Chemistry - How does ammonium nitrate explode on its own?
Solution 1:
It is known that ammonium nitrate decompose exothermically when heated to form nitrous oxide and water. This paper1 notes that the irreversible decomposition of ammonium nitrate occurs at the temperature range of $\pu{230-260 ^\circ C}$.
$$\ce{NH4NO3 ->[t >230 ^\circ C] N2O + 2H2O}$$
They also further noted that beyond $\pu{280 ^\circ C}$, $\ce{NH4NO3}$ is capable of rapid, self-accelerating decomposition (to the point of detonation).
But at the detonation temperature, $\mathrm{t_d}$ (the temperature at which compounds detonate), ammonium nitrate fully decomposes to nitrogen, oxygen and water releasing a tremendous amount of energy.
$$\ce{2NH4NO3 ->[t_d] 2N2 + O2 + 4H2O}$$
In the context of Beirut explosion, the question that raised was "when did ammonium nitrate reached detonation temperature, and why did it suddenly explode?". According to a news report from cnet.com:
When heated to above 170 degrees Fahrenheit, ammonium nitrate begins to undergo decomposition. But with rapid heating or detonation, a chemical reaction can occur that converts ammonium nitrate to nitrogen and oxygen gas and water vapor. The products of the reaction are harmless -- they're found in our atmosphere -- but the process releases huge amounts of energy.[...]
Additionally, in the explosion, not all of the ammonium nitrate is used up and exploded. Some of it decomposes slowly creating toxic gases like nitrogen oxides. It's these gases that are responsible for the red-brown plume of smoke seen in the aftermath of the Beirut explosion, Rae said.
So, my theory is that ammonium nitrate started heating (from the fire) releasing all sorts of nitrogen oxides (the red fumes). This fire further accelerated the reaction, further heating the remaining ammonium nitrate to the point of detonation and that's when ammonium nitrate exploded instantaneously releasing tremendous amount of energy which send shockwaves around the site along with a white mushroom shaped cloud (from @DDuck's comment) which could probably be nitrogen and/or water vapours where the humid air (water vapor laden air) condensed due to the explosion(@StianYttervik) with release of nitrogen. It is a sad and quite devastating incident.
References
- On the Thermal Decomposition of Ammonium Nitrate. Steady-state Reaction Temperatures and Reaction Rate By George Feick and R. M. Hainer, 1954 (PDF)
- Reaction Rates of Ammonium Nitrate in Detonation Melvin A. Cook, Earle B. Mayfield, and William S. Partridge The Journal of Physical Chemistry 1955 59 (8), 675-680 DOI: 10.1021/j150530a002 (PDF)
- https://en.wikipedia.org/wiki/2020_Beirut_explosions
Solution 2:
Ammonium nitrate ($\ce{NH4NO3}$) is widely used in the fertilizer industry and is one of the most concentrated forms of nitrogen fertilizer (35% of $\ce{N}$). At the same time, it has also been widely used as an explosive material for detonation in mines. Because of its explosiveness, $\ce{NH4NO3}$ is associated with various hazards including fire and explosion, which have occurred repeatedly in the past (more than 70 incidents during 20th century, more than half of them occurred in the US soil). Regardless, $\ce{NH4NO3}$ is not considered a flammable or combustible material at ambient temperature and pressure (Ref.1). However, it is a strong oxidizing agent that can detonate under certain conditions such as temperature, fire, confinement, and presence of impurities (e.g., $\ce{KCl}$), which can be act as a promoter to detonate (Ref.2).
To use as an explosive or a blasting reagent, $\ce{NH4NO3}$ is mixed with fuel oil, which is called ammonium nitrate fuel-oil (ANFO; Ref.1). According to Ref.2, during the explosion, following exothermic reaction would take place (hydrocarbon is represented by $\ce{CH2}$):
$$\ce{3NH4NO3 + CH2 -> 3N2 + 7 H2O + CO2} \quad \Delta H = \pu{-4017 kJ/kg} \tag1$$
Interestingly, this can be compared with TNT, the heat of combustion of which is $\Delta H = \pu{-4196 kJ/kg}$. Without fuel oil, can be detonated under certain conditions. It is believed that the vaporization of molten $\ce{NH4NO3}$ leads to the formation of ammonia and nitric acid, which could initiate the decomposition of $\ce{NH4NO3}$ through following reaction:
$$\ce{NH4NO3 <=> HNO3 + NH3} \quad \Delta H = \pu{176 kJ/mol} \tag2$$
At higher temperatures (i.e., between $\pu{170 ^\circ C}$ and $\pu{280 ^\circ C}$) exothermic irreversible reactions (equations $(3)-(5)$) occur:
$$\ce{NH4NO3 -> N2O + 2H2O } \quad \Delta H = \pu{-59 kJ/mol} \tag3$$ $$\ce{NH4NO3 -> 1/2N2 + NO + 2H2O } \quad \Delta H = \pu{-2597 kJ/mol} \tag4$$ $$\ce{NH4NO3 -> 3/4N2 + 1/2NO2 + 2H2O } \quad \Delta H = \pu{-944 kJ/mol} \tag5$$
If the material is suddenly heated up, there will be explosive decompositions as shown in equations $(6)$ and $(7)$):
