Fire Dynamics I

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Fire Dynamics I

• Lecture 12
• Smouldering and
• Spontaneous Combustion
• Jim Mehaffey
• 82.575 or CVG7300

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• Smouldering and Spontaneous Combustion
• Outline
• Smouldering combustion
• Glowing combustion
• Spontaneous combustion (ignition) of solids
• General principles
• Insulating fibreboard
• Mathematical models
• Related phenomenon

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• Smouldering Combustion
• Slow, low-temperature, non-flaming combustion
  sustained by heat evolved when O2 reacts directly
  with surface of a solid
• Smouldering is a serious fire hazard because
• it typically generates a substantially higher
  yield of toxic compounds than flaming (though at
  a slower rate)
• it provides a gateway to flaming that can be
  initiated by heat sources too weak to directly
  produce a flame.

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• Smouldering Combustion
• Characteristics of materials (e.g. cigarettes)
  that experience smouldering
• solid has a large surface area per unit mass,
  which facilitates surface oxidation
• solid is permeable, which permits O2 to reach
  reaction site by diffusion and/or convection
• solid is a good thermal insulator, which reduces
  heat losses and permits sustained combustion
  despite low heat heat release rates
• solid forms a carbonaceous char when it undergoes
  pyrolysis. Char has a high heat of combustion
  and is susceptible to oxidation at moderate
  temperatures (gt 670 K 400C).

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• Smouldering Combustion
• Rate of propagation of smouldering is controlled
  by supply rate of oxygen
• Reverse propagation When oxygen diffuses through
  unburnt fuel toward reaction front
• Example 1D upward smouldering through sawdust
• Typical rate of upward advance is slow. It has
  been found to take 2 weeks to propagate upwards 1
  m
• When air is supplied from above, rate of advance
  increases. Yield (mole fraction) of CO is 6 - 7
  for PU and 10 - 22 for cellulose insulation.

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• Smouldering Combustion
• Forward Oxygen flow is in same direction as
  advance of smoulder front
• Example cigarette during a draw
• Rate of advance of smouldering still not fast
• Yield (mole fraction) of CO is 9 for cellulose
  insulation

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• Smouldering Combustion
• Can occur in porous materials which form
  carbonaceous char when heated
• Examples paper, cellulosic fabrics, sawdust,
  fibreboard, latex rubber and some thermosetting
  foamed plastics
• Materials which melt and shrink away from a heat
  source do not exhibit smouldering combustion

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• Initiation of Smouldering
• Can follow flaming combustion of cellulosic
  materials if flame is extinguished
• Can be induced by another smouldering source such
  as a cigarette
• Radiant or conductive heating in the absence of a
  pilot
• Self-heating leading to spontaneous combustion

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• Smouldering along a Horizontal Cellulose Rod

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• Smouldering along a Horizontal Cellulose Rod
• Zone 1 Pyrolysis zone in which there is a steep
  temp rise and an outflow of visible airborne
  products.
• Zone 2 A charred zone where temp is maximum,
  evolution of visible products stops and glowing
  occurs.
• Zone 3 A zone of porous residual char and/or ash
  which no longer glows and whose temp falls slowly.

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• Smouldering along a Horizontal Cellulose Rod
• Heat is released in Zone 2 where surface
  oxidation of char occurs and temps are 600 -
  750C. Heat of combustion of char is high.
  Oxygen reaches Zone 2 because of permeability of
  specimen.
• Heat is conducted from Zone 2 into Zone 1 causing
  thermal decomposition of fuel to produce
  volatiles and carbonaceous char. Flow of hot gas
  (when smoker draws on cigarette helps. Temp must
  be above 250-300C for this to take place.
• Rate of propagation of smouldering front depends
  on rate of penetration of O2 to surface of
  reacting char (heat release rate) and rate of
  heat loss

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• Smouldering along a Horizontal Cellulose Rod
• Smoke Non-oxidized volatile products of
  pyrolysis.
• Smoke includes gaseous products, but also high
  boiling point liquids and tars which condense to
  form an aerosol.
• Smoke is combustible and, in principle, can give
  rise to a flammable atmosphere.
• Smoke may also be quite toxic.
• Smoke produced in smouldering is quite different
  from that produced in combustion.

