Ammonium Nitrate Based Fertilizer (ANBF) cargoes has been transported on ships as a bulk cargo for a long time, and its safe carriage during sea transport has been of great concern to the industry. It was one of the first cargoes to be included in the Code of Safe Practice for Solid Bulk Cargoes, which was later replaced by the mandatory International Maritime Solid Bulk Cargoes Code (IMSBC Code). There are three individual schedules for ANBF in the Code, of which ANBF (non-hazardous) is defined as Group C cargoes that are neither liable to liquefy nor to possess chemical hazards. However, since 2002 some ships carrying this type of cargo from the Europe have experienced catastrophic thermal decomposition, and therefore the industry has been questioning the appropriateness of the term “non-hazardous” and the UN test results. MSC 105 adopted amendment 06-21 of the IMSBC Code at the end of April this year to include the reclassification of ANBF (nh). This article, with reference to some previous incidents, provides a detailed introduction to reasons for the chemical reaction known as self-sustaining decomposition (SSD) and the correct cargo schedule to be applied.

I. Safety accidents

Compared with highly purified ammonium nitrate (a Class 5.1 dangerous good, the explosive that caused the Beirut blast), ANBF is more stable but more complex in composition and is prone to reactions when exposed to heat. As non-hazardous Group C cargoes, ANBFs and especially compound NPKs, when transported by sea, are still reported to have decomposed in heat and to release toxic gases, resulting in serious cases of total loss of ship and cargo, the following three being typical.

1. MV Ostedijk

In February 2007, the cargo ship loading non-hazardous NPK15-15-15 fertilizer was in transit from Norway to Spain. The fertilizer in cargo hold 2 underwent a decomposition off the coast in Northeast Spain. Initial fire fighting aboard consisted of cooling the outside of the hold with water from a nearby tug, which proved ineffective in controlling the fire. Specialist salvors later opened the hold and identified several hot spots. The hotspots were then targeted with localised water injection and the fire was declared distinguished on the seventh day.

Apart from cargo damages, all crew were evacuated with four members airlifted to hospital having been affected by the fumes. Investigation showed that the incident was an SSD event where the cargo reacted upon exposure to a heat source. The cargo owner declared that the fertilizer’s classification as Group C cargo was determined by a standardized Trough Test as detailed in the UN Manual of Tests and Criteria. The test obviously provided only limited information. Investigation report also suggested that the heat source leading to the decomposition was probably the heated fuel pipe.

2. MV Purple Beach

In May 2015, the multipurpose container vessel was loading at Antwerp with fertilizer at the bottom of cargo holds 2-5 and some general cargo at the tween deck of cargo holds 3 and 4. The ship found smoke coming out of cargo hold 3 where non-hazardous fertilizer NPK15-15-15-2S was stored while in the anchorage off the German Gulf. Following instructions of the company, the CO₂ flooding system onboard was activated to put out the fire but it was unsuccessful. A German rescue team concluded after assessment that the fertilizer was decomposing due to exposure to heat, and as the cargo was loaded in the bilge and the second deck could not be opened, the blaze was stopped by flooding through the manhole.

The accident resulted in damage to the cargo and the vessel was declared a constructive total loss. All crew were evacuated and some persons on board were airlifted for treatment for inhaling the toxic gas. The accident investigation found that as a result of misunderstanding the term “non-hazardous”, the company and the ship have neglected many requirements of the IMSBC Code, including bunkering during the loading, welding lashing rings on the second deck of hold 3, not isolating the power supply of lighting in the hold, and loading cargo at the bottom without being able to open the tween deck hatch cover.

3. MV Cheshire

In august 2017, the vessel was heading from Norway to Thailand loading non-hazardous ANBF (NPK15-15-15 and NPK16-16-16). While en route, the chief officer noticed a small amount of water drips and a smell from cargo hold 4 when conducting his daily checks. Such conditions continued in the following days and the smell became much stronger. The cargo manufacturer advised that it was a thermal decomposition taking place. After an explosion in cargo hold 4 while the ship was underway south off the Canary Islands, the decomposition transferred to the cargo in other holds. Since the toxic gas cannot be kept clear of the accommodation, the crew was evacuated by the Spain MRCC.

