Dust explosions: Fundamentals

Abstract

The phenomenon named dust explosions is easy to envisage: Any solid material that can burn in air will do so at a rate that increases with increasing degree of sub-division of the material. Therefore, if a fine dust of a combustible material is whirled up as a dense cloud in air and ignited, a flame will propagate rapidly through the cloud. Heat is then liberated at a high rate due to the reaction: fuel + oxygen → oxides + heat. The main groups of materials that can give dust explosions are: natural organic materials (grain, wood, linen, sugar, etc.), synthetic organic materials (plastics, organic pigments, pesticides, pharmaceuticals, etc.), coal and peat, and metals (aluminum, magnesium, titanium, zinc, iron, etc.). Flame propagation in a dust cloud occurs by heat conduction and convection (turbulence) and to some extent by thermal radiation. In extreme cases dust clouds can even detonate with supersonic flame front speeds, most often due to a deflagration-to-detonation-transition (DDT). A basic condition for a dust cloud to be explosible is that the dust concentration is within certain limits. Typical ranges of explosible dust concentrations are several orders of magnitude above typical dust concentrations in unpleasant dusty atmospheres in factory work rooms, causing industrial hygiene concerns. Therefore, dust clouds in industry in which dust explosions can be initiated are nearly always located inside process equipment (mills, mixers, screens, dryers, cyclones, filters, bucket elevators, hoppers, silos, aspiration ducts and pipes for pneumatic powder transport). A rapid combustion of dust clouds in such confined spaces causes a rapid pressure rise inside the confinement, which can be destructive to process plant and people. This kind of initial explosion, taking place inside some process equipment, is called a primary explosion. However, in order for such an explosion to occur, the dust cloud has to be ignited by some ignition source. The list of known sources includes: smoldering dust nests, open flames, hot surfaces, heat from mechanical impacts, electric sparks and arcs, electrostatic discharges, jets of hot combustion products, shock waves, radiation of light. It is important to emphasize that most often, the most severe consequences of dust explosions are not caused by a primary explosion, but by secondary explosions outside the initial process unit. Secondary dust explosions are caused by entrainment of dust layers outside the primary explosion enclosure, by the blast wave from the primary explosion. The resulting secondary dust cloud is then ignited by the primary dust flame following the blast wave. If the air in which the dust cloud is dispersed contains a fraction of a combustible gas or vapor, the mixture is named a hybrid mixture. Initially focus was primarily on mixtures where the combustible gas/vapor fraction was smaller than the lower explosibility limit (LEL) in air of the gas/vapor itself. Small percentages of a combustible gas in the air most often reduce the minimum ignition energy and increase the explosion violence of the explosible cloud significantly. Also, in some cases the minimum explosible dust concentration is reduced.