Self-Sustaining Decomposition of NPK Fertilisers Containing Ammonium Nitrate

Keywords: Ammonium nitrate safety, Self-sustaining decomposition, Fertiliser safety, Consequence analysis, Ammonium nitrate, Fertiliser storage.

Self-sustaining decomposition is a phenomenon in which a decomposition front initiated locally will spread gradually throughout the fertiliser mass. Ammonium and nitrate ions as RedOx components are needed together with a catalyst in a concentrated narrow zone. Additionally, a matrix is needed to act as a heat conveyor in the presence of sufficient heat. The decomposition reactions have a complex nature, thus reducing the predictability of the hazardous properties of a fertiliser. The products should be considered hazardous unless proven to be safe.

The degree of hazard associated with self-sustaining decomposition in a mass of fertiliser is related to the fertiliser composition, the velocity of propagation, the extent of mass loss, the toxicity of the gases released and the sensitivity to initiation. The classical trough test has been modified by Kemira Agro to obtain more information. Comparing the mass loss rates, the products containing high amounts of ammonium nitrate release more gases during decomposition. Using a comparison limit of 15 cm/h for a product such as 15-15-15, the same consequence is obtained for 20-5-15 with burning rates less than 5 cm/h, which is borderline Class B in terms of the trough test. When attempting to decrease the rate of decomposition of such grades the formulation, may burn with a reduced rate of decomposition, but the ignition temperature will have decreased by e.g. 50°C. Such a safety margin in production is significant and should be applied.

The consequence analysis of a self-sustaining decomposition has been done for a) small quantities initiated by e.g. friction, b) in a drier and c) in a warehouse. Even small amounts of decomposing fertilisers are able to create sufficiently large amounts of toxic gases inside a building that the use of personnel protection must be available over the whole plant area. With a high number of air changes per hour (high ventilation rates) the threshold limit for exposure is greatly affected by dilution. Decomposition in a warehouse has major environmental consequences if the first protective actions are not successful. Weather conditions play a significant role in the dispersion of the toxic gases. The main factor affecting the environmental consequences of decomposition in a drier is whether the exhaust fan can take in all the released gases. In a Fume Off the rapid decomposition causes extremely bad conditions both inside and outside the building. The safety precautions are discussed along with the different cases.

Harri Kiiski, Kemira Agro Oy, Espoo Research Centre, Finland

47 pages, 6 tables, 23 figures, 44 refs.