$$\ce{2NH4NO3 -> 2N2 + O2 + 4H2O } \quad \Delta H = \pu{-1057 kJ/mol} \tag6$$ $$\ce{8NH4NO3 -> 5N2 + 4NO + 2NO2 + 16H2O } \quad \Delta H = \pu{-600 kJ/mol} \tag7$$
Keep in mind that all of these reactions except for $(2)$ are exothermic. Also, most products are gases. I attached PDF file if Ref.2 if a reader is interested in how explosions happen during right conditions (otherwise, it is a broad field to explain). For instance, the reaction $(3)$ can be made more exothermic ($\pu{789 kJ/mol}$) with more gaseous products, if some oxidisable fuel is added such as $\ce{C}$ (Ref.3):
$$\ce{2NH4NO3 (s) + C (s) -> 2N2 (g) + CO2 (g) + 4H2O (g)} \tag8$$
It is evident from the past incidents involving $\ce{NH4NO3}$ that the presence of impurities and environmental conditions have a huge effect on the detonation of $\ce{NH4NO3}$ during storage. For example, one of the deadliest industrial incidents in US history occurred on April 16, 1947, in Texas City, Texas where an $\ce{NH4NO3}$ explosion involving $\pu{2300 tons}$ of $\ce{NH4NO3}$ caused 581 fatalities and thousands of injuries. The fire was caused by the initial explosion of $\ce{NH4NO3}$ on a ship, which resulted in subsequent chain reactions of fires and explosions in other ships and facilities nearby. The exploded $\ce{NH4NO3}$ was coated with (carbon based) wax to prevent caking (See equation $(8)$ above). After this accident, the new technologies and safe practices introduced in the 1950s eliminated the use of wax coatings (Ref.2).
References:
- Guy Marlair, Marie-Astrid Kordek, "Safety and security issues relating to low capacity storage of AN-based fertilizers," Journal of Hazardous Materials 2005, 123(1–3), 13-28 (https://doi.org/10.1016/j.jhazmat.2005.03.028).
- Zhe Han, "Thermal Stability Studies of Ammonium Nitrate," Ph.D. Dissertation, Texas A&M University, TX, 2016 (PDF).
- Alex M. Djerdjev, Pramith Priyananda, Jeff Gore, James K. Beattie, Chiara Neto, Brian S. Hawkett, "The mechanism of the spontaneous detonation of ammonium nitrate in reactive grounds," Journal of Environmental Chemical Engineering 2018, 6(1), 281-288 (https://doi.org/10.1016/j.jece.2017.12.003).
Solution 3:
First, ammonium nitrate is a kind of mixture between an oxidizer - the nitrate part - and a reducer - the ammonium one. This is at the core of your question.
The direct decomposition correctly mentioned in the answers is nevertheless a process in which something gets oxidised and something gets reduced.
In ammonium nitrate basically you have all you need - the "fuel“ and the “oxygen“ analogues of what is involved in a standard, explosive or not, combustion.
Still the other answers are valid and more detailed from a chemical mechanicist viewpoint. One point out the presence of NO2 clearly seen by its red brownish colour before the second powerful explosion.
But the straight answer to your question is that the oxidizer and the reducing stuffs are already within the salt.
Side note: ammonium nitrate can decompose by mechanical shock, so there were enough conditions to trigger the second powerful blast.
Solution 4:
The main point of this answer is to use the 2013 West Fertilizer Company explosion (USCSB animation for context) as an example of scenarios that could lead to AN detonation, and also to show that the situation can become very complex and unpredictable.
Anything in the vicinity can become fuel, especially if a fire is already in progress. This includes the containers, impurities, soot and debris from the fire, etc. Plus, ammonium nitrate's melting point is ~337 F, meaning it can become molten, possibly escaping its container, and readily mixing with fuel sources.
The US CSB West Fertilizer Explosion final report, section 4.3, outlines three possible scenarios under which the 2013 explosion in West, Texas could have occurred. Section 4.2 outlines general contributing factors.
These aren't the only ways it can explode, but they are a few examples of the types of conditions that could lead to explosion.
You should definitely read the report; my brief summary below leaves a lot of relevant analysis out.
So from section 4.2, contributing factors (FGAN = fertilizer grade ammonium nitrate):
Contamination
In fire situations, the behavior of FGAN is unpredictable, in part because of the number of endothermic and exothermic decomposition reactions that take place with increasing temperature. FGAN decomposition reactions beyond the first step have yet to be uniquely defined, and subsequent decomposition reactions of FGAN can only be assumed. When contaminants are added to AN, the decomposition reactions become increasingly more complex. Possible sources of contamination in an FGAN storage area can include ignitable liquids, finely divided metals or organic materials, chloride salts, carbons, acids, fibers, and sulfides. These contaminants can increase the explosive sensitivity of FGAN.
