Silo explosion from smoldering combustion. A case study

Smoldering combustion in power/dust deposits constitutes a significant problem in some process industries. Numerous bulk materials produce heat due to different biological and oxidation processes. Spontaneous heating occurs if the rate of heat generation by an exothermic process is fast relative to the rate of heat loss to the surroundings. Such phenomena are particularly dangerous in bulk materials storage equipment (e.g. silos, bins, hoppers, bunkers) where larger fire and dust explosions may be ignited by smoldering material. The present case study concerns an explosion that occurred in a silo containing sawdust and wood chips. Firefighters were called to extinguish a fire in the silo. During their intervention an explosion occurred. Four firefighters were injured and one of them died some months later, as a result of the explosion; the explosion caused also significant damage to the silo and minor damage to the adjacent buildings. This paper describes the investigation into the cause of the silo explosion. The CFD code FLACS was used to evaluate the consequences associated with gas explosion of pyrolysis gases produced by smoldering combustion. Simulation results showed that the most probable scenario was the explosion of pyrolysis gases accumulated in the upper part of the silo where bag filters were present. The venting system was inefficient in mitigating the explosion, due to the corrosion of metal bolts connecting the silo walls. Another factor could have been the position of relief hatches, which were located in front of the filter elements, thus not meeting the required standards.