Geology Reference
In-Depth Information
Extinguishing 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 18.1.1. Artistic impression of a smoldering fire in an abandoned coal mine and illustration of possible fire
damage and suppression attempts. Illustration by E. Burns, 2008 (commissioned by G. Rein, University of
Edinburgh).
rarely practical because of the large amount of fuel involved in subsurface coal fires. All these methods are
reviewed in detail in Chapter 16 (Kim, 2010) and are shown in Figure 18.1.1 which is an artistic representation of a
subsurface coal fire including typical damage to the environment and infrastructure. Hypothetical growth of the fire
with time is indicated as well as suppression attempts that are: (1) injection of inert gas, (2) injection of liquid, (3)
injection of slurry or foam, and (4) sealing of the mine shaft.
In order to understand suppression, it is important to consider the processes that govern the spread of smoldering
fires (Chapter 17, Rein (2010)) and how these are disrupted by different suppression methods.
For a smoldering reaction to propagate, a feedback mechanism must be established where the heat released from
the reacting fuel is greater than the sum of the heat losses from the reaction zone and the energy required to preheat
the surrounding fuel. If the heat balance is altered by increasing heat loss to the surroundings, the rate of spread will
decrease and if the losses are increased sufficiently, the smoldering combustion may be quenched.
Increasing the heat losses from the reaction zone can be achieved by introducing a suppression agent to the reaction
zone to cool the coal. The thermal properties of the suppression agent such as heat capacity and heat of vaporization
play an important role in determining its efficiency. This forced cooling is employed in suppression of subsurface
coal fires by injecting a suppression agent below the surface and allowing it to penetrate through the ground to the
seat of the fire. In order for forced cooling to be effective, it is necessary to ensure that the temperature of the coal
everywhere in the reaction zone is reduced below the re-ignition temperature. The re-ignition temperature is
dependent on the coal properties and the geometry and geology of the coal seam but is on the order of 50°C
(Kuenzer et al., 2007; Zhang et al., 2007). Given the size of coal seams, this method requires large quantities of the
suppression agent and it is challenging to ensure that sufficient quantities reach the seat of the fire as the
suppression agent disperses through the subsurface layers. More invasive cooling methods include total or partial
flooding of the affected area.
Smothering of the fire exploits the experimental observations by Ohlemiller (2002), Walther et al. (2000), and
Palmer (1957), which show that if the concentration of oxygen is reduced below a critical level, the exothermic
oxidation reaction will stop and insufficient heat will be released to overcome losses, quenching the reaction. The
critical concentration below which a smoldering reaction in coal will not propagate is currently unknown but
research indicates it would be around 16% as measured by Belcher et al. (2010) for peat. Smothering can be
attempted by covering the area of the fire with a seal or cap (surface sealing) to prevent oxygen ingress from the
free surface or displacement of the oxygen surrounding the fire with an inert fluid.
