Autoignition temperature


The autoignition temperature or kindling point of a substance is the lowest temperature in which it spontaneously ignites in 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 ignites 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 flask placed in a temperature-controlled oven in accordance with the procedure described in ASTM E659.
When measured for plastics, autoignition temperature can be also 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.

Autoignition equation

The time it takes for a material to reach its autoignition temperature when exposed to a heat flux is given by the following equation:
where k = thermal conductivity, ρ = density, and c = specific heat capacity of the material of interest, is the initial temperature of the material.

Autoignition point of selected substances

Temperatures vary widely in the literature and should only be used as estimates. Factors that may cause variation include partial pressure of oxygen, altitude, humidity, and amount of time required for ignition. Generally the autoignition temperature for hydrocarbon/air mixtures decreases with increasing molecular mass and increasing chain length. The autoignition temperature is also higher for branched-chain hydrocarbons than for straight-chain hydrocarbons.
SubstanceAutoignitionNote
Barium550±90
Bismuth735±20
Butane
Calcium790±10
Carbon disulfide
Diesel or Jet A-1
Diethyl ether
Ethanol
Gasoline
Hydrogen
Iron1315±20
Lead850±5
Leather / parchment
Magnesium635±5
Magnesium
Molybdenum780±5
Paper
Phosphorus
Silaneor below
Strontium1075±120
Tin940±25
Triethylborane