Is Ammonium Nitrate Flammable? (A Comprehensive Overview)

Is Ammonium Nitrate Flammable?

Ammonium nitrate is not flammable but it poses a very serious fire hazard. Ammonium nitrate composes about 94% of the explosive known as ANFO (ammonium nitrate/fuel oil), also known as AN-foh or AN/FO.

Ammonium nitrate is a strong oxidizing substance, in a typical ANFO explosion, ammonium nitrate will oxidize long chain hydrocarbon compounds in a combustion reaction.

In this explosive, ammonium nitrate is the oxidant of the combustion reaction. And the fuel oil, which can be diesel, kerosene or other hydrocarbons, is the flammable substance (the substance that is oxidized in the combustion reaction).

According to the NFPA 400 classifications of 2016, ammonium nitrate is classified as a Class 2 oxidizer. That being said, it is believed by some that certain properties of ammonium nitrate are not fully understood. 

According to the International Fire Code, ammonium nitrate falls into Class 1 of solid oxidizers, meaning it is an explosive material.

A number of fire incidents in the past have been related to ammonium nitrate. 

The form in which ammonium nitrate is has a major influence on its explosive capacity. The two most common forms are the technical grade (TGAN) and the fertilizer grade (FGAN).  In the technical grade ammonium nitrate has a higher porosity to facilitate fuel oil absorption.

The TGAN is therefore overall more likely to cause explosions than FGAN, but both these forms can.

Ammonium nitrate is also available as an aqueous solution.

Ammonium Nitrate Fire Hazards

The main fire hazard of ammonium nitrate is its potential to cause explosions. It is known that in the presence of a flammable substance and heat explosions will occur. 

There is reason to believe that ammonium nitrate is also capable of causing explosions under other specific conditions. Given the relatively high number of incidents that have been related to ammonium nitrate in the past.

In its use in explosives ammonium nitrate undergoes a combustion reaction action as the oxidizing agent. Most everyday life fires have oxygen gas as the oxidizing agent. The difference between them is that ammonium nitrate can easily be packed in high amounts since it is a solid, while oxygen is a gas.

The other known reaction ammonium nitrate is known to undergo is decomposition at high temperatures.

Combustions in Which ammonium Nitrate is the Oxidizer

Most everyday fires occur through the oxidation of a given material by the oxygen from the air, those fires are produced by the energy released in a combustion reaction. A combustion reaction is one in which a substance is oxidized generating heat.

Ammonium nitrate is not considered flammable because it is not easily oxidized, on the contrary it can oxidize other substances relatively easily. In the ANFO explosive, ammonium nitrate oxidizes a given flammable material (usually diesel or kerosene).

For this process to start (the combustion reaction) heat is necessary. Once started heat will be produced as one of the products of the combustion reaction. This heat is consumed to give continuation to the combustion process.

Under common conditions though, the combustion of ammonium nitrate will also lead to formation of carbon monoxide (CO) and nitrogen oxides (NOx).

The combustion of ammonium nitrate in the presence of a flammable material also falls in the definition of a decomposition reaction, that is a reaction in which a substance has its atoms rearranged into more than one product (NH4NO3 to N2 CO2 and H2O in this case).

Decomposition of Ammonium Nitrate Reactions

Besides reacting with fuels to cause explosions or fires, ammonium nitrate can also react by itself by decomposition at high temperatures. Decomposition may occur under different chemical pathways.

At temperatures above 180 ºC up to around 300 ºC, ammonium nitrate can decompose into nitrous oxide (N2O), which is also known as laughing gas, and water. 

Both these decomposition products are relatively non hazardous, nonetheless the reaction liberates heat and can also increase the pressure if occurring in a closed environment. Meaning there is a certain risk of explosion.

At temperatures above 300 ºC another decomposition reaction may occur in parallel. In this case, the formed gases are nitrogen gas, oxygen gas and water. All of which are  not hazardous, however, for being gases they can increase the pressure of the environment.

Such reactions occur because ammonium nitrate is a salt composed of an nitrate anion (NO3) and an ammonium cátion (NH4+) when enough heat (energy) is applied the nitrate ion oxidizes the ammonium ion.

Under completely isolated conditions only the above products can be formed (N2O, N2, O2 and H2O). However when decomposition occurs in most environments, other species will be formed such as nitroxide (NO).

Chemical Reactivity of Ammonium Nitrate

Ammonium nitrate be explosive if in contact with the following chemicals:

Aluminium chloride (AlCl3)Potassium permanganate (KMnO4)Sugars
Ammonia (NH3)Rust (Fe2O3.nH2O, FeO(OH), Fe(OH)3)Urea ((NH2)2CO
Barium nitrate (Ba(NO3)2)Sodium hypochlorite (NaClO)Wood/Sawdust
Hypochlorous acid (HClO)Sulfur (S8)

Ammonium nitrate can also react strongly with the following metals:

Aluminum (Al0)Chromium (Cr0)Magnesium (Mg0)Sodium (Na0)
Antimony (Sb0)Copper (Cu0)Manganese (Mn0)Tin (Sn0)
Bismuth (Bi0)Iron (Fe0)Nickel (Ni0)Zinc (Zn0)
Cadmium (Cd0)Lead (Pb0)Potassium (K0)

Additional Informations Regarding Ammonium Nitrate and Related Fire

  • Appearance: crystalline powder.
  • Color: varies from white to brown.
  • If an already existing fire goes in contact with ammonium nitrate the fire will likely intensify given the highly oxidizing character of ammonium nitrate.
  • Fumes from fires involving ammonium nitrate may liberate nitrogen oxides (NOx) and other hazardous vapors.
  • The explosion velocity is around 3000 m/s.
  • Explosions involving ammonium nitrate often have a dark yellow color, which is from the formation of nitroxide (NO) which is produced by the incomplete decomposition of ammonium nitrate.

