Geology Reference
In-Depth Information
17.1. Smoldering and Coal
Combustion
Guillermo Rein
Smoldering charcoal briquettes.
Photo by J.B. Nielsen, Public Domain, Wikimedia Commons, 2009.
Introduction
S
moldering fires are rare events at the local scale but occur regularly at a global scale. Once ignited, they are
particularly difficult to extinguish despite extensive rains or firefighting attempts and persist for long periods of
time, spreading over extensive areas and deep into the ground. Smoldering subsurface fires in coal deposits are
the longest continuously burning fires on Earth and pose significant economic, social, and environmental
threats (Stracher and Taylor, 2004). A hypothetical smoldering-coal fire and suppression attempts are shown in
Figure 17.1.1. Geologists are not alone in their study of smoldering fires; these are a hazard of importance to
several other scientific disciplines (Rein, 2009). Ecologists and forest scientists study smoldering wildfires
because they destroy large amounts of biomass and cause greater damage to the soil ecosystem than flaming
fires (Frandsen, 1997). Atmospheric scientists have studied acute pollution episodes caused by the Indonesian
peat fires in 1997 and the destruction of vast amounts of stored carbon in the soil (Page et al., 2002). In the
realm of fire safety engineering, smoldering is an important threat because it is the leading cause of deaths in
residential fires and causes economics losses of the order of $350 million per year in property damage in the
United States alone (Hall, 2007).
Smoldering is a slow, low-temperature, flameless form of combustion of a solid fuel (Ohlemiller, 2002).
Whereas flaming combustion has been widely studied and is the aim of hundreds of papers per year,
smoldering combustion has received very little attention. Combustion and fire scientists were the first to
research the topic in the 1950s. The first widely available scientific work of merit on the topic was published in
1957 (Palmer, 1957). Palmer
s seminal work consisted of a collection of observations from simple experiments
involving burning piles of dust. This pioneering work was followed by research on dust, fibrous materials,
polymeric foams, cigarettes, polyurethane foam, and cellulose. It is only in the last decade that smoldering
phenomena has been studied significantly by other scientific fields such as geology, ecology, and atmospheric
sciences.
'
This chapter attempts to synthesize a comprehensive view of smoldering combustion bringing together contribu-
tions from other scientific disciplines. For an in-depth review, the reader is refereed to the work of Ohlemiller
(1985), which today still stands as the only review of the fundamental scientific concepts behind smoldering
combustion. A more recent and general review on smoldering phenomena was carried out by Rein (2009), upon
which this chapter is based.
Overall Characteristics
S
moldering is a fundamental combustion problem involving heterogeneous chemical reactions and the transport
of heat, mass, and momentum in the gas and solid phases. The cover of this chapter shows a most familiar example
smoldering-charcoal briquettes.
The fundamental difference between smoldering and flaming combustion is that, in the former, the oxidation
reaction and the heat released occur on the solid surface of the fuel or porous matrix and, in the latter, these occur
