The science behind the Beirut Blast: the chemical detonation that shook the Middle East

On August 4, a blast in the port city of Beirut, the Paris of the East, has devastated the lives of 300,000 people by sheer destruction of life and property. The chemical blast has led to a loss of over 172 lives, injuries of over 6000 people, many traumatized by the explosion. This article delves into the what, how, and whys behind the catastrophic incident.

Introduction

           One of the worst humanitarian crises has hit the nation of Lebanon. A blast in the port city of Beirut, the Paris of the East, has devastated the lives of 300,000 people by sheer destruction of life and property. The chemical blast has led to a loss of over 172 lives, injuries of over 6000 people, many traumatized by the explosion. The blast has been so strong that it shook various parts of Europe and West Asia, including Turkey, Syria, Israel, and even the Mediterranean island of Cyprus, equivalent to a 3.3 on the Richter Scale Earthquake, reported by USGS Agency. The explosion, however, cannot and should not be directly compared to the earthquake of the same magnitude as the nature of shockwaves produced due to the blasts and due to the earthquake are way apart. However, the seismic waves generated at the surface are not efficiently generated when compared to the same in earthquakes. If the same explosion had occurred underground, it would have led to even more muscular tremors. 

            The blast has led to a shock in the political climate of the country with the country Government Cabinet Officials, including the Prime Minister resigning their respective positions. The chemical behind the blast can be described as Ammonium Nitrate, which is constructively used in the manufacture of Fertilizers and controlled explosions. But when not stored in a secured or a safe place, the same chemical can act as a detonator.

          According to official sources, around 2750 tonnes of ammonium nitrate was stored at one of the warehouses in the Beirut Port. If all of this ammonium nitrate were to catch fire and explode, it would lead to an energy release equivalent to that 1.1 kilotonnes of TNT.

             The Before and After pictures of the blasts indicate shear the extent of the devastation. The explosion led to the formation of a large crater. The explosion eventually destroyed almost all the life and property with a devastating shockwave. To understand how this happened, we need to understand the chronology of the events.

The chain of events:

                 Before the stored ammonium nitrate caught fire, the blast was A Twin blast first blast at the fireworks company that led to a much more robust and devastative blast involving Ammonium Nitrate. This blast led to the formation of a mushroom cloud and a shockwave as powerful as the 3.3 Rictor scale earthquake. The USGS agency reported the estimates of the severity.

                    The exact timeline of the blast includes an initial explosion at a fireworks facility in Beirut Port. The depiction of the same is in the Photograph captured below. It is visible that the first much smaller explosion has led to large smoke clouds surrounding the port facility.

First, a fire broke out at one of the warehouses that had firecrackers in the Port of Beirut. It is evident from the below picture. The fire was burning violently with the release of enormous smoke clouds that dispersed the skies of Beirut. There were vast amounts of Ammonium Nitrate stored in a nearby warehouse, which was kept under the dark from the citizens of Beirut.

Fig 1. Fire at a Fireworks Facility at one of the warehouses in Beirut Port

            It was another beautiful evening in Beirut; the ammonium nitrate stored was about to catch fire. It was the moment when Murphy's Law was about to get right. "Anything that can go wrong will go wrong."The fire from the firework had come in contact with the poorly stored ammonium nitrate. Ammonium Nitrate is not flammable, but it supports burning, and if the conditions of pressure and heat are high enough, it can lead to massive-scale detonation. A similar occurrence is supposed to have happened in the warehouse. The exothermic reaction of the combustion of the chemical then led to the formation of a Fireball (Fig. 2) consisting of the exhausted gases from the destructive chemical reaction.

  Fig 2. Formation of Fireball due to combustion of Ammonium Nitrate

            The fireball then led to a subsequent formation of a Mushroom Cloud, and the high amount of moisture present in the air made things worse, leading to the generation of a destructive shockwave. Quantian has estimated the velocities of the gases released from the explosion to be around 3000m/s from the video footage, which is way more than the speed of sound, about 9 times. The broad availability of moisture in the air due to humid conditions led to a massive detonation of the infrastructure of the Capital City. The distinctive reddish-brown color of the exhausted gases can be supposed to be Nitrogen Dioxide, which is a combustion product of Ammonium Nitrate.

Fig 3. The formation of Shockwave and Mushroom Cloud

What led to the enormous storage of Ammonium Nitrate at the port facility?

