NOTE: I received this really detailed analysis of at least one potential explanation of why the Athens, TX ammonium nitrate facility did not explode Thursday like the one did in West, TX a year ago. It came to me in an email from Jim Overman who has graciously allowed me to post it here.
I really think there needs to be a lot of technical work done on the response of AN to fire. I have gotten a lot of questions from friends about one had a devastating explosion and the other just burned. You can include the incident in Bryan, TX a few years ago as well. I'm sure that research would reveal countless others.
Certainly, there are many factors involved in what happened in each of these circumstances. I still think a significant factor is the volume to surface area ratio. As I have stated several times, 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.
As you know, the rate of reaction doubles with every 10 degree (K) increase in temperature. (I recognize this is only a rough guideline). Also, the rate of energy production of a reaction is directly related to the rate of the reaction (we are discussing exothermic reactions). 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 like "fire". It really gets exciting when the rate is high enough to use terms like deflagration and if the reaction moves fast enough through the medium to exceed the speed of sound, when refer to it as a detonation.
Essentially, in oxidation reactions like those between iron and oxygen, the only difference between rusting and burning is the rate of reaction. We all know that steel wool will burn in pure oxygen. This is an example of concentration of the reactants influencing the rate.
In general, we use water effectively on Class A fires because the evaporation of the water removes heat from the reactants (e.g., wood and oxygen) faster than it is being produced. (It is also pertinent to this discussion that the water vapor produced reduces the concentration of the other reactants.) 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 removes it and the fire will continue to burn until there are not enough reactants (concentration).
Now, look closely at pictures of the AN storage in Athens [PJC – For example here and here]. The piles are not especially deep and the bins are are more like rooms an not very high. That means that the ration of volume to surface area is low. In other words, the total surface area of a given bulk is higher in a shallow pile with higher length and width. This is the relevant heat transfer equation:
Fourier's Law express conductive heat transfer as
q = heat transfer (W, J/s, Btu/s)
A = heat transfer area (m2, ft2)
k = thermal conductivity of the material (W/m.K or W/m oC, Btu/(hr oF ft2/ft))
dT = temperature difference across the material (K or oC, oF)
s = material thickness (m, ft)
Note that the only variables that really count are A and s. As A gets smaller and s gets larger, q gets smaller. In other words, physics works and less heat gets removed from the bulk and the reaction rate goes up. As the temperature of the burning material goes up, dT actually increases so sometimes the change in dT will balance the other factors and we will have steady state burning. This explains why "cooling the pile with hose streams sometimes works. However, if the increase in dT does not balance things out, we have a recipe for disaster.
There are many other variables involved, but unless contamination is a factor, I suspect this may be the most significant. After all is said and done, standards that do not address this issue will fall short when fires actually occur.
Jim makes some very important points about the heat transfer effect in an ammonium nitrate fire. As he mentions in his closing paragraph contamination of the ammonium nitrate may also play an important role. This was noted in the Chemical Safety Board’s interim report on the West explosion. Another factor that could come into play is building collapse as that can have a direct pressure effect, reduce heat transfer and impede the flow of water to the ammonium nitrate stack.
At this point it does not look like the Chemical Safety Board will be doing an investigation of this accident. There was no loss of life and there was only limited (on a grand scale) property damage involved. As a single incident goes this just isn’t important enough to tie up the CSB’s limited resources. As we are finding more and more of these under-regulated ammonium nitrate storage facilities across the country, perhaps it would be important for the CSB to get involved in the investigation to help prevent another West Fertilizer type explosion. That would almost certainly take a congressional request, something unlikely to come from the Texas congressional delegation.