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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. It was at a seaport, too, meaning there were probably a lot of nearby things to act as fuel sources.

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.

The main point of this answer is to use the 2013 West Fertilizer Company explosion as an example of scenarios that could lead to AN detonation, and also to show that the situation can become very complex and unpredictable.

So yeah, the TLDR here is that conditions in a fire are far more complicated than ammonium nitrate just sitting around getting hotter.

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.

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.

Based on the location of the pile and the properties of the bin along with the circumstances of other fireinduced 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 pyrolysis86 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.

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.