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The Combustion Process
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OXYGEN HEAT
FUEL
THE FIRE TRIANGLE
FIREcombustion
What basic elements are needed for fire to occur?
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CO2 + H2O + HEAT (+ gases + char + ash)
(C6H10O5)n
Solar Energy + CO2 + H2O
+ O + Ignition Temperature
FUEL: PHOTOSYNTHESIS(storage of chemical energy)
COMBUSTION PROCESS…A chemical reaction…
Decay
COMBUSTION(conversion of stored energy to thermal, radiant, kinetic energy)
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THE PHASES OF COMBUSTION1) preignition 2) ignition3) combustion 4) extinction
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PyrolysisPreignition (“pre-heating”)…
Thermal degradation of the fuel = “heat divided” ( ~250 – 355 C )
char (low temperatures = glowing) tar (high temperatures = volatile gases = flaming) mineral ash (inorganics)
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2. IGNITIONTHE PHASES OF COMBUSTION…
Transition between preignition and combustion
- Low temps = charring glowing combustion
- High temps = gases flaming combustion
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2 types of “natural” ignition:
Lightning- High temperature within column of hot gases- 100 cloud-to-ground discharges/sec on Earth - Only 0.1 - .001 of strikes = wildfire
1. Lightning2. Spontaneous ignition
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Ignition continued…Spontaneous ignition
Microbial activity = respiration CO2 + H2O + HEATRequirements for ignition: Oxygen + formation of char
- surface oxidation of char = smoldering - smoldering heat continuous pyrolysis (flaming)
Pile heating (heat liberated faster than lost to surroundings) - chip piles: fresh chips + foliage, moisture > 20%
- pile > 1m in height
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3. COMBUSTION – 2 types:- Smoldering / glowing- Flaming
•Surface fires- Lower temps, but longer duration
• Ground fires in organic soil horizons •– smolder for mo/yrs (potential for re-ignition)
• High smoke production (particulates, CO)
Smoldering or glowing combustion
PHASES OF COMBUSTION…
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• Volatile gases mix with air = flames• High temperatures necessary (425-480 C)
• In general, fewer emissions than smoldering fires
Flaming combustion
TYPES OF COMBUSTION continued…
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4. Extinction:
THE PHASES OF COMBUSTION
- 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”)
termination of combustion
Two important factors can cause smoldering to cease:
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OXYGEN HEAT
FUEL
THE FIRE TRIANGLE
FIREcombustion
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 MethodsConduction:
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
Transfer of heat from one molecule to another.
Example: touching your hand to a hot object
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Radiation:
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
– only by thin layer at surface (rest by conduction)
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Convection:
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
Examples: heating a pot of water, smoke from a fire.
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What Factors can Increase
Heat Transfer?
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Flames are brought
closer to the fuel
due to slope.
Factors that Increase Heat Transfer
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Wind
The wind pushes the flames over...
...bringing the flames closer to the fuel.
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Turn to your Partner (TYP)
• FORMULATE an answer individually• SHARE your answer with your partner• LISTEN carefully to your partner’s
answer• CREATE a new answer through
discussion• ACCOUNT for your discussion by begin
prepared to be called upon
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