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.
West Explosion
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.
Comparative Risks
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.