fire dynamics ii - chemistry of room fire combustion lecture_5

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Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture #

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Fire Dynamics II

Lecture # 5Chemistry of Room Fire Combustion

Jim Mehaffey

82.583


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Chemistry of Room Fire CombustionOutline• Introduction• Review: Generation of products of combustion in well-

ventilated fires• Generation of products of combustion in poorly-

ventilated fires• Review: Life tenability criteriaObjectives• Predict rates at which heat & chemical species are

generated in fires in order to provide input for assessments of thermal environment & life safety

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IntroductionPerfect Combustion• Combustible burns in an excess of pure O2

• Products: net heat of combustion, CO2 and H2O Well-ventilated fires (diffusion flames in the open)• Combustible burns in open configuration in air• Products: Chemical heat of combustion, CO2, H2O,

CO, C (soot) and hc (hydrocarbons)Poorly-ventilated fires (many fires in enclosures)• Combustible burns in air, but air supply is restricted• Products: Less heat, CO2 and H2O

More CO, C (soot), hc (hydrocarbons)


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Heat Release Rate: Perfect Combustion• Net heat of combustion = HC (kJ / g)

• Get theoretical maximum heat release rate (kw)

Eqn (5-1)

• = mass loss rate of fuel (kg s-1)

••

= mHQ CMAX

•

m

3


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Heat Release Rate: Well-ventilated Fire• Well-ventilated fires (diffusion flames in open)

experience incomplete combustion• Reduction in combustion efficiency means net heat of

combustion is not released• Actual (chemical) heat release rate (kW) is

Eqn (5-2)

Hch = Actual (chemical) heat of combustion (kJ / g)

••

= mHQ ch


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Heat Release Rate: Well-ventilated Fire

• Rate heat is convected above flame (kW):

Eqn (5-3)

• Rate heat is radiated away by flame (kW):

Eqn (5-4)••

= mHQ radrad

••

= mHQ conconv

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Heat Release: Well-ventilated Fires (2)M aterial H C (kJ/g) H ch (kJ/g) H con (kJ/g) H rad (kJ/g)M ethane 50.1 49.6 42.6 7.0E thane 47.1 45.7 34.1 11.6Propane 46.0 43.7 31.2 12.5B utane 45.4 42.6 29.6 13.0H eptane 44.6 41.2 27.6 13.6O ctane 44.5 41.0 27.3 13.7G asoline * 44.6 41.2 27.6 13.6K erosene 44.1 40.3 26.2 14.1B enzene 40.1 27.6 11.0 16.5M ethanol 20.0 19.1 16.1 3.0E thanol 27.7 25.6 19.0 6.5PE 43.6 38.4 21.8 16.6PP 43.4 38.6 22.6 16.0PM M A 25.2 24.2 16.6 7.6PS 39.2 27.0 11.0 16.0PS (foam ) 38.2 25.6 9.9 15.7PU (flexible foam ) 26.2 17.8 8.6 9.2PU (rigid foam ) 26.0 16.4 6.8 9.6PV C 16.4 5.7 3.1 2.6W ood (red oak) 17.1 12.4 7.8 4.6W ood (pine) 17.9 12.4 8.7 3.7


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Generation of Species: Perfect Combustion• Only CO2 and H2O are generated

• Example: Complete combustion of methanol

2 CH3OH + 3 O2 → 2 C O2 + 4 H2O

(64 g) (96 g) (88 g) (72 g)

• Define yield of CO2 : Eqn (5-5)

• Maximum possible yield: 1.38g 64 / g 88Y2CO ==

••

= mYm2co2co

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Generation of Species: Well-ventilated Fires• Well-ventilated fire involves incomplete combustion:

CO2, H2O, CO, C (soot) and hc (hydrocarbons) are generated

• Rate of generation of chemical species is proportional to rate of generation of volatiles

(kg s-1) Eqn (5-6)

• Yi = Yield of species i (kg / kg)

•

= mY iim•


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Well-ventilated Methanol Fires

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Well-ventilated Methanol Fires

or

• Complete combustion of methanol yields maximum possible yield of CO2:

3.1Y2CO ="m 1.3"m

2CO

••

=

1.38g 64 / g 88Y2CO ==


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Yields in Well-ventilated Methanol Fires

Carbon dioxide:

Carbon monoxide:

Hydrocarbons:

Soot (carbon):

