Skip to content
Upcoming courses

Autoignition temperature

Tools
From Fire Safety Academy Wiki
(Redirected from Auto-ignition temperature)
Contents
  1. 1Definition and related temperatures
  2. 2How autoignition happens
  3. 3Typical values
  4. 4Measurement and the spread of tabulated values
  5. 5Autoignition of solids and the role of heat flux
  6. 6Use in fire safety engineering
  7. 7Limits and common misconceptions
  8. 8See also
  9. 9Notes
  10. 10References

The autoignition temperature (AIT) is the lowest temperature at which a mixture of fuel vapour or gas and air ignites by itself, without a spark, flame or other pilot source.[1][2] Below this temperature, a flammable mixture needs an external energy source before a flame can start. Above it, the mixture heats itself through its own chemical reactions until a flame appears.[3] For the same material, this spontaneous ignition generally needs a higher temperature than ignition by a pilot.[4] Tabulated values for common fuels range from about 90 °C for carbon disulfide to more than 600 °C for methane.[5][6]

The autoignition temperature tells engineers how hot a surface or a gas may become before it ignites a leak or a vapour cloud on its own. Hot steel surfaces in power plants have ignited sprayed hydraulic oil and leaking turbine oil in this way.[7][8] The value is not a fixed property of a substance, however. It depends on the test apparatus, the size of the heated volume, the pressure and the mixture composition.[9] Tables from different sources can therefore differ by 100 °C or more for the same fuel.[10][11]

Definition and related temperatures

Ignition is the onset of combustion, usually flaming, and shows itself by a sudden rise in reaction rate, heat release and temperature.[12] For a liquid or solid fuel, three conditions must be met. The fuel must first release enough combustible vapour or gas when heated. These vapours must then mix with air.[12] Finally, the mixture must either be hot enough for self-accelerating oxidation, or a pilot source must locally heat a small part of it to near flame temperature.[12] The first route is called spontaneous ignition or autoignition, and the second piloted or forced ignition.[12]

Kanury defines the autoignition temperature of a vapour and air mixture as the minimum temperature at which the mixture is self-igniting.[1] The glossary of NUREG-1805, a set of fire dynamics tools for nuclear power plants in the United States, gives two similar definitions. The first is the lowest temperature at which a fuel and oxidiser mixture can propagate a flame without a pilot, spark or flame.[13] The second is the lowest temperature at which a vapour and air mixture within its flammability limits can ignite spontaneously.[13] Introduction to Fire Safety Engineering describes the ignition temperature in general terms as the lowest temperature at which a substance ignites, and notes that it differs between substances.[14]

The autoignition temperature must not be confused with the flash point. The flash point is the liquid temperature at which the vapour just above the surface flashes briefly when a small pilot flame passes over it.[1] The fire point is slightly higher: at this temperature, the flame does not only flash but keeps burning the liquid. It typically lies about 20 to 40 °C above the flash point.[1] Both depend on a pilot.

The autoignition temperature lies much higher, because the whole mixture must reach a temperature at which it ignites without help.[15] Kerosene is an example: its closed-cup flash point is 328 K (about 55 °C), but its autoignition temperature at atmospheric pressure is 528 K (about 255 °C).[16] The gap of about 200 °C means that a liquid can be far above its flash point and still be unable to ignite without a pilot.[16]

The SFPE Handbook places these temperatures on one diagram of vapour concentration against temperature. At low temperatures, the vapour pressure of a liquid limits how much fuel can be in the air above it. At the flash point, the vapour just reaches the lower flammability limit.[17] The flammable range widens as the temperature rises. At sufficiently high temperatures autoignition occurs, and the minimum temperature at which this can happen is the autoignition temperature.[18] By the time a liquid fuel reaches this temperature, it is vaporising vigorously or even boiling.[19]

Graph of fuel concentration against temperature with the saturated vapour curve, the flammable region, the flash point and the autoignition region.
Effect of temperature on the flammable range of a combustible vapour in air, with the flash point and the autoignition temperature. Based on Beyler (2002b), p. 2-174, and Simmons (2002), p. 1-147.

