1 Spatial Modelling of the Terrestrial Environment - Georeferencial

178 Spatial Modelling of the Terrestrial Environment
complete, and therefore the actual heat produced during the combustion is but a fraction
of the potential (theoretical) heat yield (Pyne, 1984) – an important point and one that we
will return to. Combustion requires activation energy from an external source, which, as we
have previously stated, is commonly provided as a result of human activity but may also
be due to lightning, burning of underground coal deposits, volcanic activity, specific forms
of vegetation decomposition or certain other phenomena (Chandler et al., 1983). The fuel
is ignited when one of these processes applies enough heat for pyrolysis to occur (Albini,
1993; Whelan, 1995). Pyrolysis can be summarized as the chemical degradation of fuel
through thermal decomposition, and results in the release of water vapour, carbon dioxide
and other gases such as methane, methanol and hydrogen (Lobert and Warnatz, 1993).
During this process the combustion reaction shifts from being endothermic to exothermic.
A spreading fire is a more complex combustion process than a simple campfire or candle
flame; it is one in which the flaming front is heating and then igniting unburnt fuels, allowing
the fire to propagate through the fuel bed. During the heating process, fuel moisture is
initially evaporated (fuel temperature > 100 ◦ C), then the cellulose is thermally degraded
and at its breakdown temperature volatized (temperature > 200 ◦ C) and finally the volatiles
are ignited to form a visible flame (300–400 ◦ C). The combustion process in a spreading
fire also goes through three distinct stages (Pyne 1984; Albini, 1993): (i) preheating during
which the fuel ahead of the fire front is heated, dried and partially pyrolyzed; (ii) flaming
combustion, which is the result of the ignition of flammable carbohydrate gases; and (iii)
glowing combustion where any remaining charcoal burns as a solid, with oxidation taking
place on the surface, leaving a small amount of residual ash. A consideration of the processes
of pyrolysis and ignition shows that a fire can to some extent be considered as self-sustaining.
Hence, the fire is able to spread away from the originally ignited region and into the
surrounding landscape. In areas where conditions are optimum for this process, huge areas
of hundreds of thousands of hectares maybe burnt in a single fire event.
9.2.2 Heat Generation
The spreading of fire through the heating of neighbouring unignited fuel, and the measurement
of energy emitted from the fire via remote sensing, are both possible because
combustion releases energy in large quantities, most obviously in the form of radiation
(some of which is visible to the naked eye) but also via convective and other processes.
The amount of energy liberated is of great significance because it is closely related to fire
intensity, one of the most significant parameters in any wildfire. In order of importance, the
factors affecting the nature of the combustion and the amount of heat released are fuel moisture,
fuel chemistry and the surface area to volume ratio of the fuel particle (Chander et al.,
1983). The single most important element influencing the total amount of heat released is
fuel moisture content (FMC), the most variable component of a fuel’s chemistry (Lobert
and Warnatz, 1993; Whelan, 1995). FMC may vary from 2.5% (dead savanna grasslands) to
200% (fresh needles and leaves) of the vegetation’s dry weight. High FMC has the capacity
to stop a fire, or to slow down the process to a slow, intense smoldering where much of the
vegetation may remain only partly combusted. Since the heat released is used to (i) raise
fuel temperature to 100 ◦ C; (ii) separate bound water from the fuel; (iii) vaporize the water
in the fuel; and (iv) heat the water vapour to flame temperature, it is clearly a very important
characteristic. The importance of FMC is illustrated by the fact that for eucalyptus fuels,