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Limits of Inflammation

It is a matter of common knowledge that a room may smell quite strongly of coal gas without its being dangerous to strike a match within it. When this observation is pushed to its logical conclusion it is evident that a certain minimum quantity of the coal gas must be present for its inflammation to be self-supporting. This minimum quantity is termed the lower limit of inflammation of the combustible gas, and is influenced by two factors:
  1. The initial source of heat should be of sufficient volume, intensity, and duration to raise the layer of gases in its immediate vicinity to a temperature at least as high as the ignition temperature of the mixture.
  2. The heat contained in the products of combustion of this first layer must be sufficient to raise the adjacent layer to its ignition temperature - and so on.
If too low a proportion of combustible gas is present, only a small quantity of heat per unit volume of mixture is liberated when the layer surrounding the initial source of heat is inflamed, and the products of combustion have to impart heat to a considerable volume of " inert " gases. The number of collisions between molecules of combustible gas and of oxygen that are chemically fruitful is therefore small. Such collisions, resulting in combination, will occur only in the neighbourhood of the initial source of heat, around which an aureole or "cap " will form of a size dependent on the nature and quantity of the combustible gas present.

Upon increasing the proportion of combustible gas, not only is a greater quantity of heat evolved per unit of mixture, but there is a smaller volume of inert gases present to absorb it; ultimately, therefore, a point may be reached when the amount of heat contained in the products of combustion of any given layer is just sufficient to raise the adjacent layer to its ignition-temperature. Flame is then propagated from layer to layer throughout the mixture without any necessity for the continued presence of the source of heat which started the inflammation, and the mixture either inflames or explodes according to the rapidity of the propagation.

Consideration will show that there must also be a higher limit of inflammation of the combustible gas, for if its proportion over that of the oxygen be largely increased, the excess will function as a diluent, absorb heat, and tend to retard flame propagation.

Since gaseous combustion is a reciprocal phenomenon, it follows that the amount of oxygen present in this latter case is the minimum quantity supporting combustion, and may be termed the lower oxygen limit of inflammation.

It may happen that the lower oxygen limit is above that which can be realised when the combustible gas is mixed with air. In that case the gas will not normally burn in air, but may do so in an atmosphere enriched with oxygen. Ammonia vapour is a typical example. If a lighted taper is applied to a jet from which this gas is escaping, the characteristic livid flame appears side by side with the flame of the taper; but it at once dies away upon removal of the latter. If, however, the jet is surrounded by air enriched with oxygen the flame of ammonia gas becomes'self-supporting, and continues to burn even when the taper has been withdrawn.

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