How autoignition happens

Autoignition is a self-induced start of a flame in a relatively large volume of reacting gas, whereas a pilot starts a flame at one point.[19] The process concerns the vapour and air mixture only, irrespective of whether the vapour comes from a liquid, a solid or a gas leak.[19] In a heated vessel, the fuel slowly oxidises and releases heat. Part of this heat is lost to the walls of the vessel, and the balance between heat generation and heat loss decides what happens next.[3]

Chemical reaction rates increase almost exponentially with temperature, whereas heat loss increases only linearly.[20] Above a critical temperature, heat generation outruns heat loss and the temperature in the middle of the vessel rises rapidly.[3] This event is called thermal runaway, and it marks the moment of ignition in thermal theory.[3] If the fuel consumed before ignition is ignored, the event is also called a thermal explosion.[3] The same competition between heat production and heat loss governs self-heating of solid materials, such as piles of wood chips or bagasse.[21]

Graph of heat generation and heat loss against temperature; an exponential generation curve touches a straight loss line at the critical temperature.
Heat generation by the reaction rises almost exponentially with temperature, heat loss only linearly; where generation outruns loss, thermal runaway and ignition follow. Based on Gray (2002), p. 2-212, and Kanury (2002b), p. 2-196.

Simple theory shows which conditions lower the autoignition temperature. It is lower when the heated volume is larger, the heat of combustion is larger, the pressure is higher, the fuel molecules are heavier or the thermal conductivity of the mixture is smaller.[9] All these changes either increase heat production or reduce heat loss. A spherical vessel, which has a large surface area for its volume, therefore shows a higher critical temperature than a long cylinder or a flat slab of the same width.[9] The effect of pressure is large: the autoignition temperature of kerosene falls from 528 K at 1 atm to 448 K at 33 atm, a drop of 80 K.[16] For the other oils in the same table, the drop ranges from 70 to 207 K.[16]

Dumbbell chart: for nine oils the autoignition temperature at 33 atm lies 70 to 207 K below the value at 1 atm.
Autoignition temperatures of liquid fuels in air at 1 atm and at 33 atm. Based on Kanury (2002b), p. 2-191.

Time matters as well. At exactly the autoignition temperature, ignition is only just possible and would take an infinitely long time.[9] At any higher temperature, the mixture ignites after a finite delay, and the hotter the mixture, the sooner it ignites.[9] There are also upper and lower composition limits to autoignition at each temperature. The mixture that ignites most quickly is probably close to stoichiometric, which means just enough air for complete combustion.[22] Test methods such as ASTM D2155 therefore record both the autoignition temperature and the ignition delay.[15]

Typical values

The main tabulations in the source documents give autoignition temperatures for gases and vapours in air at atmospheric pressure. The table below gives values for a few common fuels from three tabulations in the source documents. The values of Kanury are printed in kelvin and are converted here to degrees Celsius.[11]

Autoignition temperatures in air at atmospheric pressure
Fuel Beyler (2002b), °C[5][10] Kanury (2002b), °C[11] IFEG (2005), °C[23][6]
Methane 540 637 640
Propane 450 450 500
Hydrogen 400 – 520
n-Hexane 225 225 223
n-Heptane 215 204 223
Benzene 560 498 580
Methanol 385 385 470
Ethanol 365 363 365

Light gases such as methane and hydrogen have high autoignition temperatures, between about 400 and 640 °C depending on the source.[10][6] Heavier liquid fuels ignite at much lower temperatures. In Kanury's table, the value falls from 910 K for methane to 498 K for n-hexane and 474 K for n-decane.[11] Within one family of fuels, a higher molecular weight generally, though not always, gives a noticeably lower autoignition temperature.[1] Kerosene, jet fuel JP-4 and diesel fuel have values of 210 to 240 °C in the SFPE Handbook.[5][10] A few substances ignite at strikingly low temperatures, such as diethyl ether at 160 °C and carbon disulfide at 90 °C.[5]

Grouped bar chart of autoignition temperatures for eight fuels from three tables; methane, hydrogen, benzene and methanol differ by 80 to 160 K.
Autoignition temperatures of eight fuels in air as given in three tabulations. Based on Beyler (2002b), pp. 2-175 to 2-176; Kanury (2002b), p. 2-192; and National Research Council of Canada et al. (2005), pp. 3.3-2 to 3.3-3.

