Pollution
Humain
Environnement
Economique

In an alfalfa drying facility, a sparks detector placed downstream of the drying oven and cyclone separator detected sparks and automatically closed the gas supply line. The technician manually shut down the alfalfa feed and activated the gas extraction system to purge the oven, thereby preventing a fire outbreak. Afterwards, smoke was seen on the building roof and fire-fighters were called to the scene; with the help of infrared detectors, they identified hot zones on the roof, whose metal cladding had been partially removed to allow sprinkling caked dust between the steel tank and the former asbestos cement roof. The oven and cyclone were also sprinkled. The fire was considered extinguished 3 hours following shutdown of the installation, which was restarted once fire-fighters had completed a 30-min monitoring round.

After 2+ hours of resumed operations, smoke was again observed on the roof, this time by the technician making his rounds. Fire-fighters returned for a 2-hour battle and, out of precaution, performed 2 rounds of monitoring 3 hours apart. According to the facility operator, this 2nd alarm was not related to the dryer since no spark had been detected; instead, it was due to residual hotspots on the roof enabling the dust to re-combust. The initial incident was caused by alfalfa ignition in the oven that produced incandescent particles subsequently discharged via the chimney onto the roof, in penetrating between metal sheets and the former roof and then igniting the layer of dust that had accumulated over the years. This incident could be attributed to both accumulated dust and the season’s exceptional drought and sunshine conditions, which reduced the alfalfa humidity to extremely low rates of 50-55%. The dryer had been set for typical humidity rates of 65-75%, with temperatures of 980°C at the input and 140°C at the output. The overheating of alfalfa had triggered ignition.

For the site to continue running, the operator proposed: lowering temperatures to 800°C at the input and 95°C at the output; introducing a chimney sprinkling device servo-controlled to spark detection; and cleaning all dust within the installations. The Inspection Authorities for Classified Facilities recorded a considerable amount of dust throughout the site and recommended that the Prefect impose administrative penalties.