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• Glowing Combustion
• Surface oxidation of carboneous materials or char
• No thermal degradation of parent fuel
• Examples
• activated charcoal
• char after flaming has ceased

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• Spontaneous Combustion within Solids
• Some combustible solids can ignite as a result of
  internal heating if an exothermic process
  liberates heat faster than it can be dissipated
• This can occur only if the material
• is sufficiently porous to allow air (O2) to
  permeate it
• yields a rigid char as result of thermal
  decomposition
• Smouldering (oxidation of char) begins inside the
  material and slowly propagates radially outward
• Phenomenon is normally associated with relatively
  large mass of material (small surface to volume
  ratio)

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• Spontaneous Combustion within Solids
• Heat generated by some exothermic process
• surface oxidation interstices of porous
  materials
• micro-biological activity
• If rate of heat release gt rate of heat loss,
  material undergoes self-heating and may ignite
  (smouldering)
• O2 must diffuse into material, so time to
  ignition (if it occurs) can be very long
• However, spontaneous combustion (ignition) may
  occur at low ambient temperatures

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• Spontaneous Combustion within Solids
• Smouldering (oxidation of char) begins inside the
  material and slowly propagates radially outward
• May cause flaming when it reaches the surface

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• Case Study Insulating Fibreboard (3)
• Forintek Royal Sun Alliance Insurance Company
• Several fires occurred during transport of
  insulating fibreboard panels from a plant in the
  prairies to retailers in southern Ontario
• Suspected insulating fibreboard panels
  experienced self-heating and spontaneous
  combustion
• Forintek asked to investigate

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• Insulating Fibreboard Panels
• Thickness 12.7 mm
• Density 230 kg m-3
• Insulating fibreboard is known to be subject to
  self-heating spontaneous combustion because
• It is rather porous so O2 can permeate it
• It is a good insulator so heat is trapped within
  it

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• Test for Self-heating Spontaneous Combustion
• Drying oven stabilized at some temperature.
• Cube of insulating fibreboard placed in oven
• Three thermocouples placed in cube
• at geometric centre
• above centre
• below centre
• Observe thermal performance of cube
• does cube experience self-heating whereby centre
  temperature exceeds oven temperature, and
• does cube experience spontaneous combustion
  smouldering starts in centre progresses
  outwards

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• 190 mm Cube _at_ 145C

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• 190 mm Cube _at_ 155C

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• 190 mm Cube _at_ 150C

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• Summary of Findings Insulating Fibreboard (2)

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• Photograph of inside of Cube after Testing

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• The Frank-Kamenetskii Model
• A thermal explosion theory developed to explain
  auto-ignition in vapour / air mixtures
• Assumptions
• Rate of heat generation / volume Hc ? A exp(- E
  / RT)
• Heat is transferred through specimen by
  conduction, and this is a slow process
• Heat transfer at surface to surroundings
  (convection radiation) is high, so that surface
  temp ambient temp
• Physical properties are independent of
  temperature

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• The Frank-Kamenetskii Model
• Provides a relationship between
• Ta,cr critical ambient temperature for ignition
  (K)
• ro dimension of sample (m). (Side of a cube
  2 ro)
• ?cr critical parameter dictated by geometry of
  sample
• ln?cr Ta,cr2 / ro2 C1 - C2 / Ta,cr
  Eqn (14-1)
• C1 lnE Hc A ? / (k R)
• C2 E / R

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• The Frank-Kamenetskii Parameter
• For large cubes, assumption that surface temp
  ambient temp is reasonable and ?cr 2.52
• For small cubes, the assumption breaks down and a
  correction factor must be applied
• For 75 mm cube ?cr 1.97
• For 190 mm cube ?cr 2.34
• For 305 mm cube ?cr 2.52

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• The Frank-Kamenetskii Model
• Using the structure of Eqn (14-1) as a guide,
  critical ambient data for the 75, 190 and 305 mm
  cubes were plotted on next slide as 1000 / Ta,cr
  vs ln?cr Ta,cr2 / ro2
• The resultant plot is nearly linear and a
  regression analysis provided the equation
• 3.662 - 0.07333 ln?cr Ta,cr2 / ro2
  1000 / Ta,cr

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• Correlation of Ignition Data
• ln?cr Ta,cr2 / ro2

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• Summary of Findings Insulating Fibreboard (2)

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• Insulating Fibreboard Panels
• An ambient temp of 104ºC is required for a pallet
  (1.22 m cube) of insulating fibreboard to
  experience spontaneous combustion
• An ambient temp of 89ºC is required for a tractor
  trailer full of insulating fibreboard (2.44 m
  cube) to experience spontaneous combustion
• Typical ambient temperatures found in a tractor
  trailer or railcar are clearly not sufficient to
  cause spontaneous combustion.
• So how did spontaneous combustion occur?

