The Chemistry behind the Tianjin Explosions

Artwork by Liu Bolin: “Tianjin Explosions”

About the artist: Liu Bolin imbeds himself and others into the photograph, declaring their position as individuals within the catastrophic incident, thus calling viewer’s attention to the aftermath and investigation of the disaster. Through recreating the imagery of the damage and devastation caused by the explosion, the project is Liu’s attempt to reveal social issues in China, as well as to reflect on the complex relationship between the past and the present, the reality and the illusion, as well as individuality and society. Visit Liu Bolin’s gallery page here.

As a Chemistry teacher, my initial reaction to the enormous explosions at a hazardous chemicals storage facility in Tianjin, China this week was a need to find out what exploded and why. As soon as the news broke, I started following #Tianjin on Twitter and getting alerts from Google News. Here’s what I’ve learned about the Chemistry behind these two fatal blasts. We know there were several dangerous chemicals on site. We also know that firefighters were present at the facility putting out a fire before the first explosion. The second explosion was much larger than the first, with the two blasts measuring the equivalent of 3 and 21 tons of TNT, respectively. The second, larger blast was so powerful that it caused a magnitude 2.9 earthquake in the surrounding area. For a surface explosion to cause a measurable earthquake is rare.

Here’s my understanding of what happened.

Stage 1: Fire

An unknown substance caught fire inside one of the storage containers at the facility. Firefighters arrived at the scene to douse the flames with water.

Stage 2: Water touches calcium carbide, producing acetylene gas

CaC₂(s) + 2H₂O(l) → Ca(OH)₂(s) + C₂H₂(g) ΔH = -127.7 kJ/mol

Calcium carbide, CaC₂(s), is an unstable compound that’s used in the production of acetylene (ethyne) and also in steelmaking. When water (or moist air) touches calcium carbide, it fizzes gently, releasing acetylene gas, C₂H₂(g), which, when mixed appropriately with air, explodes upon ignition. The reaction above is only slightly exothermic, and the ethyne gas released is colourless and odourless: it’s possible that the firefighters didn’t even notice that the gas was being produced.

Stage 3: Flames ignite the acetylene gas, causing the first explosion

After the ethyne had mixed sufficiently with the surrounding air, one part of this explosive gas mixture was ignited by the pre-existing flames, causing the first explosion.

C₂H₂(g) + 5/2O₂(g) → 2CO₂(g) + H₂O(g) ΔH = -1299 kJ/mol

Eyewitness reports have estimated this first explosion to be equivalent to 3 tons of TNT, which equates to 12.5 million kilojoules of energy. Using n = E/ΔH, we find that around 9662 moles of ethyne appears to have exploded. Using V = n×V_M, we can calculate that at 25°C and 1 atm of pressure, that explosive gas would have occupied a volume of 236719 litres. Using r = (3V÷4π)^1/3, we can approximate the ethyne gas to have occupied a sphere 76 metres in diameter, which is (very approximately) consistent with what we’ve seen in the video footage.

Interestingly, we can do a simple stoichiometric calculation using m = n×M_r and calculate the initial mass of calcium carbide that decomposed: 9662 × 64.1 = 619 kilograms. At a density of 2.22 g/cm³, those 619 kilograms would have occupied 279 litres in powdered form: this is about the same size as three large luggage cases.

A quick search on Chinese wholesale directory Alibaba.com shows that very few companies offer calcium carbide in such small quantities, which might help narrow down which company was responsible. Interestingly, the raw material for that first explosion was worth a mere US$400 at 2015 wholesale prices… but the consequential damage was far more costly.

Stage 4: High temperatures caused nearby ammonium nitrate to detonate at >240°C, causing the second explosion

Temperatures of over 3000°C were generated by the combustion of the ethyne in stage 3. The immense heat from that initial fireball heated the surrounding containers to above 240°C, which initiated a runaway decomposition reaction of ammonium nitrate, NH₄NO₃(s), which was stored nearby. The reaction is shown below.

NH₄NO₃(s) → N₂(g) + 2H₂O(g) + 1/2O₂(g)  (ΔH uncertain)

The enthalpy change for the reaction above wasn’t easy to find, but this book by Sam Mannan claims it to be 0.175 million kilocalories per tonne, or 732,000 kilojoules per tonne. Analysis of the video recordings have estimated this second explosion to be around 21 tons of TNT equivalent, which equates to 88 million kilojoules of energy. Using calorimetry formula m = E/heat of combustion, we can estimate the mass of ammonium nitrate in this second explosion to be 8300 tonnes, which seems extremely high: four times as big as the Texas City Disaster of 1947. Either the second Tianjin explosion was the biggest ammonium nitrate disaster in history, or I’ve made an error in this part of the analysis. Let’s wait for more information and see.

The above reaction has caused hundreds of fatalities worldwide in the last 100 years. Smaller incidents occur every 2 or 3 years worldwide, around half of which are fatal. Gases are produced under extreme temperature and pressure, which expand outwards and destroy almost everything in their path. Ammonium nitrate is supposed to be handled and stored under very strict government regulations. These rules aren’t always followed (or understood) in rapidly-developing countries such as China.

The Aftermath

The products of these two explosions are calcium hydroxide, carbon dioxide, water vapour, nitrogen and oxygen, which pose zero risk to nearby residents. However, the main concern now is that other (non-flammable) hazardous chemicals such as sodium cyanide, NaCN(s), might have been tossed into the air following the first two explosions. Residents living within 3 kilometres of the blast site have been evacuated as a precaution.

Fortunately, satellite imagery shows that almost all of the smoke plume was blown eastwards over the ocean, and not back westwards and back onto the city. We’ll get a clearer picture when China’s chemical experts report their findings in the next few days or weeks.