Explolding the Myth

Ammonium Nitrate Fertilizer-Exploding the Myth

Ammonium Nitrate Fertilizer - Exploding the Myth

by Maurice Greiner

When ammonium nitrate fertilizer is involved in a fire or spill there is sometimes a great deal of apprehension and confusion on the part of emergency response personnel concerning the hazards of the product. Fears are deeply instilled in many that prilled or granulated ammonium nitrate fertilizer in its pure form is extremely sensitive to fire, heat, or impact and that it is easily triggered into an explosion. This paper reviews some of the myths that have contributed to this thinking and then takes a close look at the characteristics of this material and its behavior in heat, fire, and pressure conditions. It also outlines emergency response procedures and identifies some real dangers and concerns which surround this product when it is improperly stored, handled, or involved in fire.

A recent newspaper account typifies quite accurately the fear and misunderstanding that continues to exist concerning the effect of fire upon ammonium nitrate fertilizer.

A fire had involved a small shed which contained four tons of ammonium nitrate fertilizer. The newspaper described the "near disaster" from which their city had been spared because firefighters had managed to stop the fire before it came into contact with the "pelletized bomb" - ammonium nitrate fertilizer. The article made several additional references to the "bomb in the shed" and the four tons of "highly explosive ammonium nitrate.". One fire official was quoted as saying that "such an explosion might have devastated an area of 2500 square feet in all directions, probably killing everyone within this radius". Fire officials also described the urgency of their fire control actions as essential to prevent "the unleashing of a killer explosion.”

In 1960 a large fire loss was suffered in a warehouse because it was reported that a fire had reached 20 tons of ammonium nitrate fertilizer. The firefighters feared that an explosion was imminent and abandoned firefighting activity.

To emphasize the reality of the impact myth, especially as it pertains to the fertilizer product in a non-fire situation, I once received a letter from a fertilizer dealer who wanted to jackhammer some ammonium nitrate fertilizer in a storage bin but was not sure that it was safe. He was under the impression that the pure fertilizer might explode from the jackhammer's impact.

In another example, a transport truck carrying ammonium nitrate fertilizer overturned spilling the product onto the highway. The main concern of the local fire department and state police at that time was that the impact of other highway traffic passing over the product would actually trigger an explosion.

There exists also a fear that the dust from ammonium nitrate will explode with violence in a fire much the same as grain dust and other organic dusts. As an example, an OSHA inspector visited a retail fertilizer distribution plant where several hundred tons of ammonium nitrate fertilizer product was stored. He cited the company for not having explosion-proof light fixtures because he considered ammonium nitrate dust highly explosive. The citation was contested and won on the basis that AN fertilizer dust is not a highly explosive dust and, in fact, has a very low dust explosion hazard.

This paper will help to dispel some of the myths that have been perpetuated and at the same time prevent us in the industry from taking an oversimplified approach to the characteristics and behavior of ammonium nitrate fertilizer in fires and spills. In particular, I have identified some real concerns such as the sensitivity of molten AN, contamination, oxidation, and the phenomena of self-sustained decomposition in fertilizer mixtures containing AN. I have researched numerous papers and recent studies including the five year study completed in 1982 by the Mining Department of Queens University on "The Explosion Hazards of AN and AN-based Fertilizer Compositions".

Characteristics of Ammonium Nitrate Fertilizer
Ammonium nitrate fertilizer is made by chemically combining ammonia with nitric acid in a water solution. Water formed during the reaction is evaporated, leaving a concentrated ammonium nitrate melt. The hot melt is then processed in one of several ways, depending on plant design, into prills or granules. The finished product is then coated with a conditioning agent, usually a clay, to prevent it from caking.

Ammonium nitrate fertilizer is an oxidizer, a substance that oxidizes readily to stimulate the combustion of organic matter or other fuels. It is classified as an oxidizer by the United States Department of Transportation.

