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Contents
  1. 1How a flammable smoke cloud forms
  2. 2Flammability limits and mixtures of gases
  3. 3Ignition and deflagration
  4. 4Pressures and consequences
  5. 5Related phenomena
  6. 6Electric vehicles and battery gases
  7. 7Limits of prediction
  8. 8See also
  9. 9Notes
  10. 10References

A smoke explosion is the very rapid burning of a cloud of smoke and air in which flammable gases from a fire are already mixed with oxygen before they are ignited. The flammable gases come from the decomposition of fuels or from incomplete combustion.[1] A typical condition is that the gases and the surrounding air are reasonably well premixed and that the mixture lies between its lower and upper explosion limits. It can then be ignited by autoignition, when the gas reaches its autoignition temperature, or by a spark or a glowing particle from the fire.[1]

The event is also called a smoke gas explosion or fire gas explosion.[2][3] A guide of the US Nuclear Regulatory Commission describes how a smouldering fire can produce enough unburned fuel and carbon monoxide to form a premixed combustible atmosphere. If the fire then heats the mixture to its autoignition temperature, the cloud deflagrates.[4] Such explosions have been observed in smouldering fires of polyurethane foam.[4] Unlike most fires, a smoke explosion can produce pressures high enough to damage a building.[2] It differs from a backdraft, which is triggered by a sudden supply of air, and from rollover, in which the smoke layer burns at its boundary with fresh air.[5][6]

How a flammable smoke cloud forms

Smoke is a mixture of solid particles, liquid droplets and gases. Besides combustion products and entrained air, it contains decomposition gases of the fuel.[7] When these unburnt pyrolysis gases mix with oxygen, they can ignite if the temperature is high enough or an ignition source is present.[7] The amount of flammable gas in smoke therefore depends on how completely the fuel has burned.

Two kinds of fire produce smoke rich in fuel gases. The first is a ventilation-controlled fire. In such a fire the burning rate is limited by the supply of oxygen, and unburnt pyrolysis gases can escape into the smoke. Mixed with oxygen, they form a combustible mixture that needs only ignition energy to burn.[8]

In full-scale room fires, carbon monoxide levels of several per cent have been measured.[9] In a worked example of a furniture fire short of air, the upper layer held 2.8 per cent carbon monoxide but only 0.5 per cent oxygen.[10] Such a layer contains fuel but too little oxygen to burn on its own.

The second kind is a smouldering fire. Smouldering is a slow combustion process with low gas temperatures and no flames, and it can generate high levels of carbon monoxide.[9] Without flames, the unburned fuel and carbon monoxide from a smouldering fire can build up to form a premixed combustible atmosphere.[4] This is the route described for smoke explosions in polyurethane foam fires.[4]

Three stages of one room: smouldering sofa filling the room with smoke, a spark igniting the mixed cloud, and the explosion breaking the window.
A smoke explosion in three stages: unburned gases from a smouldering fire mix with the room air, the premixed cloud is ignited, and the deflagration raises the pressure. Based on Iqbal and Salley (2004), pp. 15-3–15-4, and Van Rossum (2027), §4.10.

The flammable gases in such smoke are products of incomplete combustion. In a handbook example of a hot smoke layer, they were unburned hydrocarbons, carbon monoxide and hydrogen.[11] A Dutch study lists methane, carbon monoxide, carbon dioxide and hydrogen among the usual combustion products released when the battery of an electric vehicle burns.[12] Before such a cloud can explode, it must be diluted with enough air to fall within its flammable range, but not so much that it falls below it.[4]

Flammability limits and mixtures of gases

A premixed gas can only carry a flame within a certain range of composition. The boundaries of this range are the lower and upper explosion limits, also called the lower and upper flammability limits.[4] Outside the range the mixture is not explosive. At the limits it burns with only a weak flame, and somewhere between the limits the flame spreads fastest.[4] For methane in air at normal temperature and pressure the limits are 5 and 15 per cent by volume. For most simple hydrocarbons the lower and upper limits correspond to equivalence ratios of about 0.5 and 3.[13]

