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# Autoignition temperature

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 detonate decreases as the pressure increases 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]. 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, and a variety of other characteristics).

## 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

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

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

## 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. ^ Principles of Fire Behavior. ISBN 0-8273-7732-0. 1998.