Thermodynamic Analysis of the Flammability Limits of Fuel, Oxygen and Inert Mixtures

In Sheldon's [1] presentation of the flammability limit of a methane-oxygen-nitrogen mixture, he did not consider the effect of other inert gases. In this paper, Sheldon's idea is generalized by considering the presence of inert gases. Three important parameters appear in the conservation of mass equation describing the mixture; the molar stoichiometric ratio of oxygen to fuel, the molar ratio of inert gases to oxygen and the molar ratio of the excess gases to the stoichiometric oxidizing mixture. Other literature data on the lower flammability limit for a number of hydrocarbon compounds in both oxygen and air was utilized to show that lean limit values are almost identical. The flammable envelope is defined by a region in a plot of the reciprocal of the molar fuel concentration versus the molar excess gases to stoichiometric oxidizing mixture ratio. A similar region is defined by a plot of the molar inert oxygen ratio versus the molar excess gases to stoichiometric oxidizing mixture ratio. It is shown that the flammable region of any fuel can be defined by considering the flammability limits of a fuel in pure oxygen.