An explosion April 17 at a fertilizer plant in West, TX that killed at least 14 people and injured more than 200, is under investigation by various federal agencies.

The explosion at the West Fertilizer Co., where according to numerous sources, more than 500,000 tons of ammonium nitrate were stored, left a large crater, and did severe damage to the surrounding area.

Powder/Bulk Solids asked some of its industry contributors to share their thoughts about the hazard potential of fertilizer, its safe handling, and to offer other thoughts on the explosion in Texas.

Ashok Ghose Dastidar is the vice president of dust and flammability testing and consulting services with Illinois-based Fauske & Associates. “Nitrogen rich fertilizer is very explosible and it would have contributed to the magnitude of the explosion,” said Dastidar. “Ammonium nitrate fertilizer and fuel oil are commonly used in the mining industry as an explosive in place of dynamite. Additionally, terrorists use it as well – think Timothy McVeigh and Oklahoma City Bombing back in 1995.”

Dastidar added that a blast involving fertilizer can be more powerful than an explosion occurring as a result of a product such as sugar dust.

Bob Gravell, principal process safety consultant with DuPont Engineering’s Explosion Hazards Laboratory, cautioned that it is premature to draw any conclusions about what happened in Texas. Speaking generally about fertilizer, Gravell wrote that “forensic evidence, primarily estimations of blast pressures at various distances based on observed damage, will likely be used to determine whether the event was the result of an ammonium nitrate detonation or something else.”

Gravell added that there is a long history of ammonium nitrate incidents throughout the world, most notably the 1947 SS Grandcamp explosion in Galveston Bay and the 2001 explosion in the AZF plant at Toulouse, France.

According to Gravell, one of the main reasons that fertilizer can be so explosive is that problems usually occur when a large quantity of ammonium nitrate, under some degree of confinement, is exposed to an external fire. “The molecules will rapidly ‘unzip’ when the autodecomposition temperature is reached (~210-220 C), releasing a lot of energy in a very short time period (detonation).  This situation can be exacerbated by the presence of organic contaminants,” Gravell said.

Gary Q. Johnson, a consultant with Workplace Exposure Solutions, also shared his thoughts about the dangers of ammonium nitrate. “Ammonium nitrate can be used as a fertilizer and as an ingredient in common explosives,” he said. “It is a reactive chemical hazard, not a combustible dust hazard except for extreme conditions of high temperature or pressure. Reactive chemicals, when interacting with incompatible materials, lead to runaway reactions that generate heat and hot gases like an explosion. It is a strong oxidizer and if mixed with incompatible organic materials, it can detonate as an explosive. In addition, it can become shock sensitive during decomposition.”

According to Johnson, protection comes from preventing contact with the incompatible chemicals or processing conditions listed on the MSDS or in databases. Care must be taken to prevent overheating or ignition and when fighting fires as it can be water reactive under the wrong conditions.

The U.S. Chemical Safety Board completed a general chemical reactivity hazards study a few years ago. Johnson said a startling conclusion was that 2/3 of chemical reactivity incidents did not happen in a reactor. They happen due to handling practices that led to mixing with incompatible materials. The Board has sent a team to investigate this incident and will issue recommendations based on its findings. Sites storing these chemicals need to review the hazards with the chemical manufacturers and assess whether the site has adequate protection for the business and the community around it, according to Johnson.

Richard Prugh, a principal process safety specialist at Chilworth Technology, provided some general thoughts about fertilizer and its hazardous potential. His comments are not intended to provide an explanation for what happened at West Fertilizer.

“Ammonium nitrate itself is unlikely to explode unless it is confined,” said Prugh. “Fertilizer companies may coat the ammonium nitrate prills (small spheres) with an organic material, such as urea, and this combination could explode (and perhaps detonate) if exposed to fire.”

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