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Contents
  1. 1Piloted, non-piloted and self-induced ignition
  2. 2Fuel gases, liquids and solids
  3. 3Heat exposure and ignition time
  4. 4Autoignition and critical conditions
  5. 5Ignition sources and the fire triangle
  6. 6Measurements and material data
  7. 7Prevention and engineering assessment
  8. 8Limits and common misunderstandings
  9. 9See also
  10. 10Notes
  11. 11References

Ignition is the transition at which a fuel-oxidiser system becomes capable of sustaining an exothermic combustion reaction. It marks a change from heating, decomposition or a local reaction to combustion that can continue and potentially grow. The transition depends on fuel production, mixing with oxygen, temperature, reaction rate and heat loss.[1]

An ignition source and an ignitable material are therefore not sufficient as separate labels. Their interaction must produce suitable conditions in the same place and at the same time. A spark can fail near a poorly mixed vapour cloud, while a weaker source can succeed within a suitable mixture.[2][3]

Ignition is also a stage within fire development rather than a complete description of the later fire. It distinguishes the start of combustion from subsequent flame spread, enclosure growth and possible transitions between combustion modes in different fuel packages and settings. Each requires separate assessment and evidence. Persistent flaming requires continuing fuel and heat feedback, while transient ignition can produce a flame that disappears when external heating is removed. Prevention, detection and suppression depend on which of these conditions has been reached.[2]

Piloted, non-piloted and self-induced ignition

Piloted ignition uses a local hot object or energy source such as a flame or spark to initiate combustion in a prepared fuel-air mixture. The pilot is not necessarily responsible for heating the entire solid or producing all its fuel gases. Its principal role is to start a reaction that can then propagate.[3][2]

Three routes: pilot, hot mixture without pilot and internal self-heating.
Three ignition modes with different initiating conditions. Based on Australian Building Codes Board et al. (2005), p. 2.4-4.

Non-piloted ignition occurs when the fuel-vapour and oxygen mixture reaches a condition in which it ignites without that local pilot. Temperature and composition remain essential, and heat losses must not prevent the reaction from accelerating. The absence of a visible flame before ignition does not mean that no energy was supplied.[3][4]

Self-induced ignition begins with oxidation inside a solid material. These reactions release energy, increasing decomposition and temperature until the critical condition is crossed. The process is often associated with heat being retained by poor conduction through the material or its container.[3][5]

Self-heating describes this internal rise caused by chemical heat generation exceeding heat loss. Larger bodies and higher ambient temperatures can favour criticality by reducing relative heat loss. The result depends on material chemistry, size, shape and initial temperature rather than on one universal ambient threshold.[5][6]

Fuel gases, liquids and solids

For gaseous fuels, composition and mixing are immediate concerns. Fuel and oxidiser must occur within a flammable range, and the ignition source must exceed the mixture's requirements. The flame can then propagate through the connected mixture rather than remaining only at the source.[7][8]

Liquids usually have to evaporate before flaming combustion. The vapour above the surface mixes with air and can flash when piloted. Sustained burning requires enough heat feedback to maintain evaporation after the initial flash. Liquid bulk temperature, volatility, ventilation and container geometry all affect this sequence.[9][10]

Many solids require Pyrolysis before they can support a gas-phase flame. Heating decomposes the material and releases volatile products, often leaving char. These volatiles mix with oxygen in the boundary layer and ignite when their composition and thermal condition are suitable.[11][4]

The solid's thermal response therefore forms only one part of ignition. Kanury groups the problem into production of combustible gases, their mixing with oxidant and induction of a sufficiently rapid, self-supporting reaction. Material properties, geometry and gas flow influence all three stages.[2]

Smouldering and glowing are different combustion modes. Smouldering involves subsurface or surface oxidation in a porous fuel, while flaming occurs in the gas phase. A fire scenario should not assume that every ignition immediately produces a visible flame.[2][12]

Heat exposure and ignition time

Critical heat flux for ignition refers to the threshold exposure associated with ignition under specified material and test conditions. It is not an independent material constant detached from exposure time, heat losses or geometry. A surface can absorb energy while also losing it through conduction, convection and radiation.[13]

At the start of radiant heating, energy arriving at a solid is divided between conduction into the interior, reradiation and convective heating of nearby gas. Moisture evaporation and later pyrolysis change this partition. The surface and interior consequently follow different temperature histories.[4]

Radiant heat dividing into solid conduction, reradiation and convection before pyrolysis.
Energy partition during heating and ignition. Based on Kanury (2002), p. 2-232.

