Ammonium Nitrate: Lessons learned and safer approaches

Every year, almost 20 million tons of ammonium nitrate are produced worldwide. It is so far the most potent, economical and convenient fertilizer on the market, supporting farmers in feeding billions. However, the recent catastrophic explosion in Beirut is a reminder of the hazards associated with improper storage and transportation of this common chemical. How can we harness the power of ammonium nitrate while ensuring that tragedies like Beirut are avoided in the future?

The Beirut disaster

On August 4, 2020, a large amount of ammonium nitrate in a storage hangar at the Port of Beirut exploded, causing over 300 deaths and 6,000 injuries, as well as enormous property damage, leaving an estimated 300,000 people homeless. The catastrophic explosion in Beirut was one of a few large-scale accidents caused by ammonium nitrate, comparable only to explosions in Texas City, United States (1947); Toulouse, France (2001); and Tianjin, China (2015).

Since the beginning of the 20th century, there have been nearly 40 large accidents, with over half of these occurring in the last 20 years. Eleven of these accidents involved more than 1,000 tonnes of ammonium nitrate or caused more than 30 deaths (Figure 1). Over 80% of these accidents occurred during the storage or transportation of ammonium nitrate (Figure 2). Analysis of these accidents shows that ammonium nitrate explosions are almost always preceded by fires or other ignition sources.

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Can ammonium nitrate be tamed?

What can be done to ensure that tragedies like Beirut are avoided in the future? The key to driving future innovation in this space is understanding the molecular structure and reactions that cause these powerful explosions. Ammonium nitrate is made by reaction of ammonia with nitric acid in water followed by careful evaporation of the water.

Ammonia is most often prepared from atmospheric nitrogen, while nitric acid is prepared from the combustion of ammonia, and so ammonium nitrate is most conveniently manufactured where ammonia is made. The formation of ammonium nitrate gives off significant amounts of heat, thus it is made in aqueous solution to better dissipate that heat. The water must be then removed from the solution to yield solid ammonium nitrate, most often by evaporation. While ammonium nitrate is stable at the usual temperatures of its manufacture and purification, additives are often used reduce the hazards. For example, potassium compounds like nitrogen-potassium and nitrogen-phosphorus-potassium fertilizers have been used to increase the stability of ammonium nitrate to phase transition and decrease its sensitivity to explosion. Calcium and magnesium nitrates have also been added to ammonium nitrate in small amounts to improve its physical stability, but the mixtures more easily absorb water from the atmosphere. Calcium ammonium nitrate (CAN), prepared from ammonium nitrate and calcium carbonate, is used in some cases as an alternative to ammonium nitrate. Aluminum sulfate has also been used as a drying additive for ammonium nitrate. While these alternatives have helped, it is important to note that unintended impurities in the manufacturing process may reduce its stability or facilitate explosion.

Ammonium nitrate innovation focused on safety

An in-depth analysis of the CAS content collection reveals a few key areas that have begun to emerge in the Intellectual property landscape, outlining efforts to prevent accidental explosion or misuse. These include advancements around mixtures, coatings, slow release formulations, particle size control, and hygroscopicity approaches that minimize the potential for misuse and increase stability for safer storage. For example, a coating for a fertilizer has been invented that prevents the infiltration of hydrocarbons, such as fuel oil, into the pores of fertilizer, thereby reducing its efficacy as an oxidizer (WO2003106377). Alternatively, hygroscopic approaches have also been explored by adding a key ingredient such as a urea double salt which forms a stable fertilizer with ammonium nitrate (US20150218058). So, some alternatives have emerged that provide users with safe, cost effective options but there are still significant tradeoffs either in safety or nitrogen content, so more innovation is still required.

Seeking alternative nitrogenous fertilizers

Although ammonium nitrate is an important fertilizer because of its high nitrogen content, its explosive hazards limits its application, and it has even been banned in some areas. Reducing the concentration of ammonium nitrate, finding alternative compounds and developing safer forms are strategies that may help increase the safety of nitrogen fertilizers. Table 1 lists alternative nitrogenous fertilizers that have emerged as safer options that could provide substitutes to ammonium nitrate. Unfortunately, the alternative with the highest nitrogen content is anhydrous ammonia, which is a toxic gas at ambient temperature. Other alternatives with high nitrogen content, such as ammonium hydroxide solutions and urea, are attractive due to their low cost and effectiveness; however, they are volatile. Slow-release formulations of urea, as well as modified lignins and hydrogels, have been developed to overcome this challenge.

Table 1. List of alternative nitrogenous fertilizers

Handling ammonium nitrate safely

Previous disasters have heightened public awareness of the explosive nature of ammonium nitrate, and the consequences of its unsafe storage and use. In response, many regulations, rules and guidelines for ammonium nitrate, especially regarding its storage and handling, have emerged in different areas to improve safety. The primary U.S. regulations for ammonium nitrate are “OSHA 29 CFR 1910.109(i) – Storage of Ammonium Nitrate”, and the Memorandum of “Guidance on the Ammonium Nitrate Storage Requirements in 29 CFR 1910.109(i)” published in 2014 after the Adair Grain Inc., DBA West Fertilizer Company explosion in 2013. Noteworthy, however, the diligent enforcement of the safety rules and regulations is the key for avoiding future accidents such as the Beirut disaster.

The requirements to safely store ammonium nitrate can be summarized as follows
(Figure 3):

• Ventilation: capable of adequate ventilation in case of fire
• Isolation from combustion: noncombustible and AN-resistant materials are required
• Temperature control: storage temperatures should be below 130F
• Volume: storage limits should be less than 2270 tonnes
• Fire protection/prevention: strict fire protections are necessary
• Avoidance of combustible contaminants: dividing storage from other products

Despite safety concerns, ammonium nitrate remains an important fertilizer worldwide as it is effective, cheap and easy to manufacture. However, as has been highlighted once more by the Beirut incident, ammonium nitrate’s susceptibility to fire and explosion due to poor storage practices or contamination present significant risks. Inconsistent regulations and a lack of sustained awareness on how to safely handle and store ammonium nitrate within the global supply chain are key challenges that must be overcome through education and advocacy to prevent another global tragedy.

See how scientists are making ammonium nitrate safer in our most recent white paper Lessons learned from Ammonium Nitrate.