[ppt]powerpoint presentation - natural resource ecology … · web viewthe combustion process * * *...
TRANSCRIPT
The Combustion Process
OXYGEN HEAT
FUEL
THE FIRE TRIANGLE
FIREcombustion
What basic elements are needed for fire to occur?
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)
THE PHASES OF COMBUSTION1) preignition 2) ignition3) combustion 4) extinction
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)
2. IGNITIONTHE PHASES OF COMBUSTION…
Transition between preignition and combustion
- Low temps = charring glowing combustion
- High temps = gases flaming combustion
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
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
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…
• 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…
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:
OXYGEN HEAT
FUEL
THE FIRE TRIANGLE
FIREcombustion
What basic elements are needed for fire to occur?
• Three methods of heat transfer…
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
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)
Important for pre-heating of shrub layers and crown canopy
Examples: heating a pot of water, smoke from a fire.
What Factors can Increase
Heat Transfer?
Flames are brought
closer to the fuel
due to slope.
Factors that Increase Heat Transfer
Wind
The wind pushes the flames over...
...bringing the flames closer to the fuel.
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