3.1Y2CO =

0010.0YCO =

0.0~Yhc

0.0~YS

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Chemical Species: Well-ventilated Fires (2)M ateria l Y C O 2(g /g ) Y C O (g /g ) Y h c (g /g ) Y S (g /g )M ethane 2 .7 2 - - -E th ane 2 .8 5 0 .0 01 0 .0 01 0 .0 13P ro pan e 2 .8 5 0 .0 05 0 .0 01 0 .0 24B utan e 2 .8 5 0 .0 07 0 .0 03 0 .0 29H ep tan e 2 .8 5 0 .0 10 0 .0 04 0 .0 37O ctane 2 .8 4 0 .0 11 0 .0 04 0 .0 38G aso line * 2 .8 5 0 .0 10 0 .0 04 0 .0 37K ero sen e 2 .8 3 0 .0 12 0 .0 04 0 .0 42B en zene 2 .3 3 0 .0 67 0 .0 18 0 .1 81M ethano l 1 .3 1 0 .0 01 - -E th ano l 1 .7 7 0 .0 01 0 .0 01 0 .0 08P E 2 .7 6 0 .0 24 0 .0 07 0 .0 60P P 2 .7 9 0 .0 24 0 .0 06 0 .0 59P M M A 2.1 2 0 .0 10 0 .0 01 0 .0 22P S 2 .3 3 0 .0 60 0 .0 14 0 .1 64P S (foam ) 2 .3 0 0 .0 65 0 .0 16 0 .2 10P U (flex ib le foam ) 1 .5 5 0 .0 10 0 .0 02 0 .1 31P U (rig id foam ) 1 .5 2 0 .0 31 0 .0 03 0 .1 30P V C 0.4 6 0 .0 63 0 .0 23 0 .1 72W o od (red o ak ) 1 .2 7 0 .0 04 0 .0 01 0 .0 15W o od (p in e) 1 .3 3 0 .0 05 0 .0 01 0 .0 15


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Well-ventilated vs. poorly-ventilated fires

• = mass flow rate of air into flame (kg s-1)

• = mass flow rate of volatiles into flame (kg s-1)

• r = stoichiometric air requirement (kg / kg) r kg of air are required for completecombustion of 1 kg of fuel

• Wood: r ~ 5.7 (page 184 Drysdale)• Wood volatiles: r ~ 4.6 (page 184 Drysdale)• PMMA: r = 8.27 (page 2-36 Fire Dynamics I)• polystyrene: r = 13.25• Polyethylene: r = 14.76

am•

m•

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Φ = Equivalence Ratio

Eqn (5-7)

• Φ < 1 ⇒ well-ventilated fire (fuel lean)• Φ = 1 ⇒ stoichiometric mixture• Φ > 1 ⇒ poorly-ventilated fire (fuel rich)

• Steward ⇒ for turbulent diffusion flames Φ ≤ 0.25

• For wood & plastics:at LFL Φ ~ 0.05at UFL Φ ~ 4.0

r/ m

m

m

mr

aa•

•

•

•

==Φ


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Example Calculation of Equivalence RatioPost-flashover Fires Involving Wooden Cribs

• Harmathy (1972) identified two burning regimes for room fires involving wooden cribs: ventilation-controlled & fuel-surface controlled

• = mass loss rate of fuel (kg s-1)

• Θ = ventilation parameter (kg s-1)

=

• Af = exposed surface area of fuel (m2)

•

= mR

hA 3.76h gA =Oρ

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• Post-flashover fire is ventilation-controlled if

Θ / Af < 0.63 kg m-2 s-1

Eqn (5-8)

• Fuel mass loss rate is

Eqn (5-9)

1/2f m 0.07AhA <

1

1

s kg hA 0.09m

s kg 0.0236m−

•

−•

=

Θ=

10


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Rate of Entry of Air - From Lecture 4

• C = 0.68; ρa = 1.2 kg m-3; g = 9.8 m s-2 and A = b h

2/3

32

3/23/1a

11

1 g TT1 2 hh b Cm

f

aa

+

+

−=

•

•

•

aa

f

m

mTT

ρ

2/3

3/23/1

11

1 TT1 hA .42m

f

aa

+

+

−=

•

•

•

aa

f

m

mTT


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The Coefficient C1

hA Cm 1a =•

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Assume Tf ~ 1000°C = 1273 K

• Solve by iteration: 1st guess:

• Find:

3/22/3a

a

mm1 1.61

hA 2.1 m

++

=••

•

1s kg hA 0.09m −•

=

1s kg hA 0.50ma−

•

=

1s kg hA 0.45ma−

•

=


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• For ventilation-controlled post-flashover fire

r = 4.6

• Equivalence ratio is Φ ~ 0.92

1s kg hA 0.09m −•

=

1s kg hA 0.45ma−

•

=

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Post-flashover Fires Involving Wood, PMMA & PE


