Firebombs and Fertilizer

On August 4, 2020 a warehouse in Beirut, Lebanon blew up. It was the same explosive that was used by the domestic terrorist who blew up the federal building in Oklahoma City in 1995. An even larger explosion destroyed much of the gulf port of Texas City in 1947, killing over five hundred. The explosive? Ammonium Nitrate, used mainly as a fertilizer but also as occasionally employed as an explosive.

Roughly ten to fifteen thousand years ago humanity began the transition from a hunter-gather tradition to agriculture. The transition is ongoing to this day but is complete for all but the most isolated primitive societies. One thing learned about agriculture early on is the plants do better with fertilizer – traditionally animal manure. It provides a micronutrient – Nitrogen, necessary for building protein.

Our atmosphere has an abundant supply but it is in a form that has limited use. Some plants such as legumes have nodules in their roots that contain bacteria that can convert Nitrogen from the atmosphere to the form, Nitrate, that plants can take up from the soil.

If you are growing grain however, and especially in a northern climate, additional Nitrogen from other sources is important. A rich source was Chilean Nitrate. The Atacama desert, the highest driest spot on earth has for countless ages accumulated Nitrate. The source is slow but atmospheric – lightening. Lightening converts atmospheric Nitrogen in very small amounts to nitrate, which in the dry desert slowly accumulates. But it is a slow high energy process. The Beagle of Charles Darwin’s fame visited the Atacama to assess the supplies of nitrate available.

Early in the twentieth century as the demand for nitrate accelerated, two German Chemists, Fritz Haber and Carl Bosch developed the chemistry to produce nitrate from atmospheric Nitrogen, a process called artificial fixation of Nitrogen. With natural gas as a source of energy and Hydrogen, atmospheric Nitrogen is reduced to Ammonia and thence converted to nitrate.

The chemists received the Nobel Prize for their work. Although the process has been used to this day to produce fertilizer, Ammonium Nitrate was used by Germany during WWI as an explosive. So what the heck turns fertilizer into an explosive? Concentration and pressure.

Whether the Ammonium Nitrate is for a bomb in a Ryder Truck, or a storage warehouse in Beirut the conditions are essentially the same. In Oklahoma City, the bomb was in barrels saturated with diesel fuel which increased the explosive density by solution. In Beirut, the extant condition was age. The material had been there far too long in a humid environment which again allowed some dissolution and hence concentration.

The explosion was triggered by a fire in an adjacent warehouse. The blast killed hundreds and made homeless hundreds of thousands more. It blew ships out of the water, demolished nearby grain silos, and created a several hundred feet wide crater.

Dr. Bob Allen is Emeritus Professor of Chemistry, Arkansas Tech University.

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