Hazmat: Understanding Ammonium Nitrate

Ammonium nitrate fertilizer, chemically classified as an oxidizer, is commonly used in agriculture and for commercial and residential turf grass. Approximately 7 million tons of ammonium nitrate are produced annually in the U.S. and, under normal conditions, it is a relatively safe material in storage and use.

Ammonium nitrate has been in the news during the past two decades when it was used as an oxidizer for homemade chemical explosives by domestic and foreign terrorists. Ammonium nitrate also has been involved in accidents and responsible for the deaths of emergency responders and civilians. This year in West, TX, a fire and explosion at a retail fertilizer facility killed 15 people, mostly emergency responders fighting the fire. Ammonium nitrate is also used as a chemical oxidizer in the manufacturing of the commercial explosive ANFO, a chemical mixture of ammonium nitrate and No. 2 fuel oil.

Handle with care

Ammonium nitrate, NH4NO3, is a strong inorganic oxidizer that can be an explosive by itself under certain conditions. It is primarily used as an agricultural fertilizer for its nitrogen content. Response personnel should deal with ammonium nitrate incidents with a great deal of caution. Ammonium nitrate is a colorless or white-to-gray crystal that is soluble in water. It decomposes at 210°C (392°F), releasing nitrous oxide gas and ammonia.

Although ammonium nitrate itself does not burn, as an oxidizer it supports and enhances combustion. When it comes in contact with other combustible materials, the fire hazard is increased. A fire involving ammonium nitrate in an enclosed space can lead to an explosion. Because it is an oxidizer, a fire involving ammonium nitrate can occur in the absence of atmospheric oxygen. Ammonium nitrate may explode when exposed to strong shock or high temperatures under confinement.

Contaminants may increase the explosion hazard of ammonium nitrate. Organic materials, such as chlorides and some metals, such as chromium, copper, cobalt and nickel, can make explosions involving ammonium nitrate more energetic.

Under National Fire Protection Association (NFPA) 704, Standard System for the Identification of the Hazards of Materials for Emergency Response, hazards for ammonium nitrate are health 1, fire 0, reactivity 3 and special information OX for oxidizer. The four-digit United Nations identification number is 1942 with an organic coating and 2067 as the fertilizer grade. Other mixtures of ammonium nitrate have four-digit numbers; they can be found in the Department of Transportation (DOT) Hazardous Materials Tables and Emergency Response Guide.

Factors in explosions

Several factors must be in place for an explosion to occur involving a chemical explosive such as ammonium nitrate. An explosion involving a chemical explosive is in actuality a fast-moving fire. In simple terms, an explosive that functions via a chemical reaction creates a rapidly burning fire that is made possible by the presence of a chemical oxidizer. Atmospheric oxygen does not provide enough oxygen for a chemical explosion to take place.

As a review of fire science, in order to have a fire, you need a fuel, oxygen and heat, which when combined under the right physical conditions create an ongoing chemical chain reaction. This process continues until the fuel is consumed, the heat is reduced, the oxygen is removed or something interrupts the chemical chain reaction.

Components that allow a fire to burn rapidly enough to produce an explosion are the presence of a fuel; instead of oxygen, you need a chemical oxidizer; plus heat or some type of initiator that creates the heat. Unlike fire, you need the chemical mixture to be confined for an explosion to occur. Confinement can be accomplished with the use of a piece of pipe, plastic tube, cardboard tube or any type of substantial container that will accomplish confinement. The material itself, if in an appropriate-size volume, can provide the confinement necessary for the explosion to occur.

Deadly explosions

Several major disasters involving ammonium nitrate have occurred over the years. In Texas City, TX, on April 16, 1947, the SS Grandcamp was at the port taking on a load of ammonium nitrate fertilizer to be shipped to Europe as part of the rebuilding process following World War II. Approximately 17 million pounds of ammonium nitrate was loaded onto the ship. Also in the harbor that day was the SS High Flyer, located approximately 600 feet from the Grandcamp on the same dock and loaded with 2 million pounds of ammonium nitrate. (By comparison, the bomb used in the terrorist bombing of the Murrah Federal Building in Oklahoma City, OK, in 1995 contained 5,000 pounds of ammonium nitrate. The deadly explosion at West, TX, this year involved approximately 60,000 pounds of ammonium nitrate.)