The molten FGAN at the WFC likely came in contact with contaminants that were stored in the fertilizer warehouse or were produced during the fire that preceded the explosion. Seed materials, zinc, and other organic products, including the wood-constructed bins, were present near the FGAN storage area or could have come in contact with molten FGAN. During the fire, soot from the smoke and also collapsing wood and roofing material might have mixed with the FGAN pile.
Poor Ventilation
The limited ventilation increased the quantity of soot in the smoke and the potential contamination of the FGAN pile. ...
At some point around 5 to 6 minutes before the detonation, the character of the fire changed, according to eyewitness accounts and photographic evidence (Figure 40). This change was most likely caused by increased ventilation through an opening low in the building, possibly when the fire burned through the seed room doors or the roof. The fire also might have been enhanced by oxidizing gases from the heated FGAN pile...
The additional ventilation caused a marked decrease in dark smoke and probably was accompanied by a major increase in heat radiation inside the fertilizer building because of increased oxygen availability to the burning wood and other fuels. With the dark smoke inside of the structure reduced, radiant heat would reach the surface of the FGAN in the bin, and the increased airflow through the building would greatly increase the radiant heat flux by raising the temperature of the burning wood. The surface of the FGAN, covered with soot or molten asphalt, would absorb the heat flux and cause a very rapid heating of the surface of the FGAN pile. The very hot and contaminated surface of the pile was then sensitive to detonation.
And from section 4.3, a few detonation scenarios:
- Scenario 1: Detonation from the top of the FGAN pile.
- Scenario 2: Detonation in heated FGAN along exterior wall exposed to fire.
- Scenario 3: Detonation in elevator pit that spread to main FGAN bin
Scenario 1: Detonation from top of pile
Based on the location of the pile and the properties of the bin along with the circumstances of other fire induced incidents, one possible scenario is that a period of contamination with soot and other organics (possibly including molten asphalt and plastic dripping from the burning composite shingle roof and PVC drop pipe from the elevator mechanism) was followed by about 5 to 6 minutes of intense radiant heating from the flames above and adjacent to the main FGAN bin. During this time, a layer of very hot, contaminated, and sensitive liquid FGAN could have built up on the pile. The foaming FGAN likely produced oxidizing gases, and those mixed with flammable smoke to produce a detonable gas cloud over the FGAN pile in the main bin and possibly in an adjoining bin linked to the main bin through a series of holes cut in the partition between the bins. The cloud consisted of powerful oxidizers that would be expected when FGAN undergoes thermal decomposition—such as NO2, O2, and HNO3 as wells as fuelrich smoke and pyrolysis products off-gassing from the molten FGAN. The gas cloud then might have ignited from above, undergoing a gas-phase deflagration-to-detonation transition (DDT) in the confinement of the bin.
Scenario 2: Detonation along fire line
This scenario involved heating of the FGAN through the walls and is noted as being highly unlikely, so to keep this short I'm not going to quote it here. See section 4.3.2 for details.
Scenario 3: Detonation in elevator pit
Another possible detonation scenario focuses on the elevator pit near the FGAN bin. A fiberglass lid covered the pit, and the floor sloped away from the pit to prevent runoff from entering it, but the fire might have melted the cover, and FGAN remnants could have been in the pit. ...
If the detonation began in the pit, then the most feasible mechanism would be a collapse of the west wall of the bin, spilling FGAN into a mixture of burning rubber from the melted elevator belt and residual FGAN in the bottom of the pit. The mass of the falling FGAN, combined with the strong confinement of the concrete pit walls, might have provided the conditions for a solid phase DDT beginning in the bottom of pit and spreading into the main pile.
TLDR
So yeah, the TLDR here is that conditions in a fire can be extremely complicated and unpredictable, giving rise to a lot of opportunities for contamination and detonation.
In particular:
- AN can melt and the liquid can do unpredictable things.
- The containers can be destroyed by fire allowing AN to escape into unpredictable places.
- Even if the AN was contaminate free under normal conditions, anything in the area can become a fuel source including the containers themselves, fire debris, smoke, soot, collapsed building parts, etc.
In Beirut we saw that there was already a fire burning for a significant amount of time before the explosion, as well as a smaller explosion that occurred < 30 seconds before the main one. There were also flashes and bangs and a lot of other stuff going on there (reportedly there were fireworks stored in the same warehouse). It was at a seaport, too, meaning there were probably a lot of nearby things to act as fuel sources.
It is very, very conceivable that the AN became both heated enough and contaminated enough during this time to detonate.
Here is a list of other AN accidents that you could research on your own to find out about other scenarios that can lead to detonations. Most notable:
- BASF, Oppau, Germany, 1921
- Texas City, Texas, USA, 1947
- AZF, Toulouse, France, 2001
- Ryongchŏn, North Korea, 2004
- Tianjin, China, 2015
Also, you may find some of the theories regarding the 1988 PEPCON accident in Nevada, USA interesting as well. That was not ammonium nitrate (it was ammonium perchlorate, another oxidizer), but the possible scenarios are similar and it also illustrates the complexity of those kinds of situations.