I will freely admit that before last night, I had never heard of West, Texas. My Texas travels have been limited, mostly consigned to cursing Dallas’ patchwork tollroad system and visiting family in San Antonio.

But chemistry is something I know a few things about, and so the explosion yesterday caught my attention.

Web searches indicate that West Fertilizer Co., of West, Texas, primarily dealt with anhydrous ammonia (NH₃), although there are mixed reports that indicate ammonium nitrate may have been on site as well. Ammonium nitrate (NH₄NO₃) is responsible for the Texas City Disaster (by itself) and the Oklahoma City bombing (where it was combined with another chemical).

Looking at an example MSDS for anhydrous ammonia, there are a few key points of interest:

Physical Data

Boiling Point: -28°F at 1 atm
pH: N/A
Specific Gravity of Gas (air = 1): 0.596 at 32°F
Specific Gravity of Liquid (water = 1): 0.682 at -28°F (compared to water at 39°F)
Percent Volatile: 100% at 212°F
Appearance and Odor: Colorless liquid or gas with pungent odor

Here, we can see that the boiling point of ammonia is below freezing - meaning that tanks are either refrigerated (unlikely) or under pressure (substantially more likely) to keep the product as a liquid, which is much more cost effective to store and transport. Liquids are also relatively incompressible compared to gases. However, if the pressure is suddenly removed, the liquid can suddenly transition to a gas with the subsequent expansion of volume (gases take up a lot more space) forcing it outward.

However, in the paperwork filed by West Fertilizer with the EPA, the worst risk they invisioned was:

[…] a 10-minute release of ammonia gas that would kill or injure no one.

The second worst possibility projected was a leak from a broken hose used to transfer the product, again causing no injuries.

Checking the MSDS, we can see what the risks of overexposure to ammonia gas are:

Eye: May cause severe irritation, eye burns or permanent eye damage.
Skin: Irritation, corrosive burns, blister formation may result. Contact with liquid may produce a caustic burn and frostbite.
Inhalation: Exposure may result in severe irritation and / or burns of the nose, throat and respiratory tract. It may cause bronchospasm, pulmonary edema or respiratory arrest. Extreme exposure may result in death from spasm, inflammation or edema. Brief inhalation exposure to 5,000 ppm may be fatal.

Well. Maybe they assumed that since the tanks were outside, the levels following a leak would never hit the levels required for overexposure. Still, their worst case scenario seems rather mild. For example:

The fertilizer plant that exploded Wednesday night in West, Texas, reported to the Environmental Protection Agency and local public safety officials that it presented no risk of fire or explosion, documents show.

Back to the handy MSDS:

Fire and Explosion Hazard Data

Flashpoint: None
Flammable Limits in Air: LEL/UEL 16% to 25% (listed as 15% to 28% in the NIOSH Pocket Guide to Chemical Hazards.)
Extinguishing Media: Dry Chemical, CO2, water spray or alcohol-resistant foam if gas flow cannot be stopped
Auto Ignition Temperature: 1,204°F (If catalyzed), 1,570°F (If un-catalyzed)

Special Fire-Fighting Procedure
Must wear protective clothing and a positive pressure SCBA. Stop source if possible. If a portable container (such as a cylinder or trailer) can be moved from the fire area without risk to the individual, do so to prevent the pressure relief valve of the trailer from discharging or the cylinder from rupturing. Fight fires using dry chemical, carbon dioxide, water spray or alcohol-resistant foam. Cool fire-exposed containers with water spray. Stay upwind when containers are threatened. Use water spray to knock down vapor and dilute.

Unusual Fire and Explosion Hazards
• Outdoors, ammonia is not generally a fire hazard. Indoors, in confined areas, ammonia may be a fire hazard, especially if oil and other combustible materials are present. Combustion may form toxic nitrogen oxides.
• If relief valves are inoperative, heat exposed storage containers may become explosion hazards due to over pressurization.

It does appear that, alone, out in the open, anhydrous ammonia does not present an immediate fire or explosion hazard. However, it seems likely that in this case, it was stored in pressurized tanks (to keep it liquid). Therefore, a fire around these tanks could very well result in an explosion hazard, as highlighted in the bottom section. Of additional interest are the flammable limits in air: this represents the air-to-fuel mixing ratio that is needed for combustion to occur. Recall that above, when pressure is suddenly removed, the liquid can ‘flash’ to a gas and expand out suddenly. One of the side effects of this expansion is a mixing of air and ammonia. Once the lower flammable limit is reached, if a source of ignition is present, you have the potential for a very messy situation.

Of course, this assumes as well that temperatures in the enclosed containers had not reached the point of decomposition:

Hazardous Decomposition Products
Anhydrous ammonia decomposes to hydrogen and nitrogen gases above 450°C (842°F). Decomposition temperatures may be lowered by contact with certain metals, such as iron, nickel and zinc and by catalytic surfaces such as porcelain and pumice.

It seems pretty clear that, although certainly not as dangerous as its better known cousin ammonium nitrate, anhydrous ammonia still has a substantial ability to wreak havoc in a worst case scenario. Which is exactly why this plant was inspected all of once in at least the past 7 years:

There has not been a complaint about the plant since 2006, and therefore it has not been inspected, Covar said.

[…]

Covar said that the facility has been in existence since 1962 and was grandfathered “up to a certain point. I believe in 2004 they were supposed to come in and get reauthorized. They failed to do so.”

Covar said he didn’t know why the facility didn’t get reauthorized. But in resolving the 2006 complaint, he said, “we got them into compliance.

Of course, there is no way of knowing yet if this was a freak accident or something more. There are many questions unanswered, some of which may stay that way forever. Even if we assume the fire was an unavoidable accident, though, other questions remain.

How much did the volunteer firefighters know about the risk? What kind of incident response plan did they have to deal with the specific hazards that this plant presented? Evidence certainly suggests that they were well aware of the plant:

Adair said his father, Donald Adair, has owned the grain and fertilizer distribution plant for about 7 or 8 years. About a dozen employees work there, including a volunteer firefighter, he said.

The plant closes at 5 p.m. so no employees were present when the fire broke out. He said no machinery is left on, leaving him stumped at what may have caused the initial blaze.

Reports from the press conference indicate that the time from the initial call on the fire to the explosion was 24 minutes. It may unfortunately be impossible to completely reconstruct what the firefighters did during those 24 minutes, but I would like to hope they were at least aware of the hazard involved, even if so many of the people seemed unaware.

The proximity of schools, homes, and other facilities to the plant also raises other questions: what type of zoning laws exist in this area, if any? What order were things built? How aware were people of the hazards? At which point is it acceptable for the government to step in and notify residents of the hazards nearby?

The cause of the fire is almost academic in the light of these questions. An explosion that damages and destroys property is devastating but rarely a tragedy. An explosion that takes lives, especially through ignorance (willful or not) of the hazards involved - that’s a tragedy.