Peatland fires

Fires burning in peat and other organic terrain are predominantly characterised by smouldering combustion. Such fires are spreading slowly, flameless and burn at low temperatures. Smouldering is dominated by pyrolysis and heterogeneous oxidation while flaming requires gas-phase oxidation. In their natural state, peatlands are too humid to burn. In Southeast Asia peat fires are commonly the result of human intervention.

Key characteristics:

• Peat is particularly sensitive to drought conditions: the severity of peatland fires is foremost related to soil moisture, second to the mineral content of the peat. This means peat drainage makes the soil more flammable; not only does it reduce the moisture available, it also creates channels whereby atmospheric oxygen can follow, thus encouraging smouldering at depth. Because smouldering fires transfer heat at depth, the fuel consumption of peat fires can be two orders of magnitude larger than flaming fires.

• Smouldering fires are hard to suppress, requiring large amounts of water. Moreover, the nature of smouldering (as opposed to flaming fires), means lower oxygen concentrations are needed (about 10% rather than 16% for flaming fires) and oxygen removal is inefficient until the fuel bed is cooled enough to prevent re-ignition. Finally, because lower temperatures are involved (450 to 700°C rather than 1500°C), dry peat is much more susceptible to smouldering fires and ignition is easier; dry peat is also at risk of self-ignition. The main issue when combating smouldering fires is that standard firefighting techniques which work for flaming fires (e.g. fire-breaks) cannot be relied upon to extinguish peat fires: prevention is much more efficient.

• Flaming and smouldering interact, such that smouldering fires typically succeed flaming fires, and under the right conditions, smouldering fires can also provide an ignition source for flaming fires to emerge. This means an integrated fire management approach is required for locations at risks of smouldering fires such as dry peatlands.

• The chemical composition of emissions from peat fires significantly differs from that of typical flaming fires. Incomplete combustion is prevalent, with more reduced compounds emitted. For instance, the CO/CO₂ ratio in smouldering fires is about 0.4 (about 0.1 for flaming combustion).

• Because little heat is generated, pollutants are emitted and accumulated close to the surface, which encourages their transport to population centers where they seriously threaten health.