Solids do not have a single autoignition temperature, because they must first decompose by pyrolysis before a flammable gas can form.[24] For solids, the ignition temperature is the surface temperature at which ignition occurs. It can be treated as a property of the solid, but it varies with the rate of heating.[25] The International Fire Engineering Guidelines (IFEG, 2005) give average autoignition temperatures of 457 ± 63 °C for thermoplastics and 514 ± 92 °C for thermosetting plastics.[26]

The same table gives 250 to 400 °C for wood, paper and cotton, and 353 ± 56 °C for elastomers.[26] NUREG-1805 quotes typical values for wood of 350 °C for piloted ignition and 600 °C for autoignition.[25] Experiments by Akita on convectively heated wood gave about 765 K for spontaneous and 725 K for piloted ignition.[27]

Clouds of combustible dust behave in a similar way. Their minimum ignition temperature is measured by blowing dust through a preheated vertical tube, the Godbert-Greenwald furnace.[28] Most dusts in the SFPE Handbook have cloud ignition temperatures of 440 to 750 °C.[28] A layer of the same dust on a hot plate often ignites at a significantly lower temperature. Heat escapes much more easily from a dispersed cloud than from the inside of a layer.[29]

Measurement and the spread of tabulated values

Many test methods exist for the spontaneous ignition temperature of liquids. The reaction vessel may be a crucible, a flask, a furnace, a pressure bomb or a compression device.[15] In the American test standard ASTM D2155, the liquid is placed in a flask that is heated in a furnace. The test records the autoignition temperature and the ignition delay.[15] Because the result depends on how quickly heat leaves the reacting gas, it depends on the apparatus. Flasks with highly conductive walls give higher autoignition temperatures, for example.[16]

This explains why tables disagree. For methane, the SFPE Handbook gives 540 °C in one chapter and 910 K (637 °C) in another, and the IFEG gives 640 °C.[10][11][6] For hydrogen, the values are 400 °C in the SFPE Handbook, 520 °C in the IFEG and 560 °C in a NUREG/CR-6850 appendix on hydrogen fires.[10][6][30] For other fuels, such as n-hexane and ethanol, the three tables agree within a few degrees.[10][11][6] A tabulated value is therefore a guide rather than a precise limit.

Gray adds a further warning. Many organic materials, including hydrocarbons, show more than one autoignition temperature. Many also ignite when the surrounding temperature is lowered rather than raised.[31] Older tabulations do not recognise these peculiarities and should be used with great caution.[31]

Autoignition of solids and the role of heat flux

Solids can be heated to ignition by radiation from flames or hot gases, by direct flame contact or by contact with hot gases. The severity of this heating is expressed as a heat flux in kW/m².[25] Ignition in the absence of a pilot is also called spontaneous ignition or autoignition for solids.[32] The gases released by the hot surface mix with air in a thin boundary layer. Spontaneous ignition occurs when this mixture is within its flammability limits and hot enough to react on its own.[33]

It is often more convenient to think in terms of the critical heat flux for ignition, the minimum heat flux that must fall on a material before it ignites.[26] For wood heated for less than 30 minutes, the IFEG lists 12 kW/m² for piloted ignition and 20 kW/m² for autoignition.[34] For flexible polyurethane, the values are below 10 to 16 kW/m² with a pilot and 16 kW/m² without.[26] When wood is heated for several hours, both values fall to 4.3 kW/m².[34] At the lowest heat flux that can ignite wood, a surface temperature of about 250 °C suffices for piloted ignition and for autoignition alike.[26]

Fire tests on office workstations for the investigation of the World Trade Center disaster showed how autoignition can drive fire growth. A large burner flame bathed the workstation in radiant heat, and simulations suggest peak fluxes of the order of 30 kW/m² or more on the desk.[35] Yet the burner did not act as a pilot, because it was too far from the heated surfaces.[35] Instead, the top sheets of paper on the desk charred, curled up, glowed and then burst into flame.[36] The burning paper then piloted the ignition of the computer monitor, the desk and the chair. The report notes that bare wood needs a radiant flux of about 40 kW/m² for non-piloted ignition.[36]

Use in fire safety engineering

One use of the autoignition temperature is to judge whether hot surfaces can ignite leaking fuels. NUREG-1805 describes a fire in 1987 at the Fort St. Vrain nuclear power plant in the United States. A failed filter sprayed hydraulic oil at about 3,000 psig onto hot exposed steel.[7] The oil ignited because the hot surfaces were above its autoignition point.[7] A list of turbine generator fires in nuclear power plants includes similar events, such as turbine oil igniting on hot surfaces at Muhleberg in Switzerland in 1971.[8]

