Autoignition temperature

The autoignition temperature or self-ignition temperature, often called spontaneous ignition temperature or minimum ignition temperature (or shortly ignition temperature) and formerly also known as kindling point, of a substance is the lowest temperature at which it spontaneously ignites 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 ignites decreases as the pressure is decreased.

• Substances which spontaneously ignite in a normal atmosphere at naturally ambient temperatures are termed pyrophoric.

Autoignition temperatures of liquid chemicals are typically measured using a Template:Convert flask placed in a temperature-controlled oven in accordance with the procedure described in ASTM E659.

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.

Autoignition time equation

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

$t_{\text{ig}}=\frac{\pi }{4}k\rho c{\left[\frac{T_{\text{ig}}-T_0}{q^{″}}\right]}^2,$

where k = thermal conductivity, ρ = density, and c = specific heat capacity of the material of interest, $T_0$ is the initial temperature of the material (or the temperature of the bulk material).

Autoignition temperature 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.

Substance	Autoignition[D]	Note
Barium	Template:Convert	550±90[C]
Bismuth	Template:Convert	735±20[C]
Butane	Template:Convert
Calcium	Template:Convert	790±10[C]
Carbon disulfide	Template:Convert
Diesel or Jet A-1	Template:Convert
Diethyl ether	Template:Convert
Ethanol	Template:Convert
Gasoline (Petrol)	Template:Convert
Hydrogen	Template:Convert
Iron	Template:Convert	1315±20[C]
Lead	Template:Convert	850±5[C]
Leather / parchment	Template:Convert
Magnesium	Template:Convert	635±5[B][C]
Magnesium	Template:Convert	[B]
Methane	Template:Convert
Molybdenum	Template:Convert	780±5[C]
Paper	Template:Convert
Phosphorus (white)	Template:Convert	[A][B]
Silane	Template:Convert	or below
Strontium	Template:Convert	1075±120[C]
Tin	Template:Convert	940±25[C]
Triethylborane	Template:Convert

A On contact with an organic substance, melts otherwise.

B There are two distinct results in the published literature. Both are separately listed in this table.

C At 1 atm. The ignition temperature depends on the air pressure.

D Under standard conditions for pressure.

See also

• Fire point
• Flash point
• Gas burner (for flame temperatures, combustion heat energy values and ignition temperatures)
• Spontaneous combustion

External links

• Analysis of Effective Thermal Properties of Thermally Thick Materials.