Limits of some gases found in smoke are listed in the SFPE Handbook. Carbon monoxide burns in air between 12.5 and 74 per cent, hydrogen between 4.0 and 75 per cent, and hydrogen cyanide between 5.6 and 40 per cent.[14] The autoignition temperatures in the same table are 400 °C for hydrogen and 540 °C for methane.[14] The limits widen as the temperature of the mixture rises.[15] More oxygen raises the upper limit, while the lower limit hardly changes, because a lean mixture already has more oxygen than it needs.[15]

Horizontal bars of the flammable ranges in air of methane, carbon monoxide, hydrogen and hydrogen cyanide, with the lower limit of a mixture.
Flammable ranges in air of four gases found in fire smoke at normal temperature and pressure, and the lower limit of a mixture of methane, carbon monoxide and hydrogen. Based on Beyler (2002b), pp. 2-174–2-176.

Fire gases are mixtures of several fuels. The lower flammability limit of such a mixture can be estimated with an empirical rule of Le Chatelier.[15] Each fuel gas contributes to the flammability in proportion to its share of the fuel mixture, divided by its own lower limit:

LFLm=100∑i=1nCf,i/LFLi

Symbol Description Unit
LFLm lower flammability limit of the fuel gas mixture in air vol %
Cf,i share of fuel gas i in the fuel gas mixture (the shares add up to 100) vol %
LFLi lower flammability limit of fuel gas i alone in air vol %
n number of fuel gases in the mixture –

The handbook gives a worked example for a fuel gas of 50 per cent methane, 25 per cent carbon monoxide and 25 per cent hydrogen. All three gases occur in fire gases, for example in the gases released by burning vehicle batteries.[12] Their lower limits are 5.0, 12.5 and 4.0 per cent respectively.[15]

LFLm=10050/5+25/12.5+25/4=10018.25=5.48 %

At the lower limit, the mixture therefore contains 2.74 per cent methane, 1.37 per cent carbon monoxide, 1.37 per cent hydrogen and 94.5 per cent air.[16] Carbon monoxide alone would need 12.5 per cent before it could burn, so the more reactive gases lower the limit of the mixture considerably.[14] The rule applies to mixtures of flammable gases in air. Smoke also contains inert gases such as carbon dioxide and nitrogen, and the flammability limits depend on the oxygen and inert gas concentrations as well as on the fuel.[17][11]

Inert gases set a second limit. Below a limiting oxygen concentration, a premixed flame cannot spread whatever the amount of fuel.[18] With nitrogen as diluent this concentration is generally 10 to 12 per cent oxygen, lower for carbon monoxide and hydrogen, and 2 to 3 per cent higher with carbon dioxide as diluent.[18] A smoke cloud must therefore contain both enough fuel and enough oxygen to be within its flammable range.[18]

Ignition and deflagration

A smoke explosion needs the same three elements as any deflagration in a building: a flammable mixture of fuel and oxygen, a means of ignition and an enclosure.[19] The mixture can ignite by autoignition, for example when a smouldering fire heats it to its autoignition temperature.[4] It can also be ignited by a spark from an electrical installation or by glowing particles from the fire.[1] In a study of smoke explosions in a reduced-scale enclosure with a smouldering wood crib, the change from smouldering to flaming was suggested as the trigger. The evidence for this was indirect.[20]

Once ignited, the flame runs through the premixed cloud as a premixed flame, like the flame of a gas burner in which fuel and air are mixed before ignition.[21] In a deflagration, the flame front travels below the speed of sound, which is about 330 m/s in air at 20 °C. In a detonation it moves at about 2000 m/s and drives a shock wave.[19] Laminar burning velocities of fuels are far lower, about 40 cm/s for methane and 312 cm/s for hydrogen.[22] The burning gas expands as it heats up, and in an enclosed space this expansion raises the pressure.[23][24]

The time to reach the maximum pressure depends on the size of the enclosure and the fuel, and can be up to a few hundred milliseconds.[19] Normal openings in an enclosure usually vent some gas but are usually too small to prevent destructive pressures.[19] When gas escapes through a door or window, the resulting turbulence can produce a second pressure peak.[24]

Pressures and consequences

Ordinary fires seldom create high pressures. The NRC guide notes that, apart from smoke gas explosions, fires rarely reach the pressures that injure people.[2] If a premixed gas-air mixture is ignited inside a building, the pressure can reach a maximum of about 8 bar.[2]

