While ammonium nitrate is noncombustible, it has a melting point near 170oC. Once heated past its melting point by an external heat source (e.g. a nearby fire), the decomposition reactions are all exothermic thus they all contribute to heat generation, further melting the solid ammonium nitrate, and presenting possible runaway conditions. This may lead to the explosive detonation reaction seen in the port of Beirut, the port of Tianjin, and West, Texas.
Click here to watch the “Unraveling the Mysteries of AN Detonations: Why Beirut, Tianjin, and West Texas should not occur” webinar.
The resultant release of energy may generate a temperature rise of approximately 1,800°C to 2,000°C (3,272°F to 3,632°F) in the immediate vicinity. The rapid volume increases from near instantaneous expansion of gasses will typically produce a shock wave that can result in considerable area damage.
Despite the potentially hazardous decomposition of ammonium nitrate, large scale burn tests involving several tons of ammonium nitrate have not resulted in an explosion decomposition. FM Global reports that there have been a number of fire incidents involving quantities of ammonium nitrate exceeding 50 tons, which only burned and did not result in an explosion. Data suggests that detonations are promoted by severe shock or by heating with confinement.
The sensitivity of ammonium nitrate to explosion increases with the presence of organic material such as oil, sulfur, grease, charcoal, and combustible dust.
So, what can we do to prevent and mitigate these events? Find out in the third part of our ammonium nitrate blog.
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