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Example Calculation of Equivalence RatioPost-flashover Fires Involving PMMA Cribs


r = 8.27


1s kg hA 0.09m −•

=

1s kg hA 0.45ma−

•

=

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Example Calculation of Equivalence RatioPost-flashover Fires Involving PE


r = 14.76


1s kg hA 0.09m −•

=

1s kg hA 0.45ma−

•

=


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Impact of Ventilation on Combustion Dynamics

• Many small-scale experiments have been conducted to assess the impact of ventilation on heat release and generation of chemical species employing– FMRC Flammability apparatus (small-scale)– Fire Research Institute enclosure (0.022 m3)

• Limited full-scale experimental data

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Impact of Ventilation on Heat Release


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Impact of Ventilation on Heat Release

Eqn (5-10)

• Hch(Φ<<1) = well-ventilated limit of the chemical heat of combustion

• Experimental data ⇒ correlation of the form

Eqn (5-11)

• Correlation holds for non-halogenated polymers. For halogenated polymers like PVC, a different correlation applies.

)1(H)(H

ch

ch<<Φ

Φ=chξ

( )2.1 2.5exp 0.971 −Φ−−=chξ

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Impact of Ventilation on Convective Heat Release


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Impact of Ventilation on Convective Heat Release

Eqn (5-12)

• Hconv(Φ<<1) = well-ventilated limit of convective heat of combustion


Eqn (5-13)

• Higher fraction of chemical heat of combustion is converted to radiative heat of combustion as move from well-ventilated to poorly-ventilated conditions

• For halogenated polymers (PVC), different correlation

)1(H)(H

convconv

<<ΦΦ=conξ

( )8.2 2.5exp 1 −Φ−−=conξ

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Impact of Ventilation on Consumption of O2


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Impact of Ventilation on Consumption of O2

Eqn (5-14)

• CO2 = mass of O2 consumed per mass of fuel


Eqn (5-15)

• Compare Eqn (5-15) with Eqn (5-11)


( )2.1 2.5exp 0.9712

−Φ−−=Oξ

)1(C)(C

2

2

O

OO2 <<Φ

Φ=ξ

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Impact of Ventilation on Generation of CO2


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Impact of Ventilation on Generation of CO2

Eqn (5-16)

• YCO2 = Yield of CO2 = mass CO2 generated / mass of fuel


Eqn (5-17)

• Compare Eqn (5-17) with Eqns (5-15) & Eqn (5-11)


( )2.1 2.5exp 12

−Φ−−=COξ

)1(Y)(Y

2

2

CO

COCO2 <<Φ

Φ=ξ

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Impact of Ventilation on Generation of CO


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Eqn (5-18)

• YCO = Yield of CO = mass of CO generated / mass of fuel


Eqn (5-19)

)1(Y)(Y

CO

COCO <<Φ

Φ=ξ

( )βαξ −Φ−+= 2.5exp 1CO

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Combustible α βPolystyrene (PS) 2 2.5Polypropylene (PP) 10 2.8Polyethylene (PE) 26 2.8Nylon 36 3.0PMMA 43 3.2Wood 44 3.5


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Impact of Ventilation on Generation of Hydrocarbons

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Eqn (5-20)

• Yhc = Yield of hyrocarbons


Eqn (5-21)

)1(Y)(Y

hc

hchc <<Φ

Φ=ξ

( )βαξ −Φ−+= 5.0exp 1hc


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Combustible α βPolystyrene (PS) 25 1.8Polypropylene (PP) 220 2.5Polyethylene (PE) 220 2.5Nylon 1200 3.2PMMA 1800 3.5Wood 200 1.9

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Impact of Ventilation on Generation of Soot


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Eqn (5-22)

• YS = Yield of soot = mass of soot generated / mass of fuel


Eqn (5-23)

)1(Y)(Y

S

SS <<Φ

Φ=ξ

( )βαξ −Φ−+= 2.5exp 1S

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Combustible α βPolystyrene (PS) 2.8 1.3Polypropylene (PP) 2.2 1.0Polyethylene (PE) 2.2 1.0Nylon 1.7 0.8PMMA 1.6 0.6Wood 2.5 1.2


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Engineering PredictionsYields of Chemical Species in Fire

• During all stages of fire, yield of most species (CO2, soot, HCl and HCN in real-scale scenario is same as in bench-scale tests (same Φ)

• During early stages of room fire, yield of CO in a real-scale scenario is similar to bench-scale tests (same Φ)

• Following flashover, yield of CO is independent of chemical structure of fuel. Bench-scale tests cannot accurately predict CO yields in post-flashover fires.

