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
Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
at which it
spontaneously ignites in a normal atmosphere without an external source of ignition, such as a
flame
A flame () is the visible, gaseous part of a fire. It is caused by a highly exothermic chemical reaction made in a thin zone. When flames are hot enough to have ionized gaseous components of sufficient density, they are then considered plasm ...
or
spark.
This temperature is required to supply the
activation energy
In the Arrhenius model of reaction rates, activation energy is the minimum amount of energy that must be available to reactants for a chemical reaction to occur. The activation energy (''E''a) of a reaction is measured in kilojoules per mole (k ...
needed for
combustion
Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion ...
. The temperature at which a chemical ignites decreases as the
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...
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 flask placed in a temperature-controlled oven in accordance with the procedure described in
ASTM
ASTM International, formerly known as American Society for Testing and Materials, is a standards organization that develops and publishes voluntary consensus technical international standards for a wide range of materials, products, systems and s ...
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
it takes for a material to reach its autoignition temperature
when exposed to a
heat flux
In physics and engineering, heat flux or thermal flux, sometimes also referred to as heat flux density, heat-flow density or heat-flow rate intensity, is a flow of energy per unit area per unit time (physics), time. Its SI units are watts per sq ...
is given by the following equation:
:
where ''k'' =
thermal conductivity
The thermal conductivity of a material is a measure of its ability to heat conduction, conduct heat. It is commonly denoted by k, \lambda, or \kappa and is measured in W·m−1·K−1.
Heat transfer occurs at a lower rate in materials of low ...
, ''ρ'' = density, and ''c'' =
specific heat capacity
In thermodynamics, the specific heat capacity (symbol ) of a substance is the amount of heat that must be added to one unit of mass of the substance in order to cause an increase of one unit in temperature. It is also referred to as massic heat ...
of the material of interest,
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
In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal g ...
of oxygen, altitude, humidity, and amount of time required for ignition. Generally the autoignition temperature for hydrocarbon/air mixtures decreases with increasing
molecular mass
The molecular mass () is the mass of a given molecule, often expressed in units of daltons (Da). Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. The derived quan ...
and increasing
chain length. The autoignition temperature is also higher for branched-chain hydrocarbons than for straight-chain hydrocarbons.
[Zabetakis, M. G. (1965), Flammability characteristics of combustible gases and vapours, U.S. Department of Mines, Bulletin 627.]
See also
*
Fire point
*
Flash point
The flash point of a material is the "lowest liquid temperature at which, under certain standardized conditions, a liquid gives off vapours in a quantity such as to be capable of forming an ignitable vapour/air mixture".
The flash point is somet ...
*
Gas burner (for flame temperatures, combustion heat energy values and ignition temperatures)
*
Spontaneous combustion
References
External links
Analysis of Effective Thermal Properties of Thermally Thick Materials
{{DEFAULTSORT:Autoignition Temperature
Chemical properties
Fire
Threshold temperatures
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