The Combustion Process

What makes a fire burn?  Why is one fire a roaring inferno while another barely creeps along?  Fire is a chemical reaction in which energy in the form of heat is produced.  When forest fuels burn, there is a chemical combination of the oxygen in the air with woody material, pitch and other burnable elements found in the forest environment.  This process in known as “Combustion.”  Combustion is a chain reaction chemically similar to photosynthesis in reverse. 

Photosynthesis requires a large amount of heat which is furnished by the sun.  The Combustion process releases this heat.  The tremendous amount of heat that is produced in the burning process is the major reason that the suppression of wildfires is such a difficult task and why the use of prescribed fire is a complex and exacting process requiring knowledgeable and experienced people. 

The combustion process or “fire” is sometimes called “rapid oxidation.”  It is similar to the formation of rust on iron or the decay of dead wood in the forest, except that the process is drastically speeded up. 

Fire begins with ignition.  The match is a common ignition device.  Friction creates sufficient heat to ignite the phosphorus on the end of the match.  Combustion occurs and the match flames. 

Heat is necessary to begin the combustion process.  Once started, fire produces its own heat. Wild land fires originate from such sources of heat as matches, embers from cigarettes, cigars or pipes, campfires, trash fires, exhaust sparks from railroad locomotives, sparks from brake shoes or “hot-box” on railroad cars. lightning, spontaneous combustion, hot ashes and arson.

The Fire Triangle

Three things are required in proper combination before ignition and combustion can take place---Heat, Oxygen and Fuel.

  1. There must be Fuel to burn.

  2. There must be Air to supply oxygen.

  3. There must be Heat (ignition temperature) to start and continue the combustion process.


Fuel pre-heating

Pre-ignition

Flaming stage

Flaming & Residual Smoke Generation

Residual /Smoldering

Glowing

Heat is Energy

Sources of heat are:

  • Sun
  • Atomic
  • Volcanoes
  • Fires

The Sun is the major source of heat.
Heat from the sun drives our weather.

With a continuous supply of heat (furnished by the combustion process itself), the ignition of additional fuel will continue as long as there is enough oxygen present.  Thus it is obvious that these three elements must be present and satisfactorily combined before combustion can occur and continue.  For the sake of simplicity we call this the “Fire Triangle.”

Remove any one of the three sides or elements and the fire will cease to burn.  Weaken any one, and the fire will weaken.  Increase any one or more of the elements, and the fire will increase in intensity. Armed with this knowledge the fire fighter or the prescribed burner can do much to manage a fire.

Demonstration

CAUTION: the jar gets hot!  Do not touch it without protection.  Young children should not attempt this exercise without adult supervision.

There are various ways in which the fire triangle can be broken or altered.  One example of how the triangle can be broken can be done with a short candle and a jar.  Light the candle and set it on a flat surface.  After it’s burning well, place the jar upside-down over the candle.  After a short time, the candle will go out.  This occurs because all of the oxygen inside the jar has been used up by the burning candle and no additional oxygen can get to the candle because of the jar.  Before you put the jar over the burning candle, you had all the ingredients necessary for combustion; heat from the match, fuel in the candle and oxygen from the air.

Relight the candle.  This time, take a pair of scissors and cut off the wick below the flame and remove the candle.  Again, the fire will go out after a short period when the rest of the wick that was left on the scissors is consumed.  This time you had plenty of oxygen in the air but you removed the fuel.  The same principle is used in fighting wildfires.  Remove heat, oxygen or fuel and the fire goes out.

In suppression of a wildfire, the objective is to stop combustion by removing or altering one or more sides of the triangle.

Stages of combustion: flaming stages  (see pictures from lab)

Pre-heating – temperature of the fuel is raised to the point where gases start to volatize

Pre-ignition – volatile materials in the fuel are vaporized

Flaming – the ignition temperature of the fuel is reached and combustion begins

Transition – fuel is partially consumed by combustion while flaming continues in portions of the fuel resulting in initiation of smoldering and smoke generation

Smoldering – combustion of the fuel is essentially complete where oxygen is available and smoldering continues resulting in smoke generation

Glowing – a stage of combustion where oxygen is limited

The four most important stages of combustion for prescribed burners are,

pre-ignition (fuel is about to burst into flame)

flaming – active combustion

transition – smoke generation begins

smoldering – residual smoke production

Suppressing fire and smoke generation (segment on Suppression)

When a wildfire has started, we try to remove the oxygen side of the triangle by smothering the fire with a fire retardant, foam, dirt or water in a fine spray or fog.  They will replace the oxygen around the fuel affecting one side of the fire triangle.  They also absorb heat and thus also alter the heat side of the triangle.  Retardants will coat the fuel and protect it from the heat even after the water has evaporated.  They also inhibit the flaming combustion by chemical action.  Foams also coat the fuel and last longer than water.  They reduce heat as well as supply of oxygen to the fuel.  They will adhere to vertical fuel and can be easily applied by ground units.

Water absorbs vast amounts of heat, especially when applied as a fog.  Each droplet absorbs a large amount of heat which turns the water into a hot gas or vapor (steam).  The hot steam is then dispersed by the wind into the atmosphere.  However, water is heavy and it is difficult to deliver it to the fireline in inaccessible areas.  There is also the possibility of running out at the most inappropriate time and losing the fire.

In forest conditions, one of the more important approaches to suppression of wildfires is removal of the third side of the triangle--Fuel.  The fuel is removed by building a fireline thus separating the fuels.  When the wildfire burns up to the fireline, no more fuel is available and the fire goes out.  The fire line is usually constructed with a tractor-plow unit or by hand.  (In the Western part of the United States, bulldozers and handcrews are used because of the steep, rocky conditions.)  In some areas, helitack crews and specialized ground equipment are used.

Removing Fuel 

Removing the fuel source is the most common method of attacking wildfires.  This method does not extinguish the fire.  The fire continues to burn until the fuel inside the fireline is consumed.  Removal of fuel in the path of the fire prevents the fire from spreading.  A slowly advancing fire burning sparse ground  fuels may be checked by constructing a fireline down to mineral soil.  A hot, fast-running fire may require several firelines, burning out the fuel between the firelines and the fire or a combination of both. 

Good firefighting procedure is often a skillful combination of removing fuel, heat, and oxygen.  Therefore, when you tackle a going fire, you should consider how best you can use personnel and equipment to remove one or all of the sides of the fire triangle.

1.  Fire is a
2.  The heat energy given off by a fire can be viewed a stored energy from
3.  The three legs of the fire triangle are
4.  Fire is a
5.  The fire stages of fire, the flaming stages, are
6.  The two most important flaming stages for prescribed burners are:
7.  Suppression or “holding” a prescribed fire consist of