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 COMBUSTION

1) 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. 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- 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 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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