At 9:12 A.M., an explosion occurred within the hold of the Grandcamp. Instantly, all 27 members of the Texas City Volunteer Fire Department at the scene were killed. Some bodies were disintegrated by the heat and blast pressure of the explosion. All that remained of the department’s fire apparatus were piles of twisted metal. Texas City lost all but one of its firefighters and all of their apparatus in the explosion.

On Nov. 29, 1988, at approximately 03:40 A.M., the Kansas City, MO, Fire Department received a call for a fire at a highway construction site. Several explosions occurred following the arrival of the fire department. It was reported by the fire department that the first explosion involved a split load of materials in a trailer/magazine.

One compartment held approximately 3,500 pounds of ANFO. The rest of the contents were approximately 17,000 pounds of ammonium nitrate/fuel oil mixture with 5% aluminum pellets. In the second trailer/magazine there were approximately 1,000 30-pound “socks” of ammonium nitrate/fuel oil mixture with 5% aluminum pellets.

Pumper 30 was dispatched and arrived on scene at 03:52. Twenty-two minutes after Pumper 41 arrived and approximately 16 minutes after Pumper 30 arrived, the magazine exploded, killing all six firefighters assigned to Pumper 41 and Pumper 30. (For more information about the Texas City disaster, see “The Day Texas City Lost Its Fire Department” by Robert Burke in the May 2007 issue of Firehouse®.)

The 1995 terrorist bombing at the Oklahoma City federal building involved a homemade mixture of ammonium nitrate fertilizer and fuel oil. As a result of the attack, 168 people, many of them children, died and another 600 were injured.

No emergency responders were killed at either incident since the explosions occurred before their arrival, but that may not always be the case. More than 800 buildings sustained some type of damage from ground shock and blast pressure. Of the buildings damaged, 50 would have to be demolished. Windows were broken as far as two2 miles from the blast site and the blast was heard 50 miles away. It registered 3.5 on the open-ended Richter Scale in Denver, CO.

Regulated materials

Explosives in the United States are regulated by the federal Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) in fixed storage and by the DOT when in transit. Ammonium nitrate fertilizer is not regulated by the ATF because it is not an explosive. Ammonium nitrate is regulated by the Department of Homeland Security (DHS) for security purposes.

DOT classifies ammonium nitrate as an oxidizer. The NFPA covers storage requirements for ammonium nitrate and other chemicals in its Standard 400, Hazardous Materials Code.

Since the bombing of the federal building in Oklahoma City, DHS has monitored the storage and sale of bulk ammonium nitrate fertilizer used for agricultural purposes. This oversight has led to the reduction of ammonium nitrate used by farmers and stored for sale by retailers across the country. It has also caused the retailers that still handle the product to be careful about information regarding its presence. It can only be sold to licensed farmers.

(While conducting research for this article, I learned that according to the Nebraska Agri-Business Association Inc, there are 31 sites in Nebraska that store and sell ammonium nitrate fertilizer, but the association would not divulge the names or locations of those retailers. The association also advised me that if I happened to locate one of the retailers, it is likely I would be forbidden to take photographs or obtain any additional information. As I tried to contact retailers for this article, I found none who were willing to admit they had the ammonium nitrate, even though I talked to people who said they did. Though very frustrating to me as a reporter, all of this is good because it would also make it very difficult for potential terrorists or criminals to gain access to the ammonium nitrate as easily as those who used it in Oklahoma City.)

The West, TX, tragedy

On April 17, 2013, a fire and subsequent explosion occurred at the West Fertilizer Company in West, TX. Firefighters from the West Volunteer Fire Department were fighting a fire at the facility when the explosion occurred. Ammonium nitrate was located in a bin inside a seed and fertilizer building on the property. The explosion registered 2.1 on a seismograph reading in Hockley, TX, 142 miles away. Fifteen people, mostly emergency responders, were killed, more than 200 were injured and 150 buildings sustained damage.

