At 6:15 a.m., a fire was detected when a 150 m³ silo containing 150 t of activated carbon was being loaded into a production plant. No smoke or toxic emissions were observed. The fire brigade used water hoses to cool down the silo from the outside (internal silo temperature = 380 °C at 10:30 a.m.), the fluids were shut off. The silo was inerted in the late afternoon with nitrogen that had been trucked in. The Inspection authorities for classified facilities went to the site. The sub-prefecture, the Regional Health Agency and the County Territorial Directorate were informed. The investigation showed that the 3 temperature sensors installed in the silo never exceeded the 120 °C alarm threshold and that the temperature measurements had been stable for several days.

The internal investigation focused on 2 possible phenomena explaining inflammation:

• a product stored in the silo: activated carbon dust (flammable solid), subject to spontaneous ignition;
• the effect of pressure on the activated carbon inside the silo. This could have fostered the outbreak of fire, although no increase in pressure was recorded in the silo prior to the accident.

The facility operator has taken the following preventive measures:

• to prevent the first phenomenon, the silo no longer receives dust. Dust is now stored directly in IBCs. The volume of residual dust in the silo shall be limited to 25 t. The storage in IBCs has led to a reduction in the amount of dust formed by modifying the activated carbon grinding system, which is the primary source. This system is equipped with a system for recycling dust taken from the mill’s dust removal system without intermediate storage;
• the pressure sensor in the pipe of the silo’s dust suction circuit was moved to avoid the overpressures that caused the 2nd phenomenon. This sensor was poorly located, at the level of the dust collector itself. Its optimal position is in fact upstream of this equipment. At the same time, the silo overpressure warning system was optimised to trigger an overpressure alarm in the PLC’s interface and automatic adjustment of the intake flow rate until the dust collector stops.