While it has a wide use as a fertilizer, ammonium nitrate is also the principal base material in slurry explosives and lower-cost blasting agents. It is converted to an effective blasting agent by properly mixing it with a carbonaceous material such as fuel or ground walnut hulls (2). Although chemically the same as the fertilizer grade, the ammonium nitrate used for blasting purposes is of a lower density, usually less than 0.85 grams per cubic centimeter and containing small percentages of anti-caking agents. By definition, a blasting agent is any material or mixture consisting of a fuel and oxidizer intended for blasting, not otherwise classified as an explosive, provided that the finished product, as mixed and packaged for use or shipment, cannot be detonated by a No. 8 blasting cap when unconfined. Ammonium nitrate has roughly 50 percent of the strength of TNT when detonated completely. It yields an energy release of approximately 400 calories per gram. TNT when detonated yields an energy release of approximately 750 to 900 calories per gram (2).

A commercial explosive which is sometimes made with low density ammonium nitrate is ammonia dynamite. The nitrate is mixed with various fuels and sensitized by varying percentages of a high explosive such as nitroglycerin.

Behavior in Fire
Ammonium nitrate fertilizer does not explode when contacted by flame, but rather begins to melt at approximately 330°F. (166°). From 350°F. to 410°F. (177°C. to 212° C.) the fertilizer decomposes into nitrous oxide and water vapor. The nitrous oxide is a supporter of combustion, even in confined spaces where there is a lack of oxygen. This decomposition is an exothermic reaction. As the temperature rises the rate of decomposition increases and more toxic oxides are liberated.

In contrast to the heat-producing reaction, the simultaneous vaporization and dissociation of ammonium nitrate into ammonia and nitric acid is an endothermic vaporization reaction which takes place at about 175° F. to 200°F. (80°C. to 93°C.) and tends to offset or balance the heat-producing reaction. It has also been observed that when the external heat source is removed from the ammonium nitrate the temperature of the decomposing material begins to drop rapidly, illustrating the greater endothermic nature of the volatilization into ammonia and nitric acid as opposed to the exothermic decomposition into nitrous oxide and water (3_).

The only thing that interferes with this self-limiting effect during heat decomposition is an increase in pressure from very tight confinement as in the hold of a ship. At a critical pressure level the energy from the exothermic reaction overpowers the moderating effect of the vaporization reaction, thereby resulting in a rapid escalation of temperature and reaction rate which can accelerate to detonation state conditions (3). If pure ammonium nitrate fertilizer is involved in fire, the pressure build-up would have to reach many thousand pounds-per-square inch, much higher pressures than normally would be attainable in a storage facility or transportation container. Fueled or molten ammonium nitrate, however, is more sensitive and therefore requires much less pressure under heat conditions to offset the self-limiting effect and trigger a deflagration or detonation.

Case Histories of Fires
It has been demonstrated many times in actual fires and experimental situations that when the products of reaction are allowed to vent freely an explosion will not occur in the absence of explosives or reactive contaminants. This is because the high pressures necessary to achieve a runaway reaction cannot usually be attained in ordinary storage and transportation modes. An exception would be the tightly constructed hold of a ship.

