The decomposition of ammonium nitrate under fire conditions – a review of ammonium nitrate thermolysis

Since the discovery of ammonium nitrate (AN) in the late 17th century, its behavior under heating has been of interest to the scientific community. Following its introduction as a commercially important chemical in fertilizers and explosive formulations, but also for the production of nitrous oxide (anesthetics), the safety field also became attentive to this as a consequence of the underlying explosion risks associated with handling, transport and storage of the material, especially when subjected to elevated temperatures. Evidently, ammonium nitrate under fire exposure is now a well-established explosion hazard and has been responsible for some of the greatest industrial accidents to date. Accordingly, the investigative literature on AN thermolysis is immense, of which this review endeavors to present a comprehensive and contemporary compilation. Morphology, decomposition pathways and the shock-sensitivity of AN are influenced by elevated temperatures and heating rates. However, researchers have never been able to induce explosion in pure AN under controlled conditions by heat alone. Here, other factors play an important part. Of these, confinement stands out as a crucial determinant – elevated pressure increases gas-solid phase interactions under decomposition, promoting exothermic behavior, even for pure AN. This is especially an important feature to consider if bulk amounts of AN is subjected to fire, as larger heaps are likely to self-confine. Contaminants and combustible material are also well-known contributors to the explosion risks of AN. As more realistic and complex thermoanalytic and computational technologies have been developed, a greater understanding of the mechanisms behind the thermolysis of contaminated AN has emerged. For most contaminants, detrimental effects such as thermal run-aways and explosions seem to depend on a certain amount of initial heating and confinement. Due to the prominent endothermic behavior of AN dissociation, however, AN is in most cases a relatively stable compound even when heated. This means, that strictly defined conditions are necessary for achievement of deflagration-to-detonation transitions or just deflagration alone. As the critical determinants for such transitions are yet to be established, heat exposure is still the most obvious, generic and critical cause of explosion in any fire related AN explosion.