The Combustion Process

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The Combustion Process
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What basic elements are needed for fire to occur?
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O Ignition Temperature

• CO2 H2O HEAT
• ( gases char ash)

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THE PHASES OF COMBUSTION

• preignition
• ignition
• combustion
• extinction

preheating

• PREIGNITION

• pilot heat source
• temperature of fuel raised
• dehydration water evaporates (100 C)

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Pyrolysis
Preignition
Thermal degradation of the fuel heat divided
(chemical decomposition _at_ 250 355 C )
? tar (high temperatures volatile gases
flaming) ? char (low temperatures glowing) ?
mineral ash (inorganics)
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Transition between preignition and
combustion - Low temps charring ? glowing
combustion - High temps (325 C) gases ?
Flaming combustion - Heat generated
ignites new fuel continued cycle (series of
ignitions)
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2 types of ignition
1. Lightning2. Spontaneous ignition
Lightning

• High temperature within column of hot gases
• 100 cloud-to-ground discharges/sec on Earth
• Only 0.1 - .001 of strikes wildfire

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Ignition continued
Spontaneous ignition
Pile heating (heat liberated faster than lost to
surroundings) - chip piles fresh
chips foliage, moisture 20 - pile 1m in
height

• Microbial activity respiration ? CO2 H2O
  HEAT
• Requirements for ignition Oxygen formation of
  char
• surface oxidation of char smoldering
• smoldering ? heat ? continuous pyrolysis
  (flaming)

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PHASES OF COMBUSTION

• COMBUSTION 2 types

- Smoldering / glowing - Flaming
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TYPES OF COMBUSTION continued
Flaming combustion

• Volatile gases mix with air flames (oxidation)
• Temperature of gas/air mixture must reach
• 425-480 C in order to support flames
• In general, fewer emissions than smoldering
  fires

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termination of combustion
Two important factors can cause smoldering to
cease
- Inorganic materials (ash) absorb heat but
do not oxidize reduces the total amount of
heat - Not enough heat produced to cause
vaporization in in moist fuels (no more
available fuel)
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Turn to your Partner (TYP)

• FORMULATE an answer individually
• SHARE your answer with your partner
• LISTEN carefully to your partners answer
• CREATE a new answer through discussion
• ACCOUNT for your discussion by begin prepared to
  be called upon

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Explain in one sentence without using technical
terms

• What is combustion (fire)?

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What basic elements are needed for fire to occur?
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• Three methods of heat transfer

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Three Heat Transfer Methods
Transfer of heat from one molecule to another.
Conduction is the only means of transferring heat
to the interior of fuels (wood, litter, duff).
High-density fuels (green wood) vs. low-density
fuels (litter, decayed wood)
High density fuels have greater conductivity
more heat needed to raise temperature of surface
layer
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Transmission of heat by electromagnetic waves.
Examples of radiation
Heat from sun, fire place, stove
Contact between radiation source and affected
body not necessary Example preheating of fuels
ahead of fire front
Absorption of radiation by woody fuels
(conversion to thermal energy) only by thin
layer at surface (rest by conduction)
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Transfer of heat by movement of a gas or liquid
(air).
Hot air moves vertically (exceptions winds,
slopes)
Important for pre-heating of shrub layers and
crown canopy
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What Factors can Increase Heat Transfer?
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Factors that Increase Heat Transfer
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SUMMARY

• Fire triangle combustion process Heat
  transfer methods (3) Factors that increase heat
  transfer Phases of combustion (4) Factors that
  affect fuel consumption

• Next fire behavior wildland fuels