1. Introduction
2. Fire hazard
3. Decomposition hazard
4. Self-sustaining decomposition
5. Explosion and detonation hazard
6. Detonation hazard
7. Post production risks

Introduction

Ammonium nitrate is a strong oxidiser.

Under certain conditions (confined conditions like an enclosed volume, elevated pressure, contamination) it can explode. Ammonium nitrate (AN) is generally regarded in most countries as posing no unacceptable hazard when the specification of the material is fulfilled. Uncontaminated fertilizer-grade ammonium nitrate cannot be exploded by impact, by heat, or by fire alone.

However, explosivity of ammonium nitrate depends for example on what the material is mixed with, how much carbonaceous material is present. Therefore, some countries prohibit or limit production, storage, and/or application of straight ammonium nitrate. In a majority of countries, the detailed specification is regulated by law and the safe handling, transportation, and application of ammonium nitrate are ensured.

All ammonium nitrate-based fertilizers, under normal conditions, are stable materials, which in themselves present no risk. Under abnormal conditions they can give rise to certain hazards; the main ones being the enhancement of fire, thermal decomposition (with release of toxic fumes) and, under extreme conditions, explosion.

The development of safe anti-caking treatment and prilling/granulation processes, coupled with clearer guidance for safe practices helped to boost the large-scale production of high-density ammonium nitrate as prills or granules for use as a nitrogen fertilizer. This product has high resistance to detonation.

Fire hazard

Ammonium nitrate-based fertilizers are not combustible.

Although not all ammonium nitrate-based fertilizers are classified as oxidizers, they are all oxidizing in nature and, therefore, the risk of fire depends on the presence of general combustible materials, such as parts of handling equipment, fuels, lubricants and hydraulic fluids used there in, and materials stored or used in the construction of the store or bays. Experience shows that fires start in combustible materials stored near the fertilizer or in associated equipment such as trucks or belt conveyors.

When a fire involves bagged fertilizer, the bags may melt and break but they will have an insignificant effect on the fire. Polyethylene and similar packaging materials do not spread the burning, but may be oxidised by the hot or molten fertilizer. The hot spilled fertilizer may cause wooden pallets which are in contact with it to smoulder or burn.

With bulk fertilizer there are no packaging materials or pallets involved and the fire, therefore, may not penetrate into the heap. The effect on the fertilizer depends on the fierceness of the fire and the other materials present.

The capability to intensify fire depends not only on the AN content of the fertilizer but also on the nature of other components which may be present in the fertilizer and may have catalytic or thermal effect on the decomposition of AN.

Decomposition hazard

Ammonium nitrate decomposes when heated to well above its melting point, releasing water vapour, ammonia, nitric acid vapour and oxides of nitrogen by way of a number of reactions. These include (i) the endothermic and vapour-pressure-dependent reversible dissociation reaction into ammonia and nitric acid vapours and (ii) a number of exothermic reactions which are irreversible and which release gases containing water vapour, toxic oxides of nitrogen and/or nitrogen. These reactions are described in detail in literature.

The combined effect of these endothermic and exothermic reactions produces a self-limiting thermal effect up to a certain temperature, provided the gaseous products are able to freely escape. This phenomenon has been theoretically studied and experimentally verified; it shows that under adiabatic conditions and free escape of gases the self-limiting temperature in pure AN is in the region of 290˚C at atmospheric pressure. It must be emphasised that any adverse condition such as the presence of reactive or catalytic substances and/or confinement of product gases will reduce this temperature, making AN thermally less stable.

Under extreme conditions, if the gases are not able to escape, the endothermic effect can be almost totally suppressed, leading to a rapid exothermic effect and explosive behaviour. This is of particular significance for hot work on equipment which has been used for handling or processing AN and which may still contain deposits of AN due to inadequate cleaning and/or inspection.

Certain substances, notably chlorides, chromates, copper and zinc salts enhance the rate of decomposition of ammonium nitrate. Acid conditions also exert a similar effect.

Ammonium nitrate-based fertilisers are thermally stable and are not prone dangerously to self-heat in normal conditions of storage. They require input of external heat to initiate decomposition. Consideration of the potential decomposition hazard is important for ammonium nitrate-based compound fertilisers, which contain chloride, e.g., in the form of KCl.

During decomposition of compound fertilisers, for example under heating, copious amounts of fumes are given off which contain water vapour and various toxic gases such as oxides of nitrogen, hydrogen chloride, ammonia and chlorine depending on the composition of the fertiliser. The fumes may also contain ammonium chloride and ammonium nitrate, which along with the water vapour can markedly reduce visibility. The decomposition is accompanied by release of heat with temperatures in the decomposing mass sometimes reaching 300-500°C. Decomposition can start when the fertiliser is in the solid state. Melting may also occur with fertilisers containing high levels of ammonium nitrate.

