By Dr. Vyto Babrauskas
2020年8月4日，黎巴嫩大部分贝鲁特港口，由于巨大的ammonium nitrate (AN)explosion. On April 17, 2013, a portion of the town of West, Texas, was leveled when an ammonium nitrate storage facility exploded. Based on initial information concerning the Beirut disaster, we shall examine here what lessons could have been learned from Texas that might have averted the Beirut disaster.
硝酸铵灾难和安全措施
在20世纪早期,用于regul灾害arly happen in the manufacturing of ammonium nitrate. But in more recent times, manufacturers learned to be safety conscious and there have been very few accidents in connection with the actual manufacturing process. Where disasters continue to recur with regularity is when the material is either in transport or in storage. A study I did a few years ago demonstrated that 100 percent of the AN accidents in storage or in transport have been due to a single cause: an uncontrolled fire (1). There have been no other causes.
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But if this is the case, it should not be hard to see that the solution needed is simple and robust. No accidents will occur if we make an uncontrolled fire to be an impossibility. This is not hard; in fact, it is fairly easy. One needs to do only two things:
- Only store the material in a fully non-combustible building.
- Make sure there are no combustible goods nearby which could ignite and burn.
It is not a challenging engineering problem to accomplish these things. We have known how to build fully non-combustible structures since the 1880s (2). And it again requires no advanced technical knowledge to make sure there is nothing combustible nearby that could threaten the storage with its flames, whilst burning.
Other safety precautions can be marginally helpful(3), but some are genuinely misguided. Foremost of these is the concept of setback distances, intended as a safety zone surrounding an explosion. Looked at very superficially, this might seem to be a good idea, but a closer examination reveals the opposite. Consider first if the management of combustibles is inadequate, there remains a risk of explosion. Obviously, if an explosion is immanent, the surrounding area needs to be cleared of all individuals. This means first-responders need to be sent into an area where they may be blown up. This is not consistent with protecting the lives of the first responders themselves. On the other hand, if combustibles have been successfully managed so that there is no risk of explosion, then explosion-safety setback distances clearly serve no purpose.
的Sad History of Disasters
Unfortunately, entirely preventable disasters keep recurring. Over the last century, there have been about 70 major fires involving AN in storage or in transport. Of these, 30 percent resulted in explosions, while 15 percent resulted in loss of life(4). Since enough data are available, these statistics can be taken as a direct indication of expected consequences of what will happen if a serious fire threatens stored AN. I consider that any activity which entails a 15 percent risk of involving fatalities to be deemed unacceptably risky, but evidently more education is needed to bring this message across.
在美国，最近发生的一场灾难发生在2013年4月17日，当时德克萨斯州西部的一个约有30吨的仓库着火，炸死，爆炸了15人被杀，约260人受伤。该镇的很大一部分被炸毁了，其中包括两所受到严重破坏的学校，需要拆除它们（5）。这场灾难尤其令人沮丧，因为大多数被杀的人是消防员，要么是直接做出回应以协助当地志愿消防部门努力的人。在此之后，我发表了九篇有关AN火/爆炸危害以及如何预防此类灾难的文章。显然，这些信息都没有到达黎巴嫩贝鲁特的官员，因为他们知道死亡的可能性15％，并且只是接受了这种风险，这似乎是不合理的。而且，当然，贝鲁特悲剧的数量级比德克萨斯州的悲剧差，这是非常悲惨的。
Texas vs. Beirut: Some Similarities and Some Differences
的two incidents shared a crucial similarity: In both cases, it was clear that uncontrolled fire was the proximate cause of the explosion. In both incidents, videos clearly show flaming from a rapidly worsening fire just prior to a detonation. The combustible materials in Texas were a wooden building plus diverse combustible agricultural commodities. In the Beirut incident, photographs indicate that building itself was not combustible. The early reports have not elucidated all the conditions, but it appears that piles of fireworks were burning. It is not clear what else was burning, but it is clear that the explosion was preceded by—and triggered by—a massive fire.
的AN in Texas was a fertilizer product, intended for sale to local farmers. The AN product in Beirut was confiscated from a vessel and was intended to be shipping to a mining operation in Mozambique, possibly as an ingredient for making ammonium nitrate/fuel oil (ANFO). This means the physical specifications of the prills were slightly different, but this would not affect the propensity for the material to explode. The biggest difference, of course, is that some 30 tons sufficed to level a sizable portion of West, Texas, whereas in Beirut some 2,750 tons were involved. There was also a substantial difference in casualties. In Texas, 15 persons died in the incident. In Beirut, the official tally is not yet available but initial estimates suggest about 150. Since there were nearly two orders of magnitude more material in Beirut, it is fortunate indeed that there were not two orders of magnitude more of fatalities.