The Ammonium Nitrate was stored in the port warehouse without proper safety measures for the previous six years, after having been confiscated by the Lebanese authorities from the abandoned ship MV Rhosus. A disaster was there to happen; it just needed date and time. The properties of the compound make the compound dangerous for storage under high humidity conditions.

Why did Ammonium Nitrate explode?

Properties of Ammonium Nitrate

                 Ammonium Nitrate is a white crystalline solid consisting of ions of ammonium and nitrate. It is highly soluble in water and hygroscopic as a solid, although it does not form hydrates.

History of Ammonium Nitrate

                      It has been predominantly used in agriculture as a high-nitrogen fertilizer. Its other primary use is as a component of explosive mixtures used in mining, quarrying, and civil construction. It is also the principal constituent of ANFO, a famous industrial explosive that accounts for 80% of explosives used in North America. Some similar formulations have been used in improvised explosive devices that have been misused for terrorism. That's why many countries have been raising concerns over its use by the civilian population. The industrial disasters caused due to Ammonium Nitrate have led to significant losses in terms of life and property since the early 20th century. Ammonium nitrate was first developed as a fertilizer, and over the years, the blasting industry found out that this could even work as a blasting agent. It takes a high explosive to set off ammonium nitrate, but if there is a massive fire that starts near it. That can also set off ammonium nitrate. 

Dependence on External Factors

         Moisture is one of the factors that pose a threat to the safe storage of Ammonium Nitrate. Ammonium Nitrate being hygroscopic, tends to absorb moisture present when the relative humidity of the surrounding air crosses the critical relative humidity value of 59.4%.

Therefore, it is crucial to store ammonium nitrate in a tightly sealed container or an enclosed warehouse with dry conditions. The Relative Humidity of the surroundings is generally always higher than 69.4% in the vicinity of the port. Thus, if the precautionary measures are not taken, the chemical in such conditions can coalesce into a large, solid mass, which when absorbs enough moisture, it can lead to liquefaction.

      The Molten ammonium nitrate is very sensitive to shocks and detonations, significantly when it becomes contaminated with materials such as combustibles, flammable liquids, chlorides, acids, chlorates, sulfur, metals, charcoal, and sawdust. Thus the relative humidity always needs to be in check in the storage facility.

What are the possibilities when Ammonium Nitrate can explode or detonate?

          When the chemical is stored in a confined, well-secured place, the chemical cannot catch fire on its own. For safer storage, the chemical is generally stored in a fire-resistant storage facility that is exclusively built from non-flammable materials. The reason behind this is that the chemical is not flammable on its own.

     Generally, when ammonium nitrate is heated, it decomposes in a non-explosive manner giving out O2, N2, and H20. However, it needs to be induced by an external stimulus to cause a massive detonation.

    One of the instances is when there is a shock to the detonation transition. The initiation can happen by an explosive charge that goes off in the mass, by the detonation of a shell thrown into the mass, or by the detonation of an explosive mixture that gets in contact with the mass. 

     The other can be when there is a fire in the vicinity of the stored ammonium nitrate or a mixture of ammonium nitrate. The fire needs to be in a confined location where the fire can transition into a massive detonation. The phenomenon is also known as DDT(Deflagration to Detonation Transition)

              In the case of Beirut Blast, as per the graphic video evidence, initially, there was a fire in a neighboring fireworks warehouse. The fire in this warehouse could have led to an initial deflagration in the stored ammonium nitrate. Also, due to higher relative humidity and confined area, the fire would have transformed into a massive detonation. The above instance could be drawn one of the most probable reasons behind the massive exothermic detonation.

The explosive nature of Ammonium Nitrate

               Once the solid ammonium nitrate gets exposed to an explosive material or fire, it gets decomposed into nitrous oxide and water vapor for temperatures below around 300 °C,

NH4NO3 → N2O + 2H2O

The below reaction predominates for temperatures above 300 ℃.

2NH4NO3 → 2N2 + O2 + 4H2O

             Both decomposition reactions are exothermic, and their products are gaseous. Under certain conditions of pressure and temperatures, this can lead to a runaway reaction, with the decomposition process becoming explosive.

Some previous significant instances of such explosions include the 1947 Ship Explosion in the Harbour of Texas City (Texas, USA), the 2015 Tianjin Explosion (Port of Tianjin, China ), and the current 2020 Beirut Port Explosion in the Warehouse of Beirut Port ( Beirut City, Lebanon).

