cool fire and remove oxygen– 1.5 meter range- can be dangerous for large electrical fires, esp if visibility is

limited due to smoke• Type ABC – Ammonium Phosphate

– Releases ammonia which removes oxygen and yields phosphoric acid which induces char which releases fewer volatiles

• Type ABC Halon– Eliminates free radicals, displaces oxygen

• Type D fire extinguisher– A bucket of sand for metal fires

Fires are classified according to the material, which is being burned. The four classes of fires, with the American and International symbols, are as follows:

Class A: Ordinary Combustibles - Cloth, Wood, Paper, Rubber, many plastics.                                                      Extinguisher:Pressurized water (it removes Heat) suitable for use on Class A only. Dry chemical: mono-ammonium phosphate, (it removes contact between Oxygen and Fuel), rated for Class A, B, and C fires.Extinguishers suitable for Class A fires should be identified by a green triangle containing the letter "A" and the pictograph shown above. 

http://www.fireadesource.com/faqs.html

Class B:

Flammable Liquids - Gasoline, Oil, Oil-based paint, Cooking Oil

                                                      

Extinguisher:1) Carbon dioxide (it displaces Oxygen but dissipates quickly; the combustible surface, if hot, may re-ignite).2) Dry Chemical (it removes Oxygen from the Fuel by coating the surface inhibiting the release of combustible vapors): mono ammonium phosphate, rated for Class A, B, and C fires; Sodium Bicarbonate and Potassium Bicarbonate, for Class B and C, preferred for cooking oil fires.3) Halon: it interferes with the fire chemical reaction by quenching free radicals. Production has been banned (Montreal, 1998) because Halon has been found to be an ozone-depleting substance.Extinguishers that are suitable for Class B fires are identified by a red square containing the letter B and the pictograph shown above.                                  

http://www.fireadesource.com/faqs.html

Extinguisher:1) Carbon dioxide (it removes Oxygen but dissipates quickly; the combustible surface, if still hot, may re-ignite).2) Dry Chemical (it removes Oxygen from the Fuel by coating the surface and inhibiting the release of combustible vapors): mono Ammonium Phosphate, rated for Class A, B, and C fires; Sodium Bicarbonate and Potassium Bicarbonate, for Class B and C, preferred for cooking oil fires.3) Halon: it interferes with the fire chemical reaction by quenching free radicals. Production has been banned (Montreal, 1998) because Halon has been found to be an ozone-depleting substance.Extinguishers suitable for Class C fires are identified by a blue circle containing the letter C and the pictograph shown above. 

Class C: Energized electrical equipment, including appliances, wiring, circuit breakers, and fuse boxes.                      

http://www.fireadesource.com/faqs.html

Class D:Combustible metal such as Mg, Na, Li, powdered Al, etc.                                   Extinguisher:Extinguishers rated for class D fires have a label, which list the types of metal, on which the extinguisher may be used. The extinguishing medium must not react with the burning metal. Extinguishers suitable for Class D fires are identified by a yellow star containing the letter D.

http://www.fireadesource.com/faqs.html

Fire Retardants• Barrier theory- chemicals form a glassy barrier on exposure to heat

• Thermal theory – chemicals change the thermal property of the wood to dissapate (conduct) or absorb (heat capacity)heat - sodium silicates, chemicals with waters of hydration

• Noncombustable gas theory – chemicals release nonflamable gases interfering with combustion - borax (soduim tetraborate decahydrate) releases large quantities of water following pyrolysis

• Free radical trap theories – chemicals release free radical inhibitors at pyrolytic temperatures interrupting chain propagation – halogens attack free radicals formed

• Increased char theories – temperature of pyrolysis is lowered, directing degradation towards charring instead of burning, lower volatile gases – borax, NH3PO4

• Most fire retardants operate using several of these mechanisms(H2O) x10

NH3 H2P04Levan, Chemistry of Fire Retardancy in The Chemistry of Solid Wood http://www.fpl.fs.fed.us/documnts/pdf1984/levan84a.pdf

