Smoldering Combustion

Smoldering combustion is the least efficient phase of combustion and produces the most smoke per unit of fuel consumed. This phase lacks flame, and is associated with conditions where oxygen is limited – either by char of fuels (particularly those with large surface to volume ratios) or by tightly packed fuels like duff and organic soils or in wet fuels. As such, smoldering combustion dominates during ground fires. The smoldering phase can be distinguished from glowing combustion (the final phase of combustion) by the presence of smoke.

Smoldering combustion is “a self-sustaining, low temperature combustion process involving pyrolysis of the substrate ahead of a solid-phase combustion front” (Shafizadeh et al. 1982). The main features include thermal degradation and charring of the fuel with evolution of smoke (Moussa et al. 1976). A build-up of char on the surface of fuels is usually the reason for the transition from flaming combustion to the smoldering phase. Char is residual carbon that does not fully break down from pyrolysis, and therefore builds up on the surface of fuels. If the levels of char that inhibit volatile release are reduced in particular regions, fuels in the smoldering phase can re-ignite.

Smoldering fire can burn in duff and other ground fuels for long periods of time after flames have ended, and can be responsible for high levels of fuel degradation. Smoldering also tends to occur in tightly packed materials that lack the high levels of oxygen needed to maintain flaming conditions, or in fuels that have high moisture content and therefore do not readily ignite. Surface temperatures of fuels in the smoldering phase can exceed 500^oC due to the energy release from pyrolysis in the fuel (Pyne et al. 1996). The smoldering process is important in ecosystems that require removal of duff for plant regeneration.

The main factors that influence the smoldering rate of spread are time of day, wood chemical and physical composition, and log or duff location on the ground (Rabelo et al. 2004). Moisture content seems to play a less significant role in the speed of propagation than other combined parameters. Fire burning in high drought conditions can be self-sustaining and extend deep into the duff. Smoldering combustion varies with fuel type; it is more prevalent in duff, organic soil, and rotten logs, and less in fuel with high surface to volume ratios, like grasses, shrubs, and small diameter woody fuel (Sandberg and Dost 1990). See also: Ground Fires.

Products of smoldering combustion

Smoke released by the smoldering phase usually consists of liquids with high boiling points and tars that combine into an aerosol smoke (Pyne et al. 1996). Smoldering produces large quantities of this aerosol smoke and particulate matter, especially in wet fuels. Smoldering produced under severe drought conditions can sometimes be sustained for days and weeks.

The smoldering phase releases several times more fine particles than flaming combustion. Production of CO increases and reaches a maximum following the cessation of flaming combustion. This release of CO can continue for a few minutes after the flames die down (Ward 1998).