FIRE CATAGORIES
Forest fires can be categorized by many factors such as sizes, vegetation species, and burned part. The category of forest fires which are used regularly can be classified into three categories depending on what part of the forests they burn which are Crown fires, Surface fires, and Ground fires. (Brown and Devis, 1973).
Crown fire is a forest fire which occurs on top of trees, and in some case can “jump” from one tree to another; as well as, their fast spreading behaviour, making this type of fires the most aggressive and the highest risk so far. The height of this fire is also very high; at least 10-30 metres, in some cases, it can reach 40-50 metres. Crown fire can also be divided into two types depending on the influence of surface fire which are Dependent Crown Fire; this fire depends on surface fire to spread because of the lack of tree density, and another is Running Crown Fire where it appears in a very dense forest.
The second category of forest fires is Surface fire. This fire is the most tamed fire and can be put out easily. It occurs due to natural fuels on the forest such as leaves, branches, and bushes, etcand appears on the surface of the forest up to 1.3 metres high. It is the most seen fire in the world but it causes the least amount of destruction to the forests.
The last one is Ground fire which occur below materials such as leaves and peat. These fires are slow-moving, but if they are left unattended, they are able to take out large areas. This type of fires is dangerous because they can hole up below the surface during winter and reemerge once the weather gets warm. They are also the fires that cause the most destruction to the forest since they destroy the roots of trees that cause the trees to die later even though, there is no fire. This type of fire is rare, but when it happens it can last long and spread onto larger territories. It is also the hardest fire to be extinguished. These types of fires can be found in tropical areas such as Kalimantan, Indonesia or Narathiwat, Thailand. This research will focus on the peat fires which are located in tropical areas so they can be considered as Ground fires.
Commonly, Ground fire first occurs on the surface, and later on they go deeper into the underground. This fire can be divided into two types which are True ground fire, and Semi-ground fire. The former is the fire which burns in the underground. It is very difficult to detect this type of fire without special equipment such as a heat detector. Sometimes the fire already burned for years before it was found, so the burning just kept spreading and creating larger territories. The example of True Ground Fire is Coal Seam fire which is located in Kalimantan, Indonesia.
The other fire is Semi-ground fire meaning that the fire can spread on the surface as same as Surface fire, but parts of the flames will spread deep into the soil or peat at least a few feet. Semi-ground fire can be found in peat swamp forest in Sumatra, and Kalimantan, Indonesia; as well as, Narathiwat, Thailand.
PEAT FIRE PROCESS
Peat fire in Indonesia is considered to be Ground fire which occurs below materials such as leaves, and peat. This fire is slow-moving, but if it is left unattended, it will be able to take out a large area. This type of fire is dangerous because it can hole up below the surface during winter, and reemerge once the weather gets warm causing the most destruction to the soil properties, and biological system. It is also notable as the most difficult fire to be extinguished.
SMOULDERING VS
FLAMING
Wildfires can also be broadly divided into two types which are Flaming (Homogeneous Combustion), and Smouldering (Heterogeneous Combustion). Both of them are very important, and although they are structurally different, Smouldering can lead to Flaming.
There are different characteristics between Flaming and Smouldering. Flaming combustion is the process occurring when visible flames and plumes of a fire are visible. The main concept of Flaming Combustion is the fuel has to be in “Gaseous stage” in order to react with the oxidizer which is already in the gaseous stage. Flaming fire consumes surface fuel vegetation and tree crowns, so it consumes only thin layers of ground fuels. Flaming wildfire also releases a strong fire plume of black or dark grey colour with a higher concentration of soot. Crown fire and Surface fire are considered to be Flaming Combustion.
The other type of combustion is Smouldering. Naturally, Smouldering fire burns two types of biomass categorised by a thermal time compared to fuels; the first is thick fuels such as tree branches, or logs, the other is organic soil like duff or peat. For the fire in Indonesia, it can be considered as Smouldering wildfires with peat as a fuel. Smouldering Combustion in peatland is different from the flames in wildfires. When compared to Flaming, Smouldering is slower, lower-temperature, flameless, and the most persistence type of combustion since it is more difficult to suppress than Flaming combustion. Ground fire can be considered as Smouldering wildfires.
The spread of Smouldering Combustion is a multi-dimensional process; horizontal, and vertical. Most Smouldering wildfires begin on the ground surface by many factors such as flaming fires, lightning strikes, hot particles, and even self-heating. However, there are two mechanisms controlling the spread of Smouldering combustion; Heat losses, and Oxygen supply. After the ignition on the free surface, the fire can spread along the free surface, and vertically downward into peat layers. Recent peat fire experimentsshowed that the horizontal Smouldering spread reduces because of increasing moisture content or MC; while, vertical Smouldering spread is controlled by Oxygen supply. It means that the direction of downward Smouldering fire will follow where the Oxygen is. To be concluded, the Smouldering fire can spread to the underground, but the depth of spreading depends on the lack of Oxygen, and moisture which might be affected by underground water.
In some cases, Smouldering peat fire can also spread upward if the ignition is below the surface. For example, when the surface fire is extinguished; while, the deep fire is still burning, the hidden fire can spread upward during the drought season. Smouldering fire can also self-ignited, and it affects the fire suppression strategy; however, there are limited studies on the upward smouldering spread. Even current understanding of the whole Smouldering combustion is limited and less advanced than Flaming combustion due to the complexity with chemical processes.
Smouldering is much more hazardous to the ecosystem than Flaming since it occurs both above and below ground. It is very difficult to detect due to its low-temperature, flameless, and when it is detected, it is also well-known for difficulty to be extinguished without requiring a large amount of water.