· Independence, Kansas, 1949 This fire involved nearly 1400 tons of wax-coated, bagged ammonium nitrate fertilizer, the same type of ammonium nitrate that was in the hold of the ship that blew up at Texas City in 1947. The limited water supplies made firefighting difficult. The fire went out-of-control and after several hours of burning the building was completely destroyed and alI but 100 to 150 tons of the nearly 1400 tons of fertilizer-grade ammonium nitrate had been consumed. The building was well vented during the fire allowing for the release of the products of combustion. No explosion took place in spite of the fact that the ammonium nitrate was coated with wax and that a number of other contaminants had come in contact with the molten nitrate including aluminum, copper, iron, zinc, and carbonaceous materials such as wood and asphalt. Quantities of aluminum and copper had melted during the fire and since the melting point of copper is approximately 1901°C. it would appear that flame temperatures were at least at or above this temperature (5_). The temperature of the ammonium nitrate was self-limiting, even in a runaway fire, because of the powerful endothermic action of the volatizing material.
Barksdale, Wisconsin, 1920 30,000 pounds of ammonium nitrate were destroyed in a well-vented storage building fire. No explosion.
Brooklyn, New York, 1920 1,944,500 pounds of ammonium nitrate were consumed in a fire in the holds of the Norwegian freighter, the SS "Hallfried". No explosion.
Muscle Shoals, Alabama, 1925 Two boxcars filled with ammonium nitrate were destroyed by fire. No explosion.
Gibbstown, New Jersey, 1940 147,000 pounds of uncoated ammonium nitrate in paper sacks was consumed in a warehouse fire. No explosion.
Presque Isle, Maine, 1947 80,000 pounds of mixed fertilizer containing ammonium nitrate and superphosphate and potassium salts were involved in a plant fire. No explosion.
St. Stephen, Canada, 1947 400 tons of ammonium nitrate fertilizer were consumed in a fire. No explosion.
Railroad Boxcars, 1946 to 1949 From 1946 to 1949 there were 13 separate railroad cars involved in fire, each containing 80,000 to 100,000 pounds of ammonium nitrate fertilizer. There were no explosions during any of these fires.
Boron, California, 1960 20 tons of prilled ammonium nitrate were destroyed in a burning warehouse. At the peak of the fire the heat was so intense the steel roof glowed red and collapsed on the burning and molten ammonium nitrate. No explosion.
Potosi, Wisconsin, 1967 A boxcar loaded with 50 tons of ammonium nitrate caught fire. The boxcar was steel with a wooden interior. All of the wood burned out of the car, melting the nitrate and leaving a large crusted mass. No explosion.
Rocky Mountain, North Carolina, 1978 A storage facility containing 500 tons of ammonium nitrate fertilizer was destroyed by fire. No explosion.
Moreland, Idaho, 1979 Fire involved the wood framework and belting of the overhead conveyor system in a fertilizer distribution plant while it was being used to unload a railroad car of ammonium nitrate fertilizer. The fire spread from the conveyor system to the roof. About 200 tons of ammonium nitrate was involved. There was no explosion.

There are many more cases which could be cited where fire has involved pure ammonium nitrate fertilizer. The temperature in decomposing nitrate piles or bags where normal ventilation is achieved tends to be self-limiting because of the endothermic reaction. Also as I pointed out earlier, the high pressure necessary to bring about a runaway reaction is difficult to attain in ordinary storage and transportation facilities.

Hazards Of Ammonium Nitrate Fertilizer In Fires
There are some very real dangers, however, which should be identified when ammonium nitrate is involved in fire. The first is the shock and heat sensitivity of the molten material. Through thermal expansion the density of the normally insensitive fertilizer product is lowered and the sensitivity thereby increased. Tests have shown that at temperatures greater than 245°C. the shock sensitivity is quite high and that at 260°C. it can be initiated by impact pressures of approximately 6 to 7 kilobars (_3). Large explosive devices or large diameter projectiles travelling at 200 meters per second could possibly trigger an explosion. However, it was speculated by those conducting these tests that the molten ammonium nitrate would not likely be initiated by a falling steel beam, falling roof, or even by small fragments from exploding gasoline tanks (6J . It was also concluded that when ammonium nitrate fertilizer is involved in a typical fertilizer warehouse fire the accumulation of large pools of molten ammonium nitrate would not be readily possible unless the structure has a pit or a basement area. Pooling would be likely during fires in ships holds, however (5j. In advanced fires where the ammonium nitrate is directly involved firefighters must try and determine if the product is in a molten state and forming molten pools. Ventilation and the application of water will quickly desensitize and stabilize the hot material. Water has a powerful endothermic effect upon ammonium nitrate (cold paks for first aid application are made with ammonium nitrate and water).

