An unexpected explosion while fumigating a grain silo: It wasn't the dust

Abstract Large‐scale fumigation of grain silos is a strategy to mitigate infestation of the bulk material. A common fumigant precursor for grain processes is aluminum phosphide, which exothermically decomposes in the presence of moisture (from the grain or air) to the toxic fumigant: phosphine gas. To overcome the hazards of exothermic decomposition and phosphine exposure, aluminum phosphide pellets can be metered into the silo's grain feed to distribute them throughout the grain more evenly. This approach decreases the decomposition rate of aluminum phosphide, better distributes heat generation to avoid igniting grain, and sustains a phosphine gas concentration well below the pyrophoric concentration—all while achieving the fumigation objectives. During a fumigation activity, a large explosion occurred within a group of silos at a grain elevator complex. Dust explosions are a common hazard for grain handling facilities, but this incident was caused by the autoignition of a phosphine gas cloud inside the conveyor tunnels. It was only through post‐incident evaluations of the grain flow dynamics and pellet addition activities that a gap between the desired pellet distribution and the incident conditions was identified. As a result, a new insight into bulk grain handling and safe fumigation was developed.