In many cases the decomposition, initiated by an external heat source, will stop when the heat source is removed. With some fertilisers, however, the decomposition will continue and spread deep into the mass of material even when the heat source is removed. This is the phenomenon of self-sustaining decomposition, sometimes referred to as ‘cigar burning’ where the decomposition propagates through the mass of the material.

Ammonia gas can be liberated from ammonium nitrate-based fertilisers (as from all ammonium salts) when they come into contact with alkaline materials such as lime. Ammonia is a toxic gas; it is colourless, but its presence can be detected by its characteristic strong smell.

Self-sustaining decomposition

Compound fertilisers containing ammonium nitrate may be subject to propagating decomposition called ‘cigar burning,’ when ignited.

Once ignited the decomposition propagates through the mass of material at rates of 5 – 50 cm per hour and with a temperature in the decomposition zone of 300-500°C, often producing red-brown hazardous fumes. Compound fertilisers containing ammonium nitrate, having a content of 4% or more of KCl, are susceptible to this phenomenon.

The reaction may be inhibited or retarded by the presence of ammonium phosphates; therefore, NPK grades containing ammonium nitrate and ammonium phosphate may be less liable to this hazard.

In case of a self-sustaining decomposition its characteristics, for example the speed of propagation, the temperature in the decomposition zone and the amount of gas produced, depend on the composition of the fertiliser and on the extent of melting. The presence of compounds of trace elements such as copper salts and impurities such as chromium salts can increase this decomposition.

The speed of propagation (as well as the temperature in the decomposition zone) can be assessed by the so-called trough test.

With this type of fertiliser, the bulk form of handling presents a greater risk than does packaged handling due to the relatively greater ease of initiation from exposure to heat sources and the ability to propagate the decomposition throughout the heap. Minor heat sources such as a buried inspection lamp or self-heating resulting from contamination can be sufficient for the initiation of the decomposition.

Explosion and detonation hazard

An explosion is a sudden release of pressure energy, which can take place as a result of rupture of a pressurised system. In detonation the substance reacts at a supersonic speed and generates a shock wave.

Ammonium nitrate-based fertiliser is difficult to detonate, requiring very high energetic shocks. Neither flame, nor spark, nor friction can cause detonation in uncontaminated material. The risk of detonation is relevant to fertilisers with high concentrations (> 80 %) of ammonium nitrate. In practice, the fertiliser products are made to have a high bulk density and low porosity to give high resistance to detonation and contain additives to give good anti-caking properties and thermal stability against breakdown due to temperature cycling.

In the EU, a resistance to detonation test has been prescribed in the fertiliser regulation for AN based fertilisers containing more than 28 % nitrogen derived from ammonium nitrate (equivalent to 80%). The fertilisers, which pass the resistance to detonation test, have a very high resistance to detonation.

If the fertiliser is not properly handled, a number of factors can decrease this resistance. These include contamination with incompatible substances, a reduction in particle size, increase in temperature and thermal cycling (which increases the porosity and causes breakdown of the prill or granule structure).

Detonation hazard

There are four main mechanisms which, in theory, can cause a detonation in an ammonium nitrate fertiliser stack or bulk heap:

(i) shock initiation by another explosive; an explosive detonating close to stored ammonium nitrate could initiate a detonation or cause a sympathetic detonation in the fertiliser. The use of explosives to break-up badly caked fertiliser as well as the storage of explosives near ammonium nitrate have been banned after some serious accidents several decades ago.

(ii) shock initiation by a high velocity projectile; an initiation by impact from a high velocity projectile is improbable and very unlikely to happen; the required combination of conditions necessary for such an event are difficult to meet, neither do high velocity projectiles occur readily nor is the fertiliser simultaneously sufficiently sensitised due to higher temperatures, a molten state or contamination. High velocity projectiles can be generated in a fire when ammonium nitrate-based fertiliser is confined in hollow sections of equipment such as conveyor rollers and components of shovels. The rupture of welding equipment such as gas cylinders can have a similar effect. These projectiles would not have sufficient energy to initiate a detonation in normal solid products but molten and/or contaminated fertilisers would be more susceptible. Roof beams or building structures which may collapse in a severe fire are not expected to have sufficient impact energy to initiate a detonation even in molten decomposing fertiliser.

(iii) thermal decomposition; in a fire the transition of any rapid decomposition (deflagration) to detonation in a stack or bulk heap is very unlikely because the necessary severe conditions of increased pressure (confinement) are not met in practice. The risk is increased if the molten material is contaminated.

