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Contents
  1. 1Formation and transport
  2. 2Ignition by firebrands
  3. 3Firebrands in wildfires and building fires
  4. 4Roofs and testing
  5. 5Limitations and open questions
  6. 6See also
  7. 7Notes
  8. 8References

Firebrands are flaming or smouldering airborne objects that emerge from a fire and can sometimes ignite combustible material at a distance from it.[1] The terms flying brands, burning brands and embers are also used.[2][3][4] An ember is a particle of solid material that emits radiant energy because of its temperature or because it is burning at its surface.[5] Firebrands rise with the hot gases above a fire and can be carried by the wind before they fall onto combustible material.[4][3]

Burning embers and other debris are one of three ways in which fire spreads from one building or property to another, besides radiation and direct flame contact.[6] In forest fires, wind currents can carry large embers over great distances and start spot fires at other locations.[2] In areas prone to bushfires or wildfires, the spread of flying brands and embers can therefore have consequences for building design.[7] The distance a burning brand can travel is difficult to establish with any certainty, which makes firebrands hard to quantify in design.[3]

Formation and transport

Transport starts in the fire plume. Above an open fire, about 70% of the heat leaves by convection and about 30% by radiation, so most of the energy goes into a rising column of hot gas.[8] Convective forces in this plume lift burning brands, which fall onto combustible materials elsewhere. This lofting has been observed in warehouse fires and in wildland fires, both of which involve cellulosic materials.[4] While it rises and falls, a brand may be carried by the wind. Introduction to Fire Safety Engineering notes that wind is difficult to predict, and the International Fire Engineering Guidelines state that the travel distance of a brand rising in the plume is difficult to establish.[9][3]

Computer models treat firebrands as particles moving through the gas. In the Fire Dynamics Simulator, a solid-phase reaction can produce material that leaves the burning surface as particles, which the documentation calls a firebrand.[10] Lagrangian particles can represent airborne vegetation such as burning brands. Each particle feels gravity and a drag force from the surrounding gas, which depends on its size, shape and speed relative to the flow.[11] Drag coefficients are given for cylindrical and for thin disk-shaped particles.[11] The United States Forest Service supported the development of such sub-models to simulate fire spread at the wildland-urban interface.[12]

Ignition by firebrands

A firebrand starts a new fire when it lands on a receptive fuel and causes ignition. A glowing brand can act as a pilot source. Piloted ignition is ignition in the presence of a small flame, a heated wire, an electric spark or an incandescent particle, as opposed to spontaneous ignition without such a source.[13][14] The pilot source is usually so small that its contribution to heating the solid can be ignored. Its role is to start combustion in a flammable mixture of fuel gases and air near the surface.[14]

Ignition can take three forms. Combustion is called smouldering when it takes place within the solid below the surface, glowing when it takes place at the surface, and flaming when it takes place in the gas phase.[13] Because a firebrand may be smouldering rather than flaming, smouldering combustion is part of the picture.[1] Smouldering can turn into flaming when the supply of oxygen is increased, for example by airflow over the fuel.[15] In experiments by Palmer on layers of wood sawdust, the transition occurred at airflow velocities of about 0.9 to 1.7 m/s, but only when the air moved in the same direction as the smoulder front. Ohlemiller compares this with starting a camp fire from tinder and sparks.[15]

The time to flaming can be long. In a literature survey of smouldering in upholstered chairs and beds, about two-thirds of the items tested changed to flaming combustion, after between 22 and 306 minutes.[15] Smouldering ignition can therefore take a long time to develop into a flaming fire. Glowing particles from a fire can also ignite a flammable mixture of pyrolysis gases inside a space and cause a smoke gas explosion.[16] For comparison, most materials ignite at temperatures between about 200 and 500 °C when they are heated by a hot smoke layer.[17]

Firebrands in wildfires and building fires

In a wildfire in a forest, flame spread depends on radiant heat transfer and on convective heating by wind or by the slope of the terrain. In severe fires the crowns of the trees also burn.[2] Wind currents can then carry large embers great distances from the fire, where they start spot fires. Breaks in the fuel can be used to control the spread, but the break needed to interrupt the fire depends on the size of the fire and its heat transfer.[2]

In building fires, firebrands are one route of fire spread between buildings. The International Fire Engineering Guidelines list burning embers and other debris, radiation and direct flame contact as the ways in which fire can spread to an adjacent enclosure, building or property boundary.[6] Depending on how a building and its contents burn, the distribution of flying brands and embers may need to be considered if fire spread to adjoining property is to be limited.[7] Protection of adjacent buildings against flying brands is also listed among the objectives that fire service intervention and firefighting facilities support.[18]

Firebrands can also be produced by a building collapse. When the North Tower of the World Trade Center collapsed in 2001, most burning material would have been extinguished. NIST considered it possible, however, that many potential ignition sources survived and were spread with the dust- and debris-laden air. These flows were many storeys tall and passed around and beyond World Trade Center 7.[19] Not every fire spreads by brands: in NIST's experiments with burning office workstations, the video record showed no evidence that flying brands played a role in flame spread.[4]

