On August 4, 2020, a massive explosion shook Beirut, the capital city of Lebanon, a small west-Asian nation located on the Mediterranean coast. The scale of this disaster was catastrophic, and sites as far as 250 kilometres felt tremors. A lot of questions arise from this cataclysm that took the entire world by surprise and caused appalling losses of life and property. What chemical was at the root of an explosion of such a massive scale, and what triggered it? As chemical engineers, what should we be most concerned about? We will try to answer some of these questions in the paragraphs to follow.
Ammonium Nitrate – A stockpile of disaster
The notorious chemical behind the catastrophic explosion is ammonium nitrate (), as primary investigations of the blast videos and eye-witnesses’ reports had speculated. The two telltale signs of an ammonium nitrate explosion (detonation, to be precise) – dark orange smoke rising from the explosion site, indicative of nitrogen dioxide gas () and massive shockwave engulfing the neighbouring areas – couldn’t have been more conspicuous in the case of the Lebanese fiasco.
Ammonium nitrate, widely used in fertilizers and explosives, contains nitrogen in two different oxidation states. An exothermic redox reaction occurs upon supplying energy to it, with ammonium acting as the reducing agent and nitrate as the oxidizing agent. Under normal conditions, it will gradually decompose into nitrous oxide () and water, however, at high temperatures (read external heating, as in case of a fire), the dominant reaction would be-
The above reaction is exothermic and can result in deflagration (a combustion reaction propagating through a medium such as air at subsonic speeds) or even destructive detonation (deflagration at supersonic speed), with the sight of explosion shockingly similar to a nuclear blast and giving rise to shockwaves and ‘mushroom’ clouds. The orange clouds, as mentioned previously, are attributed to nitrogen dioxide, which is one of the alternative products in a common side reaction.
Bonus – Shockwaves and Mushroom Clouds
Mushroom clouds, omens of a nuclear explosion, are not limited to atomic blasts. Any natural or artificial reaction resulting in an explosive release of energy and hot gases in a humid environment, such as volcanoes, can result in such a phenomenon. In a typical exothermic reaction, a shockwave is triggered by the detonation, which causes water vapour to condense under high pressure as it sweeps across the surrounding medium, and quickly evaporate as the pressure difference reduces resulting in the expanding white dome seen just after the detonation. After the blast, hot gases start rising rapidly and are met by cooler, more dense layers of air above, which cause them to curve outwards and downwards. The narrow stem of rising hot gases and the curving vortices resembles a mushroom, hence the name mushroom clouds.
Storage of hazardous materials – an industrial and domestic concern
The 2,700 tonnes of ammonium nitrate stored in the Beirut dock were perilously lying around for half a decade, prone to a cataclysmic detonation. The lethargy of systems to act upon pressing matters of safety and overlooking suggested norms and regulations the primary reasons for such catastrophes. In India too, a consignment of around quarter the size of that in Beirut was found near Chennai (fortunately, it was promptly auctioned off and transported to a Hyderabad based firm).
Very often, fires are the triggers for such incidents. Firecrackers stored in the vicinity are alleged to be the cause of the fire in Beirut. All regulatory organizations have strict fire norms for the storage of inflammable materials, and a sophisticated approach is imperative to avoid any accidents. Ammonium nitrate needs to be stored in sealed containers (called superstacks, which can hold up to a ton of the substance) to prevent the absorption of moisture. The detonation is a consequence of heating in confined environments, so containers need to be stacked with sufficient airflow in between them, necessitating a well-ventilated storage facility away from urban establishments. In terms of international maritime security, governments focus on piracy and terrorism, ignoring the much-needed prevention of port accidents. Thus, it becomes crucial to revisit these rules, with emphasis on punitive action for abandoning vessels and improper transport and storage of hazardous materials. If we do not take these measures seriously, history will only repeat itself, and we will have no other option than lamenting over our losses.
In recent times, a lot of industrial accidents have come to light in India as well. Very often, such incidents are a result of overlooking safety norms, unintentionally or intentionally. In the articles to come, we will explore the issues of industrial and domestic safety in the context of chemical engineering.