With all the cargo decomposed, the vessel was declared a constructive total loss. The Norwegian Court of Appeal ruled on 15 November 2021 that the cause of the incident was solely due to the negligent actions and omissions of the crew on the grounds that: a) despite clear and repeated instructions, the crew failed to isolate all the heated source within the cargo holds – the hold light system was not properly disconnected prior to loading; b) the crew did not make early and effective attempts to arrest the decomposition until several days later.

II. ANBF and main products

Inorganic fertilizers, the main source of nutrients for plants today, can provide primary nutrients including nitrogen(N), phosphorus (P) and potassium (K); secondary nutrients including calcium (Ca), magnesium (Mg) and sulphur (S); and minor elements including chlorine (Cl), iron (Fe), manganese (Mn), boron (B), selenium (Se), zinc (Zn), copper (Cu), molybdenum (Mo). While nitrogen is used to enhance plant growth, ammonium nitrate is widely used in fertilizers due to its high nitrogen content and simple manufacturing. To meet various nutritional requirements, compound fertilizers such as NPK and NPSB are made combining ammonium nitrate and other elements.

ANBF refers to fertilizers that contain both NH4+ and NO3-, regardless of its sources and mainly includes:

• straight nitrogen fertilizers such as ammonium nitrate (AN, NH₄NO₃), calcium ammonium nitrate (CAN, NH₄NO₃/CaCO₃ /CaMg(CO₃)²/CaSO₄/MgSO₄) and ammonium sulphate nitrate (ASN, NH₄NO₃/(NH₄)²SO₄);
• nitrate-based compound fertilizers such as NP, NK, NPK and NPSB, whose name often denotes the ratio of components (NPK15-15-15, NPK16-16-16, NPSB12-20-18-5S-1B₂O₃).

ANBF is divided in the International Maritime Dangerous Goods (IMDG) Code and the International Maritime Solid Bulk Cargoes (IMSBC) Code into three individual schedules based on the content of ammonium nitrate and combustibles and on test results according to the UN Manual of Tests and Criteria: UN2067 Oxidizer Class 5.1; UN2071 Class 9 NPK/NP/NK compound fertilizers capable of undergoing self-sustaining decomposition; and non-hazardous Group C cargo.

III. ANBF hazards

Pure ammonium nitrate is inherently stable and non-combustible at room temperature and pressure, but it is an oxidizer that can accelerate fire or explosions in contact with combustible materials and sources of heat. The fact that the high purity ammonium nitrate that exploded at the port of Beirut was kept in a poorly managed warehouse for six years was a proof. Ammonium nitrate fertilizer is less susceptible to combustible materials due to its higher density and is more stable due to the manufacturing process. ANBF cargoes are hence stable under normal storage and transport conditions and are not susceptible to chemical hazards. However, some incompatible substances such as phosphate, potassium chloride or other contaminants may be mixed with the compound fertilizer during manufacturing, or components themselves may be incompatible with each other. In such cases and when the cargoes are heated, there can be significant risks of fire, decomposition, toxic fumes and even explosion under certain conditions.

• Fire hazard

ANBF by itself does not burn, but the ammonium nitrate in it is an oxidizer that can support and intensify a fire even in the absence of air. CO₂, dry powder, or foam extinguishers, sand or other smothering agents are ineffective and will probably make things worse. The likelihood of a fire (or decomposition) increases if the cargo becomes contaminated with combustibles such as coal, grain, sawdust, oil, grease, or sulphur. When it is caught in a fire or heated strongly, it will give off toxic gases such as nitrogen oxides and ammonia.

• Decomposition

Ammonium nitrate melts at around 170°C and decomposes between 200°C and 260°C, with the release of heat and gases. Temperature of the reaction can sometimes reach 300°C-500°C. It begins with endothermic dissociation to NOx, ammonia, hydrogen chloride and nitric acid, and eventually develops into an exothermic reaction.

ANBF is thermally stable and not prone to self-heating under normal storage conditions. It decomposes on heating and stops when the heat source is removed. However, decomposition of some fertilizers may propagate through the mass of materials even if the heat source is removed. Such cargoes in bulk present a greater risk than those in packages as they are subject to greater exposure to heat and greater propagating decomposition. Tiny heat sources such as hold lights buried inside cargoes or a self-heating reaction caused by contamination are sufficient to initiate a decomposition. Some compound fertilizers (NPK/NP/NK) are capable of self-sustaining decomposition, while straight nitrogen fertilizers such as AN, CAN, ASN and other compound fertilizers may decompose only when contaminated with sensitive substances such as acids, chromates, chlorinated chemicals and metals such as zinc and copper and their salts.

