Reference
Smoldering Combustion
By Mark Hughes, P. Eng.
© 1998 Sintra Engineering Inc.
Many fire investigators intuitively understand the nature and manner of smoldering combustion. Unfortunately expressing the concept as words often leads to problems when the fire investigator is called upon to describe in detail the difference between flaming and smoldering combustion.
Fire investigators talk about the fire triangle: heat, fuel and an oxidizing agent (most commonly oxygen). In truth, the fire triangle should really be a square as there is a fourth requirement for the fuel and oxidizer to interact in a self-sustaining reaction in order to achieve a fire. This fourth component is the key to understanding the difference between smoldering and flaming combustion.
Flaming combustion involves gaseous fuel mixing with air, reacting with oxygen and releasing heat. The combustion process produces a variety of gases (CO2, CO, H2O etc.) as well as particulate matter known generally as smoke. Regardless of whether the fuel was originally a liquid or solid, the overall burning process must gasify the fuel. With liquids, the supply of gaseous fuel is a result of evaporation at the surface from the heat generated by the flames. Solids entail a significantly more complex process involving chemical decomposition (pyrolysis) of large polymeric molecules. Certain combustible solids such as sodium, potassium, phosphorus, and magnesium can even be oxidized directly by oxygen in the air without the need of pyrolysis.
Smoldering combustion a condition where all four elements of a fire are present, but are imbalanced. Smoldering combustion exists in condition where there is a self-sustaining reaction, but the reaction is limited by some of the constituents of the fire "square". A smoldering fire will not easily change to flaming combustion. To understand why one must understand how smoldering works.
Smoldering is self-sustaining reaction in which the heat released by surface oxidation causes pyrolysis of the unaffected fuel adjacent to the reaction zone, which in turn yields a rigid char which will subsequently undergo surface oxidation. The heat, oxygen or fuel is sufficient to maintain a continuous reaction but not enough for the fire to grow. An example of smoldering combustion is a cigarette. Typically smoldering combustion has reduced temperatures, limited ventilation and/or in an environment where the generated heat can be partially dissipated. In order to convert smoldering combustion into flaming combustion one of these three items must be altered. For example a cigarette will not flame unless additional oxygen is added and once this is removed, the flame dissipates. To have a cigarette change into flaming combustion, typically the method of dissipating the heat generated must be altered. An example of this is dropping a cigarette into a sofa. This prevents the dissipation of the heat generated by the cigarette and allows the smoldering to convert into flaming combustion.
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