Opposite effects of typical solid inertants on flame propagation in Mg dust clouds versus dust layers

Abstract

Adding solid inertants to combustible dust is an effective means of reducing dust explosion risk through both preventive and mitigative measures. However, since dust layers and dust clouds differ in their flame spread mechanisms, they may also differ in their response to solid inertants. In this paper, inerting efficacy of four commonly used solid inertants in Mg dust layers and dust clouds was investigated. As concentration increased, all four inertants effectively suppressed burning in Mg dust clouds. However, CaCO3, NaHCO3, and SiO2 greatly increased the fire hazard in Mg dust layers. CO2 produced by thermal decomposition of CaCO3 destroyed the MgO layer formed on the surface of Mg dust layers and caused more burning of Mg vapor in the gas phase. The water vapor produced by NaHCO3 reacted with Mg to generate hydrogen which intensified combustion. SiO2 emitted heat through a substitution reaction with Mg powder to thrust the mixed dust layer upwards, causing violent combustion similar to that in dust clouds. As inertant proportion increased, the fire hazard in dust layers was initially enhanced but was later decreased, although the fire hazard at 70% inertant remained much higher than that with pure Mg dust. Only NaCl, which is chemically stable and does not react with metal dust, can effectively inert both Mg dust layers and dust clouds. Research on the use of solid inertant technology to prevent or mitigate dust explosions should therefore consider the impact on both dust clouds and dust layers.