There have been many questions as to how the ammonium nitrate at the West, TX fertilizer distributor could have blown up. Investigators have still not stated with 100% certainty that the explosion involved AN, but it is my belief they are just crossing all their “t’s” and dotting all their “i’s” before stating this. Unless the facility had some other energetic materials on site that they failed to report, this explosion was caused by AN exposure to the fire conditions at the facility. This posting is to try and explain the conditions in which AN fertilizer can become explosive as it decomposes. One theory the investigation team has to be looking at is what else was stored in that building that could have been INCOMPATIBLE with the AN storage. It is a known fact that the energy necessary to initiate thermal decomposition of AN decreases when the AN is sensitized. There are several conditions that can sensitize AN and increase the hazards of thermal decomposition, deflagration and detonation. They are:
- Concentration – AN explosions have occurred on solid product or in solutions with concentrations greater than 95%. However, violent decomposition reactions of AN solutions have been documented with concentrations as low as 80% ammonium nitrate by weight in water. AN fertilizer is usually sold in solid form at 99-100% concentration.
- pH – As AN becomes more acidic, AN stability decreases. Free acid in AN solution is a significant source of or a contributing factor to AN solution decomposition.
- Temperature – Increased temperatures can result in increased sensitivity. Reduced temperatures can result in AN crystallization (salting out), and solid AN may be less difficult to initiate than the 83% AN solution. At this point in the investigation, it is believed that the fire at the facility played a MAJOR role in this tragic accident.
- Contamination – AN contaminated with organic materials is more easily detonated. Inorganic contaminants also sensitize AN and include chlorides and some metals, such as chromium, copper, cobalt and nickel. These contaminants reduce the energy required to initiate a critical ammonium nitrate decomposition reaction. Chlorides react synergistically with some metals to further reduce energies needed to initiate thermal decomposition. Studies show that synergistic catalysis with chloride generally requires a metal capable of forming chloro complexes of reasonable stability in two oxidation states differing by one electron unit, and at the same time are capable of forming chloro complexes of moderate stability in these states. Chromium was considered a special case because oxo complexes were believed to be involved. Metals that react synergistically with chlorides to further reduce thermal decomposition reaction temperatures include chromium, copper, silver, nickel, iron, palladium, gold, cerium, and cobalt. Other sensitizing materials include hypophosphites and thiosulfates.
- Confinement – The higher the degree of confinement, the greater the possibility of deflagration/detonation. The degree of confinement necessary to support detonation decreases, as the presence of other sensitizing factors increase. The mass of the ammonium nitrate may create sufficient confinement if the AN is sufficiently sensitized and the threshold temperature for thermal decomposition is reached. It is my personal belief at this stage of the investigation that CONFINEMENT played a role. Based on the radio traffic from the responders on scene, it was stated a “tank blew up”. This is hard to mistake – so I am leaning to the belief that the facility was receiving their AN by rail (a known fact now) and that they had some type of tanks/bins/containers they used to store the AN. The facility had two choices for storage: 1) in tightly sealed bins/tanks/containers, or 2) in open piles inside a climate controlled warehouse. I have seen no signs that the warehouse was climate controlled and based on the radio traffic from the surviving responders I believe the facility was using bins/tanks/containers to store their AN. This is KEY as CONFINEMENT is the one condition that can be easily explained at this facility based on these two storage methods and radio traffic. Throw in #3 above “Temperature” and we have TWO CRITICAL CONDITIONS for AN detonation.
- Low density areas – Low density pockets in the ammonium nitrate melt, such as gas bubbles, reduce the energy required to initiate decomposition reactions. Bubbles also enhance thermal decomposition propagation in the ammonium nitrate solution through adiabatic compression as the decomposition pressure wave moves through the media, generating temperatures as high as 2,000°F.
Source of these 6 items: CHEMICAL ACCIDENT INVESTIGATION REPORT – TERRA INDUSTRIES, INC. NITROGEN FERTILIZER FACILITY PORT NEAL, IOWA
PLEASE NOTE I do NOT claim to be an explosives expert or an expert on AN. There are REAL EXPERTS in the mining and demolition industry that work with AN on a daily basis. I am merely providing some factual information so that those interested in learning about how the West, TX tragedy may have happened and how it COULD happen in their community. AN is a relatively SAFE product as compared to many other HAZMATS; but all HAZMATS come with hazards and risks.