Secondly, contamination and confinement are hazards. Under conditions of fire exposure, ammonium nitrate fertilizer contamination could possibly occur because of the material inherent in most storage structures. For example, charcoal (from wood, paper, etc.), organic fuels, such as diesel oil, plastics, polyethylene (from bags, etc.), aluminum (from conduit or wiring), and copper (from wiring, etc.) can increase the shock sensitivity of the ammonium nitrate, and under fire conditions form deflagrating compositions or mixtures with the ammonium nitrate fertilizer. Also, if molten ammonium nitrate aerates with oxidation materials such as an asphalt base or fuels the shock characteristics may be altered further. The presence of free acid will also promote reactive combustion. An explosion hazard could exist if contaminated, molten ammonium nitrate became confined in drains or sewers. Also, in the presence of copper the molten ammonium nitrate causes formation of tetramine cupric nitrate which can decompose explosively when heated strongly (2). Cupric nitrate also appears to be able to promote a rapid exothermic oxidation of metals such as aluminum which normally would not react too readily with molten ammonium nitrate (_3). A conclusion reached in the Queen's University study was that the presence of copper represents the single biggest contamination hazard with respect to accidental detonation of ammonium nitrate under conditions of fire exposure" (3). In any event, water can be used to desensitize a potentially dangerous situation.

The fertilizer industry has experienced ammonium nitrate detonations where contaminants or explosives have been involved. Some examples are Texas City (1947), Emporium, Pennsylvania (1925), Brest, France (1947), Mount Vernon, Missouri (1966), and the Cherokee Nitrogen Plant at Pryor, Oklahoma (1973).

The Texas City disaster involved a cargo vessel which was carrying large quantities of fuel oil and wax-coated ammonium nitrate when fire broke out. The molten ammonium nitrate was aerated with the oxidation materials and tightly confined in the ship's hold. Water was not applied to help desensitize the contaminated molten material. Also, the vessel was not vented to relieve the pressure.

A third hazard to consider when ammonium nitrate fertilizer is involved in fire is the oxidization effects. Toxic products of decomposition are produced at the higher temperatures. The primary health hazard is the toxic gaseous oxides of nitrogen. Concentrations of more than 200 parts per million may be fatal even after short exposures. Their effects may be delayed for as much as 48 hours. Exposure victims should be hospitalized as quickly as possible. Nitrous oxide, one of the toxic decomposition products, will also support combustion even in poorly ventilated structures and will continue to feed a fire. Also, molten ammonium nitrate is a powerful oxidizer capable of igniting some combustible materials with which it comes in contact and of reacting explosively with others as mentioned earlier.

A fourth hazard under fire exposure is the phenomena of self-sustaining decomposition. This can occur when mixed fertilizer compounds such as ammonium nitrate and nitrogen phosphate, nitrogen potassium phosphate, or nitrogen potassium are involved in fire. Through progressive heating as from a fire, the surface temperature of the compound can be raised to a point (without melting) where exothermic decomposition may take place. If the source of heat is then suppressed, the decomposition will stop. However, if the mixture contains micronutrients such as copper, iron, or other finely divided metals, sulphur, or potash (which liberates chlorine as it decomposes) the decomposition mechanism may be catalyzed into a self-sustaining internal exothermic reaction which will penetrate deep into the fertilizer pile creating a zone of decomposition. This reaction may continue even after the actual fire is suppressed and spread through the entire fertilizer mass

External temperatures of between 190°C. to 230°C. are considered sufficient to initiate a self-sustained decomposition in some ammonium nitrate fertilizer mixes (7j. The phenomenon has been recorded from prolonged heating at temperatures as low as 130°C (7). Zonal decomposition may also be initiated to a self-sustained reaction by small heat sources such as light bulbs, welding torches, etc. The energy for this type of "burning" or decomposition is provided internally by the ammonium nitrate. The reaction therefore cannot be controlled by smothering, but rather by directing straight stream nozzles of water into the center of the decomposition zone. Special lances may have to be used to penetrate the surface hardened by the hot decomposition process. Small zones of decomposition may be physically removed from the main body of fertilizer with mechanical equipment and taken outdoors where the hot material can be quenched with water. While there is little explosion hazard from zonal decomposition unless the contaminated fertilizer is very tightly confined as in a drain or sewer, or explosive devices are present, there will be the release of hot toxic products of decomposition, posing a serious health hazard to firefighters and downwind areas.