Time to ignition is the exposure duration needed to cross the ignition condition. It depends on heating intensity, material response, thickness, initial temperature and whether transient or persistent ignition is sought. Longer exposure at lower intensity cannot always be traded directly against shorter exposure at higher intensity.[2][13]

Kanury's discussion of Martin's map distinguishes nonignition, glowing ignition, transient ignition and persistent ignition regions for the tested cellulosic arrangements. Thin specimens can ignite quickly without producing persistent flaming, while thick specimens can lose energy into their interior. At very high exposure, surface ablation can occur without ignition in the mapped conditions.[13]

The 2005 International Fire Engineering Guidelines note that direct flame contact with a second item is sometimes treated conservatively as immediate ignition. This simplification ignores the time actually required for pyrolysis and heating of decomposition products. It is a modelling choice for that context, not a universal physical observation.[3]

Autoignition and critical conditions

Autoignition temperature is associated with ignition of a gas or vapour mixture without a pilot under specified test conditions. It differs from the temperature of a hot surface or from the bulk temperature of every reacting solid. Test geometry, pressure and exposure can affect the measured result.[14]

Self-heating bodies have a critical condition representing a watershed between stable subcritical behaviour and thermal runaway. Chemical reaction rates can rise steeply with temperature, while heat loss changes more gradually. Once generation exceeds loss sufficiently, internal temperature can accelerate upwards.[5][6]

Critical ambient temperature is used when body size is fixed, while critical size or diameter is relevant when ambient temperature is fixed. Preheated materials introduce a further stacking-temperature consideration. These variables show why one material cannot be assigned a universal spontaneous-ignition temperature in every storage arrangement.[5]

The location of runaway is often internal. Gray describes charring or ash at the centre while the exterior remains comparatively intact in some spontaneously ignited bodies. Such deep-seated combustion can be difficult to extinguish and can reignite after apparent suppression.[6]

The persistence criterion also changes how test results should be compared. One test may establish the first flash, while another establishes whether burning continues after the external source is removed. Reporting both as a single ignition temperature hides the different outcomes and can make materials appear directly comparable when the tests do not support that conclusion.[2]

Persistent ignition also depends on what happens beyond the initial reaction zone. Heat must reach new fuel or maintain decomposition, while oxygen and combustion products continue to move. A local flame can therefore be self-sustaining for a period without establishing a growing compartment fire.[13][15]

Flash point describes a liquid temperature at which the vapour-air mixture just above the surface supports a momentary flash under a specified piloted test. The flame can sweep through the mixture and then go out. It does not yet demonstrate persistent combustion of the liquid.[10]

Fire point instead requires self-sustained burning. This distinction mirrors Kanury's broader separation of transient and persistent ignition. An engineering assessment should therefore state whether it concerns the first observable reaction, a propagating flame or a fire that continues after the source is removed.[10][2]

Persistent solid-fuel flaming requires continuing pyrolyzate production and heat feedback. Heating the whole body can help sustain this production in some configurations, while conductive loss into a thick body can suppress it in others. Appearance of a short-lived flame is not conclusive evidence of this later state.[13]

Smouldering can also precede later flaming. Transition requires a flammable pyrolysis-gas mixture and sufficient heat to ignite it at the same location and time. Enhanced oxygen supply, reduced heat loss and local geometry can promote this change, but it is not an automatic stage of every smouldering fire.[16]

Ignition sources and the fire triangle

An Ignition source supplies the local energy or reactive condition needed to initiate combustion. Examples include a small flame, heated wire, electric spark or incandescent particle. The source's effectiveness depends on the mixture it contacts and on heat lost to the surroundings.[2][4]

The Fire triangle places ignition source within the wider combination of fuel, oxygen and heat. Most building environments already contain fuel and oxygen, so qualitative prevention often focuses on separating them from open flames, sparks and sufficiently hot surfaces. The model remains a checklist rather than an ignition-time calculation.[3]

For gases and vapours, avoiding flammable concentrations is a principal strategy. The 2005 guidelines recommend keeping concentrations well below the lower flammable limit rather than relying on a rich mixture remaining above the upper limit. Dilution can move a rich mixture back into the ignitable range.[8]

Ignition can also pass from one object to another. NUREG-1805 uses the example of a burning wastebasket igniting a chair, which can then ignite further fuel. The initial igniter and subsequent burning objects become sources in a sequence, so the fire history is not limited to the first device.[17]

Measurements and material data

Ignition tests impose controlled radiant or convective exposure and observe the response. The measured temperature, heat flux and time depend on apparatus, specimen thickness, backing and pilot arrangement. Results should retain these conditions instead of being reduced to an unqualified material label.[1][13]

The 2005 guidelines list grouped ignition-temperature ranges for several solid categories. Wood, paper and cotton are listed at 250 to 365 °C for piloted ignition and 250 to 400 °C for autoignition. These older grouped values illustrate variability and should not be used as universal thresholds for every product and exposure.[18]

Wood, paper and cotton group: piloted 250 to 365 °C and autoignition 250 to 400 °C.
Grouped ignition-temperature ranges in the 2005 IFEG. Based on Australian Building Codes Board et al. (2005), p. 3.3-3.

