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Parameters for Characterizing Fuel-Air Mixture

Various parameters are used to describe the flammability characteristics of various fuels, including gases and vapors, liquids and solids. Some of these are briefly discussed below.

Ignition Temperature

The ignition temperature or auto-ignition temperature is the minimum temperature of a flammable mixture that is required to initiate or cause self-sustained combustion without ignition from an external source of energy such as a spark or flame (ASTM D2155).

Flammability Limits

The lower flammable limit (LFL) or lower explosive limit (LEL) is the minimum concentration of vapor in air below which a flame is not propagated when an ignition source is present. Below this concentration, the mixture is considered too lean to burn.

The upper flammable limit (UFL) or upper explosive limit (UEL) is the maximum vapor concentration in air at which a flame can propagate. Above this concentration, the mixture is too rich to burn, i.e. the oxygen is consumed in the combustion of one particle and there is insufficient oxygen to burn the adjacent particle of fuel. Products of combustion surrounding the first particle tend to quench the flame.

The flammable range or explosive range (EL) consists of all concentrations between the lower flammable limit and the upper flammable limit. Flammable limits usually refer to flowing materials; explosive limits usually refer to confined or stagnant mixtures. Values for LFL and LEL, or UFL and UEL, are identical.

The stoichiometric concentration is that mixture of fuel and oxidant, usually air, that produces fully oxidized combustion products, chiefly water and carbon dioxide, following ignition. Such mixtures typically are the most easily ignited and produce the highest temperature and pressure at the greatest rates.

The Flammability Limits is related to the Minimum Oxygen Concentration and the Minimum Ignition Energy. Click here for more information.

Minimum Oxygen Concentration (MOC)

The oxygen content in atmospheric air is 21% by volume. Oxygen is the key ingredient in the fire triangle and there is a minimum oxygen concentration required to propagate a flame. This is also known as the limiting oxygen concentration. It is the concentration below which combustion, usually in air diluted with an inert gas such as nitrogen or carbon dioxide, does not propagate in a mixture of gases or vapors.

This is an especially useful result, because explosions and fires are preventable by reducing the oxygen concentration regardless of the concentration of the fuel. This is the basis of the prevention technique of inerting (see later).

The MOC has units of % oxygen in air plus fuel. Below the MOC, the reaction cannot generate enough energy to heat the entire mixture of gases (including the inerts) to the extent required for the self-propagation of the flame. The MOC for several chemicals are shown in the Table.

Selected values of MOC

Typically, when the oxygen concentration within most fuel mixtures fall below about 10 volume % no combustion can occur. As this is only an indicative value, when reliable inerting is required the exact value of the MOC has to be determined. This is measured for an optimum fuel concentration using a very strong ignition source which supplies spark discharge energies in the range of 2 - 10 kJ, depending on the volume of the test apparatus.

The MOC is related to the Flammability Limits and the Minimum Ignition Energy. Click here for more information.

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Minimum Ignition Temperature (MIT)

The minimum ignition temperature is the lowest temperature of a hot surface at which a fuel-air mixture of the most easily ignitable composition can just be stimulated to burn with a visible flame. There is no linear relationship between the concentration ratio (or mole fraction) of the fuels and the ignition temperature for the mixture. The ignition temperature values for fuel mixtures must therefore be determined for every individual case.

Also, the ignition temperature for a given fuel in oxidizing media other than air may sometimes be quite low. This is especially so for pure oxygen and chlorine, for example: toluene-air (535 oC) vs. toluene-chlorine (175 oC).

Minimum Ignition Energy (MIE)

The minimum ignition energy is the minimum energy input required to initiate combustion. It is the smallest amount of energy stored in a capacitor, that is just sufficient, when discharged across a spark gap, to ignite the most ignitable explosive mixture. All flammables (including dusts) have MIEs. The MIE depends on the specific chemical or mixture, the concentration, pressure, and temperature. A few MIEs are given in the Table.

Minimum Ignition Energies

As can be seen from the Table, many hydrocarbons have MIEs of about 0.25 mJ. This is low when compared to sources of ignition. For example, a static discharge of 22 mJ is initiated by walking across a rug, and an ordinary spark plug has a discharge energy of 25 mJ. Electrostatic discharges, as a result of fluid flow, also have energy levels exceeding the MIEs of flammables and can provide a ignition source, and thus possibly contributing to plant explosions.

The MIE is related to the Flammability Limits and the Minimum Oxygen Concentration. Click here for more information.

Minimum Ignition Current

This parameter is used as a measure of the ignitability of electrical (inductive) sparks. It is the smallest current flowing in a circuit of a certain inductance, such that the spark produced when the current flow is interrupted (in an explosive mixture of optimal ignitability) just ignites the mixture.

If the minimum ignition current is known, electrical circuits can be designed so that no igniting sparks are created by switching, or on failures such as short-circuit. This design approach is employed in explosion-resistant electrical equipment rated "instrinsically-safe".

Maximum Experimental Safe Gap (MESG)

The propagation of an explosion from a part of the plant featuring explosion-resistant design into upstream or downstream sections that are not so designed, or that are not vented, must be effectively prevented. A parameter that measures of the ability of exploding gas mixtures to propagate through a narrow gap is the maximum experimental safe gap.

On the basis of the MESG, fuels are classified into various explosion groups. The MESG is used in the design of flame arresters or explosion-resistant electrical equipment with "flame-proof" enclosure.


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