The Sensitivity Myth
Contrary to the thinking of many, ammonium nitrate fertilizer is quite insensitive to initiation by impact and is extremely difficult to detonate in the granular or prill form (8) (3). In my workshops on ammonium nitrate" fertilizer safety I have impacted the high density fertilizer product with a heavy stainless steel device many times to demonstrate its insensitivity. One of the more dramatic illustrations I have of ammonium nitrate insensitivity is a ruptured 12-gauge shotgun barrel plugged solid with ammonium nitrate fertilizer. The gun was being used by a fertilizer plant manager in Colorado on May 14, 1974 to break up chunks of ammonium nitrate that were blocking the down-spout of a metal fertilizer storage bin. The manager had used this method successfully on previous occasions, however, this time as he pulled the trigger the barrel was full of ammonium nitrate fertilizer. It ruptured much the same as if the barrel had been filled with sand or mud. The manager was only slightly injured and there was no explosion of the ammonium nitrate despite the very tight confinement of the shotgun barrel and high pressure produced by the 12-gauge shell.

The Dust Explosion Myth
I earlier made reference to some of the concerns that exist regarding the dust explosion hazard of pure ammonium nitrate fertilizer. I have demonstrated many times in my own workshops with simple dust explosion devices that the ammonium nitrate fertilizer dust is not explosive. The Bureau of Mines conducted extensive tests in 1968 on the explosibility of miscellaneous dusts (1). They concluded that pure oxidizers do not present a dust explosion hazard and that in some instances they may even serve as inerting agents to inhibit explosion. The inclusion of small amounts of carbonaceous materials such as carbon or fuel oil will increase the sensitivity of the ammonium nitrate dust to a rapid ignition but not a true dust explosion.

Fire Control Techniques
For fires involving ammonium nitrate fertilizer in storage or in transportation firefighters should keep in mind these basic facts and techniques as they consider their fire-fighting strategy.

1. Pure ammonium nitrate fertilizer prills or granules will not explode when exposed to fire. They will, however, begin to melt and decompose into toxic oxides.

2. In the absence of heat the high density fertilizer prills or granules are not sensitive to impact.

3. There is no reason to fear a dust explosion. The ammonium nitrate fertilizer dust is not explosive.

4. Ammonium nitrate fertilizer is an oxidizer, not an explosive. Under fire conditions it will begin to decompose into nitrous oxide which will sustain combustion in the absence of oxygen. At high temperatures and when mixed with certain contaminants it may undergo a deflagration to detonation transition. Also, at higher temperatures more toxic oxides are produced creating a severe health hazard.

5. The shock sensitivity of molten ammonium nitrate is a hazard at temperatures above 245°C. This makes it especially dangerous if it is allowed to form large pools as in the hold of a ship. In such cases ventilation and the application of water will quickly desensitize and stabilize the hot material.

6. Contamination of either the ammonium nitrate prills or molten material with metals, organic fuels, or combustibles will increase the sensitivity of the material and add to the explosion danger. The aeration of the molten nitrate with oxidation products will greatly increase the hazard for a deflagration or explosion. Again, ventilation and the application of water will cool and desensitize the hot ammonium nitrate and greatly reduce the hazard.

7. Molten ammonium nitrate is a powerful oxidizer capable of igniting some combustible materials.

8. Fertilizer compounds containing ammonium nitrate may be heated externally to the point where internal exothermic decomposition can take place and eventually self-sustain itself and penetrate deep into fertilizer piles. Large straight stream nozzles directed to the center of the fire will quickly cool the material and extinguish the fire. If the zone of decomposition is small and easily accessible it may be removed from the main body of the fertilizer pile by some mechanical means and taken outside where it can be cooled with water.

