Smouldering Fires and Soils

Smouldering is the slow, low temperature, flameless burning of porous fuels and the most persistent type of combustion phenomenon. It is the driving mechanism in the combustion of organic soils, such as peat. Different from surface and crown wildfires, which produce flames, spread faster, and released more heat, smouldering fires pose greater threats in term of fuel consumption, emissions, and damage to the soil system. Range of mass consumption during peat fires could be from hundreds to billions of tons. This scale leads to massive emission of carbon stored in peatland to the atmosphere. Once peat is ignited on the free surface, smouldering propagation develops in two dimensions; the lateral (horizontal) and the in-depth spread. In lateral direction, spread rate is 3.1 ± 1.1 cm/h. In in-depth direction, spread rate is 1 cm/h. Due to the slow spread and high persistency, smouldering fires lead to enhanced heat transfer to the soil for much longer durations (i.e., in the order of one hour) than flaming fires, and the soil can reach peak temperatures as high as 500 °C. These thermal conditions are more severe than, for example, medical sterilization treatments, and imply severe thermal effects and require longer recovery post fire than flaming. Reported data of previous peat fires events show boreal peat fires have a lower carbon emission flux than tropical peat fires. In average, depth of burn and total quantity of carbon emitted from peat fire are 39 cm and 841.7 Mt-C, per event. This data shows the enormous magnitude of carbon footprints from peat fire. While flaming wildfires have disproportionally received more attention than smouldering fires, this chapter aims at reversing the trend and hopes to bring new forward-thinking efforts and ideas into the important study of flameless fires.