The fire triangle best represents non-flaming combustion. It is comprised of three elements represented as sides of the triangle:

• Reducing agent (fuel);
• Oxidizing Agent (usually oxygen); and
• Energy (in the form of heat).

All three must be present for combustion to occur. For flaming combustion, there must also be a sustained chemical chain reaction.

Non-Flaming Combustion

When oxidation reactions occur, either flaming or non-flaming combustion processes can take place.

Non-flaming (or "smouldering") combustion occurs when solid fuels, particularly those that are porous or can char, undergo exothermic oxidation reactions at their surface. The oxidizer is a gas, but the fuel is primarily still the solid.

The heat released by the reactions results in pyrolysis (which will be discussed in more detail later) of fuel near the reacting zone, which in turn chars and sustains the process of surface oxidation. Typically, the temperatures in non-flaming combustion are low, the supply of oxidizer to the reaction zone is limited and the products of combustion are quite different than those that occur during flaming combustion.

If the conditions change appropriately, non-flaming combustion processes can undergo a transition to flaming combustion with subsequent development of an open flaming fire. Figure 6, shows the opposite occurrence: the surface oxidation of a popsicle stick that was initially flaming. In this case, flaming combustion has died out, and non-flaming combustion is occurring through continued oxidation of the remaining surfaces as the stick curls around and eventually burns through.

The processes and chemistry involved in non-flaming combustion are not nearly as well understood as those of flaming combustion. This sometimes makes it hard to predict when this situation might occur.