For direct comparison, there is a need to have a standard through which two explosions can be compared. Equivalent TNT or ton of TNT is a unit of energy that is defined by convention to be 4.184 gigajoules, which is the approximate energy released in the detonation of a metric ton (1,000 kilograms) of TNT. Thus, whether it's a conventional explosive, asteroid impact, nuclear or atomic explosion, non-nuclear chemical explosion, or even an earthquake, they are more commonly measured using the Equivalent TNT.

Why the explosion led to the formation of the mushroom cloud and the shockwave?

Major Types of Explosions – Detonations and Deflagrations

       An explosion is very different from a regular fire. An explosion involves a rapid increase in volume and release of energy with the development of high temperatures and the release of gases. Explosions can be classified based on how do the gases travel.

Deflagration:

                  Deflagration (Lat: de + flagrare, "to burn down") is subsonic combustion that propagates through heat transfer; here, the hot burning material passes the heat to the next layer of cold material and ignites it. Most "fires" found in daily life, from flames to explosions such as that of black powder, are deflagrations.

In short, it is an explosion, where the gases arising from the explosion travel at lesser speeds than the speed of sound. Thus Shockwaves that travel at velocities greater than the speed of sound don't arise due to this explosion.

                Fig 4. Deflagration arose due to a fire spreading over an array of wood logs

Detonation:

               A detonation is way more fatal than deflagration. Detonation (from Latin detonare, meaning 'to thunder down/forth') is a type of combustion involving a supersonic exothermic front of flame that accelerates through a medium that eventually drives a shock front propagating directly in front of it. The velocity of the gases arisen from detonation has larger velocities than the velocity of sound in those conditions.

Thus, how the fire spreads determines the extent and strength of the resulting explosion. A deflagration can, without any signs, transform into a detonation when it comes in contact with explosive material.

                   Fig.5   Detonation accompanying the formation of a Mushroom Cloud

Further Reading: Helmenstine, Anne Marie, Ph.D. "The Difference Between Deflagration and Detonation." ThoughtCo. https://www.thoughtco.com/explosions-deflagration-versus-detonation-607316 (accessed September 12, 2020).

Vsause video about Detonation vs. Deflagration

What is a mushroom cloud, and what led to its formation?

        A Mushroom cloud is not a thing you should be seeing. These clouds are the effects of massive explosions where large changes in pressure and temperature occur. The source of a Mushroom Cloud doesn't always need to be nuclear. It instead needs to be a mighty one. Violent Forest fires, Regular non-nuclear explosion like chemical blasts, and violent volcanic eruptions can result in enormous mushroom clouds, which can have the potential to cross the atmospheric barrier.

       When a massive explosion occurs, a fireball is generally formed, which is composed of vast amounts of energy, and the temperatures of it can even reach up to the temperatures that we observe at the center of the sun: millions of degrees Celsius. The air from the explosion is hotter than the surrounding air making it less dense. However, the cooler atmospheric air surrounds the hotter air that gets released from the site of the explosion. Due to the density gradient, the hotter air tends to move upwards due to buoyancy leading to the Rayleigh-Taylor Instability. As the gases or fluids released from the explosion zone or explosion are hotter than the atmospheric air, they have relatively lower densities when compared to the surrounding air. The updraft leads to a buoyant driving force that results in momentum gain and, thus, the velocity of the explosive exhaust gases. The Rayleigh-Taylor Instabilities initially dominate the instabilities in the fluids due to a more considerable density difference. This effect eventually leads to a bulge, illustrated in the below depiction.

Fig 6. Rayleigh-Taylor instability and Kelvin-Helmholtz instability

                  Due to this, an updraft, powerful enough to pick up dust and moisture, is created that forms the stem of the mushroom cloud. As the velocity difference increase due to the updraft, gases in the interface of the hot and cold air in the sideways undergo rolling motion in the lateral direction resulting in the Kelvin-Helmholtz Instabilities, depicted in the above illustration( Second from left). On the other hand, the same updraft has an opposite effect by the resistive forces of the surrounding air. It is a well-known fact that as the altitude increases, the density of the air decreases. Thus the strength of the updraft force decreases with increasing altitude leading to flattening or lateral spread of the exhaust gases, which form the distinctive cap of the mushroom cloud; Final depiction in the above figure.

Fig. 7 Mushroom Cloud Formation

          In the above figure, the red arrows represent the hot air while the blue arrows represent the cooler atmospheric air.

