Methane Explosibility Curve
If you want to find out the flammability of methane-air mixtures, explosibility curve of methane is extremely useful.
There is proper oxygen balance with methane content of 9.8 % by volume in the air. Because of this reason, at this concentration of methane i.e., 9.8 %, the mixture is most explosive.
Another important point which you should remember is that, the explosions caused due to methane gas are not as violent as those of commercial explosives. This is because of the density difference.
The density of methane-air mixtures is around 1.15 kg/m3.
The density of gun powder is 1000 kg/m3 and for nitroglycerine it is 1600 kg/m3.
The explosible range for methane in air is 5.4 % to 14.8% by volume. As told earlier, most explosive mixture of methane gas occurs at 9.8 %.
The lower flammable limit of methane gas is almost constant whereas the upper limit reduces with decrease in the oxygen percentage in the air.
The diagram shown below was initially developed by scientist H. F. Coward in 1928. This diagram helps us to find the flammability of methane-air mixtures as the composition changes. It is also popularly known as Coward's Diagram.
Some of the salient points to be observed from the explosibility curve of methane are:
In region A, the mixture is not flammable. However, it may become flammable if further methane is added to it. This is possible, when that particular portion of the mine is sealed off.
In region B, the mixture is explosive with a minimum nose value at 12.2 % oxygen (The nose limit represents the most dangerous mixture as this mixture requires the largest amount of excess nitrogen to render it non-flammable. The ratio of the percentage of excess nitrogen to the percentage of a combustible gas at the nose limit is called ‘Inert Factor’).
Region C and D illustrate mixtures that may exist in sealed areas at all.
A mixture in region C will become explosive if the mixture is diluted by incoming air.
In region D, the mixture can’t become explosive.
Region E is impossible mixture in air.
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