In closed vessel tests, ten of twelve flammable gases and vapours reached maximum overpressures of 6.8 to 8.1 bar. Methane reached 7.1 bar.[22] Calculated worst-case pressures of 8 to 9.6 atmospheres absolute occur at mixtures slightly richer than stoichiometric. Near the flammability limits, measured pressures are much lower than calculated, because only part of the gas burns and heat is lost to the walls.[23]

Buildings fail long before such pressures are reached. Typical failure pressures are 2 to 7 kPa for glass windows, 2 to 3 kPa for room doors and 2 to 5 kPa for light partition walls.[25] Unrestrained brick walls fail at about 7 to 15 kPa.[25] A pressure of 8 bar equals 800 kPa, more than a hundred times the failure pressure of a window. Even a partial explosion of a smoke cloud can therefore break windows, doors and partitions.

Logarithmic pressure scale from 1 to 1000 kPa with failure ranges of windows, doors and walls, injury levels and the 800 kPa explosion maximum.
Failure pressures of building elements and pressure effects on people compared with the maximum pressure of a premixed gas explosion inside a building. Based on Iqbal and Salley (2004), pp. 15-12–15-13.

The effects on people depend on the pressure and on what is thrown about. The limit for eardrum rupture is about 35 kPa and for lung damage about 70 kPa, while 1 per cent mortality is expected at about 180 kPa.[2] When people are killed by blast waves, it is usually by falling objects rather than by the pressure itself.[2] After ignition, the flame front travels through the whole mixture.[26] The hazards of a deflagration include the ejection of flame and unburned product into the surroundings, possible secondary explosions and personal injury.[19]

Related phenomena

Backdraft and smoke explosion both involve fuel-rich smoke and air, but in a different order. In a backdraft, gases build up in a closed room during an oxygen-starved fire. When fresh air suddenly enters, for example through an opened door, the gases burn explosively and can produce a large fireball.[5] If the smoke is cool, ignition is delayed until fresh air has mixed with the smoke and reached the fire, which then acts as the ignition source.[4] The NRC guide notes that a backdraft explosion has the characteristics of a premixed fuel and air deflagration.[4]

In a smoke explosion, the fuel and air are already mixed before any ignition source acts.[1] No sudden opening is needed, and the cloud can be ignited by its own temperature or by a spark.[4] Rollover, by contrast, is the burning of fuel-rich gases at the boundary between the smoke layer and the air below it. It occurs during underventilated burning and does not involve a premixed cloud.[6]

Three rows of steps comparing smoke explosion, backdraft and rollover by when fuel gases, air and ignition come together.
Smoke explosion, backdraft and rollover compared by the order in which fuel gases, air and ignition come together. Based on Iqbal and Salley (2004), p. 15-4; Van Rossum (2027), §4.9 and §4.10; Gottuk and Lattimer (2002), p. 2-55.

The NRC guide groups gas explosions and backdraft explosions with dust explosions as combustion explosions.[27] All of them involve a rapid release of high-pressure gas that forms a pressure wave.[27] Physical explosions, such as the failure of a tank heated from outside, form a separate group in which the high-pressure gas is produced without a chemical reaction.[27]

Electric vehicles and battery gases

A newer source of explosive fire gases is the thermal runaway of lithium-ion batteries. A Dutch study by the Netherlands Institute for Public Safety (2021) describes how battery cells release gases when they go into thermal runaway.[3] If these gases are not ignited at once, an explosive mixture of fire gases can build up in a confined, enclosed or poorly ventilated space. When the lower explosion limit is reached and the mixture is ignited, a fire gas explosion can occur.[3]

The same study notes that gases escaping from a battery pack can lead to a flare fire or a gas cloud explosion when they are in the right mixing ratio.[28] Fires in electric vehicles can involve repeated minor explosions, flying parts and flash fire.[29] A thermal runaway without direct ignition of its gases can result in a fire gas explosion once the lower explosion limit is reached and an ignition source is present.[29] These findings concern indoor car parks in the Netherlands.[3]