Roofs and testing

Several test standards expose roof coverings to brands. British Standard BS 476 Part 3 describes a test of the ignitability and fire spread of external roof surfaces. It exposes the roof to radiation from a fire in an adjacent building and to piloted ignition from flying brands.[3] In the United States, the roof covering tests ASTM E108 and UL 790 include a burning brand test and a flying brand test besides tests for flame spread and intermittent flame.[20]

The American roof covering standard NFPA 256 classifies roofs by the flame spread in the test. Class A allows less than 1.82 m (6 ft), class B less than 2.44 m (8 ft) and class C less than 3.96 m (13 ft).[21] For all classes there may be no significant lateral flame spread and no flaming on the underside of the deck. Flying brands or particles from the roof covering may not continue to flame or glow after reaching the floor.[21]

Limitations and open questions

The fire safety engineering documents used for this article treat firebrands only briefly. They define the phenomenon, describe how brands are lofted and carried, and name roof tests and design considerations, but give no measured data on brand sizes, numbers or travel distances.[3][1] The International Fire Engineering Guidelines state plainly that the distance a burning brand may be carried cannot be established with any certainty.[3] In these sources, firebrand exposure is addressed through roof covering tests and design considerations for bushfire-prone areas rather than through calculation methods.[7][21]

Numerical models can follow individual brands as particles, but their results depend on the assumed shape, size and drag of each particle and on the wind field.[11] Whether a smouldering brand leads to flaming also depends on the airflow over the fuel.[15] Firebrands are not limited to forest fires. The same mechanism has been observed in warehouse fires, and it is one of the routes by which fire spreads between buildings.[4][6]

See also

Notes

  1. ↑ 1.0 1.1 1.2 Iqbal and Salley (2004), p. F-24.
  2. ↑ 2.0 2.1 2.2 2.3 Quintiere (2002b), p. 2-255.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 National Research Council of Canada et al. (2005), p. 2.6-12.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 Hamins, Maranghides, McGrattan, Ohlemiller, and Anleitner (2005), p. 30.
  5. ↑ Iqbal and Salley (2004), p. F-22.
  6. ↑ 6.0 6.1 6.2 National Research Council of Canada et al. (2005), p. 1.6-7.
  7. ↑ 7.0 7.1 7.2 National Research Council of Canada et al. (2005), p. 1.6-5.
  8. ↑ Van Rossum (2027), §5.3.
  9. ↑ Van Rossum (2027), §6.6.
  10. ↑ McGrattan et al. (2026), p. 222.
  11. ↑ 11.0 11.1 11.2 McGrattan et al. (2026), pp. 95–96.
  12. ↑ McGrattan et al. (2026), p. ix.
  13. ↑ 13.0 13.1 Kanury (2002a), p. 2-230.
  14. ↑ 14.0 14.1 Kanury (2002a), p. 2-232.
  15. ↑ 15.0 15.1 15.2 15.3 Ohlemiller (2002), p. 2-208.
  16. ↑ Van Rossum (2027), §4.10.
  17. ↑ Van Rossum (2027), §9.3 (Results from zone models).
  18. ↑ National Research Council of Canada et al. (2005), p. 2.9-3.
  19. ↑ McAllister et al. (2008), p. 189.
  20. ↑ Iqbal and Salley (2004), p. C-12.
  21. ↑ 21.0 21.1 21.2 Iqbal and Salley (2004), p. C-11.

References

  • Hamins, A., Maranghides, A., McGrattan, K. B., Ohlemiller, T. J., & Anleitner, R. L. (2005). Experiments and modeling of multiple workstations burning in a compartment (NIST NCSTAR 1-5E). National Institute of Standards and Technology.
  • Iqbal, N., & Salley, M. H. (2004). Fire Dynamics Tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission Fire Protection Inspection Program (NUREG-1805). U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation.
  • Kanury, A. M. (2002a). Flaming ignition of solid fuels. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-229–2-245). National Fire Protection Association; Society of Fire Protection Engineers.
  • McAllister, T. P., Gann, R. G., Averill, J. D., Gross, J. L., Grosshandler, W. L., Lawson, J. R., McGrattan, K. B., Pitts, W. M., Prasad, K. R., Sadek, F. H., & Nelson, H. E. (2008). Structural fire response and probable collapse sequence of World Trade Center Building 7 (NIST NCSTAR 1-9, Vols. 1–2). National Institute of Standards and Technology.
  • McGrattan, K., Hostikka, S., Floyd, J., McDermott, R., Vanella, M., Mueller, E., & Paul, C. (2026). Fire Dynamics Simulator technical reference guide: Vol. 1. Mathematical model (6th ed., NIST Special Publication 1018-1). National Institute of Standards and Technology. https://doi.org/10.6028/NIST.SP.1018
  • National Research Council of Canada, International Code Council, Department of Building and Housing, & Australian Building Codes Board. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
  • Ohlemiller, T. J. (2002). Smoldering combustion. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-200–2-210). National Fire Protection Association; Society of Fire Protection Engineers.
  • Quintiere, J. G. (2002b). Surface flame spread. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-246–2-257). National Fire Protection Association; Society of Fire Protection Engineers.
  • Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.
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