Autoignition temperature
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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 mixtureSimilar 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_\, ] it takes for a material to reach its autoignition temperature [T_\, ] when exposed to a heat flux [q''\,] is given by the following equation
[t_ = \left ( \frac \right ) \left (k \rho c \right )\left [ frac-T_infty} right ] ] Principles of Fire Behavior. ISBN: 0827377320. 1998.
where [ k = thermal\ conductivity \left ( \frac \right )\, ], [ \rho = density \left ( \frac \right )\, ], and [ c = specific\ heat\ capacity \left ( \frac \right )\, ] of the material of interest. [ T_\infty\,] is the temperature, in Kelvin, the material starts at (or the temperature of the bulk material), and [ q''\left ( \frac \right )\,] is the heat flux incident to the material.
References
Further Reading
- Analysis of Effective Thermal Properties of Thermally Thick Materials.[link]
See also
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