# Autoignition temperature

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The autoignition temperature or kindling point of a substance is the lowest temperature at which it will spontaneously ignite in a normal atmosphere without an external source of ignition, such as a flame or spark. This temperature is required to supply the activation energy needed for combustion. The temperature at which a chemical will ignite decreases as the pressure or oxygen concentration increases. It is usually applied to a combustible fuel mixture.

Autoignition temperatures of liquid chemicals are typically measured using a 500 mL flask placed in a temperature controlled oven in accordance with the procedure described in ASTM E659.[1]

When measured for plastics, autoignition temperature can also be measured under elevated pressure and at 100% oxygen concentration. The resulting value is used as a predictor of viability for high-oxygen service. The main testing standard for this is ASTM G72.[2]

## Autoignition equation

The time $t_{ig}\,$ it takes for a material to reach its autoignition temperature $T_{ig}\,$ when exposed to a heat flux $q''\,$ is given by the following equation

$t_{ig} = \left ( \frac{\pi}{4} \right ) \left (k \rho c \right )\left [ \frac{T_{ig}-T_{o}}{q''} \right]^2$[3]

where k = thermal conductivity (W/(m·K)), ρ = density (kg/m³), and c = specific heat capacity (J/(kg·K)) of the material of interest. $T_{o}\,$ is the temperature, in Kelvin, the material starts at (or the temperature of the bulk material), and $q''\,$ is the heat flux (W/m²) incident to the material.

## Autoignition point of selected substances

Temperatures vary widely in the literature and should only be used as estimates. Factors which may cause variation include partial pressure of oxygen, altitude, humidity, and amount of time required for ignition. Generally the auto-ignition temperature for hydrocarbon/air mixtures decreases with increasing molecular weight and increasing chain length. The auto-ignition temperature is also higher for branched chain hydrocarbons than for straight chain hydrocarbons.[4]

Substance Autoignition point (°C) Autoignition point (°F) Note
Triethylborane −20 °C −4 °F
Silane 21 °C 70 °F Or below
White phosphorus 34 °C 93 °F Ignites on contact with an organic substance, melts otherwise
Carbon disulfide 90 °C 194 °F
Diethyl ether 160 °C 320 °F[5]
Diesel or Jet A-1 210 °C 410 °F
Gasoline (Petrol) 247–280 °C 477–536 °F[6]
Ethanol 363 °C 685 °F[6]
Butane 405 °C 761 °F[7]
Paper 218–246 °C 424–475 °F[8]
Magnesium 473 °C 883 °F
Hydrogen 536 °C 997 °F[9]

For paper, there is considerable variation between sources, mainly because there are many physical variables over different kinds of paper, like thickness, density and composition; in addition, it takes longer for the combustion of paper to start at lower temperatures.[10]

## References

1. ^ E659 – 78 (Reapproved 2000), "Standard Test Method for Autoignition Temperature of Liquid Chemicals", ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959
2. ^ S. Grynko, "Material Properties Explained" (2012), ISBN 1-4700-7991-7, p. 46.
3. ^ Principles of Fire Behavior. ISBN 0-8273-7732-0. 1998.
4. ^ Zabetakis, M.G. (1965), Flammability characteristics of combustible gases and vapours, U.S. Department of Mines, Bulletin 627.
5. ^ "Diethyl Ether - Safety Properties". Wolfram|Alpha.
6. ^ a b Fuels and Chemicals - Autoignition Temperatures, engineeringtoolbox.com
7. ^ "Butane - Safety Properties". Wolfram|Alpha.
8. ^ Tony Cafe. "Physical Constants for Investigators". Journal of Australian Fire Investigators. (Reproduced from "Firepoint" magazine)
9. ^ "Hydrogen - Safety Properties". Wolfram|Alpha.
10. ^ Forest Products Laboratory (1964). "Ignition and charring temperatures of wood". Forest Service U. S. Department of Agriculture.