Volume-to-Surface

Certainly, many factors are involved in an ammonium nitrate (AN) detonation as occurred in West, TX. One of the most significant is the volume-to-surface area ratio.

The rate of reaction is dependent on concentration of the reactants, which in the case of the gas phase of any reaction is definitely related to pressure, and the temperature. A rough guideline is that the rate of reaction doubles with every 10 degrees Kelvin increase in temperature. Also, the rate of energy production of a reaction is directly related to the rate of an exothermic reaction.

This is one reason we have runaway reactions. We utilize this intentionally with commercial explosives. If the rate is very slow, we can use such common terms as corrosion in reference to the reaction. If the reaction is more vigorous, we use terms such as “fire.” It really gets exciting when the rate is high enough to use terms such as deflagration.

Basically, a fire takes place at the interface between fuel and oxygen. The larger that interface, the more fire we have and the more rapidly the fuel is consumed. Thus, a deflagration or low order explosion is really a very fast burning fire. If we initiate such a reaction and feed fuel to it at a rate equal to the rate of burning, say as with a gas burner, we have what amounts to a continuous explosion.

If the reaction moves fast enough to exceed the speed of sound, we have a detonation.

Essentially, in oxidation reactions such as those between iron and oxygen, the only difference between rusting and burning is the rate of reaction. Steel wool will burn in pure oxygen, the concentration of the reactants influencing the rate. Water is effective on Class A fires because the evaporation removes heat from the reactants (e.g., wood and oxygen) faster than it is being produced. Now consider fires in bulk materials. If we can’t get water to the hot reactants fast enough, the reaction continues to produce heat faster than the evaporating water remove it. The fire will continue to burn until there are not enough reactants, unless the reactant itself is water reactive.

Worse, the AN is not a solid but granulated. Because there is more reactive area, the fire will consume the fuel faster. The amount of heat liberated will be the same since the available fuel has not changed.

Early gun powder makers understood the relationship between surface area and the volume of fuel. Hence, they manufactured black powder in grains of various sizes for use in different firearms and as an explosive. But that led to a new problem. As the fuel burns, it gets smaller. As it gets smaller, the amount of surface area decreases and the reaction rate slows.

To counteract this, powder makers developed a process called “corning” in which the powder is formed into grains having a hollow center. As these grains burn, the outside surface decreases but the surface on the interior of the hollow center increases, meaning the rate of burning is nearly constant. Propelling gases are produced over a longer time span and at a more constant rate.

The same principle has been applied to fertilizers, i.e., AN, to reduce the possibility of a dust explosion during processing or field application.

In the Athens, TX, AN fire, the piles were not especially deep and the bins, more like separate room, were not very high. That means the ration of volume to surface area was low. In other words, the total surface are of a given bulk is higher in a shallow pile with a higher length and width. Less heat gets removed from the bulk and the reaction rate goes up. As the temperature of the burning material goes up, the temperature difference across the material actually increases so sometimes the change will balance the other factors, causing steady state burning.

This explains why cooling the pile with hose streams sometimes works. However, if the increase in temperature difference across the material does not balance things out, we have a recipe for disaster.

Unless contamination is a factor, temperature difference may be the most significant factor. Any new standard that do not address this issue will fall short when fires occur.

Because AN is an oxidizer, it does not need atmospheric oxygen to burn. Attempting to smother that reaction like a normal structure fire not only does not work, it may lead to catastrophic results. Smothering may well have the effect of reducing the temperature difference by trapping the heat. There is no advantage to reducing the oxygen in the container. Likewise, there is no advantage to smothering the fire. The impact of trapping heat and steam in the container would be the same as if you had a fire in a ship and closed the hatches to apply steam. Students of history will recognize the tragic comparison to the 1947 Texas City disaster.

AN is extremely soluble in water, exhibiting a high negative heat of solution, explaining its use as the cooling agent in cold packs used by EMTs and athletic trainers. It is also extremely hydroscopic, meaning it will absorb moisture from the atmosphere and harden like a rock. Using dynamite to break-up hardened piles caused for some of the first industrial explosions involving AN, such as the 1921 blast in Oppau, Germany that killed 561.

When ammonium nitrate undergoes this moistening and drying sequence, the concentration of reactants changes and so do the chemical and energetic properties. It is commonly thought that the formation of a rock-like crust of ammonium nitrate sealed the reactants from the atmosphere and contributed to previous tragic explosions. For this reason, modern fertilizer manufacturers package ammonium nitrate in small pellets or “prills” that are then coated with clay to reduce caking due to atmospheric moisture.

Formerly prills were coated with paraffin wax but it was soon realized that this practice was counterproductive and had the same effect as the oil added to AN to make ANFO (Ammonium Nitrate and Fuel Oil), commonly used as an explosive in mining and quarrying operations.

Any storage standards should require a way to rapidly provide sufficient vertical ventilation once a fire starts. This issue is complex and involves competing priorities. What we have to avoid is a rush to standards that do not lead to solutions but make the situation worse.