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Engineering PredictionsPrediction of Yield of CO

• Important because CO inhalation is most common cause of death in fires (USA)

• Death patterns ⇒ need need CO prediction methods for post-flashover fires {0.5 < Φ < 3.0}

• For post-flashover fires assume (within the enclosure)

YCO = 0.2 Eqn (5-24)


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Prediction of Yield of CO (outside Enclosure)• Consider flame exiting room (post-flashover fire)

– If flame rises vertically, does not impinge physical obstacles, and is in an area of plentiful O2,• CO is incinerated. YCO = well-ventilated limit

since Φ is small.

– If flame is flattened horizontally against a ceiling, impinges obstacles (heat sinks) or gets air from a long corridor• Little incineration of CO. YCO = 0.2 as in room.

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Life Safety Considerations

• Smoke = solids, liquids & gases• Hazards presented by smoke:

– toxicity– obscure visibility– excessive thermal exposure

• Consider exposure conditions which may prevent occupants of average susceptibility from escaping unassisted

• Adverse effects following exposure not considered


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Carbon Monoxide

• Suppose CO is the only toxicant present

• Maximum time, t (min), that the average human can remain in an atmosphere with high levels of CO {concentration VCO of CO in ppm} is

t = 35,000 / VCO Eqn (5-25)

• If the concentration of CO is time dependent, which it usually is, then

Eqn (5-26)35,000dt' )(t't

0COV =∫

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Visibility in Smoke

S = visibility (m)

For light-emitting signs: KS = 8For light-reflecting signs: KS = 3 Eqn (5-27)

Data based on subjects viewing smoke through glass so irritant effect of smoke eliminated, so visibility may be reduced compared with Eqn (5-27).


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Extinction Coefficient K

• Proportional to mass concentration of sootK = Km Cs Eqn (5-28)

Km = specific extinction coefficient (m2 / g)Cs = mass concentration of smoke (g / m3)

• For flaming combustion of wood & plasticsKm ~ 7.6 m2 / g Eqn (5-29)

• For pyrolysis (no flaming) of wood & plasticsKm ~ 4.4 m2 / g Eqn (5-30)

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For a Closed System

• Mass concentration of smoke (g / m3)Cs = ms / V = Ys mf / V Eqn (5-31)

ms = mass of soot produced (g)V = volume occupied by smoke (m3)Ys = yield of soot (g / g)mf = mass of fuel volatilized (g)


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For an Open (Flowing) System

• Mass concentration of smoke (g / m3)

Eqn (5-32)•

•

•

•

==V

m Y

V

mC SS

S

)s (m smoke of rate flow volumetricV

soot of yieldY)s (g fuel of rate loss massm

)s (gsoot of rate generation massm

13

S

1

1S

−•

−•

−•

=

==

=

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Exposure of Skin to Convection• Tenability limit for exposure of skin to convected heat

is 120ºC, above which pain and burns occur quickly.• Depending on length of exposure, convected heat

below 120ºC may also cause hyperthermia.• For fully clothed people, time for incapacitation (t in

min) is given in terms of T (ºC )

t = (4.1 x 108) T-3.61 Eqn (5-33)

• For unclothed or lightly clothed people, time for incapacitation (t in min) is given in terms of T (ºC )

t = (5 x 107) T-3.4 Eqn (5-34)


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Radiant Exposure of Skin• Tenability limit for exposure of skin to radiant heat is

< 2.5 kW m-2 Eqn (5-35)• Below 2.5 kW m-2, exposure can be tolerated for 30

min without affecting the time available for for escape

• Above 2.5 kW m-2, the time to burning of skin(t in min),

due to radiant heating ( in kW m-2) decreases rapidly as follows

t = 4 { }-1.35 Eqn (5-36)

q" d21

•

−

q" d21

•

−

q" d21

•

−

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References• D. Drysdale, An Introduction to Fire Dynamics,Wiley, 1999, Chap 1• A. Tewardson, ”Generation of Heat and Chemical Compounds in

Fires" Section 3 / Chapter 4, SFPE Handbook, 2nd Ed. (1995)• ISO/DTS 13571, “Life threat from fires - guidance on the estimation

of the time available for escape using fire data”.

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