Limits of prediction

The pressure produced by an explosion is usually difficult to predict, and its effects on people depend strongly on nearby objects that can be thrown.[2] The flammability limits in tables apply to well-mixed gases at normal temperature and pressure. They change with temperature, oxygen content and inert gases.[15] Smoke that flows away from a fire is cooled by mixing with surrounding gases and by heat losses to the walls, so its temperature and composition change along the way.[30]

Temperature also shifts the limits. A mixture can burn outside its flammability limits only if energy is supplied throughout its volume.[17] Preheating makes leaner mixtures burnable. In one experiment, a mixture of 1 per cent methane in air burned when preheated to 1270 K, although the lower limit at room temperature is 5 per cent.[16] Hot smoke can therefore be flammable at lower fuel concentrations than tables for room temperature suggest.[15]

See also

Notes

  1. ↑ 1.0 1.1 1.2 1.3 Van Rossum (2027), §4.10.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 Iqbal and Salley (2004), p. 15-12.
  3. ↑ 3.0 3.1 3.2 3.3 Van der Graaf et al. (2021/2023), p. 27.
  4. ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 Iqbal and Salley (2004), p. 15-4.
  5. ↑ 5.0 5.1 Van Rossum (2027), §4.9.
  6. ↑ 6.0 6.1 Gottuk and Lattimer (2002), p. 2-55.
  7. ↑ 7.0 7.1 Van Rossum (2027), §6.1.
  8. ↑ Van Rossum (2027), §4.8.
  9. ↑ 9.0 9.1 Gottuk and Lattimer (2002), p. 2-54.
  10. ↑ Gottuk and Lattimer (2002), p. 2-80.
  11. ↑ 11.0 11.1 Beyler (2002b), p. 2-184.
  12. ↑ 12.0 12.1 Van der Graaf et al. (2021/2023), p. 26.
  13. ↑ Beyler (2002b), p. 2-172.
  14. ↑ 14.0 14.1 14.2 Beyler (2002b), pp. 2-175–2-176.
  15. ↑ 15.0 15.1 15.2 15.3 15.4 15.5 Beyler (2002b), p. 2-174.
  16. ↑ 16.0 16.1 Beyler (2002b), p. 2-177.
  17. ↑ 17.0 17.1 Beyler (2002b), p. 2-173.
  18. ↑ 18.0 18.1 18.2 Beyler (2002b), p. 2-179.
  19. ↑ 19.0 19.1 19.2 19.3 19.4 Iqbal and Salley (2004), p. 15-3.
  20. ↑ Ohlemiller (2002), p. 2-208.
  21. ↑ Van Rossum (2027), §4.3.
  22. ↑ 22.0 22.1 Zalosh (2002), p. 3-407.
  23. ↑ 23.0 23.1 Zalosh (2002), p. 3-406.
  24. ↑ 24.0 24.1 Iqbal and Salley (2004), p. 15-5.
  25. ↑ 25.0 25.1 Iqbal and Salley (2004), p. 15-13.
  26. ↑ Iqbal and Salley (2004), p. 15-6.
  27. ↑ 27.0 27.1 27.2 Iqbal and Salley (2004), p. 15-2.
  28. ↑ Van der Graaf et al. (2021/2023), p. 22.
  29. ↑ 29.0 29.1 Van der Graaf et al. (2021/2023), p. 29.
  30. ↑ Gottuk and Lattimer (2002), p. 2-71.

References

  • Beyler, C. L. (2002b). Flammability limits of premixed and diffusion flames. 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. 2-172–2-187). National Fire Protection Association; Society of Fire Protection Engineers.
  • Gottuk, D. T., & Lattimer, B. Y. (2002). Effect of combustion conditions on species production. 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. 2-54–2-82). National Fire Protection Association; Society of Fire Protection Engineers.
  • 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.
  • 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.
  • Van der Graaf, J., Hessels, T., & Rosmuller, N. (2023). Fire safety of indoor car parks accommodating electrically powered vehicles. Nederlands Instituut Publieke Veiligheid. (Original work published 2021)
  • Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.
  • Zalosh, R. (2002). Explosion protection. 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. 3-402–3-421). National Fire Protection Association; Society of Fire Protection Engineers.
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