Investigators confirmed that ammonium nitrate was the source of the explosion. According to the U.S. Environmental Protection Agency (EPA), there was a report of 240 tons of ammonium nitrate on the site in 2012. According to DHS, the company had not disclosed to the agency its ammonium nitrate stock. Federal law requires that DHS be notified whenever any facility has more than one ton of ammonium nitrate on hand, or 400 pounds if the ammonium nitrate is combined with combustible material

The fire and explosion at West were investigated by the U.S. Chemical Safety Board (CSB). Listed below are some of the observations and preliminary findings following the initial investigation (for a complete listing and to monitor the investigation of the West explosion, see www.csb.gov):

• The explosion at West Fertilizer resulted from an intense fire in a wooden warehouse building that led to the detonation of approximately 30 tons of ammonium nitrate stored inside the wooden bins. Not only were the warehouse and bins combustible, but the building also contained significant amounts of combustible seeds, which likely contributed to the intensity of the fire.

• The building lacked a sprinkler system or other systems to automatically detect or suppress fire, especially when the building was unoccupied after hours. By the time firefighters reached the site, the fire was intense and out of control. The detonation occurred just 20 minutes after the first notification to the West Volunteer Fire Department.

• Although some U.S. distributors have constructed fire-resistant concrete structures for storing ammonium nitrate, fertilizer industry officials have reported to the CSB that wooden buildings are still the norm for the distribution of ammonium nitrate fertilizer across the U.S.

• No federal, state or local standards have been identified that restrict the sitting of ammonium nitrate storage facilities in the vicinity of homes, schools, businesses and healthcare facilities. In West, there were hundreds of such buildings within a mile radius, which were exposed to serious or life-threatening hazards when the explosion occurred on April 17.

• West volunteer firefighters were not made aware of the explosion hazard from ammonium nitrate stored at West Fertilizer and were caught in harm’s way when the blast occurred. The NFPA recommends that firefighters evacuate from ammonium nitrate fires of "massive and uncontrollable proportions." Federal DOT guidance contained in the Emergency Response Guidebook, which is widely used by firefighters, suggests fighting even large ammonium nitrate fertilizer fires by “flood(ing) the area with water from a distance.” However, the response guidance appears to be vague since terms such as “massive,” “uncontrollable,” “large” and “distance” are not clearly defined. All of these provisions should be reviewed by fire departments and other emergency response agencies in light of the West disaster to ensure that firefighters and other responders are adequately protected and are not put into danger protecting property alone.

Rural responses

Responders in agricultural communities should be aware of the types of fertilizers stored at local facilities. Ammonium nitrate is not one of the extremely hazardous substances covered by the Emergency Planning and Community Right to Know Act (EPCRA). However, fire departments have the right to information involving chemicals at a facility for the purposes of pre-planning, even if the chemicals are not regulated under the act. The State Emergency Response Commission (SERC) in each state and the Local Emergency Planning Committee (LEPC) may also be of assistance.

When responding to fixed-facility or transportation incidents in or around construction sites, mining operations or facilities that retail agricultural fertilizers, be on the lookout for explosives or chemical oxidizers such as ammonium nitrate or commercial-grade ANFO. When responding to transportation incidents, always consider the possibility of explosives or oxidizers being present.

Fire is the principal cause of accidents involving explosive materials. Look for explosive signs, such as placards and labels. Evacuate the area according to the distances listed in the DOT Emergency Response Guidebook orange section. If no other evacuation information is available, a 2,000-foot minimum distance should be observed, according to the NFPA Fire Protection Handbook.

There is one rule of thumb in responding to incidents where explosives or chemical oxidizers are involved: DO NOT FIGHT FIRES IF THE FIRE HAS REACHED THE EXPLOSIVE STORAGE AREA OR CARGO. THE SAME APPLIES TO CHEMICAL OXIDIZERS SUCH AS AMMONIUM NITRATE.