Page 410 - Advanced Mine Ventilation
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Gas and Dust Explosions 379
Vapor: It emanates from a substance that is liquid at standard temperature and
pressure.
Flammability Limits: It is variously called flammable, explosive, or explosion
limits. In some books, such limits refer to flammability limits of gas and vapors
only. Dust is explosive, and it also has an upper and a lower limit. However, many
books use flammability and explosive to mean the same thing.
Explosion: An explosion is the result, not the cause of rapid expansion of gases
caused by a physical change or a chemical reaction.
Deflagration: It is a reaction that propagates in the unreacted material at a speed that
is less than the speed of sound.
Detonation: It is an exothermic reaction that propagates in the unreacted material at
a speed greater than the speed of sound.
23.1.2 Flammability Limits of GaseAir Mixtures
Coward and Jones [3] produced a flammability curve for methane and air as shown in
Fig. 23.1. The lower and upper limits of methane in air are 5% and 14.5%, respec-
tively. It is famously known as “Coward’s Diagram.” It clearly shows that if methane
concentration is less than 5% or oxygen concentration is less than 12%, the mixture
cannot be flammable. Mixtures containing more than 14.5% methane can become
explosive if mixed with air. The flammable/explosive mixture is shown as a triangle.
In a typical mine on fire, the mine atmosphere may contain methane, hydrogen, and
carbon monoxide as combustible gases and nitrogen and carbon dioxide as inert gases.
Zabetakis [4] developed a flammability diagram for such gas mixtures. All combus-
tible gases were combined and called
Effective Combustibles ¼½CH 4 % þ 1:25 H 2 % þ 0:4 CO% (23.1)
The factors 1.25 and 0.4 are the ratios of lower flammable limit for CH 4 to that of H 2
and CO, respectively. Likewise, all inerts were combined and called
Effective Inerts ¼ ½Excess N 2 % þ 1:5%CO 2 (23.2)
where excess N 2 is defined as
Excess N 2 % ¼½100 CH 4 % Air% (23.3)
Fig. 23.2 shows a flammability diagram for all these five gases.
A new term, composition point, R is defined as
CH 4 %
R ¼ (23.4)
CH 4 % þ H 2 % þ CO%
