Metal dust combustion: Explosion limits, pressures, and temperatures

Abstract

Measurements are reported for the explosibility behavior in air of a variety of metallic and other elemental dusts. The data are useful for evaluating the hazards involved in their manufacture, transport, storage, and use. This study was also designed to help resolve, a fundamental issue of whether the combustion reactions in those dust flames proceed homogeneously in the gas phase or heterogeneously at particle surfaces. Data are reported for 14 dusts: from the more volatile metals such as magnesium and zinc, to the refractory metals such as tantalum, tungsten and niobium. Also included are some intermediate metals: hafnium, titanium, aluminum, iron, lead and copper; as well as the nonmetallic elements: boron, silicon, and carbon. Explosion pressures, rates of pressure rise, and continuum radiation temperatures were measured as a function of the dispersed dust concentration, using the Bureau of Mines 20-L explosibility test chamber and strong pyrotechnic ignitors. The lean limit fuel equivalence ratios varied from as low as 0.13 for the more reactive and volatile dusts to well in excess of 1.0 for the less reactive and less volatile ones. The copper and lead dusts were nonexplosible. For some dusts, preliminary data on particle size dependencies were obtained. The data are analyzed in terms of calculated adiabatic flame temperatures and the equilibrium vapor pressures or nonequilibrium vapor fluxes at those temperatures. An inverse correlation is observed between the measured lean limit equivalence ratio and the calculated equilibrium flame zone volatility for the dust. The correlation is analogous to one previously established for the carbonaceous dusts. Estimated absolute vaporization fluxes for the more flammable dusts are adequate to support a homogenous mechanism. Some refractory dusts, however, exhibit marginal (but finite) explosibility for the finer particle sizes even though their estimated vaporization fluxes appear to be too low to support a homogeneous mechanism.