Geology Reference
In-Depth Information
Ignition and Propagation Experiments
The effect of particle size on the ignition and propagation dynamics was investigated. Particle size affects the
porosity of the coal bed, the bed flow permeability, the effective thermal conductivity and the surface area available
for the heterogeneous reactions. Figure 18.1.4 shows the time to ignition for samples of different particle size.
Ignition is deemed to occur when temperatures measured 50mm from the igniter exceed those at 20 mm from the
igniter. This criterion was chosen as it is independent of heat transfer effects from to the igniter and infers a
chemical reaction with net heat release has been established. It can be seen in Figure 18.1.4 that the ignition time
decreases from 180 minutes for particles of 15mm to 140 minutes for particles of 30 mm in size, but then begins to
increase as heat transfer to the larger coal becomes more significant up to 220 minutes for particles of 45mm. It
was not possible to ignite particles smaller that 7mm in the time scale of these experiments (150 minutes) due to
the low flow permeability of the bed which prevents sufficient oxygen flow. Particles larger than 45 mm had a very
high thermal inertia and poor contact with the heater, which prevented ignition in this time scale. This suggests that
for small particles, the reaction is limited by oxygen flow through the bed and for larger particles, heat transfer to
the coal plays an important role in the ignition.
The maximum smoldering temperature was observed to be constant for particles larger than 15mm with tempera-
tures in the range 720 - 930°C. Lower temperatures observed for smaller particles suggest that the lower flow
permeability for fine particles significantly hinders the intensity of the fire.
Suppression Experiments
Three suppression mechanisms were considered to study their effectiveness: single point injection, shower, and
spray. Water was used as the extinguishing agent. The single point injection was a pipe buried 10 mm below
the surface of the sample. Water was allowed to flow freely through the pipe into the coal. The shower was
generated by allowing water to flow through holes of 4mm diameter in the bottom of a liquid reservoir and the
spray was generated using a hand operated atomizer. The water flow rates of each method are 75 ml·s
1
,
10 ml·s
1
, and 2 ml·s
1
, respectively.
The fire was deemed extinguished when all the temperature measurements in the box were below 50°C, since
this is below the self ignition temperature found by Kuenzer et al. (2007) and Zhang et al. (2007). Measure-
ments of the amount of water required for suppression were made by measuring the time for the application of
each and multiplying by the flowrate. The run-off generated was collected and measured immediately after
extinguishing. Run-off gives an indication of efficiency as large values mean that more water has to be applied
to achieve the same extinguishing effect. A method which results in low run-off will
therefore be more
efficient.
(
)
Figure 18.1.4. Experimentally observed relationship between time to ignition and particle size.
