ABSTRACT
Electrostatic spark is a common ignition stimulus for both condensed and gasphase ƒammable compounds (e.g., Glor 1985; Glor 2003; Glor 2005; Walther and Schacke 2008). Multiple tests, standards, and evaluation methodologies have been and continue being developed to investigate ignition behavior of different substances struck by a spark (Dahn and Dastidar 2003; Smallwood 2005; Perrin et. al. 2007; von Pidoll 2009). Electrostatic discharge (ESD) ignition of ƒammable gases was studied in great detail, motivated by both safety requirements for handling ƒammable gas mixtures and the convenience of using spark discharge as a controlled energy igniter for fundamental ƒammability studies. Effects of electrode materials, electrode shape, discharge duration, and other similar parameters have been quanti ed and reported. Reviews by Mellor et al. (1990), and Mellor and Baker (1994) described these trends and suggested that a similar systematic study would be necessary to establish a scienti cally sound test of ESD sensitivity of powders. Ignition of various aerosolized powders (or ƒammable dust clouds) has been studied by many investigators (e.g., Ryzhik 1978; Ryzhik et. al. 1980; Kim and Colver 1990). Important in their own right, such studies, however, do not provide insight for ignition mechanisms and ƒammability limits for powders stored in containers or spread on solid surfaces, while both situations are ubiquitous in many industries. On the other hand, experimental tests of ESD ignition sensitivity for nonaerosolized powders are among the most commonly used safety assessments for powdered materials in the areas of agriculture (Kao and Duh 2002), food (Glor 1985), textiles (Wu et al. 2003), pharmaceuticals (Tunnicliffe and Thomson 2003), plastics (Marmo and Cavallero 2008; Gao et al. 2006), metallics (Senecal 1991; Matsuda and Yamaguma 2000; Ebadat and Pilkington 1995), and, of course, energetic components (Zeman and Koči 2000; Skinner et al. 1998). Typically, a spark is generated by a high-voltage capacitor discharging over a gap between a sharpened electrode and a powder bed. The sharpened electrode can approach the surface or be xed at a certain distance from it. The minimum capacitor energy at which the powder ignites is speci ed as the minimum ignition energy (MIE), a parameter de ning the sensitivity of a powder to ESD ignition stimulation (Glor 1985, 2003). It has been suggested (Skinner et al. 1998) that the spark represents primarily a thermal source capable of raising the temperature of a ƒammable powder above the point at which thermal runaway occurs. However, it remains unclear how the spark heats the powder particles, what portion of the spark energy is being transferred to the powder, and by which mechanism. For example, the spark’s plasma can heat the powder surface directly while the current of the spark discharge can result in Joule heating distributed in the powder volume, along the current path. It remains unclear whether the polarity of the spark discharge is a factor affecting the ignition energy. The transport properties of powders including their thermal and electrical conductivities are initially governed by the respective contact resistances between the particles.
