The fundamental difference between smouldering andflaming combustion is that smouldering occurs on the surface of the solid rather than in the gas phase. Smouldering is a surface phenomenon but can propagate to the interior of aporous fuel if it is permeable to flow. The characteristictemperature andheat 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]
Polyurethane foam sample from the NASA smouldering experiments.
Many materials can sustain a smouldering reaction, includingcoal,tobacco, decayingwood andsawdust,biomass fuels on the forest surface (duff) and subsurface (peat),cotton clothing and string, and polymeric foams (e.g.,upholstery andbedding 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 asthermal insulation, reducing heat losses. The most studied materials to date arecellulose andpolyurethane foams.
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.
Smoke and pollution from fires in Borneo, 1997.Fire safety: The main hazards posed by smouldering arise from the fact that it can be easily initiated (by heat sources too weak to ignite flames) and is difficult to detect. Fire statistics draw attention to the magnitude of smouldering combustion as the leading cause of fire deaths in residential areas (more than 25% of the fire deaths in theUnited States are attributed to smoulder-initiated fires, with similar figures in other developed countries). A particularly common fire scenario is acigarette igniting a piece ofupholstered furniture. This ignition leads to a smouldering fire that lasts for a long period of time (in the order of hours), spreading slowly and silently until critical conditions are attained and flames suddenly erupt;[4]fire-resistant cigarettes have been developed to reduce the risk of fire due to smouldering. Smouldering combustion is also a fire-safety concern aboard space facilities (e.g.,International Space Station), because theabsence of gravity is thought to promote smouldering ignition and propagation.
The smouldering pile of debris after theSeptember 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 theforest ecosystem. Smouldering ofbiomass can linger for days or weeks after flaming has ceased, resulting in large quantities of fuels consumed and becoming a global source ofemissions to the atmosphere.[5] The slow propagation leads to prolonged heating[6] and might cause sterilizations of the soil or the killing ofroots,seeds, andplant 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 incoal mines,peat lands and landfills are rare events, but when active they can smoulder for very long periods of time (months or years), emitting enormous quantities of combustion gases into the atmosphere, causing deterioration ofair 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, fracturedstrata, cracks, openings or abandonedmine 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 1997peatland 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]
World Trade Center debris: After the attack, fire and subsequentcollapse of the Twin Towers onSeptember 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]
Smouldering combustion has some beneficial applications.
Biochar is thecharcoal produced from the smouldering and/orpyrolysis of biomass. It has the potential to be a short-term solution to lower CO2 concentrations in the atmosphere. Charcoal is a stable solid and rich in carbon content, and thus, it could be used to lock carbon in the soil. The natural decomposition and burning of trees andagricultural waste contributes with a large amount of CO2 released to the atmosphere. Biochar could be used to store part of this carbon content in the ground, at the same time that its presence in the earth increases soil productivity. Biochar hascarbon negative applications for energy production.
Inwildfire management, smoulderingcontrolled 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 oftire recycling.
^G Rein, Smouldering Combustion Phenomena in Science and Technology, International Review of Chemical Engineering 1, pp. 3-18, 2009http://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]