Insulating oils are also characterised by their autoignition temperature. The mineral oil in the transformers of World Trade Center 7 had an autoignition temperature of about 250 °C.[37] A high autoignition temperature does not make a gas safe, however. NUREG/CR-6850, a fire risk method for nuclear power plants, warns that metals and other materials near a hydrogen release can lower its autoignition temperature significantly through catalytic effects.[30] Hydrogen can also be ignited by very weak ignition sources. Its flammable range of 4.1 to 74% by volume means that almost any hydrogen and air mixture can be assumed to be ignitable.[30]

In buildings, the autoignition temperature helps explain several fire phenomena. Introduction to Fire Safety Engineering lists the cold zone near the floor exceeding about 300 to 350 °C as one condition for flashover. At that point, many combustible materials on the floor become hot enough to ignite spontaneously.[38] A smoke explosion can start in the same way. A premixed cloud of pyrolysis gases and air ignites when it reaches its autoignition temperature, or when a spark or glowing particle ignites it.[39] NUREG-1805 adds that smouldering fires involving polyurethane foam have produced such explosions.[40]

The concept also appears in computer fire models. By default, the Fire Dynamics Simulator lets fuel and air burn as soon as they meet, so that no ignition source has to be modelled.[41] In some scenarios this leads to unrealistic burning at the boundary of an oxygen-starved room. The user can then specify an autoignition temperature, and combustion is suppressed in grid cells that are colder than this value.[41]

Methods to prevent an explosion include avoiding flammable mixtures, excluding ignition sources and enclosing ignition sources so that a flame cannot escape.[42] Places where a flammable atmosphere may form are the subject of hazardous area classification. Some low-power electrical equipment for such places is designed to be intrinsically safe, so that even its worst fault cannot release enough energy to ignite the gas.[42] Other equipment may be placed in an explosion-proof enclosure, which stops a flame from leaving it.[42]

Limits and common misconceptions

A common misconception is that a material cannot ignite below its autoignition temperature. Self-heating materials show the opposite. Gray reports that liquids with very high flash points can ignite spontaneously at temperatures more than a hundred degrees below their flash point or autoignition temperature.[43] Drying oils with flash points around 230 °C, spread on cotton rags, sometimes ignite at room temperature.[43]

Hydraulic fluids that leak onto mineral wool or other porous insulation can also ignite spontaneously.[43] In these cases, the large surface area of the porous material allows slow oxidation, and the heat it produces is retained inside the material.[21] Ignition may then follow after hours or even days.[43]

Wood shows the same effect of time. One chapter of the SFPE Handbook gives a normal ignition temperature for wood of 204 to 260 °C, but reports ignition after four days of heating at 109 °C.[44] The IFEG states that self-heating materials have no unique ignition temperature. For wood heated over long periods, a maximum temperature of 77 °C has been recommended.[26]

A second misconception is that the autoignition temperature is a fixed material constant. It depends on the size of the heated volume, the pressure, the composition of the mixture and the heat loss in the test apparatus.[9][16] Values measured in small laboratory flasks may therefore overestimate the temperature at which a large volume of vapour ignites. Because tables can disagree by 100 K or more, a single tabulated value means little without its test method and conditions.[10][6]

