The fundamental difference between smouldering and
porous fuel if it is permeable to flow. The characteristic temperature and heat released during smouldering are low compared to those in the flaming combustion. Smouldering propagates in a creeping fashion, around 0.1 mm/s (0.0039 in/s), which is about ten times slower than flames spread over a solid. In spite of its weak combustion characteristics, smouldering is a significant fire hazard. Smouldering emits toxic gases (e.g., carbon monoxide) at a higher yield than flaming fires and leaves behind a significant amount of solid residue. The emitted gases are flammable and could later be ignited in the gas phase, triggering the transition to flaming combustion.[3]
Smouldering materials
Polyurethane foam sample from the NASA smouldering experiments.
Many materials can sustain a smouldering reaction, including
coal, tobacco, decaying wood and sawdust, biomass fuels on the forest surface (duff) and subsurface (peat), cotton clothing and string, and polymeric foams (e.g., upholstery and bedding materials). Smouldering fuels are generally porous, permeable to flow and formed by aggregates (particulates, grains, fibres or of cellular structure). These aggregates facilitate the surface reaction with oxygen by allowing gas flow through the fuel and providing a large surface area per unit volume. They also act as thermal insulation, reducing heat losses. The most studied materials to date are cellulose and polyurethane
foams.
Threats from smouldering
The characteristics of smouldering fires make them a threat of new dimensions, taking the form of colossal underground fires or silent fire safety risks, as summarized below.
absence of gravity
is thought to promote smouldering ignition and propagation.
The smouldering pile of debris after the September 11 attacks, Manhattan, New York.Wildfires: Smouldering combustion of the forest ground does not have the visual impact of flaming combustion; however, it has important consequences for the forest ecosystem. Smouldering of biomass can linger for days or weeks after flaming has ceased, resulting in large quantities of fuels consumed and becoming a global source of emissions to the atmosphere.[5] The slow propagation leads to prolonged heating[6] and might cause sterilizations of the soil or the killing of roots, seeds, and plant stems at the ground level.
Subsurface fires: Fires occurring many meters below the surface are a type of smouldering event of colossal magnitude. Subsurface fires in
air quality and subsequent health problems. The oldest and largest fires in the world, burning for centuries, are smouldering fires.[citation needed] These fires are fed by the oxygen in the small but continuous flow of air through natural pipe networks, fractured strata, cracks, openings or abandoned mine shafts which permit the air to circulate into the subsurface. The reduced heat losses and high thermal inertia of the underground together with high fuel availability promote long-term smouldering combustion and allow for creeping but extensive propagation. These fires prove difficult to detect, and frustrate most efforts to extinguish them. The dramatic 1997 peatland fires in Borneo caused the recognition of subsurface smouldering fires as a global threat with significant economic, social and ecological impacts.[7] The summer of 2006 saw the resurgence of the Borneo peat fires.[8]
September 11, 2001, the colossal pile (1.8 million tons) of debris left on the site smouldered for more than five months.[9] It resisted attempts by fire fighters to extinguish it until most of the rubble was removed. The effects of the gaseous and aerosolized products of smouldering on the health of the emergency workers were significant.[10]
Beneficial applications
Smouldering combustion has some beneficial applications.
carbon negative
applications for energy production.
In wildfire management, smouldering controlled burns can be used to reduce shallow layers of natural fuels at a slow propagation rate.[11] These fires have two benefits when kept in very shallow layers: they are easy to control and result in little damage to the forest stand
Smouldering of tires producing tar and energy at the same time, is one method of tire recycling.
oil recovery
when traditional extraction methods prove inefficient or too costly.
In situ smouldering combustion is being explored as a novel remediation technology for soil contamination.[12]
^G Rein, Smouldering Combustion Phenomena in Science and Technology, International Review of Chemical Engineering 1, pp. 3-18, 2009 http://hdl.handle.net/1842/2678
^J. R. Hall, 2004, The Smoking-Material Fire Problem, Fire Analysis and Research Division of The National Fire Protection Association, Quincy, MA (USA). November 2004.
^I.T. Bertschi, R.J. Yokelson, D.E. Ward, R.E. Babbitt, R.A. Susott, J.G. Goode, W.M. Hao, 2003, Trace gas and particle emissions from fires in large diameter and belowground biomass fuels, Journal of Geophysical Research 108 (D13), pp. 8.1-8.12.
^S.E. Page, F. Siegert, J.O. Rieley, H.-D.V. Boehm, A. Jaya, S. Limin, 2002, The amount of carbon released from peat and forest fires in Indonesia during 1997, Nature 420, pp. 61-61.
^J. Beard, Ground Zero's fires still burning, NewScientific, 3 December 2001.
^J.D. Pleil, W.E. Funk, S.M. Rappaport, 2006, Residual Indoor Contamination from World Trade Center Rubble Fires as Indicated by Polycyclic Aromatic Hydrocarbon Profiles, Environmental Science & Technology 40 (2006) 1172-1177.
^H.H. Biswell, Prescribed Burning in California Wildlands Vegetation Management (University of California Press, Berkeley, 1989)
^P Pironi, C Switzer, G Rein, JI Gerhard, JL Torero, A Fuentes, Small-Scale Forward Smouldering Experiments for Remediation of Coal Tar in Inert Media, Proceedings of the Combustion Institute 32 (2), pp. 1957-1964, 2009. [1]
External links
Look up smolder in Wiktionary, the free dictionary.