• Detonation

ANBF is generally unlikely to detonate, and fertilizers of standard quality are highly resistant to detonation and cannot be detonated by flame, sparks or friction. It can only be detonated under certain conditions, such as when in confinement and when heated strongly, for example in a fire. Contamination of the fertilizer with combustibles and other reacting substances may increase the risk of explosion.

IV. SSD reactions

For carriage of ANBF, especially non-hazardous ANBF cargoes, the risk of fire and explosion is not as high as that of an SSD event, which is a phenomenon where  a decomposition initiated locally will spread gradually throughout the fertilizer mass. During the process, the weight of the fertilizer will drop considerably as fumes of toxic gases are produced. The reaction generally requires:

① RedOx components (NH₄+ + NO₃-);

② Catalyst (Cl-, Mn, Co, Cu, ...);

③ Combination of the first two components;

④ A matrix to act as a heat conveyor;

⑤ A confined space with sufficient heat and good thermal conductivity.

The decomposition of ammonium nitrate in ANBF can be non-catalytic or catalytic. In the case of non-catalytic decomposition, ammonium nitrate is reduced to ammonia and nitric acid, which is further decomposed into N₂O and N₂. Whereas chlorides are present in most compound ANBF (e.g. NH₄NO₃ + NH₄H₂PO₄ + KCl in NPK), catalytic decomposition is more common in such cases and the reaction goes as follows.

NH₄NO₃ ^CL-→NH₃+HNO₃→N₂+H₂O+HNO₃+NOx

V. The misleading schedule for ANBF (nh)

The several thermal decomposition accidents during the transportation of ANBF at sea have suggested that the term “non-hazardous” in the individual schedule for ANBF can be misleading.

(1) Misled by the term “non-hazardous”, anyone along the industry chain, including shippers, shipowners, crews, stevedores, may neglect the hazards, the precautions and the emergency procedures as required by the relevant provisions.

(2) Although the IMSBC Code specifies that cargoes designated as Group C are neither liable to liquefy nor to possess chemical hazards, the individual schedule for ANBF (nh) warns that “even though this cargo is classified as non-hazardous, it will behave in the same way as the ammonium nitrate based fertilizers classified in class 9 under UN2071 when heated strongly, by decomposing and giving off toxic gases… there will be a risk of toxic fumes in the cargo space and on deck if the cargo is strongly heated.”

(3) Same as Group B ANBF, non-hazardous ANBF should not be stowed immediately adjacent to any tank or double bottom containing fuel oil heated to more than 50°C. However, the compliance certificate issued by the class may not always contain specific requirements for loading and carriage of ANBF (nh) since it is a Group C cargo.

(4) A fire as a process of burning requires heat, combustible materials, and oxygen, without any one of which the process cannot continue. Thermal decomposition as a process of chemical reaction requires only heat and combustible materials, and the absence of oxygen does not really matter. Without knowledge of such reactions, the personnel involved may be unaware of the hazards with respect to carriage of ANBF (nh) and the proper way of handling such cargoes. This may also be overlooked in fire safety trainings for crews on board ships.

(5) The Group C non-hazardous ANBF cargoes should not be liable to self-sustaining decomposition as determined by the result of testing in the trough test (see IMSBC Code Appendix 2, Section 4). The cargoes carried on the ships in all three accidents above, however, are liable to self-sustaining decomposition, indicating that the results of trough test may not always be accurate.

- Trough test described in UN S.1 was originally used to determine the tendency to suffer self-sustaining decomposition of dangerous goods transported in package, and later it was also used for classification of bulk cargoes in the IMSBC Code.

-The trough test is carried out in an open-top apparatus consisting of a trough of internal dimensions (15x15x50 cm). A sample of approximately 11-12 kg of the fertilizer is taken. With heat and gases escaping from the six dimensions of the apparatus, the reaction will slow down following the consumption of energy and will be eventually discontinued if there is no propagation. However, in the case of bulk carriers, propagation may occur throughout the cargoes as the heat cannot be easily removed from the bulk in the holds.

