We still need the ‘combustibles’. What the heck was burning?
Down to: Accident or Terrorism
ANFO: The Tool of Destruction 10/01/1995
BY FRANK L. FIRE
ANFO is an acronym for the most popular blasting agents in use today. It stands for ammonium nitrate, fuel oil; and it is a simple mixture of the two. Ammonium nitrate is an oxidizing agent that does more than just supply oxygen to support the combustion of a fuel—it also is itself an explosive substance. Since it is an oxidizer and a material that burns (explodes), this combination of two legs of the fire triangle (the third, of course, is energy) can prove deadly. Indeed, on occasion, ammonium nitrate has itself detonated, causing loss of life and considerable property damage.
Ammonium nitrate`s explosive properties were recognized as early as 1867, when a patent was issued for its use with various sensitizers and nitroglycerin. Its first widespread use was to replace some of the nitroglycerin used in dynamite. Its first use as an ingredient in nondynamite explosive materials was in a blasting agent called Nitramon, introduced by Du Pont in 1935. In 1955 a patent was issued for a blasting agent using fertilizer-grade ammonium nitrate (FGAN) with a solid fuel sensitizer. Coal was one of the solid fuel sensitizers used, and the solid fuels later were replaced with liquids such as fuel oil.
Ammonium nitrate`s explosive nature has been known to scientists and industrialists for more than 130 years but is also known to the general public because of the many disasters its accidental detonation has caused. The most widely known disaster is the 1947 Texas City explosion, which caused nearly 500 deaths and extensive property damage.
ANFO is amazingly easy to produce and therefore is a favorite tool of terrorists. It is safer and easier to make than black powder, the previous choice of terrorists. ANFO is relatively stable, whereas black powder is extremely sensitive and prone to accidental explosion. ANFO is classified as a blasting agent; black powder is classified as an explosive.
MAKING THE BOMB
To make an ANFO bomb, you can purchase ammonium nitrate in quantity from any business that services the agriculture industry. Farmers routinely buy quantities of it because it makes a good fertilizer. You blend two to five percent common kerosene, household fuel oil, or diesel fuel with 95 to 98 percent ammonium nitrate, mixing it well. The resulting ANFO is so insensitive to shock that it requires a high explosive such as dynamite to detonate it (as opposed to the slight friction needed to explode black powder). The dynamite is detonated with a timing device so the bomber can escape before the explosion. As long as the ANFO is in a container or contained in some manner, the detonation will be quite destructive.
The energy liberated by pure ammonium nitrate is 2,620 kiloJoules per kilogram (kJ/kg), while common explosives liberate between 2,500 and 7,000 kJ/kg (nitroglycerine is 6,790 kJ/kg). The gas volume generated by exploding ammonium nitrate is 980 decimeters cubed per kilogram (dm3/kg); for nitroglycerine, it is 716 dm3/kg. The detonation velocity of pure ammonium nitrate is 2,700 meters per second (m/s), while that of ANFO is around 4,200 m/s.
“FERTILIZER” IS MISNOMER
The majority of ammonium nitrate manufactured is for use as a fertilizer. Since ammonium nitrate is the principal ingredient in ANFO, people often refer to ANFO as a fertilizer instead of a blasting agent or explosive material. Ammonium nitrate is a fertilizer, and it will explode under certain conditions. It is a very stable chemical compound, but when anything that will burn (a fuel) is added to it, ammonium nitrate becomes “sensitized” and can be detonated. Since there is so much ammonium nitrate in ANFO, the industry has begun referring to ANFO as a fertilizer.
Blasters also have gotten into the habit of calling ANFO a fertilizer. They could be sensitive to the possibility of vandals want- ing to get their hands on a relatively insensitive explosive material. Even though the semitrailers transporting ANFO must be placarded with “Blasting Agent,” the placards are removed as soon as the shipment reaches its destination. Over-the-road trailers, with some minor modifications, can be classified as Class 5 magazines, approved for the storage of blasting agents. In almost every case, the trailer used to transport the ANFO to its destination will be used to store the material.
Some individuals working for construction companies, government agencies, and even blasting companies have no idea of what a blasting agent is. Since they hear others refer to it as a fertilizer, they do, too.
DANGER TO RESPONDERS
Lack of “Blasting Agent” placards on trailers at construction sites, lack of warning signs indicating the presence of explosive materials (a common practice), and lack of education of employees concerning the hazards contained in trailers all contribute to an apparent disregard for the safety of personnel responding to an emergency at the site.
The U.S. Department of Transportation classifies ANFO as a blasting agent, not an explosive. The difference in definitions between the two is technical. Blasting agents are explosive materials, and they are designed to explode, producing predetermined results. n
FRANK L. FIRE is the vice president of marketing for Americhem Inc. in Cuyahoga Falls, Ohio. He`s an instructor of hazardous-materials chemistry at the University of Akron as well as an adjunct instructor of haz mats at the National Fire Academy. Fire is the author of The Common Sense Approach to Hazardous Materials and an accompanying study guide, Combustibility of Plastics, and Chemical Data Notebook: A User`s Manual, published by Fire Engineering Books. He is an editorial advisory board member of Fire Engineering.