Smouldering combustion in glowing embers of barbecue coal briquettes
Smouldering combustion in glowing embers of barbecue coal briquettes

Smouldering (British English) or smoldering (American English; see spelling differences) is the slow, flameless form of combustion, sustained by the heat evolved when oxygen directly attacks the surface of a condensed-phase fuel.[1] Many solid materials can sustain a smouldering reaction, including coal, cellulose, wood, cotton, tobacco, cannabis, peat, plant litter, humus, synthetic foams, charring polymers including polyurethane foam and some types of dust. Common examples of smouldering phenomena are the initiation of residential fires on upholstered furniture by weak heat sources (e.g., a cigarette, a short-circuited wire), and the persistent combustion of biomass behind the flaming front of wildfires.[2]


A smouldering cigarette.
A smouldering cigarette.

The fundamental difference between smouldering and flaming 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 a 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.
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.

Beneficial applications

Smouldering combustion has some beneficial applications.

See also


  1. ^ "Smoldering Combustion" by T.J. Ohlemiller, SFPE Handbook of Fire Protection Engineering (3rd Edition),2002
  2. ^ G Rein, Smouldering Combustion Phenomena in Science and Technology, International Review of Chemical Engineering 1, pp. 3-18, 2009
  3. ^ "Smoldering Combustion" by T.J. Ohlemiller, SFPE Handbook of Fire Protection Engineering (3rd Edition),2002.
  4. ^ 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.
  5. ^ 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.
  6. ^ G. Rein; N. Cleaver; C. Ashton; P. Pironi; J.L. Torero (2008). "The Severity of Smouldering Peat Fires and Damage to the Forest Soil". Catena. 74 (3): 304–309. doi:10.1016/j.catena.2008.05.008. hdl:1842/2480.
  7. ^ 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.
  8. ^ Forest fire haze brings misery to Indonesia and beyond, The Guardian, October 6, 2006.,,1889323,00.html
  9. ^ J. Beard, Ground Zero's fires still burning, NewScientific, 3 December 2001.
  10. ^ 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.
  11. ^ H.H. Biswell, Prescribed Burning in California Wildlands Vegetation Management (University of California Press, Berkeley, 1989)
  12. ^ 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]