LESSON 2 - FUNDAMENTALS OF FOREST FIRE CONTROL

2.4 Stages of Burning

The combustion process consists of:
1. Preheating
2. Fuel break down
3. Combustion

The completion of these three stages will only occur if all three elements of the fire triangle are present and if enough heat is applied.

Preheating. In this stage, the fuel absorbs heat; fuel temperature rises and the moisture contained in the fuel is evaporated. There is no visible change in the fuel.

Fuel break down. In this stage, the preheated fuel begins to break down into gases and charcoal. There are visible signs of a change-taking place as the fuel begins to char.

Combustion. During this stage, the fuels that have been broken into gases and charcoal burn; gases with open flame, charcoal by glowing combustion (no visible flame).

In most instances, all three stages of burning will be occurring simultaneously in the same piece of fuel and the transition between one stage and the next is difficult to determine precisely.

Ignition

Ignition temperature is defined as the temperature at which fuels will ignite and continue to burn without any additional heat from another source.

Three important factors in determining if a fuel will reach its ignition temperature are:

1. Size of fuel.
2. Amount of heat applied.
3. Moisture content of the fuel.

Small, fine fuels require less energy (heat) to cause ignition. Therefore a small amount of heat for a short amount of time will cause a grass fire, and a sustained heat source will be required over a longer period of time to ignite a fire in heavy fuels.

Moisture content will impact ignition since moisture is a cooling agent and fuels of any size will require more heat to reach ignition. Smaller, fine fuels will require less time to dry out and therefore will be available for ignition much faster than larger coarse fuels.

Fuel consumption occurs on three levels:

Ground Fire – occurs below the surface; subsurface fuels are roots, duff and other partially decomposed organic materials.

Surface Fire – occurs on the surface; surface fuels are slash, brush, grass, twigs, litter and dry leaves.

Crown Fire – occurs in the upper portion of trees; includes leaves, needles, cones and limbs.

A forest fire may exhibit consumption on all levels if conditions are favourable for the development