Aluminum Phosphide-Based Fumigants as an Ignition Source


John L. Schumacher, MChE, PE, CFI, CFPS
Zachary J. Jason, PE, CFEI
Advanced Engineering Investigations Corporation, USA

Presented at International Symposium on Fire Investigation, 2012


Raw agricultural commodities, such as corn, soybean, rice and wheat, are typically stored in bins and silos prior to shipment. During storage, it is often necessary to protect the commodities from damage by insects and pests. A common protection method utilized is the addition of solid fumigant pellets or tablets to the commodity.

One of the most common solid fumigants employed is a blend of aluminum phosphide, ammonium carbamate and other inert ingredients. Aluminum phosphide reacts with atmospheric water and moisture in the commodity based on the following equation:

AlP + 3H2O = Al(OH)3 + PH3 + Heat

The reaction yields phosphine gas (PH3), which is highly toxic to insects, pests and humans. The reaction is exothermic, which means heat is generated alongside the other products. Phosphine gas has a lower flammable limit (LFL) of about 1.8% gas in air and can ignite spontaneously at concentrations above the LFL. The ammonium carbamate is added to the mixture to reduce the potential fire hazard by generating ammonia and carbon dioxide, which act as inerting gases. The carbon dioxide reduces the tendency of phosphine to auto-ignite in air. The decomposition reaction is as follows:

NH2COONH4 = 2NH3 + CO2

Improper application of the fumigant tablets or pellets can lead to fires. This paper provides basic product information, and discusses the chemistry, application methods, previous testing, and ignition scenarios associated with solid fumigants containing aluminum phosphide. A case study of a fire that occurred in a metal grain bin containing wheat will be presented.

Presented at International Symposium on Fire Investigation, 2012