Geology Reference
In-Depth Information
Exitingushing attempts
Damage to environment
Damage to infrastructure
1 year
1
2
3
1 month
1 decade
1 week
4
1. Insert gas
2. Liquid
3. Slurry
4. Backfilling
© E Burns Commissioned by G Rein
Figure 17.1.1. Artistic rendition of a smoldering fire that began in an abandoned coal mine and subsequently
spread. Fire-fighting efforts (left) are ineffective because the fire was not accurately located. The vertical mine shaft
is being backfilled to prevent the ingress of oxygen. Hypothetical damage to the forest and buildings is shown.
Illustration by Emma Burns, 2008; commissioned by Guillermo Rein, 2008, University of Edinburgh (Rein, 2009).
in the gas phase surrounding the fuel (i.e., the flame). Figure 17.1.2 shows the two burning regimes for forest fuels.
The characteristic temperature, spread rate, and heat released during smoldering are low compared to those in the
flaming combustion of a solid. Typical values in smoldering at ambient conditions are around 500
-
1000°C for the
peak temperature and 6
-
12 kJ/g for the average heat of combustion; whereas typical values during flaming are
around 1500
30 kJ/g, respectively. Because of these characteristics, smoldering propagates at
creeping velocities, typically around 10
-
1800°C and 16
-
-
30mm/hour, approximately two orders of magnitude lower than the
velocity of typical flame-spread.
Because of its low temperature, smoldering is characteristically an incomplete oxidation reaction and thus emits a
mixture of unburned fuel, toxic, asphyxiant, and irritant gases and particulates at a higher yield than flaming fires. It
favors CO
2
to CO ratios around 1 (as opposed to ratios around 10 in flaming combustion), so CO is an important
toxic factor in smoldering fires (Purser, 2002; Rein et al., 2009).
Porous material as in coal piles or fractured coal seams are susceptible to smoldering combustion. The porous
nature of coal pile allows air to feed the exothermic reaction while protecting the reaction zone from heat losses to
the surroundings. But many other solid materials can sustain a smoldering reaction, including cotton, tobacco, dust,
paper, peat, duff and humus, wood, board of organic fibers, synthetic foams, and charring polymers including
foams. Smoldering fuels are characterized by having a significantly greater characteristic thermal time than fine
fuels but allow oxygen transport to the surface. These characteristics lead to the slow but persistent burning typical
of smoldering combustion. In general terms, a smoldering fuel consists of an aggregate and permeable medium
formed by particulates, grains, fibers, or a porous matrix. These aggregate fuel elements facilitate the surface
reaction with oxygen by providing a large surface area per unit volume.
It is well established that the two limiting factors in smoldering propagation are the oxidizer flow to, and the heat
losses from, the reaction zone. Thus, porous organic materials foster propagation because they act as thermal
insulation that reduce heat loss but, at the same time, permit oxygen transport to the reaction sites by convection
and diffusion. Smoldering ignition requires the supply of heat flux to the solid fuel. The subsequent temperature
increase of the solid first sets off the thermal degradation reactions (mainly pyrolysis) and then oxidations,
until the net heat released by oxidation is high enough to balance the heat required for propagation. This net
heat released by the reactions is partially transferred by conduction, convection, and radiation ahead of the