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• Absorption of Water Vapour by Fibreboard
• Wood and cellulosic fibres are hygroscopic
• Absorption of water vapour by wood products is
  known to be an exothermic process.
• Could this exothermicity play a role?
• In the summer, it is much more humid in southern
  Ontario than in the prairies.
• An experiment was devised to investigate this
  possibility.

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• Effect of Absorption of Water Vapour

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• Experiment Absorption of Water Vapour by
  Fibreboard
• Measure moisture content of 75 mm and 305 mm
  cubes. Found to be 5.7
• Dry 75 mm and 305 mm cubes in chamber at 50C and
  low relative humidity for 28 hours. Moisture
  content of 75 mm cube dropped to 1.5.
• 305 mm cube allowed to cool overnight in chamber
  at 20C and RH lt 2.
• Next morning, ambient conditions in chamber
  changed to 30C and RH 76. Evidence of
  self-heating due to absorption of water vapour
• Following morning, ambient conditions in chamber
  were changed to 32C and RH 99. Again
  evidence of self-heating due to absorption of
  water vapour
• Measure moisture content of 305 mm cube. MC
  6.9

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• Absorption of Water Vapour by Fibreboard
• When stored at 32C and RH 95, moisture
  content of fibreboard can be 23-24
• In summer in closed tractor trailer, may be 65C
  and RH 95. Moisture content of fibreboard
  very high
• Sweden If fibreboard at 20C and RH 70 with
  M.C. 10 suddenly absorbs enough water vapour
  so M.C. increases by 4.5, temp of board may
  increase to 100C under adiabatic conditions
• Board manufactured in prairies often has M.C.
  1-2
• Potential for problems

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• Absorption of Water Vapour by Fibreboard
• Fibreboard manufactured in prairies with M.C.
  1-2 shipped to Ontario where temp RH are much
  higher can absorb enough water vapour to drive
  temp in load of fibreboard in an enclosed vehicle
  to over 100C
• This is hot enough so that oxidation within the
  load will commence and spontaneous combustion may
  occur
• Prevention Remoisten board with liquid water
  after production. Energetics not so unfavourable
  

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• Spontaneous Combustion
• of Animal Feedstuff (4)
• Animal feedstuff processed byproduct from
  distillery

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• Spontaneous Combustion
• of Skimmed Milk Powder (4)

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• Spontaneous Combustion
• Yeast-Based Powder (4)

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• Time to Ignition Chemically active carbon (1)
• Problems with cargo of active carbon (4-14
  tonnes) in holds of ships passing through tropics
  (38C)
• Time to ignition appeared to be very long

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• Time to Ignition Chemically active carbon (1)
• Times associated with observations slightly less
  than extrapolation from small-scale data

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• Related Phenomena
• Linseed oil on cotton rags. Oil dispersed on
  rigid porous structure. Large surface area on
  which oxidation occurs but heat is not quickly
  dissipated.
• In chemical plants where thermal insulation, or
  lagging, is applied to pipes carrying hot process
  fluids. Leakage from faulty flange into lagging
  can lead to a lagging fire. Boundary
  conditions are different hot against pipe, cool
  against air

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• Spontaneous Combustion in Haystacks
• Stage 1 If hay is very wet (63-92 by weight
  water), micro-biological activity (bacteria) can
  raise temp to 70C
• Stage 2 Chemical oxidation causes self heating.
  Water may be a catalysis for such low temp
  oxidation.
• Stage 3 Ignition
• At typical ambient temperatures, critical size
  for cubical haystack is 2ro 2 m
• Bales of hay do not experience spontaneous
  ignition however, haystacks do

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• References
• 1. D. Drysdale, An Introduction to Fire Dynamics,
  Chapter 8.
• 2. T.J. Ohlemiller, Smoldering Combustion,
  Section 2 / Chapter 11, SFPE Handbook, 2nd Ed.
  (1995)
• 3. J.R Mehaffey, L.R. Richardson, M. Batista and
  S. Gueorguiev, Self-heating and spontaneous
  ignition of fibreboard insulating panels, Fire
  Technology Vol 36, 226-235 (2000)
• 4. P.F. Beever, Self-heating and spontaneous
  combustion, Section 2 / Chapter 12, SFPE
  Handbook, 2nd Ed. (1995)