Geology Reference
In-Depth Information
Runoff
Effective water
3
2
1
0
Pipe
Shower
Spray
Figure 18.1.5. Comparison of suppression methods showing effective or absorbed water (grey) and run-off
(blue) for coal particles with a 30 mm diameter.
The results of water required and run-off for each suppression method can be seen in Figure 18.1.5. The most
efficient method with respect to total water required is the shower however, using a spray results in less water run-
off and better control of the flow. The injection pipe is significantly less efficient requiring three times more water
than a spray of which more than 80% is lost as run-off.
The reason for the poor performance of the injection pipe is due to channeling of the liquid through the coal bed,
which was observed in the experiments. The channeling arises when the majority of the water takes the same flow
path through the bed. The result of this is that there is little global evaporation as the contact surface area between
the water and the coal is small; resulting in poor heat transfer. This coupled with a low residence time of water
results in large quantities of water being required.
The shower reduces this problem by applying the water over a larger area. This results in the coal being cooled
more uniformly and the generation of steam occurs throughout the bed to displace oxygen. However, the large
volumetric flow rate results in high liquid velocities through the bed, which again results in channeling, larger run-
off, and poor control of the flow. Using the spray allows greater control of the water application and more uniform
application across the free surface of the coal. It is believed that it is the good distribution of water over the coal
surface and subsequent slow flow through the bed that leads to the enhanced suppression properties of the spray
and not the droplet form of the water which plays a smaller role.
Effect of Particle Size on Suppression
Figure 18.1.6 shows the amount of water required using a spray to extinguish the small-scale smoldering fires. The
water required is expressed per unit mass of burning coal, assuming that the entire bed is burning at the time
suppression is attempted. This is a good assumption confirmed by visual observation and temperature measure-
ments in all experiments. The trend suggests that the amount of water per mass of coal decreases with particle size
and levels off for larger particles.
The volume of water per unit mass of burning coal required to extinguish a smoldering fire is in the range from 1 to
2 l·kg
1
. A figure larger than this could be required in field-scale fires to account for the water lost due to the
complex flow path through the subsurface and cooling of surrounding rock.
Using the thermal properties of coal and water and the average temperature at the time of suppression, the
theoretical quantity of water required per mass of burning coal can be estimated by equation 18.1.1, where m
w
is the mass of water required, m
c
is the mass of coal undergoing combustion, C
p,c
is the specific heat capacity of
coal,
Δ
T
c
is the temperature difference between the average coal temperature at the time of suppression and the
