Many are familiar with Ammonium Nitrate as a component of Ammonium Nitrate and Fuel Oil (ANFO). This is the tertiary explosive that was used in the Murrah Federal building terrorist attack in Oklahoma City, Oklahoma on April 15, 1995. But fires and explosions involving ammonium nitrate are not new. In 1988, six Kansas City, Missouri firefighters died in the line of duty fighting a fire at a construction site that involved Ammonium Nitrate and fuel oil. On April 16, 1947, one of the largest non-nuclear explosions occurred in Texas City, Texas and involved Ammonium Nitrate.
A cargo ship, the S.S. Grandcamp, was moored in its dock when smoke was observed from the cargo hold at around 8:00 a.m. The Grandcamp was loaded with over 2,300 tons of ammonium nitrate. Over the next hour, attempts to extinguish the fire failed. Shortly before 9:00 a.m., the captain ordered his men to steam the hold, a fire fighting method where steam is piped in to put out fires in the hope of preserving the cargo. Meanwhile the fire had attracted a crowd of spectators along the shoreline, who believed they were a safe distance away. Spectators noted that the water around the docked ship was already boiling from the heat, and the splashing water touching the hull of the ship was vaporized into steam. The cargo hold and deck began to bulge as the pressure of the steam increased inside. At 9:12 a.m., the Ammonium Nitrate reached an explosive threshold and the vessel detonated, causing great destruction throughout the port.
The tremendous blast sent a 15-foot wave that was detectable nearly 100 miles off the Texas shoreline. The blast leveled nearly 1,000 buildings on land. The Grandcamp explosion destroyed the Monsanto Chemical Company plant and resulted in ignition of refineries and chemical tanks on the waterfront. Falling bales of burning twine added to the damage while the Grandcamp’s anchor was hurled across the city. Sightseeing airplanes flying nearby had their wings shorn off, forcing them out of the sky. Ten miles away, people in Galveston were forced to their knees; windows were shattered in Houston, Texas, 40 miles away. People felt the shock 100 miles away in Louisiana.
The explosion blew almost 6,350 tons of the ship’s steel into the air, some at supersonic speed. Official casualty estimates came to a total of 567, including all the crewmen who remained onboard the Grandcamp, but many victims were burned to ashes or blown to bits, and the official total is believed to be an under count. All but one member of the Texas City volunteer fire department were killed in the initial explosion on the docks while fighting the shipboard fire, and with the fires raging, first responders from other areas were initially unable to reach the site of the disaster.
You may be wondering by now, why Ammonium Nitrate even exists, since it has been the cause of so many deaths. Ammonium Nitrate is an odorless, colorless or white, crystal salt produced by the reaction of ammonia and nitric acid. Its chemical formula is NH4NO3 which is simply Nitric Acid (HNO3) plus Ammonia (NH3). Ammonium Nitrate is extremely beneficial in agriculture as it is an important component in fertilizer mixtures. Ammonia Nitrate’s nitrogen content is readily absorbable by plant life and vegetation. Small quantities are also sold as an additive for mining explosives and other nonagricultural uses.
As previously mentioned, Ammonium Nitrate and Fuel Oil otherwise known as ANFO is a tertiary explosive. A tertiary explosive does not readily detonate without other materials. Ammonium Nitrate’s NFPA 704 marking with a blue zero, red zero, yellow three, and OX (for oxidizer) present in the white section.
Ammonium Nitrate is generally not harmful. However, inhaling large concentrations of Ammonium Nitrate dust can cause respiratory problems or even suffocation. Moreover, if high concentrations of Ammonium Nitrate are swallowed, it can cause cardiac irregularities, vomiting, convulsions, collapse and suffocation. When mixed with water, Ammonium Nitrate forms a mild acid which can cause irritation to the eyes, nose and skin. Some effects of exposure can be delayed up to 48 hours.
