Fire Safety Science Digital Archive

Ignition and extinguishment of sodium fires was investigated in air diluted by nitrogen using a coaxial burner. Oxygen concentration and temperature were changed from 0.5% to 21% and from 110ºC to 700ºC, respectively. In undiluted air, ignition occurred near the crust of sodium oxides at temperatures below 600ºC. At temperatures higher than 625ºC, ignition was observed in a vapour layer and a conical diffusion flame appeared on a sodium pool at 700ºC. The flame was observed for only one second after the ignition before a dense cloud from burning sodium obscured vision. In this study, the lowest ignition temperature of sodium was 115ºC under an air flow rate of 5 L/min. There were extinguishing limits of oxygen concentrations for burning sodium. The limits decreased linearly with increasing sodium temperature up to a temperature of 700ºC. Anomalous ignition was found in a region of oxygen concentrations lower than the limits. One example was an observed ignition occurring in the region of oxygen concentrations below 3% and temperatures above 400ºC. The ignition was observed on the crust of sodium oxides after relatively long induction periods, but no stable combustion followed. Another anomaly was re-ignition of a cooled combustion residue exposed to fresh air. Such ignition was even observed below 50ºC. The oxidized sodium prepared in a low oxygen atmosphere also ignited at the same temperature. Both types of anomalous ignition were attributed to production of sodium peroxide. Thus, the extinguishment of sodium fires by nitrogen requires consideration of methods to block the production of sodium peroxides.