硝酸铵特性
Since AN is not in widespread general usage, it is useful to understand some of its basic properties and traits. Ammonium nitrate (NH4NO3）是一种不可燃烧的化学物质。因此，报道说存储的“点燃”是不正确的，因为只有燃料可以点燃，并且是氧化剂，而不是燃料（6）。但是，罐子爆炸（7），如果数量很大，爆炸可能会大大巨大。现在需要考虑一些细节。由于A是氧化剂，因此可以与各种燃料反应，在此过程中产生热量。但这不是爆炸的方式。典型的爆炸是由于分解过快。除了成为氧化剂外，一个分子还不稳定，并且可以分解，也就是说，分解成碎片。分解可以通过各种方式引起，但到目前为止，最常见的是热。这称为热分解（8）。 Next one can ask: Where does the heat come from? Again, in theory, there are many possibilities. But in practice, what usually happens is that a large, uncontrolled fire occurs nearby, and this proceeds to heat the AN.
If the burning material is very close, then an additional complication arises. With normal fires, heat rises upwards. Then, if there is a ceiling above, the hot gas plume gets stopped by the ceiling and turned sideways. This is how fires spread in a wooden structure, for example. But AN has a special property. If a pile of AN is being heated by an adjacent fire, the material will get heated up, and will eventually melt at 169.6ºC (337.3ºF). Molten AN flows like a liquid. Like any liquid, it will flow downwards and establish a flow path governed by gravity. If this “river” of flowing AN then encounters some combustible, such as wood or cardboard, the fuel will ignite hypergolically. Hypergolic ignition is what happens with certain very reactive chemicals when ignition occurs upon contact of that chemical with some other material, without needing any external ignition source. Thus, if there is a fire involving wood, cardboard, or other fuels next to a pile of AN, fire spread will be substantively accelerated. Instead of just spreading up-and-across, fire will also spread along a second path, that of the flowing AN “river.” The result can be a much faster fire development than might have been expected for the particular fuel that is burning.
Ammonium nitrate is primarily used in two industries: as an ingredient for explosives-making and as a fertilizer in agriculture. For both applications, the substance is typically sold in the form of “prills,” which are small spherules of about 2 mm diameter. Chemically, the material is identical for both industries, though there are some physical differences, primarily in density and porosity, of the grades sold for explosives-making and for agriculture. But either grade is susceptible to explosion, including its most severe form, detonation. Detonation occurs when a reaction wave moves at a velocity greater than the local speed of sound in the unreacted material.
What might be surprising to individuals who do not work with AN is that it is not classified as an explosive, even though it can explode and these explosions can produce massive loss of life. The U.S. Federal government has a narrow definition (9) of what constitutes an ”explosive,” and AN is not classified as an “explosive” but, rather an “oxidizer.” This practice is the same worldwide, according to guidelines set forth by the UN (10), which specify that AN is classed as an oxidizer and not an explosive, unless the product contains combustible material in excess of a certain limit value. This is for two reasons: (1) AN is a less sensitive explosive than TNT or RDX. This means it requires significantly more energy to be put into the material to initiate an explosion. However, once an explosion is proceeding, this limitation will not offer much solace to the victims. (2) Classifying AN as an explosive is strongly resisted by the agricultural industry, since this would make it more difficult for them to use AN as a fertilizer. This, however, is also a poor reason, since a very similar product, calcium ammonium nitrate (CAN) is available for fertilizer use which shows no significant risk of explosion (11).
Médard（7）在他的评论中得出结论，没有发现纯粹，未加热的纯粹爆炸的案件。另一方面，在实验室研究中，很容易通过大量污染物混合来爆炸（8）。但是，这种污染只是在现实中不会发生。(It is sometimes claimed that the large explosion of AN which occurred in 2001 in Toulouse, France, was due to contamination. However, Guiochon (12), the world’s leading expert on decomposition of AN, found instead that the actual cause was due to a buried, unexploded bomb which detonated beneath the factory). What is inarguable is that the governing cause of AN explosions has always been uncontrolled fire.
