The Combustion
Process
What basic elements are needed for fire to occur?
THE FIRE TRIANGLE
HEAT
OXYGEN
FIRE
combustion
FUEL
COMBUSTION PROCESS…
A chemical reaction…
FUEL: PHOTOSYNTHESIS
(storage of chemical energy)
Solar Energy + CO2 + H2O
(C6H10O5)n
Decay
+ O + Ignition
CO2 + H2O + HEAT
Temperature (+ gases + char + ash)
COMBUSTION
(conversion of stored energy to thermal,
radiant, kinetic energy)
THE PHASES OF COMBUSTION
1) preignition
2) ignition
3) combustion
4) extinction
Preignition (“pre-heating”)…
Pyrolysis
Thermal degradation of the fuel = “heat divided”
( ~250 – 355 C )
 char (low temperatures = glowing)
 tar (high temperatures = volatile gases = flaming)
 mineral ash (inorganics)
THE PHASES OF COMBUSTION…
2. IGNITION
Transition between preignition and combustion
- Low temps = charring  glowing combustion
- High temps = gases  flaming combustion
2 types of “natural” ignition:
1. Lightning
2. 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
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)
PHASES OF COMBUSTION…
3. COMBUSTION – 2 types:
- Smoldering / glowing
- Flaming
Smoldering or glowing combustion
•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)
TYPES OF COMBUSTION continued…
Flaming combustion
• Volatile gases mix with air = flames
• High temperatures necessary (425-480 C)
• In general, fewer emissions than smoldering fires
THE PHASES OF COMBUSTION
4. Extinction: 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”)
What basic elements are needed for fire to occur?
THE FIRE TRIANGLE
HEAT
OXYGEN
FIRE
combustion
FUEL
•
Three methods of heat
transfer…
Three Heat Transfer Methods
Conduction:
Transfer of heat from one molecule to another.
Example: touching your hand to a hot object
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
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)
Convection:
Transfer of heat by
movement of a gas
or liquid (air).
Hot air moves vertically
(exceptions: winds, slopes)
Examples: heating a pot of
water, smoke from a fire.
Important for pre-heating of
shrub layers and crown
canopy
What Factors
can Increase
Heat Transfer?
Factors that Increase Heat Transfer
Flames are brought
closer to the fuel
due to slope.
The wind pushes the
flames over...
Wind
...bringing the
flames closer
to the fuel.
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