Geology Reference
In-Depth Information
18.1. Burning and Suppression
Experiments
Rory Hadden
Guillermo Rein
Small-scale apparatus developed to study
the burning characteristics and suppres-
sion of smoldering coal
Photo by Rory Hadden, BRE Centre for Fire Safety Engineering, University of Edinburgh, 2009.
Introduction
U
nderstanding the burning and suppression behavior of subsurface coal fires is essential if these fires are going to
be monitored and extinguished in a scientific and cost effective manner. In this chapter, small-scale experiments are
presented in which the burning and suppression behavior of smoldering coal is studied.
The laboratory experiments presented here are undertaken at a small scale (0.1 m). Some in-situ large scale
experiments are reported in detail in Chapter 16 (Kim, 2010). The advantage of reducing the scale and the scope of
the problem by studying suppression in the laboratory is that most of the complexities of in-situ experiments such
as locating and mapping the fire, and the delivery of the suppression agent to the fire seat are eliminated.
Meanwhile, the important variables (fire size, particle size, thermal and flow conditions, etc) can be controlled. The
results presented here are only a first step in understanding the suppression of coal fires once they have been
detected and located, and the liquid water used as suppression agent has been delivered to the fire seat. Application
of the results to larger scales requires field trials.
Subsurface coal fires are driven by smoldering combustion which is defined as a slow, low temperature flameless
form of combustion sustained by the heat evolved when oxygen directly attacks the surface of a condensed phase
fuel (Ohlemiller, 2002). Therefore, a subsurface coal fire propagates at a low speed governed primarily by the rate
at which oxygen can diffuse through the ground to the reaction zone as well as the heat losses from the system and
the thermal properties of the coal (Ohlemiller, 2002; Rein, 2009). Material properties such as moisture content,
carbon content, heat transfer properties and flow permeability are important parameters dictating the ignition and
spread of subsurface fires. Typical smoldering temperatures of coal are in the range 700-1000°C and the
propagation speed is on the order of 0.1mm·min
1
(Rein et al., 2008).
However, although the general behavior is known, little experimental work on the combustion behavior of
smoldering coal is present in the literature. This knowledge is essential to aid understanding of the phenomena
and to prevent future events occurring. This chapter presents research which, for the first time, studies the burning
and suppression behavior of smoldering coal in small-scale laboratory experiments.
Suppression of Smoldering Coal Fires
I
n the United States alone, it is estimated that subsurface coal fires affect over 400 ha and estimated reclamation
costs exceed $42 million (Kim, 2004). Kim (2004) and Colaizzi (2004) show that extinguishing coal fires is a
costly, resource intensive procedure which often fails to produce the desired results. Common techniques of
suppression often include sealing of the affected area or injection of foams to prevent oxygen ingress thereby
smothering the fire, injection of inert gas to displace oxygen and water injection to cool the coal. Additional
methods such as injection of a cryogenic liquid have been investigated (Kim, 2004) however, the advantages may
be outweighed by high cost. All of these methods require long timescales and large quantities of materials.
Alternatively, the most successful method to suppress a smoldering fire is the removal of the fuel, though this is
