IEEE Power & Energy Magazine - January/February 2022 - 25

still occur at the surface of the char left behind by the pyrolysis
(similar to charcoal in a barbecue). The heat produced is
much less, but if conditions are favorable (small heat losses
and enough oxygen), it might be enough to maintain pyrolysis.
In porous fuel beds, such as grounds covered by pine
needles, or in peat soils, such smoldering fires can survive
and grow for long periods. If conditions again change, the
pyrolysis gases can reignite and transition to flaming may
take place, such as happened in the Oakland Hills Fire of
California in 1991.
Ignition of Wildfires
As described, a series of physical processes must take place to
ignite a wildfire. Sustained ignition can only be achieved if the
heat from the flame produces more gaseous fuel by heating
more solid fuel (in simpler words, the fire must then be able to
spread). Even under very favorable conditions (e.g., Santa Ana
winds in California), about half of ignited fires have limited
spread and do not grow to become large wildfires.
Natural ignition sources are limited to lightning and volcanism.
Self-heating of natural fuels, such as peat, may also
cause ignition. Since peat is a porous organic material, air
can penetrate the soil and cause spontaneous exothermic
oxidation. This oxidation process is very slow at ambient
temperatures, but since heat cannot escape, temperature
builds up and ignition of self-sustained smoldering combustion
may occur.
However, according to a 2017 article in Proceedings of the
National Academy of Sciences by Balch et al, the predominant
cause of ignition around the globe (>80%) is a consequence of
human activity. The ignition can be due to arson, negligence,
or accidents. Among the latter, powerlines play an outsized
role. Powerline-involved wildfires
typically occur under worse fire
conditions (high winds, low moisture/humidity)
resulting in rapid
growth. As a result, powerlinecaused
fires are typically larger
and faster spreading compared to
the average wildfire.
Types of Wildfires
Since vegetation (live or dead)
feeds wildfires, the nature of the
resulting fires is as diverse as
vegetation itself. A fire spreading
over grasslands will produce
shorter flames than one propagating
through densely packed shrubs.
The rate of propagation, on the
other hand, will be significantly
higher than in the latter. This results
in a shorter residence time, which
reduces the time larger objects are
exposed to heat. This fact is crucial
january/february 2022
for the characteristics of wildfires propagating in woodlands
and forests, where large trees coexist with shrubs and smaller
vegetation and where the ground is covered with combustible
material that has fallen from the trees. The wildfire type is also
important in terms of the risk it poses to power networks, as
not all fires would potentially produce the same damage.
Surface Fires
If wildfires occur regularly in woodlands and forests, consuming
the dead organic matter accumulated over a relatively
short time, they will spread quickly over the dry fuel
lying on the ground and have a low intensity (short flames
and a low rate of heat release as shown in Figure 8). If such a
fire passes below powerlines, it is less likely to cause significant
damage to them, as it does not interact closely enough
with the conductors.
Larger shrubs and trees require more heat to ignite. Their
size also allows the vegetation to retain moisture longer than
smaller fuels, such as dry grass or twigs.
A variety of reasons, however, can lead to intensified
ground fires, such as drought periods that lower the moisture
content of shrubs and other larger vegetation. Another reason
could be poor land management causing accumulation
of dry organic matter with a consequently larger energetic
content. There is evidence that weather extremes, such as
abundant rain followed by prolonged drought, can result in
excessive vegetation with very low moisture content, which
is a hazardous combination in terms of wildfire.
Canopy (Crown) Fires
When surface fires, which under normal circumstances do not
pose a problem to trees or powerlines, grow in intensity, their
figure 8. A low-intensity surface fire (backfire set by firefighters to contain fire).
Short flames and high spread velocity cause little damage. (©Getty Images/Philip
Pacheco; used with permission.)
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IEEE Power & Energy Magazine - January/February 2022

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