Estimation of LOC (limiting oxygen concentration) of fuel–air–inert mixtures at elevated temperatures by means of adiabatic flame temperatures

The limiting values of fuel concentration in a flammable fuel–air mixture are the LEL (lower explosion limit) and UEL (upper explosion limit). The addition of an inert component to fuel/air mixtures determines the increase of LEL and decrease of UEL, until these values finally merge at the inerting point. The maximum oxygen amount of a non-flammable fuel–air–inert mixture is the LOC (limiting oxygen concentration), an important safety characteristics. The investigation of a comprehensive set of flammability data at elevated temperatures and ambient pressure taken from literature sources was made for systems containing nitrogen, carbon dioxide and water(vapour) as inert components, at 100°, 200° and 250 °C. The adiabatic flame temperatures at LEL (CAFTLEL) and LOC (CAFTLOC) were calculated by taking into account the dissociation of gases within the flame. A linear correlation of CAFTLOC versus CAFTLEL was empirically derived for the examined systems. The slope and intercept of the correlation are dependent on temperature and on nature of the inert gas. The correlation allows the development of a simple procedure for estimating LOC, when the LEL of fuel–air and the equivalence ratio of the fuel–air–inert mixture at the inerting point are known. Knowing the scarce information concerning the flammability of fuel–air–inert mixtures at temperatures higher than ambient, the proposed procedure brings about an useful tool for estimation of LOC.