9. Firefighters should wear self-contained breathing apparatus and protective clothing. This equipment will protect them from the toxic fumes of decomposing ammonium nitrate, and the clothing will guard against molten nitrate splashes.

10. Immediately ventilate a building or vessel under fire where ammonium nitrate is stored. This will prevent pressure build-up and will allow the endothermic nature of the volatization reaction to offset the heat produced by the exothermic decomposition of the ammonium nitrate.

11. Residents downwind should be evacuated to protect them from the toxic vapors.

12. Spectators should be kept away and employees removed from the burning area.

13. Approach the fire from upwind.

14. Firefighters should have a two-fold objective: (i) They must extinguish the surrounding fire - the source of heat to the ammonium nitrate; and (ii) cool and desensitize the hot ammonium nitrate piles or molten material with straight streams of water. Water has a powerful endothermic effect when in combination with ammonium nitrate. (This is how cold paks are made for first aid applications. )

15. Firefighters should operate from behind protective barriers to protect themselves against molten nitrate splashes when applying water to the f ire.

16. Steam, carbon dioxide, dry chemicals or other smothering agents should never be used on fires involving ammonium nitrate. Any attempts to smother decomposing ammonium nitrate will simply accelerate the exothermic activity of the decomposing material.

17. Care should be taken to keep nitrate out of confined spaces. Particularly, molten ammonium nitrate should be kept away from areas such as sewers and drains.

18. If a fire is out-of-control and firefighters are unable to effectively apply water to desensitize the hot nitrate, or there is a suspicion of the presence of contamination or explosive devices in the burning building the entire area should be evacuated and the fire-fighting efforts abandoned in anticipation of a possible explosion.

19. Run-off water from a fire should be contained and prevented from entering streams, lakes, or wells. Nitrate ions can be toxic to humans, animals and marine life.

20. After a fire or spill involving ammonium nitrate fertilizer clean-up activities should begin. Contaminated material should be properly disposed of; all exposed areas should be scrubbed and flushed - especially wood - to ensure that ammonium nitrate residue is dissolved. Wet, empty bags should be removed, permitted to dry, and properly disposed of. Metal in contact with the residues of ammonium nitrate should be thoroughly cleaned with water to prevent corrosion.

Conclusion
Although ammonium nitrate fertilizer has a good safety record in storage and transportation, and firefighters can approach an ammonium nitrate fertilizer fire in its early stages with confidence there are hazards which can develop from contamination, confinement, exploding pressure vessels and fuel storage tanks. Shock and heat sensitivity and the aeration of molten ammonium nitrate and the phenomenon of self-sustained decomposition are also matters for concern. The firefighters should be aware of all of these and know how to deal with them. Immediate ventilation and the application of water will be a major deterrent to potentially dangerous situations.

References:
1. "Explosibility of Miscellaneous Dusts", Bureau of Mines Report 7208, (1968).
2. 1C 8746 Bureau of Mines Information Circular (1977).
3. "The Explosion Hazards of Ammonium Nitrate", Department of Mining Engineering, Queen's University, (Kingston, Ontario), 1982.
4. "Explosion Hazards of Ammonium Nitrate Under Fire Exposure", U.S. Bureau of Mines, Rl #6773 (1966).
5. "A Review of Accidents With Ammonium Nitrate", Mining Engineering, Queen's University, (Kingston, Ontario), 1977.
6. "Critical Parameters for Low Amplitude Shock Initiation of Molten Ammonium Nitrate and Ammonium Nitrate Solutions", Department of Mining Engineering, Queen's University, (Kingston, Ontario), (1982).
7. "The Storage of Compound Fertilizers Containing Ammonium Nitrate", International Superphosphate and Compound Manufacturers' Association Limited, (1971).
8. Sax, n. Irving, Dangerous Properties of Industrial Materials, Fourth Edition.