The Fire Triangle and Combustion

Fire Triangle

The fire triangle illustrates the relationship between the three basic elements needed to establish fire: oxygen (O₂), combustible fuel (a form of (C₆H₁₀O₅)_n), and heat to initiate and sustain combustion. These three elements combined together in the proper ratio lead to fuel ignition and combustion. The progression of combustion in wildland fires is generally divided into four phases: pre-ignition, flaming, smoldering, glowing, and extinction. Here, the chemical reaction of combustion is explained.

In the combustion process, oxygen “oxidizes” (combines with) carbon and hydrogen molecules as heat breaks chemical bonds in carbon chains during combustion. When vegetation is burned, heat is released in the form of thermal, radiant, and kinetic energy.

Heat is supplied initially from an external source (lightning, matches, etc.) that begins breaking chemical bonds in the organic compounds. Ignition occurs when the external heat source has broken enough carbon bonds and released enough heat from these bonds to sustain combustion without the external heat source. Thereafter, heat generated by the breaking of bonds maintains combustion as a “feedback loop” until the combustible fuel is exhausted or removed, or the heat produced is inadequate to further break additional bonds, or oxygen supply becomes limiting.

The general reaction formula for combustion is the reverse of photosynthesis, and is represented as:

(C₆H₁₀O₅)_n = a plant-synthesized organic compound

O₂ = gaseous oxygen

CO₂ = carbon dioxide

H₂O = water vapor

PICCs = products of incomplete combustion (multi-carbon molecules pyrolyzed from longer compounds)

Combustion can be high-speed (as in rocket motors, gas turbines or internal combustion engines) or low-speed (forest fires and candles). In low-speed combustion, a feedback loop exists between gases above the fuel and the fuel itself. In the gaseous phase, oxygen in the air drives combustion of the gaseous fuels and heat is released exothermically. Some of the heat sustains combustion of the gases and some is transferred back to the condensed phase (vegetative fuels), where it causes endothermic (heat/energy absorbing) gasification of the vegetative fuel. The feedback of heat from gas phase flames to condensed phase fuel is a crucial mechanism for sustaining the combustion process.

With the heat losses, combustion of plant material in open air is seldom complete; if it were, all carbon molecules would be oxidized to CO₂. Products of incomplete combustion include carbon monoxide (CO), nitrous oxides (NOx), sulphur oxides (SOx), dioxin, a variety of multi-carbon molecules and soot (Ward 2001).

Wildland fuels vary in their combustibility depending on type, chemical composition, size, arrangement, moisture content, and quantity. Vegetative fuels are generally composed of cellulose, hemicellulose, other sugars, lignin, resins, and other organic compounds (see Chemical Fuel Properties). Dead plant material generally contains less moisture (due to a cessation of water uptake), a different arrangement of moisture (less intra-cellular water), and less long-chain organic compounds (due to decomposition), all of which influence combustibility (or fuel availability).