(6) Most fertilizer manufacturers will conduct trough tests annually on their products or additional tests if grading changes may affect product safety, but these tests are conducted only before the delivery of cargo. If the cargo is stockpiled for a long period, there may be a change in its tendency to suffer self-sustaining decomposition when mixed with other substances including combustible organics (e.g. pigeon droppings) or inorganic materials (e.g. oils). Another trough test for such cargoes before loading is not yet required by any parties.

VI. IMO Amendments

The Maritime Safety Committee (MSC 105) met from 22-29 April 2022 and adopted amendment 06-21 of the IMSBC Code, which included a reclassification of ANBF (nh) cargo.

(1) The individual schedule for “AMMONIUM NITRATE BASED FERTILIZER (non-hazardous)” is deleted to avoid misunderstandings. ANBF is now classified as either “AMMONIUM NITRATE BASED FERTILIZER” or “AMMONIUM NITRATE BASED FERTILIZER MHB” depending on composition limits (as indicated in the table below).

(2) The shipper shall declare the ammonium nitrate content and the chloride content in accordance with 4.2 of the IMSBC Code.

(3) Following investigations on previous accidents involving carriage of ANBF (nh), guidance is provided in the revised schedule: a) there shall be a daily assessment of  the trends of the cargo temperature and oxygen concentration in the cargo spaces throughout the voyage; b) the hatches of the cargo spaces, including those of tween decks should be kept free at all times; c)revised emergency procedures: emphasizing that the ship’s fixed gas fire-fighting installation will be ineffective. Injection to the seat of decomposition is the first control measure (e.g. using Victor lance).

(4) Circular with respect to carriage of ANBF (nh) (CCC.1/Circ 4) will be revised.

 VII. Advice to Members

(1) Amendment 06-21 of the IMSBC Code may be applied by Administrations on a voluntary basis beginning 1 January 2023 and will enter into force on 1 January 2024. Before that, shipowners intending to load ANBF should inquire the applicable rules at the port states and replace the certificate of compliance with the IMSBC Code in time. Companies should promptly revise the guidance for shipment of such cargoes and provide adequate training to crew members on the changes. Where the current rules still apply, crew members should be fully aware of the hazards of Group C ANBF (non-hazardous) cargoes.

(2) Requirements for safely transporting ANBF cargoes can be complicated. Prior to loading, it is important for shipowners and crew members to full assess cargo worthiness considering the shipper’s declaration, the MSDS and the certificates of compliance, and applicable requirements imposed by international conventions or by the company. Precautions necessary to the safe stowage and loading of the cargo should be taken and ask the shipper/the charterer for clarification immediately if the group of cargo or the cargo name (BCSN) disagree with what the IMSBC Code provides.

(3) Remove external heat sources that could facilitate an SSD event, including but not limited to cargo lights, heating systems, steam pipes, electric motors, electric cables and open flames. All lights and other heat sources in the cargo holds should be switched off and isolated prior to loading and until the cargo is discharged. No welding or any other form of hot work which may affect the temperature of the cargo shall be carried out during the entire voyage. Smoking in cargo spaces should be prohibited to all personnel including crew members, stevedores, and visitors.

(4) Make sure the ANBF cargo is not contaminated with organic (e.g. packaging materials, grains) or inorganic substances (e.g. chlorides and metals including aluminium powder, chromium, copper, alloys of copper, cobalt and nickel). Clean the hold well before loading and remove all cargo residues, especially when grains or ore concentrates have been loaded for previous voyages. Oil and bunkers are also potential sources of contamination.

(5) Have a constant monitoring of the temperature and oxygen concentration in the hold so that early actions can be taken in the event of cargo decomposition. It is also advisable to have additional fire-fighting equipment on board as well as temperature measuring equipment, ideally Victor lances, so that the crew may effectively prevent propagation of heat.

(6) To stop a thermal decomposition that does not require the presence of oxygen, it is only effective to remove the heat from the reaction area, either by vigorous ventilation or by injecting water directly to the seat of decomposition with a Victor lance. Where large quantities of water are required, consider flooding the affected holds, provided the strength and stability of the vessel is calculated.

(7) With ANBF cargo giving off toxic fumes during decomposition, fire-fighting equipment should be donned by personnel handling the cargo. The vessel should also be manoeuvred in a way to drive the fumes away from accommodation. Evacuate in time if the vessel cannot stay clear of the fumes.

For more information, please contact Managers of the Association.