Smouldering is a non-flaming, self-sustained combustion process, often described as a heterogeneous combustion, because the reaction occurs on the surface of the solid fuel, rather than in the gaseous phase of the released flammable vapours. It is a combination of partial combustion and pyrolysis. Most fuels that can undergo smouldering combustion are typically porous; cotton, tobacco, sawdust and peat are all good examples. Smouldering fires can self-propagate for many hours before transitioning to flame. Some wilderness smouldering fires have been burning for in excess of 6000 years, and given the correct conditions would still transition to flaming fire.
There has been a large amount of research into the transition from smouldering to flaming combustion. A large number of fatalities in house fires can be attributed to smouldering fires that have transitioned to flaming fires after several hours (and everyone is asleep).
A couple of excellent references examining this transition mechanism include:
There are three requirements to transition from smouldering to flaming combustion:
- the rate of release of combustible vapours must be sufficient to support flame
- the ratio of air (oxidant containing medium):vapourised fuel is high enough to support flame.
- heat of the system can support flame combustion.
You might note that these three factors are in fact the key ingredients in the fire triangle.
In smouldering fires, there is a complicated balance between the direction of smoulder travel, and direction and velocity of gas flow. Pathways of pyrolysis (which is endothermic) also serve to compete with heat production.
As the fuels are generally somewhat porous, there is an airflow created the burning solid fuel which influences heat dissipation and oxygen supply. Convections created by the smoulder travel introduce the gas flow. This may mean that the air flow pushes heat, volatile fuels and oxygen into a zone that will allow for flame combustion. Often the directions of travel are contradictory, and so can oppose each other. Air/gas flows can push volatiles away from the combustion zone, and also dissipate heat. Whilst the actual trigger mechanism appears to depend on the fuel type, composition and airflow of the system under study, it is suspected that a critical point is reached where the balance between oxygen content, fuel levels and heat is reached. It appears that oxygen concentration is the critical key to this process; not necessarily the existence of other flammable fuels.
Smouldering fires are usually low temperature fires, and sufficient heat must be present to support flaming combustion. Again, oxygen concentration is critical here, and plays a key role in the char oxidation (solid fuel combustion) which provides a rapid boost in heat to support transition to flame. Char oxidation is usually a second phase oxidation; it occurs at some point after initial partial combustion of the starting fuel.