Since the explosion last month in West, TX there has been an awful lot of talk in the press (and amongst politicians) about the dangers associated with ammonium nitrate (See Twitter @Chemicalsafetyboard for the most comprehensive set of links to such news reports). The devastation in West, TX notwithstanding, ammonium nitrate fertilizer is not an explosive; to become an explosive it must be adulterated with other flammable/combustible material.
Ammonium Nitrate Explosives
Ammonium nitrate the explosive is a mixture of ammonium nitrate and fuel oil, more commonly referred to in the industry as ANFO. The ammonium nitrate explosive used in the attack in Oklahoma City was a mixture of ammonium nitrate fertilizer and racing fuel. The ammonium nitrate contributes two things to this ‘explosive’ mixture. First it is an oxidizer, upon decomposition (from heating for example) it produces oxygen gas which makes other flammable things burn faster. Secondly it increases the burnable surface area of the organic liquid by distributing it (absorbing it) throughout its bulk placing it in close proximity to the oxygen produced by decomposition. This greatly increases the speed of burning turning a flammable/combustible liquid into an explosive.
Other oxidizers can do the same thing. I mentioned in an earlier blog post ‘sugar bombs’ made by mixing either potassium chlorate or sodium chlorate with sugar. Again, the oxidizer provides both a matrix and an oxygen source for the explosive.
As far as we know (and we may never know because of the bureaucratic infighting between ATF and the Chemical Safety Board; mostly on the ATF side from what we have heard in the news) no one deliberately added any combustible/flammable liquids in the stored ammonium nitrate at West Fertilizer. There may have been other combustible organic material in the area (seeds, wood construction building, wood constructed storage bins, etc) that the oxidative properties of the ammonium nitrate turned into explosives. There was after-all something burning in the area and oxidizers don’t really burn.
Self Accelerating Decomposition Reaction
Nearly all molecules, if heated to a high enough temperature, will decompose into small molecules and/or atoms. There is a class of molecules, however, that when they begin decomposing produce heat that will accelerate the decomposition process through a self-accelerating decomposition reaction (SADR). Ammonium nitrate is one of these molecules.
Since gasses are typically the end product of these SADRs a great deal of pressure can build-up during the decomposition process if the material is in a confined space, such as a container. Since heat has a tendency to weaken the strength of the confining material, these pressure buildups from SADRs frequently result in the violently catastrophic failure of the container. That looks to most people like an explosion.
In large bulk storage of ammonium nitrate the material itself may act as the container, particularly if it is in a structured storage situation like bins. There are even reports that something falling onto a large bulk of heated ammonium nitrate may be enough to cause this type of pressure explosion.
So, ammonium nitrate is not an explosive, it is an oxidizer. As with other oxidizers you keep it away from combustible materials and you generally do not have any problems. This is clearly reflected in the large amounts of this material that are handled every day in this country in very large quantities and the very small number of explosive incidents that do occur.
To put this in perspective, let’s look at some other readily available chemicals that people handle every day that have the potential for causing explosions much larger than the one in West Texas. Gasoline for instance; under the proper circumstances the fumes from a gasoline spill may form just the right fuel air mixture to become a deadly fuel-air explosive (see the 2009 Catano Oil Refinery explosion in Puerto Rico or the 2013 PEMEX refinery explosion). Under the proper circumstances the amount of fuel in a gasoline tanker truck could easily be enough to produce an explosion comparable to West Fertilizer. These trucks drive major city streets every day.
Natural gas is another flammable, a gas this time instead of a liquid, that under the proper circumstances can produce a devastating explosion. The Chemical Safety Board has investigated a number of these (see The Little General Store) as has the NTSB (see San Bruno). Again, under the proper circumstances a significant, but hardly catastrophic, pipeline leak can also produce an explosion comparable to West Fertilizer. These pipelines run through neighborhoods.
None of this decreases the problems seen in the West Fertilizer situation. Neither the EPA nor OSHA has taken any actions on regulating SADR type situations (See T-2 Labs Explosion) as strongly recommended by the TSB. Congress has done nothing to provide support to the CSB in this matter and continues to under-fund and under-staff the agency.
Maybe the West Fertilizer explosion and all of its publicity will help change this situation. Probably not.