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Monday, December 20, 2010

Fertilizer fire aboard cargo ship

A recent journal paper titled "Small-scale experiments of self-sustaining decomposition of NPK fertilizer and application to the events aboard the Ostedijk in 2007" has published in Journal of Hazardous Materials. Its content is presented here.

The global fertilizer industry produces 170 million tonnes of fertilizer annually. As the global population increases and countries develop, this is expected to rise. Production sites are limited to locations with good availability of key raw materials. Therefore, large quantities are required to be shipped to the point of use.

Fertilizers contain three main ingredients essential for plant growth: nitrogen, phosphorous and potassium (NPK). These are present in various forms, however it is the presence of ammonium nitrate that constitutes the biggest risk. Ammonium nitrate is classified as a Dangerous Good by the UN Recommendations on the Transport of Dangerous Goods. This is because in the presence of an initiating event, ammonium nitrate will undergo self-sustaining decomposition. This is a chain reaction that occurs when a molecule of ammonium nitrate breaks down and releases heat which allows the decomposition of further molecules. In the presence of organic material this may result in explosion as in Texas City (1947) in which 581 people were killed.

Figure: The Ostedijk on 21st February (the 5th day) after the hold was opened and before specialized fire-fighting activities had commenced. Derived from photograph courtesy of Agencia EFE.

The research presented here gives an experimental insight into the decomposition of NPK fertilizers, highlights some of the limitations of the current UN Recommendations and applies the results to the events aboard the cargo ship Ostedijk in 2007.

The Ostedijk was carrying a cargo on NPK fertilizer from Norway to Spain when an accidental decomposition reaction occurred. The decomposition continued for seven days before it was stopped by partial flooding of the cargo hold as previous attempts to cool the cargo had been unsuccessful. During this time, a large plume of toxic gases formed and the crew had to be evacuated from the ship.

This unique set of experiments was performed in the laboratory using NPK 16.16.16, an industrially available fertilizer, and three different apparatus. The propagation behaviour was studied in an apparatus similar to that proposed by the UN test. Thermo-gravimetric analysis was performed to identify the reactions occurring and investigate the reaction mechanism. Finally, the state of the art for testing reactive materials, the Fire Propagation Apparatus, was used to find the conditions under which the reaction would become self-sustaining and to measure the heat of reaction.

The experiments showed beyond doubt that NPK 16.16.16 can undergo a self-sustaining decomposition reaction. This results in temperatures up to 350°C and releases heat at a rate of 1.8 MJ/kg of reacting fertilizer. This is in contradiction to the UN classification that the material is free from the hazard of self-sustaining decomposition. The paper allows us to understand and quantify some of the observations during the accidental event aboard the Ostedijk.

Figure: (a) Unreacted fertilizer granules and (b) cross section showing partially reacted sample with 4 phases visible.

These experiments are important as there is very little research in the open literature regarding decomposition of ammonium nitrate containing fertilizers and this is the first time such measurements have been applied to a real scenario. They also provide an insight into this complex risk and the controlling mechanisms. The data and experimental methods can be used to further investigations into other incidents which may help in identifying causes of, and reduce losses from, this phenomenon.

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