Exposure duration changes the appropriate data. The guidelines note that many wood studies used exposures of 20 minutes or less and provide a lower heat-flux result from a day-long exposure. Long-term self-heating requires a different treatment again, including the size of the stored aggregation.[18]

Material properties also change during heating. Moisture leaves, char forms and some polymers melt before decomposition. These changes affect heat transport and volatile release, so an ignition model based on unchanged properties can apply only within its stated assumptions.[4][19]

Prevention and engineering assessment

Prevention can address fuel condition, oxidiser concentration, ignition energy and heat retention. Keeping work areas free of unnecessary combustibles, controlling flammable atmospheres and separating hot surfaces from fuel each acts on a different part of the ignition problem. Their suitability depends on the scenario.[3][8]

For subsequent items, radiant exposure depends on the burning item and separation distance. The 2005 guidelines describe a simplified pre-flashover method in which radiation from the upper layer and room surfaces is neglected and only flame radiation from the first item is included. That assumption must not be transferred to a developed compartment fire.[3]

Probabilistic studies can use fire statistics for ignition frequency by building class and ignited material. The 2005 guidelines state that general quantitative methods for predicting ignition potential were not available in that document. Statistical frequencies support analysis but do not replace scenario-specific assessment of sources and fuels.[3]

Self-heating requires particular caution in extrapolation. Gray warns that time to ignition is highly sensitive to material, size, initial temperature and distance from the critical condition. Hot stacked material can ignite much sooner than a simple ambient-temperature model predicts.[6][20]

Limits and common misunderstandings

Ignition is not the same as heating or smoke production. Forced pyrolysis can release smoke without self-sustained oxidation, and a hot material can remain below criticality. The defining issue is whether combustion can sustain itself under the stated conditions.[12][1]

A visible flame is not the only ignition outcome. Glowing and smouldering can occur without gas-phase flame, and some flames are difficult to see. Assessment based only on appearance can therefore miss active combustion.[2][15]

There is no single ignition temperature for every configuration of a material. Thickness, heating rate, heat loss, pressure and pilot conditions can all change the measured threshold. Tables are evidence for their stated tests, not universal constants.[13][18]

Finally, ignition does not guarantee unlimited fire growth. Fuel depletion, oxygen limitation and heat losses can stop a fire after initial reaction. Persistent combustion and subsequent spread require their own continuing balances.[2][21]

See also

Notes

  1. ↑ 1.0 1.1 1.2 Kanury (2002), p. 2-229.
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 Kanury (2002), p. 2-230.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 Australian Building Codes Board et al. (2005), p. 2.4-4.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 Kanury (2002), p. 2-232.
  5. ↑ 5.0 5.1 5.2 5.3 Gray (2002), p. 2-211.
  6. ↑ 6.0 6.1 6.2 6.3 Gray (2002), p. 2-212.
  7. ↑ Beyler (2002), p. 2-172.
  8. ↑ 8.0 8.1 8.2 Australian Building Codes Board et al. (2005), p. 3.3-1.
  9. ↑ Van Rossum (2027), §4.1.
  10. ↑ 10.0 10.1 10.2 Kanury (2002), p. 2-190.
  11. ↑ Van Rossum (2027), §3.1.
  12. ↑ 12.0 12.1 Ohlemiller (2002), p. 2-200.
  13. ↑ 13.0 13.1 13.2 13.3 13.4 13.5 13.6 Kanury (2002), pp. 2-231–2-232.
  14. ↑ Kanury (2002), pp. 2-189–2-190.
  15. ↑ 15.0 15.1 Van Rossum (2027), §4.3.
  16. ↑ Ohlemiller (2002), p. 2-208.
  17. ↑ U.S. Nuclear Regulatory Commission (2005), p. F-36.
  18. ↑ 18.0 18.1 18.2 Australian Building Codes Board et al. (2005), p. 3.3-3.
  19. ↑ Beyler and Hirschler (2002), p. 1-112.
  20. ↑ Gray (2002), p. 2-213.
  21. ↑ Drysdale (2002), p. 1-97.

References

  • Australian Building Codes Board, National Research Council of Canada, International Code Council, & Department of Building and Housing New Zealand. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
  • Beyler, C. L. (2002). Flammability limits of premixed and diffusion flames. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
  • Beyler, C. L., & Hirschler, M. M. (2002). Thermal decomposition of polymers. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
  • Drysdale, D. D. (2002). Thermochemistry. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
  • Gray, B. (2002). Spontaneous combustion and self-heating. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
  • Kanury, A. M. (2002). Flaming ignition of solid fuels. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
  • Kanury, A. M. (2002). Ignition of liquid fuels. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
  • Ohlemiller, T. J. (2002). Smoldering combustion. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
  • U.S. Nuclear Regulatory Commission. (2005). Fire dynamics tools: Quantitative fire hazard analysis methods for the U.S. nuclear regulatory commission fire protection inspection program (NUREG-1805). Author.
  • Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.
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