Introduction

The autoignition temperature, or the ignition temperature of a substance is the lowest temperature at which a chemical 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 detonate decreases as the pressure increases or oxygen concentration increases. It is usually applied to in a combustible fuel mixture

Similar to the autoignition temperature is the flash point, which is the lowest temperature at which a substance can form an ignitable mixture with air. This point is always less than the autoignition temperature, but the activation energy needed for combustion can be supplied by an external source of ignition, such as a spark.

Autoignition temperatures are measured using the same closed cup apparatus used for measuring flash points. The commonly accepted autoignition temperature of paper, 451 °F (233 °C), is well known because of the popular novel Fahrenheit 451 by author Ray Bradbury (although the actual autoignition temperature depends on the type of pulp used in the paper's manufacture, chemical content, paper thickness, etc.)

Autoignition Point of Selected Substances

• Silane: <21°C (70°F)
• White phosphorus: 34°C (93°F)
• Carbon disulfide: 100°C (212°F)
• Gasoline: 257°C (495°F)
• Butane: 430°C (806°F)
• Magnesium: 473°C (883°F)
• Hydrogen: 571°C (1060°F)

Autoignition Equation

The time $t\_\{ig\}\backslash ,$ it takes for a material to reach its autoignition temperature $T\_\{ig\}\backslash ,$ when exposed to a heat flux $q\text{'}\text{'}\backslash ,$ is given by the following equation

$t\_\{ig\}\; =\; \backslash left\; (\; \backslash frac\{\backslash pi\}\{4\}\; \backslash right\; )\; \backslash left\; (k\; \backslash rho\; c\; \backslash right\; )\backslash left\backslash frac\{T\_\{ig\}-T\_\backslash infty\}\{q\text{'}\text{'}\}\; \backslash right$ Principles of Fire Behavior. ISBN: 0827377320. 1998.

where $k\; =\; thermal\backslash \; conductivity\; \backslash left\; (\; \backslash frac\{J\}\{s\; \backslash cdot\; m\; \backslash cdot\; K\}\; \backslash right\; )\backslash ,$, $\backslash rho\; =\; density\; \backslash left\; (\; \backslash frac\{kg\}\{m^3\}\; \backslash right\; )\backslash ,$, and $c\; =\; specific\backslash \; heat\backslash \; capacity\; \backslash left\; (\; \backslash frac\{J\}\{kg\; \backslash cdot\; K\}\; \backslash right\; )\backslash ,$ of the material of interest. $T\_\backslash infty\backslash ,$ is the temperature, in Kelvin, the material starts at (or the temperature of the bulk material), and $q\text{'}\text{'}\backslash left\; (\; \backslash frac\{J\}\{s\; \backslash cdot\; m^2\}\; \backslash right\; )\backslash ,$ is the heat flux incident to the material.

References

Further Reading

• Analysis of Effective Thermal Properties of Thermally Thick Materials.^*

See also

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• Pyrolysis

Chemical properties | Fire

Teplota vznícení | Température d'auto-inflammation | Temperatura de auto-ignição | Температура самовоспламенения | Itsesyttymislämpötila