Flame Propagation Mechanisms in Hybrid-Mixture Explosions

Hybrid mixtures, i.e. mixtures of flammable gas and combustible dust, often occur in chemical and processing operations like paint manufacturing, pharmaceutical industries, coal mines, and grain elevators. Research into dust- and gas-air explosions has been extensive so they are well understood, but hybrid-mixture explosions have not been as extensively investigated so data on them is rather scarce. They pose a significant hazard because their combined Lower Explosion Limit (LEL) has been shown to lie below that of the single components.

The overall objectives of this research grant are to: 1) elucidate the mechanisms of flame propagation in hybrid mixture explosions; 2) better understand and quantify the factors that influence the minimum explosible concentration (MEC) of dust flames and the LEL of hybrid mixtures; and 3) predict the boundary between explosion and no-explosion.

Italian researchers identified five distinct regimes for hybrid mixtures: 1) dual-fuel explosions where the gas concentration is above its lower flammable limit (LFL) and the dust concentration is above its MEC; 2) dust-driven explosions where the dust is above its MEC and the gas is below its LFL; 3) gas-driven explosions where the dust is below its MEC and the gas is above its LFL; 4) synergistic explosions in which both dust and gas are below their lower limits; and 5) a region where both dust and gas are below their lower limits and no explosion occurs.

Both gas- and dust flame propagation have been extensively studied. However, no comprehensive study of hybrid-mixture flame propagation in each of the four explosion regimes dust-driven, gas-driven, dual fuel, and synergistic has been undertaken. Likewise, no definitive explanation of synergistic explosions has been given nor has the boundary between synergistic explosions and no explosion been conclusively defined. The MEC of dusts is complex since it depends upon the particle size distribution, nature of the dust, turbulence level in the dust cloud, and ignition type and strength as well as the equipment in which it is measured. To further complicate matters, the lower explosion limit (LEL) of hybrid mixtures also depends upon the nature of the gas in addition to all the factors that influence the MEC of dusts.

Flame propagation mechanisms for the different regimes will be investigated using two novel techniques. The first is a unique explosion chamber that creates a hybrid mixture of varying gas/dust concentrations close to the LEL of the mixture by sublimation, i.e. where the vapour turns directly to solid just like when a CO2 fire extinguisher is discharged and forms dry ice. The secondly is a special spray chamber to form hybrid mixtures by rapid cooling of liquid droplets in a cold flammable gas. A clearer understanding of hybrid-mixture explosions will lead to better estimates of risk and improved mitigation protocols thereby leading to enhanced safety in process plants.