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The Combustion Process

The Combustion Process. THE FIRE TRIANGLE. HEAT. OXYGEN. FIRE combustion. FUEL. What basic elements are needed for fire to occur?. COMBUSTION PROCESS… A chemical reaction…. FUEL: PHOTOSYNTHESIS (storage of chemical energy). Solar Energy + CO 2 + H 2 O. (C 6 H 10 O 5 ) n. COMBUSTION

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The Combustion Process

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  1. The Combustion Process

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

  3. COMBUSTION PROCESS… A chemical reaction… FUEL: PHOTOSYNTHESIS (storage of chemical energy) Solar Energy + CO2 + H2O (C6H10O5)n COMBUSTION (conversion of stored energy to thermal, radiant, kinetic energy) Decay + O +Ignition Temperature • CO2 +H2O + HEAT (+ gases + char + ash)

  4. THE PHASES OF COMBUSTION • preignition • ignition • combustion • extinction

  5. 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)

  6. THE PHASES OF COMBUSTION… 2. IGNITION Transition between preignition and combustion - Low temps = charring  glowing combustion - High temps = gases  flaming combustion

  7. 2 types of “natural” ignition: 1. Lightning2. 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

  8. 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)

  9. 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) PHASES OF COMBUSTION… • COMBUSTION – 2 types: - Smoldering / glowing - Flaming

  10. 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

  11. 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”)

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

  13. Three methods of heat transfer…

  14. Conduction: Three Heat Transfer Methods 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

  15. 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)

  16. 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

  17. What Factors can Increase Heat Transfer?

  18. Flames are brought closer to the fuel due to slope. Factors that Increase Heat Transfer

  19. The wind pushes the flames over... Wind ...bringing the flames closer to the fuel.

  20. 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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