Heat Transfer

• Three mechanisms– Conduction, convection, radiation

• Conductive heating– Takes place within solids– Rate is dependant on

» Thermal conductivity heat transfer within a material» Heat capacity heat required to raise the

temp of a substance 1 degree C» Density g/cm3

Thermal Conductivity

• Thermal intertia density, heat capacity, thermal conductivity– At equilibrium, density and heat capacity

become unimportant– Thermal conductivity rules

• Pipes and metal fittings produce fire spread and structural damage

– Thus thermal intertia is maintly important in the early stages of a fire

Thermal properties of selected materials

material Thermal conductivity

W/mK

Density

kg/m3

Heat capacity

J/kgK

copper 387 8940 380

concrete 0.8 1900 880

pine 0.14 640 2850

polyethylene 0.35 940 1900

NFPA 921-14

Convection

• Transfer of heat energy through the movement of liquids or gases

• Heat is then transferred to a cooler solid– Rate is a ftn of

• Temperature• Surface area• Velocity of gases

– Convection is extremely important in the early stages of a fire

• Hot gases rise to upper portions of the room• Then they mushroom down• As heat builds, flashover occurs and entire room ignites• Hot gases then spread fire through the rest of the building

Radiation

• Transfer of heat through infrared energy• Radiative power = σ(T)4

– where σ = 5.67 x 10-8 (watts/m2)/K4

– Stefan’s law of radiation

• Thus

• Radiative power becomes highly significant at elevated temperatures

Fire development

• There is a sequence of events which begin as a fire evolves1. Incipient stage

2. Ceiling layer development

3. Preflashover -

4. Flashover

5. Post flashover

Not all fires will go through the entire process

Incipient phase

• Incipient – room doesn’t heat– Can be short time – accellerant– Or long- spontaneous combustion

• Oily rags (linseed oil) dust or even grass clipping• Build up of heat due to chemical or bacterial action

– For ignition to occur material must be • In a gaseous state• At sufficient concentration to form a flammable air/gas

mixture• Exposed to activation energy of

– Match, spark, friction

Incipient stage

• Small flames progress upwards and produce hot gases.

• Smoke begins to accumulate

• Average temperature is just above ambient

Emergent Smolder• Fuel vapors must be raised to higher than ignition temperature• Some solid materials begin to burn by smoldering

– A hazardous situation as incomplete combustion release of CO and other toxins

– Smoldering is a pyrolytic process in which chemical bonds begin to break, gases are released and free radicals form

Growth (open burning)• Room begins to heat up• Oxygen concentration still high or unchanged• Fire burns up and out as it moves across the ceiling looking for a

way up

Ceiling layer development• Also called the growth phase. • Smoke increases and begins

to accumulate at the ceiling level

• Room heats up and other items begin to burn

• Hot smoke creates a negative pressure in the room.

• There is essentially two layers of heat in the room, a hot upper layer and a cooler rest of the room.

Preflashover

• Smoke and hot gas layer at ceiling reaches 400-500 C

• Rate of heat transfer increases

• Burning rate is fuel controlles and sufficient oxygen is present

• Items in the room begin to pyrolyze – notice smoke given off by chair

Flashover• Temperature in the room rises to the point that all materials spontaneously

combust• Flashover can simulate arson fire as multiple points in the room ignite• Windows break due to thermal stress, Floor to ceiling charring will occur due

to radiative heating of all exposed surfaces• Freeburning occurs until ventilation is limited• Fires can selflimit if nearby fuel isnt present of it initial fire is too small to

ignite adjacent materials

Flashover

• Hot gas reached a critical temperature of 600 C and ignites, significantly increasing the radiant heat transferred to floor

