In a factory producing biofuels (bioethanol) from the fermentation of corn, a dust explosion occurred at 12 am at the spent grain dryer – fibre fractions generated during the production of grain alcohol and recycled into animal feed. The explosion cracked the inlet joint of the superheated condensation in the drying tube and caused several discharge traps to open. The dryer’s safety systems (water injection and steam into the drum) were activated automatically to quickly limit this incident. Emergency services went to the site but did not have to intervene. The operator notified the Classified Facilities Inspectorate, which reached the site at 3 pm.

Property damage was minimal. Two employees suffered minor injuries. The dust-laden installation cooling water was collected and treated.

The dryer was in the process of restarting following a stop due to a product jam at the end of the line. The non-supply of wet spent grains from the hopper of the dryer then caused an air intake and excess of oxygen in the drum, which was normally lacking in O2. The hopper scale, which was faulty, did not activate the safety devices and the burner ignited despite the lack of product, causing spontaneous combustion of the wet spent grains inside it followed by the explosion. According to the operator, the malfunction of the weighing system was due to a deviation in the calibration of the hopper’s three weighing scales: it was empty, whereas the control room console indicated the presence of 200 kg of spent grain; the level alarm for which the threshold limit was set at 100 kg did not activate. In addition, the explosion occurred at a temperature below the automatic control threshold of the two temperature sensors.

The automatic controller of the drying unit was modified: a lowering of the threshold limit for the automatic control by the thermal sensors; automatic scanning of the steam before each firing of the burner to remove excess air; restarting of the burner authorised from 1 tonne of spent grains in the hopper, with a stop threshold of 500 kg; control of the twin-screw intensity at the hopper outlet with the shutdown of a burner if too low; a minimum flow intake of the horizontal decanter to obtain the wet spent grains; an increase in the maintenance frequency and replacement of joints between gas/gas exchangers and rotary dryers; and a new frequency for the weighing scale calibration. The operator studied the technical feasibility of measuring the O2 concentration and moisture in the dryer in order to control the atmosphere of drums and prevent deviations. The Inspectorate requested a revision to the zoning of this drying unit with respect to ATEX regulations, while the Labour Inspectorate asked to conduct a “HAZOP” study on the same unit to define additional safety needs. This accident was reported in the local press.