The Combustion Process

OXYGEN HEAT FUEL THE FIRE TRIANGLE FIRE combustion What basic elements are needed for fire to occur?

 CO 2 + H 2 O + HEAT (+ gases + char + ash) (C 6 H 10 O 5 ) n Solar Energy + CO 2 + H 2 O + 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 COMBUSTION 1)preignition 2)ignition 3)combustion 4)extinction

Pyrolysis Preignition (“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. IGNITION THE 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 cloud-to-ground discharges/sec on Earth - Only of strikes = wildfire 1. Lightning 2. Spontaneous ignition

Ignition continued… Spontaneous ignition Microbial activity = respiration  CO2 + H2O + HEAT Requirements 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 ( 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 FIRE combustion What basic elements are needed for fire to occur?

Three methods of heat transfer…

Three Heat Transfer Methods Conduction: 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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