      In simpler terms, the Rayleigh-Taylor instabilities occur due to the difference between densities of fluids while the Kelvin-Helmholtz Instabilities occur when there is a difference in velocities. These instabilities are not exclusive to the Mushroom clouds but can occur where the respective conditions are fulfilled. A perfect mushroom cloud from the top can be visualized as a donut or a mendu Wada of rotating winds. 

What is a shockwave, and what are the conditions that lead to its formation?

              Shock waves were only recognized as a natural phenomenon for more than a century ago; still, they are not widely understood. They are responsible for the crash of thunder, as well as the bang of a gunshot, the boom of fireworks, the sonic boom produced due to supersonic jets, or the blast from a chemical or nuclear explosion. Sound waves and Shockwaves are both Pressure Waves. However, they are not loud sounds but are different from what sound is. A medium can thus produce shockwaves due to the sudden dissipation of mechanical energy in a small enclosed space. For a shockwave to form in a chemical explosion, the explosive gases need to travel with velocities higher than the speed of sound in that medium. As discussed earlier, the velocities of the exhaust gases were about 9 times the speed of sound.

      The shockwave can travel in a matter of milliseconds and has the potential to destroy the medium through which it travels with a sudden and violent change in stress, density, and temperature. However, the intensity of the shockwave decreases inversely with the distance from the source of the explosion. The Shrimp can generate localized shockwaves that can stun its prey using its claws.

       Generally, high energy explosions always lead to detonating shockwaves. As the shockwaves are not generally visible, they are difficult to predict. However, special imaging techniques like Schlieren and Shadow imaging can visualize the onset of a Shockwave at the lab scale. Shockwaves are sometimes visible if they travel through the air that has varying refractive indices.

      Fig. 8 A schlieren photograph that captures the bursting of a balloon as the skin shreds very rapidly. This reveals a balloon-shaped bubble that consists of compressed air from the inside. The occurrence leads to the formation of a circular-shaped shockwave. (Photograph courtesy of Gary S. Settles)

                   According to Brian Castner, a weapons investigator for Amnesty International, in his exact words, says that "When there's a detonation, what's caused is a shockwave and that shockwave is the air pushing itself into itself, essentially, and it sets up a front—kind of like a storm front [although] this is many, many, many times more than that. But as the air pushes into itself when the air is humid, you can see the shockwave, and it forms that cloud". He further adds that "So there's always a shockwave (with detonations), the only difference is I think because it was warm, and because it was by the ocean, and because it was humid the conditions were right for you to see the shockwave this time—when oftentimes you don't see it." The below video depicts the actual incident with shockwave formation and the subsequent cracking of the ground near the site of the Beirut Blast.

                     Some other visual indication of the onset of the shockwave is the formation of Wilson Clouds, which form due to low-pressure waves that follow the shockwave. Wilson Cloud and Mushroom Cloud are composed of the same starting material: condensed vapor. As the region of the explosion was a port with higher humidity, the extent of the Wilson Cloud, which forms due to condensation of moisture in the air, was more significant. Thus, Wilson clouds result in the cooling of air marking the termination of the shockwave up to a particular region.

                       Fig.9 Wilson Cloud Formation after the Beirut Blast

Ending Notes

The catastrophe that occurred at the Port of Beirut was avoidable, in my opinion. Governments, especially that of the developing nations, should ensure that the regulations of the chemical manufacturing and storage facilities are meeting the highest possible standards with stringent monitoring of the situations. The storage of large amounts of hazardous chemicals should be avoided as much as possible as they present human health and safety issues. In-situ manufacturing, Process Intensified Operations, and processes can lead to safer usage and handling of Hazardous Chemicals. 

One might argue that with stringent monitoring and abidance to safety protocols, hazardous chemicals can be stored and processed more safely. We would be answering the same in the next blog, so stay tuned.

Sources:

1. Enthalpy of the reaction of Ammonium Nitrate
2. Imaging of Shockwaves
3. Shockwaves in Slow motion (Video)
4. Shockwave hits camera
5. Wired.me Article
6. Reuters Graphics
7. Kelvin–Helmholtz_instability Wiki
8. Rayleigh–Taylor_instability Wiki
9. American Chemical Society(ACS) Article
10. Popular Mechanics Article
11. CEN Article
12. Mushroom Cloud Wiki
13. LANL Article
14. Another Wired.me Article
15. Bellingcat Article
16.  Imaging of Shockwaves