THE COMTHERM ON-LINE TECHNICAL MANUAL

INFLAMMABILITY LIMITS

A fuel gas will not burn without oxygen, usually in the form of air; likewise air alone will not combust without a fuel, usually in the form of hydrogen or a hydrocarbon. It can be easily appreciated that it is necessary to produce a fuel gas: oxygen mixture in order to allow the exothermic chemical reactions that take place during combustion to take place. Not all mixtures of a fuel with air or oxygen will burn continuously in self-sustained combustion. An inflammable mixture of gas is one which is able to propagate flame indefinitely at a given temperature and pressure when the unburned mixture is maintained at that temperature and pressure.

Gases, when mixed with air, have well defined upper and lower limits of inflammability between which the mixtures are capable of flame self-propagation. These mixture limits are expressed as the per cent of fuel in a mixture with air. Within these limits the mixture liberates enough energy to continue burning. The zone between the upper and lower limits of flame propagation is known as 'range of inflammability. These limits of inflammability vary with different gases. Outside these limits the mixture cannot generate sufficient heat at a high enough temperature to maintain combustion.

Actual limits of inflammability vary depending on physical circumstances and the experimental methods used. Comtherm table CT-9a gives typical limits as percentage by volume in air of some common fuel gases:-
A graphic comparison of the limits of inflammability of some common fuel gases is provided in Comtherm fig CT-9b.

Information with regard to limits of inflammability of most common industrial chemicals and solvents can be found in the publications of the U.S.A. National Fire Prevention Association

The lower limit of inflammability represents the smallest proportion of gas which, when mixed with air, will burn without the continuous application of heat from an external source. This minimum gas concentration for combustion is referred to as the lower explosion limit (LEL). The maximum gas concentration for combustion is referred to as the upper explosion limit (UEL).

Above the upper limit the large amount of combustible gas present acts as a dilutent and again combustion cannot be maintained. Mixtures above the upper limit may burn on contact with additional external air. Even within the inflammability limits, there are certain practical limitations to the fuel-air mixtures that may be used. One of these is the problem of unsafe gases being produced; for example, poisonous carbon monoxide (CO) is normally produced by a gas rich flame. Another limit may be the temperature or atmosphere required for certain processes.

Hydrogen and carbon monoxide have a wide range of inflammability whilst methane, propane and butane have the narrowest. Town gas being a mixture of such gases, has a 'medium' range of inflammability. A typical natural gas may have a range of 5.0% to 14.5% between lower and upper limit. These percentage figures expressed in another manner indicate an air/gas ratio between 19:1 to 5.8:1 where the mixture is inflammable.

An example of the impact of limits of inflamability on burner design is illustrated by the fact that most common primary aerated burners use about 50% primary aeration because this results in optimum flame stability. In the case of towns gas (G4) this mixture would be approximately 2.2:1 air : gas ratio and within the limits of inflamability; however this mixture would be well outside the limits in the case of methane where 50% aeration would require a 5:1 air : gas mixture ratio.

Certain conditions cause a change in the limits, either increasing or decreasing the spread between them. These conditions include: ignition source, turbulence, humidity, pressure, physical factors such as pipe diameter or containing vessel dimensions, direction of flame travel, dilutants (which affect the oxygen concentration) and temperature.

Pressure changes have different effects on the limits of inflammability with different fuel gases. With hydrogen and carbon monoxide increasing the pressure of gas/air mixtures narrows the higher and lower limits. With methane and other paraffin hydrocarbons however the lower limit narrows whilst the upper limit extends. In all cases the pressure must be increased substantially to produce a significant effect; for example a five fold increase in pressure on a methane/air mixture increases the upper limit from 13.3% to 13.8%). With a reduction of pressure the limits are narrowed as the pressure is decreased until finally a critical pressure is reached below which the gas/air mixture will not ignite.

Physical factors such as the dimensions of the combustion enclosure or mixture tube affect the Limits of inflamability. Normally limits are measured in the laboratory by filling a glass tube with a known gas/air mixture and igniting the mixture at the bottom of the tube. If the mixture is inflammable then the flame can be seen moving up the tube; if no flame is observed or just a flicker then the mixture is not combustible. The observed limits will vary depending on the size and shape of the glass tube; the larger the tube diameter the wider the limits although this affect stops with tube diameters above 7.5cm. The influence of tube diameter on flame propagation is believed to be due to the change in heat losses to the tube walls.

Direction of mixture flow also affects the speed of flame propagation through a tube or pipe; also dilutants such as nitrogen and carbon dioxide, if present in the gas air mixture affect the limits of inflamability. Temperature changes of the gas/air mixture also has an affect on the limits of inflamability; as would be expected, increasing the mixture temperature has the effect of widening the limits.

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