A chemical reaction process began by feeding a combustible fine powder into a reactor through a charging hopper. The reactor and the charging hopper were fitted with a nitrogen inerting system to prevent dust explosions. Surprisingly, a dust explosion occurred when feeding the powder into the reactor from a flexible intermediate bulk container (FIBC). On-site investigations found that the design of the inerting system allowed a sufficient amount of air to enter the charging hopper. There were no mechanical, electrical, and thermal sources of ignition inside the closed reactor-charging hopper system before the explosion. Electrostatic discharges appeared to be the ignition source; however, the ignition source could not be identified without further analysis by conducting tests. Therefore, various dust explosion and electrostatic tests were conducted to determine (1) the minimum amount of electrostatic discharge energy required to ignite the dust cloud and (2) the likelihood of various types of electrostatic discharges to ignite the dust cloud under the existing design and process conditions. The test results showed that the powder suspended in air at the optimal dust concentration requires < 3 mJ of spark energy to ignite. A glass-lined pipe at the bottom of the charging hopper was found to be the most likely ignition source. Laboratory tests demonstrated that the ignition was the result of a propagating brush discharge from the glass-lined pipe. (c) 2005 American Institute of Chemical Engineers Process Saf Prog 25: 58-63, 2006.