by - rapid combustion (combination of O[subs]2[/subs] with carbon, hydrogen, and other elements in a chemical reaction that produces light, heat, and flame
- exothermic reaction (releases heat) that is the opposite of photosynthesis
- photosynthesis: solar energy + 6CO[subs]2[/subs] + 6H[subs]2[/subs]O ⟶ C[subs]6[/subs]H[subs]12[/subs]O[subs]6[/subs] + 6O[subs]2[/subs]
- fire: C[subs]6[/subs]H[subs]12[/subs]O[subs]6[/subs] + 6O[subs]2[/subs] ⟶ CO[subs]2[/subs] + 6H[subs]2[/subs]O + heat
Fire Stages (all these stages are ongoing at once in different parts of the fire)
- Pre-heating:
- water must first be expelled from wood/fuel
- raising the temperature of the fuel will drive off the water
- this is accomplished through:
- flames (kindling)
- long periods of dryness (drought)
- Pyrolysis:
- Thermal degradation of the wood cellulose
- Cellulose is stable to about 615°F (325 °C)
- Above that, cellulose breaks down and expands
- Gives off flammable gases & water vapor
- Plus mineral residue, tars, etc. ⟶ ash
- gas movement causes cracks in the wood
- in the presence of O[subs]2[/subs] those gases ignite to form flames
- Flaming combustion:
- pyrolized wood burns hot and fast
- stage of greatest energy release
- heat transfer in several ways:
- conduction - heat moves inward through wood by physical contact of wood molecules (inefficient because wood is a poor conductor)
- radiation - energy released directly from flames in the form of electromagnetic (EM) radiation
- convection - warming of the air molecules surrounding the wood
- highly efficient and predominates in windy environments
- wind accelerates fire spread by:
- bringing in more oxygen
- heating up air to the point of igniting other fuel
- spreading material already ignited
- Glowing combustion:
- after the active flames die off (combustible gas is depleted)
- "coals" stage
- wood is slowly consumed in an oxidation reaction (lower temperature)
6,565.41 