Additional Aspects
Since the Beirut explosion, I have received queries about toxicity hazards from AN explosion incidents. AN explosions are caused by very rapid decomposition of the material. The chemical reactions involved in decomposition are complex and numerous8. In principle, the decomposition of ammonium nitrate produces some reaction products which can be toxic. These include not only nitric acid and ammonia, but also various oxides of nitrogen. In large concentrations, any of these would be toxic and could even be lethal, but the blast effect is such that these substances are dispersed very widely and do not accumulate in any particular location. So while there will be trace readings that can be obtained, there is not likely to be any surviving person who would be exposed to injurious concentration levels of the toxic chemicals. In the West (TX) disaster, for example, numerous individuals who were not close enough to the warehouse to be killed were injured and many of them hospitalized. But the injuries were primarily blunt trauma (from falling objects) or from flying glass. There were not any cases identified of injuries due to chemical poisoning. One might also ask about the dangers of being exposed to unreacted prills of AN. Here, again, the danger is very modest. AN can be an irritant, especially to mucous membranes, and would be toxic if ingested in sufficient quantities (13), but the latter seems highly improbable. Thus, while there are numerous extremely serious concerns about AN explosions, toxicity is generally not one of the primary concerns.
Conclusions
Ammonium nitrate is a chemical which can be highly dangerous, but only under certain circumstances. As with any explosible substance, it can detonated by an existing explosion reaction. In practice, however, the most likely danger is from fire. If an uncontrolled fire impinges on stored ammonium nitrate, the material can explode. The safety takeaway, which needs to be disseminated as widely as possible, is this:
请勿将硝酸铵转移到无法安全处理材料的任何存储设施上。存储设施的主要要求是该结构是不可燃烧的，并且没有可燃的商品或可燃结构靠近存储设施，因为这种情况可能会导致无法控制的曝光火灾。
ENDNOTES
1. Babrauskas, V., “Explosions of Ammonium Nitrate Fertilizer in Storage or Transportation Are Preventable Accidents”,J.危险材料304，2016，134-149。
2. Freitag，J。K.，的Fireproofing of Steel Buildings, Wiley, New York, 1899.
3. Babrauskas，V。，“西部，德克萨斯州硝酸铵爆炸：监管失败”，J.消防科学35, 2017, 396-414.
4. Babrauskas, V.,“Will Firefighters Be Any Safer Under the New Hazardous Materials Code?”,188金宝搏是正规吗168：11，2015年11月，66-70。
5. West Fertilizer Company Fire and Explosion (15 Fatalities, More Than 260 Injured), Report 2013-02-I-TX, U.S. Chemical Safety and Hazard Investigation Board, Washington, 2016.
6. Babrauskas, V.,Ignition Handbook, Fire Science Publishers/Society of Fire Protection Engineers, Issaquah, Washington, 2003.
7. Médard, L. A.,Accidental Explosions，2卷，埃利斯·霍伍德，英格兰奇切斯特，1989年。
8. Babrauskas，V。和Leggett，D。，“硝酸铵的热分解”，火与材料44、2020、250-268。
9. Code of Federal Regulations, 49 CFR 172.101 Hazardous Materials Table.
10.关于危险货物运输的建议。模型法规。卷。1，联合国，2019年。
11. Babrauskas, V., “The Ammonium Nitrate Explosion at West, Texas: A Disaster That Could Have Been Avoided,”火与材料42, 2018, 164-172 .
12. Guiochon, G., “On the Catastrophic Explosion of the AZF Plant in Toulouse, September 21, 2001,”8th全球过程安全大会AIChE,休斯顿,德克萨斯州,2012,737 - 759。
13. Wexler, P., ed.,Encyclopedia of Toxicology，3rded., Academic Press, London, 2014.
VYTO BABRAUSKAS,Ph.D., earned degrees in physics and structural engineering and a Ph.D. in fire safety. As a researcher at the National Institute of Standards and Technology, he developed devices to measure the heat release rate of products and developed a computer program for modeling the development of room fires. He founded a consulting firm in 1993 and provides fire safety science expertise to fire investigation and litigation. HisIgnition Handbookis widely used in the fire services, and he has two books forthcoming:Electrical Fires and ExplosionsandSmoldering Fires. He is currently based in New York City, where he is affiliated with the John Jay College of Criminal Justice.