• Whole room is suddenly and completely engulfed in flame

• Transition lasts only a few seconds

Full Room Invovement Ventilation Control Backdraft

• Room transitions to oxygen regulated smoldering

– The point at which the amount of O2 regulates the fire

– Fire itself is slow smoldering producing large amounts of CO– If a door is opened at this point, hot CO combines explosively

with O2

• Effect can be confused with explosives however char pattern will occur only at the top of the room

Example: Station Nightclub Fire simulation by NIST

http://www.youtube.com/watch?v=IxiOXZ55hbc

At first, there was no panic. Everybody just kind of turned. Most people still just stood there. In the other rooms, the smoke hadn't gotten to them, the flame wasn't that bad, they didn't think anything of it. Well, I guess once we all started to turn toward the door, and we got bottle-necked into the front door, people just kept pushing, and eventually everyone popped out of the door, including myself

The Station nightclub fire occurred beginning at 11:07 PM EST, on Thursday, February 20, 2003, at

The Station, a glam metal and rock n roll themed nightclub located at 211 Cowesett Ave in West

Warwick, Rhode Island, United States; it is considered to be the fourth deadliest nightclub fire

in American history, killing 100 people, four of whom died after being admitted to local hospitals. The fire was caused when pyrotechnic sparks, set off by the

tour manager of the evening's headlining band, Great White, ignited flammable sound insulation foam in the walls and ceilings around the stage, creating a flash fire that engulfed the club in 5½

minutes. Some 230 people were injured

Incipient phase

• Incipient – room doesn’t heat– Can be short time – accellerant– Or long- spontaneous combustion

• Oily rags (linseed oil) dust or even grass clipping• Build up of heat due to chemical or bacterial action

– For ignition to occur material must be • In a gaseous state• At sufficient concentration to form a flammable air/gas

mixture• Exposed to activation energy of

– Match, spark, friction

Emergent Smolder

• Fuel vapors must be raised to higher than ignition temperature

• Some solid materials begin to burn by smoldering– A hazardous situation as incomplete

combustion release of CO and other toxins– Smoldering is a pyrolytic process in which

chemical bonds begin to break, gases are released and free radicals form

Flashover

• Temperature in the room rises to the point that all materials spontaneously combust

• Flashover can simulate arson fire as multiple points in the room ignite

• Freeburning occurs until ventilation is limited

• Fires can selflimit if nearby fuel isnt present of it initial fire is too small to ignite adjacent materials

Post flashover

• Also called full room involvement

• Every piece of combustable material in room burns

• Areas under furniture may be relatively spared, also materials near the influx of oxygen

• Examples http://faberc.org/Images/NIST/Flashoverx3/LivingRoomFlashover.wmv

http://faberc.org/Images/NIST/Flashoverx3/ScotchPine.wmv

Backdraft

• Oxygen regulated smoldering– The point at which the amount of O2 regulates

the fire– Fire itself is slow smoldering producing large

amounts of CO– If a door is opened at this point, hot CO

combines explosively with O2

• Windows will blow out• Effect can be confused with explosives however

char pattern will occur only at the top of the room

Accelerated vs Nonaccelerated fires

• Originally thought that accelerated fires would burn hotter. This is not true

• Actually modern homes are composed of a lot of plastic, which basically burns just like gasoline. (also gasoline burns at the same temperature as wood)

• Only real difference is a faster rate of room temperature increase. This is due to the faster rate of heat release with gasoline

Effects of Fire/Scene Reconstruction

• Damage to ceiling is 5x that of Floor• Damage is usually heaviest near origin

– Aligator char is deepest– Scales are smaller

• Char burn rate = 1” in 40 min @1400-1600 F

– Glass melts @1200F becomes running @1600F• Ovid cracks in glass rapid heating• Verticle cracks slow fire

– Light bulbs above 40W will expand towards origin due to melting and expansion of gas inside bulb