Ammonium Nitrate is not flammable. However, it is a strong oxidizer that can contribute to the ignition of surrounding combustibles. Ammonium Nitrate will only explode under extreme heat and pressure in a confined space. An explosion will create a visible cloud of ammonia, carbon dioxide and nitrogen oxides. Because Ammonium Nitrate is a strong oxidizer, it can burn and support the burning of other surrounding combustibles even in the absence of oxygen. This makes extinguishment of fires involving Ammonium Nitrate extremely difficult at best.
When responding to an Ammonium Nitrate release with fire, flooding amounts of water should be used in the early stages of the fire. When massive fires occur involving Ammonium Nitrate, operational emphasis should be placed on public protective actions in the event an explosion occurs. Ammonium Nitrate decomposes during combustion and produces toxic oxides of Nitrogen. When water and dissolved Ammonium Nitrate are heated, Nitrous Oxide is produced. Nitrous Oxide is used as an anesthetic (known as laughing gas). It, too, is an oxidizer that will support combustion and is dangerously explosive. Solid streams of water may be ineffective on Nitrous Oxide
The proper personal protective equipment to wear during an Ammonium Nitrate fire is difficult to determine. Structural fire fighting clothing provides limited protection from Oxides of Nitrogen produced during the fire. However, chemical protective clothing provides little to no thermal protection from fire.
As in all hazardous materials research, multiple sources should be examined and the worst-case scenario should be considered appropriate. The CHRIS manual recommends that SCBA be worn and the surrounding area be evacuated. The CHRIS manual also recommends that fire fighting only be conducted from a protected location with unmanned hose holder or monitor nozzles.
The Department of Transportation Emergency Response Guide (ERG) concurs with the CHRIS manual. Fighting Ammonium Nitrate fires that are large should be conducted from maximum distance using unmanned hose holders or monitor nozzles. If this is impossible, withdraw from the area and let the fire burn. The Wireless Information System for Emergency Responders from the National Library of Medicine also concurs with the CHRIS manual and ERG. Smothering agents — such as inert gases, steam, foam, dry chemicals or sand — will have no effect as Ammonium Nitrate (as an oxidizer) creates its own oxygen to support combustion. Remember that in the Texas City disaster, the ship’s captain attempted to steam the hold to extinguish the fire. This failed to prevent the massive explosion that occurred.
Ammonium Nitrate can be detonated if the initiating source is sufficiently large or if it is heated under sufficient confinement. The degree of confinement necessary is dependent upon the purity of the material. United Nations/North American Number 1942 is used to designate technical grade Ammonium Nitrate while UN/NA Number 2067 is used for fertilizer grade. Technical grade may begin decomposition at lower temperature due to chemical additives.
This decomposition generally follows three stages. First, ammonia and nitric acid will dominate the release during decomposition. As temperatures are increased, Nitrous Oxide will be the main decomposition product. Finally, at temperatures above 260 degrees Celsius (or 500 degrees Fahrenheit), Nitrogen Oxide gases are formed in considerable amounts.
The National Fire Protection Association Standard Number 490 is titled the “Code for the Storage of Ammonium Nitrate.” The latest published version is from 2002 and it covers the building construction, pile sizes, spacing, and separation from contaminating material that could increase its sensitivity during a fire.
Fighting large fires involving Ammonium Nitrate parallels attempting to fight a tractor-trailer fire where the contents are dynamite and the trailer is already 25 percent involved on your arrival. The key is to identify the occupancies in your jurisdiction — and any surrounding jurisdictions to which you may be called to respond on “one of those days” — that store Ammonium Nitrate. While industries may be easier to identify through their Tier II reporting, farms are likely harder to identify.
Make no mistake, just because they are not sending in Tier II reports or otherwise not informing you, does not mean that they don’t have a large amount of Ammonium Nitrate storage. You do not want to find out about their large storage when it catches fire. Instead, work with your local industries and farmers to identify Ammonium Nitrate storage beforehand. Develop a pre-incident survey that includes a worst-case scenario. In that scenario, plans for public protective actions as well as the protection of firefighters, should be the primary, and perhaps one of the very few, priorities.
Be safe and do good.