What is Smouldering?

Smouldering is a slow, flame-less form of combustion that can persist for long periods of time. It can spread over extensive areas, spreading through sub-surface layers of soil with high organic matter (e.g. peat, humus, duff).

When peat is dry smouldering can be initiated by weak sources of heat (e.g. flaming vegetation). Once ignited smouldering fires are difficult to extinguish and can abruptly start flaming fires.

The main visible consequences of a smouldering fire are:
• smoke haze,
• removal of soil layers,
• ground destabilisation
• local subsidence

Several of these cause habitat loss, damage to root systems (pictured right) and produce important carbon emissions. Peat fires can affect especially drained peatlands and forests or plantations with a shallow layer of peat.

Table 1: Some differences between smouldering and flaming combustion

	Impact	Flame	Temperature (degrees C)	Propagation	Spread Rate (cm/hour)
Flaming	vegetation	yes	1500	fast	100
Smouldering	soil	no	500-700	slow	<5

References

Smouldering Fires and Natural Fuels, by G Rein, Chapter 2 in: Fire Phenomena in the Earth System - An Interdisciplinary Approach to Fire Science, C Belcher (editor). Wiley and Sons, 2013.