Comparison of behavior and microscopic characteristics of first and secondary explosions of coal dust

Differences in behaviors of primary and secondary explosions of a bituminous coal dust were investigated using a standard 20 L spherical test vessel. The aim was to compare the explosibility and microstructures of residues of first and secondary explosions to obtain reference data for investigation of accidental explosions. The residues were analyzed by energy-dispersive spectrometry, scanning electron microscopy, and automatic surface area and pore analyzer. The first explosion showed a higher rate of heat release and higher total heat release, which indicated greater severity of explosion. The spatial distributions of C, Cl, and Si in the residues were uniform, whereas those of Ca, O, and Al occurred as discrete points. The carbon content decreased and Ca, Al, Si, Cl, and Fe contents increased as the reaction proceeded. Unexpectedly, the oxygen content was reduced by 4.41% during the primary explosion, but increased by 9.69% during the secondary explosion. The scanning electron microscopy picture of the secondary explosion residue exhibited a higher degree of fragmentation and a more developed pore structure. All isotherms of dust samples were of Type II and the shapes of the hysteresis loops were of Type H3 or Mixed Type H3-H4. Change of fractal dimension D2 illustrated increasingly complicated pore structures with advance of the explosion process.