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 in which it spontaneously ignites in a normal atmosphere without an external source of ignition, such as a flame or spark. [1] This temperature is required to supply the activation energy needed for combustion. The temperature at which a chemical ignites decreases as the pressure is increased.
Autoignition temperatures of liquid chemicals are typically measured using a 500-millilitre (18 imp fl oz; 17 US fl oz) flask placed in a temperature-controlled oven in accordance with the procedure described in ASTM E659. [2]
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. [3]
The time it takes for a material to reach its autoignition temperature when exposed to a heat flux is given by the following equation: [4]
where k = thermal conductivity, ρ = density, and c = specific heat capacity of the material of interest, is the initial temperature of the material (or the temperature of the bulk material).
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. [5]
| Substance | Autoignition [D] | Note |
|---|---|---|
| Barium | 550 °C (1,022 °F) | 550±90 [1] [C] |
| Bismuth | 735 °C (1,355 °F) | 735±20 [1] [C] |
| Butane | 405 °C (761 °F) | [6] |
| Calcium | 790 °C (1,450 °F) | 790±10 [1] [C] |
| Carbon disulfide | 90 °C (194 °F) | [7] |
| Diesel or Jet A-1 | 210 °C (410 °F) | [8] |
| Diethyl ether | 160 °C (320 °F) | [9] |
| Ethanol | 365 °C (689 °F) | [7] |
| Gasoline (Petrol) | 247–280 °C (477–536 °F) | [7] |
| Hydrogen | 535 °C (995 °F) | [10] |
| Iron | 1,315 °C (2,399 °F) | 1315±20 [1] [C] |
| Lead | 850 °C (1,560 °F) | 850±5 [1] [C] |
| Leather / parchment | 200–212 °C (392–414 °F) | [8] [11] |
| Magnesium | 635 °C (1,175 °F) | 635±5 [1] [B] [C] |
| Magnesium | 473 °C (883 °F) | [7] [B] |
| Molybdenum | 780 °C (1,440 °F) | 780±5 [1] [C] |
| Paper | 218–246 °C (424–475 °F) | [8] [12] |
| Phosphorus (white) | 34 °C (93 °F) | [7] [A] [B] |
| Silane | 21 °C (70 °F) | [7] or below |
| Strontium | 1,075 °C (1,967 °F) | 1075±120 [1] [C] |
| Tin | 940 °C (1,720 °F) | 940±25 [1] [C] |
| Triethylborane | −20 °C (−4 °F) | [7] |
| 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. |
Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion does not always result in fire, because a flame is only visible when substances undergoing combustion vaporize, but when it does, a flame is a characteristic indicator of the reaction. While the activation energy must be overcome to initiate combustion, the heat from a flame may provide enough energy to make the reaction self-sustaining.
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