AT 7:50PM AN IGNITION OF COAL DUST OCCURED WITH THE #3 KILN COAL FIRING SYSTEM. THE PRESSURE RELIEF VENT WAS RUPTURED AS IS DESIGNED TO DO. THE IGNITION SOURCE IS NOT KNOWN AT THIS TIME. A FUEL BLEND OF RECYCLED PAPER PRODUCT & COAL WAS BEING USED. NO INJURIES OR PROPERTY DAMAGE.