Inside a chemical plant, an electrostatic filter for removing dust, containing 696 plates sized 17.5 m x 7.5 m x 18 m on a 116-MW coal-fired boiler, exploded. The accident occurred during a restart procedure after a two-week shutdown for maintenance; it resulted from the accumulation of 440 m³ of gas inside the boiler following failure to close the backup burner feed line (300 m³/hr), which had been opened 1 hour and 20 minutes before the accident and which was discovered 90 minutes after the accident. One manual valve and two automatic gate valves stayed open (no visual control of valve position, automated shutoff of gate valves while continuing to inject compressed air, warning message ignored). The explosion caused 1 death and 8 injuries among the technician staff. Shattered window panes and projectiles were observed up to 250 m away. Total property damage was estimated at 20 MF.