Other Ammonium Nitrate Health Hazards

  • Lethal oral dose (LD50) in rats is 2.950 mg/kg.
  • Lethal dose by inhalation (LD50) in rats for 4 h is equal or higher to 88,8 mg/l.
  • Lethal dermal dose (LD50) in rats is equal or higher to 5.000 mg/kg.

Ammonium Nitrate Storage Recommendations

The ammonium nitrate storage standards should follow the NFPA 400 regulations and the IFC regulations.

Incidents Caused by Ammonium Nitrate

The causes for unintended ammonium nitrate related explosions can be divided in two classifications.

  • The first is by shock followed by explosion. The shock can be caused by an initial small explosion, a high impact on ammonium nitrate, or other mechanical processes.
  • The second is caused by flames, sparks or heating being subjected to ammonium nitrate. In the event of an explosion being caused by a spreading fire, the term ‘deflagration to detonation transition’ or DDT is used.

The above classifications do not include the incidents in which the possible cause was ammonium nitrate decomposition by itself. For such cases there is still unknown data.

Explosives that Contain Ammonium Nitrate in their Composition

Some explosives other than ANFO that have ammonium nitrate along with other materials: Astrolite, Amatol, Ammonal, amatex, DBX, Goma-2, Minol, Tovex.

Conclusion

This article gave insights to the question ‘is ammonium nitrate flammable?’.

Ammonium nitrate is a very important substance in today’s society for its huge usage as fertilizer. At the same time a considerable number of casualties have been caused by explosions involving ammonium nitrate.

This substance handling, transportation and usage is of considerable risk.

Frequently Asked Questions (FAQ): Is Ammonium Nitrate Flammable?

Does ammonium nitrate explode or burn?

Ammonium nitrate does not burn by itself, but it can lead to ignition of flammable materials. But by itself ammonium nitrate can cause explosions if heated to temperatures above 180 ºC. When heated to such temperatures ammonium nitrate can decompose into gases that will lead to an increase in the ambient pressure. This can lead to explosions.

Can fire make ammonium nitrate explode?

Yes, an explosion of nitrate can happen if it is submitted to high temperatures. The danger is considerably higher if a flammable material, such as wood, cloth, alcohol, gasoline, diesel, coal, molasses, sugars are in the vicinity.

Is ammonia flammable or explosive?

Ammonia (NH3) is a gas at normal temperature and pressure and is not flammable by definition. However it poses significant explosion hazards because it is a highly reactive chemical, ammonia can react violently with many common materials such as cleaning, products, cosmetics, water, oxygen, ammonium nitrate. Ammonia is also often kept in pressurized cylinders, damaging or heating ammonia containing cylinders can also lead to explosions.

Why is nitrogen explosive?

Many, but not all, substances that contain nitrogen atoms in their molecules are prone to be explosive because when they decompose they often produce nitrogen gas as one of the products of an eventual combustion reaction. The relation between formation of nitrogen gas and explosivity can be explained for two main reasons.

Nitrogen gas is a very stable molecule, which translates to its formation producing high amounts of energy; and to the quick increasing of the volume or pressure of the ambient in which nitrogen gas was produced.

Can nitrogen gas catch fire?

Nitrogen gas, also known as dinitrogen, is the molecule represented by N2 and is one of the most stable molecules on Earth. There are no feasible means to cause it to catch on fire.

However, there are many nitrogen-containing materials (a substance with the element nitrogen in them is very different from the element nitrogen by itself) in gas, solid or liquid form that are flammable or explosive.

Is dry ice liquid nitrogen?

No, dry ice is how carbon dioxide in solid form is commonly known. Liquid nitrogen is obtained after nitrogen gas is pressurized or cooled into liquid, it remains the same substance, but in a different physical state. Nitrogen turns into liquid at -196 ºC at normal pressure. Carbon dioxide turns into solid at -78 ºC.

References

https://www.nfpa.org/News-and-Research/Data-research-and-tools/Hazardous-Materials/Variables-Associated-with-the-Classification-of-Ammonium-Nitrate (Accessed March 30th, 2022)

https://codes.iccsafe.org/content/IFC2015/appendix-e-hazard-categories (Accessed March 30th, 2022)

https://www.tfi.org/sites/default/files/documents/fact_sheet_west_fertilizer_-_april_20_-_kom.pdf (Accessed March 30th, 2022)

https://cen.acs.org/safety/industrial-safety/chemistry-behind-Beirut-explosion/98/web/2020/08 (Accessed March 31th, 2022)

S.B. DuTeaux, Texas City Disaster, Editor(s): Philip Wexler, Encyclopedia of Toxicology (Third Edition), Academic Press, 2014, Pages 519-520, https://doi.org/10.1016/B978-0-12-386454-3.00086-5.

P.S. Rao, Ammonium Nitrate, Editor(s): Philip Wexler, Encyclopedia of Toxicology (Third Edition), Academic Press, 2014, Pages 209-211, https://doi.org/10.1016/B978-0-12-386454-3.00235-9

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