An Unexpected Explosion While Fumigating a Grain Silo – It Wasn’t the Dust

Large scale fumigation of grain silos is a sometimes necessary activity to mitigate infestation of the bulk material. Aluminum phosphide is a common fumigant chemical, which is added to grain in a silo. Aluminum phosphide reacts with moisture in the grain or air to liberate heat and decompose to phosphine gas, which is highly toxic and pyrophoric at high concentrations. Aside from toxicity, a recognized silo fumigation hazard is the self-heating and spontaneous ignition of grain inside silos from piled aluminum phosphide pellets. To overcome these adversities, aluminum phosphide pellets can be metered into grain as it is fed into a silo. In this way, the pellets are distributed throughout the grain. The pellets decay slowly, heat is distributed and kept below the grain ignition temperature, and the concentration of phosphine is intended to remain well below the pyrophoric concentration while achieving the fumigation objectives.

This manuscript will discuss the investigation, findings, and lessons learned from a large explosion while fumigating 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 autoignition of a phosphine gas cloud inside the conveyor tunnels. The fumigation plan, the pellet addition, and pre-job risk analysis did not hint at the hazard of phosphine gas accumulation and concentration above the lower explosive limit. It was only through evaluation of the grain flow dynamics in combination with the grain elevation and conveyor configurations, and pellet addition activities that a gap between the desired pellet distribution and the incident conditions were elucidated. As a result, new insight into bulk grain handling and safe fumigation was developed.