See also

Notes

  1. ↑ 1.0 1.1 1.2 1.3 1.4 Kanury (2002b), p. 2-190.
  2. ↑ Iqbal and Salley (2004), p. B-21.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 Kanury (2002b), p. 2-196.
  4. ↑ Tien et al. (2002), p. 1-87.
  5. ↑ 5.0 5.1 5.2 5.3 Beyler (2002b), p. 2-175.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 6.5 6.6 National Research Council of Canada et al. (2005), p. 3.3-3.
  7. ↑ 7.0 7.1 7.2 Iqbal and Salley (2004), p. H-14.
  8. ↑ 8.0 8.1 Nowlen et al. (2005b), p. O-2.
  9. ↑ 9.0 9.1 9.2 9.3 9.4 9.5 Kanury (2002b), p. 2-197.
  10. ↑ 10.0 10.1 10.2 10.3 10.4 10.5 10.6 10.7 Beyler (2002b), p. 2-176.
  11. ↑ 11.0 11.1 11.2 11.3 11.4 11.5 Kanury (2002b), p. 2-192.
  12. ↑ 12.0 12.1 12.2 12.3 Kanury (2002b), p. 2-188.
  13. ↑ 13.0 13.1 Iqbal and Salley (2004), p. F-6.
  14. ↑ Van Rossum (2027), §4.2.
  15. ↑ 15.0 15.1 15.2 15.3 Kanury (2002b), p. 2-189.
  16. ↑ 16.0 16.1 16.2 16.3 16.4 16.5 Kanury (2002b), p. 2-191.
  17. ↑ Beyler (2002b), p. 2-174.
  18. ↑ Simmons (2002), p. 1-147.
  19. ↑ 19.0 19.1 19.2 Kanury (2002b), p. 2-195.
  20. ↑ Gray (2002), p. 2-212.
  21. ↑ 21.0 21.1 Gray (2002), p. 2-211.
  22. ↑ Kanury (2002b), p. 2-198.
  23. ↑ National Research Council of Canada et al. (2005), p. 3.3-2.
  24. ↑ Iqbal and Salley (2004), p. 6-1.
  25. ↑ 25.0 25.1 25.2 Iqbal and Salley (2004), p. 6-2.
  26. ↑ 26.0 26.1 26.2 26.3 26.4 26.5 National Research Council of Canada et al. (2005), p. 3.3-4.
  27. ↑ Kanury (2002a), p. 2-239.
  28. ↑ 28.0 28.1 Zalosh (2002), p. 3-404.
  29. ↑ Zalosh (2002), p. 3-405.
  30. ↑ 30.0 30.1 30.2 Nowlen et al. (2005b), p. N-2.
  31. ↑ 31.0 31.1 Gray (2002), p. 2-213.
  32. ↑ Kanury (2002a), p. 2-230.
  33. ↑ Kanury (2002a), p. 2-232.
  34. ↑ 34.0 34.1 National Research Council of Canada et al. (2005), p. 3.3-5.
  35. ↑ 35.0 35.1 Ohlemiller et al. (2005), p. 40.
  36. ↑ 36.0 36.1 Ohlemiller et al. (2005), p. 41.
  37. ↑ McAllister et al. (2008), p. 641.
  38. ↑ Van Rossum (2027), §4.7.
  39. ↑ Van Rossum (2027), §4.10.
  40. ↑ Iqbal and Salley (2004), p. 15-4.
  41. ↑ 41.0 41.1 McGrattan et al. (2026), p. 64.
  42. ↑ 42.0 42.1 42.2 Beyler (2002b), p. 2-181.
  43. ↑ 43.0 43.1 43.2 43.3 Gray (2002), p. 2-224.
  44. ↑ Barry (2002), p. 5-191.

References

  • Barry, T. F. (2002). Quantitative risk assessment in chemical process industries. 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. 5-176–5-213). National Fire Protection Association; Society of Fire Protection Engineers.
  • 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.
  • Gray, B. (2002). Spontaneous combustion and self-heating. 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. 2-211–2-228). 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.
  • 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.
  • Kanury, A. M. (2002b). Ignition of liquid 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. 2-188–2-199). 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.
  • Nowlen, S. P., Najafi, B., Wyant, F. J., Forester, J., Kazarians, M., Kolaczkowski, A., Joglar, F., Funk, D., Hannaman, G., & Anoba, R. (2005b). EPRI/NRC-RES fire PRA methodology for nuclear power facilities: Vol. 2. Detailed methodology (NUREG/CR-6850; EPRI 1011989). Electric Power Research Institute; U.S. Nuclear Regulatory Commission, Office of Nuclear Regulatory Research.
  • Ohlemiller, T. J., Mulholland, G. W., Maranghides, A., Filliben, J. J., & Gann, R. G. (2005). Fire tests of single office workstations (NIST NCSTAR 1-5C). National Institute of Standards and Technology.
  • Simmons, R. F. (2002). Premixed burning. 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. 1-144–1-154). National Fire Protection Association; Society of Fire Protection Engineers.
  • Tien, C. L., Lee, K. Y., & Stretton, A. J. (2002). Radiation heat transfer. 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. 1-73–1-89). National Fire Protection Association; Society of Fire Protection Engineers.
  • 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.
Retrieved from "https://fire-safety.academy/w/index.php?title=Autoignition_temperature&oldid=2831"