– Burn patterns can help indicate origin

Liquid fuel properties

• Melting and boiling points– Increase with number of carbons within HC class– Branching and cyclic groups decrease melting and

boiling points (increased disorder)– Double bonds decrease melting and boiling points– Aromaticity increases melting and boiling points due

to increased polarity– Alcohol groups greatly increase melting and boiling

points

BP 254

BP 229 alcohol

BP 174

BP160 branched BP 171 alkene

BP 148

BP 183aromatic

BP 181cyclic

BP 171

Specific Gravity• Compounds lighter than water have lower

specific gravity gm/cc = 1 for H2O

• Petroleum products generally have a low specific gravity and float on water (up to asphalt)

• General trends-– Increasing with carbon number for n-alkanes– Aromatics tend to have higher SG than alkanes– Compounds with Cl or S tend to have high SG

Vapor density

• Volume of vapor or gas compared to air (Air = 29 g/mol = vapor density of 1)– Air is 78.1% N2, 21% O2, 1% Ar, 0.03% CO2

– This is 21.9g/mol N2 6.7 g/mol O2

• If the vapor density of a gas is below 1 it will rise. If above 1 if settles at floor level

Vapor Density

• Only 14 gases and vapors have a vapor density less than 1– acetylene, ammonia, CO, diborane, H2, He,

HCN,HF, CH4, methyl lithium, Ne, N2, H2O

– 9 are flammable,

• Many other gases are heavier than air – – Methanol, propane, butane, acetone,

pentane, toluene, etc.

Too lean

Effect of vapor density

Combustion possible

Too Rich

Stove

Flammability limits

Lower flammability limit upper flammability limit• Methane 5.3% 14%• Propane 2.2% 9.5%• Acetylene 2.5% 81%• Butane 1.9% 8.5%• Gasoline 1.5% 7.6%• Kerosene 1% 5%• Diesel 0.5% 4.1%• Carbon monoxide 12.5% 74%• H 4% 75%• HS 4.3% 45%

• NH3 15.5% 45.5%

Vapor pressure of a liquid mixture

• How to calculate if in an explosion LFL was reached?

• Use Raoult’s law – Ptotal = Sum(Pn χn)

– vapor pressure of mixture time sthe molar fraction of the liquid in the mixture

Flash Point calculations

• Flash point is the lowest temperature at which a substance produces sufficient vapor to form an ignitable mixture (pilot light)

• Gas ignites and then extinguishes. The concept is important as this is the lowest temperature at which a risk of fire exists.

• Flash points are temperature dependent

• 1000/(Tf+273) = B0 + B1log P25

• Where B0 and B1 are constants (see book) and P25 is the vapor pressure at 25C

Summary of concepts

• Melting point• Boiling point• Specific gravity• Vapor density• Flammability limits –lFL, UFL• Vapor pressure• Flash point – will pop• Fire point – sustains a fire• Ignition temperature – will ignite• Autoignition temperature – will ignite with no source

Thermal Conductivity

• Thermal intertia density, heat capacity, thermal conductivity– At equilibrium, density and heat capacity

become unimportant– Thermal conductivity rules

• Pipes and metal fittings produce fire spread and structural damage

– Thus thermal intertia is maintly important in the early stages of a fire

Thermal properties of selected materials

material Thermal conductivity

W/mK

Density

kg/m3

Heat capacity

J/kgK

copper 387 8940 380

concrete 0.8 1900 880

pine 0.14 640 2850

polyethylene 0.35 940 1900

NFPA 921-14

Convection

• Transfer of heat energy through the movement of liquids or gases

• Heat is then transferred to a cooler solid– Rate is a ftn of

• Temperature• Surface area• Velocity of gases

– Convection is extremely important in the early stages of a fire

• Hot gases rise to upper portions of the room• Then they mushroom down• As heat builds, flashover occurs and entire room ignites• Hot gases then spread fire through the rest of the building

Radiation

• Transfer of heat through infrared energy• Radiative power = σ(T)4

– where σ = 5.67 x 10-8 (watts/m2)/K4

– Stefan’s law of radiation

• Thus

• Radiative power becomes highly significant at elevated temperatures