Explosion due to heating and confinement; an explosion hazard can also arise with trapped or confined AN. When heated strongly under confined conditions, for example, in a fire, during hot work on equipment, or heating of solidified ammonium nitrate in pipes, the material can decompose violently causing an explosion. Contamination of the fertiliser with combustible and other reactive substances increases this risk.

(iv) reaction with a highly reactive substance; in theory certain substances can react very rapidly and energetically.

Adherence to good practice as given in numerous guidance, recommendations and legal requirements minimises the risk of contamination and degradation and therewith the risk of the above described hazards.

Post production risks

Shah, 2008, undertook an analysis of 80 AN related events between 1991 and 2007. This showed that the type of activity involved was: storage: 41%, transport 30%, production 24% and maintenance: 5%. Points of interest to note are that only about a quarter of the accidents took place in the production-related activities; effective process control, good training and a high level of awareness are likely to be factors in this. Nearly three-quarters took place post production (i.e. in storage and transport) with a higher percentage taken by storage. An important factor for this seemed to be inadequate controls or standards.

From this study the author drew the following conclusions:

• On production sites AN solution pumps, even those handling low concentration solutions or liquors, can present explosion hazards. They require appropriate safe-guards e.g. low flow alarm and/or high temperature trip.
• Throughout the production and distribution chain, safe and effective management of off-spec and reject materials should be given a high priority.
• Contamination seems to be a major factor in many accidents, particularly where the material is handled in bulk. A higher level of awareness of the potential hazards and likely consequences in workers and supervisors who are involved in the handling operations is required. More effective procedures for checks and extra care are essential.
• Much guidance in the form of codes of practice and safety ‘rules’ is available to those involved in production, storage and distribution. Despite this, several accidents have happened in distribution operations. The most likely causes include application of heat (e.g. buried electrical lamps, electrical faults, proximity of combustible materials) and contamination. More effort is needed to ‘educate’ and train workers and supervisors in order to improve adherence to good practices.

It is important to note that:

• There has been no major AN explosion accident in sea transport for more than sixty years, during which period millions of tonnes of AN have been moved, thanks to the development of good safety rules and their successful implementation and compliance.
• There has been no major explosion accident with in-spec marketable fertiliser grade AN product in an industrial store in at least four decades.

References:

H. Kiiski. (2000). Self-sustaining decomposition of NPK fertilisers containing ammonium nitrate, Proceedings International Fertiliser Society, 450.

Fertilizers Europe. (2022). Guidance for the storage, handling and transportation of solid mineral fertilizers, second edition.

Links to related IFS Proceedings

110, (1969), Production of Ammonium Nitrate including Handling and Safety, R W R Carter, A G Roberts

124, (1971), Thermal Stability of Fertilisers Containing Ammonium Nitrate, G Perbal

137, (1973), Safety in Works, G Perbal

137, (1973), Safety of Products and Raw Materials, V P England

137, (1973), Safety Systems and Legal Requirements, J F Killeen

207, (1982), Surface treatments – Symposium on Materials of Construction in Fertiliser Plants. Session 2, R S Hullcoop

207, (1982), Protecting Fertiliser Manufacturing Plant:  The Role of Surface Coatings, D W May

207, (1982), Corrosion and Protection of Concrete in an Ammonium Nitrate Environment, P Furnival

384, (1996), Safety of Ammonium Nitrate Fertilisers, K D Shah

448, (2000), Ammonium Nitrate: Safety Aspects of Blended and Granulated Compound Fertilisers based on Ammonium Nitrate, K D Shah, D C Thompson

492, (2002), Safety Legislation and the Fertiliser Industry, K D Shah

496, (2002), Ammonium Nitrate: Toxic Fume Risk from Fires in Storage, G Atkinson, W D Adams

508, (2003), Product Stewardship Applied to Fertilisers, H Kiiski, R J Milborne

516, (2003), Safety Assessment of Materials used in Construction and Equipment for Ammonium Nitrate Production and Storage, K D Shah

534, (2004), Review of Recent Legislation Affecting the Fertiliser Industry, D J Heather, J A M van Balken

580, (2006), Safety Testing of Ammonium Nitrate Products, R J A Kersten, E I V van den Hengel, A C van der Steen

581, (2006), Ammonium Nitrate Handling Operations: Guidance for Safe Practice, K D Shah, J A M van Balken

585, (2006), Ammonium Nitrate Transport: Accidents and Investigations, K D Shah, J F D Chys

628, (2008), Safety Assessment of Bitumen-based Asphalt (Tarmac) Flooring in Ammonium Nitrate Fertiliser Stores, R H Dyson, P Waller, K D Shah

629, (2008), Ammonium Nitrate Production, Storage and Distribution: Accidents and Investigations, K D Shah

Links to external resources

International Fertiliser Association Safety Handbook. Establishing and Maintaining Positive Safety Management Practices in the Work Place